Quote:
Originally Posted by theunchosen
You are both arguing slightly different things.
ROFL you are saying that the sheer heat is not likely to get to a temperature high enough to damage the cat for a long period of time. That is true.
Chris is saying that if the cat manages to get hot during the engines warm up status when its flowing very fuel rich the fuel could ignite.
Which would be outside "normal operating parameters" and in the outlier region for a research paper.
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Umm... The fuel igniting is how a cat works. That's what oxidation is. It's a totally normal operating parameter.
Quote:
Originally Posted by theunchosen
That said, cat's catching fire is not unheard of uncommon. It happens. Chris says its happened to him twice. I find that reasonable since my grandfather owns and is one of the main mechanics for a garage and I know this happens sometimes.
What usually happens is for some reason or other the cat gets too hot when the engine is spooling up and dumping fuel. the fuel ignites as it enters the cat(see backpressure= increased heat) and gets obstructed as it tries to expand in the cat. the still rich fuel mixture coming in keeps it going much like a Gas Turbine engine and a "flame holder" (It's actually the exact same thing just the GT does not use any fancy metals on its skin). It would only take a few seconds before that kind of heat would do damage maybe not to the cat directly but surrounding components. . .i.e. the floorboards catching fire or melting nearby components.
Also the enclosed gasoline burn in the enclosed space of the cat would be much greater than its outside burn temperature. definitely enough to burn through floor boards. Seen it.
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They're always on fire, well, at least causing them. What you're referring to is when the engine runs really rich for whatever reason and coking leads to an exhaust restriction, which leads to the driver putting their foot down farther to get the same amount of power, which leads to even more unburnt HC in the exhaust stream. Rinse and repeat and the end result is a really hot, maybe melting, coked up cat that'll heat up itself and anything near it.
A condition rich enough to cause this will not happen if a vehicle is operating properly, even if the cat is already warm while the engine is cold. Otoh a carburettor where the float isn't set right, a malfunctioning oxygen sensor, in the case of open loop for OBD-I/earlier running WOT really rich through screwing w/ the AFM/whatever measures the amount of air coming in, no ignition in one or more cylinders, etc... could result in the cat getting too hot and coked up/melted from a constant rich exhaust stream.