ENGINE PARTS AND FUNCTIONS The principal elements of a gas engine are not difficult to understand and their functions are easily defined. In place of the barrel of the gun one has a smoothly machined cylinder in which a small cylindrical or barrel-shaped element fitting the bore closely may be likened to a bullet or cannon ball. It differs in this important respect, however, as while the shot is discharged from the mouth of the cannon the piston member sliding inside of the main cylinder cannot leave it, as its movements back and forth from the open to the closed end and back again are limited by simple mechanical connection or linkage which comprises crank and connection rod. It is by this means that the reciprocating movement of the piston is transformed into a rotary motion of the crank-shaft. The fly-wheel is a heavy member attached to the crank-shaft of an automobile engine which has energy stored in its rim as the member revolves, and the momentum of this revolving mass tends to equalize the intermittent pushes on the piston head produced by the explosion of the gas in the cylinder. In aviation engines, the weight of the propeller or that of rotating cylinders themselves performs the duty of a fly-wheel, so no separate member is needed. If some explosive is placed in the chamber formed by the piston and closed end of the cylinder and exploded, the piston would be the only part that would yield to the pressure which would produce a downward movement. As this is forced down the crank-shaft is turned by the connecting rod, and as this part is hinged at both ends it is free to oscillate as the crank turns, and thus the piston may slide back and forth while the crank-shaft is rotating or describing a curvilinear path. Large
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(135 kB). Fig. 21 Fig. 21.—Side Sectional View of Typical Airplane Engine, Showing Parts and Their Relation to Each Other. This Engine is an Aeromarine Design and Utilizes a Distinctive Concentric Valve Construction. In addition to the simple elements described it is evident that a gasoline engine must have other parts. The most important of these are the valves, of which there are generally two to each cylinder. One closes the passage connecting to the gas supply and opens during one stroke of the piston in order to let the explosive gas into the combustion chamber. The other member, or exhaust valve, serves as a cover for the opening through which the burned gases can leave the cylinder after their work is done. The spark plug is a simple device which may be compared to the fuse or percussion cap of the cannon. It permits one to produce an electric spark in the cylinder when the piston is at the best point to utilize the pressure which obtains when the compressed gas is fired. The valves are open one at a time, the inlet valve being lifted from its seat while the cylinder is filling and the exhaust valve is opened when the cylinder is being cleared. They are normally kept seated by means of compression springs. In the simple motor shown at Fig. 5, the exhaust valve is operated by means of a pivoted bell crank rocked by a cam which turns at half the speed of the crank-shaft. The inlet valve operates automatically, as will be explained in proper sequence. In order to obtain a perfectly tight combustion chamber, both intake and exhaust valves are closed before the gas is ignited, because all of the pressure produced by the exploding gas is to be directed against the top of the movable piston. When the piston reaches the bottom of its power stroke, the exhaust valve is lifted by means of the bell crank which is rocked because of the point or lift on the cam. The cam-shaft is driven by positive gearing and revolves at half the engine speed. The exhaust valve remains open during the whole of the return stroke of the piston, and as this member moves toward the closed end of the cylinder it forces out burned gases ahead of it, through the passage controlled by the exhaust valve. The cam-shaft is revolved at half the engine speed because the exhaust valve is raised from its seat during only one stroke out of four, or only once every two revolutions. Obviously, if the cam was turned at the same speed as the crank-shaft it would remain open once every revolution, whereas the burned gases are expelled from the individual cylinders only once in two turns of the crank-shaft. WHY MULTIPLE CYLINDER FORMS ARE BEST Owing to the vibration which obtains from the heavy explosion in the large single-cylinder engines used for stationary power other forms were evolved in which the cylinder was smaller and power obtained by running the engine faster, but these are suitable only for very low powers. When a single-cylinder engine is employed a very heavy fly-wheel is needed to carry the moving parts through idle strokes necessary to obtain a power impulse. For this reason automobile