Study On Optimizing And Matching Of Combustion System Of High Thermal Efficiency Gasoline Engine

At present,with energy and environmental problems becoming more and more serious,efficient energy saving has become the design goal pursued in automotive industry.The GDI engine with high compression ratio and miller cycle is one of the effective design schemes to improve the fuel economy.The reasonable design of combustion system can optimize the distribution of turbulent kinetic energy and equivalence ratio in cylinder,increase the intensity of turbulence in cylinder and improve the combustion performance of gasoline engine.CFD simulation technology can effectively shorten the total time-consuming of engine design and has become an important means of engine design.The 2.0L naturally aspirated GDI engine optimized from a MPI engine is the research object in this paper.In order to achieve rapid combustion and reduce the knock tendency,the influence of the structure of intake port and the relative parameters of injector on the flow and combustion in cylinder of the gasoline engine is studied by CFD simulation.Firstly,in order to explore the influence of ultra-high tumble on flow and combustion in cylinder of GDI engine,three groups of different intake ports are designed.After the core-box fabrication,the differences of these three groups of intake ports are verified by intake ports steady flow test.And four calculation schemes are designed.In order to eliminate the influence of mass flow-rate caused by the change of discharge coefficient,the intake ports of the scheme NO.3 and the inlet boundary of the scheme NO.2 are used in the scheme NO.4.It is found that the magnitude of tumble ratio affects the distribution of mixture and the intensity of turbulent kinetic energy.The higher tumble ratio can retain more energy until the TDC,improving the average turbulent kinetic energy in cylinder at the time of ignition,accelerating the flame propagation speed and shortening the duration of combustion.But the ultrahigh tumble ratio has no obvious impact on engine performance improvement in this combustion system.Based on the analysis,the Intake port 2 is selected as the intake port design for the follow-up study considering the discharge coefficient.Secondly,the influence of 20 MPa and 35 MPa injection pressure on combustion systems is studied based on the Intake port 2 and the piston.The results show that the 35 MPa injection pressure scheme has advantages in fuel atomization and fuel vaporization.However,the turbulent kinetic energy and the tumble ratio in cylinder with 35 MPa injection pressure are relatively low.Besides,the average oil film thickness of the 35 MPa injection pressure scheme at earlier stage increases.Thus,the combustion of 35 MPa injection pressure scheme in this combustion system is poor.The adjustment of spray pressure needs to consider other factors comprehensively.Then,in order to optimize the matching of the spray,the piston and the intake port,four injection direction schemes are designed.The influence of the injection direction on the flow field in cylinder with 35 MPa injection pressure is studied.It is found that the turbulent kinetic energy and tumble ratio in cylinder of the Inject 3 scheme are significantly improved,and the turbulent kinetic energy in the late stage of the Inject 2 scheme is also improved,but the over rich area still appears in the two schemes.Compared with the other schemes,the Inject 4 scheme has the most homogeneous equivalent ratio distribution.The Inject 4 performs best through the comprehensive analysis considering the flow field,fuel evaporation rate and oil film thickness.Finally,the preliminary test of prototype designed with 35 MPa injection pressure,Inject 4 injection direction scheme and Intake port 2 is carried out.The engine fuel economy and the combustion performance under the corresponding operating conditions are analyzed,providing a basis for the optimization design of engine with high thermal efficiency.