Current hybrid vehicles have a gasoline engine that is assisted by an electric motor, which helps it save gas. A typical hybrid vehicle conserves fuel a few different ways. It recaptures energy lost through braking and decelerating. This recaptured energy is stored in a battery pack as electricity. As demand warrants, the stored electricity is used to power an electric motor that assists the vehicle's gasoline engine--usually during acceleration.
While a conventional hybrid vehicle may travel short distances in pure-electric mode, plug-in hybrids are designed to travel extended distances with little or no assistance from the gasoline engine. Even before the charge is depleted, the gasoline engine may be called on to provide additional power for recharging the battery, accelerating, passing, and merging.
While operating on the additional plug-in charge, a plug-in hybrid more-or-less works the opposite of a conventional hybrid, with the electric motor acting as the primary power source, and the gasoline engine providing supplemental motivation. In the case of the Vue, once the initial charge is depleted, it would operate just as conventional hybrid does, using the gas engine. The Chevrolet Volt Concept is designed to use only its electric motor, using gasoline only to aid in battery recharging, not driving.
To keep vehicle weight inline with a conventional hybrid, plug-in hybrids would have little or no additional battery capacity. As such, the distance a plug-in hybrid will travel in pure-electric mode will be relatively modest. Saturn has suggested distances as great 40 miles, and as low as 20; GM estimates that the Volt Concept could travel an average of 40 miles per charge.
In the next section, we'll detail the benefits of plug-in hybrid cars and take a sneak peak at pure plug-in concepts from Chevrolet and Ford.