+ — T1770 a cw pe. | Cornell University Library OF THE Mew Work State College of Ugriculture 3514 The gas engine, THE GAS ENGINE BY CECIL P. POOLE EDITOR OF POWER AND THE ENGINEER AUTHOR OF ‘ ‘THE WIRING HANDBOOK,” ‘‘DIAGRAMS OF ELECTRICAL CONNECTIONS,” ‘“DESIGNS FOR SMALL DYNAMOS AND MOTORS,”’ ETC. YUU UU UC UU CUCU CUC UU UU TC UU UU UC UC UU UU UU UU UU UU UU UY UW UCU UU UV CUUUUUUUUUUCUUUTE Published by the McGraw-Hill Book Company Fs 5 Fs 7 : New York E Fs r Fj : Successons to the Book Departments of the ; | McGraw Publishing Company Hill Publishing Company is | 5 Publishers of Books for ; 8} Electrical World . The Engineering and Mining Journal RB} The Engineering Record Power and The Engineer [f | Electric Railway Journal American Machinist |F oe > 5 ¢ DAD OA MDDOOOMADADADDADDADDNDDDDAADAODDDODA DODD A DOA A DDODDAOAOAMAAAADAADDDAAADDAL THE GAS ENGINE BY CECIL P. POOLE EDITOR OF POWER AND THE ENGINEER AUFHOR OF ‘“PHE WIRING HANDBOOK,’’ ‘‘DIAGRAMS OF ELECTRICAL CONNECTIONS,” ‘“ DESIGNS FOR SMALL DYNAMOS AND MOTORS,” ETC. 1909 HILL PUBLISHING COMPANY 505 PEARL STREET, NEW YORK 6 BOUVERIE STREET, LONDON, E. C. Power and The Engineer — American Machinist — The Engineering and Mining Journal e “TT TTO ee Pa 624 Copryrieut, 1909, sy THE Hitt PuBLisuine Company Hill Publishing Company, New York, U. S. A. PREFACE Tus book is not intended as a complete treatise on the subject. The object of the author is to present the principles governing the salient features of gas-engine construction and operation in as simple a manner as possible, and to that end academic discussions of the characteristics of gases and of hypothetical heat-energy cycles, of the character commonly found in text-books, have been avoided. Since the pressures, temperatures, and energy transformations which occur in a gas-engine cylinder cannot be adequately explained without the use of algebraic equations which appear complex to a beginner, such equations have been employed in that connec- tion, but their use has been restricted to a single chapter. This chapter may be omitted without sacrifice by readers who wish merely general, rudi- mentary information, but not by real students of the subject. C, -B. PB. New York, January, 1909 CONTENTS CHAPTER I Bue ELEMENTARY PRINCIPLES. ‘i ‘ . . ‘ 3 ‘ Fi ‘ 3 . 3-15 The Working Medium, 3. The Four-stroke Cycle, 5. The Two-stroke Cycle, 10. CHAPTER II PRESSURES AND TEMPERATURES . ‘ : : : : : . : : . 16-29 Compression, 16. Combustion, 19. Expansion and Exhaust, 22. Mean Effective Pressure, 28. CHAPTER III Cootine anp Heat Loss. ‘ . : : . . ‘ 3 ; . . 30-34 CHAPTER IV VALVES AND VALVE GEAR . ; : : ‘ : . z e ~ Z . 35-39 The Mixing Valve, 38. CHAPTER V IGNITION ; % . a : s s F : : : ‘ : ; ‘ . 89-50 Make-and-Break System, 40. Jump-spark System, 43. Automatic Ig- nition, 44. Timing the Ignition, 46. CHAPTER VI Mrxine Lieuip FUEL with AIR Z : ‘ : : A ‘i s : . 51-55 CHAPTER VII METHODS OF GOVERNING. ‘ : ; : 3 ‘ 5 : : . 56-64 Hit-and-Miss, 56. Variable Quantity of Intake, 58. Varying the Quality of Mixture, 63. Combination Methods, 63. CHAPTER VIII Some CoNSIDERATIONS OF DESIGN. : : : ’ : : 4 : . 65-65 Cylinder Construction, 65. Valves and Operating Gear, 66. e vl CONTENTS CHAPTER IX PAGES CarRE AND MANAGEMENT oF ENGINES ‘ ‘ j . 67-72 Starting an Engine, 67. Running an Engine, 68. Shutting Down, 71. Troubles, 71. CHAPTER X 73-93 PRESSURE, TEMPERATURE, AND OuTpuT CALCULATIONS. Gases, 73. Heat in Cylinder Contents, 77. Work Done per sais 80. Indicated Horsepower, 82. Practical Output Estimation, 83. Effi- ciency, 89. TINDER ok od OR: de @. a Ba & cM ahs cm: oe cH ok - . 95-97 THE GAS ENGINE I ELEMENTARY PRINCIPLES THE Wornine Mepivumu Gas and oil engines differ from other forms of heat engine chiefly in that the pressure which gives the engine its power is produced within the cylinder by the combustion of the gas or oil. For the operation of all other heat engines the working medium (steam, hot air, etc.) is raised to a pressure much higher than that of the atmosphere before it is delivered into the cylinder; after entering the cylinder, the working medium is expanded to a low pressure, the expansion driving the piston forward. The working medium of the gas or oil engine is delivered to the cylinder at about atmospheric pressure and there compressed and ignited; the rise of temperature produced by the combustion causes a corresponding rise of pressure and the high-pressure gases are then expanded behind the piston in the same manner that steam expands in a steam-engine cylinder, and with similar results. In order to burn anything, no matter how inflammable it may be, it is necessary for oxygen to be brought into contact with the substance to be burned, because combustion is nothing more than the union of oxygen par- ticles with the combustible particles of the substance “ burned,’ under the influence of heat. Air, which consists of oxygen and nitrogen, is the only free source of oxygen and is therefore universally used to supply the oxygen required for combustion of any kind. Hence, the gas or oil used as fuel in an engine is always mixed with air either immediately before it enters the engine cylinder or immediately afterwards. The proportion of air to gas or to oil is of great practical importance; if too little air is supplied, combustion is slow and incomplete, and if the proportion of air is too large, the inflammability of the mixture is reduced and combustion is retarded. Again, it is important that the mixing of the air and fuel should be thorough; otherwise some of the gas or oil either will not be burned at all or will burn too late to do much good in the way of producing pressure behind the piston. Gas and oil burn quietly in the open air chiefly because the gases pro- duced by the combustion can expand as rapidly as they are formed, having a) ELEMENTARY PRINCIPLES 5 the whole universe into which to expand and being restrained only by the moderate pressure of the atmosphere. Moreover, the atoms of gas or oil are not mixed intimately with the atoms of oxygen in the air before burn- ing begins, so that the combustion is very gradual. When gas and air or oil vapor and air are thoroughly mixed and highly compressed in a closed vessel, however, igniting the mixture will produce almost instantaneous combustion of the whole mass, resulting in a sudden rise of temperature and pressure amounting practically to an explosion. This is what happens in the cylinder of a gas or oil engine when conditions are right. The explosion of the mixture occurs when the piston is at one end of its travel, the mixture being compressed in the clearance space between the piston and the near cylinder head; and as soon as the crank passes the dead center, the burned and burning gases expand, forcing the piston away from the end of the cylinder. At the end of that stroke, the spent gases are exhausted into the atmosphere, just like the expanded steam in a non- condensing steam-engine cylinder. Tuer Four-STROKE CYCLE There are two general classes of gas and oil engines; one works on the four-stroke cycle and the other on the two-stroke cycle. All small engines of both classes are single-acting: the piston is commonly of the trunk type, as illustrated in Fig. 1, and the combustion of fuel occurs in one end only of the cylinder. The four-stroke cycle is more widely used than the two-stroke, for rea- sons which will be explained later on. The cycle comprises five events, namely: Admission, compression, combustion or explosion, expansion and exhaust. In this type of engine the charge is taken into the cylinder at. atmospheric pressure, and is therefore necessarily drawn or “sucked” in by the piston of the engine, acting for the time as a pump piston. This occurs during one out-stroke of the piston (from position 4 to position B, Fig. 2), during which the inlet valve is held open either by the valve gear or by the atmospheric pressure. At the end of this stroke, commonly called the “suction ” stroke, the inlet valve is closed, and when the piston comes back on the return stroke (B to C, Fig. 2) it compresses the cylinderful - of mixed air and gas (or air and oil vapor) into the clearance space, which is relatively much larger than in a steam-engine cylinder. When the piston has reached the end of the compression stroke, and " while the particles of mixed air and fuel are compressed into intimate con- tact, the mixture is ignited and burns explosively, as already explained, producing a sudden rise of pressure behind the piston. This enhanced ‘pressure drives the piston forward on its power stroke (C to D, Fig. 2a), during which the pressure is gradually reduced by the expansion of the THE GAS ENGINE Inlet Valve THE SUCTION STROKE. Exhaust Valve Inlet Valve ms ‘Inlet Valve THE COMPRESSION STROKE. FIG. 2. —ILLUSTRATING THE FOUR-STROKE CYCLE. ELEMENTARY PRINCIPLES Inlet Valve Exhaust Valve Inlet Valve THE EXPANSION STROKE. Exhaust Valve ‘Inlet Valve THE EXHAUST STROKE. FIG. 2a.— ILLUSTRATING THE FOUR-STROKE CYCLE, 8 THE GAS ENGINE hot gases, the valves remaining closed, of course, until the stroke is almost completed; then the exhaust valve is opened by the valve gear and the burned gases allowed to expand through the exhaust port down to atmos- . pheric pressure. On the return stroke (D to #, Fig. 2a), the piston drives the remaining ,hot gases out of the cylinder, except what remain in the clearance space at the completion of the stroke (position H). It is evident from the foregoing that the five events in the engine cylin- der occur during four strokes of the piston: the suction stroke, compression stroke, expansion or power stroke, and the exhaust or expulsion stroke ; hence the term “ four-stroke cycle.” Fig. 3 is an indicator diagram made 240 4 Py 230 + 220 + 210 5 200 + 190 4 ¢ 190+ ar - 3 1 Combustion 0 Pg" 87 Lbs, 804 0-7 60- 50-4 Pym 43 Lbs. O. 405 SEP ression : ons 305 20 bs gy Ne — 14.5, 10 ke - respecte aos Piast, |. a y Pa 0 : 2 FIG. 3.— INDICATOR DIAGRAM FROM A GAS ENGINE WORKING on. ; THE FOUR-STROKE CYCLE. Ignition occurs Absolute Pressure per S: by a gas engine working on the four-stroke cycle. The line s from po ‘to Pa was traced by the indicator during the admission or suction stroke of the piston, and lies a trifle below the line e, which is at practically atmos- pheric pressure. The reason for this is that the piston, moving away from the cylinder head, forms a slight vacuum behind it before the fresh mixture begins to enter the cylinder, and this partial vacuum is maintained through- out the remainder of the suction stroke. Consequently the entering charge is at a slightly lower pressure than that of the outside atmosphere, as shown by the admission line s on the diagram. This is necessary, of course, because if the pressure within the cylinder were not lower than that of the atmosphere the mixture of air and gas would not enter, unless pre* viously compressed to a pressure higher than the atmospheric pressure. The degree of vacuum required to draw in the charge depends on the resistance offered to the charge by the passages and inlet-valve port ‘through ELEMENTARY PRINCIPLES 9 which it reaches the cylinder, and, in order to keep down the work done in “sucking ” the charge through these passages and port, their area is made as large as practicable and all bends are of as large radius as constructional considerations will permit. On the diagram here reproduced the admis- sion pressure was 13} lbs. (inaccuracy in redrawing it has made it appear higher) ; the degree of vacuum, therefore, was 1.2 lbs. per square inch or 2.43 ins. of mercury. The curve marked “ Compression ” shows the rise of pressure produced by compressing the mixture in the cylinder during the return piston stroke. If the mixture had not been ignited until the compression stroke was com- pleted, the curve would have been continued to the point p,, as indicated by the dotted extension of the curve, and if ignition had then occurred, the combustion line would have started abruptly upward, as indicated by the vertical dotted line. But it has been found advisable in practice to ignite the mixture just before the end of the compression stroke, and that is what was done in this case, producing an upward change in the com- pression curve at the point marked “Ignition occurs.” The reasons for igniting the mixture before the piston completes the compression stroke are fully explained in the chapter on Ignition. It will be noticed that the lower part of the combustion line is strictly vertical and that the line leans over slightly toward its upper end. That is due to the fact that combustion was not instantaneous, but continued after the piston had started forward on its power stroke. Absolutely in- ” stantaneous combustion is not obtained in an actual engine because of the impossibility of getting a perfect mixture and inflaming the whole of it at the_same instant. he expansion curve of the diagram will be jocaonned as very similar . to the expansion curve of a steam-engine indicator diagram. It drops more rapidly, however, than the curve of steam expansion, and at the point. pe its direction changes rather abruptly; this is due to the fact that the exhaust valve opens before the piston completes the expansion stroke, which is necessary in order to give the hot gases time to expand to atmospheric pressure before the piston starts back on the return (expulsion) stroke, and thereby avoid excessive baclt pressure during the first part of that stroke. The expansion of the burned gases to atmospheric pressure is represented by the reverse curve from the point pe to the extreme “toe” of the dia- gram, and although the exhaust valve is open while the pressure is falling from the release pressure pe to atmospheric pressure, the expanding gases do some work on the piston. If it were practicable to have an exhaust- valve port large enough to let the gases drop instantaneously to atmospheric pressure when it was opened, the expansion curve could be continued to the end of the stroke, as indicated by the dotted extension, but this is, of course, impossible. 10 THE GAS ENGINE The line e, at practically atmospheric pressure, is traced during the exhaust or expulsion stroke of the piston. The actual pressure is very slightly above the atmosphere, of course, owing to the resistance to the flow of the burned gases presented by the exhaust port and channel and the piping leading away from the engine, but the difference is not meas- urable on an indicator diagram. Tue Two-sTRoKE CYCLE In a two-stroke-cycle engine the five events just described also occur, but they are crowded into the limits of two piston strokes instead of four; hence the name of the cycle. This type of engine, however, does not draw its charge into the working cylinder by piston suction, but receives it either from a separate pump or from a reservoir to which it has been pumped, and at a pressure, therefore, above atmospheric. There are several forms of two-stroke-cycle engine; one of the simplest is that illustrated in Fig. 4. There are no valves in the cylinder wall; the inlet and exhaust ports, indicated by the letters J and EZ, are covered by the piston during all of its movement except a small proportion near the outer end of the travel. The inlet port J is connected by the annular passage B and ports C with a chamber D formed in the front end of the cylinder, which is provided with a stuffing-box through which the piston rod passes. This chamber D is really a pump cylinder. When the piston moves from the position shown toward the rear end of the cylinder, it draws a mixture of gas and air into the chamber D through the valve A, the channel B and the ports C (there are several of these ports spaced at equal distances around the wall of the chamber D).. At the same time mixture previously taken into the power end of the cylinder is compressed between the piston and the cylinder head, just as in the four-stroke-cycle engine. 