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.

[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)

[joecar]

Then say the commanded AFR is 13.3, so
fuelmass = airmass / AFR = 0.6667[g] / 13.3 = 0.0501[g]

say the IFR is 4.175 g/s, then
injector pulse width (IPW) = fuelmass / IFR = 0.0501[g] / 4.175[g/s] = 0.012[s] = 12.0[ms]

and [say] engine speed is 6000 RPM (100 rev/s),
for sequential injection, each injector fires once every 2 revs (4 strokes), so
4 stroke period = 2[rev] / 100[rev/s] = 0.02[s] = 20.0[ms]
so injector duty cycle (IDC) = IPW / period * 100% = 12.0[ms] / 20.0[ms] * 100% = 60.0%.

The IDC is the end result of obtaining the airmass from the VE table,
and computing the fuelmass needed for the commanded AFR,
and computing how long to open the injectors to spray that amount of fuel.

Just so you see the "big" picture... ;)

[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.

[5.7ute]

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 believe that is uses RPM in either maf or SD mode along with tables
B4512
B4514
B4515
B4521
I am happy to be corrected though as I have only just started looking into these tables.

[Dale]

4521 limits how much the pcm will correct. Its part of what I'm dealing with, but not the fully what I was looking for in the original post.

The others I dont understand.

[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]

When the MAF is not funtional how does the PCM know how much fuel to add.

There never was a question on how does the pcm know how much fuel to add.

The question was about Desired Airflow.

[Gelf VXR]

But its relevant

[Dale]

The desired airflow affects the ve table. Yes.

I'm trying to figure out how the desired airflow is calculated when you do the refaig(sp?).

[joecar]

I forgot what the question was...? :doh:

Here it is...

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

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

[Dale]

ok, tried the new maf out last night.

I dont feel that refaig is determined by the maf.

[dfe1]

I think you're getting cause and effect confused. The entries in the "Desired Air Flow" table tell the PCM how much air flow is desired to achieve the specified idle speed. The PCM then adjusts throttle opening or IAC position (depending on whether electronic or cable throttle). Desired air flow varies according to engine displacement, cam profile, throttle body diameter, desired idle speed, temperature and a number of other variables, so the entries in the table are established by the person doing the tuning, not the PCM. The purpose of monitoring RAFIG is to determine the difference between specified and actual air flow. Once this value is known, you can change the entries in the "Desired Air Flow" table so that the PCM has to do a minimum amount of correcting to achieve the desired idle speed.

[5.7ute]

The purpose of monitoring RAFIG is to determine the difference between specified and actual air flow. Once this value is known, you can change the entries in the "Desired Air Flow" table so that the PCM has to do a minimum amount of correcting to achieve the desired idle speed.
Rafig is not the difference between actual & desired airflow as I understand it. It is an airflow correction based on the difference between commanded & actual idle speed.
The amount of correction added or removed from the {B4307} desired airflow table is governed by a couple of other tables.
If the idle speed difference is less than {B4508} then the added airflow correction in {B4514} is used. The maximum short term correction is set in {B4521}.
An idle speed difference of more than {B4509} or when the AC is used will use {B4515} as the added airflow correction.
As I said earlier I could be wrong about this & the STIT & LTIT could use different tables that we cannot manipulate. i might have to have a look over a few old logs & see if I can verify this.

[dfe1]

RAFIG- Running Air Flow In Gear is the sum of long term and short term idle air correction. So in a sense, it is the difference between desired and actual air flow because if they were equal, there would be no correction. The PCM uses a variety of tables to determine the amount of air correction required, but the intent of using RAFIG data is to establish desired air flow data that minimizes the amount of correction required. As you alter desired air flow table data, RAFIG numbers will change to reflect those alterations. As an example, if RAFIG is positive and you increase the desired air flow numbers, the RAFIG values will become less positive or negative, depending on the amount you increased the desired air flow numbers.

I think all this makes sense, but as Joecar said, "I forgot what the original question was...?"

[stigmundfreud]

If you were sticking with a MAF system rather than scale the MAF table what would happen if you adjusted the VE table?

[macca_779]

If you were sticking with a MAF system rather than scale the MAF table what would happen if you adjusted the VE table?

it will effect fueling where MAF and MAP are blended but will have no effect above the default 4000rpm that is exclusive MAF fueling.

[stigmundfreud]

it will effect fueling where MAF and MAP are blended but will have no effect above the default 4000rpm that is exclusive MAF fueling.

so under 4k you could adjust the ve and above 4000 adjust the scaling. Of course that could lead to a jump around 4k+.

Its more for a case of the VE is dialed in now and the MAF scaled but to fine tune with the 2 was just wondering about the effect adjusting the ve would be

[5.7ute]

RAFIG- Running Air Flow In Gear is the sum of long term and short term idle air correction. So in a sense, it is the difference between desired and actual air flow because if they were equal, there would be no correction. The PCM uses a variety of tables to determine the amount of air correction required, but the intent of using RAFIG data is to establish desired air flow data that minimizes the amount of correction required. As you alter desired air flow table data, RAFIG numbers will change to reflect those alterations. As an example, if RAFIG is positive and you increase the desired air flow numbers, the RAFIG values will become less positive or negative, depending on the amount you increased the desired air flow numbers.

I think all this makes sense, but as Joecar said, "I forgot what the original question was...?"
The question is what sensors are used in the rafig process to determine desired airflow.
Lets get this right. Maf or Speed density has no relation at all to this calculation. it is a difference to actual & desired idle speed & nothing else.

[dfe1]

The question is what sensors are used in the rafig process to determine desired airflow. Lets get this right. Maf or Speed density has no relation at all to this calculation. it is a difference to actual & desired idle speed & nothing else.
I think there's more confusion here than anything else. When you go thorugh the RAFIG process, what you're trying to do is establish the proper desired air flow for the specified desired idle speed. If you look at the RAFIG PID, you'll see that it is simply the sum of long and short term idle air correction. If desired air flow data was perfectly correlated to desired idle speed, RAFIG would equal zero because both long term and short term IAC correction would be zero.

[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 :cheers:

Ok, thanks, I added this to the following thread: showthread.php?14188-Summary-Notes (http://forum.efilive.com/showthread.php?14188-Summary-Notes)

[ one day I'll organize all the info :hihi: ]