Numerical Investigation on an Injection Strategy Optimization for Dual Fuel Marine Engine Fuelled by Ammonia

In the context of greenhouse gas reduction (GHG) 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 combustion limitations. In this work a wide examination of diesel injection strategies is numerically presented, while ammonia acts as the primary fuel with energy shares 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 distributions 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 in a closed-valve cylinder; the initial conditions are obtained from a one-dimensional model able to simulate the entire engine. The results demonstrate that with an appropriate strategy it is possible to maintain the nominal value of the indicated mean effective gross pressure (IMEPg) 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. Being ammonia a compound of nitrogen, particular attention is paid to NOx and N2O emissions, providing a quantification of their emission index.