15 replies to this topic

[Joe Bosworth]

Over in the thread titled “V8 – V10 Torque” there has been a long running un-meeting of the minds on the relative importance of Power vs Torque as developed by an IC engine.

There has started a side discussion that is really OT to the original topic so I thought I would start a new thread from that side discussion so that a discussion could take place, hopefully without the points scoring exercise that V8 – V10 has degenerated into.

NTSOS in the V8 – V10 thread asked people to check out SAE Paper #942476, “Dynamometer Testing of a Formula 1 Engine” etc. He concluded with a direct quote from that paper, “Torque – Obviously, the most important quantity to be measured is torque.”

Paul Ransom came back with, “And do you think they work towards the biggest torque figure they can make?”


NTSOS came back with, “Well generally speaking, yes and no!” NTSOS went on to explain the yes part with a discussion about engine mapping on a dyno.

I would like to come back now with my clarification of NTSOS’ post and say yes and yes and to explain why.

Let’s start with a typical mundane go to market world vehicle, the VW Jetta. Its 2.0 litre engine has a technical efficiency, (TE), of 26.5% and develops its max torque at 2600 RPM which is 50% of the max power RPM. This 2 to 1 spread of torque and power points is pretty typical of low efficiency, go to market, vehicles. They will spend 99% of their lives near or below peak torque revs and the designers want them to be easy to drive and last a whole bunch of miles/kms.

You ask what is this TE thing? Well, it is a measure of the max power the engine is rated at compared to how much power it could make if it got all of its swept capacity of inlet air in, (a 100% volumetric efficiency – VE), and was able to burn its fuel without any losses in the way of heat, friction, pumping losses and the like. I could use BMEP for this discussion but to most this is just some number that doesn’t mean much. By comparison, most people can visualise 26.5% efficiency as having an inefficiency of 73.5%.

How good/bad is the Jetta’s TE? Pretty much the norm for such vehicles. By comparison, a 2008 standard model Corvette of 6.2 litres and 430 HP at 5900 RPM has a TE of 28.2%. The 7 litre C6R Vette has a TE of 34.3%. Really full race stuff like F1 engines go up to just over 40%. That over 40% sets the upper boundary of what we can find for the most highly developed atmospheric IC engine today and into the near future.

One of the interesting things about this TE figure is that as engines are designed to operate at higher TEs, the peak torque RPM more closely approaches the peak power RPM. In fact quite a few years one group of Japanese motorcycle engineers chased power to the point that one GP bike found its peak torque and power RPMs at exactly the same point. The fact that no one could ride it to a fully competitive lap time should come as little surprise.

With the standard Corvette’s TE we find the peak torque at 75% of peak power RPM. With the C6R this has risen to 85%. I can give you a great deal more examples but please take the above as a sign that the higher the TE the closer the torque and power peak revs come together.

So what you say. Well here we come to the point. When your Jetta engine torque comes at 50% of power and low efficiency the engine designer can juggle all kinds of things and the peak torque and power figures have little to do with one another. As the RPM for torque and power come closer together the more that a change to one factor alters the other.

Now we get to the F1 engine. I once had a good source of F1 engine data which has since dried up. In fact the F1 guys keep data so close to their chests that few get a look at much real data. However one can make make some intelligent (??) estimates.

One of the interesting things with F1 is that for over the last dozen or so years the engineers were developing more and more power primarily by increasing the rev limits of the engines. A 5% increase of revs, even with a 2.5% loss of torque at those revs saw a power increase of 2.5%. Not that they weren’t chasing torque as well but revs as a route to power seemed to be the in thing. But now the rules have changed. They have to work within the constraint of software fixed limit of 18,000 RPM. The only way to now get more power is to increase torque.

So now we hear that the top engines develop about 850 HP. I have seen no data that tells us where that power comes but we can use known TE data to make a pretty good estimate. Peak F1 power is likely to be developed at between 16,500 and 17,000 RPM. A central figure of 16,750 is most likely. If any one out there has any better data lets here about it.

