EFI University

SunsetMoto · Joined: 29 Mar 2007 Posts: 12 Location: UK

Volumetric efficiency can be defied as:

(Mass flow rate of air into cylinder)/(Mass of air contained in swept volume (at reference temperature and pressure) x unit time)

Its a measure (estimate) of the breathing efficiency of the engine at some point in the engine operating range. Predicting it is not an exact science as there are many influences that change throughout the working range of the engine.

Using unity brackets we can resolve the equation from mass to volume and eventually a dimensionless value that represents efficiency

There are many ways of describing most things and as long as all the interested parties in a conversation or exchange understand the context of the information and the units used, does it matter?
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sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

Turboivo · Joined: 09 Mar 2006 Posts: 669 Location: Bulgaria

OK, I'm starting to think that VE in terms of volumes (not mass) is meaningless at least. I've met somewhere and I'm agree that actually there is no other value except 100% VE (volumes). What I mean is if we let some small volume of air into cylinder it will spread all over the cyl volume, not just in part of it!? Am I correct?

From what I saw here, VE could be also a measure of engine breathing restrictions and inertia/impulse tuning.
So, if we have a highly tuned N/A engine with 110%VE (mass). Then we turbocharge it with 2bar (gauge, not absolute), does that means that VE also jumps to almost 300% (assuming very efficient cooler and turbo), OR VE is still 110%?
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Power-Tripp · Joined: 26 Nov 2008 Posts: 311 Location: Alabaster, AL, USA

Let's make this simple.
Without measuring pressures in each cylinder, think or air and fuel in pounds per hour. Everything else will fall into place.
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MarcoV6T · Joined: 09 Jan 2007 Posts: 159 Location: Belgium, Brussels

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

SunsetMoto · Joined: 29 Mar 2007 Posts: 12 Location: UK

[quote="MarcoV6T"]

you need to suck some mass into it, if you want to calculate volumetric efficiency.

I agree.

Lets assume a cylinder with a swept volume of 500cc (I work in SI units wherever possible) at standard temperature and pressure (STP) which is 20 deg C and 100kPa (or 101.3 if you want to be pedantic) - not to be confused with normal temperatuire and pressure which can have different temperature constant depending on your geography (15degC in the UK). We will assume the cylinder is rigid and is operating under adiabatic conditions so will have a constant volume of 500cc. We will also assume that we are pumping air with no fuel or exhaust end gas present to complicate things. Density of air at STP is 1.2 kg per cubic meter or 0.0012 kg per liter so the contents of our cylinder weigh 0.6 g. Now supose that the pressure in the inlet manifold is 75kPa due to throttling at the instant the inlet valve closes (this point may seem unimportant but is significant) - we will assume that the temperature has stayed constant at 20degC. the 500cc cylinder now contains a mass of 0.45g. Now if we calculate the volume of cylinder that would be needed to contain air with a mass of 0.45g at STP we find that it is 375cc. If we divide 375 by 500 and multiply by 100 we get 75 which is a VE of 75%.

Hope this helps make sense of the term VE

VE is just an measure of the breathing efficiency of the engine and because it is a derived perameter there can be errors in estimation. In an engine management system it can be used as a numerical scalar against injector opening time to determine pulse width, doesn't really matter if its not absolutely accurate as long as we remember bigger numbers mean more fuel and visa-versa. Its also a usefull perameter to measure changes made to inlet and exhaust in an engine simulation program but that's not realy relevent to this thread.

I agree with Sam@TDI that we can't interchange volume and mass, the point of my comment on all parties understanding information comes from working with racers who may not have the advantage of an engineers vocabulary and I find it easier to adapt my understanding to their description than to insist they use the correct term.

I'm all for being pedantic (I've learnt to be a pragmatist as well) but it can sometimes lead to procrastination and as we all know procrastination is the thief of time - the guy with the chequered flag doesn't care anyway!!

Sean
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Turboivo · Joined: 09 Mar 2006 Posts: 669 Location: Bulgaria

And I still find (over the net) huge misunderstandinding of VE therm/meaning. Even in some popular books I see VE comments that state something like: " boosting an engine rises its VE" or " ...a 15 psi boost actually doubles your VE to almost 200%..."

What I meant in the begining of this thread was the wrong airflow(mass) estimation using wrong/doubled VE values.

Mass airflow =(MAP *VE*rpm/2*Vd) / (R*(460+Tint.manifold)
So having 29,7psia (15psi boost) and 2 for VE (200% /100) they are actually doubling their results for air mass flow. This way all kind of inflated dyno results could be explained.

Tomak · Posted: Wed Dec 21, 2011 2:09 pm Post subject:

I didn't read the thread, but you would in fact use 110% (the engines NA VE).

VE is VOLUME. If it's mass based, most modern references call that load (measured mass divided by calculated displaced mass at stp)

So, how can you have 110% volume? Easy, you are increasing density.

Standard VE assumes air at standard temperature and pressure.