and aircraft designers must use more than one cylinder, and the tendency is to produce power by frequently occurring light impulses rather than by a smaller number of explosions having greater force. When a single-cylinder motor is employed the construction is heavier than is needed with a multiple-cylinder form. Using two or more cylinders conduces to steady power generation and a lessening of vibration. Most modern motor cars employ four-cylinder engines because a power impulse may be secured twice every revolution of the crank-shaft, or a total of four power strokes during two revolutions. The parts are so arranged that while the charge of gas in one cylinder is exploding, those which come next in firing order are compressing, discharging the inert gases and drawing in a fresh charge respectively. When the power stroke is completed in one cylinder, the piston in that member in which a charge of gas has just been compressed has reached the top of its stroke and when the gas is exploded the piston is reciprocated and keeps the crank-shaft turning. When a multiple-cylinder engine is used the fly-wheel can be made much lighter than that of the simpler form and eliminated altogether in some designs. In fact, many modern multiple-cylinder engines developing 300 horse-power weigh less than the early single- and double-cylinder forms which developed but one-tenth or one-twentieth that amount of energy. DESCRIBING SEQUENCE OF OPERATIONS Referring to Fig. 22, A, the sequence of operation in a single-cylinder motor can be easily understood. Assuming that the crank-shaft is turning in the direction of the arrow, it will be seen that the intake stroke comes first, then the compression, which is followed by the power impulse, and lastly the exhaust stroke. If two cylinders are used, it is possible to balance the explosions in such a way that one will occur each revolution. This is true with either one of two forms of four-cycle motors. At B, a two-cylinder vertical engine using a crank-shaft in which the crank-pins are on the same plane is shown. The two pistons move up and down simultaneously. Referring to the diagram describing the strokes, and assuming that the outer circle represents the cycle of operations in one cylinder while the inner circle represents the sequence of events in the other cylinder, while cylinder No. 1 is taking in a fresh charge of gas, cylinder No. 2 is exploding. When cylinder No. 1 is compressing, cylinder No. 2 is exhausting. During the time that the charge in cylinder No. 1 is exploded, cylinder No. 2 is being filled with fresh gas. While the exhaust gases are being discharged from cylinder No. 1, cylinder No. 2 is compressing the gas previously taken. Fig. 22 Fig. 22.—Diagrams Illustrating Sequence of Cycles in One- and Two-Cylinder Engines Showing More Uniform Turning Effort on Crank-Shaft with Two-Cylinder Motors. The same condition obtains when the crank-pins are arranged at one hundred and eighty degrees and the cylinders are opposed, as shown at C. The reason that the two-cylinder opposed motor is more popular than that having two vertical cylinders is that it is difficult to balance the construction shown at B, so that the vibration will not be excessive. The two-cylinder opposed motor has much less vibration than the other form, and as the explosions occur evenly and the motor is a simple one to construct, it has been very popular in the past on light cars and has received limited application on some early, light airplanes. Fig. 23 Fig. 23.—Diagrams Demonstrating Clearly Advantages which Obtain when Multiple-Cylinder Motors are Used as Power Plants. To demonstrate very clearly the advantages of multiple-cylinder engines the diagrams at Fig. 23 have been prepared. At A, a three-cylinder motor, having crank-pins at one hundred and twenty degrees, which means that they are spaced at thirds of the circle, we have a form of construction that gives a more even turning than that possible with a two-cylinder engine. Instead of one explosion per revolution of the crank-shaft, one will obtain three explosions in two revolutions. The manner in which the explosion strokes occur and the manner they overlap strokes in the other cylinder is shown at A. Assuming that the cylinders fire in the following order, first No. 1, then No. 2, and last No. 3, we will see that while cylinder No. 1, represented by the outer circle, is on the power stroke, cylinder No. 3 has completed the last two-thirds of its exhaust stroke and has started on its intake stroke. Cylinder No. 2, represented by the middle circle, during this same period has completed its intake stroke and two-thirds of its compression stroke. A study of the diagram will show that there is an appreciable lapse of time between each explosion. Three-cylinder engines are not used on aircraft at the present time, though Bleriot’s flight across the British Channel was