3 When the piston reaches the end of the compression stroke, the com- pressed mixture is burned, producing a rise in pressure behind the piston, as in the four-stroke cycle, and this pressure drives the piston forward on its power stroke. The forward movement compresses the mixture just drawn into the chamber D, with the result that when the piston passes the inlet port J the mixture rushes from the chamber D into the power end of the cylinder. In order to permit the fresh charge to enter the cylin- der when the inlet port is uncovered, the exhaust port # is so located that the piston uncovers it before the inlet port is uncovered. This allows the burned gases to expand almost to atmospheric pressure before the fresh charge is admitted; therefore, the pressure to. which this charge is com- pressed in the pump chamber D need be only a few pounds above that of the atmosphere. From the foregoing it will be evident that a fresh charge is drawn into 11 ELEMENTARY PRINCIPLES ‘ ‘TIOAO AMOULS-OML AHL NO DNIMHOM ANIONT SVD ONILOV-HTIONIS — % “DIE ] ee > SS N N N Sc WN FALL) Le ae N | RL NNR iis NYS VA a | oa LWT Vj AS ANE SSS v H i] TREES ele Le \ a Yj ERS SR pac ULL TLL yy || Y ITT Ls BV emma) ®VE WS 1 re UZ ici VS [iS N y eK) 0 ¥ 4 | SS Ps) es ] rh Ly Lp Lif ZL, rps rs YZ DP Ly Zs [are OUELLETTE LY SILELLLILD 12 THE GAS ENGINE the pump chamber D, and the previous charge in the cylinder is simul- taneously compressed, every time the piston makes a backward stroke; that the compressed charge is fired every time the piston reaches the end of the instroke; and that the heated gases expand behind the piston and a new charge is slightly compressed in the pump chamber every time the piston makes a forward stroke, so that every forward stroke is a power stroke. It will naturally occur to the student that while both the inlet and exhaust ports are uncovered, during the movement of the piston head from the edge of the inlet port to the end of the stroke and back again, the incoming charge, being above atmospheric pressure, will tend to pass out through the open exhaust port. This is true, and it is only by most skillful proportioning of the two ports with relation to the pump pressure and the piston speed that the escape of a considerable part of the fresh mixture with the burned gases can be avoided. It is easily conceivable that with a certain combination of port areas, pumping pressure and piston speed, the burned gases will expand to a pressure below the pump delivery pressure, but not to atmospheric pressure, — by the time the inlet port is uncovered, and that they will not get down to atmospheric pressure until the inlet port has been covered again by the piston on its return stroke. Under such conditions, the burned gases would form, a sort of barrier between the fresh mixture and the exhaust port. Of course, there is some mixing of the fresh charge with. the burned gases, but the incoming mixture is deflected toward the cylinder head by the baffle G on the piston head, so that, under the ideal combination of con- ditions just outlined, the burned gases with which the fresh charge mixes will be retained in the cylinder after the exhaust port is covered. It will be clear, however, that unless proper relations are obtained | between the port areas, moments of opening, pump delivery pressure and piston speed, either a good deal of the fresh mixture will escape with thé exhaust gases or else too large a proportion of the burned gases will be trapped in the cylinder and reduce unnecessarily the quantity of fresh mixture that can be taken in. - Two indicator diagrams are necessary to show all of the phases through which the working medium passes during each cycle; one for the power end of the cylinder and another for the pump end, or the separate pump if one be used. Fig. 5 is a power diagram from a two-stroke-cycle engine of the type just described, and Fig. 6 is the corresponding pump diagram. The power diagram corresponds to the large loop of the four-stroke dia- gram, and the pump diagram corresponds roughly to the small “ negative ” loop of the four-stroke diagram. The right-hand end of the power diagram, Fig. 5, will sno seem a little confused until the student becomes accustomed to the fact that the exhaust and inlet ports are open simultaneously during a small part of ELEMENTARY PRINCIPLES 13 the piston travel. The vertical dotted line c—d shows where the piston begins to uncover the exhaust port on its outstroke (at the point pe) and has just covered it on the instroke (at the point pa). The dotted line a—b indicates similarly the point in the piston travel when the edge of 230 4 230 4 moo | ft PE 2 Lbs. 210 + 200 4 Combustion Pg~ 88 Lbs, Ignition occurs ih 30-4 5 204 | Atmospheric Pressure —— . 7 jez, ——__-- ——-- — Piston Travel— - —__ - —__ -__“ b_, FIG. 5. — INDICATOR DIAGRAM FROM AN ILLUMINATING-GAS ENGINE WORKING ON THE TWO-STROKE CYCLE. the piston is at the edge of the inlet port, beginning to uncover it at pj and completing its closure at the corresponding point on the lower curve. Starting at the -point p;, on the diagram, the charge begins to enter the cylinder from the pump chamber and continues to enter during the remainder of the outstroke and that part of the instroke represented by . the line from the toe of the diagram to the point cut by the dotted line a—b; the exhaust port is closed at ya, and compression then begins, as indicated by the rise of the lower curve. The compression, ignition, com- oa t is 3 ns a It | | “Al | Atmospheric Line + b g s —— FIG. 6.— PUMP DIAGRAM, TWO-STROKE CYCLE. bustion and expansion are the same as in the four-stroke cycle, down to the point pe, where the piston begins to uncover the exhaust port. From here on, the pressure drops abruptly until the inlet port begins to open at p; and the inrush of the slightly compressed charge from the pump 14 THE GAS ENGINE chamber keeps the pressure up. The burned gases continue to escape during the remainder of the outstroke and that part of the return stroke from the toe of the diagram to the point pa where the exhaust port is covered by the piston. Referring to the pump diagram, Fig. 6, the vertical dotted line a—b, crossing the upper and lower curves, corresponds with the same line in Fig. 5, showing the points at which the piston covers and uncovers the inlet port of the power cylinder. Beginning at b, when the piston has just covered the port and stopped the escape of mixture to the cylinder, the line s is drawn while the piston is drawing fresh mixture into the pump cham- ber through the admission valve A, Fig. +; this occurs simultaneously with the drawing of the compression curve of Fig. 5. Then the piston is driven forward, after the explosion in the power end of the cylinder, by the expanding gases, and compresses the mixture in the pump chamber, making the curve from g to a@ on the pump diagram and the expansion curve down to p; on the power diagram. At a the piston uncovers the inlet port of the cylinder and the rush of mixture from the pump chamber into the cylinder relieves the pump pressure, causing the drop from a to f on the diagram. As the piston returns (from right to left in Figs. 4, 5, and 6), the pressure in the pump chamber drops rapidly until the cyl- inder port is covered (at 0), stopping the escape of the mixture from the pump chamber; then the drop continues more gradually until the pressure falls below that of the atmosphere, when the automatic admission valve A, Fig. 4, is opened by the atmospheric pressure and another charge is drawn into the pump chamber, giving the suction line of the succeeding pump diagram. It should be noted that the pressure scale in Fig. 6 is much lower than that of Figs. 5 and 3, so that the area inclosed by the loop is much larger than if the same scale had been used in all of the diagrams. . The diagrams, Figs. 3 and 5, have been slightly idealized at the “ toe” in order to explain more clearly what occurs at the end of the expansion stroke in both types of engine. An accurate clean-cut “toe” cannot be obtained with the average indicator unless the speed of the engine is very slow, because the inertia of the indicator mechanism prevents the- pencil from following the abrupt changes of pressure which actually occur. An ordinary diagram shows a rounded “ toe ” as illustrated by Figs. 25 to 30. From the foregoing it will be evident that an engine working on the four-stroke cycle has the advantage of forcing nearly all of the burned gases out with its piston during an entire stroke and taking in a full piston displacement of fresh mixture during another entire stroke, whereas an engine working on the two-stroke cycle must take in its fresh mixture and exhaust its burned gases simultaneously and during a very small portion of two strokes. > ELEMENTARY PRINCIPLES 15 On the other hand, an engine working on the two-stroke cycle has the, advantage that one half of its piston strokes are power strokes, while the other engine gives only one power stroke in every four, the other three being devoted to intake, compression, and expulsion. Modifications of the original type of two-stroke-cycle engine have been built, moreover, in which only compressed air is admitted to the cylinder while the exhaust port is open, the fuel being delivered under pressure after the piston has closed the exhaust port. In this form of engine, it is manifestly easy to sweep the cylinder clean of burned gases and take in a cylinderful of. fresh mixture without losing any of the latter. The work of air pumping, however, is a serious item in such an engine, since the air pressuré must be fairly high in order to clear out the burned gases during the brief period of time available while the exhaust port is uncovered by the piston. This complete sweeping out of the burned gases is termed “scavenging,” and an engine in which this is accomplished is designated a “scavenging” engine. The difficulty of admitting a fresh charge and exhausting spent gases simultaneously and during an extremely brief interval of time, with econ- omy, and the greater pump work required have prevented the two-stroke cycle from being as extensively applied as the four-stroke cycle. — Il PRESSURES AND TEMPERATURES COMPRESSION In both four-stroke and two-stroke engines the pressure obtained at -the end of the compression stroke is of great importance; the higher this pressure the higher will be the maximum pressure produced by combustion and the mean effective pressure of the cycle, up to a certain point. Beyond that point, which varies with differing working conditions, increasing the compression pressure does not produce any increase in the mean effective pressure, and a considerable increase will cause a decrease in mean effective pressure. ee The compression pressure is determined chiefly by the relation between the volume of the clearance space (the space between the piston face and the nearest cylinder head when the crank is on the dead center) and the » volume of the space “swept out” by the compression stroke of the piston. It is also influenced by the pressure which exists in the cylinder imme- diately before compression begins and by the loss of heat through the cylinder walls during the compression stroke; the higher the precompres- sion pressure, the higher will be the compression pressure; the less the heat loss through the cylinder walls, the higher will be the compression pressure. These relations are simply expressed by the rule: Pressure before compression X Volume before compression -- Tempera- ture before compression = Pressure after compression X Volume after compression — Temperature after compression. It is even simpler, however, to express these relations in the shape of a formula, thus: Pa X Va es Pe X V, in z in which Pa = Absolute pressure per square inch in the cyl- inder, Before Va = Volume, in cubic feet, behind the piston, Compression. T, = Absolute temperature, | 16 PRESSURES AND TEMPERATURES 17 pe = Absolute pressure, V. = Volume, in cubic feet, Bate a T., = Absolute temperature, pression. Absolute pressure is the gauge pressure plus the atmospheric pressure, and is commonly taken at 14.7 pounds per square inch above the gauge pressure. ; The volume “behind the piston” is the total space between the piston head and the cylinder head under consideration. The absolute temperature is the Fahrenheit thermometer temperature -++ 460, because the absolute zero of gases is 460° below the zero of the Fahrenheit thermometer. Reference to Fig. 7 will doubtless help the reader to grasp the meaning of the formula more readily. At A is represented a cylinder and piston: FIG. 7. — ILLUSTRATING COMPRESSION EFFECTS. with a space of 12 cu. ft. between the piston and closed end of the cylinder, that is, “behind the piston.” Suppose this space to be filled with air at 14 Ibs. absolute pressure per square inch and 500 degrees absolute tempera- ture (40 degrees thermometer temperature). Now suppose the piston were forced down until the space beneath it were reduced to 2 cu. ft., as indicated at B. If the piston and the walls of the cylinder were abso- lutely nonconductive to heat, so that no heat could escape, the pressure 18 THE GAS ENGINE under the piston would rise to 175 Ibs. absolute. Then, according to the formula: x1 ins x2 oo - a and, consequently, - 500. f, fixie is x2 which obviously reduces to: 500 Te 168 ~ 850 Therefore, the absolute temperature after compression would be: aX 350 = 1041.7 degrees. Now, abandon the supposition that the piston and cylinder do not conduct heat, and assume, for example, that the escape of heat during compression was such that the temperature after compression was 900 degrees. Then the following relations would exist, according to the for- mula: 14 & 12 Pe x 2 500 st— Te", re being the expansion ratio, and n the exponent of the expansion curve. The temperature of the gases at the moment of release is LHP ge rh, By transposing these formulas their similarity to the compression for- mulas will be more apparent, thus: Pe X Te" = Pz and TeX rei T,. Since the release pressure and temperature are determined by the ex- plosion pressure and temperature, however, the first form is the correct one for both formulas. Table 4 gives release pressures and Table 5 gives Table 4. Absolute Pressures per Square Inch at Release CorrEsPonrING TO ExpLoston PressurES COMMONLY OBTAINED. ‘ Note:—The expansion ratios in the left-hand column are based on the volume behind the piston when the exhaust valve begins to open. Exe n,= 1.29 n, = 1.32 pan- Ratt atio. px = | pxr=| px= | px=| px= || px= | px = | px = | px = | Px = Ir, | 240 | 270 | 300 | 330 | 360 || 240 | 270 | 300 | 330 | 360 3.00 | 58.2 | 65.4 | 72.7 | 80.0 | 87.2 || 56.3 | 63.3 | 70.4 | 77.4 | 84.4 3.05 | 56.9 | 64.0 | 71.2 | 78.3 | 85.4 || 55.1 | 62.0 | 68.8 | 75.7 | 82.6 3.10 | 55.8 | 62.7 | 69.7 | 76.7 | 83.6 || 53.9 | 60.6 | 67.4 | 74.1 | 80.9 3.15 | 54.6 | 61.4 | 68.3 | 75.1 | 81.9 || 52.8 | 59.4 | 66.0 | 72.6 | 79.2 3.20 | 53.5 | 60.2 | 66.9] 73.6 | 80.3 || 51.7 | 58.2 | 64.6 | 71.1 | 77.5 3.25 | 52.5 | 59.0 | 65.6 | 72.1 | 78.7 || 50.6 | 57.0 | 63.3 | 69.6 | 76.0 3.30 | 51.4 | 57.9 | 64.3 | 70.7 | 77.2 || 49.6 | 55.8 | 62.0 | 68.3 | 74.5 3.35 | 50.5 | 56.8 | 63.1 | 69.4 | 75.7 || 48.7 | 54.7 | 60.8 | 66.9 | 73.0 3.40 | 49.5 | 55.7 | 61.9 | 68.1 | 74.2 || 47.7 | 53.7 | 59.6 | 65.6 | 71.6 3.45 | 48.6 | 54.6 | 60.7 | 66.8 | 72.9 || 46.8 | 52.7 | 58.5 | 64.4 | 70.2 3.50 | 47.7 | 53.6 | 59.6 | 65.6 | 71.5 || 45.9 | 51.7 | 57.4 | 63.1 | 68.9 3.55 | 46.8 | 52.7 | 58.5 | 64.4 | 70.2 || 45.1 | 50.7 | 56.3 | 62.0 | 67.6 3.60 | 46.0 | 51.7 | 57.5 | 63.2 | 68.9 || 44.2 | 49.8] 55.3 | 60.8 | 66.4 3.65 | 45.2 | 50.8 | 56.5 | 62.1 | 67.8 || 43.4 | 48.9 | 54.3 | 59.7 | 65.2 3.70 | 44.4 | 49.9 | 55.5 | 61.0 | 66.6 || 42.7 | 48.0 | 53.3 | 58.7 | 64.0 3.75 | 43.6 | 49.1 | 54.5 | 60.0 | 65.4 || 41.9 | 47.2 | 52.4 | 57.7 | 62.9 3.80 | 42.9 | 48.2 | 53.6] 58.9 | 64.3 || 41.2 | 46.4 | 51.5 | 56.7 | 61.8 3.85 | 42.2 | 47.4 | 52.7 | 58.0 | 63.2 || 40.5 | 45.6 | 50.6 | 55.7 | 60.7 3.90 | 41.5 | 46.7 | 51.8 | 57.0 | 62.2 || 39.8 | 44.8 | 49.8 | 54.7 | 59.7 3.95 | 40.8 | 45,9 | 51.0 | 56.1 | 61.2 |} 39.1 | 44.0 | 48.9 | 53.8 | 58.7 4.00 | 40.1 | 45.2 | 50.2 | 55.2 | 60.2 || 38.5 | 43.3 | 48.1 | 52.9 | 57.8 4.05 | 39.5 | 44.4 | 49.4 | 54.3 | 59.2 || 37.9 | 42.6 | 47.3 | 52.1 | 56.8 4.10 | 38.9 | 43.7 | 48.6 | 53.5 | 58.3 |} 37.3 | 41.9 | 46.6 | 51.2 | 55.9 4.15 | 38.3 | 43.1 | 47.8 |] 52.6 | 57.4 || 36.7 | 41.3 | 45.8 | 50.4 | 55.0 4.20 | 37.7 | 42.4 | 47.1 | 51.8 | 56.5 || 36.1 | 40.6 | 45.1 | 49.6 | 54.2 4.25 | 37.1 | 41.8 | 46.4 | 51.0 | 55.7 |} 35.5 | 40.0 | 44.4 | 48.9 | 53.3 4.30 | 36.6 | 41.1 | 45.7 | 50.3 | 54.8 || 35.0 | 39.4] 43.7 | 48.1 | 52.5. 4.35 | 36.0 | 40.5 | 45.0] 49.5 | 54.0 || 34.5 | 38.8 | 43.1 | 47.4 | 51.7 4.40 | 35.5 | 39.9 | 44.4 | 48.8 | 53.2 || 34.0 | 38.2 | 42.4] 46.7 | 50.9 4.45 | 35.0 | 39.4 | 43.7 | 48.1 | 52.5 || 33.4 | 37.6 | 41.8 | 46.0 | 50.2 4.50 | 34.5 | 38.8 | 43.1 | 47.4 | 51.7 |] 33.0 | 37.1 | 41.2 | 45.3 | 49.4 4.55 | 34.0 | 38.2 | 42.5 | 46.7 | 51.0 || 32.5 | 36.5 | 40.6 | 44.7 | 48.7 4.60 | 33.5 | 37.7 | 41.9 | 46.1 | 50.3 || 32.0 | 36.0 | 40.0 | 44.0 | 48.0 4.65 | 33.1 | 37.2 | 41.3 | 45.4 | 49.6 || 31.6 | 35.5 | 39.5 | 43.4 | 47.3 4.70 | 32.6 | 36.7 | 40.7 | 44.8 | 48.9 || 31.1 | 35.0 | 38.9 | 42.8 | 46.7 475 | 32.2 | 36.2 | 40.2 | 44.2 | 48.2 || 30.7 | 34.5 | 38.4 | 42.2 | 46.0 4.80 | 31.7 | 35.7 | 39.6 | 43.6 | 47.6 || 30.3 | 34.1 | 37.8] 41.6 | 45.4 4.85 | 31.3 | 35.2 | 39.1 | 43.0 | 46.9 || 29.9 | 33.6 | 37.3 | 41.1 | 44.8 4.90 | 30.9 | 34.8 | 38.6 | 42.5 | 46.3 || 29.5 | 33.1 | 36.8 | 40.5 | 44.2 4.95 | 30.5 | 34.3 | 38.1 | 41.9 | 45.7 || 29.1 | 32.7 | 36.3 | 40.0 | 43.6 5.00 | 30.1 | 33.9 | 37.6 | 41.4 | 45.1 || 28.7 | 32.3 | 35.8 | 39.4 | 43.0 5.10 | 29.3 | 33.0 | 36.7 | 40.3 | 44.0 || 27.9 | 31.4 | 34.9 | 38.4 | 41.9. 5.20 | 28.6 | 32.2 | 35.8 | 39.3 | 42.9 || 27.2 | 30.6 | 34.0 | 37.4 | 40.8 5.30 | 27.9 | 31.4 | 34.9 | 38.4 | 41.9 || 26.6 | 29.9 | 33.2 | 36.6 | 39.8 5.40 | 27.3 | 30.7 | 34.1 | 37.5 | 40.9 || 25.9 | 29.1 | 32.4 | 35.6 | 38.9 5.50 | 26.6 | 29.9 | 33.3 | 36.6 | 39.9 || 25.3 | 28.5 | 31.6 | 34.8 | 37.9 5.60 | 26.0 | 29.3 | 32.5 | 35.8 | 39.0 || 24.7 | 27.8 | 30.9 | 34.0 | 37.0 5.70 | 25.4 | 28.6 | 31.8 | 34.9 | 38.1 24.1 | 27.1 | 30.2 | 33.2 | 36.2 5.80 | 24.9 |} 28.0 | 31.1 | 34.2 | 37.3 || 23.6 | 26.6 | 29.5 | 32.4 | 35.4 5.90 | 24.3 | 27.3 | 30.4 | 33.4 | 36.5 || 23.0 | 25.9 | 28.8] 31.7 | 34.6 6.00 | 23.8 | 26.8 | 29.7 | 32.7 | 35.7 || 22.5 | 25.4 | 28.2 | 31.0 | 33.8 6.10 | 23.3 | 26.2 | 29.1 | 32.0 | 34.9 || 22.1 | 24.8 ] 27.6 |] 30.3 | 33.1 6.20 | 22.8 | 25.7 | 28.5 | 31.4 | 34.2 |] 21.6 | 24.3 | 27.0 | 29.7 | 32.4 6.30 | 22.3 | 25.1 | 27.9 | 30.7 | 33.5 || 21.1 | 23.8 | 26.4 29,1 | 31.7 6.40 | 21.9 | 24.6 | 27.4 | 30.1 | 32.8 || 20.7 } 23.3 | 25.9 | 28.5 | 31.1 6.50 | 21.5} 24.1 | 26.8] 29.5 | 32.2 || 20.3 | 22.8 | 25.4 | 27.9 | 30.4 6.60 | 21.0 | 23.7 | 26.3 | 28.9 | 31.6 19.9 | 22.4 | 24.8 | 27.3 | 29.8 6.70 | 20.6 | 23.2 | 25.8 | 28.4 | 30.9 19.5 | 21.9 | 24.4 | 26.8 | 29.2 6.80 | 20.2 | 22.8 | 25.3 | 27.8 | 30.4 19.1 | 21,5 | 23.9 | 26.3 | 28.7 6.90 | 19.9 | 22.3 | 24.8] 27.3 | 29.8 || 18.7 } 21.1 | 23.4 | 25.8 | 28.1 7.00 | 19.5 | 21.9 | 24.4] 26.8 | 29.2 18.4 | 20.7 | 23.0 | 25.3 | 27.6 7.10 | 19.1 |] 21.5 | 23.9 | 26.3 | 28.7 18.1 | 20.3 | 22.6 | 24.8 | 27.1 7.20 | 18.8 | 21.2 | 23.5] 25.9 | 28.2 17.7 | 19.9 | 22.2 | 24.4 | 26.6 7.30 | 18.5 | 20.8 | 23.1 | 25.4 | 27.7 || 17.4 | 19.6 | 21.8 | 23.9 | 26.1 7.40 | 18.2 | 20.4 | 22.7 | 25.0 | 27.2 17.1 | 19.2 | 21.4 | 23.5 | 25.6 7.50 | 17.8 | 20.1 | 22.3 | 24.5 | 26.8 16.8 | 18.9 | 21.0 | 23.1 | 25.2 7.60 | 17.5 | 19.7 | 21.9 | 24.1 | 26.3 16.5 | 18.6 | 20.6 | 22.7 | 24.8 7.70 | 17.2 | 19.4 | 21.6 | 23.7 | 25.9 16.2 | 18.2 | 20.3 | 22.3 | 24.3 7.80 | 17.0 | 19.1 | 21.2 | 23.3 | 25.4 15.9 | 17.9 | 19.9 | 21.9 | 23.9 7.90 | 16.7 | 18.8 | 20.9 | 22.9 | 25.0 15.7 | 17.6 | 19.6 | 21.6 | 23.5 8.00 | 16.4 | 18.5 | 20.5 | 22.6 | 24.6 15.4 | 17.3 | 19.3 | 21.2 | 23.1 Table 5. Absolute Temperatures at Release CorRESPONDING TO Explosion TEMPERATURES COMMONLY OBTAINED. Note:—The expansion ratio is based on the volume behind the piston when the exhaust valve begins to open. Ex- n, = 1.29 n, = 1.32 Ppan= sion Ratio) 7. — | 7. =|te =| Tr =|) =|] Tr = | =| Ts = | eH I Tr. | 1800 | 2100 | 2400 | 2700 | 3000 || 1800 | 2100 | 2400 | 2700 | 3000 3.00 | 1309 | 1527 | 1745 | 1963 | 2182 |] 1266 | 1478 | 1689 | 1900 | 2121 3.05 | 1303 | 1520 | 1737 | 1954 | 2171 || 1260 | 1470 | 1680 | 1890 | 2100 3.10 | 1297 | 1513 | 1729 | 1945 | 2161 || 1253 | 1462 | 1671 | 1880 | 2089 3:13 | 1290 | 1505 | 1721 | 1936 | 2151 1) 1247 | 1455 | 1662 | 1870 | 2078 3.20 | 1285 | 1499 | 1713 | 1927 | 2141 || 1241 | 1447 | 1654 | 1861 | 2068 3.25 | 1279 | 1492 | 1705 | 1918 | 2131 || 1234 | 1440 | 1646 | 1852 | 2057 3130 | 1273 | 1485 | 1698 | 1910 | 2129 || 1228 | 1433 | 1638 | 1843 | 2047 3135 | 1268 | 1479 | 1690 | 1902 | 2113 || 1223 | 1426 | 1630 | 1834 | 2038 3.40 | 1262 | 1473 | 1683 | 1894 | 2104 || 1217 | 1420 | 1622 | 1825 | 2008 3145 | 1257 | 1466 | 1676 | 1885 | 2095 || 1211 | 1413 | 1615 | 1817 | 2018 3150 | 1252 | 1460 | 1669 | 1878 | 2086 |] 1205 | 1406 | 1607 | 1808 | 2009 3155 | 1246 | 1454 | 1662 | 1871 | 2077 || 1200 | 1400 | 1600 | 1800 | 2000 3.60 | 1241 | 1448 | 1655 | 1862 | 2069 |/ 1195 | 1394 | 1593 | 1792 | 1991 365 | 1237 | 1443 | 1649 | 1855 | 2061 || 1189 | 1388 | 1586 | 1784 | 1982 3:70 | 1232 | 1437 | 1642 | 1848 | 2053 || 1184 | 1382 | 1579 | 1776 | 1974 3175 | 1227 | 1431 | 1636 | 1840 | 2045 || 1179 | 1376 | 1572 | 1769 | 1965 380 | 1222 | 1496 | 1630 | 1833 | 2037 || 1174 | 1370 | 1566 | 1761 | 1957 3185 | 1218 | 1420 | 1623 | 1826 | 2029 || 1169 1559 | 1754 | 1949 390 | 1213 | 1415 | 1617 | 1819 | 2022 || 1164 | 1359 | 1553 | 1747 | 1941 3:95 | 1209 | 1410 | 1611 | 1813 | 2014 || 1160 | 1353 | 1546 | 1740 | 1933 4.00 | 1204 | 1405 | 1606 | 1806 | 2007 || 1155 | 1348 1733 | 1925 4.05 | 1200 | 1400 | 1600 | 1800 | 2000 || 1151 | 1342 | 1534 | 1726 | 1918 4.10.