Another Look at the Diagnostics for Exhaust Systems with Dual Urea

To meet the low NOx standard for heavy duty engines deployed by CARB for 27MY and 31MY, many manufacturers are resorting to an exhaust layout with 2 SCR catalysts and 2 urea injectors, where the 2 SCR catalysts are separated by another non-NOx converting catalyst. The first SCR allows for best NOx conversion in cold start and low load conditions, whereas the second SCR addresses NOx conversion at full load. The regulation allows monitoring the SCR catalysts as individual components or as a system, where CARB staff has indicated that monitoring the individual catalysts entails some further requirements. In this presentation we will evaluate the benefits and drawbacks of both options under some realistic aging assumptions. The findings will be supported by simulations of various aging scenarios.