Aspects of Mixture Formation and Combustion in GDI Engines

The internal mixture formation within SI engines using fuel direct injection has a significant potential regarding the reduction of bsfc and pollutant emission. However the short time available for injection and spray distribution, as well as the complexity of the fluid dynamic conditions, amplified in a wide load and speed range, form a different base for the combustion process than using external mixture formation. The intend of the present study is to develop a method for modeling and optimization of mixture formation and combustion using a general approach for the fuel direct injection, which consist in the modulation of the injection rate, independently on the engine speed. In the first stage of modeling, the optimum combination between mixture formation elements as fuel pressure history, injection timing, spray characteristics, injector location or combustion chamber design is of great importance, forming the conditions for the subsequent combustion process. Moreover the interaction between fuel and air flow within the cylinder during the mixture formation is determining for the flame propagation. The model was developed in base of the commercial code FLUENT. Different combinations of fuel quantity, injection timing, fuel/air interaction and engine speed are represented as time- and space- related sequences of mixture formation. The program calibration is made by spray visualization for distinct cases, as a base of reliable simulation in a wide range of parameters.