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.
The four stroke cycle engine, some times improperly called the “four cycle” engine is the most widely used type for all classes of service, except possibly for marine work. Its extended use is due to its superior scavenging, high efficiency and reliability, although it is somewhat more complicated than the two stroke cycle type. Its ability to function properly under a wide variation of speed has driven the two stroke cycle type out of the automobile field, and its many admirable characteristics have cut a wide swath in the marine field, the stronghold of the two stroke cycle type.
A four stroke cycle engine performs the cycle of events in four strokes or two revolutions, only one of the strokes being a power of working stroke. In a single cylinder engine the explosion in the working strokes supplies enough power to the fly-wheel to carry the engine and its load through the remaining three strokes. Thus the energy stored in the fly wheel is sufficient to carry not only the load during the idle strokes but to “inhale” and compress the charge as well. Due to the long interval that exists between explosions, they are corresponding heavy and are productive of heavy strains in the engine and are the cause of considerable vibration.
To reduce the ill effects of the heavy intermittent blows, the majority of automobile and stationary engines are provided with two or more cylinders, the power being equally divided among them. In a four cylinder engine, there are four times as many impulses as in a single cylinder engine and the blow dealt by the individual cylinder is only one-quarter as great. While a single cylinder engine has an impulse only once in every other revolution, the four cylinder has two impulses in one revolution. Besides the advantages gained by increasing the impulses, the mechanical balance of a multiple cylinder engine is always better than that of a single and is also much lighter in weight since less material is required to resist shocks of the explosions.