Optimizing the On Board Diagnostic System (OBD) to Monitor for Reduction of the SCR Catalyst Conversion Efficiency using the NOx Sensor

Analyzing the European directives that are legislating the OBD system, the NOx sensor must be monitored by the emissions control system, guaranteeing faults detection in the aftertreatment system that could increase the emissions. According to the annex 4 of the directive 2005/78/EC item 3.3.2.1 (requirements for the OBD system Stage 2), it is needed to monitor the catalyst efficiency as an isolated component of the system.

In case of efficiency reduction of the aftertreatment system, due to the low quality of the SCR reagent liquid, physical damage or catalyst aged, the current softwares are ready to detect the faults, but providing a generic and inaccurate diagnostic.

The SCR catalyst aging, due to the high level of sulfur in the diesel, to the lubricant oil burning or to the high exhaust gas temperature, will be verified in real applications when the SCR system is implemented. When it is aged, the main feature of the catalyst (the ammonia storage capacity) is reduced, which can cause ammonia emissions (slip) in the ambient. The treatment of this particular feature can also be found in sophisticated software strategies, but applying a theoric factor to the catalyst aging based on the system operating conditions to be applied as a correction factor in the closed-loop dosing control strategy.

This work consists in the development of a software algorithm to be integrated to the OBD system that allows diagnosing, through a dynamic functional test, a NOx conversion efficiency reduction in the SCR catalyst as an isolated component of the complete system. With this diagnostic, applied within the strategies for catalyst aging detection, the dosing controller can correct the reagent liquid injection, preventing ammonia emissions in the ambient, and getting a faster and easier fault resolution.