Power to Weight versus Torque to Weight

[Bilbo]

Hi Gary G

 

Yes Peter is correct, well almost, this is o'level but his later education gave more insight in how its applied in practice. So his comments is a bit unfair.

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

 

 

 

Edited by - bilbo on 19 Sep 2002 22:19:57

[martinwhitcher]

WOW ....was that the start!!.....now losses?!?! Bloody hell I think this tread could (already has ) turn into one of the longest ever 😬

If I read it all, can I collect my degree 😬 😬

 

 

 

Martin

 

MW 51 CAT

[Bilbo]

Hi Highlander

 

Some of my posts may not be expertly written as I have to do it covertly without the project manager seeing!

 

Well may be not... I would have no problem as long as you did the rest of your work *biggrin*

 

Then again thats why I only post in the evenings and let me do some if not all of the design as well

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

[Paul Ranson]

Well they do produce power but I did say they only produce torque as an OUTPUT again power= rev x TORQUE.

An engine produces power, unless it's producing power you cannot measure a torque and rpm. Power is the same as torque*rpm, it's not some abstract calculated relationship, it's equivalence.

 

Power is what accelerates a vehicle. To go from 30-60mph requires an amount of energy, the power output of the engine determines how long it takes. When you are trying to undo the crank pulley you may apply 200lbft to the system for some time. The car doesn't end up doing 60mph down your drive, which is a good thing considering you just undid the crank pulley...

 

Paul

[Highlander]

Martin,

 

For the reecord this is the longest post, and by some margine !! 😬

[Highlander]

Power is what accelerates a vehicle. To go from 30-60mph requires an amount of energy, the power output of the engine determines how long it takes. When you are trying to undo the crank pulley you may apply 200lbft to the system for some time. The car doesn't end up doing 60mph down your drive, which is a good thing considering you just undid the crank pulley...

---

 

Exactly. The difference between Static and Dynamic torque.

[Bilbo]

Hi Paul

 

Please read "OUTPUT" thats what you measure. Look at the sites I posted. Dont listen to me read what they say as on whats developed / measured.

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

[Alias]

Going back to the original title of the thread...

 

I assume that there is no argument that the lighter a vehicle is, the more easy it is to accelerate it.

 

We would all prefer full torque and power curves to be given on every vehicle in Evo, perhaps we could suggest this. However the information given by most car reviews is limited at best.

 

I am with Peter C on this (though have to confess that some of his equations lost me completely), on the basis of the following.

 

You have two cars of identical weight. Car (a) develops 100 lb/ft per tonne and car (b) develops 75 lb/ft per tonne. Both cars develop maximum torque at maximum rpm. Which car would you choose? On this information you would choose car (a).

 

If it transpired that car (a) had maximum rpm at 5000 and car (b) had maximum rpm at 10000 then you could theoretically gear car (b) to be half of car (a). If you half the gearing you effectively double the torque - thus giving you 150 lb/ft.

 

If you were given the respective bhps of these vehicles, car (a) would develop 95.2 and car (b) 142.8 On this information you would choose car (b). Thus if you were only given one piece of information about a vehicle I would suggest that bhp is generally more helpful than torque.

 

I think highlander is trying to say that torque at the wheels is what causes the vehicle to accelerate and therefore the greater the torque here, the faster the vehicle will accelerate. I don't think anyone is disagreeing here, but when Evo state torque they tend to mean "at the flywheel"

 

On a personal note - I've been wondering about this torque / power thing for ages and now feel I've got the hang of it. Thank you all.

[Paul Ranson]

Exactly. The difference between Static and Dynamic torque.

?

 

To 'Bilbo', you measure power, otherwise there's no point. Whether this is 'direct' or by measuring a torque and speed doesn't matter. It's still power, it makes you go or boils your kettle.

 

Paul

[trailerman]

this is all terrific stuff - thanks to everyone.

can I just ask a couple of ignorant questions :

 

various people from DAF onwards have tried to implement 'stepless' or constant-velocity transmissions (as opposed to fixed gears).

