and the exhaust gases are escaping from the exhaust port e, as shown by the dotted arrows, while the fresh charge is entering through the by-pass b and port o, and is thrown by means of the deflecting plate d toward the cylinder head, as shown by the full arrows. The momentum of the column of exhaust gas as it leaves the cylinder is so great that, unless there is considerable resistance in the exhaust passage, the pressure falls below that of the atmosphere, as shown by the small loop w, and is raised slightly, as shown by the loop y, when the fresh charge enters from the crank-case. If the engine is properly proportioned, none of the new mixture will escape at the exhaust port e, as it will be closed before the fresh charge has reached it. During this part of the stroke, the pressure in the crank-case rises from c' to c and then drops to V', when the transfer port is opened. The following inward stroke compresses the new mixture in the cylinder and draws a new charge into the crank-case, as illustrated in Fig. 18, thus beginning a new cycle.
54. The series of operations taking place during the two-stroke cycle are as follows:
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TWO-STROKE CYCLE
Cylinder Crank-Case
First Stroke, Inward
Compression; pressure rises; ignition near end of stroke, followed by explosion and rapid rise of pressure.
Suction; inlet valve open; pressure falls below atmosphere.
Second Stroke, Outward
Expansion; pressure falls; exhaust followed by entrance of fresh mixture from crank-case.
Compression; pressure rises to from 4 to 8 pounds; charging cylinder; pressure falls to atmospheric pressure.
55. Three-Port Two-Cycle Engine. — In some forms of two-cycle engines, especially those designed for high speeds, a piston-opened port is used instead of the inlet-valve s shown in Figs. 18, 19, and 20. This port is so located that it is opened by the crank-case end of the piston as the
