Engines draw in air on the outward movement of the piston, and compress on the next stroke. Burning occurs at or near the inner dead center and expansion follows on the third stroke. Near the end of this stroke the exhaust valve is opened, the burnt gases being swept out on the fourth stroke. In some large engines working on low value fuel, such as furnace gases, air from a low pressure blower is admitted towards the end of the exhaust stroke. This assists in clearing the combustion space of dead gases, preventing further dilution of the next charge. The Four-stroke Cycle is by far the most popular, practically all car, truck and small aero engines. as well as the bulk of small and medium commercial engines, being of this class.
Cycle - In this the charge is compressed on the inward stroke, burning occurs near the inner center and expansion follows for the large part of the next stroke. Near the end of this second stroke, large ports or valves are opened and the gases are allowed to escape, so that the pressure in the cylinder falls to a low figure before the stroke is completed. Other ports are then opened, allowing air from a blower to enter, and the time of opening, size, and direction of the ports must be such that the residual exhaust is largely swept out and replaced by a clean charge before the piston has traveled far on the next (third) stroke, which then becomes the compression stroke for the second cycle. Curiously enough, two-stroke engines are usually found either in the very small class, such as is used in light motor cycles, lawn mowers, etc., using the underside of the piston and the crankcase as an air pump and running on a petrol-oil mixture, or in the very large class, mostly marine engines running on heavy oil and with elaborate scavenging arrangements. The reason, no doubt, is that the very simple ported cylinder crankcase compression two-stroke engine, while very convenient in small sizes is not economical in fuel, and introduces many problems, thermal and otherwise, when made in larger sizes. It is usually not worth while providing the blower equipment, piston cooling, etc., until such dimensions are reached that a considerable overall saving in weight, etc., is effected by the doubling of the working strokes. The two-stroke engine was very popular for the marine work in both single- and double-acting forms. Oil engines are to be found in the medium power category using piston controlled scavenge ports and poppet exhaust valves, e.g. General Motors, Foden, Napier 'Deltic', and Tilling-Stevens (with opposed pistons).