Re: Muffler Madness

["Robert Myers" <rmyers@inetone.net>]

Sean,

The "cat" or catalytic converter functions to catalyze the oxidation of
unburnt fuel to form CO2.  It only functions relatively efficiently at
elevated temperatures.  While the cat is cold, it simply passes the unburnt
fuel on through to the environment via the exhaust.  It has virtually no
"storage space" for the crud.

This is why the sniff test for hydrocarbons is done at running temperature
not with a cold engine.

Once the cat is heated sufficiently by hot exhaust gases it will further
heat itself as the hydrocarbons are oxidized on the surface of the platinum
 impregnated catalytic surface.  This is the same principle you see in
action with some of these propane heaters which produce lots of heat but no
flames.  They just glow as the propane reacts with oxygen and produces heat.

The cat will eventually reach a more or less steady state condition in
which heat lost from the cat via radiation and convection and wind cooling
balances the heat from the exhaust plus the combustion inside the cat and
the temperature stabilizes.  That's why a heat shield is needed for the
cat.  It gets *seriously* HOT!  Hot enough to ignite dry grass under an
idling car or to melt the foam backing of carpeting over the cat or...

At 08:34 AM 2/13/98 -0500, you wrote:
>
>
>To:   quattro
>Jim,
>
>This is all guesswork, and I'm sure the more technically advanced among us
>will either correct me, or give a better explanation, but doesn't the cat
>block a bit of the more noxious emissions when cold? I may be confusing
>this process with another car, but I thought the cat held back some really
>dirty stuff until it was warm and then let loose once it was able to better
>clean up the exhaust. Anyone feel like spanking that theory?
>
>Sean Ford           '92 Audi 100CS 5-spd 34k mi
>Sean_Ford@idx.com        '98 Honda CR-V EX auto 0K mi
>IDX,Boston,MA       http://www.geocities.com/MotorCity/Downs/5528
>

---Jim Griffin story snipped---


___
   Bob
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