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Turboivo
Joined: 09 Mar 2006
Posts: 669
Location: Bulgaria
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 Posted: Mon Aug 08, 2011 7:50 am Post subject: Let's talk VE and maf estimation |
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Hello all,
VE and MAF estimations are often source of huge disputes and fights over the forums, especially when inflated power figures have to be proved. The need of MAF estimation is because usually no one is using MAF meter in his high power application. Recently I got myself involved in similar disput and still can't be 100% sure what's correct.
So, about the VE which states "VOLUMETRIC". 100%VE means 0.5L cyl swept volume is filled with 0.5L air. Some highly tuned NA engines are quoted with 110% VE though. OK, we know about the inertia/resonance filling. My understanding is that "volume" VE is still 100% but the "MASS" VE is 110% and we have ingested more air by weight (i.e. kind of natural boost).
Looking at the Garret's website, in the "Turbo Tech 103 (Expert)" we see the formula for MAF estimation:
Wa=(MAP*VE*N/2*Vd)/(R*(460+Tm))
where Wa is the airflow in lb/min.
If my thinking of VE is correct (I mean volume and mass VEs) which VE is in this formula??? Why!? Here is why....If we assume that this is the "volume" VE with max 100%(1.0) and 40psi absolute MAP we'll get... some number for mass airflow (and respectively guessed HP level for that flow).
If we assume that this is the "mass" VE with max 110% (1.1) and 40psi absolute, we'll get higher MAF number and respectively proof for inflated power numbers. I've seen quoted VE up to 180%!!??
Hope it is understandable 
Thanks! |
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Matt Cramer
Joined: 25 Sep 2007
Posts: 274
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| Posted: Mon Aug 08, 2011 9:34 am Post subject: |
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You would use the mass VE in that equation.
A "VE" number over 180% sounds as if it's off an ECU that uses a "VE" based fuel table but isn't correcting for air density.
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Aurélien
Joined: 30 Jun 2009
Posts: 137
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| Posted: Tue Aug 09, 2011 7:51 am Post subject: |
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Hum.
100% VE means you fill your 0.5L cylinder with 0.5L of air in the intake manifold.
You will ALWAYS fill your cylinder with 0.5L ! You can't change that !
VE = Mass air in the cylinder / ( Displaced volume * Air density in the manifold )
You can also rewrite this equation with airflow (g/s) and engine speed. |
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sam@tdi
Joined: 11 Sep 2006
Posts: 648
Location: London, England
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| Posted: Tue Aug 09, 2011 8:17 am Post subject: |
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| Aurélien wrote: | Hum.
100% VE means you fill your 0.5L cylinder with 0.5L of air in the intake manifold.
You will ALWAYS fill your cylinder with 0.5L ! You can't change that !
VE = Mass air in the cylinder / ( Displaced volume * Air density in the manifold )
You can also rewrite this equation with airflow (g/s) and engine speed. |
Errr have I missed something? what you just described would be MASSemetric efficiency.
VE stands for Volumetric Efficiency
And yes through creative uses of intake/exhaust gas inertia and valve overlap you can certainly achieve volume displacement higher than that of the swept volume, I.E greater than 100% VE, but 180%! only if you measure before a turbo and the system has a leak maybe 
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Turboivo
Joined: 09 Mar 2006
Posts: 669
Location: Bulgaria
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| Posted: Tue Aug 09, 2011 12:57 pm Post subject: |
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May be the problem lies In the kind of VE is used in that formula. As I 've found
VEboosted=VEunboosted*((boost gauge pressure/atmo pressure)+1)^(1/1.7)
i.e. 180% for boosted VE is normal value at about 25psi boost if we measure at the turbo inlet instead of inlet manifold. So, if we put these 180% or 1.8 into the mentioned formula we'll get enormous amount of mass air flow. Imho that is because of double accounting of boost pressure, first time by VE boosted and second time by MAP value.
My conclusion is that correct VE value used should be the VE unboosted, i.e. N/A VE. Am I correct ?
I believe that all power numbers should have some approx physical and math. confirmation/explanation. What do you all think about these results:
EVO8 2.0L(or 2.2) engine highly tuned, 814hp @ 11300rpm, 15psi boost, stock fuel.
