During the last—the second inward—stroke (fig. 9) the exhaust valve is opened, and the returning piston sweeps all the burnt gases (the product of combustion) out into the exhaust pipe and so into the atmosphere. This completes the cycle, and the piston, crank, and valves are in the same relative positions as formerly, and the same series of operations is repeated again and again. Of course, it is not always the case that both air and gas valve are opened on the charging stroke; that depends upon the method employed to govern the speed of the engine. Supposing it were governed on the hit and miss principle (to be explained hereafter), the gas valve would be allowed to remain closed during the charging stroke, and air alone would be drawn into the cylinder, then compressed, but not being explosive would simply expand again on the working stroke, giving back nearly all the energy which was absorbed in compressing it, and finally be exhausted in the same manner as the burnt gases are.
Fig. 8.—Second out stroke, showing position of valves during working stroke.
Fig. 9.—Second inward stroke, showing position of valves during the exhaust stroke.
Fig. 10.—First out-stroke, showing position of valves during the charging stroke.
Fig. 10 shows diagrammatically the position of crank, piston, and valves during the charging stroke.
