piston approaches the inner end of the stroke. During that portion of the inward stroke before this port is uncovered, the piston produces a partial vacuum in the crank-case, which is filled during the short interval in the two strokes that the port is uncovered.
This type of two-cycle engine is illustrated in Fig. 21, in which s indicates the third port, while the remainder of the engine is practically the same as in the engine already shown and the parts are lettered the same. In the three-port type of engine, the indicator card for the crank-case is of slightly different form. During the inward stroke, the pressure within the crank-case falls until the point m is reached. When the piston passes the outer edge of the port s, the mixture rushes in, causing the pressure to rise to that of the atmosphere at the point n, just about the time the outer edge of the port s is covered on the outward stroke of the piston. The pressures in the crank-case that are below atmospheric pressure are shown below O'V' , and the number of pounds below the pressure of the atmosphere are marked with the minus sign, as — 1, — 2, etc., at the left of the crank-case diagram. The diagram within the cylinder is the same as with the check-valve type shown in Figs. 18, 19, and 20.
56. Variations of the Two-Stroke Cycle. — The enclosed crank-case in the types of two-cycle engine described is nothing more nor less than an air pump. The variations of this form of engine are principally in the air pump and the connections of the air pump to the cylinder. In some forms of the enclosed crank-case type, the mixture from the case is carried through a long by-pass to the top of the cylinder and admitted through a valve located in the top of the cylinder head. This valve is held against its seat with a light spring, the pressure of which is overcome by the pressure in the crank-case, and the deflecting plate is omitted. Still another variation is made by enclosing the front end of the cylinder, using a piston rod and cross-head and guides, and admitting the mixture either through a piston-opened port or through a slide valve.
