Selective catalytic reduction (SCR): Notes

In addition to the engine-internal measures for exhaust re-treatment, the required exhaust emission standards are also met by the SCR system subjecting exhaust gases to chemical aftertreatment.

In combination with the AdBlue urea/water mixture, the pollutants (nitrogen oxides) in the SCR catalytic converter are converted into harmless nitrogen and water by chemical reactions.

NOTE: The acronym DEF (Diesel Exhaust Fluid) is also used in the United States of America.

To continue to comply with the tighter statutory exhaust-emission regulations, the SCR system sprays a urea/water mixture called AdBlue into the SCR catalytic converter. This AdBlue spray reacts with the hot exhaust gas and reduces the proportion of harmful NO_x emissions (nitrogen oxide) in the exhaust gas. The chemical reaction takes place in the SCR catalytic converter downstream of the diesel particulate filter. This chemical reaction can only take place when the SCR catalytic converter has reach a required minimum temperature. The SCR system remains inactive until this temperature (around 200 °C in the SCR catalytic converter) is reached.

Schematic diagram:

Fig 1: Catalytic Reduction Schematic Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

Item	Explanation
1	Digital diesel electronics (DDE)
2	Selective catalytic reduction (SCR control unit)
3	Temperature sensor
4	Passive tank fill level sensor
5	Passive tank
6	Transfer pump
7	Active tank
8	Temperature sensor
9	Active tank fill level sensor
10	Active tank heating element
11	Feed pump
12	Return pump
13	Pressure line heating element
14	Nitrogen oxide sensor evaluation electronics
15	Nitrogen oxide sensor after SCR catalytic converter
16	SCR catalytic converter
17	Temperature sensor, SCR catalytic converter
18	SCR metering module
19	Nitrogen oxide sensor before SCR catalytic converter
20	Oxidation catalytic converter and diesel particulate filter
21	Nitrogen oxide sensor evaluation electronics

Example installation position F30 with N47D20O1:

Fig 2: Identifying Fuel Filler Pipe, SCR Catalytic Converter And SCR Delivery Module

Courtesy of BMW OF NORTH AMERICA, INC.

Item	Explanation
1	Active tank
2	Tank ventilation line
3	Fuel filler pipe
4	Passive tank
5	Recirculating pump unit line
6	SCR delivery module
7	SCR catalytic converter
8	SCR metering module
9	Oxidation catalytic converter with diesel particulate filter
10	Pressure line

NOTE: Depending on the series and motorization, the installation position of the active tank and passive tank may vary! There are also vehicles with just an active tank. The volumes of the active and passive tanks also varies depending on series and motorization.

The Digital Diesel Electronics (DDE) sends the request with the calculated volume of AdBlue and the release to the SCR control unit via the powertrain CAN. AdBlue is metered depending on the load (flow rate and exhaust-gas temperature), the temperature profile and the load profile.

The DDE must ensure a sufficiently high exhaust-gas temperature and take measures where necessary to increase the exhaust-gas temperature. The NO_x conversion rates that can be achieved depend on the temperature in the SCR catalytic converter. The effectiveness of the conversion rate is influenced by:

Ratio of nitrogen oxide NO₂ to nitrogen monoxide
Length of time that nitrogen oxide spends in the SCR catalytic converter
Presence of unburned hydrocarbon CH
Amount of ammonia NH₃ stored in the SCR catalytic converter