• HEV Types

    The type of HEV is determined by how the powertrain propels the vehicle down the road and may be considered either series, parallel, or series-parallel.

  • Hybrid Drivetrain Animation

    Click on the thumbnail above to see a hybrid drivetrain animation courtesy of the Hybrid Center,
    a project of the Union of Concerned Scientists.

  • Video shows how hybrid vehicles work, featuring the 2015 Hyundai Sonata Hybrid.

    Video explains the differences between a series hybrid (extended-range) and plug-in hybrid (PHEV)


    Energy Flow Diagrams and Descriptions

    Series Hybrid diagram

    Series (Extended-Range) Hybrid

    A series hybrid is like a battery electric vehicle (BEV) in design. Here, the combustion engine drives an electric generator instead of directly driving the wheels. The generator both charges a battery and powers an electric motor that moves the vehicle. When large amounts of power are required, the motor draws electricity from both the battery and the generator. Series hybrids may also be referred to as extended-range electric vehicles (EREVs) or range-extended electric vehicles (REEVs) since the gas engine only generates electricity to be used by the electric motor and never directly drives the wheels. Modern examples include the Cadillac ELR, Chevrolet Volt, and Fisker Karma.

    Parallel Hybrid diagram

    Parallel Hybrid

    A parallel hybrid is propelled by both an internal combustion engine (ICE) and an electric motor connected to a mechanical transmission. Power distribution between the engine and the motor is varied so both run in their optimum operating region as much as possible. There is no separate generator in a parallel hybrid. Whenever the generator's operation is needed, the motor functions as generator. In a parallel mild hybrid, the vehicle can never drive in pure electric mode. The electric motor turns on only when a boost is needed. 

    Series Parallel Hybrid diagram


    The vehicle can be powered by the gasoline engine working alone, the electric motor by itself, or by both energy converters working together. Power distribution between the engine and motor is designed so that the engine can run in its optimum operating range as much as possible.

    Note: All configurations above may use more than one motor to drive the vehicle.