bhp question..

[Demon]

So think of it as..:

If first car is at 2 rpm and second car is at 6 rpm then, each car is producing x torque / y length of time it's being produced for.
So car 1 produces 200 / (1/2) = 400 acceleration (1 second / 2 rpm) while car 2 produces 100 / (1/6) = 600 acceleration



On a side note, am I right in thinking that the most fuel-efficient acceleration is throughout the 'high-torque' band?

Except, as I thought, Torque as measured by a Dyno (which is what we are referring to) is a mean value.. (See above, I linked to an article discussing it).

Unless anyone has some evidence to suggest otherwise?

 

[Dr Who]

This whole discussion is irrelevant because too many other factors such as gearing, torque curve over the increased rpm (rpm is not static if accelerating) etc etc.

 

[Dr Who]

I'm very confused, I thought it was really simple, isn't the measured Torque value a 'mean' over the entire revolution of the engine? and not 'peak' Torque of each detonation?

If so, then the 200lbft@2K would accelerate quicker?

Here's some info (if valid)
http://www.epi-eng.com/piston_engine_technology/torsional_excitation_from_piston_engines.htm

The key line being

OFC the torque is a mean value over 1 revolution, but its rate of work is over 2000 revolutions or 6000 revolutions you cannot look at torque at a given rpm in isolation for acceleration, as it it merely a measurement of turning force.

 

[Dogbreath]

Except, as I thought, Torque as measured by a Dyno (which is what we are referring to) is a mean value.. (See above, I linked to an article discussing it).

Unless anyone has some evidence to suggest otherwise?

You are now adding an yet another layer of confusion to the thread! Unless you have a single cylinder engine, the ratio between peak crank torque and mean crank torque will be much lower. The more cylinders the engine has, the more constant the torque will be. However, none of this has got anything to do with the original question.

I'm honestly at a loss as to why people find this such a difficult concept to grasp; every thread I have ever read on people discussing torque and power ends up the same way.

 

[Dr Who]

every thread I have ever read on people discussing torque and power ends up the same way.

This is so true

 

[Fantom]

Great thread!

 

[miniyazz]

Except, as I thought, Torque as measured by a Dyno (which is what we are referring to) is a mean value.. (See above, I linked to an article discussing it).

Unless anyone has some evidence to suggest otherwise?

I think (although may be wrong, as I haven't found the specific area of text) that they refer to the mean torque throughout each rev - i.e., instead of giving the peak torque per rev per rpm, they give the mean torque per rev per rpm, which is a much more meaningful value (and therefore your typical torque curve plots the mean torque per rev for each rpm). In which case my point stands.

 

[Demon]

You are now adding an yet another layer of confusion to the thread! Unless you have a single cylinder engine, the ratio between peak crank torque and mean crank torque will be much lower. The more cylinders the engine has, the more constant the torque will be. However, none of this has got anything to do with the original question.

I'm honestly at a loss as to why people find this such a difficult concept to grasp; every thread I have ever read on people discussing torque and power ends up the same way.

Ignore the number of cylinders, it's irrelevant if we are being told the mean torque value, which we are, the article was just to confirm the one point which is that the Torque figure quoted by the OP and manufacturers is a mean value, and not 'peak', therefore engine speed is irrelevant.

The misunderstanding is that people are assuming that the Torque value has to be multipled by engine speed to indicate accelerative force, which I'm now sure is not the case.

 

[Demon]

I think (although may be wrong, as I haven't found the specific area of text) that they refer to the mean torque throughout each rev - i.e., instead of giving the peak torque per rev per rpm, they give the mean torque per rev per rpm, which is a much more meaningful value (and therefore your typical torque curve plots the mean torque per rev for each rpm). In which case my point stands.

No I don't think this is true, the mean value is an average (of course!) and only applies to 'per revolution', time does not come into it, if both examples where fully loaded and stayed at those RPM values idefinitely the average 'torque' produced would always be 200 and 100lbft not matter how long you left them..

I admit the notion of more pulses of torque sounds good, but clearly if Torque is always measured as a mean, it takes the number of pulses out of the equation.

 

[paddy]

every thread I have ever read on people discussing torque and power ends up the same way.

It's a popular torqueing point

 

[Jonnycoupe]

its the same car, same engine, same gear but is it more accelerative at 2krpm or 6krpm

If it is only cos the air resistance would be 9 times less.

 

[Django x2]

It may well be right, but its absolutely no reason to laugh at the bloke in the pub in his crap box diesel because, given gearing and the fact you have to pass through 6k of rpm to get to your acceleration peak, the crapbox diesel will already be half way down the road because it reached its peak acceleration 4krpm earlier.