Are these generally intended to keep the engine at peak power or peak torque (at the flywheel or wherever)? And why?

 

seems to me that general understanding (especially mine) of this stuff is clouded by the fact that reciprocating engines transmit their output via rotating masses. Any chance of relating this to, say rocket engines?

They are usually rated, as I recall, by max thrust produced. Why not power?

I had assumed (before this thread) the reason to be that the *force* was more 'fundamental' somehow than the capacity for work.

 

(but what do I know?)

 

[Highlander]

Trailerman

I posted this earlier, (not wanting to contradict myself)

Torque is the only thing

that a driver feels, and horsepower is just sort of an esoteric measurement

in that context. 300 foot pounds of torque will accelerate you just as hard

at 2000 rpm as it would if you were making that torque at 4000 rpm in the

same gear, yet, per the formula, the horsepower would be *double* at 4000

rpm. Therefore, horsepower isn't particularly meaningful from a driver's

perspective.

---

So for a given gear ratio maximum thrust would be available at the torque peak (as detailed in the above statement), and as the CVT is varying the gear ratio I would summise that CVTs aim to keep the engine turning at peak torque.

 

But wait . . . !

 

Captain!

Yes Sulu.

Klingon photon torpedo incoming!

Red Alert Mr Chekhov, brace for impact.

 

Well it is Friday

[Highlander]

Oops missed a bit.

 

As to rocket thrust, I'm sure I've seen an power equivolent quoted somewhere, but I wouldn't pretend to know how to work it out.

 

Yet.

 

wrong smilie as well should have been

[kjp]

No

 

Peak power for the CVT for best accelaration.

 

 

 

 

[Highlander]

Hmm, had a rethink and for maximum acceleration, you'd have the CVT set for peak power.

But you wouldn't want your engine screaming at 6500rpm or wherever PP was, so its more likely that it's be set for peak torque.

[kjp]

Peak power for best accelaration.

 

All other times you want to match road load with minimum losses. So you slide down the power curve and run at at WOT (minimum pumping losses) at low engine speed (where the power curve matches the road load).

 

Interesting thing is when running at steady speed, low engine speed and WOT, the only way to speed up is to to slow down first....... almost an intresting control problem!

 

 

[Highlander]

Sorry, WOT ?

[kjp]

Sorry, Wide Open Throttle (WOT)

[Bilbo]

Hi Paul

 

Don't expect any sensible posts from me tonight as I had to cancel Cadwell today because of work and I am currently sulking...... ☹️

 

"To 'Bilbo', you measure power, otherwise there's no point. Whether this is 'direct' or by measuring a torque and speed doesn't matter. It's still power, it makes you go or boils your kettle"

 

Think your missing the point it all started which was more important torque or power verses performance I and other moved it on to what's actually measured and understanding WHY. This lead on to RR and dyno readings. I was making the case that we all talk about engine not car outputs but its a miss guided as the engine produces torque the car, a machine, has a power output. The only true reading is off a dyno lots make claims of coast down readings for power and Dave Walker is very good at it, others are not. Still they are all derived from torque measured. I am only arguing on the point of what's measured rather than derived about an engines output.

 

So continue to leave your head buried in the sand, don't accept that the input to an engine is fuel/oxygen and the output is heat, noise and TORQUE with combustion products as a by-product (its still an output} is the OUTPUT.

 

Not all engines are put on a dyno to measure power, some are only done to measure torque... a gearless lift motor is a very good example. However, that's nothing to do with the car industry. Actually its not, the only desired output from a starter motor, that would be of interest, is torque the rotational force to overcome the resistance of the engine.

 

One again, just to say it one more time, you MEASURE torque that's the OUTPUT then you CALCULATE power PLEASE read the manufactures literature. The fact that YOU are only interested in power is irrelevant and the fact that its derived by calculation seems to be also irrelevant to YOU. As I said before I like to have both torque and power curves from a RR or dyno see sig.

