Torque and RPM relation

[xxChrisxx]

Oh, for ---'s sake, people, have we forgotten already? Torque is energy!

It most certainly isn't.

Rotational mechanics is just a dressed up version of linear mechanics.
You'd never say that 'force is energy' because it isn't.
A force acting over a distance is work.


The dimensions for torque and work might be the same but there is the crucial difference:

Work (energy) is a scalar quantity.
Torque is a vector quantity.

So in the same way that
linear force * distance through which it acts = work
W = Fd

Work = torque * angle through which it acts.
W = T*theta


EDIT: But everyone apart from WB clearly knows this to be the case!!

[xpensive]

Power equals Propulsion force times Speed, which is why you get wheelspin at standstill, the engine torque itself is irrelevant.

I neg voted you for this on the basis that it's factually wrong for the following reasons.

By saying engine output torque is irrelevant, you are effectively saying we don't care about the shape of the torque curve and by extension we don't care about the shape of the power curve (as the two are intrinsically linked)

As i said in an earlier post; in maths world you are fine to use a power figure and force x speed, as we don't need to worry about fixed gearing. Your tractive effort curve would asymptote at 0rpm and decay away to zero at infinite speed.

All would be well, and you would get a pretty decent, though optimistic, approximation the the car's performance.
...

Oh you did did you now, Good Lord, I'm tempted return the favor for the following reasons;

- The power curve is not the issue here, the ambition was to show analytically that the engine torque itself is irrelevant.

- Theoretically and with stepless variomatic gearing, we don't need to worry about any fixed gears or the power curve,
while Force will decrease as speed increases until said Force balances air- and rolling resistance, this is called terminal speed.

- In the other end, att zero speed and same conditions, Force will be theoretically infinite if you can apply all the power,
why there's wheel-spin up to a certain speed when traction balances Force, something a dragracer should be all too aware about.

Finally, I'm mighty surprised to note members downvoting opponents in a technical debate, most disappointing actually.

[strad]

I run a Jet powered dragster. Zero torque but pretty quick.

Then you know that is entirely different.

[Cold Fussion]

Oh, for ---'s sake, people, have we forgotten already? Torque is energy!

It most certainly isn't.

He is quoting a most esteemed member, for they want on ad-infinitum about how torque is energy.

[Brian Coat]

Isn't this thread a repeat?

A good explanation of the key principles and terms was already covered in the postings by Zak_Newton (1680) and JamesW (1782), among others.

[strad]

I run a Jet powered dragster. Zero torque but pretty quick.

Then you know that is entirely different.

Whats the name of the car?
We might know some of the same people.
They still limit you right? That's a shame .

[gruntguru]

Sorry Strad. My tongue was firmly planted in my cheek while typing that!

[Jersey Tom]

Ah yes, an ever popular discussion. Obviously, engine power, gearing, and torque at the driven wheels are all inseparable and equally crucial for lap speed.

If you had to pick an engine to use though - all other things being open to change (like final drive ratio) - power is ultimately the measure of how quickly the chemical potential energy in your fuel can get transformed into mechanical shaft energy going through the powertrain. Can you measure that directly? No, you do it via torque and speed, but that doesn't make it less important.

If you're a race team and you go from one season to another and change engines to something that makes double the torque at half the RPM - you're not going to lap twice as fast. You might accelerate like hell coming off the corners, but then run out of steam and have half the top speed - not good. So then you change your final drive ratio by a factor of two and ta-da, you're back to where you started - with the same torque at the wheels.

[xxChrisxx]

Good post that JT.

If you're a race team and you go from one season to another and change engines to something that makes double the torque at half the RPM - you're not going to lap twice as fast. You might accelerate like hell coming off the corners

Or constantly or end up with a box full of neutrals because you only designed the gear teeth for half the load!

Convesely, if you had an engine with half the torque at double the speed you end up cooking the gearbox due the high input speed.

[xpensive]

..., but then run out of steam and have half the top speed - not good. So then you change your final drive ratio by a factor of two and ta-da, you're back to where you started - with the same torque at the wheels.

You left out the all important conclusion Chris, this was why I upvoted him.

[gruntguru]

If you had to pick an engine to use though - all other things being open to change (like final drive ratio) - power is ultimately the measure of how quickly the chemical potential energy in your fuel can get transformed into mechanical shaft energy going through the powertrain. Can you measure that directly? No . . . .

Sure you can. Use a generator or alternator as your dynamometer and measure the electrical power produced. (If you measure the rpm you can even calculate the torque as well.)

Not the kind of dyno you buy for tuning engines, but it is done a lot.

[xxChrisxx]

..., but then run out of steam and have half the top speed - not good. So then you change your final drive ratio by a factor of two and ta-da, you're back to where you started - with the same torque at the wheels.

You left out the all important conclusion Chris, this was why I upvoted him.

Hmm my assuming that the fundamental law of conservation of energy was obvious and common knowledge was, indeed, an oversight.

Don't be obtuse.

[xpensive]

Not really common knowledge no, obviously not to all anyways, which is the very reason for this debate see.

[ringo]

This thread is a headache lol.

If torque is more fundamental, then why does an engine need fuel?
and why would the fuel energy content matter?

Power gives the capacity to create a torque. You can create torque by harnessing power. However It doesn't work the other way round. You require energy input for any form of motion.

[jz11]

If you had to pick an engine to use though - all other things being open to change (like final drive ratio) - power is ultimately the measure of how quickly the chemical potential energy in your fuel can get transformed into mechanical shaft energy going through the powertrain. Can you measure that directly? No . . . .

Sure you can. Use a generator or alternator as your dynamometer and measure the electrical power produced. (If you measure the rpm you can even calculate the torque as well.)

Not the kind of dyno you buy for tuning engines, but it is done a lot.

that is the thing - you can't measure torque of a running engine directly, you get time involved, and then you have power, and from the amount of power the engine made you derive average torque, this average might be over quite small period of time, but still an average calculation of some work that was done - torque in its sense is "instantaneous" and has no time component <- good calculated measurement for different sorts of expressions, but not so great when you want to do actual work, which is when time again gets involved and torque loses sense and power emerges

edit: changed my wording a little