Public-road testing by Volvo Car Group of its flywheel technology has shown that when coupled with a four-cylinder turbocharged engine it has the potential to reduce fuel consumption by up to 25% compared with a six-cylinder turbo engine at a comparable performance level. The KERS (kinetic energy recovery system) is fitted to the rear axle. The carbon-fiber flywheel spins in a vacuum at up to 60,000 rpm, and the kinetic energy is released through a specially designed transmission back to the wheels. The flywheel weighs about 6 kg (13 lb) and has a diameter of about 20 cm (7.9 in). "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," said Derek Crabb, Vice President - Powertrain Engineering, Volvo Car Group. The research is being carried out with SKF.