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
The story goes that Watt was working with ponies lifting coal
at a coal mine, and he wanted a way to talk about the power
available from one of these animals. He found that, on average,
a mine pony could do 22,000 foot-pounds of work in a minute.
He then increased that number by 50 percent and pegged the
measurement of horsepower at 33,000 foot-pounds of work in
one minute. It is that arbitrary unit of measure that has
made its way down through the centuries and now appears on
your car, your lawn mower, your chain saw and even in some
cases your vacuum cleaner!
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.
You can probably imagine that you would not want to load 33,000
pounds of coal in the bucket and ask the horse to move it
1 foot in a minute because the horse couldn't budge that big
a load. You can probably also imagine that you would not want
to put 1 pound of coal in the bucket and ask the horse to
run 33,000 feet in one minute, since that translates into
375 miles per hour and horses can't run that fast. However,
if you have read How a Block and Tackle Works, you know that
with a block and tackle you can easily trade perceived weight
for distance using an arrangement of pulleys. So you could
create a block and tackle system that puts a comfortable amount
of weight on the horse at a comfortable speed no matter how
much weight is actually in the bucket.
Horsepower can be converted into other units as well. For
· 1 horsepower is equivalent to 746 watts. So if you
took a 1-horsepower horse and put it on a treadmill, it could
operate a generator producing a continuous 746 watts.
· 1 horsepower (over the course of an hour) is equivalent
to 2,545 BTU (British thermal units). If you took that 746
watts and ran it through an electric heater for an hour, it
would produce 2,545 BTU (where a BTU is the amount of energy
needed to raise the temperature of 1 pound of water 1 degree
· One BTU is equal to 1,055 joules, or 252 gram-calories
or 0.252 food Calories. Presumably, a horse producing 1 horsepower
would burn 641 Calories in one hour if it were 100-percent
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.
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
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
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.
Another thing you can see from a car's horsepower curve is
the place where the engine has maximum power. When you are
trying to accelerate quickly, you want to try to keep the
engine close to its maximum horsepower point on the curve.
That is why you often downshift to accelerate -- by downshifting,
you increase engine rpm, which typically moves you closer
to the peak horsepower point on the curve. If you want to
"launch" your car from a traffic light, you would
typically rev the engine to get the engine right at its peak
horsepower rpm and then release the clutch to dump maximum
power to the tires.
Horsepower in High-Performance Cars
A car is considered to be "high performance" if
it has a lot of power relative to the weight of the car. This
makes sense -- the more weight you have, the more power it
takes to accelerate it. For a given amount of power you want
to minimize the weight in order to maximize the acceleration.
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