COMPRESSION.--When the piston starts on its return stroke, or upward movement, it compresses the charge thus received, and when the piston nears the upper end of its stroke the sparking mechanism again explodes it, so that the cycle is formed by the two operations, performed by a single turn of the crank shaft. This latter type of engine is not used to a great extent. It has the advantage that no valves are used, except the one at the inlet of the gas to the crank case, and no stems, push rods, cam shafts, or springs are required to control the movements of the fresh and burnt gases. Aside from that such engines weigh considerably less than the four cycle type. ECONOMY OF FOUR-CYCLE ENGINE.--On the other hand, the four cycle is more economical, because there is more time for the admission of the fuel, and for exhausting the gases. Furthermore, it is obvious that in a two cycle engine more or less of the fresh fuel gas is mixed with and is discharged from the cylinder with the burnt gases. As the discharge of the burnt gases and the admission of a fresh charge, is practically simultaneous, the opening of the discharge is placed in the cylinder at such a point that the pressure of the gases cannot be utilized for the full downward stroke, as is the case with the four cycle type. [Illustration: THE FOUR-CYCLE ENGINE Fig. 63. Firing Position. Fig. 64. Return First Cycle.] VALVE MOVEMENTS.--Before proceeding to explain the engine in detail, the different valve movements of a four cycle cylinder are shown, and this will be of service in explaining the different parts as they are referred to. In the construction of engines, as will be more particularly pointed out hereinafter, the inlet and exhaust valves are usually operated by mechanical means, but certain engines are so constructed that the inlet valve is automatic in its operation, and the exhaust valve only is actuated mechanically. In the drawings, Figs. 63 to 66, inclusive, both valves are operated from cams on a secondary shaft, and in the first of these four figures the crank has just turned the point where the piston is at its highest limit, and is about to descend. Both valves A B are closed, and the spark fires the charge, driving down the piston to its lowest limit. In Fig. 64 the crank is shown about to move the piston upwardly, and just as it turns the dead center the cam C, on the secondary shaft, unseats the valve B, through the stem D. As the piston moves upwardly, the burnt gases are forced out past the valve B. When the piston reaches the highest point in its first revolution, as shown in Fig. 65, the stem D drops off the cam C, thus closing the discharge, and immediately the valve A is opened by the cam E moving the valve stem F upwardly, and as the piston now descends, fuel is now drawn in until the piston reaches its lowest point.