Research Progress on Ammonia-Fueled Marine Engines

Amidst the escalating global greenhouse effect, the shipping industry faces mounting pressure to decarbonize. Consequently, ammonia has emerged as a promising zero-carbon fuel for marine engines. This review systematically categorizes current research on ammonia-fueled marine engines into compression ignition (CI) and spark ignition (SI) types. CI engines, including homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and in-cylinder high-pressure direct-injection dual-fuel (HPDF) configurations, face challenges such as limited operating ranges, difficulties in emission control (particularly NOx and NH3 slip), and power constraints. Conversely, SI engines encompass conventional spark plug and pre-chamber jet ignition systems. While conventional SI engines suffer from limited power output and rely heavily on reactive fuel blending, such as hydrogen, pre-chamber jet ignition shows potential to enhance combustion stability and flame speed. Despite notable progress, widespread application of ammonia fuel in marine engines is hindered by persistent technical bottlenecks, including low reactivity, slow combustion kinetics, and the formation of fuel-bound NOx and unburned NH3 emissions. A key unresolved technical gap is the lack of an integrated combustion–after-treatment control strategy capable of dynamically maintaining an appropriate engine-out NH3/NOx balance under variable marine operating conditions while simultaneously suppressing NOx, NH3 slip, and secondary N2O formation, thus accelerating the practical implementation of ammonia-powered marine propulsion.