Ammonia, a carbon-free fuel, can make an important contribution towards the decarbonization of on- and off-road internal combustion (IC) engines. However, the research on the ammonia use in IC engines is still in initial stages. The goal of this study was to investigate the effect of the equivalence ratio on an ammonia-dedicated engine performance. To observe how the equivalence ratio of the air-ammonia mixture affects in-cylinder flame speed and the reaction pathway of pollutant formation (therefore, how it affects engine efficiency and emissions), a multi-dimensional computational fluid dynamics simulation of a single-cylinder diesel engine converted to dedicated ammonia spark ignition operation was built. The numerical results suggested that stoichiometric operation was better than lean operation for this engine. In detail, lean operation reduced the combustion efficiency and increased engine-out NOX and ammonia emissions, therefore potentially discarding the known thermal efficiency benefit of lean operation in IC engines. In addition, a rich equivalence ratio drastically reduced combustion and thermal efficiency, and increased fuel consumption rates and unburned engine-out ammonia emissions. Finally, while stoichiometric operation produced the best overall performance in terms of efficiency and emissions, attention must be paid to N2O and unburned NH3 engine-out emissions.
Keywords internal combustion engine, zero-carbon ammonia fuel, spark ignition, 3D CFD simulation, air-fuel ratio, engine combustion and emission formation