Overview:

Full hybrids use a gasoline engine as the primary source of power, and an electric motor provides additional power when needed.

In addition, full hybrids can use the electric motor as the sole source of propulsion for low-speed, low-acceleration driving, such as in stop-and-go traffic or for backing up.

This electric-only driving mode can further increase fuel efficiency under some driving conditions.

Starting
When a full hybrid vehicle is initially started, the battery typically powers all accessories.

The gasoline engine only starts if the battery needs to be charged or the accessories require more power than available from the battery.

Low Speed
For initial acceleration and slow-speed driving, as well as reverse, the electric motor uses electricity from the battery to power the vehicle.

If the battery needs to be recharged, the generator starts the engine and converts energy from the engine into electricity, which is stored in the battery.

Cruising
At speeds above mid-range, both the engine and electric motor are used to propel the vehicle.

The gasoline engine provides power to the drive-train directly and to the electric motor via the generator.

The generator can also convert energy from the engine into electricity and send it to the battery for storage.

Passing
During heavy accelerating or when additional power is needed, the gasoline engine and electric motor are both used to propel the vehicle.

Additional electricity from the battery may be used to power the electric motor.

Braking
Regenerative braking converts otherwise wasted energy from braking into electricity and stores it in the battery.

In regenerative braking, the electric motor is reversed so that, instead of using electricity to turn the wheels, the rotating wheels turn the motor and create electricity. Using energy from the wheels to turn the motor slows the vehicle down.

If additional stopping power is needed, conventional friction brakes (e.g., disc brakes) are also applied automatically.

Stopped
When the vehicle is stopped, such as at a red light, the gasoline engine and electric motor shut off automatically so that energy is not wasted in idling.

All other systems, including the electric air conditioning, continue to run.

Source: www.fueleconomy.gov