Metering Strategy
The metering strategy is an integral part of the SCR control that calculates how much urea-water solution is to be injected at what time.
| Index | Explanation |
|---|---|
| A | Value from NOx sensor |
| B | Injected quantity of urea-water solution |
| 1 | Too-little urea-water solution injected |
| 2 | Correct quantity of urea-water solution injected |
| 3 | Too-much urea-water solution injected |
During normal operation, the signal from the NOx sensor before the SCR catalytic converter is used for the purpose of calculating the quantity. This sensor determines the quantity of nitrogen oxide in the exhaust gas and sends the corresponding value to the DDE.
However, the NOx sensor must reach its operating temperature before it can start measuring. Depending on the temperature, this can take up to 15 minutes. Until then the DDE uses a substitute value to determine the amount of nitrogen oxide in the exhaust gas.
A second NOx sensor is installed after the SCR catalytic converter for the purpose of monitoring the system. It measures whether there are still nitrogen oxides in the exhaust gas. If so the injected quantity of the urea-water solution is correspondingly adapted.
The NOx sensor, however, measures not only nitrogen oxides but also ammonia but cannot distinguish between them. If too much urea-water solution is injected, although the nitrogen oxides are completely reduced so-called "ammonia slip" occurs, i.e. ammonia emerges from the SCR catalytic converter. This in turn causes a rise in the value measured by the NOx sensor. The aim, therefore, is to achieve a minimum of the sensor value.
This, however, is a long-term adaptation and not a short-term control process as the SCR catalytic converter performs a storage function for ammonia.