Performance and emission characteristics of marine ammonia/diesel dual-fuel engines at different diesel substitution rates

This study evaluates the feasibility of using zero-carbon ammonia as marine engine fuel via a premixed ammonia and diesel direct injection (PADD) mode. Experimental and numerical investigations on the performance and emissions of a marine PADD engine were conducted at 1200 and 1500 RPM with loads of 25 %, 50 %, and 75 %, and varying diesel substitution rates (DSRs). The results reveal that the maximum stable DSR exceeds 90 % at relatively high loads. Ammonia blending alters in-cylinder combustion, shifting the heat release rate from dualpeak to single-peak, particularly at low speeds and loads. In the 0-40 % DSR range, diesel spray shape aligns with the high-temperature region of early ignition combustion, indicating the spray distribution's significant influence on initial ignition. At 60 % DSR, early ignition deviates from the spray shape, appearing mainly at the spray tip. Ammonia blending causes ignition delay and more concentrated heat release, with increasing DSR reducing indicated thermal efficiency. Regarding emissions, as DSR increases, THC and CO emissions initially rise then fall, NO emissions initially decrease then increase, and unburned ammonia emissions exceed 10,000 ppm. Despite relatively low absolute GHG emissions, the indicated equivalent GHG concentration initially increases then decreases with increasing DSR, mainly due to N2O emissions. In-cylinder concentration distribution shows N2O forms ahead of and is consumed behind the flame front, influenced by flame area and position.