Engine Torque vs Power

[Ratbag]

Gidday Ate

Thanks RB, not quite what I had in mind.........

Considering how I am feeling today, you're lucky to get that ... :biggrin: :rotfl:.

 

[Red XS]

For the T=9.5 X P/rpm

Forget everything about engine designs.

Look at a power torque graph.
Pick any point on the power curve.
Divide that torque value by the rpm value and multiply by 9.5
That value is your Torque value at that set rpm.

I'm not talking about an engines max power and max torque. I'm talking about the relationship between power and torque at a constant rpm.

 

[Ratbag]

^ I think I get it now.

The equation will only apply to a specific, given engine (regardless of what that engine is), but cannot be used to compare two different engines, even engines from the same 'family'.

This would also explain the lack of any variables that account for engine design, technology changes or differences. So the relationship should hold true for an Austin Healey (big) 4 banger, or an EJ-253, but would make no attempt to explain the differences in the torque curves, power output or torque between the two engines.

Have I got this right now?

 

[Dedman]

Torque is proportional to power divided by revs. (e.g. Torque (N.m) = 9.5 X Power (KW) / revs (rpm)).

Finally some fact on this threat.

The whole power vs torque debate is one of my very few pet hates and 90% of people seem to have no idea what they are talking about. When I was studying even many of the engineering tutors used to have no clue and now as an engineer I still have to correct people all the time

POWER = TORQUE X RATE OF ROTATION

Depending on the units you use there will also be another factor in there (the 9.5 Red XS mentioned above)

Torque and power are Directly related to each other. You cant have more of one without the other at a given rpm

Power is the rate energy is being added and is directly proportional to the acceleration of the car.

Torque is the turning force and on its own has NO relation to acceleration at all.

When people argue that an engine has more torque in terms of off road driving or how nice it is to drive around town etc they really are referring to the fact that it has more power at a lower rpm.

In the automotive industry the manufacturers quote maximum torque and power which whilst giving a small amount of information about performance is only useful when operating at the rpm they are listed for.

Unfortunately unless you are driving a car with a CVT the engine can not operate exactly in the peak power output during acceleration and will vary in rpm as it changes through the gears. This is where the shape of a torque curve comes in to play as Ratbag often points out.
What will give you the best indication of acceleration is the area under a torque - rpm curve for the given range of rpm you wish to use for your acceleration as this will be proportional to the energy added by the engine during acceleration and the more energy added the better the acceleration. A flat torque curve will lead to constant acceleration if drag and other losses are ignored which is what is desirable in day to day driving.

The final point I will add is that gearing that matches the engine you have is most important of all as it is your gearbox and drive train that multiplies the torque output of your engine to something usable . The concept of a CVT trans is the ultimate for performance because it will always have the perfect gear, which is probably why my sisters 1.5L honda jazz can accelerate like a bat out of hell for a 1.5L The argument that a motorcycle would be lousy for towing but has lots of power again is entirely down to gearing. If you ran a motorcycle motor through a low range box and had it screaming away at 20000rpm im sure it would tow bloody well just as my gutless 2.0L engine tows pretty well when screaming away at 5000rpm. This however is not practical/desirable which is one of the main reasons that diesels are desirable as the produce high torque and consequently comparably high POWER at low rpm

 

[idw]

you've all got it wrong, it's the amount of unicorn's blood you put into the fuel tank.

 

[Ratbag]

^ Gidday DM

Thanks for that too.

If this equation is absolute, then it appears to me that the torque curve and power curve should have a constant relationship to each other for any given engine. Am I misunderstanding this?

If this is so, then how is the fact that they do not appear to have a direct, constant relationship with each other.

This is why I asked Red about the differences between the EJ-251 and EJ-253, two superficially very similar engines with very similar maxima for power and torque (albeit at different revs), yet having very different power/torque characteristics from each other.

All engines for which I have seen these curves, they exhibit significant differences between the power and torque curves. Different shapes, different relationships to one another. While I haven't got them to hand ( :rotfl: ), I do have them in my head, collected over a lifetime of observation. However, I did not foresee this discussion, so didn't photocopy them with the photocopiers that hadn't yet been invented ...

A further point I would like explained is that if power and torque bear a linear relationship to each other, why have two different measures and units for them?

