Power vs Torque Questionnaire -RESULTS

[Stradivarius]

I do not agree - CI & SI mills as shown in the M-B have quite distinctive differences in feel..

The short punchy rev range of the diesel is marked - in comparison to the creamy long-legged petrol mill.

& the non-throttle CI engine has a characteristic disconnected non-linear 'floaty' powerband/turbo response, too.

The rev range is a relative term, you can't conclude anything from the absolute values. Let's imagine that someone had installed a 1:2 gear and then measured the torque to generate the curve. The curve would then reach half the original torque and twice the original rpm. You may say that this would make the rev range creamy and long-legged, but that has nothing to do with the engine performance.

If you look at the curves posted above, you see that they are short and punchy in 1st gear and long-legged in 6th gear. But the engine is the same. An engine that delivers 500 Nm between 2000 rpm and 3000 rpm performs identically to an engine that delivers 250 Nm between 4000 rpm and 6000 rpm. So whether it is short and punchy or long-legged, doesn't matter.

[Cold Fussion]

If you look at the curves posted above, you see that they are short and punchy in 1st gear and long-legged in 6th gear. But the engine is the same. An engine that delivers 500 Nm between 2000 rpm and 3000 rpm performs identically to an engine that delivers 250 Nm between 4000 rpm and 6000 rpm. So whether it is short and punchy or long-legged, doesn't matter.

That's if you were confined to that rev range. If you started both the diesel and petrol merc from a very slow rolling start (just fast enough to have the clutch fully disengaged and not stalling), you would have to pass through the low rpm, low power zone on a petrol engine so under these conditions you would see a difference in performance (so long as they weren't traction limited).

[machin]

If you started both the diesel and petrol merc from a very slow rolling start (just fast enough to have the clutch fully disengaged and not stalling), you would have to pass through the low rpm, low power zone on a petrol engine so under these conditions you would see a difference in performance (so long as they weren't traction limited).

I'll say it again: the diesel engines that I work with only rev to 1300rpm and idle at 230rpm. So it is not correct to say that "2000rpm is low"... it depends on what the idle speed and max speed of the engine in question are....

It is almost certain that the Merc Diesel will idle at a lower RPM than the Merc petrol engine.. so from a gentle rolling start as in your scenario the diesel will probably start from say 800rpm (20% of max), whereas the petrol engine will start at 1200rpm (also 20% of max)... if you look at the power curves both engines make about the same power at those rpms, so in reality both have the same performance potential (or capacity to spin their wheels depending on the traction available!!!); exactly as the performance figures tell us!

I think you might be projecting your personal experiences of what the "typical petrol engine" feels like and not using the specific data available: the petrol engine in question is a turbo-charged engine which has almost identical power characteristics to the diesel engine in question; which is exactly why I chose those two engines for the comparison.

[CBeck113]

If you started both the diesel and petrol merc from a very slow rolling start (just fast enough to have the clutch fully disengaged and not stalling), you would have to pass through the low rpm, low power zone on a petrol engine so under these conditions you would see a difference in performance (so long as they weren't traction limited).

I'll say it again: the diesel engines that I work with only rev to 1300rpm and idle at 230rpm. So it is not correct to say that "2000rpm is low"... it depends on what the idle speed and max speed of the engine in question are....

It is almost certain that the Merc Diesel will idle at a lower RPM than the Merc petrol engine.. so from a gentle rolling start as in your scenario the diesel will probably start from say 800rpm (20% of max), whereas the petrol engine will start at 1200rpm (also 20% of max)... if you look at the power curves both engines make about the same power at those rpms, so in reality both have the same performance potential (or capacity to spin their wheels depending on the traction available!!!); exactly as the performance figures tell us!

I think you might be projecting your personal experiences of what the "typical petrol engine" feels like and not using the specific data available: the petrol engine in question is a turbo-charged engine which has almost identical power characteristics to the diesel engine in question; which is exactly why I chose those two engines for the comparison.