 

How's that for sounding grumpy

 

Just to prove I not really grumpy but just bored no racing today

 

As to using a kettle as an allergy of the use of power this is poor logic

 

Well the INPUT is POWER the OUTPUT is HEAT after exciting the molecules in the element then transmitting their heat OUTPUT to the water, then the desired and very calculated reading is a nice cup of tea . (for gearbox/transmission read tea leaves)

 

So the kettles INPUT is power the OUTPUT is heat and the desired effect/ reading is TEA.

 

Similarly the a cars engine INPUT is ENERGY the OUTPUT is TORQUE the desired effect/ reading after the addition of the gearbox/transmission in your case is POWER (BHP). The fact it has other outputs is irrelevant to you because you chose not to measure them.

 

Very very off subject... I am off to witness/approve the testing of some reasonably big chillers (machine) 4 x 0.7 MW in France very soon. If anyone starts telling me the power output from the motor/compressor (machine read car) in BHP I am likely to throttle them after this .

 

Actually to be fair I am interested in the machine's motors output as power but the starting torque is also a very interesting. I will know that the electrical power input is derived from torque that gives the kW output from the motor into the compressor. What turns the compressor torque.

 

As to the point of all this the machines OUTPUTS measured will not just be power in kW of cooling. I have other interests noise being one of the major ones the other import one is Coefficient of Performance (COP) that's how much it will cost the client to run the machine. If the COP is as bad as our car engines the manufactures going to have a very very red face. The starting torque is another that gives a guide to starting input power (money) and longevity/bearing wear (maintainance ie money).

 

The point you are making with power is that's what YOU are interested in, the fact its derived from torque you chose to ignore. The engine develops torque the machine (read car) develops power.

 

"a man hears what he wants to hear and disregards the rest!"

 

As to what's more important Power verses Torque to performance... well Torque is what the engine gives Power is what the machine (car) provides seems to me its just a case of how you chose/can/appreciate the difference and effect.

 

Oh Highlander yet another impressive degree....forgot to post this last night.

 

Ps I am not really grumpy

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

 

Edited by - bilbo on 20 Sep 2002 21:48:09

[Paul Ranson]

Bilbo, an engine makes power, this may be decomposed to torque and rpm. This was the point of my comment about tightening your hub nuts or undoing your crank pulley where substantial torques are involved but negligible power.

 

Even in terms of engine development you may have the choice of improving torque a

at relatively low rpm or relatively high. The only way you can determine which is preferable is by looking at the power.

 

My Vauxhall race engine makes 292BHP and 180lbft, I almost always run it well above the torque peak. Here is a practice run at Shelsley Walsh for example.

 

Paul

[AMMO]

 

 

Edited by - Ammo on 20 Sep 2002 22:50:21

[trailerman]

kjp:

>Interesting thing is when running at steady speed, low engine speed and WOT, the

>only way to speed up is to to slow down first....... almost an intresting control problem!

 

 

mmm - sounds like a case for disassociating the accelerator pedal from the throttle.

 

in an ideal world where your engine is *always* doing its best, the fly-by-wire 'loud pedal' under your right foot would deliver, instantly, on demand, - what?

 

oomph.

grunt.

welly.

 

I'll shut up now. back to the big picture.

( "that's not my department", says Wernher von Braun )

 

 

[Morls]

*idea*I am fat but cannot pedal very fast.

*idea*Most pro cyclists are thinner but can pedal fast.

*idea*Most bikes have lots of gears.

 

If I stand on the end of my pedal crank, of say; 300mm length I generate a torque of about 100Kg x 0.3M

 

If pro cyclist stands on the end of the same crank he generates a torque of around 70Kg x 0.3M

 

I generate the most torque

 

Yet presumably he can accelerate faster than me ☹️

 

 

Therefore I remain convinced that Power is more important than torque, in almost every respect (in sporty vehicles, not watches, wrenches etc.)......

 

 

 

Can we do losses now?

 

And to get my two penneth in first...

 

If we were to remain obsessed by peak torque, would we not be ignoring the engine and tranmission speed component that is used for top speed?

