Study Of Coupling Relationship And Simulation Method Between In-cylinder Working Process And Heat Transfer In GDI Engine

GDI has been an essential modern gasoline technology for passenger car with the increasingly stringent fuel consumption and emissions regulations.CAE has been widely used to in-cylinder working process and heat transfer simulation of engine,and greater demands are being placed on the computational accuracy and efficiency of CAE.However,in-cylinder flow and combustion process of GDI engine is an extremely complicated process of physical and chemical reactions,which is accompanied by heat transfer with surrounding parts and cooling systems,the combustion system surface is the bridge of coupling relationship between in-cylinder working process and heat transfer.In order to accurately simulate the in-cylinder combustion process,accurate wall temperature distribution is necessary,in turn,it is also extremely beneficial for heat transfer process simulation and temperature prediction to obtain more accurate gas-side thermal boundary from in-cylinder combustion process.This thesis took a 2.0TGDI gasoline engine as the research object to study the above coupling relationship under typical part load and rated conditions by using an iterative method.First,engine bench and temperature measurement tests were carried out simultaneously,and 1D thermodynamic model was calibrated to extract transient boundary conditions for subsequent 3D CFD in-cylinder flow and combustion simulation which was based on the empirical and uniform wall temperature;next,the time-averaged thermal boundary of gas-side was obtained and mapped to conjugate heat transfer(CHT)model and piston heat transfer(PHT)model;then the whole engine temperature field was solved,from which the non-uniform wall temperature distribution of combustion system surface can be extracted and was reversely mapped to 3D CFD model,and the in-cylinder flow and combustion process was recalculated;finally,the whole engine temperature field was iteratively solved with updated gas-side thermal boundary for CHT and PHT models.Compared with the traditional FEA-CFD method,the above method has obvious advantages of stronger grid structure adaptability,fast calculation speed and high accuracy.According to this method,the following conclusions can be obtained:1)The wall temperature of GDI engine combustion system has important influence on the evaporation,mixing,combustion and emission under partial load and rated conditions;the prediction accuracy of in-cylinder pressure,heat release rate and in-cylinder mean temperature can be improved by taking the non-uniform wall temperature into account the in-cylinder working process simulation;2)Relative deviation of CHT and PHT simulation based on combustion results of uniform wall temperature and hardness test results is mostly within 10%,while the relative error after iteration can be within 5% which is close to the precision of hardness plug,so there is little improvement room for temperature and in-cylinder process prediction by continuing to iterate;3)The change trend of major emissions(except Soot)after iteration is closer to the experimental value in the in-cylinder working process simulation,the wall temperature of GDI engine combustion system has a great impact on the formation and distribution of major emissions,mainly for the following aspects: under different operating conditions,the amount of NOx generation at uniform wall temperature is high,but the difference of the final production is not significant;the HC production of non-uniform wall temperature distribution under rated condition is much higher than that of uniform wall temperature,while HC production is not very sensitive to wall temperature under partial load;the impact trend of wall temperature on Soot production is opposite for partial load and rated condition.