Coming back to reality: A proposal for real-world accuracy requirements for vehicle on-board fuel and energy consumption monitoring
To monitor the gap between official fuel consumption values and those observed during real-world driving, the European Commission plans to use real-world data from on-board fuel and energy consumption monitoring (OBFCM) devices. For obtaining reliable real-world data, it is necessary that OBFCM data be accurate. Data accuracy that can reasonably be required for verification testing is affected by three main factors: the accuracy of the OBFCM device itself when tested at reference conditions, uncertainty of the method used to verify OBFCM accuracy, and uncertainties introduced by differences between verification test conditions and reference conditions. This study provides evidence on the level of uncertainty of these elements and develops recommendations for future OBFCM data accuracy requirements.
To generate independent data for this analysis, an extensive vehicle test program was performed on three modern passenger cars equipped with OBFCM devices. The vehicles were tested on chassis dynamometer and on public roads. During all tests, the OBFCM values were recorded together with fuel consumption and driven distance measured independently by different verification methods. For estimating the effect of using market fuel instead of reference fuel and variability of wheel dimensions on the OBFCM data accuracy, a theoretical analysis was conducted.
The analysis suggests the combined OBFCM fuel consumption and distance accuracy requirements presented in the figure below. Depending on whether the OBFCM accuracy verification tests are performed on chassis dynamometer or on public roads, and whether the tests are performed under reference conditions, different uncertainties are considered. For plug-in hybrid electric vehicles (PHEVs), the analysis suggests that the same accuracy requirements should be used for operating mode-specific fuel consumption and distance parameters.
The insights gained during the project suggests the following recommendations to amend the regulation for making OBFCM data more robust and reliable: Accumulated second-by-second OBFCM values should equal the OBFCM lifetime values, OBFCM values should not have a systematic offset, and it should be clarified that the volumetric OBFCM fuel consumption relates to a fuel temperature of 15°C.