ABSTRACT 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. Also developed is the vehicle velocity limit for both the 2-MT and the THS-II transmissions under battery electric vehicle (BEV) operation. INTRODUCTION The front-wheel drive version of GM's 2-MT includes two electric machines; two planetaries, one a compound planetary, and the other a simple planetary; two transfer clutches, and two braking clutches. References [ 1], and [ 3], [4], [5], [6] give detailed descriptions of this latest version of the 2-MT. As discussed in [2], the activation of specific pairs of these four clutches provides four fixed ratios between the input and output shafts of the 2-MT. This fixed-gear-ratio operation is similar to a four-speed automatic transmission excluding the torque converter. In addition, activation ofspecific single clutches provides two operating modes whereby the IC engine speed on the transmission input shaft is decoupled from the vehicle velocity. This decoupling allows for the IC engine to idle without a torque converter. These two modes achieve electrical continuously variable transmission (ECVT) operation by splitting the IC engine output power into two paths to the transmission output shaft. One path maintains the IC engine output power in its original mechanical power form. The second path involves a double energy conversion whereby the IC engine's output mechanical power is converted to electrical power and this electrical power is reconverted back to mechanical power. Thus ECVT operation is achieved at the expense of this mechanical-to-electrical-to-mechanical double energy conversion of a portion of the IC engine output power. Reference [ 2] develops a dimensionless variable, termed a separation factor that determines the fraction of the IC engine's output power that takes each of these two paths through the transmission. This separation factor is formulated for each of the two ECVT modes. For these two modes the separation factors involve the ratio of the vehicle velocity to the engine speed, similar to the speed ratio defined in [ 9]. Since ECVT operation allows for a decoupling of IC engine speed and vehicle velocity, the value of each of these two separation factors can be adjusted by controlling the engine speed for a given vehicle velocity. Thus in both modes the split of IC engine power between the two paths can be controlled. In general, there is a tradeoff that involves the fact that transmitting 100% of the IC engine power over the mechanical path, yielding the highest transmission efficiency by avoiding the double energy conversion, results in higher less efficient IC engine speeds. So the 2-MT efficiency must be balanced against the IC engine efficiency in the two ECVT modes. Kinematic Study of the GM Front-Wheel Drive Two-Mode Transmission and the Toyota Hybrid System THS-II Transmission2011-01-0876 Published 04/12/2011 Jerome Meisel Georgia Institute of Technology Copyright © 2011 SAE International doi:10.4271/2011-01-0876 SAE Int. J. Engines | Volume