Volumetric efficiency

[Howard Chu]

> From: "Camden Lindsay" <cjl169 at hotmail.com>

> How does one go about approximating volumetric efficiency at
> differet rpms?

> I ask because the excellent spreadsheet posted a couple of weeks ago
> (turbosize.xls) requires it as input, as does the megasquirt fuel system.

> I (think i) know that the VE of a n/a motor is peaky, whereas a forced
> induction motor is mutch flatter (??due to less reliance on
> tuning frequency of intake/exhaust manifolds, tracts, valves, etc??)

> could i use some type of mass meter affixed to the intake of a n/a motor
> while running and compute VE using the appx air mov't (volume *rpm/2, i
> think)divided by the output of the meter?  Or is there a simpler
> way which i am missing?

> Thanks for any help
> Camden Lindsay

I calculated the VE of my engine using my OBD-II scanner, logging
MAF/IAT/RPM across the RPM range at WOT. I don't know of any simpler way
than what you point out. Use the temperature to get air density, and convert
mass to volume. Divide this volume by theoretical max volume at the given
RPM and you have your efficiency at that RPM. To be more precise you should
also have your barometric pressure and relative humidity readings factored
into the air density. All of the formulas required for air density are
already on the spreadsheet, so it should be simple enough for you to
generate the required numbers.

As an alternative, if you already have reliable dyno plots of your engine,
you can approximate the VE curve based on your torque curve. I believe the
two are directly proportional. In this case, look at your torque peak and
set the "Mech Eff" fudge factor in the sheet so that the calculated HP and
torque matches, using an initial guess of VE between 80-100%. A modern DOHC
engine design should hit at least 90% at its torque peak, older designs will
be less. Once you have this value set, you can work backwards along the rest
of your dyno plot to get the VE across the range. The resulting curve will
be the right shape; it will be directly proportional to your true VE curve.
But without a true mass or volume measurement, you have no way to nail it
down to an actual value.
  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc

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