ABSTRACT The Chevrolet Volt is an electric vehicle (EV) that operates exclusively on battery power as long as useful energy is available in the battery pack under normal conditions. After the battery is depleted of available energy, extended-range (ER) driving uses fuel energy in an internal combustion engine (ICE), an on-board generator, and a large electric driving motor. This extended-range electric vehicle (EREV) utilizes electric energy in an automobile more effectively than a plug-in hybrid electric vehicle (PHEV), which characteristically blends electric and engine power together during driving. A specialized EREV powertrain, called the “Voltec”, drives the Volt through its entire range of speed and acceleration with battery power alone, within the limit of battery energy, thereby displacing more fuel with electricity, emitting less CO 2 , and producing less cold-start emissions than a PHEV operating in real world conditions. The Voltec powertrain architecture provides four modes of operation, including two that are unique and maximize the Volt's efficiency and performance. The specialized electric transaxle, known as the 4ET50, enables patented operating modes both to improve electric driving range when operating as a Battery Electric Vehicle (BEV) and to reduce fuel consumption when extending range by operating with the ICE. Historically, most EVs have used a single-speed electric transaxle with one motor and a fixed gear reduction. The single-speed gear reduction is a simple arrangement that takes advantage of the wide speed range of electric motors. While this arrangement can work well, the wide speed range of most electric motors comes at the price of a well known loss of efficiency at higher motor speeds for most types of motors. Consequently, two-speed transmissions have been proposedfor BEVs that can improve both tractive effort and efficiency although with the attendant additional extra gear shift hardware and controls. The Voltec 4ET50 multi-mode electric transaxle introduces a unique two-motor EV driving mode that allows both the driving motor and the generator to simultaneously provide tractive effort while reducing electric motor speeds and the total associated electric motor losses. This new operating mode, however, does not introduce the torque discontinuities associated with a two-speed EV drive. For ER operation, the Voltec transaxle uses the same hardware and controls that enable one-motor and two-motor EV operation to provide both the completely decoupled action of a pure series hybrid, as well as a more efficient powerflow with decoupled action for driving at light loads and high vehicle speed. Construction of the GM Voltec 4ET50 transaxle employs significant re-use of the GM front wheel drive Two-Mode Hybrid 2MT70 transaxle with modifications to enable all- speed and full-power EV operation. A new high power driving motor, optimized generator, and modified control elements allow the two EV driving modes and the two ER driving modes to be realized in the Chevrolet Volt. INTRODUCTION VEHICLE TYPES AND ELECTRIC UTILITY Hybrid Electric Vehicle (HEV) A hybrid is defined by SAE [ 1] as: “A vehicle with two or more energy storage systems both of which must provide propulsion power - either together or independently.” In practice, hybrid vehicles typically require a combination both sources to provide full vehicle capability. The engine is also The GM “Voltec” 4ET50 Multi-Mode Electric Transaxle2011-01-0887 Published 04/12/2011 Michael A. Miller, Alan G. Holmes, Brendan M. Conlon and Peter J. Savagian General Motors Company Copyright © 2011 SAE International doi:10.4271/2011-01-0887 SAE Int. J. Engines | Volume 4 | Issue 1 1102 typically the larger of the two propulsion sources, being sized to provide most of the power during high power vehicle events. The motor is typically the smaller of the two propulsion sources, being sized for transient events to maximize the amount of energy that