UEGO AFR Precision

[Gar Willis]

On Sat, 07 Aug 1999 03:03:04 +1000 Phil Lamovie <injec at ains.net.au>
wrote:

>Be wary of any O2 sensor voltage vs  A/F ratio inferences. If you
>don't correct for temp there is a drift of 1.5 A/F over 400 deg
>c. And that's for the expensive BROAD BAND device.

Whoa, not so fast; you wanna always QUALIFY *which* so-called UEGO
you're refering to. The last generation Bosch non-current-pump wideband
varieties, such as the LSM-11, I wouldn't be too surprised if they DID
vary 1.5AFR over 400degC (their spec is only given for a 250degC range,
so I dunno), but that does NOT apply to the NTK current-pump sensors,
nor the Bosch LSU current-pump sensor. Hell, that'd be bloody 10%, mate!

I hope that's not what you're getting from some broadband UEGO equipment
you are using!

Sadly, Bosch hasn't published much of anything on their "LSU" sensor
itself AFAIK, so I dunno what the raw sensor specs are like, but I have
seen a operator's manual for their LA-3 meter that's based on it, and
their box, unlike most of the NTK-based devices, DOES regulate the
heater voltage, using no less than a PID controller (for those
unfamiliar, that's control system jargon for a "Proportional Integral
Differential" controller), basically a fairly powerful controller
method, just for the heater current/temps. But the NTK data seems to
indicate that this isn't really necessary until you wanna reach "lab
equipment" precisions. The rumor someone spread a couple years ago, BTW,
that the NTK sensor's heater would burn out if connected directly to
batt. is a myth, a veritable automotive urban legend. Course I imagine
if you DO tightly regulate smack at 12V, you're right in the middle of
the operating range temp-wise AND you've eliminated most of the temp
dependant variations. That's noice. :)  Bosch may have also opted for a
construction on the LSU that minimizes exh. pressure dependencies, at
the expense of temp. variations (see below), and that's maybe WHY they
need to go so such extremes in heater regulation, for their AFR meters.

Since you're a confessed SAE addict, you probly have the seminal SAE
paper on the NTK/NGK sensors, and in there (#920234), you'll find a
graph on pg. 30 (figs. 14 & 15) where the AFR is within a band of
+-0.2AFR on both lean & rich sides of stoich, over the usable operating
temp range of the sensor (600degC to 950degC), which is also within the
heater's range of temp authority over a varying batt. voltage of
10-14.5V. I've confirmed these AFR delta specs in testing the
soon-to-emerge EGOR systems, but then again, I haven't tested a
bazillion diff. sensors, admittedly.

Because of the strongly positive temp. coefficient of the heater
element, even if the heater voltage were regulated, the heater current
would adaptively increase/decrease with corresponding decrease/increase
in EGTs, so I think that graph is meant to demonstrate the sensitivity
of those sensors to tip temperature overall, as well as the variation IN
tip temperature that can be expected over varying battery voltage alone.

The article, I have to acknowledge is kinda old ('92), and it appears
that the sensor "design" they actually deployed, in OEM applications at
least, is NOT as sensitive to exhaust pressure as they indicate. I wanna
test a lot more sensors before releasing any exh. pressure compensation
data for EGOR, so that's all I'll say at this point. They mention in
this article that they have a tradeoff point they must pick when
constructing the sensor, between temp sensitivity and pressure
sensitivity, and that they opted for reduced temp sensitivity. That
discussion is on the bottom of page 28, and is pretty interesting. We
haven't found NEARLY the level of exhaust gas pressure dependency that
the paper suggests in their graph on the same page (fig. 11), so I think
they must have made some 'other' tradeoffs in the process of actual mfg.
& deployment in the field. Suffice it to say, it doesn't appear they've
done anything to mess with the low temp sensitivity, tho.

When EGOR-the-module get's announced here shortly (real soonium, now),
I'll discuss some of the details on the testing apparatus we've used on
the Honda/NTK sensors, for both temp & pressure, AND some special
surprise tricks on calibration if you wanna get REAL high precision with
EGOR.

>DISCLAIMER .......I am a hopeless addict of the SAE book store
>and would buy everything if I could.

I'm sure you'd be welcome to join the nearest SAE bookaholic's anonymous
chapter. You'd find there, MANY souls just like yerself, from all walks
of life. :)  [Just too damn bad the drugs are soooo blinkin spendy!]

Gar