Liquid ammonia is an ideal zero-carbon fuel for internal combustion engines. High-pressure injection is a key technology in organizing ammonia combustion. Characteristics of high-pressure liquid ammonia injection is lack of research. Spray behaviors are likely to change when a high-pressure diesel injector uses liquid ammonia as its fuel. This study uses high-speed imaging with a DBI method to investigate the liquid penetration, width, and spray tip velocity of high-pressure liquid ammonia injection up to 100 MPa. Non-flash and flash boiling conditions were included in the experimental conditions. Simulation was also used to evaluate the results. In non-flash boiling conditions, the Hiroyasu model provided better accuracy than the Siebers model. In flash boiling conditions, a phenomenon was found that liquid penetration and spray tip velocity were strongly suppressed in the initial stage of the injection process, this being the “spray resistance phenomenon”. The “spray resistance phenomenon” was observed when ambient pressure was below 0.7 MPa during 0–0.05 ms ASOI and was highly related to the superheated degree. The shape of near-nozzle sprays abruptly changed at 0.05 ms ASOI, indicating that strong cavitation inside the nozzle caused by needle lift effects is the key reason for the “spray resistance phenomenon”.

Adopting zero-carbon fuels, like hydrogen, can significantly reduce environmental harm and pave the way for a decarbonised trajectory with zero carbon emissions. The hydrogen internal combustion engine (ICE) technology has demonstrated its reliability and capacity to seamlessly integrate into the current ICE platform, originally designed for diesel and gasoline operation. The direct utilisation of pure hydrogen eradicates steady-state carbon dioxide and hydrocarbon emissions. It is important to highlight that efforts to comprehend and comprehensively tackle NO_x emissions are underway. A comprehensive study was carried out to assess the NO_x emissions for a hydrogen ICE with different injection modes from gasoline. The study involved varying the relative air-to-fuel ratio (AFR) from stoichiometric to the lean-burn limit in a boosted spark ignition (SI) engine fuelled with gasoline or hydrogen. A fast NO_x emissions analyser was employed to measure the instantaneous NO and NO₂ emissions in the engine exhaust. The study provides a detailed analysis of NO_x emissions, including steady-state averaged emissions, average crank angle domain NO_x distribution and emissions, in-cylinder pressure analysis, as well as time and cycle analyses of NO_x emissions’ temporal and cyclic variations. The primary discovery was that NO_x emissions are almost zero between lambda 2.75 and 3.7, and hydrogen produces 13.8% less NO_x emissions than gasoline at stoichiometric operation. Finally, the full NO_x time analysis revealed that the consistency of NO_x emissions is higher with hydrogen than with gasoline by using a novel approach by identifying the coefficient of variation of the NO_x emission of each cycle.