Powrót starej technologii

When Andre Lotterer took the checkered flag at this year's Le Mans 24 race, the German motorsport driver made history.

Not content with powering Audi to its 11th win in the grueling endurance race, his car also became the first "hybrid" to take the top position at Circuit de la Sarthe. But what is perhaps even more remarkable is that his diesel hybrid car draws on a technology more often associated with 19th Century machinery than racing cars.

Inside his 200mph (320km/h) V6 racer was a device known as a flywheel, a rotating mechanical device, traditionally used to store energy from steam engines. They can be thought of as a mechanical battery. In their simplest form, they are just a heavy disc mounted on a shaft that store rotational energy. Spin the disc up, and it gains momentum and keeps spinning. Couple the spinning disc to something, say the powertrain of a Le Mans sports car, and it can be used for a quick energy kick.

"Under acceleration it is able to deliver that energy back to wheels and give an acceleration boost that ideally you use at the beginning of a straight or as you come out of a corner to reduce the lap time," says David Greenwood, head of hybrid and electric systems product group, at engineering firm Ricardo UK, who was not involved in the Audi system.

Coupled with their ability to reduce fuel consumption, flywheels are making a comeback. Now, they are being incorporated into everything from Formula 1 cars to city buses. And in a few years, they could be in your new saloon.

Sound barrier

Today, the most common type of hybrid system is a petrol or diesel engine paired with one or more electric motors or generators, along with a battery pack. When the vehicle coasts or when the driver brakes, a generator charges the cells. When the driver accelerates the same generator is run in reverse as an electric motor, and helps propel the vehicle forwards.

But battery packs are big, heavy, expensive and have a "low power density", meaning they store little power compared to the space they take up. Traditionally flywheels have also been big and bulky, but new materials mean they can be made smaller, able to spin faster. Crucially, they also have a high power density making them ideal for lightweight vehicles from passenger cars to track-bound speed machines.

In a vehicle, the flywheel is used to store energy when coasting or braking, by spinning the disc to ever faster speeds. When power is needed, a gearbox helps transfer it from the disc to the drive wheels. Unlike a battery-electric hybrid, the energy captured doesn't have to be converted from mechanical to electrical, and then to chemical. It is stored as mechanical energy, and fewer conversions means fewer losses.

The system used in Lotterer's Audi was at the cutting edge of today's flywheel designs. It was part of a so-called kinetic energy recovery system (Kers), designed by the Formula 1 team Williams. It is made of high-tech carbon composites and has no physical connection between the wheel and the power trains it drives - instead relying on an electrical connection. It is charged when the car brakes.

The technology was first developed by Williams for the 2009 Formula 1 season when Kers was first allowed. However, the Although it is still allowed, and used in some cars, many teams don't use it - preferring to use well understood battery alternatives.

In the meantime, Williams and other firms have set up divisions to push the technology towards more mainstream use. Greenwood's firm, for example, has built one that uses a carbon fibre disk about 30cm in diameter. It spins at up to 60,000rpm, meaning the edge of the disc is moving at about Mach 3, or three times the speed of sound.

"Clearly to do that we have to run the flywheel in a vacuum, or the air friction that we would get on the outside of the flywheel would cause very high energy loss," says Greenwood. "It would also heat up the composite to the point at which it would disintegrate."

To maintain the vacuum the flywheel is in a sealed chamber, and a magnetic coupling is used.

"We take all of the drive out with a set of magnets which sit on the outside of the flywheel casing, and because of the clever way that we have arranged the magnets you get a gear ratio between the two," he says.

"That means that although everything is rotating very fast inside the vacuum chamber, everything outside is rotating at relatively normal speeds."

That reduces any drag and air resistance. And because the flywheel is rotating so fast, the engineers do not need very much torque to get a very high power from it, which reduces the strain on the magnetic coupling.

Lighting up

One of the first mass uses of fly wheels might be the "Flybus" - not, as the name would suggest, an airport shuttle - but a bus fitted with a flywheel. Buses have a long history of using flywheels stretching back to the 1940s when Swiss firm Oerlikon introduced the Gyrobus. But early designs needed fixed charging points at bus stops and junctions to give the flywheel a boost. But, like with cars, the ability to recover energy from the braking process means they are now more viable. And because their routes are by design stop-start, energy recovery could improve fuel consumption.

"It's a technology which lends itself very well to working at very high power, but not needing to store energy for a very long time," says Greenwood.

It is a sector that Williams also has its eye on with its GyroDrive technology. It claims that a flywheel - either retrofitted or built in to a new bus - could save up to 30% of fuel, along with associated CO2. The firm also envisages the system being used to power onboard lights and electrical systems.

And once it has proven itself to be safe, reliable, cheap, and effective, the technology may then begin to trickle down into other use. Experts currently believe it is about five years until flywheel technology being incorporated into passenger cars. When it is, it could offer an improvement in fuel consumption of 12-15%, for about half the price of a conventional hybrid, not to mention the performance gains that are so attractive to motorsports teams.

Source: bbc.co.uk