Knock suppression strategies of methanol/diesel dual fuel engines using three-dimensional model

Abstract

With the increasing stringency of international decarbonization regulations, the maritime industry is accelerating the application of low-carbon alternative fuels. Among them, methanol is considered one of the most promising alternatives due to its low carbon content, clean combustion and easy-storage characteristics. When applied to marine engines, methanol is typically used in combination with a certain amount of diesel to ensure stable ignition and combustion, resulting in a methanol/diesel dual fuel engine configuration. For methanol/diesel dual fuel engines operating under port injection (PI) mode, abnormal combustion phenomena—such as misfire at low loads and knock at high loads—often occur, which limit further increases in the methanol substitution ratio (MSR). In this study, a three-dimensional simulation model was developed and validated by using experiment data from a natural gas/diesel dual fuel engine, which is then modified to a methanol/diesel dual fuel engine model considering the differences in fuel property and combustion reaction kinetics mechanism. The modified engine model is employed to investigate the effects of MSR on the combustion and emission characteristics under various operating conditions. The characteristics of abnormal combustion phenomenon—misfire and knock—were analyzed, whilst the maximum MSR under various engine operating loads were predicted. Additionally, the effects of three knock-related factors, that is, initial gas temperature (IGT), exhaust gas recirculation (EGR), and start of injection (SOI), were analyzed under the commonly used 85% operating load. Subsequently, a multi-objective optimization was performed to determine the optimal combination of the abovementioned three factors. Results indicate that the optimal performance was achieved when EGR = 12%, SOI = 2.5°CA, and IGT = 348 K. Under this strategy, the ringing intensity (RI) was reduced by 95.07%, enabling stable combustion at 50% MSR. Simultaneously, NOx emissions were reduced by 73.95%. Although the output power of methanol/diesel dual fuel engine decreased by 6.89% compared to that before the optimization, it still showed an 11.75% improvement over the baseline output power of natural gas/diesel dual fuel engine at 85% load.