Power vs Torque Questionnaire -RESULTS

[J.A.W.]

For a 'from the horses mouth' view, I'd suggest you could ask the M-B test guys..

From my own empirical perspective, there is a marked difference in that the perceived propulsive
urge provided by the shorter rpm powerband diesel - it feels like a harder hit per gear - that runs out sooner..

Side by side acceleration-wise you'd see the diesel initially gain ground on the petrol,
- only to lose it at each again at each ( earlier) gearshift point - as it runs out of rpm.

Diesels also have that deceptive non-linear feel to the 'throttle' pedal - whereby a steady application can
result in an seemingly imperceptible ( but actionable) gain in rpm/speed - when driven off the cruise control.

Diesel vehicles tend to have a unpleasant sour smell about them too..

[Stradivarius]

For a 'from the horses mouth' view, I'd suggest you could ask the M-B test guys..

From my own empirical perspective, there is a marked difference in that the perceived propulsive
urge provided by the shorter rpm powerband diesel - it feels like a harder hit per gear - that runs out sooner..

Side by side acceleration-wise you'd see the diesel initially gain ground on the petrol,
- only to lose it at each again at each ( earlier) gearshift point - as it runs out of rpm.

Diesels also have that deceptive non-linear feel to the 'throttle' pedal - whereby a steady application can
result in an seemingly imperceptible ( but actionable) gain in rpm/speed - when driven off the cruise control.

Diesel vehicles tend to have a unpleasant sour smell about them too..

If you look at those two curves I just posted, everything you say seems to be transferable to those curves. The problem is only that it is incorrect. The diesel doesn't have a shorter rpm powerband because it is only the relative rpm that matters. If you increase the rpm from 1500 rpm to 3000 rpm with a diesel engine, you will double your speed, there is no way around it, unless you have shifted gears in the mean time. If you increase the rpm from 3000 rpm to 6000 rpm with a petrol engine, you will also double your speed even though the absolute increase in engine speed is greater. The petrol engine increases the rpm by 3000, while the diesel engine increases the rpm by only 1500. Yet, they both accomplish exactly the same thing.

So it is not correct that the diesel will gain ground initially. And it is also not true that the diesel will have to shift earlier. The only way this would happen is if the diesel starts in a lower gear, while the petrol starts in a higher gear. Then the diesel would gain ground initially and have to shift earlier. But that goes both ways. If the diesel starts in a higher gear, it will lose ground initially, but will be able to delay the gear shift. If they both start in the same gear (relative to their respective torque curves), there won't be any difference.

But I some how think this tells something about the perception many people have of diesel engines. If they tend to run in lower gears with diesel engines compared relatively to petrol engines, they will feel exactly what you are describing when they hit the throttle. The diesel engine will come right away, while the petrol engine will not come until the rpm has increased. But the reason doesn't have anything to do with the engine itself, it's simply due to the driver's choice of running at relatively high rpms with the diesel engine and relatively low rpms with the petrol engine in the first place.

If you run the diesel engine at around 1500 rpm, this corresponds to 3000 rpm on the petrol engine. But in my experience many drivers like to hover around 2000 rpm whether it's a diesel engine or a petrol engine. And that may be the source of many of these misunderstandings. We have seen many examples of it in this thread already that people try to compare engine revs directly, not realizing it's the relative engine revs that matter.

[J.A.W.]

& yet the empirical comparison between the two M-Bs still shows a statistically significant time-to-distance
acceleration advantage to the petrol machine..

If they were both lined up side by side on a simultaneous acceleration test, this would clearly been seen
as a gain by the petrol car, which would match the same advantage shown in the force chart graphs..

Similarly - this motion would be experienced viscerally by the drivers of the competing vehicles..

[hollus]

The empirical test shows that the car with an ~3% more powerful engine and ~3% lighter accelerates somewhat faster. No surprised there.
I think we have a group of people here talking about two hypothetical cars (only they are not that hypothetical) with power, weight and gears evenly matched.
On the other hand we have a group of people comparing the average petrol car from the last 20 years, many of which were sporty, with a high theoretical top speed; with the average diesel car from the last 20 years, many of which were heavy and bulky utilitary vehicles with limited top speeds built for economy... and geared accordingly.
In the second case, of course there will be a difference in feeling and noise and, yes, smell.
It would help if people say whether they are talking of the first situation, as Machin is, or of the second situation and many others seem to be.

[J.A.W.]

What do you mean by "more powerful" in this instance H?
Diesel torque or petrol/high rpm horsepower? L.O.L...

Are the two M-Bs listed - not direct alternatives?

[Stradivarius]

What do you mean by "more powerful" in this instance H?
Diesel torque or petrol/high rpm horsepower? L.O.L...

Are the two M-Bs listed - not direct alternatives?

The more powerful engine is the engine with more power, i.e. the petrol engine. As the calculations presented in this thread show, it is the power that determines the acceleration for a given mass. The calculations we see here that consider the torque need to take into account either the gear exchange ratio and velocity, or the engine rpm, which effectively means that the torque is only used to determine the power and thereby finding the acceleration for a given mass.

