O2 sensor is actually a temperature sensor???

[Garfield Willis]

On Thu, 20 Jul 2000 00:19:52 -0500, "Eric Payne"
<uniteknik at earthlink.net> wrote:

It was suggested in an earlier post,
>>Basically, either they taught you wrong, or you remembered wrong.

That's quite possible, of course, OR, he remembers from "wayback"; if he
studied this at some point in the '80s, and then his training was
arrested and he hasn't been keeping up, his syllabus may have been
written during a time when the titanium coatings were being heavily
explored. You can sure see that from the patent history. MANY
titanium-based patents in the late '70, early-mid '80s, but by the early
'90s activity seems to have died out. It's still an oversimplification
to say these sensors are mere "temperature sensors", tho.

The reason why you don't hear of "titanium" vrs "zirconium" O2 sensors
much is that they all start out built on a substrate of solid
electrolytic zirconium (least all that I've ever read about). The
"titanium variety" has to do with using a layer of some so-called
"nonstoichiometric metal oxide" (and I gather a few different metals
other than TiO2 have also been tried) placed atop the ZrO2. Hell, if you
look at the patents for all these sensors, both early and late, you'll
find a number of elaborate multilayer ideas were explored.

I gather in the early days, there were two problems with the simple
platinum on zirconium constructions: the platinum would wear away if
used too thinly, and would migrate and plug the ZrO2 if used too
thickly, and the pore size necessary for useful difussion rates was
small enough that combustion byproducts (carbon and lead) could block up
the sensor. (Remember I'm not a physical chemist, as I've often said, so
I'm just trying to convey the gist of what I've read hope I understood).
Researchers started looking for ways to deal with these problems, and
one of them was the use of an intermediate layer of TiO2, which wasn't
as sensitive to lead & other contaminants (it's useable pore sizes were
larger?) and because of it's electrochemical nature, it didn't need to
reach as high a temperature as the ordinary Zirc sensor designs.

For some reason I haven't ever heard or read, the titanium layer
contruction seems to have fallen out of favor fairly early in the patent
history AND in the mfg. of actual commercial sensors, UNTIL only
recently! (Toyota and Nissan both use a few new ones hereNthere). Since
I really have only scratched the surface of this "history O2 sensor
technology", my best guess is that the recent focus on improving
cold-start and warmup emissions has brought back an interest in these
so-called "titanium" O2 sensors, because they are usable at a couple
100degs-C lower temps than the conventional "zirconium" O2 sensors, as
Eric alludes to below (BTW, I think you meant 600degC?)

>This is correct in the zirconium type oxygen sensors, however the titania
>type sensors are constructed differently and have a few different
>properties. For instance, the Titania sensor does not have to be heated to
>600 farenheit in order to output a reliable reading.  Also, instead of using
>the current flow between two platinum plates, it uses a semiconductor that
>changes resistance in relation to oxygen content in the exhaust.
>
>I guess nitpicking isn't the best way to make my first post, oh well, I'm
>not like this all the time. =)

Not if the nits are possibly important. I'd like to hear more about the
history of these variations that were tried and then seemed to die out,
only to resurface way down the road, when suddenly one of their
properties becomes of greater interest again. The recent emergence of
the 4-wire WBO2 sensor (a "Zirc" sensor, BTW, and NOT the same as a
temperature-compensated conventional HEGO, like the Bosch LSM-11) is
also a case in point; their electrics is more complicated even than the
O2-pump style 5-wire sensors, because they have to be run in "phases"
relating to the amount they are warmed up or "activated" (to use the
parlance of their vernacular), BUT because they can be made to detect
stoich zero-crossings at an earlier time than the 5-wire type, they seem
to be taking over as a sensor that has the combination of eventually
being useable for some actual AFR measurement once high temps are
reached AND can also help out the OEM vehicle mfg. with his cold-start &
warmup emissions requirements. Because the sooner the O2 sensor can be
used to at least find stoich, the sooner the ECU can be placed in
closed-loop. Soooo, altho these 4-wire WBO2 sensors aren't apparently as
accurate for AFR measurement purposes, they represent a better tradeoff
for smog. Hence Honda themselves switched from the NTK 5-wire sensor, to
a Denso 4-wire WBO2, in the '96 and later models of the Civic HX.
They're about the same price as repair parts, so it wasn't likely an
"economy" move, but rather a shift in focus of priorities, and this
4-wire WB technology became an overall better choice for them.

Just some thots on a fascinating subject, IMHO.

Gar


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