Picture a CFM meter at the inlet of and NA engine (ONLY think about it this way in NA). At some point around peak torque it can in fact show that a 500cc NA engine ingested 550 cfm air at stp. However, what ends up IN the cylinder is still obviously 500 cc, but at elevated pressure.

So think of VE as an NA engines capacity to move volume, VE on a boosted engine is rarely higher than a NA engine ( a turbo can slightly lift VE at higher rpm).

ECU software never uses VE for fuelling. The VE curve is one input, then the ecu uses pressure and temperature to convert this flow into a mass. A true "VE" based software will never deviate much in terms of numbers from the 100kpa line to the 200 kpa line.

As an example, if its 110% VE at 4500 rpm, and 97% at 6500 rpm NA (100 kpa), then at say 200 kpa, you MAY see 110 % VE at 4500, and maybe 105 at 6500).

That's "true" VE.

I have seen crappy software where the "VE" table would need to actually double at double the pressure. This is incorrect in the truest sense.

This is probably one reason OEM manufacturers went to pure mass based systems. Less confusion lol.

I usually do it the quick and dirty way. mass = Pressure ratio x cfm of engine (na) x .0764 lb/cf (denisty of air at stp) x (460+60)/(460+T))

Where CFM is displacement (cubic inches) x (rpm/2) / 1728 x VE (na)....

In other words, calculate the mass the NA engine moves, then multiply it for pressure ratio, and correct for drop in density due to temperature rise.
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Turboivo · Joined: 09 Mar 2006 Posts: 669 Location: Bulgaria

So your opinion is that generally VE is pressure independent (as I think too)? It's a measure only for inlet/outlet restrictions and available resonance tuning!?

Tomak · Posted: Wed Dec 21, 2011 3:06 pm Post subject:

TurboNova · Posted: Wed Dec 21, 2011 3:51 pm Post subject:

Roberto Arano · Joined: 22 Nov 2005 Posts: 358 Location: colorado

I'm not going to read the entire thread, but typically, these discussions go back and forth because no one has posted an accepted definition (mathematically with units (unless unitless Wink

) of the definition.

So who initially coined the term and where is the reference to it's definition.

Once everyone agrees on the source definition....then you can explain it to those who don't.
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Tomak · Posted: Wed Dec 21, 2011 6:17 pm Post subject:

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

I really don't understand all this confusion Confused

To weigh something without using a set of scales you need to know the volume it takes up and the it's density. Once you have volume and density you have the mass, really simple.

If any engine displaces it's entire "swept volume" during one engine cycle then we would say that it has achieved 100% VE .... however if by using the intake and exhaust gases inertia and some valve overlap this engine managed to display it's entire swept volume, plus in addition some of it's un-swept volume (the combustion chamber), then we can say that it has achieved a VE of above 100% ... but it'll never be likely to exceed 100% by very much.

Ultimately of course it's Mass flow in to the engine which is the variable dominating our ignition angle and fuel delivery estimations not volumetric efficiency.
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Turboivo · Joined: 09 Mar 2006 Posts: 669 Location: Bulgaria

I think the bigger confusion comes from missing small part of VE definition, i.e. "atmospheric conditions" or "standard atmo conditions". A cyl volume at 15psi boost is actually x2 (approx) at standard atmo conditions. The air mass inside the cyl at 15psi boost is also twice greater compared to atmo conditions. So if our MAF/CFM measures doubled mass/volume at 15psi we should divide it to the ("theoretical" or "ideal") doubled mass/volume inside the cyl (at 15psi) to obtain VE number, instead to mass/vol at atmo conditions. IMHO VE is not volume or mass. It has no units. It is just koefficient.
Here are definitions copied from J.Heywood's book "Internal Combustion Engine Fundamentals" which are a bit too "scientific" to me.

sam@tdi · Joined: 11 Sep 2006 Posts: 648 Location: London, England

Ok I can now see where this confusion is coming from...

Why 'O why would someone look to use the words "volumetric efficiency" to express a method of trying to gauge overall engine cycle effectiveness when we already use the words in the industry to express something else entirely (as stated above)? ... that's just crazy.
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Turboivo · Joined: 09 Mar 2006 Posts: 669 Location: Bulgaria

Sam, it all started (at least for me) when I saw huge dyno power claims in EvolutionM mitsu forum, from a guy who lives in a neighbour country. He shows dyno plots (hub dyno but not Rototest) etc... For explanation of his "results" he claims VE of 145% in a N/A Cosworth engine and 150-sky..? % for another turbo engine . In my second post here I put one of his claims - 2.0L Evo, 15psi, 11500rpm, 813 crank hp!?@ pump gas (100 Oct). So I started to think (as many other do) could that be possible at all? After some basic math about air flow and all conditions almost ideal (but not real) I could prove on paper only about 600hp with VE of IIRC 115-120% and assumption that approx 1lbs of air is needed for 10hp. Also at low 0.45 BSFC.