Effects of split injection proportion and the second injection time on the mixture formation in a GDI engine under catalyst heating mode using stratified charge strategy

• Numerical simulation of GDI working process under catalyst heating mode.
• Effects of injection proportion and SOI2 on the mixture formation were studied.
• Elaborating how injection proportion and injection timing influence the mixture.
• Determined the optimum injection proportion and SOI2 to form a suitable mixture.

The effects of split injection proportion and start of injection for the second injection event (SOI2) are investigated with multi-dimensional computational fluid dynamics in a gasoline direct-injection (GDI) engine under catalyst heating mode using stratified stoichiometric charge strategy. Results show that when the injection timing is fixed, the equivalence ratio in the spark plug at ignition time decreases with the decrease in the second injection proportion. The homogeneity of the overall mixtures increases at the same time. The 25%–75% case can form an ideal ignitable mixture and exhibits good combustion performance, which results in rapid catalyst light-off and low soot emissions. NO emissions are relatively high but can be addressed by a three-way catalyst. When the split injection proportion is fixed, the cases with SOI2 of 610° CA, 620° CA, 630° CA, and 660° CA can form an ideal mixture at ignition time. The cases with SOI2 of 610° CA and 620° CA produce a more homogeneous mixture and thus exhibit better combustion performance; however, the detonation tendency is obvious. The mixture of the case with SOI2 of 630° CA is more homogeneous than that of the case with SOI2 of 660° CA. The former exhibits better combustion performance and has a lower amount of soot emission. Although the NO emissions of 630° CA SOI2 are slightly high, it contributes to rapid catalyst light-off. Therefore, 630° CA is the ideal SOI2 in the present conditions.