Numerical Study of an EGR Dilution in a Pre-Chamber Spark Ignited Engine Fuelled by Natural Gas

Exhaust gas recirculation (EGR) is a proven strategy for the reduction of NO_X emissions in spark ignited (SI) engines and compression ignition engines, especially in lean burn conditions where the increase of thermal efficiency is obtained. The dilution level of the mixture with EGR is in a conventional SI engine limited by the increase of combustion instability (CoV IMEP). A possible method to extend the EGR dilution level and ensure stable combustion is the implementation of an active pre-chamber combustion system. The pre-chamber spark ignited (PCSI) engine enables fast and stable combustion of lean mixtures in the main chamber by utilizing high ignition energy of multiple flame jets penetrating from the pre-chamber to the main chamber. In this paper, as an initial research step, a numerical analysis is performed by employing the 0D/1D simulation model, validated with the initial experimental and 3D-CFD results. The simulation model is used for the prediction of possible benefits of using EGR dilution in the methane fuelled PCSI engine, and to determine the possible operating window that will be experimentally investigated in the next phase. The obtained results indicate that the combined approach of diluting the mixture with both air and EGR can result with further increase of indicated efficiency when compared to the air dilution only approach. While it is not possible to reduce the NO_X emissions under the regulation limit when targeting MBT optimized spark timings, it can be achieved by retarding the spark timing at the expanse of efficiency. The resulting efficiency at such operation is still higher than that obtained with the air dilution approach at MBT optimized spark timings, proving the potential of the investigated approach.