How do hybrid cars work?
Most popular hybrid cars produced by Toyota, Ford and Lexus use two motors: an electric motor and an internal combustion engine. Each engine can individually drive the car or their power can be combined. For example, at some point in time 40% of the power can be from the electric motor and 60% can be from the combustion engine.
When a car starts or moves slowly it normally operates on electric power alone. The maximum speed and distance over which electric-only operation is possible varies, but most cars currently set it to 30 mph and 1 mile. On the highway the primary power source is the internal combustion engine. When maximum power is required, for example to overtake, the electric motor is used to assist maximizing the available power for a short period.
Hybrid control software shuts the engine off while stopped at traffic signals and automatically restarts it again when the driver releases the brake pedal. Eliminating the fuel waste of an idling gas engine causes overall mpg to climb significantly.
Regenerative braking is used to generate electricity to recharge the battery. It absorbs a portion of the vehicle's momentum when slowing or coasting downhill. Normal cars waste all of their excess momentum as heat in the brakes. Regenerative braking is insufficient to stop a car quickly, so conventional hydraulic brakes are still necessary.
If the on-board batteries fall below a certain level of charge, the gas engine will automatically turn on to charge the batteries and run the electrical motor if needed. That's why a standard hybrid car never needs to be plugged into an electrical outlet. (Unless you actually want it and bought a plug-in hybrid).