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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Horsepower is, unsurprisingly, a measure of power. 1 horsepower is equal to 745.7 Watts. Mostly for traditional reasons we still use “horsepower” to describe the power output of certain things, like cars, but you could just as easily use “Watts” to describe the power instead (and I personally think that would be better but I digress). For example, a typical car may have something like 200 horsepower, which, you could convert to Watts by multiplying 200hp by 745.7W/hp to get 149,140W. Or, if you want a more convenient number, you could round to the nearest thousand and convert to KiloWatts (KW) by dividing by 1000. So you could say a 200hp car is also a 150KW car, it would mean the same thing. I would prefer this because Watts are the standard metric for measuring any form of power, heat, electricity, light output, etc.
Now a better question may be “what is power”? Power measures how much energy something converts into a different form of energy over a period of time. If we are talking about Watts, it means 1 joule of energy every second. For example, a typical 60 Watt light bulb turns 60 joules of electrical energy into 60 joules of light and heat energy every second. Intuitively you would then imagine a 100 Watt light bulb would be brighter because it is converting more electrical energy per second into light and heat energy (depending on the bulb the ratio between light and heat may change, an incandescent bulb creates more heat than light, while an LED creates more light than heat, which is why you need a smaller Watt LED to create the same amount of light as a larger Watt incandescent. It’s also why incandescent bulbs get so hot, most of their energy is turned into heat not light.)
Now, if we want to talk about engines the same logic applies. I’ll be assuming a car engine, but it would be the same idea for any engine. If a car engine is rated at 200hp (150KW) that means its PEAK power is 200hp, it does not mean that it is always outputting 200hp. If the car is just idling it won’t be outputting much power at all, it would be just a 1hp or so just to keep the engine running, and most of that power would be turned into waste heat. However, if you were push the gas pedal to the floor, then suddenly the car should be getting around its maximum 200hp (150KW). This means it should be turning about 150,000 Joules of energy into another form of energy. In the case of the car (assuming a gas car) it would be turning 150,000 joules of the chemical energy of the gasoline in its engine into the kinetic energy of the car to drive it forward (as well as waste heat). Now just like the light bulb example, the more horsepower you have, the more energy per second the car is able to turn for chemical energy into kinetic energy. For example, if you were to race a 100hp car vs a 200hp car, (with all other things being equal) the 200hp car would win because it is turning twice as much chemical energy from its gas into kinetic energy to move forward. In car terms, this would mean a car with more horsepower will have faster acceleration because it’s turning more chemical energy into kinetic.
One thing to keep in mind though, the relationship between a car’s horsepower and its acceleration isn’t typically linear. You may expect that if you go from a 100hp car to a 200hp car (again all other things being equal) that since you are converting twice as much chemical energy into other kinds of energy, that you would accelerate twice as fast. However the reality is that much of the chemical energy those cars are burning in their engines is wasted, a very large portion gets turned into heat either through the burning of the fuel itself or through friction with the car’s components, etc. This is why car engines get so hot after they’ve run for a while, it’s also why you need oil in your car, to lubricate components and reduce friction. All in all this means that doubling your horsepower doesn’t double your acceleration, it may only increase your acceleration by 50% or something.
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