The regulations for mobile applications will become stricter in Euro 6 and further emission levels and require the use of active aftertreatment methods for NOX and particulate matter. SCR and LNT have been both used commercially for mobile NOX removal. An alternative system is based on the combination of these two technologies. Developments of catalysts and whole systems as well as final vehicle demonstrations are discussed in this study.
The small and full-size catalyst development experiments resulted in PtRh/LNT with optimized noble metal loadings and Cu-SCR catalyst having a high durability and ammonia adsorption capacity. For this study, an aftertreatment system consisting of LNT plus exhaust bypass, passive SCR and engine independent reductant supply by on-board exhaust fuel reforming was developed and investigated. The concept definition considers NOX conversion, CO2 drawback and system complexity. The passive SCR significantly contributes to the total NOX conversion over a broad temperature range. In contrary to conventional LNT applications, the LNT NOX slip does not directly reduce the system NOX conversion but can be converted over the passive SCR. Thereby, the NOX adsorption duration might be prolonged, resulting in lower LNT regeneration frequency and less fuel consumption, resp. CO2 emissions. Additionally, the engine independent LNT enrichment is beneficial regarding fuel consumption in comparison to conventional engine internal enrichment.
The system has been tested in a vehicle in combination with a highly efficient Diesel engine. Results regarding CO2 and NOX as well as their dependency on temperatures are shown here.