[Diy_efi] WB O2 system

[Garfield Willis]

On Thu, 13 Jun 2002 10:02:29 -0700, Dennis Gearon <gearond at cvc.net>
wrote:

>I would think that the WB systems would be able to tell if you are
>stoicometric(sp) and using a recorder taking in rpm, map, and
>accelleration, you could get best acceleration in areas that you don't
>run stoichometric(sp).
>
>Anything not being stoichometric is for power purposes only, right? And
>off of all main 'putt putt' speeds?
>
>that assumes a street car, not a race car.

Yes, the first part is right, since 'stoich is stoich'. No excess
oxygen, no partial combustion byproducts, so for every fuel, the sensor
is doing the same thing at stoich, namely NUTHIN.

But if you want to talk in comparable numbers of lambda or AFR, when
changing your fuels, and be sure you're in the 'safety zone' at WOT, you
need to correct for the effects of the fuel on lambda/AFR. Otherwise,
you can be WAY off.

So let's say you knew the stoich AFR for your particular fuel, like
TomS's example of gasohol (10% eth + petrol). OK, it's roughly 14.2AFR,
so you think, pieceOcake, I'll just scale or maybe shift the AFR curve
down 0.5AFR, and voila, afr meter recal'd for gasohol. Sorry, BZZTT.
Neither one is even horse-shoes close.

The reason is pretty obvious once you consider the additional effect of
the oxygenates in the fuel, which not only affect the combustion ratios,
but ALSO alter the amount of exhausted water vapor, which since it's in
gaseous/vapor form, dilutes the other components the sensor is sensitive
to. The alcohols are prodigious water vapor producers. This happens both
rich and lean of stoich, to different degrees. So all the partial
pressures of the components are also skewed by the wetness of the
exhaust. This is ALSO true of petrol combustion to a lesser degree, but
the effect is already built into the sensor's curve for petrol, because
the amount of H2O byproduct is known for petrol combustion. But change
the fueling, and especially when adding oxygenates, and that's no longer
the same at all.

Just an example of not making things 'simpler than possible'. It's not
rocket science (hmm, well maybe it is sorta in this case, rocket engine
science anyway :), but it IS a bit more complicated than simple
conversion factors or multipliers or rulesOthumb.

If you need to understant these issues, and don't have a meter/mfg that
supports your alternate fuels, you need to find yourself a good chemist
that knows MRM Combustion Stoichiometry, and explain your situation to
him. Plan on spending quite a few beers (or bucks) in return for the
imparted knowledge. Then get ready for alot of hand calcs if your meter
doesn't support them. That's assuming you have a real honest2gawd AFR
meter.

HTH,

Gar Willis
Principal Engineer
EGOR Techno
3491 Edison Way
Menlo Park, CA 94025
650-216-9874
garwillis at msn.com (e-mail & PayPal transfers)
www.egortech.com (best viewed with IE)


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