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What is “Horsepower”?

The term “horsepower” has its origins in the days when horses were integral to the successful activities of human beings.

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 ( or 14,968 kilogram) 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.

Compared to one horsepower, my physical effort is very feeble.  I can pick up a box weighing 15 kg from the ground and raise it two feet off the ground in a second or two.  Multipy that out to cover 60 seconds, and it means perhaps something like 900 kg one foot or more over one minute.  In other words, if I am interpreting things correctly, my work capacity is 6 percent ( 900 divided by 14,968 ) of what one horse can do, in the example pictured below.

Scottish Engineer and inventor, James Watt (1736-1819) used the term “horse power” to indicate the rate at which an engine can deliver work. As such, “horse power” is a measure of power, that is, of energy produced or work done by an engine per unit of time.   Watt also looked at ponies lifting coal at a coal mine, and wondered how much power they exerted, and if he could use that power potential to describe and sell his improved steam engines (the Watts steam engines or Boulton and Watts steam engines).   Steam engines were used to drive locomotives (trains) and can generate around 2,000 horsepower (hp).


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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 ground 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.

So, if an engine can push 33,000 Lb of something one foot in one minute, we say that is a one-horsepower engine.  Currently, there are two systems of units used: one is the metric system used internationally, and the other one is the English system used mostly in the US.  Horsepower (hp), as a unit of power, belongs to the English system. The unit for power in the metric system is Watt (W), named after James Watt.   One horsepower is equivalent to 746 watts.

One horsepower also equates to 550 pounds-foot-second, or 249 kilograms-foot-second.  One horsepower is the power needed to lift 550 pounds one foot off the ground in exactly one second.  Horsepower is a unit of measurement to consider when looking at mid and upper-range engine responses.

While the calculation is still used today, it’s important to realize that no horse can sustain a horsepower for a long period of time; therefore a horsepower in a car engine is actually the equivalent of a Superman version of a horse.

The horsepower (hp) of an engine is derived from a measurement of its torque (twisting force) and its RPM.  RPM or Revolutions Per Minute, in this case, means how many times per minute that the crankshaft is rotating, at which power is available from the engine at its maximum.  If you are a physicist, or a budding physicist, the following information about such may help you.  If you are not, like me, then the following calculations will probably mystify and bewilder you.


What I have found in the interim is the following formula, which measures the brake horsepower.   Torque is measured using a machine called a Dynanometer.

Horsepower = (Torque X Engine RPM) / 5250

I have heard that coupes and sedans usually have between 110-150hp in brake horsepower, and I have found an interesting website, linked below, that lists the Horsepowers of a variety of cars and appliances.  The list includes the good old Ford Fiesta, which I believe must be quite similar in hp to the Ford Festiva that I drove for 13 years, until I retired it.


Nowadays the “horsepower” of a car engine often refers to the engine size in cc (cubic centimetres), so a 1500 cc engine is said to have a horsepower of 1500, which is not the true horsepower measurement of the brake horsepower.  New cars may have manuals which state the engine capacity in terms of both cc and horsepower.

If not, then, to measure the brake horsepower, you will need to find someone whom can help you put in the values into the formula above, to calculate it.  Good luck, and best wishes to the horses that started all of this, and to all the horses that we human beings have used for work and other purposes.  I hope that we look after our horses, as we would like to be looked after, especially because horses have been so critical in shaping (and entertaining) our lives.





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