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There are only a few reasons engines will intentionally run rich. Most of those occur when something is broken and the engine is trying to cool itself, but one occurs somewhat regularly.
If an engine can burn lean and it kicks back into stoich or rich or if its running stoich and it kicks into rich its doing it because cat temps are dropping. Cats are very simple, think bee honeycomb coated in expensive exotic materials. Its a very tight mesh hex comb because that traps fuel deposits against the materials that are very hot. . .fuel heat. . .fire. This removes the HCs the exotics are to remove other noxious compounds. If it for regulations cats could run as cool as you wanted, but the epa dictates that the cat operate at x temperature to make sure all HCs get consumed(even if there are 0 HCs. . .)
I don't think its so much the EPA is corrupt. . .more just misguided entirely. To avoid thread jacking I'll just say they don't understand necessary evils and ban the least of all evil options(R-12 for example) just because its most prevalent and causes eco damage. . .(134 does more, uses more fuel and does a crappy job).
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