Recently, as regulations on greenhouse gas emissions have become stricter, driven by global warming, there is increasing interest in engines utilizing environmentally friendly fuels. In this context, ammonia is attracting attention as a potential alternative to fossil fuels in the future. However, due to its distinct fuel properties compared to conventional fuels, research is being conducted on utilizing diesel as an ignition source for ammonia. In this study, the effects of diesel injector fuel flow rate, and micro-pilot (MP) diesel injection timing on combustion and exhaust emission characteristics were analyzed in a single cylinder 12L marine ammonia-diesel dual-fuel engine. Two types of diesel micro-pilot injectors were tested. The first one was high flow rate micro-pilot injector (HMPI) and the second one was low flow rate micro-pilot injector (LMPI). HMPI injector had 66% more number of fuel injector nozzle hole and 250% larger fuel flow rate. Therefore, HMPI injector could distribute diesel more widely within the combustion chamber in a short injection duration, which led to advantages such as an increased ratio of premixed combustion in diesel, improved oxygen utilization in the combustion cylinder, and enhanced ignitability of ammonia.
To maintain a constant energy ratio between ammonia and diesel under steady engine load conditions, the injection durations were adjusted, and MP diesel injection timing was varied in increments of 5 crank angle degrees (CAD) to evaluate performance and emission characteristics. The experimental results showed that HMPI demonstrated higher thermal efficiency and lower unburned NH3 and N2O emission levels compared to LMPI. HMPI also showed improved overall performances under advanced MP diesel injection timing, however, performance of LMPI was also improved under the same conditions due to reduced interference between ammonia and MP diesel injection spray compared to the conditions under delayed MP diesel injection timing.