Modeling the Effects of High EGR Rates in Conjunction with Optimum Multiple Injection Techniques in a Heavy Duty DI Diesel Engine

Exhaust Gas Recirculation (EGR) is an effective pre-treatment technique, which has been widely used to decrease the amount of the oxides of nitrogen (NOx) emission from diesel engines. However, the use of high EGR rates leads to the reduction in oxygen availability in the burning regions of the combustion chamber which impairs the soot oxidation process. Consequently, higher soot generated by EGR leads to long-term usage problems inside the engines such as higher carbon deposits, lubricating oil degradation and enhanced engine wear. In this study, CFD modeling has been carried out to analyze the effects of high EGR rates in conjunction with optimum multiple injection strategies. A heavy-duty DI Diesel engine has been modeled to study the engine performance and emissions with various EGR rates (from 0% to 40%). The selected operating points have been achieved with the same injection profile including a main and post injection for all considered cases. The results showed the effectiveness of multiple injections at controlling soot emission under high EGR conditions. By using multiple injections the soot formation is accrued in the multiple regions in the combustion chamber and thus has more area for oxidation. The fuel that is pulsed into the combustion chamber after main injection ignites rapidly and thus will not contribute significantly to soot formation in high temperature rich regions. Even though the EGR reduces some of the intake oxygen content, the heat added to the intake air enhances the soot oxidation to some extent which leads to reduction of soot emission. It was found that, the optimum separation for simultaneous reduction of NOx with low soot emissions under high EGR rates can be obtained by using 25 CA dwell delay between the injection pulses.