What is a Series Hybrid, and How Does it Work?

Jessica Shea Choksey | Aug 17, 2021

Hybrid vehicles have been around for a while now, and many people have become familiar with how they operate. These vehicles combine internal combustion engines (ICE) with electric motors to achieve greater fuel efficiency than cars that run on gasoline alone.

2019 BMW i3 120Ah Powertrain Cutaway Euro spec

Hybrid vehicles are classified as either series or parallel based on their drivetrain configuration—the system of components that delivers propulsion from a vehicle's power source (engine or electric motor) to the vehicle's wheels. Of the two, series hybrid systems are the most straightforward in terms of design and operation.

Hybrid Drivetrain Classifications

There are three main types of hybrid drivetrain configurations: series, parallel, and series/parallel (a combination of the two). Understanding how a series hybrid works entails defining the fundamental differences between the three drivetrain classifications:

  • Series hybrids send power to the wheels solely from the electric motor, which draws energy from a battery and an ICE-powered generator.
  • Parallel hybrids can simultaneously send power to the wheels from the electric motor and ICE for combined output; the two power sources result in a parallel setup.
  • Series/parallel hybrids allow the electric motor and ICE to provide power independently or in conjunction for combined output.

How Does a Series Hybrid Work?

The components of a series hybrid system include an electric motor, a conventional gasoline engine, a battery, and a generator. A computerized electronic control unit (ECU) manages energy flow.

A series hybrid design is much like a battery electric vehicle (BEV) setup, except for the presence of a gasoline engine. In a series hybrid, the electric motor is the only source of propulsion to the wheels. The ICE does not send any power directly to the wheels.

The electric motor receives its power from two sources: the battery pack and the ICE-powered generator. An ECU determines whether power comes from the battery or the engine/generator. When large amounts of power are required, the motor may draw electricity from both the battery and the generator simultaneously. Each source's energy output will vary as the computer optimizes power flow to achieve maximum efficiency.

In this system, the generator does two things: charges the battery and powers the electric motor. The battery pack also recharges by way of regenerative braking.

Although series hybrids are less prevalent than parallel hybrids, some well-known models include the Karma Revero (formerly Fisker Karma) and the BMW i3 with range extender. The Chevrolet Volt (now discontinued) also operates as a series hybrid vehicle but includes a mechanical link from the engine to the wheels, which engages at highway speeds.

Advantages

Since the gasoline engine in a series hybrid does not drive the wheels, it is subject to less power demand and does not have to operate through a wide rev range. This means the engine is typically smaller and more efficient than engines found in conventional gas-powered vehicles and parallel hybrids. This characteristic also makes series hybrid vehicles more suited for stop-and-go conditions and thus better for in-town driving where traffic lights and stop signs are commonplace.

Furthermore, the greater reliance on battery electric power and the smaller gasoline engine results in fewer carbon emissions, making series hybrid vehicles more eco-friendly overall.

Disadvantages

Series hybrid vehicles, equipped with smaller, less powerful engines, need more potent battery packs to make up for the remaining power required to drive the wheels. This larger battery and electric motor make series hybrid vehicles more costly to produce than parallel hybrids. That higher cost is passed on to consumers.

Since the engine does not connect directly to the wheels, series hybrids do not have the option to draw extra power if desired. Series hybrids are also not as efficient as parallel hybrids in highway driving. The most popular hybrids—Chevy MalibuFord Fusion, and Toyota Prius—are parallel hybrid vehicles for these and cost reasons.

Summary

A hybrid vehicle improves mileage, burns less gas, and reduces carbon emissions. A series hybrid accomplishes these things with a simple design and straightforward operation. Whether it's the right kind of hybrid for a buyer depends on their budget and the type of driving they plan to do.

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