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Volumetric Efficiency 101

by Brian Barnhill

This can actually be a quite tricky subject, mostly due to confusion and differing opinions among many people. Volumetric efficiency (VE) is typically defined as "the actual amount of air being pumped by the engine as compared to its theoretical maximum."

Basically, VE is a measure of how "full" the cylinders are.

As most of us will know from basic science, gas will expand to fill its container. Seemingly, that would suggest that the cylinder is always full. And, in the pure volumetric sense, that is correct. A 0.5 Liter cylinder will always have 0.5 liters of air in it. The measure we are looking for here is air density. A cylinder with 500 mols/liter of air in it is said to me "more full" than one with 400 mols/liter.

Now, where is this air density measured?

This is one of the points of disagreement. The point at which air density is measured is crucial. Many will claim that you must take the measurement at a standard, such atmospheric density. This, however, can cause many issues with VE measurements. Forced induction cars will have skewed VE values due to the simple fact that they are forcing more air into the manifold. With more air available to the engine, it will receive a larger/more dense amount. This is not a pure measurement of the efficiency of the engine,

To correct for these factors, air density available at the intake manifold should be used. This will correctly measure the VE based on the amount of air available to the engine. As a simple example: Take a 4 cylinder, 2.0 Liter engine (assume even flow to each cylinder) each cylinder will be 0.5 liters. If the intake manifold has a density of 100 mols/liter (this gives 25 mols/cyl), at 100% VE, the cylinder will have 25 mols/Liter. This comes from the equation:

VE = Densitycylinder/Densitymanifold * 100%

Lets look at this another way. Say the cylinder in a single cylinder engine has 186 mols/Liter. Now, the density of at the manifold is measured at 213 mols/Liter. The calculation of VE gives: VE = 286/213 * 100% or 87.32%

It is upon this principle that variable valve timing and similar technologies rely.

They will change the flow aspects of the engine to best match the particular RPM range. An engine is typically only maximized for a particular rpm range. By allowing the change in parameters, this can be overcome. This can easily be seen when looking at DYNO charts for any Vtec equipped engine (the S2000 is a good example). In these charts there will be a "double peak." The horsepower will begin to fall off at one point, and then climb again. This rpm point will correspond to the "Vtec" point.

Volumetric Efficiency plays a large role in how your engine operates. By understanding this parameter one can begin to grasp the details required to properly tune any engine.

About the Author

Brian Barnhill is a Mechanical Engineer currently completing his masters at Worcester Polytechnic Institute where he is treasurer for the WPI Motorsports Club, driver for the Formula SAE team, captain of the fencing team and serves as manager for sales and technical support for Tuner Tools, llc.

 

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