2021 Audi A3 Sportback Twin Dosing Technology
11The catalytic converters and the particulate filter are the key components in the purification of diesel exhaust gases. The catalytic converters work best in a certain temperature range. The EA 288 Evo actually features three catalytic converters – an oxidising catalytic converter fitted directly to the engine, and the first SCR catalytic converter inside a component with the diesel particulate filter directly behind it. The second SCR catalytic converter is located in the underbody of the vehicle. Since the distance to the engine is greater, the exhaust gas temperature upstream of the second catalytic converter can be as much as 100oC lower. As a result, one of the catalytic converters is always operating in the optimum temperature range, depending on the vehicle operating mode, to facilitate extra-efficient exhaust gas aftertreatment.
Ammonia is needed to reduce harmful nitrogen oxides. A metering module injects the substance into the exhaust gas upstream of the relevant SCR catalytic converter as AdBlue. Here, the solution evaporates and the reducing agent (urea) is broken down, combining with steam to form ammonia. In the catalytic converter, the ammonia then reacts with the nitrogen oxides on a special coating to form water and harmless nitrogen.
The special feature here is that the twin dosing process is effective at almost all temperatures owing to the two catalytic converters. The first SCR catalytic converter nearest the engine warms up quickly so begins to work quickly once the engine is started and it reaches peak performance when the vehicle is in motion and operating at low loads, while the catalytic converter in the underbody comes into its own when the car is operating at high loads – for instance on motorways or when towing a trailer. The increased efficiency of the dosing process means that AdBlue consumption remains at the same level despite being injected twice.
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