While flywheels appear somewhat outdated, there is a number of good arguments why they are at least competitive against electric solutions: Since there is no conversion loss when the kinetic energy is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 technology is at its most effective during driving featuring repeated stops and starts. In other words, the fuel savings will be greatest when driving in busy urban traffic and during active driving.
If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra 80 horsepower and, thanks to the swift torque build-up, this translates into rapid acceleration, cutting 0 to 100 km/h figures by seconds. The experimental car, a Volvo S60, accelerates from 0 to 100 km/h in 5.5 seconds.
Flywheel propulsion assistance was tested in a Volvo 260 back in the 1980s, and flywheels made of steel have been evaluated by various manufacturers in recent times. However, since a unit made of steel is large and heavy and has rather limited rotational capacity, this is not a viable option.
The flywheel that Volvo Cars used in the experimental system is made of carbon fibre. It weighs about six kilograms and has a diameter of 20 centimetres. The carbon fibre wheel spins in a vacuum to minimise frictional losses.
Volvo tests fywheel-based KERS technology /en/volvo-tests-flywheel-based-kers-technology.html?cmp_id=7&news_id=222901593