Why is it so hard for people to understand torque vs power??
Discussion
Sam_68 said:
deeen said:
...I would say c7xlg's statement is correct - "A car's acceleration is directly linked to the torque AT THE WHEELS".
Go figure out what is happening at Vmax, then....If we're doing stupid questions...
Another pre-GCSE go figure question for you... how fast would a car accelerate if the engine isn't producing any torque?
I forgot about wheelspin, but otherwise the point stands; the torque at the wheels (hub if you prefer) is directly related to the thrust at the contact patch; they're just 1 lever apart (the wheel/tyre combo), you can't get a much more direct connection than that.
I'll copy and paste and add to what I put in that thread:
Power = Torque * Rotational velocity
Acceleration = Force/Mass
Force from wheels = Torque at wheels/wheel radius
Force is proportional to torque at the wheels, hence acceleration is proportional to torque at the wheels.
Gear ratios transform torque and rotational velocity. So to satisfy conservation of energy:
power in = power out (neglecting losses)
200Nm at 5000rpm into a 10:1 gear ratio gives the exact same torque at the wheels as 400Nm at 2500rpm into a 5:1 gear ratio. The conclusion from this is that peak torque is an irrelevant figure as it must be geared accordingly, torque*speed (i.e. power) is the quantity that defines performance.
What this rather simplistic analysis doesn't take into account is torque spread. As most transmissions have a set of discrete gear ratios, it is impossible to keep the engine at its peak power constantly whilst accelerating, which is why (for the same maximum power) a peaky power delivery is inferior to a more spread power delivery. As mentioned earlier, the range of rotational velocity at which a good proportion of maximum torque is available is hugely important in how a car performs in the real world. This is where diesels have an advantage, due to the shape of their torque/speed characteristic, and why a 200bhp diesel will usually feel faster than a 200bhp small capacity NA petrol.
Power = Torque * Rotational velocity
Acceleration = Force/Mass
Force from wheels = Torque at wheels/wheel radius
Force is proportional to torque at the wheels, hence acceleration is proportional to torque at the wheels.
Gear ratios transform torque and rotational velocity. So to satisfy conservation of energy:
power in = power out (neglecting losses)
200Nm at 5000rpm into a 10:1 gear ratio gives the exact same torque at the wheels as 400Nm at 2500rpm into a 5:1 gear ratio. The conclusion from this is that peak torque is an irrelevant figure as it must be geared accordingly, torque*speed (i.e. power) is the quantity that defines performance.
What this rather simplistic analysis doesn't take into account is torque spread. As most transmissions have a set of discrete gear ratios, it is impossible to keep the engine at its peak power constantly whilst accelerating, which is why (for the same maximum power) a peaky power delivery is inferior to a more spread power delivery. As mentioned earlier, the range of rotational velocity at which a good proportion of maximum torque is available is hugely important in how a car performs in the real world. This is where diesels have an advantage, due to the shape of their torque/speed characteristic, and why a 200bhp diesel will usually feel faster than a 200bhp small capacity NA petrol.
deeen said:
Wheelspin?
Nope. Try again. What you and many other people on this thread are failing to recognise is the differences and relationships between work and effort.
deeen said:
If we're doing stupid questions...
Another pre-GCSE go figure question for you... how fast would a car accelerate if the engine isn't producing any torque?
In this case...Another pre-GCSE go figure question for you... how fast would a car accelerate if the engine isn't producing any torque?
...surprisingly quickly
deeen said:
Sam_68 said:
c7xlg said:
1) A cars acceleration is directly linked to the torque AT THE WHEELS and the mass of the car.
Wrong!!!!!A cars acceleration is linked to the thrust at the wheels (tyre contact patch), which can be calculated from either flywheel torque or flywheel power (multiplied by transmission efficiency) and gearing.
"Thrust" at the tyre contact patch is directly linked to torque at the wheel hub, so I would say c7xlg's statement is correct - "A car's acceleration is directly linked to the torque AT THE WHEELS".
One of my cars has a constant 258lb ft from 1900-4000rpm - which means that in any one gear, it pulls equally hard all the way through that range. So that's the effect of the @flywheel torque figure - in any one particular gear, the torque available at the current revs determines how hard the car accelerates.
At 35 miles per hour, I have a choice of 3rd gear and 2000rpm or 2nd gear and 3000rpm. I have the same amount of torque at the flywheel in either gear, but the car will accelerate far harder in the lower gear - the ability to continue to make that amount of torque at higher revs means that I can use a lower gear for the same road speed and thus further multiply the torque to the wheels. By this means, being able to make torque at higher revs enables me to accelerate harder. That's why the product of torque and revs - which is power - is useful. 258lbft @ 2000rpm gives me 98bhp. 258lbft@3000rpm gives me 147bhp.
Vladimir said:
TaffRichie said:
Torque is what people who drive diesels bang on about trying to justify there fuel choice...
