Effects of exhaust gas recirculation on diesel particulate matter morphology and NOx emissions

Diesel particulate morphology and nitrogen oxides (NO,) emissions were investigated in detail to reveal the effects of exhaust gas recirculation (EGR). The different rates of EGR were precisely controlled by using a customized engine control unit in a 1.7 l turbocharged common-rail direct-injection diesel engine. The tests, which combined two different EGR modes (i.e. constant boost pressure (CBP) and constant oxygen-to-fuel ratio (COFR)), were designed to decouple the effects of EGR thermal and dilution processes. Particulate samples were collected directly from the raw engine exhaust by using a novel thermophoretic soot-sampling system. The samples were examined and imaged with a high-resolution transmission electron microscope and quantitatively analysed by using a customized image-processing/data-acquisition system. Results showed that the particulate dimensions, number density of primary particles, and soot yield all changed significantly under various EGR rates. The NO, emissions also varied significantly as the EGR rate changed, showing a typical trade-off with respect to the data measured for particulate emissions. At low EGR rates, the thermal effect was the dominant phenomenon that affected the changes of the measured morphological characters, while at higher EGR rates the dilution effect became more important. However, the fractal geometry of diesel particulates did not change significantly between the two EGR modes, suggesting that the influence of EGR dilution was less than that of the thermal process. EGR operation providing a COFR at the same EGR rate yielded a significant benefit in particulate emissions and engine power output, while still maintaining the reduction of NO, emissions at a satisfactory level.