Holistic Analysis of a Mild Hybrid Waste Heat Recovery System for Commercial Vehicles

To further reduce fuel consumption and CO2 emissions of heavy-duty vehicles, recovering waste heat from the engine's exhaust gases is a promising method. By means of an Organic Rankine Cycle (ORC), the thermal energy of the exhaust gases is converted into useable energy to support the powertrain. The integration of such a waste heat recovery (WHR) system into the powertrain as well as the transient operation presents several challenges: The interactions between the WHR system and the powertrain have to be analyzed, and their effect on fuel consumption has to be quantified in order to provide reliable fuel-saving potentials. In this article, a co-simulation model that couples the cooling system, the combustion engine, the vehicle's longitudinal dynamics including the control system, and the WHR system is presented. The latter uses a turbine generator to supply electrical energy to a 48 V board net connected to an electric motor that is used to both support the powertrain and to recuperate the brake energy (mild hybrid concept). The presented study uses three route profiles for the detailed investigation of the ORC's efficiencies, the comparison of two mild hybrid control strategies, and the optimization of both the 48 V battery capacity and the electrical engine power. Furthermore, the effect of the increased counter-pressure in the combustion engine's exhaust path and the additional cooling load caused by the ORC's condenser are analyzed. A 3% to 4% of fuel-saving potential is possible with the presented WHR system, depending on the ambient temperature, the total vehicle weight, and the route profile.