On combustion and emission characteristics of ammonia/hydrogen engines: Emphasis on energy ratio and equivalence ratio

Ammonia is a promising alternative carbon-free fuel, but its applications in engines encounters great challenges from poor combustion and heavy emissions. Hydrogen can effectively enhance ammonia reactivity as a carbon–neutral solution. Hydrogen energy ratio and equivalence ratio exhibit significant impacts on engine performance, but the comprehensive characteristics and underlying mechanisms remain not fully understood. In this work, we investigated the engine performance, combustion evolutions, and nitrogen-based emissions of ammonia/hydrogen blends in an optical engine, addressing energy ratio and equivalence ratio. Six levels of hydrogen energy ratio and three groups of equivalence ratio were conducted under different spark-ignition timing conditions. Results show that hydrogen addition significantly improve ammonia combustion stability, particularly for low-level hydrogen energy ratios at stoichiometric conditions. Optimal thermal efficiency is achieved at a hydrogen energy ratio of 10% at stoichiometric conditions and higher hydrogen addition under leaner or richer conditions. Combustion evolutions show that the initial flame kernel and flame development become intensified at higher levels of hydrogen energy ratio, resulting in advanced combustion phasing. The tendency becomes more pronounced at the stoichiometric conditions. Regarding emissions characteristics, unburned NH3, NO2, and N2O emissions decrease as hydrogen energy ratio is raised, particular for low-level hydrogen addition, whereas NO emissions show an opposite tendency. Unburned NH3, NO2, and N2O emissions achieve the optimal at stoichiometric conditions, but NO emissions become lower at rich-burned conditions.