| In view of the global energy shortage and environmental pollution problems increasied,modern society further improves vehicle gasoline engine economic and emissions requirements simultaneously in the pursuit of its power.Gasoline direct injection engine with its good transient response and fuel economy has undoubtedly become the mainstream of today’s passenger power source.However,compared with the conventional port injection gasoline engine,the problem of particulate emissions in GDI engine has hindered its further development.Fuel is directly injected into the cylinder;part of the fuel hit the wall which causes the formation of fuel films.Due to the presence of fuel films,"pool fires" are produced during the combustion---the diffusive combustion at the wall films directly affects the formation of particles in the cylinder.Therefore,this article,with the help of the project of "Optical Research on Particulate Matter Generation Process under GDI Engine Transient Conditions",focuses on the fuel film formed by the cylinder wall wetting.The CFD simulation method is used to research the particle generation and emission of the GDI engine.The main contents of this paper include the following five aspects:1.Simulation platform was constructed and model validity was verified.This article completed to build a three-dimensional solid model using Pro/E and the model was meshed by Hypermesh and AVL-Fire.The model was validated using the cylinder pressure data from the bench test and the spray images taken by the visualization platform to ensure the reliability of the later simulation results.2.The characteristics of fuel film generation and the law of in-cylinder combustion and particulate matter emission under typical operating conditions were studied.The simulation results show that fuel films are mainly located on the side wall of the cylinder(liner),the top of the piston and the cylinder head(divided into the cylinder chamber and the intake valves).The distribution of fuel films are different,their characteristics are different: The evaporation rate of wall film on cylinder liner is highest,the middle of films are thick and the edge are thin;Due to the high temperature of the piston and its flat top structure,the films on piston exist for a short time;the films on intake valves are thick,their evaporation rate is almost zero.The films which are still retained in the cylinder at ignition time directly affect the final soot generation.The fuel films with different characteristics have different effects on the soot generation: the wall films on cylinder liner with low thickness and large spreading area contribute to soot with high mass fraction and large particle size;the fuel films on intake valves with high thickness and small spreading area contribute to soot with high number density and small particle size.3.The study of the different fuel injection time impact on fuel film characteristics and particulate matter emission were conducted.The fuel injection time changes,the generation order of the films on each part is different.As fuel injection timing advanced,fuel first hit the intake valves,then hit the top of the piston,and then arrive at the cylinder liner.Fuel first hit the intake valves and then hit the cylinder liner when the injection timing delayed.The wall film on the cylinder is mainly located on the cylinder liner and intake valves at ignition time.With the injection time delayed,the total films mass first decrease and then increase.With the injection time delayed,soot average size increases,soot mass fraction and number density decreased first and then increased.Spraying too late leads to a substantial increase in soot mass fraction and number density.4.The study of the different fuel injection pressure impact on fuel film characteristics and particulate matter emission were conducted.The different fuel injection pressures lead to different mass and thickness of wall films.As injection pressure increases,mass and thickness of films on cylinder chamber increase,the cylinder liner films mass increases and then decreases,the thickness of films on cylinder liner decreases.The wall film on the cylinder is mainly located on the intake valves at ignition time.The mass of films on intake valves decreases then increases with the injection pressure increased.Soot mass fraction showed a downward trend with injection pressure increased from 10 MPa to 20 MPa,soot mass fraction rose slightly when the injection pressure increased from 15 MPa to 20 MPa.With the injection pressure increased,soot number density showed a trend of going down.Soot particle size change is not obvious.5.The study of the different fuel injector angle impact on fuel film characteristics and particulate matter emission were conducted.The change of the injector angle directly affects the placement of the fuel drops.With the injector angle reduced,the mass of films on cylinder chamber and intake valves increases significantly,and the thickness of films on intake valves is significantly increased.While the injector angle is too small,soot mass fraction and number density increase significantly.In summary:The impact of fuel film on soot generation is critical.Injection parameters,injector structure and the location of the injector and intake valves should be taken a full consideration to minimize the formation of fuel films,so as to achieve the purpose of reducing particulate emissions.Besides,with the injection pressure increased,the soot mass fraction increases slightly but the soot number density greatly reduces which is in accordance with GDI engines development trend. |
