Catalytic Converter: Description and Operation

16 00 - Function description of Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction is a system for reducing nitrogen oxide in the exhaust gas. A reducing agent (urea/water mixture) is used which is injected directly into the exhaust gas after the diesel particulate filter.
The urea-water mixture is carried in two reservoirs in the vehicle. The quantity is configured in such a way as to be sufficient for one oil change interval.
The graphic below shows a simplified representation of the system:









The urea/water mixture is injected via the metering module ahead of the SCR catalytic converter into the exhaust pipe. DDE calculates the amount which has to be injected. The nitrogen oxide content in the exhaust gas is determined via the nitrogen oxide sensor ahead of the SCR catalytic converter. In accordance with this value exactly the right amount of urea/water mixture is injected to reduce the nitrogen oxide completely.
The urea/water mixture in the exhaust pipe is converted into ammonia. The ammonia reacts in the SCR catalytic converter with the nitrogen oxide to create nitrogen and water.
A further nitrogen oxide sensor, which monitors the function, is located after the SCR catalytic converter.
A temperature sensor in the exhaust pipe ahead of the metering valve measures the exhaust-gas temperature. Injection of the urea/water mixture starts when at least 200°C is measured here.