Torque - In Diesel and Petrol engine

Hello Friends,

Is it true that torque in Diesel engine is different (generally more) than the Petrol one? Why so? Also what is the meaning Torque - for a layman? I have tried to do some 'Google' for Torque but there are very much technical details in that? May someone explain what does the Torque mean? And how does it help the driving pleasure?

Thanx

Have a look at this

In addition to Binoy's post (which explains almost everything), to put it in short, torque means "twisting force" - Imagine something rotating about an axis. Torque then means how much force needs to be applied to stop the rotation. Under this condition the torque and external force cancel out, so the external force applied is a measure of the torque. Now, greater torque does not necessarily mean greater RPM - think about a small tape motor, the one you find in home audio systems (high RPM, low torque) v/s a large pump motor (low RPM, high torque).

I re-read my post and found it to be a bit ambiguos. So here's the improved version

Imagine a disc rotating about its axis. Torque then means how much force needs to be applied at the disk's edge (tangentially) to stop the rotation. Under this condition the torque and external force cancel out, so the external force applied is a measure of the torque.

If we denote the external force required to stop the disc as 'F', torque being applied at the disc's centre as 'T', and 'r' as the radius of the disc. Then, F = T/r (i.e., the larger the disc, the lesser force it will take to stop the disc for the same torque being applied at the centre). Imagine how easy it would be to stop a fan by its blades than by its rotor.

Conversely, if an external force F is applied to a non-rotating disc of radius r, then the torque developed at the center of the disc is given by, T = F*r (i.e., the larger the disc, the greater is the torque developed at the center for the same external force). Imagine using a long spanner and a short one to unscrew a nut - which makes it easier? The long one (greater length - extend that to the disc we get a greater radius).

Now, greater torque does not necessarily mean greater RPM - think about a small tape motor, the one you find in home audio systems (high RPM, low torque) v/s a large pump motor (low RPM, high torque).

As for the question of how torque affects driving pleasure, everything else being the same an engine with greater torque means that the vehicle can move greater load or move up a slope better than one that has lower torque.

Consider a beam ABC of length 2m with the distance between A and B being m and the distance between B and C being m, with B being the point at the centre. If a force N is applied at A, then the force at point A is N and torque is zero. The force at B is zero and torque is N*m, at point C, the force is zero and torque is N*2m.

Torque= Force * distance

Hope this has helped you to understand what is torque.

. 

Rachit, torsion is not twisting force but the amount of distortion an object undergoes as a result of applying a torque. Imagine a fixed rod. Now with a plier of length 'r' apply a force 'F' to the plier in a direction perpendicular to the rod. Now the torque or twisting force developed at the rod is F*r while the amount of distortion the rod experiences is the torsion. See http://en.wikipedia.org/wiki/Torsion_(mechanics)

Edit: I see you have removed that line

Yes, coz when I added that I didn't see your post.

Hi All,

I just wanted to know whether higher torque at low RPM helps to drive a car in better way. For example Indica V2 Disel delivere a 108Nm torque @1800RPM and Indica Petrol deliver a 12.4 Kgm torque @ 2600RPM. So which one is better to drive.

Does high torque at low RPM require less gear shifting? If yes then I think disel cars are better to drive as compared to petrol but many of my friends says that Petrol cars are better to drive.

Please suggest.

Torque/power is always better if you get it at lower RPM's. It means lesser gear shifts.

Petrol cars being better to drive was a myth which was kinda true in the last decade. However with the emergence of turbo/VGT in diesel cars, things have changed.

Thank you Binoy, Thank you Rohit, Thank you Rachit,

I guess your easy language/explanation cleared the ‘Torque’ question. That Wikipedia link was a googly to me!

I have one more query, earlier I used to drive Swift VDi and then my company gave away that Diesel car to a colleague of mine and bought D’Zire VXi for me. To be honest with you, I find the Diesel version of Swift was better than the petrol D’Zire, however, I am not finding any pick up problem with air conditioning on and of course the mileage part won’t come in consideration since fuel is on company’s account – I am coming to the point – when I leave the gas paddle of petrol, the rpm and the speed immediately starts reducing, but was not the case in the Diesel one. In the DDiS, even if I ease the foot pressure on the accelerator, the speed did not wash out very soon, but very very gradually.

Ø  What is the reason for this difference?

Ø  Which ones better? The sudden decrease of Petrol engine or the smooth deceleration of Diesel one?

Ø  Why the hell the Petrol engine of D’Zire is not as powerful as the DDiS?

Cheers!

Kabir