Catalytic converter temps

[Edward Hernandez R]

"Logic dictates that if you are running lean, there would be less
combustible by-products, and the catcon would not have "fuel" to react
with, thus the internal temperature of the catcon would be lower.
(Jeez, I sound like Spock there :)  If, however, you run rich, the
catcon has an abundance of fuel, which would cause it to overheat and
fuse it's internals."

Be careful with your logic, Spock:). Running lean enough can produce lots of combustible by-products. This is exactly what happens during a lean misfire: Little or nothing burns and you send raw fuel and lots
of oxygen into the exhaust system. Guess what happens when it reaches
the cat?

Experience has shown that to reduce excessive catalytic con-
verter temperatures you must run rich. Running at or leaner than
stoich at WOT causes substrate temps to skyrocket. Having an air in-
jection checkvalve stuck open(so air flows after light-off) causes
the same thing and has been known to burn things other than just the
substrate.

A cat literally shuts down when rich. It needs oxygen to reduce and
oxidize the exhaust gas constituents; running rich does not supply
enough oxygen and running rich also lowers inlet temps into the cat,
both of which contribute to it "shutting down". Running lean, which
increases the amount of axygen available and increases inlet temps
into the cat, can and will raise cat temps to the point where the
precious metals themselves will sinter, producing larger and less
effective crystals. The cat becomes less and less effective until it
permanently shuts down.

Looking at a cat efficiency conversion chart tells a lot: as you go
lean, HC conversion efficiency approaches 100%, a pretty good indica-
tion of how much heat gets generated when you go lean. Such a chart
can be found in Heywood's book, if you have one. Damn good book!

Ed Hernandez
Ford Motor Company
ehernan3 at ed8719.pto.ford.com