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Horsepower
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.
Big Block Chevy Horse Power
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 It has since been replaced
by the kilowatt but the EEC where horsepower was still used in advertisements as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.
How Does it Work
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