Volvo evaluates flywheel hybrid drive - fuel savings of up to 25%

April 25, 2013 // By Christoph Hammerschmidt
In hybrid cars, several technologies are available to harvest excess kinetic energy; converting it into electricity and storing it a battery or supercap is currently the most popular approach. Kinetic flywheels offer an alternative. Carmaker Volvo has tested this technology under everyday driving conditions on public roads - and appears to be pleased about the outcome.

While flywheels appear somewhat outdated, there is a number of good arguments why they are at least competitive against electric solutions: Since there is no conversion loss when the kinetic energy is transformed into electric energy and back, the overall efficiency level can be very good. Volvo finds that the flywheel technology is a light, cheap and very eco-efficient solution.

"The results show that this technology combined with a four-cylinder turbo engine has the potential to reduce fuel consumption by up to 25 per cent compared with a six-cylinder turbo engine at a comparable performance level," says Derek Crabb, Vice President Powertrain Engineering at Volvo Car Group, "Giving the driver an extra 80 horsepower, it makes car with a four-cylinder engine accelerate like one with a six-cylinder unit."

The experimental system, known as Flywheel KERS (Kinetic Energy Recovery System), is fitted to the rear axle. During retardation, the braking energy causes the flywheel to spin at up to 60,000 revs per minute. When the car starts moving off again, the flywheel's rotation is transferred to the rear wheels via a specially designed transmission.

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.

According to Volvo expert Crabb, the flywheel's stored energy is sufficient to power the car for short periods. "This has a major impact on fuel consumption", Crabb said. "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 (NEDC)," explains Derek Crabb.

Since the flywheel is activated by braking, and the duration of the energy storage - that is to say the length of time the flywheel spins - is limited, the