Impact of oxygen enrichment on ammonia combustion in spark-ignition engines under partial load conditions

NH3-fueled internal combustion engines are a promising technology in the perspective of decarbonization. However, when NH3 is used as fuel, its high minimum ignition energy and slow flame propagation severely limit operating conditions, making its application in internal combustion engines challenging. The low reactivity of NH3/air mixtures results in high unburned NH3 emissions and low combustion efficiency. In this context, oxygen-assisted combustion is a promising technique for improving both the NH3/O2/N2 mixture reactivity and the performance of internal combustion engines. This study experimentally explores the feasibility of using NH3 in a fully premixed spark ignition single-cylinder engine with a compression ratio of 12:1 at 1000 rpm under both medium and low load conditions. The oxygen (O2) content in the intake gas mixture was varied to achieve stable combustion without any misfires. At medium load, i.e. with an indicated mean effective pressure of 5.8 bar, stable combustion was achieved with 22.4% vol. of O2 (relative to the O2/N2 mixture) and a significant reduction in ignition delay time and combustion duration was observed. 1-D numerical simulations of adiabatic, unstretched, freely propagating flat premixed flame were performed, using Ansys Chemkin-Pro 2024, implementing the chemical kinetic mechanism for NH3 oxidation (31 species and 203 reactions) proposed by Stagni et al. 2023, to investigate the influence of O2 additions on the laminar flame speed of NH3/O2/N2 mixtures under spark timing thermodynamic conditions. The addition of O2 enhances the laminar flame speed by 47% when O2 concentration is 26.8% in the O2/N2 mixture. Its role in the chain-branching reaction, O2 + H ↔ O + OH, is crucial for the production of O and OH radicals. Moreover, H-abstraction involving the OH radical (NH3 + OH ↔ NH2 + H2O) plays a key role in NH3 oxidation even when O2 is added to the mixture. Finally, to further investigate the role of NH3 combustion in cycle-to-cycle variability, the intake pressure was reduced to reach lower load, resulting in highly unstable combustion in the absence of O2 enrichment. As expected, the O2 enrichment stabilizes the combustion process. However, as the engine load decreases until 4.3 bar, the amount of O2 needed to achieve stable combustion increases significantly, up to 32.5% vol.