Volkswagen will soon introduce the new SCR system to all its models with 2.0 TDI Evo engines and also later on in the new Golf, that will have its world premiere soon.
The second SCR catalytic converter is the need for the twin dosing process. The exhaust temperature upstream of the second catalytic converter can be as much as 100 degree Celsius lower as the distance to the engine is greater. This expands the window for after treatment of exhaust gases: even at exhaust gas temperatures close to the engine of +500 degree Celsius, the system is still able to achieve very high conversion rates. Also, a blocking catalytic converter downstream of the SCR system prevents excess ammonia slip.
Ammonia helps in reducing the nitrogen oxides produced in diesel engines. It is injected as an aqueous reducing agent (AdBlue) via a dosing module into the exhaust gas upstream of an SCR catalytic converter. The solution evaporates; the reducing agent is split, combining with steam to form ammonia. The ammonia in the SCR catalytic converter reacts on a special coating with the nitrogen oxides to form water and harmless nitrogen.
In existing exhaust gas treatment systems, a close-coupled SCR catalyst is located between the turbocharger, the diesel oxidation catalytic converter – which converts uncombusted hydrocarbons – and the flexible connecting piece to the silencer pipe. The SCR coating is applied to the honeycomb structure of the diesel particulate filter, thereby enabling a single component to perform several functions. The close-coupled arrangement means that the exhaust gas temperatures required for high conversion rates can be achieved quickly after a cold start – the ideal range for conversion rates of more than 90 percent is between +220°C and +350°C. These conditions are met in many operating situations.
Conversion rates do not drop above +350°C – thanks to twin dosing-system. Temperatures at this level occur for example when driving at high speeds on the motorway, at high engine speeds over prolonged periods of time and when driving uphill, especially if the vehicle is fully loaded or towing a trailer.