FOR nearly as long as man has used wheels to turn all sorts of energy efficiently into motion, he has been performing the reverse trick with flywheels. From spindle whorls to steam engines, they have served to harvest and store energy for use in the (immediate) future. Now, they are gearing up to energise hybrid cars.

The physics of a flywheel is pretty basic. Take a disk that is free to rotate. Apply torque and it spins, gaining momentum in the process. Once the initial torque is taken away, the wheel will keep going. Some momentum is subsequently lost to friction on the bearings and to air resistance. Whatever remains can be put to work, powering whatever gubbins is connected to it.

A flywheel's momentum can be increased either by making it heavier or by getting it to spin faster. In the past flywheels used for more ambitious energy-storing purposes have tended to be bulky. That is because at speeds above several thousand revolutions per minute (rpm) the materials they were made of could disintegrate. This made them practical for applications in which size does not matter much, such as balancing loads across power grids. They have only found one widespread use in transport: on trains, where they propel some locomotives across gaps in the power rail. Typically, that calls for wheels one metre across, weighing over 100kg—not counting the hefty casket in which they are encased for safety reasons.

There have been attempts to use flywheels on big buses and trucks, but most involved devices which were only slightly less cumbersome. In smaller cars the extra weight negated any fuel-efficiency savings they might have brought. That, though, is finally beginning to change. One reason is that modern flywheels are increasingly being made of carbon fibre, a material much stronger than steel. This lets them whirr at over 60,000rpm without falling apart.

Carbon fibre is not just strong. It is also extremely light. But a flywheel's energy is proportional to its mass, and proportional to the square of its rotational speed. In other words, doubling the mass merely doubles the energy stored, but doubling the speed quadruples it. Increasing the speed makes it possible to ramp up a flywheel's energy-storage capacity while reducing its mass and size to something more manageable, says Andy Atkins, chief engineer at Ricardo, a British company working on flywheel technology.

Road tests have shown that, thanks to modern materials and clever design, a flywheel as small as a hockey puck can reduce fuel consumption by more than one-fifth. Some industry-watchers are even predicting that flywheels will promptly replace electric hybrids as the technology of choice for green vehicles.

The two approaches to energy saving have much in common. When you step on the brakes in a hybrid electric car such as a Toyota Prius, the electric motor that is used to drive its wheels runs in reverse as a generator, turning the car's kinetic energy into electrical energy that is stored in its on-board battery. Step on the accelerator, and this energy can be fed back to the motor, causing the car to pick up speed again. This is called “regenerative braking”.

Flywheels can do a similar thing by acting as a temporary store of energy. But they can do it much more efficiently. In electric hybrids only 35% of the kinetic energy lost during braking is retrievable. With flywheels more than 70% is, according to Dick Elsy, boss of Torotrak, another British firm working on the technology. That is because regenerative braking converts kinetic energy into electrical energy, and then into chemical potential energy in the battery. Flywheels, by contrast, merely turn one sort of kinetic energy (of the wheels) directly into another (of the flywheel), which is far less wasteful.

Moreover, unlike batteries, which need to be replaced every few years, flywheels are designed to last the lifetime of the vehicle, and contain no nasty chemicals which need to be disposed of. Small wonder, then, that carmakers have at last begun to sit up and take notice.

Origins of a revolutionary idea

As is often the case with newfangled technologies, high-tech flywheels debuted in Formula 1 cars. In 2009 the Fédération Internationale de l'Automobile (FIA), the sport's governing body, permitted teams to deploy kinetic-energy-recovery systems (KERS) in cars. The move was intended in part to show that petrolheads, too, can embrace environmentally friendly technologies. But it also injected new vim into rivalry on the track, letting drivers boost acceleration in short bursts when trying to overtake or avoid being overtaken.

Some teams plumped for battery-based KERS akin to those in electric hybrids. A few, though, including Williams, chose to work with flywheels instead, giving the technology's image a makeover. No longer synonymous with lumbering buses and locomotives, flywheels were modern and high-tech. Firms like Volvo and Jaguar are now testing them for use in ordinary cars.

