Enhancing pure ammonia engine combustion by pre-chamber ignition coupled with dual spark ignition strategies

Advanced ignition strategies are essential for overcoming the poor ignitability and low burning velocity of ammonia. Multi-point spark ignition can increase the combustion rate by shortening the flame propagation distance, while pre-chamber ignition (PCI) can enhance main-chamber (MC) combustion through distributed ignition sites and jet-induced turbulence. In this study, the combustion characteristics of pure ammonia under different ignition strategies were systematically investigated in a single-cylinder optical engine. A novel pre-chamber ignition coupled with dual peripheral spark ignition strategy (PCI-DPSI) was proposed to achieve coordinated combustion enhancement through the interaction between peripheral flames and pre-chamber jet flames, and the effect of pre-chamber spark delay relative to the peripheral spark timing was examined. The results show that the dual peripheral spark ignition strategy exhibits poorer combustion performance than the central single-spark ignition strategy because of near-wall heat transfer losses. In contrast, the triple-spark ignition (TSI) strategy effectively improves the combustion performance by reducing the flame propagation distance. A clear synergistic effect is observed in the PCI-DPSI mode between the peripheral flames and the pre-chamber jet flames. Appropriately delaying the pre-chamber spark timing allows the early peripheral flames to establish a more favorable thermochemical environment in the MC, thereby promoting subsequent pre-chamber ignition and intensifying MC combustion. Compared with the TSI strategy, the PCI-DPSI strategy achieves a higher peak heat release rate and faster flame propagation. These findings demonstrate that the PCI-DPSI strategy is an effective approach for improving pure ammonia engine combustion.