Effect of ignition timing on the combustion characteristics of liquid ammonia spray in a lean premixed H₂/Air

Combustion of ammonia has recently been emerging as a promising approach for carbon mitigation. However, monitoring ignition of ammonia is a technique challenge due to its low reactivity. This work experimentally investigates the ignition and combustion behaviors of ammonia spray injected into a lean hydrogen air mixture, with emphasis on the ignition timing effect on the ignition and flame evolution behaviors as well as the pressure evolution and heat release. Results show that the ignition timing leads to profound alteration of the ammonia spray ignition and flame evolution due to the mixture status induced by different spray evolution time. Specifically, the flame is the most significantly affected by spray injection at ignition timings close to the start and end of injection and the heat loss is reduced, compared with the premixed fuel due to the lower heat release from evaporation and incomplete combustion. Additionally, for 10 ms pre-ignition timing, at the instant when the spray reaches the flame, the unburned gas assists in the evaporation and diffusion of the liquid ammonia, leading to a sudden increase in flame speed and a higher pressure of combustion. While, finally, for the 10 ms post-ignition timing case, the stratified fuel concentration distribution leads to accelerated heat release rate and a reduced overall combustion duration. The special spray characteristics of liquid ammonia and the high latent heat of evaporation are the main factors leading to the reduction of the combustion efficiency of liquid ammonia spray, which can be effectively solved by precombustion heating or homogeneous mixing. This study is believed to be beneficial for organizing better combustion for this zero-carbon fuel in SI engines.