Beth396

This equation for injection pulse width was given to me by a tuner that frequents the Vettenet email list:

[VE*MAP*displacement]
PW= ----------------------------------------
[#cyl*(IAT*CLT/2)*AFR*flowrate]

He said that CLT was coolant temperture. At one time I looked over the source code for the 91 ECM and it looked like CLT was some other parameter. Does anyone know for sure?

Beth

[Modified by Beth396, 4:32 PM 11/11/2003]


[Modified by Beth396, 4:33 PM 11/11/2003]

Beth396

I've tried twice to fix the format of the above equation. Can someone fix it? It's about a clear as mud..

DOCTOR J

Um, I can't read your equation. Presuming you are looking at the $8D mask for a 727,
fuel flow (inj PW) is calculated using fixed inj size & two variables:
Calculated air flow & Commanded AFR.

Commanded AFR is the sum of stoich plus AE & PE,
plus any other fuel modifiers in the main program.

Calculated air flow is the ideal gas law (displacement X rpm) including compensation
for baro pressure, MAT, and EGR. It's expressed in grams/sec.

Baro compensation is taken from a MAP reading just before engine start.
EGR compensation is a subtraction from the partial pressure of air when EGR is active (look-up table).

Temperature compensation is sort of convoluted in the way GM handled the math.
The MAT term can come from 2 tables in the program:

Depending on the setting of a switch, the 'Inv. MAT Term Lookup Delta
Multiplier Vs. Airflow' table may or may not be used.

If this table is disabled then the MAT Compensation Counts Vs. MAT table is
indexed with the normal value of MAT. If this table is enabled, then the
table index for the MAT Compensation Counts table is a corrected value of
MAT. The corrected value is calculated as follows:

Corrected MAT = MAT + Table Value * (Coolant Temp - MAT)
where 'Table Value' is the value looked up from the Inv. MAT Term Lookup
Delta Multiplier table.

The Inverse MAT term is then calculated as:

Inverse MAT Term = Cylinder Volume / (((Table Value ** 0.5) + 233) * 128)
where 'Table Value' is the value looked up from the MAT Compensation
Counts table (indexed by either MAT or the corrected MAT depending on the option switch).

This value is then used in the calculation of the grams of air per cylinder
as follows:

grams air per cylinder = Volumetric Efficiency * (MAP - EGR Partial
Pressure) * Inv. MAT Term

This value and engine RPM are then used to calculate air flow which is then
used to calculate the base pulse width.


Not sure if that answers your Q, but generally that's what is going on
IIRC (it's been a while since I tried to read the hack).

The base fuel calc is updated ~80 times/sec in a 727.

During steady-state operation there is a further embellishment: the pw
is constantly being adjusted via the lambda feedback loop (the
so-called adaptive tuning loop) from the EGO.

Hope that helps.

edited for readability


[Modified by DOCTOR J, 1:42 PM 11/11/2003]

Skippy Stone

I hope Beth396 will chime in with more information on the equation. However, I want to thank you Doctor J (not THE 76er Doctor J, I presume), for some good information. You have confirmed a suspicion of mine about the EGR. Thus making certain running conditions unstable when the EGR is gone. And the explaination on the MAT and Mat/inv tables has also helped me.

Thank you

Beth396

Doctor!

Yes this helps, my intenion was to get some discussion going in this tune section. I don't want to scare anyone with a bunch of technical stuff but some might be interested. Ideal gas law...PV=nRT...cool. I'm going to go over your post and comment, but right now I have to get ready for a meeting in a couple of hours.

Maybe you and Skip can derive the PW equation as a function of injector flow rate, calculated air flow and commanded AFR from the ideal gas law in the mean time..just kidding. Later, Beth

Skippy Stone

If you could pass around that source code for that 91, Beth396, I'll hack anything you need!

Well, at least I'll find somebody who can. :blueangel:

DOCTOR J

Commented hacks of the GM programs have been publicly available for several
years. One of the most vererable $8D copies is on the site of the late 'ECMGuy':
http://www.geocities.com/MotorCity/S...g/do_tune.html

GM only makes reading their programs complicated in order to limit those who
actually understand how the computer works to the technically literate :).


There is no need then to derive the pw equation - the calculation is there in
black-letter code. Rather than belabor it, the highlights are these:

Generally, GM treats a high-impedence injector as a fixed-weight device,
although in fact it's a fixed-volume flow controller. GM uses voltage & low-pw
compensation (offset) tables to correct injector opening-time latency, trying to
achieve linearity for their equation.

Based then on some nominal fuel density, the (compensated) pw for the injectors is
calculated to give a stoichiometric weight balance of air to fuel. Stoich is generally
taken as 14.7:1, though it is adjustable in the code (within small limits). It's also
worth noting GM allows for different stoich ratios at idle and under load, just to make
tuning the engines that much more elegant.

BTW, the GM code also contains the complete definition of the ALDL output stream.

Thus the tuning & scanner info needed to work with these ECMs is already de facto
public-domained - which is not surprising for a near-20 year old computer system.

Have fun.

edit for typo


[Modified by DOCTOR J, 3:53 PM 11/11/2003]

Beth396

Skip, DoctorJ provided the link. Good thing because I couldn't remember where it was.

Beth396

[VE*MAP*displacement]

PW= ----------------------------------------

[#cyl*(IAT*CLT/2)*AFR*flowrate]

I can't figure out how to get the format of this equation correct so it looks right when I post it. Volumetric efficiency (VE), MAP and displacement are in the numerator and # of cylinders, intake air temperature (IAT), CLT (?), air/fuel ratio (AFR), and injector flowrate are in the denominator.

I don't really feel like getting into the level of detail of the algorithm that DrJ went into. If the equation is basically correct, I'm happy with that. The equation is nice because it shows mathmatically that the injector PW will increase if the IAT is decreased. Cooler air is more dense and the fuel will be increased. If you increase your injector flow rate (bigger injectors or more fuel pressure) the injector PW will decrease.

That's why you don't want to go too large on injectors...you might not be able to shut them down enough at low RPM and MAP. Also, if you increase your MAP (less vacuum) or increase the displacement you'll increase the PW.

Beth

HighHopes85

Beth...use [cod e] equation here [/cod e] without the spaces in 'code'

<BLOCKQUOTE>code:<HR><PRE>
[VE*MAP*displacement]
PW= ---------------------------------
[#cyl*(IAT*CLT/2)*AFR*flowrate]
</PRE>[/QUOTE]


[Modified by HighHopes85, 9:00 PM 11/12/2003]

Skippy Stone

A technology pup!

that's younger than the original Mac OS
which is a bunch younger than DOS (Microsoft still uses it)
which is a bunch younger than UNIX.

But then the auto technology is rather primative. You could get DOHCs on something like a 1908 or 1910 Alfa. I forget which year.

Thanks for the link. I forgot about that.
------------------------------------------------

Thanks HighHopes85 for the clear equation.