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What is torque?

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matt_london | 14:43 Tue 30th Oct 2007 | Science
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And why is it used as a specification on which to advertise a car?
How is it different to BHP?
Thanks

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Torque is best thought of as "twisting power"

1 kg is about 10 Newtons force - imagine holding a 1 metre stick with 1 kg on the end. The twisting force is 10 newton metres.

The BHP figure on a car is to do with the power that can get to the wheels, that doesn't necessarily tell you much about how that power is used.

I would disagree with jake in that the value of torque discussed in relation to an engine needs to take into consideration the example used by jake in his 1 meter in length stick. What he doesn't say is that the same amount of force applied to a longer stick will produce more torque.
In the example of internal combustion engines one needs to understand that two engines capable of producing the same amount of horsepower can produce widely varying amounts of torque. This is especailly true in considering diesel vs. gasoline (petrol for you Brits) engines. Generally, a diesel engine is designed to produce higher torque at much lower rotational speeds and achieves this mainly by having a longer, or more accurately, larger diameter crankshaft, therefore taking advantage of leverage, so to speak. Obviously, there are many more factors involved, but torque and horsepower aren't neccessarily related...
mathematically, torque and power are always related.

torque = power/angular speeed
That is very true but remember too that we're talking cars not planes so you've got the gearbox, differential and even the size of the wheels to take into consideration.

We're talking about the torque of the car's wheels rather than just the torque of the engine alone.

I guess the bottom line is that the torque a car produces (together with it's weight) gives you a better idea of it's ability to accelerate than just the BHP figure
An engine produces torque over a range of rpm. It is the product of Torque and RPM and a constant that gives you power. Gearboxes are (if friction is ignored) torque amplifiers. The input speed is fixed over a range by the engine but the output speed varies considerably from very low speed to high speed and is affected by the gear ratio. The other important thing is that the reflected inertia of the load (car) is reduced by the gear ratio squared. For example when starting off in 1st gear the vehicle will have a certain acceleration rate. Try the same in say 5th gear and the accelleration rate is poor. Acceleration is Torque divided by inertia in rotary motion and therefore the inertia of the load(car) has a big effect. Peak acceleration is always when there is lots of torque. In the petrol engine the torque drops off with high revs and there is no point in holding that gear if the torque is dropping off...you will start to slow down. Diesels have buckets of torque low down in rev range and therefore can accelerate very well without screaming its head off. But dont hold on until the revs hit the red line...torque droop can be more significant on a deisel than a petrol engine. Want a good motor?? Try a DC electric motor...Full torque at zero rpm and how fast would you like it to go...no gearbox needed. Lots of food for thought for you

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