Effect of Homogenous-Stratified Mixture Combustion on Performance and Emission Characteristics of a Spray-Guided GDI Engine - A CFD Study 2020-01-0785

Today, gasoline direct injection (GDI) engine is one of the best strategies to meet the requirement of low pollutant emissions and fuel consumption. Generally, the GDI engine operates in stratified mixture mode at part-load conditions and homogeneous mixture mode at full-load conditions. But, at part-loads, soot emissions are found to be high because of improper air-fuel mixing. To overcome the above issue, a homogenous-stratified mixture (a combination of the overall homogeneous lean mixture with a combustible mixture at the location of the spark plug) is found to be better to reduce soot emissions compared to a stratified mixture mode. It will also help reduce fuel consumption. In this study, the analysis has been done to evaluate the effect of homogeneous-stratified mixture combustion on the performance and emission characteristics of a spray-guided GDI engine under various conditions using computational fluid dynamics (CFD). Here, for the analysis, overall equivalence ratios of 0.5 to 0.8 are considered. The constant engine speed of 2000 rev/min. and fuel injection pressure of 200 bar is used. Two direct fuel injections (the first in the suction stroke and the second in the compression stroke) are used to get the homogeneous-stratified mixture. The split ratios used among the two injections of 20:80, 30:70, 40:60, and 50:50 are tried. The comparisons of combustion, performance and emission characteristics at various split ratios are done with that of the single injection case using the stratified mixture.

It is found that with split injection, homogenous-stratified mixture formation is very effective with a higher equivalence ratio and higher quantity of fuel in the first injection. In the above conditions, the in-cylinder pressures are comparable to that of the single injection case. Also, soot emissions are lower by about 95-99% compared to that of the single injection case. Also, with lower equivalence ratio and higher quantity of fuel in the first injection, NOx emissions are lower by about 15% compared to that of the single injection, whereas HC emissions are higher.