Research on parameter adjustment strategies for combustion control in dual-fuel engines with high ammonia...

The key to green shipping lies in the transition of oil-fueled vessels, which account for 98.4% of the total fleet, to renewable and clean fuel types. This shift is crucial for achieving the greenhouse gas (GHG) emission reduction targets set by the International Maritime Organization (IMO) and the European Union (EU), namely reaching climate neutrality by 2050. In the medium to long term, ammonia holds promise as a renewable and low-carbon fuel. However, during the ammonia combustion cycle, nitrogen oxide (NO_x) emissions increase concurrently, which are strictly regulated by the MARPOL 73/78 Convention. Therefore, it is imperative to implement solutions that optimize the combustion cycle to reduce NO_x emissions. This paper conducts a numerical study on the adjustment of diesel and ammonia injection timings for a marine medium-speed engine MAN L23/30H with intake-port premixed ammonia and in-cylinder direct-injected diesel pilot ignition under high ammonia substitution rates. In preliminary experiments, it was observed that at a 90% substitution rate, the combustion efficiency and thermal efficiency were only around 71% and 24% respectively under 100% load conditions, and approximately 35% and 13% respectively under 50% load conditions, indicating a state of near misfire. Consequently, this study focuses on adjusting combustion control parameters to analyze their influence on combustion characteristics under high substitution rates. In summary, both combustion efficiency and indicated thermal efficiency exhibited an initial increase followed by a decrease under both 100% and 50% load conditions. This suggests that appropriate heating of the intake air is beneficial for efficient combustion of ammonia/diesel dual fuel, but excessively high intake temperatures have a detrimental effect, particularly under heavy load conditions, where excessively high intake temperatures lead to a significant reduction in combustion efficiency and indicated thermal efficiency. The optimal initial temperatures for 100% and 50% load conditions were determined to be 375 K and 425 K, respectively. Furthermore, it was inferred that for high ammonia substitution rates, a more efficient fuel injection strategy involves controlling the pilot diesel injection timing within the range of - 10 to - 20° CA.