Power vs Torque Questionnaire -RESULTS

[bhall II]

Can we all agree torque is invisible?

It's energy, dammit!

[Phil]

EDIT: Corrected version.

Yeah, I agree. Though to be fair, quite a few magazines (at least in Germany) post actual power curves of the engines. But it's usually so small it's easy to ignore and most people tend to simply compare maximum figures.

When comparing cars, I like torque at the wheel (after the gear train). When doing that, you can actually see which car has more "punch" at which point within which gear compared to another. In fact, I programmed a little tool to messure how much force you have at the wheel:

http://temp.conceptics.ch/performance/

With actual data comparing two cars:

http://temp.conceptics.ch/performance/? ... 85;380;355

(click the link, then click on "draw chart"). This produces the following calculated graph:



This compares my car (Bemani Exige) against the newer Exige V6. The former has around 275bhp with max torque of 253Nm at 930kg with the newer V6 Exige with 350bhp and 400Nm at 1176kg. On paper, both cars have roughly around the same power/weight ratio.

The graph shows which car has more "punch" over all gears.

1st gear, it's the Exige V6, but gear 2, 3 and 4th, mine is faster up until the drop-off when I need to change into the next higher gear. The graph is all in good fun and assumes equal drag. Technically, the graph should drop off more as the speed increases due to wind resistance, but this would make the overal calculation quite a bit more complex. But as an example to show the correlation between *force at the wheel* over the gear-ratio, it's rather nice to play around with.


Anyone feel free to play around with the application; just beware to enter figures in Nm and gear ratio seperated by a dot (.) not a comma (,), or else you'll be faced with devision by zero errors. It was just a quick and dirty programming, not ment for sharing.

Cheers

[Phil]

To keep this in line with the questionnaire, I tried to find the data for the two Mercedes on your site, machin:

lets recap what we know:

Mercedes e250


Engine power curves:


I converted the LB-FT figures into NM for my performance calculator. I also searched for gear-ratios - not sure if completely correct, as I found the manual transmission data for both models, though not sure if the year model is accurate.

Mercedes e250 CDI (Diesel):
http://www.daimler.com/Projects/c2c/cha ... ata_en.pdf

Mercedes e250 CGI (Petrol) (5 gear manual):
http://www.automobile-catalog.com/car/2 ... matic.html

Code: Select all

               e250cdi (diesel)     e250 cgi (petrol)
-----------------------------------------------------------
final drive:   2.47                 3.07
1st:           5.10                 3.95
2nd:           2.78                 2.42
3rd:           1.75                 1.49
4th:           1.25                 1.00
5th:           1.00                 0.83
6th:           0.81
-----------------------------------------------------------

Tyres are assumed to be identical, as per your website.

This comes up with the following graph:


Link: http://temp.conceptics.ch/performance/? ... ;272;250;0

What this shows us, is that the diesel car (black) has a lot more force at the wheel in 1st gear up to the drop off point at around 45km/h. That would be at close to max rev on the diesel (4000rpm). At that point, the petrol e250 is only at midrange of its rpm range and continues to rev until it hits 70km/h. From 45km/h to 70km/h it is quicker, because in the Diesel, you will already be in 2nd gear where higher friction through the transmission will see less force at the wheel.

In the later gears, both cars are quite similar, when comparing diesel 4th gear to the petrols 3rd, 5th gear to 4th, and 6th gear to 5th.

It also shows us quite simple that in the lower rev range, the Diesel produces more force at the wheel, but due to its narrow rev range (only to 4000rpm) it needs to go into the next higher gear earlier, at which point the petrol can still keep on going.

The above graph shows nicely how gear ratios can influence how much force you are left with at the wheel. It also shows at which point within the rev-range and at which gear a car has more force, compared to another. A peaky engine (like a car with a lot of torque) will deliever a lot of load / acceleration at that point, where as a more linear power delievery will be more constant over a wider rev range. Not as extreme, but over the entirety of the rev-range, perhaps quicker.

[machin]

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....

[Phil]

They already are in the 6-speed vs 5-speed constellation above. The diesel in the above example is quicker to 45km/h, but then loses time from 45-70 when the petrol keeps on reving to its max and continues to be quicker in the higher gears due to the offset in gears. Above 120km/h they are fairly identical.

Do you have the exact gear ratios of the 7 speed gearbox? Would be interesting to see how the chart comes up then...

EDIT: I know what you mean with the final gear. If the final gear is 3.5 in the petrol car and all other gears remain identical to each other, the graph is nigh on match. That is only because the petrol and diesel engine have very similar curves, just spread over a slightly wider rev-range in the petrol. If the engine characteristics were more different, it would be a bit more difficult to compare. Here's the link with identical gear ratios and a slightly different final drive ratio (3.5) for the petroled car:

http://temp.conceptics.ch/performance/? ... ;272;250;0

(click -> then "draw chart")

[Stradivarius]

What this shows us, is that the diesel car (black) has a lot more force at the wheel in 1st gear up to the drop off point at around 45km/h. That would be at close to max rev on the diesel (4000rpm). At that point, the petrol e250 is only at midrange of its rpm range and continues to rev until it hits 70km/h. From 45km/h to 70km/h it is quicker, because in the Diesel, you will already be in 2nd gear where higher friction through the transmission will see less force at the wheel.

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.

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.

[machin]

I know what you mean with the final gear. If the final gear is 3.5 in the petrol car and all other gears remain identical to each other, the graph is nigh on match. That is only because the petrol and diesel engine have very similar curves, just spread over a slightly wider rev-range in the petrol.

The chart from your link is the be-all and end-all of the discussion:-



Because the two engines have power curves of similar shape and magnitude the torque/motive force/"Punch" that each car can generate at the wheels is also similar, regardless of the proportions of RPM/Torque that each generates at the flywheel.

If one engine had a "bigger"/"larger"/"Fuller" power curve it would be able to generate more torque/motive force/"Punch" at the wheels, regardless of the proportions of RPM/Torque that each generates at the flywheel.

The actual gear ratios are not important to the discussion; only that each one is optimised to account for the prime mover (Engine) rev range; which can be achieved with a simple final-drive ration change, as demonstrated in your chart above.

[Phil]

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.

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)



...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.

[CBeck113]

Engine power curves:
http://www.competition-car-engineering. ... e_comp.jpg

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.

[Stradivarius]

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]

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.

[machin]

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.

[J.A.W.]

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.

[strad]

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

[langwadt]

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