192 STEAM ENGINES 



General Rule for Calculating I. H. P. Knowing the dimen- 

 sions and speed of the engine and the mean effective pressure 

 on the piston, all the data for finding the rate of work done 

 in the engine cylinder expressed in horsepower are at hand. 

 Let H indicated horsepower of engine; 



P= M. E. P., in pounds per square inch; 

 A = area of piston, in square inches; 

 L = length of stroke, in feet; 

 N = number of working strokes per minute. 

 PLAN 



Then - H= 



In a double-acting engine, or one in which the steam acts 

 alternately on both sides of the piston, the number of working 

 strokes per minute is twice the number of revolutions per 

 minute. For example, if a double-acting engine runs at a 

 speed of 210 R. P. M. there are 420 working strokes per minute. 

 A few types of engines, however, are single-acting; that is, 

 the steam acts on only one side of the piston. Such are the 

 Westinghouse, the Willans, and others. In this case, only 

 one stroke per revolution does work, and, consequently, the 

 number of strokes per minute to be used in the foregoing 

 formula is the same as the number of revolutions per minute. 

 Unless it is specifically stated that an engine is single-acting, 

 it is always understood, when the dimensions of a steam engine 

 are given, that a double-acting engine is meant. 



Piston Speed. The total distance traveled by the piston 

 in 1 min. is called the piston speed. It is customary to take 

 the stroke in inches. Then, to find the piston speed, multiply 

 the stroke in inches by the number of strokes and divide by 12; 



or, letting 5 represent the piston speed, S = rr^t where / is 



the stroke in inches. But N =2 R, where R represents the 

 number of revolutions per minute. Hence, 



_M _IX2JR l_R 



~ 12~~12~ " 6 



EXAMPLE. An engine with a 52-in. stroke runs at a speed 

 of (id R. P. M. What is the piston speed? 



52X06 

 SOLUTION. By the formula, 5- =572 ft. per rr.in. 



