Why Do We Measure Engines in Terms of Horsepower?

Why Do We Measure Engines in Terms of Horsepower?

If you’ve ever bought a car (or even watched a car commercial on TV), you’ve no doubt heard the term “horsepower”.  It’s a unit of measure that also gets applied to other types of machinery, like lawnmowers, chainsaws, snow blowers, and vacuums. But in this age of technology we live in, doesn’t it seem a bit odd that so many engines and other machines are compared to the power of horses? Where did this all come from? Why do we measure engines in terms of horsepower in the first place?

The History of Horsepower

Scottish inventor James Watt is generally credited with coining the term “horsepower.” In 1776, he had just invented a steam engine that was far better than the Newcomen steam engines that were being used at the time. He needed a way to market his product though, so he came up with a new unit of measurement. Since most people at the time used horses for a variety of different things, including work, he decided this was the best way to get them to understand how powerful his steam engine was.

While the details are a bit vague, the story goes that Watt was working at a mine with some ponies. They were lifting coal, and he observed that one horse could do approximately 33,000 foot-pounds of work a minute. He took it a step further and calculated that a draft horse could probably lift 33,000 pounds of material 1 foot in 1 minute, which is the equivalent of 3,300 pounds of material 10 feet in one minute.

Watt’s horsepower numbers are a bit abstract, and it’s actually not entirely clear how he ultimately came up with those digits. What’s more, the numbers were incredibly generous. Most horses probably couldn’t maintain that power throughout an entire workday. But in this case, Watt wasn’t really trying to be exact. He was trying to make his steam engine look more appealing than any others out there. So his unit of measure stuck nonetheless.

Why Do We Measure Engines in Terms of Horsepower?

Ultimately, “horsepower” became a popular unit of measure because it was easy for people to understand during the time period. Horses were common, engines were not – so Watt devised a way for the layman to understand just how powerful his steam engine was. And his technique would come to be applied to other machinery, like automobile engines at the turn of the 20th century.

Old traditions die hard, and we still use the horsepower measurement today when talking about the output of mechanical items. But there is no question it is most common to hear about horsepower when discussing cars. To measure the horsepower of an engine today, it is placed on a dynamometer, which works by placing a load on an engine and then measuring how much power the engine produces against that load.

If a car has a lot of power relative to the weight of the vehicle, it is considered to be “high performance.” Which is logical if you think about it. If a car is heavier, it will take more power to get it to accelerate. To make the car accelerate faster, you want the car to be lighter.

Horsepower Defined

While horsepower is still commonly used in relation to cars and other commercial products, most countries now use the SI unit watt for the measurement of power. Horsepower might be easy for people to visualize and understand, but it has always been a bit hard to define.

Two common definitions being used today are the mechanical horsepower, which is about 745.7 watts, and the metric horsepower, which is approximately 735.5 watts. Furthermore, there exist definitions for other types of horsepower, like Electrical horsepower, Boiler horsepower, Hydraulic horsepower, and Air horsepower.

Horsepower was originally developed as a way for an inventor to convince people that they should buy his engine. Watt’s idea worked, and horsepower became a common unit of measure that survives today. It helps us understand what a machine is capable of, and the more horsepower something has, the more power the engine is capable of.


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