I’m confused about the meaning of this word. Is it how much a horse can pull translated into how much the engine can pull? Also, what is the actual “metric”? Why do we still use this? It seems archaic.
Also, what type of horse was originally used to get the measurement?
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If you are engineering an engine for some reason, you are usually worried first about torque. Torque is a unit that measure twisting power, in SI we call it a newton meter. Or, one newton of force applied to an arm that is one meter long. The more of those newton meters, the more you can twist. The more you can twist, the heavier the vehicle you can move.
That is all well and good, but it leaves out how we measure how fast two vehicles can go assuming similar weight and similar torque. That measure is horsepower which is calculated as h = torque * RPM / 5252. So if you have an engine that for each rotation of the crank, produces X amount of torque and can max out at 7,000 RPM, that vehicle will have higher horsepower and will go faster than an engine that produces X amount of torque but taps out at 5,200 RPM. Race engines, famously, rev really high. I think an indy car revs up to 20,000 RPM.
There is more to this, like if you are designing a diesel that should go 85 mph while towing 7,000 kilos, that engine will likely have something like 350-400 horsepower (don’t need more than that to go 85 mph) but will produce something like 1200 newton meters of torque and taps out at some low RPM. It doesn’t benefit you at all to have twice the horsepower and half the torque, the stated purpose is to tow.
Now you are the engineer developing a 5.0 liter 10 cylinder motor for the Audi R8. You have the inverse requirement, you will literally never tow so you only really need enough torque to handle the weight of the vehicle plus two passengers and gas. You need horsepower to make it go nice and fast. So that engine produces 600 horsepower and 565 newton meters of torque while it red-lines at 8,100 RPM.
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