12 THE ENGINES OF THE HUMAN BODY 



explosive mixture of petrolised air (fig. 2a). If now the 

 piston is made to rise in the cylinder, by turning the crank- 

 pin half a turn more, the petrolised air will be squeezed or 

 compressed at the head of the cylinder and the engine will 

 be thus loaded (fig. 2b). The first downward movement of 

 the piston, which sucks in the petrol mixture, is called the 

 " charge " stroke ; the first upward movement which com- 

 presses the charge is named the " compression " stroke. 

 Thus two -strokes are spent in loading the engine. 



b- heF 



Fio. 2A. Fig. 2B. Fig. 2c. 



Charge stroke. Compression stroke. Driving stroke. 



Fig. 2D. 

 Exhaust stroke. 



When thus loaded, the charge is fired and the second 

 downward stroke of the piston takes place ; it is the 

 "explosion," "effective," or "driving" stroke (fig. 2c). 

 All the driving power of the engine lies in this stroke. With 

 the descent of the piston the cylinder is filled with exploded 

 or burnt gas. It is evident that before the engine can be 

 charged again the burnt gas must be cleared out from its 

 cylinder. The inventor got rid of it most ingeniously by 

 using two devices. He first made an outlet or exhaust 

 and set a valve there which opened automatically as soon 

 as the explosion was over and the driving stroke had 



