Series vs. Parallel Hybrid Cars: What's the Difference

Hybrid automobiles employ a combination of electric generator output and old-fashioned gasoline engines for power.

Andy Stonehouse | 

Mar 21, 2024 | 4 min read

Toyota

With hybrid vehicles now well established in the automotive marketplace, consumers clearly understand the basics of the technology — or at least its ability to reduce fuel consumption when compared with a gas-only engine. As a middle ground between gas and electric, hybrids make motoring more energy efficient and environmentally friendly by pairing a traditional internal-combustion engine with an electric motor.

But not all hybrids use those components in the same way. The differences lie in how power is created and put into motion on the road. There are three types of hybrid systems, known as series, parallel, and series-parallel.

A Series Hybrid Motor Is Directly Connected to the Transmission

In a series hybrid, only the electric motor is directly connected to the transmission, making series hybrids more akin to fully electric vehicles than other types of hybrids. In fact, they're often referred to as extended-range electric vehicles.

Instead of directly powering the drivetrain, the gas engine serves as a generator that powers the electric motor and, in combination with regenerative braking, recharges the battery. The car's computer determines how much power comes from the battery pack and how much comes from the generator based on driving conditions.

The gas engine operates only when necessary. In slow or stop-and-go traffic, where series hybrids typically excel, the computer may opt to power the vehicle entirely by its battery pack, saving the gas engine for situations where it's the more efficient option.

The gas engine in a series hybrid is typically small since the electric engine does much of the heavy lifting. Series hybrids employ traction motors rather than an old-fashioned mechanical transmission and clutch, so they tend to be easier to manufacture, but the large and more complicated battery and motor can make them more expensive for consumers.

Series hybrid systems have been used in vehicles including the luxury Fisker Karma Revero electric sports sedan, as well as the first-generation Chevrolet Volt sold through the 2015 model year. The tech dates back more than a century, though gas-only powertrains proved decisive victors for most of the 1900s.

A Parallel Hybrid Motor and Engine Power the Drivetrain Together

In a parallel hybrid, the engine and motor work together — in parallel, as the name indicates — to power the drivetrain. A computer splits the power demands between the two systems as needed to optimize efficiency.

Because the engine is connected directly to the transmission, parallel hybrids don't have to convert the engine's mechanical energy into electrical energy and then back into mechanical energy through the motor, the way a series hybrid does. This improves efficiency during highway driving but reduces efficiency somewhat in stop-and-go traffic.

These vehicles typically use larger engines and smaller battery packs than series hybrids. They rely mostly on regenerative braking to recharge the battery, but when demands on the motor are low enough, it can also be used for charging, much like a conventional car's alternator.

A parallel hybrid cannot drive in fully electric mode.

Parallel systems were first offered to the mass market in Honda vehicles, with the Integrated Motor Assist system introduced in the Insight hybrid in 1999.

While conventional parallel hybrid systems have fallen by the wayside, the basic tech gave birth to through-the-road hybrid systems consisting of an additional electric motor on the axle not driven by the gas engine. Through-the-road hybrid systems in turn give all-wheel drive traction with the promise of improved fuel economy.

The Series-Parallel Hybrid Engine and Motor Can Work Independently

The series-parallel combination hybrid system, found on the Toyota Prius since 2001, allows the vehicle to operate entirely on either the gas engine or the generator/battery system, independently.

Series-parallel hybrids — also sometimes called power-split hybrids — offer the flexibility of both power sources working either independently or in conjunction with each other. That equates to added range and boost from the refillable gasoline engine, plus tailpipe emissions-free electric-only driving when possible, such as at lower speeds or under light-load cruising situations.

Andy Stonehouse

Andy Stonehouse literally fell into the world of auto writing while working as a ski-town journalist, and has not looked back since. A childhood spent dealing with the eccentricities of a 1976 MG Midget has made any subsequent auto experience a more safe and reliable drive. He has been blessed with nearby mountain trails and snowy roads in Colorado to do TV-adventure-styled test drives on a weekly basis.