The European Union (EU) has defined legally-binding targets for the fleet of new cars allowing 95 grams CO2 per kilometer in 2021. It is already under discussion to reduce average emissions of the EU car fleet by further 15% in 2025 and again by 30% in 2030 compared to 2021 goal. Therefore, improvement of fuel economy is becoming one of the most important issues for the car manufacturers. Today’s conventional car powertrain systems are reaching their technical limits and will not be able to meet future fuel economy targets without further development of additional measures.
This paper presents the analysis of a Rankine cycle unit applied to improve the overall efficiency of a hybrid electric vehicle (HEV). The authors propose a new concept for recovering a considerable part of exhaust waste heat from an HEV with spark ignition internal combustion engine (ICE) by applying a bottoming Rankine cycle with a Ruths storage tank. It enables the storage of discontinuously available exhaust waste heat from the internal combustion engine as sensible heat in a pressurized working fluid. Thus, the vapor generating process is temporally decoupled from the transient engine operating condition and its unsteady flow of exhaust heat energy. The major benefits of this proposed concept are the utilization of recovered waste heat energy within the hybrid electric powertrain and faster engine warm up resulting in optimum ICE operating conditions.
The potential for fuel economy improvement for the proposed powertrain concept is demonstrated in standard certification cycles by applying vehicle simulation. The simulation models used in this study were validated by experiments.