The use of flywheels has yet to be commercially successful in production vehicles. Decades ago they were tried as an energy source powered up before the vehicle went on its way. That proved expensive, heavy and sometimes dangerous but the technology has reappeared as mechanical storage of braking energy and therefore potentially useful in both pure electric and hybrid electric vehicles. For example, in 2011, the Chinese company Volvo is pursuing Formula 1-style Kinetic Energy Recovery System KERS flywheel technology as a means to boosting eco-efficiency in its V70 hybrid family cars. A 20% improvement in fuel consumption is claimed to be possible and it should also make a "four-cylinder feel like a six-cylinder" - enhancing the driving experience while saving the planet, says the company. This is at the top end of claimed savings from regenerative electrical braking typically by allowing AC electric traction motors to work in reverse as brakes because now power conversion is involved.
The Volvo flywheel KERS is only 20cm across and fitted to the rear axle of the front wheel drive vehicle. Braking energy is transferred to this device, causing it to spin at up to 60,000rpm, storing the energy that would otherwise be lost. This energy can then be released to help the car accelerate again, bringing potentially significant economy gains in both stop-start - i.e. city-centre - and more spirited driving. It will be synchronised with shutting down the conventional combustion engine under the bonnet when decelerating to maximise fuel economy. The energy release from the flywheel is expected to be equivalent to an extra 60 kW (80hp for the nostalgic), accompanied by a "swift torque build-up", meaning rapid acceleration and lower 0-62mph times. Volvo's system differs in the way the flywheel is attached directly to the rear axle to drive the rear wheels, while the front wheels remain driven by a regular combustion engine. Volvo is also using carbon fibre to construct the flywheel, rather than conventional steel. This increases the speed of rotation while also keeping weight down to around 6kg. The carbon fibre wheel spins in a vacuum to minimise frictional losses. Volvo has high hopes of an imminent real world impact - and to see the flywheel KERS in production within the next few years.
A similar system is used on the Porsche 911 GT3 R Hybrid racing car, which took its first outright race victory in mid 2011 and other carmakers have been evaluating the technology in recent years, but Volvo will be amongst the very first to test it on the public road. Vice President Volvo Car Corporation Powertrain Engineering, Derek Crabb enthuses:
"The flywheel technology is relatively cheap. It can be used within a much larger volume of our cars than top-of-the-line technology such as the plug-in hybrid. This means that it has potential to play a major role in our CO2-cutting DRIVe Towards Zero strategy."
'DRIVe Towards Zero' is Volvo Cars' vision for developing cars entirely free from harmful exhaust emissions and environment-impacting carbon dioxide.
Certainly there is still scope for mechanical features in electrical vehicles. At the forthcoming Electric Vehicles Land Sea Air in Stuttgart 28-29 June, Antonov will present on how its gear trains designed specifically for electric vehicles improve efficiency and performance for example. Indeed Torotrak has confirmed that its continuously variable transmission (CVT) technology forms part of the Volvo Car's KERS project.
For more read, Energy Harvesting for Electric Vehicles 2011-2021 .
