ELEMENTS OF AVIATION ENGINES ELEMENTS OF AVIATION ENGINES By JOHN B. F. BACON, PH. B. Instructor, Engines Department U. S. School of Military Aeronautics Berkeley, California PAUL ELDER AND COMPANY SAN FRANCISCO M CM XVIII d COPYRIGHT, 1918, BY JOHN B. F. BACON BERKELEY, CAL. ^503535 SEP g-ni)*) CONTENTS PAGE Introduction VII CHAPTER I The Aviation Engine 3 CHAPTER II Application of the Basic Principle .... 7 CHAPTER III Engine Specifications 16 CHAPTER IV Engine Parts 22 CHAPTER V Carburetion 47 CHAPTER VI Ignition 57 CHAPTER VII Lubrication 71 CHAPTER VIII Cooling 80 CHAPTER IX Rotary Engines 84 CHAPTER X The Liberty Motor 96 Index 105 III ILLUSTRATIONS facing page Thrust Bearings [36 Diagram to Illustrate the Curtiss Ox Valve Action 42 The Miller Aviation Carburetor . . .50 A Half Section View of a Zenith Carburetor . 52 l Diagrams to Illustrate the Location of the Core in a Shuttle Type Magneto 58 Wiring Diagram of a Magneto System . . 62 - Diagram to Illustrate the Principle of Revolv- ing Poles on the Dixie Magneto .... 64 Diagram to Illustrate Position of Rotor in the Dixie Magneto when the Core is Magnetized 66 ' Diagram to Illustrate Position of Rotor in the Dixie Magneto when the Core is Demagne- tized .66 Diagram of a Battery System of Ignition with a Non Vibrating Coil 68 Gear Pump 76 Diagram to Illustrate the Operation of a Vane Pump .... 76 Centrifugal Pump 82 Diagram to Illustrate the Principle of a Rotary Engine 84 1/ [V INTRODUCTION Having been forcibly' impressed with the fact that many of those who take up the study of aviation are not familiar with gasoline engines and have little mechanical inclination, it has been the endeavor of the writer to explain in a simple way some of the points that appear to cause beginners the greatest amount of trouble. While it may aid those who are conscientiously reviewing the subject, it is far from the purpose of this book to provide a short cut to passing marks on examination papers. All of the information herein contained has been before the engineering public at one time or another. Realizing that certain new develop- ments must not appear in print during this critical period every precaution has been taken to observe strict avoidance of revealing confiden- tial information. The writer wishes to express his gratitude to the members of the Engines Department in the S. M. A. of Berkeley for their assistance. Special thanks is due Mr. James Irvine for his sugges- tions which have resulted in many improvements. JOHN B.F.BACON, 818th Aero Depot Squadron, U. S. A. Berkeley, Cal., August, 1918. [VII] ELEMENTS OF AVIATION ENGINES ELEMENTS OF AVIATION ENGINES CHAPTER I THE AVIATION ENGINE In taking up a new subject it is often best to fix clearly in mind just what is meant by the name of the subject, so in beginning a dis- cussion upon aviation engines it seems well to start with a rough definition of the term avia- tion engine. A simple statement that an in- ternal combustion engine so designed that it is capable of lifting from the ground and sustain- ing in flight a heavier than air flying machine will suffice as a definition for our subject. By the term internal combustion engine is com- monly meant simply a gasoline engine, because in such an engine the power is derived from the force of an explosion within a cylinder. This will make clear what we mean by our subject. The question at once arises : Why must avia- tion engines be internal combustion engines in- stead of steam engines, and why not propel aeroplanes by aid of electricity? The answer is simply that maximum power and minimum [3] ELEMENTS OF AVIATION ENGINES weight can be best obtained with the internal combustion engine. In the study of aeronau- tics weight is a tremendous factor, and it is in- teresting to note that not until the gasoline engine had reached its modern development was human flight practical. On account of the unlimited use of gasoline as a motive power and the increasing interest of technical men in the problems of aviation, the gasoline engine has been developed to such a point that it may deliver 1 H.P. for every 1.8 pounds of its weight. To a mechanical mind this seems one of the greatest achievements of the twentieth century. Since gasoline engines have been used so ex- tensively and with such marked success in automobiles, the aviation student will at once involuntarily compare the aviation engine with that in an automobile, and oftentimes he com- pares them wrongly by stating that the avia- tion engine develops a vastly greater speed than the engine of an automobile is capable of attaining. This is incorrect and is a poor way of comparing the two. The main difference is that of lightness. Aviation engines are of the lightest possible construction and are designed to run continuously at their highest speed. [4] THE AVIATION ENGINE Seldom are the frail supporting members for the engines in a horizontal plane, and often the engine is called upon to do its work while com- pletely inverted. These are conditions that the automobile engine does not have to meet. In order to attain a construction that will fulfill the requirements imposed upon aviation en- gines, it