Numerical Investigation of Mixture Preparation in a GDI Engine

The purpose of the present paper is to develop an engine simulation tool in a commercial CFD code to study the spray and mixing process that can be used to access the performance of a Gasoline Direct Injection (GDI) engine. The ignition, combustion and pollutant formation are strongly dependent on the quality of the fuel-air mixture. The fuel is injected directly into the combustion chamber by high-pressure fuel injector. The fuel atomization and evaporation process takes place due to the interaction of the small fuel particles generated by the injector and the in-cylinder air motion. Experimental study on the spray and mixing process is difficult and expensive, which has been recognized as a major obstacle towards the optimization of the combustion chamber geometry, engine components and the injection strategies. This study is part of an effort to develop a means to determine injection spray characteristics in a unique combustion chamber to help define injector specifications through analysis instead of costly repetitive dyno tests. The program will be used as a selection tool to determine injector requirements for a specific customer combustion chamber.

In the current study, the engine intake flow and fuel spray are simulated using a commercial CFD code. The fuel jet is tracked by a discrete phase model in the Lagrangian scheme, in which the liquid fuel jet is modeled by groups of droplets. The interaction of the jet and the gas flow is simulated to predict the breakup and evaporation of the liquid phase. The spray model constants are calibrated against a spray bench provided by Delphi. The spray model is applied to the fuel injection and mixing simulation of the GDI engine. The equivalence ratio distribution is calculated in the combustion chamber, and the spark plug location is picked to monitor the local air-fuel ratio as a parameter for evaluating the ignition and combustion performance. The injection pressure and injection timing are studied to understand the effects of different engine operating conditions on the quality of the combustion process.