Oddly enough coming from someone with a nearly torqueless Toyota Celica...myles1972 said:
Dave Hedgehog said:
lets look at the science of this properly
diesel = w
ker rep car
petrol = hero wagon
i hope that clears it up
Dave, that's all I need to know.diesel = w
ker rep carpetrol = hero wagon
i hope that clears it up
ks posted aboveif petrol and diesel where both 1p a litre we would all be running around in petrol 427 V8s
Edited by Dave Hedgehog on Monday 16th January 20:40
Sam_68 said:
In this case...
...surprisingly quickly
Not much "thrust" at the tyre contact patch, either!...surprisingly quickly
I think you are missing the point that the OP highlighted torque AT THE WHEELS, which is directly related to power at the wheels (I think the formula was mentioned earlier in the thread, except to calculate one from the other AT THE WHEEL you would need the speed the wheel is turning, NOT the engine), and "thrust" at the tyre contact patch.
ETA: See Chris above, simple, lucid and correct
Edited by deeen on Monday 16th January 20:41
otolith said:
And more to the point, given the freedom to choose the gearing, the factor which sets the upper limit on torque at the wheels at any given road speed is the peak power of the engine.
One of my cars has a constant 258lb ft from 1900-4000rpm - which means that in any one gear, it pulls equally hard all the way through that range. So that's the effect of the @flywheel torque figure - in any one particular gear, the torque available at the current revs determines how hard the car accelerates.
At 35 miles per hour, I have a choice of 3rd gear and 2000rpm or 2nd gear and 3000rpm. I have the same amount of torque at the flywheel in either gear, but the car will accelerate far harder in the lower gear - the ability to continue to make that amount of torque at higher revs means that I can use a lower gear for the same road speed and thus further multiply the torque to the wheels. By this means, being able to make torque at higher revs enables me to accelerate harder. That's why the product of torque and revs - which is power - is useful. 258lbft @ 2000rpm gives me 98bhp. 258lbft@3000rpm gives me 147bhp.
Agreed with all, but the quote is "torque AT THE WHEELS"!One of my cars has a constant 258lb ft from 1900-4000rpm - which means that in any one gear, it pulls equally hard all the way through that range. So that's the effect of the @flywheel torque figure - in any one particular gear, the torque available at the current revs determines how hard the car accelerates.
At 35 miles per hour, I have a choice of 3rd gear and 2000rpm or 2nd gear and 3000rpm. I have the same amount of torque at the flywheel in either gear, but the car will accelerate far harder in the lower gear - the ability to continue to make that amount of torque at higher revs means that I can use a lower gear for the same road speed and thus further multiply the torque to the wheels. By this means, being able to make torque at higher revs enables me to accelerate harder. That's why the product of torque and revs - which is power - is useful. 258lbft @ 2000rpm gives me 98bhp. 258lbft@3000rpm gives me 147bhp.
We left the flywheel behind at the other end of the transmission!
Actually Chris's explanation on the previous page is very simple, lucid and correct.
Edited by deeen on Monday 16th January 20:39
Regardless of all the above and reference diesel M cars etc..,
Q. Given the choice + somebody else paying for your fuel, which would you choose, a diesel M car with bigger torque figure or petrol M car with equal or larger bhp figure.
A. Your right - the petrol one!
Reason being they may both offer similar power, ( all be it at different revs ), but turbo diesels have a narrow rev range and ultimately have switch like power delivery,i.e, nothing > full on > nothing > change gear. IMHO this does not make for the best performance driving experience as you loose a lot of the flexibilty and more importantly FUN that a wide rev range offers.
No doubt a diesel can be quick as recent LeMans wins testify, but, balancing the car on the throttle, holding revs whilst you flick through the gears, hanging on to a gear till the roar tells you to change, ( que Braveheart - hold, holD,hoLD, hOLD, HOLD ), means the petrol engine offers a more enjoyable and often quicker experience. I currently hillclimb, ( many hills not disimilar to english B roads ), a 1370cc GSXR powered Jedi and can't belive that a diesel car with same power to weight ratio, ( I should imagine rare unless your name is McNish ), could be anywhere near as quick because you can't keep them on the boil in the same way because of that on off power deliver.
Q. Given the choice + somebody else paying for your fuel, which would you choose, a diesel M car with bigger torque figure or petrol M car with equal or larger bhp figure.
A. Your right - the petrol one!
Reason being they may both offer similar power, ( all be it at different revs ), but turbo diesels have a narrow rev range and ultimately have switch like power delivery,i.e, nothing > full on > nothing > change gear. IMHO this does not make for the best performance driving experience as you loose a lot of the flexibilty and more importantly FUN that a wide rev range offers.
No doubt a diesel can be quick as recent LeMans wins testify, but, balancing the car on the throttle, holding revs whilst you flick through the gears, hanging on to a gear till the roar tells you to change, ( que Braveheart - hold, holD,hoLD, hOLD, HOLD ), means the petrol engine offers a more enjoyable and often quicker experience. I currently hillclimb, ( many hills not disimilar to english B roads ), a 1370cc GSXR powered Jedi and can't belive that a diesel car with same power to weight ratio, ( I should imagine rare unless your name is McNish ), could be anywhere near as quick because you can't keep them on the boil in the same way because of that on off power deliver.
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