I would really appreciate a clear explanation of these points. I am always seeking to expand (and correct ... ) my knowledge. I can't help this. Nor can I take any credit for having this drive - it's an in-built thing over which I have no control! If/when we meet up in person, I will explain it to you, if you are interested.

 

[Red XS]

Well said Dedman

If this equation is absolute, then it appears to me that the torque curve and power curve should have a constant relationship to each other for any given engine. Am I misunderstanding this?

Correct. Doesn't matter if your driving a Holden or Ford, if your doing 100Km/hr, you'll travel 100Km in 1 hour. Doesn't matter if you have an EJ253 or an electric motor, if your making 100KW at 5000rpm your also making 9.5X100,000/5000=190Nm at that 5000rpm

A further point I would like explained is that if power and torque bear a linear relationship to each other, why have two different measures and units for them?

No purpose in having both graphs. But if you give 2 figures, 1 for max torque, 1 for max power, you'll get an idea of the amount of push the engine has at 2 different rpms.

I would really appreciate a clear explanation of these points. I am always seeking to expand (and correct ... ) my knowledge. I can't help this. Nor can I take any credit for having this drive - it's an in-built thing over which I have no control! If/when we meet up in person, I will explain it to you, if you are interested.

I've got that too. I didn't discover this concept at uni, but by me looking stuff up.

 

[Dedman]

If this equation is absolute, then it appears to me that the torque curve and power curve should have a constant relationship to each other for any given engine. Am I misunderstanding this?

They are related through rate or rotation (engine rpm) so for example a perfectly flat torque curve will always have a perfectly straight line for a power curve, the slope of the line is proportional to the torque (higher torque = steeper power curve)
This is a good example of this

If this is so, then how is the fact that they do not appear to have a direct, constant relationship with each other.

Does the above make this more clear. Power and torque are not the only variables in the equation, rotational speed is also a variable.

This is why I asked Red about the differences between the EJ-251 and EJ-253, two superficially very similar engines with very similar maxima for power and torque (albeit at different revs), yet having very different power/torque characteristics from each other.

The fact that the max torque/power is at different rpm gives a good indication that the torque curves are different for these engines due to the differences between engines and explains the difference in performance.

A further point I would like explained is that if power and torque bear a linear relationship to each other, why have two different measures and units for them?

Using engineering units
Power has the units of Joules/Second
Torque is in Newton Metres
And rotational speed (angular velocity) is in radians/second

The equation is
Power = Torque x Angular velocity

Using the above units you get

Joules/Second = Newton Meter x Radians/Second

Whilst these units dont look consistent, Torque X rotation (Units: Newton Meter x Radians) is work done which has the unit of Joules. There is probably a way to prove this but I cant think of an easy one at the minute.

Hope this make things more clear

 

[Red XS]

Having a look at this image you'll notice that the little bumps and dips on the torque curve are all directly above or directly below the same bumps and dips on the power curve.

If you where to "shear" this image (I don't know how to do that on my computer) i.e. drag the right hand side of the image down, while the left side stays in the same spot. You'll notice that the power curve will look a lot like the unsheared torque curve.

 

[guzzla]

Liberty 2.2L N/A donk produces 98 KW at 5,600 rpm and 193 Nm at 4,000 rpm according to the brochure I have here.

The Gen1 Liberty that had the EJ-22 had 100kW. That dropped to 94kW in the Gen2 Liberty but later models of that Gen2 had 98kW.

I have a friend who had the 100kW Gen1 Liberty and my memories of it are that it didn't have a torquey bottom end and needed to be revved to produce the kind of performance you'd expect of 100kW. My 93kW / 205Nm Magna at the time felt much more powerful but only because it produced more peak torque earlier in the rev range (2400rpm I think). She replaced that first Liberty after 546,000km with a Gen3 Liberty with the 92kW EJ-20 engine and complained for every one of the 300,000km that she travelled in it. The increase in weight (it was also AWD) and less power and torque really made a big difference. (My memories are that the Gen3 thankfully had a nicer gearbox though as it certainly needed to be used more often).

 

[guzzla]

I have noticed subtle differences between the EJ-253 in SWMBO's series I SH and the one in my series II SG. I have never seen a torque curve for that last of the EJ-253 engines, but I would bet the family 'jewels' that it is flatter and broader again than that of the EJ-253 in my car.

