Quantitative Parametrization of Mixture Distribution in GDI Engines: A CFD Analysis

This paper presents an objective classification of mixture distribution in the combustion chamber of a gasoline direct injection (GDI) engine into homogeneous and non-homogeneous types. The non-homogeneous mixture distribution is further classified as properly stratified, improperly stratified and mal-distributed types. Based on this classification, four types of properly stratified mixture distributions viz., random, linear, Gaussian and parabolic are virtually simulated in the combustion chamber of a GDI engine using computational fluid dynamics to identify the mixture that results in maximum indicated mean effective pressure (IMEP). It is found that the IMEP is highest for the parabolic mixture distribution which is followed by Gaussian, linear and random types. The performance and emission characteristics of the virtual mixture distributions are compared with a late fuel injection case at different over all equivalence ratios ranging from 0.3 to 0.7. Then the variation of mixture equivalence ratio with the distance from the spark plug is parametrized for different virtual mixture distribution cases and expressed using a parameter called the stratification index. It is found that the stratification index based on Gaussian variation gives maximum information about the mixture distribution in the combustion chamber. Finally the stratification index of different virtual mixture distributions is compared with the late fuel injection case at various overall equivalence ratios. It is found that the late fuel injection case tends to produce highest IMEP when the stratification index is close to unity.