Quote:
Originally Posted by swamp2
Quite a few factors involved actually. But I think we are both wrong...
-Combustion temperature
-Combustion rate
-Combustion energy transferred to engine (and hence engine temperature)
-EGT, exhaust gas temperature
-Greatest power from combustion
-Engine timing
-A stoichiometric mix actually makes the hottest flame.
-A rich mixture will always make the most power.
-Both a rich and lean mixture will burn with a cooler combustion temperature than stoichiometric. Rich does so because the uncombusted fuel absorbs energy when vaporized (heat of vaporization), lean does so simply because there is less fuel energy available and hence less combustion.
-A lean mixture will also burn slower which exposes the combustion chamber to the heat for longer causing the engine to heat up more.
Both the engine and cats need to be properly warmed up and warmed up quickly to reduce emissions. Thus a lean mixture is probably employed. It probably also employs a substantially retarded timing (which also increases exhaust gas temps and hence cat temps). Probably the latter causes the rough idle more than the AFR.
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I'm not sure you have it all right here.... I'm pretty sure a lean mixture burns hotter.
I was under the impression it runs richer because it's easier to get a cold/high compression engine started with a rich mixture - think back to the days of a choke on a carburetor, it cut off air so the mixture was richer. Also a rich mixture dumps more unburned fuel into the cats which then burns there and heats them up quicker.