Power vs Torque Questionnaire -RESULTS

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Phil
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Re: Power vs Torque Questionnaire -RESULTS

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Stradivarius wrote:Interesting stuff! This is a great effort, and I hope you don't mind a couple of comments: At speeds below 45 km/h I would think that wheel spin/traction is likely to be the limiting factor of the car's acceleration, so the force advantage of the diesel engine in first gear isn't necessarily real, at least not until approaching 45 km/h. At 45 km/h, the engine is capable of an acceleration higher 0.6 g and I am not sure if the tires are capable of much more.
Thanks! Yeah, traction limit is probably a big issue. But it's hard to quantify (different tyre compound or surface grip could offset how much force you can apply before the wheels spin). Even drag too. In reality, you would need to account for traction and at higher speed the drag coefficient that would lead to an overal reduction in force as you gain speed, but progressively more air resistance. I find the graph great though for comparing in-gear acceleration between different cars. It's easy to see where which car at which rev-range and gear would have an advantage or not.
Stradivarius wrote:By the way, you write that from 45 km/h to 70 km/h, the petrol car is quicker because the diesel car is already in 2nd gear. This is correct, but it has nothing to do with friction. As far as I can understand, your calculations don't even account for friction. The reason why the diesel has less force in 2nd gear is the simple fact that a higher gear (exchange ratio) is trading torque for rotational velocity. So even though the engine torque actually is higher right after shifting to 2nd gear compared to right before the gear shift, the torque on the wheels is smaller due to the gear exchange ratio. This is why I think it's more convenient to forget about the torque and instead consider the power. Then you directly see the exact reason why the force drops when shifting to a higher gear: The engine power drops because you move to an engine rpm range with less power. Additionally, the force always decreases as the speed increases due to the simple equation: Power = force x speed.
Yeah. What I ment with friction is what you just explained.


EDIT:

Just to add, if I add the Lotus Exige (sorry for me keep bringing up this car, but I already have the data for it, so it's convinient)

Image

...it's easy to see it's in a completely different league performance wise. But what is even more interesting that if I decided to launch the car off the line in 3rd gear (lets assume a rolling start for arguments sake), I'd be slower then both Mercedes up until 70km/h when the petrol e250 reaches it's rev limit in 1st gear and goes into 2nd gear. It also shows that if I ever were to be in a drag race from say 40-100km/h and I was stuck in 5th gear, I'd lose out to both Mercedes going through the gears maximizing their potential.
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CBeck113
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machin wrote:Both cars have the same 7 speed gearbox: only the final drive ratios are adjusted to account for the different rev ranges of the two cars... if you re-do the charts based on that it will be very telling why the performance of the two cars is very similar....
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.
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Stradivarius
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Re: Power vs Torque Questionnaire -RESULTS

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Phil wrote:
Stradivarius wrote:Interesting stuff! This is a great effort, and I hope you don't mind a couple of comments: At speeds below 45 km/h I would think that wheel spin/traction is likely to be the limiting factor of the car's acceleration, so the force advantage of the diesel engine in first gear isn't necessarily real, at least not until approaching 45 km/h. At 45 km/h, the engine is capable of an acceleration higher 0.6 g and I am not sure if the tires are capable of much more.
Thanks! Yeah, traction limit is probably a big issue. But it's hard to quantify (different tyre compound or surface grip could offset how much force you can apply before the wheels spin). Even drag too. In reality, you would need to account for traction and at higher speed the drag coefficient that would lead to an overal reduction in force as you gain speed, but progressively more air resistance. I find the graph great though for comparing in-gear acceleration between different cars. It's easy to see where which car at which rev-range and gear would have an advantage or not.
I don't think it is necessary to start estimating the grip based on data such as tire compound, surface parameters and temperatures. These are two almost identical cars with different engines, so I guess it's safe to assume they have similar traction. My point was merely that in 1st gear, the engine is not the limiting factor and the engine performance is thus not interesting when considering 1st gear performance. At least not for the diesel engine as it seems to have very low exchange ratio in 1st gear. The petrol engine probably becomes the limiting factor before changing to 2nd gear, as this won't happen until the speed is around 70 km/h. But unless the cars have different traction, there is no reason why they won't perform equal at low speeds, let's say below 40 km/h.

It's interesting to observe how you get 6 different torque curves, one for each gear, which all tell the same story about the engine performance. It really demonstrates why I think power is a better suited property for evaluating engine performance. You start with information about the torque at a specific engine speed and then you use the gear exchange ratio and wheel dimenions to determine the force. By doing so you implicitly determine the power. This is equivalent to starting with information about the power at a specific engine speed and then simply divide it by the car's speed in order to get the force. If you want to, you can also use the ratio between the different gear exchange ratios to produce one curve for each gear.

For the record: Friction represents a power loss or energy loss if you integrated it over time, which has nothing to do with changing the gear exchange ratio, which ideally does not involve any energy loss. Actually, if we were to consider the friction, a car at a given speed will have smaller friction losses in 2nd gear than 1st gear because the engine and all moving parts before the gear box will rotate faster, while everything else will be equal.

Stradivarius
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Re: Power vs Torque Questionnaire -RESULTS

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CBeck113 wrote: 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.

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machin
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CBeck113 wrote: 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).
Be careful of falling into the trap of paying too much attention to the rpm numbers. I work for a Diesel engine manufacturer and some of our engines only rev to 1300rpm FLAT OUT.... which is about idle speed on some automotive petrol engines, and nowhere near idle on a gas turbine engine.... So it is wrong to say that 2000rpm is the "low end" in every case... in the Merc example above 2000rpm is actually 50% of max.... whereas 2000 rpm on the petrol is only 30% of max. We're conditioned to think that 2000rpm is "low"... but only because the majority of us drive around in petrol engined cars with 6000-7000rpm red lines.

If both cars have good noise and vibration damping, and I gave you a rev counter without any numbers on it (just "idle", "50%" and "Max" marked on it) I can promise you that you would be very hard pushed to know which engine you were using since they both have pretty much the same power curve shape and magnitude. You would quite naturally drive both cars around at about the same % of max RPM which might be 1500rpm on the Diesel and 2250rpm on the petrol engine where the two engines make about the same power.
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J.A.W.
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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.
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strad
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J.A.W.'s post sums it up better than most any in this thread.
Anyone that thinks you can drive a diesel or a petrol engine and not feel the difference is out in left field. imo
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langwadt
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strad wrote:J.A.W.'s post sums it up better than most any in this thread.
Anyone that thinks you can drive a diesel or a petrol engine and not feel the difference is out in left field. imo
if you couldn't hear the engine and didn't have a rev counter it might not be so easy

Stradivarius
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J.A.W. wrote: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
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Stradivarius wrote: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).

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machin
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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.
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CBeck113
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machin wrote:
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.
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CBeck113
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Stradivarius wrote:
CBeck113 wrote: 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!
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machin
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CBeck113 wrote: 2. Have you driven both?
Have you?! :lol:

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:

Image
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CBeck113
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machin wrote:
CBeck113 wrote: 2. Have you driven both?
Have you?! :lol:

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.
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