Wednesday, January 20, 2010

How Horsepower Works

How Horsepower Works

Whenever someone mentions horsepower the first thing that comes to mind is cars. Fast cars with extreme torque, vehicles with large power and extreme speed. But what makes horsepower measuring so meaningful? And why is it called horsepower? The word horsepower was created in 1782 by James Watt, a famous 19th century engineer while he was developing a way to improve the power of a steam engine. While watching horses haul coal out of a coal mine he came up with the idea of defining the power exerted by these animals. He calculated that a normal horse attached to a mill that ground corn or cut wood walked in a circle that was 24 feet in diameter. He then calculated that the horse pulled with a force of 180 pounds. Watt noticed the horse could make 144 trips around this circle in an hour. That is 2.4 trips per minute. Using these numbers he calculated that the horse traveled about 180.96 foot per minute then rounded up and came up with 181 foot per minute. He then multiplied the 181 foot per minute by 180 pounds of force the horse exerted and came up with the number 32,580 foot pounds per minute then rounded up again to the number 33,000 foot pounds per minute. This number equals one horsepower.

Mr. Watt used his new found term to rate the power of the steam engine. Since most people were unfamiliar with the steam engine he had to come up with a comparison measurement that the normal farmer of the day would understand. As with any measurement there are different variations and different methods of measuring horsepower or hp. The normal measurement of horsepower is called mechanical horsepower.
Metric Horsepower
In Europe horsepower is measured a slightly different way. Horsepower is known by the various countries literal translation of the word horsepower. Some acronyms include PS, CV, pk and so on. Depending on the origin of the engine in question its horsepower is measured by that country's standards. Metric horsepower is defined as .73549875 kW. This is roughly 98.6% of mechanical horsepower.
Horsepower in Europe
Other countries have their own ways to measure horsepower. PS stands for Pferdestarke or horse strength. This is the German equivalent of horsepower. It is no longer used in Germany but it is in some other counties. It has since been replaced by the kilowatt but the EEC where horsepower was still used in advertisements as most people do not know the use of the kilowatt as a power measurement for combustion engines. Mathematically a PS = .73549875 kW = 0.9863201652997627 hp. The Dutch have the paardenkracht (pk). The Swedish have the hastkraft (hk). The Finnish have the hevosvoima (hv). All of these are equal to the German (ps). RAC horsepower, or taxable horsepower, is a British standard measurement of an automobile's power. It was adopted by the Royal Automobile Club. Taxable horsepower does not reflect true horsepower but it is a calculation based on the engine's bore size, the number of cylinders and a presumption of engine efficiency. The figure is no longer used as a standard in the UK but it is still put to use for the calculation of a vehicle's tax. The equation is RACh.p.= D squared * n/2.5 where D is the diameter of the bore of the cylinder in inches and n is the number of cylinders.

How Horsepower is used Measure the Power of an Engine?

1970 Chevy Chevelle SS

There are several names used to give an idea on how powerful an engine is. They are Nominal horsepower, Indicated horsepower and SAE gross horsepower. Nominal horsepower (nhp) is an early 19th century standard for estimating the power of steam engines. NHP = 7 x area of the piston x equivalent piston speed/33,000. It is measured by the size of the engine and the piston speed. Indicated horsepower (ihp) is calculated from the pressure in the cylinder. It is a misleading measurement as the actual power may be 79% to 90% of the indicated power. This was used to measure steam engine power. This measurement was open to interpretation so numbers varied. SAE gross horsepower was used by most car makers prior to 1972. It was measured with a test engine that had no accessories attached. It was a measurement of the maximum value of the engine and was often over rated to influence the sales of muscle cars. An engine that has its horsepower rating approved by the SAE will always be true to its number.  

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