Too true. It also has extra power - 126kW v 121kW but has to contend with more weight and I notice Subaru claim the SG to be faster. I did see a brochure that they published showing how the SH engine was changed and it included a power and torque graph. They also modified the EJ-255 engine in the XT too and while keeping the same peak power and torque - 169kW and 320Nm - the peak torque came at much lower revs. Once again, extra weight meant it was slower.

 

[Davidov]

Any and all cars especially used no matter what will always have variation in how they perform. Ive experienced an SG 2.5 that was gutless as hell, to my old EJ25 (Tazas) that now is rebuilt is a torque freak. Cant really just jump from one car to another and say that the way they perform is representative of them all. Sometimes its not even close.

Tazas brumby has easily double the power of mine.

 

[NachaLuva]

I remembered power = torque x rpm but didnt know about the 9.5 factor. the differences are a tricky one that arent easy easy to understand. Never thought of power as being the rate that energy is added...interesting!

Note with the graph its not a smooth curve, thats due to inefficiencies in the engine, esp resonance. I'm sure the other engineers can explain better.

The increase in weight (it was also AWD) and less power and torque really made a big difference.

Remember also that power at the flywheel is much more than at the wheels due to powertrain losses. IIRC its ~30% for a normal 2wd, more for an AWD (has to turn 3 diffs instead of just one)

 

[Red XS]

Remember also that power at the flywheel is much more than at the wheels due to powertrain losses. IIRC its ~30% for a normal 2wd, more for an AWD (has to turn 3 diffs instead of just one)

Another point I'll add to that is that the Subarus symmetrical AWD loses less torque then most AWD's do. This is because most AWD's (Rav 4's, X-trails ect.) place the engine in sideways, which means that the shafts have to make a series of bends before they make it to the main drivetrain. While the subarus boxer engine is connected more directly. Still not as efficient as a 2wd, but much more efficient than Rav 4's, X-trails ect.

I noticed this when driving my mates X-trail. Even though he had greater power and torque figures, it still felt gutless to drive and didn't have the power at the wheels to climb sanddunes that my "less powerful" Forrie cruised up.

I remembered power = torque x rpm but didnt know about the 9.5 factor.

The 9.5 factor is only for KW-Nm. It is a different number if use use Hp or lb.ft

Never thought of power as being the rate that energy is added...interesting!

1watt (of power) = 1 Joule (of energy) per second (of time)

 

[duncanm]

They are related through rate or rotation (engine rpm) so for example a perfectly flat torque curve will always have a perfectly straight line for a power curve, the slope of the line is proportional to the torque (higher torque = steeper power curve)
This is a good example of this

Does the above make this more clear. Power and torque are not the only variables in the equation, rotational speed is also a variable.



The fact that the max torque/power is at different rpm gives a good indication that the torque curves are different for these engines due to the differences between engines and explains the difference in performance.



Using engineering units
Power has the units of Joules/Second
Torque is in Newton Metres
And rotational speed (angular velocity) is in radians/second

The equation is
Power = Torque x Angular velocity

Using the above units you get

Joules/Second = Newton Meter x Radians/Second

Whilst these units dont look consistent, Torque X rotation (Units: Newton Meter x Radians) is work done which has the unit of Joules. There is probably a way to prove this but I cant think of an easy one at the minute.

Hope this make things more clear

and this is where the 9.5 magic number comes from

Power = torque (Nm) x rotational velocity (radians/s)

velocity (rad/s) = (rpm / 60) x 2 x pi = rpm / 9.5 (approx)

so Power = torque (Nm) x rpm/9.5

Torque = Power x 9.5 (at a fixed rpm)

 

[NachaLuva]

I noticed this when driving my mates X-trail. Even though he had greater power and torque figures, it still felt gutless to drive and didn't have the power at the wheels to climb sanddunes that my "less powerful" Forrie cruised up.

I'm often surprised by how well my na 2.0L does on steep hills & sand

1watt (of power) = 1 Joule (of energy) per second (of time)

D'oh...of course. I'd forgotten that lol. I guess I was thinking of engine power differently than say electrical power, but its all power!

 

[Venom]

If you have a H6 there's no need to think about any of this :lol:

 

[NachaLuva]

If you have a H6 there's no need to think about any of this :lol:

:rotfl:

Although I remember a certain H6 having to be towed up a certain track in Walhalla

 

[taza]

But its still not turbo. Look at the turbo H6 in Gramps (MCM). Producing almost 300wkw!

 

[Venom]

It was only towed because it was the only car with no low range