| 1196 | 1395 | 1594 | 1793 | 1993 || 1146 | 1337 | 1528 | 1719 | 1910 4.15 | 1191 1389 | 1787 | 1986 || 1142 | 1332 | 1522 | 1712 | 1903 4.20 | 1187 | 1385 | 1383 | 1781 | 1978 || 1137 | 1327 | 1516 | 1706 | 1895 4.25 | 1183 | 1380 | 1578 | 1775 | 1972 || 1133 | 1322 | 1510 | 1699 | 1888 4.30 | 1179 | 1376 | 1572 | 1769 | 1965 |} 1129 | 1317 | 1505 | 1693 | 1881 4.35 | 1175 | 1371 | 1367 | 1763 | 1959 || 1124 | 1312 | 1499 | 1687 | 1874 4.40 | 1177 | 1366 | 1562 | 1757 | 1952 || 1120 | 1307 | 1494 | 1681 | 1867 4.45 | 1167 | 1362 | 1357 | 1751 | 1946 || 1116 | 1302 | 1488 | 1675 | 1861 4.50 | 1164 } 1358 | 1532 | 1746 | 1940 |] 1112 | 1298 | 1483 | 1668 | 1854 4.35 | 1160 | 1333 | 1547 | 1740 | 1933 || 1108 | 1293 | 1478 | 1663 | 1847 4.60 | 1156 | 1349 | 1542 | 1734 | 1927 || 1105 | 1289 | 1473 | 1657 | 1841 4.65 | 1153 | 1345 | 1337 | 1729 | 1921 || 1101 | 1284 | 1468 | 1651 | 1835 4.70 | 1149 | 1341 | 1532 | 1724 | 1915 || 1097 | 1280 | 1463 | 1645 | 1828 4.75 | 1146 | 1337 | 1328 | 1719 | 1910 || 1093 | 1275 | 1458 | 1640 | 1822 4:80 | 1142 | 1332 | 1323 | 1713 | 1904 |} 1090 | 1271 | 1453 | 1634 | 1816 4/85 | 1139 | 1328 | 1518 | 1708 | 1898 |) 1086 | 1267 | 1448 | 1629 | 1810 4.90 | 1135 | 1324 | 1514 | 1703 | 1892 || 1082 | 1263 | 1443 | 1624 4 4.95 | 1132 | 1321 | 1509 | 1698 | 1887 || 1079 | 1259 | 1439 | 1618 | 1798 5.00 | 1120 | 1317 | 1505 | 1693 | 1881 || 1075 | 1255 | 1434 | 1613 | 1792 3.10 | 1122 | 1309 | 1496 | 1683 | 1870 || 1069 | 1247 | 1425 | 1603 | 1781 5:20 | 1116 | 1302 | 1488 | 1674 | 1860 || 1062 | 1239 | 1416 | 1593 | 1770 5.30 | 1110 | 1295 | 1480 | 1665 | 1850 || 1056 | 1232 | 1407 1759 340 | 1104 | 1288 | 1472 | 1656 | 1840 || 1049 | 1224 | 1399 | 1574 | 1749 5750 | 1098 | 1281 | 1464 | 1647 | 1830 || 1043 | 1217 | 1391 | 1565 | 1739 5.60 | 1092 | 1274 | 1486 | 1638 | 1820 || 1037 | 1210 | 1383 | 1556 | 1729 5:70 | 1087 | 1268 | 1449 | 1630 | 1811 || 1031 | 1203 | 1375 | 1547 | 1719 5/80 | 1081 | 1261 | 1441 | 1622 | 1802 || 1026 | 1197 | 1367 | 1538 | 1709 B90 | 1076 | 1285 | 1434 | 1614 | 1793 || 1020 | 1190 | 1360 | 1530 | 1700 6.00 | 1070 | 1249 | 1427 | 1606 | 1784 || 1015 | 1184 | 1353 | 1522 | 1691 6.10 | 1065 | 1243 | 1421 | 1598 | 1776 || 1009 | 1177 | 1346 | 1514 | 1682 6.20 | 1060 | 1237 | 1414 | 1591 | 1767 || 1004 | 1171 | 1339 | 1506 | 1673 6.30 | 1056 | 1231 | 1407 | 1583 | 1759 || 999 } 1165 | 1332 | 1498 | 1665 6.40 | 1051 | 1226 | 1401 | 1576 | 1751 || 994 | 1159 | 1325 | 1491 | 1656 6.50 | 1046 | 1220 | 1395 | 1569 | 1743 || 989 | 1154 | 1318 | 1483 | 1648 6.80 | 1041 | 1215 | 1389 | 1562 | 1736 || 984 | 1148 | 1312 | 1476 | 1640 6.70 | 1037 | 1210 | 1382 | 1555 | 1728 || 979 | 1143 | 1306 | 1469 | 1632 680 | 1032 | 1204 | 1376 | 1549 | 1721 || 975 | 1137 | 1300 | 1462 | 1625 690 | 1028 | 1199 | 1371 | 1542 | 1713 || 970 | 1132 | 1294 | 1455 | 1617 7:00 | 1024 | 1194 | 1365 | 1536 | 1706 || 966 | 1127 | 1288 | 1449 | 1610 710 | 1020 | 1189 | 1359 | 1529 | 1699 || 961 | 1122 | 1282 | 1442 | 1602 720 | 1015 | 1185 | 1354 | 1523 | 1692 || 957 | 1117 | 1276 | 1436 | 1595 7°30 | 1011 | 1180 | 1348 | 1517 | 1686 || 953 | 1112 | 1270 | 1429 | 1588 4°40 | 1007 | 1175 | 1343 | 1511 | 1679 || 949 | 1107 | 1265 | 1423 | 1581 4:30 | 1003 | 1171 | 1338 | 1505 | 1672 || 945 | 1102 | 1259 | 1417 | 1574 7160 | 1 i166 | 1333 | 1499 | 1666 || 941 | 1097 | 1254 | 1411 7°79 | 996 | 1162 | 1328 | 1494 | 1660 || 937 | 1093 | 1249 | 1405 | 1561 7-231 992 | 1157 | 1323 | 1488 | 1654 || 933 | 1088 | 1244 | 1399 | 1555 490 | 988 | 1153 | 1318 | 1483 | 1647 |) 929 | 1084 9 | 1394 | 1548 $00 | 985 | 1149 | 1313 | 1477 | 1641 || 925 | 1079 | 1234 | 138s | 1 28 © THE GAS ENGINE release temperatures commonly obtained as results of the explosion pres- sures and temperatures stated at. the heads of the columns and the expan- sion ratios given in the side columngof each table. These tables are based on values of 1.29 and 1.32 for the exponent n of the expansion curve. Mean EFFECTIVE PRESSURE The mean effective pressure of a complete four-stroke eycle is the difference between the average pressure during expansion and the average pressure during compression, if we ignore the small loop of the diagram due to the formation of a partial vacuum behind the piston during the suction stroke. This loop is usually so small in comparison with the work area of the diagram that it is not worth considering. Table 6. Probable Mean Effective Pressures Fuel Engine Compression Pressure, Pounds per Sqaare Inch, Absolute ue! ee 100 | 115 | 130 | 145 | 160 E 10 55 | 60 65 |..... . Suction 25 60 65 70 1D: Vintecsate Anthracite 50 65 70 75 80 80 Producer 100 70 75 80 85 85 Gas 250 75 | 80 85 90 90 500 80 | 85 | 90 90 90 De cae haute Os eaalnd eee 65 65 oa ere Mond i 25 60 65 65} 70) 75 Producer 50 65 | 70 70 | 75 | 80 Gas , 100 65 70 75 | 80! 85 250 7 75 | 80} 85 90 500 75 | 80 85 90 90 5 10 Natural 25 and 50 Illuminating Gases 100 250 500 5 50 55 60 Kerosene -10 55 60 65 Spray 25 60 65 70 50 65 70 75 5 | 70 75 | 80 Gasolene 10 75 80 85 Vapor 25 80 85 90 50 85 90 95 _ In the two-stroke diagram there is no negative loop, but the mean effective pressure of the pump diagram should be subtracted from that of the work diagram to get at the true mean effective pressure of the cycle. As the pump delivery pressure is usually from 4 to 8 lbs. above the atmosphere, the mean effective pressure of the pump diagram is not negligible. Since the mean effective pressure of the power diagram depends on the mean pressures of the two curves and both of these involve many uncer- PRESSURES AND TEMPERATURES 29 tainties, it is impossible to calculate the mean effective pressure with as close an approach to accuracy as can be done for a steam engine. The most one can do is to assume average conditions and base the estimate of mean effective pressure, or of the horse power which it produces, on those conditions. Even then it is necessary to know just what quality of gas or oil will be used and what the proportion of air to gas or to oil vapor is going to be, in order to make the roughest estimate of the mean effective pressure or the horse power. Table 6 gives average mean effective pres- sures for engines of different sizes, working with compression pressures of 65 to 160 lbs. absolute, and different classes of fuel, the most effective mixture of fuel and air being assumed. These figures are merely a rough general guide; the actual mean effective pressure obtained with any com- bination of the stated conditions might be considerably higher or very much lower than the pressure value given in the table. III COOLING AND HEAT LOSS As explained in the discussion of combustion pressures and tempera- tures, the maximum absolute temperature of the gases in a gas-engine evlinder may be as high as 3,000 degrees, and is ordinarily around 2,300 to 2,500 degrees; a thermometric temperature of 2,000 degrees is quite common. A moment’s reflection will convince the reader that if the cylinder walls were allowed to reach any such temperature operation would be impossible. Lubricating oil would be instantly decomposed, and, even disregarding the effect of the heat upon the iron itself, the piston would soon jam by lack of lubrication. In order to make it possible for such enormous temperatures to exist intermittently in the cylinder without preventing operation, the escape of heat through the cylinder walls is assisted by jacketing the cylinder and passing water through the jacket space continuously. Reference to Figs. 1, 2 and 4 will show that the cylinders illustrated are provided with double walls; the outer of these walls constitutes the water jacket. In Fig. 1 the water in the jacket is also represented. The water is usually admitted to the jacket of a horizontal engine at the bottom and near the combustion end of the cylinder, and is discharged at the top near the outer end of the piston travel. The object of this arrangement is to apply the coldest water at the hottest part of the cylinder wall, which is that part surrounding the clearance space, where combustion occurs. Moreover, as the water becomes heated it tends to rise, and this tendency is assisted by putting the entrance at the bottom and the exit at the top of the jacket. The circulating water, or jacket water, as it is commonly called, usually carries away from one third to one half of the heat liberated by the com- bustion of the gases in the cylinder. The ideal operation would be to pro- portion the water flow so that the amount of heat taken away will be just sufficient to keep the surface temperature of the cylinder walls and piston below the point at which the lubricating oil will burn or decompose. Of course, the temperature of the cylinder-wall surface cannot be measured directly, but an experienced engine runner can judge pretty well as to how 30 COOLING AND IIEAT LOSS 31 the‘heat inside is affecting the cylinder oil by watching the exhaust and listening to the working of the piston. The faster the water passes through the jacket, the more heat will it take away and the lower will be the temperature of the water at the point of discharge, if the initial temperature remains unchanged ; or, at a given rate of flow, and a given quantity of heat taken out per minute or per hour, the discharge temperature will depend entirely on the temperature of the water as it enters the jacket. Consequently, no hard-and-fast rule can be laid down as to the temperature which the water should show as it emerges from the jacket. Ordinarily, this temperature is from 50 to 100 degrees above the temperature at which the water entered the jacket. Water Tank FIG. 11.— COOLING SYSTEM WITH PUMP. As a rule, the cooling water is pumped through the jacket, then cooled off and returned to the pump by gravity to be forced through again. Such an arrangement is illustrated elementarily in Fig. 11. In the case of a small engine, however, it is often unnecessary to use a pump. The heating of the water produces a circulation sufficient to keep the temperature within bounds. An elementary arrangement of this sort is illustrated in Fig. 12. In both illustrations the arrows indicate the course of the cooling water. In Fig. 12 it will be noticed that the delivery pipe a, leading from the water jacket of the engine to the cooling tank, enters the tank below the water level. This is necessary in order that the height of water in the tank may be equal to the height of water in the vertical pipe and engine jacket, up to the horizontal pipe a. The water in the tank is cooler than that in the vertical pipe and engine jacket, and therefore heavier ; consequently, it forces the hot water upward and out of the pipe a. The level in the - 32 THE GAS ENGINE tank does not need to be greatly higher than the outlet of the pipe a; it is wise, however, to keep it several inches above the outlet of the pipe in order that evaporation of water may not bring the level down far enough to stop the circulation before it is noticed. In order to prevent air locks and to allow any water vapor to escape without going into the tank, a vent b is provided. The cocks c and c are for the purpose of shutting off the tank whenever it may be necessary to disconnect the circulating pipes, and thereby avoiding the emptying of the tank. The valve d is to drain all the water from the engine jacket and the connecting pipes when the engine b FIG. 12.— COOLING SYSTEM WITHOUT PUMP. is to be shut down in very cold weather; then the cocks ¢ and ¢ are closed, of course. In some small engines the excess heat is disposed of by means of thin flanges on the outside of the cylinder, which is made with a single wall, of course; the heat is radiated from these flanges to the surrounding air and carried away by the natural air currents or by a stronger draught pro- duced by a fan blowing against the cylinder. Fig. 13 is a sectional view of a small vertical engine of this type. Air cooling has been found im- practical with cylinders of more than 5 inches diameter of bore, and the usual limit is 4 inches. Double-acting engines are always provided with hollow pistons of the short type and hollow piston rods and tail rods, and cooling water is forced through these as well as the cylinder jackets. This is necessary because both ends of the cylinder are closed and explosions occur in both ends; consequently, there is no opportunity to radiate any heat to the outer air from the piston, and the number of explosions per revolution of the crank shaft is twice as great as in the single-acting engine, giving COOLING AND HEAT LOSS 33 Hit ‘Ribs C) Heat-radiating Ribs Ribs t FIG. 13.— A VERTICAL AIR-COOLED ENGINE. Inlet Valve Chamber Inlet Valve Chamber NSS FESS Ss A J N R\ LA eer SWE EY rE ZZ VW d= Fa {TI 4 SE ES ee ae SQ [| —S Gq GJ MN 2 a aes L | Sy mm NNW) N AN Exhaust FIG. 14.— CYLINDER AND PISTON OF A DOUBLE-ACTING ENGINE. 4 an ZEEE) ERY ll PY Ws 34 THE GAS ENGINE twice the quantity of heat to be got out of the cylinder per revolution. Fig. 14 illustrates this construction of piston and rods, and the cylinder construction of a well-known German double-acting engine. The spaces marked J are all connected together and constitute the water jacket of the cylinder. The inlet and exhaust valves are omitted in order to simplify the drawing. IV VALVES AND VALVE GEAR THE inlet and exhaust valves used in gas and oil engines are of the poppet type for the reason that the high temperatures to which the valve faces and seats are subjected make the use of sliding valves impracticable. Since it is impossible to connect the stem of a poppet valve rigidly with the valve gear and adjust the connection so as to seat the valve accurately when it is closed, it is the universal practice to mount the valve in a “ cage ” which carries a guide for the valve stem, provide a spring which will con- FIG. 15.-- INLET AND EXHAUST VALVES; EXHAUST-VALVE ROCKER ARM AND PUSH ROD. stantly press the valve toward its seat, and arrange the valve gear so that a push rod or the end of a lever will press against the outer end of the valve stem at the proper moment and lift the valve from its seat. This sort of construction is illustrated in Fig. 15, where # indicates the exhaust valve and I the inlet valve. The exhaust valve is opened by the rocker arm A when the long end of the arm is pushed back by the valve rod, the latter being moved by a cam not shown in the drawing. When the cam 35 36 THE GAS ENGINE releases the unseen end of the valve rod, the spring s slides the rod back away from the rocker arm A and the heavier spring S presses the valve back to its seat. In this engine, the inlet valve J is of the “ automatic” type; it is held on its seat by a weak spring and when the piston moves forward on the suction stroke, forming a partial vacuum behind it, the pressure of the atmosphere opens the valve. “The valve rod, in the case of the exhaust valve illustrated in Fig. 15, is pushed to the left by a rocker arm and cam arranged as shown in Fig. 16. The shaft on which the cam is mounted is driven by gears from the crank shaft of the engine, which works on the four-stroke cycle; as the FIG. 16.—CAM, ROCKER, AND PUSH ROD. exhaust valve is to be opened only once every two revolutions of the crank shaft, the cam shaft is geared down to one half the speed of the crank shaft. (This is true of the valve-gear shaft of every four-stroke-cycle engine.) The ends of the valve rod in Fig. 15 are provided with hardened steel buffer pins, as indicated at a, Fig. 16, and the rocker arms are fitted with corre- sponding hardened steel plates, as at b. The rocker arm B is also provided with a hardened roller r against which the cam C presses in opening the valve;.the object, of course, is to reduce the friction as much as possible. This is only one of many forms of valve-operating mechanism, but nearly all are based on the general principles embodied in the form shown. Some large engines are equipped with eccentrics instead of cams on the half-speed shaft, but some form of cam must be interposed between the eccentric rod and the valve stem, as a rule. Fig. 1% illustrates an American valve gear of this class in which one eccentric operates both the inlet and the exhaust valve. The motion of each eccentric rod is trans- mitted to its valve stem by a pair of “wiper cams.” In the drawing, the valve gear is shown in that position where the inlet wiper cams are VALVES AND VALVE GEAR 37 farthest apart and the exhaust wipers are at the “full open” position. This mechanism has the advantage of opening the valves and allowing them to close without serious shock or pounding. When the wiper cams first Tolet Rocker tt xz Admission Water Jacket SS Piston Nut Faced Flusb with Piston ‘Air Starting Inlet and Eccentric both Valves Drive o Exhaust Rocker 9 FIG. 17.