A Practical Pathway to Downsize the On-Board Hydrogen Generation System for Near-Zero Emissions Mono-Fuel Ammonia Operation Using a Long-Stroke Engine

Ammonia-fuelled engines have emerged as a promising route toward net-zero emission targets due to ammonia’s carbon-free nature, ease of storage, and established handling infrastructure. However, the low burning speed and narrow flammability limits of ammonia pose a significant combustion challenge, which can be addressed through hydrogen co-fuelling. For practical implementation, on-board hydrogen production via thermal catalytic cracking of ammonia is an attractive solution, as it eliminates the need for external hydrogen storage and associated handling and capital costs. Previous studies by the present authors identified a lean operating strategy that achieves an equal molar ratio of NOx and unburned ammonia (NH3), enabling complete conversion to nitrogen and water vapour when coupled with a Selective Catalytic Reduction (SCR) system. This strategy was further validated using cracked ammonia-derived hydrogen in place of bottled H2 through an on-board cracker, thereby representing a realistic real-world configuration. The hydrogen requirement, however, is strongly influenced by engine architecture and operating conditions. The present study investigates the effect of stroke length on hydrogen energy fraction requirements to achieve an optimum α-ratio (NH₃/NOx) of unity. Results demonstrate that long-stroke engines require a substantially lower hydrogen energy fraction compared to short-stroke configurations. Furthermore, at higher engine loads and speeds, the hydrogen requirement approaches zero, effectively enabling standalone ammonia operation with near-zero tailpipe emissions. These results suggest that long-stroke engine architectures, when coupled with optimized operating conditions, can substantially reduce or even eliminate hydrogen demand at high engine speeds and loads, while maintaining gross thermal efficiencies exceeding 45% in ammonia-fuelled engines. Consequently, smaller on-board crackers are sufficient for mid-load operation, while high-load operation may not require hydrogen assistance, thereby reducing system complexity, parasitic losses, and overall integration challenges.