Improvement of SI engine combustion with ammonia as fuel: Effect of ammonia dissociation prior to combustion

Although recent studies have shown the possibility of running ‘standard’ spark-ignition engines with pure ammonia, the operating range remains limited mainly due to the unfavorable characteristics of ammonia for premixed combustion and often requires the addition of a complementary fuel such as H₂ to extend it. As the best way to add H₂ is to crack ammonia directly on-board, this paper focuses on the impact of the upstream cracking level of ammonia on the performance and emissions of a single cylinder spark ignition engine. Experiments were performed over several equivalence ratios, dissociation rates and load conditions. It is confirmed that only a slight rate of ammonia dissociation (10%) upstream of the combustion considerably enhances the engine's operating range thanks to a better combustion stability. In terms of pollutant emissions, the partial dissociation of ammonia, especially for slightly lean mixtures induces a very clear trade-off between high NO_x and high unburned ammonia level for high and low ammonia dissociation rates, respectively. Therefore, cracking NH₃ does not only improve the operating range of ammonia-fueled spark ignition engines but can also help to reduce NH₃. However, to reach the same engine output work, higher ammonia fuel consumption will be necessary since the global system efficiency is lower using fuel dissociation. In addition, the global warming effect is increased with dissociation level since a higher level of N₂O is generated by the hydrogen contribution.