— ECCENTRIC AND WIPER-CAM VALVE GEAR. engage, the leverage is all in favor of the eccentric-rod cam, which is moving much faster at its tip than the valve-stem cam first moves at its tip; consequently, the valve is started from its seat very slowly and the pressure against the valve mechanism beyond the wipers is much less than 38 THE GAS ENGINE if a direct cam movement were employed as in Fig. 16. After the valve is off its seat, and the pressure on its disk relieved, its motion becomes quicker with relation to that of the eccentric rod and it reaches the “ full open” position very quickly. In closing, the reverse relations are true; the leverage is in favor of the valve stem, and the valve begins to close rapidly. As it nears its seat, the leverage having shifted back again, its motion is retarded so that when it is finally released by the cams it is seated with the force due to the elasticity of its spring only, instead of having high momentum to increase the seating force. Tuer Mixine VALVE Besides the exhaust valve and the main inlet valve, there is frequently employed what is termed a “mixing valve,” the function of which is to produce a more intimate mingling of the particles of gas and air than would occur if the two constituents were merely turned into the inlet \ FIG. 18.— ONE "FORM OF MIXING VALVE. passage together from their respective sources. The general principle of all mixing valves is the same; the object is to break up the stream of gas into particles and to mix those with the particles of the incoming stream of air, and the general method is to force the gas and air to pass through a number of small openings together and to change their direction of flow \ VALVES AND VALVE GEAR 39 abruptly in passing through these openings. Fig. 18 illustrates the form of mixing valve in use by one of the prominent American builders. The air-supply pipe is enlarged near the engine intake and a cylindrical chamber is secured in the center of this enlarged part; the gas is led in to the central chamber and passes from it into the air pipe through slots, as indicated by the arrows g, g; in the air pipe it meets the stream of air coming up from below as indicated by the arrows a and a, and the air and gas together flow into the engine intake passage through narrow ports in the upper end of the vertical pipe, as indicated by the arrows m and m. The butterfly valve 7' is a throttle, controlled by the governor to regulate the quantity of mixture admitted to the cylinder. In all gas engines, provision is made for adjusting the proportion of air to gas in the mixture. Almost always the adjustment is made by means of a hand valve in the gas-supply pipe, the flow of air being unimpeded until the mixture reaches the throttle, if there be a throttle. On some engines, however, a special valve is used which simultaneously throttles the gas supply and opens up the air-supply passage to make the mixture “poorer,” and vice versa. . IGNITION In early gas engines the mixture was ignited by means of a “ hot tube,” consisting of a tube of relatively small diameter opening into the cylinder at one end and closed: at the other; this tube being located within a housing and kept hot by a Bunsen burner. The compression stroke of the piston _ forced mixture up into the tube and the heat of the latter fired the mixture. . During the suction stroke the burned gases in the tube expanded and ~ prevented the fresh mixture from entering it. This arrangement has the serious disadvantages that any change in the relation between: the length of the tube and the compression of the engine will change the timing of ignition. It is almost universal practice now to ignite the mixture in the cylinder of a gas engine by means of an electric spark. Ignition in most gasolene engines is accomplished by this means also, but there are many kerosene engines in which other methods are employed. Maxkr-AND-BREAK SYSTEM There are two general systems of electric ignition: the “ make-and- break” and the “jump-spark” systems. In the former, which is used in most stationary engines, electric current is passed through two separable contacts located in the cylinder and connected in series with an inductive (not induction) or “sparking ” coil, and at the proper instant the contacts are separated by a mechanism operated from the valve-gear shaft. Upon the separation of the contacts, the current forms a spark between them, and this spark is greatly enhanced by the inductive coil in the circuit. Fig. 19 is a diagram of the ignition circuit of this system in which the separable contacts A and B are of the hammer type, held in contact with each other by a spring; 7 is a trigger attached to the same spindle as the pivoted contact A and located with its tip in the path of a trip rod F, which is lifted at the proper moment by a cam or some other form of mechanism mounted on or operated from the valve-gear shaft. As the rod F is lifted, the stop J presses it away from the end of the trigger 7’, allowing the latter to be drawn back into place by the helical spring shown. The end 40° IGNITION 41 of the rod F is constantly pressed toward the stop J by the flat spring 8. Just before the rod F is lifted to move the pivoted contact 4 away from the stationary contact B, the electric circuit is closed by the cam C’ wiping against the spring D, and the contacts are separated while the cam and w $ a Sm e Spark Coil. &~ : : =_ el FIG. 19. — ELEMENTARY DIAGRAM OF A MAKE-AND-BREAK IGNITION SYSTEM. spring are in contact and the circuit closed.. When the contacts separate, opening the circuit, the spark coil gives an inductive “kick,” and produces a much larger spark at the contacts than would occur if it were not in the circuit. The contact pieces 4 and B are, of. course, located inside the cvlinder; the stud of B and the spindle of A pass through an insulating block to the outside, where the electrical connections are made and the trigger T is attached. The source of current is indicated diagrammatically FIG. 20. —IGNITER OF A MAKE-AND-BREAK SYSTEM. at E as an electric battery, but small electric generators are largely used for this purpose. In most make-and-break systems the circuit-closing cam and spring C and D are unnecessary, the circuit being first closed and then opened by the sparking contacts. Fig. 20 illustrates the elementary principles of such a system, in which the rocking member .1 is brought into contact with 42. THE GAS ENGINE the stationary terminal B by means of the cam C and tongue D, a coil spring S being interposed between the hub G and the spindle of the contact A. The use of this spring makes it possible to insure firm contact between the tips A and B without extremely accurate proportioning of the cam C and corresponding setting of the hub G; it also gives a sudden separation of the contacts when the cam passes the tip of the tongue D, and this enhances the action of the inductive or spark soil. The wires w and w lead to the spark coil and the battery or generator, and correspond to the wires w, win Fig. 19. The block H is made of metal, and the spindle of the contact A is not insulated from it; consequently, the current can pass from the binding post through the block H to the contact FIG. 21 piece A. The stud J of the electrode B passes through a bushing of insulating material such as vitrified porcelain, so that there is no direct electrical connection between A and B when their tips are separated. Fig. 21 illustrates the working principle of a class of make-and-break *‘ systems which is now more generally used than either of the two just described. In these systems two moving members are incorporated in the igniter proper, the rocking electrode and another lever which controls it, this actuating lever being operated by a cam, finger, push rod or other contrivance linked or geared to the valve-gear shaft, or else by an electro- magnet. The rocking electrode A is normally held away from the sta- tionary electrode B by the spring S, through the medium of the rocker D and push rod F acting on the lever G, which is fastened to the outside end of the electrode spindle. At the proper point in the cycle the finger OC, mounted on a shaft which revolves in unison with the valve-gear shaft, carries the rocker D away from its stop, withdrawing the push rod F from the lever @ and allowing the weak spring K to pull the rocking electrode into contact with the stationary one, closing the electric circuit and ener- gizing the spark coil. An instant later the finger C rotates beyond the IGNITION 43 end of the rocker D, releasing it, and the spring S snaps the rocker back to the position shown; as it moves back, the push rod F strikes the lever G a sharp blow, knocking the electrodes apart very suddenly and pro- ducing the spark between their points. The sketch does not indicate the construction actually used in any sys- tem known to the author; it only illustrates the principle of operation. In most cases the rocker D is mounted on a stud through the center of which the electrode spindle passes; a lateral lug on the actuating rocker takes “the place of the push rod F. In some igniters a rotating cam is used, as at C; in others a reciprocating trigger is used, its end being pushed away from the rocker D by a roller partly in its path. JUMP-SPARK SYSTEM In the jump-spark system, an induction coil is used instead of a simple spark coil and the terminals inside the cylinder are both stationary, with their tips very close together so that the induced spark can jump across; the usual practice is to space the spark points or tips 0.03 to 0.05 of an inch \ > y rs oy Induction Coil J FIG, 22; — ELEMENTARY JUMP-SPARK SYSTEM. apart, more often the smaller distance. Fig. 22 illustrates the principles of the jump-spark system. The sparking points 4 and B are mounted in an iron plug, G, from which they are insulated by porcelain bushings through which they pass. From these, wires extend to the secondary ter- minals of an induction coil. The primary winding of the induction coil receives current from a battery or other electrical source, H, through a circuit-closing cam and tongue C and D and a vibrator V. The latter consists of a steel spring carrying a soft-iron piece which is in line with 44 THE GAS ENGINE the end of the iron core of the induction coil. When the primary circuit is open, the vibrator spring rests against a contact screw A, closing the circuit at that point, When the cam C' comes into contact with the tongue D, the primary circuit is completed ; the iron core of the coil is magnetized by the passage of current in the primary winding and attracts the vibrator V, drawing it away from the contact K, and opening the circuit there; then, the iron core being demagnetized by the cessation of current in the primary winding, the spring rebounds into contact with the screw A, closing the circuit again. These actions occur several times within the brief period of time while the cam C is rubbing against the tongue D, and the successive magnetization and de- magnetization of the iron core of the coil induces a rapidly alternating electromotive force in the secondary windings; this pressure is so high that it readily jumps across the small air gap between the terminals A and B in the cylinder, and produces a series of flashes or sparks there which ignite the mixture. There are, of course, many variants of the two sys- tems, but it is impracticable to describe here all of the arrangements actually in use. The jump-spark system as generally applied, however, does not include two I insulated terminals or spark points in the cylinder. B _ One of these is mounted in the metal plug and the other FIG. ee insulated from it, as illustrated in Fig. 22a, where the : point A is shown set into the plug G and the point B is insulated from it by the porcelain sleeve J, J. One of the wires from the secondary of the induction coil is attached to the outer end of the insulated sparking point, as shown in the drawing, and the other may be attached to any part of the engine that is in permanent metallic contact with the cylinder, because the plug G is screwed into the cylinder wall. | = = = | SSS SSG eeceeren SS URRY SS AUTOMATIC IGNITION There are some oil engines which are not provided with any auxiliary apparatus for igniting the charge, because none is needed. The engine illustrated in Fig. 23 is one of these. It is designed to burn kerosene oil, which is injected by a small pump into a cylindrical chamber, known as a vaporizer, which is connected to the engine cylinder by a constricted passage, V. Air alone is drawn into the cylinder through the inlet valve during the suction stroke, and at the same time the oil pump discharges a jet of kerosene into the vaporizer, which is kept hot by the repeated ex- plosions. The oil is vaporized by the heat of the chamber, and when the piston returns on the compression stroke, the air is forced back into the 45 IGNITION "YRHANITAO HHL OLNI ONINAdO UAZIUOdVA V HLIIM INIDNGY ANWSOUAM — “86 “OLA LOLI UV YY Y} Z) Z; LYE: j | ————————————————— BSISKSSS 46 THE GAS ENGINE vaporizer through the neck N, mixing with the oil vapor to form an ex- plosive mixture. The compression of the air and the temperature of the vaporizer are so related that at the end of the compression stroke the mixture ignites by reason of the increase in temperature due to compres- sion. The rear end of the vaporizer is covered by a hood, H, to prevent its being cooled by outside currents of air, while the end near the neck is water-jacketed like the cylinder. For starting the engine up cold, the vaporizer is heated by applying a blow torch or similar lamp at the mouth of the hood, beneath the vaporizer. Fig. 24 shows another engine in which the oil is vaporized by the heat of a combustion chamber opening into the cylinder through a constricted passage. The neck of the firing chamber V is extended on the lower side to form a lip LZ, on which the oil is sprayed through a nozzle N at the beginning of the compression stroke. The lip is kept hot by the explosions and vaporizes the oil; the oil vapor and the air are forced back into the bulb V by compression and the heat of the bulb added to that of compres- sion fires the mixture. In another kerosene-burning engine a thin metal plate is fastened by a small stud in the clearance space of the cylinder and a spray of oil is squirted on this plate through an atomizing nozzle by the oil pump, just as the piston completes the compression stroke. The plate, being thin and not in direct contact with any cooling medium, remains constantly at red heat, so that the oil is ignited as soon as it touches the plate, the élearance being filled with compressed air to support combustion. Still another method of obtaining automatic ignition consists of com- pressing air alone in the engine cylinder to such a degree as to raise its temperature above the ignition temperature of oil, and injecting the oil into the clearance space in the form of a very fine spray. No hot plate is required because the compressed air is hot enough to ignite the particles of oil as fast as they are sprayed into the clearance space. The oil is forced in by means of a jet of compressed air supplied by a separate pump; delivery into the cylinder begins at the completion of the compression stroke and continues for a brief time after the piston has started on its power stroke. TIMING THE IGNITION Practically all gas engines and most gasolene engines are provided - with mechanism whereby the moment of ignition can be adjusted through a considerable range, because it is usually necessary to ignite the mixture before the piston reaches the end of the compression stroke, and the extent to which ignition is advanced varies according to the character of the mixture, the speed of the piston, and other working conditions. It is impracticable to obtain a perfect mixture of the air and gas—that is, a 47 IGNITION mixture in which every atom of the gas is in contact with its proportion of the oxygen in the air, and there is no excess air; and a certain length of time, though very brief, is required for the flame to spread throughout ‘LVAH @INE-YaAZIYOdVA AW ONILINDI ANIOND ANAUSOUAM — "bs “DIT Tt : mY 7. \ yy 4 4 y Uj rm ae, (, eam SSSR OY vy Vs 5 NH es man’ é s hy ESSN ESSN sy 7 * /3 SE SS a Lt Sy Ss &S 3 SENET a 7 bi OS MEY o & the mixture, this time interval being increased by the imperfection of the mixture just mentioned. Therefore, in order that complete inflammation may be obtained at the desired moment (when the piston is at the end of the stroke) the mixture must be ignited a little before that moment. 