is natural to expect that some sacri- fice must be made. This accounts for their low degree of durability. When we examine the heavy construction of a 400 H.P. marine gaso- line engine and then regard the frail parts of a 400 H.P. aviation engine there is not the slightest doubt which engine will continue longer in its operations. However, since light construction is an absolute necessity, it is use- less to expect much in the way of durability, and as a means of knowing what an aviation engine will stand it is interesting to note that after every seventy-five hours of operation the engine should be rebuilt. As a compact and light power plant the avia- tion engine is the highest attainment of me- chanical genius. It has been developed from the type that propels the automobiles, and just as the old types of automobile engines do not resemble in appearance the types used to- [5] ELEMENTS OF AVIATION ENGINES day, so the first aviation engines have little resemblance to those of the present time. The development has been rapid, and it is difficult to predict what will be the effect upon aviation if the rapid strides taken during the past ten years continue to add to the efficiency and re- liability of the aviation engine during the next ten years to come. [6] CHAPTER II APPLICATION OF THE BASIC PRINCIPLE The working principle of an aviation en- gine is identically the same as that of the ordinary gasoline engine. In the middle of the nineteenth century it was satisfactorily proven that the explosive force of gasoline could be used to actuate a piston, and this has given rise to the adoption of a new form of motive power. Since that time gasoline engines have been developed along two lines, one being called the two-stroke cycle engine, and the other the four-stroke cycle engine, but since the former has not been used extensively in aviation work little attention will be given to it here. A two-stroke cycle engine is one in which an explosion takes place in the cylinder every time the crank shaft makes one revolution. A charge of combustible gas is slightly com- pressed within the crank case by the piston traveling downward. Near the bottom of this downward stroke the piston uncovers a port in the cylinder wall allowing some of the com- pressed gas to enter the cylinder. Then the [7] ELEMENTS OF AVIATION ENGINES piston moves upward, closing the port and compressing the gas. The charge is ignited when the piston is near the end of its upward stroke, and the result is that the force of the explosion violently drives the piston down- ward. An exhaust port on the opposite side of the cylinder from the intake port is uncovered as the piston sweeps downward, and the force of the explosion starts the burnt gas rushing out of the cylinder. The intake port having also been uncovered by this time will allow a fresh charge to enter. By using a deflector on the piston head the fresh charge is hindered from rushing straight to the exhaust port and is diverted upward, serving admirably to expel the remaining burnt gases. Now the piston is ready to go upward again, and the same opera- tions are repeated. In this way the piston makes two strokes to complete a cycle, hence it is spoken of as the two-stroke cycle engine. Some confusion may be caused by not knowing the exact meaning of the word cycle, so it may be well to insert here a definition. A complete series of events occurring in regular sequence and ending so that the same opera- tion can be repeated in the same order is called a cycle. [8] APPLICATION OF THE BASIC PRINCIPLE The four-stroke cycle engine has proven the more satisfactory of the two types, and since it is the one used in connection with aviation, it is very desirable to fully understand it. This type differs from the two-stroke cycle in that it has two distinct mechanically-operated valves in the cylinder which, of course, necessi- tate a few more working parts. Instead of the gas being stored and compressed within the crank case, this engine draws its explosive charge directly from the carburetor by opening the inlet valve as the piston goes downward and making use of the suction thus exerted. The charge is compressed by the reversal of the piston's motion and the closing of the inlet valve. Near the end of this compression stroke the charge is ignited, resulting in an explosive force being exerted on the piston when it is ready to go downward again. Near the end of this succeeding downward stroke the exhaust valve is opened permitting the force of the ex- plosion to give the burnt gases their initial outward impulse. The valve remains open during the entire upward stroke of the piston to insure all of the burnt gases being expelled. The clearing out of the cylinder is often re- ferred to as scavaging the cylinder. Generally [91 ELEMENTS OF AVIATION ENGINES the exhaust valve closes after the piston has reached its uppermost position. This brings us to the opening of the inlet valve and with that the sequence of events is repeated. By the stroke of the piston is meant the movement of the piston in one direction. It follows from this that the length of the stroke is the linear distance the piston