Numerical Simulation Study Of Mixing And Combustion Process Of Dual Fuel Low-speed Two-stroke Engine

As the International Maritime Organization has increasingly stringent emission regulations,especially since 2016,Tier III standard would be implemented in controlled areas,NO_x emission is required to be reduced by over 80%from Tier I,making traditional low-speed two-stroke engine to meet the requirements of emissions needs to pay higher costs.Gas/diesel dual-fuel engine has attracted the attention from the ship industry due to its advantages of low pollutant emission.The low-pressure injection gas/diesel dual-fuel low-speed marine engine is simulated in three dimensional domains,and the mixing and combustion process of the engine are analyzed.Through optimizing the radial and longitudinal angles of the scavenging port,it has been found that,prior to pilot fuel injection,when the radial angle is small(θ<20°),the distribution of natural gas in the cylinder has"upper and lower"stratification.When the radial angle is large(θ≥20°),there will be"left and right"stratification in the cylinder,and natural gas will escape at the same time.Moreover,a moderate radial angle(θ=25°)is conducive to improving the combustion rate and reducing the consumption of natural gas.When the longitudinal inclination angle is too small(α<85°),the natural gas in the cylinder is asymmetrical on both sides,which is not conducive to the propagation of fuel flame,thus reducing the indicating power.When moderately reduces the longitudinal inclination angle of the scavenging port(α=85°),it is conducive to promote the propagation of ignition flame,and reducing the consumption rate of natural gas.Increasing the radial angle of the scavenging port will promote the flow of natural gas toward pre-combustion chamber and cylinder head,which is beneficial to the combustion.When the radial angle is too large,natural gas would escape easily,so the combustion strategy of coupling the radial angle of scavenging port with the angle of natural gas injection,the timing of exhaust valve closing,and the timing of fuel injection is designed.It is found that the gas consumption is lower when the scavenging radial angle is similar with the gas injection radial angle.Delaying the closing time of the exhaust valve can promote the movement of the mixture towards the cylinder head,then promote the mixture combustion;however,a too large delay could lead to large amounts of gas escaping.Proper pre-injection timing can effectively improve the combustion of mixture gas in the cylinder.At a small scavenging angle,earlier injection timing can reduce the natural gas consumption of the engine.In contrast,at a large scavenging angle(θ>25°),it is impossible to reduce the natural gas consumption rate by changing the injection timing.