OTHER FUELS 



163 



Third or Power Stroke. 



Ignition by the spark (/I) of the compressed fuel charge produces explosion 

 and expansion of the gases, forcing the piston outwards, away from the cylinder 

 head (Al) during the power stroke. Both inlet and exhaust valves (H) and 

 (HI), respectively, are closed. 



Fourth or Exhaust Stroke. 



The piston (B), moving inward, towards the cylinder head (Al), drives the 

 burned gases out through exhaust valve (HI). Inlet valve (H) is closed. 



FIG. 132. 



Thus the four strokes of the piston, i.e., one power stroke fol- 

 lowed by three idle strokes, complete the cycle of events; hence we 

 have a four^stroke cycle. 



In a one-cylinder gas engine the flywheel thus makes two revolu- 

 tions for every power stroke. In a two-cylinder engine, with alter- 

 nate explosions in the two cylinders, one power stroke occurs for 

 every revolution of the flywheel. 



