19 replies to this topic

[shaun979]

Which of the following is correct?

BMEP = IMEP - FMEP - PMEP

or

BMEP = IMEP - FMEP

I seem to come across these two definitions and am not entirely sure if PMEP is already included in FMEP or not.

=======

Slight digression..is this following statement correct?

"Peak volumetric flow through a 4 stroke SI IC gasoline engine can be at the point of peak power, redline, or anywhere in between these two points."

At first it would seem that at peak power would be the point of peak volumetric flow, however in the case of an extremely high revving engine that had very high FMEP, isn't is possible that all of the additional airflow beyond the point of peak power is being consumed as FMEP, so it doesn't yield higher BMEP?

[Greg Locock]

Second bit, no.

On typical engines peak torque is very close to peak VE. But your first equation demonstrates why this is not quite true, and we could fudge the friction curve to make it totally untrue.

[McGuire]

Originally posted by shaun979
Which of the following is correct?

BMEP = IMEP - FMEP - PMEP

or

BMEP = IMEP - FMEP

I seem to come across these two definitions and am not entirely sure if PMEP is already included in FMEP or not.

Well, they are both correct in context... though they do seem mutually contradictory in that two definitions of FMEP would seem to be in circulation: one which regards PMEP in separate category and one which does not.

In the second statement we can probably assume PMEP is still there; it just hasn't been broken out of the FMEP value. Which is technically correct in that PMEP is a kind of FMEP: air friction. And until recently it wasn't such an accessible figure. I guess this is another case where in order to understand the info we must regard the context.

Originally posted by shaun979
Slight digression..is this following statement correct?

"Peak volumetric flow through a 4 stroke SI IC gasoline engine can be at the point of peak power, redline, or anywhere in between these two points."

At first it would seem that at peak power would be the point of peak volumetric flow, however in the case of an extremely high revving engine that had very high FMEP, isn't is possible that all of the additional airflow beyond the point of peak power is being consumed as FMEP, so it doesn't yield higher BMEP?

The statement is absolutely correct in principle, and so is your example. There are your apparent special cases...like if you artificially impose an extreme airflow restriction to high rpm. Such as the NASCAR carburetor restrictor plate, where four holes roughly the size of US quarters strangle an 850 hp, 9000 rpm engine down to 425 hp at 6400 rpm. We can assume that peak power occurs at around maximum net airflow but we have done a number of other things of interest to the engine as well. With plate engines, suddenly everyone is interested in frictional losses and you can see why. (Dan Davis of Ford calls these the most expensive 425 hp engines in the world and you can see why that is too.)

Your example is entirely plausible, especially when we depart from paper engines and get into the real ones. There was a time not so long ago when an engine might be perfectly capable of 8500 rpm in flow volume but the ignition system was all done at 6500 rpm and combustion started falling off. Or the fuel distribution started to go to hell. Or the crankshaft started to wobble and eat itself up on the bearings. I am not involved in current F1 engine development but I expect not much has really changed there, only the numbers.

* * *

And we have to distinguish between volumetric flow and volumetric efficiency, which are two rather different things. If you put aside exhaust scavenge tuning, wave inlet tuning, and air speed-density (ram air) charging, torque and BMEP will track the Ve curve. At peak Ve the engine will be pumping its maximum air volume per crankshaft rotation. And if everything else goes right, there the engine will produce peak BMEP and torque.

However, with respect to horsepower, what is relevant is not maximum flow volume per crank rotation but per unit of time. The foregoing paragraphs nothwithstanding, an engine is "just" an air pump and while Ve is certainly a factor, it's still a constant-displacement machine. The faster you turn it the more air it will pump per unit of time. Let's say a 350 CID engine produces peak torque and 100% Ve at 5000 rpm, and peak hp at 8000 rpm, at 90% Ve. So at 100% Ve, it's pumping 506 cubic feet of air per minute. But at max hp, it's still pumping 730 cubic feet per minute even though Ve is 90% and falling.

