Why can two engines have the same horsepower, but one will produce more torque?

In: Engineering

Think about this. You and I are gonna have a race, we are standing on a roof and we each have a rope attached to a 100 lbs weight down on the ground below. Your plan is to grab the rope and walk backwards, pulling as hard as you can. I however have a winch with a large gear reduction to haul up the load. We race and what are the odds, we tie!!! We each hauled the load the same distance in the same amount of time. Because of this we both produced the exact same horsepower, we each accomplished the same amount of work. Let’s look at how it felt for each of us to do this. You pulling with your body had to push and pull very hard to pull the weight and you made very slow progress. With my winch I didn’t have to push all that hard. The gear reduction made getting the load moving much easier, however I had to spin the handle on my winch really fast to keep up with you. The handle covered a much greater distance than you did. In that situation, even though we know we made the same amount of horsepower, you had to produce much more pushing force (torque) than I did. To make up for my low torque, I had to do my easy work much faster to keep up.

This one is pretty simple math:

Power = torque x RPM (speed)

If power is constant, an increase in torque = decrease in RPM.

So if we have 1 power, 1 torque, and 1 RPM (ignoring units for simplicity):

1 = 1×1

If you have another engine with 2 torque but 0.5 RPM:

2 x 0.5 = 1

Same power.