----- Original Message -----
From: "Scot Sealander" <Sealand at clarityconnect.com>
To: <gmecm at diy-efi.org>
Sent: Tuesday, October 03, 2000 6:36 PM
Subject: Re: Calculating VE from MAF and MAP sensors
| VE = m_dot / ( N * Vol * Rho / 120 ) * 100
|
| where: VE - volumetric efficiency (%)
| m_dot - mass air flow (g/s)
| N - crank speed (rpm)
| Vol - Engine Displacement (litres)
| Rho - Manifold Air Density (g/litre)
|
| where: Rho = 1.202 * MAP / 101.325
| MAP = Manifold Absolute Pressure (kPa)
|
| You should also compensate the air density for temperature too since
| the 1.202 is air density at 20_C and inlet temperature is elevated due
| to heat transfer from the manifold surfaces etc.
|
|
| That was posted by Andrew Rabbitt from Orbital, a while ago.
| Much simpler, and the variables are available in the ECM.
Looks like what we came up with - we just used the ideal
gas law to calculate Rho, so already took temp into account.
|
| Where are you getting "T" (air temp) in your setup? How accurate
| do you think it is?
On an LT1, using the IAT (incoming air temperature sensor). It is
located a bout 4" from the TB, and excepting possibly heatsoak issues
it should be farily accurate. It tends to run just a bit hotter than ambient.
|
| I make the ECM calculate VE real time, then save it in added
| non-vol RAM. It took a while, but the results are very
| gratifying now.... ;-)
What did you do about coasting issues? I had to write a parser
to take out all samples that had negative acceleration - the VE values
I would calculate in these cells were WAYY off - running about 160-
200% (since MAP was VERY low - ~20KPA, while airflow
still scaled with rpm somewhat.
Once I parsed out these cells the VE chart definitley looks good,
just need to get enough samples in the higher rpm ranges.
Chris Bennight
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