And HP is the quanitity of torque produced per minute. When the engine can't produce any more torque per minute, that is the rpm of peak horsepower, he said redundantly. In a nutshell, there is the relationship between Ve, MEP and torque; air flow rate and horsepower; and torque, horsepower and rpm.

[shaun979]

Originally posted by Greg Locock
Second bit, no.

On typical engines peak torque is very close to peak VE. But your first equation demonstrates why this is not quite true, and we could fudge the friction curve to make it totally untrue.

Thanks Greg. I was referring however, to peak volumetric flow, not peak volumetric efficiency, as McG has differentiated between.

[shaun979]

Originally posted by McGuire


Well, they are both correct in context... though they do seem mutually contradictory in that two definitions of FMEP would seem to be in circulation: one which regards PMEP in separate category and one which does not.

In the second statement we can probably assume PMEP is still there; it just hasn't been broken out of the FMEP value. Which is technically correct in that PMEP is a kind of FMEP: air friction. And until recently it wasn't such an accessible figure. I guess this is another case where in order to understand the info we must regard the context.

Hmm yes, I just wonder what the official SAE definition (in the context of the usual gasoline 4st SI engines) is though- just so in the future I can be technically correct.

The statement is absolutely correct in principle, and so is your example. There are your apparent special cases...like if you artificially impose an extreme airflow restriction to high rpm. Such as the NASCAR carburetor restrictor plate, where four holes roughly the size of US quarters strangle an 850 hp, 9000 rpm engine down to 425 hp at 6400 rpm. We can assume that peak power occurs at around maximum net airflow but we have done a number of other things of interest to the engine as well. With plate engines, suddenly everyone is interested in frictional losses and you can see why. (Dan Davis of Ford calls these the most expensive 425 hp engines in the world and you can see why that is too.)

It is exactly restrictor plate engines that started me along mentioned line of thought because I had heard that at higher RPM and increased draw frequency on the SEOs, that more air could actually be flowed but that power would not benefit because it was all getting consumed as friction at that RPM. So then it followed that the same situation might be encountered with unrestricted but very high RPM engines.

Your example is entirely plausible, especially when we depart from paper engines and get into the real ones. There was a time not so long ago when an engine might be perfectly capable of 8500 rpm in flow volume but the ignition system was all done at 6500 rpm and combustion started falling off. Or the fuel distribution started to go to hell. Or the crankshaft started to wobble and eat itself up on the bearings. I am not involved in current F1 engine development but I expect not much has really changed there, only the numbers.

I hadn't previously thought of the situations that you mention - ignition and fuelling accuracy, component deformation - affecting output. Thanks for the insight!

And we have to distinguish between volumetric flow and volumetric efficiency, which are two rather different things. If you put aside exhaust scavenge tuning, wave inlet tuning, and air speed-density (ram air) charging, torque and BMEP will track the Ve curve. At peak Ve the engine will be pumping its maximum air volume per crankshaft rotation. And if everything else goes right, there the engine will produce peak BMEP and torque.

While most engines would have peak BMEP at or close to peak VE, since BMEP (torque) = IMEP - FMEP, then isn't it possible in some cases (say very high RPM applications that have tracts and valvetrain setup for peak charging efficiency at very high RPM as well) that BMEP might not be highest at peak VE? In other words, IMEP may be peak, but FMEP at that RPM may be so large that BMEP isn't peak. I think Fat Boy mentioned this recently.

However, with respect to horsepower, what is relevant is not maximum flow volume per crank rotation but per unit of time. The foregoing paragraphs nothwithstanding, an engine is "just" an air pump and while Ve is certainly a factor, it's still a constant-displacement machine. The faster you turn it the more air it will pump per unit of time. Let's say a 350 CID engine produces peak torque and 100% Ve at 5000 rpm, and peak hp at 8000 rpm, at 90% Ve. So at 100% Ve, it's pumping 506 cubic feet of air per minute. But at max hp, it's still pumping 730 cubic feet per minute even though Ve is 90% and falling.

