Pre + Post O2s

[Gary Derian]

The first cats were oxidizing only.  Engines ran lean so there was excess O2
to promote oxidation, HC and CO became H2O and CO2.  This left NOx untreated
because it requires a reduction reaction which is not possible in an
oxidizing atmosphere.  Running a cat at stochiometric and adding rhodium and
palladium to the mix permitted both ox and red reactions to take place.  The
3 way name is because it treats all 3 regulated emissions.  HC and CO become
H2O and CO2 and NOx becomes N2 and the oxygen is used in the HC and CO
reactions.  Overall, cats add heat.

The cat has some O2 storage capacity, the post cat sensor therefore swings
more slowly than the pre cat sensor.  As the cat becomes ineffective, the
post cat sensor swings with the pre cat sensor setting off the SES light.

Gary Derian <gderian at oh.verio.com>

> As I understand things, the cat converters are a reducing chemical
reaction,
> so
> they consum calories to react.  (True or False?).
>
> The rear O2 sensor would run cooler then the front.
>
> So, the rear O2 in order to measure the effectiveness of the cat would
need
> to
> operate at least at the same temp as the front.  So it would need a
"better"
> heater.
>
> Since it might not track the front too well, then it's "range" would need
to
> be greater.
>
> Only have any documents to compare the above to, or does it make sense
> to someone other than me?.
> Bashful
>