NOx-Particulate Filter (NPF): Evaluation of an After-Treatment Concept to Meet Future Diesel Emission Standards

Simultaneous particulate and NOx reduction represents the next step to the reduction of diesel emissions. One of the most promising concepts to achieve this target involves the combination of two technologies already in use in the after-treatment technology - Diesel Particulate Filter and NOx Storage Catalyst - in the same component. The major issue to be solved is the design of a complex thermal strategy, for the regeneration of NOx emissions, particulate matter and possibly sulfates. For this set-up to function properly the engine must periodically generate a rich spike to induce the NOx desorption process. The system must also increase the exhaust gas temperature to induce the soot oxidation process. Complicating matters further, the regeneration process of the filter must also be controlled to avoid substrate or washcoat damage.

To fully understand the process an extensive investigation of an NPF (NOx Particulate Filter) was conducted using a 6-cylinder diesel engine on a test bed. To support de-NOx and induce the soot regeneration conditions an in-house developed diesel fuel vaporizer was used, which was placed upstream of the DOC-NPF assembly. To enable the system to be tested, a complete regeneration strategy for soot and NOx after-treatment was then developed and tested.

The main objective of this work was to verify if these three technologies could be reasonably combined in order to obtain a reliable diesel after-treatment concept. The investigations showed interesting benefits in combining NSC and DPF technology, like an increased NOx storage capacity in presence of soot. Important effects of the regeneration strategy design over the thermal behavior of the NPF are also outlined.