Power and Torque

[dgrev]

Can someone please give me a clear explanation of the difference between power and torque?

When I have asked mechanics in the past, they immediately start muttering and are unable to explain it without resorting to "you know, torque is torque and not the same as power". They don't seem to know themselves, only that the bigger the number the better.

Also, why is it that diesels of a cubic capacity and power rating to a petrol engine, lets say both are 250 cu in, are far more "powerful" than the petrol engine of the same size?

Should say, that I bought the diesel Pajero as it produces more torque but less power than the petrol motor. Salesman was very adamant about me buying diesel due to
having better trade in value.

Thanks
Doug

[WogsRus]

power is how fast you hit the wall, torque is how far you take the wall with you.

Basically more power means faster acceleration, more torque means more pulling ability, so ie towing.

[Aussie_Dan]

Have a read of this one Doug:

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

Power is a product of torque and rpm.
Diesels have a large amount of torque but are quite a slow revving engine by nature, which therefore limits maximum power.
The larger amount torque produced by diesels (as compared to a similar capacity petrol) is to do with the higher compression ratio of a diesel, making them more efficient at extracting as much energy as possible out of the fuel that is burnt.

Here's another good read:

http://en.wikipedia.org/wiki/Diesel_engine

Hope this helps!
Dan

[David Anderson]

I could explain it in vehicle terms.

A Honda S2000 has a lot of power for a non-turbo 2 liter engine, but not a lot of torque.
The torque it does have is high in the rev range as well.
If you keep the engine running hard between 6000 & 9000 RPM it hammers, but come off the boil and it requires you drop gears to get it going again.
It's a hard car to drive because of it's torque characteristics, but very exciting because of it's power.

Our Volvo doesn't have a lot of power at 147 KW, but has good torque in a wide band from 1500 - 4500 RPM so it always seems willing to get up and go.
You wouldn't race it, but it's an easy car to drive because of it's torque.
You wouldn't complain if it had more power though.

The numbers on paper for both vehicles don't do the driving experience justice IMHO.

The NT diesel has a lot of torque @440 NM for a 4 cylinder and pretty good power @ 147 kw.
Also, because of it's good torque it's a missile when up & moving and is brilliant for overtaking.

Forgetting re-sale, price and fuel economy, I picked the diesel over the petrol for it's torque and how easy it makes the Pajero to drive in most situations.

FWIW, the NT diesel is a brilliant engine..

[Stevie-Ray]

Prefer good torque...

...but if you don't have enough power, you're still having to forever change gears to keep rpm within the power-band, whereas all power & little torque (classic example are early rotary engines) has you revving your way up & down the gears to keep it moving. Ultimately a torquey motor with a bit of grunt is best as you don't have to drive it hard so you have good economy but have something in reserve for passing, towing & off-roading.

Steve

[WogsRus]

Forgetting re-sale, price and fuel economy, I picked the diesel over the petrol for it's torque and how easy it makes the Pajero to drive in most situations.

FWIW, the NT diesel is a brilliant engine..

Well the petrol is no slouch i but does suffer on torque, mine got dynoed about 3 months ago and made about the claimed 187kW at the fly after the new engine went in.

I made about 330Nm at the wheels or so.

Only have one more year left with warranty, and then the supercharger goes on. Seen some awsome results in Saudi, so heres hoping.

[David Anderson]

Well the petrol is no slouch

Not wrong, and they're quiet as well.

[Silver Streak]

Can someone please give me a clear explanation of the difference between power and torque?

Should say, that I bought the diesel Pajero as it produces more torque but less power than the petrol motor. Salesman was very adamant about me buying diesel due to having better trade in value.

Thanks
Doug

Excellent question Doug.