The main conclusion that can be drawn from the two M-Bs listed is that they have similar mass and similar power and thus similar performance (top speed and acceleration). The torque, however, is far from similar, clearly demonstrating its insignificanse.

[Phil]

I'd argue that "power" is just a momentary figure. A power output rating at a specific point in time (rpm). It doesn't necessarely tell us anything about acceleration because acceleration happens over a wider rev range, not at a idle point. The torque curve shows us the characteristic, which leads to over which band of the rev range the engine produces how much power.

As an example; We could have two engines both producing a maximum of 200bhp, where as in one engine achieves this in a linear fasion (half revs = 100bhp, max rev = 200bhp) and the other perhaps with more torque in the midrange so at half-revs it may already be producing 150bhp (50 more) but peaks as well at 200bhp at maximum revs. In this case, the latter will lead to a faster acceleration because you will have more power over the entirety of the rev-range.

Again: The two Mercedes have nigh on identical acceleration figures because they have a very similar power delievery. That the diesel spreads it (with more torque) over a more narrow range vs. the petrol is utterly irrelevant as this is normalized by the final gear ratio. In essence, the diesel will rev exactly within the same time to its rev-limit as the petrol engine and the gearing will ensure that the same speeds are reached at the same time.

[machin]

I'd argue that "power" is just a momentary figure. A power output rating at a specific point in time (rpm). It doesn't necessarely tell us anything about acceleration because acceleration happens over a wider rev range, not at a idle point. The torque curve shows us the characteristic, which leads to over which band of the rev range the engine produces how much power.

But "Power" (Which you could just think of as "Torque x Speed divided by an arbitrary number to make the numbers more manageable") can (and should) be considered in a curve and is therefore no more "momentary" than a single torque figure at a single RPM...

[rjsa]

People need to understand that power, torque and rpm are just three dimensions of one very same physical phenomenon. You can bitch and moan all that you like, but the three are inter joined and with two of them you always know the value of the third, no mater what.

[Stradivarius]

I'd argue that "power" is just a momentary figure. A power output rating at a specific point in time (rpm). It doesn't necessarely tell us anything about acceleration because acceleration happens over a wider rev range, not at a idle point. The torque curve shows us the characteristic, which leads to over which band of the rev range the engine produces how much power.

In that case, speed, acceleration, engine rpm and torque are also momentary figures. They change with time. But the momentary power divided by the momentary speed always gives the momentary motive force, i.e. at every moment in time the current motive force is equal to the current engine power output divided by the current speed.

When you use the torque curve, the gear exchange ratios, the final drive and the tyre- and wheel dimensions to calculate the motive force as a function of the vehicle speed, you are implicitly defining the power. So basically, the only use you make of the torque is to determine the power.

On the other hand, let's say that we have the following information about a car:

-The power curve with power as function of relative engine speed (in % of maximum engine rev speed, not in rpm)
-The relative difference between the gear exchange ratios of two consecutive gears, i.e. the ratio of 2nd gear divided by the ratio of 1st gear, the ratio of 3rd gear divided by the ratio of 2nd gear and so on.
-On point on the above mentioned power curve where we know the vehicle speed and which gear is engaged

With this information I can calculate the motive force of the car under all circumstances. I will know everything there is to know about the performance of this engine. If I also get information about the car such as mass, center of gravity, aerodynamic parameters etc. I will know everything there is to know about the car's performance. But I don't know the following information, and I have no way of determining the information about:

-Engine torque
-Engine rpm (absolute figure)
-Gear exchange ratios
-Final drive
-Tyre- and wheel dimensions

Let's call the former set of information data set 1 and the latter set of information data set 2. Both data sets can be used alone to determine the engine performance. But data set 1 cannot be used to determine data set 2. However, data set 2 can be used to determine data set 1. That means data set 2 contains more information than data set 1. But since data set 1 alone contains all the information we need, and data set 2 contains more information than that, data set 2 must necessarily contain some insignificant information.

Imagine that you need to paint a building. You then need to know how much paint you need to buy. You know that one liter of paint is enough to cover 10 square meters, so you then start to collect data. You measure the length of each wall, the height and the ridge angle of the gable walls. Then you start measuring the windows doors that do not need to be painted. With all this data you will be able to calculate how much paint you need. But there is really just one parameter you need to know: The total area of the surface that is to be painted.

I would argue that the surface area is the only essential parameter in this case, given the 10 square meters per liter figure. You may well argue that the height of the building is essential because it affects the surface area, but the truth is that you don't need to know it, you only need to know the surface area.

[Phil]

When you use the torque curve, the gear exchange ratios, the final drive and the tyre- and wheel dimensions to calculate the motive force as a function of the vehicle speed, you are implicitly defining the power. So basically, the only use you make of the torque is to determine the power.

