CHART FOR DETERMINING COMPRESSION PRESSURES
Fig. 16.—Chart Showing Relation Between Compression Volume and Pressure.
A very useful chart ([Fig. 16]) for determining compression pressures in gasoline-engine cylinders for various ratios of compression space to total cylinder volume is given by P. S. Tice, and described in the Chilton Automobile Directory by the originator as follows:
It is many times desirable to have at hand a convenient means for at once determining with accuracy what the compression pressure will be in a gasoline-engine cylinder, the relationship between the volume of the compression space and the total cylinder volume or that swept by the piston being known. The curve at [Fig. 16] is offered as such a means. It is based on empirical data gathered from upward of two dozen modern automobile engines and represents what may be taken to be the results as found in practice. It is usual for the designer to find compression pressure values, knowing the volumes from the equation
| P2 = P1 ( | V1 | )1.4 | 1 |
| V2 |
which is for adiabatic compression of air. Equation (1) is right enough in general form but gives results which are entirely too high, as almost all designers know from experience. The trouble lies in the interchange of heat between the compressed gases and the cylinder walls, in the diminution of the exponent (1.4 in the above) due to the lesser ratio of specific heat of gasoline vapor and in the transfer of heat from the gases which are being compressed to whatever fuel may enter the cylinder in an unvaporized condition. Also, there is always some piston leakage, and, if the form of the equation (1) is to be retained, this also tends to lower the value of the exponent. From experience with many engines, it appears that compression reaches its highest value in the cylinder for but a short range of motor speeds, usually during the mid-range. Also, it appears that, at those speeds at which compression shows its highest values, the initial pressure at the start of the compression stroke is from .5 to .9 lb. below atmospheric. Taking this latter loss value, which shows more often than those of lesser value, the compression is seen to start from an initial pressure of 13.9 lbs. per sq. in. absolute.
Also, experiment shows that if the exponent be given the value 1.26, instead of 1.4, the equation will embrace all heat losses in the compressed gas, and compensate for the changed ratio of specific heats for the mixture and also for all piston leakage, in the average engine with rings in good condition and tight. In the light of the foregoing, and in view of results obtained from its use, the above curve is offered—values of P2 being found from the equation
| P2 = 13.8 ( | V1 | )1.26 |
| V2 |
In using this curve it must be remembered that pressures are absolute. Thus: suppose it is desired to know the volumetric relationships of the cylinder for a compression pressure of 75 lbs. gauge. Add atmospheric pressure to the desired gauge pressure 14.7 + 75 = 89.7 lbs. absolute. Locate this pressure on the scale of ordinates and follow horizontally across to the curve and then vertically downward to the scale of abscissas, where the ratio of the combustion chamber volume to the total cylinder volume is given, which latter is equal to the sum of the combustion chamber volume and that of the piston sweep. In the above case it is found that the combustion space for a compression pressure of 75 lbs. gauge will be .225 of the total cylinder volume, or .225 ÷ .775 = .2905 of the piston sweep volume. Conversely, knowing the volumetric ratios, compression pressure can be read directly by proceeding from the scale of abscissas vertically to the curve and thence horizontally to the scale of ordinates.
CAUSES OF HEAT LOSS AND INEFFICIENCY IN EXPLOSIVE MOTORS
The difference realized in the practical operation of an internal combustion heat engine from the computed effect derived from the values of the explosive elements is probably the most serious difficulty that engineers have encountered in their endeavors to arrive at a rational conclusion as to where the losses were located, and the ways and means of design that would eliminate the causes of loss and raise the efficiency step by step to a reasonable percentage of the total efficiency of a perfect cycle.
An authority on the relative condition of the chemical elements under combustion in closed cylinders attributes the variation of temperature shown in the fall of the expansion curve, and the suppression or retarded evolution of heat, entirely to the cooling action of the cylinder walls, and to this nearly all the phenomena hitherto obscure in the cylinder of a gas-engine. Others attribute the great difference between the theoretical temperature of combustion and the actual temperature realized in the practical operation of the gas-engine, a loss of more than one-half of the total heat energy of the combustibles, partly to the dissociation of the elements of combustion at extremely high temperatures and their reassociation by expansion in the cylinder, to account for the supposed continued combustion and extra adiabatic curve of the expansion line on the indicator card.
Fig. 17.—The Thompson Indicator, an Instrument for Determining Compressions and Explosion Pressure Values and Recording Them on Chart.
