VOLVO has been conducting trials on UK roads of a new form of hybrid energy system which it says can cut fuel consumption by 25 per cent.
And it is doing it without relying on heavy batteries to store the energy.
Volvo has spent four years doing trials of a flywheel-system called Flybrid KERS, which both boosts power by up to 80hp while cutting fuel consumption by up to a quarter.
The Swedish car maker says the system is lightweight, financially viable and very efficient.
Volvo is the first full-scale trial of a rear-axle mounted flywheel system in a front-wheel-drive passenger car.
Jon Hilton, founder of Flybrid Automotive which is now part of Torotrak plc, said: "This car is a great demonstration of how close to market this technology could be, and how class-leading fuel efficiency can be combined with real performance and driver enjoyment."
The Flybrid KERS (Kinetic Energy Recovery System) is fitted to the rear axle of an S60 powered by a 254hp five-cylinder T5 petrol engine.
Under braking, kinetic energy which would otherwise be lost as heat is transferred from the wheels to the KERS, and is used to spin a six kilogramme carbon fibre flywheel at up to 60,000 revs per minute in a vacuum to reduce frictional losses. 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 petrol engine powering 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.
Alternatively, it can be used to power the vehicle once it reaches cruising speed.
Derek Crabb, Vice President of Powertrain Engineering at Volvo Car Group, said: "The flywheel's stored energy is sufficient to power the car for short periods.
"This has a major impact on fuel consumption."
Since the flywheel is activated by braking, and the length of time the flywheel spins is limited, the technology is at its most effective during a lot of braking and accelerating, making it ideal for city journeys.
The KERS drive to the rear wheels also offers the experimental car part-time four-wheel drive to add extra traction and stability under acceleration.
The system has been under development for years.
It was briefly considered by Formula One teams before being discarded.
This was partly because of the danger of having extra kinetic energy to dissipate during a high speed racing crash.