The Role of EGR in PM Emissions from Gasoline Engines

A dilute spark-ignited engine concept has been developed as a potential low cost competitor to diesel engines by Southwest Research Institute (SwRI), with a goal of diesel-like efficiency and torque for light- and medium-duty applications and low-cost aftertreatment. The targeted aftertreatment method is a traditional three-way catalyst, which offers both an efficiency and cost advantage over typical diesel aftertreatment systems. High levels of exhaust gas recirculation (EGR) have been realized using advanced ignition systems and improved combustion, with significant improvements in emissions, efficiency, and torque resulting from using high levels of EGR. The primary motivation for this work was to understand the impact high levels of EGR would have on particulate matter (PM) formation in a port fuel injected (PFI) engine. While there are no proposed regulations for PFI engine PM levels, the potential exists for future regulations, both on a size and mass basis. To determine the effect of EGR on PM emissions, a 2.4L, 4-cylinder PFI engine was run at a variety of part and full load conditions. PM and gaseous emissions were measured at each condition, with the PM measurements consisting of both mass and particle size/number measurements.

This work indicated that an engine operating in the fuel enrichment region produces a significant amount of PM. As modern gasoline engines move to higher specific power levels, the size of the enrichment region, in terms of percent of the total engine performance map, increases significantly. By using EGR to reduce combustion temperatures and eliminate the need for enrichment, the PM mass and number emissions were reduced substantially. Comparisons in the enrichment regime indicated the primary benefit of EGR was from eliminating enrichment, but a significant benefit was found from using EGR even at rich conditions. EGR also reduced PM emissions at part load conditions, particularly those above 50% load. An analysis of the PM measurements combined with the gaseous emissions indicated that the addition of EGR reduces combustion temperatures, suppressing the PM formation mechanism more than the oxidation / destruction mechanisms, resulting in an overall PM emission reduction.