Assessment of NOx concentration levels during periodic technical inspection idle test of SCR-equipped diesel vehicles

Background

The European Union implements periodic technical inspections (PTI) to ensure the safety and environmental performance of vehicles throughout their operational lifespan. Inspections cover both light-duty (LD) and heavy-duty (HD) vehicles. Current inspections do not cover nitrogen oxide (NOx) emissions, a major air pollutant. To address this, the European Commission is assessing the introduction of a methodology to detect malfunctions in modern Diesel vehicle selective catalytic reduction (SCR) units. Previous studies provided a procedure for PTI, where NOx is measured during idling (NOx–PTI idle test) after the vehicle’s exhaust system has been warmed-up. This study investigates which range of NOx concentration would allow detecting vehicles with SCR system malfunctions during PTI.

Results

We analysed NOx concentration recordings from 99 light-duty (LD) and 88 heavy-duty (HD) vehicles during idle tests at various European workshops and the European Commission’s Joint Research Centre. The results showed that well-functioning SCR systems had NOx concentrations below 13 ppm for LD vehicles and 31 ppm for HD vehicles. In contrast, malfunctioning systems had concentrations above 47 ppm for LD vehicles and 45 ppm for HD vehicles. The findings suggest that a reliable threshold for NOx concentration could be established towards the upper end of the identified ranges, allowing for the precise identification of malfunctions while providing a sufficient time buffer for inspection operators to carry out the 30 s test protocol (15 s analyser stabilization period followed by a 15 s NOx measurement).

Conclusions

This study provides insights for establishing a sound NOx concentration threshold for distinguishing between functioning and faulty SCR systems in modern Diesel vehicles type-approved Euro 6d-TEMP (LD)/Euro VI (HD) and onwards. The results presented could support current revisions of the PTI directive in ensuring accurate detection of malfunctions while allowing for effective integration of the NOx–PTI idle test into existing inspection procedures. The findings have potential implications for improving air quality and public health by maintaining low NOx emissions from Diesel vehicles throughout their operational lifespan.