made with a three-cylinder Anzani motor. It was not a conventional form, however. The three-cylinder engine is practically obsolete at this time for any purpose except “penguins” or school machines that are incapable of flight and which are used in some French training schools for aviators. FOUR- AND SIX-CYLINDER ENGINES In the four-cylinder engine operation which is shown at Fig. 23, B, it will be seen that the power strokes follow each other without loss of time, and one cylinder begins to fire and the piston moves down just as soon as the member ahead of it has completed its power stroke. In a four-cylinder motor, the crank-pins are placed at one hundred and eighty degrees, or on the halves of the crank circle. The crank-pins for cylinders No. 1 and No. 4 are on the same plane, while those for cylinders No. 2 and No. 3 also move in unison. The diagram describing sequence of operations in each cylinder is based on a firing order of one, two, four, three. The outer circle, as in previous instances, represents the cycle of operations in cylinder one. The next one toward the center, cylinder No. 2, the third circle represents the sequence of events in cylinder No. 3, while the inner circle outlines the strokes in cylinder four. The various cylinders are working as follows: | 1. | 2. | 3. | 4. | | Explosion | Compression | Exhaust | Intake | | Exhaust | Explosion | Intake | Compression | | Intake | Exhaust | Compression | Explosion | | Compression | Intake | Explosion | Exhaust | It will be obvious that regardless of the method of construction, or the number of cylinders employed, exactly the same number of parts must be used in each cylinder assembly and one can conveniently compare any multiple-cylinder power plant as a series of single-cylinder engines joined one behind the other and so coupled that one will deliver power and produce useful energy at the crank-shaft where the other leaves off. The same fundamental laws governing the action of a single cylinder obtain when a number are employed, and the sequence of operation is the same in all members, except that the necessary functions take place at different times. If, for instance, all the cylinders of a four-cylinder motor were fired at the same time, one would obtain the same effect as though a one-piston engine was used, which had a piston displacement equal to that of the four smaller members. As is the case with a single-cylinder engine, the motor would be out of correct mechanical balance because all the connecting rods would be placed on crank-pins that lie in the same plane. A very large fly-wheel would be necessary to carry the piston through the idle strokes, and large balance weights would be fitted to the crank-shaft in an effort to compensate for the weight of the four pistons, and thus reduce vibratory stresses which obtain when parts are not in correct balance. There would be no advantage gained by using four cylinders in this manner, and there would be more loss of heat and more power consumed in friction than in a one-piston motor of the same capacity. This is the reason that when four cylinders are used the arrangement of crank-pins is always as shown at Fig. 23, B—i.e., two pistons are up, while the other two are at the bottom of the stroke. With this construction, we have seen that it is possible to string out the explosions so that there will always be one cylinder applying power to the crank-shaft. The explosions are spaced equally. The parts are in correct mechanical balance because two pistons are on the upstroke while the other two are descending. Care is taken to have one set of moving members weigh exactly the same as the other. With a four-cylinder engine one has correct balance and continuous application of energy. This insures a smoother running motor which has greater efficiency than the simpler one-, two-, and three-cylinder forms previously described. Eliminating the stresses which would obtain if we had an unbalanced mechanism and irregular power application makes for longer life. Obviously a large number of relatively light explosions will produce less wear and strain than would a lesser number of powerful ones. As the parts can be built lighter if the explosions are not heavy, the engine can be operated at higher rotative speeds than when large and cumbersome members are utilized. Four-cylinder engines intended for aviation work have been built according to the designs shown at Fig. 24, but these forms are unconventional and seldom if ever used. Fig. 24 Fig. 24.—Showing Three Possible Though Unconventional Arrangements of Four-Cylinder Engines. The six-cylinder type of motor, the action of which is shown at Fig. 23, C, is superior to the four-cylinder, inasmuch as the power strokes overlap, and instead of having two explosions each revolution we have three explosions. The conventional crank-shaft arrangement in a six-cylinder engine is just the same as though one used two three-cylinder shafts fastened together, so pistons 1 and 6 are on the same plane as are pistons 2 and 5. Pistons 3 and 4 also travel together. With the cranks arranged as outlined at Fig. 23, C, the firing order is one, five, three, six, two, four. The manner in which the power strokes overlap is clearly shown in the diagram. An interesting comparison is also made in the diagrams at Fig. 25 and in the upper corner of Fig. 23, C. Large
