Horsepower-types

The concept of 'horse power' was originally developed for machines (initially for steam engines) and was estimated at 33,000 foot-pounds per minute, which is exactly 550 foot-pounds per second. However, with the development of other types of engines (e.g. electrical engines), slightly different figures were used. This has resulted in varioius different 'horse power' measures, as follows:

HP Measure Definition Watt Equivanent
Mechanical HP 550 foot-pounds/second 745.699872
Electrical HP see below 746
Metric HP 75 kg-meters/second 735.49875

Some notes on the above:

  • In addition to defining the measure horsepower, due to his many contributions to practical science, the term 'watt' was named after the scientist and inventor James Watt.
  • With the development of electrical motors, their input is in terms of electrical watts so it is reasonable to define their output in terms of watts. For simplicity, the watt equivalent of mechanical HP was rounded off to 746 watts.
  • As the Imperial/British/USA system uses measures (foot, pound) which are not available in the metric system, users of the metric system created a measure (metric HP) which uses metric measures (kilogram, meter) and is approximately the same value. In Europe and some other countries, the term PS is often used instead of metric HP, as the Germans were the first to widely use metric HP and consequently popularized the German term (In German, Pferde = horse and Stärke=power). Equivalent abbreviations in other languages include pk (Dutch), ch (French), hk (Finnish), hv (Norwegian/Danish), LE (Hungarian), k/ks (Czech and Slovak), KS (Croatian and Servian), KM (Slovenian), CP (Romanian), KC (Macedonian).

There are a number of other variations of HP, which are not in common use. These include: 

  • Boiler HP. For steam engines and steam turbines, the term boiler HP was created. It is related to the amount of water which can be evaporated per unit time. However, as the efficiency of steam engines varies, the actual power output is not closely related to the amount of steam produced, so this measure has largely fallen out of use.
  • Drawbar HP. This is typically used as a measure of the drawing power of railway locomotives and agricultural tractors.

Point of Measurement

The power of an engine can be measured at various points. For example, with a car, one could measure HP at the engine or at the drive shaft or at the wheels. The highest figure would be at the engine itself, somewhat lower at the drive shaft and lowest of all at the wheels (due to frictional and mechanical losses associated with intermediate mechanical components). Consequently, a car manufacturer measuring HP at the engine output will get higher figures than a car manufacturer measuring HP at the wheels. To prevent such discrepancies, the term 'horse power' can be preceded by a qualifier to indicate where it is measured. These include:

  • Crankshaft or Brake or Net. Crankshaft horsepower is the power delivered to the crankshaft and measured at this point. It is commonly referred to at brake horsepower (named after the device called a 'brake', which is used to measure the torque and rotational speed of the crankshaft and thus calculate the HP) and is often abbreviated to BHP (see below section 'BHP abbreviation'). As the measurement is made at the crankshaft, it measures power before losses to elements such as the gearbox, differential, alternator, muffler, cooling system and so on.
  • Shaft. Shaft horsepower is the power measured at the output shaft of the transmission. It is commonly used for ships and airplanes but seldom for cars.
  • Effective or True. Effective horsepower or true horsepower (abbreviated to THP) is the actual power at the wheels. It is less than the brake or shaft horsepower, as it is measured after the various mechanical losses between the engine and the wheels. However, it is a more meaningful measure of the power available for the primary function of the car, given rise to the qualifier 'true'. This measurement is also referred to as 'wheel horsepower' or simply WHP.

Two other qualifiers, originally developed for steam engines as an approximate indication of power, but which have fallen out of use due to their inaccuracy are:

  • Nominal. The term 'nominal HP' or 'nominal horsepower' is a theoretical calculation of the horsepower based on the engine size and piston speed. As such it is a calculated value rather than a measured value, with the result that it is imprecise and consequently has fallen out of use.
  • Indicated or Gross. Like nominal horse power, indicated horsepower or gross horsepower is an estimate based on engine calculations. The actual power may be only 70% to 90% of the estimated power.

Standardization

As the above discussion on 'point of measurement' shows, the engine rating in HP varies depending on where the measurement is done. The values can also be affected by other variables such as temperature or atmospheric pressure, which affect the combustion efficiency of the engine. Furthermore, in the case of 'true horse power', the figures will be affected depending on how many optional accessories are being used. 

Consequently, depending on the test conditions, the resulting figures can vary. To ensure consistent measures, especially between different manufacturers, a number of organisations have developed a set of standards which define exactly how horsepower should be measured and under what conditions. These include the Society of Automotive Engineers (SAE), the German Deutsches Institut für Normung (DIN), the Economic Commission for Europe R24 (ECE R24), 80/1269/EEC, the International Standards Organisation (ISO), and the Japanese Industrial Standard D 1001 (JIS D 101). Measurement values based on these standards may include the standard abbreviation to indicate the basis of the measurement. For example, 'SAE Net Horsepower' is a calculation of 'Net Horsepower' (see above section 'point of measurement'), corresponding to the SAE measurement standard.

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