Pourquoi mesure-t-on la puissance des moteurs en chevaux ? Eh bien, la raison est tout simplement historique. Explications.
À la fin du XVIIIe siècle, les chevaux étaient le moyen le plus utilisé pour déplacer des charges et servaient aussi à actionner des machines, par exemple pour remonter l’eau des puits. Avec l’invention de la machine à vapeur, il a fallu établir une conversion entre la puissance de ce moteur et celle que l’on peut obtenir d’un cheval. Le but était, par exemple, de répondre à ce genre de question : quelle puissance doit avoir une machine à vapeur pour remplacer un cheval extrayant l’eau d’un puits ?
James Watt et le horsepower
Le grand savant britannique James Watt s’est, comme d’autres avant lui, attaqué à ce problème et il lui a trouvé une solution. Il a pris comme référence la puissance nécessaire à un cheval pour faire tourner un moulin à grain et a utilisé les unités de son pays : les livres (lb en anglais), les pieds (feet, ou ft) et les secondes. Ainsi naquit le « horsepower ».
Valeur d’un horsepower
L’unité de puissance est, plus tard, devenue… le watt (W). Celui-ci est défini dans le SI (système international) comme valant un newton.mètre par seconde.
Un « horsepower », ou hp, vaut 745,699.872 watts.
Le fardier (servant à transporter de lourdes charges) est le premier véhicule automobile à moteur à vapeur. Il a été réalisé en 1771 par l’ingénieur militaire Nicolas-Joseph Cugnot (ici, le deuxième modèle, avec sa taille définitive, conservé au musée des Arts et métiers, à Paris). Au cours du siècle suivant, la généralisation des machines à vapeur, remplaçant peu à peu les chevaux, a conduit à adopter une unité de puissance plus parlante que le watt du SI (système international d’unités) : le « cheval ». L’habitude survivra jusqu’au XXIesiècle… © Roby, GNU
Valeur d’un cheval-vapeur
Le reste du monde, utilisant le système métrique, a défini le « cheval-vapeur » comme la puissance développée par un cheval pour remonter de 1 m une masse de 75 kg en 1 s. Une masse de 1 kg pèse 9,81 newtons, car la valeur du newton est donnée par l’accélération de la pesanteur (due à la gravité terrestre) et vaut, par convention (c’est une moyenne), 9,81 m/s/s.
Un cheval-vapeur du système métrique, ou cheval, vaut donc 1 s x 75 kg x 9,81 m/s/s, soit 735,498.75 W.
Le « cheval-vapeur » (The metric horsepower, en anglais sur le schéma) est la puissance développée par un cheval pour remonter de 1 m une masse de 75 kg en 1 s, soit 735,498.75 W. (Attention : le cheval-vapeur du système métrique ne doit pas être confondu avec le horsepower, évoqué plus haut, qui lui vaut 745,699.872 W.) © Wikipédia, CC by-sa 3,0
Chevaux fiscaux, chevaux-vapeur…
Lorsque les moteurs à vapeur ont commencé à remplacer les chevaux, leur puissance a été exprimée en horsepowers ou en chevaux-vapeur. L’habitude perdure, et nous conservons les hp et les chevaux-vapeur, auxquels se sont ajoutés le watt électrique, les chevaux fiscaux (CV, une valeur administrative) et même, pour les automobiles, les chevaux SAE et les chevaux DIN… Alors qu’il suffirait que tout le monde compte en watts !
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