Quote:
Originally Posted by Isaac Zackary
Isn't stoichiometric the hottest?
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stoich would be hottest with perfect mixture/atomization characteristics, but that isn't the case with any engine that I'm aware of(or at least, not outside of a very narrow RPM/load range). you'll always have some fuel somewhere in the chamber that isn't quite mixing and combusting at the same time as the air/fuel charge around it. essentially delayed combustion, which lowers the overall flame temperature a bit from it absorbing heat but not combusting.
with a little less fuel(say 15.4:1 vs 14.7:1), that pocket of fuel may not exist, preventing it from effecting the overall flame temp, causing higher overall combustion temps. now you just have air that didn't mix with fuel, which isn't nearly as detrimental to the flame temperature as air that didn't mix with fuel AND fuel that didn't mix with air.
either side of whatever ratio your engine burns a charge the fastest at will burn cooler. if you were really curious and happened to have control over commanded air/fuel ratio, you could install an EGT probe pre-catalyst and see what ratio causes the highest EGTs. there is some variance to be accounted for, different ratios require different amounts of spark advance to make best power per fuel unit, so being able to control air/fuel ratio and spark advance on the fly would be advisable. overly rich AFRs and retarded timing can do some really interesting things to exhaust temps though, since that is a recipe for sending a still burning air/fuel charge out of the exhaust port, wreaking havoc on everything along the way.
I've run everywhere from stoich up to 17:1(on E10) with no EGR, normal amounts of EGR and HUGE amounts of EGR(as much as the valve could flow) up to the point of lean misfires, EGR and lean burn do accomplish much of the same effect when used correctly as they both allow the introduction of excessive amounts of air(and "air") that isn't used for combustion. for reasons of cost, complexity and emissions requirements, EGR has been fairly universal and lean-burn much less so until recently.
can't speak for water injection though, never tried it. I see the appeal for certain circumstances though.