Comparative study on combustion and flame characteristics of NH3 dual-fuel engine under CI vs SI combustion modes

Ammonia can be used for engines to reduce carbon emissions, while its poor combustion performance limits commercial applications. Thus, combustion promotors are needed to improve the combustion properties of NH3. In this study, the combustion and flame characteristics of NH3-heptane dual-fuel engines under CI vs SI combustion modes are comparably studied. The initial flame formation and its development are addressed by high-speed direct photography, whereas simultaneous pressure acquisition is applied to quantify the engine performance. The results show N-heptane can significantly improve the combustion of NH3 under both SI and CI modes, while the effective energy fraction of N-heptane is beyond 30%. Besides, a critical direct injection timing exists for the effect of N-heptane on NH3 dual-fuel CI combustion, before/after which the combustion phase will delay. Flame images show that the color of ammonia-based fuel flame tends to feature an orange-yellow color. When comparing the SI and CI mode, the thermal efficiency of CI combustion is much higher because of the multipoint auto-ignition combustion in CI mode. Statistical analysis of flame shows that the early flame duration of CI mode is short and the corresponding cycle-to-cycle variation is low. More specifically, the flame of CI is nearly symmetrical distribution and the high probability is at the center, while that of SI is at the side.