Managing EGR for MPG

[omgwtfbyobbq]

MetroMPG, from what I've gathered, the appropriate amount of EGR reduces peak flame temperatures/NOx during combustion because it minimizes the amount of oxygen in the combustion chamber, but it has limits because if too much exhaust/heat goes back into the cylinder, the fuel will ignite on it's own after hot spots start to develop. So it's a fine line to walk so to speak, fuel with higher octane allows for more EGR, which reduces pumping losses by a proportional amount while resisting detonation that would happen if gasoline was used.

Originally Posted by Wikipedia

Reduced heat rejection. Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.