FARM MOTORS 347 



ance space at the end of the cylinder. This operation is 

 necessary in order to get the full power out of the fuel. 



Third, the expansion stroke. Just before the end of the 

 compression stroke the ignitor acts so that combustion takes 

 place; and at the end of this stroke there is a high pressure 

 ready to act under the piston, pushing it forward, thus doing 

 the work. 



Fourth, the exhaust stroke, during which the piston 

 returns toward the closed end of the cylinder and the exhaust 

 gases are pushed out through the exhaust valve. At. the end 

 of this stroke the piston is again at the beginning of the suc- 

 tion stroke. To complete the cycle it is noticed that two 

 entire revolutions of the crank shaft and fly wheels have been 

 required and that only one of these four strokes is a working 

 stroke, or a stroke during which the engine is receiving power. 

 During the other three strokes the fly wheels must furnish the 

 energy to keep the engine in motion. 



Two-Stroke Cycle Engine. The two-stroke cycle engine 

 is an attempt to increase the number of working strokes by 

 providing an auxiliary chamber in which the gasoline or fuel 

 mixture is given such an initial compression that at the end 

 of the exhaust stroke these fresh, unburned gases under com- 

 pression readily displace the burned gases. This displace- 

 ment takes place so quickly that it is possible to compress the 

 fresh gases during the return stroke. These operations are 

 shown in Fig. 222, which shows in outline an engine using the 

 crank case as a compression chamber. Owing to the larger 

 number of working strokes for a certain rotative speed the 

 two-cycle engine has the advantage of light weight. 



As the events in the two-cycle engine occur in every revo- 

 lution instead of once in two revolutions, the two-cycle engine 

 is of more simple construction. A secondary shaft operated 

 by a reducing gear for opening the valves and making one 



