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Why do we measure power in horsepower?

Что такое Лошадиные силы

Horsepower (Russian designation: L, English: hp, German: PS, French: CV) is an extrasystem unit of power. In the world, there are several units of measurement called "horsepower". In Ukraine, as a rule, horsepower means the so-called "metric horsepower", which is exactly 735,49875 watts.

Self-reliant carts long ago replaced horses, but we still continue to measure the power of their motors in horsepower. But why? Why not get rid of this archaism?

At present, in Ukraine, formally horsepower is taken out of use, but it is still used to calculate the transport tax and compulsory motor TPL insurance. In Ukraine and in many other countries, it is still very widespread in an environment where internal combustion engines are used (cars, motorcycles, tractor equipment, motorcycles, trimmers).

In the International System of Units (SI), the official unit of power measurement is watt.

In the English (Imperial) system of measures, a unit of power is considered pound-foot per second, but in reality it is no longer used in England, and in the US it is used exceptionally.

The International Organization of Legal Metrology (OIML), in its recommendations, refers the metric horsepower to units of measurement "that must be removed from circulation as soon as possible where they are currently used and that should not be introduced if they are not used."

Standards of horsepower

In Ukraine, the horsepower is set at 735,499 watts .

In most European countries, horsepower is defined as 75 kgf · m / s , that is, as the power expended for a uniform vertical lifting of a load of 75 kg at a speed of 1 meter per second with a standard acceleration of free fall ( 9,80665 m / s² ). In this case 1 liter. from. is exactly 735.49875 W , which is sometimes called the metric horsepower (the designation is German PS, ff ch, nderl pk ), although it is not included in the metric system of units.

In the United States and Great Britain, in the automotive industry, horsepower is more often equated to 745.69988145 W ( designation English hp ), which is 1.01386967887 metric horsepower.

The United States also uses electric horsepower and boiler horsepower (Boiler horsepower - used in industry and energy).


Name Formula Power in watts
Metric horsepower ≡ 75 kgf · m / s = 735.49875 W (exactly)
Mechanical horsepower
= Indicator horsepower
≡ 33,000 ft · lb f / min
= 550 ft · lb f / s
= 745.69987158227022 W
Electric horsepower ≡ 746 W
Kettle horse power ≡ 33,475 BTU / h = 9809.5 W

Horsepower, of course, is one of the most ironic units of measurement. Its presence in the mechanical form practically eliminated the need to use "biological" horsepower, but we still measure the power of any cars in good old "horses". Imagine if we still estimated the brightness of a light bulb in candles! So where did this term come from?

It turns out that it was invented by James Watt - the famous Scottish engineer and inventor. In 1763 he was asked to repair the operating model of the steam engine Newcomen. The model was equipped with a two-inch cylinder and had a piston stroke of six inches. After a series of experiments, Watt replaced the metal cylinder with linseed oil and dried in a wood oven, and also reduced the amount of water lifted in one cycle.

The mockup worked, and the inventor was convinced of the inefficiency of the steam engine and made numerous improvements in the design, which increased the productivity by more than four times. As a result, Watt's work initiated the industrial revolution first in England, and then in the whole world. But that the steam engines sold well, the customers needed to clearly and easily explain their advantages.

For example, to demonstrate how many horses can replace these same steam engines. At that time in England, barrels with a volume from 140.9 to 190.9 liters were used to lift coal, water and people from mines. One barrel weighed 172.4 kilograms, and two horses could pull out such a barrel by the rope, thrown across the block. The effort of an average horse for 8 hours of work is 15% of its weight or 75 kg of force with a horse weight of 500 kg.

Watt came to the conclusion that a barrel weighing 180 kilograms could draw two horses from the mine at a speed of 2 mph. Then, multiplying ½ barrel by 2 mph, we get that one horsepower is 1 barrel · mph. Rounding up the calculations in lb-ft for a minute, the inventor decided that the horsepower would be 33,000 lb-ft. In most European countries and Russia, horsepower is defined as 75 kgf · m / s, that is, as the power expended for a uniform vertical lifting of a 75 kg load at a speed of 1 m / s and a standard acceleration of gravity.

In this case, 1 hp. is exactly 735.49875 watts - this amount is also called "metric horsepower . " At the Second Congress of the British Scientific Association in 1882, a new unit of power measurement - "watt", named, in fact, in honor of James Watt, was adopted. But instead of virtual watts, we prefer to use the good old horsepower. Agree, it is much more understandable.