On the potentially misleading evaluation of CO2 emissions for hybrid-electric vehicles

The adoption of hybrid electric vehicles (HEVs) is becoming more popular during the last few years due to government incentives and favourable legislation both for automotive companies and final users. This type of vehicle claims very low carbon dioxide emissions while eliminating the range anxiety associated with battery electric vehicles thanks to the on-board range extender being able to recharge the battery throughout the journey. Unfortunately, the low emissions values are more representative of the particular mathematical model implemented by the legislation than the measured real driving emissions. Specifically, the legislation does not take into account the CO2 embedded in production of the batteries or of the electrical energy stored in it. This work analyses these aspects by means of a numerical model of the BMW i3 94Ah vehicle. The results obtained are collected from simulations conducted over the Worldwide harmonized Light vehicles Test Cycle (WLTC) by using the commercial software GT-Power. The numerical model has been built with parameters presented in literature and validated against experimental data originally published as part of an Argonne National Laboratories research project. The model assessed all the most important energetic and emissions parameters over the regulated driving cycle such as the instantaneous fuel and electrical energy consumptions together with the CO2 produced by the fuel, the instantaneous vehicle power and the total energy needed to complete the driving cycle. The standard CO2 results are then compared with the same one also considering the CO2 embedded in the battery. Finally, an extreme interpretation of the analysis illustrates the misleading approach taken by the legislation in the definition of the CO2 emission for a HEV and discusses a more suitable way to evaluate them.