14. An eccentric (Fig. 163), is a round disc mounted a little to one side of its center, _A_, on the engine shaft _B_. A band, _C_, on the circumference of the disc is connected by a rod, _D_, with the slide valve in the steam chest. How is the rotary motion of the shaft changed into a backward and forward motion of the slide valve? (4) HEAT ENGINES =196. The Gas Engine.=--One of the heat engines in common use to-day is the gasoline engine. It is used to propel automobiles and motor boats, to drive machinery, etc. The construction and action of a gasoline engine may be understood by studying a working model, or by proper diagrams. [Illustration: FIG. 164.--Cut away view of a modern automobile engine, with parts requiring attention most frequently, indicated. (Courtesy of the "Automobile Journal")] The common gasoline or gas engine is called a four-cycle (better four-part cycle) engine (see Fig. 164), since it requires four movements of the piston to complete one cycle or series of changes. This is illustrated in Fig. 165 =1=, which represents a cross-section of the _cylinder_ of the gasoline engine with the _piston_ moving downward. At the upper end of the cylinder are two _ports_ or openings. One, the _exhaust_ port, is closed, the _inlet_ port is open and a mixture of gas and air is entering. Fig. 165 =2= shows the piston returning; both ports are closed and the "charge" of air and gas is being compressed. As the piston reaches the end of its stroke in compressing the charge, an electric spark explodes or "fires" the charge of gas and air. The hot burning gas expands suddenly driving the piston downward with great force (Fig. 165 =3=). The piston rod is attached to the crank of a heavy fly-wheel and this is given sufficient energy or momentum to keep it going through the next three strokes. Fig. 165 =4= represents the returning piston pushing out the burnt "charge" through the open exhaust valve _e_. On the next downward motion of the piston the valve _e_ closes. It opens, and new charges of gas and air enter and the "cycle" is repeated. [Illustration: FIG. 165.--The four strokes of a gas engine cycle.]