1. I haven't seen a diesel engine in a passenger car that idles at 230 and has a limit of 1300 rpm, and we are talking about two passenger cars here - like you stated. Those numbers are either ship or generator engines, which also have distinctively different usages, hence the different rpm ranges.
2. Have you driven both? Or even a modern diesel and modern petrol engined car? The only time you could confuse a petrol engine for a diesel is when it's supercharged (and not turbo charged, they are usually tuned for the higher rpm range, otherwise they would negatively impact the fuel consumption), simply because of the amount of power available in the lower rpm range. Yes, they are geared differently - because of the power they can be geared that way - to make the most of this power difference.

[CBeck113]

The rev range is the key to the power "feel" - the diesel generate almost 50% more power at 2000rpm (this is the "low-end torque" which is actually low-end power and the root cause/ root-of-all-evil for all the power vs torque discussions). Change the scale on the x-axis and you'd see engines with very different characteristics. This is also the main advantage of the diesel - more power at a lower rpm means better fuel efficiency, and makes it better for pulling tasks to boot (you need to get the rpms up and let the clutch slip to get a heavy trailer moving). Based on those reasons I would always take a car with the power max at the lower rpm, unless the power deficit is too large.

It is a valid and important point that diesel engines have better fuel efficiency in everyday use. When you don't want full power from a petrol engine, you use the throttle, to restrict the intake to the cylinder, which causes an energy loss not seen in diesel engines because they directly limit the amount of fuel going into the cylinder. So on partial loading, diesel engines are more efficient, but I don't follow you when you say they are better for pulling tasks. It is true that the clutch on a diesel car works at smaller rpm, but it also transfers a higher torque when comparing to a petrol engine, so I don't understand why one would be any better than the other.

With this I was refering to the rpm delta to get the cars moving - the petrol needs a higher rpm to reach the necessary power band to get moving, which causes higher wear on the clutch plates - nothing else!

[machin]

2. Have you driven both?

Have you?!

But seriously though, I can see that you are letting your previous experiences of the "general" characteristics of the two types of engine cloud your judgement, rather than looking at the data available for these two specific engines...

....You don't need to take the step of showing the motive force available once you get experienced at reading power curves, but it does show it rather clearly; both engines have near identical capacity for performance/wheel spin in every gear at every road speed due to the similarity of the power curve shape and magnitudes, as shown in Phil's chart:

[CBeck113]

2. Have you driven both?

Have you?!

But seriously though, I can see that you are letting your previous experiences of the "general" characteristics of the two types of engine cloud your judgement, rather than looking at the data available for these two specific engines...

....You don't need to take the step of showing the motive force available once you get experienced at reading power curves, but it does show it rather clearly; both engines have near identical capacity for performance/wheel spin in every gear at every road speed due to the similarity of the power curve shape and magnitudes, as shown in Phil's chart:

http://i90.photobucket.com/albums/k248/ ... lwyxfr.jpg

No, I haven't - just the E250cdi estate and a C250 estate, since I just bought a new can, and wanted to see it the extra price for a premium car was worth it - in my opinion it isn't, but has nothing to do with this.

We are talking about two different things here - you are referring to the fact that the entire drivetrains are tuned through the gearing to perform equally (if that's the engineering goal, they can do it), while I am referring to the power / rpm ratio, which is much higher on the diesel in the lower rpm range, and no comparison is possible in the upper because the diesel just doesn't go there. My point is that the driver sets a rate of acceleration by using the pedal, and the engine responds by sending power through the drive train to the driven wheels. As the power builds up to overcome the resistance, the difference is felt because the diesel delivers the power earlier than the petrol. This may not have an effect on the performance due to the gearing, but it can be felt, and, if wanted, exploited by the engineering team to enhance the performance in the lower rpm range. Example: you can get the 250cdi moving without touching the gas pedal, due to the available power at idle, making the car easier to drive and less prone to stalling (good for the old men who drive these things), while the petrol will always stall, unless you very carefully play with the clutch.