48 ’THE GAS ENGINE I The fraction of the piston stroke by which the moment of ignition is advanced depends chiefly upon the inflammability of the mixture and the speed of the piston. A thoroughly mixed and well-proportioned charge of gas and air will burn much more rapidly than a poorly mixed charge of correct proportions or a well-mixed charge of incorrect proportions, while a poorly mixed charge of unfavorable proportions will be comparatively © sluggish in becoming fully aflame. Now, in order to get the most out of the fuel, the whole mass should be aflame when the crank is just over the exact dead center, and a mixture that is slow-burning must be ignited farther ahead of the dead-center position of the crank than one that burns more rapidly. For example, suppose that a certain mixture requires, under the conditions of operation, one two-hundredth of a second for the flame to spread throughout its entire mass after being ignited; then it should be - ignited when the piston is at such a point in the compression stroke that one two-hundredth of a second will be occupied by the crank in reaching the position just barely over the exact dead center. In practice it is not necessary to measure the time required for a mixture to become completely aflame because the proper setting of the igniter can be readily ascertained by a little experimenting. The “timing” of the igniting spark in gas and gasolene engines is highly important. If the mixture is ignited too soon, the rise of pressure - due to combustion progresses too far before the crank reaches the dead center, producing wasteful‘and perhaps dangerous back pressure. If igni- tion occurs too late, the explosion pressure does not rise to the proper height and the power of the expansion stroke is reduced. Figs. 25 to 30 inclusive are reproductions of actual indicator diagrams taken from an engine run- ning on natural gas and with the ignition varied from 25 per cent. ahead of the end of the compression stroke to 10 per cent. of the power stroke of the piston. That is, the diagram of Fig. 25 was taken with the igniter timed to produce the spark when the piston still had one fourth of its stroke to make before the crank reached the dead center, and Fig. 30 was taken with the igniter set to make the spark when the piston had traveled one tenth of the expansion stroke. These diagrams illustrate very clearly the effects of different timing. In Fig. 25 ignition occurred so early that: the combustion pressure attained its maximum before the end of the compression stroke, causing the negative loop at the top of the diagram. This is also true of Fig. 26, but to a less extent. In both cases pounding was caused by the premature rise of pressure, and this was so vicious in the first case that it would have been dangerous to continue running for any length of time. In Fig. 27 the timing is excellent so far as smoothness of operation is concerned, but it was still too early to get the greatest power out of the fuel. In Fig. 28 this latter result was obtained, the mean effective Poy “\ or ez “DI a ie a 0! OI8T ZOT WOTgTMS] ergy £g TOAST "83 “OLA ‘La COTM Alave Zep-uopyasyL A110 YOT WOTUS] IGNITION 9% “OLL "SZ COLT Arxee Y0g WOTTAST spree yoz uowasy 50 THE GAS ENGINE pressure being 93 lbs. per square inch. Curious'y enough, this was exceeded by the diagram of Fig. 26, but the two diagrams were taken some time apart and it is more than likely that the higher mean pressure of Fig. 26 was due to a better combustible mixture. _ Figs. 29 and 80 show the effects of late timing; these are a late rise of pressure and an unduly high exhaust pressure. In Fig. 30 the exhaust pressure was so high that the gases could not drop to atmospheric pressure until the piston had returned to about one fourth of its expulsion stroke. The means of adjusting the time of ignition depends to a great extent upon the kind of ignition system employed. It is impracticable to describe, within the scope of this discussion, the different arrangements used in practice. The principle is the same in all; the position of one member ‘of the igniter mechanism is made adjustable with respect to the member with which it codperates in producing the spark which ignites the mixture. For example, in Fig. 19, the trigger 7 could be made adjustable on its spindle so that the upper end of the rod F would strike it sooner or later with respect to the dead-center position of the engine crank. In Fig. 20, the collar in which the cam C is set is adjustable around the shaft which drives it. In Fig. 21 the finger C would be made adjustable around its shaft or the gearing between this shaft and the valve-gear shaft would he adjustable to alter the angular relation between the two. In Fig. 22 the arm on the end of which the spring D is mounted is pivoted on the end of a journal box in which the cam-shaft revolves, so that the arm may be swung around, concentric with the shaft, and the moment at which the circuit-closing cam touches the spring thereby changed; this type of mech- anism for timing the ignition is universally used with the jump-spark sys- tem, in both gas and gasolene engines. : The ignition point in most kerosene-burning engines is fixed by the builders and cannot be adjusted while the engine is running, because vary- ing the time of ignition is not usually necessary with kerosene engines; the explanation of this is that the character of the mixture does not vary widely and, because of the high temperature required to vaporize kerosene, vaporization and ignition are commonly effected by contact with large hot surfaces or with the heated body of air in the clearance space, while in gas and gasolene engines the mixture is ignited at one point only and time is required for the flame to spread throughout the mass. VI MIXING LIQUID FUEL WITH AIR THE principle of mixing gas and air by passing them through small openings and causing abrupt changes in the direction of flow, described in ‘the chapter on valves, cannot well be applied to liquid fuels because the telative volume of the fuel is too small. In order to obtain anything like a thoroagh mixture, the oil must be either atomized—broken up into a fine mist—by means of a special nozzle or vaporized by means of leat. Gasolene evaporates at low temperatures, and it is therefore customary to vaporize it and mix the vapor with the air just before the charge enters To Engine Inlet Valve Gasolene Supply Pipe KZ Air Intake FIG. 31. — ELEMENTARY GASOLENE-AIR MIXER. the cylinder. Kerosene requires to be raised to a rather high temperature for evaporation, and it is therefore the common practice to inject the liquid either into the cylinder or into-a hot chamber connecting with the cylinder, 51 é 52 THE GAS ENGINE as described in the preceding chapter, thereby securing vaporization with- out the application of a special heater, which would be required for mixing the charge entirely outside the cylinder. Fig. 31 illustrates the general principle on which is based the operation of all efficient devices for mixing gasolene and air. The gasolene supply pipe terminates in a spraying nozzle located in the center of the air- intake pipe, not far from the inlet valve of the engine. A small pump Spring to hold . Needle Valve in position, Section A-A. Section D-D é FIG. 32. —CONSTANT-LEVEL (OVERFLOW) MIXING VAPORIZER FOR GASOLENE. driven from the valve-gear shaft forces a charge of gasolene out of the nozzle during the suction stroke of the engine, and the air that is being drawn into the cylinder catches up the gasolene spray, evaporates it, and carries the vapor along into the cylinder. The bore of the air pipe is usually reduced where the gasolene nozzle is located, as here shown, in order to inerease the velocity of the air at that point and insure the pickings up of all of the gasolene spray, but this construction is not always em- - ployed. The gasolene, being sprayed into the air current, is very thor-., oughly distributed, and the air is usually drawn from a point near the exhaust pipe of the engine, so that it is warm enough to yaporize the gasolene; the mixture, therefore, is very good. In a few form: of mixing vaporizer, however, a disk fan is pivoted in the air pipe just beyond the gasolene nozzle, and revolved by the rush of air past its blades; this gives the air and gasolene vapor a whirling motion and greatly increases the intimacy and efficiency of the mixture. This statement is based on actual tests made by the author with a well-designed vaporizer, applied with and without the fan. Many gasolene engines are. provided with a mixing vaporizer in which the gasolene is delivered to the spray nozzle at constant pressure, so that’ the fuel mixture may be regulated by fine gradations and with great accu- MIXING LIQUID FUEL WITH AIR 58 racy. The constant pressure is obtained by supplying gasolene to the nozzle from a small chamber in which the gasolene is maintained at a constant level. This level is slightly below the level of the spray orifice, and the gasolene is drawn from the nozzle by the suction of the engine piston. Fig. 32 shows sectional views of such a vaporizer; the supply chamber is provided with an overflow pipe leading back to the main gasolene tank, and the upper end of this pipe is set at the level desired. - The gasolene is pumped to the level chamber at a rate considerably higher than the engine requires, and the surplus goes back through the overflow pipe to the tank by “gravity. A needle valve is provided at the spray orifice by which to regulate the quantity of gasolene drawn out by each Gasolene Supply Vent To Engine Intake, FIG. 33.-- CONSTANT-LEVEL (FLOAT-VALVE) MIXING VAPORIZER FOR GASOLENE. suction stroke of the piston. This valve also serves to spread the gasolene - into a thin conical sheet and thereby promote its mixture with the air as it rushes by.. The butterfly valve T is a throttle, by which the charge taken in by the engine is regulated; the greater the quantity of air admitted by this valve, the greater will be its velocity at the spray nozzle and, con- sequently, the greater will be the quantity of gasolene sucked out, and vice versa. Vaporizers of the type just described are practical enough for stationary engines, but not for engines used on automobiles and boats, because of the constant changes in position. For these classes of service the “ float feed ” type of vaporizer and mixer is almost universally used. Fig. 33 illustrates _ the general principle on which this type of vaporizer is based. The only important difference from the type in Fig. 32 is that the gasolene is main- 54 THE GAS ENGINE tained at constant level by means of a float valve. Gasolene is delivered from a main supply tank into a small chamber in which is a float, mounted on or attached to the stem of a needle valve controlling the inlet orifice. The float maintains a substantially constant level of gasolene in the appa- ratus, and this level is a little below the tip of the spray nozzle in the air pipe. The engine suction draws the gasolene out of the nozzle and the mixing is effected as in the previous case. The proportion of gasolene to air is adjustable by means of a needle valve located within the spray nozzle in this case. The float chamber is provided with a small vent, or more accurately, an equalizing orifice, through which atmospheric. pressure may act on the gasolene and force it out when the air, rushing past the nozzle, forms a partial vacuum near it. This vent also serves to prevent the development of undue pressure in the reservoir by the evaporation of gasolene in very warm surroundings. Air enters the nozzle chamber through a series of holes in the wall below the level of the nozzle. Very few kerosene engines built in this country are equipped with devices for vaporizing the oil and mixing it with the air outside the cylinder xhaust from Engine ULL LLL LLL Lk N Le ZZ VA il d FIG. 34. — KEROSENE-AIR MIXING CHAMBER, Ll SSSSSSS G or some extension of the cylinder. One well-known kerosene engine, how- ever, has a mixing vaporizer heated by the exhaust gases from the engine, of which Fig. 34 shows a longitudinal section. Air is admitted freely to the mixing chamber, and oil is pumped into it through an atomizing nozzle. The engine draws in the mixture by piston suction, as in the gas or gaso- lene engine. The mixing chamber is enveloped entirely by a jacket through which the exhaust gases from the engine pass to the exhaust pipe. The next step is to vaporize the oil in a hot chamber opening perma- nently into the cylinder, as illustrated by Fig. 23, and described in the chapter on ignition. The third method, that of injecting the oil directly into the cylinder and vaporizing it by contact with a hot plate, was also described in that chapter. Then there is a combination of the two methods just described; the engine in which it is applied is illustrated by Fig. 24. The oil is injected ye MIXING LIQUID FUEL WITH AIR «5S by a small pump P through the nozzle N into the cylinder and strikes the lip Z of a hot bulb V. This occurs early in the compression stroke of the’ piston, and as compression progresses the oil vapor is forced back into the hot bulb V, where it and the air are compressed and then fired as in the engine illustrated in Fig. 23. The torch shown beneath the bulb V is for the purpose of heating the bulb before starting the engine. After it is running the explosions keep the bulb hot. The fourth method is that employed in the Diesel engine, which works with very high compression. The piston draws air alone into the cylinder and compresses it to a pressure of several hundred pounds per square inch. The oil is blown into the cylinder through a suitable nozzle by a jet of compressed air, which breaks it up into a very fine spray, and the heat of the highly compressed air in the cylinder ignites the particles of atomized oil as rapidly as they enter. This combustion is not explosive, as in other internal-combustion engines, but gradual, continuing as long as the spray enters, and this period is varied by the governor according to the load requirements. Vil METHODS OF GOVERNING Hir-anp-Miss THERE are three fundamental methods of governing the speed of a gas or oil engine and several combinations of these. The oldest-is the “ hit- and-miss ” method, which consists of causing the engine to stop taking in charges of mixture when the speed drops back to normal. One way of accomplishing this on a gas engine is to provide a valve in the gas-supply pipe which is opened regularly by the valve gear during the suction strokes as long as the speed remains normal and allowed to remain closed when the speed exceeds the normal, the governor being arranged to control the actuation of the valve. The corresponding method with an oil engine is to arrange the governor so as to stop the pump or allow it to be oper- ated, according to the speed. This application of hit-and-miss regulation to a gas engine is illustrated by Fig. 35. The rocker arm A is moved by the cam C at the proper moment to open the gas valve. To the left- hand end of the rocker arm is pivoted a finger, commonly called a “ pick blade,” and a link from the governor mechanism holds this pick blade either into or out of line with a hardened extension of the gas-valve stem. So ° long as.the speed is normal, the governor keeps the pick blade in line with the valve stem, but if the speed exceeds the normal rate, the blade is drawn to the right far enough to miss the end of the valve stem when the rocker arm is operated by the cam; consequently, the gas valve remains closed during that suction stroke and the engine takes in air alone. There is no explosion, therefore, after the compression stroke is completed, and the engine speed is reduced by this failure to get a power impulse for the expansion stroke. : The principle embodied in the construction shown in Fig. 35 is applica- ble to an oil engine by merely substituting the oil pump for the gas valve, arranging the pick blade to strike the end of the pump plunger stem. nor- mally, and providing a spring to return the plunger after each delivery. stroke. This arrangement, however, cannot be used with any form of vaporizer which contains a fuel reservoir, as in Fig. 32 or Fig. 33, because the missing of a pump stroke would not cause the engine to miss its fuel 56 METHODS OF GOVERNING 57 charge immediately. It is only practicable where a vaporizer is used with- out any constant level supply, as in Figs. 31 and 34, or where the fuel is injected directly into the cylinder or an extension of it, as in Figs. 23 and 24. The application of the hit-and-miss method illustrated in Fig. 35 has the advantage that whenever the governor cuts out a charge the piston draws in pure air which mingles with the burned gases in the clearance space and greatly reduces the quantity of useless material remaining in the — =* FIG. 35.— A HIT-AND-MISS GOVERNOR MECHANISM. cylinder when the next charge is taken in. It also cools the cylinder con- tents and permits a greater weight of mixture to be taken in during the next effective suction stroke. Under some conditions the features just mentioned may be disadvan- tageous instead of desirable. For example, if the engine is running with a rather small load, so that it misses more explosions than it gets, the cylinder may be cooled down sufficiently to impair the efficiency of the engine or even to cause it to discontinue firing the occasional charges, in the case of a kerosene engine; furthermore, if the mixture is adjusted for maximum effectiveness at full load when there are few or no “ misses” and the clearance space is filled with hot burned gases almost every time the cylinder takes in a fresh charge, then the cylinder contents will not x - Po 58 THE GAS ENGINE have maximum effectiveness at light loads when the clearance is filled with almost pure air at each admission of mixture and the temperature is lower. However, it is a simple matter to adjust the proportion of gas or oil to air when the engine is running at its average load; then the discrepancy at greater or smaller loads will not be so objectionable. An application of hit-and-miss regulation that has been used to a con- siderable extent is illustrated elementarily in Fig. 36. The engine is equipped with an automatic inlet valve and the governor is arranged so that when the speed is too high it moves a detent d in position to catch a dog e on the exhaust-valve stem after the valve has been opened by the push rod, and thereby hold the valve open. The consequence is that when the piston moves forward on its suction stroke it cannot form a vacuum in the cylinder and the inlet valve is not opened. The cylinder, therefore, does not get a charge of fresh mixture, and misses the explosion that would | Governor Link oS d IN Exhuust Valve Rod FIG. 36.