Phenomenological micro-pilot ignition model for medium-speed dual-fuel engines

The medium-speed dual-fuel engine has become popular in the marine industry for its advantages of fulfilling the stringent emission regulations and relative affordability of natural gas. In such engines, the ignition process importantly influences the subsequent combustion processes and engine performance. This work developed a phenomenological micro-pilot ignition model with a minimal number of tuning parameters aiming to improve the understanding of the ignition event and enable better control of the dual-fuel engine. The model comprises of a spray and a chemistry submodel to accurately capture the interaction between the direct injection of a small amount of diesel fuel (called micro-pilot) and a two-stage ignition of the diesel fuel mixed with the surrounding reactive charge in relatively low temperature. A 1D transient spray model is adapted to reproduce the micro-pilot spray characteristics by assuming a realistic trapezoidal fuel injection profile and the varying discharge coefficient during the transient spray period. The chemical reactions are modeled with a 0D transient flamelet approach based on an opposed flow reactor. The model is validated using three sets of experimental data, namely ECN Spray A (constant volume chamber), RCEM with optical accessibility, and finally, medium-speed dual-fuel engine. Quantitatively good predictions of the spray formation, ignition delay, and ignition location over broad conditions ranging from the conventional diesel ignition to the micro-pilot ignition in the dual-fuel engine are demonstrated. Finally, the developed model is used to explore the characteristics of micro-pilot ignition under conditions relevant to the medium-speed dual-fuel engines.