Voltec Electric Drive System Overview
Exceeding customer expectations was top of mind for the Chevrolet Volt engineers as
they developed the Voltec electric drive system. Since the Volt is the industry’s first
electric vehicle with extended-range capability, the team focused on delivering the most
efficient, yet fun-to-drive experience in both electric and extended-range operation.
VOLTECELECTRICDRIVE
Kinematic Architecture
Battery Pack
Inverter
Traction Motor
Planetary Sun Gear
Generator
Planetary Ring Gear
Planetary Carrier
C1
C3
Final Drive Gearing
C2
2.16 ratio
Axle Differential
Unlike most battery electric vehicles, which use a single motor with a fixed reduction
gear that steps down the motor speed to synchronize with the wheel speed, the
Chevrolet Volt uses a unique electric drive unit to boost efficiency. The efficiency of any
electric motor always drops off as it approaches its maximum rotational speed. The Volt
drive unit consists of two motors, three clutches and a planetary gear set that improve
overall efficiency by reducing the combined rotational speed of the electric motors. This
configuration reduces battery drain at highway speeds, adding up to two miles of
additional EV range.
The Volt’s motors and gear-set are mounted in-line with the industry’s first rangeextending internal combustion engine. Two of the clutches are used to either lock the
ring gear of the planetary gear-set or connect it to the generator/motor depending on the
mode. The third clutch connects the internal combustion engine to the generator/motor
to provide range extension capability.
This innovative new drive unit has four distinct operating modes:
ELECTRICDRIVING
Low Speed
(1-Motor)
3.24 ratio
C1
C3
C2
2.16 ratio
Single Motor EV Driving
In this mode, the primary traction motor provides all propulsion at lower vehicle speeds
and hard accelerations, drawing all of its energy from the battery. The ring gear is
locked and the generator/motor is decoupled from both the engine and gear-set. The
traction motor can use up to 111 kilowatts of power and deliver 273 pound-feet of
smooth, direct torque to provide spirited off-the-line acceleration.
ELECTRICDRIVING
High Speed
(2-Motor)
3.24 ratio
C1
C3
1.45 ratio
C2
2.16 ratio
Two Motor EV Driving
As vehicle speed increases, the ring gear is unlocked and coupled to the
generator/motor. This allows the two motors to work in tandem to provide blended
output with higher electrical efficiency. Using the blended output of the slower running
motors allowed the team to extract up to two extra miles of pure electric operating range
in highway driving.
EXTENDED RANGE DRIVING
Low Speed
(1-Motor Series)
3.24 ratio
C1
C3
C2
2.16 ratio
Single Motor Extended-Range Driving
Once the battery has reached its minimum state of charge, the 1.4-liter engine is
coupled to the motor/generator via the third clutch. At lower speeds and hard
accelerations the Volt is propelled by the traction motor alone with the ring gear locked.
The engine-driven generator and battery provides electricity to the traction motor via the
inverter. On an average basis, the engine-driven generator will maintain the battery at a
minimum state of charge for extended range operation.
Since the most efficient way to charge the Volt’s battery is to plug it in, the generator is
only used to maintain minimum battery state of charge. If the battery is drawn down
below the minimum level during acceleration or when mountain mode is engaged below
about 45 percent charge, the generator will charge the battery up to its minimum state
of charge and then maintain it there.
EXTENDED RANGE DRIVING
High Speed
(2-Motor Combined)
3.24 ratio
C1
C3
1.45 ratio
C2
2.16 ratio
Two Motor Extended-Range Combined Driving
The blended two motor electric propulsion strategy used at higher speeds in EV driving
has also been adapted for extended-range driving. The clutches that connect the
generator/motor to both the engine and the ring gear are engaged, combining the
engine and both motors to drive the Volt via the planetary gear set. All of the propulsion
energy is seamlessly blended via the planetary gear and sent to the final drive.
Even in combined extended-range driving, the Volt uses electric power delivered by the
traction motor -- the engine cannot propel the Volt unless the traction motor is also
powered. That is because, in order for a planetary gear-set to transmit torque, two of
the three main elements (ring gear, sun gear, planet carrier) must be driven, or one
locked, with the third element providing the output. In the case of the Volt, the traction
motor is used in this mode to provide reaction torque for the sun gear, enabling
propulsion of the vehicle.
By seamlessly blending power from the engine and two electric motors, this
sophisticated technology allows the Volt to achieve 10-15 percent better efficiency at
highway speeds than would have been possible by using only the single traction motor.
At the same time, the Volt always delivers a solid electric driving experience, even in
extended range driving, by utilizing the large battery for full electric launches even
during brisk accelerations.
Contacts:
Kevin M. Kelly
Manager, Electric Vehicle and Hybrid Communications
Desk: (313) 665-0898
Mobile: (313) 316-9742
kevin.m.kelly@gm.com
Randy Fox
Phone 313-667-4128
Mobile 313-378-5872
randal.l.fox@gm.com