Search Results

Hybrid-electric powertrains for passenger vehicles and light trucks are generally being designed with two different configurations described as follows: The Toyota Hybrid System, THS-II, implemented in the 2004 Prius, the Lexus 400-H, and the Ford Hybrid Escape, is a power-split approach involving two electric machines and an internal combustion engine (ICE) mechanically coupled by a three-shaft planetary gear train. The second leading approach is a parallel hybrid-electric powertrain that generally includes a single electric machine and an ICE with a mating multi-ratio transmission. These parallel configurations are further divided as weak parallel and strong parallel. Honda uses a weak parallel powertrain in their Insight and Hybrid Civic. At Georgia Tech a strong (full), split-parallel hybrid powertrain has been implemented in a Ford Explorer. The vehicle is referred to as the Model GT.

General Motors has introduced a Two-Mode Transmission (2-MT) that provides significant improvements over the Toyota THS-II transmission. These improvements are achieved by employing additional planetaries with clutches and brakes to switch from a Mode-1 to Mode-2 as vehicle speed increases. In addition the 2-MT has four fixed-gear ratios that provide for a purely mechanical energy path from the IC engine to the driven wheels with the electric machines also able to provide additional driving torque. The purpose of this present paper is to extend the methodology in a previous paper [1] to include the 2-MT, thereby presenting an analytic foundation for its operation. The main contribution in this analysis is in the definition of dimensionless separation factors, defined in each mode that govern the power split between the parallel mechanical and electrical energy paths from the IC engine to the driven wheels.

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.

General Motors has recently developed a front-wheel drive version of its two planetary two-mode transmission (2-MT) for a hybrid-electric vehicle powertrain [1]. This newer transmission includes two planetary gears with two transfer clutches and two braking clutches. With activation of designated pairs of these four clutches, four fixed-gear ratios between the transmission's input shaft and output shaft are obtained. In addition, activation of specific individual clutches gives two modes of operation whereby the IC engine speed is decoupled from the vehicle velocity thus providing an electrical continuously variable transmission (ECVT). This present paper extends the power-split analysis in [2] by deriving a safe-operating region (SOR) in the plane of IC engine speed vs. vehicle velocity for the four fixed-gear and two ECVT modes. This SOR is bounded by the speed limitations of the 2-MT components. Similar results are presented for the Toyota Hybrid System II (THS-II) transmission.