The term ‘horsepower’ (hp), which represents the unit of power, was first introduced by James Watt in 1782 in order to help market his improved version of steam engine. James Watt (1736-1819) was a Scottish scientist, who brought the fundamental changes in the steam engine. This started off the Industrial Revolution in England. The background history of the term ‘horsepower’ goes like this:
In olden days the mill wheel used to be turned by the horses. James Watt observed that a wheel of 12 feet in radius could be turned 144 times in an hour by one horse. This meant that in one minute the horse traveled 2.4 × 2π × 12 feet. He calculated that the horse could pull with a force of 180 lbs.
This historical value, originally derived from the unit power of a horse, was converted to SI unit for power, the Watt (W). However, the concept of horsepower continued to exists particularly in automotive industry as a legacy term for measuring the maximum power the internal-combustion engine can generate.
Measuring the Value:
There are several points in the transmission channel (from its generation to wheels to its application on ground surface) where the power of engine can be measured. The following are the various names given to the development of power at various stages in this process.
Indicated Horsepower (ihp):
This was most commonly used for steam engines during 19th century. The “Indicated horsepower” is applied to engines which convert the energy generated by the expanding gases within the cylinders. Therefore it is calculated from the pressure that develops within the cylinders and is measured by an Engine Indicator device – hence called Indicated Horsepower.
Brake Horsepower (bhp):
The net ratings given by the “American Society of Automotive Engineers” (SAE), which is more accurate than gross ratings, represent the power of engine at the flywheel, and does not measure power at the drive wheels. Brake horsepower in fact is the measure of a horsepower of engine without the loss in power caused by the gearbox, generator/alternator, differential, water-pump and other auxiliaries. The corollary is that the horsepower delivered to the driving wheels is much less than the one generated at the engine end. Therefore, the engine will have to be tested all over again in altogether another system so as to obtain an authentic rating.
Shaft Horsepower (shp):
The Shaft horsepower relates to the power delivered to the propelling shafts of an aeroplane or a ship as therefore this metric does not pertain to automobiles as such.
Effective Horsepower (ehp) also called Wheel Horsepower(whp):
Effective horsepower is the power converted into some sort of useful work . For example the power generated by the engine in vehicle is converted into forward motion. This effective horsepower is often referred to as the ‘Wheel Horsepower’ in automobiles. The wheel horsepower in vehicles is mostly measured by the automotive dynamo-meters. The net or Brake horsepower at the engine is then calculated once a conversion factor has been applied upon. Because of a loss through the drive-train, the Brake Horsepower ratings will often be 5-15 % higher than the Wheel Horsepower.