Research On In-cylinder Heat Transfer Model For Internal Combustion Engines

The in-cylinder heat transfer dramatically affects the operation and emissions characteristics of internal combustion engines,thus it is significant to establish an accurate heat transfer model for engine design.A new heat transfer model was proposed for internal combustion engines,it was derived from the energy equation for compressible viscous fluid,continuity equation and ideal-gas-law.The influences of laminar Prandtl number and the practical meaning of thermal wall function in boundary layers were taken into account,and the gas turbulence boundary layer was subdivided into the viscous sublayer,buffer layer,and the outer layer according to the characteristics of the actual flows.A piecewise function to describe the characteristics of fluid heat transfer in the boundary layer.In order to verify the stability and accuracy of the new heat transfer model,the KIVA-3V was coupled with the new heat transfer model to simulate heat flux and in-cylinder pressure in the cylinder under different loads and different CA50 conditions for three combustion strategies including traditional diesel compression ignition(CDC),homogeneous compression ignition(HCCI)and reaction control compression ignition(RCCI).The results are compared with the experimental data and the predictions of Han-Reitz heat transfer model and Rakopoulos heat transfer model,which have been widely used in the engine heat transfer simulation.The comparisons show that the new heat transfer model is more accurate than Han-Reitz heat transfer model and Rakopoulos heat transfer model in predicting the heat flux.What's more,the flaws in the above two models and in their thermal wall functions were discussed,the reasons for its low prediction accuracy were pointed out.Finally,the new model is used to analyze the distributions of heat transfer,turbulent kinetic energy,equivalence ratio and thermal wall functions in internal combustion engines under different combustion strategies and different loads.The results explain why the low temperature combustion technology has better economy than CDC.