We can use the torque at peak power figure of 267 foot pounds and factor from some other known high output engines to make a peak torque estimate of an F1 engine today of between 275 and 280 foot pounds and further estimate that it comes at within 10% of peak power revs or over 15,250 revs.

The real situation with the torque and power peak revs being so close together is that you cannot alter the power at peak without having a direct influence on torque at peak and visa-versa.

Being locked into a max revs by regulations there is hardly anywhere for the F1 engine developers to go but to make discrete changes within the regs that result in more torque in the narrow rev band that they are locked into.

Hence my response to Paul Ransom, “Yes, most emphatically I think that they do work towards the biggest torque figure they can make!”

There are some other performance related things that I can point to but enough for now.

Regards

[Paul Ranson]

Torque is meaningless for our purposes (accelerating a car, for the record...) unless there is an associated rpm. In which case you are considering 'power'. I don't know why this causes so much controversy.

FWIW your torque figures look, I think, a little optimistic.

IIRC fuel defines an upper bound to torque and valve area to power. Folk memory tells me that if you see a claim of more than 90lbft/litre on pump fuel the nose should start twitching. Happy to be corrected on that.

Paul

[Greg Locock]

A minor linguistic quibble

“Torque – Obviously, the most important quantity to be measured is torque.”

That is correct. Dynamometers measure torque and rpm, not power. Is torque more important than ECT to the guy running the dyno? yes. Is it more important than power? We can't answer the question, the dyno doesn't measure power. Minor quibble, you could try and measure power directly, I don't think it would be very accurate.

So, if we are going to base a 100 post thread on that sentence I'll bow out now.

[Dmitriy_Guller]

Originally posted by Paul Ranson
Torque is meaningless for our purposes (accelerating a car, for the record...) unless there is an associated rpm. In which case you are considering 'power'. I don't know why this causes so much controversy.

Yep, that's all there is to it.

[gruntguru]

Originally posted by Joe Bosworth
There has started a side discussion that is really OT to the original topic so I thought I would start a new thread from that side discussion so that a discussion could take place, hopefully without the points scoring exercise that V8 – V10 has degenerated into.

Good luck!!!

Let’s start with a typical mundane go to market world vehicle, the VW Jetta. Its 2.0 litre engine has a technical efficiency, (TE), of 26.5%

TE is "Thermal Efficiency"

You ask what is this TE thing? Well, it is a measure of the max power the engine is rated at compared to how much power it could make if it got all of its swept capacity of inlet air in, (a 100% volumetric efficiency – VE), and was able to burn its fuel without any losses in the way of heat, friction, pumping losses and the like. I could use BMEP for this discussion but to most this is just some number that doesn’t mean much. By comparison, most people can visualise 26.5% efficiency as having an inefficiency of 73.5%.

Thermal efficiency has nothing to do with volumetric efficiency. Thermal efficiency is the ratio of crankshaft work output to the heating potential of the fuel going in - ie what percentage of the energy we put in is being converted to useful work (the rest will all be converted to waste heat in the exhaust or cooling system(s) and unburned fuel.

Volumetric efficiency indicates how much air the engine can breathe and therefore how much fuel it can burn. So you can see VE sets the upper limit for heat energy input and TE determines how much of that heat energy becomes useful work.

[gruntguru]

Originally posted by Paul Ranson
Folk memory tells me that if you see a claim of more than 90lbft/litre on pump fuel the nose should start twitching. Happy to be corrected on that.
Paul

Ahhh lbft/litre - sounds like BMEP to me - pity about the mixed units.

I wonder how many people make the connection between specific torque (torque per unit engine displacement) and MEP?

[Todd]

Originally posted by Paul Ranson
Torque is meaningless for our purposes (accelerating a car, for the record...) unless there is an associated rpm. In which case you are considering 'power'. I don't know why this causes so much controversy.

FWIW your torque figures look, I think, a little optimistic.

IIRC fuel defines an upper bound to torque and valve area to power. Folk memory tells me that if you see a claim of more than 90lbft/litre on pump fuel the nose should start twitching. Happy to be corrected on that.

