Functions of the SCR System: Notes
Selective catalytic reduction is currently the most effective system for reducing nitrogen oxides (NOx ). During operation, it achieves an efficiency of almost 100% and approximately 90% over the entire vehicle operating range. The difference is attributed to the time the system requires until it is fully operative after a cold start.
| Index | Explanation |
|---|---|
| 1 | NOx sensor, pre catalyst |
| 2 | Metering module |
| 3 | NOx sensor, post catalyst |
| 4 | Temperature sensor after DPF |
This system carries a reducing agent, urea-water solution, in the vehicle. The urea-water solution is injected into the exhaust pipe by the metering module upstream of the SCR catalytic converter.
The DDE calculates the quantity that needs to be injected. The nitrogen oxide content in the exhaust gas is determined by the NOx sensor before the SCR catalytic converter.
Corresponding to this value, the exact quantity of the urea-water solution required to fully reduce the nitrogen oxides is injected. The urea-water solution converts to ammonia in the exhaust pipe.
In the SCR catalytic converter, the ammonia reacts with the nitrogen oxides to produce nitrogen (N2) and water (H2 O).
A further NOx sensor that monitors this function is located downstream of the SCR catalytic converter.
A temperature sensor in the exhaust pipe after the diesel particulate filter (i.e. before the SCR catalytic converter) and the metering module also influences this function. This is because injection of the urea-water solution only begins at a minimum temperature of 200°C (392°F).