California’s LEV III is the current OEM target for future motor vehicle emission regulations for Diesel vehicles. The formidable challenge represented by these limitations will likely be met through combustion process improvements, and hence, reduced raw emissions. But this will, likely not be enough. Improvements in exhaust gas aftertreatment efficiency and the use of enhanced control logic will also become necessary.
The requirements of Diesel vehicle on-board diagnostic systems (OBD) are becoming more challenging, as well. The currently employed monitoring strategies can no longer guarantee reliable detection of partially failed systems. Monitoring of aftertreatment components will become particularly complex with higher system efficiencies, since the margin between a part at the tolerance limit and a properly functioning component will be much smaller. With this in mind, OBD can impact the tailored selection of future exhaust aftertreatment systems.
Challenges for OBD strategies
To fulfill the LEV III thresholds, combined NOx aftertreatment systems (for example, NOx storage catalysts (NSC) plus an SCR system) represent an attractive and comprehensive approach. The split of NOx conversion, in general, represents a noticeable benefit with regard to diagnosis of the aftertreatment system, since the component efficiency loss that must be detected is decisively lower, even for high overall NOx reduction.
However, the increasing complexity of aftertreatment systems requires an appropriate adaption of the existing monitoring strategies as well. For instance, low NOx concentrations downstream of the NSC represent a risk for robust and frequent SCR monitoring due to limited accuracy of the NOx sensors.
The on-board diagnostic system must ensure ongoing improvement in the monitoring concepts in order to support compliance with future emission legislations. Therefore, FEV GmbH is developing potential OBD diagnosis strategies for various innovative aftertreatment systems with regard to regulation compliance and robustness, optimally using existing sensor concepts. Due to increased interaction, enhanced exchange of information between the monitoring functions as well as the application of active strategies are needed to design an intelligent and holistic diagnosis system for the aftertreatment system. Monitoring for increased component efficiencies must not only be guaranteed by advanced software based approaches. In addition, this goal must be realized by minimizing the number of sensors and adaptation of a cost-oriented calibration effort.