The Four-Cycle Engine.—The most approved type is that which is known as the four-cycle. This will also be fully diagrammed so as to enable us to point out the distinctive difference.
Figs. 26 and 27 show sections of a typical four-cycle engine, in which the inlet and the exhaust valves are mechanically operated. The cylinder A is either cast with or separate from the crank case B, and has a removable head C. The upper end of the cylinder has a water space formed by the jacket D.
The inlet port E and the discharge port F are both at the upper end of the cylinder. The crank shaft G passes horizontally through the crank case, and it is not necessary, as in the case of the two-cycle-engine, to have the case closed tight.
The piston H is attached to the connecting rod I, which is coupled to the crank, as shown. The crank shaft has a small gear J, which meshes with two gears of double size on opposite sides of the crank shaft, one of the gears K, being designed to carry the cam L for actuating the stem L´, which opens the valve M in the port that admits the carbureted air.
The other large gear N is mounted on a shaft which carries a cam O that engages the lower end of a push rod P, to open the valve Q in the discharge port F. It should be observed that the stems L´, P, are made in two parts, with interposing springs R, so the valves may be firmly seated when the stems drop from the cams.