Effects of Exhaust Gas Recirculation on Particulate Morphology for a Light-Duty Diesel Engine 2005-01-0195

Exhaust gas recirculation (EGR) is a commonly used technique for the reduction of Nitrogen oxide (NO_x) emissions from internal combustion engines. However, it is generally known that the use of EGR will cause an increase in emissions of particulate matter (PM). The effects of EGR operating mode on particulate morphology were investigated for a 1.7-liter light-duty diesel engine. This engine was equipped with a turbocharged and inter-cooled air induction system, a common-rail direct fuel injection system, and an EGR system. A rapid prototyping electronic control system (RPECS) was developed to operate this engine at various EGR rates under different conditions (i.e. constant boost pressure, constant oxygen-to-fuel ratio (OFR)). A unique thermophoretic sampling system was employed to collect particulates directly from exhaust manifold after exhaust valves. The collected samples were later analyzed by using a high-resolution transmission electron microscope (TEM) and an image processing/data acquisition system. Diesel particulates were characterized in their morphological characteristics, such as primary particle size, aggregate particle size and fractal geometry. From analyses for the soot samples collected at 2500 rpm and 50% load, it was revealed that primary particle sizes (d_p) increase with the increasing EGR rate, in general. The EGR dilution effect has more influence on primary particle sizes than thermal effect at low EGR rate. The aggregate particle sizes (radius of gyration, R_g) were measured within a narrow size range at different EGR rates. The fractal dimensions, which represent the complex geometry of diesel particulates, were measured in the range of 1.78 to 1.95. These results revealed that particulate geometry is sensitive to the changes of engine EGR rates. From this investigation, comprehensive information was obtained for effects of EGR on particulate morphology. These results will offer essential data to designers of diesel engines and aftertreatment systems.