In order to comply with the limits laid down by the Euro 6 emissions standard, necessitating a substantial reduction in nitrogen oxide emissions, Audi has converted its TDI engines to clean diesel technology. In most cases this requires measures in the engine and the exhaust tract; for the more compact engines and vehicle models a Nox storage-type catalytic converter is sufficient for the purpose.
The technology for the larger models and engines is more complex. The new 3.0 TDI, for example, has an enlarged oxidation catalyst, which in the 160 kW (218 hp) version is electrically heated. The diesel particulate filter is installed directly downstream of the oxidation catalyst. The filter walls have a coating which additionally converts the nitrogen oxides in the exhaust gas based on the SCR technique. To ensure rapid activation, the exhaust gas cleaning components are located as close to the engine as possible.
The next step by Audi will be in the 3.0 TDI scheduled for launch in 2015. It will feature a new NOx storage-type catalytic converter instead of the oxidation catalyst. This stores the nitrogen oxides until it is full; the cleaning is effected by enrichment of the mixture in the engine. So as to minimize any increase in fuel consumption, the new catalytic converter only activates at low exhaust gas temperatures, after starting the engine and under low load. In all other situations the SCR-coated diesel particulate filter converts the NOx.