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# Why do we measure horsepower?

Horsepower (Russian designation: HP; English: hp; German: PS; French: CV) is an off-system power unit. In the world there are several units of measurement called "horsepower." In Ukraine, as a rule, the horsepower refers to the so-called "metric horsepower", equal to exactly 735.49875 watts.

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

Currently, in Ukraine, horsepower is formally taken out of use, however, it is still used to calculate 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, motor mowers, trimmers).

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

In the English ("Imperial") system of measures, the unit of power is considered to be lb-ft per second, but in reality in England it is no longer used, and in the USA it is used only rarely.

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

## Standards horsepower

In Ukraine, the value of horsepower is set equal to 735.499 watts .

In most European countries, horsepower is defined as 75 kgf · m / s , that is, the power expended with a uniform vertical lifting of a mass of 75 kg at a speed of 1 meter per second with standard acceleration of free fall ( 9.80665 m / s² ). In this case, 1 l. with. it is exactly 735.49875 W , which is sometimes called the metric horsepower ( designation is German. PS, fr. ch, niderl. pk ), although it is not included in the metric system of units.

In the USA and the UK, in the automotive industry, horsepower is still more often equated to 745.69988145 W (the designation is English hp ), which is equal to 1.0186967887 metric horsepower.

In the USA, electric horsepower and boiler horsepower are also used (Boiler horsepower - used in industry and energy).

## Ratios

Title Formula Watt power
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
Boiler horsepower ≡ 33,475 BTU / h = 9809.5 W

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

It turns out that he came up with James Watt - the famous Scottish engineer and inventor. In 1763, he was asked to repair the current layout of the Newcomen steam engine. The model was equipped with a two-inch cylinder and had a stroke of six inches. After a series of experiments, the Watt replaced the metal cylinder with oiled wooden linseed oil and also reduced the amount of water raised in one cycle.

The model earned, and the inventor was convinced of the inefficiency of the steam engine and made numerous improvements to the design, which raised the performance more than four times. As a result, the work of Watt marked the beginning of the industrial revolution, first in England and then around the world. But in order for steam engines to sell well, buyers needed to clearly and clearly explain their merits.

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

Watt came to the conclusion that a 180-pound barrel can pull two horses from the mine at a speed of 2 mph. Then, multiplying the ½ barrel by 2 mph, we get that one horsepower is equal to 1 barrel · mile / h. Rounding up calculations in lb-ft per minute, the inventor decided that the horsepower would be 33,000 lb-ft per minute. In most European countries and Russia, horsepower is defined as 75 kgf · m / s, that is, as the power expended with a uniform vertical lifting of a mass of 75 kg at a speed of 1 m / s and standard acceleration of free fall.

In this case, 1 hp makes exactly 735,49875 watts - this value is also called "metric horsepower" . At the Second Congress of the British Scientific Association in 1882, a new unit of power was adopted - "watt", named after James Watt. But instead of virtual watts, we prefer to use the good old horsepower. Agree, so much clearer.