Modelling the transient heat transfer in the ceramic combustion chamber walls of a low heat rejection diesel engine

Abstract

A new hybrid finite-element thermostructural model is developed and applied for the investigation of the thermal effects of various insulation configurations on the combustion chamber surfaces of a DI (direct injection) diesel engine, under transient operating conditions. The model uses a comprehensive thermodynamic engine cycle simulation model in combination with a detailed structural analysis model, which allows the study of the effect of engine geometry and construction parameters on its performance. The separate representation of the various component subregions by the hybrid model makes possible the quantitative estimation of the effect of contact resistances on the amount of heat rejected to the combustion chamber walls. Connection between the resulting submodels is accomplished via the adequate use of the heat balance method. The model is applied for two of the most commonly used engine insulation configurations under transient operation (load increase), with the variation of the thermal characteristics of the fluids surrounding the combustion chamber simulated in detail. An engine transient event was revealed to consist of two different characteristic thermal stages, which are distinguished and analysed. The importance of the rate of a specific variation towards the development of sharp temperature gradients (thermal shock) inside the sensitive ceramic materials is clearly revealed. A satisfactory degree of agreement is found between theoretical predictions and experimental measurements at the initial and final stages of the transient variation, confirming the models validity.