Horsepower is a familiar measurement to anyone with an interest in cars. But what precisely is horsepower, and where did it come from? And does a horse always exert one horsepower?


What is horsepower?

Developed by engineer James Watt, horsepower is a quantity intended to measure the power output of a steam engine. His improved design for a steam engine was much more efficient than previous designs, requiring much less fuel. So, he developed the horsepower as a way to demonstrate to customers who hadn't yet switched from horses to steam engines that it was a good investment.

He calculated that, over an average day's work, a horse could turn a 24ft mill wheel around 2.5 times per minute. Power is defined as the work done per unit time, where work is a measure of energy transferred, calculated by multiplying the force applied by the distance travelled.

Watt, who is the namesake for the unit of power in the metric system, estimated the amount of force that the horse applied to turn the mill wheel. Using this, he then calculated how much energy it applied, and therefore the power.

Watt knew that the number he got was only an estimate, so he chose to round his calculation for his final result.

By Watt's definition, one horsepower is 33,000 foot-pounds per minute. This is approximately equal to 746 Watts (W, or Joules per second).

Modern car manufacturers use both Watt's definition, and a similar quantity known as metric horsepower. Metric horsepower is defined as the power required to raise a 75-kilogram mass against gravity over a distance of one metre in one second. This works out to about 735W.

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How much horsepower does a horse have?

Watt's estimates were fairly accurate: one horsepower is roughly the average rate of work a healthy draught horse can do over a full day, confirmed by data from the 1925 Iowa State Fair and by English veterinary surgeon William Youatt.

In 1993, biologists R. D. Stevenson and R. J. Wassersug published a letter to Nature in which they estimated the maximum power output a horse can muster. Previous studies had shown that the maximum sustainable mechanical power per kilogram of muscle is between 100 and 200W. Using the lower bound, they calculated a theoretical peak performance of 18,000W, or around 24 horsepower.

Looking at data from the 1925 Iowa State Fair, they found a much lower real-world value. Over a short period of time, they calculate, a horse can exert up to 14.9 horsepower.

Why do cars lose horsepower as they age?

Things tend to get dirty and worn over time. Air filters and exhausts can clog up, blocking airflow to and from the engine; fuel injectors and spark plugs can get mucky, causing less efficient firing; and fuel pumps can wear out and no longer pump fuel so well. Meanwhile, combustion byproducts can form deposits, stopping engine valves closing, reducing pressure generated by the burning fuel and causing backfires.

More seriously, inside the engine the piston rings can wear down, reducing the pressure in the cylinders and lowering the power output. Keep your car serviced regularly, though, and a modern engine should only lose a few per cent of horsepower over hundreds of thousands of miles. - Dr Peter Bentley

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.