Effect of Injection Timing on Ammonia–Natural Gas Co-Combustion Performance of Marine Low-Speed Two-Stroke High-Pressure Direct Injection Engines

The development of a marine ammonia–natural gas co-combustion engine aims to achieve high power with low carbon and low nitrogen oxide (NOx) emissions. Using AVL Fire software, a numerical model of a marine dual-fuel engine with a large cylinder diameter and diesel ignition for ammonia and natural gas co-combustion was constructed. The effects of ammonia injection timing (AI) and natural gas injection timing (NGI) on the combustion process, as well as on NOx and carbon dioxide (CO2) emissions, were investigated. The results indicate that, under full-load, low-speed engine conditions, when the energy fractions of three fuels are constant, the heat release rate and pressure are more sensitive to AI. Advancing the injection timing of NH3 and CH4 can achieve higher indicated mean effective pressure and indicated thermal efficiency. Under various ammonia and natural gas injection strategies, the marine engine meets the NOx emission requirements of International Maritime Organization’s Tier III. Furthermore, advancing AI or NGI reduces greenhouse gas emissions. Specifically, when ammonia is injected at a high pressure before the top dead center (TDC), advancing injection by every 2 degrees of the crank angle (°CA) reduces equivalent CO2 emissions by 1.3%. Similarly, when natural gas is injected at a high pressure before the TDC, advancing injection by every 2 °CA reduces equivalent CO2 emissions by 1.7%.