[Diy_efi] WB O2 system

[Garfield Willis]

On Mon, 10 Jun 2002 12:52:29 -0700, Dennis Gearon <gearond at cvc.net>
wrote:

>What does 'wide band' mean anyway?

A "marketing term", not originally used by the developers of these
sensors able to measure AFR/mixture, but introduced later as an aid to
distinguishing between the 'switching' type sensors that can only be
used to detect either lean-of or rich-of stoichiometric. Over time the
O2 sensors that could measure wide ranges of AFR well-lean and well-rich
of stoich, came to be called 'WB' and the standard emissions O2 sensors
that could only detect stoich crossings came to be called 'narrow band'
or NB sensors.

Originally, the term used by mfgs of these sensors was "UEGO", or
'universal exhaust gas oxygen sensors', and were originally focused on
lean-side measurement for lean-burn vehicles, but probly because the
term wasn't very descriptive and also was harder to communicate,
'wideband' or WB has caught on now in the trade. Somewhere in this
timeframe, IIRC Bosch also introduced the term 'lambda sensor' to ID the
WB sensors it was developing. Besides, most Euro's talk in lambda's
rather than AFRs.

>Is it fast? Could I put it into the
>header collector, and using timing delays, calculate the mixtures of
>different cylinders?

Variously spec'd at from 100-200mS response time (depending on how
measured, among other things), the original NTK/SAE paper suggested it
was feasible to do what you've asked above, at least at modest rpms, but
the numbers don't seem to work well at the WOT rpms where such a
scenario would likely be most needed . But with those caveats, yes, at
least to be able to see any imbalance. But there are other factors that
mitigate against this being very practical. One is pressure effects; as
soon as you try sensing instantaneous AFR of the exhaust gas 'slugs'
from each cylinder, you also begin to see the effects of the
instantaneous pressure pulses associated with these slugs. And those
effects are both significant and not easily corrected for. Add to the
fact that there is always a fair amount of mixing by the time you reach
the collector, and removing the pressure effects so you are seeing the
actual AFR per cylinder, and ... you rapidly reach the point where it's
just easier and more time/cost-effective to put a sensor in each runner.

So, depending on your interests...if you are a serious racing
practitioner, and NEED per-cyl tuning, the cost per sensor and per
channel of electronics has each come down dramatically in the last
couple years, to the point where, for example, our Medusa product for
dyno shops & per-cyl tuning, is now around $150/channel for electrics
and $125/sensor. That's down from around $1000/channel just a couple
years back.

OTOH, if you're a hobbyist or experimenter, by all means give it a go.
I'm sure it would be fun and intriguing. But I think you'll find the
fact that AFAIK, no commercial AFR sensing vendor provides this sort of
'time-multiplexed multichannel sensing' by attempting to separate out
each cyl's signal at a collection point, is indicative of the overall
viability (or lack thereof) of this approach. Too many variables, not
the least of which are interesting oddities like the fact that exhaust
waves are 'finite amplitude' pressure waves (much higher pressures than
normal sound waves), and so travel at speeds not only well above the
mass-transport speed of the exhaust gas slug, but also exhibit
characteristics quite unlike mere sound wave propagation. So the science
of unravelling all these effects in the time domain becomes natty.

But per-cylinder AFR sensing IS definitely coming. As the cost
per-channel of electronics begins to drop below the cost per-cylinder
for each sensor, it becomes increasingly viable, and there ARE some real
insights to be gained by looking at and tuning each cylinder
individually.

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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