Torque vs power – what’s best?

Posted: 28th January, 2011 in Cars
Tags: , , , , , , , , ,

I’m talking about that age old argument between diesel fans (who like to talk about torque) and petrol heads who claim power is most important.  My vote is power all the way, and here is why…

Torque:

Say we need to tighten a nut up.  Doing this without a spanner will not get you very far, why? because you need lots of torque.  Torque includes two things-  force AND distance.  For example, slamming a door by pushing on it next to the hinge will not make it slam, that is because the force may be large but the distance between point of rotation and acting force is small, so the torque is low.  Infact even an incredibly strong man with no tool will not be able to tighten a nut as much as a child with a spanner.

So our strong man ‘Arnie’ tightens the nut using only his very strong fingers.  Now a child comes along to undo the nut, without a spanner he’s helpless.  With a spanner, the child now applies a comparitively small force to the spanner but this force acts a long way away from the centre of the nut the longer the spanner is, the easier (or less forece) required to undo the nut.  Try this with a door.  See how hard it is to close the door if you push near the hinges, compared to if you push near the handle.

Taking this example and applying it to a car, the engine has now become our strong man/weak child and  the car’s gearbox is considered to be the length of the spanner.  The nut we were tightening is of course the road applying an opposing force on the car’s wheels.

Power:

Say we need to tighten up 100 nuts, engineers refer to this job as ‘work done’ (which is equivalent to ‘energy’).  Doing this without a spanner would probably be a lot quicker, why, because you don’t have to tighten the nut up by travelling through a large arch/distance.  Speed is the relationship of distance : time.  If we have less distance to cover and we turn nuts at the same rotational speed it will always use less time.

If you don’t believe me, drive 10 miles at 10 miles an hour and then do only 1 mile at 10 miles an hour- the latter surely won’t use up as much time!

So if Arnie and the child both complete the same amount of work in the same amount of time, they are said to have the same power.

Transition to the car example.  Two cars of equal weight have to tow a caravan up a hill.  One car has a lovely (sarcasm) torquey diesel engine, the other has a racy motorbike engine.  The work done will be the same, (same hill, equal weights).  So which one will get there first?  Well it’s all about power.  Power is of course, work done per unit time.  So if you get your work done quicker you finish first.

Interestingly, these vehicles make power in very different ways.  Rember Arnie and the child, well the diesel engine is our strong man- he can afford to spin things slowly because he has lots of torque (strength).  The motorbike engine and child have relatively little strength but a gearbox/spanner provide a torque/strength multiplication.  Like all things in engineering, if you fix one issue, another one is introduced.  By using the gearbox as a torque multiplier you must operate more quickly to keep your work output high.  This is why motorbike engines have to rev so much.

For those interested, a new school SI unit engineer like myself uses the following formula to work out power in kW:

Power [kW] = Torque [Nm]  * EngSpeed [RPM] * (2*3.142/60000)

In the brackets above we are converting engine speed into radians per second and watts into kilo-watts.

To convert kW into brake horsepower, BHP = kW * 1.34

So, What’s Best?

If you want to win a drag race, you need lots of power.  Simple, that is the answer to the question.  If you want to tow your caravan to the top of the hill before all the other caravans you still need to have the most power!  The subtly is the gearbox that denotes the relationship between road speed and engine speed.  Towing caravans up hills requires lots of torque at the wheels even to get rolling.  This can be done by a weak or low torque engine geared to do many more revolutions for each turn of the cars wheels.  Or perhaps a high torque engine, geared to do similar revolutions as the wheels it is driving.  In a towing race, the higher powered engine will always win!  And it doesn’t matter which engine produces more torque (provided they both have a suitable gear box) .

Most diesel fans will claim that this means lots of revving of the less torquey engine to keep up.  Yes this is true, but so what- thats why you have a gearbox and a red line almost double that of the average diesel!  Also, in favour of the smaller less torquey petrol engine is the weight advantage.  By minimising weight you reduce the amount of work needed to complete a race/sprint.

