Engine Horse Power

What is Horse Power? How to calculate & use it effectively?

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What is Horse Power?

The power, in simple terms, is ‘rate or speed of doing work’. The work is defined as a  'force operating thru' a distance'. The automotive engines use the term 'horse power' (hp) to measure its power. This term comes from the erstwhile practice of measuring the rate of work done by a horse which equals to 33,000 ft-lb of 'work done' per minute.

1 hp (mechanical) = 33,000 ft·lbf/min = 550 ft·lbf/s

Engine Horse Power
Engine Horse Power

A conventional engine produces its power by burning the fuel. The combustion process releases the heat energy which results in the generation of expanding forces known as power. The engine’s power is commonly measured in Brake-Horse-Power (bhp), Pferdestärke (PS) which is the German translation of horsepower. Metric system uses kilowatt (kW) unit of measurement while in the case of the imperial/British system, it is 'feet-pound-force-per-second' (fps).

Engine loses some of its generated power while overcoming its own friction known as  'Frictional Horse Power'. 'Indicated Horse power' is the actual power developed by the engine whereas Brake Horse Power is the available power which is almost about 70 to 85 % of the actual power developed inside the engine.

You can calculate the Indicated Horse Power of a 4-stroke engine by the following formula -

I.H.P = ((PLAN x n) / 2) ÷ 4500

(where, P = Mean effective pressure in kg/cm², L = Length of  stroke, A = Area of  piston  in cm², N = RPM of crankshaft, n = Number of cylinders)

B.H.P = (Torque x RPM) ÷ 5252


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