Traceable Uncertainty of Exhaust Flow Meters Embedded in Portable Emission Measurement Systems

Portable emissions measurement systems (PEMS) are used in the type approval tests of diesel- and petrol-fuelled light-duty vehicles. PEMS measure the amount of pollutants emitted during on-road, real driving, and testing of a vehicle. PEMS are comprised of gas analyzers, which measure the concentration of pollutants in the exhaust gas, and exhaust flow meters (EFMs), which measure the amount of emitted exhaust gas during an on-road test. With the adoption of the real driving emissions (RDE) legislation in 2016–2018, usage of PEMS during on-road tests became mandatory in the European Union for the type approval of light-duty vehicles. For accurate assessment of pollutant emissions, SI-traceable calibration of PEMS equipment is required, which is typically performed under the carefully controlled conditions of a calibration laboratory. However, when using the PEMS equipment in real driving conditions on the road, additional factors contribute to the measurement uncertainty with respect to the laboratory measurement uncertainty as established during the SI-traceable calibration. For this reason, the RDE prescribes the usage of conformity factors to account for additional measurement uncertainty in real-driving-type approval tests. The pollutant emissions measurements are a combination of the gas analyzer measurements and the EFM flow measurement. The EFM measurement uncertainty contributes strongly to the overall pollutant emissions measurement uncertainties. Further, the EFM measurement uncertainty in on-road conditions is not fully characterized. Consequently, SI-traceable, quantitative information on on-road factors affecting measurement uncertainty is needed. Important factors potentially affecting EFM measurement uncertainty are exhaust flow transients, flow pulsations, gas composition, temperature effects, and flow meter drift. For an improved understanding of representative, SI-traceable, EFM measurement uncertainty (I) a generic uncertainty analysis is presented, which is obtained from performing an uncertainty analysis on state-of-the-art knowledge stemming from literature. This is followed by (II) presenting quantitative and partly traceable measurement results of experiments dedicated to the assessment of uncertainty from potentially significant factors contributing to the overall measurement uncertainty of the EFM in on-road conditions. Experimental results indicate that the EFM can match the (laboratory) accuracy limits as defined in the RDE. Other experimental results indicate that specific potential on-road conditions can affect the EFM such that the resulting overall measurement uncertainty exceeds the limit from the employment of the RDE-prescribed conformity factor.