Prediction of Heat Transfer to the Walls in Direct Injection (DI) Diesel Engines

摘要

This paper presents an analysis of the heat transfer from gases to the cylinder walls of Direct Injection (DI) Diesel engines by means of Computational Fluid Dynamics (CFD). The aim of this study is to evaluate the parameters that are significant in the heat transfer and to improve the predictions obtained with a zero-dimensional thermodynamic model currently used in the automotive industry for combustion diagnosis. First, several tests with retarded start of injection have been carried out on two geometries, in order to study the heat transfer to the walls during the compression stroke before ignition. The comparison of these CFD results with those obtained from the zero-dimensional model showed that the latter over-predicted or under-predicted the heat transfer depending on the swirl level in the engine. The knowledge gained from the CFD results was used to obtain better correlations to represent the influence of the swirl and thus improve the zero-dimensional heat transfer model in the pre-combustion phase. The capability for the CFD calculations to predict heat transfer during the whole closed engine cycle taking into account the combustion has then been explored. To avoid large calculations including the reactive combustion process, the effects of the combustion in terms of heat release and changes in species concentrations is simulated by means of source terms in the corresponding equations. With this methodology it is possible to accurately predict the pressure and temperature of the gases enclosed in the cylinder and to obtain a good estimation of the heat transfer to the walls.