Impact of injection settings operating with the gasoline Partially Premixed Combustion concept in a 2-stroke HSDI compression ignition engine

Partially Premixed Combustion (PPC) using gasoline-like fuels has proven its potential to control or even break the NOx and soot emissions trade-off, retaining the high efficiency levels characteristic of the conventional diesel combustion (CDC) concept. However, selecting an appropriate fuel and a suitable injection strategy is essential to assure a successful PPC operation in the full engine map. Additionally, extending the limit of PPC beyond 10 bar IMEP was not possible due to excessively high pressure gradients and onset of knocking-like combustion, so the CDC concept has to be adopted and the conventional trade-off between NOx and soot emissions was recovered. Present investigation focuses on evaluating the use of a multiple injection strategy for extending the load range of the PPC concept to medium/high load conditions, when using a commercial RON95 gasoline in a 2-stroke engine under development. Experimental results confirm how with a fine tuned triple injection strategy it is possible to reach extremely low NOx and soot levels keeping combustion efficiency over 96%, while indicated efficiency is improved compared to a well-optimized point obtained operating with the CDC concept. Finally, the research work is completed by including 3D-CFD modeling activities that are carried out to contribute to the understanding on how the mixture preparation and stratification prior to the start of combustion impacts its development and particularly the experimentally observed pollutant emissions trends.