Take an F1 engine, its got heaps of power, but virtually no torque, especially low down in the rev range. Usually a big bore, but short stroke engine. As you see on the tele sometimes, they rev between 16,000 and 19000 RPM and develop a huge amount of horse-power at high revs. Not very practical for a road car and especially a 4WD.
With our 4WD's we want only to lope along most times, but want good torque (pulling power) low down, to get you off the line easily, especially if you have a van in tow.
Also, what do you want when you're driving along a creek-bed, or in the sand dunes, or on the beach, at lowish speed, you want an engine that is almost just above idle, but has got the grunt right there at low revs.
As you would know, especially noticeable in a manual petrol vehicle, if you've a a fair load on the back, you've got to almost slip the clutch to grab the torque that available at maybe 3000 RPM. Once you're rolling, your off and the the combination of power and torque take over. in the higher rev range.
But with a diesel, because of the very high compression ratio and a long stroking engine, it's got that GRUNT a very low RPM, and you just pull away easily, with almost no fuss.
Jump on you bike, and feel those initial few pumps of the pedals, that's grunt, that's turning force at virtually no revs. Once you get moving, it gets easier, doesn't it. Also, when riding up a hill, the riders with the big thumping thighs have the advantage. More torque in their legs, but once on the flat, it's those guys with the high revving lighter legs that come into play.
So as been said, torque= turning or twisting force, measured in Newton Metres or Foot Pounds. A measurement of force (pounds) with a bar a foot long. Like when you are torquing up a bolt.
Even when you pump iron to get those bigger torquier biceps, that's a tuning moment, or force. Nothing to do with RPM in that case.
Anyway, apart from all that drivel, more torque at low RPM is what you want. the power will take care of itself, once the revs rise.
I hear you say, what about those big lazy V8's, plenty of low down torque and power eventually, but at what cost....a big thirsty gas guzzler, that's what. Where a diesel, especially a turbo diesel these days, has the best of both worlds, good low down torque and once the turbo kicks in, reasonable power, in the higher rev range.
It's probably not a well known fact, but a 6 cylinder engine with the same capacity as the V8 engine, has more torque. Mainly again, due to the longer stroke needed to achieve the same capacity engine, with less cylinders. But it'll be a slow revving plugger in comparison, to the free-er revving V8. Horses for courses of course. if you want to tow a horse float, get the big 6. But there is a limit to the bore and stroke ratio. Hence bigger engines.....more cylinders!!
Also, how many ships, trains and prime-movers do you get with a petrol engine....none, probably.
My 2c.

[dgrev]

My thanks to everyone for their help and their individual ways of explaining it
both with real world examples and theoretically.

The link to aviation article that explains the relationship of torque and power in one reply is also very good.

Regards
Doug

[nj swb]

My 2c:

Trying to speak in engineering terms:

Torque is a measure of how much work an engine can do, and power is how fast it can do it.

More torque can pull a larger load, which is where diesels are good. Power determines how fast the load can be moved (in the real world, with aerodynamic drag, and presuming the gearing is optimised).

For the physics oriented folk, the units of torque (Newton-metres) and the units of work (Joules) are literally the same. Physics-wise, power is defined as how many Joules per second. Engine wise, power is how many Newton-metres per second the engine generates. Peak power is where torque starts to drop faster than revs are rising.

Yes, I'm a very trivial person.

[nj swb]

It's probably not a well known fact, but a 6 cylinder engine with the same capacity as the V8 engine, has more torque. Mainly again, due to the longer stroke needed to achieve the same capacity engine, with less cylinders.

Unless the 6 has achieved the same capacity by increasing the bore rather than the stroke.

My uncle, a mechanical engineer with 20 years experience in the automotive industry, believes that peak torque is predominantly a function of engine capacity (I haven't spent a lot of time testing his theory). Of course, this presumes comparing apples with apples - petrol with petrol, diesel with diesel, once you add boost then boost pressure alters effective capacity.

In my experience, issues such as compression ratio, bore to stroke ratio and rod to stroke ratios affect engine characteristics more than outright capacity.

But it'll be a slow revving plugger in comparison, to the free-er revving V8. Horses for courses of course. if you want to tow a horse float, get the big 6. But there is a limit to the bore and stroke ratio. Hence bigger engines.....more cylinders!!

Yes.

Generally speaking, a short stroke motor will rev more easily (due to lower peak piston speeds) but has less low speed torque - peak torque will typically be higher in the rev range. A long stroke motor generally produces better torque at low rpm, but it needs to be much better designed to work efficiently at high rpm (which need to accelerate the pistons to higher speeds each turn of the crank). Hence, the good old Falcon 250 inline 6 had lots of lazy bottom torque, but wouldn't rev.

Also, how many ships, trains and prime-movers do you get with a petrol engine....none, probably.

I expect you're right. But this would also be partly due to liquid diesel's higher energy capacity per litre, and that diesel engines are inherently better at making use of that energy. That is, they are more energy efficient (which we all know) and this is important when you're using huge amounts of fuel.

[4ePikanini]

It's pretty simple

KW = NM x rpm

So.... the more torque you have the less rpm you need to get the same power.

The torque comes from the arm on the crank. See my quick sketch below. The long arm gives more leverage on diesels - like using a long strongarm to tighten a nut but lowers your speed.

[Boertjie]

......Only have one more year left with warranty, and then the supercharger goes on. Seen some awsome results in Saudi, so heres hoping.

As long as somebody else pick up the fuel bill it will be lots of fun.

[WogsRus]

fuel wise it will be OK when off boost. LOL, then i think i will transform my 6x4 to a mini tanker.

Seriously, stock trim maps on the MIVEC are atrocious, i have seen fuel points as rich as 9.3 to 1 so next on the map is a remap of the ECU.

[Zed]

Well the petrol is no slouch i but does suffer on torque.

You need to drive something with some power, the petrol do alright but I wouldn't say they are powerfull
supercharged will give them a boost but costly.