78 POWE2, AND THE PLOW 



On its return trip it compresses the charge into a small clearance 

 space at the head of the cylinder. At a point usually previous 

 to the end of the compression stroke, the charge is ignited. By 

 the time the piston starts outward on the third, or power, 

 stroke, the full force of the explosion is exerted against the 

 piston head. As the flywheel again carries the piston back- 

 ward, the exhaust valve opens and the burned gases are expelled 

 from the cylinder, thus completing the cycle. There is but 

 one power stroke to four strokes of the piston; hence the name. 

 It will be seen that on the suction stroke the exhaust valve is 

 closed and the inlet valve open. On the exhaust stroke the 

 inlet valve is closed and the exhaust open. One automobile 

 manufacturer has aptly compared the four-stroke cycle to the 

 operations involved in firing an old muzzle-loading rifle. First, 

 the charge is admitted, and next it is rammed home. The 

 third step is the firing and the fourth consists of swabbing out 

 the gun barrel. 



DESIGN OF GAS TRACTORS 



The gas tractor consists essentially of power plant and traction 

 mechanism. The former consists of the same essential parts 

 as a stationary gas engine, while the latter includes the support- 

 ing frame, wheels, shafting and gearing for transmitting power 

 to the traction wheels. At the present time there are probably 

 one hundred distinct types and sizes of gas tractor. Variations 

 in design are so great that a strict classification would be 

 extremely complicated, and for general purposes they are 

 roughly divided into low, medium, and high speed types, with 

 a further class for those of special nature. For tractors of 

 standard types the above classes correspond quite closely to 

 the single, double, and four cylinder classes, and to a less extent 

 represent the power developed, the single-cylinder engines 

 as a class being the smallest and the four-cylinder the largest. 



The engines on single-cylinder tractors usually run at from 

 220 to 300 revolutions of the crankshaft per minute. The two- 



