Horse power was first used by James Watt to define the equivalent work a steam engine could do compared to a horse. Watt defined 1 horsepower as 'the amount of work required from a horse to pull a 150lb weight up a shaft that was 220 feet deep'
.
Later, John Smeaton estimated that a horse could produce 22,916-foot-pounds per minute, and John Desaguliers increased that to 27,500-foot-pounds per minute.
Foot-pounds is used to describe torque i.e. twisting power. As work performed by a force is the product of force and displacement of the body, 33,000 foot-pounds represented the work performed by the horse.
Indicated horse power, usually abbreviated to HP is determined by the pressure in the cylinder(s) and will be greater than BHP as it excludes friction and other losses.
A more useful measure of how much work an engine can produce is when the losses due to friction etc. are removed.
The term 'brake' in Brake Horse Power (BHP) describes how a turning shaft from the engine has a brake applied to it and the torque exerted on the brake and is measured on a
dynomometer. The measurement can be taken at the flywheel or at the driven wheel(s).
BHP = torque x r.p.m.
So, the HP of an engine is the
theoretical maximum power excluding the losses described above and BHP is the
actual power it produces.
As per the conventions, 1 BHP is equal to:
- 745.5 watts
- 1.01389 ps
- 33,000 ft lbf/min
- 42.2 BTU/min