Series vs. parallel hybrids

Hybrid electric vehicles, or HEVs, are equipped with an internal combustion engine (ICE) and an electric motor. These two distinct power sources can work together in various ways to extend the range of the vehicle, reduce its emissions or act as a kind of electric turbo boost.

Depending on how the engine and the motor are connected, the HEV is either a series hybrid, or a parallel hybrid. We’ll look at each option in more detail below. You can think of the two systems as opposite ends of the hybrid spectrum – and the configuration does make a significant difference to the vehicle’s drivetrain in several ways.

What’s the Difference?

There are numerous differences in design between series and parallel hybrids. Some of the most common features of series hybrids include:

• A smaller engine
• A larger battery and electric motor
• The ICE powers a generator to run the motor

Compare this with the features of a parallel hybrid:

• A smaller motor and larger engine
• The electric motor acts as the generator
• The ICE is coupled directly to the wheels

However, there are some commonalities. Both series and parallel hybrids contain both an ICE and an electric motor, a rechargeable battery for the motor and a power inverter.

How does a Series Hybrid Work?

Just like in electronic circuits, ”series” means the hybrid drivetrain components are connected in a single line. The electric motor is ultimately responsible for driving the wheels, whereas the ICE runs a generator to charge the motor.

In a series hybrid, a controller in the vehicle’s transmission detects how much power the car needs at any one time, and will draw that power either from the ICE or the battery pack. Either way, the wheels are turned only by the electric motor and never directly by the ICE.

While driving, the car may switch between power sources at any time, often turning to ICE power as the charge remaining in the battery pack runs low. This means the car runs with fewer emissions but allows a seamless transition to petrol power when needed.

How does a Parallel Hybrid Work?

Whereas series means ”in a line”, parallel means ‘side by side”. In this configuration, the vehicle’s wheels can be driven by the motor, the engine, or both. The major difference is that the ICE is not only there to drive the electric motor, but can power the car directly.

There is still a controller in the car’s transmission, which decides when to use the ICE and when to run the electric motor. This decision is based on the circumstances at any one time, and is designed to ensure that the vehicle runs on the most appropriate fuel source for those conditions.

Parallel hybrids offer several capabilities to suit the manoeuvre that the motorist is trying to carry out:

• Immediate torque via electric motor when starting from a standstill
• Extra power from the ICE, e.g. for merging onto a motorway at speed
• Option to run the ICE as a generator to put charge back into the battery

A parallel hybrid doesn’t necessarily mean that the motor and ICE can be used simultaneously during acceleration. Instead, a parallel configuration may still require the transmission controller to choose only one power source at any given time.

Energy Recovery During Braking

Whether a hybrid EV uses a series configuration or a parallel setup, it can still be fitted with regenerative brakes, which recover some of the kinetic energy from the wheels under braking. This energy is passed back to the electric motor and, in turn, is used to recharge the batteries.

As a result, less energy is wasted when the driver needs to brake. Turning the motor helps dissipate the braking energy faster, which can help hybrids to brake sooner. By putting charge back into the battery, the max range of the car also increases – reducing the reliance on the ICE.

Which is more Efficient?

Series and parallel hybrids have some clear differences – so what does this mean for energy efficiency? In the real world, it comes down to the road and traffic conditions imposed upon the vehicle, so to decide between series and parallel, you need to think about the type of journey you make most often:

• Series hybrids are better for urban stop-start driving with less wear and tear
• Parallel hybrids are more efficient for steady speed, fast motorway driving

This is perhaps not surprising – the series configuration is more straightforward and less versatile. In contrast, the more complex parallel configuration can respond uniquely to the challenges of driving in high-speed traffic.

What is a Hybrid?

Vehicles like the Toyota Prius combine series and parallel drivetrains into a single car, making it a hybrid vehicle. Whereas a parallel hybrid, strictly speaking, must choose between the ICE or the electric motor at any one time, a series-parallel hybrid car can be powered by both simultaneously.

In the Prius, a gearbox connects the electric motor, generator and ICE into a single unit. This gearbox responds to the engine’s required power when the car is driven. As far as possible, the Prius runs from the electric motor and generator. But when extra power is needed, the hybrid configuration means that the ICE can contribute too, enabling the car to carry out high-power manoeuvres seamlessly.

The Future of Hybrids

HEVs were popular when all-electric BEVs could not offer the maximum ranges motorists needed, and recharging infrastructure was scarce and slow. With the advent of much larger battery packs and ultra-rapid charging stations, BEVs are becoming the dominant technology.

Once internal combustion engines are banned from the new cars market, hybrids – both series and parallel will also be consigned to the second-hand market. While they remain popular among many motorists, the transition to fully electric BEVs will likely spell the end for these innovative powertrains in the decades ahead.