Volumetric Efficiency

One of the most important processes that govern how much power and performance can be obtained from an engine is getting the maximum amount of air into the cylinder during each cycle. More air means more fuel can be burned and more energy can be converted to output power. Getting the relatively small volume of liquid fuel into the cylinders is much easier than getting the large volume of gaseous air needed to react with fuel. Ideally, a mass of air equal to the density of atmospheric air times the displacement volume of the cylinder should be ingested each cycle. However, because of the short cycle time available and the flow restrictions presented by the air cleaner, carburetor (if any), intake manifold, and intake valve(s), less than this ideal amount of air enters the cylinder. Volumetric efficiency is defined as:

VE=(number of rev/cycle*mass flowrate of air) / (air density*Displacement Volume*Engine Speed)

Where
Rev/cycle = 2 for 4-stroke
Air density evaluated at atmospheric conditions outside the engine


Reference:
Fundamentals of the Internal Combustion Engine Second Edition, Pulkrabek, Willard W. pg. 69


And for a side note.. remember Volume = mass times density

 

Thank you, that was very clear and succinct.

 

Clear but it leaves unmentioned the distinction between air ingested and air retained for combustion. E.g., if a well -tuned single cylinder engine (lets say a mid-sixties 500 cc./30.5 c.i. Norton Manx) was booming away at 6,000 RPM & WOT on the dyno, and an inlet airflow meter showed it to be ingesting 53 CFM (lets ignore density effects for simplicity), we would know it was attaining 100% V.E.: 30.5 x [(6000/2)]/1728 = 53 CFM.
Now, if we were to flip a switch which diverted the exhaust outlet to a large reservoir evacuated down to say 20 "Hg. vacuum, what would happen to the airflow reading? Would the increase represent a higher V.E.? The engine is ingesting more air, even if the excess is blowing straight out the exhaust..

 

That is a good point and is the reason very small overlap is desired in FI engines

 

This is where the confusion lies. The reason is because the values in the ls1 pcm's VE table use air density inside the intake manifold instead of atmospheric conditions. This means by the definition you referenced, VE can be changed by simply by changing manifold pressure, eg opening the throttle, supercharging, etc. While you can still change what the pcm thinks of as VE by doing those things, it is different because the "PCM" VE change is small, the real difference is in the manifold pressure change.

If what i'm saying isn't clear, think of it this way. In Miami you could be at 50% throttle at a certain rpm and have 80kpa manifold pressure. To get the same manifold pressure (same rpm) in Colorado Springs would take 100% throttle because that's atmospheric pressure at that altitude. By the book's definition the same engine at 50% throttle in Miami would have lower VE than at 100% throttle in Colorado even though the manifold pressure is the same. But according to the pcm, the VE would be exactly the same in both situations. Hope that makes sense.

I'm not arguing with your definition, just explaining the source of confusion for people who may not be familiar with ls1 tuning. Maybe there should be a different term for "VE referenced to intake manifold conditions".

 

Tuning the VE tables on an LSx is a discussion that has been run through the ringer. Check this out -
https://ls1tech.com/forums/showthrea...+table+cracked

WARNING - this is 12 pages long and will probably make your head hurt.

There are reasons for calculating VE other than tuning though. One example would be picking an appropriate turbo for your engine.

 

Any efficiency calculation is generally expressed as efficiency = actual/theoretical the debate is whether or not when calculating engine volumetric efficiency, atmospheric pressure is used as a reference density, or intake manifold pressure. Almost every reference on the internet regarding volumetric efficiency uses the atmospheric reference, and then states that the volumetric efficiency can be above 100% for a forced induction application due, of course, to the pressurisation of the inlet to the cylinder. Any "efficiency" calculation giving a result over 100% is generally bogus. The ENGINES volumetric efficiency is no way affected by a supercharger or turbocharger. A better way to state the volumetric efficiency everybody bandies about on the Internet is "volumetric efficiency relative to a naturally aspirated engine". The volumetric efficiency as calculated by the PCM is far more correct. Any calculation of volumetric efficiency for a positive displacement pump uses the inlet pressure as a reference. For example, if we used the "volumetric efficiency" formula as defined at the beginning of the thread, and I had a gas compressor with an suction line pressure of 400 PSI (gauge) or so, and the cylinder internal suction pressure (after the pressure drops across valving, etc.) was 390 PSI (gauge), I would have a compressor with a volumetric efficiency of ~2750% [(390+14.7)/14.7] assuming an atmospheric pressure of 14.7 PSI. If I had more or less suction pressure the efficiency of my compressor would change accordingly. I hope this demonstrates why the volumetric efficiency formula referenced to atmosphere (where the cylinder inlet is not at atmospheric pressure) makes no sense. When it comes to ENGINE control, the PCM can measure the manifold pressure directly, and along the measured engine speed and known cylinder displacement can calculate fuel required for that cylinder, providing it also has correct VE value for that particular MAP and RPM. There are in addition to the VE formulas referenced to atmospheric and inlet pressures, VE formulas that take into account the displacement of the injected fuel, and inlet flow out of exhaust valves due to valve overlap. My engineering books are all buried in the shed; here are links to a few engineering papers with references to volumetric efficiency. Note that all these papers refer to manifold pressure, the first link even mentions the use of a volumetric efficiency calculation referenced to atmospheric pressure on a turbocharged engine under section "2.3.1 Definitions"

