Effect of EGR on Particle Emissions from a GDI Engine

Gasoline direct injected (GDI) engines are becoming a concern with respect to particulate matter (PM) emissions. The upcoming 2014 Euro 6 regulations may require a drastic reduction in solid particle number emissions from GDI engines and the proposed California Air Resources Board (CARB) LEV III regulations for 2014 and 2017 will also require some PM reduction measures. As a result, it is necessary to characterize PM emissions from GDI engines and investigate strategies that suppress particle formation during combustion. The main focus of this work was on using exhaust gas recirculation (EGR) as a means to reduce engine-out particle emissions from a GDI engine with an overall stoichiometric fuel to air mixture. A small displacement, turbocharged GDI engine was operated at a variety of steady-state conditions with differing levels of EGR to characterize total (solid plus volatile) and solid particle emissions with respect to size, number, and soot or black carbon mass. Cooled external and un-cooled internal EGR were both investigated to provide further insight into the role of bulk temperature on PM formation. Analysis of the data helped to identify the dominate mechanisms that contribute to particulate emissions in GDI engines at different operating conditions and determine what EGR strategies would be most effective at reducing engine-out particulate matter. In general, cooled external EGR was very effective in reducing soot mass by as much as 65 percent, compared to engine baseline. Solid particle number followed the soot mass trend, but showed an opposite trend for some engine operation. These findings present some complexity in achieving both soot mass and solid particle number reduction with one strategy. Internal EGR was much more effective than cooled external EGR in reducing soot mass and solid particle number at light load, low temperature combustion. EGR increased engine-out volatile particles that are mainly formed during dilution and cooling of hot exhaust. Other work showed that a three-way-catalyst can be very effective in reducing the precursor of volatile particle formation.