MOTOR-CAR PRINCIPLES
The action of a steam, gasoline, or hot-air engine depends on the principle that when air or other gas is heated it expands, and that if it is confined in a space that will not permit it to expand, in striving to do so it creates pressure against all parts of the chamber in which it is contained. The more a gas is heated, the more it will expand if it is free to do so, and if not free, the greater will be the pressure that it will exert in striving to expand. Pressure may thus be generated by heat, and following along similar lines, heat may be produced by pressure, for when the pressure of a gas is increased by compressing it, or forcing it to occupy a smaller space, the gas will become heated. The reverse is also true, that when a gas is cooled, its volume is reduced, which reduces the pressure that it exerts; similarly, reducing the pressure by permitting the gas to expand reduces its temperature.
To state these principles in another form, to create pressure in a gas it must either be heated or compressed into a smaller space, and to reduce its pressure it must either be cooled or permitted to expand.
The action of a locomotive, the most familiar type of steam engine, is no mystery, and the production of steam in the boiler, its passage to the cylinder, and the application of its steady pressure against first one side of the piston and then the other, resulting in the turning of the driving wheels, are well understood. Water being converted into steam in the boiler, pressure is created because of the tendency of the steam to expand, but the only place in which it may expand is the cylinder, where in so doing it moves the piston.
Fig. 1.—Engine Actions.
A gasoline engine is similar to a steam engine in that its piston is moved by the pressure exerted by a heated and expanding gas; it is different in that the pressure is produced inside of the cylinder by the combustion of an inflammable mixture of gasoline vapor, instead of being generated in a boiler away from the cylinder. The heat of the combustion creates great pressure, and as the piston is the only part that can give before it, it is moved from one end of the cylinder to the other, this motion being utilized in the turning of the crank shaft. The combustion, which is so rapid that the generally accepted term for it is explosion, can occur only after the mixture has been drawn into the cylinder, and so prepared that it ignites quickly and burns completely, with the object of obtaining the greatest possible heat from it in the shortest possible time. In order that one explosion may be followed by another, the burned and useless products of combustion must be expelled to make place for a fresh charge of the inflammable mixture.
These successive events, forming a cycle, must be performed as long as the engine runs, and the constantly changing pressure in the cylinder due to the movement of the piston allows a fresh charge to enter, prepares it, and expels the products of combustion after the pressure that they have exerted has been utilized.
While in the great majority of steam engines the steam acts first on one side of the piston and then on the other, in an automobile gasoline engine the pressure is exerted on only one side, the combustion of the mixture taking place between the piston and the closed end, or head, of the cylinder. The other end of the cylinder is open, and the piston slides between the ends, its movement from one end to the other, called a stroke, corresponding to a half revolution of the crank shaft.
Gasoline engines are divided into two classes, according to the number of strokes of the piston that are necessary to accomplish the cycle; in the most usual type, four strokes are necessary, the class being called the four-stroke-cycle, or four-cycle, in distinction to the two-stroke-cycle, or two-cycle, in which but two strokes are necessary.
Of the five events that compose the cycle, three (the inlet, during which the fresh mixture enters the cylinder, its compression or preparation, and the exhaust of the burned gases) are performed by the piston; during the power event the piston is moved by the pressure resulting from the combustion, while the combustion event is due to an outside source. In the four-cycle type of engine, which is in almost universal use for automobiles, the events are considered with reference to the movement made by the piston during which they are performed, and may be called the inlet, compression-combustion, power, and exhaust strokes. In order that the engine may continue to run, it is obvious that the events must be performed in the correct order, and that the failure of one will affect all the others.
During the inlet stroke, a charge of fresh mixture enters the cylinder as the piston makes an outward stroke from the closed toward the open end. When the piston makes the following inward stroke, the mixture is compressed and combustion occurs, the pressure from which drives the piston outward on the power stroke. This is followed by another inward stroke, which pushes the burned gases out of the cylinder. It will be seen that power is developed during only one stroke of the four, the other three being required in the preparation for the following power stroke. The movement of the piston over these three dead strokes is secured by attaching to the crank shaft a heavy fly wheel, the momentum of which, acquired during the power stroke, keeps the crank shaft revolving and the piston in motion while the events are performed.
Fig. 2.—Gasoline Engine Cycle.
The space between the piston and cylinder head in which the combustion occurs is called the combustion space, and the inlet and exhaust valves open into it, the first being that by which the fresh mixture enters, and the second that by which the products of combustion escape. The device for igniting the mixture projects into the combustion space, and the means of ignition in universal use for automobile engines is an electric spark.
