Definition
The term horsepower was invented by the engineer James
Watt. Watt lived from 1736 to 1819 and is most famous for his work
on improving the performance of steam engines. We are also reminded
of him every day when we talk about 60-watt light bulbs.
What horsepower means is this: In Watt's judgement, one horse can
do 33,000 foot-pounds of work every minute. So, imagine a horse
raising coal out of a coal mine as shown above. A horse exerting 1
horsepower can raise 330 pounds of coal 100 feet in a minute, or 33
pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in
one minute. You can make up whatever combination of feet and pounds
you like. As long as the product is 33,000 foot-pounds in one
minute, you have a horsepower.
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If you want to know the
horsepower of an engine, you hook the engine up to a
dynamometer. A dynamometer places a load on the
engine and measures the amount of power that the engine
can produce against the load.
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Torque
Imagine that you have a big
socket wrench with a 2-foot-long handle on it, and
you apply 50 pounds of force to that 2-foot handle.
What you are doing is applying a torque, or
turning force, of 100 pound-feet (50 pounds to a
2-foot-long handle) to the bolt. You could get the
same 100 pound-feet of torque by applying 1 pound of
force to the end of a 100-foot handle or 100 pounds
of force to a 1-foot handle.
Similarly, if you attach a shaft to an engine,
the engine can apply torque to the shaft. A
dynamometer measures this torque. You can easily
convert torque to horsepower by multiplying torque
by rpm/5,252. |
You can get an idea of how a dynamometer works in the
following way: Imagine that you turn on a car engine,
put it in neutral and floor it. The engine would run so
fast it would explode. That's no good, so on a
dynamometer you apply a load to the floored engine and
measure the load the engine can handle at different
engine speeds. You might hook an engine to a
dynamometer, floor it and use the dynamometer to apply
enough of a load to the engine to keep it at, say, 7,000
rpm. You record how much load the engine can handle.
Then you apply additional load to knock the engine speed
down to 6,500 rpm and record the load there. Then you
apply additional load to get it down to 6,000 rpm, and
so on. You can do the same thing starting down at 500 or
1,000 rpm and working your way up. What dynamometers
actually measure is torque (in pound-feet), and to
convert torque to horsepower you simply multiply torque
by rpm/5,252.
If you plot the horsepower versus the rpm values for
the engine, what you end up with is a horsepower
curve for the engine. A typical horsepower curve for
a high-performance engine might look like this (this
happens to be the curve for the 300-horsepower engine in
the Mitsubishi 3000 bi-turbo):
What a graph like this points out is that any engine
has a peak horsepower -- an rpm value at which
the power available from the engine is at its maximum.
An engine also has a peak torque at a specific
rpm. You will often see this expressed in a brochure or
a review in a magazine as "320 HP @ 6500 rpm, 290 lb-ft
torque @ 5000 rpm" (the figures for the 1999 Shelby
Series 1). When people say an engine has "lots of
low-end torque," what they mean is that the peak torque
occurs at a fairly low rpm value, like 2,000 or 3,000
rpm.
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