F. E. Nvpher — JS on- condensing Steam Engine. 283 



/ d(lH P ) \ / d(Pnp) \ _ 2nBHn 



\ dP )n "' \ dP )n ~ ~33000 



/d(Iup)\ _ ItzRHP 



\ dn )r 



'P 33000 



/ d(£Hp) \ _ 27tRH(P-P i 



') 



Inrw 



\ dn ) P 33000 33000 



. In fig. 1, oP' and oA" are the axes of pressure, HP, AA" is 

 the line of atmospheric pressure, and W" is the vacuum line. 

 The lines op" and P^p' are rectilinear elements in the surfaces 

 of Thp and i?HP at constant speed, the ordinates Pp" and Pp' 

 representing simultaneous values. If the mean effective pres- 

 sure were reduced to zero, the engine being driven at the same 

 speed by means of the belt, the power required is represented 

 by od. Line VJB represents £h.y as function of boiler pres- 

 sures, OP and OP ! being simultaneous values of mean effec- 

 tive and boiler pressure, measured from the atmospheric line. 



V V represents the belt-power required to drive the engine if 

 boiler and mean effective pressure were zero. 



Calling h = the atmospheric pressure, and P'= boiler pres- 

 sure measured from atmospheric pressure, we have 



h+P _ h+P 

 h+P ~ h+P' 

 or 



*"=-*+ &£<A+J\) ■ • • (6) 



In this equation the value of P is known from (1). P is de- 

 termined by means of the indicator. It remains to determine 

 P\, the boiler pressure required to drive the engine at the 

 fixed speed represented in fig. 1, when w — o. 



If the engine were driven at a very slow speed, the cylinder 

 pressure would be identical with boiler pressure until the point 

 of cut-off. The mean effective pressure would be less, and 

 the back pressure would be h. Increasing the boiler pressure, 

 the back pressure increases by a quantity which is proportional 

 to the speed. 



Measuring P\ from the atmospheric line, 



P' = cn+P +E+bn . . . (7) 



where en is the mean back pressure, P the mean effective 

 pressure, E a function of the inverse expansion ratio, and on 

 is a term applying to the entry port which is entirely analogous 

 to en. The constants c and b depend upon the size of the 

 ports, b also depending to a less extent upon the pipes connect- 

 ing the steam -chest and boiler. In a throttle governor, the 



