The operation of a four-cycle machine may be understood by studying the four different steps in the working of the engine. There are two openings or valves in the cylinder - an inlet valve for the mixture to enter, and an exhaust valve for the disposal of the gases. When the piston is at its highest position, the valves are closed. As soon as the engine is running, the motion of the fly-wheel carries the piston down, and the partial vacuum created behind causes the inlet valve to open because the outside atmospheric pressure is greater than the inside pressure. Many up-to-date engines have a mechanical inlet, and do not depend upon atmospheric pressure to open the inlet valve. The explosive mixture of air and gas enters and fills the cylinder. The momentum of the fly-wheel is sufficient to keep the piston moving. The greatest power is derived from an engine when the gas explodes just before the piston reaches the highest point, because the speed of the piston makes it necessary to ignite the gas at the top of the stroke in order to have complete combustion. The spark-plug, screwed into the opening, gives off a spark which explodes the mixture. As the piston rises again, the exhaust valve opens mechanically and the burnt gases, still very hot, escape through the

Fig. 177. - The Cylinder and Water Jacket (in section) of a GasEngine. Designed to prevent premature ignition of the mixture and to assist in lubricating the piston. The water surrounds the cylinder in the section marked "Water." exhaust pipe. The piston passes through the cylinder four times, twice in each direction. The first mixture of air and gas is drawn in during one stroke; then the mixture is exploded; the force of the explosion starts the next stroke, and on the return the burnt gases are driven out.

The heat generated by the burning of the oil is so great that the walls of the cylinders would become red hot if water were not circulated over them by a pump. The cranks of the engine revolve in an oil-tight case and are dipped in oil so that it will splash up into the cylinder and in this way keep the piston well lubricated.