Cycle-resolved emission measurements of ammonia-hydrogen combustion in a spark-ignition engine

Abstract

Ammonia–hydrogen mixtures are promising carbon-free fuels for internal combustion engines, but their unconventional combustion behavior complicates stable operation and emission control, particularly for NO, N₂O, and unburned NH₃. To characterize these emissions and their underlying formation pathways, diagnostics with high temporal resolution and strong species selectivity are required. This study employs high-speed, in-situ tunable diode laser absorption spectroscopy (TDLAS) to measure H₂O, NH₃, NO, and N₂O at 1 kHz directly downstream of the exhaust valves of a single-cylinder spark-ignition engine. The technique resolves the temporal emission characteristic of individual cycles and provides insight into how cycle-to-cycle combustion behavior influences pollutant formation. Analysis across the tested operating points shows that increasing hydrogen content improves combustion stability and reduces NH₃ slip but elevates NO formation. NO correlates positively with mean in-cylinder pressure, whereas N₂O exhibits an inverse trend, reflecting their distinct temperature-dependent production and decomposition pathways. During warm-up, the measurements capture a sudden advance in combustion phasing under otherwise constant boundary conditions. Zero-dimensional reactor simulations suggest a plausible explanation, indicating that temperature- and pressure-dependent NH₃ cracking can increase in-situ H₂ concentrations and thereby accelerate combustion. By combining cycle-resolved measurements with thermochemical simulations, this work clarifies how transient combustion dynamics, mixture reactivity, and boundary-temperature effects shape the formation of NO, N₂O, and NH₃ in ammonia–hydrogen engines. The results support the development of stable and low-emission operating strategies and suggest that, under sufficiently high temperature, pressure and residence times, the cracking of NH₃ within the cylinder can provide enough H₂ to enable stable pure-ammonia combustion.