Summary, because I started to ramble a bit: max performance can be tuned through the power train to be equal, as in the example, but who drives for max performance constantly, other than race drivers and a hand full of idiots who end up wrapping themselves around trees? The diesel has the advantage in the real world.

[Phil]

Summary, because I started to ramble a bit: max performance can be tuned through the power train to be equal, as in the example, but who drives for max performance constantly, other than race drivers and a hand full of idiots who end up wrapping themselves around trees? The diesel has the advantage in the real world.

This is a good point, but IMO not accurate in this particular case and example, where in both petrol and diesel variant have the same gearing ratios and the final gear is optimized. In this case, both will perform identically, irregardless if the petrol spreads it's range over 6000rpm and the diesel over 4000rpm. The power delievery at the wheel is nigh on identical.

For most cases and in general, yes, the Diesel will have an advantage due to their max-torque at very low rpm (relatively), meaning that in typical rev-range, the engine is constantly in a range where it is "good to go", compared to a petrol, if even non turbo car, that needs to be revved for maximum effect.

Example: you can get the 250cdi moving without touching the gas pedal, due to the available power at idle, making the car easier to drive and less prone to stalling (good for the old men who drive these things), while the petrol will always stall, unless you very carefully play with the clutch.

This may be a bit off-topic, but I was under the belief that just about any car, if properly adjusted, maintained, should not stall when in idle and in first gear without the clutch engaged. Of course, letting the clutch go in an abrupt manner will most of the time always result in a stalled engine, even a diesel. I always thought that a diesel may be a bit more resistant to it, has to do with the higher compression and lower rpm (but at idle still higher relatively speaking) than a petrol based engine?

[machin]

not accurate in this particular case and example, where in both petrol and diesel variant have the same gearing ratios and the final gear is optimized. In this case, both will perform identically, irregardless if the petrol spreads it's range over 6000rpm and the diesel over 4000rpm. The power delievery at the wheel is nigh on identical.

Exactly. Maybe if we referred to the two engines as "Engine A" and "Engine B" it would be easier for people to leave their prejudices behind and judge the two based on the facts alone... which really is the whole point of this thread: Base an engine on the relevant facts and you will be able to judge the performance potential of the vehicle it is fitted in very accurately. Base your performance prediction on irrelevant data or pre-conceived ideas and you will be wrong a large number of times.

[Stradivarius]

If you look at the curves posted above, you see that they are short and punchy in 1st gear and long-legged in 6th gear. But the engine is the same. An engine that delivers 500 Nm between 2000 rpm and 3000 rpm performs identically to an engine that delivers 250 Nm between 4000 rpm and 6000 rpm. So whether it is short and punchy or long-legged, doesn't matter.

That's if you were confined to that rev range. If you started both the diesel and petrol merc from a very slow rolling start (just fast enough to have the clutch fully disengaged and not stalling), you would have to pass through the low rpm, low power zone on a petrol engine so under these conditions you would see a difference in performance (so long as they weren't traction limited).

Ok, I think I understand what you are saying. Let's define two very simple but complete torque curves then: One engine is able to drive with the clutch fully engaged at 600 rpm where it produces 150 Nm. The torque then increases linearily from 600 rpm to 2000 rpm where it reaches 500 Nm and then stays at 500 Nm until 3000 rpm. Then the torque starts droping, but that is not relevant to this particular discussion. The other engine is able to drive with the clutch fully engaged at 1200 rpm where it produces 75 Nm. The torque then increases linearily from 1200 rpm to 4000 rpm where it reaches 250 Nm and then stays at 250 Nm until 6000 rpm. Again, what happens at higher revs is not in discussion.

What you are saying is that since the rev delta for the first engine is only 1400 rpm (2000 rpm - 600 rpm), while the rev delta for the second engine is 2800 rpm (4000 rpm - 1200 rpm), the first engine will reach maximum torque before the second engine does so and because of that it will perform better than the second engine at lower revs. Have I understood you correctly?

