Injection Pressure Fluctuations Model Applied to a Multi-Dimensional Code for Diesel Engines Simulation

In the last decade, the development of automotive Diesel engines has been strongly influenced by the stringent regulations on pollutant emissions, which are directly related to the quality of the combustion process. The control of the combustion process requires primarily an understanding of its physics and chemistry as well as the capability to modify one or more of the interdependent process parameters in a given direction. Since many parameters have to be considered, numerical approaches are necessary to reduce time and costs, leaving to experimental analyses the final choice among few configurations. In this context, the research group of the University of Rome “Tor Vergata” has developed a multidimensional code for Diesel engines simulation based on the KIVA 3V code. This paper deals with the pressure supply fluctuations that occur in the fuel rail and in the injector on the immediate upstream side of the atomizer. It has been experimentally demonstrated that this has a considerable effect on the instantaneous mass flow and spray velocity thus affecting the spray hard-core structure and the atomisation process. Moreover, considering the oscillatory nature of the phenomenon, the effects on a given spray can change from one injection to the other (in modern multijet systems) and from one engine cycle to the next. The needed computing time and the uncertainty related to a detailed simulation of the injection system fluid dynamics would be unacceptable in a multidimensional computational code. Therefore a simplified model able to predict fuel pressure supply fluctuations on the upstream side of the injector has been developed and is described in this paper. The model is validated and calibrated by comparing numerical results with available experimental data. Some numerical results on a fuel spray injected in a vessel at constant pressure are finally presented, in order to quantify the impact of a given level of pressure fluctuations on fuel spray characteristics.