On the Potential of Low Heat Rejection DI Diesel Engines to Reduce Tail-Pipe Emissions

Heat transfer to the combustion chamber walls constitutes a significant portion of the overall energy losses over the working cycle of a direct injection (DI) diesel engine. In the last few decades, numerous research efforts have been devoted to investigating the prospects of boosting efficiency by insulating the combustion chamber. Relatively few studies have focused on the prospects of reducing emissions by applying combustion chamber insulation. A main purpose of this study is to assess the potential of reducing in-cylinder soot as well as boosting aftertreatment performance by means of partially insulating the combustion chamber. Based on the findings from a conceptual study, a Low Heat Rejection (LHR) design, featuring a Nimonic 80A insert into an Aluminum piston, was developed and tested experimentally at various loads in a single-cylinder Hatz-engine. The piston wall temperature, which was measured using an infrared pyrometer technique, was approximately 250 K higher using the Nimonic-insert compared with the conventional aluminum piston. For various EGR levels combustion in the tested low heat rejection engine was characterized by a shorter ignition delay followed by a prolonged diffusion burn. Replacing the conventional piston with the low heat rejection one, IMEP was slightly improved, emissions of CO and NO_x were generally reduced, whereas the trend for soot and hydrocarbons was found to be more complex. In order to realize the full potential of the developed Low Heat Rejection concept, a re-optimization of the combustion system has to be undertaken.