GAS, OIL AND ALCOHOL ENGINES 



403 



in 1876 the first engine of commercial value. All present-day types 

 work on the same principle as Otto's, but under fewer practical 

 difficulties. 



555- Types of gasoline engines. Otto was the first to 

 put into practice the idea of compressing the gas and air 

 mixture before igniting. This gave rise to the name of 

 Otto cycle, which is now used in all engines. Com- 

 pression is one of the important things which determine 

 the economy of the engine ; theoretically, the efficiency 

 of the engine depends upon the compression pressure. 

 However, it is not possible to increase the compression 

 pressure indefinitely because the charge pre-ignites and 

 causes the engine to pound. It is desirable, however, to 

 use as high a compression as possible. 



As stated before, practically all the engines used to- 

 day are designed to follow the Otto cycle. However, 

 they are divided into two distinct types, four-cycle en- 

 gines and two-cycle engines. 



556. Four-cycle engines. The term cycle is applied to 

 the entire operation of converting heat into mechanical 

 energy. In the four-cycle engine four strokes of the piston 



FIG. 28l SUCTION STROKE OF FOUR-CYCLE ENGINE 



