Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(36) Diagram of Four Stroke Cycle Engine.

By referring to paragraph 25, Chapter III, it will be seen that the five events of suction, compression, ignition, expansion and exhaust are accomplished in four strokes, in the following order:

Stroke 1.		Suction—(Mixture drawn into cylinder).
Stroke 2.		Compression—(Mixture compressed).
Stroke 3.	{	Ignition.
		Expansion (working stroke).
Stroke 4.		Exhaust—(Scavenging stroke).

These events with the pressures incident to each drawn to some relative scale are shown graphically in Fig. 10 by four lines representing the four strokes of the piston. In order to show the relation between the diagram and the piston, a sketch of the cylinder with a stroke equal to the length of the diagram is shown directly beneath the curve. The vertical line IJ is the scale of pressures (somewhat exaggerated in order that the small vacuum and scavenging pressures shall be clearly shown). The line marked “atmosphere” represents atmospheric pressure and it is from this line that all measurements of pressure are taken.

Figs. 10–11–12. Showing Respectively a Typical Four Stroke Diagram, Retarded Combustion and Retarded Spark.

Consider the piston starting on the suction stroke, the piston moving from the position L to K, or from left to right. The movement creates a partial vacuum in the combustion chamber N which is shown on the diagram as the distance OA, equal to 2 pounds below atmosphere according to the pressure scale. The suction line remains at this distance below the atmospheric line until within a short distance of the end of the stroke when it rises to meet the atmospheric line at B when the piston reaches the end of the stroke at K. This rise at the end of the stroke is due to the fact that the piston moves more slowly when approaching the end of the stroke while the velocity of the incoming gases remains nearly constant so that the piston exerts no pull nor suction. On the diagram the entire suction stroke is represented by AB.

The piston now returns on the compression stroke from K to J compressing the mixture in the combustion chamber N. On the diagram this stroke is shown beginning at B, with the pressure slowly rising until the pressure is a maximum at the point C at the end of the stroke. During the compression, the pressure has risen from that of the atmosphere at B to 125 pounds pressure at C as shown by the scale. At a point slightly before C is reached, ignition occurs, and the pressure rapidly rises from C to D, due to the expansion of the heated gas. In this case the combustion is practically instantaneous as shown by the straight, vertical combustion line CD.

At D the piston starts on the working stroke from left to right increasing the volume of the gas and at the same time diminishing the pressure because of the expansion until the maximum pressure of 400 pounds per square inch at D is reduced to 30 pounds per square inch at E, the line DE being called the expansion line. During this time the heated gas has been performing work on the piston. At E the exhaust valve opens and the pressure drops from E to T, a point still about 10 pounds above atmospheric pressure. Theoretically the pressure should drop instantly from E to atmosphere, or from 30 pounds per square inch to zero, but practically this is impossible because of the back pressure due the slow escape of the exhaust gases through the comparatively small valve openings and exhaust pipes. Since considerable pressure is exerted by the piston on the return stroke in forcing the gases out of the exhaust valve, the exhaust line TO on the diagram is nearly 10 pounds above the atmospheric pressure from T to O. At a point near O, the piston slows up on nearing the end of the stroke so the gases have more time to escape through the valves, and the pressure drops to the atmosphere, ready for the succeeding suction stroke.

It should be noted that the points A, B, E, and F represent periods of valve action. At A the inlet valve opens; at B the inlet closes; at E the exhaust opens; at F the exhaust closes, and at A the inlet again opens at the beginning of the suction stroke AB. That this is true is apparent from the fact the inlet must open at the beginning of the suction stroke, and both valves must be closed from the point B to the point E in order to prevent the escape of the compressed charge and expanded gases from the cylinder. At the end of the working stroke the exhaust valve must liberate the gases and remain open to the end of the scavenging stroke to eliminate the residual gas while the closed inlet valve prevents the burnt gases from being forced through the inlet pipe and carburetor.

As shown on the diagram, the exhaust valve closes at the same time that the inlet opens, as F, and O both occur on the same vertical line DL. This is true theoretically, but owing to the different conditions met in practice, the actual setting of the valves may vary slightly from that shown on the diagram. Some makers of high speed engines open the inlet slightly before the exhaust closes as it is claimed that the inertia of the exhaust gas passing through the exhaust pipe creates a slight vacuum that is an aid in filling the cylinder with a fresh charge. It should be borne in mind that this condition only exists when the piston has come to rest and exerts no pressure on the exhaust gas. The vacuum is due to the velocity inertia of the gas after it has been reduced to atmospheric pressure. Other makers close the exhaust valve a very little before the inlet opens, but no matter what the setting, the difference in the time of opening and closing is very small, and the results obtained probably differ by an almost negligible amount.

During the suction and scavenging strokes, the fly wheel of the engine is expending energy on the gas since it is moving a considerable volume at a fairly high pressure. In the case of the scavenging stroke, the piston is working against 10 pounds back pressure, which on a 10 inch piston would amount to a force of 785 pounds. With the 2 pound vacuum the drag on the piston would amount to 157 pounds, no small item when the velocity of the piston is considered. Of course the pressure of 10 pounds per square inch is rather high, but it is often attained with long and dirty exhaust pipes. It is items of this nature that cut into the efficiency of the engine, and increase the fuel bills, and it is only by the indicator that we can determine the extent of such “leaks” and remedy them.

Since the area of the indicator card represents the power of the engine, it is evident that we lose the power represented by the area included in the rectangle FEBO on the scavenging stroke plus the area BOA on the suction stroke. The area included in BCO represents the work taken from the engine in compressing the charge, but this is returned to us during the next stroke plus the benefits gained by compressing the mixture. The arrows show the direction in which the piston is moving during that event.

An actual engine does not follow the form of the diagram shown by Fig. 10 exactly because of certain conditions met with in practice such as imperfect mixtures, faulty valve and ignition timing, small valve areas or leakage. The combustion in the real engine is neither instantaneous nor complete but it approximates the “IDEAL” cycle just described more or less closely with a high compression and a fairly well proportioned mixture.