Combustion and emission characteristics of an ammonia-hydrogen engine using hydrogen-nitrogen jet ignition

Abstract

Ammonia is a promising carbon-free fuel for internal combustion engines, but its low reactivity poses significant ignition challenges. Hydrogen jet ignition has emerged as a potential solution. While using ammonia decomposition as a hydrogen source is safer and more practical than hydrogen cylinders, the ignition performance of its decomposition gas (a hydrogen-nitrogen mixture with a molar ratio of 3:1) remains poorly studied. This study investigated the performance of an ammonia-hydrogen engine using a hydrogen-nitrogen mixture with a molar ratio of 3:1 as the ignition gas for active jet ignition under two categories of injection strategy: varying injection times and durations. The results showed that, compared to pure hydrogen, the hydrogen-nitrogen mixture increases the density inside the jet chamber, resulting in an increase in jet flame penetration length and flame ignition area. Moderately reducing the hydrogen energy ratio (${\alpha }_{H_2}$) could reduce heat loss and improve indicated thermal efficiency (ITE). Additionally, slightly lean combustion could achieve better thermal efficiency. The maximum ITE of 44.2 % was obtained with an excess air ratio (λ) at 1.18 and ${\alpha }_{H_2}$ = 1.1 %. Slightly lean burn improved ITE but too lean mixture led to unstable combustion. As λ increased, NO_x emissions first increased and then decreased at the maximum ITE operating condition under the same injection strategy and ${\alpha }_{H_2}$. Under ${\alpha }_{H_2}$ = 3.5 % and λ = 1.0, the minimum NO_x was measured at 2796 ppm. This study validates the ignition capability of ammonia decomposition products and proposes optimized combustion strategies for improving efficiency and reducing emissions.