— ANOTHER HIT-AND-MISS METHOD. ordinarily next occur. This arrangement is open to the objection that when the governor blocks the exhaust valve open during a suction stroke the piston sucks in hot burned gases from the exhaust pipe. It has the ad- vantage, however, of being applicable to oil engines having constant-level mixing vaporizers because, as the main inlet remains closed during the suction stroke, no fuel can be drawn in. The hit-and-miss method of regulation has the sole advantage of high fuel economy. The engine takes in practically a full piston displacement of mixture every time it takes a charge, and the mixture is of uniform quality; consequently, it burns with high efficiency. On the other hand, the power impulses do not occur at regular intervals unless the engine is carrying the maximum possible load, and this necessitates a very heavy flywheel in order to prevent too great a drop in speed between impulses. Moreover, when running at moderate load the working parts are subjected to just as violent stresses due to the sudden rise of pressure after ignition as when the engine is carrying its maximum load. VARIABLE QUANTITY OF INTAKE , The next simplest method of speed regulation is that of throttling the mixture, just as a throttling steam engine controls the quantity of steam admitted to the cylinder. This is usually accomplished by putting a butter- ’ METHODS OF GOVERNING 59 fly valve in the inlet pipe and controlling its position by the governor, as — indicated in Fig. 18. The arrangement is so simple and obvious that further illustration of it is unnecessary. The throttle valve is usually located just as near the main inlet valve as possible, in order to obtain prompt response to a change in its position. With gas engines this is advisable for the additional reason that the quality of the mixture is main- tained more nearly constant than it would be if the throttle were in the air-intake pipe. The advantages of throttling the mixture are simplicity of mechanical construction and the maintenance of a practically constant ratio of gas or oil to air. The disadvantage is that the compression in the cylinder changes with the load. Thus, at light load, the governor admits a smaller quantity of mixture during each suction stroke than at full load, and the compression pressure is therefore lower. This means that the particles of fuel and air are not so intimately compressed, and the result is slower combustion. At very light loads it frequently occurs that the compression is so low that a charge is either not completely burned or not even ignited. In either case there is a loss of fuel, and when ignition fails to occur there is also likely to be an explosion in the exhaust pipe of the charge which passed unburned through the cylinder. This is called “ after-firing ” and sometimes produces disaster by wrecking the flue through which the exhaust gases pass to the atmosphere. The low compression at light loads also causes a serious falling-off in efficiency; this is discussed in the chapter dealing with the calculation of pressures, output, etc. Another method of varying the quantity of mixture taken by the engine during each suction stroke is by closing the admission valve sooner or later in the suction stroke, according to the demands of the load—in other words, “cutting off” exactly as in the automatic cut-off steam engine. Fig. 37 illustrates a form of cut-off gear that has been applied to an engine having separate gas and air inlet valves. The two valves are on one stem, the air valve being rigidly mounted and the gas valve spring-mounted. The valve stem is depressed by the rocker arm 4 when its outer end is raised by the push rod, and the nose of the latter is pressed out from under the rocker arm by the tripping plate P, allowing the valves to be seated by the upper spring on the valve stem. The trip P is mounted on one arm of a bell crank which is tilted forward more or less by the governor, thereby deter- mining the point of cut-off and consequently the amount of mixture taken in during the suction stroke. The push rod is moved up and down by an eccentric on the valve-gear shaft. Fig. 38 shows another form of cut-off gear embodying the same prin- ciple as illustrated by Fig. 3%. The air valve A and the gas valve @ are connected rigidly, however, by a barrel D, which encircles the lower end of the spring that seats the valves. When the air valve is down on its seat, 60 THE GAS ENGINE the gas valve closes the gas port, which is tapered, with a sufficient approach to absolute tightness to prevent appreciable gas leakage. The main inlet valve J is opened by a cam on the valve-gear shaft at the beginning of the inhalation stroke and held open throughout the stroke. The stem of the \ Gds—> Air—> Gas Valve Air Valve t \ ‘ Aes ; van te Exhaust Barr %. 4+ Outlet " , NA \eshaust Rocker Arm FIG. 37.—AN AUTOMATIC OUT-OFF GOVERNOR MECHANISM. inlet valve is linked to a short rocker arm R, Fig. 38, to the other end of which is pivoted a block arranged to slide vertically in a guide. To this block is hung the pivoted latch Z (shown in Fig. 39), the end of which nor- mally engages a dog on the block which is screwed on the upper end of the stem of the cut-off valve. When the main inlet valve is opened, the latch METHODS OF GOVERNING! 61 L lifts the cut-off valves, allowing air and gas to pass to the mixing cham- ber M through the ports shown closed by the valve disks A and G, respect- ively. From the mixing chamber the mixture passes into the cylinder through the port of the main inlet valve 7; at the point in the suction stroke determined by the governor, the cam C, in Fig. 39, engages a lug and draws the drag link over, thereby pulling out the latch Z and allowing \ BA Air Inlet Md DES Gas Inlet LH M M N Y, Me 3 Uy FIG. 38.—SECTIONAL ELEVATION OF INLET AND CUT-OFF VALVES AND MECHANISM. the cut-off valve to drop. The drag link is pivotally attached to a lug on the latch Z, and its other end is curved around the cut-off shaft S, the upper leg of the bend resting on the journal box and holding the link. in place as it slides back and forth. Before the succeeding suction stroke begins, the cam (’ has turned to the “low” side and the latch Z is thrown into engagement with the valve-stem dog by a small helical spring. When the cut-off valve drops, it is cushioned by the inverted cup # (Fig. 38), 62 THE GAS ENGINE acting as a dash pot, the boss F constituting the plunger. The cut-off cam shaft S rotates continuously at the same speed as the main valve-gear shaft, and its angular position with respect to that of the valve-gear shaft is adjusted by the governor through a “ floating ” bevel gear. yd) Lal | Y = =e iy Y, a paid ae | ay L_—__ DA | Combustion Chamber FIG. 39.—END ELEVATION OF INLET AND CUT-OFF VALVE MECHANISM. The cone-shaped disk B, immediately beneath the gas port in Fig. 38, is a baffle to control the proportion of gas to air in the mixture. It is adjustable toward and away from the gas port by means of the knurled METHODS OF GOVERNING 63 head N, which is locked in the proper position by the set screw s. The shank of the baffling disk serves also as a guide for the lower end of the stem of the cut-off valve disks A and G. The automatic cut-off method of regulating the speed involves the same advantages and disadvantages that were mentioned as features of the throttling method. There is a slight advantage, however, in cutting off as compared with throttling in that the negative work done by the piston during the suction stroke is a trifle less. This is due to the fact that the throttling engine at underloads draws in its charge past a partly closed throttle during the whole suction stroke, whereas the cut-off engine draws in its charge through wide-open passages and during less than the full piston stroke. VARYING THE QUALITY oF MIXTURE The third fundamental method of governing is to vary the quantity of fuel alone, keeping the total quantity of cylinder contents constant. This is done in gas engines by putting a throttle in the gas-supply pipe and controlling it with the governor, or by arranging a cut-off valve in the . gas passage, opening it at the beginning of the stroke when the air valve opens and closing it earlier or later in the stroke by means of the governor. Both of these methods are open to the serious objection that the quality of the mixture is varied as the load changes. An engine so regulated will not work under widely varying loads because neither a very rich mixture nor a very poor mixture will ignite. Where the load does not vary much within short periods of time this method of governing is fairly satisfactory, but it involves constant watchfulness to see that the proportion of air to gas does not become. too large or too small. The only advantage of “ quality regulation,” as it is called, is that the compression. remains constant at all loads, giving better efficiency below full loads than if the compression varied. This method is the one generally used on kerosene engines and on gasolene engines which feed the fuel to the cylinder with a pump. It is not used with constant-level vaporizers. COMBINATION METHODS With a view to overcoming, avoiding or minimizing the disadvantages of the three fundamental methods, several combinations of them have been tried. One engine builder has used quality regulation at full load and for under-loads as far as possible, and at smaller loads than this method would handle the hit-and-miss method came automatically into play. The most successful compromise thus far attempted, however, seems to be that of admitting air during the complete suction stroke and varying 64 THE GAS ENGINE the time at which gas begins to enter, continuing the intake of gas with the air to the end of the stroke. Thus, if the load is such as to require the admission of gas during one half of the stroke, the gas valve remains closed until the piston has traveled halfway of the suction stroke and then it is opened and kept open until the end of the stroke. While this method may seem at first thought to be the exact caribaea of admitting gas during the first half of the stroke and then cutting it off, and therefore mere “quality regulation,” it is not at all so in practice. When gas is admitted from the beginning of the stroke until some point considerably short of full stroke, say three fourths of the stroke, the air which continues to enter during the remainder of the stroke tends to sweep the gas away from the vicinity of the inlet port and to diffuse it throughout the cylinder contents of air and burned gases. The result is a weak mix- ture and possibly an absence immediately around the inlet port of -any combustible at all. Consequently, if the igniter be located near the inlet port, which is usually the case, the mixture may not be ignited at all, and _certainly will not inflame rapidly even if ignited. On the other hand, when the gas is admitted during the latter part of the stroke, there is always a good mixture near the inlet port at the end of the compression stroke and there is much less tendency for the gas to be diffused throughout the whole mass of air and form a poor mixture. Since air is admitted through- out the entire suction stroke the combined volume of air and gas taken in is always the same, as in thé case of simple “quality regulation ”; consequently, the compression is the same at small loads as at full load. So far as the author knows, this method has not been applied to gaso- lene engines, though there is no reason why it should not be applicable when a constant-level vaporizer is not used. Vil ‘SOME CONSIDERATIONS OF DESIGN GAS-ENGINE design is yet far from being an “exact science” or any- thing like an approximation to it. Experience thus far gained, however, has developed some hard-and-fast rules which are of great assistance to young designers. The more important of these follow. CYLINDER CONSTRUCTION The inner wall of a cylinder should not be made to transmit axial stresses if the requisite strength necessitates the use of a very thick wall. When the thickness greatly exceeds three inches, the retardation of heat transmission is liable to cause serious troubles, such as deterioration of the inner surface by the prolonged action of excessive heat, unequal expan- sion of the inner and outer parts of the inner wall, irregular combustion due to ineffective cooling, ete. The interior of the cylinder should not contain any recesses of great depth relative to their cross section, or pockets which open into the main part of the cylinder through restricted passages. The hot bulb used on some kerosene engines is, of course, an exception to this rule, but the very fact of its applicability to the ignition of the mixture is a forceful demon= stration of the objectionable influence of such pockets in cylinders where the mixture is intended to be ignited by other means. They interfere by storing hot gases which pre-ignite the fresh mixture (before compression is complete), and may even cause back-firing, or the ignition of the incom- ing mixture while the inlet valve is still open. . The contour of the cylinder should be as simple and symmetrical as possible in order to avoid inequalitics in expansion when the engine heats up. and since the heat energy in one British thermal unit is equivalent to 778 ft. Ibs. of mechanical energy, the mechanical energy added to the gas will be V78 Ce. Therefore 2116.3 V= 778 C., if the heat added be sufficient to raise the temperature one degree and expand the gas against atmospheric pressure. If the pressure be anything else and the temperature elevation be more or less than one degree, and if the weight of gas be W pounds, the case is covered by representing the pressure and temperature rise by symbols, thus: PV="%8 ( t W, in which ¢ = the temperature rise in Fahrenheit degrees. Absolute pressures and temperatures are used in considering the be- havior of gases for reasons given in the next paragraph. Absolute pressure is gauge pressure + atmospheric pressure, and is taken at 14.69 lbs. per square inch or 2116.3 lbs. per square foot. Absolute temperature is taken at Fahrenheit thermometer temperature + 460, the absolute zero being 460 degrees below the Fahrenheit thermometer zero. Volumes are in cubic feet throughout this discussion. All calculations are based on absolute pressures and temperatures be- cause it is considered that a gas in any condition has been expanded to that condition from zero pressuxe, temperature, and volume. Consequently, no matter what the pressure P and temperature 7, and the resulting vol- ume TV’, it is considered that the gas has increased its volume from zero to V cubic feet against a constant pressure of P pounds per square foot due to a gain of heat which has also raised the temperature from zero to T degrees, absolute. It follows, therefore, that PV=1%8 0. T W, (a) no matter what the values of P, V, T and W may be. This being accepted as a working hypothesis, it is evident that when the condition of a given weight of gas is changed, as by compression, combustion, or expansion under artificial influences, since fr = 118 Ce W, PRESSURE, TEMPERATURE, AND OUTPUT CALCULATIONS 75 and C, and W are constant, the ratio PV T remains constant through the change in condition. Indicating the condi- tion before the change by the subscript , and the condition after the change by 2, Pe We = Pe Vi pe . A» CoMPRESSION The first change to which the gases of an engine are subjected in work- ing through a cycle is that due to compression. Using the subscript . to indicate the conditions of the cylinder contents just before compression begins and the subscript , to indicate the conditions when compression is complete, P.Ve = Pa Va Ds From this fundamental equation are derived the formulas: Pe= Pate, (c)* and P= Ties (d)* in which r, is the compression ratio, as explained on page 18. * Derivation of Equations (c) and (d): PeVe — Pa Va ; : T. T; consequently, ROT Vo fe es Now, with adiabatic compression, : T. Va k-1 a ae ( ¥ ) ; in which k= Cp : Ge therefore - - (Gt): Compression, however, is not adiabatic; heat is lost to the jacket, so that an ex- ponent of smaller value than & must be used; this is usually represented by n. Making this change r. oe 7 ak Ve ) =e P. = ( Vo y= ree Ps Ve Perey These obviously transpose into and T. = Ta re—. (d) d 5 eS Py = Pere" x a (c) 76 THE GAS ENGINE If combustion of the mixture were complete and instantaneous while the piston was stationary at the end of the compression stroke, and if there were no loss of heat to the water jacket,.the rise of temperature pro- duced by combustion would be given by the formula HT Cy =7,—Ty, in which H =B.t.u. evolved by combustion, per pound of cylinder contents, Cy = Specific heat of cylinder contents at constant volume, T, = Maximum temperature attained by. cylinder contents. But complete and instantaneous combustion is out of the question prac- tically, as already explained in the earlier chapter on pressures and tem- peratures, and heat must be taken away by the jacket water in order to keep the cylinder from becoming red-hot. It is impossible to predict just how much these imperfections will reduce the temperature rise, but by using a symbol to- represent the proportion of the ideal temperature rise actually obtained, a definite formula may be deduced for the actual tem- perature rise, thus: Hu Cy in which uw represents the ratio of actual temperature rise to that which would be obtained with perfect and instantaneous combustion and no heat =T,—T-e, ) loss. Thus, if a were equal to 3000 and the actual rise of temperature were 1620, the value of u would be 0.54. As a general rule, with gases of high heat value the value of w is lower than with gases of low heat value, if the proportion of air to gas be equally favorable in both cases. One reason for this is that engines using rich gases cannot work with high compression because of the liability to premature ignition; consequently, . their clearances are larger than those of engines which burn producer or blast-furnace gases, and the cooling surfaces are larger. This allows a relatively greater loss of heat during combustion and lowers the efficiency of the process, thereby reducing the value of u, which covers heat loss as well as slowness of combustion. Moreover, it is generally the case that a mixture formed with rich gases is neither as well-proportioned nor as thor- oughly compounded as one made with lean gases, and this causes relatively sluggish combustion. With oil fuels the value of w is usually much higher than with gaseous fuels. Whether or not this is because oil atoms are more easily brought into contact with their proportion of oxygen than gas atoms the author does not know, but it is certain that the rise of pressure is relatively much more violent with gasolene than with any gaseous fuel. Explosion pres- PRESSURE, TEMPERATURE, AND OUTPUT CALCULATIONS 77 sures of 350 to 400 Ibs. are often obtained with compression pressures of 80 to 90 Ibs. per square inch, using gasolene, while with illuminating and natural gases compression pressures of 100 to 125 lbs. are usually accompanied by explosion pressures of 250 to 350 lbs. The cylinder contents in a nonscavenging engine (few engines actually clear out all of the burned gases) consist of a fresh mixture of air and gas or air and oil vapor and the burned gases remaining over from the preceding explosion. The volume of the fresh mixture in a four-stroke- cycle engine is usually equal, before compression, to the piston displace- ment during the suction stroke because the clearance space remains filled with burned gases at the end of the expulsion stroke. Consequently the heat per pound of cylinder contents will be the total heat in the fresh mixture divided by the weight of the cylinder contents. The total heat in the mixture is practically equal to h aha | consequently, the number of heat units per pound of cvlinder contents is h V. tg Wo » ff) in which h =B.tu. per cubic ‘foot of gas alone at the temperature of the mixture immediately before compression, a@ = Cubic feet of air per cubic foot of gas in the fresh mixture, Vs = Cubic feet of piston displacement, W = Pounds of cvlinder contents. Substituting this equivalent for H in equation (e), transposing and making the substitutions explained in the footnote, the result is the. formula: 1 Ta T,=T.