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!
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Torque is a measured unit of force. An ICE engine will have a specific range of rpm where it puts out the most torque. It’s actually a curve that goes up, usually levels out a bit, and goes back down.
Work is force over a displacement (apply a force to move something ten feet, you’ve done work).
Horsepower is a calculated unit of power, which is work over time, in this case one minute. The raw equation is complicated, but when using lb-ft it is distilled down to (rpm * torque)/5252.
So how can engines be different? One big factor is the bore: the diameter vs. the stroke (how far the cylinder travels). Remember, the displacement of an engine is calculated based on the bore and the stroke (creates a cylinder, which has a volume).
Take an old WWII Jeep. It produced 60 hp and 105 lb-ft of torque from a 2.2l four cylinder. It had fairly thin cylinders a stroke much longer than the diameter, so each boom in the engine provided a lot of force over a long distance on the cylinder to produce the torque at the crank shaft. It produced its maximum torque at a rather low rpm. The curve went quickly from zero to max torque at about a 2,000 rpm peak, and then it immediately dropped off to about 70 ft-lb at the 4,500 rpm redline.
So peak torque goes into the equation at 2,000 rpm, but the rpm is so low that the horsepower number comes out low, maybe 40 hp. As we rev higher, torque drops but rpm more than doubles, so with the about 70 ft-lb torque at redline we’re finally making 60 hp. But the redline is low so we can’t go any higher to get any more horsepower.
Compare to a more modern car, say an average 2.2l four cylinder, which is going to have a bore and stroke somewhere about equal. But overall the engine is more efficient so all numbers will be larger (fuel injection, etc.). The modern engine revs a lot higher, helped by that shorter piston travel (less stuff going back and forth, easier to go higher rpm).
So instead of that torque peak, you get a softer plateau around 4,000 rpm at maybe 150 ft-lb, slowly trailing to about 75 at 8,000 rpm. With the higher rpm, horsepower slowly rises to peak around 6,000 rpm at 150 hp, trailing to about 120 at redline. Notice peak horsepower and torque are the same, they just happen in different places.
For a bigger contrast within gas engines, you can think of an old Ferrari V8. The one I’m thinking about was only 2.9 liters for a V8, so it had small cylinders, and each cylinder was much wider than the stroke. That’s low reciprocating mass and distance, less work being done on each stroke, but it could rev high to produce a lot of horsepower. Its torque number was always much lower than the horsepower number.
So we design an engine to put out a lot of torque at lower rpm to haul stuff, not worrying much about horsepower. Or we design an engine that’s balanced between torque and horsepower. Or we design an engine that foregoes getting a lot of torque so it can rev to very high rpm to get a lot of horsepower. You still need some torque in the equation, but as long as the rpm goes up faster than the torque goes down, you keep getting more horsepower.
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