Originally Posted By: mechtech2
Everyone says it does.
But your highest temps are at the correct [stoichiometric ratio].
Running richer or leaner has cooler combustion temps.
So I submit that lean mixtures burn cooler than normal.
And burn SLOWER, to boot!
Lean will NOT not burn valves or piston tops.
It is not so much the hot combustion temps, it is the temp of the piston crowns and chamber that matter. Running richer provides a boundary layer (mix of unburned fuel and air) on the combustion surfaces, a little insulation layer. The mixture on the combustuion surfaces never gets hot enough to actually burn. The layer lowers the overall amount of heat that makes it into the engine, but plays h*ll with hydrocarbon emissions.
Lean mixtures burn cooler than stoich.....yes, absolutely. Do lean mixtures push more heat into a combustion chamber/valves.....yes.
Lean does burn pistons and valves, but not directly. Lean conditions spike chamber temps, increased chamber temps cause detonation/pinging (near the intake valve head by the cylinder wall), the detonation blows away any boundary layer present in the chamber, more heat makes it into the combustion surfaces, and so on. It is a vicious cycle. That is why you see piston edges burned/melted and top rings siezed in the gap. Gets bad enough, you get a burned valve.