THE INTERNAL -COMBUSTION TRACTOR 77 



The fundamental principles of the internal-combustion 

 engine are the same, no matter what fuel is used, and for con- 

 venience we may speak of it as the gas engine. The class may 

 first be divided into the two-cycle and the four-cycle types. By 

 a cycle we mean a complete series of events in which one work- 

 ing stroke occurs. In the two-cycle engine the charge is drawn 

 into a separate chamber, usually the crank case, and there 

 partially compressed by the outward movement of the piston. 

 At the end of its working stroke the piston uncovers a port 

 in the cylinder which is connected by a side passage with the 

 crank case, and forces a charge into the combustion chamber. 

 At the same time it opens a port on the opposite side of the 

 cylinder, out of which the exhaust gases rush while the incoming 

 charge is filling up the combustion chamber. A projection on the 

 end of the piston deflects the new charge toward the head end of 

 the cylinder, with the result that the cylinder is more effec- 

 tively scavenged of the burned gases, which would tend to 

 dilute the fresh mixture. As the piston is carried back through 

 the force of the flywheel, the charge is compressed in the 

 cylinder and ignited at the proper time. Thus a working stroke 

 occurs to each revolution of the flywheel, or to each two strokes 

 of the piston. 



The two-cycle principle is used largely on marine engines, 

 where lightness and compactness are the prime essentials. It 

 is used on tractors to a very limited extent. Having twice 

 the number of power strokes at a given speed, the two-cycle 

 engine will naturally deliver more power than the four-cycle. 

 However, owing to the difficulty in combining the suction, 

 power and exhaust strokes in one, the power developed is only 

 from one third to three fourths more than would be obtained 

 from a four-cycle engine of the same dimensions and speed. 

 This type is also less economical of fuel. 



In the four-cycle engine, which is practically universal on 

 tractors, the piston moves outward on what is known as the 

 suction stroke, during which a charge is drawn into the cylinder. 



