Numerical Investigation On The Combustion Process For Large-bore Low-speed Two-stroke Marine Diesel Engine

Low-speed two-stroke marine diesel engines have the advantages of good economy,high reliability,and use of inferior fuels,and are widely used as the main engines in large ships.Aiming at the IMO third-phase emission restrictions,meeting the Tier Ⅲ emission regulations is a serious challenge for Chinese marine diesel engines.Therefore,it is urgent to optimize the combustion process from the aspects of oil and gas,to reveal the mixing mechanism of the oil and gas in the cylinder of the diesel engine,and to ultimately achieve efficient and clean combustion.Based on CFD software,a three-dimensional simulation for a low-speed two-stroke marine diesel engine is performed in this study.The structure of the scavenging port（horizontal inclination）,the shape of the combustion chamber,the EGR,the Miller cycle,the injection angle,and the segmented injection are explored under transient conditions.Strategies（pre-injection volume,pre-main injection interval,three injections）and other technologies that influence gas-air mixing,combustion,and emission processes in the cylinders of low-speed diesel engines are analyzed.Feasible technical routes to meet Tier Ⅲ emission standards based on multiple factors are explored.The study found that as the horizontal tilt angle of the scavenging port increases,the scavenging efficiency increases and then decreases.When the horizontal tilt angleα=13（?）,the scavenging efficiency reaches the maximum（92.64%）.A too large horizontal tilt angle will affect the mixing of exhaust gas and fresh air,making the scavenging performance worse.With deep-pit and pit-type combustion chambers,the vortex ratio in the cylinder is significantly higher than that of shallow-type combustion chambers.However,when the pit-type structure is used,the fuel-rich zone is near the cylinder wall;in the deep-pit combustion chamber,fuel-rich zone will appear;and fuel/air mixture is more uniform in shallowωcombustion chamber.The injection angle directly affects the formation of the air/fuel mixture in the cylinder.If the injection angle is too small,the fuel sprays will overlap at the center of the cylinder to form a fuel-rich zone.Then,the air utilization rate in the cylinder will be reduced.If the angle is too large,the fuel spray and the flame will impinge the wall.The heat loss of the cylinder wall will increase.In contrast,and a moderate injection angle（20（?））is conducive to increasing power and to reduce fuel consumption.The high-intensity Miller cycle will cause the diesel engine pressure to decrease significantly,the peak value of the heat release rate in the cylinder will be significantly shifted back,the combustion heat release duration will increase,and the pre-combustion ratio will increase.When M5 is used,the NO_x generation will decrease by 24.28%compared with the original engine,ISFC will increase by 3.66%.In addition,coupled with the segmented injection technology,the combustion duration can be shortened,and the center of gravity of combustion is closer to the top dead center,which improves the power of the diesel engine,but this technical route cannot make NO_x emissions meet Tier Ⅲ emission regulations.The use of 30%EGR can meet Tier Ⅲ emission regulations,but the fuel consumption will significantly deteriorate.Multiple injection and injection angles will be optimized through synergy（30%EGR+10%pre-injection amount+Δt=15（?）+20（?）will reduce NO_x by 65.5%and increase fuel consumption by only 1.24%）,which is a reasonable technical route to meet Tier Ⅲ emission regulations.The further use of triple injection technology can improve the trade-off relationship between fuel consumption and NO_x emissions（usingα=20（?）,m₁=5%,t₁=22（?）,m₂=10%,and t₂=15（?） to further reduce 0.41%）.