Performance and Emission Comparison between a Conventional Euro VI Diesel Engine and an Optimized PCCI Version and Effect of EGR Cooler Fouling on PCCI Combustion

Premixed charge compression ignition (PCCI) is an advanced combustion mode that has the aim of simultaneously reducing particulate matter and nitrogen oxide exhaust emissions, compared with conventional diesel combustion, thanks to a partially premixed charge and low temperature combustion. In this work, PCCI combustion has been implemented by means of an early single-injection strategy and large amounts of recirculated exhaust gas. Starting from a commercial Euro VI on-road engine, the engine hardware has been modified to optimize PCCI operations. This has involved adopting a smaller turbo group, a new combustion chamber and injectors, and a dedicated high-pressure exhaust gas recirculation system. The results, in terms of engine performance and exhaust emissions, under steady-state operation conditions, are presented in this work, where the original Euro VI calibration of the conventional engine has been compared with the PCCI calibration of the optimized hardware engine. The obtained results show that the engine-out nitrogen oxides and soot were dramatically reduced, and this can offer the possibility of reviewing the after-treatment system. The penalties, in terms of brake specific fuel consumption, were generally below 10%, compared with the Euro VI configuration. The main limitations were derived from the combustion noise and the hydrocarbon and carbon monoxide emissions at the exhaust, which could represent an issue, especially when the exhaust temperature is not high enough to allow the diesel oxidation catalyst to work with high conversion efficiencies. Furthermore, the effect of EGR cooler fouling on the performance and emissions has been presented and discussed. The increased pressure drop across a fouled EGR cooler results in a reduced amount of exhaust gas recirculation, thus posing a serious problem for PCCI calibration activity.