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(84 kB). Fig. 25 Fig. 25.—Diagrams Outlining Advantages of Multiple Cylinder Motors, and Why They Deliver Power More Evenly Than Single Cylinder Types. A rectangle is divided into four columns; each of these corresponds to one hundred and eighty degrees, or half a revolution. Thus the first revolution of the crank-shaft is represented by the first two columns, while the second revolution is represented by the last two. Taking the portion of the diagram which shows the power impulse in a one-cylinder engine, we see that during the first revolution there has been no power impulse. During the first half of the second revolution, however, an explosion takes place and a power impulse is obtained. The last portion of the second revolution is devoted to exhausting the burned gases, so that there are three idle strokes and but one power stroke. The effect when two cylinders are employed is shown immediately below. Here we have one explosion during the first half of the first revolution in one cylinder and another during the first half of the second revolution in the other cylinder. With a four-cylinder engine there is an explosion each half revolution, while in a six-cylinder engine there is one and one-half explosions during each half revolution. When six cylinders are used there is no lapse of time between power impulses, as these overlap and a continuous and smooth-turning movement is imparted to the crank shaft. The diagram shown at Fig. 26, prepared by E. P. Pulley, can be studied to advantage in securing an idea of the coordination of effort that takes place in an engine of the six-cylinder type. ACTUAL DURATION OF DIFFERENT STROKES Fig. 27 Fig. 27.—Diagram Showing Actual Duration of Different Strokes in Degrees. In the diagrams previously presented the writer has assumed, for the sake of simplicity, that each stroke takes place during half of one revolution of the crank-shaft, which corresponds to a crank-pin travel of one hundred and eighty degrees. The actual duration of these strokes is somewhat different. For example, the inlet stroke is usually a trifle more than a half revolution, and the exhaust is always considerably more. The diagram showing the comparative duration of the strokes is shown at Fig. 27. The inlet valve opens ten degrees after the piston starts to go down and remains open thirty degrees after the piston has reached the bottom of its stroke. This means that the suction stroke corresponds to a crank-pin travel of two hundred degrees, while the compression stroke is measured by a movement of but one hundred and fifty degrees. It is common practice to open the exhaust valve before the piston reaches the end of the power stroke so that the actual duration of the power stroke is about one hundred and forty degrees, while the exhaust stroke corresponds to a crank-pin travel of two hundred and twenty-five degrees. In this diagram, which represents proper time for the valves to open and close, the dimensions in inches given are measured on the fly-wheel and apply only to a certain automobile motor. If the fly-wheel were smaller ten degrees would take up less than the dimensions given, while if the fly-wheel was larger a greater space on its circumference would represent the same crank-pin travel. Aviation engines are timed by using a timing disc attached to the crank-shaft as they are not provided with fly-wheels. Obviously, the distance measured in inches will depend upon the diameter of the disc, though the number of degrees interval would not change. Fig. 28 Fig. 28.—Another Diagram to Facilitate Understanding Sequence of Functions in Six-Cylinder Engine. EIGHT- AND TWELVE-CYLINDER V ENGINES Those who have followed the development of the gasoline engine will recall the arguments that were made when the six-cylinder motor was introduced at a time that the four-cylinder type was considered standard. The arrival of the eight-cylinder has created similar futile discussion of its practicability as this is so clearly established as to be accepted without question. It has been a standard power plant for aeroplanes for many years, early exponents having been the Antoinette, the Woolsley, the Renault, the E. N. V. in Europe and the Curtiss in the United States. Fig. 29 Fig. 29.