STEAM ENGINES 



371 



haust port by having it open in time for the exhaust 

 steam to be sufficiently released so that at the instant 

 the piston starts on the return stroke there is no back 

 pressure. Fig. 248 shows the lead, both inlet and ex- 



Eccenrno 



FIG. 249 



haust. Fig. 249 shows the valve when at its end of the 

 stroke, showing that the exhaust port is completely 

 opened, but that the inlet is not necessarily so. Fig. 250 

 represents the position of the valve when the piston is 

 at the opposite end of the stroke. It will be noticed that 



OanK 



FIG. 250 



This 



the lead in this case is the same as that in Fig. 248. 

 should be true in all engines. 



A lead of 1/32 inch is about proper for most engines. 

 Too much lead in a valve allows steam to enter the cylin- 

 der so soon that the piston has to complete its stroke 

 against boiler pressure, hence a loss of energy. Also 



