Study On The Effect Of Multiple Injections On Oil Film Characteristics And Soot Emission In GDI Engine With Cold Start

GDI engine has become the mainstream of application due to its advantages in power and economy.However,the fuel spray has a large penetration distance and limited cylinder space,and inevitably formed wall oil film in the cylinder.The oil film on the wall can promote the formation of fuel mixture to a certain extent through evaporation,but it may also worsen the emission of the engine,especially in the cold start-up stage.Due to the low temperature in the cylinder and the poor evaporation of the oil film,the existence of the oil film will make the emission deterioration more obvious.In order to further reduce the emissions in the process of cold start and warm-up of GDI engine,it is necessary to study the formation and development of the oil film on the wall at the injection time and injection proportion,and then explore the relationship between the oil film and the fuel mixture,Soot.In this paper,by means of fluid simulation and bench test,the effects of different secondary injection time and injection proportion,as well as different tertiary injection time and injection proportion on fuel mixture formation,wall oil film and Soot emission in the cold start-up stage are explored,and the changes of oil film and the characteristics of Soot distribution at different oil film positions are further explored.In general,the increase of fuel injection times can effectively enhance the space atomization effect of fuel,which is an effective way to reduce fuel impingement,and can effectively avoid pool fire in the later stage of combustion.Therefore,multiple fuel injection plays an important role in improving Soot emissions.With the increase of fuel injection times,fuel impingement quality and oil film thickness are gradually reduced,and with the injection time the effect of air flow on the formation of oil film is more and more obvious.With the delay of the second injection time,the quality of the oil film on the wall first decreases and then increases,and the oil film on the wall mainly increases on the cylinder wall.With the delay of the second injection time,the quality of the oil film on the piston top,the intake valve and the top of the combustion chamber first decreases and then increases,but the quality of the oil film on the cylinder wall has been increasing.When the secondary injection time is 150 ° CA,the oil film on the piston top,intake valve and combustion chamber top is the least,and the oil film mass on the cylinder wall increases less.With the increase of the proportion of the first injection,the mass of the oil film on the wall first decreases and then increases when the exhaust valve is opened,and the evaporation rate of the oil film on the piston top is obviously faster than that on the cylinder wall.From the point of view of the position of oil film formation,with the delay of the second injection time and the increase of the second injection proportion,the influence of air flow on the oil film formation is gradually enhanced,and the delay of the second injection time and the increase of the second injection proportion make the amount of oil film on the cylinder wall increase significantly.Fixed the first and second injection time,with the delay of the third injection time,the quality of the oil film on the inner wall of the cylinder decreases first and then increases.When the third injection time is 210 ° CA,the quality of the oil film is the smallest when the exhaust valve is opened.Fixed the proportion of the first injection,compared with the amount of oil film formation under different injection proportion,when the proportion of the third injection is large,the amount of oil film formation increases correspondingly,but because the evaporation and atomization time of the oil film is short,the amount of oil film residue increases,and the overall uniformity of the mixture becomes poor.With the delay of the second injection time,both the maximum value of the Soot mass fraction in the cylinder and the Soot mass fraction when the exhaust valve is opened follow the law of decreasing first and then increasing.When the second injection time is earlier,when the exhaust valve is open,the Soot is mainly distributed on the top of the piston and the side wall of the cylinder,while other areas are relatively less.When the second injection time is relatively late,the Soot distribution is similar to the oil film distribution when the exhaust valve is opened,and it gradually transits from the piston top and the cylinder side wall to the cylinder side wall and the slot area at the intake valve.In the process of cold start-up,Soot is mainly generated due to the incomplete combustion of fuel due to the uneven mixture caused by the poor atomization of fuel space and oil film on the wall.Therefore,it is not conducive to reduce the emissions of Soot when the second injection time is delayed more.With the decrease of the second injection ratio,the Soot mass fraction first decreases and then increases when the exhaust valve is opened.When the second injection ratio becomes smaller,the Soot distribution area gradually transits from the slit area at the intake valve and the cylinder side wall to the slit area at the top of the piston,the intake valve and the cylinder side wall.Therefore,the appropriate injection ratio can greatly promote the mixture formation and reduce the mixture concentration area.Fixed the first and second injection time,with the delay of the third injection time,the formation time of the mixture and the evaporation time of the oil film in the cylinder are shorter,more fuel is injected on the piston,which cannot completely evaporate,and pool fire is easy to occur near the oil film in the combustion process.The convertible top gradually becomes the Soot generation area,with a certain amount of pool fire carbon smoke generation.With the increase of the third injection ratio,the oil film on the piston top cannot evaporate effectively,so there are more Soot on the piston top and less Soot in the slot area of the intake valve.In order to further study the relationship between the oil film and the Soot,an oil film was added in the combustion chamber under the condition of homogeneous mixture,and the relationship between the thickness and position of the oil film and the emission of the Soot was studied.At different positions,the evaporation rate of oil film is different,among them,the evaporation rate of oil film on the piston top is the fastest,and the evaporation rate in the top of chamber is the slowest.When the oil film cannot completely evaporate or there are more residues,there are more Soot near the oil film when the exhaust valve is opened.