A hybrid electric vehicle (HEV) includes two means of moving the car: a conventional gasoline-powered internal combustion engine (ICE) plus an electric motor.
In a parallel hybrid like the Toyota Prius, both the ICE and the electric motor can be mechanically connected to the wheels through a planetary gear system.
When a full hybrid vehicle is first started, it is propelled by the electric motor using electricity from a battery bank. For extra power to accelerate the vehicle or to climb steep hills, the ICE is turned on. The car also has a generator connected to the ICE, so the ICE can recharge the battery under some driving conditions.
When the driver hits the brakes, the electric motor becomes a generator, converting braking energy that previously heated the brake pads of conventional friction brakes into electricity that charges the battery. This braking energy can then be used later to power the car....a process called regenerative braking, as shown in this DOE animation.
A gasoline hybrid has higher fuel economy than a regular car for several reasons:
The ICE on an HEV operates over a much smaller range of torque and RPM (revolutions per minute...the speed of engine rotation), remaining close to its “sweet spot” of high efficiency, whereas the ICE on a regular car must provide power at all torque and RPM rates required by the driver. The ICE efficiency is very low at low speeds and low torque.
The ICE is typically turned off when the car brakes are applied and whenever the car is moving slowly in congested traffic or stopped at a traffic light.
Energy is saved by reusing regenerative braking energy from the battery.
This higher fuel economy cuts oil consumption and pollution.
General Motors is now selling a series hybrid vehicle, the Chevy Volt. GM calls their vehicle an “ERev” for extended range electric vehicle. GM originally stated that there would be no mechanical connection between the ICE and the wheels in a series hybrid. The wheels were to be driven exclusively by an electric motor or motors. The ICE would be coupled mechanically to a generator that either recharges the car battery or provides electricity directly to the traction motor. The VOLT will also be a plug-in HEV, and GM is planning on a fuel cell version of the Volt, where a fuel cell replaces both the ICE and the electric generator. However, earlier press reports claimed that the Volt ICE is mechanically coupled to the wheels, much like the Toyota Prius, so the Volt was said to be a parallel hybrid, not a series hybrid as originally planned. However, a later press report (12/9/2010) corrected this misrepresentation, clarifying that the VOLT is a series plug-in hybrid, with the wheels powered only by electric motors; the confusion apparently arose because the electricity generated by the ICE/generator does sometimes bypass the battery and go directly to the motors to avoid the efficiency penalty of passing current into and out of the battery pack.