Potential of ORC-Based Waste Heat Recovery in Conventional and Hybrid Heavy-Duty Powertrains

Heavy-duty vehicles are major contributors to greenhouse gas emissions and urban air pollution, particularly under cold-start and transient operating conditions where engine efficiency and exhaust aftertreatment effectiveness are reduced. In this context, waste heat recovery (WHR) technologies represent a practical pathway for improving fuel efficiency and reducing CO₂ emissions in real-world heavy-duty applications. This study investigates the integration of an Organic Rankine Cycle (ORC)–based WHR system within conventional and hybrid electric powertrain architectures applied to a reference heavy-duty truck (ISUZU FTR850). A sensitivity analysis is performed on four WHR configurations, combining shell-and-tube and plate heat exchangers with simple and regenerative ORC layouts. For the parallel hybrid configuration, two engine sizes and two energy management strategies are compared: a fuel-minimizing fuzzy logic rule-based strategy and a constant-power strategy aimed at stabilizing exhaust thermal conditions to enhance WHR effectiveness. A scalable quasi-static simulation framework, validated against real-world vehicle data, is used to predict fuel consumption and exhaust gas properties over representative duty cycles. Off-design ORC behavior is characterized through two-dimensional performance maps as a function of exhaust gas temperature and mass flow rate, enabling control-oriented integration at the vehicle level. Results indicate that the optimal WHR configuration depends strongly on both the degree of hybridization and the adopted energy management strategy, underscoring the need for coordinated powertrain and WHR design. For the conventional powertrain, a shell-and-tube heat exchanger provides the best robustness-to-benefit trade-off, yielding approximately 2 kWh of recovered electrical energy over an eight-hour duty cycle, corresponding to a measurable reduction in engine load and fuel consumption. In the hybrid electric configuration, a simple ORC combined with a shell-and-tube heat exchanger enables the recovery of up to 3.5 kWh when paired with the constant-power strategy, translating into additional electric energy available for onboard auxiliaries or battery charging and a further reduction in fuel demand or the adoption of a smaller battery. Overall, the results demonstrate that ORC-based WHR can deliver tangible fuel savings and associated CO₂ emission reductions in production-oriented heavy-duty hybrid powertrains, particularly when exhaust thermal conditions are actively managed.