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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Yes there are mechanical design elements that affect the torque of an engine, for instance a piston engine with a longer crankshaft offset acts like a larger lever on the output so it makes more torque. There are many more factors that go into engine design, but that’s why HP and torque aren’t directly linked.
Torque = how much force can be applied. Related to how much fuel combusts at once.
Horsepower = how quickly that power can be applied over time. Related to how quickly the engine shaft spins – generally related to how many times per second combustion occurs (in eli5 terms)
Engines with large cylinders and slow rotating output shafts have a lot of torque, and require high gear ratios to spin wheels quickly
Engines with small cylinders and quickly rotating output shafts have low torque, and require low gear ratios to not spin wheels too quickly.
There are practical materials engineering limits for size of cylinder vs speed of rotation. Also in transmission gear wear vs power. And so we see the common related bands of torque vs horse power in modern cars.
The 1955-1990’s Chevrolet V8 family had a block where a wide variety of components could be swapped around for various combinations of bore and stroke. Of course the bore is cast into the block, and the stroke is the configuration that can be changed easily by the home builder.
The 350 cubic inch displacement version was probably the most common (roughly 4-inch bore and 3.5-inch stroke). For this example, the blocks could be had in a 400-cubic in displacement.
If you used a 400 block and swapped-in a 350 crank, you had a big-bore/short stroke engine at 377 ci, and the large valves that were allowed because of the largebore created an engine that could easily rev to 7,000 RPM’s and make a high peak horsepower. However, at low RPM’s it had less crankshaft torque than the 350 due to poor cylinder filling.
Reversing it, you could take a 350 block and swap-in a 400 crank, creating a small bore/long-stroke 383. Peak horsepower was down compared to the 350, but it had more torque across the entire RPM range, with a very noticeable improvement in low-RPM torque…
It’s because power has the time element to it. You take two engines that make the same torque, and one can produce that torque at double the RPM you are effectively applying that force twice as often in any given time period, hence more power.
Torque does not take time or rate into consideration, it is purely a force applied. No matter how quickly or how slowly you apply it the torque is the same. It’s also why the old school phrase “HP sells cars but torque wins races” is bullshit. The extreme example of that is if I put a 3 foot breaker bar on a crankshaft I can pretty easily put 600 Nm of torque through the crank but I’m still only making like 0.5 kW of power at most.
Horsepower and torque are related. You an convert one to the other with a few other factors.
Engine torque is proportional to horsepower/engine rpm. So engine torque is effectively how much power you get out of a single engine cycle, where horsepower is how much total power you get in a minute.
The transmission comes into play when talking about power at the wheel. Think of a large truck with a wide gear ratio – it can go into a very low gear to ‘concentrate’ that power into a single rotation of the wheel, or a higher gear to turn the wheel faster. You lose some power due to system inefficiencies, but you’re otherwise running that problem backward by using the transmission to measure how much torque there is off of the wheel, rather than how much torque is being applied to the drive shaft by the engine. The latter isn’t changing as you shift gears, but the former is.
Engine torque does vary based on the engine RPM because of the rate at which the pistons are firing, valves are opening, fuel mix, temperature, etc. shifts the efficiency of the engine causing torque to vary – lower at low and high RPM and higher at mid RPM. This is a sort of efficiency of an engine to generate power. Really good engines (racing cars) are more consistent at generating torque across engine RPM than less good engines (your car).
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.
power is torque times speed (rpm)
so an engine that has lower torque but spins faster can have the same power as an engine with more torque that can’t spin as fast
Torque * RPM is HP
I think this helps you understand that torque is really telling you how hard the pistons push, since if you break it down to one cycle, then the energy from the piston, times the distance is torque, so torque is a measure of how much force comes from one cycle in the engine.
When you add in RPM, then the pistons push that much per second, so that’s total power. So for a particular engine, you’d expect torque to be relativity flat over RPM, while HP goes up linearly with RPM. Obviously, due to a lot of reasons, it’s not quite true, but you can help understand what’s going on.
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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