Quote:
Originally Posted by ever_green
PLUS lean conditions such as 17+AFR create less nox than at stoich.
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Except your cat doesn't work at >stoichiometric and so your NOx out the tailpipe is way higher. Stoichiometric is cleanest overall precisely because the cat can react the remaining CO and NOx together.
Lean burn reduces HC and CO up till a certain point but at the cost of a lot more NOx. I am not sure if the EPA is right to freak out over NOx so badly but it is what it is with 3-way cats.