Quote:
Originally Posted by RustyLugNut
I agree that flame velocity is slower with increasingly lean mixtures but you then say it has higher thermal mass? And higher reaction rate? Remember, you have less fuel to provide the heat.
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Yes. This is comparing the same quantity of fuel, but more air (higher load, higher MAP).
There is excess air that absorbs heat. Same quantity of fuel => same quantity of heat, but more air. That decreases alpha.
There is excess oxygen. Reaction rate is proportional to concentration^order, you're increasing oxygen concentration while keeping fuel concentration the same, since the cylinder is still the same volume but you have higher partial pressure of oxygen. That increases omega (with the dot over it).
Going from 1.0 to 1.2 lambda isn't a massive change all things considered, so assuming the effects cancel out is approximately correct. You can always use less timing advance to be safe.
Side note: In the case of adding EGR, reaction rate is not affected much since the concentration of CO and NO is tiny, but you're also raising the temperature considerably which speeds up the reaction.