I'm a little confused by how catalytic converter overtemp protection works. I've read in a number of threads that the car dumps extra fuel (ie: runs rich) to help cool that cats down.

Since the job of the cats is to BURN any such UNBURNED fuel, how does that help?

I know running rich cools the EGTs, and maybe it really is that simple, but I'd like think the cats would light off that fuel and get even hotter.

Can anyone explain this seeming paradox?

 

https://forum.efilive.com/showthread...emp-Protection

 

Thanks, but they really just confirm the paradox by saying:

1) Overly rich causes the cats to burn the excess fuel causing the cats to glow bright orange/red
2) Converter over-temp protection utilizes a relatively rich mixture to lower EGTs

Unless it's as simple as "A little richer cools things down unless you don't go TOO rich"?

 

I think you zeroed in on it Dave. I would file it under a generally workable but not perfect solution to a real problem. As long as the overly rich mixture is still consumed before it reaches the cats it will reduce the EGT and contribute less to cat heating. But if too many unburned hydrocarbon products make it through your cats will be going through the same process modern diesel DPF filters go through in their self cleaning cycle except the cats aren't designed to withstand those temps.

It would be nice if the cats could be temporarily bypassed under brief high stress conditions; if it were legal a "NPP like option" that could bypass when needed would be popular but I don't think the EPA would give their blessing even though drag strips and road race courses should fall under the off-road operation exemption.

This cooling technique probably resulted from the process where engineering was asked to provide the best solution and was told afterwards, "that was interesting. Now give us your cheap solution." I give GM credit for using a 9th injector in their diesel pickups to inject raw fuel just ahead of the DPF when it calls for a cleaning cycle. Ford uses the regular engine injectors to dump fuel on the exhaust stroke to over-fuel the DPF during cleaning and in the process generally dilutes the engine oil with diesel fuel which is OK according to Ford.