volumetric efficiency

So I've been playing around with compressor maps and calculations for estimating different airflows, temperatures etc, but many calculations require the volumetric efficiency. I haven't found anything about a good way to calculate or estimate the VE of a particular engine. I've seen it stated tons of times that a NA engine "is usually around 85%", although this is usually in reference to a small block Chevy or Ford which I would think is going to be quite different from a four valve motor like the 4G63. So can anybody explain a way to estimate VE from some simple measurements and calculations (besides throwing sensors on a running motor)? I'm not expecting precise numbers, but something better than "probably around 85%." Anybody know what the actual VE is for a stock 4G63 without boost?

 

[Dialcaliper]

I would guess around 75-80% stock, if you're going to boil the system down to the throttle body inward, and take out all of the forced induction setup for calculations. You might reach as much as 85-90% on a highly modified race engine. Most 2-valve V8's are probably in the 70-80% range.

N/A engines are usually talking about the whole intake system, which includes ram-air effects, intake resonance, and plenty of other tricks that result in higher than atmospheric intake pressures. By this measurement, some of the import 4-bangers are quoted at over 100% VE. The S2000 engine is apparently running at 110-112% N/A

Comparing those numbers to a turbo engine is comparing apples to oranges, unless you consider the whole system for both, in which case the stock 4G63T is running around 125-135% (10psi boost at 75-80% base VE)

The "base" VE of a turbo engine without considering boost is necessarily lower than a N/A engine, for several reasons like reduced cam overlap that works better once FI is introduced, and the fact that the intake tract is much more restrictive.

A good approximation would simply be taking a logger output of airflow converted to lbs/min, divide by boost pressure ratio and RPM, and compare to displacement/rev (around 1L).

 

How did I know you would probably be the one to respond?

Yes, "base VE" is what I was looking for. I was thinking that a four valve design would have a VE higher than a two valve motor, but wasn't thinking of cam overlap and intake restriction.

For clarification...you say "A good approximation would simply be taking a logger output of airflow converted to lbs/min, divide by boost pressure ratio and RPM, and compare to displacement/rev (around 1L)."

Let me see if I understand this right... Airflow / pressure ratio X rpm would give me the actual amount of air going in. Dividing that by the displacement would give me a figure such as .80 which would be 80% VE, right?

What exactly do you mean by "compare to displacement/rev (around 1L)."

Thanks

 

It is a 2.0 liter engine, which means that each revolution is 1.0 liter of displacement. Each cylinder only fills/fires every other revolution.

 

[Dialcaliper]

Sorry for not clarifying - what Keydiver said - each revolution has only two cylinders sucking in air, while the others are compressing/combusting, so it comes out to about 998.5 cc's per rev of the crankshaft. Also, lbs/min isn't quite the right measure to use - that would be more like "mass efficiency". Convert to liters/min or cc's/min and the logger formula will work for "volumetric" efficiency (since you're not taking into account temperature and density).

Here is a good conversion from MAF Hz output to L/min for a 2G MAF, which incidentally credits Jeff /ubbthreads/images/graemlins/smile.gif . There are others out there for 1G and 3G/Evo MAF Sensors as well. If you're using a translator, you can get pretty close by just using the converted output.

Note that the VE quoted is usually a "peak VE" for bragging rights. The VE will be different for every part of the RPM curve (and throttle setting, but I doubt you want to go there)

If you're trying to use this to set up a speed density system, I'd suggest getting the car running and tuned using some type of mitsubishi MAF first, since the Karman type is one of the most accurate as far as airflow sensors go. Take the logger readings from all points in the RPM range and plug them into a spreadsheet.

For example, 835 Hz on a 2G MAF is about 6000 liters/min. At 4000 RPM, that's 1.5L/rev. If you're running 15psi in the intake manifold (PR of 2.02), that gives you a base VE of about 74.2% (if you include the turbo, your overall VE is 150%), but only in reference to the engine at 4000RPM with 15psi boost - changing the boost level can change the VE, since we're talking about airflow through tubes, pipes, intake, head, etc. There is still error in the calculation (it could be off by 2-3%) but it should be decently close.

There's no good way of calcuating the base VE of the engine without all the intake piping, short of attaching the MAF directly to the throttle body (which of course eliminates the ability to take into account the losses from forced induction)

 

Thanks again for all the info. Guess I had a brain fart about the 1L of displacement per revolution, since every other formula I've been using includes dividing by 2 for that same reason. I do know that the VE will change at different rpm points, temperatures, and densities, and I'll probably be playing with those numbers later. Not using it for speed density, really just playing with lots of numbers and making calculated estimates for various setups for the sake of learning how to do it and applying it later.