Effects Of Wall Film On The Unburned HC And Soot Emission Of A Direct Injection Gasoline Engine With Cold Start

GDI engine is popular because of its unique advantages,while the phenomenon of the spray/wall interaction is inevitable,which will form the wall-film.Especially in the cold start stage,this phenomenon will be more obvious because of the lower temperature in the cylinder and the worse atomization,which is the main source of the unburned hydrocarbon and soot.In order to further optimize the performance of in-cylinder direct-injection gasoline engines to achieve energy conservation and emission reduction,it is necessary to study the formation and development of the wall film.This article takes the EA888 second-generation engine as the research object,and uses the combination of CFD simulation and bench experiment to explore the impact of different injection strategies on unburned hydrocarbon and soot emissions during the cold start phase.And then seek effective measures to reduce emissions during the cold start phase.The research indicates:By increasing the engine starting speed,the flame propagation speed can be significantly increased,the air flow in the cylinder can be enhanced,the suction effect of the hot air flow in the cylinder can be enhanced,and the evaporation and atomization of the fuel can be increased.The above results have played an important role in improving fuel collisions,avoiding serious pool fires and improving emissions.As a result,the amount of oil hitting the wall and the area and thickness of the wall film are gradually reduced.With the delay of injection timing,the quality of the wall film on the top of the piston and the cylinder wall first decreases and then rises,while the intake valve and the cylinder head increase first and then decrease.At the time of ignition,as the injection timing is delayed,the total wall film in the cylinder first decreases and then rises.At 70° CA injection,the total oil film is the lowest.With the reduction of the injection duration,the quality of the wall film at the top of the piston decreases first,then increases.The wall film on the cylinder wall and the intake valve gradually decreases,and the wall film quality of the cylinder head increases first and then decreases.At the time of ignition,with the decrease of the injection duration,the total amount of wall film in the cylinder showed a tendency of first decrease and then increase.The total amount of wall film was the least when the injection duration was at 25°CA;With the delay of the fuel injection time,the overall homogeneity of the air-fuel mixture in the cylinder deteriorates,and the concentration of the air-fuel mixture near the spark plug deteriorates.There is a thicker area at the inlet and exhaust valve slits,which is not good for combustion and emissions.The peak pressure of the cylinder pressure curve gradually decreases,and the peak phase gradually falls behind.When the injection time is at 50°CA and 70°CA,the combustion effect is better,and it is closer to the constant volume combustion;when the injection duration is reduced,the overall distribution of the air-fuel mixture in the cylinder is good,the combustion volume in the cylinder is high,and the heat release is more concentrated.Cylinder pressure peak performance first increased and then decreased,the maximum peak cylinder pressure was at 25 ° CA;When the injection time is earlier,after the end of combustion,HC mainly exists on the top of the piston and the side wall of the cylinder,while other areas are relatively less.When the fuel injection time is relatively late,the HC distribution area gradually transits from the top of the piston and the side wall of the cylinder to the slit area at the side wall of the cylinder and the intake valve.The HC production mainly includes incomplete combustion,wall quenching,and wall film.Therefore delaying the injection timing is unfavorable for unburned HC emissions.As the injection duration becomes shorter,the generation of HC at the intake valve is gradually reduced after the combustion is completed,which is favorable for reducing the carbon deposition phenomenon;As the injection timing is delayed,the mixing time in the cylinder is shortened,and more fuel is injected on the cylinder wall,and the temperature of the cylinder wall is relatively low relative to other parts of the cylinder.Therefore,the wall film area becomes larger,and the wall film on the side wall of the cylinder easily generates a pool fires phenomenon during the combustion process,resulting in the formation of more pool fires soot.As the duration of injection is shortened,the momentum is higher after the fuel is sprayed from the nozzle hole,and the oil particle size becomes finer,thus increasing the surface area that interacts with the air.This will result in better mixing of oil and gas at the time of ignition and a decrease of wall film on the wall,which will reduce the generation of soot in the pool fires.The injection strategy is more effective in improving the cold start combustion and emissions of the engine.Secondary injection can effectively reduce the total amount of wall film attached to the wall.The secondary injection results in a decrease in the thickness and area of the wall film,which is more favorable for evaporation of the wall film.It plays an important role in reducing the amount of wall film attached to the wall;The secondary spray cylinder has a higher peak pressure and the peak phase is more forward.According to the distribution of the air-fuel mixture in the cylinder,the overall distribution of the air-fuel mixture in the cylinder during the secondary injection is better,the combustion volume in the cylinder is higher,and the heat release is more concentrated;In the second injection,the mixing of the air-fuel mixture in the cylinder is more uniform,the wall film is less,and the generation of the pool fires soot is reduced.