The only real advantage of high torque is longevity.  By creating  power at low engine speeds, service life is dramatically enhanced.  It is also possible to observe fuel savings by keeping engine speed low, as frictional losses are less significant at low speeds, and are zero when the engine is stopped!

In Summary

Cars, trucks boats have all been designed to speed things up.  Whether it’s getting to work quicker in a car compared to walking.  Or perhaps transporting building materials from A to B.  These jobs took a lot longer before the benefit of powerful engines.  Low power means less work done for a given period.

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Comments
  1. sartenada says:

    Great post, really!

    I got a turbo powered car with 2.8 liter. That means power and torque, I guess. More info in my post: “Cars-my blue car”.

  2. Joe Diacono says:

    Torque is the force that causes something to rotate and horsepower is the rate at which torque can be applied over a given distance. Horsepower is merely a measure of the rate of torque production,

    For example a giant diesel engine in a container ship may produce 1 million lb/ft. of torque but only 20,000 h.p. Why? because the engine only turns 300 r.p.m. Conversely a motorcycle engine may produce 150 h.p. but only 75 lb/ft. torque but it produces it’s maximum torque at 10,000 r.p.m. Horsepower is a number in an equation, nothing more. It’s more a matter of what torque curve would work best for a given application.

    • thoughtdraw says:

      I agree with your comments and yes power is a function of torque curve. For the sake of argument, if you want to get something done quickly, whether at sea or on land, more power is a wise choice, discounting other factors like cost. So in a pub argument between petrol or diesel, I would choose high power petrol vs. a high torque diesel car.

  3. traveler says:

    thank you for the article, however i would like to confirm my thinking here : in the drag race exemple you mentioned, you estaplished that the most powerful car wins regardless of it being the one with the more torque or not.. but in case of let’s say two cars, being the same model, same weight and the only difference is the engine : same power diesel and petrol (but by definition more torque for the diesel model), i think here that torque is the deciding factor..because we would have more torque done at X rate compared to less torque done the same X rate with X being the horsepower, even if peak values for both are attained much low at the rev range (diesel).. so the diesel car provided with same power and weight as the petrol car, would outperform it in everyday driving and even in drag race… am i correct ?

    • thoughtdraw says:

      All things being equal, the equal power vehicles will finish at the same time regardless of their peak torque, since they are doing the same amount of work (moving a mass over a given distance) in the same amount of time, power = [work done] / [time taken]

      The issue is complicated since it is often not feasible to operate at peak power all of the time, think gear changes and torque & power curves for each engine. So the real life winner would be the engine that was able to operate closest to peak power for more of the time – which engine is capable of that, not sure! http://www.parkers.co.uk/cars/reviews/facts-and-figures/saab/9-3/saloon-2002/ a selection of different engine & gearbox combos in the same vehicle and for say 147bhp petrol manual 0-60 is 9.0secs and the 147bhp diesel manual again 9.0 secs kind of proving the point that power is key, and something like the auto box robs efficiency and therefore adds time to the 0-60 sprint.

  4. briank101 says:

    I think if it was just peak power that was all that mattered we’d all be driving around in noisy cars that sounded like highly strung motorcycle engines with short engine life with poor fuel economy. Why I believe an engine with more power at lower RPMs (i.e. a “high torque” engine), is because it’s the RPM range a driver will be 99% of the time. An engine with high peak horsepower say 150 hp at 7000 RPM but with only 10% of the peak power 15 hp available at 2000 RPM will feel very under powered without changing down 2 gears, in other words driving it like you stole it. The excitement of driving such a car would get old very quickly, kind of like a teenager who drops the suspension on their Civic. Unfortunately too many customers and journalist salivate at the peak horsepower number, but it’s a power level seldom experienced. On the other hand a car with high torque at lower RPM (which is precisely the same as high power at low RPM), the driver will be rewarded with more power at 2000 RPM, more power at 3000 RPM and more power at 4000 RPM. Guess who will experience more power most of the time and effortless power most of the time? Guess whose engine and transmission will last longer? Guess whose car won’t sound like it’s trying to win a race just to keep up with traffic? Yes the car with higher torque (higher power) at lower RPM.

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