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.
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.