A Simplified Model for the Spatial Distribution of Temperature in a Motored DI Diesel Engine

The purpose of this paper is to present an alternative method to predict the temperature and flow field in a motored internal combustion engine with bowl in piston. For the fluid flow it is used a phenomenological model which is coupled to a computational fluid dynamic method to solve the energy conservation equation and therefore the temperature field. The proposed method has the advantage of simplicity and low computational time. The computational procedure solves the energy conservation equation by a finite volume method, using a simplified air motion model (estimating axial and radial velocities) to calculate the flow field. The finite volume discretization employs the implicit temporal and hybrid central upwind spatial differencing. The grid used contracts and expands following the piston motion, and the number of nodes in the direction of piston motion vary depending on the crank angle. The mean cylinder pressure, the local temperature distribution and the flow field are calculated at each crank angle. Experiments have been conducted in our Laboratory, on a DI diesel engine with bowl in piston at various speeds and the experimental compression curve is compared with the theoretical one. A very good agreement between the predicted and the experimental cylinder pressure is observed. The results obtained provide information concerning the distribution of gas temperature and gas velocity. The current model can be used either to examine the combustion mechanism in homogeneous charge engines or it can be combined with a jet model to develop a sophisticated but always simple model for the air-fuel mixing mechanism. This allow us to examine the combustion and pollutant formation mechanisms on an engine cycle basis, which is extremely difficult when using sophisticated CFD models. Thus the current proposal seems to be a compromise between detailed CFD models and sophisticated multi-zone phenomenological ones, offering the advantage of low computational time and examining in a more fundamental way compared to the phenomenological models the various processes taking place inside the engine cylinder. In the present work are presented the results of the first step which is necessary before expanding the analysis to describe the fired part of the engine as well.