 

And is it not at top speed were the windage / oil drag losses in the gearbox output shaft / prop / diff / driveshafts etc. will peak?

 

Therefore the effect of losses would surely be most significant when at peak power (OK actually max attainable revs in top), rather than peak torque.

 

So again the torque issue is secondary. IMHO

 

 

 

 

Bilbo,

 

Wow , they are big chillers. What are they for?

 

I think some secretly located similar ones are used for treating the wind coming off the sea, before blowing around our village each winter?

[Bilbo]

Hi Paul

 

OK I give up (sigh), my only argument is about what is measure and you refuse to read what the manufacture's of dyno say they measure. Pick one of the ones I posted ring them up and tell them they have got it all wrong. Tell them your going to prosecute under the trades descriptions act.

 

The only point I am trying to make is the difference between the engine and the car as a machine. I thought this was relevant to the discussion as lots seem to be arguing the car overall as an engine without realising/appreciating the difference.

 

"a man hears what he wants to hear and disregards the rest!" I thought you would at what some one with the same forename said .

 

Hi Mark

 

Well very off subject...............

 

These are not BIG ones they are screw compressors, these go up to 2.5 MW each aaaaaaaah that's cooling load normally you would go for 2 or 3 as a minimum. Like to pay the electrical bill on a few of these the power in is about 1/4 of the cooling load sounds like something for nothing but its not, please don't ask me to explain the reason is very long and toooo much like work. Centrifugal chillers are bigger up to 7 MW cooling with the same min number. The explanation of why you chose one against the other is even longer and I not going to post a 10 page explanation, plus the fact there are other types.

 

Currently working on 20 MW of steam heating I have designed 3 installations this size the other 2 were bigger but this is the first steam one this big, the actual loads about 2/3rds. Similarly so are the chillers (in current cooling case its near actual load more to go in later) bit different to a car engine these plants are expected to have full capacity when maintenance is being carried out not just off the road awaiting parts .

 

 

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

 

Edited by - bilbo on 21 Sep 2002 19:53:58

[Paul Ranson]

OK I give up (sigh), my only argument is about what is measure and you refuse to read what the manufacture's of dyno say they measure.

If you can find a dyno manufacturer who doesn't measure rpm or speed then I'll take your 'argument' more seriously.

 

Torquexrpm IS power.

 

Paul

[Tony Martyr]

We arent talking about engine losses are we 'cos that get tricky.

Vehicle losses are usually summed within the Road Load Equation that is particular to that vehicle.

If, for the moment, we ignore gradient there are three terms to the RLE.

F0 is a function of vehicle weight and tyre charactoristic and is normally made up of k0=m.g.a

where m is mass in kg

g is acceleration due to gravity

a is a term used to differentiate between tyres. I use 0.018 for crossply and 0.013 for radial.

F1 is the term that describes increase of internal friction of running gear with speed you can think of it as thrashing resistance.

F2 is the term that kills Caterhams as it describes the exponential rise of air resistance with speed - it uses the drag factor k2 and can be used as:

k2 is 1/2.rho.A.d

where A is frontal area of car - not easy to estimate.

rho is the air density

d is the drag coefficient.

With a true RLE for any vehicle a modern chassis dyno can simulate the forward forces very accurately. The outcome aligns with common sense thought experiments:

if you have soft tyres it takes more effort to push the car

top speed is dictated by the air resistance - open all windows on the motorway and prove it.

Just a thought about the thread on dynamometers measuring torque of power.

There are three major types of engine dynamometer:

Water Brakes which are hydrokinetic devices which do not work at zero or near zero speed so are converting POWER and measuring its component parts - speed and torque.

Eddy Current Dynos - disk or drum - which require a minimum rotational speed to generate torque by cutting their magnetic flux paths and so are converting POWER and measuring its component parts - speed and torque.

ac or dc motor based dynos which can measure down to stall torque and so could for one single part of their envelope could be considered as a torque only measuring device.