Horsepower

Car fans often talk proudly about the number of ‘horses under the hood’ of a car without thinking why engine power is measured in hp. We live in the 21st century. Robots work instead of humans, high tech, wireless transmission, and space expeditions are all around… But we still use horsepower as a unit of the car power. Why this artifact of the past is still in use as a measurement unit? And what is this ‘horsepower’?

Horsepower as a unit of power was introduced in 1789 by Scottish James Watt, a mechanic, and inventor of the universal steam engine.

A good publicity gimmick

Watt’s task was to measure the power of a horse doing its job. Why did he do that? Well, to prove that his steam engine can do the job better than the strongest horse! His invention made it faster and cheaper to lift coal and workers as well as water accumulated at the bottom of a coal mine onto the top of the ground. The owners of the mines used to use horses for this work in the old days. They didn’t want to buy Watt’s steam engine. That’s why he compared his engine to a horse!

He detected the speed of drudge horses that pulled a barrel of coal (weighing 1 barrel, 380 pounds, or 172.4 kg) out of the mine. Pulling out such a barrel by the rope hitched round a lead block took two horses. Their speed was about 2 mph (3.6 km/h). So, 1 horse at 1 mile per hour pulls out 1 barrel or 33,000 lbs. Watt calculated that the force of an average horse within 8 hours is about 15% of its weight. That’s 75 kgf for a horse weighing 500 kilograms. If we convert it into the International System of Units, it turns out that in 8 hours, a horse having this force can travel 28.8 km at a speed of 3.6 km/h (or 1 m/s). Thus, a new unit of measurement has appeared – a horsepower, equal to 75 kgfm/s. Obviously, Watt found his success in selling steam engines after such an illustrative example as they were stronger than the strongest horse! James Watt was not only able to demonstrate to mine owners how his miracle could replace horses. He also started the industrial revolution in the UK. And the term “horsepower” became so lucky that it took root in languages around the world for centuries.

What a horsepower is equal to?

One hp is equal to 75 kgfm/s. This means that lifting a load of 75 kg at 1 m/s takes a force equal to 1 horsepower. In English it is designated as hp; in German as PS; in French as CV, in Russian as l.s. All these are translated as ‘horsepower’. One metric horsepower is exactly 735.49875 W. In the USA and UK, in the automotive industry, 1 hp is equal to 745.69988145 W. After replacing drudge horses, Watt chased them away into the world of units. Watt (W) is the international official designation for the power unit. By the way, Watt is named after the same Jumal Watt, who introduced the term ‘horsepower’. 1 Watt is the power at which the work in 1 J is done in 1 second. Of course, traditionally in the UK and the US, the unit of measurement of power is still considered to be a pound-feet per second, which is the energy it takes to lift a body weighing 1 pound (453.6 g) to a height of 1 foot. But in reality, pound-feet as a unit of power is not used in fact.

Horsepowers and taxation

Yeah, since 1960, everyone had to officially use watts to measure power. And horsepower, as morally outdated, is recommended to be removed from use. But it is still used, for example, in Russia, to calculate a vehicle tax. The more horsepower, the higher is the tax. Moreover, the engine power in kW is taken and specially recalculated in hp by simple multiplication by 1,35962. In Israel, the transport tax depends on the ecological safety of the car instead of power and ranges from 10 to 92% of the car cost. Once a year (or once every two years if the car is new) one should pass a technical inspection, which also means the renewal of the vehicle certificate of ownership and the car tax. In the past, in the UK, Germany, Belgium, France, Spain, transport tax also depended on horsepowers. But then, some countries simply refused to use power to calculate the car tax and began to apply other parameters, such as the size of the car, as in the UK. And other countries began to use kilowatts instead of horsepowers to calculate the transport tax, such as France. But the expressions ‘Caballo fiscal’ and ‘Cheval fiscal’ are still used for ‘taxable horsepower’.

Though it is not customary to write the power on the body, on some European-type trucks, it is indicated either above the front axle wheel arch, or on the front of the cab.

So, it is correct in the modern world to measure the power of the car engine in kW, not in horsepower. The standard power measurement method adopted in Europe uses kilowatts. And yet… horsepower is very often found, maybe because it demonstrates the power and strength of the car as clearly as an image of a herd of horses.