travels from its uppermost position to its lowest position or vice versa. The term stroke has come to mean simply the number of inches between top cen- ter and bottom center, thus designating the two extreme positions of the piston. To make clear the four strokes of the piston in a four- stroke cycle engine, the first one in which the piston goes down and draws in a charge is called the intake stroke. The next upward motion is the compression stroke. Then comes the explosion which drives the piston down- ward. This is the power stroke. Finally the expulsion of the burnt gases is the exhaust stroke, and this completes the cycle. In aviation engines it is customary to ignite the charge near the end of the compression stroke instead of at the beginning of the power stroke. The speed of the engine justifies this. If ignition were to take place when the piston [10] APPLICATION OF THE BASIC PRINCIPLE was at top center or a little afterward, the force of the explosion would be exerted upon the piston head at such a late time that the piston could not deliver its maximum impulse to the crank shaft. When the piston is nearing bottom center its effectiveness for transmitting force is negligible. Consequently by opening the exhaust valve at the end of the power stroke instead of at the beginning of the ex- haust stroke, the force of the explosion serves to start the burnt gases rushing outward without losing power. The exhaust valve is generally held open until the beginning of the intake stroke. This aids in scavaging the cylinder as it permits more time for the operation, and the danger of retaining some of the burnt gases is avoided since the out-going exhaust will possess a certain amount of inertia. Different makes of engines have different times for open- ing the intake valve. On some there is a small interval between the closing of the exhaust valve and the opening of the inlet valve, as is the case with the Curtiss OX and the Hall-Scott. This permits the downward motion of the piston to establish somewhat of a rarefication within the cylinder, so that when the inlet valve is opened there will be a ten- [111 ELEMENTS OF AVIATION ENGINES dency for the gas to enter more promptly. The closing of the inlet valve occurs at the be- ginning of the compression stroke. The gas passing through the manifold will have some inertia which will maintain a flow into the cylinder during the first part of ensuing up- ward stroke. By thus keeping the valve open past bottom center a larger amount of gas is placed in the cylinder. The question often arises : Why are not two- stroke cycle engines used for aviation work, on account of the decrease in weight due to the less number of working parts, the more fre- quent power impulses, and the need of an engine that will do its best work when running at top speed? The two-stroke cycle engine ful- fills all the requirements demanded of an aviation engine except for the fact it will not ordinarily run satisfactorily at low enough speeds to allow the propeller to idle. Since a successful aviation engine must be able to run slow enough without stopping to allow the plane to glide, it can be easily seen that the present form of two-stroke cycle engine is poorly suited for aviation work. So far in explaining the different operations involved in a cycle, only one cylinder has been U2 1 APPLICATION OF THE BASIC PRINCIPLE considered. It is advisable to have frequent power impulses and to avoid vibration as much as possible. This is accomplished by using a number of cylinders which decreases the weight of the reciprocating parts. Vibration is due to the shifting of the cen- ters of gravity of pistons and connecting rods. In a single cylinder engine of required power turning at a speed suitable to drive a propeller, the amount of vibration would be prohibitive. The greatest bearing pressure in an engine at high speeds comes not so much from the ex- plosion, but from the effort of starting and stopping the weight of the piston and connect- ing rod. To decrease this reciprocating weight it is necessary to resort to the basic law of volumes and areas. If we make a body half the dimensions of another, it will have but one quarter of the area and only one-eighth of the weight. This can be applied to pistons. Thus a piston can be replaced by four smaller ones half as large, and the area of the four will equal that of the larger one. However, these four pistons will weigh practically one-half as much as the original single piston. This illustrates the way reciprocating weight is lessened and shows plain- ly the demand for a larger number of cylinders. [13] ELEMENTS OF AVIATION ENGINES The way the cylinders are arranged serves as a means of classifying aviation engines. If the cylinders stay in a fixed position in respect to the crank shaft, it is spoken of as a fixed cylin- der engine, but if the cylinders revolve about the crank shaft it is called a rotary engine. Various difficulties in construction are encoun- tered when the number of cylinders is increased, so fixed-cylinder engines are not confined to the vertical style but are often built in a V form to permit a shorter crank shaft. A