– Horsepower vs. Torque in automobile engines.

Question

I took college physics. I learned that power is unit work per unit time, which can be expressed as newton-meters per second. Torque is a cross-product quantifying rotational force accounting for a lever arm, which is expressed as newton-meters. I know that the distance in the measurement of torque is perpendicular to the direction of rotational motion whereas the distance in measuring power is parallel to the direction of motion, so these are not the same “meters” at all. But both of these involve a measure of force – more force means more power and it means more torque. However, when it comes to car engines, two engines can have the same horsepower but very different torque. Why do HP and torque not increase in lock-step? Is this just a matter of available gear ratios in the transmission? Or is there a way to build an engine deliberately to make torque vs. Deliberately to make horsepower independent of the transmission? Thanks!

In: 2

Answer 1

Work= Force × Distance

Power=Force × Distance per unit of time

Distance per unit of time= RPM × 2πr

Torque= Force × r

Force= Torque/r

Power= Torque/r × RPM × 2πr

Power= Torque × RPM × 2π

Horsepower= basically the same as Power

as you can see Horsepower is effected by both Torque and RPM

so a high Torque and low RPM engine can have the same Horsepower as a low Torque and high RPM engine.

>is there a way to build an engine deliberately to make torque Vs. Deliberately to make horsepower independent of the transmission?

Yes. That’s exactly how we build our engines. each engine has an output torque and output horsepower , the output Torque and Horsepower are two inherent properties that describes the engine itself, independent of the transmission, or the whole car.

the output Torque of an engine when applied on wheels, can be changed through transmission, so the torque of the whole car can be changed with transmissions. The horsepower, on the other hand, can not.

well if the torque after going through transmissions can be changed then why does it matter the torque of an engine?

because if you put a higher torque engine in a *same* car with a *same* transmission, in replace of a lower torque engine, the output torque of the whole car will be higher, because the source torque is higher.