Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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.
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 NOx 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:
| 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:
| 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 |
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 NOx 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
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.