I just cant find any reasonable math behind these numbers. What about you? |
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Liborek
Joined: 10 Apr 2010
Posts: 10
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| Posted: Tue Aug 09, 2011 4:42 pm Post subject: |
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| sam@tdi wrote: | | And yes through creative uses of intake/exhaust gas inertia and valve overlap you can certainly achieve volume displacement higher than that of the swept volume, I.E greater than 100% VE |
Well, air will always fill entire cylinder and combustion chamber volume, so IMHO, VE can only reach certain numerical value based on sum of swept volume and combustion chamber volume. Clearly this isn´t what is used in all these formulas.
What is considered as VE in these equations is density ratio between cylinder and intake manifold - plenum. Or at least this is what I found in most documents regarding "volumetric efficiency" |
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Liborek
Joined: 10 Apr 2010
Posts: 10
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| Posted: Tue Aug 09, 2011 5:08 pm Post subject: |
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| Turboivo wrote: | I believe that all power numbers should have some approx physical and math. confirmation/explanation. What do you all think about these results:
EVO8 2.0L(or 2.2) engine highly tuned, 814hp @ 11300rpm, 15psi boost, stock fuel.
I just cant find any reasonable math behind these numbers. What about you? |
What do you mean by stock fuel?
Is it some sort of claim or has it some build reference? I saw something similar on evolution forum... |
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RootesRacer
Joined: 04 Apr 2005
Posts: 460
Location: Arvada, Colorado
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| Posted: Tue Aug 09, 2011 5:18 pm Post subject: |
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| Liborek wrote: | | sam@tdi wrote: | | And yes through creative uses of intake/exhaust gas inertia and valve overlap you can certainly achieve volume displacement higher than that of the swept volume, I.E greater than 100% VE |
Well, air will always fill entire cylinder and combustion chamber volume, so IMHO, VE can only reach certain numerical value based on sum of swept volume and combustion chamber volume. Clearly this isn´t what is used in all these formulas.
What is considered as VE in these equations is density ratio between cylinder and intake manifold - plenum. Or at least this is what I found in most documents regarding "volumetric efficiency" |
There is more to it then that.
The cam timing and induction inertial effects will effect the ability for the cylinder to fill.
Think of it like this, at STP, a 500cc swept volume would like to fill that combustion chamber to 500 standard cc (at a higher pressure), but the valve overlap prevents some of that swept volume from compressing to the theoretical compression pressure. The ratio of the compression pressure to the (barometric pressure * CR) is VE (IMO).
Like for instance a 10/1 CR should yield 10bars of compression pressure at 100% VE. If you net only 5 bars, your VE would be 50%.
Resonance can allow the actual compression pressure to exceed the theoretical compression pressure, this is a VE of greater than 100%. |
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Power-Tripp
Joined: 26 Nov 2008
Posts: 311
Location: Alabaster, AL, USA
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| Posted: Tue Aug 09, 2011 6:13 pm Post subject: |
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Roots,
There are a couple of issues with your VE theory.
If you look at a "500cc" cylinder with 2:1 static compression ratio, and then increase compression to 12:1, the piston is actually taking up cylinder volume.
Yes, cylinder VE is altered by resonance, but inertia plays a part too. And this part becomes a bigger and bigger part, as rpm climbs.
VE is like BSFC, a poor way to do much more than come up with an approximation. If you want to KNOW what is actually occurring, the best we can do is to measure actual cylinder pressures in the running engine in crank degrees. In the near future, this will make dynos obsolete.
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RootesRacer
Joined: 04 Apr 2005
Posts: 460
Location: Arvada, Colorado
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| Posted: Tue Aug 09, 2011 6:46 pm Post subject: |
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| Power-Tripp wrote: | Roots,
There are a couple of issues with your VE theory.
If you look at a "500cc" cylinder with 2:1 static compression ratio, and then increase compression to 12:1, the piston is actually taking up cylinder volume.
Yes, cylinder VE is altered by resonance, but inertia plays a part too. And this part becomes a bigger and bigger part, as rpm climbs. |
You are going to have to splain the 2:1, 12:1 CR and how the piston is taking up volume.
To me, static CR = (Combustion volume + swept volume)/Combustion volume.
