A Novel CFD Approach for an Improved Prediction of Particulate Emissions in GDI Engines by Considering the Spray-Cooling on the Piston 2015-01-0385

The emission of particulate matter from future GDI engines has to be optimized, to comply with more stringent emission standards such as EU6. Therefore, the mechanisms responsible for the formation of particles have to be analyzed in detail. The understanding of the in-cylinder processes, necessary for this purpose, can only be achieved by a complementary use of optically accessible single-cylinder engines as well as the numerical simulation. This however leads to great demands on the 3D flow simulation.

In this paper the complete CFD approach, incorporating a detailed description of the entire underlying model chain is shown. Particularly the wall surface temperature and the temperature drop due to the interaction with liquid fuel spray were identified as important parameters influencing the spray-wall interaction and thus also the particulate emissions. Nevertheless, in conventional CFD models, the spray cooling cannot be captured because of an assumed constant wall temperature. In this case, the calculated wall film mass is considerably too small. In order to enable the CFD calculation to consider the temperature drop due to the spray-cooling, the novel approach of calculating heat conduction in thin walls was used and validated with surface temperature measurements on the piston of a fired single-cylinder engine.

In summary, this study will reveal that with this improved and validated CFD approach the impacts of different parameters influencing the mixture formation and combustion in GDI engines on the particulate emissions can be evaluated. Thus the prediction of particulate emissions by numerical simulation in a GDI engine could be improved considerably.