Effects of intake parameters and compression ratio on ammonia combustion and emissions in SI engines

Abstract

Ammonia is an attractive hydrogen carrier and zero-carbon fuel for achieving de-carbonization targets. However, realistic engines with pure ammonia often suffer from serious issues like ignition instabilities, combustion variations, and heavy emissions. It has shown that ammonia combustion is highly sensitive to combustion boundary conditions. However, comprehensive parametric studies on combustion and emissions of ammonia engines are still lacking currently. In this work, ammonia combustion and emissions were numerically investigated in a spark-ignition engine under various operating conditions, emphasis on pressure characteristics, combustion flow field, and NOx and unburned NH3 emissions. The results show that ammonia combustion can be promoted significantly at elevated intake temperatures and pressures and compression ratios, manifesting increased peak pressure, accelerated flame propagation, and shortened combustion duration. Indicated power increases at elevated compression ratios and intake pressures while decreasing at elevated intake temperatures. Equivalence ratios affect ammonia combustion nonmonotonically, and the highest peak pressure and thermal efficiency are achieved at Φ = 1.2 and 1.1, respectively. Besides, equivalence ratios have large impacts on the shortening of late-stage combustion duration (i.e., CA50-CA90). Regarding emission characteristics, it shows that fuel-NO is dominant while thermal-NO is minimal. Equivalence ratios can result in pronounced NO emissions under lean-burning conditions. Unburned NH3 decreases while NO2 and N2O remain low levels at an elevated compression ratio, intake temperature and pressure and equivalence ratio of around 1.1. This work demonstrates the significance of active thermo-atmosphere in the optimization of ammonia engine performance.