[AT] RE: Vintage tractor pulling advice/ horsepower

[Danny Tabor]

I found this article on the website Automath. It has
never been clear to me how HP was determined. Hope
others find it as interesting as I have.

Danny Tabor

What is Horsepower?


How do you define horsepower? Ask a car enthusiast and
most of the time you'll get a blank look, a shrug of
the shoulders and maybe a guess along the lines of
"What a horse can do!".

That answer begs the question: What horse? A
thoroghbred race horse that can carry the small weight
of a jockey with a lot of speed, or a working horse
that can pull heavy loads albeit slowly? Obviously
there is a more precise answer. Car manufacturers,
despite their reputation for being creative regarding
the horsepower ratings of their products for marketing
reasons, require a more stable definition.

Horsepower definition Horsepower is defined as work
done over time. The exact definition of one horsepower
is 33,000 lb.ft./minute. Put another way, if you were
to lift 33,000 pounds one foot over a period of one
minute, you would have been working at the rate of one
horsepower. In this case, you'd have expended one
horsepower-minute of energy.

Even more interesting is how the definition came to
be. It was originated by James Watt, (1736-1819) the
inventor of the steam engine and the man whose name
has been immortalized by the definition of Watt as a
unit of power. The next time you complain about the
landlord using only 20 watt light bulbs in the hall,
you are honoring the same man.

To help sell his steam engines, Watt needed a way of
rating their capabilities. The engines were replacing
horses, the usual source of industrial power of the
day. The typical horse, attached to a mill that
grinded corn or cut wood, walked a 24 foot diameter
(about 75.4 feet circumference) circle. Watt
calculated that the horse pulled with a force of 180
pounds, although how he came up with the figure is not
known. Watt observed that a horse typically made 144
trips around the circle in an hour, or about 2.4 per
minute. This meant that the horse traveled at a speed
of 180.96 feet per minute. Watt rounded off the speed
to 181 feet per minute and multiplied that by the 180
pounds of force the horse pulled (181 x 180) and came
up with 32,580 ft.-lbs./minute. That was rounded off
to 33,000 ft.-lbs./minute, the figure we use today.

Put into perspective, a healthy human can sustain
about 0.1 horsepower. Most observers familiar with
horses and their capabilities estimate that Watt was a
bit optimistic; few horses could maintain that effort
for long.

Although the standard for rating horsepower has been
available for over 200 years, clever car manufacturers
have found ways to change the ratings of their engines
to suit their needs. During the famous horsepower wars
of the 1960s, manufacturers could get higher figures
by testing without auxiliary items such as alternators
or even water pumps. High ratings backfired when
insurance companies noticed them and started to charge
more for what they saw as a higher risk. Manufacturers
sometimes responded by listing lower horsepower
figures, forcing enthusiasts to look at the magazine
test reports to determine what was going on. In the
early seventies the SAE (Society of Automotive
Engineers) stepped in with standardized test
procedures and the fiqures were more consistent.

Between 1922 and 1947, the Royal Automobile Club used
a horsepower rating that was the basis for an
automobile tax. The horsepower of an engine was
determined by multiplying the square of the cylinder
diameter in inches by the number of cylinders and then
dividing that figure by 2.5. Using this dubious
method, What we know of as a 385 horsepower motor
found in the Z06 Corvette would be rated at only 48.67
hp!

There is a metric horsepower rating, although it is
rarely used. The two methods are close, with one SAE
horsepower equal to 1.0138697 metric horsepower.

One horsepower also equals 745.699 watts. This means
that if you really want to confuse people, you could
complain about the .02682 horsepower light bulb your
landlord has in the hallway as opposed to the mundane
20 watt measurement.




		
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