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What is Horsepower? A Study of the Term and its Relation With Torque

What is Horsepower?
HP, short for horsepower, can be defined as the amount of work completed in a unit or specified amount of time. This ScienceStruck article lists different types of horsepower.
Shashank Nakate
Last Updated: Apr 2, 2018
The term 'horsepower' is generally defined as, "the work done at the rate of 550 foot-pounds per second." However, there is no standard definition for this term. It is the British unit of power, also known as imperial unit. This unit is not in use anymore as it was replaced by watt, an SI unit. SI is an abbreviated form of Système International d'Unités.

Although the unit is not used today, it was applied to compare the power of steam engines with those of the draft horses. In addition to this unit, horsepower-hour is an unit of work. It is equal to the amount of work completed using one horsepower in 60 minutes (1 hour).

Mechanical Horsepower
The word 'horsepower' was coined by James Watt, a Scottish inventor and a mechanical engineer. The origin of this word has an interesting history behind it. Ponies and horses were used for the generation of power before the invention of the engine. James Watt studied the amount of power used by the ponies to lift the material in coal mines. After several experiments and calculations, he found that a pony was able to complete 22,000 foot-pounds of work per minute. That is the pony lifted 22,000 pounds to a height of one foot in one minute. Going by the estimate that a horse is 50% more powerful than a pony, Mr. Watt arbitrarily estimated the value of a mechanical horsepower to be 33,000 foot-pounds per minute. Thereafter, the term 'horsepower' came into popular use. The exact value of the mechanical horsepower calculated by James Watt was 32,400 foot-pounds per minute. However, it was afterwards rounded off to the value of 33,000 foot-pounds per minute.

The term 'metric horsepower' originated in Germany and became popular the world over. The various units that are used to measure mechanical power are 'PS', 'CV' and 'ch'. The unit 'PS' is used in South America, Europe, and Japan. Metric power amounts to 98.6% of the mechanical horsepower. CV is the French equivalent of the German unit PS.

The term is used to measure the power of boilers i.e., the work done per unit time. One boiler horsepower is the amount of energy required for the evaporation of 15.65 kg water in a single hour. The evaporation of water in the process to measure the boiler horsepower is done at a temperature of 100°C. The term, 'boiler horsepower' was first used in 1876.

Measurement of power of various electrical devices is done by means of electrical horsepower. The value of an electrical horsepower is 746 Watts.

Difference Between Torque and Horsepower
Torque is defined as the engine's ability to perform work. It is not the same as horsepower. On the contrary, it is a component of horsepower. Horsepower, as discussed above, is the rate at which an engine operates.

How to Convert Torque to Horsepower

The formula for horsepower is as given below:

Horsepower = [ (Revolutions per minute × Torque ) ÷ 5252 ]

Changing the equation to derive torque, we get -
Torque = [ Horsepower × 5252) ÷ Revolutions per minute ]

Drawbar Horsepower
It is the power required by a railway locomotive to pull a train equals to a drawbar horsepower. Instead of calculating or deriving this value, it is actually measured. A 'dynamometer car' was used to measure the drawbar horsepower. In the process of measuring horsepower, the dynamometer car records the pull exerted on the locomotive.

Today, the unit used is watts, instead of horsepower. But car enthusiasts and truck owners still prefer using the 'horsepower language'. The origin of the word horsepower and the related facts make it an interesting concept to study.