Autoignition enhancement of ammonia spray under engine-relevant conditions via hydrogen addition: Thermal,chemical,and charge cooling effects

With the growing trend of decarbonization in ground transportation, low and zero-carbon fuels have attracted extensive research interest. Liquid ammonia is a promising alternative fuel due to its relatively high volumetric energy density, mature production and distribution infrastructure, convenience of storage, and zero carbon emissions. However, ammonia combustion also suffers from low flame speed and weak chemical reactivity. In this work, we computationally investigate the suitable engine-relevant thermochemical conditions for auto-ignition of constant volume ammonia spray, as well as its spray dynamics, vaporization, flash boiling effects, and emissions. The simulation is first validated by comparing it against available experimental data from a vaporizing ammonia spray and is then extended to chemically reactive conditions. Results show that ammonia sprays under engine-relevant conditions (60 bar and 1200 K) can only successfully auto-ignite for cases with ambient hydrogen addition, through enhancement of thermal condition and chemical reactivity. A chemical flux analysis is conducted to further understand the important species and reactions that promote ammonia auto-ignition from hydrogen, which potentially can be introduced via H₂ solubility, exhaust gas recirculation, and onboard ammonia thermal decomposition. Furthermore, results have indicated that charge cooling effects can further decrease the temperature in the flow field and make auto-ignition more difficult. This study provided useful insights for the application of ammonia as a zero-carbon diesel fuel for ground transportation.