The Effects of Mixture Stratification on Combustion in a Constant-Volume Combustion Vessel

The role of mixture stratification on combustion rate has been investigated in a constant volume combustion vessel in which mixtures of different equivalence ratios can be added in a spatially and temporally controlled fashion. The experiments were performed in a regime of low fluid motion to avoid the complicating effects of turbulence generated by the injection of different masses of fluid. Different mixture combinations were investigated while maintaining a constant overall equivalence ratio and initial pressure. The results indicate that the highest combustion rate for an overall lean mixture is obtained when all of the fuel is contained in a stoichiometric mixture in the vicinity of the ignition source. This is the result of the high burning velocity of these mixtures, and the complete oxidation which releases the full chemical energy. The total energy release of stratified mixtures with a rich mixture near the ignition source were limited by the mixing of the rich products with the available oxidizer before the mixture cools below the temperature required for oxidation. These results suggest that for direct-injection engines operating at light load the target fuel distribution in the chamber is a homogeneous stoichiometric mixture near the spark gap, and that sufficient fluid mechanical mixing is necessary during the expansion stroke to promote mixing and oxidation of any rich combustion products.