Efficient and clean: Aftertreatment for gasoline and diesel engines
Audi is responsible for the basic development of the V6 TDI engines of the new Evo 3 generation within the Volkswagen Group. As the requirements imposed by stricter emission standards keep increasing, the engineers have to improve the efficiency of exhaust gas purification. In the case of exhaust aftertreatment, this requires larger design volumes of the catalytic converters, among other things. With the current new development for the V6 TDI, Audi has achieved a compact combination of all technologies. The flow of the two exhaust gas tracts on the outside of the two cylinder-bank sides converges behind the engine in front of the firewall, where the exhaust gas turbocharger is located. Directly downstream in the exhaust system is an oxidation catalyst, referred to as NSC. The name stands for NOx Storage Catalyst. It is directly followed by an SCR-coated diesel particulate filter (SDPF). The abbreviation SCR stands for Selective Catalytic Reduction. The second SCR catalyst is located further downstream in the exhaust system below the vehicle’s underfloor.
The oxidation catalyst (NSC) close to the engine can temporarily store nitrogen oxides up until the regeneration stage. This catalyst is effective even at low engine operating temperatures, for instance following a cold start. Regeneration occurs by means of a short-term mixture enrichment triggered by the engine control unit. Thus, besides storing and subsequently neutralizing the nitrogen oxides, the catalyst oxidizes unburned hydrocarbons and carbon monoxide into carbon dioxide and water vapor, using the oxygen molecules of the temporarily stored NOx.
Another stage to reduce nitrogen oxides is triggered by the injection of the AdBlue additive. Because this aqueous urea solution is injected into the exhaust system at two points where temperatures differ, using one dosing module at each point, the total system is referred to as twin dosing. Subsequently, the chemical process of urea thermolysis occurs in the exhaust system, which converts the AdBlue additive into ammonia. The ammonia reacts with the SCR-coated diesel particulate filter (SDPF) close to the engine, and on the second SCR catalyst, located further downstream in the exhaust system, with nitrogen oxides that have not been converted yet. This results in the formation of water and elemental nitrogen, which accounts for about four fifths of our Earth’s atmosphere.
Twin dosing technology in the V6 TDI will be used from the Evo 3 generation onwards. It is available in diesel engines with three liters of displacement in three different performance classes and will be installed for the new year in all models featuring this engine.