pecu- liar style of fixed-cylinder engine is that with an additional row of cylinders between the two rows that go to make up the V. This is the design of the Sunbeam Engine. Another style of fixed-cylinder engine is one in which the cylinders radiate from the crank case allowing the force of all explosions to be exerted upon the same crank pin. The Anzani engine is of this design. The rotary engines have not so many variations. As a means of increasing the number of cylinders a second bank of cylinders is often added, which of course necessitates two throws on the crank shaft. Rotary engines are limited to those having one and two banks. In both the fixed-cylinder and the rotary types the growing demand for an increased number [14] APPLICATION OF THE BASIC PRINCIPLE of cylinders has resulted in the adoption of en- gines of the designs just referred to for aviation work. [15] CHAPTER III ENGINE SPECIFICATIONS As a basis of comparing aviation engines _t\. certain specifications are used. It must be remembered that all engines are not called up- on to do the same work, and furthermore that they are not all designed by one man or even by a group of men holding the same views on various mechanical problems. This will ac- count for the wide range in specifications. In order to become familiar with the points where engines differ, a few items will be taken up here. The first point to consider is whether the engine has fixed cylinders or is a rotary. If it is a fixed-cylinder engine, the arrangement of the cylinders should be noted. Generally speaking, rotary engines are used for very fast but brief flights, while fixed-cylinder engines serve better for long flights where speed is not so important. The horse-power of an engine is probably the matter of greatest interest. All planes are not of the same size and weight, so there is need for engines of different power. One horse- power is the power required to lift 33,000 [161 ENGINE SPECIFICATION pounds a distance of one foot in one minute. The horse-power necessary to operate a plane is calculated by multiplying the total air re- sistance of the plane, expressed in pounds, by the speed in feet per second, then by 60 sec- onds in a minute, and dividing the product by 33,000. The actual horse-power that an engine develops is spoken of as brake horse-power. It may be found by measuring the torque exerted by the engine running with a propeller attached. By torque is meant the moment of tangential effort, or to put it more roughly, a force tending to produce rotation. The torque is allowed to be exerted upon an arm which delivers the force to a platform balance. By multiplying the force in pounds by the dis- tance in feet through which it acts in one revo- lution by the R.P.M. and dividing the product by 33,000, the actual horse-power is obtained. The distance through which the force acts is the circumference of a circle having the power arm as a radius. This distance will be 6.2832 times the arm's length, so if we make the arm exactly 534 feet long, the distance through which the force acts will be 33 feet. This per- mits us to reduce our fraction to the lowest terms, making the denominator 1,000 instead [17] ELEMENTS OF AVIATION ENGINES of 33,000. The horse- power can then be ob- tained by multiplying the torque expressed in pounds by the R.P.M. and then dividing by 1,000, which simply amounts to moving the decimal point three places to the left. The weight of an engine is of great import- ance, for it determines the engine's fitness. As has been said before, aviation work requires maximum power for minimum weight. Light- ness is the keynote of the whole engine, so the aviation engine is devoid of all unnecessary equipment. Self-starters are seldom used on account of their weight and mufflers never, on account of their weight and resistance also. Aviation engines avoid the use of a fly-wheel, on account of the large number of cylinders and also on account of the steadying effect of the propeller. In speaking of the weight of an engine, the weights of tanks and radiators are not included, nor does oil or water enter into the engine's weight. By dividing the weight by the horse-power the weight per horse-power is obtained. This is a very significant figure and is widely used in comparing engines. The most modern types of aviation engines range from two to three pounds in weight for every horse-power developed. [18] ENGINE SPECIFICATION The speed of most aviation engines is gener- ally about 1,400 R.P.M. being a compromise between the most efficient propeller speed and the most efficient engine speed. An ordinary propeller will do its best work when turning from 900 to 1,000 R.P.M. If it is driven con- siderably faster than that, it will cause what is known as cavitation, which means that the blades are working in an unfavorable medium so far as their usefulness is concerned. This will show the undesirability of having pro- pellers turn at speeds which a high-grade auto- mobile motor can easily attain. Consequently since the speed of an engine is normally greater than 900 or 1,000 R.P.M. it is advis- able to compromise by driving the propeller a little faster