Anatomy of the cooled EGR effects on soot emission reduction in boosted spark-ignited direct-injection engines

- Soot emissions are reduced by up to 50% with cooled EGR in SIDI engines.
- The mechanism of EGR effects on soot emission reduction is clarified.
- Lowered temperature of “pool fire” by EGR is the primary contributor to soot emission reduction.
- Improved fuel-air mixing due to EGR also plays positive roles in soot emissions reduction.

It has been extensively examined that cooled exhaust gas recirculation (EGR) is effective not only to reduce NOx emissions but also to improve fuel conversion efficiency in spark ignition (SI) engines. Recently, the benefit of cooled EGR on soot particle emission reduction in SI direct injection (DI) gasoline engines attracts increased attention. However, the mechanism behind the phenomena needs to be further clarified. The objective of this study is to anatomize the cooled EGR effects on soot emission reduction in SIDI gasoline engines by the experiments along with the computational fluid dynamics (CFD) simulation. The soot emission reductions up to 48% by cooled EGR are firstly demonstrated at three typical high load conditions of a 2.0 L boosted SIDI engine. Since the engine-out soot emissions are the combined results of soot particle formation and oxidation, the effects of cooled EGR on these two processes are respectively considered. It is found that cooled EGR plays adverse effects on soot oxidation, owing to the reduced in-cylinder oxygen availability and lowered reaction temperature. Although the increased intake boost pressure and reduced fuel injection quantity with cooled EGR can help reduce remarkably the wall wetting during the intake stroke, the quantity of liquid fuel on the wall is only slightly lower than the case without EGR during the combustion processes. The in-cylinder mixture distribution and equivalence ratio (ϕ)-temperature (T) plots obtained by the CFD simulations show that the lowered temperature of “pool fire” is the primary contributor to the reduced soot formation with cooled EGR, while improvement of the local rich pockets in the cylinder and the dilution effect of EGR may also play positive roles in suppressing the soot formation.