Study on Methods of Coupling Numerical Simulation of Conjugate Heat Transfer and In-Cylinder Combustion Process in GDI Engine

Wall temperature in GDI engine is influenced by both water jacket and gas heat source. In turn, wall temperature affects evaporation and mixing characteristics of impingement spray as well as combustion process and emissions. Therefore, in order to accurately simulate combustion process, accurate wall temperature is essential, which can be obtained by conjugate heat transfer (CHT) and piston heat transfer (PHT) models based on mapping combustion results. This CHT model considers temporal interaction between solid parts and cooling water. This paper presents an integrated methodology to reliably predict in-cylinder combustion process and temperature field of a 2.0L GDI engine which includes engine head/block/gasket and water jacket components. A two-way coupling numerical procedure on the basis of this integrated methodology is as follows. With SAGE detailed chemical kinetics model, 3D CFD was applied to simulate the in-cylinder combustion and predict emissions with transient pressure and temperature boundary conditions from 1D simulation model to obtain gas side time-averaged HTC and temperature distribution of combustion system walls. These thermal boundary conditions were mapped to CHT model and piston heat transfer (PHT) model to directly solve the temperature field in both solid and fluid domains. The updated wall temperature results were then conversely mapped to 3D CFD model to iteratively simulate combustion process. Then recalculate the component temperatures in CHT and PHT models. It indicates that wall temperatures of GDI engine have great significant effects on fuel evaporation and mixing, combustion process and emission formation. The iterative 3D CFD simulation results are closer to experimental data while there is a great difference between the calculated temperature distribution from CHT model and assumptive uniform wall temperatures. The predictive component temperatures using CHT model based on mapping thermal boundary conditions also agree well with the measured values by hardness plug. Therefore, it is necessary to modify the wall temperatures to improve the prediction accuracy of in-cylinder flow and combustion process.