HORSE POWER OF STEAM ENGINES. 457 



HORSE POWER OF STEAM ENGINES. 



PKOFESSOR R. C. CARPENTER. 



[From Rural New Yorker.] 



Meaning of " horse power ;" horses and engines as motive powers ; mode of ascertain- 

 ing horse-poiver ; steam expansion ; nominal and actual horse-poiver ; horse-power of 

 an engine varies with the conditions ; strength of horses ; strength of a horse equals 

 two-thirds a horse-power of an engine : variations in power of a horse ; power of 

 machines ; absurd misrepresentations. 



The work done by a steam engine is usually expressed in units of work 

 called "horse-power." This term was invented by James Watt, who defined 

 it as the ability to lift 33,000 pounds one foot each minute or a less number 

 of pounds an equivalent number of feet per minute. In obtaining this unit, 

 the actual work of a large number of horses was measured, from which it was 

 found that the work of the average horse, working 10 hours per day, was 

 equivalent to 22,000 pounds lifted one foot each minute. From this it is 

 seen that the unit adopted for the steam engine is 50 per cent greater than 

 the work of the average horse. This I think is in accordance with experi- 

 ence where steam has been substituted for horses in driving the machinery. I 

 have made many inquiries of some threshers, receiving the inevitable answer 

 that a ten horse-power engine gave very much more power than 10 horses. 

 It is, no doubt, true that many have met with experience very different from 

 this, especially where engines nominally of two or three horse-power have 

 been substituted for the same number of horses. This probably cannot be 

 accounted for except in a general way; but, no doubt, it comes from the fact 

 that either exceptionally strong horses had been used, or that the engine did 

 not develop its nominal horse-power. 



The nominal horse-power of an engine is calculated by multiplying the 

 average steam pressure acting on each square inch of the piston, by its area 

 and by the space passed through in feet per minute and dividing that result 

 by 33,000. This, of course, is somewhat greater than the power actually 

 developed, as it makes no allowance for friction and other losses. As the 

 engine becomes smaller these losses become a greater and greater proportion 

 of the work of the engine. The average steam pressure acting on the piston 

 is much less than the boiler pressure. This is due to two facts: first, the 

 steam is expanded to a greater or less degree after it gets into the cylinder, 

 and, second, there is more or less of pressure in conveying the steam from 

 the boiler to the cylinder. 



In obtaining the nominal horse-power of small engines, manufacturers 

 usually count on an average pressure of 40 pounds per square inch on the 

 piston. To secure this pressure on the piston a boiler pressure much higher 

 will be needed, depending on the expansion of the steam after it reaches the 

 cylinder and on the losses. When an engine is sold the manufacturers should 

 state clearly what steam pressure should be carried in the boiler to develop 

 the nominal horse-power. The governor which is furnished with the engine 

 should be set so as to give the proper speed. The expansion of the steam in 

 the c^-linder is accomplished by closing the port that admits steam before the 

 piston has finished its stroke. In large engines this port is sometimes closed 

 58 



