F. E. Nipher — Non-condensing Steam Engine. 281 



Art. XL. — On the Output of the Non-condensing Steam 

 Engine, as a Function of Speed and Pressure;* by 

 Francis E. Nipher. 



In the discussion which follows, the engine is supposed to be 

 running at a fixed cut-off, and without change in the throttle. 

 The pressure changes required to produce a change of speed 

 are supposed to be effected by a change in boiler pressure. 

 The effect of the throttle or the governor with automatic cut- 

 off will be pointed out as we proceed. 



The difference between the two belt-pulls, or the load on the 

 brake, is represented by w, r being the brake-arm, or radius of 

 the driving-wheel. If the belt-pulls are F' and F", then F' — 

 F' = w. It is supposed also that the mean effective pressure 

 required to drive the engine when w = is constant for all 

 speeds. In an engine with balanced valves and where the 

 amount of lubrication used increases with the speed, this 

 assumption may be tolerated for a general treatment of the 

 case, although the peculiarities of engines will doubtless cause 

 them to depart from this assumption in a more or less irregular 

 way. Engines are usually built for definite speeds, and often 

 behave poorly when run at widely different speeds from those 

 for which they were designed. 



For these reasons, some portions of this treatment cannot lay 

 claim to very great precision. It wi]l serve mainly to present 

 the general conditions of the problem, and may serve as a basis 

 for investigating the peculiarities of individual engines. 



Let P a = mean effective pressure when w = 0, 



-P ' = boiler pressure above atmospheric pressure when w = 0. 



P= mean effective pressure with load 10. 



R = piston radius. 



I = stroke, n = revolutions per minute. 



Then, during one stroke of the engine at uniform speed, 



7tR\P-P )l=7rrw. 



P = F -+Wi <*> 



-,.... . . . , 27t RHn 



Multiplying this equation by 



-r -27tRHnP n Inrnw . % 



Jhp = - + ... (2) 



33000 33000 v ' 



The indicated horse-power is equal to the brake horse-power 

 plus the power required to drive the engine alone. 



In the equation leading to (1) the second member should 

 strictly contain a term =f{F'+F") the exact form of which 

 * Read before the St. Louis Academy of Science, May 20th, 1889. 



