Analysis and visualization of micro-pilot diesel injection timing in a high-pressure direct injection Ammonia-Diesel dual-fuel marine engine

• Experimental study on HPDI ammonia-diesel dual-fuel combustion for marine engines. • Three-stage combustion model clarifies ammonia ignition and combustion behavior. • The in-cylinder diesel flame region changes with MP diesel injection timing. • Ammonia combustion efficiency directly governs thermal efficiency and emission trends. • High-speed flame imaging links combustion with in-cylinder flame development. This study investigates the effect of micro-pilot (MP) diesel injection timing on ammonia combustion and emissions in a 12.5 L-class single-cylinder high-pressure direct-injection (HPDI) dual-fuel marine engine. Ammonia and diesel were directly injected through separate injectors, ammonia via a centrally mounted mechanical injector, and diesel via a side-mounted, common-rail injector. Diesel served as the ignition source and was injected earlier than ammonia. Experiments were conducted at 25 % of the engine’s full load, with diesel injection timing varied from −31.5 to −11.5 crank angle degrees (CAD) relative to the fixed ammonia injection timing at 5 CAD intervals. Key performance indicators, including indicated thermal efficiency (ITE), ammonia combustion efficiency ( η comb . N H 3 ), and NO X , N 2 O, and NH 3 emissions, were analyzed. The combustion process was divided into three stages: diesel premixed combustion, ammonia ignition delay, and ammonia main combustion, to interpret the effects of injection timing. High speed flame imaging through an endoscope provided visual insight into ammonia flame structure, location, and color. Results indicated that the optimal MP diesel injection timing was around ref.-21.5 CAD, yielding the best ammonia utilization and lowest N 2 O emission. Overly advanced or delayed timings (ref.-31.5 CAD or ref.-11.5 CAD) led to performance deterioration due to misalignment between diesel and sprays and combustion instability. This study provides new experimental evidence clarifying how MP diesel injection timing governs the staged combustion process and its trade-off between efficiency and emissions in HPDI ammonia-diesel marine engines.