Ammonia engine combustion performance under varied pre-chamber geometries and partial ammonia cracking conditions

An appropriate pre-chamber (PC) design is essential for improving the combustion performance of ammonia-fueled PC engines. However, the effects of pre-chamber geometry and the underlying mechanisms under partially cracked ammonia conditions remain insufficiently understood. In this study, four passive pre-chambers were designed, and natural flame luminosity imaging was employed to systematically investigate the flame development of different PC geometries under different ammonia cracking ratios (ACRs). Furthermore, under the ACR of 10% conditions, the combustion behaviors of different PC geometries were examined at the respective optimal spark timings to identify the most favorable PC geometry. The results show that increasing the ACR from 0% to 10% significantly enhances the combustion performance for all PC cases. For the 5 mm-throat pre-chambers, the small-orifice PC exhibits inferior combustion behavior at low ACR conditions due to stronger contraction effects. Increasing ACR to 10%, the overall combustion performance of both PC cases becomes comparable. The small-orifice PC achieves a higher jet penetration velocity due to a larger-pressure difference (ΔP), while the larger-orifice PC has a faster overall flame propagation through a more uniform jet flame distribution. For the 3 mm-throat pre-chambers, although increased flow restriction increases the peak ΔP, it leads to severe flame quenching in both PCs. These findings demonstrate that the combustion performance of partially cracked ammonia strongly depends on the coupled effects of throat and orifice geometries, in which jet flame quenching is a key factor. Once flame quenching happens, an increased ΔP fails to promote flame propagation in the main chamber.