Combustion System Optimal Design Of Turbocharged Downsizing Gasoline Engine Based On CFD

At present,serious energy and environment problems make energy-saving and emission reduction task increasingly important.As for gasoline engine,Turbocharged-Downsizing technology is one of the effective means and gets extensive attention.This thesis based on a 1.0 liter 3 cylinder turbocharged gasoline engine,when gasoline engine adapts to turbocharging and downsizing technologies,the knock attendance rises and the compression ratio of the engine was limited.From the perspective of optimizing in cylinder flow field,CFD method was used to simulate multiple combustion systems and the port geometry,chamber structure,piston appearance were studied to reveal it's effects and influence on promoting tumble and turbulent kinetic energy(TKE).First,based on port design principal,3 ports were designed and were assembled with basic chamber.Transient simulation under 5500r/min show that PortC can produce the strongest tumble motion,the transient tumble ratio and average turbulent kinetic energy can reach 2.85 and 377 m~2/s~2 respectively.While the compression end TKE of the 3 combustion system all reach 200m~2/s~2,The TKE center of the three combustion systems all sit on the intake side,which may induce dissymmetrical flame propagation and result in higher knock possibility.Second,a deeply study was carried on chamber masking.A comparison of PortA+masking combustion system transient simulation with bench tests during prototype developing stage was conducted to verify the accuracy of calculation model.Then,based on optimized PortC,the effects of chamber masking geometry on promoting in cylinder tumble ratio and normalizing TKE position was studied.3masking chambers were designed based on basic chamber and were assembled with PortC.5500r/min and 2000r/min full load operation points transient simulation results show that C03 combustion system with intake and exhaust masking can promote tumble ratio and TKE by 10%and 6.9%when compared with PortC+basic chamber.Adapting masking geometry alone can't make TKE on the chamber center on both typical operation conditions.Third,based on PortC and chamber C03,3 newly designed pistons were simulated and the flow field,TKE position were analyzed.According to the results,two optimal pistons were designed,which adjust the bowl geometry and the height of piston slope.The simulation results under 5500r/min and 2000r/min show that while P04 piston can normalize the TKE around the spark plug under both engine speed,P05 piston can sit the TKE position on the chamber center under the 2000r/min only.Combustion system with bowl piston geometry can decrease in cylinder tumble ratio and TKE value.Finally,combustion simulations were conducted on the 3 stage optimization results:PortC,C03 and P04 combustion systems.The results show that while the flame propagation speed in C03 combustion system is faster than PortC and P04 combustion system,the intake side flame propagation speed is faster than exhaust side in C03 and PortC combustion system.P04 combustion system has a symmetrical flame propagation both on intake and exhaust sides.Through port,cylinder chamber,piston appearance optimization,the tumble ratio and turbulent intensity can be improved,which can promote flame propagation speed.On the other side,reasonable TKE distribution is favorable for symmetrical flame propagation,both of them are helpful for turbocharged downsizing gasoline engine reducing knock tendency and promoting the engine performance.