THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES GIFT OF Join S.Prell GAS ENGINE THEORY AND DESIGN A. C. MEHRTENS, M.E. INSTRUCTOR IN MECHANICAL ENGINEERING ENGINEERING SCHOOL MICHIGAN AGRICULTURAL COLLEGE FIRST EDITION FIRST THOUSAND NEW YORK JOHN WILEY & SONS LONDON: CHAPMAN & HALL, LIMITED 1909 Copyright, 19(19 BY A. C. MEHRTENS Electrotyped and Printed by the Publishers Printing Co., New York, U. S. A. Engineering Library TJ 110 CONTENTS * 3 CHAPTER PAR. I. GENERAL PRINCIPLES OF OPERATION l-7a II. HISTORICAL 8-14a III. APPLICATIONS OF THE GAS ENGINE 15-15a IV. HEAT THERMODYNAMICS 16-336 V. COMBUSTION 34-55 VI. FUELS 56-72 VII. LAWS OF GASES 73-91 VIII. GAS-ENGINE EFFICIENCY 92-97 IX. EXPLOSIVE MIXTURES 98-104 X. MIXING VALVES AND CARBURETERS 105-107 XI. GOVERNING 108-112 XII. IGNITION 113-123 XIII. COOLING 124-130 XIV. EXHAUST 131-134 XV. SELECTION OF TYPE 135-140 XVI. DETERMINATION OF THE PRINCIPAL DIMENSIONS.. . . 141-147 XVII. FORCES ACTING IN THE GAS ENGINE 148-158 XVIII. DESIGN AND DIMENSIONS OF PARTS 159-183 XIX. GAS-ENGINE MANIPULATION 184-188 XX. TESTING 189-200 XXI. DESIGNS . . . 201-202 TABLES N} PAR. 33a 60a 71a 91 146a ^ 183 ' 147 S. VOLUME, PRESSURE AND TEMPERATURE CURVES 149 1. PHYSICAL PROPERTIES OF MATERIALS 2. PETROLEUM DISTILLATES 3. PROPERTIES OF FUEL GASES 4. VOLUMES AND SPECIFIC HEATS OF GASES. 5. EFFICIENCIES AT DIFFERENT ALTITUDES. . , 6. STRENGTH OF MATERIALS 7. HEAT AND POWER UNITS. . . ...... 713773 Engineering Library PRELIMINARY IT has been the aim of the author to prepare a book for all who are interested in gas engines students, draughts- men, engineers, as well as the men who operate gas engines of any kind, and wish to become better acquainted with the theory and the why of many things. The book should be of special interest to the technical student, and was, in fact, first prepared for the engineering classes at the Michigan Agricultural College, since no suit- able text-book could be found. The reading-matter throughout has been arranged care- fully and with a definite object in view. The large number of figures illustrating the text have been made as simple as possible. It has also been the aim of the author to make the treatment clear and concise, and for this reason every paragraph should be studied not merely read over. It is hoped that this book will enable every earnest stu- dent to acquire a foundation upon which he may eventually build a broad and comprehensive knowledge of the subject. Acknowledgment is due Professor L. L. Appleyard for his kindly criticism and assistance in reading the proofs. GAS-ENGINE THEORY AND DESIGN CHAPTER I (SKXKRAL PRINCIPLES OF OPERATION 1. THE HEAT ENGINE may be defined as a machine which converts heat into mechanical energy. The heat sets the engine in motion and the engine, by virtue of this motion, can drive machinery to which it is connected. The two principal classes of heat engine are the Steam Engine and the Gas Engine. 2. THE GAS ENGINE, or internal-combustion engine, is a machine in which the fuel is burned directly in the engine cylinder. Every internal-combustion engine is a gas en- gine, no matter whether the fuel applied is a gas or a liquid, since, in the act of burning, a liquid fuel is first converted into a gas. 3. A GENERAL CLASSIFICATION of gas engines is as follows : (a) According to the fuel : Gas engines, operating on fuel in the form of gas; oil engines, engines operating on fuel in the form of oils heavier than gasolene such as kerosene, fuel oil, crude oil; gasolene engines, engines operating on gasolene; alcohol engines, engines operating on alcohol. (6) According to the R. P.M. (revolutions per minute): High speed, for example, an automobile engine running at i GAS-ENGINE THEORY AND DESIGN FIG. 1. 1,2(M) R.P.M.; slow speed, a stationary engine running at KM) R.P.M. The piston speed in feet per minute may be the same for both a high-speed and a slow-speed engine. This can readily be seen by assuming that the automobile engine mentioned above has a 4" stroke and the stationary en- gine a 48" stroke. (c) According to the strokes required to complete a working cycle: Two-stroke cycle; four- stroke cycle. Gas engines can also be classified according to the me- chanical construction as single-cylinder, multiple-cylinder, horizontal, vertical, etc. 4. A CYCLE is a complete succession of events, or chain of events. The following paragraphs will make clear the term " cycle" as applied in this chapter. 5. GENERAL PRINCIPLES OF OPERATION. In Fig. 1 we have a cylinder C, a piston P which can readily lx i pushed back and forth, but fits so closely to the cylinder that air cannot leak past. The space A is filled with a mixture of air and fuel gas, or air and the vapor of some liquid fuel, such as gasolene. This mixture is at atmospheric pressure and when ignited the re- sulting combustion raises the tem- perature, and consequently the pressure in A, causing the mixture to expand and push the piston out to the position indicated by the dotted lines, or until the pressure again drops to atmospheric. Now, it has been found that if the mixture of air and fuel is first compressed by moving the piston in, as shown GENERAL PRINCIPLES OF OPERATION W in Fig. 2, and then ignited, the piston will be driven out with much greater force than before and more work is gotten out of the same amount of fuel. All gas engines to-day work on the compression principle. The matters of compression, ignition, combustion, etc., will be consid- ered more fully later on. 6. THE TWO-STROKE CYCLE. The next four figures illus- trate the operation of the two-cycle engine. Fig. 3 shows the piston P ready to start on its down stroke. The combustion chamber S is filled with a compressed mixture of air and com- bustible which has been compressed by the preced- ing up stroke of the pis- ton. This mixture is now ignited (usually by means of an electric spark) and the expansion of the burn- ing gases drives the piston down. As P nears the end of its down stroke it un- covers the exhaust port E through which the burnt gases flow from the cylin- der. A little later the inlet port 7, leading from the crank chamber to the cylinder, is uncovered by P. The crank chamber C is air-tight, and in moving down P covers up the air-inlet port A and compresses the combustible charge in C. When / is uncovered by P this compressed charge flows from C into the cylinder, partly filling it with a fresh combustible charge and helping to expel some of the remaining burnt gases. The baffle plate B deflects the FIG. 3. 4 GAS-ENGINE THEORY AND DESIGN incoming charge, as shown by the arrows, preventing it from passing directly across the cylinder and out through the exhaust port. Fig. 4 shows the piston ready to start on its up stroke. In moving up P covers A, E, and /, and compresses the charge in the cylinder. As soon as A is uncovered the motion of P sucks a charge of air and combus- tible into C and this is again compressed on the next down stroke. The air and combustible must be thoroughly mixed be- fore passing into C. \Yhen P again reaches the posi- tion shown in Fig. 3 it has completed a working cycle in two strokes, or one rev- olution. Ignition now oc- curs and the foregoing operations are repeated. The piston is kept in mo- tion during the time it receives no working impulse by means of a heavy fly-wheel. The circle in Fig. "> represents the travel of the crank-pin D, shows what takes place above the piston and approx- imately what part of a revolution is required for the various operations. Fig. 6 shows what takes place in the crank chamber during one revolution. In large two-cycle gas engines the preliminary compres- sion of air and fuel takes place in separate pumps Instead of in a closed crank chamber. FIG. 4. GENERAL PRINCIPLES OF OPERATION The space W is filled with water which circulates through the jacket and absorbs and carries away surplus heat so as to prevent the cylinder from overheating, since the tem- FIG. 5. perature of combustion is veiy high. The water is usually circulated by means of a pump. A two-cycle engine of the type illustrated can run in either direction. The direction of rotation is indicated by the arrows. 7. THE FOUR-STROKE CYCLE. The next six figures illustrate the operation of the four-cycle engine. Fig. 7 shows the piston ready to start on its down stroke. The air-inlet valve V is open, the exhaust valve V is closed; as the piston moves down it draws a charge of air and combustible into the cylinder. Fig. 8 shows P ready to start on its up stroke. Both valves are closed and the charge in S is compressed as P moves up. Fig. 9 shows P ready to start on its second down stroke. Both valves are FIG. 7. 6 GAS-ENGINE THEORY AND DESIGN closed, the compressed charge is ignited, and the resulting expansion drives P down. Fig. 10 shows P ready to start on its second up stroke. V is closed, V is open, and as P moves up it drives the burnt gases from the cylinder. One working cycle has now been completed in four piston strokes, or two revolutions, and this cycle is repeated in- definitely until the engine stops running. FIG. 8. FIG. 9. FIG. 10. Figs. 11 and 12 show approximately during what periods of the crank-pin travel these operations take place. The four-cycle engine will run in one direction only with the valve gearing arranged in the ordinary manner. la. RESUME. The student has learned from the fore- going that: In the two-cycle engine there is one power impulse for every revolution of the crank-pin ; in the four- cycle engine there is one power impulse for every two revolutions of the crank-pin ; the operation of the gas engine consists of; GENERAL PRINCIPLES OF OPERATION (a) Causing a charge of air and combustible to flow into the engine cylinder; (6) Compressing this charge; (c) Igniting the compressed charge and driving the piston out by means of the expansion of the highly heated gases; (d) Discharging the burnt gases from the cylinder. FIG. 11. FIG. 12. Also, that a gas engine must be provided with means for mixing the air and combustible before the charge passes into the cylinder; That means must be provided for igniting the combus- tible charge at the end of the compression stroke; That the cylinder wall must be water-cooled to prevent overheating ; That a fly-wheel must be provided to keep the engine running during the idle strokes and furnish the necessary power for compressing the combustible charge. In the four-cycle engine the valves are opened and closed at the proper time by springs and cams operated by gearing driven from the crank-shaft. CHAPTER II HISTORICAL 8. Very little is known of early attempts to produce a heat engine, and it is doubtful if such machines were ever constructed in ancient times. Several nations attained a high degree of civilization many centuries ago, and now and then the investigator comes across a toy set in motion by steam or hot air, but there seems to be no record of a machine powerful enough to do work. In 1774 James Watt, in England, completed the first commercially successful steam engine. This was applied to the driving of machin- ery, pumping, etc. After a while locomotives were devel- oped and the steam engine was applied to navigation. It early occurred to investigators that if fuel could be burned directly in the engine cylinder a great deal of the heat loss, which occurs in the roundabout way of applying heat in the steam engine, could be avoided. The simplicity and economy of a first-class gas engine as compared to the steam-power plant bears out the correctness of these early ideas. The first successful type of internal-combustion engine was the gas engine proper, i.e., one using gas as fuel. The manufacture of gas from coal for illuminating purposes had become fairly well established by the middle of the nineteenth century. In 1804 the Lyceum Theatre, in London, was illuminated by gas, and in 1810 the first public gas-lighting plant was installed. In 1823 gas-lighting was introduced into New York City. As time went on improve- ments in the manner of manufacturing gas from coal, cul- HISTORICAL 9 minating in the gas-producer of to-day, were made. In 1859 petroleum was discovered in the United States in great quantities, and conditions became ripe for the devel- opment of the internal-combustion engine, since suitable fuels (gas and oil) were now at hand. The great advance made in America in refining mineral oil stirred up Russia, the next largest producer of petroleum in the world, and American and Russian oils were soon carried to all parts of the globe. All the various attempts to produce gas en- gines of which we have a record will not be given here, but only some of the most important investigators mentioned. 9. THE LEXOIR CYCLE. In 1860 some work was done on the gas engine by Lenoir in France. In the Lenoir engine air and gas were drawn into the cylinder during a part of the suction stroke, the inlet valve was then closed and the mixture fired, the resulting expansion of the burning gases driving the piston out during the remainder of the stroke. During the return stroke the cylinder was cleared of the burnt gases. This was a non-compression engine. 10. THE BRAYTON CYCLE.- In 1873 Brayton, in America, brought out an engine in which combustion took place without a rise in pressure. Air and gas were compressed by means of a pump into a reservoir which communicated with the engine cylinder. During about one-half of the out stroke of the piston the charge was allowed to flow into the engine cylinder from the reservoir, and then it was ignited. Wire netting prevented the flame from going back into the reservoir. At about half stroke the reservoir was cut off and the stroke completed by the expansive working of the gas. 1 1 . THE BEAU DE ROCHAS CYCLE. The compression cycle, on which the present-day engines operate, was first sug- gested by Beau de Rochas, in France, in 1862. In 1877, after many years of hard work, Dr. N. A. Otto, in Germany, brought out the first commercially successful gas engine 10 GAS-ENGINE THEORY AND DESIGN operating on the Beau de Rochas cycle. This engine was a great improvement upon the attempts of his predecessors, and the development of the gas engine from now on was rapid. This cycle (see "Two-" and "Four-Cycle" in the preceding chapter) is often called the "Otto Cycle." 12. In 1880 Dugald Clerk, in England, brought out the first successful "two-cycle" engine operating on the Beau de Rochas cycle. Engines working on fuels other tnan gas now followed. 13. In 1884 Daimler, in Germany, brought out a light- weight, high-speed oil engine. In 1895 he introduced a perfected light-weight, high-speed gasolene engine suitable for automobiles and motor-boats. This engine was quickly adopted by the European manufacturers, especially in France, and gave a great impetus to the development of automobiling and motor-boating in fact, made the pres- ent development of these industries possible. The im- provements made in carbureter design and electric ignition were also important factors in this connection. 14. THE DIESEL CYCLE. Another great step in the development of the gas engine was made in 1897, when Rudolf Diesel, a German engineer, brought out an internal- combustion engine which surpassed all previous heat engines in the matter of fuel economy, chiefly by means of very high compression. The Diesel cycle is really the ordinary compression cycle, but differs from it in the manner of handling the fuel. A charge of air alone is compressed to 500 or 600 Ibs. per square inch. The fuel (oil) is injected at the end of the compression stroke by means of air compressed to about 800 Ibs. in a small two-stage compressor. The fuel is in- jected from the time the piston has passed the dead centre until it has completed about one-tenth of the expansion stroke so that there is a gradual combustion, and the com- HISTORICAL 11 bustion does not increase the pressure in the cylinder, but the pressure drops throughout the expansion stroke. The temperature at the completion of the compression stroke is 1000 or more, so that the oil is burned directly it is in- jected and without the aid of any ignition apparatus. When handled in this manner petroleum and its distillates can be burned completely without carbonizing. 14a. More work has been done on large gas engines in Europe than here, since fuel economy is of greater im- portance there. The first 1000-H.-P. gas engine was built in 1898 by the J. Cockerill Co., of Seraing, Belgium. At that time the building of such a large gas engine was looked upon as a very doubtful venture. To-day gas engines are in operation developing over 5000 H.-P. in single units. One great factor in the development of the gas engine has been the utilization of blast-furnace gas. It was found that, by increasing the compression, gases that are very poor in quality could be burned in the gas engine, and mil- lions of cubic feet of blast furnace and coke-oven gas which were formerly wasted at the great steel works are now converted into power by means of the gas engine. Automobiles and motor-boats have practically been developed during the past twelve years, and large gas engines during the past nine years. The smaller com- mercial gas engine is barely thirty years old. The growth of the gas-engine industry has increased at a tremendous pace in the past few years, and a few figures may prove interesting: In 1881 the gas engines in 200 European cities aggregated a total of 2,442 H.-P., and in 1902 this had reached a total of 123,000 H.-P. The total of large gas engines in operation in Germany at the present time ex- ceeds 400,000 H.-P. The present gas-engine power of the world has been estimated as about 4,000,000 H.-P. CHAPTER III APPLICATIONS OF THE GAS ENGINE 1"). A brief review of the principal applications and of some of the large gas-power installations in this country may prove interesting. In 1896 the Westinghouse Machine Company put on the market a vertical multiple-cylinder gas engine suitable for driving generators and general power work. Up to the present time this company has installed gas engines aggre- gating about 100,000 H.-P. The DC La Vergne Machine Company has installed at the Lackawanna Steel Company's plant two-cycle Koerting gas engines operating on blast-furnace gas and aggregating 43,200 H.-P. This company has also installed at the Bald- win Locomotive Works oil engines, operating on cheap fuel oils, aggregating 5,315 H.-P. The Allis-Chalmers Company has under construction at the time of writing the following large units: 25 2,000 K.W. units direct-connected to generators (about 70,000 H.-P.); 12 3,000-H.-P. blowers; 9 1,000 K.W. units con- nected to generators. A number of the above units are for the Indiana Steel Company, which will install in its plant at Gary, Indiana, gas engines aggregating over 100,000 H.-P. Gas power will be used for driving generators and the electric current will be used for driving rolling-mill and other machinery. Gas power will also be used for operating the blowers. The San Mateo Power Company, at its Martin Station, 12 '/ 14 r,AS-KNY5IXE THEORY AND DESIGN California, has in operation three gas-power units driving generators, and two more units will be installed. These engines were built by the Snow Steam Pump Works, operate on gas made from crude oil by the Lowe process, and have a maximum capacity of about 5,300 H.-P. each. Besides the foregoing there are, of course, a great num- IXT of smaller installations of gas engines driving generators, pumps, mill and factory machinery, etc. For small station- ary power purposes the gas engine has the field to itself. Not so many years ago there were practically no auto- mobiles in the United States. According to statistics that have been compiled the total value of pleasure automobiles which have been produced in the United States in 1907 was $105,669,572. The capital invested that year in the industry was 94,200,000, and the number of employees 58,000. It is estimated that the number of employees in factories turning out automobile accessories was 29,000, and the capital invested 36,700,000. There were 2,151 sales offices and garages which employed 21,500 persons and the capital invested in these garages w r as 57,500,000. Over 40,000 automobiles have been registered in New York State, and of these about 25,000 are in New York City. The motor-cycle industry is rapidly growing. The fastest mile ever made by man, viz., in 26f seconds, was done on a racing motor-cycle. . In this country the number of business and pleasure boats propelled by gasolene and oil engines has been placed at 400,000, and these are increasing at the rate of 75,000 a year. The value of the marine gas engine has been recognized by several governments, and the use of motor-boats for general government and naval service is constantly increasing. The Imperial Russian Marine has in service ten torpedo boats,each propelled by gasolene motors of 600H.-P. These torpedo boats are capable of a speed of 21 knots (24.2 1 16 GAS-ENGINE THEORY AND DESIGN miles) an hour. Several governments have in operation gasolene-propelled submarine boats. In 190.") the Gregory, a motor-boat 91 feet long and equipped with two 300 H.-P. Standard engines, crossed the Atlantic the first gas-power boat to perform this feat. At the present time there arc 1 several motor-boats that can do close to 30 miles an hour. The gas engine, in connection with the gas-producer, will doubtless be extensively applied to navigation in the near future 1 . Several beginnings have already been made 1 in this direction both here and abroad. A gas-producer occupies about one-fourth of the space of a water-tube boiler, and the coal consumption is about one-third. There is consequently a great saving in coal space 1 , boiler space, and cost of running. In one instance the cost of carrying freight by a gas boat was only one-fifth of that of transport- ing it by rail. The oil engine is being applied more and more to general portable, contracting, and agricultural work. Thc> oil traction-engine is used for plowing, threshing, pumping, anel hauling the produce of the farm to the market over considerable distances. With power-driven machinery a few me 1 !! can take care of a thousand acres of wheat land. 15a. Gasolene motor-cars are being used by several rail- mads for short-run traffic. The Union Pacific has 9 of these cars in operation at the present time, and is building 22 more. One of the cars is 5.5 feet long, has a seating capacity of 75, is equipped with a 200 H.-P. 6-cylinder gasolene engine, and can easily make 65 miles an hour. The car can be starteel and stopped quicker than an electric car. The cost of oper- ation is below that of either steam or electric power. Gasolene engines weighing about two and a half pounds per H.-P. have been constructed for flying-machine and racing purposes. APPLICATIONS OF THE GAS ENGINE 17 18 GAS-ENGINE THEORY AND DESIGN APPLICATIONS OF THE GAS ENGINE 19 The difference in design between a high-speed engine and a slow-speed stationary engine is strikingly shown by their respective weights. Many stationary gas engines average 500 Ibs. per H.-P., and even more, while an auto- FIG. 17. Warren Gas Engine. 20 GAS-ENGINE THEORY AND DESIGN mobile engine averages 10 Ibs. per H.P. A 50-H.-P. engine of the type first mentioned would weigh 25,000 Ibs., while the automobile engine would weigh 500 Ibs. A word might here be said about the cost of operation. This depends largely upon local conditions which determine the kind and cost of the fuel to be used. The author has in mind an instance where some oil engines consumed two- thirds of a pint of crude oil per H.-P. hour. The oil was supplied at a net cost of 2 cents per gallon. The cost of fuel per H.-P. hour was therefore one-sixth of a cent. In conclusion, it might be mentioned that there are at the present time over 300 builders of gas engines in this country. Fig. 13 shows an arrangement which is now the standard for the large Allis-Chalmers gas engines. The engine illus- trated is a four-cycle doublet-acting twin tandem machine driving a 2,500 K.W. generator. The maximum rating of the engine on producer gas is 4,500 H.-P., running at 83 R.P.M. The floor space occupied is 69 ft. by 35 ft. The cylinder dimensions are 44" x 54" stroke. The diameter of the crank-shaft is 30", and the crank-pin 20". The length of the main bearing is 54". The flywheel is 23 ft. in diam- eter, and the weight varies according to conditions. The weight of the main frame is 90 tons. Figs. 15, 16, and 17 show clearly the constructive details of the Warren heavy-duty tandem gas engine built by the Struthers Wells Co. Fig. 18 is a photographic reproduction of a 90-H.-P. ver- tical engine and 100-H.-P. producer built by this company. The general over-all dimensions of the engine are: Height, 9'0"; length HXO"; width 7'0". The producer occupies a floor space about 9' square, and the highest point is 14' from the ground. Some tests on Warren engines are given under "Testing." CHAPTER IV HKAT. THERMODYNAMICS 16. Heat may be defined as a form of energy which enables us to do work. 17. THEORY OF HKAT. The sensation of heat is-supposed to be caused by the rapid vibration of the molecules of a body. The hotter a body the more rapid these vibrations, and the colder the body the less rapid the vibrations. IS. EXPAXSIOX. The terms "heat" and "expansion" are practically synonymous. That heat will expand the things that are heated is one of the most patent facts in our every-day life. When we heat one end of an iron bar to a red, or white, heat, the hot end is larger than the cold one and the difference in size is plainly visible. When water is heated it expands. When air is heated it expands and occupies more space than before, weighing less, of course, than the same bulk of cold air. We here have examples of a solid, a liquid, and a gas expanding through heating, and this increase in volume due to heating is taken advantage of in the heat engine to convert heat energy into mechanical energy it is the fundamental principle of the working of all heat engines. 10. TEMPERATURE. When one body is at a higher tem- perature than another, i.e., when it is warmer, heat tends to flow from the warm body to the cold one until both are at the same temperature. When hot water, for example, is poured into a vessel containing cold water an interchange of heat takes place, the hot water loses some of its heat 22 HEAT. THERMODYNAMICS 23 and the cold water absorbs it, and this interchange goes on until all the water is at the same temperature. We find this tendency toward an equilibrium of heat on every hand, it is universal, and it is of the greatest importance in the working of the heat engine. Those materials that feel cold to the touch, like metals, simply conduct the heat away more rapidly than other materials. Temperature is an indication of the intensity and not of the amount of heat. A boiler may contain water at a temperature of 200, and the temperature of a blow-pipe flame may be 2000, but the amount of heat in the boiler may be several thousand times that of the blow-pipe flame. Temperature depends upon the rate of combustion and not upon the total amount of heat. 20. SOURCES OF HEAT. Heat is obtained in various ways from the sun, by the combustion of fuels, by means of the electric current, friction, percussion, etc. The heat used in the various heat engines is invariably obtained from the combustion of a fuel. 21. TRANSFER OF HEAT. Heat is transferred by con- duction, radiation, and convection. (a) Conduction is the transfer of heat in a body from a higher to a lower temperature. When one end of an iron bar is heated some of the heat is transferred along the bar by conduction. (6) Radiation is the transfer of heat from a hot body to a colder one across an intervening space. The sun heating the earth is an example of this. The intervening medium may not necessarily become heated. (c) Convection is the transfer of heat by the motion of the heated matter. Liquids and gases heated from below are examples of this. The hot currents of fluid rise and warm the surrounding fluid, the colder fluid sinks to the bottom of the containing vessel, and upon becoming 24 GAS-ENGINE THEORY AND DESIGN heated rises to the top. When heated from above liquids and gases are very poor conductors and cannot heat by convection. 22. RADIATION OF HEAT. The intensity of heat radiated from any source varies, as: The temperature of the source ; Inversely, as the square of the distance from the source ; Changes, as the inclination of rays to the surface changes. A polished surface will give out less heat, and absorb less heat, than a non-polished surface. It is sometimes very important that the radiation of heat be prevented as much as possible, as in the case of a steam- engine cylinder, and the cylinder is then covered with some material that is a poor conductor of heat. Among the best materials in common use for this purpose are mineral wool and asbestos. Air is a poor conductor of heat when it has no chance to circulate, and for this reason a material of a woolly nature will constitute a good non- conductor, provided it will withstand the required tem- perature. 23. HEAT UXIT. A heat unit is the amount of heat required to raise the temperature of a pound of water 1. This is termed a British Thermal Unit, or B.T.U. 24. MECHANICAL EQUIVALENT OF HEAT. Joule's experi- ments (1843-50) proved that heat and mechanical energy are mutually convertible and that there is a constant and definite relation between the amount of work that can be done by a certain amount of heat, and vice versa. Joule placed some paddles in a vessel filled with water in such a manner that the falling of a weight revolved the paddles and the friction caused by the motion of the paddles raised the temperature of the water. From these and later ex- HEAT. THERMODYNAMICS 25 periments the value of the B.T.U. was established which is now generally accepted as 778 foot-pounds (a foot- pound is the work done in lifting vertically 1 pound 1 foot), i.e., it requires 778 foot-pounds of work to raise the tem- perature of 1 pound of water 1. 25. SENSIBLE AND INSENSIBLE HEAT. Sensible heat is that which can be perceived by the senses. Insensible heat is that which cannot be perceived by the senses. The latent heat of water is an example of the latter. 26. SPECIFIC HEAT. Different bodies have different capacities for storing heat. Specific heat is the amount of heat required to raise the temperature of a unit mass of a body 1 as compared with some standard. The amount of heat required to raise the temperature of 1 pound of water 1 from 32 F. (the freezing-point of water) is taken as the standard. For high temperatures the specific heats are somewhat greater than the values given in Tables I and IV. 27. LATENT HEAT. Latent heat is the extra amount of heat necessary to force the molecules of a body farther apart, and overcome the forces of cohesion, in order to change the state of the body. 28. LATENT HEAT OF FUSION. This is the extra amount of heat necessary to convert a solid into a liquid. 29. LATENT HEAT OF VAPORIZATION. This is the extra amount of heat necessary to convert a liquid into a gas. Water furnishes a good example of the three states of matter solid, liquid, and gaseous, and how latent heat affects these states. Ice melts at 32 and atmospheric pressure. The mole- cules of the ice are held together by a certain force and a certain amount of heat is necessary to overcome this force (142 B.T.U. per pound) and convert the ice into water. This heat is called the latent heat of fusion and apparently 26 GAS-ENGINE THEORY AND DESIGN disappears as the temperature of the resulting water will be 32, and it is therefore insensible heat since it cannot be perceived by the senses. Upon freezing this heat is again restored, or liberated. When all the ice has been melted, if the heating is continued, the temperature of the water will rise until it reaches 212 (180 B.T.U. are required to raise the temperature of one pound of water from 32 to 212), when a large amount of heat (966 B.T.U. per pound) is necessary to convert the water into steam, viz., a gas. The heat which here apparently disappears is the latent heat of vaporization, and the temperature of the steam does not rise above 212, at atmospheric pressure, until all the water has been converted into steam. When the steam is condensed this latent heat is restored. The latent heats vary with the pressure. 