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Horsepower
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Everything about Horse Power totally explainedHorsepower ( hp or HP) is the name of several non-metric units of power. In scientific discourse, the term "horsepower" is rarely used because of its various definitions and the already existent SI unit for power, the watt (W). However, use of the term "horsepower" persists as a legacy in many languages and industries, particularly as a units of measurement of the maximum power output of internal-combustion engines of automobiles, and often of trucks, buses and ships. The use of 'HP' is being slowly replaced by kW (kilowatt) and MW (megawatt).
There are two important factors to consider when evaluating the measurement of "horsepower":
- The inconsistent definitions of the "horsepower" unit itself
- The various standards used in measuring the value of "horsepower"
These factors can be combined in unexpected ways — the power output for an engine rated at "100 horsepower" might not be what a reader expects. For this reason, various groups have attempted to standardize not only the definition of "horsepower" but the measurement of "horsepower". In the interim, more confusion may surface.
Current definitions
The following definitions have been widely used:
Hydraulic horsepower is equivalent to mechanical horsepower. The formula given above is for conversion to mechanical horsepower from the factors acting on a hydraulic system.
Additionally, the term "horsepower" has been applied to calculated (estimated rather than measured) metrics:
RAC horsepower is based solely on the dimensions of a piston engine (1 litre of engine displacement is equal to 10 RAC horsepower)
Mechanical horsepower » See History of the term "horsepower"
The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower for example 33,000 foot-pounds of work per minute.
Assuming the third CGPM (1901, CR 70) definition of standard gravity, g, and the international avoirdupois pound (1958), one mechanical horsepower is:
» .
This was rounded to an even 33,000 ft·lbf/min.
Others recount that Watt determined that a pony could lift an average 220 pounds 100 feet (30 m) per minute over a four-hour working shift. Watt then judged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure.
Engineering in History recounts that John Smeaton initially estimated that a horse could produce 22,916-foot-pounds per minute. John Desaguliers increased that to 27,500-foot-pounds per minute. "Watt found by experiment in 1782 that a 'brewery horse' was able to produce 32,400-foot-pounds per minute". James Watt and Matthew Boulton standardized that figure at 33,000 the next year.
Put into perspective, a healthy human can produce about 1.2 hp briefly (see Orders of magnitude (power)) and sustain about 0.1 hp indefinitely, and trained athletes can manage up to about 0.3 horsepower for a period of several hours.
Most observers familiar with horses and their capabilities estimate that Watt was either a bit optimistic or intended to under promise and over deliver; few horses can maintain that effort for long. Regardless, comparison to a horse proved to be an enduring marketing tool.
Horsepower from a horse
R. D. Stevenson and R. J. Wasserzug published an article in Nature 364, 195-195 (15 July 1993) calculating the upper limit to an animal's power output. The peak power over a few seconds has been measured to be as high as 14.9 hp. However, for longer periods an average horse produces less than one horsepower.
Further Information
Get more info on 'Horse Power'.
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