Atomization and evaporation characteristics of liquid ammonia spray under engine intake stroke conditions

The utilization of ammonia as an alternative fuel in combustion engines is gaining attention due to its carbon-free nature. However, the conventional method of introducing gaseous ammonia into the intake manifold reduces engine volumetric efficiency and power output. Using liquid ammonia (LNH₃) injection can mitigate these issues, but the atomization and evaporation characteristics of LNH₃ sprays are not fully understood due to flash boiling and aerodynamic interaction. In this study, high-speed diffused back-illumination (DBI) and Schlieren techniques were employed to capture the liquid and vapor phase evolutions, while high-resolution microscopic drop size imaging (MDSI) and laser-induced breakdown spectroscopy (LIBS) were used to measure droplet size distribution and fuel concentration. The results show that the ambient pressure significantly affects spray morphology, with lower pressures leading to broader dispersion due to higher superheat levels. A bimodal drop size distribution is found under flare-flashing conditions, transitioning to a more uniform distribution as ambient pressure increases. Additionally, ambient pressure plays a crucial role in fuel-air mixing compared to injection pressure, whose influence is nearly negligible under flare-flashing conditions. These findings provide valuable insights into the behavior of LNH₃ sprays, which are essential for optimizing ammonia injection strategies and improving engine performance.