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Im looking a two cars, both are the same model but ones got a 2.0L turbo petrol engine and ones got a 3.0L turbo diesel engine.

These are the full specs

2.0L Petrol

enter image description here

3.0L Diesel

enter image description here

what im wandering is that although the 0-60 speed is similar the torque is almost 50% more. Ive driven the diesel and what i really liked about its 'effortless' acceleration, is this down to the torque, rather than the speed - if that makes sense ?

sorry if thats a very obvious question my knowledge of physics is 0.

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Their transmissions are probably just geared differently. –  Ben Crowell Jun 12 '13 at 22:56
    
The torque/RPM curves of gas and diesel engines differ. Diesel produces more torque at lower RPM, but is unable to produce as much torque (relatively speaking) at high RPM. A gas engine's torque tends to increase sorta-linearly with RPM. –  Hot Licks Aug 12 at 21:32
    
PS. Torque us in lbs*ft not lbs/ft. Who ever published those numbers needs some edumacation. –  ja72 Aug 12 at 21:42

5 Answers 5

The answer to this question is not at all obvious for a number of reasons. Here are a few:

  1. The torque output of an engine depends on the engine RPM at any given instant. This results in a torque curve that looks generically like THIS one.

  2. When the engine torque is quoted, it usually refers to the maximum torque on that curve.

  3. The torque that is delivered to the tires, not the engine torque, more-or-less determines the acceleration of the car at any given instant.

  4. The relationship between the torque output of the engine and the torque delivered to the tires depends on what gear you're in.

  5. The 0-60 time only tells you information about the average acceleration of the car during that time interval.

If you combine all of these things, then you'll see that there is no simple relationship between the max engine torque, and the average acceleration during 0-60. You'd either need to perform a complicated computer model, or just go test the car, to determine what's going to happen given certain figures. That's precisely why they list all of those numbers; you can't easily predict one from the others.

Cheers!

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The figures used in adverts for cars typically show peak torque, as it actually varies over the entire range of engine speeds.

Simply put, torque is what gives you acceleration at low engine revs, but gearing and maximum engine revs are also essential. And in fact drag and other factors come in at higher speeds. And over all that you need to look at the weight of the car, as acceleration = force / mass - despite being the same model, they will have different weights and the gearing will almost certainly be different as diesel and petrol engines have peak torque at different revs.

So from the question there is not enough data to say what is the core component in the case of these two vehicles

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The kinetic energy E of a mass M moving with velocity V is $$E=\frac12 MV^2.$$ Taking the derivative of both sides with respect to time T gives $$P = \frac{dE}{dT} = MV\frac{dV}{dT}.$$ $P = \frac{dE}{dT}$ is power, and $\frac {dV}{dT}$ is acceleration A. Rearranging, we get $${P\over MV} = A.$$ I.e. at a given velocity and mass, the acceleration A is proportional to power P. Since force $F=MA$, this last equation is equivalent to the familiar $$P=FV.$$

Conclusion: Instantaneous acceleration is proportional to instantaneous power. With a continuously variable transmission, you get the greatest acceleration by setting the gear ratio at the engine rpm giving peak horsepower. With a transmission having a finite number of gears, at a given velocity each gear puts the engine at a particular rpm. You get the greatest acceleration by being in the gear choosing the rpm having greatest horsepower.

The engine delivering the greater horsepower will give greater acceleration.

Peak torque is not as good a predictor of acceleration. After all, you could run a hand crank through a gearbox that reduces the rpm by a large ratio. The gearbox will increase the torque by the same ratio, so it would output a high torque but at extremely low rpm. Because of conservation of energy, the power output is no greater than power input. This 'engine' would not give good acceleration for a car, even though it has high peak torque.

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Hi jack512 and welcome to Physics.SE! We use MathJaX, a online TeX solution, here to typeset formulae. A basic tutorial is here, if you want to write pretty-looking formulae on your own. –  ACuriousMind Aug 12 at 15:28

The diesel while having similar max hp as the petrol, will have significantly more hp at 2000 rpm than the petrol. So from 1000 to 4500 rpm the diesel will have significantly more acceleration than the petrol in the same 1000 to 4500 rpm range with equivalent gearing, hence the more effortless acceleration. So in 99% of everyday acceleration scenarios, the diesel will be far more satisfying and peppy. Now if one accelerates 0 to 60 mph at max rpm, where one lets out the clutch at 5000 rpm then the petrol will win that race, but if the same race is conducted with both engines at idle as they normally would be in everyday driving, the diesel will pull far ahead at the start if both cars have equal gearing. So in the cars you mentioned the petrol is putting out 82 hp at 2200 rpm, and the diesel is putting out 120 hp at 2200 rpm. Guess which rpm a car will be closer to most of the time. So 99% of the time the diesel will feel more satisfying and 1 % of the time the petrol will feel more satisfying if you like making a lot of RPMS to impress people.

As someone else put it you have to drive a petrol like you stole it, to get good acceleration.

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You should tie in torque and power into your answer and explain what effect they have and why. –  Brandon Enright Jun 12 '13 at 20:29

Peak torque is a pretty useless number. You can gear torque up or down anyway you like. What sets the limit is the ability to provide torque at speed which is power. In fact, it is the power to weight ratio that is primary indicator of performance.

Imagine an electric motor that makes 5000 lbs*ft when its not spinning, but the torque drops down to zero at 1000 rpm. How does it compare? Peak power of the motor is 238 hp which should give similar acceleration to the cars above when 2nd gear does 600 rpm at 62 mph.

Now the petrol and diesel engines have different torque characteristics which changes how the car feels in each gear, but when looking at overall performance, peak torque is meaningless. What is important for feel is how wide is the engine sweet spot where it feels eager and responsive and does it have linear power delivery (aka steady torque output). In this respect modern turbo diesel engines excel very well.

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