December 9 2011

Voltec Electric Drive System Overview

Exceeding customer expectations was top of mind for the Volt engineers as they developed the Voltec electric drive system.  Since 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. 

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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, 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 improves overall efficiency by reducing the combined rotational speed of the electric motors.  This configuration reduces battery drain at highway speeds, adding up to 3.5 kilometres of additional EV range.

Volt’s motors and gear-set are mounted in-line with the industry’s first range-extending 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:

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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-kW of power and deliver 370-Nm of smooth, direct torque to provide spirited off-the-line acceleration.

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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.

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Single-Motor Extended-Range Driving
Once the battery has reached its minimum state of charge, the 1.4 litre engine is coupled to the motor/generator via the third clutch.  At lower speeds and hard accelerations 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 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.

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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.

Regardless of which drive mode is used, the Volt is always propelled with electric power delivered by the traction motor. The engine cannot propel the Volt unless the traction motor is also running.  In order for a planetary gear-set to transmit torque, at least one of the three main elements (ring gear, sun gear, or planet carrier) must be able to be locked or held.  Since there is no clutch to lock the sun gear, the traction motor must be used to provide the reaction torque needed for propulsion.

This unique propulsion architecture 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, Volt always delivers a solid electric driving experience, even in extended range driving, utilizing the large battery for full electric launches even during brisk accelerations.