are you've heard about horsepower. Just about every car ad
on TV mentions it, people talking about their cars bandy the
word about and even most lawn mowers have a big sticker on
them to tell you the horsepower rating.
is horsepower, and what does the horsepower rating mean in
terms of performance? In this article, you'll learn exactly
what horsepower is and how you can apply it to your everyday
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.
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
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.
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.
can be converted into other units as well. For example:
- 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 F).
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
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.
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.
: 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.
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
Horsepower : 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 twin-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.
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.
in High-performance Cars
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.
table shows you the horsepower and weight for several high-performance
cars (and one low-performance car for comparison). In the
chart you can see the peak horsepower, the weight of the car,
the power-to-weight ratio (horsepower divided by the weight),
the number of seconds the car takes to accelerate from zero
to 60 mph, and the price.
see a very definite correlation between the power-to-weight
ratio and the 0-to-60 time -- in most cases, a higher ratio
indicates a quicker car. Interestingly, there is less of a
correlation between speed and price. The Viper actually looks
like a pretty good value on this particular table!
want a fast car, you want a good power-to-weight ratio. You
want lots of power and minimal weight. So the first place
to start is by cleaning out your trunk.
transformers, potential transformers, voltage transformers, motors,
ac motors, dc motors, 2 stroke engines, two stroke engines, diesel
engines, turbochargers, steam engines, rocket engines, manual transmissions,
horsepower, gas turbine engines, fuel injection systems, car engines,
car cooling systems