Too low to be true? How to measure fuel consumption and CO2 emissions of plug-in hybrid vehicles, today and in the future

Published: 2017.07.28

Iddo Riemersma, Peter Mock

The global market for electric vehicles is growing rapidly and plug-in hybrid electric vehicles (PHEVs) play a particularly important role given their relatively high market share – accounting for 13% of all new passenger cars in Norway and 5% in the Netherlands in 2016, for example. From the perspective of vehicle manufacturers, type-approval authorities, and consumers, PHEVs provide a specific challenge: they use two different energy sources, fuel and electricity, and the relative shares between these two energy types strongly depend on how the vehicle is driven and recharged in practice. Therefore, it is no surprise that some consumers struggle to understand why the fuel consumption values advertised for their PHEVs are so much lower and the advertised electric range is so much higher than what they experience in everyday driving.

This document first outlines the current procedure for the determination of fuel consumption, carbon dioxide (CO2) emissions, electric energy consumption, and electric range, specifically for PHEVs in Europe. It then highlights the most relevant changes expected with the introduction of the new Worldwide Harmonized Light Vehicles Test Procedure (WLTP), which will come into effect in the European Union (EU) in September 2017. Finally, key differences between the EU and U.S. test procedures for PHEVs are briefly discussed.

Key conclusions include that the electric driving range of a PHEV strongly influences the fuel consumption and CO2 emission values that are used for the official reporting as well as for communication to consumers. In the current vehicle testing procedure the electric range of PHEV tends to be estimated in a rather optimistic way. In the real world, the CO2 emission level of a PHEV strongly depends on factors such as the driving distance, consumer behavior, ambient temperature, and recharging behavior. Beginning in September 2017, as part of the new WLTP test procedure, a utility factor is introduced that is expected to better reflect the real-world energy consumption and electric range of PHEVs. Nevertheless, even under the WLTP, official and real-world values for PHEV will continue to strongly deviate for some customers. In the future, the specific driving behavior of individual customers could be taken into account for estimating fuel consumption and CO2 emission values that more accurately reflect the everyday driving experience and that allow for a fairer comparison between individual vehicle types.