In the context of greenhouse gas emissions (GHG) reduction the most viable short-term solution in the maritime sector is the use of renewable carbon-free fuels. Among these, ammonia represents a possible alternative in compression ignition (CI) engines operating in dual fuel (DF) mode. Although, such fuel features low chemical reactivity, especially in lean mixtures, resulting in poor combustion efficiency, exhaust ammonia slip and low engine performance, DF combustion can be an interesting strategy to overcome such limitations. In this work a wide numerical examination of diesel injection strategies is presented, while ammonia acts as the primary fuel with energy supply around 80%. Since the original marine engine, fuelled with natural gas (NG), presents a single diesel injection, firstly, a pilot injection is added and different diesel mass shares between pilot and main are investigated, by varying the injection rate shape and the pilot start of injection (SOI).
Calculations are performed with a CFD approach using ANSYS Forte® code on a closed-valve cylinder domain. The results demonstrate that with an appropriate strategy it is possible to maintain the nominal value of the indicated mean effective pressure (IMEP) with limited ammonia exhaust losses by adopting a split injection and an adequate shape of the injection profile, a parameter with a great influence on the spray evolution. Namely, a pilot SOI of 20° BTDC, with a total diesel mass of 80 mg split into two equal injections with a sine-shaped injection rate, leads to better results in terms of IMEP and ammonia emissions. Being ammonia a compound of nitrogen, particular attention is paid to NOx and N2O emissions, providing a quantification of its emission index for all simulated cases.