The High Potential for Waste Heat Recovery in Hybrid Vehicles: A Comparison Between the Potential in Conventional and Hybrid Powertrains

In future mobility, the mix of different drive trains will probably be much more diverse than it is today. According to a large number of scenario analyses, a predominant number of vehicles will continue to be based on the internal combustion engine (ICE), while an increasing number of hybrid vehicles are expected. To achieve the required reductions in CO2 emissions it is necessary to investigate all potential technologies for efficiency improvement. Therefore in this work the potential of waste heat recovery is examined for conventional and hybrid vehicles. Due to the fact that in an internal combustion engine approximately 2/3 of the fuel's chemical energy dissipates as waste heat, the potential for the recovery of this energy in all ICE driven powertrains is, in principle, high. The results of this work show that in hybrid vehicles the highest share of the energy supplied by the fuel is lost in the exhaust gas. In order to further elaborate this result, we conduct an exemplary examination of two comparable vehicles of the compact class within the worldwide harmonized light duty test cycle. Measurement data from the two vehicles at the roller dynamometer is used. The result shows that the averaged exhaust gas heat flow of the conventional vehicle is 5.0 kW. For the hybrid vehicle, driving in the charge sustaining mode, the averaged exhaust gas heat flow results in 8.1 kW. The comparison shows that the temperature level of this exhaust gas is even higher than that of the conventional vehicle. In addition, this work shows that through the higher temperatures, the exergy in the exhaust gas is higher in hybrid vehicles even if the combustion engine works with a higher efficiency. In the exemplary comparison the averaged exergy of the exhaust gas is 3.2 kW for the conventional and 5.7 kW for the hybrid vehicle. As a result of this work, the high potential for waste heat recovery in hybrid vehicles could be demonstrated.