The loss of heat to the walls of the cylinder, piston, and clearance space, as regards the proportion of wall surface to the volume, has gradually brought this point to its smallest ratio in the concave piston-head and globular cylinder-head, with the smallest possible space in the inlet and exhaust passage. The wall surface of a cylindrical clearance space or combustion chamber of one-half its unit diameter in length is equal to 3.1416 square units, its volume but 0.3927 of a cubic unit; while the same wall surface in a spherical form has a volume of 0.5236 of a cubic unit. It will be readily seen that the volume is increased 331⁄3 per cent. in a spherical over a cylindrical form for equal wall surfaces at the moment of explosion, when it is desirable that the greatest amount of heat is generated, and carrying with it the greatest possible pressure from which the expansion takes place by the movement of the piston.
Fig. 18.—Spherical Combustion Chamber.
Fig. 19.—Enlarged Combustion Chamber.
The spherical form cannot continue during the stroke for mechanical reasons; therefore some proportion of piston stroke of cylinder volume must be found to correspond with a spherical form of the combustion chamber to produce the least loss of heat through the walls during the combustion and expansion part of the stroke. This idea is illustrated in [Figs. 18] and [19], showing how the relative volumes of cylinder stroke and combustion chamber may be varied to suit the requirements due to the quality of the elements of combustion.
Although the concave piston-head shows economy in regard to the relation of the clearance volume to the wall area at the moment of explosive combustion, it may be clearly seen that its concavity increases its surface area and its capacity for absorbing heat, for which there is no provision for cooling the piston, save its contact with the walls of the cylinder and the slight air cooling of its back by its reciprocal motion. For this reason the concave piston-head has not been generally adopted and the concave cylinder-head, as shown in [Fig. 19], with a flat piston-head is the latest and best practice in airplane engine construction.
Fig. 20.—Mercedes Aviation Engine Cylinder Section Showing Approximately Spherical Combustion Chamber and Concave Piston Top.
The practical application of the principle just outlined to one of the most efficient airplane motors ever designed, the Mercedes, is clearly outlined at [Fig. 20].
HEAT LOSSES TO COOLING WATER
The mean temperature of the wall surface of the combustion chamber and cylinder, as indicated by the temperatures of the circulating water, has been found to be an important item in the economy of the gas-engine. Dugald Clerk, in England, a high authority in practical work with the gas-engine, found that 10 per cent. of the gas for a stated amount of power was saved by using water at a temperature in which the ejected water from the cylinder-jacket was near the boiling-point, and ventures the opinion that a still higher temperature for the circulating water may be used as a source of economy. This could be made practical in the case of aviation engines by adjusting the air-cooling surface of the radiator so as to maintain the inlet water at just below the boiling point, and by the rapid circulation induced by the pump pressure, to return the water from the cylinder-jacket a few degrees above the boiling point. The thermal displacement systems of cooling employed in automobiles are working under more favorable temperature conditions than those engines in which cooling is more energetic.
For a given amount of heat taken from the cylinder by the largest volume of circulating water, the difference in temperature between inlet and outlet of the water-jacket should be the least possible, and this condition of the water circulation gives a more even temperature to all parts of the cylinder; while, on the contrary, a cold-water supply, say at 60° F., so slow as to allow the ejected water to flow off at a temperature near the boiling-point, must make a great difference in temperature between the bottom and top of the cylinder, with a loss in economy in gas and other fuels, as well as in water, if it is obtained by measurement.
From the foregoing considerations of losses and inefficiencies, we find that the practice in motor design and construction has not yet reached the desired perfection in its cycular operation. Step by step improvements have been made with many changes in design though many have been without merit as an improvement, farther than to gratify the longings of designers for something different from the other thing, and to establish a special construction of their own. These efforts may in time produce a motor of normal or standard design for each kind of fuel that will give the highest possible efficiency for all conditions of service.
CHAPTER IV
[Engine Parts and Functions]—[Why Multiple Cylinder Engines Are Best]—[Describing Sequence of Operations]—[Simple Engines]—[Four and Six Cylinder Vertical Tandem Engines]—[Eight and Twelve Cylinder V Engines]—[Radial Cylinder Arrangement]—[Rotary Cylinder Forms].
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.
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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.—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.—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.—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.
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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.
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Fig. 26.—Diagrams Showing Duration of Events for a Four-Stroke Cycle, Six-Cylinder Engine.
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.