During the stroke (Fig. 2), the piston is moved outward by the crank shaft, which is revolved either by hand or by the momentum of the fly wheel. This movement increases the size of the combustion space, thereby reducing the pressure in it, and the higher pressure of the atmosphere outside of the cylinder will force fresh mixture into the combustion space, the inlet valve being open to admit it. If the piston moves slowly, the mixture will be able to enter fast enough to keep the pressure in the combustion space equal to that outside, but at the high speed at which a gasoline engine is run the piston will reach the end of its stroke before a complete charge has had time to enter, so that the pressure in the combustion space will still be below that of the atmosphere. If the inlet valve closed at this point so that no more mixture could enter, the combustion of the partial charge would result in a lower pressure than would be possible with a full charge; the inlet valve should therefore remain open until the piston reaches the point of its next inward stroke at which the pressure in the cylinder equals that outside.
The compression and the combustion of the charge occur during the next inward stroke of the piston.
The period between the bringing together of the liquid gasoline and air and its admission to the cylinder is too brief to secure perfect combination, and the mixture that results is not satisfactory. A portion of the air will not have been able to come into contact with the gasoline, and much of the liquid will not have been vaporized; what passes into the cylinder consists of pure air, liquid gasoline, and a more or less perfect mixture of the two. The combustion of this would be slow and incomplete, resulting in loss of power and waste of fuel. In order to render the mixture more perfect, advantage is taken of the heat that is produced by compression; the inward stroke of the piston raises the temperature of the mixture by compressing it, the heat rendering the gasoline more volatile, and the compression forcing it into combination with the air. Even this does not result in the formation of a perfect mixture, for the period is too short to effect it. The failure of an engine to deliver full power may often be traced to this condition, for the air and gasoline vapor, instead of being thoroughly combined and mixed, will be in layers, so to speak, and the combustion will be slow and uneven. Future development of the internal combustion engine will no doubt eradicate this, to the increase of efficiency and economy.
The charge of inflammable mixture can produce a certain amount of heat, and the more rapidly and completely this heat is obtained, the greater and more sudden will be the rise in pressure. The pressure will be greater when the mixture is contained in a small space than when in a large, and as the combustion space is smallest when the piston is at its inmost point, the greatest pressure will be obtained if combustion is complete at this point. If the combustion of the mixture were instantaneous, it should be ignited at this point; but even though very rapid, it nevertheless burns slowly enough to make it necessary to ignite it sufficiently before the end of the stroke to have the combustion complete as the piston comes into position to move outward. The instant at which the mixture must be ignited in order to produce this result depends on the speed of the piston, for the interval between the ignition of a good mixture and its complete combustion does not vary to any great extent. When the piston is moving slowly, the mixture may be ignited toward the end of the compression stroke, for there will be sufficient time for complete combustion by the time the stroke is ended; but when moving at high speed, ignition must occur much earlier in the stroke, as otherwise the piston will have completed the compression stroke and begun to move outward on the power stroke before the mixture is entirely burned. The instant at which ignition occurs also depends on the mixture that is used, for its quality and proper combination make a difference in the rapidity with which it burns. The better the quality of the mixture, the faster and more completely it will burn, and ignition may occur later in the stroke than would be possible with a mixture of poor quality. As the mixture is ignited by the passing of an electric spark in the combustion space, the difference in the instant at which it occurs may be secured by permitting the spark to pass earlier or later, and this is under the control of the driver.
When ignition occurs early in the compression stroke, the spark is said to be advanced, in distinction to a retarded spark, which passes when the compression stroke is more nearly complete.
If the spark is advanced too much, combustion will be complete before the piston has reached the end of the compression stroke, and it will be necessary to force it to the end of the stroke against the pressure by the momentum of the fly wheel, in order that it may get into position to move outward on the power stroke. In such a case, the momentum may not be sufficient to overcome the pressure, and the piston will be brought to a stop. A retarded spark results in the combustion of the mixture being completed after the piston has begun to move outward on the power stroke, and the pressure will then be reduced because it is exerted in a larger space, the piston consequently being moved with less force; if the spark is still further retarded, the combustion will not be complete by the time the exhaust begins, and the heat from only a portion of the mixture will be utilized, because it will still be burning as it is forced out of the cylinder.
