The present investigation explored the effects of increasing in-cylinder mixing on DI diesel emissions using auxiliary gas injection (AGI). Four gas injection parameters were examined: AGI direction, oxygen concentration, intensity, and timing. Intensity was used to describe the amount of fuel-air mixing brought about by the gas jet. By exploring these parameters, mechanisms by which AGI affects emissions were identified.
The main soot reduction mechanism identified in the present study was the increase in fuel-air mixing produced by the turbulent gas jet. The parameters controlling this mechanism are still not well understood despite attempts to link the amount of mixing to either the momentum or kinetic energy of the gas jet. A second mechanism identified in this study was the change in the composition of gas brought into the fuel spray. This mechanism was responsible for changes in both soot and NO emissions, which was either beneficial or detrimental depending on the composition of the injected gas. The effect of adding relatively cold gases to the combustion chamber on in-cylinder temperatures constituted the third mechanism identified in this investigation. This cooling effect brought about a decrease in NO emissions and an increase in soot emissions. The final mechanism responsible for the effect of AGI on emissions was the obstruction of the fuel spray by the gas jet. The presence of this mechanism depended on the direction and momentum flux of the gas jet (compared to that of the fuel spray). This mechanism caused an increase in soot emissions and possibly a decrease in NO emissions.
Aside from identifying the mechanisms responsible for changes in emissions in this system, this study was used to explore to feasibility of using AGI with intake air nitrogen-enrichment to overcome the soot-NOx tradeoff curve. It was found that there are operating ranges where the soot-NO x tradeoff can be overcome. It was also discovered that this window was larger with early gas injection.