A comprehensive experimental and numerical analysis of two state-of-the-art diesel AfterTreatment Systems (ATS) for automotive applications is presented in this work.
Both systems, designed to fulfill Euro 6 emissions regulations standards, consist of a closed-coupled Diesel Oxidation Catalyst (DOC) followed by a Selective Catalytic Reduction (SCR) catalyst coated on a Diesel Particulate Filter (DPF), also known as SCR on Filter (SCRoF or SCRF). While the two systems feature the same Urea Water Solution (UWS) injector, major differences could be observed in the UWS mixing device, which is placed upstream of the SCRoF, whose design represents a crucial challenge due to the severe flow uniformity and compact packaging requirements.
First, both the ATS were experimentally characterized to determine the physical-chemical properties of the catalysts, the UWS spray characteristics (i.e. liquid penetration, droplets size) and to evaluate the NOx conversion efficiency under steady state flow conditions, representative of type-approval operating conditions. The experiments highlighted significant differences in terms of NOx conversion efficiency between the two ATS, especially at low temperature operation.
In order to highlight the root causes of these differences, numerical analyses were then conducted by means of a commercial 3D CFD code. A systematic methodology was developed to build a robust and reliable simulation setup, for both the ATS, and validated over the experimental data. The resulting simulation model was then used as a virtual test bench to understand the UWS spray evolution, breakup and mixing, the amount and location of liquid film and to evaluate the SCRoF inlet conditions in terms of species composition and flow uniformity indexes. The numerical analysis demonstrates that the different conversion of NOx at low temperature is mainly due to the different mixing systems design, which could enhance the production of ammonia from the UWS when the temperature conditions are far from the optimum, providing useful insights for future design and optimization of similar ATS.