General Motors has revealed that the Chevrolet Volt extended-range electric vehicle (E-REV) can powered by no less than three motors in certain circumstances – and as a result, has struck controversy with its claim that the Volt is always electrically driven and not a hybrid.
GM has previously concealed the inner detail of the Volt's propulsion arrangements because, it says, it has been seeking a patent for its Voltec drive system. According to GM's director of hybrid powertrain engineering Barry Nitz, the system is "patent-protected now, so we can talk about it."
The additional motor in the system is the Volt's generator, which can also become a motor and operate in tandem with the gasoline engine under the heavy demands of high speed cruising, assisting the main traction motor that always drives the car.
The reason for bringing in a second electric motor is energy consumption – at high cruising speeds, the efficiency of an electric motor drops off, placing heavier demands on the battery. To overcome this, a planetary gearset blends the outputs of both the combined generator/motor and the gas engine, and the tractive motor. GM describes this as "indirect electro-mechanical drive."
As proof that it is not the combined generator and gas engine that are directly driving the wheels, the company points out that if the tractive motor were removed from the system, the Volt would not drive. On this basis, it argues that the main tractive motor is always the driver of the wheels, and for this reason its claim that the Volt is an extended-range EV still holds.
Whatever the semantics of the argument, the result, says Nitz, is a "10-15 percent improvement in fuel economy, and a mile or two of extra range" in pure EV mode when the gas engine is dormant.
The planetary gearset blends the outputs of the two electric motors, allowing the main motor's revs to drop from the 6,500rpm needed to maintain the Volt's maximum speed of 100mph to 3,200rpm, where it operates more efficiently, while the generator/motor runs at 1,500rpm. it is this ingenious arrangement that brings about the fuel savings, and it's these drive kinematics, says GM, that it described in a patent application made in autumn 2007. The desire to hide this system from competitors is why it has not explained more about the system up to now, according to the company.
GM has also revealed that the efficiency of the battery, developed in partnership with LG Chem, now allows the pack to be depleted by 65 percent rather than the 50 percent originally planned, this allowing an improved electric-only range of "25 to 50 miles," says Micky Bly, GM's director of global electrical systems. Up to now, GM has been claiming a maximum range of 40 miles for the Volt.
GM's eight-year, 100,000-mile battery warranty is unaffected by the increased battery depletion.