30. THERMOMETRY. The thermometer is an instrument for measuring differences in temperature. The Fahrenheit (abbreviated F.) thermometer is gener- ally used in English-speaking countries, while the Centi- grade (C.) thermometer is used in the countries which have adopted the metric system.* On the Fahrenheit thermometer the freezing-point of water is taken at 32, and the boiling-point at 212, the intervening space being divided into 180 equal parts, or degrees. Water is taken as the standard for the sake of convenience, and at a pressure of one atmosphere at the sea level. On the Centigrade thermometer the freezing-point of water is taken at and the boiling-point at 100. Below are given examples of the method of converting Fahrenheit readings into Centigrade, and vice versa: Q. What temperature C. corresponds to 152 F.? A. (152 32) x = 66f C. * The Fahrenheit scale is used in this book. HEAT. THERMODYNAMICS 27 Q. What temperature F. corresponds to 90 C.? A. (90 X f ) + 32 = 194 F. The substances commonly used in thermometers mer- cury and alcohol have a limited range, freezing at low and vaporizing at high temperatures, so that where a greater range is necessary other instruments and methods must be employed. 31. ABSOLUTE TEMPERATURE AND ABSOLUTE ZERO. In the air thermometer the temperature is indicated by a drop of mercury resting on a column of air. If this air is exposed to atmospheric pressure at the freezing-point, and the position of the mercury marked, and then the posi- tion of the mercury is marked at the boiling-point of water, it will be found that the expansion of the original column of air amounts to 36.65 per cent. This gives an expansion for each degree of 0.3665/180, or 0.002036. If we now assume that the volume of air will decrease I/. 002036 for each degree F. in cooling, i.e., in the same ratio, then a temperature must finally be reached where a further re- duction in volume by a reduction of temperature is im- possible. This is called the absolute zero, and is 491.13 below the melting-point of ice, or 459.13 below F. This has been computed for a perfect gas as 492.66 F., or 273.7 C. This absolute temperature, as will be seen later, is of great importance in thermodynamic calculations. 32. PYROMETRY. Pyrometers are appliances for measur- ing, or observing, high temperatures. One form suitable for determining the temperature of exhaust gases contains compressed nitrogen in a tube above mercury. This is .suitable for temperatures up to 950 F. 33. THERMODYNAMICS. Thermodynamics is the name given to the science of heat energy. The two laws given below are used constantly in heat calculations. First Law. The first law of thermodynamics is that heat 28 GAS-ENGINE THEORY AND DESIGN energy and mechanical energy arc mutually convertible in a definite ratio, viz.: 1 heat unit (B.T.U.) equals 778 foot-pounds. Second Law. The second law may be expressed alge- braically as follows: Q,-Q 2 T.-T, where Q, and T\ equal the quantity and absolute tem- perature of the heat received, and Q 2 and T 2 equal the quantity and absolute temperature of the heat rejected. The heat used in a heat engine is the difference between the heat received and the heat rejected, and this, divided by the heat received, equals the thermodynamic efficiency of the machine. The thermal efficiency is therefore pro- portional to the absolute temperatures and expresses the percentage of the heat used. 33a. TABLK I PHYSICAL PROPERTIES OF .MATERIALS Solids Material Specific Gravity Specific Heat Weight per Cubic Foot Coefficient of Linear Expansion Silver 100 Thermal conduc- tivity Tempera- ture of Fusion F. Aluminum 2 . 56 0.2143 160 0.0000130 31.3 1220 Brass 8.32 0.0939 520 '0.00001037 1900 Bronze 8.83 550 0.00000986 1700 Copper 8.82 0.0951 557 0.00000955 73.6 2000 Iron, cast 7.20 ! 0.1298 450 0.00000617 11.9 2192 Iron, wrought .. 7.7 i 0.1138 480 0.00000686 8.5 2912 Lead 11.37 0.0314 710 0.0000162 8.5 626 Steel, soft 7.8 i 0.1165 490 0.00000599 11.6 2520 Steel, hard . 7.8 ; 0.1175 490 0.00000702 2570 Tin 7.29 0.0562 455 0.00001230 15.2 446 Zinc 7.15 0.0956 430 0.00001634 28.1 786 HEAT THERMODYNAMICS Liquids 29 Specific Gravity Specific Heat Weight per Cubic Foot Tempera- ture Fusion Tempera- ture Vapor- ization Latent Heat Va- porization Water. . . Alcohol 1.0000 0.794 1.0000 0.6200 62.4 49.6 32 -202.9 212 173 966 B.T.U. 372 336. Density = specific weight = weight of a unit volume. Vapor is a gas below the critical temperature, i.e., it can be reduced to a liquid by pressure alone. CHAPTER V COMBUSTION 34. CHEMISTRY OF COMBUSTION'. An elementary knowl- edge of chemistry is necessary for the understanding of the process of combustion. An element is a substance that cannot be separated into anything else. Example: Iron, carbon, oxygen. A compound is a substance that can be separated into elements. Example: Y\'ater can be separated into oxygen and hydrogen. An atom is the smallest particle of matter that can exist. A molecule is the smallest quantity into which a mass of matter can be divided without changing its chemical nature. Every molecule consists of two or more atoms. A mechanical mixture is one in which substances are not chemically combined. Example: Salt water. A physical change is one in which the nature of the sub- stance is not changed. Example: Water converted into steam. A chemical change is one in which the nature of the sub- stance is changed. Example : The burning of a piece of coal. Atomic weight is the weight of an atom of any element as compared with hydrogen, the lightest known element. Chemical nomenclature. Abbreviations are used in chem- istry for the names of the various elements, in writing the reactions. Some of these are given in the following para- graphs. Chemical action is most energetic between dissimilar COMBUSTION 31 substances and takes place under certain conditions only. It is very difficult to bring about chemical action by mechanical means, as it is effective at insensible distances only, and for this reason the agents usually employed are solvents and heat. Chemical combination always takes place in certain definite proportions; for example, when hydrogen and oxygen are brought together and chemical action is started by heating, 2 atoms of hydrogen will unite with 1 atom of oxygen (written 2H+0 = H,O) forming water, and the elements will not combine with each other in any other proportion. 35. COMBUSTION may be defined as the chemical com- bination of one or more elements with oxygen, taking place with sufficient rapidity to be accompanied by heat and light, Let us examine this matter of chemical action and com- bustion more closely. In order to burn a substance let us take a piece of coal for example it must be heated to a certain temperature before it will ignite. This temperature of ignition varies with different substances. When the coal is first heated gases are formed on the outside (where the heat acts first) and these gases, combining with the oxygen of the air, undergo the chemical transformation called "combustion." The heat now given off by the part of the coal that is burning will heat the rest of the coal to the temperature required for ignition, and thus the com- bustion goes on until all the coal has been consumed. In place of a piece of coal we now have a certain bulk of very hot gas and a little ash. In burning nothing is destroyed, combustion is simply a chemical change in which the burning elements enter into new combinations which we now have in the form of hot gases. These gases and the ash weigh just as much as the coal and air consumed did, nothing has been lost, but the heat stored in the coal has 32 GAS-ENGINE THEORY AND DESIGN boon liberated. Air is a mechanical mixture of oxygen and nitrogen, the proportion being about one part of the former to three parts of the latter by weight. Oxygen, as men- tioned before, possesses the property of entering into chemical combination with many substances after they have been heated to a sufficiently high temperature, and it is for this reason that air is necessary for combustion since it furnishes the required oxygen. This changing of a fuel, first into gas by raising its temperature, and then burning the gas, takes place no matter whether the fuel is a solid or a liquid. When carbon is burned completely it burns to a gas called carbon dioxide. Each atom of carbon combines with two atoms of oxygen, and the reaction is written as follows: C + 20 = C0 2 If the carbon is in combination with another element, the preliminary heating weakens the force which holds it in combination and the C and O pull together as a magnet and a piece of iron pull together. Because heating weakens the force holding the elements together it is necessary to heat most fuels before they will burn. To separate the C and after combustion requires just as much heat as was liberated during the combustion. Since the sun's heat in the first place brought about the chemical changes by which fuels were formed, the sun is the source of the energy stored in the fuels. 36. A FLAME is a current of hot gas carrying with it solid particles at such a temperature as to glow and give out heat and light. 37. IGNITION is the first step in combustion, i.e., it is the beginning of the chemical combination. 38. SMOKE is a current of burnt gases carrying with it particles of unburnt carbon. COMBUSTION 33 39. An EXPLOSION is extremely rapid combustion. 40. SPONTANEOUS COMBUSTION occurs when a body ab- sorbs oxygen so rapidly that the chemical combination raises the temperature sufficiently so that it will burst into a flame. 41. COMPLETE COMBUSTION of a fuel element is its com- bination with that amount of oxygen which produces the most stable compound. Example: C burning to C0 2 . During complete combustion no flame is visible. 42. INCOMPLETE COMBUSTION of a fuel element is its combination with oxygen in such proportions as to form an unstable compound. Example: C burning to CO. 43. A FUEL is a substance containing elements which will combine with oxygen under proper conditions and in so doing produce heat. A fuel may be a solid, a liquid, or a gas. 44. CALORIFIC POWER. The complete combustion of a unit weight of any fuel element produces a definite quan- tity of heat. This is called its calorific power and is ex- pressed in heat units. 45. ASH is incombustible matter contained in a fuel. 46. SPEED OF COMBUSTION. This depends upon a num- ber of conditions and therefore varies greatly. It depends upon: (a) The temperature before ignition, increasing with this temperature; (6) The elements of which the fuel is composed, some elements burning faster than others; (c) Proportion of diluents, etc., decreasing as an excess of air is provided; (d) The more intimately the gases and oxygen are mixed the quicker the combustion ; (e) The greater the compression the quicker the com- bustion. 3 34 GAS-ENGINE THEORY AND DESIGN 47. COMPOSITION OF Am. Air contains oxygen ami nitro- gen in the following proportions at 32, and at atmospheric pressure: By Weight. By Volume. Oxygen. .. .0.236 0.213 Nitrogen. ..0.764 0.787 48. AIR REQUIRED FOR THE COMBUSTION OF CARBON TO CARBON DIOXIDE. C + 20 = C0 2 12 + 32 =44 12 is the atomic weight of C, and 16 is the atomic weight of O. For burning 1 Ib. of C, 32/12 Ibs. of are required, or 2.66 Ibs. The amount of air required will be 32/12x 100/23 or 11.6 Ibs. The products of combustion are: N 8.93 Ibs. C0 2 3.66 " Total 12.6 Ibs. Nitrogen is inert so far as combustion is concerned. At a temperature of 32, and a pressure of 1 atmosphere, 1 Ib. of air occupies 12.39 cu.ft. The total amount of air required in cu.ft. is: 12.39 X 11.6 = 143, or about 153 cu. ft. at 62. 49. AIR REQUIRED FOR THE COMBUSTION OF CARBON MONOXIDE TO CARBON DIOXIDE. CO + 0=C0 2 28 + 16=44 The weight of air required will be : 16/28 X 100/23: 2. 48 Ibs. The air required in cu. ft. will be: 12. 39X2. 48 = 30. 75 at 32. COMBUSTION 36 The products of combustion are: N 1.91 Ibs. C0 2 1.57 " Total 3.48 Ibs. 50. Am REQUIRED FOR THE COMBUSTION OF HYDROGEN TO H 2 0. 2H + = H 2 2 +16 = 18 The weight of air required will be: 100/23X8 = 34. 8 Ibs. The air required in cu. ft. will be: 12.39 X 34.8=431.52 The products of combustion are: N 26.8 Ibs. H 2 O . . 9 Total 35.8 Ibs. The H 2 is in the form of superheated steam. 51. AIR REQUIRED FOR THE COMBUSTION OF SULPHUR TO SULPHUR DIOXIDE. S + 20 = S0 2 32+32 =64 The weight of air required will be: 100/23 X 1=4. 34 Ibs. The air required in cu. ft. will be: 12.39 X 4. 34 = 53 The products of combustion are: N.. 3.35 Ibs. S0 2 2 Total 5.35 Ibs. 36 GAS-ENGINE THEORY AND DESIGN 52. COMBUSTION OF A COMPOUND. Experiment has shown that: When a fuel contains 0, then so mueh less will be re- quired for complete combustion; When a fuel contains both H and O, then only the surplus H (if any) need be considered. The H and of the fuel used, in forming H 2 O, will not affect the calorific power of the fuel and may be neglected. This gives a high ".ml a low heating value for such a fuel, the low value being the true one. Example: A sample of anthracite coal has been analyzed and found to be as follows: C 94 per cent. H and O in proportions to form water. . . 4 " II available for combustion 2 " " The cu. ft. of air required for the combustion of C will be 143 X 94 100 and for H 431 X2 100 The analysis gives percentages by weight. The cu. ft. of air required can be found direct without first finding the weight of air, but the student should not confuse weights and volumes. A formula often used for finding the weight of air re- quired is: (1) Weight of air in Ibs. = 12 C + 36( -) 52a. THE CALORIFIC POWER OF A COMPOUND can be figured according to the percentage of the elements. The H and O of the fuel must be treated as described in the preceding paragraph. COMBUSTION 37 Note. It must be understood that C and may exist in a fuel without being in the form of CO, or C0 2 , and that H and S may also be in a fuel without being combined with 0. More air is always supplied in gas engines than is theoret- ically required for combustion. 53. VOLUME OF THE PRODUCTS OF COMBUSTION. It will be shown later that VT = VT. Where V is the volume of the gas at the freezing-point. T " temperature abs. V " actual volume. T " temperature. This is true only when no chemical reactions are going on during expansion. The combined volume after com- bustion may not be the same as the sum of the volumes before combustion, even when the final and initial tem- peratures and pressures are the same. For example: 1 cu. ft. of H requires 2.37 cu. ft, of air, total 3.37 cu. ft. The volume after combustion will be 2.87 cu. ft. 1 cu. ft. of CO requires 2.37 cu. ft. of air, total 3.37 cu. ft. The volume after combustion will be 2.87 cu. ft. 1 cu. ft. of CH 4 requires 9.50 cu. ft. of air, total 10.50 cu. ft. The volume after combustion will be 10.50 cu. ft. The Law of Avogadro must here be taken into account. This law is as follows: Equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules. (2) Weight in Ibs. per cu. ft. at 32 - Molecular weight X 0.00559 2 Following are the volumes in cu. ft. per Ib. of some gases at 62: 38 GAS-ENGINE THEORY AND DESIGN CO, 8.6 cu. ft. H." 190 S0 2 5.85 " N 13.5 " 54. THEORETICAL TEMPERATURES OF COMBUSTION. a. When C burns to C0 2 Number of B. T. U. available = 14,500 per Ib. of C. For every degree rise in temperature N requires 8.93 X 0.244 B. T. U. And CO, requires 3.66 X 0.216 B. T. U. (multiplying the weight in Ibs. by the specific heat) then t [(8.93x0.244) + (3.66x0.216)] = 14,500 t= 4872 = increase in temperature. If we assume the temperature of the air before com- bustion to have been 62, then the final temperature will be 4872 +62 =4934. b. When CO burns to CO., Number of B.T. U. available =4,320 per Ib. of CO 4320 = (1.9 X 0.244) + (1.57 X0.21 6 r 538 '" temperature. c. W lien H burns to H 2 Here the temperature of the water must first be raised to the boiling-point. Latent heat must be supplied to convert the water into steam. The steam must be raised to the final temperature. The N must be raised to the final temperature. Number of B. T. U. available = 62,000 per Ib. of H. 62,000 -(9X966) =53,306 (212-t.) 9+ (t-212)(9x0.48) + (t-t 1 ) (26.8 X 0.244) = 53,306 t=4910 t, = 65= temperature of air before combustion. COMBUSTION 39 The theoretical temperatures are never attained because: the combustion is seldom complete; an excess of air is always supplied; there are radiation losses; some moisture is usually present in the fuel; dissociation takes place. 55. DISSOCIATION. The tendency of carbon to combine with oxygen increases with the temperature, as has been stated before, until' it reaches a certain limit, and it then decreases with a further increase in temperature, the affinity between the elements finally becoming zero, and a still further increase in temperature results in a separation of the CO or C0 2 , or other combinations with 0. This breaking up of the chemical combination with is called "dissociation," and takes place with a corresponding ab- sorption of heat, lowering the temperature of the fire and decreasing the calorific power of the fuel, since it takes just as much heat to break up an oxygen compound as was liberated in the formation of that compound. The H 2 present is also broken up. When the temperature has fallen sufficiently the oxygen compounds again form and re- supply the lost heat, but in a gas engine this re-combination, or "after- burning," may occur so late in the stroke as to constitute a heat loss. Dissociation is supposed to take place at temperatures ranging from 2,500 up. The tem- perature varies, of course, with conditions and cannot be accurately determined. CHAPTER VI FUELS 56. The fuels commonly used in cdnnection with heat engines are: solid, anthracite and bituminous coal; liquid, petroleum and its distillates, alcohol ; gas, the various gases made from coal, natural gas; other fuels are coke, charcoal, wood, peat, etc., but since they are used only to a limited extent they will not be further considered here. 57. COAL. Regarding the origin of coal, geology teaches us that many ages ago the earth was very different from what it is to-day. Continents slowly emerged from seas and great changes on land and water took place. During one era, called the carboniferous era, the air was very hot and moist and the land was covered with immense tropical forests and other luxuriant vegetation. During succeeding ages further changes took place and these forests were even- tually buried beneath deposits of sand, rock, etc., until a great crust had been formed over them. Decomposition of the vegetable matter produced heat and the heat and pressure resulted in a partial combustion and distillation of the vegetable products, changing them into coal. This coal, as we all know, is now obtained by mining. Coal is generally classified as anthracite, or hard, and bituminous, or soft. In this country Pennsylvania is the great anthra- cite-producing State. The output for 1905 was 78,731,523 tons. About 28 States produce bituminous coal, and the total output for the same year was 310,040,644 tons. The production of coal in the United States during the calendar year 1907 was 428,895,914 tons of 2,240 Ibs. each. Great 40 FUELS 41 Britain and Germany rank next as coal-producing coun- tries. The chief reason why greater progress has been made in Europe with large gas engines than here is because fuel is so much scarcer there and the natural resources, in general, are more limited, necessitating greater economy. Since the composition of coal varies much the calorific power varies accordingly. In order to obtain a fair average of the heating value of coal from a certain locality several samples should be analyzed. 58. Anthracite is a hard coal, burning with little or no smoke. Its calorific power is usually greater than that of soft coal. It contains a certain amount of incombustible matter commonly termed "ash." An average composition is as follows: Carbon 90 per cent by weight. H, orOandN 5 " " Water 1 " " Ash 4 " " " B. T. U. per pound 13,000 to 14,000. 59. Bituminous, or soft coal, ignites more readily than the anthracite, and during combustion (under a boiler) usually gives off a large amount of black smoke. A "good average" analysis of bituminous coal cannot be given since it varies from grades rich in heating value to grades almost unfit for use as a fuel. The following, however, may be taken as a guide: Carbon 50-80 per cent by weight. H, 0, N, etc 10-40 " " Sulphur 1- 3 " " Ash, or earthy matter, 2-20 " " " B.T.U. per pound 9,000 to 14,000. There are, of course, other kinds of coal such as semi- bituminous, lignite, etc., but these need not be discussed here. 42 GAS-ENGINE THEORY AND DESIGN 60. PETROLEUM. Petroleum is also called mineral and crude oil. With respect to its origin opinions differ, but it seems that this oil is the result of the decomposition of animal matter which was buried in a similar manner to the coal. The United States is the greatest oil-producer in the world, with Russia second. The oil is obtained by boring wells in the oil districts. Sometimes the petroleum has to be pumped out, but frequently it gushes out of the well with considerable force. Petroleum is found in this country in Pennsylvania, Texas, California, and some other States. The annual production in the United States in 1905 reached a total of 117,090,772 barrels of 42 gallons each. During the calendar year 1907 the production of petroleum was 6,976,004,070 gallons. In order to obtain the various oils known as gasolene, kerosene, etc., the crude oil is subjected to a distillation process. The lighter oils are driven off first and the result of the distillation, and general properties of the distillates, are given in Table II. 60a. TABLE II PETROLEUM DISTILLATES Tempera- ture Distillate Per Cent Specific Gravity Flashing Point B. T. U. per Ib. Baume Degree at 60 F. 113 113-140 140-160 160-250 250-350 340 480 Rhigolene Chymogene Gasolene Benzine Naphtha Kerosene Lub. oil Paraffin Residue Traces Traces 1.5 2-10 2-10 50 15 2 16 .59-. 62 .59-. 62 .65-. 72 .74 .74 .98 .90 40^70 14-32 14-32 100-160 230 18,000 18,000 18,000 22,000 "QS 48 Distillate Crude oil .80-. 90 .88 17,000-20,000 19,000-22,000 22 30 FUELS 43 The specific gravity of a substance is the weight of a unit volume as compared with the weight of a unit volume of water. The flashing-point of a substance is the temperature at which it gives off an ignitible vapor. Fractional distillation is the separation of different con- stituents from a substance. An example of this is the dis- tillation of petroleum. Destructive distillation is the heating of a substance from which air has been excluded. This is employed in making coke and charcoal. Hydrocarbons is the name given to the various combina- tions of hydrogen and carbon. To this class belong the petroleum products, alcohol and natural gases. Petroleum has an average composition of:* Carbon 85 per cent. Hydrogen 13 " " and impurities 2 " " Crude petroleum gives off a disagreeable odor and its volatile constituents make it dangerous in confined spaces. On account of its impurities, and the difficulty of obtaining complete combustion, it is difficult to use in the engine cylinder direct except with high compression as in the case of the Diesel engine. The gummy residue resulting from incomplete combustion would soon cause trouble in the engine cylinder. Engines are sometimes operated on gas made from the crude oil (see Oil Water Gas). The great advantage of crude oil is its cheapness. 61. Fuel oil, or fuel distillate, is the product left over when distillation is stopped after the kerosene has been obtained. It is safer than the crude oil since the more volatile constituent shave been driven off. The problems of carburation (where a carbureter is used) and of ignition, 44 GAS-ENGINE THEORY AND DESIGN however, are difficult ones. Unless complete combustion is obtained the exhaust will be smoky and give off disagree- able odors. There is also danger from carbon deposits in the engine cylinder. If practically perfect combustion can be obtained it forms a safe and cheap fuel. There are so many grades of fuel oil that, when the oil is wanted for a gas engine, this fact should be stated in ordering, since some fuel oils, as well as some crude oils, are absolutely unfit for use in the engine cylinder. Since the fuel oil and distillates contain various com- binations of hydrogen and carbon, it is difficult to give the chemical composition. 62. Kerosene is a cheap and safe fuel and can be used to good advantage in the gas engine by increasing the com- pression somewhat over that commonly used for gasolene, and by properly carbureting. Its use as a fuel for small motors is spreading rapidly. While the probable percent- age of kerosene in petroleum is about 50, only 30 per cent is actually obtained by distillation. One pound of kerosene vapor, at atmospheric pressure, occupies a space of 2.47 cu. ft. ; 188 cu. ft. of air are required for its combustion, or volumes in a ratio of 1 to 76 ; it is about five times as heavy as air. A number of the so-called "kerosene" engines are first started on gasolene and when the engine is warmed up the gasolene is turned off and the kerosene on. 63. Gasolene has been used for automobile and the smaller marine and stationary engines, almost exclusively because of the following advantages: It is easily carbur- eted, since it is volatile ; it is readily ignited by the electric spark; good combustion is more easily obtained than in the case of the heavier oils; in short, it is cleaner and easier to handle. It possesses a number of disadvantages, however, among FUELS 45 which are the following: It is volatile, therefore dangerous unless carefully handled; more expensive than the other oils; in some localities the cost is almost prohibitive; considerable loss from evaporation occurs in warm locali- ties; cannot be procured at all in some places; in some of the South American countries its use is forbidden by law. One pound of gasolene vapor, at atmospheric pressure, occupies a space of 4.06 cu. ft. ; 181 cu. ft. of air are required for its combustion, or volumes in a ratio of 46.6 to 1 ; it is about 3.05 times as heavy as air. There are several grades of gasolene and this fact must be considered in specifying it for fuel purposes. 64. ALCOHOL. There are two kinds of alcohol methylic, or wood alcohol, C 2 H 4 O 2 , made by dry distillation of wood in iron retorts, and ethyl alcohol, C 2 H 6 2 , made by dis- tillation from the fermented infusions of substances con- taining starch, such as potatoes, corn, rice, barley, wheat, etc., or substances containing sugar, such as sugar beets, sugar cane, molasses, etc. Waste products can be used, such as diseased potatoes, bitter molasses, etc. For fuel purposes the ethyl alcohol is denaturized so as to make it unfit for human consumption. This is done by adding benzine, wood alcohol, gasolene, and other sub- stances. There are also electrical methods of manufacturing alco- hol, and for these a great deal is claimed. Following are the principal properties of ethyl alcohol: Specific gravity, 0.79; freezing-point, about 200 below zero when pure; calorific power, 28,000 B.T.U. per Ib. when pure; when diluted this may run down to 12,000 B.T.U. ; has a strong affinity for water. A somewhat higher compression than for gasolene is usually required on account of the water in the alcohol. 46 GAS-ENGINE THEORY AND DESIGN Until quite recently the extensive manufacture of alcohol was prevented by an excessive tax, but this has been re- moved, so that alcohol can now be manufactured in quantity, although under certain restrictions. When produced in large quantities it can be sold at a price that compares favorably with that of gasolene or kerosene. The Treasury Department gives the following figures for denatured al- cohol: For the six months ending June 30th, 1907, whole- sale dealers received 1,724,062 wine gallons (this includes alcohol received from other dealers) and sold and removed 1,441,360 gallons. Among its principal advantages as a gas-engine fuel are: It can be produced in any quantity and at a comparatively low cost; it is safe, fires can be extinguished with water; this cannot be done in the case of oil, where water simply spreads the flames; it is cleaner than the oils and leaves no deposits in the engine cylinder; there is no danger from explosions in case of leaky connections. The consumption in pounds per H.-P. hour, however, is greater than for the fuel oils when the alcohol is greatly diluted. Under certain conditions acids are formed during com- bustion which will corrode metallic surfaces. Alcohol is an ideal fuel for small engines properly de- signed for its use. 65. NATURAL GAS is, or was, formed by underground dis- tillation. It is obtained, as in the case of petroleum, by drilling to the subterranean accumulations. Where obtain- able it forms an ideal fuel, especially for the larger station- ary gas engines. The calorific power can readily be cal- culated when the chemical composition is known. 66. COAL GAS, or ILLUMINATING GAS, is obtained by de- structive distillation of coal. The coal is placed in closed iron retorts which are heated from the outside. The gases FUELS 47 which are driven off during this heating are filtered and cooled. The product remaining in the retorts is called coke. This gas has been largely superseded by water gas. 67. WATER GAS is formed by blowing air and steam alter- nately through a mass of incandescent carbon. It is similar to the product called "producer gas." For illuminating purposes the water gas is enriched by the addition of car- buretted hydrocarbon vapors. The illuminating gas is "fixed" (made a stable compound) the same as coal gas, by passing it through a superheater. An illuminating gas is not so good for power purposes as a power gas. 68. OIL WATER GAS is sometimes made from petroleum by heating the oil in a retort into which highly superheated steam is passed in such a manner as to ultimately mix the constituents. While this gas has been used as an engine fuel, it is more of an illuminating than a power gas. 69. BLAST-FURNACE GAS. In the modern blast furnace for the reduction of iron ore less than one-third of the car- bon burns to C0 2 , so that the discharged gas consists largely of CO. The CO has a calorific power of about 100 B.T.U. per cu. ft., or about 1,280 B.T.U. per pound. By in- creasing the compression sufficiently this gas can be used in the gas engine (see Par. 144). About 0.80 cu. ft. of air is required per cu.ft. of gas, and the calorific power of the mixture is about oo B.T.U. per cu.ft. The following figures were furnished by the Lackawanna Steel Company, and represent the average blast-furnace practice per ton of iron: Charge Production Ore... . 3,600 Ibs. Iron 2,240 Ibs. Coke 2,000 " Gas 10,600 " Limestone 1,200" Slag 1,210" Air 7,250 " 14, 050 Ibs. 14,050 Ibs. 48 GAS-ENGINE THEORY AND DESIGN Average analysis of gas by weight: Per cent by Weight Nitrogen, N 52.18 Carbon monoxide, CO 26.83 Carbon dioxide, CO 2 18 . 23 Methane, CH 4 38 Hydrogen, H 08 Water vapor, H S O... . 