FWIU it varies depending on how much Oxygen is detected by the O2 sensor. The ECM will pulse the EGR module to maintain its AFR demand. Even if you don't have an SES light, the rolling dyno will fail your vehicle for high NOx.
This is where it gets a bit confusing.
With AFR there is a lean flammability above which the mixture will not ignite with a spark. The excess of air implies an excess of oxygen.
EGR contains no oxygen so shouldn't have any effect on AFR from the oxygen content perspective.
From the Wiki page:
''In a typical automotive
spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the need of the mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns''
And that kind of puts the kybosh on leaner mixtures - or you're adding to your initial flammability problem.
(With EGR it obviously isn't an excess of oxygen leading to a 'lean flammability' limiting condition but EGR does inhibit initial combustion and therefor only the additional dilution can be the cause - preventing the oxygen from the intimate contact with fuel that it requires (fuel attached to EGR not oxygen requires more fuel to find the oxygen)).
Stoich for gasoline is an AFR of 14.7. You are at TPS sub 50% running AFR of 16.7. 16.7/14.7 = 1.14. You are providing 14% excess air to reduce throttling losses which compares very well with the quoted '5% to 15% ' of EGR in the Wiki article.
My conclusion is that EGR is used to mitigate throttling losses not because it is necessarily more effective but because (due to running AFRs closer to stoich) the fuelling is easier (and possibly the combustion products the cat has to deal with).
