rapidly, and during the short time at the end of the stroke when the piston is practically at rest the pressure rises from g to h. This stroke is called the compression stroke.
34. In Fig. 10 is shown the expansion stroke, during which the pressure of the heated gases drives the piston toward the right, the pressure falling as the piston moves forwards, as shown by the drop of the line hj. When the expansion stroke has been nearly completed, the exhaust valve e is opened, and from j to V the drop of pressure is due both to expansion and to the escape of the gas through the exhaust valve. By the time the end of the stroke is reached, the pressure has fallen very nearly to that of the atmosphere, and the expanding gas has done its work.
35. In Fig. 11, the piston is returning, the exhaust valve e being open, and the gases are driven from the cylinder to prepare it for the reception of a new charge. There is a small rise of pressure during this stroke, due to the driving of the gas from the cylinder, and this is indicated by the line w. At the end of the exhaust stroke, the exhaust valve closes, and the succeeding outward stroke begins a new cycle with the suction of a fresh charge of gas and air.
36. The series of operations that take place during the four-stroke cycle are as follows:
FOUR-STROKE CYCLE
First Revolution
First Stroke. — Outwards; suction; inlet valve open; pressure falls below atmosphere. Second Stroke. — Inwards; compression; both valves closed; pressure rises; ignition before end of stroke, followed by explosion and rapid rise of pressure.
Second Revolution
Third Stroke. — Outwards; expansion; both valves closed; pressure falls; exhaust valve opens near end of stroke. Fourth Stroke. — Inwards; exhaust; exhaust valve open; pressure rises very little above that of the atmosphere.