As I said, IMO VE is the ratio in compression pressure to the barometric, I dont see it getting much simpler than that. |
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Aurélien
Joined: 30 Jun 2009
Posts: 137
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| Posted: Wed Aug 10, 2011 12:24 am Post subject: |
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| Power-Tripp wrote: | Roots,
There are a couple of issues with your VE theory.
If you look at a "500cc" cylinder with 2:1 static compression ratio, and then increase compression to 12:1, the piston is actually taking up cylinder volume.
Yes, cylinder VE is altered by resonance, but inertia plays a part too. And this part becomes a bigger and bigger part, as rpm climbs.
VE is like BSFC, a poor way to do much more than come up with an approximation. If you want to KNOW what is actually occurring, the best we can do is to measure actual cylinder pressures in the running engine in crank degrees. In the near future, this will make dynos obsolete. |
For sure in cylinder pressure sensor + angular encoder is a really good tool.
But currently expenssive.  |
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Aurélien
Joined: 30 Jun 2009
Posts: 137
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| Posted: Wed Aug 10, 2011 12:36 am Post subject: |
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| sam@tdi wrote: | | Aurélien wrote: | Hum.
100% VE means you fill your 0.5L cylinder with 0.5L of air in the intake manifold.
You will ALWAYS fill your cylinder with 0.5L ! You can't change that !
VE = Mass air in the cylinder / ( Displaced volume * Air density in the manifold )
You can also rewrite this equation with airflow (g/s) and engine speed. |
Errr have I missed something? what you just described would be MASSemetric efficiency.
VE stands for Volumetric Efficiency
And yes through creative uses of intake/exhaust gas inertia and valve overlap you can certainly achieve volume displacement higher than that of the swept volume, I.E greater than 100% VE, but 180%! only if you measure before a turbo and the system has a leak maybe |
This is how heywood define volumetric efficiency.  |
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sam@tdi
Joined: 11 Sep 2006
Posts: 648
Location: London, England
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| Posted: Wed Aug 10, 2011 1:37 am Post subject: |
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I don't know who Heywood are, sorry
But I do know the meaning of the word volume in the scientific sense, and it's an expression of "spacial dimension", a chunk of space ....
Volume is a size, not an amount.
Example, it's possible to accurately express the exact "Volume" of the room that you currently sit in, WITHOUT mentioning whether or not there is anything in it.
The internal volume of a box is simple to calculate W x D x H =Volume;
for instance 2m x 2m x 2m = 8 cubic meters.
The moment you introduce something filling the volume, be it a liquid or a gas and then attribute a "density" or "mass" to that substance you are talking about "an amount" of something and immediately you have moved on from talking about volume to something else entirely.
I can't think of any other way of making this clearer.
I think the title of the thread is a little confused also, if you know MAF and you know swept volume then it's VE which your are estimating not MAF.
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Sam Borgman
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Turboivo
Joined: 09 Mar 2006
Posts: 669
Location: Bulgaria
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| Posted: Wed Aug 10, 2011 3:54 am Post subject: |
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| Liborek wrote: | What do you mean by stock fuel?
Is it some sort of claim or has it some build reference? I saw something similar on evolution forum... |
There was no additional info but I guess 100RoN from the gas station at least. Yes, it is from that forum and quoted numbers and dyno graphs are for engine power I hope. |
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Power-Tripp
Joined: 26 Nov 2008
Posts: 311
Location: Alabaster, AL, USA
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| Posted: Wed Aug 10, 2011 4:27 am Post subject: |
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Most do not realize that a 2.0L 4-cylinder engine doesn't always actually have cylinders with 500cc of displacement.
The problem with the way most calculate VE, or displacement, is based on calculated volume (not actual measured volume). The problem is that the actual cylinder volume is not what many think it is. Do not confuse bore and stroke calculations with actual cylinder volume or displacement.
| RootesRacer wrote: |
You are going to have to splain the 2:1, 12:1 CR and how the piston is taking up volume.
To me, static CR = (Combustion volume + swept volume)/Combustion volume.
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My example of 2:1 -vs- 12:1 is a bit extreme. Let's make this more realistic.
