How Does a Hybrid Electric Vehicle Work?

Hybrid electric vehicles are powered by two energy sources—an energy conversion unit (such as an internal combustion engine or fuel cell) and an energy storage device (such as batteries or ultracapacitors). The energy conversion unit can be powered by gasoline, diesel, compressed natural gas, hydrogen, or other fuels.
Hybrid electric vehicles have the potential to be two to three times more fuel-efficient than conventional vehicles. 

Hybrid Electric Vehicles

Animation of a vehicle with a battery, electric motor, and internal combustion engine inside and an icon depicting conventional or alternative fuel outside.

Fuel-Efficient Vehicle

Most hybrid electric vehicles have an internal combustion engine and electric motor. These vehicles are powered by an alternative fuel or a conventional fuel, such as gasoline, and a battery, which is charged by regenerative braking.

Hybrid electric vehicles (HEVs) are powered by an internal combustion engine or other propulsion source that can be run on conventional or alternative fuel and an electric motor that uses energy stored in a battery. HEVs combine the benefits of high fuel economy and low emissions with the power and range of conventional vehicles.
A wide variety of hybrid electric vehicles is currently available. Although HEVs are often more expensive than similar conventional vehicles, some cost may be recovered through fuel savings.

Help from an Electric Motor

Hybrid electric vehicles are powered by an internal combustion engine and an electric motor, which uses energy stored in batteries. The extra power provided by the electric motor allows for a smaller engine. Additionally, the battery can power auxiliary loads like sound systems and headlights and reduce engine idling when stopped. Together, these features result in better fuel economy without sacrificing performance.

Regenerative Braking

A hybrid electric vehicle cannot plug into off-board sources of electricity to charge the battery. Instead, the vehicle uses regenerative braking and the internal combustion engine to charge. The vehicle captures energy normally lost during braking by using the electric motor as a generator and storing the captured energy in the battery. The energy from the battery provides extra power during acceleration.

Fuel-Efficient System Design

HEVs can be either mild or full hybrids, and full hybrids can be designed in series or parallel configurations.

• Mild hybrids—also called micro hybrids—use a battery and electric motor to help power the vehicle and can allow the engine to shut off when the vehicle stops (such as at traffic lights or in stop-and-go traffic), further improving fuel economy. Mild hybrid systems cannot power the vehicle using electricity alone. These vehicles generally cost less than full hybrids but provide less substantial fuel economy benefits than full hybrids.
• Full hybrids have more powerful electric motors and larger batteries, which can drive the vehicle on just electric power for short distances and at low speeds. These systems cost more than mild hybrids but provide better fuel economy benefits.

There are different ways to combine the power from the electric motor and the engine. Parallel hybrids—the most common HEV design—connect the engine and the electric motor to the wheels through mechanical coupling. Both the electric motor and the internal combustion engine drive the wheels directly.