White paper

Plug-in hybrid vehicle CO2 emissions: How they are affected by ambient conditions and driver mode selection

Recent studies show that plug-in hybrid electric vehicles (PHEVs) show a large gap between real-world and official type-approval CO2 emissions. The gap is to a great extent attributed to less frequent charging than anticipated by the type-approval regulation, but also to the use of heating and air-conditioning, which are not assessed during type-approval. This study assesses, via testing of a representative PHEV, the effect of ambient temperature and use of air-conditioning on CO2 emissions, electric energy consumption, and electric ranges. The vehicle was also tested using different plug-in hybrid operating modes to evaluate their effect on the CO2 emissions. Furthermore, it was assessed how on-board fuel- and energy-consumption monitoring (OBFCM) data recorded by the vehicle during the testing program can be used for determining the real-world usage of PHEVs.

The study found that the electric energy consumed by the air-conditioning compressor and cabin heater reduces the energy available for powering the vehicle and thereby the distance attributed to electric energy. When driving the speed profile of the worldwide harmonized light vehicles test cycle (WTLC), an equivalent of 46 km can be driven using electric energy at 23 °C. At -5 °C, however, the distance is almost halved to 24 km and reduces by 11% to 41 km at 35 °C.

When starting with a fully charged battery at 23 °C and 35 °C, the vehicle first operates purely electric until the battery is almost depleted. This results in low charge-depleting CO2 emissions of 2 and 10 g/km respectively. At -5 °C, however, the combustion engine is used intermittently from the start of the test to warm up the catalyst. In combination with the lower available electric energy for propulsion, this change in operating strategy results in charge-depleting CO2 emissions at -5 °C of 94 g/km, or more than 40 times higher than at 23 °C. Together with a 30% increase in charge-sustaining CO2 emissions from 155 g/km to 201 g/km, the weighted, combined CO2 value at -5 °C almost triples compared to 23 °C, from 43 g/km to 122 g/km. At 35 °C, the charge-sustaining CO2 emissions are about 190 g/km and the weighted, combined CO2 emissions are 57 g/km, which is 34% higher than at 23 °C.

The vehicle was further tested in user-selectable charge-increasing PHEV mode, finding that primarily the combustion engine is used to charge the battery until full. The additional fuel consumed for battery charging increased the WLTC CO2 emissions by 60%, from 154 g/km to 246 g/km, compared to operation in charge-sustaining mode. Compared to charging the vehicle battery with EU grid energy, 2.5 to 2.8 times more CO2 is emitted when using the charge-increasing mode. Unexpectedly, the combustion engine was also used to charge the battery when sports vehicle- and transmission-mode was selected, even when charge-increasing mode was not engaged.