Second Revolution
Third Stroke. — Expansion nearly to atmospheric pressure; both valves closed; exhaust valve opens slightly before end of stroke.
Fourth Stroke. — Exhaust; exhaust valve open.
48. It should be noted that, in all the engines thus far described, the power impulse is received by the piston on one end only. This style of engine is known as a single- acting engine, no matter how many cylinders it may have. In the double-acting engine the cylinder is closed at both ends and the power is transmitted through a piston rod and a crosshead. In the early days of gas-engine practice, it was thought impracticable to use a piston rod because the heat from the burning gas would cause the rod to cut and wear rapidly. It has been found, however, that this difficulty may be overcome by using a hollow rod and a hollow piston and passing water through them. At present, some of the larger high-powered engines are made double-acting, with valves at each end of the cylinder. In general construction, these engines are of the ordinary four-cycle automatic cut-off type, or are two-cycle engines with independent pumps, which will be described later.
TWO-CYCLE ENGINE
PRINCIPLES OP OPERATION
49. As explained in Art. 28, two-stroke cycle, or, briefly, two-cycle engines are those which require two strokes of the piston, or one revolution of the crank-shaft, to complete the cycle. This is accomplished by eliminating the suction stroke and the exhaust stroke of the four-cycle type of engine. When the expansion stroke is nearly completed, the piston uncovers an opening in the cylinder wall, through which the exhaust gases readily escape, owing to their having a pressure of about 40 pounds above that of the atmosphere. By the time the piston has reached the end of the stroke, the pressure within the cylinder has fallen to that
