Transportation on Land 



263 



In the ordinary gas engine, the explosive gas is admitted on 

 one side of the piston, is compressed by the piston itself and then 

 is exploded by an electric spark. The force of this explosion 

 drives the piston head violently back, turning a heavy wheel 

 whose momentum drives it forward again. In automobile engines, 

 cylinders usually are connected in series. A study of the four- 

 cylinder motor will give an idea of the working of the gas engine. 



The four cylinders are set in a line. Each cylinder makes 

 four different kinds of strokes : the intake stroke to draw in new 

 gas ; the compression stroke ; the explosion, or power, stroke ; and 

 the exhaust stroke to force out used gas. Two of these strokes 



A STATIONARY GAS ENGINE 



are made in one direction and two in the other. Such an engine 

 is called a four-cycle engine. 



Gasoline entering drop by drop from the gasoline tank into 

 the carburetor, or mixer, vaporizes and is mixed with air to form 

 a highly explosive mixture. This mixture is drawn into each 

 cylinder above the piston. On the first upward stroke it is com- 

 pressed. At the proper instant, by means of a timing device con- 

 nected with the cylinder, an electric current is sent through a spark 

 plug in the head of the cylinder and is forced to jump a tiny gap, 

 thus causing a spark which explodes the vapor. The explosion 



