Light Duty Diesel
Diesel exhaust aftertreatment systems for efficient reduction of NOx, CO, HC, and particulate emissions
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 particulate emissions for their mass (PM) and number (PN). Therefore, all systems require 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 significant challenges for 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), 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 standard 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 using ammonia which is dosed as aqueous urea solution (AUS) into the exhaust line.
Generally, the NSC has significant advantages for effective NOx-reduction, especially at low exhaust temperatures and during city driving. NOx aftertreatment with a broad operation window is needed to meet stringent future NOx emission legislation, including testing under real driving conditions. This takes into account high NOx mass flows in a broad temperature range. Therefore, new and innovative exhaust aftertreatment systems are key 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.
A one-size-fits-all-solution is not reasonable because different vehicle classes have individual boundary conditions regarding system costs, NOx abatement, weight and packaging. In this context other NOx aftertreatment systems offer distinctive advantages. However, with its pronounced low-temperature activity, the NOx storage catalyst 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.
One must take into account the following boundary conditions 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