+ Ky (1—— ) a (g)* in which kR-l1)hu Kya" eS (h) * Derivation of Equation (g): The heat units per pound of cylinder contents are given by the equation h iF a xX = f fe 8 yp ee (f) substituting this equivalent for H in equation (e) gives the equation hulY; Cata) (+a) Tr T, —T:, 78 THE GAS ENGINE In this last equation The relation between compression and explosion temperatures and pressures is Pek oles Pe Es which transposes to hut Urls Te = Te. + Guitar (t) Equation (a) furnishes an equivalent for the product of C, W which is easier to evaluate in practice. Thus, equation (a) may be transposed into Pa Va W- ETC’ and é Ce = Cp Cy = 0, (AE), which reduces to C = Ho \G C. = G (Ge - 1) or Ce. =C, (k—-1). Substituting this equivalent for C. and transposing C, to get it with the W, produces the equation Pa Va 778 Ta (k—1) P.Va 778 T. (& — 1) Cc, W = Now substituting in the place of C, W in formula (t) above, for explosion temperature, gives: hu 778 (k—-1) f. VY, l+a -s Po * Va T,=T. + This may be made more manageable by substituting the equivalent of V, + Va in terms of the compression ratio. Thus: V. — Va-—V- See fa Va Va But Ve — 1 7 ee so that V, ry = 1 Ve Te Making this final substitution gives, for the explosion temperature formula, 778 (k~1)huT, (1-1) (1+ a) Pu : which is identical with equation (g) when equation (h) is applied. Te = T. + PRESSURE, TEMPERATURE, AND OUTPUT CALCULATIONS 79 - Accepting the equivalent of T, given by equation (g) and making the transpositions and substitutions explained in the footnote, the following for- mula for maximum pressure of combustion is obtained: P,=P.+ Ky (re — 1). (i)* So far as it affects the pressure and temperature at the moment of release, the expansion ratio is Ves r= = ) zt and since TV, = Ve, i ey —_— > Ve In a two-stroke engine ve is practically equal to 7¢; in a four-stroke engine the relation between the two, so far as it affects the release pressure and temperature, is re=f(re-1I +1 in which f is the fraction of the piston stroke that is completed when the exhaust valve opens. Thus, if the exhaust valve opened at 90 per cent. *® Derivation of Equation (i): Applying formula (b) to the pressure increase of the combustion process, and repre- senting the maximum pressure of combustion or explosion by P. , Pe V5. Pe-Ve — Sey T: . -e and as the volume is practically constant, 7, = V.. Consequently, PB, T. Te and this transposes to Pe T3 a Accepting the equivalent of T. given by equation (g), dividing it by T. and multiplying by P. gives the following: Ky P. zy oS. P. ¢ + Pa T. But, according to equation (b), Pe a oe Be ee so that r. can be put in the numerator of the fraction and the two temperatures and pressures eliminated. And since 1— a is already there and e Te x (1- —— =r. —1, Te the f la becomes: : et Py =P, + Ky =v. @) The student should not confuse & with the exponent », but remember that k =C, + C, always, while n is always less than that. 80 THE GAS ENGINE of the expansion stroke and the compression ratio were 4, the expansion ratio would be: 0.9 (4-1) + 1=3.7. The pressure at the moment of exhaust opening is Pee Pye te ~ and the gas temperature at the same moment is Lo Ty te, (k) Tables 4 and 5 give the release pressures and temperatures corresponding to several usual explosion pressures and temperatures, assuming expansion exponents of 1.29 and 1.32; these are common practical values, the former for double-acting or other thoroughly cooled engines and the latter for single-acting or other moderately cooled engines as they are usually operated. Work Done Per Cycle The work done in compressing the cylinder contents is Pe Ve — Pa Va __ L a rt ft.-lbs. ¢,. and the work done by the gases on the piston during expansion is P, Ve — P. Ve Ne — 1 = ft.-lbs.e. In this case it is not fair to the four-stroke-cycle engine to consider the point at which the exhaust valve begins to open as the end of the expansion stroke, because, as explained in connection with Fig. 3, the pres- sure does not drop at once to atmospheric. It is more nearly accurate to assume that the valve opens at a point about halfway between the point of actual opening and the end of the piston travel. For most of the indi- cator diagrams which the author has measured and checked up, the assump- tion that the exhaust valve opened at a point of the piston stroke four tenths of the remaining travel beyond the point of actual opening, and then reduced the pressure instantaneously to atmosphere, gave calculated diagram areas almost exactly agreeing with the actual diagram areas. Table 7 has been computed on this basis, the formula being mete — 0.6 (1 — f) (ro - 1). (1) The volume V,, therefore, is that volume and the pressure P, is that pressure which would exist behind the piston at the moment of release if the actions were those described above; that is, : Ve=VeXt'es 0.80 WAHKWANTOSWAAHANOHNMOWOMMHAOC OSwoxt DOUMAMWDNOSAMOMOMEAO RRBBESSS AA RARRS IGE BS TERR GRRARSSARARRHAGIOS SOREN AGARSSOMAAAH TA QOSRRAMLAaCS = EU OVE ON. EN 015 019 19 615 019 07 C19 01D E19 19 C19 619 1D 61D EF 1D 019 9 1D 6D 9 CID CD SL SH RH st gH nt Std tt tt Ht 19 5 9 1 09 2 6 9 1 16) 1 1D AD 20 ADAG 191 19 1919.1 OO 0.81 PHROMONOMIMOAPADONO TAHOMA NO HINO Mr AO 19 4 © O79 5 673 5 6 5 ED GO CO 1 CSE RH RE St St Tt te Et tt et at tt 1 105 1659 26 1 9 2 9 1 2 1 AG 19 1 A 1G} 2 11 1G OO OL 0.82 OOD HOO 6D Dy HO OID HOO co oot 0.83 nN DOOMWAADM NO HOI OBAMA GM MIOWHAONAMSHOOIAHHONAO SxSR BAAD DOORN OW AASOOM MA ED OS WIDHOSRNHHHASOS HAG oe ©9015 09 26 1015 20 15 1 1 15 05 09 19 1 19 181 1}. IGIDIHOOGO 0.84 0.85 0.86 CO OO NI Ds m4 OO 19 HO COE NO BOOM AAD MOAKH HO NNO HAM OMIM AD HOOMORDHWDMNOANMMOOWMWOY HMMM NAD 1 ar DUNT TOFD 61D 1D CVD CVD 1D CFD CVD CVD. VD EVD YD YD CVD GYD CVD CVD C¥D GYD CVD YD CVD SH SHY eth SH cH ett sett tt ett St ed Se Se et Se ett ett ett tt Hf 205 09 209 209 9 9 9, 1 1 A.D BD 1D 10D UD AH AM Ag wwrcwwowes 0.87 TROP OOENENOOWOWAMWMMALM IO OW DP 0) CO C9 Be CVD tS 19 OH OD OD CO CD P= NO OOM A HAM OC MAO HOOMWAAWAMOARNS HOOU UIT FD £19 C19 019 C19 C19. CD CVD. CVD. CVD CVD. VD CVD CVD VD CVD CVD CVD CVD CVD VD SRE Se SHY SH sett sett sett ett att et st tt ct et se tt se se eh Se et 5 2099 2 89 19 18 29 9D DD 1D 1.1 IAD IOIO WLI INO OOOOOOS Expansion Ratios 0.88 WOOMAYAM OMA HOOMWIS Na MIOONMm AO 1 o> OO be QU Pe HOODS HD HOO OND 16 119 O10 HD MOrAOnd Ct awsa Be EE OER RBS SERN RAR SS ARRAS GBR SE RR ERAS RE SA AANRR SERBS SSRRERSSSSAARARNA DUNT OD 0D VD CD. CVD 1D EVD CVD ED CVD YD CVD EVD 1D CVD CVD EVD. CVD FD ED CHD EE RA Set eH tt Sct seit at Sc tt st ct ch sett ct ett St tt Sc SH ett 1 1.1919 1018 1918 191919 1D 1D INLD LHI IO IN MN OOOOOOOOE Table 7. 0.89 60 64 9 74 78 83 ae ee FeO NS ORR Sa aM Om eA 24 CO OO “WOR ROO rie 69 679 0) SH HAD ED COCO IE WO ADOSCOMMANA CD OD St oc EE tt tc tt 1 05 158 10 ad 16 1 18 19.118 19 AB GAH IMAG AMT GOGSOGOGOS 0.90 00 9 Be CU Bs et © wt © DD © HOD AH 00 019.00 09 Be C0 Be et © et 19 © 19 © Hd HO 09 CO ON BS OI Be tO 119 O19 HD HOM WARN AO IOWA WHO OW OONENO A OOrws DABOSAMAN cds POSSE RBRRSSaMANA OE FEBS SRLGBRAHSSAARATTATHHOORE MOOS OSIMNAN TS NIT ER VD PD 01D CVD 1D CVD CVD CID CVD 1D CVD CVD CVD GYD GYD CVD CPD CFD CVD CFD CVD eH SH ats St et a St et ett tt tt Ht ett tt? Vidi gig tid SHIH ISIN HHH HG SHH oHHsSGoOoSSOOSESS POINT OF EXPANSION STROKE AT WHICH THE EXHAUST VALVE BEGINS TO OPEN 0.91 BH 0900 ARN GD HO BD OD OMA HOM WO RARN GO HO FIDO AMA MOM OMNRANR HO HOON OMI AMANO MONARAM HO MOOMOWHUNS BAS SSA AAAHSFRBZSSRN RD SAROOM NAGY GT PBGSOON ROHARACO MANAG AM DOORN HOA AOOM AAG NG TS ENT 61D 079 019 01D CVD FD CFD CD 01D EVD CVD CFD CD CFD CFD CVD OND ED CHD CVD OD Et HH sh ett aH eit Ht it ett tt ct cH tt ttt cf tt Ht 0 09.19 19 19 1910 1D 1D IN LH DH IHL How wNunwonoocooooowoovoe 0.92 SHQwata on RARARNOnO CEN 69 69.09.09 09 00 09 09 09 69.6 69 O96 OO UD © 1D Go SH OD SH 09 00 COE NIE NO IDO HO OID OID AAPA MOM OMERABNA DHS BPN OWOW AH HOO COC EO BESSA RAN RS ITO SSRREZSSS SS = AARGASIQHSSRRSSSSSSSAAANGA Se POR HY et cH ett et tt tt tt tt th tH 09 00 101910 10191 1 DD LO IH IHW WOM OOHNONHOGOOOOOSOOSOOE 3.71 3.90 CORRESPONDING TO DIFFERENT COMPRESSION RATIOS AND EXHAUST VALVE TIMING; .FOUR-STROKE CYCLE 0.93 ANOnd HD <0) 00. OPAL AK HOH OOWOMOPAAHWHOMWMMONARARHOnND 19 APYAMWMONBAKRNO HO He BESS AA RARIA IIS SSSRCASASSS AM AAHHS FOHASSNNGHAASS AAR TG A GSOON MD TSSSSMA NES Bae NUE 1D OPP CPD OV. 19 CFD CV CVD CHD CVD EVD CFD CFD CFD CFD. CFD EVD EVD CVD EVD. OVD SH SH et ett eH cH a Se ett ett etl ett tt tt cit eh a ett ett eft 09 19 10 1918 10 11D ID ID AD INAH IN I INI WMIMNMWOGGSHOSOOHOOOOOE Com- pression Tatio I. 0o WOwowOowow Swowownononow SCwWOWSWOSWMOWOCWONMSHMOMOwWwOowow BRR RRR ee Be BE eB 8S RAAB SBR SBR RRA BS SAAR ASIA BS SRRGSS ASCH RAKES BESSON GYD EVD EVD OVD CVD CV CVD 1D CVD CPD EVD CFD EVD EVD CPD CVD CD CVD CFD CYS SHH ERY SY Se Se se cH etl tt et ft ett ce ei eH tt tt Hf 109 1915 101 19 10 ID ED IGUAL AIH ININIO OW WOOOHOOOOOOOOOOOOE PRESSURE, TEMPERATURE, AND OUTPUT CALCULATIONS 81 in which r’, is the effective expansion ratio as given by equation (1) or Table 7. The net work done on the piston is the difference between the work of expansion and the work of compression, thus: Ft.-lbs. ¢ — ft-lbs. ¢ = net indicated ft.-lbs. per cycle. From this fundamental equation is derived the more conveniently ap- *plicable equation : ; V ‘ : Pz Ke—P. Kc) ——“— = ind. ft.-lbs. per cycle. m)* eae Pp y Cc in which 1 K.= 1 Ne th 1 — Nhe —1 : and : 1 ps R = Ie n—1 = Ne — 1 * Derivation of Equation (m): The net work done on the piston per cycle is Ry tae Ye P. Ve een Pe Ve = jt.-lbs. per cycle, and since V, =V, r’- and Va =Ve Te ; & ( Pe Pere eas Poa Pats )x Ve =jt.-lbs. per cycle. Now, i Pots = e “ , and p Pate = = <__ ; consequently and, correspondingly, Pr - Pere = Ps (1 Moreover, v V. gn ae Sy Le _ Substituting these three equivalents, the result is ie Ly se E [= ~ pel JP, te xs = T — ind. foot-pounds per cycle. —I] -—1 . which becomes identical with equation (m) upon substituting K. and K. for the oe eted fractions. 82 THE GAS ENGINE The numerical values of A, and A’, are so nearly equal in practice that, in view of the other uncertainties involved in output calculations, it is sufficient to consider them equal. For example, if fo = 5, Mo = 1.35, 2's = 4.85, and te = 1.32, then I, = 1.23044, and Ke = 1.23966, the difference being less than one per cent. of the smaller of the two values. Sometimes the value of K, is larger than that of Ke; for example, if te = 5, Me = 1.3, re = 4.75, and me = 1.27, then Ke = 1.2767, while Ke = 1.2718. However, as will be explained further along, these constants have to be modified to fit actual engine performances, so that the small difference between their values is of no importance. The theoretical equation for indicated work per cycle may just as well be condensed into Ky (Po — P.) Vs a = ind. ft.-lbs. per cycle. Inspection of equation (i) shows that P,— P, “fe—1 and substituting this equivalent in the work formula gives as the final theoretical equation : Ky Ke Vs = ind. ft.-lbs. per cycle. (n) InpicateD HorsE PowER The horse power of any engine is equal to Foot-pounds per minute 33,000 and the number of useful foot-pounds per minute developed in a gas-engine cylinder is equal to the foot-pounds per cycle X number of explosions per minute. Consequently, if the number of explosions per minute be repre- sented by Nz, the indicated power of an engine will be, theoretically, Ky, K. Vs Ne 33,000 = 1.H.P. PRESSURE, TEMPERATURE. AND OUTPUT CALCULATIONS 83 The relation of this equation to the old steam-engine formula based on mean effective pressure will be obvious upon analysis. The mean effect- ive pressure per square foot is equal to the foot-pounds per cycle divided by the piston displacement, so that the mean effective per square inch is, theoretically, , _ Ky Ke Hee The piston displacement in cubic feet is equal to piston area X stroke, in foot measure, so that LA=144 V,, and it will be found that if these equivalents be substituted in the steam- engine formula pm LA Ne 33,000 the result will be the gas engine equation above. In all of the equations for pressures, temperatures and work obtained after ignition is accomplished, the factor K, necessarily appears, because it includes all of the elemental factors which affect the rise of pressure and temperature above the compression point. Referring again to equation (g), it will be found that Ky, is the product of oe — 1, the mixture heat v value ea? and the “utilization” factor u, all multiplied by 778 to reduce heat to foot-pounds. The values of Cp and C, in this case are those of the entire cylinder contents, consisting of fresh mixture and burned gases remaining in the clearance from the previous explosion. The calcu- lation of these values is very tedious and, moreover, it is difficult to deter- mine the data required to make the computation. The calculation of the ‘heat value (h) of the gas is also tedious and the proper value to assign to a (cubic feet of air per cubic foot of gas) is a matter largely of guess- work. As a matter of fact, the ratio of air to gas actually obtained in an engine cylinder is almost impossible of accurate determination; the author has never seen a set of test records which gave any evidence that the air ratio had been correctly ascertained. =LH.P., Practicat Output EstiMATION In view of the foregoing difficulties, the most practical method of esti- mating engine output is either to base the estimate on the rise of pressure which experience has shown can be obtained reliably with the given fuel and operating conditions (compression pressure, cooling water heat waste, shape of combustion chamber, timing of ignition, etc.), substituting Ps — Pe aa 84 THE GAS ENGINE h la which experience has shown to be readily obtainable. In order to facilitate the application of the former of the two methods, Tables 3-A to 3-D have been prepared, the values therein given being based on a large number of indicator diagram analyses. In selecting these values the average values shown by indicator diagrams were not taken, because in rating a gas or oil engine the benefit of all doubt must be accorded to the engine. The values in the table, therefore, will be found somewhat lower than one should be able to obtain under average conditions. In order to apply the second method of estimating, Tables 8, 9 and 10 are provided. From Table 8 one can readily compute the properties of any fuel mixture if the analysis of the fuel is known. In making such computations the results will more nearly agree with practice if the quan- tity of air per unit of gas be taken at 20 per cent. above the theoretical requirement. for oil fuels; 30 per cent. for natural gas; 40 per cent. for less rich gases, such as coke-oven gas, oil-water gas, and ordinary illumi- nating gas; and 50 per cent. for producer and blast-furnace gases. These percentages are not all in accord with theoretical reasoning, but they seem to be justified by practical experience. Table 9 has been computed on this basis, using some representative fuel analyses. ‘The heat values in this table and Table 8 will seem low to anyone accustomed to heat values based on 82° F., or 0° C.; the explanation will be found in the subheadings of the tables—the figures are for gases at 700° absolute temperature (240° F.) and 13} lbs. per square inch absolute pressure. The temperature and pressure have been selected as coming nearest to average conditions. In a well-cooled cylinder, with a cooled piston and cooled exhaust valves, the initial mixture temperature may readily be as low as 630° or 640° absolute, but in a cylinder moderately well cooled and having solid valves, the tem- perature may easily be 800°, especially if the inlet and exhaust ports are near each other. The next stumbling-block in estimating the power of an engine is the fact that, even if the actual values of all of the other factors were known beforehand, the use of the factor K, would rarely give correct results. The explanation of this is simple: the compression and expansion curves do not follow strictly the exponential law which is applied to them for lack of something better. Even when the ratio of compression to precompression pressures agrees with equation (c) and the ratio of explosion to release pressures conforms to equation (j), the indicator diagram will usually be -from ten to forty per cent. larger in area than called for by either equation (m) or equation (n). This is because the compression curve is slightly more concave and the expansion curve very much less concave than are the corresponding curves plotted by the formulas. The discrepancy between for Ky in the equations, or else to assume values for k, uw and PRESSURE, TEMPERATURE, Table 8& Properties of Gas Engine Fuel Mixture Constituents Ong Cusic Foot, at 700 DeGREES TEMPERATURE AND 1,944 Pounps PRESSURE. Note:—1,944 Ibs. per sq. ft.