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Originally Posted by RobertISaar
#2 is the cat itself... supposedly they operate more efficiently when they are put into situations of slighty rich followed by slightly lean.... i really don't understand why, but it's been suggested that it needs some extra oxygen inside it to help the reactions. if this were the case, an AIR system that pumps air into the cat nearly all of the time would probably work quite well. or at least get it hot and keep it that way very quickly, though it wouldn't hurt to estimate cat temp so not to overheat it.
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This is primarily why modern engine computers have AFR swing around stoich as you described. It's to get rid of air injector pumps that would be driven as an engine accessory - one less drain off the engine's output. The extra oxygen is pumped into the cat substrate during a lean cycle, and is then used to reduce the slight amounts of NOx and CO in the exhaust stream. This stored oxygen does not normally get depleted under normal operating conditions, but will get depleted when AFR goes rich (as with hard acceleration).