ABSTRACT 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 aredetermined 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. INTRODUCTION The three fundamental HEV architectures in use today are series, parallel and power-split. The operation of the IC engine and electric machines (EMs) in each architecture ultimately dictates the powertrain's efficiency. Since IC engines are more variable in their efficiency than EMs, it suffices to consider the IC engine operation when qualitatively assessing overall powertrain efficiency. Parallel architectures employ a highly-efficient mechanical path to transmit input IC engine power to the driven wheels. Engine speed and efficiency is therefore constrained by a finite number of fixed-gear (FG) ratios or their equivalents. IC engine operation is best suited for steady power delivery (e.g., highway driving at constant cruising speed), as opposed to dynamic power delivery (e.g., urban or variable driving). For dynamic power delivery over a large range of vehicle speeds, a finite number of FG ratios can result in potentially inefficient engine operation. In series architectures, the IC engine operates independently of the road-load conditions providing steady, highly-efficient operation. Series electro- mechanical power delivery is best suited for urban driving with significant vehicle speed fluctuations. However, electro- mechanical power delivery is less efficient than mechanical due to energy conversion losses. Power-split designs combine the advantages of single path architectures, such as parallel and series, by providing two power paths between the IC engine and the driven wheels through gearing and EMs. Backward-Looking Simulation of the Toyota Prius and General Motors Two-Mode Power-Split HEV Powertrains2011-01-0948 Published 04/12/2011 John Arata, Michael J. Leamy, Jerome Meisel, Kenneth Cunefare and David Taylor Georgia Institute of Technology Copyright © 2011 SAE International doi:10.4271/2011-01-0948 SAE Int. J. Engines | Volume 4 | Issue 1 1281Downloaded from