Effect of Mixture Distribution on Combustion and Emission Characteristics in a GDI Engine - A CFD Analysis 2017-24-0036
Mixture distribution in the combustion chamber of gasoline direct injection (GDI) engines significantly affects combustion, performance and emission characteristics. The mixture distribution in the engine cylinder, in turn, depends on many parameters viz., fuel injector hole diameter and orientation, fuel injection pressure, the start of fuel injection, in-cylinder fluid dynamics etc. In these engines, the mixture distribution is broadly classified as homogeneous and stratified. However, with currently available engine parameters, it is difficult to objectively classify the type of mixture distribution. In this study, an attempt is made to objectively classify the mixture distribution in GDI engines using a parameter called the “stratification index”. The analysis is carried out on a four-stroke wall-guided GDI engine using computational fluid dynamics (CFD). All CFD sub-models used, in this study, are validated with the available experimental and CFD results from the literature before carrying out the analysis. Three types of mixture distributions viz., ideally homogeneous, ideally stratified and mal-distributed mixtures are defined and their effect on combustion, performance and emission characteristics of the engine are analyzed. Further, the effect of fuel injector orientation on the mixture distribution in the combustion chamber is analyzed for three different orientations of the fuel injector viz., -15, 0 and 15° with the vertical. From the results, it is found that the early fuel injection doesn’t produce an ideally homogeneous mixture. Also, among the cases of the stratified mixtures, it is found that, the fuel injector orientation of 15° results in a mixture that is closer to the ideally stratified one. This is characterized by the value of stratification index that is close to 1.