Light Duty Diesel
Diesel exhaust aftertreatment systems for efficient reduction of NOx, CO, HC and particulate emissions
In order to comply with actual and future emission limits, catalytic systems for light duty diesel vehicles consist of a component capable of oxidizing Carbon Monoxide (CO) and Hydrocarbons. Typically this is done by a Diesel Oxidation Catalyst (DOC) or a NOx Storage Catalyst (NSC). Additionally, all modern Diesel engines need a device to reduce the particulate emissions with respect to their mass (PM) and their number (PN). Therefore, all systems need a particulate filter which is typically coated either with an oxidation function for CO and HC (cDPF) or a function for the selective reduction of nitrous oxides – NOx (SDPF).
Reduction of NOx
One of the big challenges with respect to emission control for Diesel engines is the reduction of NOx under lean exhaust gas conditions (i.e. excess oxygen).
Two NOx exhaust reduction technologies have been proven successfully under diesel exhaust conditions:
- One is the NOx Storage-Catalyst (NSC), which is typically used in a position close-coupled to the engine and which, in addition to NOx-abatement, converts hydrocarbons (HC) and carbon monoxide (CO) by oxidation into carbon dioxide (CO2).
- The other common technology is the Selective Catalytic Reduction Catalyst (SCR). It is used on a flow-through substrate or as an integrated device with the DPF on a wall-flow filter substrate (SDPF) downstream of a DOC or NSC. It converts nitrous oxides continuously by use of ammonia which is dosed as aqueous urea solution (AUS) into the exhaust line.
Generally the NSC has significant advantages with respect to effective NOx-reduction especially at low exhaust temperatures and during city driving. In order to meet stringent future NOx emission legislation, including testing under real driving conditions, NOx aftertreatment with a broad operation window is needed. This takes into account high NOx mass flows in a broad temperature range. Therefore, new and innovative exhaust aftertreatment systems are a key element in reaching the challenging goal for future Diesel engine applications, of fulfilling the next emissions limits for the reduction of nitrogen oxides (NOx) without penalizing CO2 emissions.
Because different vehicle classes have individual boundary conditions regarding system costs, NOx abatement, weight and packaging, a one-fits-all-solution is not reasonable. In this context different NOx aftertreatment systems offer distinctive advantages. However, the NOx storage catalyst with its pronounced low temperature activity can be part of the solution for every vehicle segment. In addition, the performance of NSC based systems can be increased by adding an SCR in various positions.
The following boundary conditions have to be taken into account for the final definition of NOx aftertreatment systems:
- Emission feasibility
- Fuel Consumption
- Reliability and robustness
- Layout and installation impacts
- Variable costs
- Total cost of ownership
- Synergies with other solutions within the same or other engine families