SCR Overview - Simplified
Selective catalytic reduction is a system for reducing nitrogen oxides (NOx ) in the exhaust gas. For this purpose, a reducing agent (urea/water solution) is injected into exhaust gas downstream of the diesel particulate filter.
The nitrogen oxide reduction reaction then takes place in the SCR catalytic converter. The urea-water solution is carried in two reservoirs in the vehicle. The quantity is measured out such that it is sufficient for one oil change interval.
The following graphic shows a simplified representation of the system:
| Index | Explanation |
|---|---|
| 1 | Passive reservoir |
| 2 | Level sensors |
| 3 | Filler pipe, passive tank |
| 4 | Metering line |
| 5 | Metering line heater |
| 6 | Pump |
| 7 | Function unit |
| 8 | Heater, in active tank |
| 9 | Active tank |
| 10 | Transfer pump |
| 11 | Filter |
| 12 | Transfer line |
| 13 | Metering module |
| 14 | Level sensor |
| 15 | Filler pipe, active reservoir |
| 16 | Exhaust system |
| 17 | SCR catalytic converter |
The reason for using two reservoirs is that the urea-water solution freezes at a temperature of -11°C (12.2°F). For this reason, the smaller "active" reservoir is heated but the larger passive reservoir is not. In this way, the entire volume of the urea-water solution need not be heated, thus saving energy. The amount in the active tank is sufficient, however, to cover large distances.
The small, heated reservoir is referred to as the active reservoir. A pump conveys the urea-water solution from this reservoir to the metering module. This line is also heated.
The larger, unheated reservoir is the passive reservoir. A transfer pump regularly conveys the urea-water solution from the passive reservoir to the active reservoir.