2.30 Total 100.00 The calorific power of the above gas equals 1,283 B.T.U. per Ib. About 50 per cent of this is available as a fuel for gas engines, or 5,300 Ibs. of gas, containing 1,283 B.T.U. per Ib., for every ton of pig iron produced. At one time these gases were considered waste products and discharged directly into the atmosphere. One of the chief problems in connection with the use of blast-furnace, as well as coke-oven and producer gas, is the proper cleaning of the gas before it reaches the engine cylinder. A centrifugal cleaner for blast-furnace gas con- sists of a drum which revolves rapidly inside of a casing and is so arranged that it throws both water and the gas against the inside of the casing by centrifugal force. The water picks up the dust in the gas and as it drains off at the bottom of the casing it carries the impurities with it. The cooled and cleaned gas passes on to the engine. 70. PRODUCERS AND PRODUCER GAS. Producers are of two kinds: pressure producers, used for large power in- stallations, and suction producers, used for the smaller installations. In the pressure producer compressed air is introduced into the ash-pit and the pressure throughout the system is greater than atmospheric. This necessitates an auxiliary air-compressing system, a gas-holder, and is also open to the objection that if there should be a leak the CO will escape. This gas is a deadly poison. The use of a gas- FUELS 49 holder has several advantages: the action of the producer is not affected by the pulsations of the engine, a supply of gas is always on hand for quick starting, and the engines can be run independently of the rate at which the producer is generating gas. The general way in which the suction gas-producer operates is shown in Fig. 19. A, the producer proper, consists of a steel shell lined with firebrick, and is provided with a fire- grate, ash-pit, etc. The producer is charged with coal from FIG. 19. above and a fire started at the bottom. As the mass of coal above the fire becomes heated gases are driven off, and when the engine is running it sucks these gases into the cylinder and burns them in the usual manner. When the gases leave A they are at a high temperature and contain many impurities. They first pass through the boiler B where they are cooled by water circulating through vertical pipes. The hot gases generate steam in B and some of this steam passes with the air through the fire in A, where it helps to control the combustion by lowering the tempera- ture and where it also enriches the gas. From B the gases pass through a wet scrubber C. This scrubber contains several layers of coke on which water is sprayed contin- 4 50 GAS-ENGINE THEORY AND DESIGN uously. The object is to further cool the gases and remove impurities such as tar, pitch, etc., which sink to the bottom of C. It is important that the gases should be as clean as possible before entering the engine cylinder as impurities will cause the piston and valves to stick and wear rapidly. From C the gases pass through the dry scrubber D which contains excelsior, removes further impurities, and also prevents pulsations of the engine from reaching A. From D the gases pass to the engine. The chief problem in connection with producer gas is the proper cleaning of the gas. As mentioned in the fore- going, when the gas leaves the producer proper (A) it carries along tar, ammonia, sulphur, dust, etc. In order to avoid stoppages and irregular running of the engine due to clogging up and wear of parts, the gas should be dry and froe from dust and other impurities, and, of course, cool in order that the volume may be reduced to a minimum. The design of producers and cleaning apparatus is rapidly chang- ing, and it will doubtless be some time before anything like a standard of construction is reached. The use of hard coal in the producer does not now present any difficulties which cannot be, or rather which have not been, successfully mastered. The use of soft coal, on account of tar in the gas, the tendency to cake and form slag and adhere to the sides of the producer, is more difficult. Doubtless distinct types of producers for the various kinds of fuels will eventually be evolved. The double- zone producer, a description of which can be found in books on gas-producers, furnishes a gas free from tar. One advantage of the producer which cannot be over- estimated is that the poorest kinds of fuels can be used. Among the fuels successfully used are lignite, peat, wood, straw, mine culm, garbage, and many waste products. 71. CHEMICAL REACTIONS IN THE PRODUCER. The FUELS 51 chemical reactions, both in the producer and in the engine cylinder, are really very complex, depending upon a number of ever- varying conditions such as composition of the fuel, amount of air and steam supplied, temperature of combus- tion, etc. Ihe air first burns to C0 2 , then, when this gas strikes the carbon above where no O reaches it, it is decomposed into CO. When steam is passed through the fire the O separates from the H and combines with carbon, forming CO. The CO and H then pass along with the other gases to the engine. When carbon burns to carbon monoxide the following reactions take place: C + = CO 12 + 16 = 28 16/ 12 X 100/23 = weight of air required. The products of combustion are: N 4.43 Ibs. CO.. . 2.34 " Total 6.78 Ibs. Now, 2.34 Ibs. of CO burning to C0 2 furnish 4,320x2.34 = 10,080 B.T.U. Carbon burning to C0 2 furnishes . . . 14,500 B.T.U. CO burning to C0 2 furnishes 10,080 " The loss equals 4,420 B.T.U. or about 30 per cent. The heat absorbed in separating 1 Ib. of H from the in CO OQA H 2 is 53,300 B.T.U., then - - = 5.33 Ibs. carbon 43.20 X 2.33 required, or 12.4 Ibs. of CO. One pound of CO burning to CO 2 requires about 2 J Ibs. of air, and 12.4 Ibs. require 31 Ibs. of air. In place of this we now have 1 Ib. of H of 62 GAS-ENGINE THEORY AND DESIGN high calorific power, with a decrease in the heat loss of about 15 per cent. The amount of steam that can be used, however, is limited, since if the temperature of the fire is lowered too much the H 2 will not be broken up and a loss instead of a gain results. The limit of the ratio of steam to coal by weight is about 1 to 40. The producer gas carries about 85 per cent of the calo- rific power of the coal. An average analysis is as follows: By Volume Hard Coal Soft Coal CO H... CH< CH 2 N... 27% 12 1.2 2.5 57.3 27% 12 2.5 2.0 56.5 80 Producer gas contains 110-150 B.T.U. per cu. ft cu. ft. of gas should be furnished by 1 Ib. of coal. The amount of coal burned to C0 2 , and which furnishes the heat required to operate the producer, is usually 5 per cent by weight of the fuel consumed. The loss in radia- tion, ashes, tarry products, etc., may run up to 10 per cent in some cases. The properties of various fuel gases are given in Table III. 71a. TABLE III PROPERTIES OF FUEL GASES B.T.U. Wei K ht Cu. Ft. Air B.T.U. per Cu. Ft. per Cu. Ft. per Cu. Ft. Fuel per Cu. Ft. Mix- ture Symbol Natural Gas .... 1,000 0.045 11 85 Coal Gas 650 0.041 8 81 Water Gas 630 0.052 6 90 Oil Water Gas 1,000 0.062 9-10 100 Blast-Furnace Gas 100 0.085 .80-1 55 Producer Gas 110-150 0.072 .90-1.25 60-70 Ethvlene or Olefiant Gas. 1,500 0.076 14.3 100 rViii Methane or Marsh Gas. . . 930 0.043 9.5 90 CH 4 Acetylene 1,550 0.081 14 110 C a H 2 FUELS 53 716. ENRICHMENT OF FUELS. Numerous attempts have been made to enrich the liquid fuels., i. e., to increase their heating value per pound, by adding various sub- stances. Thus far but little progress has been made in this direction. In Germany fair results have been obtained with benzol, C 6 H 6 (coal-tar benzine), which is generally mixed with gasolene in varying quantities. The object is to in- crease the heating value of the fuel with but little or no increase in cost. 72. CALORIMETRY. A calorimeter is an apparatus for determining the calorific, or heating, power of a fuel. In the Mahler calorimeter the liquid, or solid, fuel is put into a vessel filled with oxygen; this vessel is placed in another containing water so that it is entirely surrounded by water, the fuel is then electrically ignited and the heat resulting from combustion (which is complete, as an excess of oxygen is provided) is absorbed by the water. The rise "in the temperature of the water, the weight of which is known, plus the heat absorbed by the closed vessel (which amount is determined beforehand) gives the calorific power of the fuel. The Junker gas calorimeter consists of a cylinder con- taining a burner, similar to the Bunsen burner, through which air and gas flows. The hot gases pass out through tubes surrounded by water the tubes and water jacket are outside the first cylinder and the cubic feet of gas consumed in a given time, and the amount of water heated to a cer- tain temperature, will give the calorific power of the fuel. There are also a number of calorimeters in which chem- icals are used in the determination of the calorific power of the fuel. These possess a number of advantages over the older forms described above. It is sometimes desirable to analyze exhaust gases, and chemical calorimeters for this purpose are on the market. CHAPTER VII LAWS OF GASES 73. THE INDICATOR DIAGRAM. In Fig. 20 the vertical lines, called ordinates, represent pressures, and the hori- zontal lines, called abscissas, represent volumes. is the point of no pressure and no volume. Let us assume that: (a) The piston of a gas engine compresses its charge of air and gas from a to b without an increase in pressure ; (b) The mixture is then ignited while the piston is on its inner dead centre and the pressure rises during combustion from b to c; (c) The piston now moves out and enough heat is sup- plied to keep the pressure constant, giving the line cd; (d) At the end of the stroke the exhaust valve opens and the pressure in the cylinder falls from d to a, then The total pressure on the piston in pounds, multiplied by the distance ab in feet, equals the work done in foot- pounds. Therefore the area abed represents the work done by the piston on its out stroke. Such a diagram is called an in- dicator diagram. The actual indicator diagram is very different from the 54 LAWS OF GASES FIG. 21. above in form, although the principles are the same, since it is impossible to compress at constant pressure and con- sequently along a straight line and to expand a gas in the same way. Fig. 21 shows the form of an actual indi- cator diagram. The line 0V represents zero pressure, i.e., 14.7 Ibs. below atmospheric pressure. The total pressure from zero is called absolute pressure, and is measured in either pounds per square inch, or in atmospheres 1 atmosphere being equivalent to 14.7 Ibs. The line oaf re- presents the atmospheric pressure. This indicator diagram is read as follows: (a) The piston starts to compress the charge at a and the pressure rises during compression to the point 6; (6) The charge is now ignited, and as the piston starts to move out the pressure rises, due to the combustion going on, to the point c, and from there on it falls as the com- bustion stops and the volume increases; (c) At the point d the exhaust valve opens and in rush- ing out the gases expand to atmospheric pressure. The area enclosed by the irregular outline abed repre- sents the work done by the burning charge in expanding, just as the area in Fig. 20 represents the work done. This area in square inches, divided by the distance ab, gives the mean height of a rectangle having the same area as the diagram, as indicated by the dotted lines. If, for example, the scale of pressure is 200 Ibs. per inch, and this mean height is \ inch, then the mean pressure throughout the stroke is 100 Ibs. This mean effective pressure is abbre- viated M.E.P. 5(5 GAS-ENGINE THEORY AND DESIGN The work done by the expanding charge is now figured as follows: PLAN = "33^000" where P=M.E.P. A = area of piston in sq. in. L = stroke of piston in feet. N = R.P.M. (power strokes). In a single-cylinder four-cycle engine the power strokes are only one-half of the total R.P.M. The planimcter is an instrument for measuring the area of a surface having an irregular outline like the indicator diagram. The shaded portion in Fig. 21 represents the work done by the fly-wheel during compression. This is again restored to the fly-wheel by the gas during expansion and does not affect the indicator diagram proper, and so is neglected in our present calculations. Fig. 22 is an indicator diagram from a four-cycle engine. During the exhaust stroke the pressure in the cylinder rises a little above v ^ 1>9 atmospheric due to back pressure of the gases, and during the suction stroke the pressure drops somewhat below atmospheric due to wire-drawing effect. 74. THE INDICATOR. Fig. 23 illustrates the principles on which the indicator works. The card C, on which the indicator diagram is drawn by the pencil-point T, moves back and forth to correspond with the movement of the engine piston P. The cylinder D f communicates with the engine cylinder D. In D' there is a small piston P' (area LAWS OF GASES 1 sq. in.) which works against a stiff spring so graduated that it registers on C the pressure against pistons P and P f to some definite scale. If a pressure of 100 Ibs. will com- press the spring I", then a point on the indicator curve 1" above atmospheric press- ure shows that the press- ure in the piston was 100 Ibs. at that instant, Now it can easily be seen that as P moves back and forth, P' moves up and down, and the changes of pressure and volume in D and D' are traced on C by the point T. The indicator card thus furnishes a record of the change of pressure and volume in the engine cylinder, and consequently of the work done. The horse- power computed from the indicator diagram is called the indicated horse-power and is abbreviated I.H.-P. The actual indicator carries a drum on which the card is mounted, and instead of moving back and forth this drum revolves through a certain angle. A reducing mech- anism is provided between the drum and piston rod, since the movement of the drum is very small compared with the piston travel. 75. CHANGES IN A GAS. The state of a gas may be changed by: adding to or subtracting heat from it; doing external work upon it. This will bring about changes in: volume, temperature, pressure, specific heat, intrinsic energy, entropy. In the following pages a permanent gas, air, will be con- FIG. 23. 58 GAS-ENGINE THEORY AND DESIGN sidered, but the laws, with the substitution of the proper constants, apply to all gases. Certain laws regarding changes in pressure, volume, and temperature of gases have been determined by experiment, and these will be briefly mentioned. 76. LAW OF GAY-LUSSAC OR CHARLES: VOLUME AND TEMPERATURE. The pressure remaining constant, the vol- ume of a perfect gas is proportional to its absolute tem- perature. V, = T, V t T, or, the volume remaining constant, the pressure must vary directly with the absolute temperature 1 P, = T, PI T, also, V, = V (H-at) where T = abs. temp. V = volume in cu. ft. P = pressure in pounds per sq. ft. a = 1/493 = 0.002035. t = temp, above 32. The volume of a perfect gas increases 1/493 for each degree increase in temperature. 77. LAW OF MARIOTTE OR BOYLE: VOLUME AND PRESS- URE. The temperature remaining constant, the volume must vary inversely as the pressure, and directly as the density (since density varies inversely with the volume). 78. COMBINING OF LAWS. A combination of the fore- going laws gives: PV P.V, P 2 V 2 LAWS OF GASES = K, a constant. T T t T 2 PV = TK Note. The mathematical proof for the above, and a number of other formulas, will not be given, but can be found in books on thermodynamics. For air K , 12.39 X (144X14.7)^ 493 ternal work done for each degree increase in temperature. If we had a closed cylinder containing 12.39 cu. ft of air (1 Ib.) at atmospheric pressure, behind a piston having an area of 1 sq. ft. (144 sq. in.), then, if this body of air is heated so that the temperature rises 1, its volume will increase 1/493 of 12.39 cu. ft, and the piston will be pushed forward 1/493 of 12.39 linear feet against a pressure of 144 X 14.7 Ibs. The value of K changes for different substances, for example: For superheated steam K = 104.64. " ammonia K = 162.60. 79. INTRINSIC ENERGY. In order to have a gas do work it must be heated, and it can be seen from the above that the capacity of a gas for doing work depends upon its specific heat, its weight, and its absolute temperature; therefore intrinsic energy = G C T where G = weight in Ibs. C = specific heat T = abs. temperature. 80. AVAILABLE ENERGY. Since, in order to expand, the gas must do work against atmospheric pressure, the energy expended in doing this work is measured by the product of the weight of the gas X its specific heat X the difference between the initial and final absolute temperatures, or Available energy = G X C X (T t -T 2 ). 60 GAS-ENGINE THEORY AND DESIGN 81. EXPANSION. The expansion may be isopiestic, iso- thermal, adiabatic, according to the law PVn=a constant. 82. ISOTHERMAL EXPANSION takes place at constant tem- perature. When a gas expands under ordinary conditions its temperature falls. In order to expand isothermally heat would have to be supplied. In Fig. 24 let the curve represent the isothermal expansion of a gas from a to b, then V =the initial volume, V 1 =the final volume, P =the initial pressure, P^the final pressure. Since the temperature does not change the expansion follows the law The isothermal curve is an equilateral hyperbola and is expressed as follows: where AY = the work done, log e is the hyperbolic log. 83. ADIABATIC EXPANSION. In expanding adiabatically a gas does not receive heat from, or give out heat to, any external body. The external work done during expansion LAWS OF GASES 61 is done at the expense of the intrinsic energy of the gas. There is a fall in temperature and pressure. In Fig. 25 let the curve ab represent isothermal expan- sion, and the curve ac adiabatic expansion. Since no heat is added during adiabatic expansion the final pressure will be lower than for isothermal expansion. The PV of the FIG. 26. FIG. 27. isothermal expansion becomes PVn in adiabatic expansion. The n is the ratio of the two specific heats of the gas and expresses the ratio of the change in pressure and volume in adiabatic expansion. Isothermal expansion would be more wasteful than adia- batic since the gases would be exhausted at a much higher temperature. 8i. ISOMETRIC LINES. In Fig. 26 the line be represents an increase in pressure without an increase in volume. Such a rise in pressure would take place when a piston is held stationary while the charge explodes. 85. ISOPIESTIC LINES OR ISOBARS. In Fig. 27 the line be represents an increase in volume without a change in pressure. Such lines are called isobars. 86. SPECIFIC HEAT AT CONSTANT VOLUME AND AT CON- STANT PRESSURE. Specific heat has already been defined as the amount of heat necessary to raise the temperature of 02 GAS-ENGINE THEORY AND DESIGN a unit weight of a substance 1. When air is heated and allowed to expand a certain amount of heat is necessary to raise the temperature 1 under these conditions. If the same amount of air is confined in a closed vessel so that it cannot expand, and the same amount of heat is applied, the temperature will obviously be higher than in the first case. A gas therefore has two specific heats. Experiment has shown that the Specific heat of air at constant pressure (Cp) is 0.2375. Specific heat of air at constant volume (Cv) is 0. 1691. Cp .2375 = = 1.405 =n, a constant. For isothermal expansion n = 1. For adiabatic n = 1.405. One Ib. of air raised 1 at constant pressure requires 778 X .2375 - 184.77 ft.-lbs. One Ib. of air raised 1 at constant volume requires 778 X. 1695 = 131.56 ft.-lbs. The real specific heat of the products of combustion is a very uncertain quantity and differs more or less from the theoretical specific heats. It can be approximated by an analysis of the products of combustion by taking the mean of the various specific heats. In Table IV. the specific heats and volumes of some gases are given. 87. EXPANSION ACCORDING TO THE LAW PVn = A CON- STANT. Isothermal and adiabatic expansions are possible only theoretically and can never be realized in practice. They are very useful in the development of the theory of thermodynamics. The actual expansion of a gas takes place according to the formula PVn = a constant, in which n is the ratio of the specific heats of the gas. This formula may be written as follows: PV n = P 1 V 1 n and -= LAWS OF GASES 63 or, the pressure varies inversely with the nth power of the volume. In practice, for obvious reasons, the value of n will vary more or less from the theoretical value 1.405 (for air), but the actual value of n for an engine under given conditions can be computed from its indicator card. The actual compression curve on the indicator card will usually lie between the isothermal and adiabatic curves (see Fig. 25), and follows the law PV n = K, the value of n being 1.35 or 1.33. 88. COMPRESSION. The laws and formulas for expansion apply equally well to compression since compression is simply the reverse of expansion. 89. COMPRESSION IN Two. STAGES. The work of com- pression is lessened if the work is carried on in two, or more, stages so that the air can be inter-cooled; for example, a body of air is to be compressed to 10 per cent of its orig- inal volume. If it is compressed in the first stage (first cyl- inder) to 55 per cent, the temperature will have increased to something like 400. On its way to the second cylinder the air is cooled back to its initial temperature, say 70, and now much less energy is required to compress it from 55 per cent to 10 per cent of the original volume than would have been the case if the heat had not been withdrawn. 90. THE CARNOT CYCLE. The term " cycle" here refers to a succession of heat changes in the gas. In the theoretical cycle of maximum efficiency proposed by Carnot it is assumed that: (a) There is a heat reservoir of unlimited capacity so that heat can be supplied without a change in temperature ; (6) There is a refrigerator of unlimited capacity so that heat can be withdrawn without a change in temperature; (c) The engine cylinder and piston are non-conducting so that heat cannot escape that way; r,4 GAS-ENGINE THEORY AND DESIGN (d) The engine is connected to both heat reservoir and refrigerator so that heat can be received and discharged. The cycle then operates (Fig. 28) as follows: (a) The engine is connected to the heat reservoir and heat flows into the cylinder so that the gas expands isothermally from c to d; (b) The connection is closed and the expansion continues adiabatically from d to a; FIG. 28. (c) The engine is now connected with the condenser which withdraws heat isothermally while the piston moves from a to b; (d} The connection to the condenser is closed and the gas is compressed adiabatically from b to c so that when the point c is reached the gas is in exactly the same condition as at the beginning of the stroke. All heat transfers have been made at maximum efficiency so that the efficiency of the cycle is expressed as follows: T T, = Efficiency. This cycle is reversible. It is assumed, of course, that cylinder and piston are insulated so that they will not al> sorb any heat. LAWS OF GASES or, The actual gas-engine cycles necessarily differ greatly from the Carnot since the latter imposes conditions which can never be realized in practice, but it points out the lines along which the greatest thermal efficiency could be secured. Other cycles, such as the Beau de Rochas, Lenoir, Bray- ton, Diesel, etc., have already been described. 91. -TABLE IV VOLUMES AND SPECIFIC HEATS OF GASES Specif] cHeat Gas Vol. at 32 Constant Pressure Constant Volume Air 12.39 0.2375 1690 Carbon monoxide 12.77 2479 1758 Carbon dioxide 8 12 2170 1535 Hydrogen. . . . 178 80 3 4090 2 4122 Nitrogen. . . .... 12 77 2438 1727 Oxygen. . . 11 20 2175 1550 Superheated steam. 4805 3460 Alcohol. 4534 3200 Ammonia . 508 299 CHAPTER VIII GAS-ENGINE EFFICIENCY 92. In a discussion of the efficiency of the gas engine certain factors must be considered. Among the factors to be discussed are: reliability, economy, advantages, dis- advantages. 93. RELIABILITY. The desirability of installing gas power depends much upon the reliability of the engine, and into the question of reliability enter a number of con- siderations, among which are cost, overload and underload capacity, proper handling, etc. The only way to improve the design of any machine is to acknowledge its faults and then work to overcome them. The advantages of the gas engine are many; its disad- vantages should be carefully studied with a view to min- imizing or overcoming them altogether. In the matter of reliability, there is still much room for improvement, espe- cially in connection with some of the apparatus connected with the operation of the engine. Without reliability any heat engine is of little use. Gas-engine design presents a much more difficult problem than the steam engine, for a number of reasons. The gas engine has no reservoir in which energy can be stored; the power is delivered inter- mittently to the crank-pin; high temperatures, pressures, and fuel conditions make a reliable performance under widely varying conditions something that will result only from careful designing, good workmanship, and proper hand- ling. It has been customary on the part of some manufac- 66 GAS-ENGINE EFFICIENCY 67 turers, especially of the smaller engines, to lead the cus- tomer to believe that a gas engine will run itself without care and attention. This is far from being the case. Gas engines, both large and small, must be handled intelligently in order to give satisfactory results. There is no reason, however, why a fairly intelligent man possessing some mechanical ability should not become competent to handle a producer-gas plant, for example, with a reasonable amount of training. When a power-user installs a steam plant he knows perfectly well that he must have competent men to take care of it, and it is just as important that a gas engine, no matter whether large or small, should receive proper attention, although the help required is less for a gas- power plant than for a steam-power plant. The user of steam power knows that he can buy a cheap engine and boiler outfit and will get exactly what he pays for. He also knows that if he wants a thoroughly reliable steam plant, especially where the conditions are very exacting, he must pay accordingly. He knows that he cannot get a high-class steam-power outfit at bargain prices. Precisely the same conditions hold good for the gas engine. If a power-user wants a good reliable gas-power outfit, no matter whether large or small, he must pay accordingly and see that proper care is taken of the plant after installa- tion. A cheap engine cannot be expected to be reliable, or to develop the full power at which it is rated. Nor will it run without attention, no matter what an over-anxious salesman may say. Much trouble has been caused in the past by poorly designed and cheaply constructed engines which would look better in a museum than in a shop. In connection with producer and blast-furnace gas engines much trouble has been caused by improper cleaning-ap- paratus for the gas. In order that the engine may work successfully the entire plant must work successfully. An 68 GAS-ENGINE THEORY AND DESIGN engine cannot be expected to properly perform its work when improper fuel is delivered to it. Too many engines and producers have been installed without having been properly tested. Where power only is wanted the gas engine has much in its favor, but where steam is needed for heating during the winter, or for manufacturing purposes as in paper mills, textile mills, etc., a special heating system would have to be installed in connection with the gas engine, and under these conditions the steam engine possesses some advan- tages. In the large electric light and power plants, on the other hand, a vast amount of heat is wasted in exhaust steam, and this waste could be largely avoided by the use of a high-grade gas engine. Attempts have been made to use the exhaust gases of a gas engine for heating purposes, or generating steam, and it is claimed that 10 per cent of the heat has been saved in this way. The steam engine will carry a large overload and will pull hard under varying loads, since it has a large reservoir of energy to draw from. In the case of the gas engine the obstacles to be overcome in order to achieve the same results are great. Unless a gas engine is designed for an overload capacity it will simply slow down and stop when overloaded to any extent, and while it may be very efficient under full load it may drop greatly in efficiency when run- ning under three-quarter or half load, and when the load is much less than one-half it may again stop. A gas engine should be designed first for reliability under the conditions under which it is to work. A good design alone will not produce a good engine. Unless the work in the shop is right a poor engine will result. A well-built engine of poor design will give some results, while a well designed but poorly built engine will never give results. GAS-ENGINE EFFICIENCY 69 94. ECONOMY. The thermal efficiency of different gas engines varies, of course, with the type of engine and the goodness of the design. Automobile, marine, and the smaller engines have efficiencies of 15 to 20 per cent. The large gas engines have efficiencies ranging from 15 to 30, and even 35 per cent (brake efficiency). The theoretical heating value of a fuel can never be realized in a gas engine, but it has been customary to figure the thermal efficiency by taking the theoretical values as 100 per cent. The heat losses are about as follows: Heat absorbed by the water jacket, 30-50 per cent; heat converted into work (indicated), 15-40 per cent; heat lost in the exhaust, 30-40 per cent. From the heat converted into work must be subtracted the work done in overcoming engine friction, which may run from 1| to 10 per cent, and the remainder will be the output of the engine. The water-jacket loss can be reduced by circulating the water slowly so that the temperature of the cylinder walls and other parts is kept just below the danger-point. This is a difficult thing to do in practice. The heat loss in the exhaust can be reduced by providing for complete combustion and early ignition so that the pressure and temperature in the cylinder will be as low as possible when the exhaust valve opens. The efficiency of the engine, in general, can be increased by: having the greatest piston speed practicable; the greatest possible expansion; increasing the compression as much as fuel conditions will allow; rapid and complete combustion; keeping the excess of air over that required for combustion as low as possible. It may be stated as a general rule that economy will increase directly with the compression. A striking example of this is found in the Diesel engine, where a thermal 70 GAS-ENGINE THEORY AND DESIGN efficiency of 38 per cent, or more, is obtained chiefly by high compression. Another example is found in the Banki engine. The manner in which the fuel is handled puts a practical limit on the compression, for if a mixture of air and gas (or vapor) is heated by compression beyond the ignition temperature it will ignite too early and a back explosion results. If the fuel is forced into the cylinder after the piston has completed its compression stroke, then the degree of compression is limited only by the mechanical construction. The compression limit is raised in the larger engines by the use of water-cooled pistons, valves, etc. A large engine consumes less fuel per H.-P. hour than a small engine. Many of the large gas engines are sold under a guarantee to develop their full rated B.H.P. on a con- sumption of 10,000 effective B.T.U. per H.-P. hour, and at 50 per cent loads with a consumption of 13,000 B.T.U. The actual consumption of some engines under full load is 8,500 B.T.U. per H.-P. hour. Oil engines are usually guaranteed to develop their rated B.H.P. on a consumption of 1 Ib. of oil per H.-P. hour under full load. In a number of engines of more than 20 H.