426 



FARM MOTORS 



567. Indicated horse power. The formula for indicated horse 

 power in the gasoline engine is : 



PLAN 



H. P. = 



where 



33,000 



P = mean effective pressure, 



L = length of stroke in feet, 



A = area of piston in square inches, 



N number of explosions a minute. 



It will be seen that this formula is the same as that for steam 

 engines, excepting that N represents the number of single explosions 

 a minute in the gasoline engine formula, instead of the number of 

 revolutions a minute, with two impulses for each revolution, as in 

 the steam engine. 



568. Testing. To make a complete test of a gasoline 

 engine requires a great deal of expensive apparatus. Not 

 only is this apparatus needed, but the one doing the test- 

 ing must have a very good knowledge of science as far 

 as it pertains to heat and engines. However, a very sim- 

 ple apparatus can be arranged so that any farmer, if he 

 cares to take the trouble, can make a test which will cover 

 all matters as far as he is concerned. The formula for 

 B.H.P. is the same as that given in Chapter I. for 

 steam engines, and the same brake can be used. A speed 

 indicator can be procured 'for a dollar, and a set of scales 

 or spring balances can be easily secured. It will require 

 two men, who must work simultaneously. Before start- 

 ing to make the test it will be well to draw up a form 

 about as follows : 



Test i 2 * 



