FrancisJeffries
Well-known member
On the thread about the Audi A3 ETron, there is discussion about the power/ton and torque/ton which rather, IMHO, shows the author to misunderstand the relationship between power and torque.
Power is a rate of working. Torque is a measure of rotary force, not a rate. Power is torque X speed (in radians/second). So the i3, at zero speed, has a declared torque of 250nm. (I shall ignore the Rex because it's not relevant to the argument here.)
Torque divided by tyre radius, gives the forward force at the road surface which accelerates the car, and the poster there is right to compare the torque/ton, but only if the tyre radii are the same. For the i3, with that constant torque up to about 57mph, that means that it's almost constant acceleration up to about 57mph.
But notice that at standstill, that torque is supplied at zero horse power! When the torque starts to decline at about 57mph, the power being used is 170bhp, or 127 kW. 127kW is a BIG rate of working, and would - if sustained - flatten a full battery in about 12 minutes! In practice, that rate of working (i.e. power) is sustained only very briefly while accelerating.
The Rex version contributes nothing to these calculations, btw, and the torque and speed values given for the Rex motor are irrelevant to the car's performance and merely a sop to petrol heads who think that these numbers have relevance.
Power is a rate of working. Torque is a measure of rotary force, not a rate. Power is torque X speed (in radians/second). So the i3, at zero speed, has a declared torque of 250nm. (I shall ignore the Rex because it's not relevant to the argument here.)
Torque divided by tyre radius, gives the forward force at the road surface which accelerates the car, and the poster there is right to compare the torque/ton, but only if the tyre radii are the same. For the i3, with that constant torque up to about 57mph, that means that it's almost constant acceleration up to about 57mph.
But notice that at standstill, that torque is supplied at zero horse power! When the torque starts to decline at about 57mph, the power being used is 170bhp, or 127 kW. 127kW is a BIG rate of working, and would - if sustained - flatten a full battery in about 12 minutes! In practice, that rate of working (i.e. power) is sustained only very briefly while accelerating.
The Rex version contributes nothing to these calculations, btw, and the torque and speed values given for the Rex motor are irrelevant to the car's performance and merely a sop to petrol heads who think that these numbers have relevance.
