Properties Of Oil Film And Its Effects On Soot Emission Under The Cold Start Condition Of Direct Injection Gasoline Engine

As a main development direction of internal combustion engine, Gasoline Direct Injection(GDI) has an advantage in improving engine performance of economy and power. GDI engine inevitably leads to spray impingement while directly fuel injection into the cylinder is adopted. Though helpful to secondary breakup of oil drop, spray impingement easily results in wall film on combustion chamber wall, especially in cold start period of engine. More soot is generated because of higher density of local mixture for lower temperature of chamber and wall, lower quality of fuel atomization and more difficult evaporation of wall film. As to the issue above, through properties analysis of wall film and soot under the condition of different strategies of injection, this paper investigates mechanism and influencing factors of the generation of wall film and soot, based on which further research on the influence of wall film on soot is conducted.To meet the research purpose described above, firstly the geometry model of air ports and combustion chamber were built. Then a CFD model was built by AVL FIRE, whose initial and boundary conditions were set up and validity was proved. Finally, the processes of spray and combustion emission with different angles, timings and pressures of injection were simulated and simulation results were analyzed.Research results show that thin wall film only emerges on the top of piston and the emission of soot is low when the angle between injector and cylinder axis is 0°. However, when the injection angle is 30° and 45°, thicker wall film emerges on both cylinder wall and top of piston, and the combustion heat release is delayed, and the emission of soot is higher. As injection timing advances in compression stroke, the thickness of wall film obviously decreases and full combustion is achieved and the emission of soot is lower. If ignition happens instantly when injection is finished, stratified dense mixture forms around the spark plug, leading to higher emission of soot. As injection pressure increases, more wall film forms on the top of piston, with smaller diameter of oil drop and more homogeneous mixture, consequently causing less emission of soot. Soot at the location of wall film mainly forms in the late combustion stage, when oxygen is insufficient and soot is not fully oxidized, so soot as a emission of engine pollutes the environment. More evaporation of wall film leads to more emission of soot, which bears no certain relation to the thickness of wall film.