than it ought to turn and running the engine at a reduced speed. The efficiency of an engine, which roughly speaking is the proportion between the energy received as work and the energy supplied as fuel, can be increased if the engine is permitted to run faster than 1,400 R.P.M. Since the propeller speed has limitations, engines running at higher speeds must have a gear reduction re- garding the propeller. This is ordinarily ac- complished by driving a jack shaft carrying [19] ELEMENTS OF AVIATION ENGINES the propeller by spur gears one above the other. Sometimes internal gears are used and then the propeller will turn in the same direc- tion that the engine turns. The disadvantages of a geared propeller are that more weight is added and a slight amount of power is consumed by the gears. The direction of rotation of an engine should be considered. When standing directly in front of the propeller and noting that it turns coun- ter-clockwise, the engine is spoken of as having a normal rotation. Should the propeller turn clockwise the engine has an anti-normal rota- tion. One reason for building both normal and anti-normal engines is that in case a plane has two engines as is sometimes the case with bombing planes, then normal and anti-normal engines are used to equalize the torque effect. The number of cylinders and their bore, meaning the internal diameter, is an important item. The stroke of the piston which has been mentioned before is often spoken of in connec- tion with the bore. Various engines use differ- ent strokes with different bores, but for the sake of illustration, the stroke averages about one and one-quarter times the bore. If both the bore and the stroke are large, there will be [20] ENGINE SPECIFICATION a tendency to develop heat on the compression stroke providing the compression chamber is small. The total piston displacement is calcu- lated by squaring half the bore, multiplying by 3.1416, then multiplying by the stroke, and finally by the number of cylinders. The result will be in cubic inches. The horse-power per cubic inch of piston displacement, which is obtained by dividing the horse-power by the displacement, is a figure of much interest. Efficient motors will give from .17 to .27 H.P. for each cubic inch of displacement. Ignition, carburetion, and cooling enter into the specifications of an engine, but since separ- ate chapters are devoted to them later, they need not be dealt with here. [21] CHAPTER IV ENGINE PARTS To take up all the parts of an engine and describe them fully would be a big under- taking, and might not prove interesting to those beginning this subject. Consequently only the principal parts will be included and dealt with in a very brief manner. The cylinders of a gasoline engine are vari- ously constructed. They may be made as in- dividual units, or several may be cast in block. The advantage of the former method of con- struction is that more complete jacketing can be accomplished, while rigidity is the advan- tage of the latter type. In case an engine had four cylinders cast in block and one became damaged, then the three good ones would have to be discarded in order to replace the one cylinder that caused the trouble. This waste is not encountered when each cylinder is a separate replaceable unit. However, from the standpoint of compactness the block construc- tion is by far the more preferable. Individual cylinders are made of cast iron, semi-steel, and steel. When cast in block their material is [22] ENGINE PARTS usually aluminum alloy. A peculiar form of construction is that used in the Curtiss cylin- ders, where each cylinder is of cast iron with a band of some non-corrosive metal such as monel metal to act as a water jacket. The cylinders of the Hispano-Suiza are unusual in design, being steel thimbles that screw into an aluminum alloy water jacket designed to hold four cylinders. The Sturtevant cylinders are interesting in that they are of aluminum alloy cast in pairs with a steel liner shrunk in to act as a cylinder wall. The location of the valves determines the shape of the cylinder head. If the valves oper- ate in extensions on opposite sides of the com- bustion chamber the cylinder is said to have a T head, since its shape is that of a T. This con- struction necessitates two independent cam shafts besides being rather bulky, so is of little importance from the standpoint of aviation work. If a cylinder has only one extension in which a valve or valves work, its shape will resemble that of a Greek letter gamma or sim- ply an inverted L. It is therefore called an L head. When a cylinder has no extensions on either side but has two valves located in its head, it is called an I head cylinder. This type [23] ELEMENTS OF AVIATION ENGINES of cylinder is the most populai for aviation engines, because it does away with an irregu- larly-shaped combustion chamber. In the case of a T or L head cylinder the space above the valves may be regarded as a pocket, and very often it is difficult to scavage these pockets. The placing of both valves in the head permits the combustion chamber to be made slightly spherical in order to reduce the surface area and