Built at Hamtramck, Michigan, the U.S.-market Chevrolet Volt forms the basis for the forthcoming Opel Ampera. Only a few cosmetic and (unspecified) technical tweaks separate the two cars.
The Volt is a front-wheel-drive, four-seater ‘electric vehicle with extended-range capability’ — broadly a series-hybrid, though G.M. dislikes this description. In general terms, a series-hybrid is powered at all times by an electric motor; the battery-pack can be charged from the grid if necessary, but an internal combustion engine is fitted to act as a generator.
So the modus operandi of a series-hybrid is simple. For short trips, the car runs on battery power alone. For longer journeys, the battery-pack is replenished by an on-board combustion engine operating as a generator. When sufficient energy is available from the battery, the engine does not operate. During the electric-only phase of driving, the battery, motor, and power electronics are designed to deliver full performance on electric power alone. The car’s performance does not improve with the generator engine running, because it is not a part of the drivetrain.
The Volt’s battery-pack is a T-shaped 16kWh lithium-ion unit weighing roughly 180kg. The cells were developed and are manufactured by LG Chem; they are of the coffee-bag type. The ability of a cell to deliver high currents is defined by the electrode materials. The Volt’s battery uses carbon anodes and a manganese cathodes. The cells are assembled into modules, thence into full battery-packs, by G.M. at Brownstown Township, Michigan. The Volt’s generator is a 1.4-litre petrol engine.
Chevrolet Volt battery-pack. (Image: G.M.)
Battery modules are clamped to a battery tray and joined by flexible connectors. Relays and mechanical assemblies control the sub-assembly output voltage, temperature sensors monitor the battery coolant as it enters and leaves the pack, and manifolds and coolant lines allow for heat exchange with the cell surfaces. The connectors are designed to allow single-point entry and exit of high-voltage loads.
With a fully-charged battery and a full tank of fuel, the Volt has a driving range of over 300 miles. An electric range of 25-50 miles is quoted. As we would expect, the Volt’s braking system captures kinetic energy to recharge the battery-pack.
But a fully-charged battery is not something a user would normally encounter, because the generator engine never fully recharges the battery. This is principally because a lithium-ion battery-pack suffers long-term damage if it is persistently charged to its maximum capacity (or allowed to run very low). The pack operates more efficiently, and has a significantly longer operating life, if it works within a charge ‘window’ between around 30 per cent. and 70 per cent. charge. G.M. also states that charging the pack fully would be an unnecessary waste of petrol, though this does not in itself make sense across the unit’s life-cycle: it is battery life that is the issue in determining the maximum charge. If you’re set on charging the pack fully, you can do it by plugging the Volt into a power socket.
Although the design, development and manufacturing of the lithium-ion cells is farmed out to a specialist, the software and electronics that control the battery are the responsibility of General Motors. Similarly, the integration of the pack into the vehicle has been carried out by G.M. in Michigan.
Each lithium-ion cell measures roughly 130mm x 180mm x 7mm and weighs 450g. The fully-assembled battery-pack is around 1.7m long and fitted into a polymer-coated aluminium housing. A tab at the top of each cell is used to connect it in series with its neighbours.
Components for the control and monitoring of energy flows into and out of the battery-pack are housed within the pack.
General Motors promises that its future generations of battery-pack will offer
Lower cost, because of greater use of common parts;
Higher energy density and be more space-efficient;
Better cold-weather performance;
More efficient insulation and energy conservation;
Increased power performance.
Voltec electric drive system
General Motors has branded its series-hybrid drivetrain Voltec. It consists of two motors (a motor-generator and a primary traction motor), three clutches and a planetary gear-set. The efficiency of an electric motor drops off markedly as its rotational speed increases, so the epicyclic gearbox is used to reduce the shaft speeds of the electric motors at higher speeds. G.M. claims that the reduction gearing delivers two extra miles of E.V. range at ‘highway’ speeds — 50mph or so.
The Volt’s motors and gear-set are mounted in-line with the petrol engine. Two of the clutches are used to lock the ring gear of the planetary gear-set or to connect it to the motor-generator, depending on the driving mode. The third clutch connects the internal combustion engine to the motor-generator.
The drivetrain has four distinct operating modes:
Single-motor E.V. driving
In this mode, the primary traction motor provides all propulsion at lower vehicle speeds and under hard acceleration, drawing all of its energy from the battery. The ring gear is locked, and the motor-generator is decoupled from both the engine and the gear-set. Drive from the motor to the wheels is direct.
Two-motor E.V. driving
As vehicle speed increases, the ring gear is unlocked and coupled to the motor-generator. This allows the two motors to work in series.
Single-motor extended-range driving
Once the battery has reached its minimum state of charge, the generator engine is coupled to the electric motor-generator by way of the third clutch. At lower speeds and under hard acceleration, the Volt is propelled by the traction motor alone with the ring gear locked. The engine-driven generator and battery provide electricity to the traction motor through the inverter. The engine-driven generator will normally 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 per cent. charge — the generator will charge the battery up to its minimum state of charge and then maintain it there.
Two-motor extended-range combined driving
At higher speeds, the Volt uses what G.M. describes as a ‘blended two-motor electric propulsion strategy’. Here, the clutches that connect the motor-generator to both the engine and the ring gear are engaged, combining the petrol engine and both electric motors to transmit power through the planetary gear set. As with all other driving modes, the Volt is powered by electric traction — the petrol engine cannot propel the Volt directly. 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. The traction motor is used in this mode to provide reaction torque for the sun gear, enabling propulsion of the vehicle. By blending power from the engine and two electric motors through the epicyclic gearbox, the Volt achieves a claimed 10-15 per cent. improvement in efficiency at highway speeds compared with a drivetrain that uses only the single traction motor without reduction gearing.
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Structurally, the Volt is unremarkable: the monocoque uses galvanised steel front wings, bonnet, roof, door panels and one-piece bodyside outer panel. Wheel location at the front is by way of McPherson struts with side-loaded strut modules; at the rear, a double-walled, U-profile torsion beam is used.