the low-pressure piston is at the bottom of its stroke. When the low-pressure piston moves up, the two high-pressure pistons move down, reaching the bottom of their stroke as the low-pressure reaches the top. The inlet valves of the high-pressure cylinders are at s and s', e and e' being the exhaust valves. The low-pressure cylinder has two inlet valves S and S' and an exhaust valve E. The only difference between the cycles of the high-pressure cylinders and that of the ordinary four-stroke cycle is that the exhaust valve opens at the end of the stroke, but not before. The cycles in the two high-pressure cylinders alternate, that is, while expansion takes place in one, the other is taking in a new charge, and while compression takes place in one, the other is exhausting.
In the figure, cylinder h is compressing, with both valves closed; while cylinder h' is exhausting into the low-pressure cylinder, valve e' and valve S' being open and the piston p' moving upwards while the piston P is moving downwards. When the piston P has made about eleven-twelfths of its stroke, the exhaust valve E begins to open, and by the time the pistons reach the ends of their strokes the pressure within h' and H is down nearly to that of the atmosphere. By the time the end of the stroke is reached, the valves e' and S' have closed. The piston p' then starts on its suction stroke, while H is exhausting through the valve E.
44. The indicator diagrams obtained from this engine are shown at the top of the figure, the high-pressure diagram at d and the low-pressure diagram at D. The only difference between the diagram d and that shown in Fig. 8 is in the exhaust line. In Fig. 8, the exhaust begins before the piston reaches the end of the expansion stroke, where the pressure drops nearly to the pressure of the atmosphere and rises slightly again during the exhaust stroke. In Fig. 16, diagram d, the exhaust begins at the very end of the stroke and the pressure falls gradually to nearly atmospheric pressure during the exhaust stroke and drops to that of the atmosphere at the end. The suction, compression, and expansion lines are practically the same in both figures.
