A Novel Approach to Catalysis for NOx Reduction in Diesel Exhaust Gas

With the introduction of stringent emission standards in the US in 2007 (Tier II Bin8, 5) an exhaust aftertreatment for NOx reduction is required for compliance with the emission regulations. A new approach consists in the synergetic combination of existing technologies NSC (NOx Storage Catalyst) and SCR (Selective Catalytic Reduction) with onboard generation of ammonia as the reducing agent for the SCR. It is shown in this work that the performance of the combined system exceeds those of each one considered separately, especially after ageing. The generation of ammonia is correlated to the ammonia selectivity during the regeneration of the NSC. The selectivity is primarily dependent on the temperature, A/F ratio (Air/Fuel) and the rich time. It is shown that the development of a suitable control strategy leads to a high level of NOx reduction under transient conditions in an FTP driving cycle. Due to the complexity and high development costs of current exhaust aftertreatment systems, modelling and simulation were identified as an important aspect in the development process. A system simulation tool named ExACT (Exhaust Gas Aftertreatment Components Toolbox) developed at Daimler is presented. By using the simulation already at an early stage, specific development work can be carried out prior to the experimental work on an engine test bench.