The 1974 Dodge Charger that Richard Petty drove to his fifth and sixth Grand National/Winston Cup Series Championship in 1974 and 1975 and was used for his victory at the 1974 Daytona 500. The 2018 Dodge Charger SRT 392 cranks out 485 horsepower and 475 pound-feet of torque, says the automaker. |
Horsepower
How Horsepower Works
BY MARSHALL BRAIN
Chances 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.
But
what 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 life.
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.
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 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).
· 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 efficient.
In
this article, you'll learn all about horsepower and what it means in reference
to machines.
Measuring
Horsepower
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.
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.
Graphing 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):
Measuring
horsepower requires a power-reading dynamometer. Learn how plotting horsepower
against rpm values produces a graph called a horsepower curve.
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.
One
of the areas where people talk most about horsepower is in the area of
high-performance cars. In the next section, we'll talk about the connection
there.
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.
.
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.
The
following 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.
You
can 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!
If
you 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.
Marshall Brain, Founder
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
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