Quick question about AFR table and EGO correction

[allen22]

Hello,
I'm trying to wrap my head around the EGO correction, AFR tables and wideband 02 sensors (and autotune).

1. When using a wideband (just like a narrow band) MS is constantly trying to achieve stoich mixture, correct?

2. Unless you have an AFR table setup, then it tries to achieve whatever AFR ratio you've input into that table for that RPM and Kpa, correct?

3. If you turn on autotune then Megatune helps the MS out by adjusting the VE table so that the MS doesn't have to correct as much all the time, right?


Do I have all that right?

Thanks a bunch!
Fred

[grippo]

Hello,
I'm trying to wrap my head around the EGO correction, AFR tables and wideband 02 sensors (and autotune).

1. When using a wideband (just like a narrow band) MS is constantly trying to achieve stoich mixture, correct?

Yes as long as you set NB mode - but then your sensor calibration will get screwed up so you don't want to do this. If you have WB and you want stoich everywhere, then fill the AFR table with 14.7.

2. Unless you have an AFR table setup, then it tries to achieve whatever AFR ratio you've input into that table for that RPM and Kpa, correct?

Yes

3. If you turn on autotune then Megatune helps the MS out by adjusting the VE table so that the MS doesn't have to correct as much all the time, right?

Yes.

Do I have all that right?

Thanks a bunch!
Fred

[allen22]

Thanks! That brings up another question though. Why even have the VE table if you have a wideband? Can't the MS just read the WB's input and then modify the fuel amount based on your AFR table for that particular cell?

thanks,
Fred

[dridders]

The EGO is only a feedback after everything has already happened. If you don't keep the system stable (same load, same revs,...) once it determines the afr the combustion had... all the circumstances already changed drastically. The VE is the base, it works always. If the system is stable enough it can correct the VE's according to the feedback of the EGO, but if it isn't it simply uses the VE without EGO modification.

[allen22]

Ok, I think I understand.

1. So, the 02 reading is not really quick enough to depend on for real time engine management.

2. It's good for getting an AFR measurement then suggesting what to do about it next time your at that RPM and Kpa (not doing anything about it instantly unless your holding at that RPM and Kpa and have autotune turned on).

3. The VE table however is good for real time engine management because it's always there when the MS needs it, no slow sensor values to retrieve and compute, etc?

So then, if one had a good AFR table that was "perfect" and you used autotune for maybe a few weeks or so worth of driving (enough time to hit all RPM and Kpa cells long enough for autotune to do it's thing). Then, autotune would eventually "fix" your whole VE table and you could take out your 02 sensor completely, right? No need for EGO correction then.

But that all rides on having a very good AFR table.

thanks again,
Fred

[dridders]

Somehow correct, but EGO can also correct some errors that get into the system... like MAP slowly failing or something like that.
The EGO is of no use during accelleration/decelleration, as you cannot say to which rpm/map the reading belongs. Its mostly useful for tuning then.

[Bernard Fife]

Why even have the VE table if you have a wideband?

In addition to the transient and response time issues, there are a number of practical considerations for going to 'EGO only' control of fuelling.

Among these are:

- the sensor needs time to warm up from a cold start. Do you want to sit and wait 30 to 60 seconds while it does that (like a glow plug)?

- a sensor or sensor wiring failure would have no 'back-up' and the car would be undriveable. Other sensors can create problems too, but they aren't in a hot 800C exhaust stream.

- exhaust leaks or EGR problems could make the engine run badly or damage itself.

- rich mis-firing can indicate a lean condition to the sensor (excess oxygen), leading to an even richer mixture, and worse mis-firing, etc., in a run-away fashion.

- you would still have to tune the engine (to the optimal AFRs), so you wouldn't gain a lot in the process.

That's why MegaSquirt has limits on the EGO correction, and doesn't use EGO feedback under a number of conditions.

Lance.