What is VE?

[GlenB]

Thanks for the welcome!

I thought I'd kick things off with a question to the members to maybe get the ball rolling.

What is Volumetric Efficiency?

And....

Can it be used to help measure gains/loss in engine performance?

Can it be used to help diagnose engine breathing problems?

Can it be used to help diagnose a faulty Mass Air Flow sensor?

[ryans]

anyone care to take a crack at this?

[The Rob]

I could answer it. PNG can answer it. Maybe one or two others here know it without googling.

[GlenB]

All discussions here are open-book and no grading.

[GlenB]

Lost of views, little comments so far... so let me add a little.

When a piston is on it's intake stroke, air is filling the cylinder. In theory, the amount of air that can fill the cylinder, on a naturally aspirated engine, should equal the total amount of space inside the cylinder when the piston is at bottom dead center.

In reality however, there is not a compete filling of the cylinder.

The difference between what a cylinder can fill to, and what it actually does fill to is the engine's volumetric efficiency percentage.

Why may a cylinder not fill completely?

[ryans]

it's not completely sealed?

[GlenB]

it's not completely sealed?

Ok, let's think about that one.

Burn valves... The valve opens. Air begins to fill the cylinder through the intake valve. Would a burnt valve prevent a complete fill? I don't see how because the air will still fill the cylinder. Then during the compression stroke lets assume the intake valve is not sealing right. Air will be forced back into the intake. Ok, now we have a problem right? Some of the air that the engine worked to draw past the throttle plate and into the cylinder has now been kicked back out into the intake. Although another cylinder will draw that air in, it will be displacing some of the air that would have had to go past the throttle plate and MAF sensor. Both cylinders may have drawn the air in at full efficiency if you could somehow measure it at the intake port, but, unless you have some pretty cool equipment to do that, it will have been measured outside of the intake at the throttle plate. So, when you measure it, it may be lower if the leak is bad enough to show up. It may have to be standing open partly to measure it... but yes if it is bad enough.

Same with the exhaust valve. If it were standing open, then some of the air coming into the cylinder would be coming back in through the exhaust valve instead of the intake... so measured VE would be down even though the cylinder is actually filling at a higher percentage than what is being measured.

A hole in a piston would show up like that too, because even though the cylinder filled, it was with air from the crank case and would not get measured.

So yes, improper cylinder sealing would lower the measured VE if it is bad enough.

So this really depends on where we measure it right? If we use a scan tool to measure VE, we are looking at the engine, intake, and exhaust as a complete assembly. We're no longer looking at just how much air will fill the cylinders on intake stroke, but we are looking at the total volume of air that the engine can move past the throttle plate.

So when we step back and look at the bigger picture now... what else can do this?

[ryans]

I think I need some pictures to help me out. :)

[GlenB]

I think I need some pictures to help me out. :)

Well, how does a throttle plate control engine rpms?

[GlenB]

If it helps, here is an illustration I drew for an EGR operation article. It gives an idea of what is happening to get the air into the cylinder.

[XtremeModifier]

not bad...

[XtremeModifier]

not bad... I design EGR Systems actually, lolz.

[GlenB]

What good is it to be able to measure VE?

It can be a lot of benefit to someone who wants to perform engine mods that actually make a difference. To someone who makes a change and wants to measure it, for better or for worse, it can be very helpful.

The first VE measurement is from a Ford 3.0L engine. The dyno program is flagging it at about 90% efficient. 90% is ok, but there is still room for improvement. This engine engine could make more power because it is possible to get about 10% more flow from it.

The second VE measurement is also from a Ford 3.0L. The second is pushing 100% efficiency.

Both engines are naturally aspirated. Both are Fords. Both are 3.0L. The second will flat out smoke the first one. When comparing the 2 engines side by side, the second one is radically different from the first one.

Why such a big difference?

[GlenB]

Well, the difference between these 2 engines is the obvious fact that one can breathe a lot freer than the other one. If fact, the #2 car was breathing better through the entire RPM band. Pick any RPM on the graph, and you'll see that the #2 car was flowing more air. But are the same displacement and both were at wide open throttle.

The mechanical difference in the 2 is that #1 is a 12valve 3.0L and #2 is a 24valve 3.0L. The extra valves allowed more air flow in and out of the cylinders. Plus, the 24v engine is an OHC whereas the 12v is an OHV engine. With less parts in the valve train to flex, more precise air flow was produced in and out of the engine.

This breathing difference is shown clearly by mapping the VE out across the RPM range like this. This also would give a tuner a chance to see if engine is breathing at its best in a particular RPM band he is wanting it to run best at. He can see if his new exhaust system helped or hurt him among any other changes.

There is more that can be covered here. But, I get the feeling that interest is not real high. So I guess I'll move onto to something else. If someone would like to see what else VE can be used for, just chime in.