[Diy_efi] WB O2 system

[Garfield Willis]

On Fri, 14 Jun 2002 14:02:10 -0400, Barry Tisdale
<btisdale at cybersol.com> wrote:

>AFR meter =3D FJO WB.

OK, fine start.

>Made runs @ constant 100 kPa (2nd gear, TC locked, 1500-5400 rpms), =
tuning=20
>the F30 "VE vs RPM" table to get an AFR of 12.5:1 (arbitrary - yeah, I=20
>know, but gots to start somewhere...) as 'flat' as possible.  Got it to =
+/-=20
>0.1 AFR - good enough for me.

Good base to experiment from, a broad flat baseline that's repeatable.

>Turned the water/alky injector on - 1:1 of water/methanol, 24% by volume=
=20
>relative to fuel.  ShurFlo pump @ 225psi feeding 2 x 0.6mm Aquamist =
jets,=20
>valved in time w/ injectors.  AFRs dropped (richer) by right around 0.3=20
>across the board.
>
>Just offering some 'real world' input, ...

I'm sure you mean well and you *should* be happy you're now able to get
good repeatable trustworthy measurements with petrol, but you just
stepped on a monitor lizard of a learning curve when you started trying
to measure 'AFRs' of oxygenated fuels AND water injection with a
straight-petrol-calibrated meter. BTW, have you asked Fred about this
whole subject? You should, because ...

You must understand that you are NOT measuring AFRs anymore. You've got
'indicated AFRs', which now include alternate fuel artifacts from the
way the sensor works; this number your petrol AFR meter is showing you
is most certainly not the ratio of your fuel mass (petrol and alcohol
included) to your inducted air mass. That's what AFR is, ya know; that's
not what you've got here.

You're probly thinking, so what does that matter? Well, lemme think of
an analogy: umm, it matters about as much as worrying about temp
measurements in a known system of units. Like saying, "my IAT
post-intercooler runs 20degTisdale below inlet temps". If we don't know
what 20degT means in relation to others, all you can measure are
relative deg-T readings. If that's all you're trying to do, then the
'real world' is your little world alone. If you're just looking for
things that lower or raise the deg-T of you inlet air, then fine. But
even here, until you get used to deg-T and 'how much' hotter or cooler
1degT is, you don't even have a feel for 'how much', only 'it's cooler'
or 'it's hotter'. Even with people switching from degC to degF, it takes
a while before they get used to the differences in 'how much' is a
degree. Degrees C are significantly 'bigger' than degrees F, is usually
the felt concensus.

So, here YOU have AFRTs, or AFR-Tisdale, a unique unit of exhaust gas
measurement known only to you and your sensor. But then all you know is
more fuel moves your AFRTs lower, more air moves it higher. And compared
to actual AFRs, you don't even have a 'how much' comparison. And even in
the 'world of Tisdale', the laws of nature are going to change if you
replace that sensor with another one. Aack! Stop. A somewhat limited use
measurement technique, I would think. :)

Remember, the factory Cal R is an aggregate correction that assumes
*petrol* ratios of C:O:H. All the meters commercially made that handle
arbitrary/alternate fuel issues, calibrate their sensors for EACH
component individually. You can see why; two different sensors with
different 'compensating' sensitivities to CO & H2, for example, could
read the same overall for petrol, and have the same Cal R value, but not
read the same for some other fuel, unless the individual sensitivities
are known and corrected for individually.

Choked (or bored) yet from the firehose? If not, read on.

Since AFRs were first measured using the 'direct method' (namely
measuring fuel flow rates and mass air flow rates, and taking the
ratio), they always have MEANT something about ratio of air to fuel, so
different people can work together and compare notes (or you can change
sensors and still compare previous results with current results), and
ALSO figure out what amounts of additional fuel or air delivery are
required to move the AFR where you need it. Same ole story again about
the meaning of 'not lab grade', only this time it's not a euphemism to
hide inaccuracies, it's because you're not correcting the sensor/meter
for the alternate fuel artifacts.

>... FWIW - YMMV, etc all to hell...

Yes, I agree, that IS the question: WIW, or 'what exactly IS IT worth'?
There ARE some things you can observe qualitatively at least from what
you're doing, so it's not all confusion.

You've got THREE phenom going on there. First of all, notice that you're
increasing each: your fueling, oxygenates, and diluents. Without even
numbers/magnitudes, start this way: with your example squirts of 50:50
water/alcohol: (1) you've increased F, the fuel part (because alcohol is
combustible), so the effect should be to -lower- the indicated A/F
Ratio, ignoring all else; but now add (2), you've increased your A, well
not exactly, but you've increased the Oxy available to the fuel to burn
without having to induct it, so the effect should be to -raise- the
*indicated* AFR (the sensor thinks there's more oxygen), unless you've
increased your petrol injected also. Next add in the effect of the
diluent (water), which adds nothing to fueling or oxygenates, so is
chemically neutral there, BUT it adds water vapor to the exhaust, which
would tend to lower the partial pressure of the gas components your AFR
sensor is actually measuring, which would -raise- *indicated* AFR if on
the rich side, and lower indicated AFR if on the lean side.

See why it's important now to be able to figure the magnitudes of these
effects? Because you're got artifacts pulling in opposite directions,
until you know the magnitudes, you may not even be sure which
*direction* your readings will change, let alone how much, or what that
'how much' means.

Sidebar: You can more easily grasp the diluent effect on the sensor if
you remember that on the rich side, the sensor is sensitive to CO, H2,
CxHy, and the more of these, the richer the mixture burning. But
diluents reduce the partial pressures of ALL these components, making
the exhaust appear leaner (aka less rich) than without the diluent. Same
on the lean side; the sensor is sensitive to excess O2 partial pressure,
the more O2 the higher it's partial pressure and the leaner the exhaust.
But diluents reduce the partial pressures of all gases in this lean
exhaust, including O2 partial pressure, making the exhaust appear richer
(less lean) to the sensor than without the diluent. If you want to think
of it this way, diluents reduce the "gain" of the AFR sensor either side
of stoich, but don't change stoich; so either rich of stoich or lean of
stoich, diluents make the sensor see LESS of either actual richness or
actual leanness. Thus, diluents make rich mixtures appear less rich, and
lean mixtures appear less lean.

Now, here's the point, and then the question. You CANNOT assume that
just because your indicated "AFR" from a petrol-calibrated AFR meter
you're measuring with says 0.3AFR less, that you are *even* running
richer, let alone an actual 0.3AFR richer. It's conceivable that with
the right mix of additives, you could actually BE leaner, while your
petrol-calibrated sensor/meter INDICATES 'richer'.=20

Did you read what was previously posted about this issue of correcting
your AFR sensor for fuels (and oxygenates) other than straight petrol,
and not understand it? Those issues are much more 'real world' than your
present AFR readings in your circumstances. We're not talking 'theory'
here, but real corrections that are done in the real world of alternate
fuel racing every day.

I recommend you ask your vendor for a tutorial to help you do these
corrections by hand so you know what you're getting from what you're
measuring. After all, if you can't compare and repeat measurements with
others, you're just playin horse-shoes wiff yersef. And you might not
even know where the peg is placed. :)

Please don't misconstrue what I've said above; your FJO interface box &
NTK sensor you have are fully capable of measuring actual real AFRs even
with your fueling changes; you just need to know how to correct the
numbers from 'indicated' to 'actual'. Now that's real whirled, idnit?

Ask Fred how, he should be able to provide you with the necessaries,

HTH,

Gar

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