SCR Catalytic Converter
The effectiveness of the SCR catalytic converter is monitored by the two NOx sensors. The nitrogen mass is measured before and after the SCR catalytic converter and a sum is formed over a defined period of time. The actual reduction is compared with a calculated value that is stored in the DDE.
The following conditions must be met for this purpose:
- NOx sensors plausible
- Metering active
- Ambient temperature in defined range
- Ambient pressure in defined range
- Regeneration of diesel particulate filter not active
- SCR catalytic converter temperature in defined range (is calculated by means of exhaust temperature sensor before SCR catalytic converter)
- Flow of exhaust gas in defined range.
Monitoring involves four measuring cycles. A reversible fault is stored if the actual value is lower than the calculated value. If the fault is determined in two successive driving cycles, an irreversible fault is stored and the MIL is activated.
Long-term adaptation is implemented, where the metered quantity of urea-water solution is adapted, to ensure the effectiveness of the SCR catalytic converter over a long period of time. To execute this adaptation procedure, the signal of the NOx sensor after the SCR catalytic converter is compared with a calculated value. If variations occur, the metered quantity is correspondingly adapted in the short term.
The adaptations are evaluated and a correction factor is applied to the metered quantity.
The operating range for the long-term adaptation is the same as that for effectiveness monitoring.
A reversible fault is stored if the correction factor exceeds a defined threshold. If the fault is determined in two successive driving cycles, an irreversible fault is stored and the MIL is activated.