—Types of Eight-Cylinder Engines Showing the Advantage of the V Method of Cylinder Placing. The reason the V type shown at Fig. 29, A is favored is that the “all-in-line form” which is shown at Fig. 29, B is not practical for aircraft because of its length. Compared to the standard four-cylinder engine it is nearly twice as long and it required a much stronger and longer crank-shaft. It will be evident that it could not be located to advantage in the airplane fuselage. These undesirable factors are eliminated in the V type eight-cylinder motor, as it consists of two blocks of four cylinders each, so arranged that one set or block is at an angle of forty-five degrees from the vertical center line of the motor, or at an angle of ninety degrees with the other set. This arrangement of cylinders produces a motor that is no longer than a four-cylinder engine of half the power would be. Fig. 30 Fig. 30.—Curves Showing Torque of Various Engine Types Demonstrate Graphically Marked Advantage of the Eight-Cylinder Type. Apparently there is considerable misconception as to the advantage of the two extra cylinders of the eight as compared with the six-cylinder. It should be borne in mind that the multiplication in the number of cylinders noticed since the early days of automobile development has not been for solely increasing the power of the engine, but to secure a more even turning movement, greater flexibility and to eliminate destructive vibration. The ideal internal combustion motor, is the one having the most uniform turning movement with the least mechanical friction loss. Study of the torque outlines or plotted graphics shown at Figs. 25 and 30 will show how multiplication of cylinders will produce steady power delivery due to overlapping impulses. The most practical form would be that which more nearly conforms to the steady running produced by a steam turbine or electric motor. The advocates of the eight-cylinder engine bring up the item of uniform torque as one of the most important advantages of the eight-cylinder design. A number of torque diagrams are shown at Fig. 30. While these appear to be deeply technical, they may be very easily followed when their purpose is explained. At the top is shown the torque diagram of a single-cylinder motor of the four-cycle type. The high point in the line represents the period of greatest torque or power generation, and it will be evident that this occurs early in the first revolution of the crank-shaft. Below this diagram is shown a similar curve except that it is produced by a four-cylinder engine. Inspection will show that the turning-moment is much more uniform than in the single cylinder; similarly, the six-cylinder diagram is an improvement over the four, and the eight-cylinder diagram is an improvement over the six-cylinder. Fig. 31 Fig. 31—Diagrams Showing How Increasing Number of Cylinders Makes for More Uniform Power Application. Fig. 32 Fig. 32.—How the Angle Between the Cylinders of an Eight- and Twelve-Cylinder V Motor Varies. The reason that practically continuous torque is obtained in an eight-cylinder engine is that one cylinder fires every ninety degrees of crank-shaft rotation, and as each impulse lasts nearly seventy-five per cent. of the stroke, one can easily appreciate that an engine that will give four explosions per revolution of the crank-shaft will run more uniformly than one that gives but three explosions per revolution, as the six-cylinder does, and will be twice as smooth running as a four-cylinder, in which but two explosions occur per revolution of the crank-shaft. The comparison is so clearly shown in graphical diagrams and in Fig. 31 that further description is unnecessary. Any eight-cylinder engine may be considered a “twin-four,” twelve-cylinder engines may be considered “twin sixes.” Fig. 33 Fig. 33.—The Hall-Scott Four-Cylinder 100 Horse-Power Aviation Motor. Fig. 34 Fig. 34.—Two Views of the Duesenberg Sixteen Valve Four-Cylinder Aviation Motor. The only points in which an eight-cylinder motor differs from a four-cylinder is in the arrangement of the connecting rod, as in many designs it is necessary to have two rods working from the same crank-pin. This difficulty is easily overcome in some designs by staggering the cylinders and having the two connecting rod big ends of conventional form side by side on a common crank-pin. In other designs one rod is a forked form and works on the outside of a rod of the regular pattern. Still another method is to have a boss just above the main bearing on one connecting rod to which the lower portion of the connecting rod in the opposite cylinder is hinged. As the eight-cylinder engine may actually be made lighter than the six-cylinder of equal power, it is possible to use smaller reciprocating parts, such as pistons, connecting rods and valve gear, and obtain higher engine speed with practically no vibration. The firing order in nearly every case is the same as in a four-cylinder except that the explosions occur alternately in each set of cylinders. The firing order of an eight-cylinder motor is apt to be confusing to the motorist, especially if one considers that there are eight possible sequences. The majority of engineers favor the alternate firing from side to side. Firing orders will be considered in proper sequence. Fig. 35 Fig. 35.