The Effect of Post Injection Coupled with Extremely High Injection Pressure on Combustion Process and Emission Formation in an Off-Road Diesel Engine: A Numerical and Experimental Investigation

In this paper, a numerical and experimental assessment of post injection potential for soot emissions mitigation in an off-road diesel engine is presented, with the aim of supporting hardware selection and engine calibration processes. As a case study, a prototype off-road 3.4 liters 4-cylinder diesel engine developed by Kohler Engines was selected. In order to explore the possibility to comply with Stage V emission standards without a dedicated aftertreatment for NOx, the engine was equipped with a low pressure cooled Exhaust Gas Recirculation (EGR), allowing high EGR rates (above 30%) even at high load. To enable the exploitation of such high EGR rates with acceptable soot penalties, a two-stage turbocharger and an extremely high-pressure fuel injection system (up to 3000 bar) were adopted. Moreover, post injections events were also exploited to further mitigate soot emissions with acceptable Brake Specific Fuel Consumption (BSFC) penalties. Since the selection of the post injection calibration parameters (such as injected quantity and dwell time) is crucial to maximizing its efficacy, a 3D-CFD engine model was built and calibrated to evaluate its effects on soot emissions and fuel consumption. With this aim, different post injection strategies at rated speed / 50% load operating conditions were analyzed, highlighting the potential to achieve impressive soot mitigations (up to 50%) with negligible (< 1%) BSFC penalties. The numerical analysis also allowed a better understanding of the phenomena leading to soot emissions reduction, setting guidelines for the subsequent calibration experimental activity, which confirmed the simulation results and could be successfully extended to the whole engine operating map.