Experimental Study on Pre-Chamber Hydrogen Flame Jet Ignition of Ammonia/Air Mixture in Constant Volume Combustion Chamber

To address the issues of difficult ignition and slow combustion when ammonia is used as engine fuel, a method of igniting ammonia/air mixture with hydrogen flame jet generated by a pre-chamber is proposed. The combustion characteristics of mixtures ignited by the hydrogen flame jet were studied in a constant volume combustion chamber with high-speed video camera and pressure acquisition in the main chamber. The characteristics were compared with those ignited by the ammonia flame jet. The introduction of the hydrogen flame jet notably improved mixture combustion and expanded the lean flammability limit. Combustion with hydrogen injection demonstrated reduced pressure rise delay and combustion duration, increased average heat release rate, and sustained combustion stability. This phenomenon was more pronounced under low equivalence ratio conditions in the main combustion chamber. The hydrogen flame jet was shuttle-shaped when touched the lower surface owing to the rapid combustion speed of hydrogen, while the ammonia flame jet was spindle-shaped with the flame kernel in the center of the main-chamber. The inevitable ingress of ammonia into the pre-chamber through the orifice resulted in longer pressure rise delay at the equivalence ratio of 1.0. The combustion process initiated by the flame jet showed two distinct peaks in the heat release rate, except in the case with the equivalence ratio of 0.8 without hydrogen injection as the combustion process under this condition was similar to laminar combustion. The peaks were affected by the flame jet and flame propagation, respectively. The average apparent heat release rate during the stage dominated by the flame jet was considerably higher with hydrogen injection than without it. Furthermore, the effects of equivalence ratio on combustion were explored. The highest peaks in main chamber pressure and average heat release rate were noted at the equivalence ratio of 1.0 with hydrogen injection. This study makes it possible for ammonia, as a carbon-free fuel, to be used as engine fuel and reduce carbon emissions.