By the 1920's the old methods of estimating engine horsepower by calculation (see A.L.A.M horsepower) had been almost entirely superceded by the use of dynamometers. In one area it hung on: tractors and the sort of stationary engines used to power farm equipment. In the selection below (from page 41 of the 1928 SAE Handbook, its contents basically from the Stationary-Engine Division Report of July 1921), one can sense the state-of-the-art SAE engineers holding their noses.
General Information Only
This horsepower rating formula was developed by the 1919 Tractor Division for tractor engines, but was not adopted by the Society as it was considered a commercial rather than an engineering formula. The formula was later approved by the Stationary-Engine Division for stationary engine practice.
D = the piston diameter in inches
L = the stroke in inches
R = the revolutions per minute of the crankshaft
N = the number of cylinders
The formula is based on a piston displacement of 13,000 cu. in. per min. per hp., which is considered a fair average factor for stationary and tractor engines burning either kerosene or gasoline. Various mechanical arrangements and refinements will, of course, influence actual results.
The results obtained using this formula are almost exactly 80 per cent of the brake horsepower under average good conditions, which provides the desired 20 to 25 per cent of reserve power. It is not intended that this empirical formula shall be used in engineering calculations.
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Last revised: 4 September 2013.