Would "efficient" aircraft leaning give an appreciable improvement in performance over a long period for the engine and generally cheaper repairs during overhauls? I ask this because I was always taught to lean the mixture on taxi because of excessively rich mixtures on taxi with a lower power setting (low MP), would often leave unburned fuel and "gunk up," as they called it, the spark plugs. My thinking is maybe the unburned fuel could harden and cause excess wear on any of the other engines moving parts.
Does unlit fuel with mixed oil mix badly?
At higher power settings does the higher MP and engine temp cause better firring and total fuel burn, leaving the mixture unimportant?
Does this change with altitude? IE the richer mixture as you climb can it get to be too rich?
Oh and don't forget the very first question, that is the important one.
Thanks hope that isn't too much for one post!
O.K., let me start off by stating that I spent more than my far share turning wrenches, and have seen the affects of an improper mixture. A few things come into play, and I'll try to sum them up as simply as I can.
#1 cause of engine wear with an overly rich mixture is washing down the cyl. walls with fuel, which causes cyl. wall glazing. Cyls. are honed with a cross hatching that helps to hold the oil on the walls (more surface area for the oil to stick to). When you run an engine rich, the fuel thats not burned acts like a solvent to wash the cyl walls down, and the oil gets washed away causing excessive wear and glazing.
The second factor is the unburnt fuel will get past the rings, and into the oil, diluting it, causing engine wear. Thiner oil (i.e. 10w30 vs 20w50 or 100w), will still protect the engine fine. But if it has a solvent in it (i.e. gasoline), then you usually have metal to metal contact. When an engine starts to make metal, it's usually the beginning of the end.
one more factor is the excessive carbon buildup from a rich mixture. I'm sure you remember the two phrases heard during your systems training, detonation and pre-ignition. The two of them are very destructive conditions. Contrary to common misconceptions, detonation is actually the worse of the two. Have you ever heard your car making that "pinging" noise while climbing a hill under high power settings. Thats pre-ignition. Whats happening is the fuel is starting to light too soon, or you have hot spots causing two flame fronts to collide in the combustion chamber. It's not too bad for the engine, but is the precursor for detonation. When the spark lights the fuel in the engine, it's a gradual burn. Pre-ignition is when two fronts collide. Detonation is very, very bad for an engine. It will make the top of the piston look like someone beat it up with a hammer. You will know when you have detonation. Then engine will buck and kick. The piston has a hard time fighting the excessive pressure to make t=it to the top of the stroke. The fuel is no longer burning, it is exploding. And it will reek havoc on pistons, cyl walls, valves, bearings, crankshafts, rods, everything. It actually tries to stop the engine in it's tracks. Have you ever seen a radial operator turn the engine through by hand so they don't hydro-lock it during startup? It will bend/break con rods much like detonation will. Now back to the excessive carbon buildup from earlier. This carbon can cause hot spots that will ignite the fuel prematurely, and cause pinging (pre-ignition). If it gets bad enough, and the hot spots get big enough, it will cause detonation.
Lastly, when you run rich, and get that extra carbon buildup on the top end of the engine, it will usually make its way past the rings, and into the crank case. If you are lucky, it will stay there. If your not, it will go through the engine like little pieces of sand paper, or worse, like little rocks, and muck everything up.
Lean while taxing so the engine almost wants to quit. There will not be enough heat to hurt anything, your simply not making enough power. When in cruise, lean to max power. Cyl temps should be at their most efficient at that point, and you won't hurt anything that way either. If you can do it and can monitor all the cyls' temps, not just an overall egt., lean to 50 deg lean of peak. But only if you can get all of the cyls. temps individually, not just an overall EGT. I personally never run at peak EGT. It just creates too much heat, and like a previous poster said, ruins things. An engine, properly leaned and well maintained, should make it to TBO with never having been opened up nearly every time.
edit: The reason why automobiles now run 0w30 or 10w30 is only because the tolerances have gotten much better over the years, and the higher viscosity only makes it easier for the oil to "fill in the gaps" of the old school machining processes.
