Numerical study of effect of post injection coupled with EGR on combustion and emission performances of CRDI engine fueled with biodiesel-ethanol blends

Abstract

Energy shortage and environmental pollution are becoming more serious, biodiesel is regarded as the most promising alternative fuel for diesel engines due to its environmentally friendly and renewable characteristics. In this study, the biodiesel-ethanol blends were used in a diesel engine, and the purpose of the study was to simultaneously control the NO_x and soot emissions of the diesel engine by adjusting the injection strategy and EGR rate. A turbocharged, six-cylinder, common rail direct injection (CRDI) engine model was established using GT-Power. The effects of the main-post injection strategy and post injection coupled with exhaust gas recirculation (EGR) on combustion and emission characteristics were investigated at a maximum torque speed and a medium load. The results show that when the main-post injection strategy is employed, the combustion duration of the main injection is shortened with an increase in the main-post injection interval (MPI). When the MPI increased to more than 18 °CA, the heat release of post injection could be observed clearly from the curve of the heat release rate, NO_x emissions decreased by 5.70% and 7.12%, respectively, and soot emissions decreased by 25.56% and 30.20%, respectively. Moreover, with the increasing post injection quantity, the combustion duration of the main injection shortened, and the peak heat release rate (PHRR) of the post injection increased. When the fuel quantity for the post injection increased from 2 to 6 mg, NO_x emissions decreased from 2.33% to 9.80%, and soot emissions decreased from 16.10% to 34.97%. The effect of post injection quantity on emissions was more significant than that of the MPI. In addition, with increasing EGR rate, the ignition delay is prolonged, the peak cylinder pressure, PHRR, peak combustion temperature and NO_x emissions decrease, whereas soot emissions increase gradually. Main-post injection can improve the NO_x-soot trade-off, the optimal EGR rate is 22.86% under a post injection quantity of 4 mg and a MPI of 22 °CA.