And HP is the quanitity of torque produced per minute. When the engine can't produce any more torque per minute, that is the rpm of peak horsepower, he said redundantly. In a nutshell, there is the relationship between Ve, MEP and torque; air flow rate and horsepower; and torque, horsepower and rpm.

Thanks for taking the time to explain I did already understand this, but it is always refreshing to read a new example.

[Greg Locock]

Ooops. Good. That's all sorted.

SAE J245 says imep=bmep+fmep, in 1976, and that's the way we were told it, because we learnt about that stuff on steam engines, which don't have very meaningful pumping losses.

Also see

http://me.engin.umic...be/P2000_08.PDF

[shaun979]

That PDF should be added to the PDF sticky list at the top of the Tech Forum! Anyone have anymore that are related to engines especially racing engines?

[desmo]

Done. If anyone is aware of any interesting papers online I'd be thrilled to add them to the list.

[shaun979]

Thank you Desmo

About a month ago J.Edlund posted these two that are on the turbo era F1 engines.
http://users.du.se/~...ohed/870701.pdf
http://users.du.se/~...ohed/890877.pdf

Greg posted this one too.. http://www.diva-port...1__fulltext.pdf
Description here.. http://forums.autosp...&threadid=82583

[McGuire]

Originally posted by shaun979
While most engines would have peak BMEP at or close to peak VE, since BMEP (torque) = IMEP - FMEP, then isn't it possible in some cases (say very high RPM applications that have tracts and valvetrain setup for peak charging efficiency at very high RPM as well) that BMEP might not be highest at peak VE? In other words, IMEP may be peak, but FMEP at that RPM may be so large that BMEP isn't peak. I think Fat Boy mentioned this recently.

Thanks for taking the time to explain I did already understand this, but it is always refreshing to read a new example.

That is entirely right, and surely your focus in exploring FMEP vs. PMEP. I knew you knew all this; I was sort of speaking to the room. Or perhaps just jabbering on for no reason at all, as I am known to do.

[shaun979]

Thanks and have a great time in Australia

[TestaRoasta]

Google's not helping me with FMEP or PMEP, could someone enlighten me?

[NTSOS]

Try these in Google:

friction mean effective pressure

pumping mean effective pressure

indicated mean effective pressure

brake mean effective pressure

John

Althea's Garage

[TestaRoasta]

Thank you. Just the anachronyms got me the "Foundation for Middle East Peace" and "International Measurement Evaluation Programme"

[Powersteer]

Some fantastic parts, fabrication and welds there NTSOS. All that done at home????? That exhaust manifold look factory.

[NTSOS]

Wish I could claim credit for the exhaust/welds and bare chassis, but the exhaust was done by a guy named Chris at CPR Fabrications and of course the C4 chassis conversion is from Car Creations.

Didn't you like the cat?

John

[Powersteer]

If that cats scratching, I would not want to be the carpet. Was it a subject of growth hormones?? Is that a conversion kit to independent rear suspension?

EDIT: The shaft looks carbon fiber

[NTSOS]

Was it a subject of growth hormones??

Good question......they swear it's a stock, unmodified fat cat!

EDIT: The shaft looks carbon fiber

Yes....drive shaft also.

Is that a conversion kit to independent rear suspension?

Yes, a complete front and rear C4 conversion.

Paul Newman - Car Creations

John

[McGuire]

Originally posted by NTSOS
Althea's Garage

That is beautiful fabrication work, NTSOS. Really first rate. Many cars are not as well built as your puteroner. Someone said the exhaust was good as factory. If anybody is making anything that nice, put it in a museum.

It's almost a shame to put a body on the thing. Maybe just stick a seat on it and drive it around, it's so pretty.

[NTSOS]

Hey Mac....thanks so much for the kind words, that means alot....especially now as I age off into oblivion whilst trying to pretend that building a hotrod is still a painless venture and well worth the effort.

John