|A computer model of Volvo's flywheel, with an outer section cut away. Credit: Volvo|
In a flywheel system, energy from the wheels is used to spin a flywheel at high speeds. The flywheel continues spinning, storing energy until that motion can be transferred back to the wheels via a transmission. The idea isn't new, but it's hard to make flywheels efficient—a lot of energy can be lost to friction. In 1982, for example, GM engineered a flywheel system that was intended for its 1985 vehicles, but they canceled the project after discovering that the fuel efficiency improvements were less than half of what they'd expected. Advances in the technology now have automakers taking a second look. "Industry has gone from being skeptical to thinking it can be done, but there are enormous challenges," says Derek Crabb, vice president of powertrain engineering for Volvo.
Engineers who design Formula 1 race cars have tried to overcome the challenges of a flywheel system by using composite materials to save weight. To reduce friction, they've sealed the flywheels inside a vacuum chamber. In translating that system to passenger cars, automakers face the problem of how to maintain the vacuum, since the seals that connect the flywheel to a transmission aren't perfect.
This is fine in racing, where the system only has to last a couple of hours at a time, and can be overhauled by team mechanics. Consumer cars using a similar design would need a system to maintain the vacuum with pumps and valves—and that adds complexity and cost. In another approach, from the U.K. engineering firm Ricardo, the mechanical connection between the flywheel and the transmission is severed. Instead, energy from the flywheel is transferred to a transmission via magnets arranged around the circumference of the flywheel and in a ring outside the flywheel housing. By varying the ratio of the magnets in the flywheel to those arranged around it, it's possible to make the flywheel spin six times faster than the ring around it, which simplifies the transmission of energy.
One advantage of flywheel systems over batteries is their compact size. "Most hybrids with batteries provide a 15- to 25-kilowatt boost of power. The flywheel can deliver 60 kilowatts in a way smaller package," says Andrew Atkins, chief engineer of technology at Ricardo. The trade-off is that flywheels can't supply energy for very long.
Crabb says Volvo hasn't decided if it will use a system such as Ricardo's or something else to maintain the vacuum. Many challenges remain in bringing a flywheel hybrid to market. For instance, automakers will have to ensure that the systems can be durable, and can be manufactured on a large scale, he says. Flywheels will also have to compete with batteries and other electrical storage devices such as ultracapacitors, which are getting more powerful and less expensive. .