Experimental and numerical study of multiple injection effects on combustion and emission characteristics of natural gas-diesel dual-fuel engine

Natural gas diesel dual-fuel engines can significantly reduce soot emissions and mitigate the oil crisis. However, some drawbacks exist including low thermal efficiency, as well as high methane and carbon monoxide emissions. Multiple injection plays an important role in improving the performance and reducing emissions in dual-fuel engines. This paper presents an experimental and numerical study of multiple injection effects on the combustion and emission characteristics of a natural gas diesel dual-fuel engine at low load condition. The results showed that, when the first diesel injection timing is advanced, the indicated thermal efficiency and nitrogen oxide emissions increased first and then decreased, while the maximum pressure rise rate, carbon monoxide and methane emissions reduced initially and then increased. When the first diesel injection ratio increased, combustion rate of natural gas air mixture increased, methane and carbon monoxide emission decreased, and nitrogen oxide emission increased. When the first diesel injection timing was relatively rearward, the combustion process showed a distinct four-stage heat release characteristic. The proportion of premixed combustion of the first injected diesel and the duration of flame propagation of natural gas had considerable influence on methane emission. When the first diesel injection timing was relatively forward, the first injected diesel played a premixed role, and the start of combustion was controlled by the second injected diesel. A relatively high indicated thermal efficiency can be obtained under this injection strategy. The methane was mainly distributed in the cylinder wall and crevice area at exhaust valve opening due to the limitation of flame propagation under the multiple injection strategy.