Exhaust emission control strategies for lean-burn NH3/H2-fueled spark ignition engines

Ammonia-fueled engines are attracting increasing interest as a carbon-free energy conversion technology; however, their exhaust typically contains unburned ammonia (NH 3 ), nitrogen oxides (NO x ), and nitrous oxide (N 2 O), and achieving their simultaneous removal under oxygen-rich conditions remains a major challenge. This study investigates emission control strategies using SCR catalyst for a NH 3 /H 2 -fueled spark-ignition engine, with emphasis on the linkage between engine-out exhaust characteristics and SCR performance. Engine experiments demonstrate that the engine-out ammonia-to-NO x ratio (ANR) can be systematically controlled through mixture composition, specifically via the equivalence ratio and H 2 energy fraction, providing a practical control index for aftertreatment. Using Fe- and Cu-zeolite SCR catalysts, ANR-dependent conversion characteristics are quantified over a wide temperature range. Both catalysts exhibit a temperature-dependent optimal ANR at which near-complete simultaneous removal of NH 3 and NO x is achieved, reflecting competition between SCR reactions and NH 3 oxidation. To address sensitivity to ANR variability, two complementary and distinct approaches are demonstrated: (i) a control-based strategy employing periodic ANR swing to exploit NH 3 storage in zeolite catalysts, and (ii) a catalyst-design-based strategy using a tandem Fe/Cu SCR configuration. Both approaches achieve catalyst-out NH 3 and NO x concentrations below 10 ppm. For the tested Fe/Cu tandem system at a nominal catalyst-inlet gas temperature of 450 °C, simultaneous high NH 3 and NO x conversions were observed at all tested ANR points between 1.25 and 1.62. In addition, the effects of catalyst configuration and temperature on N 2 O formation and decomposition are clarified, highlighting trade-offs between operational robustness and greenhouse gas mitigation. The results present proof-of-concept emission-control approaches under the selected steady-state conditions.