This paper presents a comparative analysis of two different power-split hybrid-electric vehicle (HEV) powertrains using backward-looking simulations. Compared are the front-wheel drive (FWD) Toyota Hybrid System II (THS-II) and the FWD General Motors Allison Hybrid System II (GM AHS-II). The Toyota system employs a one-mode electrically variable transmission (EVT), while the GM system employs a two-mode EVT. Both powertrains are modeled with the same assumed mid-size sedan chassis parameters. Each design employs their native internal combustion (IC) engine because the transmission's characteristic ratios are designed for the respective brake specific fuel consumption (BSFC) maps. Due to the similarities (e.g., power, torque, displacement, and thermal efficiency) between the two IC engines, their fuel consumption and performance differences are neglected in this comparison. The road-load parameters defining each system are used to calculate the required mechanical power at the driven wheels necessary to follow a given drive-cycle. Admissible engine operating states are sought based on component performance limitations and the required mechanical power at the driven wheels. Each IC engine operating point defines an accompanying battery power consistent with the constraints of the electric machines. The design approach is to exhaustively search all admissible states and minimize an instantaneous cost function based on engine power and battery power, at each time instant of the drive-cycle. Two cost functions are considered which weight battery power usage using either a linear, or an inverse-tangent, function of the current battery state-of-charge (SOC). Selected operational states are then compared against each other based on the flexibility and power delivery capabilities of the powertrains. Fuel minimizing cost functions are determined with the assistance of a charge sustaining index introduced by this paper. Finally, the most fuel efficient choices are used to determine the expected efficiency of both powertrains considered.
