2009-01-1321 An Analytic Foundation for the Two-Mode Hybrid-Electric Powertrain with a Comparison to the Single-Mode Toyota Prius THS-II Powertrain Jerome Meisel Georgia Institute of Technology Copyright © 2009 SAE International ABSTRACT 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. Also included are numerical examples comparing the operation of the 2-MT and the THS-II transmissions. INTRODUCTION The General Motors Two-Mode Transmission (2-MT) represents a significant advancement over the Toyota THS-II single-mode transmission [3 - 10]. The 2-MT contains two electric machines, two or three planetary gear units, two electrically controlled clutches, and two electrically controlled brakes. The first mode of the 2-MT is very similar to the THS-II in that the speed of the engine is decoupled from the vehicle speed. Thus both transmissions function as a continuously variable transmission (CVT). Since this CVT action is accomplished electrically instead of mechanically, this operating mode is known as an electrically variable transmission (EVT). As the speed of the vehicle increases for a set engine speed, the flow of energy through the two electric machines in the 2-MT has an undesirable reversal that is eliminated by switching to the 2-MT’s second mode. The THS-II does not have this capability. In addition to these two EVT modes, the 2-MT can be switched into four set gear ratios from input to output shafts, thus constraining the engine speed to one of four values for a given vehicle speed. However, when the 2-MT is in one of these fixed ratios, one or sometimes both electric machines can supply added torque to the driven wheels. The use of the 2-MT thus incorporates the main advantage of a series architecture with the decoupling of the engine speed from the vehicle speed, and the main advantage of a parallel architecture with the synergistic summing of the engine and motor torques. The major advantages over series and parallel architectures obtained in using the 2-MT are summarized as follows [2]: /g120 Smaller electric machines and a smaller battery pack can be used. /g120 Higher efficiencies (i.e. lower fuel consumption) over a wide range of driving schedules can be achieved. /g120 A strong pure mechanical energy path exists between the engine and the driven wheels without the need for battery electric energy. ANALYSIS OF THE TWO-MODE TRANSMISSION STRUCTURE - Figure 1, shows the structure of the 2-MT with the two transfer clutches CL2 and CL4, and the The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE’s peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form o r by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. ISSN 0148-7191 Positions and opinions advanced in this paper are those of the