THE GAS ENGINE 



299 



An outward movement of the piston which draws gas and 

 air into the cylinder, or intake; inward movement and 

 explosion, or compression; outward movement and work, 

 or expansion; and inward movement, or exhaust. In the 

 other type, the " two-cycle," the explosion comes at every 

 inward movement of the piston, or once every revolution of 

 the fly-wheel. In this type the crank case is air tight, so 

 that air and gas can be admitted to the outer end of the 

 cylinder. A by-pass permits the mixture to go to the inner 

 end of the cylinder for the explosion. At the outward stroke 

 the gas and air are first compressed in the crank case and 



Fig. 73. — Gasoline tractor for plowing. 



then as the by-pass is opened by the movement of the piston, 

 the mixture is forced into the inner end of the cylinder, 

 driving ahead of it the burned gas through a port. At the 

 inward movement of the piston the mixture is compressed 

 and the explosion takes place, and at the same time gas and 

 air are drawn in to the outer end of the cylinder; then the 

 cycle is repeated. 



A very important part of the gas engine is the carburetor 

 or place where the air and gas are mixed. If the fuel is a 

 liquid at ordinary temperatures it is necessary to vaporize 

 it before an explosive mixture can be obtained. A liquid 

 like gasoline vaporizes very readily and it is only necessary 



