What is it?

For the last four years, Volvo has been working with Flybrid to develop a kinetic energy recovery system (KERS) which can boost power and improve economy for its future vehicles, at a fraction of the weight and cost of an electric hybrid drivetrain.

How does it work?

The system comprises a carbon fibre flywheel encased in a vacuum to reduce resistance, and connected to the rear wheels via a continuously variable transmission (CVT). It weighs 70kg, significantly less than the Volvo V60 Plug-in Hybrid’s 300kg battery pack.

During deceleration, the CVT uses the flywheel’s mass to slow the rear wheels, while spinning the flywheel wheel itself up to 60,000rpm. Light braking is done with the fly- wheel, rather than the mechanical brakes, and the engine is switched off to save fuel.

The process is reversed during acceleration, slowing the flywheel by transferring its energy to the rear wheels. This allows the car to be driven short distances without using any fuel, or to reduce load on the engine, or provide an 80bhp power boost. Energy is recaptured and released more quickly than a hybrid battery, but can’t be stored and is used up in a few seconds under heavy driving.

Flybrid has designed the rear axle system to fit Volvo’s next-generation cars and its new Drive-E engines. Testing has included investigating whether all accelera- tion could be done using a very powerful flywheel, switching to a very small petrol or diesel engine while coasting.

What does it feel like?

The prototype S60 T5’s KERS was set up to be inactive below 40kph to demon- strate the power it adds under hard acceleration. Flywheel assistance kicks in like a large turbocharger, and two identical acceleration tests showed the sprint to 62mph reduced from 7.5 to 6.0 seconds with the system in operation. 

It was easy to maintain a 50-60% charge in normal driving, which provided a power boost long enough for most overtaking manoeuvres.

The claims:

Like an electric hybrid, the sys- tem can offer economy and performance-tuned modes, which means drivers can increase fuel economy by 25% while accessing a substantial power increase when needed.

How much does it cost?

In mass-production volumes, Flybrid claims the rear axle system could cost between a quarter and a third of the price of an elec- tric hybrid drivetrain. Com- pact versions can also be fitted between the engine and transmission at the front of the car, and these cost less than €1,000 (£830), the manufacturer added. In both cases the system is designed to last for a million kilometres (620,000 miles).