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Re: Needed to measure AFR



On Mon, 21 Aug 2000 12:22:01 -0500, you wrote:

>6:1 for cold start and warm up.
>Alex
>----- Original Message -----
>From: Greg Hermann <bearbvd@cmn.net>
>To: <gmecm@diy-efi.org>
>Sent: Saturday, August 19, 2000 11:54 PM
>Subject: Re: Needed to measure AFR
>
>> At 12:00 AM 8/20/00, Bruce Plecan wrote:
>> >What range of AFR is really used in a common EFI engine
>> >I'd think 15.5:1 to 11.5:1 would have to cover the
>practical limit,
>> >yes-no?..
>> >For conventional engines.
>> >Bruce
>>
>> I would say go up to 16.5 or 17 to 1---this lean is useful
>for lean cruise.
>> Otherwise fine.
>>
>> Greg

Just a couple comments on the fly: first, twould be nice Alex to be
able to go down that far pig rich, but I've not seen any AFR
instruments able to measure down as low as 6:1. I think the actual
extreme practical limit on the 5-wire sensors is around 8:1, but even
there, you're pumping a LOT of current into the O2 pump portion of the
sensor, and being pretty hard on the poor ole sensor. One of the
reasons many of the commercial AFR sensors/instruments start at around
10AFR (Egor starts at 10.6AFR) is the tradeoff between practical needs
and limiting extreme treatment of the sensor. You do NEED to be able
to get down into the lower AFR ranges necessary with forced induction,
because there the ops are critical to power/safety, but accurately
measuring way down in the cold-start mixtures isn't the same kinda
criticality, unless maybe you're a smog guy.

Finally, just remember that the commanded AFRs you see during
cold-start aren't likely anywhere's NEAR the actual AFR seen by the
combustion process, since alot of that extra fuel is added to
compensate for condensation and waste, especially in wet inductions.

Since I gather Bruce's original question was about measuring during
longer-term operating ranges, I'd have to say that in all the runs
I've done with EGOR on both carb'd and FI'd engines, you DO see quite
a bit of ops in the higher AFRs. They're not "steady-state" in the
sense that you expect to see 17AFR during cruise (unless you're REALLY
good or have a lean-burn design), but for example, if you have a
carb'd setup, you will see high 16s and low 17s instead of the normal
EFI fuel-cut, when decellerating, to prevent backfire. I would spose
that guys tuning carb'd applications might want to measure these lean
AFRs to make sure they're throttle-back setup is correct. If you
consider that issue, AND the desire to be able to SEE the difference
between these extreme leans and actual fuel-cutoff, then I'd say you
would likely wanna see at least up to 17 for measurements, and THEN
give yourself another AFR of range so you can clearly see the
difference between that and actual fuel-cutoff. (e.g., EGOR blinks
when you go beyond the 18.6AFR measurement limit, and that makes it
very apparent when you've gone to fuel cutoff in an EFI system).

Just me dos centavos, amigos.

Gar


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