Why bother dialing in the VE table?

[pauly24]

Yes thats my point exactly, the fuel trims will take the BEN to 0 every time. And thats what you are trying to do by massaging the VE table with an AFR BEN.
Hence the fuel trims that have been recorded are the exact adjustments needed to fueling to bring the BEN correction to 1.

I just wanted some more info about the cons/pros about using fuel trims for dialing in the VE compared to BENs in CL mode of course.

Weathermanshawn, can you explain why you've not had as much success applying fuel trims BENs to the VE table? I understand people saying its not as good as using AFR BENS, but being told is one thing, but to believe its true I need to be given a reason why AFR BENs are better to be used then fuel trims BENs as I can't see the difference in CL mode.

[WeathermanShawn]

I could have probably stated it better.

Here is what I meant. I have had more experience applying LTFT in a MAF-Enabled car. They fall in line pretty easy. For instance if your car is averaging +10% LTFT's across the MAF Frequency..adding 10% to the MAF Table usually gets your LTFT Average to zero.

What I was trying to say is that if you tuned your VE Table 100% Open-Loop (with wideband) and then went to close-loop and attempt to use your narrowbands utilizing Trims..it is not always a 1:1 Ratio. It could be the location of the narrowbands, accuracy, or inevitable fuel/exhaust delay that makes it difficult.

In the U.S. with EPA requirements, emissions, etc., many people run 100% Closed-Loop. Some choose to retain MAF, others go MAF-less..but Closed-Loop. That is know as CLSD. If you were tuning 100% Closed-Loop (stoich portion of VE Table), then applying LTFTBENS to the VE Table has far greater success.

I do not know if I over-explained it..but here is the bottom line. If you tune Closed-Loop, techniques like STFT's and LTFT's can successfully be used. If you know how to tune Open-Loop with a wideband..you are set. Its trying to mix and match that creates problems. I.E., using Closed-Loop techniques for open-loop do not work as well.

Does that make sense?

[joecar]

The above are good reasons to make sure the VE table correctly models the engine.

Also, consider these:

The initial calculation for airmass is made (using the VE and/or MAF tables) regardless of trims... trims are applied to fuelmass later, just before calculating the injector pulsewidth.

What happens when the PCM goes to the VE table for airmass (i.e. during transients)...?

Transient air/fuel is what makes good or bad throttle response (which involves steady state VE and wall wetting/evaporation)... why should it be denied a good chance to work properly...?

What happens if for some reason the PCM faults into a OLSD mode and your VE table is not correct...?
(this has multiple side effects: with an auto trans, the running pressure will be low if the VE table is low, this allows slipping at high load).

[Redline Motorsports]

Don't loose sight to the fact that factory narrow band o2's have a accuracy of +/- 3% vs. a high quality WB. Its not unusual to get everything in line with a WB and then reable trim control and see a change.

Regardless of trims, not correctly modeling the engines airflow mapping is going to create issues everywhere. When you do a moderate throttle roll on, trims are not really working that quick vs. having the wrong reported airflow.

Your best bet is OL and use a good WB

[joecar]

In absence of a wideband, MAF seems to correct via trims ok, VE seems to be better calculated (from MAF, MAP, DAT, RPM).

[pauly24]

Thanks guys, I don't question because I'm trying to say your wrong or argue, I just question because I'm trying to learn.
Im not one to follow what someone says blindy, if someone says something to me, but they can't back it up with an explanation I usually disregard whats been said. (Which I presume what most people would do if they are trying to learn something anyways)

[WeathermanShawn]

In absence of a wideband, MAF seems to correct via trims ok, VE seems to be better calculated (from MAF, MAP, DAT, RPM).

Thats what I was trying to say...

Joe, I always like your 'Power-Point' type explanations. You use fewer words but you always get your point across.

Must be natural. My explanations never are that concise. And that was a very good way of explaining it..

[joecar]

Thanks guys, I don't question because I'm trying to say your wrong or argue, I just question because I'm trying to learn.
Im not one to follow what someone says blindy, if someone says something to me, but they can't back it up with an explanation I usually disregard whats been said. (Which I presume what most people would do if they are trying to learn something anyways)

That's fine, we can see you're asking good questions and the discussion that follows is excellent... we all learn from questioning and discussing... questions make people think. Blindly following doesn't promote learning; understanding why/how promotes learning.

Thats what I was trying to say...

Joe, I always like your 'Power-Point' type explanations. You use fewer words but you always get your point across.

Must be natural. My explanations never are that concise. And that was a very good way of explaining it..

That comes from either being an engineer or not being able to type...

[joecar]

...

Regardless of trims, not correctly modeling the engines airflow mapping is going to create issues everywhere.

...

This is a very important point that summarizes the whole point of it all.

[redhardsupra]

Heh, this question is simple, but only on the surface. The real question here is 'why is GM's ECU operating in a hybrid of open- and closed-loop fueling corrections?' But let's back up a bit first to make sure you got the basics covered.

Trims are corrective mechanisms. A fully closed-loop system doesn't have a starting condition, it is purely reactionary; it takes a measurement, compares to the desired effect, and restarts the process. In contrast, open-loop is predictive. You predict a certain outcome, and you stick to your predictions, regardless of the result.

It is easy to see the problems of both systems. Open Loop will never get better, and will never adjust to any changes in the system. Closed Loop doesn't have a good 'starting' condition, and depending on the system, the corrections might never converge on the stable solution. It is also 'naive' in the sense that a wrong batch of data will lead it astray, if it doesn't have a mechanism for sanity checks.

This is why GM came up with a hybrid of the two: you start with a prediction (airmass based on either MAF or VE/MAP/TEMP), you let the mixture burn, but then in a reactive fashion it observes what happened, and it remembers that correction for the next round. With both short and long trims, the 'remembering' process actually gets more complex, as the ECU tries to classify the observed change as either a temporary fluke, or a permanent condition. If that wasn't enough, there's sanity checks in the form of MAP and MAF and CYLAIR filters and limits.

So as per usual, a singular, simple mechanism isn't enough to catch up with the complexities of the natural phenomenons we are up against. We are forced to get creative and mix all the tricks we got, or we're not going to cover every possible condition, situation, fluke, act of nature, or sheer stupidity the ECU and its sensors will go against out in the wild. Don't fear complexity, master it instead.