# stationary engine horsepower

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1920's the old methods of estimating engine horsepower by calculation (see A.L.A.M horsepower) had been almost entirely superceded by the use of dynamometers.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.

## Stationary-Engine Rating

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.

$Nominal Engine Horsepower= 0.7854 D 2 LRN 13,000 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2Caerbbr2BIv gihfMCH12BUrgiqj3BamXvP5wqSX2qVrwzqf2zLnharyqqYLwySbsv UL2yVrwzG00uaeXatLxBI9gBaerbd9wDYLwzYbItLDharuavP1wzZb ItLDhis9wBH5garqqtubsr4rNCHbGeaGqipv0Je9sqqrpepC0xbbL8 F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepa e9pg0FirpepeKkFr0xfr=xfr=xb9adbaGaaeGadiWaamaaceGaaqaa faGbaaGcbaGaaeOtaiaab+gacaqGTbGaaeyAaiaab6gacaqGHbGaae iBaiaaysW7caqGfbGaaeOBaiaabEgacaqGPbGaaeOBaiaabwgacaaM e8Uaaeisaiaab+gacaqGYbGaae4CaiaabwgacaqGWbGaae4BaiaabE hacaqGLbGaaeOCaiabg2da9maalaaabaGaaGimaiaac6cacaaI3aGa aGioaiaaiwdacaaI0aGaamiramaaCaaaleqabaGaaGOmaaaakiaadY eacaWGsbGaamOtaaqaaiaaigdacaaIZaGaaiilaiaaicdacaaIWaGa aGimaaaaaaa@6E5A@$

where

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.