Numerical Analysis of the Effect of Fuel Supply Strategies on the Combustion and Emissions of a Methanol/Diesel Dual-Fuel Marine Low-Speed Engine

This study investigates the impact of fuel supply strategies on the combustion and emission performance of methanol/diesel dual-fuel engines, in order to promote low-carbon and green development within the shipping industry. A three-dimensional simulation model of a methanol/diesel dual-fuel low-speed engine is established using Converge CFD software, and a numerical analysis is conducted to study the effects of pilot diesel timing, the area of the methanol injection nozzle, injection pressure, and injection timing on the engine's combustion and emission performance under 100% load. The simulation findings reveal that advancing the pilot fuel timing initially leads to a decrease in the indicated thermal efficiency (ITE) and indicated mean effective pressure (IMEP), followed by an increase. The indicated specific fuel consumption (ISFC) shows the opposite trend. NOx emissions initially decrease and then increase, with the lowest NOx emissions observed when the pilot fuel timing is advanced by 3 degrees CA. Expansion of the methanol injection nozzle area decelerates the combustion rate within the cylinder, leading to a 22.6% decline in NOx emissions, a 3% rise in CO2 emissions, and an 81.3% surge in soot emissions. Progressing the methanol injection timing boosts the engine's power output but also elevates NOx emissions; conversely, postponing the methanol injection timing may reduce the in-cylinder pressure and compromise power performance. Increasing the methanol injection pressure improves ITE and IMEP by 7.8% and 7.9%, respectively, while reducing ISFC by 7.2%. However, this can lead to higher NOx emissions, and runs the risk of triggering violent combustion in the cylinder due to excessive methanol injection pressure. This study offers a new and rational solution for low-speed marine engines by optimising the fuel injection strategy to meet the requirements for reduced greenhouse gas emissions and to achieve better fuel economy.