A phenomenological model for heat release rate predictions taking into account the characteristic processes inside a direct injection gasoline engine is presented. Fuel evaporation and preparation as well as the specifics of premixed and mixing controlled combustion phase are regarded. Only a few model constants need to be set which have been fit empirically for the application in a one-cylinder research engine. This jet guided direct injection gasoline engine employs a modern common-rail injection system and runs predominantly in stratified mode.
The model allows the prediction of the influence of numerous parameter variations, e.g. injection-ignition phasing, load, engine speed, swirl, etc. on the combustion process. Furthermore efficient simulations can be carried out without using expensive three-dimensional CFD (computational fluid dynamics) calculations. Short computing time in the order of magnitude of seconds per engine cycle is achieved by efficient algorithms, which are based on a thermodynamic two-zone model.
Nevertheless extensive simulations of the gas-phase flow as well as the injection process, which have been performed with a KIVA III based code, had been necessary for the development of the model. These results provided valuable information about the processes in the region of the hollow-cone spray and thereby mainly about characteristic evaporation time scales, turbulence intensity levels, mixture preparation and dependencies of the initial flow field, e.g. swirl level variations [1].
In addition to these extensive three-dimensional computations of the flow and mixture process prior to combustion, experiments based mainly on optical methods have been utilised to understand the spray formation and the specific features of the combustion processes. Ion current sensing, endoscopic photography and two-colour-pyrometry are the most important techniques which helped to gain the fundamental understanding on which the phenomenological model is based [2]. These new insights mainly affect the propagation of the flame front, the mixture preparation in the vicinity of the spreading flame and the coexistence of diffusion and premixed flames.
The complete information has led to a model, which is not only applicable for a direct injection gasoline engine in stratified charge but also for „ordinary” homogeneous mixture SI engines.
Although most of the simulated operating conditions have been chosen in stratified charge mode, a very good agreement between experimental and numerical results with respect to the cylinder pressure evolution has been achieved for both stratified and homogeneous charge conditions.