=13 34 lbs. per sq. in. B.t un fo rang tp: Cable feet oan 7 one degree Fahr. of air uti- Constituent Weight B. t. u. Per cia lized in Constant | Constant | combus- pressure volume tion Hydrogen........ 0.003602 186.62 | 0.0122807] 0.0086901 2.41 Methane..........| 0.02874 620.55 | 0.0170393} 0.0134584 9.61 Ethylene......... 0.05034 998.45 0.0203373| 0.0087088 14.42 Carb. oxide....... 0.05032 221.18 | 0.0124742| 0.0088462 2.40 Carb. dioxide..... 0.07907 |.......... 0.0171582) 0.0121373 cteseits Nitrogen......... O.05047 = |aswiaaiesa ce 0.0123063| 0.0087174) ..... MYBO sc sc ewes 0.05750 |.......... 0.0125079} 0.0089194; ..... Ain. adage a oecdo 38 0.05204 |.......... 0.0123589| 0.0087891) ..... AND OUTPUT CALCULATIONS —§ 85 Table 9. Properties of Some Representative Fuels and Mixtures At 700 DreGREES ABSOLUTE TEMPERATURE AND 1,944 PounDs ABSOLUTE PRESSURE. Note:—1,944 Ibs. per sq. ft. = 1314 lbs. per sq. in. Air Re- : quired for Practical Mixtures Combustion B.t.u. = Fuel per 2] a cu. ft'Theo-|Prac—| B.t.u. n Tal wa Tetic-| tic- |per cu.|CP_ (k-1) h ee | ele ally | ft. |Cv Ita | wit] wlS Ss Co) : Vols. |Vols. : Gasolene vapor...... 750 |14 17 41.7 |1. 4050/16. 8885] 13,139]0 .3982 Natural gas......... 600 j12 /14 40.0 |1.3980/15.9200| 12,386/0.3753 Oil water gas........ 353.7) b.2 | & 39.3 |1.4060/15 .9558| 12,414/0.3762 Dowson producer gas. 96.7| 1.09] 1.6 | 37.2 |1.4098/15. 2445) 11,860/0.3954 Mond 93.7] 1.12] 1.7 | 34.7 |1.4085/14.1750) 11,028)/0 .3342 Taylor - « | 91.6] 1.08] 1.6 | 35.2 |1.4083/14.3722] 11,182/0.3388 Avge. suc. “ “ | 90.6] 1.03] 1.6 | 35.0 |1.4095]14.3325) 11,151/0.3379 Blast furnace « | 62.4] 0.68] 1.0 | 31.2 |1.4095)12.7760| 9,940/0.3012 ANALYSES OF THE ABOVE FUELS BY VOLUME. Gaso- Dow- Avg. | Blast Constituent leneby| Natu-| Oil son | Mond |Taylor| suc. |F’rn’ce weight| ral gas} gas gas gas gas gas gas Carbon.........-- B5 Bl canal ate eciake das | sane es heatel ees s nls cet ets Hydrogen.. 1444%| 2.2% ee ae vik 330 22 .0%]17.0%|14.0%| 2.6% Methane........../..-.+. 9 |3 5%) 2.4%) 2.5%| 0.8%]...... Ethylene.........-|---++- 90.4% : Bel la aitsca- cll ee engine | ees last ol allsheysie- site nae tar Carb. oxid@. ¢ v120<).464 8% 0.5%| 4.2%|25. 6% 16 .0%|19 .0%|26 .0%|26 .0% Carb. dioxide... ...|...1.: 0.3%| 1.4% 112 :0%| 8.4%| 5.0%| 9.4% Nitrogetie....ccsccli acess 3.7%| 5.2% )4 0%147. 0% 52. 5% 54.0% 62.0% Oxygen........-.- [e000 0.3%| 0.2% 8: 4%| 0.6%) 0.6%| 0.2%| 0.0% Table 10. Average Practical Values of K, ve h Kg = 778 Tta u (k-1) Fuel: Gasolene Kerosene Avg. Kg: 9300 - 8100 Nat’] Gas__ Ill’g Gas 7xh’ 12xh’ Nors.—h’ = B.t.u. per cubic foot of gas at 32° Fahr. Prod. Gas 40 x h’ Blast Gas 50 x h’ 86 THE GAS ENGINE the calculated and the actual curves is due chiefly to the use of formulas based on the assumption that heat is added and withdrawn in a regular manner during expansion and compression, respectively, whereas the addi- tion and withdrawal are far from regular in most cases. The heat loss during the first two thirds of the compression stroke is more rapid, rela- tively, than during the remaining third, because of the constantly decreas- ing wall surface through which the heat escapes to the jacket water. The “heat interchange during the expansion stroke is more complex. In most cases, the curve starts down about with the calculated curve; a little later it is flattened by an access of heat either from the walls or by after-burning i Compression Ratio 6. \ \ Compression Exponent 1.32 Ww K, 1.3637 Ny Initial Pressure, p,, (i. * Compression Pressure, p,, 149. ‘ Explosion “ Py, 4i1. % Release se Pe, 43. \ Expansion Ratio Te, 5.675 \ Expansion Exponent Ne, 13 % M.E.P., Actual 103. x M.E.P., Computed 15 *. M.E.P., Plotted 72 %s Kg for Actual Diagram = 1.96 Sy TRL “ = La FIG. 40.— ILLUSTRATING DISCREPANCIES BETWEEN ACTUAL AND THEORETICAL INDICATOR DIAGRAMS. of the gases. No matter what the cause is, there is obviously an irregular accession of heat which raises the curve after expansion begins. These discrepancies are illustrated in Fig. 40, in which the solid curves are reproduced from those traced by an indicator and the dotted curves were plotted by the formulas (c) and (j), using the pressures shown by the actual indicator diagram and the exponents correspond- ing to the pressure ratios. Another discrepancy lies in the fact that the “peaks ” of nearly all normal indicator diagrams are considerably rounded, whereas the power and work formulas are based on the assumption of a sharp point. If the “peak” representing the maximum explosion pressure were carried up, in the calculated diagram, to the point that it would reach with instantaneous but not adiabatic combustion, and the expansion line were started from that point, the area of the plotted diagram would doubt- less be nearer to equality with that of the actual indicator diagram, and equations containing or depending on the factor K, would approximate more uniformly to accuracy. But it is impossible to predict just where PRESSURE, TEMPERATURE, AND OUTPUT CALCULATIONS 87 the peak would go with instantaneous combustion, because neither the lib- eration nor the loss of heat during the explosion can be estimated with any confidence. In view of these discrepancies between the formulas and the actual per- formance, it is more practical to substitute a factor for K, in equation (n) which will vary generally with the compression ratio and yet allow for the absence of regularity in the compression and expansion curves and the rounding of the maximum-pressure point. Representing the substitute factor by Avg, the formula for mean effective pressure becomes hy Kk Sa ? (p) and this may also be written : i Ba IAG. pom Fe (Pea. ow The indicated horse-power formula, based on mean effective pressure, is 1th py Te ag000 LEP, (q) and if one prefers to go direct from foot-pounds per cycle to horse power, Ky Ka Vs Nz — oY “95000 (q2? It now remains to deduce a rational value for the empirical factor Ka which is supposed to include A, and also to make allowance for the dis- crepancies just discussed. From an analysis of nearly one hundred indi- cator diagrams, taken under widely varying conditions and from all classes of engines, the author has come to the conclusion that no factor can be worked out that will give calculated results corresponding to even the aver- age results obtained in practice under widely different conditions. The value of 7. —1 comes nearer the general average than any other factor that has been tried, and in many cases it has given surprisingly close results. Tables 11 and 12 will serve to illustrate the differences between actual and estimated engine performances, the estimated results being based on Tables 3-A and 3-C, Table 10, equation (p) and the use of r,"—1 for the value of Ka The data in Table 11 were obtained by tests and measure- ments of the various engines and fuels. No measurements were made of the proportion of air to fuel, the mixture in all cases being adjusted until the best results were obtained in the engine cylinder. Engine No. 5 was a tandem single-acting, No. 6 was a twin-cylinder single-acting, and No. % was one of the mammoth twin tandem double-acting engines in California, described in Power of January 14, 1908. All of the others were simple single-acting engines. 88 THE GAS ENGINE Table 11. Seven Test Records HO : ed om Test Bore gq Ay B 8 No. | and Stroke | &| Nx | ro | po | px | pm re} Fuel Ba pa 4 ae 1 54x 270/133 3.8 |75 250 89.5 6.44,Gasolene| ... 2 8.27 x16.51/218| 98.9*/6.4 |149 (341 56.5 | 15.85)/Prod. gas| 127 3 844 x1 200} 92* |4.75/113 [287 76.9 | 13.50) City gas | 585 4 9x15 202} 88.9*/5.5 |120 433 |107.35) 23.00 a 650 5 934x17}4 |180] 90 4.3 | 86.71362.7| 87.4 | 25.95 je 600 6 10x19 190} 95 4.3 | 85.3/312.3] 86.6 | 31.00 e 660 7 |42x60 88) 44 4.6 | 86 6 83.1 |767.50 se 625 * Hit-and-miss regulation. t Power developed in one end of a cylinder in double-acting engines. Table 12. Comparisons of Test Results with Estimates ea ss Compress’n Bee Pressurerise| 144 (px—po) 144 pm exponent Ks Ka Test Ps—po —_—_—_——- —- Do No. ro—1 Ke 144 Test | Est. | Test | Est Test | Est.*] Test | Est. Test Est. 1 175 | 182 9103 | 9300 | 1.432/1.493) 1.32} 1.30) 89.5 96.42 2 192 | 205 5120 | 5080 | 1.589/1.745] 1.28) 1.30} 56.5 61.56 3 174 | 183 6682 | 6780 | 1.657/1 .C98] 1.34) 1.34) 76.9 79.95 4 313 | 236 | 10016 | 7800 | 1.548]/1.668| 1.30) 1.30] 107.35 | 90.35 5 276 | 148 | 12043 | 7200 | 1.045]1.549] 1.29) 1.30] 87. 77.45 6 227 | 163 9905 | 7920 | 1.259/1.549} 1.28] 1.30} 86.6 85.20 7 17¢ | 169 6800 | 7500 | 1.760/1.630] 1.30) 1.32) 83.1 84.90 * Assumed to be equal to rn. Table 13. Horse-power Constants For Dirrerent Mean Errecrive PRESSURES M.E.P. H. P. Constant M.E.P. H. P. Constant M.E.P H. P. Constant ~ 50° 0.2182 70 0.3055 90 0.3927 51 0.2225 71 0.3098 91 0.3971 52 0.2269 72 0.3142 92 0.4015 53 0.2313 73 0.3185 93 0.4058 54 0.2356 74 0.3229 94 0.4102 55 0.2400 75 0.3273 95 0.4145 56 0.2444 76 0.3316 96 0.4189 57 0.2487 77 0.3360 97 0.4233 58 0.2531 78 0.3404 98 0.4276 59 0.2575 79 0.3447 99 0.4320 60 0.2618 80 - 0.3491 100. 0.4364 61 0.2662 81 0.3535 101 0.4407 62 0.2705 82 0.3578 102 0.4451 63 0.2749 83 0.3622 103 ° 0.4495 64 0.2793 84 0.3665 104 0.4538 65 0.2836 85 0.3709 105 0.4582 66 0.2880 86 0.3753 106 0.4625 67 0.2924 87 0.3796 107 0.4669 68 0.2967 88 0.3840 108 0.4713 69 0.38011 89 0.3884 110 0.4800 Constant < Piston Displacement in Cu. Ft. x Explosions per minute = I.H.P, Table 14-A. Piston Displacement, in Cubic Feet, or parts of a Cubic Foot, Table 15. Values of ra per Stroke o r 1.2 1.21 1.22] - 1.23 1.24) pp 1.25] - 1.26] - 1.2 38 “ ° . . 2. . . . +27 1.28) - 1.29) - 1.3 L31 1.32] 1.33) » 1.34 1.35 1.36] ge 1.37] go 1.38) ge 1.39 1.4 “e LENGTH OF PISTON STROKE HE : 7 fi ‘ i . : : , = : ' r c r ™ oOo. a 3.00] 3. 3. 3.820] 3. 3. 3.948} 3.992] 4.036) 4.081] 4.12 - ’ 26 356 7 55 -605 -65 Ee aig tetas aed ag oes bee J E 3.05 3.812) 3.855] 3.898) 3.942) 3. 4-031) 4.076) 4.122) 4.168 4215 4363 1510 4/358] 40407] 4c4g0] 4 soal dee] 4 i713] 4.705 a ine Va 4 9 48 ; . re: : . é 3 3 3. ‘ 3.97 i . : -208] 4.255] 4.: .353 -402] 4.452] 4. .55 . 606] 4.659 .Si¢ 87s a | 24 1nech | 4 inch | 34 inch| 1 inch [2 inches|3 inches|4 inches|5 inches|6 inches|7 inches|8 inches|9 inches 3.15 a. 4. 4.086 4. 4. 4.19? 4.245 4 204 4.344 4.304 4.444 400 Fei i 1658 4 tor 4761 i 038 183 | . if a 3(25| 4.114] 4/163] 4.212) 4/262 4.313] 4/364] 4.416] 4.408] 4.521] 4l574/ 4.629] 4.683] accaol 4 “$53 4csiol 4.968] & BOT] B oo8 3, 0.00102 0.00205 0.00307'0.004109)0 .00818'0.01227 0.01636/0.02045 0.024540 028630 .0327210.03682 3.80] 4.190] 4.240] 4/30] 41343] 4305] 47448) 4.501] 4953] 4.610] 4.003] 4/731] 4778] 4c8as] Acaod| dcasal 8.012] 8.083] 8 Bipsi] 8.320 376/0.00111 0 .00222\) 00333 0 .00444)0 .0US88/0 .01332)/0 .01775/0.02219|0 .02663]0.03107/0.03551|0.03995 3.35] 4. 4. 4.371) 4. a: 4.532) 4.087) 4.643] 4.700) 4.757) 4.815) 4.873) 4.982] 4.992) 5.053) 5.115 5.177] 5 5.868) 5.433 34/0 .00120,0.00240]0 00360 0 .00480]0 . 009600 .01440|0.01920|0.02400]0 .02880]0 .03361/0 038410 04321 54 a aon| a azel 4 Ae0l deel a eadl C' Gna) a onal a eaol a eeOh Eo caa] aoeeel A soe) 2700) S082] B.Lb4) 6,218) 828u) 6 Beg 90) 0 beF 35410 .00129 0..00259|0.00388 0.00513|0.0103510..01553'0.02071|0. 0258910 .03106|0 0362410 .04142)0 04659 3/60] 4.497] 4.558] 4 611] 4.600] 4.798] 4.787] 4 sds] 4909] 4971] B.038| S097] Ecker] Bidse] Cdosl Brace] Bessel Bass) 2 Boa] Bore7 37/0 .00139 0 .00278|0 00418 0.00557 0.01114]0.01670)0.02227|/0.02784/0.03341/0.03897]0.0445410.05011 3.55] 4. 4. 4.691] 4. 4. 4.873] 4.935] 4-998] 5.062] 5.126] 5.192] 5.258] 5 5: 5461 5. 5.602 5 5.819 5 S808 35% 0.00149 0.002990 00448 0. 00597|0 .01195|0 .01792]0 .02389|0.02986|0 0358410 .04181|0 .04778|0 05376 SOel ac yse| 402901 4.863] dial 4coROl B’Gael Sati Sagal 2 oeeel Sorel of ceal Seeeel Genet] B-a04| 0.305] 3.636) 0.700) Borel cise 3340 .00160/0 .00320)0 .00479|0.00639|0 .01273|0.0191810 .02557|0.03196/0.0383510.04474|0.05113)0 05753 3 70] 4.807] 41870] 4.934) 499] 5.005] 5.142] 8.199] 8.208] Saaz] S407] Bossa] Sasi] S.esa] Scoes| S.eea] Sokal Qo sbal ec 6.103] 6 244 rgd ate ee erat nares MOE ete 0.02730|0.03412]0.04095]0.04777|0.05460|0 .06142 3.75) 4-885) 4-950) 5.016) 5-082] 5.190] 5.218] 5.288] 5.358) 5-430) 5.502 5.575 5.649] 5-724] 5-800] 5/878} 5.950) 6.035] 6. 6.279] 6.363 ’ 32/0. 36 ; 5) .00727/0.0145 0.02182/0.02909|0 .03636/0. aQR6 : “05 31006545 3. 4. 5. 5. ae 5. 5: 5.2 6 5. 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B15 1714]0 .03480/0 .06960 0. 104.10'0.. 13920 0..27839]0 4175/0. 55678|0 .69598|0 .83517/0 974371 .11356)1 .25276 i ; tga ene eee eae Petes 45 173, {0 .03580|0.07160 0.10740 0. 14320 0 .28640/0..42960]0 . 57280|0.71600]0 .85920/1 .00240]1 .14560]1 . 28880 je See elae pee Nee eerily eae Elles 18 |0.03682 Dae 11045/0. 14726 0. 29453|0.44179]0. 58905|0.73631 0.883581 .03084|1 .17810}1 .32536 Lio ete _! Mes ipa ee eal Table 14-E. Piston Displacement, in Cubic Feet, or parts of a Cubic Foot, per Stroke Table 16. Values of rn—« ———— === a ai = CREE a LENGTH OF PISTON STROKE 2 3 : Ni F PISTON STROKE zs 19-2 |p 0.21) £0.22) 0.23) 0.24) 0.25) 70.26 0.27/ 0.28, 0.29) 0.3 | 0.31] - 0.32] £0.33) 7 0.34] £0 35| 0 36] 0.37] 0.38) 0.30 ro.4 = ml sin. |2 inches| 215 in. Ei re 3 Join. LE inches] 414 in. |5 inches} 51% in. ls a a inches/8 inches|9 inches 13.00] FE: 24574 25054... 27844 S875 30471. 84614 B80BIL 8453H B60 S765 560411 ane7l1 491911 agcnia anoala areal, acrcila enarvel, comll. cello SEE EP IPR Seg I Eh Re Te SE) Rg SPS LS SD: oP ST ER OEE Lp Oe Se ar ee ee, Fe Os TE ona ae eres nore ee ay tee ene a ee aaa Sees. Cn ea ee ae Ses Sats ie wu ee Soe scue ly ka oe (eee ren Urs OUE O90 Lh Vols ( (Leek IO0 Us 20210 1 §00|12.126|12 -380|12 1641/12. 906 1454/13. '026|14 320/14 .621|14.928]15 242/15 562/15 .889|16 223/16 .564]16 91 2]17. 268/17 .631|18 001/18 .379 | 1734/0 .03580|0 0716010. 10740 0. 14320|0 .28640|0 .42960|0 . 57280|0.71600|0 .85920/1 .00240]1 . 145601 28880 B00) Lee 12012880 [tee Cdl E2200 | 1S L813. 454/138..037/ 142 026/14 | pee |iS 62/15,.8 223]16 564 16 -912]17. 268/17 .631]18 .001|18.379 | | 18 |0.03682 eee en Ree 0.44179/0.58905/0.73631/0.88358)1 .03084]1 .17810]1 .32536 Table 14-E. Piston Displacement, in Cubic Feet, or parts of a Cubic Foot, per Stroke Table 16. 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‘ : : 2 4.240 7 85/1 .5100/1 .5414/1 5735/1 .6063/1 .6397|1 .6739)1 .7087/1 .7443/1 .7806)1 8177/1 .8555/1 .8941)1 .9336]1 .9738|2 .0149]2 .0569|2 .0997|2.1434|2 18802 2336/2 2801 so |1.5822 [2.3783 |s. 1643 [8.0554 |4.7405 [5.5976 |0.3287 (7.1198 (7 9100 [8.7020 |9.48a1 111.075912.G57 [14-208 | [A Mole SUL Sas0i-BEEE ESOS -Gtzet-Graah TUS ZAZA Tage -SBLOh SSM Sarat Bar ohSrelz-O1saz- 14210421 aeal 19s S382 asp 60 1.6362 |2.4543 [3.2725 [4.0906 |4.9087 |5.7268 /6.5449 |7.3630 |8.1811 /8.9992 /9.8174 /11. : . 8.00|1 5157/1 .5476)1 .5801|1 .6133/1 .6472)1 6818/1 .7171|1 .7532|1.7901|1 8277 1'8661]1 {9053|1 |9453]1 /9862|2.0279|2.0705|2 114012. 1585|2.2038|2.2501 | 2974 PRESSURE, TEMPERATURE, AND OUTPUT CALCULATIONS 89 It will be noticed by reference to Table 12 that the empirical values of Ka average up fairly close to the actual values derived from the tests, but the comparisons in Tests 2, 5, and 6 indicate the futility of trying to derive a factor that will approximate accuracy in every case. When one has a series of indicator diagrams the indicated horse power is, of course, readily computed by means of equation (q). In order to facilitate this computation, Table 13 is provided. The “ horse-power: con- stants” are merely the mean effective pressures multiplied by 144 and divided by 33,000. Having the mean effective pressure from the diagram, the corresponding constant is taken from the table, either direct or by extrapolation. Thus, if the mean effective pressure were 76.7 lbs., the factor would be 0.3316 + 0.7 of the difference between this and 0.3360, or 0.3316 + 0.00308 = 0.33468. | Tables 14-A and 14-B give piston displacements in cubic feet for diam- eters of 3 to 60 ins. and stroke lengths of 4 in. to 9 ins. Tables 15 and 16 have been prepared for the convenience of those dealing with engines which depart from the usual pressure and tempera- ture ratios so far as to escape the application of Tables 1, 2, 4, and 5. Table 16 also supplies values of Ag for making rough estimates by means of formulas (p), (p2), and (q2). EFFICIENCY The indicated conversion efficiency is the net work done on the piston per cycle divided by the work equivalent of the heat liberated ae com- bustion per cycle. Expressed as a formula: Ind. ft.-lbs. per cycle 778 < B.t.u. of combustion = indicated efficiency. The brake conversion efficiency is Ft.-lbs. per cycle at shaft ft _ _ wake efficiency. 778 X B.t.u. of combustion rake efficiency The “B.t.u. of combustion” is equal to the number of heat units per cubic foot or pound of fuel, multiplied by the number of cubic feet or pounds of fuel per minute or hour taken in by the engine and divided by the number of explosions per minute or hour. That is: h"X