Numerical study of injection strategies for marine methanol/diesel direct dual fuel stratification engine

To explore the application potential of renewable methanol and direct dual fuel stratification (DDFS) technology in marine internal combustion engines, this study conducted a fuel injection strategy research based on performance optimization for a marine methanol/diesel DDFS engine under high methanol substitution rate (95%). The results show that the fuel mixing process plays a crucial role in methanol/diesel DDFS engine combustion state switching. Premature methanol injection under methanol single-stage injection strategy causes ringing intensity (RI) to exceed the engine limit. Additionally, when start of diesel injection (SOID) occurs earlier than start of methanol injection (SOIM), the shorter methanol/diesel injection interval (MDI) leads to the deterioration of diesel premixed combustion for reason of methanol spray interference. Therefore, adopting an injection strategy with SOID earlier than SOIM under methanol single-stage strategy combined with an appropriately extended MDI achieves better comprehensive engine performance. Further optimization of methanol/diesel DDFS engine using a methanol two-stage injection strategy shows that the methanol two-stage injection strategy provides better fuel economy while maintaining acceptable RI. By increasing the methanol pre-injection ratio (MR1) and optimizing the methanol two-stage injection interval (MMI), the engine economy was further enhanced. Compared with the methanol single-stage injection strategy, the optimized methanol two-stage injection strategy reduced equivalent indicated specific fuel consumption (EISFC) by 3.95%. However, it should be noted that the combustion completeness under the methanol two-stage injection strategy decreased, leading to higher emissions of HC, CO and soot. On the other hand, this strategy resulted in lower NOX emissions due to more sufficient premixed combustion.