of the atmosphere. The outgoing gas acquires such a velocity that its momentum causes the pressure within the cylinder to fall slightly below that of the atmosphere before the port is completely uncovered by the piston. After the pressure in the cylinder has fallen to about that of the atmosphere, communication is opened to a reservoir containing a mixture of gas and air under pressure. This compressed mixture rushes in and fills the cylinder and by the time the exhaust opening is closed by the returning piston, the cylinder is charged. It will be seen that, for such a series of operations, it is necessary to provide some method of pumping the mixture into the cylinder.
50. In the simplest form of two-cycle engine, the cranks, the connecting-rod, and the end of the cylinder nearest the crank-shaft are enclosed in a case, called the crank-case. While the piston is compressing the charge in the cylinder, a partial vacuum is formed in the crank-case, into which a charge of gas and air is drawn. While the piston is making its expansion stroke, the mixture in the crank-case is compressed to a few pounds pressure above that of the atmosphere, usually from 4 to 6 pounds. When the expansion stroke has been nearly completed, the exhaust port is uncovered by the piston, and when the exhaust port is uncovered sufficiently for the pressure in the cylinder to fall to about atmospheric pressure, the piston uncovers another port, called the Inlet, or transfer, port, opposite the exhaust port and communicating with a passage, sometimes called the transfer passage, to the crank-case. This allows the mixture, already compressed in the crank-case, to rush through the inlet port and fill the cylinder with a fresh charge. This charging process, commencing before the exhaust has entirely escaped from the cylinder, assists in forcing out the exhaust gases.
51. Figs. 18, 19, and 20 are a series of illustrations showing the operation of a typical two-cycle engine, in which p is the piston; q, the crank-shaft; a, the crank; k the crankpin; r, the connecting-rod; e, the exhaust port;