lessen the amount of heat carried away at the time when an explosion takes place. Some cylinders are made so that the head may be removed without disturbing its base. This is known as a detachable head and has the advantage of providing an easy means of removing carbon and working upon the valves. However, a little more material is required in this construction, and it brings into account compression leaks and also water leaks since the cylinder heads must be jacketed. The crank case is generally divided into two parts ; the top section serving as a base for the cylinders and the bottom section carrying a supply of oil. The sump is that part which holds the oil. As a rule crank cases are alumi- num castings, and in case the motor is a V type great care is taken to strengthen the upper sec- [24] ENGINE PARTS tion by means of partitions or webs to prevent the strain exerted by explosions on opposite banks from cracking the upper section. The crank shaft bearings are generally held in the upper section. Sometimes the lower halves of the bearings are held in partitions in the lower section of the crank case, as in the Hispano- Suiza. The difficulty in this construction is that the lower section can not be removed without disturbing the crank shaft. As a means of retaining the oil in the sump when the engine is momentarily inverted, splash pans are placed in the lower section. They do not retain all of the oil, but aid in reducing the amount that would otherwise rush into the cavities of the pistons. The vents on crank cases are called breathers. These maintain atmospheric pressure in the crank case even though compression leaks are present. That ■ part of the engine which is driven downward within the cylinder by the force of an explosion is the piston. Pistons have re- ceived as much if not more attention by de- signers than any other part of the engine, and the result has been to secure satisfactory oper- ation at high speeds and at high temperatures. The material used in piston construction is [251 ELEMENTS OF AVIATION ENGINES generally aluminum alloy, although cast iron is sometimes used. The use of aluminum as piston material serves to lessen vibration and increase the speed, lessening the weight of re- ciprocating parts. Another reason for its use is the rapidity with which it conducts heat. The piston head may be either convex, flat, or concave, and all of these shapes are in use at present. The convex or domehead brings into account the ability of an arch to withstand strain. Greater strength for a given amount of material is obtained by using a convex head. The flat head is the common type. By having a flat surface less area of the piston is exposed to absorb heat. This results in a slightly cooler pis- ton, which is a big advantage, as it is impossible to cool the piston in the same way that the cyl- inder is cooled. The concave head has been ex- tensively used on rotary engines because it permits a shorter cylinder and thus lessens the centrifugal force. This shape of piston head al- lows the combustion chamber to assume a spher- ical form. By the bosses are meant the two projections within the piston that hold the wrist pin, and it follows that the upper end of the con- necting rod must fit between the two bosses. The lower portion of a piston is termed the skirt. [26] ENGINE PARTS Due to more material at the head and also on account of the top surface coming in direct contact with the heat of each explosion, it will be seen that the upper part of the piston will expand more than the skirt. This necessitates allowing more clearance between the cylinder wall and the piston at its head than at its skirt. Some idea of this difference can be had by pointing out that a five-inch piston may be cleared .020 inch at the skirt and as much as .027 at the head. To prevent compression and the force of an explosion from passing down between the pis- ton and the cylinder wall, piston rings are used. These fit in grooves in the piston and bear upon the cylinder wall. Besides prevent- ing leaks these rings prevent much oil from getting upon the piston head where it would result in the formation of carbon. The rings are made of cast iron, and each piston gener- ally requires two or three of them. When the two ends of a ring come together squarely the ring is said to have a butt joint. When the ends meet each other diagonally it is called a diagonal joint. Likewise if the ends are made so that they meet each other in the form of a step, it is called a step or lap joint. Obviously [27] ELEMENTS OF AVIATION ENGINES a ring having a step joint will offer more resis- tance to the passage of gas than those having butt or diagonal joints. A precaution to take in placing a piston in a cylinder is to make sure the joints in the rings are at equal intervals around the circumference of the piston. The wrist pin is made of steel, usually hol- low and case hardened, and is used to form a movable joint between the piston and the con- necting rod. Its length depends upon the diameter of the piston. There are three gen- eral ways of retaining the pin in its right posi- tion. It may be held rigidly in the connecting rod