Plasma-assisted intake reforming for ammonia jet ignition engine: Combustion characteristics and mechanistic insights

Ammonia serves as an efficient hydrogen energy carrier and shows great potential as an alternative fuel for internal combustion engine decarbonization. To address the ignition difficulty and low thermal efficiency in conventional spark ignition ammonia engines, this paper proposes a novel combustion strategy combining plasma intake reforming with jet ignition. The approach enhances the reactivity of ammonia mixture through plasma reforming, and organizes rapid multi-flame combustion in-cylinder using jet ignition. To elucidate the combustion enhancement mechanisms of plasma reforming, this study developed a zero-dimensional model of plasma-assisted intake reforming of ammonia based on SENKIN and ZDplaskin, and coupled with CONVERGE to characterize combustion processes in the plasma-reformed ammonia jet ignition engine. The results demonstrate that increasing nanosecond pulsed discharge frequency, number, width, and E/N under unconstrained energy input can effectively enhance ammonia combustion. Particularly, optimizing E/N and pulse width are more effective, achieving 67 % and 61 % reductions in ignition delay, along with 81 % and 86 % reductions in combustion duration respectively. In contrast, under constant energy input, increasing E/N is identified as the most effective approach to enhance the combustion of reformed ammonia. Moreover, a pre-ignition heat release stage is observed in the main chamber, and this stage is more pronounced with increased reforming intensity. Comparative analysis between thermal and chemical effects of plasma reforming indicates that both effects are indispensable for enhancing ammonia combustion. Chemical effect reduces reaction activation energy, while thermal effect amplifies chemical effect. Sustained reactions further intensify thermal effect, establishing a self-reinforcing synergy between both effects.