Take a cylinder where we want to increase static compression from 8:1 to 12:1. We are using a fixed ring pack location, wrist pin location, and can only vary gasket and chamber volume by a small amount. Lets say that to produce a static compression ratio of 8:1 in a given cylinder, you need a piston with a dish volume of 20cc. And to get a compression ratio of 12:1 we need a dome on the piston with a -20cc volume. This alone changes volume in the cylinder by 40cc.
[My 2:1 compression example would make this considerably worse.]
By increasing the compression ratio, we are forced to add volume to the piston that is taking up cylinder volume. If we cut the cylinder head, or use a thinner head gasket, again we are reducing cylinder volume. This is actual volume that is changing.
How does this alter your calculation of VE, since actual volume has changed?
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"To achieve anything in this game you must be prepared to dabble in the boundary of disaster." -Sterling Moss |
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Aurélien
Joined: 30 Jun 2009
Posts: 137
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| Posted: Wed Aug 10, 2011 5:24 am Post subject: |
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| sam@tdi wrote: | I don't know who Heywood are, sorry
But I do know the meaning of the word volume in the scientific sense, and it's an expression of "spacial dimension", a chunk of space ....
Volume is a size, not an amount.
Example, it's possible to accurately express the exact "Volume" of the room that you currently sit in, WITHOUT mentioning whether or not there is anything in it.
The internal volume of a box is simple to calculate W x D x H =Volume;
for instance 2m x 2m x 2m = 8 cubic meters.
The moment you introduce something filling the volume, be it a liquid or a gas and then attribute a "density" or "mass" to that substance you are talking about "an amount" of something and immediately you have moved on from talking about volume to something else entirely.
I can't think of any other way of making this clearer.
I think the title of the thread is a little confused also, if you know MAF and you know swept volume then it's VE which your are estimating not MAF. |
By the way it's a very good book. 
I absolutely understand what your mean, but the fact is that VE is defined just like i said.
Every engineer use that formula, no matter the name is good or what, this is what VE is. |
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Power-Tripp
Joined: 26 Nov 2008
Posts: 311
Location: Alabaster, AL, USA
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| Posted: Wed Aug 10, 2011 6:10 am Post subject: |
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| Aurélien wrote: |
By the way it's a very good book.
I absolutely understand what your mean, but the fact is that VE is defined just like i said.
Every engineer use that formula, no matter the name is good or what, this is what VE is. |
Yes, a very good book.
Just because everyone does it this way, does not mean that this is the best way. Every so often, reality steps up and slaps you in the face.
Something to consider: http://www.tfxengine.com/
The pricing is not as bad as you would think. Under USD $10K will get you started.
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"To achieve anything in this game you must be prepared to dabble in the boundary of disaster." -Sterling Moss |
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sam@tdi
Joined: 11 Sep 2006
Posts: 648
Location: London, England
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| Posted: Wed Aug 10, 2011 6:28 am Post subject: |
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| Aurélien wrote: |
By the way it's a very good book.
I absolutely understand what your mean, but the fact is that VE is defined just like i said.
Every engineer use that formula, no matter the name is good or what, this is what VE is. |
Fair enough, and I'm sure other info in the book is correct and helpful.
I just have a hard time accepting that any person educated in the sciences would confuse volume and mass, it's pretty fundamental.
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Sam Borgman
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sam@tdi
Joined: 11 Sep 2006
Posts: 648
Location: London, England
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| Posted: Wed Aug 10, 2011 6:29 am Post subject: |
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| Power-Tripp wrote: |
Just because everyone does it this way, does not mean that this is the best way. Every so often, reality steps up and slaps you in the face. |
Amen to that, days like that are the interesting ones
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Sam Borgman
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Liborek
Joined: 10 Apr 2010
Posts: 10
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| Posted: Wed Aug 10, 2011 6:54 am Post subject: |
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| Turboivo wrote: | EVO8 2.0L(or 2.2) engine highly tuned, 814hp @ 11300rpm, 15psi boost, stock fuel.
I just cant find any reasonable math behind these numbers. What about you? |
Well, engine would have to be highly developed to produce very high VE(whatever it is ), basically BMEP at atmosferic filling would have to hower around 240 psi at such speed. Going by combination of parts they use, they surely have potential to produce some very high numbers, but on the other hand, timeslips doesn´t indicate as much power as they are claiming, folowed by excuses that combination in car isn´t good for drag  |
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