Paul

If F1 engines are really producing 850 hp at 16,750 rpm, that would mean that they are producing 266.45 ft/lb of torque at that engine speed. If they're peaking at 17,600 rpm, then it would take about 254 ft/lbs of torque at 17,600 rpm. Maybe the torque numbers seem high, but then maybe F1 cars aren't really making that much power anymore.

[Paul Ranson]

Ahhh lbft/litre - sounds like BMEP to me - pity about the mixed units.

It's obviously equivalent. In the UK (at least) engines for the general public are characterised by BHP, lbft, and litres. When an engine builder is telling you a tall tale in a bar I doubt BMEP will come up. But lbft and BHP will.

Anyway according to the Peter Wright Ferrari F1 book the 2000 V10 had a max specific torque of about 84lbft/l. Which sanity checks. And suggests an upper bound for an F1 engine this year would be about 700BHP.

Paul

[gruntguru]

The following is a calculation to test the claims of 850 bhp @ 17,000 rpm for current F1 2.4 litre V8 engines. The objective is to estimate the Volumetric Efficiency (VE) and Thermal Efficiency (TE) required to acchieve these numbers.

Mass Airflow = Displacement/2 x RPM/60 x V.E. x Air Density Where:-

- Mass Airflow in kg/s
- Displacement in cu metre = 2.5/1000 (/2 in formula gives disp/rev for 4 stroke cycle)
- RPM = 17,000 (/60 converts to revs per second)
- V.E. = Volumetric efficiency = 1.15
- Air Density = 1.2 kg/cu metre

So airflow = 2.4/2000 x 17,000/60 x 1.15
= 0.391 kg/s

The maximum fuel that can be burned in this much air is 0.391/14.7 = 0.0266 kg/s (The engine would actually ingest a lot more fuel than this but all the surplus will pass through the exhaust unburned)

We can now calculate the power input to the engine (total heat energy) assuming a Heating Value for the fuel of 44 MJ/kg.
Heating Power in = fuel flow x H.V. = 0.0266 x 44 = 1.17 MJ/s (MW) (This is 1,170 kW or 1,560 Hp)
Of course only a fraction of this heat is converted to crankshaft power but in round terms, to achieve an output of 850 hp would require an efficiency of 850/1560 x 100 = 54.5%

This is an impossibly high number. Bear in mind that this is Oxygen efficiency not overall thermal efficiency since the air-fuel-ratio would likely be closer to 12:1 at peak power. Assuming 12:1 AFR gives an overall thermal efficiency of 54.5% x 12/14.7 = 44.5%. This is still implausible so either the engines are achieving a VE higher than 115% or the claims of 850 hp are exaggerated.

Essentially - for a 2.4 litre engine to make 850 hp at 17,000 rpm and 12:1 AFR it must have a TE x VE product of 0.51 (51%). So if we believe that 0.4 (40%) TE is possible, it would require a VE of 0.51/0.40=1.275 ie 127.5% for the 850hp @ 17,000 to be achieved.

[McGuire]

Originally posted by gruntguru
Ahhh lbft/litre - sounds like BMEP to me - pity about the mixed units.
I wonder how many people make the connection between specific torque (torque per unit engine displacement) and MEP?

It's rather more than a connection. BMEP is simply the gas pressure 1:1 equivalent of brake torque. Quoting a particular brake torque/liter rule of thumb is just another way of stating a BMEP rule of thumb. These modes of expression came to us straight from steam engines.

The quoted power figures for current F1 engines look pretty high to me too. However, it is an interesting fact that when you get into competitive theaters with regulated rev limits (direct or indirect) the BMEP levels soon rise beyond ye olde rules of thumb. IRL, touring cars, and sports cars are good examples. In many ways this brings race engine development more in line with production development, chasing efficiencies instead of simply wringing out another 300 rpm.

