SUMMARY
Power-split hybrid-electric vehicles (HEVs) provide two power paths between the internal combustion (IC) engine and the driven wheels using an electrically variable transmission (EVT). EVTs allow IC engine control such that rotational speed is independent of vehicle speed. If the most efficient IC engine operating point produces more power than is requested, excess IC engine power can be stored in the energy storage system (ESS) and used later. Conversely, if the most efficient IC engine operating point does not meet the required power, the ESS delivers the difference through the electric machines (EMs). Therefore with an intelligent supervisory control strategy, power-split architectures can advantageously combine traditional serial and parallel power paths. In this work two power-split HEV powertrains are compared using a 2-term cost function and steady-state backward-looking simulation (BLS). The supervisory control strategy design approach amounts to an exhaustive search over all kinematically-admissible input operating points, leading to a minimized instantaneous cost function. While the approach provides a valuable comparison of two architectures, non-ideal engine speed fluctuations result. Therefore, high-fidelity forward-looking simulation (FLS) is used next to investigate two methods for designing control strategies with refined IC engine speed transitions: i) smoothing the 2-term cost function results, and ii) introducing a 3-term cost function. Both methods achieve operable engine speed transitions, and result in fuel economy estimates which compare well to BLS results. It is further found that 3-term cost function finds more efficient operating points than the smoothed 2-term cost function approach. From the investigations carried out in this work, a two-phase control strategy development process is suggested where control strategies are generated using efficient steady-state BLS models, and then further tested and verified in high-fidelity FLS models.