by means of a clamp or a set screw which results in the pin turning in the piston bosses as the connecting rod moves back and forth. This method is used in the Curtiss OX. An- other way is to pin the wrist pin in the bosses so that it is securely held in a fixed position. The connecting rod will then turn on the wrist pin which means the bearing will be in the connecting rod. Such a construction necessi- tates a bearing at both ends of the connecting rod. The Hall-Scott A5A has its wrist pins held rigidly in the piston bosses. The floating method such as used in the Sturtevant allows the pin to move either in the bosses or in the [281 ENGINE PARTS connecting rod. Brass ends on the pin or caps over the ends of the bosses protect the cylinder walls. The connecting rod is a steel arm used to convert the reciprocating motion of the piston into the revolving motion of the crank shaft. The majority of connecting rods have a cross section resembling an I, although H and tubu- lar rods are not uncommon. In cases where the wrist pin is held in the bosses the upper end of the connecting rod is supplied with a bronze bushing that acts as a bearing surface. The lower bearing, in which works the crank pin, is given more attention. Babbitt is employed as a bearing metal and is generally backed by bronze to take its place should enough heat be developed to fuse the babbitt. Lower connect- ing rod bearings are made in two pieces to permit the crank pin being put in position. Between the two halves of the bearing are placed strips of metal called shims. These are .001, .002 and .005 inch thick and as the bearing wears away these can be removed insuring a better fit. In a V motor, when the vertical axis of opposite cylinders are in the same vertical plane, the connecting rods of opposite cylin- ders will meet the crank pin at the same point. [29] ELEMENTS OF AVIATION ENGINES This will necessitate the forked or straddled construction in which one rod works between the fork of another. It makes rather a com- plicated and costly bearing, but it is a favorite design and is being used extensively. The Hispano-Suiza has this type of lower connect- ing rod bearings. Another and simpler way is to have the cylinders "staggered" by placing the cylinders on one bank a little ahead or be- hind those on the opposite bank, thereby allowing two lower connecting rod bearings to work side by side on one crank pin. A wise precaution to take in assembling a motor is to make sure the lower connecting rod bearing is such that it allows the wrist pin to be abso- lutely parallel with the crank pin. If it is otherwise the piston will not work freely within the cylinder. The crank shaft is the driving shaft of the engine to which the power impulses are trans- mitted by the pistons and connecting rods. It is needless to say that this is the most im- portant moving part of an engine, and for this reason it is made with great precision from selected pieces of high-grade steel by drop forging and subsequent turning. The principal parts of a crank shaft are the main bearings, [30] ENGINE PARTS the cheeks, and the pins on which the connect- ing rods work. Two cheeks and a pin are spoken of as a throw. Thus the number of cylinders govern the number of throws, and also upon the number of cylinders depends the number of degrees between the throws. In a vertical motor, if the cylinders are cast separ- ately, there is generally a main bearing be- tween every two throws. Where the cylinders are cast in block there is not so much space between the pistons which often means a de- crease in the number of main bearings. The crank shaft used in a V motor is identically the same as one used in a vertical motor having half the number of cylinders. Two connecting rods are fitted to each throw, and if the cylin- ders are cast separately a main bearing is placed between every two throws. For the main bearings of a crank shaft the lining is babbitt usually backed by bronze very similar to the lower connecting rod bearings. Babbitt, which essentially consists of lead and antimony, is used as bearing metal because of its anti-friction properties, its sufficient hardness, and the ease with which it can be replaced. Lead alone possesses considerable anti-friction properties, but is impracticable on account [31] ELEMENTS OF AVIATION ENGINES of its softness. The addition of some anti- mony will materially harden the lead with- out lessening its anti-friction properties. The use of babbitt also permits the liner to be scraped to secure an exact bearing surface. By coating the journal with Prussian blue, the high spots can be detected on the liner, and these can be successively removed by scraping. To have evenly placed power impulses the throws on a crank shaft must be placed at cer- tain angles with one another. In any four- stroke cycle motor all cylinders will fire once in two revolutions of the crank shaft or once in 720 degrees. In a four-cylinder motor there would be four explosions in 720 degrees, and to get equal spacing the power impulses would have to come one-fourth of 720 or 180 degrees apart. This will explain why the angle be- tween two throws that receive