-P. this falls to j Ib. The performance of an average gas engine under average conditions is very different from that of an engine carefully adjusted and tested by an expert under the very best possible conditions. 95. ADVANTAGES AND DISADVANTAGES. The more im- portant advantages of gas engines may be summed up as follows: Small space occupied as compared with the steam plant; can be quickly started and stopped at any time; simplicity; fuel is consumed only while the engine is run- ning; economy of fuel; cheap fuels can be used; com- paratively low cost of upkeep and attendance. Among the disadvantages are: Inability to carry over- GAS-ENGINE EFFICIENCY 71 loads unless specially designed; decreased efficiency when run at less than full loads; regulation often poor; cannot be started under load; irreversibility. Most of the disadvantages can be overcome to a great extent, or altogether, by good design, workmanship and proper handling. 96. MEDIA USED IN HEAT ENGINES. Many attempts have been made to use media other than air and steam in heat engines. Experiments too numerous to mention have been made with alcohol, chloroform, ammonia, naphtha, ether, etc., but the results have been unsatisfactory for various reasons. The advantage of a low specific heat is counterbalanced, perhaps, by the greater weight of the medium required per stroke. Some of these media are expensive, others dangerous, some have an offensive odor, others are explosive, irrespirable, etc. Air and water pos- sess two immense advantages: they are abundant and safe. 97. OTHER TYPES AND CYCLES OF HEAT ENGINES. The various cycles, such as the Brayton, Lenoir, etc., have al- ready been mentioned. The limitations of the compression cycle have been pointed out in the preceding paragraphs. The other types include hot-air engines, engines using media other than air or water, etc. Hot-air engines, in which a body of air confined in a closed space is generally heated from the outside, have been used to a limited extent for small pumping outfits, but in larger powers are too bulky to be commercially successful. Naphtha engines, in which naphtha is used in the same manner as steam in a boiler, have limited application for small marine work. Compound- ing, combinations of different media used in series, etc., have been tried, but found to be impracticable. In order to be a real success an engine must be a commercial success, and if it does not answer the commercial requirements it will be a failure, no matter how perfect it may be theoretically. CHAPTER IX EXPLOSIVE MIXTURES 98. P^xplosion has been defined as extremely rapid com- bustion as practically instantaneous combustion. The burning of the charge in the gas-engine cylinder is usually so rapid that the term "explosion" is commonly applied to this combustion, but it can hardly be called an explosion in the sense that gunpowder explodes, since the combustion in the engine cylinder requires an appreciable length of time. 99. COMPRESSION. It has been mentioned in the pre- ceding chapter that the economy is increased by increasing the compression. The reasons for this are that with air compressed into a small space the combustion is more rapid and complete than with low or no compression, the heat has a better opportunity of exerting pressure against the piston before it is absorbed by the cylinder walls, and increasing the compression practically means lengthening the stroke. With low compression only a part of the fuel may be burned. Some gases are so poor that they will not burn at all with low compression, and for this reason engines running on blast-furnace gas, for example, compress to !.">() or even 200 Ibs., w r hile engines running on illuminating gas may compress to only 75 Ibs. The method of handling the fuel, however, puts a practical limit to the allowable com- pression. Note. Sec also Cooling by Water Injection. 100. METHODS OF HANDLING FUEL. There are prac- tically three methods of handling the fuel, viz. : Compressing an explosive mixture of air and fuel and igniting by the 72 EXPLOSIVE MIXTURES 73 electric spark; compressing air only and injecting the fuel upon completion of the compression stroke; compressing the air and injecting the fuel into a vaporizing chamber during the compression stroke. The simplest method of handling the fuel is to draw a charge of air and gas (or vapor) into the cylinder during the suction stroke and then compress this explosive charge. This mixture must be kept below the ignition temperature during compression in order to prevent pre-ignition, and this is the one great disadvantage of the method. In the larger engines, as mentioned before, the compression limit is raised by cooling the piston and valves as well as the cylinder and cylinder-head, and by diluting the explosive mixture with an excess of air. The method of injecting fuel into the combustion space after the compression stroke has been completed permits the compression to be increased as much as the mechanical construction will permit. This method results in high fuel economy and does away with all ignition apparatus, since the temperature of the compressed air is high enough to ignite the fuel. The disadvantages of this method are greater stresses in the machine, more friction, greater difficulty in starting, and the extra mechanism necessary for handling the fuel. One great advantage is that cheap petro- leum oils can be used, and, since the combustion is practi- cally complete, no carbon deposits in the cylinder will result. The method of injecting fuel during the compression stroke, vaporizing and igniting this fuel by means of a hot chamber, is carried out in the Hornsby-Ackroyd and several of the two-cycle oil engines. The hot chamber furnishes the necessary heat for vaporizing and igniting the heavy oils. The advantages of this method are ability to handle heavy oils, and simplicity. The disadvantages are that carbon deposits cannot be prevented (although an attempt 74 GAS-ENGINE THEORY AND DESIGN is made to confine them to the combustion chamber), and the time required for starting. 101. SCAVENGING. In the large two-cycle engines a scavenging charge is employed. Separate air and gas pumps are used. As the burnt gases pass out through the exhaust port a charge of fresh air is pumped into the cyl- inder, and following this comes the new combustible charge. The scavenging charge clears the cylinder of all burnt gases and also prevents prc-ignition, since the hot exhaust gases cannot come in contact with the fresh combustible charge. In the four-cycle engine the fresh charge cannot exceed in volume the piston displacement, and a body of burnt gases equal in volume to the compression space always remains in the cylinder. By scavenging and pumping in the fresh charge the entire cylinder is rilled with an explosive mixture and the power output of the engine is increased. 102. DILUTION OF EXPLOSIVE MIXTURES. Experiments have shown that combustion is more rapid, and the highest pressures are obtained, when the volume of air is only slightly in excess of that required for combustion. An excess of either air or gas hinders combustion besides mak- ing an extra amount of fluid to be heated. Dilution may be carried to such a point that ignition will not take place at all. As the dilution increases the rise in pressure during explosion decreases until finally there is practically no rise in pressure. Failure to ignite, and consequent stopping of the engine, may result from either too rich or too poor a charge. For rapid and complete combustion, furthermore, the mixture of air and fuel should be as intimate as possible. 103. INCOMPLETE COMBUSTION. Incomplete combustion not only means less power, and a waste of fuel, but may produce carbon deposits in the cylinder. Such deposits are liable to cause p re-ignition. They will also mix with the lubricating oil and form a gummy paste which rapidly EXPLOSIVE MIXTURES 75 wears the cylinder, piston, and valves. It is chiefly for this latter reason that the heavy oils are so difficult to handle in the internal-combustion engine. Incomplete combustion may also cause explosions in, and overheating of, the exhaust pipe, as well as causing a bad odor and a smoky exhaust. 104. THE COMBUSTION CHAMBER should have as little cooling surface as possible. There should be no projecting points or corners. These tend to become incandescent and cause pre-ignition. FIG. 29. There should be no corners, pockets, or channels in which explosive mixtures may lodge. Under certain conditions explosions in such pockets and channels may set up pulsa- tions, or explosive waves, which may become so intense as to cause serious damage. Fig. 29 shows how this condition affects the indicator card. A somewhat similar card is also produced by a non-uniform explosive mixture. Since burnt gases always remain in the cylinder unless scavenged, the igniter must be so placed that the fresh charge can reach it. Failure to ignite will obviously result if the igniter is surrounded by burnt gases. CHAPTER X MIXING VALVES AND CARBURETERS 105. MIXING VALVES. In the stationary gas engine some provision must be made for mixing the charge of air and gas thoroughly before it enters the cylinder. A non- homogeneous mixture means poor combustion. This mix- ing is usually done by means of a mixing valve. Fig. 30 shows a simple arrangement for this purpose. The gas enters at A and its flow is regulated by the valve B. The air enters at C and the mixture of air and gas passes out at D. The valve B can be arranged with a micrometer attachment for accurate adjustment. There are, of course, different styles of mixing valves, but the object in each is to secure an intimate mixture of the air and gas. All the air required for combustion may, or may not, pass through the mixing valve. 106. VAPORIZERS. In stationary engines using a liquid fuel a vapor- izer is used for atomizing the fuel and mixing it with the air. Fig. 31 shows the general arrangement of one style of vaporizer. The fuel is kept under sufficient pressure to carry it into the vaporizer either by elevating the tank or by putting the liquid under air pressure. A small pump can, of course, be used instead, if 76 MIXING VALVES AND CARBURETERS 77 desired. The fuel enters at D and its flow is regulated by the needle valve C. During the suction stroke the air enters at A and the fuel is atomized in pass- ing through the restricted opening controlled by C 1 . The overflow pipe E leads away the surplus fuel. 107. CARBURETERS. The vaporizer gives very good results for steady loads and speeds, but where the speed varies greatly, and changes quickly, a carbureter is used. Fig. 32 illustrates the general princi- ples upon which carbureters are constructed. The fuel enters at A and is kept at a constant level by the float B. FIG. 32. The float is so adjusted that it keeps the fuel level about T V" below the opening in the atomizer C. The air comes 78 GAS-ENGINE THEORY AND DESIGN in through D and, in rushing past C, creates a vacuum so that a fine atomized spray of the liquid is drawn from C and mixed with the air. The amount of car- bureted air passing through E can be regulated by the valve F. Since the vaporizing of a liquid results in lowering temperature, the incoming air should be warmed somewhat, and this can be done by flanging the pipe D and leading some of the hot exhaust gases past. The atomizer breaks the liquid up into very fine particles, but heating the fuel vaporizes it and makes a more intimate mixture of air and fuel possible, resulting in more power with a lower fuel consumption. In the case of alcohol and kerosene this preheating of air is necessary in order to obtain the best results. The air may be heated at both G and H. When kerosene and the heavier oils are used with low compression the oil seems to decompose in the engine cylinder and carbon deposits result. It is claimed that if the carbureted charge is drawn through a vaporizer at a red heat (H in Fig. 32) that no carbon deposits will form in the cylinder. Warming the incoming air accomplishes three purposes: it restores to the liquid the heat lost in evaporation and so prevents an undue cooling of the liquid; it makes a more homogeneous combustible mixture, which means more economy; it neutralizes the effect of moisture in the air and fuel. Too much moisture interferes with combustion al- though a slight amount is beneficial and trouble is some- times experienced in wet weather or when there is con- siderable water in the fuel. Gasolene, kerosene and the other oils contain more or less water, while alcohol is usually largely diluted with water. If only a part of the air necessary for combustion is drawn MIXING VALVES AND CARBURETERS 79 through the carbureter the increase in volume due to heating will amount to very little. The starting of an engine operating on alcohol or kero- sene is apt to be more difficult than in the case of gasolene unless some provision is made for warming the incoming air previous to starting. There is a considerable difference of opinion regarding the value of pre-heating as discussed above. Some ex- perimenters insist that better results are obtained by breaking up the liquid fuel as much as possible by means of atomizers, since, if the preheated charge cools on its way to the combustion chamber, the fuel will simply con- dense and no good has been accomplished by pre-heating. Other experimenters claim that good results can be ob- tained by pre-heating. Experiments made by the author lead him to believe that very good results can be obtained by pre-heating, provided care is taken that the fuel does not condense previous to ignition. The cleanest method of handling any fuel in the com- bustion chamber is doubtless by introducing it in the form of a gas free from impurities. The final solution of the method of handling heavy oils may be to gasify them. One method of doing this has been mentioned under "Oil Water Gas." A point that must be borne in mind in carbureter design is that while the mixture may be right for slow speeds, it will be too rich at high speeds, or, if right for high speeds, it may be too lean for slow speeds. Provision must there- fore be made to secure the right mixture at varying speeds. Another point is that the air should pass through D at a fairly high speed in order to obtain the best atomizing effect. A speed of 70 to 80 feet per second will give good results. A carbureter must be so designed that it will work 80 GAS-ENGINE THEORY AND DESIGN properly in spite of the jolting it may receive in an auto- mobile or boat, and when the engine is tipped up and down at different angles. The fuel must be thoroughly filtered before it roaches the carbureter. Students arc sometimes at a loss to understand how a slight variation in the adjustment of the mixing valve or carbureter may make a large difference in the horse-power. If we take the calorific power of gasolene to be 18,000 B.T.U. per pound, then this is equivalent to 14,004,000 ft.-lbs. If a four-cycle engine running at 1,000 R.P.M. con- sumes 1 pint of gasolene per H.-P. hour, then the oil con- sumption per power stroke is 1/240,000 gallon per H.-P. CHAPTER XI GOVERNING 108. FUNCTIONS OF THE GOVERNOR. An engine slows down as the load increases, and runs faster as the load de- creases; therefore a governor is necessary to take care of the variation of load by varying the power when the engine is desired to run at a constant speed as is the case in stationary practice. The governor may, of course, be ar- ranged so that it can be set for different speeds. Where both the speed and the load vary, as in automobile prac- tice, a wide range of speed and power can be obtained by throttling the charge, and by also advancing and retarding the ignition. Reliability and economy under varying loads are mat- ters of prime importance, and must be given careful con- sideration. A brief outline of the principal systems of governing will be given. 109. IMPOVERISHING THE CHARGE. Under this system the quantity of fuel used per power stroke is diminished in order to diminish the power. The advantage of this method is that the compression always remains the same, and therefore the highest efficiency so far as compression is concerned is maintained. The disadvantages are that the fuel must heat up an excess of air when running under light loads and the mixture may become so weak that it will not ignite at all in the cylinder, in which case fuel is wasted and may burn in the exhaust pipe. This method 6 81 S2 GAS-ENGINE THEORY AND DESIGN is also called quality governing, since the quality of the mixture is changed. 110. THROTTLING THE CHARGE. Under this system the governor acts on a valve which is so arranged that the charge of air and fuel is increased or diminished as the load increases or decreases. The proportions of air and fuel are not changed, but only the amount drawn into the cylinder. This method is used in the Westinghouse and a number of other stationary engines, as well as in the majority of automobile engines where the throttling is done by hand. The disadvantage of this method lies in the fact that the compression varies with the volume of the charge drawn in with a consequent decrease in economy. The economy is always less for light and overloads than for full load, but the difference in economy is frequently much more than it should be. Throttling the charge will usually allow the engine to run under lighter loads than impoverishing the charge. This second method is also called quantity govern- ing, since the quantity of explosive mixture is changed. Since the rate of combustion varies with the compression the ignition point should be advanced for light loads, therefore the ignition point as well as the throttle should GOVERNING 83 be controlled by the governor. This idea is carried out in the Rathbun and some other stationary engines. Fig. 33 shows the effect on the indicator card of varying the load in a throttling engine. 111. GOVERNING BY CUT-OFF. In this system the charge is admitted during a part of the suction stroke and then cut off by closing the valve. It is the same as throttling in so far that an incomplete charge is drawn into the cylinder at less than full load, but there is no wire-drawing, since there is no throttling. When governing by throttling or cut-off, provision should be made to prevent opening of valves due to the vacuum in the cylinder. Combinations of quality and quantity governing have been tried, so as to combine the good points of each, and are in use in several makes of engines. 112. ADVANCING AND RETARDING THE SPARK. When ignition takes place before the piston has completed its compression stroke, the spark is said to be advanced; when the spark is set past the dead center in the other direction so that ignition takes place when the expansion stroke has already begun, the spark is said to be retarded. Retarding the spark results in a late explosion diminish- ing the effective pressure on the piston, but also wasting fuel. Advancing the spark a little means earlier ignition and more complete combustion, and is of considerable im- portance in high-speed engines, but the spark may be ad- vanced so far that it will cause a back explosion and diminish the power of the engine if nothing worse. The spark may be advanced and retarded in connection with the throttling and cut-off methods of governing. Any good style of governor can be used. The governor is connected so as to operate the throttle valves and timer in whatever manner may be most convenient. CHAPTER XII IGNITION 113. Ignition may be brought about by means of a hot chamber, high compression, or the electric spark. 114. HOT-CHAMBER IGNITION. This is one of the earliest forms of ignition and is still used in some gas and oil engines. The arrangement for an oil engine is shown in Fig. 34. A cast-iron chamber, A, opens into the cylinder. Previous to starting, this chamber is heated by means of a torch to a dull-red heat and when the engine is running the oil is sprayed either into the chamber direct, or into the cylinder, and is then carried into the chamber during the com- pression stroke. The point at which ignition takes place depends upon the time at which the oil is injected and upon the general form of the hot FIG. 34. chamber. The narrow neck connecting the chamber with the cylinder delays combustion. When the engine has been running a little while the torch may be taken away, as the chamber is now kept at the proper temperature by the heat of combustion. The disadvantages of the above method are the time required for starting and the impossibility of timing the ignition with anything like accuracy, especially with a fuel of varying quality. The advantages are simplicity and that the hot chamber contains sufficient heat to vaporize and ignite the heavy oils. 84 IGNITION So In the gas engine a small porcelain tube opening the cylinder is used in place of the cast-iron chamber. 115. IGNITION BY COMPRESSION. Air may be com- pressed to such a degree that the temperature will be 1000, or even more a temperature amply sufficient to ignite any fuel. Where such high compression is used the fuel must be injected when the compression stroke has been com- pleted, otherwise pre-ignition takes place. Note. See also Cooling by Water Injection. 116. ELECTRIC IGNITION. Electric ignition is now in almost universal use on gas and gasolene engines. The great advantage of this method is that ignition can be timed with absolute certainty. The principal systems of electric ignition will be briefly described. The source of the electric current may be a primary battery, a secondary (stor- age) battery, a dynamo or a magneto. A dynamo is self-exciting, while the field of the mag- neto is composed of per- manent steel magnets. 117. THE JUMP- SPARK SYSTEM. Fig. 35 is a dia- gram of the jump-spark sys- tem. This is made up of a battery (usually several primary batteries connected in series) A which furnishes FIG. 35. the current, a revolving disc B which opens and closes the circuit (the shaded portion of the disc is a conductor, the rest is a non-conductor), an induction coil C and a spark- plug D which is screwed into the engine cylinder. In the spark-plug, as here shown, are two insulated wires 86 GAS-ENGINE THEORY AND DESIGN which project into the cylinder and have the ends so bent that the current has to jump across a small air space. In jumping across this space the current produces a spark which ignites the explosive mixture. 118. THE INDUCTION COIL. The function of the induc- tion coil is to convert the low-tension battery current into a high-tension current which is capable of jumping across the air gap between the plug terminals. A low-tension current cannot do this. The coil is composed of an iron core a, a primary winding of a few turns of heavy wire 6, a secondary winding of many turns of fine wire c, a small piece of iron d held against an adjustable screw e by a spring, and a condenser /. When the primary circuit is closed by B a low-tension direct current flows through the primary winding and magnetizes a which attracts d. As d jumps toward a it breaks the circuit, the current ceases to flow and a ceases to be a magnet, so the spring pulls d back against e. This breaking and closing of the circuit at d takes place very rapidly and con- tinues so long as the circuit is not broken at B. d is called a vibrator. The action of the pri- mary current induces a high tension alternating current in C. The condenser / consists of sev- eral sheets of tinfoil insulated from each other and its function is to store the current at one period and give it out at another, increasing the efficiency of the coil and preventing injurious sparking at d. This system is used principally on high-speed engines. 119. THE MAKE-AND-BREAK SYSTEM. Fig. 36 is a dia- FIG. 36. IGNITION ,S7 gram of the make-and-break system. The current is furnished by the battery A, the cam B pushes the rod D into the cylinder so that it makes a sliding contact with E. The object of a sliding contact is to remove any soot which may be on the contact surfaces. The soot would act as an insulator and prevent the current from passing. The cir- cuit is completed by a primary winding around the iron core C. When the cam releases D the latter is pushed back by a spring, breaking the contact with E, and as the con- FIG. 37. tact is broken a spark is produced. The current here used is a low-tension direct. The ordinary dynamo or magneto will not furnish suf- ficient current at a slow speed for a good spark, and for this reason engines are frequently started on a battery current and after they have speeded up the battery is cut out and the current is furnished by a magneto. In Fig. 36, a and b are switches, and F is a magneto, or dynamo, driven by the engine. The make-and-break system is generally used on slow- and medium-speed engines. It is more reliable and fur- nishes a better spark than the first system, but has the dis- GAS-ENGINE THEORY AND DESIGN advantage of requiring moving parts in the engine cylinder. This system is also used on alcohol and kerosene engines since these fuels are more difficult to ignite than gasolene or gas, and require a better spark than the jump-spark generally furnishes. 120. TIMERS, DISTRIBUTORS. Fig. 37 shows the wiring, etc., for a multiple-cylinder engine. A is the magneto, B is the primary winding of the induction coil, D is the timer, E is the distributor, C is the secondary winding, 1, 2, 3, and 4 are four cylinder-heads into which the sparking-plugs are screwed. In the position shown the current is flowing through the primary circuit. In the secondary circuit the plug in cylinder 1 is receiving current. The discs D and E are mounted together on one shaft and it can readily be seen how all four plugs receive cur- rent successively during one revolution of the dis- tributor shaft. The cyl- inders in this case are usually fired in the order 1, 3, 4, 2. Some six- cylinder engines are fired in the order 1,4, 2, 6, 3, 5. The ignition can be advanced or retarded by rotating the casing carrying the brushes aa and contact- points bbbb. In all electric-ignition systems proper insulation must, of course, be provided for. 121. STORAGE-BATTERY SYSTEM. In Fig. 38 (the Apple system) the dynamo A charges the storage battery B, and the current required for ignition is taken from the battery; FIG. 38 IGNITION 89 dddd are induction coils and eeee the spark-plugs. These plugs may be grounded on the frame so that only one re- turn wire is necessary. This does away with the wiring enclosed by the dotted lines. It will be noticed that the double-disc system in Fig. 37 does away with the use of a separate coil for each spark- ing-plug. The advantages of the dynamo and storage- battery com- bination are that current is always available for both ignition and lighting, and starting can be done without having a primary battery in circuit. FIG. 39. Fig. 39 shows a very simple ignition system (Atwater Kent). This consists of a primary battery, induction coil, distributor, and sparking plugs. Since the amount of cur- rent required for ignition is very small, and there is no waste of current, it is claimed that the battery will last a long time. 122. HIGH- AND LOW-TENSION MAGNETOS. Magnetos are now on the market which are so wound that no induction coil is required, and which will furnish sufficient current at a slow speed for starting, so no primary battery is re- quired. A prominent one is the Bosch. In the Bosch high-tension magneto both the primary and secondary windings are on the armature so that the magneto furnishes an alternating high-tension current 90 GAS-ENGINE THEORY AND DESIGN direct. The magneto also contains a distributor and pro- vision for advancing and retarding the spark. The arma- ture rotates and the speed of rotation depends upon the number of engine cylinders supplied with current. The Bosch low-tension magneto furnishes an alternating current. The armature oscillates instead of rotating. The low-tension current may be used with a make-and-brcak mechanism, or with a special spark-plug which is mag- netically operated. The magnetic plug permits a low- tension current to be used for high-speed work, with consequently no insulation troubles. No mechanical make-and-brcak mechanism is required. The plugs are connected to the magneto with single-wire cables. 123. CONCLUSION. In conclusion it may be stated* that a weak spark will not ignite an explosive mixture: there must be sufficient heat to start combustion; a high tem- perature of the spark is not enough if the spark is of very short duration. A poor ignition system will not furnish a good spark. The ignition should be arranged to take place near the centre of the explosive mixture. If the points of a spark-plug become coated with soot the spark cannot jump across. Engines are sometimes ar- ranged with two plugs in each cylinder so that if one set causes trouble the other set can be used, thus avoiding stoppage of the engine, or both sets can be used together. The insulation should be protected from heat, oil, water, etc., and all connections should be so arranged that they cannot work loose. CHAPTER XIII COOLING 124. In order to prevent overheating of the cylinder walls, cylinder head, valves, etc., it is necessary to make provision for carrying away a part of the heat generated during com- bustion. This is done by using either water or air as the cooling medium. 125. WATER COOLING. On account of its great heat- absorbing capacity water is generally used. Fig. 40 shows the siphon system as applied to a horizontal engine. The 1 FIG. 40. FIG. 41. cold water enters the water jacket from below and at the hottest end of the cylinder. The hot water rises to the top and establishes a circulation. When a more rapid circulation of the water is necessary a pump is used. Fig. 41 illustrates the method of cooling used for auto- mobile engines. Since a large water-tank cannot be carried a special arrangement must be provided for getting rid of 91 92 GAS-ENGINE THEORY AND DESIGN the surplus heat. The pump A is geared to the crank or cam shaft. The radiator B is of the usual honeycomb style, having a very large radiating surface. The fan C draws air through the openings in the radiator so that a large amount of heat can be abstracted in a short time. This, as most water-cooling systems, is really a combination of water and air cooling. At slow speeds, and where the water is free from injurious substances, a plunger pump may be used. In marine prac- tice, where the cooling fluid is taken from the water in which the boat moves, a centrifugal pump is generally best, since it will allow dirt and small obstacles to pass through without becoming clogged. The inlet end of the water pipe should be protected by fine wire gauze so that only very small obstacles can pass through. The rapid and free circulation of the water is usually counted upon to cany the impurities out again. A point to be borne in mind is that, while a pump may furnish sufficient water at a high speed, it may not furnish enough to prevent the engine from overheating when run- ning at a slow speed. If the pump furnishes sufficient water at a slow speed it may cool the engine too much at a high speed. 126. COOLING BY BOILING. A good method, where there is no scarcity of water, is to simply allow the water to boil away. This keeps the entire water jacket at a temperature of about 212 and the temperature of the water remains the same, no matter whether the engine is running fast or slow. The amount of water boiled away in this manner per H.-P. will be quite small. This does away with a large cooling-tank, pump, piping, etc. The steam can also be led into the exhaust passage and so help to cool it. 127. Am COOLING. In order to do away with a large radiator, pump, piping, joints which may become leaky, COOLING 93 etc., several automobile manufacturers have adopted the air-cooling system in which, Fig. 42, the engine cylinder is provided with a number of flanges, giving a large radiat- ing surface, and the cooling effect is further increased by forced-air circulation. The fuel economy is somewhat higher than in the water-cooled systems, since the temperature of the cylinder walls is higher. The cylinder requires more lubricating oil than a water-cooled cylinder. This system is not appli- cable for cylinders of more than 10 H.