Cutting the cost of hybrid technology is a target for all auto makers, and Volvo Car Corp. (VCC) working with partner SKF believes its new system for kinetic energy recovery will achieve both, while delivering fuel and emissions savings of up to 20%.
Gearless traction drive technology specialist Torotrak has confirmed that its continuously variable transmission (CVT) forms a major element of Volvo Powertrain’s mechanical Flywheel KERS (Kinetic Energy Recovery System). With a 6.57 m kronor (US$877,000) grant from the Swedish Energy Agency, Volvo plans to become one of the world’s first OEMs to test the potential of flywheel technology—already in use in motor sport—on public roads.
Torotrak CEO Dick Elsy says of the development: “Using a Torotrak variable drive transmission in conjunction with a mechanical flywheel has demonstrated the capability for double-digit improvements in fuel economy.” And Derek Crabb, VCC’s Vice President Powertrain Engineering, adds: “If the tests and technical development go as planned, we expect cars with flywheel technology to reach the showroom within a few years.”
KERS powers the rear axle
The Volvo system, with its carbon-fiber flywheel energy recovery and storage system, has also been created to meet low mass targets. It could play a significant role in engine downsizing, giving a four-cylinder unit the signature of a six-cylinder, particularly with regard to pull-away performance and available torque at very low engine speeds.
Crabb said Volvo is aiming to develop what he terms a “complete system” for kinetic energy recovery and will be testing it on public roads this fall. The Volvo Flywheel KERS is fitted to the rear axle while an ICE drives the front wheels.
Brake energy is harnessed to spin up the flywheel to at least 60,000 rpm. As the car moves away again, the flywheel’s rotation is used to power the rear wheels and accelerate the vehicle. The system can also be applied, when appropriate, to use stored energy to power the car at cruising speed. The car’s ICE is stopped as braking starts.
Detailing the application’s effect, Crabb explained that the stored energy is sufficient to power the car for “short periods” but that that is enough to provide a very significant fuel consumption bonus. “Our calculations indicate that the ICE will be able to be turned off about half the time when driving according to the official New European Driving Cycle,” he said.
The system is particularly efficient in urban environments, during short journeys with the need for repeated stops and starts, and on winding roads with regular need for vehicle braking, Crabb claimed.
Projections show that when the Flywheel KERS is combined with the combustion engine’s full capacity, it will produce close to 60 kW (80.4 hp) with rapid torque buildup to provide markedly improved acceleration times.
Mechanical flywheel propulsion has been used in buses and trams, and several OEMs and specialist engineering companies have launched research projects. Porsche raced a KERS system in its 911 GT3 R Hybrid at the Nurburgring 24-hour and other events in late 2010 and continues development in the 918 RSR, which uses a Williams Hybrid Power flywheel.
'Insignificant' gyroscopic forces
Volvo first tested a flywheel system on a road car nearly three decades ago, but one of its penalties then was high weight. This also resulted in high gyroscopic forces. Engineers say the problems have been overcome with the use of carbon fiber, providing a mass of 6 kg (13.2 lb) for a flywheel diameter of 20 cm (7.87 in).
The flywheel, supplied by U.K.-based Flybrid Systems, spins in a vacuum. A flywheel rotating at 60,000 rpm or more can be made smaller and lighter than was possible some years ago, with gyroscopic forces reduced to a level that “can be considered insignificant,” engineers claim.
Power transmission is limited only by the capability of the CVT, according to the company. Even a mundane road car is capable of very high power transfer during braking, and the salient aspect of flywheel technology is to capture as much as possible of the resultant energy.
In 2009, Flybrid Systems and Magneti Marelli announced collaboration on electric KERS energy storage for motorsport applications. Called Flywheel Capacitor, it used a carbon-fiber, high-speed flywheel connected to an electric motor-generator using technology and control electronics from the Italian company.
Recovered energy during a braking event is stored into the capacitor by speeding up the flywheel. During vehicle acceleration, the stored energy is returned to the vehicle by transforming the kinetic energy of the flywheel into electric energy via the motor generator.
At the time of the announcement, the two companies stressed that the Flywheel Capacitor would not use chemical battery-based energy storage systems. They stated the initial version would have a specification of 60 kW power output and 600 kJ (569 Btu) total storage capacity, although specification could be tailored to specific vehicle requirements.
Electric motor and flywheel rotation would be up to 60,000 rpm with the flywheel positioned in an evacuated chamber that incorporated safety features. A small electric pump could top up the vacuum, thus avoiding any need for regular maintenance.
In 2009, the complete Flywheel Capacitor including associated electronics was projected to weigh about 20 kg (44 lb).
Volvo’s system, though, is purely mechanical. Says Crabb: “The flywheel technology is relatively cheap. It can be used in a much larger volume of our cars than top-of-the-line [hybrid] technology such as plug-in.”
Other OEMs flywheeling
Other companies are working along similar lines. Jaguar Land Rover (JLR) and Ford are part of a U.K. research consortium evaluating a system that attaches Flybrid’s flywheel and Torotrak’s CVT to a vehicle’s rear axle. Pete Richings, JLR Chief Engineer, Hybrids, emphasized the importance of exploring the potential for more efficient and cost-competitive hybrid drivetrains that improve fuel economy while enhancing standards of vehicle refinement and performance.
Engineering consultancy Ricardo is also working on flywheel technology with Torotrak among others. In 2009, it was announced that Ricardo was to lead the flywheel technology KinerStor project; Williams Hybrid Power was also involved. KinerStor's goals included the demonstration of significant fuel savings at a low system on-cost, providing a route for the installation of flywheel energy systems in high-volume, price-sensitive vehicles.