Simulation Study Of Effect Of Stratified EGR On Combustion And Emission Characteristics Of Hydrogen-gasoline Engine

Hydrogen,as one of the alternative fuels with good application prospects,has a broad research space.Combining stratified EGR with hydrogen-doped engines can both use the physical and chemical properties of hydrogen to improve combustion characteristics,and it can be used on the premise of less impact on combustion The implementation of emission control not only relieves the fossil energy crisis,but also reduces the burden on the environment.This paper relies on the National Natural Science Foundation of China“Research on Quality Adjustment Mechanism of Ignited Internal Combustion Engines Based on Hydrogen/Gasoline Composite Injection and Stratified EGR Equivalent Ratio”.The CFD simulation method is used to simulate the stratified EGR form,parameters and performance of a hydrogen/gasoline hybrid injection gasoline engine equipped with stratified EGR.The specific work includes:establishing a simulation model of a hydrogen/gasoline composite injection engine;stratified EGR tumble stratified structure;the stratified effect of stratified EGR under different stratified EGR parameters,and the combustion and emission characteristics of the engine are simulated;the simulation of different hydrogen injection strategies to analyze the in-cylinder mixture and EGR;the effect of the layer effect is analyzed,and the effect on the combustion emission characteristics of the engine is analyzed.The main conclusions are as follows:(1)The stratification effect of the double-sided duct is the best among the three layered structures.Regardless of the intake stroke,compression stroke,and ignition timing,the EGR in the cylinder can form a radial“pie-like”distribution.The EGR concentration difference can reach 11%,and the stratification effect is the best.The single-sided duct can maintain radial lateral stratification at the time of the intake stroke,and the compression stroke starts,the EGR diffusion ratio is large,and for the EGR in the cylinder before the ignition timing,the concentration difference is about 7%,and the thick area overlaps with the area near the spark plug,and the layering effect is not good.Due to the arc-like area enclosed by the partition and the intake duct,the double partition will cause the EGR to enter the cylinder in multiple directions,causing the initial intake EGR to be partially distributed in the cylinder until the ignition time the difference of the EGR concentration in the cylinders is close to 2%.Similar to homogeneous EGR,the stratification effect is the worst.(2)As the EGR pressure increases within a certain range,the EGR stratification effect improves,while excessive pressure will destroy the stratified structure.The EGR stratification effect is better than 1.0 MPa under the 1.25 MPa EGR introduction this pressure,while 1.5 MPa EGR is introduced.,the stratification effect is the worst under pressure.Comparing the combustion and emission characteristics of the three types of EGR pressure,the peak value of the engine cylinder pressure is highest at 1.25MPa EGR pressure,which is 8%higher than the peak pressure of 1.0MPa EGR pressure,and the NO_x emissions are increased by 11%.SOOT emissions are reduced by 6%,that is,the better the EGR stratification effect under the same conditions,the greater the improvement of the dynamic parameters,the lower the SOOT emissions,but the greater the degree of NO_x suppression,the more NO_x generation.(3)Under the same EGR injection pressure,as the EGR rate increases,excess EGR is affected by diffusion factors,and it is difficult to achieve stratification by gas distribution.The smaller the stratification concentration difference at the time of engine ignition,the closer it is to homogeneity.Comparing the combustion and emission characteristics of the engine under the three stratified EGR rates,the engine average cylinder pressure,cylinder temperature,and heat release rate are highest at 15%EGR rate,NO_x emissions are highest,and SOOT emissions are lowest,compared to 15%at30%EGR rate.For the EGR rate,the average peak cylinder pressure drops by 7.5%and the NO_x drops by 14%.Compared with the 15%EGR rate,the average cylinder pressure peak drops by 22%and the NO_x drops by 76%.(4)The introduction of direct hydrogen injection will affect the radial distribution of EGR in the cylinder during the later stage of compression.The reason is that the hydrogen-rich region is concentrated near the spark plug at the center of the cylinder at the time of ignition,which promotes the EGR in this region to be squeezed toward the rich regions on both sides,that is,local hydrogen-rich hydrogen stratification and stratified EGR can promote each other,but after the hydrogen doping ratio exceeds a certain range,due to the excessive amount of hydrogen,an ideal stratified arrangement cannot be formed,resulting in a decrease in the EGR stratification effect,but a slight decline in the combustion characteristics of the engine.(5)As the pressure of hydrogen direct injection increases,the EGR stratification effect increases first and then decreases.At 4MPa,the hydrogen gas diffuses violently,and the distribution is relatively dispersed in the late stage of compression,failing to form a hydrogen-rich region near the spark plug,resulting in poor EGR stratification.Under 6MPa hydrogen injection pressure,the hydrogen diffusion is less,the local hydrogen-rich stratification of the spark plug is obvious,and the EGR stratification is better.When the hydrogen injection pressure was further increased to 8 MPa,the hydrogen-rich region was relatively deviated from the centerline of the cylinder,and the difference in EGR concentration was similar to 6 MPa,but the layers were uneven on both sides in the radial direction.Therefore,the injection engine cylinder pressure and heat release rate are highest under the 6MPa hydrogen injection pressure,but the NO_x emissions are also the highest.