The position at which the spark occurs is one of the means by which the speed of the engine is controlled, for the low pressure that results from a retarded spark moves the piston at low speed, while the greater pressure from an advanced spark drives the piston outward with more force and higher velocity.
While high compression of the charge improves its quality, and results in combustion being more rapid and complete, it has limits, and if carried too far the heat generated by the compression will be sufficient to ignite the mixture. This would have a bad effect on the operation of the engine, for the pressure would then be produced at the wrong point of the stroke, retarding instead of assisting the revolution of the crank shaft. Modern practice has shown that in engines that are maintained at a proper temperature the best results are obtained by compressing the mixture to from sixty to eighty pounds to the square inch; there are instances in which a higher compression is obtained, but the liability to ignite the mixture prematurely makes it undesirable.
The increasing size of the combustion chamber as the piston moves outward on the power stroke permits the gases to expand, and in doing so the temperature will fall, the pressure decreasing in consequence. A further decrease in pressure is caused by the hot gases being in contact with the metal cylinder and piston, which absorb heat. The more slowly the engine runs, the longer the gases will be in contact with the cylinder walls, and the more opportunity there will be for loss of heat from this cause; at higher speeds, there will be less time for heat to be absorbed by the cylinder walls, and more will be utilized in expanding the gases and producing work.
Even at the outmost position of the piston, the combustion space will not be large enough to permit the gases to expand until their pressure has dropped to that of the atmosphere, so that they will still be exerting pressure. By opening the exhaust valve, the gases will have an outlet for expansion, and will begin to rush out. While the pressure might be utilized against the piston to the end of the power stroke, it has been found that better results are obtained by opening the exhaust valve before the piston reaches the end of the power stroke. There is then a higher pressure forcing the gases out than there would be later in the stroke, and the greater quantity of gases that escapes leaves less to be expelled during the exhaust stroke.
The inward movement of the piston pushes out of the open exhaust valve the gases that have not escaped through their desire to expand. The exhaust valve remains open for the entire stroke, but when the engine is running at high speed the piston moves so rapidly that the gases cannot escape fast enough to prevent their being slightly compressed. When the piston is at its inmost point, the gases are still flowing through the valve because of this slight compression, and if the valve closed, a portion would be retained. The best results come from the closing of the exhaust valve not at the end of the exhaust stroke, but a short time after the piston has begun to move outward again, during which period the compression forces the gases out. The exhaust valve closes at the point when the slight compression has been reduced to the pressure of the atmosphere by the escape of the gases and the enlargement of the combustion space.
If the piston completely filled the combustion space when at its inmost point, all of the burned gases would be expelled, but the necessity for leaving a space in which combustion may take place renders this impossible, and a small portion of the burned gases therefore remains in the cylinder. The space between the cylinder head and the piston when at its inmost point, called the clearance, should be as small as possible, in order that the amount of these gases remaining in the cylinder may not be sufficient to contaminate the fresh charge and weaken the pressure of its combustion.
The passages through which the burned gases are led away from the cylinder must be large and free from obstructions, for if a free flow is not permitted back pressure will be set up, which will prevent the largest possible amount of gases from escaping, and leave a greater portion to contaminate the fresh charge.
The power that a gasoline engine is capable of developing depends on the size of the cylinder, the pressure acting on the piston, and the speed at which it operates. A steam engine, which obtains its pressure from a boiler, can do work as soon as the steam is turned into the cylinder, but a gasoline engine must be running before it can be called on to deliver power. Because of the cycle of events on which its operation depends, the piston must be forced to perform the inlet and compression strokes before pressure can be developed, and it is necessary to revolve the crank shaft by outside means until a charge of mixture has been taken into the cylinder, compressed, and ignited, when the engine begins to work by the pressure from the combustion, and takes up its cycle. Not until this has been done can it be called on to do work.
A steam engine can be made to deliver more power than it is built for by increasing the pressure acting against its piston, and the full pressure of the boiler can be utilized when extra work is necessary. The power developed by a gasoline engine being greatly dependent on its speed, and there being no reserve by which greater power can be developed in emergencies, it is necessary for an engine of this type to be perfectly adapted to the work that is desired of it. At excessive speeds the piston acquires great momentum, which must be overcome at each end of a stroke by the crank shaft, and while a speed above normal may be attained, it results in the quick destruction of the bearings and the severe straining of the engine. The best results in efficiency and long life accompany the running of the engine at the slowest speed possible for the development of the required power.