impulses, one directly after the other, is 180 degrees for a four-cylinder crank shaft. The throws in a six- cylinder crank shaft are 120 degrees apart, since there will be six power impulses in 720 degrees. In determining the order in which the cylin- ders will deliver their power impulses to the crank shaft, it is the custom to fire them so [32] ENGINE PARTS that the vibrations set up by one explosion will serve to counteract the vibrations caused by a previous explosion. To accomplish this an ex- plosion at one end of the shaft is followed by an explosion near the other end. Here we come to what is known as the firing order, which simply means the order in which the cylinders do their work. In order to discuss the firing order it is first necessary to explain how the cylinders are numbered. In American practice cylinder No. 1 is always that one at the pilot's end of the engine, and the number- ing is in regular order toward the propeller. In V engines No. 1 is the first cylinder on the left bank viewed from the pilot's cock pit. Some engines have the left bank numbered LI, L2, L3, L4, and the right bank Rl, R2, R3, R4. Others number the left bank 1, 2, 3, 4 in the regular way and then start with the cylinder nearest the propeller on the right bank calling it 1' followed by 2', 3' and 4' going toward the pilot's end. The Curtiss OX has the peculiar way of starting with No. 1 on the left bank nearest the cock pit and desig- nating as No. 2 the opposite cylinder on the right bank. No. 3 is the next one on the left bank, and in this way the odd numbers are on [331 ELEMENTS OF AVIATION ENGINES the left bank and the even numbers on the right bank. To return to firing orders, it is best to start with a four-cylinder engine. The cylinders in such an engine can be fired in a 1, 2, 4, 3 order or in a 1, 3, 4, 2 order. From this it can be seen that throws 1 and 2 are 180 degrees apart and 3 and 4 are also that distance apart. Likewise it is evident that with a four-cylinder crank shaft, pistons 1 and 4 travel together and also 2 and 3 are coming up or going down together. The two usual ways for a six-cylinder engine to fire are 1, 5, 3, 6, 2, 4, and 1, 4, 2, 6, 3, 5. Here the throws are 120 degrees apart, and the pis- tons that travel together are 1 and 6, 2 and 5, and 3 and 4. V engines use the basic four- cylinder and six-cylinder firing orders to fire the two banks. The explosions will alternate between the two banks starting with the cylin- der at the pilot's end on the left block and fol- lowed by the forward cylinder on the right block. Explosions will occur on the left bank according to either one of the two firing orders, and those on the right bank in like manner except that on the right bank we will be work- ing from the propeller end toward the pilot's end. Where an engine is numbered LI, L2, [341 ENGINE PARTS etc., and Rl, R2, etc., its firing order may be: LI, R6, L5, R2, L3, R4, L6, Rl, L2, R5, L4, R3. Where the left bank is numbered 1, 2, 3, etc., and the right bank 1', 2' ', etc., in the oppo- site direction, the firing order may be: 1,1', 5,5', 3,3', 6,6', 2,2', 4,4'. The Curtiss OX with its peculiar cylinder numbering already referred to has the follow- ing distinctive firing order for normal rota- tion: 1,2,3,4, 7,8,5,6. For an anti-normal engine it would be: 2, 1,4,3,8, 7,6,5. or to start the cycle with an explosion in cylin- der No. 1 it would be: 1,4,3,8, 7,6,5, 2. In order that the thrust exerted by the propeller upon the crank shaft may be trans- mitted to the crank case and then to the fuselage, a thrust bearing is placed upon the crank shaft very near the propeller hub. Thrust bearings are generally ball bearings hav- ing either one or two rows of balls and very often they are designed to take a load directed at right angles towards the center of the shaft as well as taking care of the thrust. In an engine [35] ELEMENTS OF AVIATION ENGINES like the Curtiss OX, where the crank shaft ex- tends several inches between the last main crank-shaft bearing and the propeller hub, the thrust bearing will be the last point where the shaft may be supported. Now if a shaft is allowed to revolve without a radial bearing at its end vibration will result and this must be avoided. Consequently on the Curtiss OX and all other engines having a nose, the thrust bearing must be capable of taking both radial and thrust loads. Some thrust bearings having a single row of balls will only take thrust in one direction. This makes it necessary to reverse the bearings if an engine is transferred from a tractor plane to a pusher plane or vice versa. The cam shaft is that part of the engine hav- ing irregularities upon its surface that open and close the valves at the proper time. The irregularities are called cams and are usually accurately shaped projections upon the shaft for imparting the necessary motion to a valve. Cam shafts are always made of high-grade steel and the cams are forged integral with the shaft. When gasoline engines were first being developed it was the practice to have as a cam shaft a plain piece of shafting with the cam keyed or pinned to it in the right position. [361 on j ///}LL-