Exploration and optimization of injection strategy for medium-speed diesel/methanol dual direct-injection marine engine under extremely high methanol energy ratio

Methanol is considered as one of the most promising efficient and clean alternative fuels for marine engines. However, increased methanol energy ratio (MER) by direct injection can easily cause extended ignition delay and thereby misfiring. To maximize MER and achieve efficient and stable combustion of medium-speed diesel/methanol dual direct-injection marine engines under extremely high MER, a 3D computational fluid dynamics (CFD) model of direct-injection diesel/methanol marine engine is established using CONVERGE/3.0 code. The suitable fuel injector configurations are explored, and the injection strategy is optimized under an MER of 96.5% based on the CFD model. The findings demonstrate that the mixing rate of diesel and methanol jets is enhanced by symmetric injectors compared to asymmetric injectors for their effects on the mechanism of spray mixing and combustion development. The indicated thermal efficiency (ITE) is increased from 45.5% to 48.3% through advancing methanol injection timing from 0°CA BTDC to 25°CA BTDC. Additionally, the optimal injection strategies characterizing the highest ITE of 48.3%, the lower global emissions, and an extremely high MER among the cases are yielded with a methanol injection duration of 40°CA and a methanol injection pressure of 56 MPa; compared to 26.4 MPa condition, ITE is improved by 15.9%.