40. In Fig. 14, the piston is shown started on the expansion stroke, during the first portion of which combustion is taking place while the fuel is entering through the valve o In the figure, the valve o has just closed, combustion has about ceased, and expansion is beginning. The pressure drops slightly, while the fuel is still burning, as shown in the diagram from h to m; but the pressure is kept above the compression line by the heat of combustion. When the piston reaches the point having the pressure marked m in the diagram, the valve o is closed and combustion ceases, but the contents of the cylinder continues to expand until the exhaust valve opens at the pressure shown in the diagram at j.
41. The following tabulation of the Diesel cycle should be carefully compared with that of the ordinary four-stroke cycle:
DIESEL CYCLE
First Revolution First Stroke. — Outwards; suction of air only; inlet valve open; pressure falls below atmosphere. Second Stroke. — Inwards; all valves closed; compression; no combustion.
Second Revolution Third Stroke. — Outwards; oil valve open for early part of stroke; combustion during admission of fuel; expansion; pressure falls slowly during combustion and rapidly during expansion; exhaust valve opens near end of stroke; beyond this point the pressure rapidly falls to that of the atmosphere at the end of the stroke. Fourth Stroke. — Inwards; exhaust; exhaust valve open; pressure rises slightly above that of atmosphere.
VARIATIONS OF FOUR-STROKE CYCLE
42. Careful observation of the diagram in Fig. 8 will show that there is considerable pressure, as shown at j, remaining in the cylinder at the time the exhaust valve opens. If by some means the stroke of the piston could be
