Effect of hybrid system battery performance on determining CO2 emissions of hybrid electric vehicles in real-world conditions

Hybrid electric vehicles (HEVs) can potentially reduce vehicle CO2 emissions by using recuperated kinetic vehicle energy stored as electric energy in a hybrid system battery (HSB). HSB performance affects the individual net HEV CO2 emissions for a given driving pattern, which is considered to be equivalent to unchanged net energy content in the HSB. The present study investigates the influence of HSB performance on the statutory correction procedure used to determine HEV CO2 emissions in Europe based on chassis dynamometer measurements with three identical in-use examples of a full HEV model featuring different mileages. Statutory and real-world driving cycles and full electric vehicle operation modes have been considered. The main observation is that the selected HEVs can only use 67-80% of the charge provided to the HSB, which distorts the outcomes of the statutory correction procedure that does not consider such irreversibility. CO2 emissions corrected according to this procedure underestimate the true net CO2 emissions of one HEV by approximately 13% in real-world urban driving. The correct CO2 emissions are only reproduced when considering the HSB performance in this driving pattern. The statutory procedure for correcting HEV CO2 emissions should, therefore, be adapted.