Electric Low-Speed [in Charge-Depleting (CD) mode]:
The simplest and most well-known of the Volt's four modes, this calls for the vehicle to use the charge from its 435-lb lithium-ion battery pack to power a 111-kW ac traction motor. When the traction motor turns, it transmits power to a sun gear in a planetary gear set, which turns the drive axle. Volt owners are expected to make heavy use of this mode - many will drive the car to work and back, charge it at night, and never use a drop of gasoline in the process.
Electric High-Speed [in Charge-Depleting (CD) mode]:
The Volt typically hits this mode at about 70 mph. Then, the supervisory controller splits the power between the large traction motor and a smaller 54-kW generator-motor (which is still operated by the battery). The planetary gear set blends the power from the two motors and sends it to the drive axle. Even as it switches to this mode, however, the Volt still isn't employing its internal combustion engine - and therefore still isn't burning any gasoline.
Extended Range Low-Speed [in Charge-Sustaining (CS) mode]:
When the battery's charge drops to an unacceptable level, the 1.4-â„“ I-4 engine makes its debut. At low speeds, the engine spins the generator-motor, which replenishes the battery.
Extended Range High-Speed [in Charge-Sustaining (CS) mode]:
This is the most complicated and controversial (see sidebar) of the Volt's four modes. With the battery depleted at higher speeds (70+ mph), the controller opts for a power split. It calls for power to be delivered through the big traction motor and through the smaller generator-motor. Unlike the other modes, however, this one calls for the generator-motor to transmit power directly into a ring gear in the vehicle's planetary gear set. GM says it has a good reason for doing this - electric motor efficiency typically plummets at higher speeds. "You're bringing down the speed of the main traction motor and supplementing it with the generator-motor," a GM spokesman says. "As a result, you're improving the efficiency."