The brake power (briefly written as B.P.) of an IC Engine is the power available at the crankshaft. The brake power of an I.C. engine is, usually, measured by means of a brake mechanism (prony brake or rope brake).

In case of prony brake, brake power of the engine,

where

W = Brake load in newtons,

l = Length of arm in metres, and

N = Speed of the engine in r.p.m.

In case of rope brake, brake power of the engine,

 where

W = Dead load in newtons,

S = Spring balance reading in newtons,

D = Diameter of the brake drum in metres,

d = Diameter of the rope in metres, and

N = Speed of the engine in r.p.m.

Note : The brake power (B.P.) of an engine is always less than the indicated power (I.P.) of an engine, because some power is lost in overcoming the engine friction (known as frictional power). Mathematically,

Frictional power, F.P. = I.P. — B.P.