Effects of Double Injection Strategy in a Diesel-Ammonia Dual-Fuel Engine

This study investigates the application of a double injection strategy in a single-cylinder marine diesel-ammonia dual-fuel engine retrofitted for experimental analysis. Diesel micro-pilot (MP) injection was used to ignite ammonia combustion, with diesel and ammonia injected separately into the cylinder through dedicated injectors. The first MP injection timing was fixed at -20 CAD before ammonia injection timing, and both early and late double MP injection strategies were implemented to evaluate their effects on ammonia combustion, engine performance, and exhaust emissions. Under all conditions, the ammonia injection timing remained constant. Early double injection strategies, with the second MP injection occurring before the first, enhanced premixed diesel combustion by raising in-cylinder temperature and pressure. However, this early heat release was ineffective for ammonia evaporation and combustion due to poor timing alignment. In contrast, late double injection strategies, with the second MP injection occurring between 65–105 CAD after the first MP injection timing, improved ammonia combustion by targeting the diesel spray at unburned ammonia in the squish region near the cylinder liner, where flame quenching typically occurs. Consequently, late MP injections reduced unburned ammonia emissions but led to higher N₂O and hydrocarbon emissions due to slower oxidation rates in the squish region. Additionally, thermal efficiency declined due to decreased work conversion efficiency. Unburned ammonia above a certain level was detected under all conditions, primarily due to injector asymmetry, which led to incomplete combustion in specific regions of the cylinder. These findings underscore the importance of optimizing diesel pilot injection timing to enhance ammonia combustion while managing trade-offs in emissions and efficiency in dual-fuel engines.