Spray Characteristics And Soot Formation Mechanisms Of Gasoline Direct Injection Engines

With the stric fuel consumption regulations,Gasoline Direct Injection(GDI)engines are widely used in light-duty vehicles because of their high fuel economy,good dynamic performance,and fast transient response.Severe emission regulations and severe atmospheric pollution issues have caused people to pay close attention to the emissions of GDI engine particulates.The degree of fuel atomization in the direct injection technology and the process of mixture formation have a crucial influence on the mass of partic-ulate matter,the amount of particulates,and the generation of carcinogenic polycyclic aromatic hydrocar-bons in GDI engine cylinders.In this paper,Computational Fluid Dynamics(CFD)simulation method and optical visualization method are used to investigate the fuel spray characteristics of GDI engine and its effect on soot formation.Firstly,this paper summarizes the mechanism of droplet disruption,wall collision,evaporation,and droplet interaction in three aspects:theory,experiment and numerical model.Secondly,the comprehen-sive development of the soot model is summarized,and the mechanisms of soot precursor formation,soot nucleation,surface growth,polymerization and oxidation processes are analyzed.The empirical models,semi-empirical models and details of carbon soot are systematically elaborated.The application of the model in the CFD simulation study analyzed the differences between the microscopic nanostructures of GDI engines and diesel engine particulates.It was found that the structure of the soot particles was mainly related to the reaction conditions.Currently,the soot model of the GDI engine cannot accurately describe the soot development process and carbon prediction.The results of smoke emissions need further im-provement.Through optical visualization experiments and CFD simulations,we studied the characteristics of GDI engine spray and cylinder mixture formation.Comprehensive analysis of the impact of environmen-tal back pressure,injection pressure,fuel temperature and other factors on the spray macroscopic mor-phology,spray penetration,Socket average diameter and other parameters of the law.Studies have shown that increasing the spray pressure can effectively improve the atomization quality of the spray;increasing the ambient back pressure will reduce the spray penetration distance,enhance the air entrainment effect on the outer edge of the sprayed oil bundle,and at the same time,the spray oil spray shrinks towards the center under the effect of high back pressure.The spray width is reduced,and the interaction between the oil bundles enhances the quality of the atomization.Combined with the existing carbon dioxide soot optical measurement results of the GDI engine,the semi-empirical soot model of low-temperature diesel was optimized and improved,and the rate of soot nucleation,coalescence,and surface growth was mainly modified.Compared with the original model,the improved soot model is closer to the experimental value in calculating the generation rate and position distribution of the soot in the cylinder of the GDI engine.Finally,through the GDI engine bench test,optical diagnosis and CFD simulation platform,this paper deeply discusses the relationship between fuel characteristics and fuel injection parameters and soot generation.It is found that the increase of aromatics and olefins content in gasoline directly leads to an increase in the amount of soot generated;in the middle and low load conditions,the spray penetration is long and hits the top of the piston under the conditions of medium and low injection pressure.It is not easy to rebound and evaporate slowly,resulting in inhomogeneous air-fuel mixture in the cylinder in-creasing the amount of soot generated in the cylinder.Increasing fuel temperature and fuel injection pres-sure,controlling injection timing,and adopting two injection techniques can effectively suppress the gen-eration of soot.