THE DIESEL CYCLE
38. A modern form of the four-stroke cycle particularly adapted to oil engines is known as the Diesel cycle, having been named after its inventor. It differs from the ordinary four-stroke cycle in that pure air only is drawn in during the suction stroke and that the compression is continued until the pressure reaches between 450 and 550 pounds. The compression heats the air to about 800° F., and any fuel forced into the cylinder at this point will take fire and burn as it enters, in the same manner as gas burns while issuing from a burner. The fuel is burned until it is cut off or until there is no longer sufficient air to support combustion. In the Diesel cycle, oil is forced into the cylinder, with air, at a pressure of from 20 to 50 pounds above the pressure in the cylinder, and just as the compression stroke is completed. After the piston has proceeded a short distance on its expansion stroke, the fuel valve is closed and the expansion takes place as in the ordinary four-stroke cycle gas engine.
39. The operation of this engine is illustrated in Figs. 13 and 14. The various parts of the engine are lettered as in Figs. 8 to 11, except that there is no igniter. There is a small oil-fuel valve shown at o. Air only enters through the inlet valve s during the suction stroke, while the fuel is forced in, under pressure, through the valve o at the end of the compression stroke, which is the position shown in Fig. 13. The small space between the piston and the cylinder head is the clearance space; by comparing this with the larger clearance space of the ordinary four-cycle engine, as shown in Figs. 8 to 11, it will be seen that the compression in this engine is carried to a much higher pressure than in the ordinary four-cycle engine. This is also shown by comparing the point h of the diagram of this engine with that of the diagrams in Figs. 8 to 11. In Fig. 13, the suction pressure in the cylinder is represented by the line v of the diagram and the exhaust pressure by the line w, the suction stroke and the exhaust stroke being practically identical with that of the ordinary four-cycle engine.
