Close coupled DOC-mixer-SCR for Tier 4 final

This paper discusses the development of a compact close-coupled DOC+mixer+SCR system, complying with the Tier 4 non-road emission legislation. Early on, it was assumed that Tier 4 final non-road engines would need aftertreatment, combining SCR and DPF. With recent advancement in boosting, fuel injection equipment and SCR aftertreatment, it is now clear that the Tier 4 final particulate limit can be reached without a DPF, when a high efficiency SCR system is used. The resulting engine-aftertreatment combination leads to a good fuel efficiency and low cost-of-ownership. To achieve high efficiencies in an SCR system, one needs a DOC for NO to NO2 conversion, to prepare the exhaust gas for the fast SCR reaction. Downstream of this DOC, AdBlue (Diesel Exhaust Fluid) is injected, evaporated and mixed with the exhaust gas, prior to entering the SCR substrates. The DOC, doser, mixer and SCR are large parts. Especially when they are designed as separate units, the complete aftertreatment system becomes large with several separate bulky units. The present paper discloses an aftertreatment system where the DOC and SCR are close-coupled in one compact unit, with integrated doser and swirl mixer. This system uses an annular/ring shaped (donut) DOC substrate. At the outlet of this ring shaped donut DOC, the AdBlue is injected along the axis of the inner tube. At the same location, swirling flow is generated. This bulk swirl with AdBlue then flows to the SCR inlet. The performance of this system in terms of (lack of) deposit formation, mixing and emission reduction is studied. The effect of various parameters such as swirl ratio, residence time, turbulence and geometry is described. This is done by a combination of CFD results, engine dynamometer lab results and natural gas burner results. The T4f actual emission performance cannot be disclosed; however, relative comparisons of different design iterations will be shown, together with actual results of an undersized (too small) T4 system.