Comparison of spark and turbulent jet ignition in a fully ammonia-fueled engine

Ammonia (NH₃) is an emerging carbon-free fuel with the potential to decarbonize the energy sector. However, its widespread adoption is hindered by challenges like low flame speed, high ignition energy, and elevated emissions of nitrogen oxides (NOx) and unburnt NH₃. These limitations necessitate innovative combustion strategies for efficient and stable engine operation. This study investigates the potential of turbulent jet ignition (TJI) to overcome these challenges through the implementation of a pre-chamber, a small auxiliary chamber equipped with a spark plug to create hot, reactive jets that propagate into the main chamber, promoting rapid combustion from distributed ignition sites. In this work, TJI operation is compared to conventional spark ignition (SI) in a Cummins ISB6.7 engine retrofitted for 100% ammonia operation. Experiments were conducted at 1200 and 1800 RPM across varying loads (25%, 50%, 75%, and 100%) with equivalence ratio and spark timing sweeps. Combustion performance, stability, and emissions (NH3, NOx, and N2O) for both ignition systems were compared. Results demonstrate that TJI significantly improves combustion stability, with the coefficient of variation in indicated mean effective pressure reduced by over 50% compared to SI at part loads. Additionally, TJI allowed operation with retarded spark timing. Emissions analysis revealed comparable NOx levels but reduced unburnt NH₃ and N2O emissions in TJI mode. These findings highlight the ability of turbulent jet ignition to address the inherent challenges of ammonia combustion, providing a pathway for its integration into practical engines. Future work will explore optimizing pre-chamber design and emissions control strategies for broader applications.