Side note apropos of not much: "Specific torque" and "specific power" originally and more accurately referred to these values with respect to bore area rather than displacement. (More steam age terms.) As such these expressions may have been of limited utility but at least they were unique. (Unlike "specific torque" which seems to be a coinage from the dept. of redundancy dept.) However, the original terms were corrupted by members of the British motoring press and the distinction was largely lost. No points for guessing who.

[NTSOS]

Originally posted by Joe Bosworth


NTSOS came back with, “Well generally speaking, yes and no!” NTSOS went on to explain the yes part with a discussion about engine mapping on a dyno.

I would like to come back now with my clarification of NTSOS’ post and say yes and yes and to explain why.

Hi Joe,

But I think you missed the reason for the "no" part!

Please read the following very carefully:

For instance, the pulsewidth (fuel/air) is first increased. If the torque increases too, the pulsewidth is increased further until the torque starts decreasing. At this point, the adjustment direction is reversed and pulsewidth is decreased to reach the torque peak value, and then a bit further until a very small drop from the maximum torque is obtained. This limit value is the optimum because with it, maximum torque is obtained with mininum fuel consumption."

"until a very SMALL DROP from the maximum torque is obtained. This limit value is the optimum because with it, maximum torque is obtained with mininum fuel consumption."

John

[McGuire]

Originally posted by NTSOS

Please read the following very carefully:

"until a very SMALL DROP from the maximum torque is obtained. This limit value is the optimum because with it, maximum torque is obtained with mininum fuel consumption."
John

Yep, this method of fuel and spark calibration is known as MBT/LBT. Mean Best Torque aka Max Best Torque and Lean Best Torque. (You could convert these values to an equivalent power value, thereby rendering them meaningless.) We also have RBT, Rich Best Torque, and RBP, Rich Best Power. All this is based on the reality that stoichiometry (~14.68:1 AFR depending on fuel) is ideal for emissions, being the sweet spot in minimizing CO, HC, and NOx, but richer is better for power (~12.5:1 or better) and leaner for economy (~15:1+). RBP is slightly richer than RBT.

The critical factors in getting there with each AFR include ignition timing to obtain peak pressure at 14-15 degrees ATDC, and obviously mixture strength is critical to flame speed. Meanwhile the piston is running away from the flame and we need full combustion before expansion phase gets away. In addition to the standard theta depiction for cylinder pressure vs. crank position, we can also plot MFB, mass fraction burned, referencing CA50, or crank angle at 50 percent burn. CA50 needs to arrive 4 to 12 degrees or so before peak due to the lag between heat release and pressure rise.

In road tuning, MBT is often best for idle (10 to 20 percent load) for smoothness with AC and PAS engaged and crisp transition. LBT is usually acceptable for light cruise at constant speed (around 30-40 percent load). Above 60 percent load with a big TPS input you can assume major acceleration is being called for, so you can throw down RBT and into RBP. WFOT at max acceleration might be 80 to 90 percent load for NA, over 100 for boosted engines.

[NTSOS]

Originally posted by McGuire


Yep, this method of fuel and spark calibration is known as MBT/LBT.

Spend lot's of time around dynos, do ya Mac?

BTW, gotta love this photo:



John

[J. Edlund]

Originally posted by Joe Bosworth
So now we hear that the top engines develop about 850 HP. I have seen no data that tells us where that power comes but we can use known TE data to make a pretty good estimate. Peak F1 power is likely to be developed at between 16,500 and 17,000 RPM. A central figure of 16,750 is most likely. If any one out there has any better data lets here about it.

If you decrease the power figure to 750 hp and bump up the peak power speed to 17500 rpm you probably have more realistic figures. That would translate into 300 Nm at peak power and thus a bmep of 15.7 bar.

[gruntguru]

Originally posted by J. Edlund


If you decrease the power figure to 750 hp and bump up the peak power speed to 17500 rpm you probably have more realistic figures. That would translate into 300 Nm at peak power and thus a bmep of 15.7 bar.

Agreed.

[xxchrisxx]

I get a bmep of about 15.5bar, so close enough I should think. I love torque-power debates, the're just so redundant. Anyway i'll leave the squabbles for the other thread.