Nitrogen oxides formation mechanisms and control strategies in ammonia high-pressure direct injection combustion of marine low-speed engines

• NO formation and consumption mechanisms in HPDI marine low-speed engines revealed. • Thermal NO formation influenced by fuel-derived nitrogen radicals. • NO and N 2 O emissions mainly originate from fuel nitrogen, over 90%. • High temperatures and H radicals play key roles in controlling N 2 O emissions. • Post-injection strategy reduces NOx by 16.8% with unchanged N 2 O and ITE. Ammonia is a promising carbon-free fuel for marine transportation, yet the understanding of nitrogen oxides formation and consumption mechanisms remains limited, particularly under high-pressure direct injection of marine low-speed engines. This study provides a mechanistic characterization of NOx and N 2 O formation in a novel ammonia–diesel stratified injection combustion mode, innovatively integrating detailed chemical-kinetic analysis, 15 N isotopic labeling, and three-dimensional reaction-path visualization based on computational fluid dynamics (CFD). The 15 N labeling approach effectively distinguishes the origins of nitrogen oxides, showing that more than 90% of NO and N 2 O emissions are derived from fuel nitrogen. Reaction pathway analysis during combustion indicates that NO formation occurs mainly via radical oxidation and third-body collision, with key reactions including NH 2 + NO 2 = H 2 NO + NO, HNO + O 2 = HO 2 + NO, NO + H (+M) = HNO (+M), and NO + OH (+M) = HONO (+M). NO is primarily consumed through DeNOx and oxidation processes involving radicals such as NH 2 , NH, and HO 2 . N 2 O is formed mainly through the conversion of NO/NO 2 by reactions such as NH + NO = N 2 O + H and NH 2 + NO 2 = N 2 O + H 2 O, while its consumption is dominated by N 2 O + H = N 2 + OH and the thermal decomposition pathway N 2 O (+M) = N 2 + O (+M). Guided by these mechanistic insights and based on the fuel-rich combustion concept that enables ultra-low NOx emission, an innovative post-injection strategy is proposed. By appropriately matching the post-injected ammonia energy and injection timing, this strategy achieves a 16.8% reduction in total NOx compared with the non-post-injection baseline, directly meeting IMO Tier III requirements, while providing a better N 2 O trade-off compared with exhaust gas recirculation (EGR) and maintaining N 2 O emissions and indicated thermal efficiency at 0.056 g/kW·h and 48%, respectively.