Optimization of Electrified Powertrains for City Cars 2011-01-2451

Sustainable and energy-efficient consumption is a main concern in contemporary society. Driven by more stringent international requirements, automobile manufacturers have shifted the focus of development into new technologies such as Hybrid Electric Vehicles (HEVs). These powertrains offer significant improvements in the efficiency of the propulsion system compared to conventional vehicles, but they also lead to higher complexities in the design process and in the control strategy. In order to obtain an optimum powertrain configuration, each component has to be laid out considering the best powertrain efficiency.

With such a perspective, a simulation study was performed for the purpose of minimizing well-to-wheel CO₂ emissions of a city car through electrification. Three different innovative systems, a Series Hybrid Electric Vehicle (SHEV), a Mixed Hybrid Electric Vehicle (MHEV) and a Battery Electric Vehicle (BEV) were compared to a conventional one. The SHEV allows the operation of the combustion engine independently from the driving situation. The MHEV combines series and parallel operation without a switchable gearbox.

The approach includes the implementation of a zero-dimensional powertrain model using the GT-Drive software with the replication of real driving behavior for the different driving profiles. Further the optimization of each powertrain and its operating strategy is based on a combination of Design of Experiment, driving cycle simulation and numerical optimization. The optimization criterion was well-to-wheel CO₂ emissions with respect to burned fuel and electric energy taken from the electric grid. These were calculated drawing on the European Union and German electric energy mixes (400 g CO₂/kWh_el and 590 g CO₂/kWh_el, respectively). In addition a number of performance constraints, such as acceleration, maximum speed, gradeability, and All Electric Range (AER) were defined and used for the optimization. In addition to the New European Driving Cycle (NEDC), four real world driving cycles were selected in order to represent the wide range of customer-relevant driving scenarios. As a result of the optimization process, for each powertrain, the optimum configuration and control strategy are provided.

In general, the MHEV has the lowest well-to-wheel CO₂ emissions as it combines the benefits of electric vehicles in urban driving cycles with those of traditional powertrains on high-speed routes. The optimal configuration is strongly dependent on the considered energy mix.