-P. since the heat cannot be carried off with sufficient rapidity. In this connection it might be men- tioned that cooling becomes a very FIQ 42 serious problem as the cylinder dimen- sions increase. This is especially so in the case of high-speed engines whose limit at present seems to be 7 x 1" or 7 x 8". 128. COOLING BY WATER INJECTION. Many experiments have been made with water injection with a view to lower- ing the water-jacket losses, the idea being that the water injected into the combustion chamber would absorb a part of the surplus heat of combustion and by its expansion (as steam) increase the power of the engine. The result has been that the temperature was lowered too much and a loss instead of a gain resulted. In the Banki motor water is used for the purpose of absorbing a part of the heat generated during compression and so decrease the work of compression, also making higher compression pressures possible without danger of pre-ignition. In this engine a water- vaporizer is located in the air-suction pipe, and fuel, water, and air are drawn into the cylinder together. The compression is about 230 Ibs., and the explosion pressure is about 600 Ibs. The thermal efficiency is about 30 per cent. 94 GAS-ENGINE THEORY AND DESIGN 129. COOLING WATER REQUIRED. The amount of water required for cooling may be calculated by figuring that the heat carried away by the jacket water is equal to the in- dicated horse-power, and then taking the difference between the temperatures of the incoming and outgoing water. The latter should be in the neighborhood of 180 and should be kept as nearly constant as possible. The tank capacity for stationary engines is usually fig- ured as 25 gal. per H.-P. In automobile engines about 1 gal. per H.-P. is figured, and the radiator must have sufficient capacity to dispose of the total B.T.U. as rapidly as they are absorbed by the water. In air cooling the amount of radiating surface required varies with the design. In the Franklin engine an auxiliary exhaust valve is used for disposing of the exhaust gases quickly. The auxiliary exhaust, which is described elsewhere, has an important bearing on the cooling of the cylinder. 130. ANTI-FREEZING SOLUTIONS. In order to prevent the jacket water from freezing during cold weather, while the engine is not running, various substances are added to the water. Among these are glycerine in proportions of half and half; also calcium chloride in proportions of 1 to 2 by weight. The filtered solutions should be used. Alcohol is sometimes used in place of water and this will freeze at a lower temperature than either of the above. Oil has also been used in place of water. Draining off the jacket water when the engine is not in use will overcome this trouble in the case of a stationary engine. Water containing lime, or any substance that will either form a coating or corrode the metal, must not be used. If this point is neglected, a reliable running of the engine cannot be expected. A.EE-X CHAPTER XIV EXHAUST 131. The noise of the exhaust should be muffled as much as possible. This is caused by the rapid expansion of the exhaust gases when they strike the atmosphere. In sta- tionary installations the gases should be carried away in such a manner that they will not cause annoyance on ac- count of noise, odor, or smoke. The exhaust should be arranged so that no damage will result from overheating of, or explosions in, the exhaust pipe. The muffling should be done without causing back press- ure. The engine exhaust pas- sage is sometimes water-jacketed to prevent overheating. Fig. 43 shows the customary method of arranging the exhaust in sta- tionary engines. The exhaust gases from the engine pass through the pipe A into a cast- iron, or riveted steel, vessel, B, which is placed some distance underground. The gases ex- pand and cool to some extent in B, and then pass out and into the atmosphere through the pipe C. By making B fairly large in proportion to the cylinder volume (at least ten times this volume) , and the pipe C long, a practically noiseless exhaust is obtained. 95 FIG. 43. 96 GAS-ENGINE THEORY AND DESIGN 132. THE MUFFLER for portable engines should be so designed as to secure a gradual expansion of the gases and consequent reduction of pressure, the speed of the exhaust gases depending upon the diameter of the exhaust passage, ,- ^ A x \ e= ^ .^ ^ 1 ^ t ^ 1 - | FIG. 44. which should be as large as convenient. Fig. 44 shows the arrangement of a muffler which may be placed near the engine. The sketch is self-explanatory. 133. A MARINE EXHAUST is shown in Fig. 45. The ex- haust gases are discharged through the bottom of the boat, and consequently under the water, doing away with noise and smoke. A valve should be provided so that the ex- haust passage can be closed when desired ; otherwise if the engine sets low in the boat, the water may back up into the cylinders when the engine is not running. The cooling FIG. 45. and consequent reduction of volume of the exhaust gases is further assisted by leading the discharging cooling-water into the exhaust passage through pipe A as shown. In the under-water exhaust care must be taken to proportion the passages so that there will be as little back pressure as possible. This is especially the case in the small two-cycle engine, which is usually so sensitive to back pressure that the engine may easily bo stopped from that cause. EXHAUST 97 134. THE AUXILIARY EXHAUST. In a number of engines an auxiliary exhaust valve operated by a cam, or an auxil- iary exhaust port uncovered by the piston toward the end of the stroke, is provided for several reasons. As the gases f)ass through the auxiliary exhaust the pressure in the cylinder falls rapidly and the exhaust valve proper is not forced open against a considerable pressure, as is ordinarily the case. The hottest gases pass out through the auxiliary port, lessening the danger of overheating the exhaust valve. The exhaust gases are disposed of quicker and a cooler cylinder results. In the larger stationary engines a bal- anced water-cooled exhaust valve dispenses with the necessity of an auxiliary exhaust. CHAPTER XV SELECTION OF TYPE 135. In selecting the type of engine to be designed the advantages and disadvantages of the various constructions must be studied and the designer can then choose the type most suitable for the work to be done. Following is the general classification according to the mechanical construc- tion : Two-cycle or four-cycle. Horizontal or vertical. Single-acting or double-acting. Single-cylinder or multiple-cylinder. 136. TWO-CYCLE OR FOUR-CYCLE. Early experiments with two-cycle engines proved unsatisfactory and the four- cycle type was built almost exclusively for a while. During the past few years the two-cycle type, in both small and large horse-powers, especially in the large, has proven very successful. In small powers the two-cycle type has the advantage of simplicity and cheapness and is at the present time very largely used in marine practice. In large powers the two-cycle type is almost a necessity since the four-cycle machine becomes excessively bulky for the power developed. Many of the objections which apply to the small two-cycle engine with crank-case compression do not apply to the large machines, where a charge of air and gas is delivered to the engine cylinder by means of separate pumps. The advantages of this latter type are that the cylinder is com- pletely charged with a combustible mixture and that there 96 SELECTION OF TYPE 99 is a power stroke for each revolution. The disadvantages are that the time during which a fresh charge can be ad- mitted, and the burnt gases exhausted, is exceedingly short, and that there is the extra pump mechanism. In the small engine with crank-case compression the volume of the fresh charge is somewhat less than the piston displacement with a consequently smaller amount of power developed. There is also more or less danger from back explosions since the hot gases and fresh charge come in contact. The advantages of the four-cycle engine are chiefly the greater length of time available for exhausting the cylinder and for filling it with a fresh charge. The disadvantages are that there is only one power stroke for every two revo- lutions, and the extra valve gearing. The claim generally made for the four-cycle engine, that it is more reliable than the two-cycle, unfortunately is often true, but does not hold good for a properly-designed two-cycle machine. In the small two-cycle engine the work done in com- pressing the charge in the crank-case, and in the large engine the work done in operating the pumps, must be deducted from the I.H.P. 137. HORIZONTAL OR VERTICAL. For stationary gas engines of large power (especially double-acting engines) the horizontal form is used almost exclusively at the present time, but the vertical type, on account of its many inherent advantages (especially in the multiple-cylinder form) is steadily gaining in favor. The horizontal type is heavier and bulkier, the cylinder is more apt to spring out of shape, the weight of the recip- rocating parts causes extra wear on the lower side of the cylinder, and it is more difficult to make the piston air-tight. The advantages are that the water-cooling of the various parts is generally more readily accomplished, the impurities (carbon, etc.) are swept out with the exhaust a very im- 100 GAS-ENGINE THEORY AND DESIGN portant point, and in the double-acting two-cycle type the arrangement of the valves is simpler. The vertical type is lighter, more compact, lends itself to multiple-cylinder construction, possesses a greater mechanical efficiency, the lubrication is better, the arrange- ment of valve gearing is simpler, it is capable of better balancing and higher speed, the various parts can be made more accessible, it is cheaper to manufacture and install. It would seem as if a vertical, double-acting, multiple- cylinder type would possess many advantages over the horizontal twin-engine construction the opinions of some experts to the contrary notwithstanding. 138. SINGLE- ACTING OR DOUBLE-ACTING. The single- acting engine is simple and easy of construction in small powers. In large powers, however, it becomes excessively bulky, the various parts become heavy and difficult to manufacture, sound castings, accurate machine work, and resulting reliability of performance are difficult to obtain. When the reciprocating and rotating parts weigh many tons they are expensive .to manufacture. The double-acting cylinder is much smaller for the same 'power, or, for the same size, there is double the power with a moderate increase in length. It has more mechanism. Since there is a crosshead there is no side thrust on the piston. The double-acting cylinder requires a better cooling arrangement for valves, piston, etc., which must be water-cooled as well as the cylinder. This type becomes a necessity for large powers. 1.39. SINGLE-CYLINDER OR MULTIPLE-CYLINDER. For a given power the single-acting cylinder is bulkier, heavier, irregular in running, and, except for small powers, more expensive. There is a greater pressure on the cylinder-head and through the mechanism. It is more difficult to cool. The objections mentioned in the preceding para- SELECTION OF TYPE 101 graph regarding size and weight of parts apply equally well here. The multiple-cylinder machine is lighter, smaller, carries a much smaller fly-wheel, and as there is a more continuous and steadier turning effort, the engine can be better bal- anced. However, the multiple-cylinder engine must be better designed and constructed so that each cylinder will do its share of the work. Whenever an engine is to be designed for a certain class of work the designer must decide for himself how far the various advantages and disadvantages enumerated in the foregoing apply and be governed accordingly. 140. SMALL UNITS vs. LARGE UNITS. A word might here be said about installing several engines in place of one unit where a certain amount of power is wanted. If, for example, 1,200 H.-P. is required in a power station, there are many advantages (aside from the question of cost, which is usually less) in having, say, four units of 300 H.-P. each in place of one unit of 1,200 H.-P. If any- thing happens and things are bound to happen and one engine must be stopped for a while, the others will continue running and the entire plant is not put out of commission. Many electric-light and power stations, pumping stations, etc., have installed a number of smaller units in preference to one of larger size. CHAPTER XVI DETERMINATION OF THE PRINCIPAL DIMENSIONS 141. POWER. The power of a gas engine depends prin- cipally upon the volume of air (or air and gas) that it can handle in a given time say the number of cubic inches per H.-P. per minute. Fuel requires a certain amount of air for complete combustion and the greater the amount of air handled per minute the greater the amount of fuel that can be burned, and the greater the amount of heat that is liberated and converted into mechanical energy in a given time. After having decided upon the type and power, then the cylinder bore, piston stroke, and R.P.M. can be deter- mined. A 4^ x 4 \" (bore and stroke) four-cylinder auto- mobile engine, running at 1,200 R.P.M., will develop as much 'power as an 8 x 12" single-cylinder engine running at about 400 R. P.M., if the fuel is the same, since the volume of air handled per minute in each case is nearly the same. The power developed by an engine of a certain bore and stroke depends to a certain extent upon the fuel used. An engine running on natural gas, a rich fuel, will develop more power than when running on blast-furnace gas. 142. COMPRESSION. The allowable compression varies with the method of handling the fuel and the method of cooling. Where a charge of air and fuel is compressed in the cylinder the danger of pre-ignition puts a practical limit on the degree of compression. The methods of increasing the compression limit by more extensive cooling, adding a surplus of air, etc., have already been discussed. In the smaller engines the compression ranges from 75 to 100 Ibs. 102 DETERMINATION OF PRINCIPAL DIMENSIONS 103 per sq. in. abs., while in the larger engines operating on lean gases the pressure runs from 150 to 200 Ibs. For small engines operating on liquid fuels, or illuminating gas, 75 to 90 Ibs. is about right. When the compression pressure has been decided upon, the size of the compression space can be calculated from the curves given in the next chapter, after the bore and stroke are known. 143. PISTON SPEED. The piston speed is limited by the mechanical construction and the strength of the materials used. Too great a speed will increase the inertia and other forces beyond safe limits, and will also cause undue wear. Some average speeds are given below: Small stationary engines 600 ft. min. Medium " * " 800 " Large " " 900 " High-speed engines 900 Automobile and marine engines are sometimes run at piston speeds of 1,200 ft. per min., and even more, but such excessive speeds cannot be recommended. The piston speed means the total piston travel back and forth. 144. CYLINDER VOLUME ; AIR REQUIRED PER H.-P. The air required in cubic inches per H.-P. minute varies with the fuel and assumed thermal efficiency. Stationary gas engines are usually guaranteed by the builders to develop their rated power on 10,000 B.T.U. per H.-P. hour, which would mean an efficiency of about 30 per cent for the I.H.P., and 25 per cent for the B.H.-P. The amount of air required for the combustion of carbon, assuming complete combus- and a thermal efficiency of 100 per cent, would be about 900 cu. in. per H.-P. minute. Assuming an actual efficiency of 20 per cent, then five times this amount, or 4,500 cu. in., would be required. The actual volume of air, or air and gas, handled per H.-P. minute varies in different engines from 104 GAS-ENGINE THEORY AND DESIGN 5,000 to 6,000 cu. in. Where an overload capacity is de- sired the cylinder volume must be increased. The amount of air required can readily be figured from the calorific power of the fuel and the air required for complete combustion of 1 Ib. of the fuel. For example, 1 H.-P. requires 42.4 B.T.U. per minute, when an efficiency of 25 per cent is assumed 170 B.T.U. are required and enough air must be furnished to burn this fuel, plus a certain percentage for fluid losses (leakage, wire-drawing, etc.) and for an overload capacity. The total cylinder volume will be the piston displace- ment plus the compression space. 145. BORE AND STROKE. Having found the amount of air to be handled per minute, and the piston speed in feet per minute, the bore and stroke can readily be computed. For example, if the piston speed is taken as 900 feet, and the engine is desired to run at 450 R.P.M., the stroke will be 12". In a four-cycle engine the cylinder volume would then be the amount of air required per minute in cu. in. 450 X 12 divided by The ratio of bore to stroke can be varied to suit conditions. In the high-speed engines this ratio is usually 1 to 1, some- times 5 to 4, although of late there is a tendency to make the stroke more than the bore; in stationary engines of ten 1 to 1.2, and in the larger engines 1 to 1.5. This last ratio is seldom exceeded. With a moderate speed, and the stroke somewhat greater than the bore, there will be more time for ignition, combustion, charging, and exhausting, and a better fuel efficiency results. In the case of an automobile engine, or other portable motor, power with light weight is the most important consideration, and fuel economy is of secondary importance. The cylinder volume is frequently figured from the M.E.P., etc., instead of from the amount of air required DETERMINATION OF PRINCIPAL DIMENSIONS 105 for an engine of a certain power, but the method here given is the simplest for beginners. 146. EFFECT OF ALTITUDE. Since the density of the air decreases with the altitude, a gas engine will develop less power at a high altitude than near the sea level, and the work of compression will be somewhat less. The effect of altitude can readily be computed from Table V. 146a. TABLE V EFFICIENCIES AT DIFFERENT ALTITUDES Altitude in Feet Barometric Pressure in Inches of Mercury Barometric Pressure in pounds per Square Inch Volumetric Efficiency of Compression Per cent Loss of Capacity Per cent Decreased Power Required per Stroke 30.00 14.75 100 1,000 28.88 14.20 97 3 1.8 2,000 27.80 13.67 93 7 3.5 3,000 26.76 13.16 90 10 5.2 4,000 25.76 12.67 87 13 6.9 5,000 24.79 12.20 84 16 8.5 6,000 23.86 11.73 81 19 10.1 7,000 22.97 11.30 78 22 11.6 8,000 22.11 10.87 76 24 13.1 9,000 21.29 10.46 73 27 14.6 10,000 20.49 10.07 70 30 i6.1 11,000 19.72 9.70 68 32 17.6 12,000 18.98 9.34 62 35 19.1 13,000 18.27 8.98 63 37 20.6 14,000 17.59 8.65 60 40 22.1 15,000 16.93 8.32 58 42 23.5 147. HEAT AND POWER UNITS, ETC. Below are given some heat, power, and other units which are very con- venient in heat-engine calculations, as well as some defini- tions of the terms commonly employed. Work is force exerted over a distance. Power is the rate of doing work; the time factor here comes in. Energy is stored work, or the capacity of performing work. Potential energy is the energy stored in a body by virtue 106 GAS-ENGINE THEORY AND DESIGN of its position. Example: A stone on the roof of a building possesses potential energy since in falling it can do work. A deflected spring and fuel both possess potential energy. Kinetic energy is the energy possessed by a body by virtue of its motion. Example: A fly-wheel possesses kinetic energy, as does also a bullet when in motion. Conservation of energy. Energy cannot be destroyed any more than matter. Energy cannot be produced except at the expense of some other form of energy; it cannot be created or destroyed, but it can change its form. 1 horse-power = 33, 000 ft.-lbs. per min. 1 " =746 watts. 1 " =42.4 B.T.U. per min. 1 " =2544.9 B.T.U. per hour. 1 " =0.746 kilowatt. 1 B.T.U. =778 ft.-lbs. 1 " =0.252 calorie. 1 caloric =3.968 B.T.U. = amount of heat required to raise the temperature of 1 kilogram of water 1 C. I kilowatt (Kw.) = 1,000 watts. -1.34H.-P. Watts = volts X amperes. I gallon = 231 cu. in. 1 gallon of water weighs 8.33 Ibs. 1 cu. ft. of water weighs 62.425 Ibs. 1 centimetre = 2.54 ins. 1 metre =3.28 ft. =39.37 ins. 1 litre =0.264 gal. 1 kilogram =2.204 Ibs. 1 in. =0.393 centimetre. 1 ft. =0.3048 metre. 1 gal. =3.785 litre. 1 pound =0.4536 kilogram. 1 atmosphere = 14.7 Ibs. per sq. in. =29.9" of mercury. CHAPTER XVII FORCES ACTING IN THE GAS ENGINE 148. In order to calculate the weight of the fly-wheel, diameter of crank-shaft, etc., it is necessary to know the character and magnitude of the forces acting in the engine, and while these cannot always be determined definitely, fairly accurate approximations can, as a rule, be made. The acting forces are due to the gas pressure on the piston and inertia of the moving parts. IDEAL INDICATOR DIAGRAM. After having decided upon the power and cylinder dimensions an ideal indicator dia- FIG. 46. gram should be laid out and kept for reference. Much can be learned from a comparison of the actual with the ideal diagram, a better understanding of the thermodynamic changes, and practical limitations of practice, will result. In Fig. 46 AB represents adiabatic compression accord- 107 108 GAS-ENGINE THEORY AND DESIGN ing to the law PV 1<41 = P 1 V 1 1 ' 41 , BC represents the rise in pressure at constant volume during explosion; CD repre- sents adiabatic expansion according to the law used for compression; DA represents the drop in pressure during exhaust. The method of plotting the PV n = K curve is given in the next paragraph. The method of calculating the theoretical temperatures of combustion has already been given, viz. : Rise in temperature = - CvXW The rise in pressure for BC can now be figured since P _T 2 P t T\" T, the absolute temperature at the point B, is found from PV PV 1 the formula = = 149. ACTUAL INDICATOR DIAGRAM. In order to figure the strains to which the engine parts arc subjected the actual indicator diagram is necessary. In designing a new engine such a diagram must be forecast as accurately as possible, although the exact diagram, of course, can only be obtained after the engine has been built and operated. A good way is to take the diagram of an engine of the same power, compression, and operating on the same fuel, when such a diagram can be obtained. Frequently this is not possible, and in such a case the following information will be helpful : In place of PV 1-41 = K the formula PV 1<35 = K is used, as this approximates more closely the actual compression and expansion curves. FORCES ACTING IN THE GAS ENGINE 109 The method of drawing these curves, as shown in Fig. 47, is as follows: The ordinates represent pressures. The abscissas represent volumes. If a line OD is drawn at angle a, then 1+tari 6 = (l+tan a) n . Find angle 6, and lay off OC. The values commonly used are given below: Factor = tan b. n tan a = .25 tan a = .15 1.35 1.41 0.352 0.37 0.2075 0.218 The smaller the angle a the more points can be plotted. The point PV being known, lay off XD = .25 XO; lay off CC X = .37 OC 1 (for n = 1.41); lay off XE at 45 and draw EE*; draw PG and lay off GG 1 at 45; draw G 1 E\ giving FIG. 47. 110 GAS-ENGINE THEORY AND DESIGN FIQ. 48. 10 20 30 40 50 60 70 80 90 100 FORCES ACTING IN THE GAS ENGINE 111 FIG. 49. 112 GAS-ENGINE THEORY AND DESIGN one point on the curve. Continue in the same way to locate the other points and draw the curve through these points. Fig. 48 is a curve plotted according to the law PV 1 ' 35 = K. Fig. 49 shows a curve plotted according to the law PV 1 - 41 = K, also a volume-temperature curve plotted according to the same law. It will be noticed that the volume first decreases rapid with a moderate increase in pressure, then the pressure increases more rapidly until the curve finally becomes nearly a vertical line. The actual explosion pressures will vary with the fuel, dilution, compression, etc., but the following may be taken as a guide : Gasolene-explosion pressure. . . . compression pressure x 4 to 4 Illuminating Gas " .... " x 3 Producer Gas " " x 2J Blast-Furnace Gas " x 2J The compression pressure for rich fuels is about 90 pounds. The compression pressure for lean fuels is about 180 pounds. 150. ANGULARITY OF CONNECTING-ROD. If we assume that the fly- wheel, and consequently the crank-pin, revolves at a constant velocity ratio, i.e., at a constant speed, then the velocity of the slider (piston or crosshead), the tan- gential effort on the crank-pin, and the side thrust on the' slider are continually varying, due to the angularity of the connecting-rod. In the following figures the ratio of length of connecting- rod to stroke has been taken as 2 to 1, the crank-pin is assumed to turn at a constant velocity ratio, and the pressure on the piston to be constant. Fig. 50 shows how the angularity of the connnecting-rod affects the slider. Angles

2. b l a l the connecting-rod; when the crank-pin is at 6 1 the slider-pin has moved from a to a 1 ; draw the line b l d. Then the distance cd represents the velocity of the slider for the position shown, as compared with the throw (one- half the stroke) which is taken as 1. This can be proved by means of the instantaneous-centre and similar triangles. The proofs for this, and some of the following statements, will not be given as they are simple geometrical and trigonometrical propositions and can be found in books on machine design. FORCES ACTING IN THE GAS ENGINE 115 Lay off a 1 / 1 equal to cd, and continue in the same manner for the other points on the semi-circle. 152. TANGENTIAL-EFFORT DIAGRAM. A part of the force that passes through the connecting-rod from the piston exerts a tangential (turning) effort, and a part of the force simply exerts a side thrust against the crankshaft bearing. In Fig. 53 let ac 1 represent to some scale the force acting on the piston, then ab represents the thrust along the con- necting-rod. Now let be represent the thrust along the connecting-rod; draw 6/at right angles to cb, i.e., tangent FIG. 54. to the circle ; complete the parallelogram ; then bf is the tangential pressure. In Fig. 54 let ai represent the pressure on the piston; draw b l d, and on cb 1 lay off b l h equal to ai; draw hg parallel to cd; gh equals the tangential pressure on the crank-pin when at the point 6 1 ; lay off a 1 / 1 equal to hg, and continue in the same manner for the other crank-pin positions. The curve on the right now shows how the tangential effort varies with a constant pressure on the piston. It must be remembered that in the actual engine the piston pressures are constantly changing and the actual pressures from the indicator diagram must be used in lay- ing out the tangential-effort diagram. 152a. OFFSET CYLINDERS. There has been a good deal of discussion in technical journals regarding the ad van- 116 GAS-ENGINE THEORY AND DESIGN tages and disadvantages of the offset cylinder. Fig. 54a, a small offset, is used by several automobile builders. The chief advantages claimed are a more equally divided side thrust equalizing wear, and a more direct thrust during the working stroke. The compression stroke is, how- ever, performed under a corresponding disad- vantage. In a single-acting steam engine this arrangement is advantageous, but in a gas engine the disadvantages seem to balance the advan- tages fairly well. The student should draw the tangential -effort diagrams (taking into account the inertia of the reciprocating parts) for an offset of \ and J of the stroke. The arrangement, as will be seen from such a diagram, is really a quick return motion. 153. SIDE THRUST ON THE SLIDER. In Fig. 55 let gb l represent the pressure on the piston; b l a l represents the thrust along the connecting-rod; then gb l represents the side thrust on the slider. FIG. 54a. FIG. 55. Figs. 56 and 57 show how this side thrust reverses during the compression and expansion strokes. The inertia of the reciprocating parts enters into the problem of sida thrust and the thrust may, in consequence, reverse during the stroke. In double-acting engines the net forward pressures must be considered in laying out these diagrams. FORCES ACTING IN THE GAS ENGINE 117 154. RATIO OF CONNECTING-ROD LENGTH TO STROKE. It can readily be seen that the longer the connecting-rod the more direct the thrust on the crank-pin until, with a connecting-rod of infinite length, the thrust would be parallel to the piston axis. The advantages of a more direct thrust, and consequently better turning effort, are overbalanced by the disadvantages of increased inertia, friction, longer engine, etc. The ratio of connecting-rod length to stroke varies from 2 to 1 in high-speed engines to 3 to 1 in slow-speed engines. 1 FIG. 56. FIG. 57. A ratio of slightly more than 2 to 1 is commonly used in automobile engines, while in stationary engines 2| to 1 is the usual practice. 155. INERTIA. A force tends to change the state of a body with respect to rest or motion. Inertia is the property of a body by virtue of which it tends to continue in its state of rest or motion until acted- upon by some force. The inertia of the reciprocating and turning parts of an engine sets up forces which affect the turning effort, balance of the moving parts, and sets up stresses in the various parts. The turning effort should be as constant as possible, and the balance should be as perfect as it can be made, although sometimes balance must be obtained at the ex- pense of turning effort, and vice versa. Perfect balance is usually impossible in the ordinary forms of construction since reciprocating parts, for instance, can be balanced only by reciprocating parts and not by turning parts. The unbalanced forces tend to rock and shake the engine. In 118 GAS-ENGINE THEORY AND DESIGN a vertical engine the reciprocation tends to lift and drop the frame alternately, and as the crankshaft is turning in one direction the frame tends to turn in the opposite direc- tion. It is a good general rule that the frame should be made stiff and the reciprocating parts as light as possible. The inertia is much greater, of course, in a high-speed engine than in a slow-speed engine, the difference on ac- count of speed may be 10-1, or even more. 156. INERTIA OF CONNECTING-ROD AND RECIPROCATING PARTS. At the dead centres the reciprocating parts and connecting-rod may be assumed to be at rest. During the first part of the stroke these parts oppose the piston press- ure as the velocity increases. During the latter part of the stroke, as the velocity decreases, these parts, on account d of their velocity and inertia, exert pressure in the direction of the piston travel. The connecting-rod partakes of both reciprocating and turning mo- tion, but one-half of the rod is usually c assumed (for the sake of convenience in calculations, and because nearly true) to have reciprocating motion and one-half to Jiavc turning motion. The following formula may be used: F =.00034 WN 2 R(l F = . 