—The Hall-Scott Six-Cylinder Aviation Engine. Fig. 36 Fig. 36.—The Curtiss Eight-Cylinder, 200 Horse-Power Aviation Engine. The demand of aircraft designers for more power has stimulated designers to work out twelve-cylinder motors. These are high-speed motors incorporating all recent features of design in securing light reciprocating parts, large valve openings, etc. The twelve-cylinder motor incorporates the best features of high-speed motor design and there is no need at this time to discuss further the pros and cons of the twelve-cylinder versus the eight or six, because it is conceded by all that there is the same degree of steady power application in the twelve over the eight as there would be in the eight over the six. The question resolves itself into having a motor of high power that will run with minimum vibration and that produces smooth action. This is well shown by diagrams at Fig. 31. It should be remembered that if an eight-cylinder engine will give four explosions per revolution of the fly-wheel, a twelve-cylinder type will give six explosions per revolution, and instead of the impulses coming 90 degrees crank travel apart, as in the case of the eight-cylinder, these will come but 60 degrees of crank travel apart in the case of the twelve-cylinder. For this reason, the cylinders of a twelve are usually separated by 60 degrees while the eight has the blocks spaced 90 degrees apart. The comparison can be easily made by comparing the sectional views of Vee engines at Fig. 32. When one realizes that the actual duration of the power stroke is considerably greater than 120 degrees crank travel, it will be apparent that the overlapping of explosions must deliver a very uniform application of power. Vee engines have been devised having the cylinders spaced but 45 degrees apart, but the explosions cannot be timed at equal intervals as when 90 degrees separate the cylinder center lines. Fig. 37 Fig. 37.—The Sturtevant Eight-Cylinder, High Speed Aviation Motor. RADIAL CYLINDER ARRANGEMENTS While the fixed cylinder forms of engines, having the cylinders in tandem in the four- and six-cylinder models as shown at Figs. 33 to 35 inclusive and the eight-cylinder V types as outlined at Figs. 36 and 37 have been generally used and are most in favor at the present time, other forms of motors having unconventional cylinder arrangements have been devised, though most of these are practically obsolete. While many methods of decreasing weight and increasing mechanical efficiency of a motor are known to designers, one of the first to be applied to the construction of aeronautical power plants was an endeavor to group the components, which in themselves were not extremely light, into a form that would be considerably lighter than the conventional design. As an example, we may consider those multiple-cylinder forms in which the cylinders are disposed around a short crank-case, either radiating from a common center as at Fig. 38 or of the fan shape shown at Fig. 39. This makes it possible to use a crank-case but slightly larger than that needed for one or two cylinders and it also permits of a corresponding decrease in length of the crank-shaft. The weight of the engine is lessened because of the reduction in crank-shaft and crank-case weight and the elimination of a number of intermediate bearings and their supporting webs which would be necessary with the usual tandem construction. While there are six power impulses to every two revolutions of the crank-shaft, in the six-cylinder engine, they are not evenly spaced as is possible with the conventional arrangement. Fig. 38.—Anzani 40-50 Horse-Power Five-Cylinder Air Cooled Engine. In the Anzani form, which is shown at Fig. 38, the crank-case is stationary and a revolving crank-shaft is employed as in conventional construction. The cylinders are five in number and the engine develops 40 to 50 H.P. with a weight of 72 kilograms or 158.4 lbs. The cylinders are of the usual air-cooled form having cooling flanges only part of the way down the cylinder. By using five cylinders it is possible to have the power impulses come regularly, they coming 145° crank-shaft travel apart, the crank-shaft making two turns to every five explosions. The balance is good and power output regular. The valves are placed directly in the cylinder head and are operated by a common pushrod. Attention is directed to the novel method of installing the carburetor which supplies the mixture to the engine base from which inlet pipes radiate to the various cylinders. This engine is used on French school machines. Fig. 39 Fig. 39.