Efficiency Improvement in a 48-Volt Mild Hybrid Vehicle Using Rankine Cycle Waste Heat Recovery

The automotive industry faces significant obstacles in its efforts to improve fuel economy and reduce carbon dioxide emissions. Current conventional automotive powertrain systems are approaching their technical limits and will not be able to meet future carbon dioxide emission targets as defined by the tank-to-wheel benchmark test. As automakers transition to low-carbon transportation solutions through electrification, there are significant challenges in managing energy and improving overall vehicle efficiency, particularly in real-world driving scenarios. While electrification offers a promising path to low-carbon transportation, it also presents significant challenges in terms of energy management and vehicle efficiency, particularly in real-world scenarios. Battery electric vehicles have a favorable tank-to-wheel balance but are constrained by limited range due to the low battery energy density inherent in their technology. This limitation has led to the development of hybrid electric powertrains, which combine an efficient internal combustion engine with a battery electric powertrain.

This paper presents the findings of a simulation study which examines the efficiency of a compact mild hybrid electric vehicle enhanced by a Rankine cycle unit. The unit employs an advanced bottoming Rankine cycle with a steam accumulator to recover exhaust heat from the internal combustion engine in a mild hybrid electric vehicle, thereby reintroducing the recovered energy into its electrical system. This potential was evaluated in a state-of-the-art Euro-6 reference vehicle with a mild hybrid powertrain to ascertain its feasibility in meeting the future European carbon dioxide and vehicle efficiency targets in the cost-sensitive C-segment mass vehicle sector. The resulting mild hybrid vehicle achieved an average vehicle efficiency of 40.84 % and a carbon dioxide figure of 93.1 g CO₂/km in the Worldwide Harmonized Light Duty Test Cycle. This result demonstrates that the vehicle under investigation could exceed the European target average vehicle cycle efficiency.