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Our Surroundings 



drives the piston back in a power stroke. On the next stroke 

 upward the exhaust valve opens so that the piston forces out the 

 burnt gas. On the next downward stroke the exhaust valve closes, 

 the vapor valve opens and vapor is drawn in by its own expansion 

 and by the vacuum created by the downward movement of the 

 piston. On the next upward movement the vapor is compressed 

 and the process continues. Each piston makes four strokes for 

 each explosion of gas, but no two explosions occur at the same 

 instant. Thus, as speed increases, there is a fairly continuous 

 application of force to the drive rod or crank shaft, with which 

 the pistons are connected. 



SPARK PLUGS 



ADMISSION 



COMPRESSION 



EXPANSION 



EXHAUST 



THE FOUR STROKES OF A GAS ENGINE 



When the engine runs at high speed explosions are very rapid 

 and the heat developed is extreme, often exceeding 3,000 de- 

 grees F. To keep this heat down, most gas engines have water 

 jackets built around the cylinders and connected by tubes with 

 the radiator at the front of the automobile. The water in the 

 jacket is heated by the cylinders and then circulates, either 

 by convection or by means of a force pump, through the radia- 

 tor where it passes through many small tubes and is cooled by 

 the air. It then returns to the cylinders to relieve them of more 

 heat. A fan, set in front of the motor, is also used to air-cool 



