Desired Airflow Question, not your normal question

[Dale]

What sensor(s) does the ecm read to determine how much airflow you need?

Maf? but what about when its in map only mode?

[Gelf VXR]

What sensor(s) does the ecm read to determine how much airflow you need?

Maf? but what about when its in map only mode?

I think you will find if its not refierencing the MAF then its ONLY looking up the Main VE table B0101 or B0200 (Speed Density)

[dfe1]

The PCM doesn't know how much air flow an engine needs, you have to tell it. That's the purpose of the "Desired Air Flow" table. As for the measurement to determine actual air flow, that's typically done by Mass Air Flow sensor readings. Of course, that brings up the question of how air flow is determned in speed density systems. The PCM accomplishes that by computation using engine displacement, engine speed and MAP sensor readings.

[Gelf VXR]

The PCM accomplishes that by computation using engine displacement, engine speed and MAP sensor readings.

Ok, its using the RPM and MAP sensors, and the main VE table.

[dfe1]

Ok, its using the RPM and MAP sensors, and the main VE table.

No, if I understand your original question correctly, the VE table is not a consideration. When you asked about the amount of air flow you need, I assumed your question was in reference to the "Desired Air Flow" table (B4307). If that's the case, that table has to do with the amount of air required for idle. The entries in this table typically have to be adjusted when a long duration camshaft is installed or any other modification that significantly alters idle air flow characteristics is done.

[Gelf VXR]

No, if I understand your original question correctly, the VE table is not a consideration. When you asked about the amount of air flow you need, I assumed your question was in reference to the "Desired Air Flow" table (B4307). If that's the case, that table has to do with the amount of air required for idle. The entries in this table typically have to be adjusted when a long duration camshaft is installed or any other modification that significantly alters idle air flow characteristics is done.

I never asked the question, but there is a long and short answer. When the MAF is not funtional how does the PCM know how much fuel to add.

Short answer, it uses the main VE Table.

Long answer, by looking at the VE Table, it will tell you it uses RPM sensor and MAP sensor, if you look at the funtion for VE it will tell you it references temperature and uses IAT sensor and ECT sensor, etc.

[Dale]

Ok, thanks. So if I have the incorrect maf sensor, then its f-ing everything up.

Ok, time to go get the propper maf and force it to fit.

[joecar]

The VE table's "real" units are g*K/kPa

where:
g is grams (mass)
K is degrees Kelvin (temperature)
kPa is kilopascals (pressure)

so each VE table cell value is the "normalized" cylinder airmass...
i.e. "normalized" for absolute termperature and absolute pressure;

as temperature goes down, airmass goes up (i.e. mass varies "per" 1/K),
as pressure goes up, airmass goes up (i.e. mass varies "per" kPa),
so the units are: g /(1/K) /kPa == g/(kPa/K) == g*K/kPa.

to calculate the cylinder airmass, the PCM reads the VE table cell value and divides by the absolute temperature and multiplies by the absolute pressure;

e.g.
very simplistically, say:
- the PCM computues (via IAT/ECT temperature blending) the air temp to be 300K (27°C, 80°F),
- VE table says 2.0 g*K/kPa,
- MAP is 100 kPa,
then airmass = 2.0[g*K/kPa] / 300[K] * 100[kPa] = 0.6667[g]

(note how the units balance out)

[bink]

The VE table's "real" units are g*K/kPa

where:
g is grams (mass)
K is degrees Kelvin (temperature)
kPa is kilopascals (pressure)

so each VE table cell value is the "normalized" cylinder airmass...
i.e. "normalized" for absolute termperature and absolute pressure;

as temperature goes down, airmass goes up (i.e. mass varies "per" 1/K),
as pressure goes up, airmass goes up (i.e. mass varies "per" kPa),
so the units are: g /(1/K) /kPa == g/(kPa/K) == g*K/kPa.

to calculate the cylinder airmass, the PCM reads the VE table cell value and divides by the absolute temperature and multiplies by the absolute pressure;

e.g.
very simplistically, say:
- PCM computues (via IAT/ECT blending) the dynamic air temp to be 300K (27°C, 80°F),
- VE table says 2.0 g*K/kPa,
- MAP is 100 kPa,
then airmass = 2.0[g*K/kPa] / 300[K] * 100[kPa] = 0.6667[g]

(note how the units balance out)

Can you organize a this info!

[joecar]

Hey Joel, how you doing

Ok, thanks, I added this to the following thread: showthread.php?14188-Summary-Notes

[ one day I'll organize all the info ]