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Horses in cars: what are they?

We often talk about horse-drawn carriages in the engine, and yet there are none, I'm talking about the animal of course but there are two breeds… uh sorry two types of horse not to be confused, the fiscal horse and the steam horse . We demystify everything here.

fiscal horses, horsepower ...
When steam engines began to replace horsepower, their power was expressed in horsepowers or horsepower. The habit continues, and we keep the hp and horsepower, to which have been added the electric watt, fiscal horses (CV, an administrative value) and even, for automobiles, SAE horses and DIN horses. While it would be enough for everyone to count in watts!


The fiscal horse


The tax horse, noted CV, comes from tax law and is used to establish registration certificates (gray card). It is an arbitrary unit of evaluation of the power of an engine for the calculation of the tax, but without physical value unlike the horsepower, noted CH, which served as a comparison between a horse pulling a payload and a machine. steam engine.

Fiscal power or fiscal power was one of the earliest systems for calculating automobile tax rates in some European countries such as Great Britain, Belgium, Germany, France and Italy.


Since 1998, the calculation of the fiscal power of a car is calculated as follows: (CO2 / 45) + (P / 40) 1.6. It is also called administrative power or fiscal horse. The terms of the calculation break down as follows: CO2 corresponds to your vehicle's carbon dioxide emissions in grams per kilometer (g / km).


To better understand this formula, I will break it down step by step so that you can easily reproduce it with another example. If this operation remains simple for those who are a little familiar with mathematics, others do not have this facility. So here is a step-by-step breakdown.

I have a 231 horsepower car in automatic which emits 197 g / km of Co2

Fiscal power = (197/45) + ((231 * 0.736) / 40) ^ 1.6

Fiscal power (info: I round the numbers to 2 digits after the decimal point) = (4.38) + ((170,02) / 40) ^ 1.6

Fiscal power = 4.38 + 4.25 ^ 1.6

Fiscal power = 4.38 + 10.13

Fiscal power = 14.51

Rounded to the nearest value we arrive at 15 fiscal horses.


As you understood, the power more or less adjusts the number of fiscal horsepower. However, Co2 emissions are not linked only to engine power, but also to the mass of the vehicle. Indeed, the same engine does not consume the same depending on whether it is in a compact or in a minivan, the emissions will therefore be different from one vehicle to another even if the engine is identical (the bill will therefore be heavier in insurance and registration card for a heavier vehicle). Another factor, the transmission: depending on whether it is manual or automatic, the emissions will once again be different. This is also why, in general, an automatic car has an additional horsepower compared to the same model in a manual gearbox.


In Algeria, the fiscal horses are used among other things: to define the customs rate, the insurance rates and the sticker



Steam horse


Steam horsepower is a unit of power that does not form part of the International System of Units, which expresses an equivalence between the power supplied by a horse pulling a load and that supplied by a steam or combustion propulsion machine. The horse was, because of its massive use, the benchmark of power for harnesses before the advent of mechanical propulsion.

ch for horsepower; it should not be confused with the CV notation for fiscal horse; hp (horsepower) for English horsepower


Electric horsepower

The European electric horsepower is defined as being equal to

1 ch = 735.5 W,

while the British electric horsepower is worth per hour under a normal atmosphere. The exact value depends on the heat of evaporation used, but the most common is 970.3 BTUIT / LB, which gives:

1 hp = 746 W.

The boiler horsepower

The United States of America has "boiler horsepower" defined as the horsepower required to evaporate 34.5 lbs (British imperial pound) of water at 100 ° C (212 ° F) per hour under normal atmosphere. The exact value depends on the heat of evaporation used, but the most common is 970.3 BTUIT / LB, which gives:

P ~ 9,810 W (a little over 13 hp).

The horsepower boiler has been used since 1876 to measure the power of steam engines and is no longer used today.

At the end of the 18th century, horses were the most used means of moving loads and were also used to operate machines, for example to raise water from wells. With the invention of the steam engine, a conversion had to be made between the power of this engine and that which can be obtained from a horse. The goal was, for example, to answer this kind of question: what power must a steam engine have to replace a horse extracting water from a well?

James Watt and horsepower

The great British scientist James Watt, like others before him, tackled this problem and found a solution. He took as a reference the power required for a horse to run a grain mill and used the units of his country: pounds (lbs), feet (or ft) and seconds. Thus was born the "horsepower".


Value of a horsepower

The unit of power later became the watt (W). This is defined in the SI (international system) as being worth one newtone.meter per second.

One "horsepower", or hp, is worth 745,699,872 watts.




The dump truck (used to transport heavy loads) was the first motor vehicle with a steam engine. It was made in 1771 by the military engineer Nicolas-Joseph Cugnot (here, the second model, with its final size, kept at the Musée des Arts et Métiers, in Paris). During the following century, the generalization of steam engines, gradually replacing horses, led to the adoption of a power unit more meaningful than the watt of the SI (international system of units): the "horse".

Value of a horsepower

The rest of the world, using the metric system, defined "horsepower" as the power developed by a horse to lift a mass of 75 kg by 1 m in 1 s. A mass of 1 kg weighs 9.81 newtons, because the value of a newton is given by the acceleration of gravity (due to terrestrial gravity) and is worth, by convention (it is an average), 9.81 m / s / s.

A horsepower of the metric system, or horse, is therefore worth 1 s x 75 kg x 9.81 m / s / s, or 735.498.75 W.





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