Motor-car principles; the gasoline automobile

by Roger B. Whitman

INLET STROKE

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.

COMPRESSION-COMBUSTION STROKE

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.