00034 WN 2 R (1-1/n) Where F is the inertia at the beginning of the stroke in Ibs., F 1 is the inertia at the completion of the stroke in Ibs., W is the weight of the reciprocating parts (piston, etc.) plus one-half of the connecting-rod weight in Ibs., N is the R.P.M., R is the radius of the crank-pin circle in feet, n is the ratio of connecting-rod length to length of crank throw. FORCES ACTING IN THE GAS ENGINE 119 Fig. 58 is an inertia diagram. F ac = - F 1 M - r is the point at which the inertia effect is zero; ab represents the length of the stroke. A is the piston area, so that ac rep- resents the inertia force in Ibs. per sq. in. of piston area. F 59 The inertia effect is zero when the angle acb, Fig. 59, is 90, therefore the point O 1 , Fig. 58, can be computed by finding the dis- tance ac in Fig. 59 (ab 2 +bc 2 = ac 2 ), arid subtracting it from the sum of the connecting-rod and crank lengths. The remainder is the distance ac in Fig. 58. The curve cod can now be drawn. This is the arc of a circle passing through the three points and approximates the true inertia curve very closely. The same scale of pressures must be used as for the indicator diagram. This inertia diagram is used in laying out the tangential- effort diagram and in computing the weight of the fly-wheel. 157. BALANCING. In a multiple-cylin- der engine the reciprocating parts pistons and connecting-rods should be balanced by weighing them on scales; the crank-shaft and fly-wheel should be FlG> 59a " balanced on knife-edges as shown in Fig. 59a. The im- portance of this balancing is obvious. To show how light the reciprocating parts are made in high-speed engines the following weights of the Franklin air-cooled automobile engine (four-cylinder, 28 H.-P.) are given: Weight of piston, rings and pin complete, 4 Ibs. 120 GAS-ENGINE THEORY AND DESIGN 6J oz.; weight of connecting- rod complete with bearings, cap screws and liners, 4 Ibs. 3 oz. 157a. COUNTERBALANCING. The connecting-rod and crankshaft may be balanced as shown in Fig. 60. Here r = r 1 = distance to centre of gravity. The weight of C I? FIG. 60. FIG. 61. FIG. 62 FIG. 63. FIG. 64. equals the weight of the crank. Then only the crank-pin and half of the connecting-rod remain to be balanced. W = W 1 + W 2 Where W = balance weight in Ibs., W 1 = weight of crank- pin, W 2 = weight of half connecting-rod. The proper way to balance is as here shown with the balance weights in the same plane with the cranks. The method of putting the weight on the fly-wheel rim (in an- FORCES ACTING IN THE GAS ENGINE 121 other plane) is not good. In order to make the latter method effective there should be two fly-wheels, and balance weights should be placed in each one. This throws out of balance a rotating part which would otherwise be balanced. In engines having three or more cylinders, especially those running at high speeds, balance weights are often omitted. The crank-pins here are 120 and 90 apart. In Fig. 61 there is not a per- fect balance since the inertia curves (Fig. 58) cross and are not the same for the cylinder po- sitions shown. There is also a couple due to the distance d. In Fig. 62 the couple is zero, but the forces are not per- fectly balanced. I ^^ f ^~^~ * In Fig. 63 the crank-pins are 120 apart, the forces are balanced, but there is a couple. In Fig. 64 there are no couples, but there are free forces. In multiple-cylinder engines care should be taken to have all pistons, connecting-rods, etc., weigh the same. 158. FOUR-CYCLE ENGINE DIAGRAMS. For a better under- standing of the preceding the following diagrams are given: Fig. 65 is an indicator card from a four-cycle single-cylinder engine. FIG. 67. Fig. 66 is the inertia diagram of the reciprocating parts ; Fig. 67 is a diagram of the piston pressures for the com- plete cycle, four strokes. 122 GAS-ENGINE THEORY AND DESIGN Fig. 68 is the combined piston pressure and inertia dia- gram. Fig. 69 is the tangential-effort diagram derived from the preceding. FIG. 68. FIG. 69. In the case of a two-cycle engine the pump diagram must be combined with the pressure diagram. In the case of a multiple-cylinder engine, the methods followed are the same as in the foregoing. CHAPTER XVIII DESIGN AND DIMENSIONS OF PARTS 159. REMARKS ON DESIGNING. The designing of a gas engine, or of any other machine, is largely a matter of ex- perience and judgment. The final form and proportioning of parts is the result of test. A number of formulas will be given in the following paragraphs, but machinery cannot be designed by formulas alone. The author has frequently found that empirical, and other, formulas would sometimes come within 500 per cent of the correct result. Designing consists of calculating the stresses acting on and strength of a part, and giving it the form best suited for the use to which it is to be put. The designer should check up his calculated dimensions with practice as far as possible. The general points to bear in mind are: (a) Each part, so far as possible, should have only one duty to perform ; (6) Each part should be as simple in form as it is possible to make it; (c) The various parts should be easy to manufacture; this includes pattern-making, foundry and machine-shop work; (d) The parts should be easy to assemble and require little fitting; (e) The wearing surfaces must admit of easy adjustment and replacement; (f) The action of every moving part should be positive; (g) Needless to say, the material and workmanship should be good. 123 124 GAS-ENGINE THEORY AND DESIGN FIG. 70. The large horizontal engines developing up to several thousand horse-power, with their heavy moving parts and high combustion pressures, present difficulties in the design and building which are peculiar to this type alone. The light, high-speed engine presents difficulties of altogether another nature. Each type re- quires a careful study of the difficulties pecul- iar to it. 160. MATERIALS OF CONSTRUCTION. The physical characteristics of the metals used for the various parts must be studied in order to make the design practical. The greater part of the gas engine consists of castings, and a knowledge of the general behavior of iron during melting, casting, and cooling is essential. The pattern-maker must make the proper allowance for shrinkage, but it is the designer's province to give the part such a form that strong castings with minimum shrinkage strains will result. In a casting having a cross section similar to Fig. 70, the thin part cools quickly and the metal hardens and becomes set. The heavy part cools slowly, the surface cools first and hardens, then the interior cools and shrinks and tends to draw in the outer portions which have al- ready cooled, creating shrinkage strains and making the interior por- ous. These shrinkage strains weaken the metal so that the casting may break when subjected to only moderate strains. Sharp corners where the thin part joins the heavy part are also a source of weakness. Very thin and FIG. 71. FIG. 72. DESIGN AND DIMENSIONS OF PARTS 125 very heavy sections should be avoided if possible. The section should be uniform throughout, or, where one sec- tion must be thinner than another, the change should be gradual, as shown in Fig. 71. The fly-wheel, Fig. 72, fur- nishes a good example of the strains set up in cooling. The arms cool first. The rim cools slowly and tends to pull out the arms, putting them under tension. The slower the castings cool the stronger they will be. If the design is good, then the matter of poor castings is up to the foundry. Scrap iron is cheap, and some of it in the frame will do no harm if cor- rectly mixed and poured, but poor metal and poor foun- dry work will result in a return of engines to the builder. FIG. 73. 161. ARRANGEMENTS OF CYLINDERS. The following fig- ures show various arrangements of gas-engine cylinders in common use to-day, and require but little explanation. u _n_ , FIG. 75. Fig. 73 is a single-acting horizontal engine, two- or four- cycle. Fig. 62 is a horizontal double-opposed engine, four-cycle. 126 GAS-ENGINE THEORY AND DESIGN Fig. 74 is a single-acting horizontal tandem engine, four- cycle. Fig. 75 is a double-acting horizontal tandem engine, four- cycle. Fig. 76 is a horizontal twin -cylinder engine, four-cycle, cranks at 90. Fig. 77 is a horizontal double-acting engine, two- or four-cycle. Fig. 78 is a horizontal double-piston two-cycle engine, Oechselhauser. Fig. 79 is a vertical single-acting engine, two- or four-cycle. Fig. 63 is a vertical single-acting engine, three-cylinder, four-cycle, cranks at 120. L n FIG. 77. Fig. 64 is a vertical single-acting engine, four-cylinder, four-cycle, cranks at 180. FIG. 78. FIG. 79. Fig. 80 is a vertical single-acting engine, six-cylinder, four-cycle, cranks at 120 ; two styles of crank-shafts shown. DESIGN AND DIMENSIONS OF PARTS 127 Fig. 81 is a vertical double-acting engine, two- or four- cycle. Fig". 82 is a four-cylinder engine with cylinders at 90, FIG. 81. FIG. 82. making a very compact arrangement, as the cylinders can be much closer together than in the ordinary multiple- cylinder type. 128 GAS-ENGINE THEORY AND DESIGN 161a. DIRECTION OF ROTATION. The horizontal engine always "turns over" as shown in Fig. 83. The vertical engine may rotate in either direction, usually counter- clockwise when the observer faces the fly-wheel. In the FIG. 83. case of a twin-screw boat the engines are right- and left- hand, i.e., turn in opposite directions, the propellers usually "turning over" and toward each other. 162. FRAMES. The drawings here given are simply in- tended to illustrate principles. B FIG. 84. During the expansion stroke the pressure in the cylinder tends to force the cylinder and crank-shaft apart and the metal in the frame which resists these strains should be as much as possible in a straight line. The conditions should approach those shown in Fig. 83. Figs. 84 and 85 are plan DESIGN AND DIMENSIONS OF PARTS 129 views of the large horizontal engine shown in Fig. 83. The outboard bearing in Fig. 84 is usually of the type shown in Fig. 108. The European builders provide three bearings for the crank-shaft, as shown in Fig. 84, while several Amer- ican builders have adopted the side crank shown in Fig. 85. In the latter arrangement there is a considerable side thrust, and, since the piston pressures are heavy, the strains are necessarily great. A very important advantage is that there are only two bearings to keep in line a thing difficult to do in the three-bearing type. When a fly-wheel weighs a trifle of fifty or eighty tons, this matter of getting FIG. 85. bearings true is an exceedingly difficult one. Unless the bearings are amply proportioned, and everything is in line, and the shaft is stiff so that there is but little deflection, the bearings will quickly wear at the edges and hot boxes will result, as well as wobbling of the fly-wheel. A fly-wheel may burst from this cause. Another point which here comes in is that, while the bearings may be in line with the machine at rest, there may be a binding of the crank- shaft when the engine is running. In Fig. 85 PL is the moment acting on the frame at the point shown; a is under tension and a^ is under compression. 130 GAS-ENGINE THEORY AND DESIGN The moment is zero at the centre x, and greatest at the extremities. Fig. 86 shows a common horizontal-frame design for small engines. The letters x designate the usual weak points, which are somewhat exaggerated in the drawing. These may be enumerated as follows: A long heavy cylinder supported only at one end; Too little metal at various points between crank-shaft and cylinder end; Bearing split on centre does not take thrust correctly. FIG. SO. Fig. 87 shows the usual vertical form and its advantages can be seen at a glance. There is no difficulty about taking care of the pressures tending to force cylinder and crank- shaft apart and to arrange for multiple-cylinder construc- ion. The section AB is treated as a beam fixed at both ends and loaded at the centre. The frame must be stiff so that there will be no bending between A and B which would tend to loosen the foundation bolts. Maximum fibre stress = Maximum total piston pressure Area smallest section of frame. The maximum fibre stress per square inch runs in dif- ferent engines from 500 to 2,500 Ibs. A low value is, of course, to be preferred. General rules for frame design are: DESIGN AND DIMENSIONS OF PARTS 131 The greatest stresses should be resisted by metal in a straight line; The bending moments should be as small as possible; The frame should take up the strains so that there is no tendency to loosen the foundation bolts; The shrinkage after casting should be as even as possible all the way through, and to this end the sections should be as uniform as they can be made. Large masses and small sections are to be avoided. The for- mer result in porous castings and the latter in excessive shrinkage strains. The materials used for frame con- struction are cast iron and cast steel. 163. CYLINDERS. The cylinder is subject not only to the explosion stresses, but to stresses resulting from rapid temperature changes which cause crystallization, i.e., the metal becomes brittle and breaks. Trouble may be also caused by the strains due to cooling after casting. In designing provision must be made to allow for expansion due to heating up of the cylinder while running; parts that are heated to a considerable extent must be free to expand. The cylinders of high-speed engines are usually bored to nearly the finished size, then set aside for a while to allow the internal strains in the metal to adjust themselves, and then finished. The cylinders of air-cooled high-speed en- gines are usually bored, then annealed to remove all in- ternal strains, and then finished to size. The cylinder must be stiff, but not too heavy, since an excess of metal not only increases the heat loss, but tends to overheat. The section should be as uniform as possible so as to give a uniform expansion. FIG. 87. 132 OAR-EXr.IXE THEORY AXD DESIC.X The allowance for re-boring is from '" up, according to the size of the cylinder. In light engines no allowance is made for re-boring. The cylinder, cylinder head and water jacket should be subjected to a hydraulic test for leaks. Fig. 88 shows cylinder, water jacket and head cast in one piece. Fig. 89 shows the cylinder and water jacket in one piece, the head (also water-jacketed) being cast separate. J FIG. 88. FIG. 89. FIG. 90. Fig. 90 shows cylinder, water jacket and frame cast separate. Such a cylinder is called a "liner," and is used in the larger engines. It is free to expand and can be made from harder metal than the frame. Better castings can be secured in this way. The normal strains in a cylinder are tension and the greatest pressures result from explosion. These range from 300 to 800 Ibs. per sq. in. In figuring the strength of the cylinder to resist rupture as shown in Fig. 91: P = p X a R = a 1 X s P = R where P=the maximum piston pressure. p=the maximum pressure per sq. in. DESIGN AND DIMENSIONS OF PARTS 133 a=the area of the piston. R= resistance offered by the metal to P. a 1 = area of cylinder wall resisting P = area d area d 1 . s = 1,500 to 5,000 Ibs per sq. in. For special mate- rials the values are higher, d and d 1 = outside and inside diameters. The greatest strains in the cylinder are those tending to produce rupture as shown in Fig. 92, since the highest FIG. 91. FIG. 92. explosion pressures are reached as the piston starts to move out, and, therefore, there is only a small area to resist the rupture parallel to the cylinder axis. Here P 1 = 2(d X c) X p. a = c X t. K, l = 2a X s. F= R 1 . Where P 1 = maximum pressure tending to produce rupture as shown. d = inside diameter of the cylinder, t = thickness of cylinder wall, c = distance between top of piston and cylinder head. R* = resistance to rupture. The tension is greater in the inner layers of the metal than in the outer layers, a fact of considerable importance in the design of large guns, but in ordinary gas-engine prac- tice this may be neglected. 134 GAS-ENGINE THEORY AND DESIGN The material for cylinders is a close-grained cast iron which can readily be machined. The cylinders, espe- cially where liners are used, may be harder than the piston so that the latter, as it can be replaced at less cost, may take the greater wear. Iron containing about 1.5 per cent of silicon possesses considerable tensile strength with a fair degree of hardness. 164. WATER JACKET. Water jackets are made thinner than the cylinder walls because they are subjected to little pressure and no wear. The water jacket is propor- tioned with a view to obtaining a sound casting. In light engines it may be f\ in. thick if cast. Sometimes the jacket is made of spun copper. In hori- zontal engines the water jacket has to carry a part of the cylinder weight, etc., and is webbed and proportioned accordingly. The space between cylinder and jacket should be such as to allow of proper cooling with a moderately slow flow of water. Air pockets must be avoided and provision made for draining off or otherwise removing impurities. The water jacket usually comes a little below the lowest point of piston travel, as shown in Fig. 88. 165. CYLINDER HEAD. The cylinder head must first of all be rigid. In small engines the thickness is about the same as that of the cylinder wall. Fig. 93 shows a separate water-cooled head. Sometimes a double head is ribbed, but the ribs arc of doubtful advantage since they interfere with expansion. Heads are sometimes corrugated in order to increase the stiffness. The shape IT FIG. 93. DESIGN AND DIMENSIONS OF PARTS 135 of the head depends, of course, upon the valve arrangement and general design. For engines of large diameter: t' = t X H t = thickness of cylinder wall. t 1 = thickness of cylinder head. Treated as a uniformly loaded plate fixed at the edges, the following formula (Grashof) may be used: d 15 2~\6 Xp X s d = cylinder diameter, p = maximum pressure per sq. in. s = working stress; this may be taken the same as the s for cylinder thickness. FIG. 94. For a stiffened (corrugated) plate t 1 is less than for the non-stiffened plate. In Fig. 94, a shows the water connection between cylin- der and head; b and c, also Fig. 93, show how the joints may be designed. A ground joint is the best since gas- kets require frequent screwing down of cylinder head in order to keep the joint gas- and water-tight. 166. BOLTS AND STUDS. The longer a bolt the better able it is to withstand shocks. If too long, however, it is difficult to keep the nuts tight. A hollow bolt is also stronger in several ways than a solid bolt. Cylinder- head bolts should be fairly close together so as to prevent 136 GAS-ENGINE THEORY AND DESIGN the head from springing. There must be room enough between the nuts so that a wrench can be freely used. Studs may be used where there is little or no occasion for taking them out ; otherwise bolts and nuts must be used, since threads in cast iron wear quickly. For simple tension: F = R R = n X a xs where F = maximum total piston pressure. s - 4,500 to 6,000 Ibs per sq. in. n = number of bolts, a = area of bolt. Bolts and studs are always made larger than theoreti- cally necessary so that they will stand considerable tight- FIG. 95. FIG. 96. FIG. 97. ening up. The strains due to screwing down a nut may be calculated by the principle of leverage as follows: S t = 2-rp't where S , = stress due to screwing up. r = radius of force applied on wrench-handle in inches, p = pressure of force applied on wrench-handle in pounds. t = threads per inch on bolt. 167. VALVE CAGES. Valve cages should be so arranged DESIGN AND DIMENSIONS OF PARTS 137 that the valve seats can be water-cooled and also easily removed for re-grinding. Several ways of arranging valves and valve cages are shown in the following three figures. Fig. 95 shows the inlet and exhaust passages cast in one piece with the cylinder. This arrangement is found in many light engines. The caps on top permit the ready removal of valves. This arrangement makes a rather expensive casting and increases the cooling surface in the combustion chamber. The inlet valves (in the case of a multiple-cylinder engine) are placed on one side and the exhaust valves on the other side. Two camshafts are therefore required for operating the valves. FIG. 98. Fig. 96 shows a simple and compact arrangement. A more direct gas flow is obtained. The cages and valves are easily removed. There is no extra space in the com- bustion chamber. In Fig. 97 the valve cages are easily removed. In the case of several cylinders one camshaft operates all valves. Pockets in which burnt gases may lodge, or places where carbon may deposit, must be avoided. The parts sub- jected to the heat of the exhaust gases are often made of nickel steel. 138 GAS-ENGINE THEORY AND DESIGN Fig. 98 shows the general arrangement of cylinder, cylinder-heads, valves, and water-cooled piston in a large horizontal double-acting engine. 168. PISTONS. The requirements here arc that the piston must not spring, it must distribute the side thrust as evenly as possible. Above 6 in. diameter, pistons are usually web-stayed. Large pistons are both webbed and water-cooled. The top of the piston, which is hottest end expands most, should be finished off smaller than the rest of the piston barrel in order to prevent binding when running hot. Fig. 99 shows a simple piston design for small engines. FIG. 99. FIG. 100. Fig. 100 shows a piston design for larger engines. The head is ribbed, providing additional stiffness, and the bosses for the piston pin are also stiffened by ribs, help- ing to obtain an even distribution of side thrust along the entire piston length. The thickness of the piston-head may be calculated according to the formula for the thickness of the cylin- der-head. The material used for pistons is a close-grained cast iron, preferably somewhat softer than the cylinder metal so that the piston, which is easily replaced, may take the greatest wear. The ratio of piston length to diameter depends alto- gether upon the design. In light engines it may be 1 to 1, DESIGN AND DIMENSIONS OF PARTS 139 in single-acting engines running at a slow speed it may be 2 to 1, and any number of ratios between these general limits are used. The longer the piston the less the side thrust per square inch. 169. SIDE THRUST ON PISTON OR CROSSHEAD. In Fig. 101 let L represent the connecting-rod, and R the crank, then p = P tan $ p = the side thrust =R P = maximum piston pressure == ab and L sin # = R sin ft $ is maximum when e = 90 p is usually taken from 10 to 50 Ibs. per sq. in. of pro- jected bearing surface. For cross- heads p runs up to 100 Ibs. and more. 170. LOCATION AND DIAMETER OF PISTON PIN. For finding the exact location of the piston pin the .,,,,,. , ' FIG. 101. side-thrust diagram must be con- structed and the inertia of the reciprocating parts also considered. As a rule, the pin is so placed that it is in the centre of the true bearing surface, the width of the rings being deducted from the piston length. The piston pin is sometimes made hollow and of hardened steel, re- ducing the weight of the reciprocating parts and providing a path for the lubricating oil. The piston pin may be treated as a simple beam supported at the ends and uniformly loaded. For a hollow pin s equals 20,000 to 60,000 Ibs. The diameter is invariably greater than it need be for strength in order to obtain sufficient bearing surface. Too small a bearing surface will prevent proper lubrication and result in rapid wear. The bearing surfaces throughout should be made as large as possible, i.e., as large as is consistent with good design. 140 GAS-ENGINE THEORY AND DESIGN 1 X d where p = pressure per sq. in. projected bearing surface. P = maximum piston pressure, d = diameter of pin. 1 = length of pin between bosses. For both the piston pin and wrist pin (crosshead) p should be kept below 1000 Ibs. if possible, although in the case of intermittent pressures, such as we have here, much higher pressures (maximum) are allowable than where the pressure is constant. FIG. lOla. FIG. lOlh. 171. PISTON-ROD DIAMETER. Where a crosshead is used (double-acting engine) the diameter of the piston rod may be calculated as follows: d = b D x/~p~ where d = diameter of the rod. b = 0.0140 for steel. D = diameter of cylinder. p = maximum unbalanced pressure per sq. in. = difference between the pressures on the two sides of the piston. 17 la. CROSSHEAD. The crosshead design follows steam- engine practice. Fig. lOla shows an arrangement which is in common use. The bearing surfaces arc lined with babbitt and can be adjusted for wear. The guiding sur- DESIGN AND DIMENSIONS OF PARTS 141 faces are rounded so as to permit a certain amount of self-centring of the mechanism. Since in double-acting engines the pressure on the cross- head is always in one direction, viz., downward, the cross- head may be arranged as shown in Fig. lOlb. 172. PISTON RINGS. Piston rings must be carefully made since their function is to prevent leakage, and with poorly fitting rings the leakage losses Ha i may become very great. The rings j ] may be lap-cut as shown in Fig. 102. Li 53 il <* When open the ring is larger than the inside cylinder circumference by the distance 2a. In order to have the ring a true circle when in the cylinder it must be sprung together and then machined to size. It is bet- ter to have more rings with the dimen- sions b and d small, than to have the ,. .. , . , , ,, FIG. 102. lewest possible rings and have them wide and stiff. For eccentric rings the following pro- portions represent good practice. d = .05 to .04 D, decreasing as D increases. b-d c = fd where D is the inside diameter of the cylinder. Eccentric rings exert a more uniform pressure all the way around than concentric rings. The rings are put in so that the lap joints bear against the cylinder at different points and pins are inserted so that the rings cannot turn. The material for piston rings is cast iron. 173. CONNECTING-ROD. The ratio of connecting-rod length to stroke has already been discussed in Par. 154. The connecting-rod is considered as a strut under com- pression from the piston pressure, and under tension from 142 GAS-ENGINE THEORY AND DESIGN the inertia pressure. The greatest piston pressure is at the beginning of the stroke. The greatest bending moment due to inertia acts at about 6/10 L from the wrist-pin, where L is the distance between centres. FIG. 103. Fig. 103 shows the I-section type used in light engines. The cross section at a distance 2/3 L may be calculated as follows: _ f X P X L 2 Ex. I = moment of inertia of the section (see Par. 183) f = factor of safety for compression 5-10. P = maximum piston pressure. E = modulus of elasticity of the metal. L = length of connecting-rod in inches. where FIG. 104. The ratio of a to b may be 1-2 or 2-3. For a round connecting-rod, Fig. 104, the following formula is given: d-ajDLVp + C where d = diameter of rod at L from piston pin. D = piston diameter in inches. L = length connecting-rod in feet. DESIGN AND DIMENSIONS OF PARTS 143 p = maximum piston pressure per sq. in. a = 0.15 C = 0.50" for a fast engine, a = 0.08 C = 0.75" for a slow engine. FIG. 105. FIG. 105a. For a rod of rectangular cross section, Fig. 105, the proportions are: h-2t Connecting-rods are usually steel drop-forgings. 174. CRANK-SHAFT. The crank-shaft is subjected to both twisting and bending moments. The maximum FIG. 106. twisting moment (Fig. 106) is obtained from the tangential- effort diagram. The bending moments are due to both piston pressure and fly-wheel weight. 144 GAS-ENGINE THEORY AND DESIGN The diameter to resist twisting may be calculated as follows. For a solid round shaft: 32 d . !32xTr also d = For a hollow round shaft: 32 where T = torsional moment in inch-pounds. L = lever arm. P = twisting force. s = safe shearing stress = 12,000 Ibs. for steel. J = polar moment of inertia. r = radius of shaft. d = diameter of shaft. d t = inside diameter of hollow shaft. The diameter to resist bending is figured as follows. For a solid round shaft: also d = .110-2 XT' \ s For a hollow round shaft: I = (d 4 d. 4 )^ c 32 d DESIGN AND DIMENSIONS OF PARTS 145 where T! = bending moment. I = moment of inertia. = moment of resistance, c The resistance of a shaft to bending is about one-half of that to resist twisting. For combined twisting and bending: For a solid round shaft: T 2 = 0.35 T,+ 0.65 VT,' + T* = -^ ' For a hollow round shaft: 4 r where T 2 = combined twisting and bending moments, r + TI= out and inside radius of hollow shaft. A hollow shaft is lighter for the same strength. For light fly-wheels d = about D For heavy fly-wheels d = about \ D where D is the cylinder bore. Since the crank-shaft has a tendency to bend when run- ning it must be designed so as to possess considerable stiffness. | p i In single-acting multiple-cylinder en- gines the crank-shaft in general need not be larger than for a single-cylinder engine since the maximum stresses are nearly the same. Crank-shafts are usually steel drop-forgings. 175. CRANK. The crank is treated as a cantilever beam. The area of the cross section is usual- ly from 1.1 to 1.4 the area of the crank-shaft cross section. 176. CRANK-PIN. The crank-pin is considered as a simple beam loaded at the middle and subjected to bending. 10 FIG. 107. 146 GAS-ENGINE THEORY AND DESIGN The crank -pin must be strong and rigid and the bearing surface must be large enough to prevent the oil from being squeezed out. In order to secure the required bearing surface the diameter is invariably made greater than it need be for strength; 1,400 Ibs. per sq. in. may be taken as the allowable pressure. The crank-pin is frequently made the same diameter as the crank-shaft. In side-crank en- gines the pin is considered as a cantilever beam (see Par. 183). 177. MAIN BEARINGS. The length of the main bearings is made from two to three times the diameter of the shaft. For heavy fly-wheels an outboard bearing, Fig. 108, should be used. This is frequently made with a spherical joint, as shown, to allow for the bending of the shaft in running. In order to prevent rapid wear and overheating the main bearings should present ample bearing surface and positive lubrication should be employed. When the main bearing is made in two pieces the general arrangement shown in Fig. 108a is followed. At ss thin DESIGN AND DIMENSIONS OF PARTS 147 pieces of sheet metal or other material (called "shims") are used, so that the cap presses against the lower part when it is screwed down. These "shims" are also used in the connecting-rod. In large engines four-piece bearings, as shown in Fig. 108c, are frequently used. Since the main bearings must be rigidly supported the frame should be arranged (in the case of a horizontal engine) as shown in 108c for a large engine, or 1086 in the design of a small or medium- sized engine. 