What is horsepower and torque and how do they relate to a car being fast or a truck having a towing capacity?
In: Engineering
Energy = Power * Time. So Power is how quickly an engine generates useful energy. 1 horsepower was originally defined to be the power to lift 550 lb at a rate of 1 foot/second. Nowadays, it is much more convenient to think of 1 HP as being about 750 Watts.
Torque is rotational equivalent of force. If you push something, you exert force. If you twist something, you exert torque. The two are related: Torque = force * distance from the center of rotation. if you put 20 pounds of force on a one foot wrench, you are exerting 20 foot-pounds of torque on the nut. If you use a cheater bar to extend the wrench to 3feet, that becomes 60 foot-pounds of torque.
The power output by an engine is the product of the torque * RPM. This is really simple and really important so it bears restating:
Power = Torque * RPM.
Gearboxes can freely change torque to RPM and vice versa. A big engine with lots of torque has no more pulling power than a small high revving engine with the same power.
The only thing useful about torque as a metric is that it tells you something about the general nature of the engine: high driveshaft torque for a given power generally implies a big, heavy engine meant for long running in applications where weight is not an issue (ships, generators, locomotives) while low driveshaft torque for a given power generally implies a small, light engine meant for applications where power-to-weight is more important than the ability run for weeks on end (aircraft, race cars, motorcycles).
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