Effects of diesel injection strategy on performance of methanol/diesel dual direct injection engine

The effects of different diesel injection strategies on the combustion characteristics, economic characteristics, and emission characteristics of a methanol/diesel dual direct injection engine at a 50.0% methanol energy substitution rate were investigated by varying the diesel injection timing and injection pressure. The results show that advancing the diesel injection timing helps improve the mixing of fuel and air in the cylinder and shorten the combustion duration, thereby increasing the indicated thermal efficiency. Appropriate advancement of injection timing combined with an increase in injection pressure (from 80 MPa to 100 MPa) can significantly enhance combustion stability. The coefficient of variation of indicated mean effective pressure (COV_IMEP) experiences a maximum reduction of 44.4% (from 3.6% to 2.0%), but this also increases the knock risk. Overall, the diesel injection timing has a greater impact on thermal efficiency than the injection pressure. In addition, through the simultaneous optimization of injection timing (-12°) and injection pressure (100 MPa), the maximum indicated thermal efficiency of the engine can reach 41.3%, while pollutant emissions are significantly improved: the specific CO emissions have a maximum reduction of 10.3% (decreasing from 7.8 g·(kW·h)^-1 to 7.0 g·(kW·h)^-1 under 80 MPa), and the specific HC and Soot emissions are significantly reduced. Although NO_x emissions increase slightly, they still remain at a low level below 5 g·(kW·h)^-1 overall.