Quote:
Originally Posted by serialk11r
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. |
Concentration.
Omega naught is proportional to the concentration of reactants. In one instance, you are reducing one reactant ( gasoline ). On the other hand you are keeping the oxidizer concentration ( oxygen ) constant. Omega naught will be smaller.
Also, you are ignoring the other variable. Temperature. With less fuel and more air, T(sub b) is smaller ( the flame front burns at a lower temperature ). The temperature relation in the parentheses will be smaller.
At just this side of lean lambda, it is known that burn rate increases. But it rapidly falls off. I worked with three Chrysler vehicles and their lean burn systems. A four, six and V8. At 18:1 AFR, all of them needed quite a bit of ignition advance (60 DCA before TDC, average). However, you are correct in that the addition of EGR increased flame speed and ignition advance could be pulled back to 35-45 BTDC.