The first all-new 2011 Chevrolet Volt electric vehicles with extended range were delivered to customers in the U.S. last month. The revolutionary five-door, four-passenger Volt provides the benefits of an electric vehicle without the range limitations associated with other electric vehicles in the market.
PROPULSION SYSTEM
The Volt is a one-of-a-kind, all-electrically driven vehicle engineered to operate in all climates. GM's Voltec propulsion system consists of a 16-kWh lithium-ion battery pack and electric drive unit that provide a pure electric range between 25 and 50 miles (40 to 80 km), depending on ambient temperature, terrain and driving technique.
When the energy level of the battery drops below the state-of-charge window, the Volt seamlessly transitions to extended-range mode. Power is inverted from a technically advanced, 1.4L 63-kW (84-hp) gasoline-powered onboard engine to the electric drive unit to provide up to 310 additional miles (500 km) of range on a full tank of fuel. When the car is plugged in and recharged, it runs again on a pure electric charge.
The electric drive unit's excellent low speed torque of 273 lb.-ft. (368 Nm) takes it from 0 to 60 mph in less than 9.0 seconds and the quarter mile in less than 17.0 seconds. The Volt can reach a top speed of 100 mph (160 km/h).
Drive Unit
Most battery electric vehicles use a single motor with a fixed reduction gear that steps down the motor speed to synchronize with the wheel speed.
The Volt uses an advanced, 111-kW (149-hp) electric drive unit to boost efficiency. (Fig. 2) The efficiency of any electric motor always drops off as it approaches its maximum rotational speed. The Volt drive unit consists of two electric 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.
Fig. 2
Towing of the Volt is to be done by a flatbed carrier only. Under no circumstances can the Volt be dinghy towed.
The Volt's motors and gear-set are mounted in-line with the industry's first range-extending internal combustion gasoline 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 gasoline engine to the generator/motor to provide range extension capability.
This innovative new drive unit has four distinct operating modes (two electric modes and two extended-range modes).
Electric Only, Engine Off (One Motor) - The vehicle operates in full-electric mode without starting the engine for approximately 25 to 50 miles. The primary traction motor provides all propulsion, drawing all of its energy from the battery.
Electric Only, Engine Off (Two Motors) - During highway cruising, if the required torque to propel the vehicle is low, the ring gear is coupled to the generator/motor. This allows the two motors to work in tandem for greater efficiency.
Electric Only, Engine On (Series) - Once the battery has reached a determined state of charge, the gasoline engine will start. It is coupled to the motor/generator via the third clutch. The engine-driven generator and battery provides electricity to the traction motor via the inverter. The engine will operate at an appropriate speed to optimize fuel consumption while maintaining output power requirements.
Because the most efficient way to charge the Volt's battery is to plug it in, the generator is used only to maintain minimum battery state of charge. If the battery is drawn down below the minimum level, the generator will charge the battery up to its minimum state of charge and then maintain it there.
Output Split, Engine On (Combined) - The blended two motor electric propulsion strategy used at higher speeds in EV driving also has been adapted for extended-range driving. During highway cruising, if the required torque to propel the vehicle is low, the clutches that connect the generator/motor to both the engine and the ring gear are engaged, combining the engine and both motors for efficient operation in an output power split configuration.
Engine Starting
Independent of the vehicle being plugged in or completely charged, when vehicle power is ON, the gasoline engine may start under the following conditions:
- Propulsion battery charge is low
- Propulsion battery temperature is hot or cold
- Cold ambient temperatures less than approximately 25° F (4° C)
This vehicle does not use a 12V starter motor to crank the gasoline engine. A much more powerful 300V motor/generator located within the transmission is utilized to crank the engine. The 300V drive motor/generator can rotate the engine to operating speed (800 rpm) within just a few hundred milliseconds. The 300V drive motor/generator allows near-instant starting of the engine.
HIGH VOLTAGE BATTERY
The Volt's high voltage battery consists of a 5.5-foot, 435-pound (198.1 kg) T-shaped, 16-kWh lithium-ion battery pack. (Fig. 3) The battery pack consists of nine modules and 288 prismatic cells.
Fig. 3
The lithium-ion high voltage battery pack holds its charge efficiently and has no memory effect (so it doesn't have to be run down completely before recharging). The owner should keep the vehicle plugged in, even when fully charged, to keep the battery temperature ready for the next drive.
Lithium-ion was chosen over nickel metal hydride for the Volt battery because it packs two to three times the power of a nickel metal hydride battery in a much smaller package. It also is more efficient and suffers little loss of charge when not in use.
