Investigation of Augmented Mixing Effects on Direct-Injection Stratified Combustion

The effects of augmented mixing through the use of an auxiliary gas injection (AGI) were investigated in a direct-injection gasoline engine operated at a 22:1 overall air-fuel ratio, but with retarded injection timing such that the combustion was occurring in a locally rich mixture as evident by the elevated CO emissions. Two AGI gas compositions, nitrogen and air, were utilized, the gas supply temperature was ambient, and a wide range of AGI timings were investigated. The injected mass was less than 10% of the total chamber mass. The injection of nitrogen during the latter portion of the heat release phase resulted in a 25% reduction in the CO emissions. This reduction is considered to be the result of the increased mixing rate of the rich combustion products with the available excess air during a time when the temperatures are high enough to promote rapid oxidation. This CO reduction was achieved despite the fact that the injected gas had a low temperature and was inert, both attributes should result in reduced oxidation rates. The effect of temperature and composition were observed by a significant reduction in the NO_x emissions at the same conditions, confirming that the AGI orientation was effecting changes in the high temperature in-cylinder processes. At the same conditions air AGI (i.e an oxidizing gas) produced a more significant CO reduction, and the oxygen content counteracted the low temperature to cause no change in the NO_x emissions. These findings suggest that a lack of late-cycle mixing limits the emissions potential of current direct-injection engines.