GAS, OIL AND ALCOHOL ENGINES 



405 



FIG. 254 EXHAUST STROKE OF FOUR-CYCLE ENGINE 



into the clearance space of the cylinder (Fig. 282). This 

 stroke is called compression. Just before the crank 

 passes dead center the charge is ignited. Owing to the 

 heat released, the gases expand, and this expansion of 

 gases acts upon the piston, driving it forward during the 

 third stroke, which is called expansion (Fig. 283). This 

 stroke is the only working stroke of the cycle. During 

 the fourth stroke the exhaust valve is forced open by 

 mechanical means and the piston crowds the burned 

 gases out. This stroke is called exhaust (Fig. 284). 



557. The two-cycle engine completes the cycle in two 

 strokes of the piston and from this fact derives its name. 

 In this type of engine there must be, besides the cylinder, 

 a compression chamber, which may be separate, which 

 may be the crank case enclosed, or which may be the front 

 end of the cylinder. To illustrate the cycle in this type 

 of engine, the enclosed crank case type is used. That is, 

 the cylinder and the crank case are both gas tight and 

 practically in one piece. However, the two chambers are 

 separated by the piston. Let the piston be at the crank 