178. BEARINGS AND LUBRICATION. The reliability and mechanical efficiency of an engine depend largely upon proper bearings and lubrication. In the design of bearings the following must be considered : (a) Friction. (6) Lubrication and lubricants. (c) Bearing-metals. (d) Form and proportion of bearings. (e) Mechanical oilers. (/) Stuffing-boxes. (a) Friction. In the case of solids, friction is due to the unevenness of the surfaces in contact. The metal sur- faces appear very rough under the microscope no matter how much they have been polished. These projections oppose the sliding of the surfaces over each other. Sliding friction depends upon the nature of the surfaces in contact, the speed, the amount of surface in contact, the nature of the lubricant, etc. The smoother the surfaces the less the friction, the greater the pressure the greater the friction, etc. Fluid friction is the internal friction of a liquid or gas. In the case of a lubricant it is independent of the pressure between the surfaces, but is dependent upon the area and speed. (6) Lubrication and Lubricants. The internal friction 148 GAS-ENGINE THEORY AND DESIGN of a fluid is much less than the surface friction of solids; therefore, with a film of oil between the rubbing surfaces the sharp edges cannot engage, and the work lost in friction becomes less. When there is a film of lubricant between the metal surfaces they obviously cannot touch. The general requirements of a lubricant are as follows: It must not become gummy; it must resist oxidation; it must not corrode metallic surfaces; it must be able to absorb and carry away the heat generated by friction; it must have a high temperature of decomposition; it must have sufficient body so that it will not be easily squeezed out; the internal friction must be low. The lubricant should be suited to the work in hand. For light pressures and high speeds a thin oil is best. For great pressures and slow speeds a heavy oil, or grease, should be used. Graphite is a good lubricant, especially if fed with oil, but in the case of the gas-engine cylinder it is apt to short- circuit the spark-plug. Where there is no danger from this source it forms an ideal lubricant since it is not affected by high temperatures. For piston lubrication a high-fire-test oil must be used. Unfortunately even the best of these oils will not stand much more than about 600. Many lubricating oils contain acids and other injurious substances. A good method of testing them for acid is to place a piece of polished steel in the oil and leave it there for several days. Needless to say, lubricating oil should be thoroughly filtered before being used. (c) Bearing-metals. When the surfaces are highly finished the wear will be greater between hard metals, as steel on steel, than when one metal is soft, as steel on babbitt. The softer metal is worked into shape more easily and by contact with the hard metal is smoothed, decreasing the DESIGN AND DIMENSIONS OF PARTS 149 friction. When the lubrication fails a soft metal, like babbitt, will melt and run out without injuring the shaft, whereas with brass or cast iron the shaft would be destroyed. The bearing-metal should carry its load without distortion, and must not heat readily. Cast iron is better for some purposes than other ma- terials. The piston furnishes an example of this. Here cast iron wears longer than brass or bronze. Brass is a copper-zinc alloy, made up in different pro- portions and sometimes with additional ingredients. Bronze is a copper-tin alloy, the proportions being about 90 per cent copper to 10 per cent of tin. It is a better bearing-metal than brass. Bronze is used where the press- ures are too high for babbitt. Phosphor bronze contains a small amount of phosphorus which improves the strength and ductility of the alloy. This bronze is used extensively for bearings. Manganese bronze is used extensively for propellers, propeller shafts, and salt-water fittings in general, since it will not corrode easily. Babbitt is a copper-tin-antimony alloy, a good grade having about the following proportions: tin 90 per cent, antimony 7 per cent, copper 3 per cent. It is used ex- tensively for the main bearings of engines. It is easily poured in place, and scraped to fit, and when the bearing overheats it will melt and run out without doing any injury. In order that the babbitt may hold to the supporting shell, grooves must be provided as shown in Fig. 108. The bab- bitt is sometimes hammered after being poured in order to make a better contact with the shell. The maximum pressure for babbitt at slow speeds is given as 1,000 Ibs. per sq. in. The maximum pressure for bronze is 5,000 Ibs. per sq. in. The maximum pressure between steel and steel, hard- 150 GAS-ENGINE THEORY AND DESIGN ened and polished, as in ball-bearings, etc., may reach 50,000 Ibs. per sq. in. For intermittent pressures higher values can be used than for constant pressures. (d) Form and Proportion of Bearings. In designing bearings the following points are to be observed: Find the direction and magnitude of the forces acting on the bearing; deter- mine the safe working pressures and speeds; provide means for forcing the WMW^^ lubricant between the bearing surfaces ; provide means for taking up wear. The bearing-metal must not change its shape when under load. In Fig. 109, if the lubricant enters at A it will take the path of least resistance and work out to the left and the rest of the bearing on the right will receive no lubrication at all. However, if the oil enters at the centre, B, it will meet as much resistance one way as the other and will work in both directions. If grooves are now cut in either the shaft, or bearing-metal, the lubri- cant will reach all parts of the bearing. When the motion reverses, the oil has a better chance to lubricate than when the motion is always in one direction. If in Fig. 110 the pressure and rota- tion are as indicated by the arrows, and the oil enters at A, the lubrication will be poor since the tendency is to squeeze out the oil where the pressure is. If the oil is forced in at B under pressure, the entire bearing will be lubricated. In Fig. Ill the oil enters at A and is worked outward DESIGN AND DIMENSIONS OF PARTS 151 by centrifugal force. A return passage is provided so that the oil can circulate. In Fig. 112A, if the bearing is made thin the metal will spring and bind the shaft as indicated by the dotted lines, causing a hot bearing. Fig. 112B shows a heavier bearing J^L FIG. 111. FIG. 112. which offers more resistance to distortion, and shows also how the metal is cut away at aa in order to prevent its seizing the shaft. (e) Mechanical Oilers. Instead of allowing the oil to feed by gravity well-designed machines now have forced lubrication, at least for the more important bearings. By means of the mechanical oiler a dozen or more bearings can be lubricated with oil under pressure and the danger of overheating is greatly lessened. A saving of lubricating oil also results since there need be no waste. (/) Stuffing-Boxes. In double-acting gas engines a stuffing-box is required for the piston-rod. For a while this was quite a problem since the type of stuffing-box used on steam engines would blow out or the pack- ing burn. Fig. 113 shows the gen- eral construction of the gas-engine stuffing-box. A number of cast- iron rings bear lightly around the shaft and against the inside of the casing. Oil is pumped through under pressure. The entire box is water-cooled. 179. VALVES AND VALVE GEARING. The poppet type of valve is used in gas engines since it is necessary to have FIG. 113. 152 GAS-ENGINE THEORY AND DESIGN a tightly fitting valve which will withstand high pressures and temperatures. The following must be studied in con- nection with valve design. (a) Valve proportions. (6) Diameter and lift. (c) Angles of valve opening. (d) Valve gearing. (e) Cams. (/) Springs. (g) Valve passages. (a) Valve Proportions. The valve proportions vary much in different designs. The thickness t (Fig. 114) must be sufficient so that the valve will not spring. This may be figured according to the formula for cylinder-head . . , thickness. In the exhaust valve t is frequently made larger than in the inlet valve. As a rule t runs from J to $ d, decreasing as the diameter increases. The valve-seat angle is usually 45. The distance a must be suffi- cient to provide a good bearing surface and usually runs from -| to T V d; t 1 may vary from \ to d for a solid stem. The metal at a 1 permits rcgrinding of the valve. (b) Diameter and Lift. The diameter and lift are fig- ured on the assumption that the valve is fully open during the entire period of valve lift and that the gases are moving in and out at a constant velocity. In a high-speed engine where the valve is lifted from 800-900 times every minute, the lift is made as small, and the diameter as large, as pos- sible. This secures a smoother action and reduces the ham- mering and jumping of the valve. The exhaust valve is sometimes made larger than the inlet valve, but usually they are of the same size. The assumed constant speed DESIGN AND DIMENSIONS OF PARTS 153 FIG. 115. is taken as 100 ft. per second for the inlet valve, and 85 ft. per second for the exhaust valve on the assumption that the exhaust gases are going out at atmospheric pressure. Again, a constant gas speed of 6,000 ft. per min. is as- sumed for both valves, but the periods of valve opening are different. The ratio of effective lift to diameter varies from 1-4 to 1-6 depending, of course, largely upon the speed of the engine. The designer must be careful not to confuse the total valve lift h with the effective valve lift h 1 . These two are quite different as can be seen from Fig. 115. To illustrate how important the time factor is, let us take a four-cycle high-speed engine, for example: The engine runs at 1,200 R.P.M. One revolution is made in .05 sec. The inlet valve is open during 180 of crank-pin travel. Total time of valve opening, .0250 sec. Valve is fully open about .0050 sec. During the total time of valve opening the full charge must be drawn in. In a slow-speed engine the conditions are not so bad, but in a two-cycle engine they may be even worse. (c) Angles of Valve Opening. - The circle in Fig. 116 represents the crank-pin travel. In engines running at medium and high speeds the periods of valve open- ing may be as here shown. The inlet valve commences to open 10 past the upper dead centre and closes about 22 past the lower dead centre. The full period of opening FIG. 116. 154 GAS-ENGINE THEORY AND DESIGN is, therefore, 212, or 106 for the camshaft which revolves at one-half of the crank-shaft speed. The reason for keep- ing the valve open after the lower dead centre has been passed is on account of the inertia of the gases which are coming into the cylinder at a high velocity and continue to come in even after the piston has started on its return stroke. By keeping the valve open a larger charge passes into the cylinder. The exhaust valve may open 40 ahead of the lower dead centre since the tangential pressure at this point is FIG. 117. FIG. 118. small and it is desirable to get rid of the hot exhaust gases as rapidly as possible. The exhaust valve is here open during 226, or 113 of the camshaft travel. These angles are varied, of course, according to the design. (d) Valve Gearing. Fig. 117 shows a typical valve gear. Rotary motion is transmitted from the crank-shaft C to the camshaft K by gearing. The cam bears against a roller R and lifts the lever L, which in turn lifts the valve. The roller reduces the friction and prevents side thrust DESIGN AND DIMENSIONS OF PARTS 155 against the valve stem. The spring S holds the valve against its seat. WW are water spaces. At a there is a little clearance, about .01*, so that the valve will seat be- tween strokes. The motion from C to K may be transmitted by three kinds of gearing spur, bevel, or spiral. In Fig. 118 A represents two spur wheels transmitting motion in opposite directions; B represents two bevel wheels transmitting motion at right angles; C represents two spiral wheels transmitting motion at right angles, but in different planes. In the case of the spiral gear a reduction of 2-1 can be obtained with wheels having the same pitch diameters. FIG. 119. FIG. 120. Unless balanced the exhaust valve is forced open against the pressure in the cylinder, which may be considerable. For this reason large exhaust valves are usually balanced so that the valve opens against a low resistance. Figs. 119 and 120 show two types of balanced and water-cooled exhaust valves. The arrows indicate the direction of water flow. (e) Cams. General rules for valve openings, which are controlled by cams, are as follows: The valve should open as rapidly as possible. The valve should close as rapidly as possible. The period of full valve opening should be as long as possible. 156 CIAS-ENGINE THEORY AND DESIGN The opening and closing motion should be as smooth as it is possible to make it. Since the piston velocity changes throughout the stroke the velocity of valve opening and closing should conform to the piston velocity. This condition can only be approx- imated in practice, but the piston-velocity diagram should be studied in connection with the cam curve. The average gas velocity has been given as 6,000 ft. per min. The maximum velocity, on account of the maximum piston velocity (since the piston draws in and discharges the gases) is much greater. Fig. 121 shows the usual cam outline. The sides of the cam are straight lines tangent to the base circle C. Such a cam is easily machined and works quite well at slow speeds, but at high speeds the valve is started rapidly from rest at d, which results in increased wear, the valve is also apt to jump at e and e l instead of following the cam curve closely, and this causes a pounding. Such a cam requires a much stiffer spring than the curve which will be described next. The angles in Fig. 121 are usually as follows: FIG. 121. a b c Exhaust valve . ... Inlet valve 42-45 35 22-24 20 42-45 35 A cam outline which will give a smooth motion at high speeds should start the valve from rest gradually, lift with increasing speed and then decrease the speed and bring the valve to rest again gradually. A body acted upon by gravity, falling from rest, travels with a uniformly increasing acceleration and the distances passed over in successive intervals are in a ratio of 1, 3, 5, 7, etc. Fig. 122 DESIGN AND DIMENSIONS OF PARTS 157 shows the method of laying out a curve following this law. The distance A represents the arc of angle a on the base circle, Fig. 121, and is divided into ten equal parts. The distance h equals one-half of the valve lift. The line on the right is divided into parts whose lengths are 1, 3, 5, 7, 9, respectively. By projecting these points over to the ver- tical lines on the left the cam curve is obtained. The ^ / x x x x x 7 % X / ^^x X x X ' _xl 1 a i 7 j 9 10 FIG. 122. upper half of the curve is simply the reverse of the lower half. The straight line on the left shows how the cam lifts the valve in Fig. 121. The curve may now be used for laying out the cam curve proper in the usual manner. When the cam bears against a roller care must be taken to draw the cam curve .so that the centre of the roller will lift according to the curve in Fig. 122. In this case the actual cam curve will be different from the one here laid out. In many of the large gas engines combinations of cams, eccentrics, rods, and levers are used for operating the valves, and these arrangements are frequently quite complicated. The ratios of base and roller circles, etc., are as follows: B = diameter of cam base circle. R = diameter of roller circle. V = valve lift. 158 GAS-ENGINE THEORY AND DESIGN Ratio of B to R is 5-3 Ratio of B to V is 5-1 to 6-1 for high speed and 4-1 for slow-speed engines. The diameter of the base circle should be as large as it can conveniently be made. Valves are drop forgings and are frequently made from nickel steel which withstands high temperatures better than ordinary steel. (/) Springs. The function of the spring is to make the valve follow the cam outline closely at all speeds and to keep the valve closed between lifts. The spring closes the valve against inertia, friction, etc. Helical springs made of round steel wire are generally used. Many turns are used, where the design permits this, so that the tension will not vary to any extent with the deflection. The force P in pounds which the spring must exert in order to close the valve according to given conditions, may be calculated as follows: 2 w h - neglecting friction 32.2 X t' where w = weight of the valve and stem. h = distance it moves through in feet, in t seconds. acceleration = ^ . \i The diameter of the wire, diameter of coil, etc., can be now found in tables given in engineering hand-books. The pressure per sq. in. of valve area may run from .5 to 5.0 Ibs. and even more. In a slow-speed engine the re- quired pressure may be 1 Ib. per sq. in. Where throttling or cut-off governing is used, the valves may open on account of the vacuum in the cylinder unless provision is made for locking the valves. (gr) Valve Passages. The diameter of inlet and exhaust pipe for each cylinder is easily figured from the assumed DESIGN AND DIMENSIONS OF PARTS 159 constant gas velocity. Where several cylinders are sup- plied by one inlet pipe, as in Fig. 123, the area of the main pipe P is not four times the area of pipes pp, since all four cylinders are not supplied at one time, but in a four-cycle engine, one cylinder after another receives a fresh charge. Consequently the diameters from p to P are increased in a moderate ratio. There should be as few turns in both inlet and exhaust passages as possible and sharp turns must be FIG. 123. avoided. A few sharp corners will have the same effect as decreasing the diameter of the inlet pipe \ or more. When a number of cylinders are supplied from one car- bureter, trouble is experienced in supplying each cylinder with an equal amount of the mixture, since the inlet passages are of different lengths, and hence having each cylinder develop the same amount of power. For this reason several carbureters are employed by some builders, each carbureter supplying two cylinders. 180. FLY-WHEEL. The fly-wheel regulates the running of the engine while the governor regulates the fuel supply according to the load. The fly-wheel absorbs energy dur- ing the expansion stroke and furnishes the necessary energy to keep the engine running during the idle strokes. It regulates the speed variations per revolution due to the changing pressures on the piston The heavier the fly- wheel the less the unsteadiness in running will be, and for 160 GAS-ENGINE THEORY AND DESIGN this reason the wheel is made much heavier than it need be for overcoming the idle strokes. While the fly-wheel is giving out energy it slows down and while it is absorb- ing energy it speeds up. W v 3 The kinetic energy of a moving body = E = - o where w = weight of body in pounds, v = velocity in feet per second, g = gravity = 32.2. Let E l represent the change of kinetic energy betw r een v { and v 2 where v t = maximum velocity of fly-wheel rim v 2 = minimum velocity of fly-wheel rim wv? wv: then E! = and Let us assume that Fig. 124 is the tangential-effort diagram for the expansion stroke. The line be is the b A FIG. 124. M.E.P. line. While the piston moves from 6 to 6 1 the fly-wheel is giving up energy equivalent to the shaded portion A, and consequently the wheel slows down. From 6 1 to c 1 the fly-wheel is absorbing energy and speeds up, the energy absorbed being equivalent to the shaded area B. From c l to c the fly-wheel is giving out energy and slows down. E 2 = greatest amount of energy in ft.-lbs. above the mean (6-c) = B DESIGN AND DIMENSIONS OF PARTS 161 then W = where w = weight of rim. V = mean velocity of rim in ft. per sec. v, v 2 greatest change in velocity K = -^r = - s i r- - - = coefficient of V mean velocity unsteadiness. In the case of several cylinders the M.E.P. is greater and the area B is consequently smaller. K =.03 to .05 for ordinary work. .08 to .10 for light fly-wheels. .01 for textile and spinning machinery. .002 to .005 for alternating and d.c. drives. Empirical formula for fly-wheel weight is 110,000,000,000 XH.P. W - -- K x d' x N 3 d = mean diameter rim in inches. N = R.P.M. The allowable rim speed for cast-iron wheels is about 5,000 ft.-min. Wheels built up from forged and rolled materials can be run at much higher speeds and possess, among other advantages, the important one of safety. Cast-iron wheels may burst from overspeeding, defective spots in the casting, and wobbling during running on account of bearings being out of line or worn. Solid cast wheels are stronger than wheels cast in sections. The safe speed for a cast wheel may also be figured as follows: -i. 6 JX \ W where V = velocity in ft. sec. w = weight of 1 cu. in. of rim material = .260 Ibs. for cast iron. 11 162 GAS-ENGINE THEORY AND DESIGN The centrifugal force tending to produce rupture in the rim as shown in Fig. 125 is _Wv' Cf ~ gR where R is the mean radius in feet. C f T = tension in any section of the rim = -^- The weight of the arms and hub usually equals one- third of the total weight of the wheel and the energy stored FIG. 125. in them for a given change in velocity is about 10 per cent of that stored in the rim. The dimensions for the cross section of the arms near the hub, Fig. 126, may be calculated as follows: d = 0.l where N = number of arms. D = diameter of cylinder in inches. B = width of rim, or the bending strength of the arms is made equal to the twisting strength of the shaft. S * r 3 ^ n S. I 2 c S = shearing strength of shaft. r = radius of shaft in inches. DESIGN AND DIMENSIONS OF PARTS 163 N = number of arms. S t = tensile strength of arms. = section modules. I = moment of inertia. C = distance from neutral axis to the outermost fibre. The arms are tapered somewhat toward the rim. This taper depends altogether upon the design. In the case of a multiple-cylinder engine the fly-wheel is smaller than for one cylinder since the area A, Fig. 69, is much less. In order to relieve the shrinkage stresses the pattern may be made with a split hub although the fly-wheel rim is solid. Large wheels are made in halves on account of the easier handling. Fig. 126A shows such a wheel. The frame is 164 GAS-ENGINE THEORY AND DESIGN cored out for the forged joints. The hub is cored out as shown in 126B, so as to relieve the shaft. The bolts, etc., are also relieved as shown. When one tapered key is used there is a tendency to throw the wheel out of true, as indicated in Fig. 126C. In order to avoid this three keys should be used in large wheels as shown in Fig. 126A. It is of the utmost impor- tance that the wheel runs true. A fly-wheel with an outboard bearing, Fig. 126E, is to be preferred to an overhung wheel, Fig. 126D. A heavy moving part, such as a fly-wheel, should be supported be- tween two bearings whenever practicable. When two overhung wheels are used, Fig. 126F, the crank-shaft is subjected to a greater torsional strain than when one wheel is used and made equal in weight to the two wheels. When a fly-wheel is made in halves the safe tensile strength of the bolts should equal the centrifugal force in the rim. The small vertical engine, running at a fairly high speed, is often discriminated against for portable and other pur- poses in favor of the much heavier horizontal engine, on the plea that the fly-wheels on the vertical engine do not possess sufficient "heft" for the work to be done. In the comparison below, the data was taken from catalogues, the only change being in the speed of the vertical engine, which has been increased somewhat. Horizontal engine, 6 H.-P., 2 fly-wheels of 200 Ibs. each, mean diameter of wheels, 3 feet, speed, 300 R.P.M., total weight of engine 1 ,400 Ibs. Vertical engine, 6 H.-P., 2 fly-wheels with a mean diam- eter of 18", speed 1,200 R.P.M. In order to have the same "heft" as in the first case the wheels should weigh 60 Ibs. each. The total weight of the engine would then be a little over 200 Ibs. DESIGN AND DIMENSIONS OF PARTS 165 Where manufacturers put out 1-, 2-, 3-, and 4-cylinder engines, the same fly-wheel is generally used for the multi- ple-cylinder engine as for the single-cylinder engine. 181. TACHOMETER. The tachometer is an instrument for recording the variation in speed per revolution. Fig. 127 shows a tachometer record covering a period of 4 cycles, or 8 revolutions. The horizontal distances represent cycles, while the vertical distances represent variations in fly- FIG. 127. wheel velocity during the cycle. An electrical tachometer will greatly exaggerate these velocity variations. The record in question shows that the engine was equipped with too light a fly-wheel for the load. The fly-wheel weight should be computed not only for full load but for other loads as well. 182. FOUNDATIONS. The method of building founda- tions for stationary engines, and the manner of securing the foundation bolts, are the same as employed in steam- engine practice and need not be described here. When an engine is installed on a floor it should be placed near a wall, preferably in a corner, in order to lessen the bending of the floor beams and decrease the vibrations. When placed on a built-up foundation the engine should be some distance from the walls of the building. A layer of min- 166 GAS-ENGINE THEORY AND DESIGN eral wool or tan bark underneath the foundation will lessen the vibrations. The weight of the foundation for an engine with heavy fly-wheels should be at least four times the weight of the engine. The area covered by the foundation will also affect the matter of vibration. A very deep foundation covering little area, or a founda- ation which covers much area but has little depth, will not give the best results. This matter of vibration, and con- sequent shaking of the building, sometimes becomes a serious one, but with a balanced engine and a fairly large foundation there will be little trouble. 183. STRENGTH OF MATERIALS. The following matter is given for the convenience of the designer. Where special materials are used, such as nickel steel, chrome nickel steel, etc., the strength in tension, compression, etc., will differ more or less from the values given, and this fact must be borne in mind and the proper values sub- stituted. STRENGTH OF MATERIALS ULTIMATE STRENGTH S IN LBS. PER SQ. IN. Cast Iron 20,000 90,000 20,000 36,000 Wrought Iron. 55,000 55,000 50,000 50,000 Steel. 100,000 150,000 70,000 120,000 Tension Compression Shear Flexure 450 0.26 WEIGHT IN LBS. PER Cu. FT. AND Cu. IN. 480 0.28 490 0.29 FACTOR OF SAFETY F. Steady str Varying ' Shocks 15,000,000 COEFFICIENT OF ELASTICITY E. 25,000,000 30,000,000 DESIGN AND DIMENSIONS OF PARTS STRENGTH OF MATERIALS (Continued) Q FOR COLUMNS 167 Cast Iron Wrought Iron Steel 1 1 1 5,000 36,000 25,000 4 4 4_ 5,000 36,000 25,000 Both ends flat or fixed Both ends round ~64 32 bd 3 12 bd' V (d 4 -d, 4 ^ Tr(d ( -di 4 ) d 2 -d> 16 64 32 "16" 12 bd 3 -b l d l 3 bd 3 -b 1 d 1 Of/ 12(6d birf 168 GAS-ENGINE THEORY AND DESIGN- STRENGTH OF MATERIALS (Continued) BENDING MOMENT M 4 iM = Vfl 1 beam 1 beam A loaded A one end M= ^~ J ~~ ' ' 1 Cantilever '//'\ ^ A uniformly loaded ^ WZ ^%^ Fixed 1 hwn /A ^ beam uniformly MSA loaded TV 7 i M = V Simple ^l ^P loaded at centre Fixed \ , _^_ A (- A loaded at '/JVA centre p loaded 7T (rf 4 rf, 4 ) TORSION-POLAR MOMENT OF INERTIA - rf H J 32 (\T~ dr ~7) ^ 32 (see Par. 174) Tension Compression Shear Flexure Torsion DESIGN AND DIMENSIONS OF PARTS 16! STRENGTH OF MATERIALS (Continued) I = Moment of inertia P = Total stress in Ibs. R = Moment of resistance I = Length in in. G a = Square of least radius of gyration W = Weight in Ibs. A = Area of cross-section in sq. in. For tension, compression, or shear where I does not exceed 10 diameters P = AS Breaking strength of beams M = SR SA For columns P = r?- FOR A SIMPLE BEAM OF UNIFORM STRENGTH At 0.1 I Depth = 0.45 d At 0.2 I Depth = 0.63 d At 0.3 I Depth = 0.77 d At 0.4 I Depth = 0.89 d CHAPTER XIX GAS-ENGINE MANIPULATION 184. Printed instructions arc usually furnished by man- ufacturers in regard to starting, stopping, cleaning, and taking apart their engines, as well as directions for installing, etc. The principal points to be observed will be briefly given. 185. STARTING. In starting the following operations are performed: Turn on the fuel; turn on the current where electric ignition is used; turn on the cooling water; turn on the L: FIG. 128. FIG. 129. lubrication; see that there is no load on the engine; start the engine ; throw on the load. 186. STARTING DEVICES. Small single-cylinder engines are usually equipped with starting-handles as shown in Figs. 128 and 129. The engine is turned over in the direc- tion in which it is to run. Considerable force is required to overcome the resis- tance of compression, and on this account relief-cocks 170 GAS-ENGINE MANIPULATION 171 are placed on the combustion chamber. The relief-cock is opened, the engine is turned over in the direction in which it is to run and after two or three revolutions an explosion should take place. When the engine is running the relief-cock is closed. High-speed and the larger stationary engines are usually equipped with a set of auxiliary cams on the exhaust- valve camshaft. The camshaft can be pushed along so that the auxiliary cams operate the exhaust valves and hold them open during a part of the compression stroke, so making the starting much easier. If a multiple-cylinder engine has not been idle long, especially if the compression is good, there is an explosive charge in one of the cylinders and the wiring can be so arranged that by pushing a button all the plugs will spark at once and the engine will start without cranking. Large multiple-cylinder engines can be equipped with a compressed-air starting-outfit. This consists of a small air compressor, air tank, valves and piping. When it is desired to start the engine the compressed air is allowed to flow into the cylinder in which the piston is on its down stroke. A quick-opening hand-operated valve is provided for each cylinder. Where electric power is available a small electric motor furnishes a convenient method for starting. Another convenient way is to have a small auxiliary engine which can be easily started by hand and when running furnishes the power required for starting the large engine. 187. STOPPING. In stopping an engine the following operations are performed: Shut off the fuel supply; turn off the ignition current; turn off the lubricators; turn off the water; throw out the clutch. 172 GAS-ENGINE THEORY AND DESIGN Open the relief -cock so that the compressed charge will not rock the fly-wheel back and forth before coming to rest when connected to a generator this rocking will injure the brushes. Another reason for opening the relief- cock is to clear the cylinders of all explosive charges and thus avoid the possibility of a back explosion in starting. 188. ENGINE TROUBLES. Assuming that the design and construction are good, half of the gas-engine troubles are caused by defective installations and the other half by careless handling. Poor foundations, small piping with many elbows, improper distance from the line shaft, wrong connections, dirt and obstacles in connections, leaky joints, etc., are often the result of installing an engine cheaply. If the engine has been properly installed then, in order to avoid trouble, the Ignition apparatus must be kept in good order. Lubrication should receive systematic attention. All water and oil joints must be tight, the oil and water must be filtered. All parts liable to become loose on account of engine vibration must be examined at regular intervals. Bearings must be examined frequently. Knocking and pounding are caused by play in the bearings. To deter- mine quickly where the noise may be the use of the stetho- scope has been suggested. Everything in connection with the fuel supply from tank to combustion chamber must be in good order. Pistons and valves must not be leaky. When pistons and valves bind, due to gumming up of lubricating oil, or carbon deposits, a little kerosene will quickly loosen the parts. The inlet and exhaust passages must be kept clean. The piping must be so arranged that any moisture which GAS-ENGINE MANIPULATION 173 accumulates can be drained off. All parts that must be cleaned at intervals, or adjusted, must be arranged so that they can be reached handily. When an engine is operated on city gas, a rubber bag, and sometimes in addition a pressure regulator, is placed between the engine and gas main in such a manner that the pulsations of the engine will not affect lights or burners in the building which are supplied from the main. A Diaphragm FIG. 129a. pressure regulator is shown in Fig. 129a. The action of this is obvious. When an engine is thoroughly tested out in the shop in the first place, is installed properly and receives intel- ligent care, there is little cause for trouble after instal- lation. In order to obtain good results a gas engine must be properly looked after, the same as a steam engine and boiler, or any other machine or apparatus. The construe tion and operation of the engine should be studied by the man who takes care of it, and this, together with a sys- tematic inspection, will overcome all ordinary troubles. The time required for keeping a gas engine in good condition is less than that required for many other machines. CHAPTER XX TESTING 189. The object of testing an engine is to determine its power, thermal and mechanical efficiency under different loads,, and to bring out and remedy defects in design, construction, and adjustment. The performance of each engine (in the case of a stock engine) should equal a stand- ard determined by careful experimental work in the way of valve-setting, timing of ignition, etc. When a new design has been completed, and an engine built, careful tests will show where improvements can be made in both efficiency and general design. Possibility of errors and wrong conclusions should be eliminated as much as possible by the use of proper ap- paratus and careful observations. The tests should be continued for a sufficient length of time to insure the engine's running in the same manner under actual working conditions. For example, an engine might show up very well during a few minutes' run, but after it has been installed in a factory and run for several hours it may overheat, or develop other troubles which a short run will not bring out. The test should include runs under different loads, say: No load. Quarter load. Half load. Three-quarter load. Full load. Overload. 