Flywheel hybrids: A prototype Jaguar XF It is not just technological advances that make flywheels attractive. The changing economics of the car industry also plays a part, explains Derek Crabb, a senior engineer at Volvo. The Swedish carmaker toyed with the idea of introducing flywheels in its models in the 1980s. But the technology was immature and reliant on heavy steel, and there was little economic incentive to pursue it, says Mr Crabb. Back then fuel was cheap and no one cared about carbon-dioxide emissions, agrees Mr Elsy. No longer. On one hand, cash-strapped consumers fretting about rising petrol prices increasingly want cars with decent fuel economy. On the other, legislation is forcing carmakers to cut their fleets' average carbon emissions.

and the Hope Racing LMP1 Meeting those demands with hybrid electric vehicles alone would be costly, argues Jon Hilton, co-founder of Flybrid Systems, a British maker of KERS technology which has been working with the car industry to make flywheels available for ordinary cars. Batteries, electric motors and complicated control systems are expensive, adding around €6,000 ($8,000) to the manufacturing cost of a car, says Mr Hilton. “All carmakers lose money on electric hybrids,” he says. “That's why they don't make many.” A flywheel system, by contrast, costs €1,500. Jaguar has already road-tested a prototype of its XF saloon featuring a Flybrid energy-storage system, and Volvo says it hopes to have a prototype by the end of the year. Technical hurdles remain, however. For a start, at 60,000rpm the flywheel's outer rim is moving at around twice the speed of sound, or over 2,000kph (1,200mph). Under such conditions, any air resistance would cause even carbon fibre to disintegrate. So the flywheel has to spin in a vacuum. That requires transmitting the kinetic energy from outside the vacuum-chamber to the inside, which is no mean feat. Flybrid's solution is to have a rotating seal. It is not fully impermeable, letting tiny quantities of air leak into the chamber. However, a small vacuum pump evacuates any stray particles automatically before they reach a dangerous level; it needs to run only once a day for about 90 seconds. Ricardo has taken a different tack. To avoid the added bulk and complexity of a vacuum pump, its Kinergy system operates its flywheel in a complete vacuum, using arrays of permanent magnets: one array in the flywheel's shaft, the other in a second, external shaft connected to the transmission. As one shaft turns its magnetic field meshes with that of the other shaft like cogs of a gear. Then there is the problem of the flywheel's constantly changing rotational speed. Torotrak solved this problem by devising a version of continuously variable transmission, used in gearless electric motors. Instead of being locked solidly together, the flywheel and transmission shafts are coupled using a viscous fluid which allows them to rotate at different rates but gently brings them into sync. This approach has proven very efficient at transferring energy, says Mr Elsy.

Finally, despite their assorted advantages over battery-based KERS, flywheels do have some drawbacks. They cannot store energy for as long as batteries. Nor can they store as much. Mr Hilton admits that the Jaguar prototype would go no farther than about half a mile on flywheel power alone.

That need not be a problem, though, argues Mr Crabb. Flywheels can save fuel indirectly, by providing a speed boost when needed. As a consequence, engines can be smaller and less gas-guzzling without sacrificing performance. When Volvo rolls out its first commercial flywheel hybrid, around 2015, its KERS should be capable of providing an 80-horsepower boost when overtaking, like in Formula 1.

“Flywheels could be a cheap way to make cars more fuel-efficient.”

Further down the line, radical flywheel designs could provide even more additional oomph. Ricardo has simulated a flywheel capable of spinning at a whopping 145,000rpm. This doughnut-shaped flywheel would reach such breakneck speeds by concentrating most of its mass in the rim, held in place with a thin carbon-fibre envelope, leaving most of the flywheel's disk hollow. The reduction in flywheel mass is more than made up for by zippier rotation. The design has yet to be tested in the real world—and might not be any time soon. So far there is little demand for this sort of performance, Mr Atkins says.

No matter how fast they rotate, though, flywheels will not drive out electric hybrids altogether. Flywheel-based designs cannot compete on range with plug-in hybrids, which have large battery packs and can travel long distances on electric power alone, says Mr Hilton. But the market for plug-in hybrids is in its infancy. Flywheel hybrids could provide a cheap way to make cars more fuel-efficient until the technology of electric vehicles matures. Firms like Volvo are hard at work designing future cars to be flywheel-ready. When these hit the road they should be no dearer than a run-of-the-mill petrol-powered car. Frugal drivers will no doubt rush to take one for a spin.