The statement is simply removing time from the equation which you can't do in a 'real world' scenario

 

[PMKeates]

It may well be right, but its absolutely no reason to laugh at the bloke in the pub in his crap box diesel because, given gearing and the fact you have to pass through 6k of rpm to get to your acceleration peak, the crapbox diesel will already be half way down the road because it reached its peak acceleration 4krpm earlier.

The statement is simply removing time from the equation which you can't do in a 'real world' scenario

Let me raise a few points:

1. A clutch or torque converter does not enforce a 1:1 relation between engine speed and transmission speed. I.e. most quick acceleration does not start at 1000 RPM, even from 0 MPH.

2. The peak acceleration/torque of a petrol engine is not at 6000 RPM. It's probably at 3000 RPM, with a typical diesel at 2000 RPM. So, you've had to go through 1000 RPM more to reach peak acceleration.

3. The duration of which the increased rate of acceleration of a diesel lasts is shorter than a petrol's decreased rate of acceleration - enough so that the overall acceleration provided is not typically a great deal more.

4. Acceleration != velocity. You can have 1000 lb-ft, but if it lasts 0.5 seconds and 100 RPM then you have not moved down the road at all.

 

[miniyazz]

No I don't think this is true, the mean value is an average (of course!) and only applies to 'per revolution', time does not come into it, if both examples where fully loaded and stayed at those RPM values idefinitely the average 'torque' produced would always be 200 and 100lbft not matter how long you left them..

I admit the notion of more pulses of torque sounds good, but clearly if Torque is always measured as a mean, it takes the number of pulses out of the equation.

I see what you mean about torque being a continuous force being produced and therefore rpm-independent. Alas, what was once crystal clear has clouded over


Edit: I think essentially what it comes down to (as has been said before) and what I'm going to leave remembering is that power = torque x rpm. To put this better, power is what pushes the car forwards.. and the area under the power/rpm curve (between the starting rpm and the ending rpm of each car) is the amount the car is pushed forwards and therefore represents how fast it will be going. Even if I can't currently understand the mechanics behind it!

 

[PMKeates]

Let's have a real example:

Ford Focus 1.8 TDCi 115 HP 280 Nm (300 Nm with overboost). 0-60 in 10.8 s, top speed of 118 MPH.

Ford Focus 1.6 Petrol 115 HP 155 Nm (Nearly half the overboost torque). 0-60 in 10.8 s, top speed of 118 MPH.

So, tell me, what's the more relevant figure for comparing maximum performance - power or torque?

In this case, the diesel should be faster - it has the same power yet additionally more torque. However, the advantage the extra torque brings is compensated for by being 10% heavier.

 

[daz]

F = MA

 

[Dogbreath]

The misunderstanding is that people are assuming that the Torque value has to be multipled by engine speed to indicate accelerative force, which I'm now sure is not the case.

Right I see what you mean. The answer is no, wheel torque is what pushes the car along, and this is proportional to engine torque (again ignoring losses). That said, engine torque of course does vary somewhat with RPM, but you don't necessarily get more acceleration at higher RPM for a given gear ratio. e.g. try accelerating in a diesel that is already running at ~4500RPM...not much tends to happen.

The bottom line is that for any given engine power output, you use the transmission to trade off wheel torque against RPM. Higher wheel torque gives more acceleration, and the most effective way to achieve this is to use a lower gear (higher ratio) to multiply the torque from the engine. This also means the engine has to turn more quickly for a gievn road speed.

 

[Django x2]

Let's have a real example:

Ford Focus 1.8 TDCi 115 HP 280 Nm (300 Nm with overboost). 0-60 in 10.8 s, top speed of 118 MPH.

Ford Focus 1.6 Petrol 115 HP 155 Nm (Nearly half the overboost torque). 0-60 in 10.8 s, top speed of 118 MPH.

So, tell me, what's the more relevant figure for comparing maximum performance - power or torque?

In this case, the diesel should be faster - it has the same power yet additionally more torque. However, the advantage the extra torque brings is compensated for by being 10% heavier.

But this is pointless. Put them in a real scenario. 50-75 on the motorway. The figures are right, there is no disputing that, but it will just never be replicated on the road.

 

[PMKeates]

But this is pointless. Put them in a real scenario. 50-75 on the motorway. The figures are right, there is no disputing that, but it will just never be replicated on the road.

Put them at 50-75 on a motorway in the gear that suits the diesel best? Sure, the diesel will pull away. Otherwise, it won't. Arguably, you're intention is pointless. You're creating a scenario that exists between two members of the motoring public who do not give a **** about going fast. Those who do, drive appropriately.

It will be replicated on the road, and is every day.

 

[Dogbreath]

But this is pointless. Put them in a real scenario. 50-75 on the motorway. The figures are right, there is no disputing that, but it will just never be replicated on the road.

As long as you are permitted to change gears (something diesel owners seem to be allergic to), performance will still be quite similar.