174 TESTING 175 The test should bring out the following: (a) Energy put into the machine. (6) Work done in the engine cylinder. (c) Outside work done by the engine. (d) Heat losses. (e) Mechanical losses. 190. ENERGY PUT INTO THE ENGINE. We will assume that the test under discussion is made under full load and continued for one hour. The energy put into the engine equals the B.T.U. per pound of fuel multiplied by the pounds of fuel used. The heating value of the fuel is de- termined by calorimeter tests. 191. W T ORK DONE IN THE ENGINE CYLINDER. This is the indicated horse-power computed from the indicator diagram. The thermal efficiency = Total R.T.U. X B.T.U. equivalent of I.H.-P. Total B.T.U. The principle of the ordinary form of indicator suitable for slow-speed engines has already been described. 192. THE MANOGRAPH. The ordinary indicator is not suitable for high-speed work, and the manograph, an indicator of special form, must here be used. By means of this instrument a diagram can be obtained at any speed. Its construction is as follows: A ray of light enters a closed box through a pin-hole. This ray is deflected by a concave mirror which concen- trates the light on a ground-glass screen. The mirror has both a horizontal and vertical movement. The horizontal movement is produced by a crank operated by the engine crankshaft. The vertical movement is produced by a diaphragm which is provided with a tube connection to the engine combustion chamber. The 176 GAS-ENGINE THEORY AND DESIGN Card 1. travel of the piston will, therefore, produce a horizontal movement of the mirror, while the pressure in the cylinder produces a vertical movement. When the engine is run- ning, a point of light travels rapidly over the glass screen, and the movement is so fast that there appears to the eye a continuous line of light. Permanent rec- ords can be made by photographing the light diagram. Some manograph cards from a Franklin automobile engine are given herewith: Card No. 1 was taken from cylinder No. 3. H.-P. lOf at 700 R. P.M. The closed lines were obtained by cutting out the spark. The light line over the main ex- pansion line shows the increased power due to a completely scavenging cylinder being fired after missing several ex- plosions. Card No. 2 was taken from cylinder No. 2. H.-P. and R.P.M. the same as above. Card No. 3 was taken from cylinder No. 3 of the engine under test. H.-P. developed, 14, at 1,000 R.P.M. The Card TESTING 177 rapid drop in pressure near the end of the stroke is due to the auxiliary exhaust. The compression pressure is 60 Ibs. gauge, and the ex- plosion pressure is about 350 Ibs. This engine has a 4" base, 4" stroke. 193. EXPLOSION RECORDER. Another valuable instru- ment for gas-engine testing is the Mathot continuous- explosion recorder. Here a paper ribbon unwinds from one drum and on to another drum. The drums are turned by clockwork and the paper travels at a certain prede- termined speed. The explosions are re- corded by a pencil set in motion as in the ordinary in- dicator. The record (see Fig. 130) shows the regularity and time of the explosions. By means of either of these two instruments it can be quickly determined whether, in a multiple-cylinder en- gine, each cylinder is doing its share of the work. Faults can be corrected and the result of changes seen at once. 194. OUTSIDE WORK DONE BY THE ENGINE. This is the brake horse-power, or the power which the engine is capable of delivering. W X L X 2 X 3.1416 X R.P.M. 33,000 where L is the length of the brake arm in feet, i.e., length from centre of rotation to centre line of scale or spring; W is the pull on the brake arm in pounds. I.H.-P.xB.H.-P. Mechanical efficiency = j p The difference between I.H.-P. and B.H.-P. is the work done in overcoming engine friction. The ordinary dynamometer and pony brake are too well known to require description here. For high-speed 12 178 GAS-ENGINE THEORY AND DESIGN engines a fan can be made into a very convenient and satisfactory dynamometer. This, of course, requires no cooling water. 195. DYNAMO DYNAMOMETER. This is an ordinary direct-current dynamo so arranged that its field swings in ball bearings. An adjustable weight, mounted on an arm fastened to the frame, balances the magnetic torque between the rotating armature and the field. B.H.-P.= W X R.P.M.X a constant. The constant is determined by dynamo tests. The effi- ciency of the dynamo does not enter into the calculations. A field theostat and a load rheostat complete the equipment. 196. HYDRAULIC BRAKE. There are several forms of the hydraulic brake, but Fig. 131 illustrates the principles v FIG. 131. involved. Several plates are fastened to the casing C, plates B are fastened to the shaft, water enters at A and circulates along the shaft. As the shaft revolves the water is forced between the plates, which are close together, and is thrown against the inside of the casing by centrifugal force. The water drains off at D. The pas- sage of the water between the plates causes considerable friction and the heat so generated is carried off by the water. As the plates fastened to the shaft revolve they TESTING 179 tend, of course, to revolve the casing, and the power is measured as in the ordinary brake. The brake will absorb more or less power according to the adjustment of the valves at A and D. 197. HEAT LOSSES. These include the following: water- jacket loss; exhaust-gases loss; losses due to imperfect combustion, radiation, etc. 198. WATER-JACKET Loss. The heat carried off by the cooling water equals the weight of the cooling water X(t' t"), where t' is the temperature of the outgoing water and t" is the temperature of the incoming water. 199. HEAT LOST IN THE EXHAUST. The heat lost in the exhaust gases equals the weight of the exhaust gases in pounds X their specific heat X their absolute tempera- ture. The specific heat of the exhaust gases can be determined more or less closely by analyzing them. From the total heat lost in the exhaust must be sub- tracted the heat in the air (or air and gas) supplied per hour. This heat equals W X T X sp. ht. The heat lost in the exhaust is also found by adding the I.H.-P. and water-jacket loss and subtracting the sum from the total heat supplied. Both of the foregoing methods are wrong, in that they will not give accurate results. In the first case the weights exhaust temperatures, and specific heats cannot be ac- curately determined, and in the second case the exhaust is charged with losses due to incomplete combustion, radiation, etc. The only way in which to accurately determine the heat lost in the exhaust is to pass the gases through a condenser and cool them to the original temperature, then find how much heat has been absorbed by the condenser water, making due allowance for the heat absorbed by the metal. 180 GAS-ENGINE THEORY AND DESIGN TESTING 181 Assuming that there has been no leakage, the difference between the total heat supplied and the I.H.-P., plus water-jacket loss, plus exhaust loss, must be charged to incomplete combustion and radiation. The testing of engines at definite, intervals is apt to result in a considerable saving of fuel and in increase in power. Instances are on record where the thermal efficiency has been increased fully 30 per cent and the power largely increased. 200. MECHANICAL LOSSES. This includes engine fric- tion, leakage, back pressure in exhausting, etc. The leak- age losses may become very large if pistons and valves do not fit well. In conclusion some indicator diagrams are given (Figs. 132 to 135) which illustrate the effects of various wrong conditions in the gas engine. The efficiency of an engine under ordinary working conditions will fall short of that determined by experts in testing perfectly adjusted engines under the best pos- sible conditions, and this must be borne in mind in figuring on the fuel consumption and maximum power of an engine for every-day, and perhaps, unfavorable, conditions. Tachometer diagrams should be also taken in order to determine the correct fly-wheel weight. CHAPTER XXI DESIGNS 201. Marine Engine. The first design (M-A, M-B, M-l to M-57) shown is that of a small canoe or boat engine designed by the author. This engine is compact and neat, has a 3" bore and 3" stroke, and at 1,000 R.P.M. will develop about 2^ H.-P. The following notes will help in the study of this design : The crank-shaft is ample in diameter, the weight of the engine complete is about 40 Ibs., the fly-wheel is heavy enough to swing a 12" propeller with a 12" or 15" pitch. With some modifications the design can be arranged for a four-cylinder engine. The screw threads are U. S. S. throughout; no pipe is used. The ports are larger than is usual in such small engines, the gas passages are as direct as possible, the exhaust port is uncovered while the piston travels the last f" of its down stroke, the cylinder inlet port is uncovered a little later, the crank-case inlet port is uncovered the same length of time as the exhaust port. Straight passages prevent wire-drawing and back pressure and so increase the power. The lubricating oil is carried along by the incoming air and lubricates all parts in the crank-case and cylinder in an efficient manner. Twelve drops per minute is suffi- cient for good lubrication. The pump is held in place by a small reverse clutch (not shown). These clutches are equipped with ball thrust bearings. 182 DESIGNS 183 The water passes from the jacket into the muffler, where it sprays on top of the exhaust pipe, thus cooling the exhaust and reducing the volume of the exhaust gases. A shoulder on the muffler casting prevents any water which may run into the exhaust pipe from blowing back into the cylinder. A three-terminal coil is used in the ignition apparatus. The wiring plan is shown in M-57. One of the secondary terminals is connected to a primary terminal inside of the coil. The return wire for the batteries is connected to the engine frame which forms a ground for both primary and secondary circuits. The water jacket is spun from 22-gauge copper, then where it covers bosses on cylinder the metal is raised and holes punched. The parts that screw into these bosses are all fitted with shoulders so as to make a tight joint. The bottom, where the ring goes, is rolled down to 5.31" and the ring shrunk on. M-10 is a good quality bronze casting. M-12 is either a steel forging or cut and turned from a bar. M-16 is made up of two aluminum castings, one-half having a shoulder and the other half being recessed cor- respondingly. The bushings are of bronze. The carbu- reter castings are aluminum or brass. The float is ad- justed by moving up and down on pin until gasolene level is T y below point of needle valve. The muffler castings are aluminum. M-32 is made of 18-gauge sheet metal, perforated and rolled. M-35 and M-57. Flexible rubber joint. The water- pump parts are of bronze. M-42. Two f" steel balls forming check valves. M-49 is a bronze casting. M-54 is made of No. 20 steel wire, outside diameter of coil I", wound four turns per %". 184 GAS-ENGINE THEORY AND DESIGN M-58 is the relief-cock. To start the engine open the carbureter needle valve one-half turn, open the relief cock, open carbureter butter- fly valve one-half turn, set commutator for a spark a little past the dead centre, turn over the fly-wheel several times until there is an explosion. The crank-case is now filled with an explosive mixture. Close the relief-cock and start M-A. DESIGNS 185 the engine. A two-cycle engine with crank-case compres- sion will not start until the crank-case is filled with an ex- plosive mixture. When the engine is running turn on the lubricating oil, adjust carbureter, and commutator for steady running. If the engine is stopped after having run for some time, it can be restarted by one turn of the starting- handle. M-B. 186 GAS-ENGINE THEORY AND DESIGN M-l.-Cylinder. DESIGNS 187 M-2. 188 GAS-ENGINE THEORY AND DESIGN M-3. Water-jacket Ring. F.A.O. DESIGNS 189 Holes for Ring Pins No. 14 Wire 190 GAS-ENGINE THEORY AND DESIGN f XT* XT -H- ..!_ j*-* 1 -* 2JS- M-7 Piston Pin. F.A.O. M M-8. Piston Pin Set Screw. M-9. Piston Ring. Three wanted. DESIGNS 191 M-10. Connecting-Rod. 192 GAS-ENGINE THEORY AND DESIGN LLLL DESIGNS 193 M-13 Fly-wheel. 194 GAS-ENGINE THEORY AND DESIGN Taper tf per foot M-14. F.A.O. DESIGNS J95 Starting Handle M-15. 196 GAS-ENGINE THEORY AND DESIGN i f- M-16. rank-Case. Right and left casting. DESIGNS 197 I-16a. Crank-Case Studs. Four wanted with nuts. M-17. Crank-Case Bolt. Two wanted with nuts. M-18. Crank-Case Bolt. Three wanted with nuts. 198 GAS-ENGINE THEORY AND DESIGN j ^ j'e Hole for Grease Cup M-19. Main Bearing Bushing. Two wanted. F.A.O. M-20. Carbureter Casting. DESIGNS 199 M-20a. Carbureter Cover. M-21. Carbureter Float. 1 M-22. Carbureter Cover Screw. Two wanted. 200 GAS-ENGINE THEORY AND DESIGN M-23. Carbureter Fitting. F.A.O. M-24. Carbureter Needle Valve. DESIGNS 201 M-25. Needle Valve Fitting. ^fjf- M-26. Needle Valve Seat. M-27. Carbureter Throttle Valve. 202 GAS-ENGINE THEORY AND DESIGN DESIGNS 203 M-30. Muffler Cover. I M-31. Muffler Cover Bolt. Two wanted. M-32. Muffler Tube. 18 gauge perforations iV wide. 204 GAS-ENGINE THEORY AND DESIGN I M-33 Exhaust Fitting. F.A.O. M-34. Exhaust Tube. Groove for Lubricant M-36. Water Pump Eccentric. F.A.O. DESIGNS 205 - M-36a. W. P. Key. M-37. Water Pump Eccentric Ring. M-39. Pump Plunger. F.A.O. 206 GAS-ENGINE THEORY AND DESIGN Drill for * 14 Wire Pin M-40. Pump Pin. M-41. Water Pump Casting. DESIGNS 207 M-43 Pump Cap. F.A.O. M-44. Pump Cap. F.A.O. 208 GAS-ENGINE THEORY AND DESIGN M-45 and M-56. For Pump Casting and Commutator Handle. Three wanted. M-46. Water Inlet Tube. K* Lizzie - 1 *| * x- > *t <-M- I M-47 and M-48. Pump Floor Plate. F.A.O. DESIGNS 209 MM M-49. Commutator Handle. 210 GAS-ENGINE THEORY AND DESIGN Commutator Bushings Fiber M-50. Commutator Bushings Fiber. Two wanted. KnurL M-52. . F. Cap. F.A.O. 211 i* - M-51. Commutator Fitting. F.A.O. M-53. C. Contact pin. Steel-hardened. M-55. C. Contact Screw. M-57 Wiring Diagram. 212 GAS-ENGINE THEORY AND DESIGN 202. Horizontal Engine. Pl&tes S-36 to S-68 show assembly views and details of a small horizontal four-cycle gas or gasolene engine, which has been partly re-designed by the author. These details are so complete that only a few words of explanation are required. The machine develops about ^ H.-P. running at 750 R.P.M. The water is allowed to boil off. The pump furnishes gasolene to a vaporizer (not shown) which is provided with an overflow pipe. The governor, in which four steel balls depress a plate, operates a throttle valve in the vaporizer. The piston is lubricated by means of an oil cup (sec S-36) mounted on a stem which is screwed into a tapped hole in the cylinder. The few small parts not shown can easily be bought in the market. The engine runs very smoothly and illustrates some excellent and interesting principles in design. The arrangement of base, sub-base, cylinder, camshaft, gover- nor, etc., is closely followed out in the design of many large horizontal gas engines. DESIGNS 213 214 GAS-ENGINE THEORY AND DESIGN DESIGNS 215 L>16 GAS-ENGINE THEORY AND DESIGN DESIGNS 217 218 GAS-ENGINE THEORY AND DESIGN DESIGNS 219 220 GAS-ENGINE THEORY AND DESIGN DESIGNS 221 222 GAS-ENGINE THEORY AND DESIGN DESIGNS 224 GAS-ENGINE THEORY AND DESIGN DESIGNS 225 226 GAS-ENGINE THEORY AND DESIGN J \ DESIGNS 227 lt_.' S J50_R.P.:.I. H f-'-> S-51. Spiral Gear. F.A.O. 228 GAS-ENGINE THEORY AND DESIGN S-52. Cylinder Head Cover. DESIGNS 229 230 GAS-ENGINE THEORY AND DESIGN fc * U, 1 ! 1 1 1 "i 1 I ; L V j>;j& i il- - iv" >k i 3 /" > S-54. C. I. Cylinder Liner. S-58. Governor Assembly. DESIGNS 231 232 GAS-ENGINE THEORY AND DESIGN g ! ?_i -"<; % DESIGNS ?33 234 GAS-ENGINE THEORY AND DESIGN "l-f H DESIGNS 235 S-60. Ball Support. S-60. Governor Cap. C.R. Steel. F.A.O. 236 GAS-ENGINE THEORY AND DESIGN S-61. Governor Plate. S-62. Governor Plate. DESIGNS 237 S-63. Cylinder Head Assembly 238 GAS-ENGINE THEORY AND DESIGN DESIGNS 239 k : 240 GAS-ENGINE THEORY AND DESIGN DESIGNS 241 242 GAS-ENGINE THEORY AND DESIGN T# III APPENDIX TESTS. Some tests of Warren gas engines are herewith given as submitted by the builders: SYNOPSIS OF THE RESULTS OBTAINED FROM THE NONPAREIL CORK WORKS' ENGINE AND PRODUCER Date Hours Run Hours. Stand. Fuel Run Con- sumed Stand. Output H.-P. Hrs. LOAD FACTOR Consump- tion Per H.-P. Hr. Max. Min. Avg. Jan. 22.. " 23. . ff s* 2100 2147 350 360 832.3 1516.9 53% SP3 35.48% 7.09 42.7% 68.95 2.5lb. 1.41 " 24.. " 25.. " 26.. lit 8i 13 12f 13 1435 l.VJS 706 355 355 355 1501 1421 730.1 89.3 80.8 53.6 9.85 11.3 20.5 67.2 66.6 42.95 .92 1.12 .967 The load factors were based upon the engine rating of 200 B.H.-P. The B.H.-P. developed was computed from 30 min. readings of am- meter and volt-meter, allowing 15 per cent loss for the belt and gen- erator. The coal charged to the producer was weighed and the magazine was kept full at all times, so that amount of coal lost over night could be very accurately determined by cleaning the fires in the morning and then filling the magazine to the top. Average Ibs. of coal per B.H.-P. per hr. . " load factor " stand-by loss " " " per H.-P. based on producer rating of 300 H.-P . . 243 1.33 Ibs. 58.7 per cent. 27.2 Ibs. coal per hr. 9 . 1 Ibs. per 100 hrs. 1M-1 APPENDIX GAS ENGINE, GAS PRODUCER AND No. 3 DYNAMO Five days run, June 10, 11, 12, 13 and 14, 1907, 11 hrs. per day. DATE Avg. Volts Avg. Am. Lbs. Pea Coal Gals. Oil June 10 225 516 2,450 2 " 11 225 524 2,800 2 " 12 225 485 2,800 2 " 13 225 492 2,800 2 " 14 225 456 2,100 2 Total 1,125 2,473 12,950 10 Average 225 495 2,590 2 Average Volts 225 Average Amperes . . . 495 Total K.W. Hours 6115 Total B.H.-P. Hrs. de- veloped by engine 10,200 1 K.W. Hour= 1.341 El. II.-P. Hours. With 88 per cent efficiency of dynamo and 92 per cent efficiency of belt, 1 K.W. Hour=1.66 Brake H.-P. Hours developed by the engine. Cost of anthracite pea coal used, per gross ton. . . . Cost of gas-engine oil used, per gallon $3.91 .21 Analysis of Coal. Moisture Volatilcs Sulphur Fixed Carbon. . Ash. . . Coal Consumption Including Stand-by Losses of 400 Ib. per Twenty-four Hours. Heating Value of Coal 12,657 B.T.U. per Ib. 1.15 per cent 4.35 per cent . 0.60 per cent . 77 . 35 per cent . 16.55 per cent 100.00 per cent Coal Consump- tion, Charge- able to Engine, Exclusive of Stand-by Losses. Total 12950 Ib. 10950 Per K.W. Hourat switchboard 2.121b. 1.7! Per B.H.-P. Hour at engine. . 1.27 Ib. 1.071b. APPENDIX 245 Operating Costs, Operating Costs Including Chargeable to En- Stand-by Losses. gine, Less Stand-by Coal per K.W. Hour 0.305c 0.258c Coal per B.H.-P. Hour 0. 183c 0. 154c Oil per K.W. Hour 0.035c 0.035c Oil per B.H.-P. Hour 0.021c 0.021c Labor per K.W. Hour 0.515c 0.515c Labor per B.H.-P. Hour .... . 310c . 310c Total per K.W. Hour . 855c . 608c Total per B.H.-P. Hour . 514c . 485c The figures above are for a load of about 80 per cent of the full capac- ity of the engine, and about 60 per cent of the capacity of the producer. At this load the engine developed one B.H.-P. on 13,500 B.T.U., meas- ured by coal consumed in the producer, and taking the efficiency of the producer as 75 per cent the engine used 10,000 B.T.U. per B.H.-P. Hour, or an amount equal to about 9,500 B.T.U. at full load on engine. REPORT OF TEST OF ONE (1) 250 H.-P. WARREN TANDEM GAS ENGINE AT POWER HOUSE OF THE CITIZENS GAS & ELECTRIC COMPANY, LORAIN, OHIO, FEBRUARY 28ra AND 29TH, 1908 ENGINE Two-Cylinder Warren Tandem Gas Engine, built by Struthers-Wells Company, Warren, Pa. Diameter of Cylinder 19 in. Stroke 25 " Diameter of Piston Rod 4J" Speed 170 R.P.M. GENERATOR Engine was belt-connected to 150 K.W. Westinghouse, 3-phase, 60- cycle, A.C. Generator, 39.5 amperes, 2,200 volts at 600 R.P.M. TEST The electrical output of the generator was transmitted partly to a water rheostat and partly to the lighting system of the Company. 246 APPENDIX The test was started at 5:00 P.M. February 28th, 1908, the load being 20 amperes per phase at 2,400 volts. The load was gradually increased until 5:30 P.M., at which time engine was carrying load of 45 amperes per phase, and at this time the Westinghouse Three-Cylin- der Vertical Engine was started, synchronized, and run in parallel with the Warren Tandem Engine. When the two engines were operating, the load on the Warren Tandem engine averaged 35 amperes and 2,300 volts per phase. This load condition continued until 9:30 P.M., at which time the load on the Warren Engine gradually decreased to 27 amperes at 2,300 volts per phase. At 10:00 P.M. the use of the West- inghouse Engine was discontinued and the entire load carried by Warren Tandem Engine, the load at that time averaging 48 amperes per phase at 2,300 volts. The night load from 10:00 P.M. of February 28th until 5:30 A.M. February 29th averaged 45 amperes at 2,300 volts per phase, equal to 179 K.V.A. Assuming at that time a power factor of the whole cir- cuit as low as 0.70, we would have an average of 125.5 K.W. at the switchboard, or an average of 201 B.H.-P. developed by the engine for seven hours and thirty minutes, or a total of 1,507.5 B.H.-P. hours. The gas consumption for this period of time was 12,650 cubic feet; 12,650 divided by 1,507.5 equals 8.4-cubic feet of gasper B.H.-P. hour. READINGS OF ENGINE AND GENERATOR PERFORMANCE AT 10:35 P.M., FEBRUARY 28ra Load on generator, 46.3 amperes, 2,300 volts equal to 184 K.V.A. Mean effective pressure in engine cylinders, 79 pounds. Engine speed, 172 R.P.M. Engine load equal to 238 I.H.-P. Assuming the mechanical efficiency of the engine at this load to be 88 per cent, we would have 209 B.H.-P. at the engine, or 132 K.W. at 132 the generator; the power factor being T^J-= 0.716. AVERAGE READING FROM 11:15 A.M., FEBRUARY 29TH TO 12:15 P.M. Average load on generator, 43 amperes, 2,470 volts, equal to 184 K.V.A. Mean effective pressure on engine cylinders, 105.8 pounds. Engine speed, 170 R.P.M. Engine load equal to 316 I.H.-P. Assuming the mechanical efficiency of the engine at this load to be 88 per cent, we would have 281 B.H.-P. at the engine, or 176 K.W. on the generator; the power factor being -=- = .957. Io4 APPENDIX 247 The gas consumed from 11:15 A.M. until 12: 15 P.M. was 2,290 cubic feet; 2,290 divided by 281 equals 8.1 cubic feet of gas per B.H.-P. hour. SPEEDS AT VARIOUS LOADS After 3:00 P.M. the load was repeatedly changed in order to ascer- tain the speed regulation, which was ascertained to be as follows: Variation from 170 R.P.M.2.35% Variation from 170 R.P.M. 1.76% Variation from 170 R.P.M. 1.18% Variation from 170 R.P.M. 0.00% Variation from 170 R.P.M. 0.59% Full load speed 170 R.P.M. Speed at no load 174 R.P.M. Speed at i load 173 R.P.M. Speed at * load 172 R.P.M. Full load speed 170 R.P.M. Speed at overload (320 B.H.P.) 169 R.P.M. Speed variation under constant load less than plus or minus 0.25 R.P.M. equals 0.15 per cent. The engine showed itself capable of carrying, without undue heat- ing or any signs of distress, a load on the generator equal to 54 am- peres at 2,150 volts, equal to 201 K.W. The M.E.P. developed in the engine cylinders at this load was 119.5 pounds at 169 R.P.M., equal to 320 B.H.-P. or 355 I.H.-P. The mechanical efficiency of the engine when carrying this load was equal to !; = 90 per cent. GOO The determination of the amount of power developed by the engine as shown by switchboard readings is based on the following data: 1 K.W. = 1.34 B.H.-P., exclusive of losses at generator or through transmission of power by belt. Assuming the average generator efficiency at 91 per cent for differ- ent loads specified, and belt efficiency (the belt being new) at 92 per cent, 1 K.W. at the switchboard would be equivalent to 1.6 B.H.-P. developed by the engine. Attached hereto are indicator cards, numbers 1 to 4, inclusive: No. 1. Showing M.E.P. in engine cylinders 67 pounds, speed at time of taking card 172 R.P.M. Generator load, 34.3 amperes per phase at 2,400 volts. No. 2. Showing M.E.P. in engine cylinders 79 pounds, speed at time of taking card 171 R.P.M. Generator load, 46.3 amperes per phase at 2,300 volts. No. 3. Showing M.E.P. in engine cylinders 106 pounds, speed at time of taking card 170 R.P.M. Generator load, 43 amperes per phase at 2,450 volts. No. 4. Showing M.E.P. in engine cylinders 119.5 pounds, speed at time of taking card 169 R.P.M. Generator load, 54 amperes per phase at 2,460 volts. 248 APPENDIX # 00 id H y ii H ol cC SS II II S5 d ^ffi APPENDIX 249 APPENDIX 5 EH 82 s I OH ffi APPENDIX 251 INDEX PAR. Adiabatic expansion 83 Advancing of spark 112-120 Advantages of the gas engine . 95 Air, composition of 47 cooling 127 required for combustion 48-52 required per H.-P 144 Alcohol 64 production of 64 Altitude, effect of 146 Anthracite coal 58 Anti-freezing solutions 130 Applications of the gas engine 15 Ash 45 Atmosphere 73, 147 Atom 34 Automobile statistics 15 Auxiliary exhaust 134, 186 Avogadro's law 53 Babbitt, brass, bronze 178 Balancing 157-157a Banki engine 128 Bearings 177-178 Beau de Rochas cycle 11 Benzol 716 Bituminous coal 59 Bolts, strength of 166 Bore and stroke 145 Brake horse-power 194 Brake, hydraulic 196 Brayton cycle 10 British thermal unit 23-147 Calorie Calorific power. Calorimetry. . . . Cams Carbureters. . . . Carnot cycle. . . . . . 147 44-52a . ... 72 . .. 179 . .. 147 .. 90 Centigrade thermometer 30 Centrifugal force 180 gas cleaner . 69 Chamber, combustion 104 Change, physical 34 chemical 34 Chemical action 34 Chemical reactions in pro- ducers 71 Classification, general 3 Clerk engine 12 Coal 57-59 production of 57 Coef. linear expansion 33a Coke 66 Combustion 34-35, 55 complete 41 compound 34 incomplete 42, 103 of a compound 52 speed of 46 Compression 88, 99, 142 two-stage 89 Conduction 21 Connecting-rod. 150, 154, 157, 173 Conservation of energy 147 Convection 21 Cooling 124 Counterbalancing 157a Crank 175 pin 176 shaft 174 Crosshead 171q Curve, PVn 149 Cycle 4-26 two-stroke 6 four-stroke 7 Cylinder head : 1 65 Cylinder volume 144 Cylinders 163 Cylinders, arrangements of . . 161 253 254 INDEX PAR. PAR. Daimler engine 13 Gas, illuminating 66 Density 33 laws of ... 73 Design of marine engine 201 natural ... 65 Design of parts 159 oil water ... 68 Design of stationary engine. . 201 Diagrams, four-cycle engine. 158 producer specific heat of ... 70 ... 91 Diesel cycle 14 volume of ... 91 Dilution of explosive mixtures 102 water ... 67 Dissociation 55 Gasolene ... 63 Distillates, petroleum 60 Gay-Lussac, law <.f ... 76 Distillation 60 Governor, cut-off' . Ill Distribution 120 function of . . 108 Dynamometer, dynamo 195 quality ... 109 Economy 94 quantity throttle ... 110 . . . 110 Efficiency 92 189 Element 34 Heat, definition of 16 Energy 147 and power units ... 147 available 87 engine 1 intrinsic 79 insensible ... 25 put into engine 190 latent . 27-29 Exhaust 131, 133, 134 losses ... 197 losses 199 mechanical equivalent of. 24 using heat of 93 sensible and insensible .. . 25 Expansion 18, 33a, 81 sources of ... 20 adiabatic 83 specific 26 , 86, 91 isothermal 82 theory of 17 PVn 87 transfer of 21 Explosion 39, 81 Heat unit . . . 23 Explosive mixtures OS, K)2 Historical . 8-14a Explosive pressures .... 148, 149 Hollow shaft . . . 174 Horizontal vs. vertical . . . ... 137 Fahrenheit thermometer. ... 30 Horse-power ... 148 Firing cylinders, order of. ... 120 Hydrocarbons . . . 60 Flame 36 Flashing-point 60 Ignition 37, 113-119 Fly-wheel 180 Indicated H.-P 74, 191 Foot-pound 24 Indicator ... 74 Forces acting in gas engine . . 148 diagram 73, 148-149 Foundations 182 Induction coil . . . 118 Four-stroke cycle 7 Inertia 155-156 cycle engine diagrams. . . 158 Frames 162 Installation of gas engine Isometric lines . 184 ... 84 Fuels 43, 56, 61, 71, 100 Isopiestic lines ... 85 enriching of 716 Isothermal expansion. . . . ... 82 Fusion 28, 30, 33a Jump spark . . . 117 Gallon 147 Gas changes in 75 Kerosene 62 blast-furnace 69 Kilowatt . . . 147 coal 66 Kinetic energy . . . 147 cleaning of 69-70 engine 2 Latent heat . 27 fuel, properties of 7 la Laws, combining of . . . 78 INDEX 255 PAR. Laws of gases 73 of thermodynamics 33 Lenoir cycle 9 3, exhaust 199 heat 197 mechanical. . . . . 200 PAH. Producer, chemical reac- tions in 70 gas 71 Products of combustion 53 Pyrometry 32 water-jacket 198 Radiation . . . . 22 Lubrication and lubricants . . 178 Recorder, explosion . . . . 193 Reliability 93 M Re'sume la Make-and-break ignition 119 Rotation, direction of. . . '.'.'.' 161a Manipulation of gas engine . . Manograph Marine exhaust 184 192 133 Scavenging Selection of type . 101 . ... 136 Mariotte, law of 77 Shaft, crank . ... 174 IfiO Side thrust 153 169 Mean effective pressure Mechanical losses 1OU 73 200 Single-acting Slider-velocity diagram . 138, 139 . ... 151 equivalent of heat Media used in heat engines. . Metric units 24 96 147 Small units . ... 140 . . . . 38 . . . . 20 Smoke Sources of heat Mixing valves 105 Spark advancing . ... 112 Mixtures, mechanical 34 Spark-plug 117, 122 Molecule 34 Specific gravity . 33a-60 Specific heat ,..86,91 Nomenclature, chemical Non-conductor 34 22 Speed Spontaneous combustion ... 3 . . . . 40 Springs ... 179 Offset cylinders 152a Starting devices of gas engine . .. 186 ... 185 Oil, fuel 61 States of matter 29 Oilers, mechanical Operation, general principles 178 Statistics, automobile 15 alcohol ... 64 of 5 coal ... 57 gas engine ... 14a Passages, valve 179 motor boat ... 15 Petroleum 60 petroleum 70 distillates 61 Stopping of gas engine . . . ... 187 production of Physical properties of ma- 60 Storage battery Strength of materials .... ... 121 ... 183 terials 33a Stroke and bore ... 145 properties of liquids 33a Studs ... 166 Piston 168 Stuffing-boxes ... 178 pin 170 Suction gas-producer .... .... 70 rings 172 rod 171 Tachometer . 181 speed Planimeter 143 73 Tangential-effort diagram ... 152 Temperature 19, 31, 54 Potential energy 147 Testing ... 189 Power 141, 147 Tests of gas engines ... 201 Preheating 107 Theoretical temperatures. .. . 54 Pressure, absolute 73 Thermal conductivity. . . . ... 33a regulator 188 Thermodynamics, laws of ... 33 Principles of operation 5 Thermometer, air ... 31 256 INDEX Thermometry Timers Troubles, engine Two-cycle vs. four-cycle. Two-stroke cycle Types of heat engines. . . Unit, heat power Valve cages. . Valves Vapor Vaporization . Vaporizers. . . .. 23, PAR. 30 120 188 136 6 97 147 147 PAR. Volumes of gases 91 of products of combustion 53 167 . . 105, 179 33 29 . 106 Water, cooling impurities in injection jacket Watt Weight, atomic of materials Work. delivered in engine cylinder Zero, absolute 125, 129 .... 130 .... 128 164, 198 .... 147 .... 34 .... 33a 147 194 191 . 31 UNIVERSITY OF CALIFORNIA LIBRARY Los Angeles Ml This book is DUE on the last date stamped beloiv NOV 3 1953 JAN 1 8 195 MAR 2 DEC 6 1954 mn. i Form L9-100m-9,'52(A3105)444