Numerical Study Of High Efficiency And Clean Combustion Control Strategy For Low Speed Dual Fuel Marine Engine

With the more stringent emission regulations of marine engines and the increasingly serious energy shortage,it is particularly important to seek appropriate marine engine fuel and realize high-efficiency and clean combustion.Natural gas is a kind of clean fuel with abundant reserves.The Diesel/natural gas dual fuel technology has a broad application prospect in marine engines.According to the different injection pressure and injection mode of natural gas,the engine can be divided into two modes of low-pressure injection and high-pressure injection.Compared with the high-pressure injection mode,the low-pressure injection mode is simpler and offers better economy and emission performances.Therefore,in this dissertation,three-dimensional numerical simulation method is used to study the high-efficiency and clean combustion strategy of low-pressure injection diesel/natural gas dual fuel combustion of a marine engine.Based on the CONVERGE software,a three-dimensional CFD model of low-pressure injection diesel/natural gas dual fuel engine was established in this dissertation.The effects of exhaust valve close timing(EVC),exhaust gas recirculation(EGR),pilot injection timing of diesel on combustion and NOx emission were studied,and the comprehensive coupling effects of various technical routes on NOx emission and fuel economy were analyzed.The results show that the effect of early EVC is more significant than that of late EVC;EGR is the most effective strategy to reduce NOx emission;retarding the pilot injection timing of diesel can delay the main combustion process and reduce NOx emission.The retarded pilot injection coupled with EGR can significantly reduce NOx emission while maintaining good fuel economy.Based on the above research,aiming at exploring the high-efficiency combustion strategy of marine dual fuel engine,the effects of high compression ratio,pilot diesel injection parameters and natural gas injection duration on combustion process were further studied,and the fuel economy and emission effect are comprehensively analyzed.The results show that the effect of increasing compression ratio to improve thermal efficiency is most obvious.However,limited by the mechanical strength of engine,the compression ratio of 12.5 in this study can obtain the best effect.Additionally,increasing the amount of pilot diesel can enhance the induced ignition effects of jet flame,extend the penetration distance of jet flow,increase ignition points,and then improve the thermal efficiency.In this study,the best range of pilot-injection diesel should be between 1.5% and 2%,and the amount of 2% is the limit of pilot-injection diesel.Increasing the injection pressure of pilot diesel is conducive to improve the thermal efficiency,but it is not as effective as increasing the amount of pilot injection diesel.Moreover,the pilot injection angle and the injection duration of natural gas have less impact on the combustion process.