Numerical Simulations and Experimental Validation of an SCR System for Ultra Low NOx Applications

Close-coupled aftertreatment systems (ATS) for automotive Diesel engines composed of DOC and SCR offer a significant potential in terms of pollutant emission control capability even with the introduction of more aggressive driving cycles and rigorous limits for type-approval tests. This is particularly important for incoming certification standards where the forecast is showing a trade-off towards ultra-low NO_x emissions values. As the SCR system NO_x conversion capability largely relies on both the UWS mixing device and on NO_x sensors used to detect the actual NH₃ slip and residual NO_x concentration, developing numerical simulation tools for the analysis of the actual flow pattern and species concentration over peculiar sections of the exhaust system is crucial to support the ATS development process. In this work, a state-of the-art complete SCR-type ATS was analyzed using CFD simulations and a hot flow bench in steady operating conditions representative of type-approval key-points in terms of flow rate, temperature and upstream NO_x concentration. The overall system NH₃ distribution maps and uniformity index over the main flow section downstream the SCR were simulated and compared with the corresponding experimental measurements to support the design process of the entire system. Further, a novel simulation methodology was developed to predict the reading accuracy of the NO_x sensors. The methodology has been applied to three NO_x sensors positioned along the exhaust line downstream the ATS and compared with the flow bench mean measurements, evidencing how the NO_x sensors actual position can significantly affect their accuracy particularly in low load operating conditions.