While many automakers have turned to turbocharged diesel and hybrid-electric propulsion in recent years to improve fuel economy and cut emissions, several companies, including Porsche, are trying out a different track. Taking a page out of Formula One racing, they are testing electromechanical kinetic energy recovery systems (KERS), which were developed to enable racecars to store braking energy in spinning flywheels for later use during acceleration. Now, Volvo Car Corp. has joined their ranks.
“We’re exploring the potential of flywheel KERS for mass-market cars,” said Derek Crabb, Volvo's Vice President for Powertrain Engineering. “We plan to develop a complete system, including flywheel and transmission, for kinetic energy recovery.”
Volvo engineers view flywheel KERS as a lightweight and potentially inexpensive way to provide a horsepower boost “to give a four-cylinder engine acceleration like a six-cylinder unit, while improving fuel economy by up to 20%,” he said. Tests of a prototype system in a Volvo car on public roads are to get under way in the fall.
Benefits on the NEDC
The system, Crabb explained, is based on a carbon-fiber flywheel that spins at up to 60,000 rpm in an evacuated chamber. At that rotation rate, the flywheel’s rim travels at Mach 2 (1522 mph) so the wheel has to be contained within a vacuum to minimize friction. The flywheel unit, which weighs about 13 lb (6 kg) and has a diameter of almost 8 in (20 cm), would be linked to a differential through a compact continuously variable transmission (CVT) transmission.
When the vehicle is braked, the normally wasted energy is used to spin up the flywheel. When the driver hits the accelerator, the revolving flywheel can be coupled to the rear axle of the car to provide a quick power boost.
In the Volvo KERS concept, the internal-combustion engine that drives the front wheels is switched off as soon as braking begins. The flywheel’s rotational energy can be used to propel the vehicle from a full stop or to add power once it reaches cruising speed.
Compared with most other automotive energy-storage systems such as electrochemical batteries, flywheels offer less energy density but they are much more power dense. And since no energy conversion is required because the stored energy stays as kinetic energy, they can deliver that energy very rapidly. This feature makes the technology highly responsive.
“The stored energy is sufficient to power the car for only short periods,” Crabb noted. But its availability can have a major impact on fuel consumption. “Our calculations indicate that the combustion engine could be turned off for about half the time when driving according to the New European Driving Cycle.”
Cost advantages vs. PHEVs
The cost-shared joint program, which includes Swedish bearing supplier SKF, is being partially financed with a million-dollar grant from the Swedish Energy Agency. SKF is supplying the solid lubricant-based bearing and the all-important vacuum-chamber seal. Two U.K.-based firms are supplying key technologies. Flybrid Systems will provide the flywheel technology, while Torotrak will deliver the specially designed CVT technology.
“We’re assembling all these components to form a complete system that we can test under real-world conditions,” Crabb stated.
Flywheel-based hybrid assist technology is not new to Volvo. In the 1980s, company engineers tested a steel flywheel system in a Volvo 240. Crabb noted that while Volvo is not the first automotive OEM to test flywheel technology, “nobody else has applied it to the rear axle of a car fitted with a combustion engine driving the front wheels.”
The technology is attractive because “it is relatively cheap compared to a plug-in hybrid system” and can be applied to a wider range of vehicle sizes, he explained.
Crabb cautioned that the benefits of flywheel technology are still theoretical and high risk, but said “if the tests and technical development go as we hope, we think that cars with flywheel technology could reach showroom floors in a few years.”