Trends and limits of WHR technologies in hybrid powertrain architectures

Typical thermal efficiencies for modern internal combustion (IC) engines range between 15 to 40% depending on operating conditions and engine design. The remaining 60 to 85% of energy available from the fuel is expelled as waste heat to the atmosphere, mainly through the vehicles cooling and exhaust systems.

Exhaust gases can contain up to 30% of total combustion energy and present an opportunity to employ waste heat recovery (WHR) technologies capable of recovering a proportion of this unused energy to raise overall powertrain efficiency.

Proposed alternatives will need to be applicable to a light-duty passenger vehicle application where aspects such as cost, packaging, weight and impact on emissions are of critical importance. Although the quality of waste heat generated by a traditional IC engine is relatively low over light-duty drive cycle, novel hybrid vehicle architectures that operate at higher engine loads offer a greater potential for energy recovery.

Hence, the aim is to evaluate the energy recovery potential for a range of WHR strategies in modern hybrid vehicle powertrain architectures. This will involve a detailed system-level study that will determine if the added cost and weight of implementing WHR technologies is offset by improved vehicle powertrain efficiency (reduced battery pack size and weight, improved fuel economy and range).