made longer for the expansion stroke than for the suction and compression strokes, this pressure could be made available and considerably more work could be obtained from the gas. The diagram would be extended as shown by the dotted lines in Fig. 15, the scale of this diagram being the
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same as in the diagram of Fig. 8; the theoretical gain is between 25 and 30 per cent. This may be accomplished in several ways. The earliest attempt was made by means of the Atkinson variable-stroke engine, a very complicated piece of mechanism and now no longer in use. Another method is known as compounding; in this, the exhaust gases pass into another cylinder of larger diameter and the expansion is continued nearly to the pressure of the atmosphere. Still another method is to close the inlet valve before the suction stroke is completed, allowing the piston to produce a partial vacuum in the cylinder, the pressure being restored on the return of the piston, and the compression above the atmosphere beginning when the piston has reached approximately a point in the suction stroke where the inlet valve was closed. This is known as the variable cut-off engine.
43. Compound Engines. — The compound engine is usually made with three cylinders, a high-pressure cylinder being located at each side of the low-pressure cylinder, as shown in Fig. 16. The engine illustrated is of the vertical type, with a low-pressure cylinder H and high-pressure cylinders h and h'. The cranks are 180° apart, so that the high-pressure pistons are together at the top of their stroke while
