Liquid ammonia direct injection for internal combustion engine application: A review

Since the 2020s, there has been renewed interest in ammonia as a valuable zero-carbon fuel for Internal combustion engines, given its wide range of potential applications, from the marine sector to off-road machinery. Ammonia's fuel characteristics differ significantly from those of conventional hydrocarbon fuels. Its phase-change properties make liquid-phase ammonia injection a promising approach, as high-pressure injection of liquid ammonia enables efficient and cleaner combustion. High-pressure direct fuel injection offers the advantage of improved atomization through droplet breakup. The subsequent evaporation of these atomized droplets is crucial for the spatial distribution of fuel vapour, local air/ammonia mixing, ignition, and combustion development. However, ammonia tends to undergo flash boiling under injection conditions relevant for an internal combustion engine. In general, flash-boiling atomization enhances liquid evaporation, reduces droplet size, increases spray velocity, and results in longer spray penetration. Since the calorific value of ammonia is approximately half that of conventional fuels, the use of larger nozzle diameters and multi-hole injectors is recommended. While flash boiling has similar effects on both single- and multi-hole injectors, the spray characteristics of multi-hole injectors are largely influenced by plume-to-plume interactions. Therefore, it is recommended to study spray collapse and flash-boiling atomization separately, as they are related but distinct phenomena. The ammonia spray development characteristics are significantly different in the early and later stages of injection, indicating that the influence mechanisms of flash boiling and conventional boiling are completely different. This review summarizes the recent state-of-the-art liquid ammonia injection for engine-relevant conditions, focusing on flash boiling atomization and some discussions about novel optical techniques needed to characterize not only at the macroscopic scale but also at the microscopic scale, to help the development of accurate modelling tools.