The current direction for Diesel combustion system development is towards homogenization, in order to reduce particulate and NOx emissions. However, a strong increase of carbon monoxide emissions (CO) is frequently noted in combination with enhanced homogenization.
Therefore, the current investigation focuses on a detailed analysis of the particulate - CO trade-off using a laser-optical and multidimensional CFD investigation of the combustion process of a swirl HSDI system. The CFD methodology involves reduced kinetics for soot formation and oxidation and a three-step CO model. These models are validated by a detailed comparison to optical measurements of flow, spray penetration and the spatial distribution of soot, temperature and oxygen concentration.
The results obtained show that high concentrations of CO occur as an intermediate combustion reaction product. Subsequently, CO and soot are oxidized in large areas of the combustion chamber. In part load operation, CO emissions are mainly caused by dilution effects in the early phases of combustion. However, the engine's soot emissions are caused by insufficient oxidization.
Soot generation and oxidation are found to be strongly impacted by the slight non-homogeneity of the swirl motion. The gaseous emissions are not affected by the small differences in local flow, which are outlined further in the text.