http://www.vehicular.isy.liu.se/Publ...EX_3379_HB.pdf
http://www.vehicular.isy.liu.se/~lar...i_2001_SAE.pdf
http://www.nd.edu/~msen/Teaching/Dir...Gas4Stroke.pdf

The volumetric efficiency of an engine can be used to compare different engines to each other or to reference changes to an engine. In addition when a turbocharger or centrifugal supercharger are added to an engine they are not positive displacement devices and the term volumetric efficiency cannot really be applied to those devices (just adiabatic efficiency) or an engine with those devices installed, although the term can still be applied to the engine itself (whether or not boost is applied). The volumetric efficiency as defined by DanO is really "volumetric efficiency referenced to atmosphere" and not the only definition of volumetric efficiency (but what I feel is the most useless and inaccurate one, especially when applied to a forced induction engine), and, in addition, is not the definition as we use it for PCM tuning.

 

First:
Bottom line is that for tuning
(number of rev/cycle*mass flowrate of air) / (air density*Displacement Volume*Engine Speed) Does not equal the value in the VE cell that the PCM is currently in.

Second:
I haven't figured out how stating that (number of rev/cycle*mass flowrate of air) / (air density*Displacement Volume*Engine Speed) = VE belongs in an advanced performance tech section without stating why it is important. That's like stating that Pi = 3.14. Sure that's important, but why?

One reason would be to determining appropriate turbo.
To plot a turbo compressor map, you need to know your engine's airflow requirements at the desired boost and also the pressure ratio. Normally you would assume 14.7psi as barometric pressure. If you calculate your engine's airflow requirements and DO NOT figure in volumetric efficiency, you will be off because your engine is NOT 100% efficient.
On my 414, if I figure I plan to spin my motor to 7000 RPM's with 10Lb's of boost, the motor will require roughly 94Lbs/min of air at 100% volumetric effieciency. Recalculate it with 82% VE, and I fall at roughly 78 Lb's/min. That's a big difference when looking at a compressor map.

 

I agree with EdmontonSS on everything but one thing, VE over 100% is possible. It's not a thermodynamic efficiency so that's why it's not bogus. But I agree that the pcm's definition makes more sense and is a more useful term than the definition up top.

Please explain. Do you mean the tune is off, because the numbers in the cells are in ls1edit units instead of %, because the formula doesn't match the physics, or something else?

 

Right. If you run that equation for a given cell and plug the number you get into the VE table, it won't work.

 

Because not everyone here understood the true definition of volumetric efficiency and there are arguments about it.

Just because the LSx VE table is based off MAP does not mean that is the true definition. This is where the arguments/discussion began.

 

I'm sorry but I have never in my life heard of "relative VE." I consider the above "bogus."

I'm sticking with the tried and true writings of CFT.

Good work DanO.

Also, ref my 3rd grade less math oriented explanation

https://ls1tech.com/forums/advanced-engineering-tech/416169-internal-combustion-engine-basics-mainly-airflow-vs-tq-hp.html

 

Gotcha DanO. Ya had me wondering why we were talkin about this.

 

my understanding is this, and most I have learned from using Megasquirt. www.megasquirt.info

VE is based on a nominal condition. 70 degrees F, 100KPA atmosphear.

an engine running with 70 degree intake air, and in Miami could be under the 'nominal' condition, then there would be 0 altitude and 0 temperature correction, using the ve value in the table for fuel calculation.

when you travel to Colorado Springs where the atmosphear is say 80KPA, the computer adjusts the VE table with a Barometer correction factor.

so the VE table still works, at any atmosphear, as long as the PCM knows the barometer reading.

So is that the confusion many people see in the VE table? Or am I my self confused?

 

Maybe this explains it better:
http://www.modularfords.com/forums/2...ger-41735.html

 

No the confusion is not whether the reference air density is to sea level standard temperature & pressure. The confusion is whether the reference air density is local atmospheric conditions (outside the engine) or intake manifold conditions (inside the engine). Read the very first post again.

Just another flaw with that definition, where is the reference condition taken, you could have different pressures and temperatures at different places outside the engine, for example right outside the air filter the air could be warmer and lower pressure than the air right in front of the ram air nostrils on the hood. Once again, i'm not arguing with the definition you guys gave, just about whether it makes sense to do it that way or not. Obviously GM engineers realized it made sense to base ve off the intake manifold air conditions.