[Stradivarius]

With this I was refering to the rpm delta to get the cars moving - the petrol needs a higher rpm to reach the necessary power band to get moving, which causes higher wear on the clutch plates - nothing else!

The wear on the clutch plates doesn't only depend on the rpm delta, it also depends on the torque which is transfered. The clutch plates on a petrol car will see a higher rpm delta, but they will see a lower torque. If the clutch plates transfer the same power while engaging, why would the wear be different?

[strad]

if you couldn't hear the engine and didn't have a rev counter it might not be so easy

First time you ran thru the gears you'd KNOW the difference.!

[machin]

Ok, I think I understand what you are saying. Let's define two very simple but complete torque curves then: One engine is able to drive with the clutch fully engaged at 600 rpm where it produces 150 Nm. The torque then increases linearily from 600 rpm to 2000 rpm where it reaches 500 Nm and then stays at 500 Nm until 3000 rpm. Then the torque starts droping, but that is not relevant to this particular discussion. The other engine is able to drive with the clutch fully engaged at 1200 rpm where it produces 75 Nm. The torque then increases linearily from 1200 rpm to 4000 rpm where it reaches 250 Nm and then stays at 250 Nm until 6000 rpm. Again, what happens at higher revs is not in discussion.

What you are saying is that since the rev delta for the first engine is only 1400 rpm (2000 rpm - 600 rpm), while the rev delta for the second engine is 2800 rpm (4000 rpm - 1200 rpm), the first engine will reach maximum torque before the second engine does so and because of that it will perform better than the second engine at lower revs. Have I understood you correctly?

if both cars start at say 20mph with their wheels turning at 250rpm (with the first engine at 600rpm making 150 Nm, and the second engine at 1200rpm making 75Nm), both cars have the same torque at the wheels of 360Nm; ((150 x600)/250, or ((75x600)/250), with a motive force at the wheels of approx. 1059 N (360 /( 0.68m / 2)). F=mA, so if both cars have the same mass (and rotating inertia) their road speeds will increase at the same rate (m/s^2 or mph per second). Since the gearing is different on the two cars (600rpm/250rpm =2.4:1 or 1200rpm/250rpm= 4.8:1 respectively) this means that the engine revs on the first engine will increase half as fast as those on the second engine. So even though the first engine's RPM only needs to increase by 600rpm to get to peak torque, compared to an increase of 1200rpm for the second engine, they will both achieve peak torque at the same time... the first engine will be turning at 2000rpm making 500Nm and the second engine is turning at 4000rpm and makes 250Nm, and the wheels on both cars will now be turning at 833rpm. The first car will generate 500 x (2000/833) = 1200Nm at the wheels, and the second engine will generate 250 x (4000/833) = 1200Nm at the wheels; i.e. exactly the same. Since the wheels on both cars have the same radius (approx 0.68/2) they will have the same motive force at the wheels; 1200 / (0.68/2) = 3529N. Since F=mA, and assuming that the two cars still have the same mass (and rotating inertia), their road speed will continue to increase at the same rate....

[J.A.W.]

if you couldn't hear the engine and didn't have a rev counter it might not be so easy

First time you ran thru the gears you'd KNOW the difference.!

Agreed, quite right Strad, & frankly - anyone who could not realize this.. has no mechanical awareness..

I'll bet the actual M-B test engineers/drivers would consider the suggestion to be laughably ignorant..

[Stradivarius]

Agreed, quite right Strad, & frankly - anyone who could not realize this.. has no mechanical awareness..

I'll bet the actual M-B test engineers/drivers would consider the suggestion to be laughably ignorant..

Can you explain how you would feel the difference?

Maybe you could make use of the following torque curves with one engine having a short and punchy rev range, as you mentioned earlier, while the other is creamy and long-legged:


How would you feel the difference? Would you notice that one accelerated faster, or that it stoped accelerating earlier, so that you would need to gear up sooner? Or would you feel it some other way? If it is so obvious that anyone would feel it, it should be possible to describe this feeling.