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

(24) Requirements of the Engine.

In order that an internal combustion engine shall operate and develop power continuously the following routine of events must occur in the cylinder in the following order, no matter what the type of engine.

(1) The cylinder must be filled with a combustible mixture of air and gaseous fuel at as nearly atmospheric pressure as possible.

(2) The mixture must be compressed in order to develop the value of the fuel.

(3) Ignition must take place at the end of the compression stroke or at the highest point of compression.

(4) Complete combustion of the fuel must follow the ignition of the charge, with an increase of temperature and pressure which will act on the piston to the end of the power stroke.

(5) After the piston has completed the working stroke the products of combustion must be ejected from the cylinder completely to make way for the admission of the new combustible mixture.

With the exception of the Diesel engine which (1) fills the cylinder with pure air without the fuel, and (2) injects the fuel after compression, all internal combustion engines not only perform each of these operations but proceed with events in the order given as well. The accomplishment of the five acts is called a “cycle of events,” or a “CYCLE,” and the series is performed in different ways in different types of engines. In the operation of the engine, the series of events occur over and over again, always in the same order, 1–2–3–4–5, 1–2–3–4–5, 1–2–3–4–5, etc. The five events are generally given in terms of the number of strokes of the piston taken to accomplish the complete routine, thus a two stroke cycle engine performs the series in two strokes, and a four stroke cycle engine in four strokes, and so on.

In order to obtain the benefits of high compression, perfect scavenging of the products of combustion from the cylinder and perfect mixtures, a great variety of engines have been developed in which the number of strokes taken to accomplish the five events varies. In some engines the cycle is accomplished in two strokes, in other engines it is accomplished in six strokes, but in the great majority of cases the cycle is performed in either two or four strokes, and as these are by far the most common routines, we will confine our description to engines of these types.