Yes. I prefer to look at torque curves however, because power is a function torque x rpm. Without torque, you have no power. But essentially, as another user noted, rpm, torque and power are all interlinked with one another. One can not exist without the other.

I'm not disagreeing with the rest you've written, it's correct as well, though I'm not sure why one would want relative power without rpm and as a percentage to mask exact torque values [except to prove a point in how all these figures are linked to one another]. I also think we are moving away from the topic. My post was merely to reflect on the fact that an absolute power value has absolutely no bearing on how that entity accelerates. Which is why I've always been talking about torque curve or diagram (which could just as well be a power curve/diagram since one can be calculated from the other).

The point therefore that two engines rated at 200bhp or 207bhp will accelerate give or take identically because they have similar [max] power-ratings is incorrect. They will achieve a similar top-speed because maximum power is detrimental to that (in combination with drag and gearing, but we're assuming these are identical), but acceleration is dependant on how the engine produces its power, hence why the curve/diagram becomes essential: case-in-point; my example with two engines with both equal maximum bhp figures, but one producing more power in the midrange. Both will have the same top-speed but one will get there faster.


PS: Strad, despite me quoting you, I'm not disagreeing with you. The message wasn't directed at you in particular but more overal towards the topic!

[hollus]

May I rephrase the question?
Given a car and an engine, lets say it is a racing car. Would you adjust your gearing and driving style
a) to say close to maximum power
or
b) to stay close to maximum torque?
Would you really try to stay around 1600-2000 rpm with either of these cars?

[mrluke]

Force = mass x acceleration

Acceleration = Force / Mass

Acceleration = Power / constant (assuming identical weight, rolling resistant etc)

Torque is irrelevant.

The only way to use torque to calculate acceleration is as follows:

Power = torque x rpm

Acceleration = (torque x rpm) / constant

So yes you can use torque and an rpm reference to calculate the available power, but it is not required when the power is already known.

Note that power =/= peak power. Knowing the available power at 9,000rpm is useless if you are changing gear at 6k. Likewise knowing torque figure at 1,500rpm is of no relevance if your power band is 6-9k.

The area under the WHP trace on a dyno is the single and best measure of available power across the working range of an engine.

I suspect the only reason peak torque figures are quoted is because it is very difficult to agree on a point in the rpm range to provide a second power measurement and having two bhp figures for a car would confuse the general public.

[Phil]

I suspect the only reason peak torque figures are quoted is because it is very difficult to agree on a point in the rpm range to provide a second power measurement and having two bhp figures for a car would confuse the general public.

How else would you label the maximum torque figure? You can't really do it with a power figure because no secondary power figure somewhere within the rev range would tell you the exact same information. It would just be some arbitrary number, like 100bhp @ 2200rpm (max being 217bhp at 6200rpm) for the Mercedes E250 (Petrol).

Perhaps a good alternative way of labling 'relevant information' would be to have power figures for 1/4, 1/2 and 3/4 range. But the peak torque figure still tells us at which point the engine, from a mechanical stand-point, produces the max turning force. A bit like when you are on a bicycle riding away - there is a point of speed at which you pedle where you have the best turning force. It's not the maximum rev and it wouldn't be where you are producing the max "power".

May I rephrase the question?
Given a car and an engine, lets say it is a racing car. Would you adjust your gearing and driving style
a) to say close to maximum power
or
b) to stay close to maximum torque?

Close to maximum power.

[Stradivarius]

Yes. I prefer to look at torque curves however, because power is a function torque x rpm. Without torque, you have no power. But essentially, as another user noted, rpm, torque and power are all interlinked with one another. One can not exist without the other.

I'm not disagreeing with the rest you've written, it's correct as well, though I'm not sure why one would want relative power without rpm and as a percentage to mask exact torque values [except to prove a point in how all these figures are linked to one another]. I also think we are moving away from the topic.

My point is to remove all the irrelevant information in order to remain with only the relevant information. It turns out the torque belongs with the former.

My post was merely to reflect on the fact that an absolute power value has absolutely no bearing on how that entity accelerates. Which is why I've always been talking about torque curve or diagram (which could just as well be a power curve/diagram since one can be calculated from the other).

I don't think anyone here is claiming that the peak power alone gives the complete picture. At least I am not. I am refering to the power curve. Note, that I am not refering to relative power. I am talking about absolute power (measured in hp or kW, not in %) as a function of relative engine rev speed (measured in % and not in rpm).

But let's try to agree on the logic here, which I think is crucial:

Do you agree with the following?

Premise: It is possible to describe the car- or engine performance completely without knowing the torque and without having the information necessary to determine the torque.

Logical conclusion: The torque is not of any significance when it comes to car- or engine performance.

Reasoning: If the torque was of any significance regarding car- or engine performance, the premise would be false. It wouldn't have been possible to describe the car- or engine performance completely without knowing the torque or having the information necessary to determine the torque.


Since you seem to believe that the torque is of significance, where is the flaw of the logic described above? By the way everyone is welcome to answer me.