The carmaker has fitted the technology to the rear axle of an S60 powered by a 254hp five-cylinder T5 petrol engine. Under braking, kinetic energy that would otherwise be lost as heat is transferred from the wheels to the KERS, and is used to spin a 6kg carbon fibre flywheel at up to 60,000 revs per minute. When the car starts moving off again, energy stored in the spinning flywheel is transferred back to the rear wheels via a specially designed transmission, and can either boost power or reduce load on the engine.

The combustion engine that drives the front wheels is switched off as soon as braking begins. The energy in the flywheel can then be used to accelerate the vehicle when it is time to move off again or to power the vehicle once it reaches cruising speed.

Volvo’s four-year trial of the technology has found that it can deliver an 80hp performance boost, together with fuel savings of up to 25%. The technology has also been found to be most efficient in city traffic.

‘The flywheel's stored energy is sufficient to power the car for short periods. This has a major impact on fuel consumption. Our calculations indicate that it will be possible to turn off the combustion engine about half the time when driving according to the official New European Driving Cycle,’ said Derek Crabb, vice president powertrain engineering at Volvo Car Group.

Although flywheel propulsion assistance was tested in a Volvo 260 back in the 1980s, and has recently been tested by other carmakers using flywheels made of steel, it has not proved a viable option. However Volvo has tested a Flybrid flywheel made with the combination of a steel hub and carbon fibre outer.

‘We are the first manufacturer that has applied flywheel technology to the rear axle of a car fitted with a combustion engine driving the front wheels. The next step after completing these successful tests is to evaluate how the technology can be implemented in our upcoming car models,’ concluded Derek Crabb.