The liquid cooled battery pack is located beneath the vehicle. The 360V direct current battery is formed from 288 cells. Each cell is 5 x 7 x 1/4 inch, and weighs about a pound. Cells are grouped together to form distinct blocks, referred to as sections.
The battery is connected to the propulsion circuit or charging circuit using unique pairs of contactors. In general terms, the main contactors will close when the ignition switch is in the ON position, and the charging contactors will close when plugged in with the ignition switch in the OFF position.
Because batteries can be sensitive to temperature changes, the Volt pack is climate-controlled via an exclusive active liquid control system that continually monitors and maintains the battery pack temperature for optimum performance and durability. Circulating liquid (consisting of coolant and distilled water) passes through a series of internal heat exchangers in the battery modules. It is designed to provide reliable battery operation when plugged in, at temperatures as low as -13° F (-25° C) and as high as 122° F (+50° C), and can be powered during driving either by the battery or the engine.
Fully charging or fully depleting a battery shortens its life. The Volt battery is controlled, or buffered, so it operates within a safe state-of-charge window. In demanding situations (such as driving in Mountain mode), the battery state of charge will raise the lower limit to ensure there is adequate power when needed. The battery's top and bottom buffer zones help ensure long life.
Charging
Charging the Volt's battery is simple and intuitive, and can be done through 120V conventional household electrical outlets, or through a dedicated 240V charging station. (Fig. 4)
Fig. 4
Charging Times
240V charging station:
Approximately 4 hours
120V household outlet:
Approximately 10 hours
Once the vehicle is plugged in, owners can schedule either immediate or delayed charges, even coordinating charging according to departure time or when electricity rates are lower. Owners also can manage and monitor the Volt remotely via computer on MyVolt.com; or with the OnStar MyLink smart phone application.
CHASSIS
The advanced design of the Volt delivers excellent ride and handling characteristics. (Fig. 5)
Fig. 5
Components of the Volt's MacPherson strut-type suspension are constructed of forged aluminum for reduced weight. Cold-formed, high-performance front springs are lighter and use less coiling than conventional springs.
A sophisticated compound crank rear axle provides quick reaction, low weight and consistent camber control. The cast iron control arms are attached to the steel cross-car beam with a patented magnetic-arc welding process.
The Volt features a ZF variable assist electric power steering system with dual pinions (one is used for steering, the other to add assist). A combined electric motor and sensing unit monitors steering angle, and delivers appropriate assist to the steering gear in all conditions. The system draws its power from a 12V battery in the rear of the vehicle.
The Volt's fully blended electro-hydraulic brake system enables 100% regenerative braking, 100% traditional disc friction braking, or any combination of both. Regenerative braking, which captures energy for transfer back to the battery, can provide up to 0.2g of deceleration (about 94% of braking events). Braking that involves more than 0.2g of deceleration is a combination of regenerative and friction braking. During a panic stop, the system seamlessly transitions to 100% friction. If the battery is fully charged, the brake controller directs the system to conduct friction braking.
Lightweight aluminum wheels weigh only 17.8 pounds (8.1 kg) each, compared to 24.2 pounds (11 kg) for typical 17-inch wheels. Goodyear Fuel Max all-season low-rolling resistance tires are optimized for electric vehicle range, noise, feel and performance. (Fig. 4)
DRIVER INTERACTION
There is much more technology inside the Volt than its electric propulsion system. Intuitive features -- high-resolution displays, connectivity and the ability to monitor and control vehicle functions remotely -- redefine how owners will interface with the vehicle. (Fig. 6)
Fig. 6
Key technologies include:
- High-resolution, seven-inch, full-color LCD reconfigurable Driver Information Center display and instrument cluster. The display shows electric-only range, fuel economy, extended-range, trip information, tire pressure information and a variety of vehicle messages.
- High-resolution, seven-inch, full-color, center stack-mounted touch screen display that serves as the interface for infotainment and cabin climate controls. An Efficiency (Leaf) button accesses power flow, charging, and energy information screens.
- Touch-control infotainment and climate control systems on the center stack.
- Remote Keyless Entry that allows drivers to remotely start the vehicle and precondition the cabin based on outside temperatures while the vehicle is plugged in, enhancing efficiency.
- An exclusive mobile app, powered by OnStar MyLink, which enables owners to engage with Volt functions using a smart phone.
- Thanks to Keith Newbury and Steve Falko
