Addressing climate change demands, energy security and resilience has necessitated replacing conventional fossil-based fuels with zero and carbon-neutral fuels/energy carriers. The most immediate solution is the partial and progressive substitution of conventional fuels in transportation. The effects of partially substituting gasoline with ammonia/hydrogen (NH3/H2) mixtures in a spark ignition (SI) engine are investigated in this paper. The utilization of NH3/H2 mixtures is a promising avenue of research since they can be produced from on-board NH3 reforming, utilising heat energy that is recovered from hot exhaust gases. Experimental results indicate that adding NH3/H2 enabled stable engine operation at lean conditions (λ = 1.4), resulting in reduced carbon-based emissions due to the non-carbon nature of NH3/H2. Utilising an integrated approach that combined a hemispherical flame geometry model with a thermodynamic model, has revealed that the introduction of NH3/H2 significantly enhanced the combustion speed during the initial phase and further improved combustion efficiency. However, nitrogen-based emissions such as NO and NO2 increased. This work also assessed the performance of a conventional three-way catalyst (TWC) and a double-function ammonia slip catalyst (ASC) in mitigating emissions. The TWC effectively controlled carbon-based emissions and NO under stoichiometric conditions but exhibited reduced efficiency under lean conditions, especially with NH3 present. The ASC demonstrated high NH3 conversion efficiency even at low temperatures, making it suitable for engine start-up and warm-up phases. Under steady-state conditions with artificially increased NH3/NOX ratios, a significant reduction in NOx emission was achieved with the ASC. However, high NH3/NOX ratios increased nitrous oxide (N2O) formation and NH3 slip.
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