The term horse power was introduced by James Watt. He chose a normal horse and found that it could do a work of 33,000 ft-lb in a minute. He adopted this measure for comparing the performance of his engines.
The actual power generated in the engine cylinder is called the indicated horse power (IHP). It is also defined as the power fed into the engine in the form of steam or calorific value of the fuel.
But all that power is not available to do useful work. This is because, part of it is used to overcome internal friction of the moving parts of the engine, work of charging, air resistance to flywheel rotation, and drive auxiliaries such as fuel pumps, governor, lubrication oil and water circulating pumps. This net power output of the engine is called brake horse power (BHP). It is 15- 30 per cent less than IHP.
The term brake horse power comes from the braking device used to measure the power output by stopping the engine.
Thus it is the amount of power that the engine can produce at a certain speed (measured as rotations per minute).
Usually engines are rated with the help of dynamometers. The device has a power absorber, such as an electric generator or a water brake, which can put different loads on the engine.
