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what are the effects of an excessively rich and lean mixture on an
-- hayden forbes, May 16, 2010
Excessively rich and the spark plugs will foul, especially when taxiing around or otherwise at low power settings. Excessively lean and the engine will stop.
-- Philip Greenspun, May 20, 2010
Also, on high-performance, often turbocharged engines, at high altitude, excessively lean mixture could cause catastrophic detonation inside the engine cylinders, which could ultimately destroy the engine.
-- Joshua Levinson, May 21, 2010
As Philip points out, a very lean mixture will simply shut the engine down, either because the fuel/air ratio gets too small to be combustible, or because the amount of fuel becomes too small to give any power.
Tests show detonation to be a fairly rare occurrence, and while turbocharged engines are more prone, it takes very high temperatures (high powersetting) and a mixture ind the best-economy-to-best-power-range to get the best "chance" of detonation. Further leaning will reduce risk of detonation.
Excessively rich and excessively lean mixtures both decrease temperatures and combustion speed. Excess fuel, as in rich mixture, cools the engine somewhat, but the effect of unburned fuel as a coolant is generally overrated. The cooling is mainly due to other effects, like lower combustion speed.
Moderately-excessive leaning wil result in rough running in many, especially carburetted engines, due to uneven fuel distribution between cylinders. The resulting vibration prohibits these engines from running at a very lean setting. In such engines one often has no choice other than running at a fairly rich mixture to avoid high engine temperatures at high power settings.
Most naturally aspirated piston engines can be run at power settings up to 65% without overheating the engine, regardless of leaning.
-- Henrik Vaeroe, May 21, 2010
... (sigh) think about it. We're in 2010, our planes are carbon fiber, our cockpits are computers, our autopilots are perfect but we STILL have to spend valuable pilot bandwidth on this leaning business. Will anyone ever crack the deadlock of aircraft engines?
-- Antoine Edde, June 21, 2010