DIY WB update

[Brian L Massey]

On Mon, 13 Aug 2001 17:10:27 -0500 jll at edge.net (John Lamb) writes:
> If you want any meaningful comparison data between your stock O2's 
> you will need to mount as near as possible to them.

Hmm, what would be the point of comparing stock O2 and WB? NB sensors
don't really give you *data* to compare, per se. Indeed, from what
reading I've done, the stoich switching point of a NB O2 sensor doesn't
so much change even as it gets old, as it either fails to switch at all
or switches *too slowly*. Placement seems to be more an issue of EGT
especially if the NB is unheated. Of course, if you put the stock O2
sensor far enough downstream, even if heated, you induce a phase lag in
the closed-loop feedback, which can cause overshoots in the AFR swings
around stoich (I'm from the mech industrial control world; ask me how I
know :). A lister recommended to me an excellent book by Heywood, called
"Fundamentals of Internal Combustion Engines". Looks from that, like you
could exploit the delay to give you a larger *swing* around stoich, which
the CAT likes, up to a point.

All in all, it seems like the placement of the WB is much less an issue
than with NBs. Soooo, what exactly were you after in your statement
above? I must be missing something.

> It would be nice to run a WB as a monitor on each side near stock 
> O2's
> to use for tuning for open loop operation.  Tune with the WB and set
> economy for 15/1 and WOT 12.5-13.  Use the WB's to watch for changes
> over time. If you were interested in this approach, you could just 
> plug them in place of your stock ones.

Umm, if you plug the WB in place of the NB, then what is going to run the
ECU? Or are you assuming during this tuning, that it's always in
open-loop?

That brings up a question I have...for BobR or anyone else familiar with
the GM highway cruise code: if the target AFR is noticeably higher than
stoich, then highway cruise I presume is maintained using open-loop
control. So my question is, has anyone measured this with a WB to see
just how sloppy the actual AFR vrs. the target is, when in highway
cruise. Seems like you would have to hit the target at least within
1/2-afr say, if you are going to avoid going too lean during hwy cruise.
I guess this also brings up another more general question; how much do VE
changes as an engine ages, *translate* into variance in the open-loop
AFRs the ECU actually hits. If you don't know for sure, take a guess. I'm
too new at this game to have much of a clue.

Tnx.

Brian

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