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.
In Fig. 66 the crank is turning the dead center, and is about to move upwardly, and the cams G E are now both in such position that the valves A B are closed, and when the piston moves up again, to complete the second revolution, the fuel gas within the cylinder is compressed, and ready to be fired the moment the crank reaches the position, shown in Fig. 63.
Fig. 65. Drawing in Charge.
Fig. 66. Compression.
The Ignition Point in the Cycle.—In practice, the firing takes place before the crank has made the turn past the dead center, and this is called pre-ignition, when the spark is advanced too far to the left. The ignition should take place slightly before the crank turns, because it takes a small interval of time for the charge to burn the gases, and during this time the crank will have passed the dead center, and started on its way downwardly.
From the diagrams it will be observed that two of the strokes, namely the first and the third, are downward, and the second and fourth are upward, and that the downward strokes take place during the admission and impulse, and the compression and exhaust while the piston moves upwardly.
The Fly-Wheel.—As the impulse in this type can take place only at each second revolution, it is obvious that some means must be provided to keep the shaft moving during the two turns, and for this purpose the fly-wheel is utilized.
Practice has found the multi-cylinder type the most valuable, in connection with the fly-wheel, as in employing two or more cylinders in line, a smaller fly wheel will be sufficient.