Fundamental study on the factors influencing the stability of ammonia combustion and the effects of pre-chamber on the combustion characteristics

Ammonia is gaining attention as a carbon-free alternative fuel for internal combustion engines. However, its poor combustion characteristics hinder its development, especially the combustion instability at low equivalence ratios. At present, most literature has not studied the factors affecting ammonia flame propagation stability under high ambient temperature and pressure. Therefore, this paper focuses on this issue, as well as the promotion of jet ignition combined with blending methane, and the prediction method of the flame propagation state under different conditions is proposed. The experimental results demonstrate that the main reason for the difficulty of stable propagation of ammonia flame at low equivalence ratios in top direct ignition mode is the buoyancy effect. The relationship between the flame propagation state and the corresponding Froude number with the flame radius of 20 mm (Fr20) is established. The flame propagation of the mixture can be stable in top ignition mode when its Fr20 is larger than 0.89, and the flame propagation of the mixture is unable in top ignition mode when its Fr20 is less than 0.50. The Fr20 increases with temperature, decreases with pressure, and first increases and then decreases with the increase of the equivalence ratio. Among the three parameters, the equivalence ratio has the greatest correlation with the Fr20. The prediction results show that under engine conditions, blending 20 % methane can expand the stable combustion limit of ammonia, but the effect is less compared with increasing the equivalence ratio to 0.55 and 0.6. In jet ignition mode, a higher equivalence ratio of the mixture in the prechamber can expand the low equivalence ratio limit of stable combustion of the mixture in the main chamber due to the acceleration of the longitudinal propagation of the flame by the jet. The combustion characteristics of the mixture in the main chamber are closely related to its properties, so at a too-low equivalence ratio, the jet has a limited promotion, in addition, the effect of blending 10 % methane is also little.