The impact of multi-injection events on a compression-ignition engine using ducted fuel injection: A numerical and experimental study

Ducted fuel injection (DFI) is a technology to curtail soot formation from diesel combustion while mitigating its typical trade-off with nitrogen oxides (NOx). Previous studies have demonstrated its potential in both vessel and compression-ignition (CI) engine applications relying on simple single-injections rather than multi-injection (MI) strategies. Consequently, injection strategy optimization remains an open question. This study examines the interaction between MI strategies and DFI through a combined approach employing analysis of experimental in-vessel data and 3D computational fluid dynamics (CFD) engine simulations. The analysis was conducted by considering the impact of pilot and post injections on DFI separately, exploring variations in dwell time and mass share. The findings indicate that, when pilot injection is introduced, DFI remains effective in curtailing engine-out soot emissions, while NOx emissions are less sensitive to pilot injection calibration than in free spray injection (FSI). Similarly, post injections can be successfully employed with DFI to further reduce soot emissions, although a different optimal calibration may be required compared to the FSI. Overall, this study suggests that DFI can be effectively coupled with MI strategies, and that DFI-oriented calibration adjustments may unlock additional benefits, representing a significant step toward its deployment in series-production CI engines.