—Unconventional Six-Cylinder Aircraft Motor of Masson Design. In the form shown at Fig. 39 six cylinders are used, all being placed above the crank-shaft center line. This engine is also of the air-cooled form and develops 50 H. P. and weighs 105 kilograms, or 231 lbs. The carburetor is connected to a manifold casting attached to the engine base from which the induction pipes radiate to the various cylinders. The propeller design and size relative to the engine is clearly shown in this view. While flights have been made with both of the engines described, this method of construction is not generally followed and has been almost entirely displaced abroad by the revolving motors or by the more conventional eight-cylinder V engines. Both of the engines shown were designed about eight years ago and would be entirely too small and weak for use in modern airplanes intended for active duty. ROTARY ENGINES Fig. 40 Fig. 40.—The Gnome Fourteen-Cylinder Revolving Motor. Rotary engines such as shown at Fig. 40 are generally associated with the idea of light construction and it is rather an interesting point that is often overlooked in connection with the application of this idea to flight motors, that the reason why rotary engines are popularly supposed to be lighter than the others is because they form their own fly-wheel, yet on aeroplanes, engines are seldom fitted with a fly-wheel at all. As a matter of fact the Gnome engine is not so light because it is a rotary motor, and it is a rotary motor because the design that has been adopted as that most conducive to lightness is also most suited to an engine working in this way. The cylinders could be fixed and crank-shaft revolve without increasing the weight to any extent. There are two prime factors governing the lightness of an engine, one being the initial design, and the other the quality of the materials employed. The consideration of reducing weight by cutting away metal is a subsidiary method that ought not to play a part in standard practice, however useful it may be in special cases. In the Gnome rotary engine the lightness is entirely due to the initial design and to the materials employed in manufacture. Thus, in the first case, the engine is a radial engine, and has its seven or nine cylinders spaced equally around a crank-chamber that is no wider or rather longer than would be required for any one of the cylinders. This shortening of the crank-chamber not only effects a considerable saving of weight on its own account, but there is a corresponding saving in the shafts and other members, the dimensions of which are governed by the size of the crank-chamber. With regard to materials, nothing but steel is used throughout, and most of the metal is forged chrome nickel steel. The beautifully steady running of the engine is largely due to the fact that there are literally no reciprocating parts in the absolute sense, the apparent reciprocation between the pistons and cylinders being solely a relative reciprocation since both travel in circular paths, that of the pistons, however, being electric by one-half of the stroke length to that of the cylinder. While the Gnome engine has many advantages, on the other hand the head resistance offered by a motor of this type is considerable; there is a large waste of lubricating oil due to the centrifugal force which tends to throw the oil away from the cylinders; the gyroscopic effect of the rotary motor is detrimental to the best working of the aeroplane, and moreover it requires about seven per cent. of the total power developed by the motor to drive the revolving cylinders around the shaft. Of necessity, the compression of this type of motor is rather low, and an additional disadvantage manifests itself in the fact that there is as yet no satisfactory way of muffling the rotary type of motor. The modern Gnome engine has been widely copied in various European countries, but its design was originated in America, the early Adams-Farwell engine being the pioneer form. It has been made in seven- and nine-cylinder types and forms of double these numbers. The engine illustrated at Fig. 40 is a fourteen-cylinder form. The simple engines have an odd number of cylinders in order to secure evenly spaced explosions. In the seven-cylinder, the impulses come 102.8° apart. In the nine-cylinder form, the power strokes are spaced 80° apart. The fourteen-cylinder engine is virtually two seven-cylinder types mounted together, the cranks being just the same as in a double cylinder opposed motor, the explosions coming 51.4° apart; while in the eighteen-cylinder model the power impulses come every 40° cylinder travel. Other rotary motors have been devised, such as the Le Rhone and the Clerget in France and several German copies of these various types. The mechanical features of these motors will be fully considered later.
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