Pre + Post O2s

[Chris Conlon]

At 07:28 PM 3/30/99 -0500, Bruce Plecan wrote:

>As I understand things, the cat converters are a reducing chemical
>reaction, so they consum calories to react.  (True or False?).

The cat basically finishes up any incomplete burning that should
have happened in the cylinder. The catalytic part of it makes the
reaction(s) proceed even without an ignition source or flame.

Because of this the cat produces heat, exactly like burning does, just
not as much heat. (Hopefully.)  Chronic misfire is hard on cats
(or so I hear) because the cat has to burn up much more air/fuel than
usual, and may overheat, melt, etc.

A 3-way cat can react an oxidizing agent (O2 or NOx) with a reducing
agent (HC or CO), hopefully yielding just CO2, H2O and N2. Or at
least this is my understanding. (I'm not clear where the "3-way"
comes from; I think it has to do with the exact reactions the cat
can and cannot catalyze. The end result is, it "finishes up" the
combustion.)

Also it supposedly has a limited sponge-like storage ability to hold
small amounts of excess CO, HC, O2 or NOx, so that the exhaust mixture
does not have to be perfectly stoich at every moment, it just has
to average out to stoich over a second or 2. Thus the whole closed
loop operation we know and love.

The above I'm pretty clear on, the rest of this is wild-guess land:

I would think that the post-cat O2 sensor wants to see a perfectly
stoich reading, or very close to it, whenever the engine ought to be
in closed loop. If it shows rich or lean, I think that would indicate
a rich or lean condition beyond the cat's ability to clean up, due
to it being too far off stoich for too long. How far, how long, I
dunno.

As EGT pre and post... the cat generates some heat, maybe a *lot* in
some apps. Of course it also sheds a lot too! If combustion in the
cylinder was perfect, and you were stoich, the cat would generate
no additional heat since there would be no fuel left over to burn.
I guess it comes down to heat generated vs. heat lost, I won't even
guess at that one.

I agree that a really precise setup should measure EGT pre and post
cat, to correct the O2 sensor readings. Let me know if anyone
actually does. ;)

And since I can never resist spouting, how long til we see OEs using
ceramic coated manifolds to speed up cat warm up time and reduce
coldstart emissions? Who's running the betting pool? ;)

   Chris C.