Why don’t we see more of flywheel hybrids?

Motorsport has had an involvement with hybrid technology for more than the past five years. The Panoz Q9 from the late 1990s, often called ‘Sparky’, hardly set the world alight, but the technology was very immature compared to the level we are at now. In the mid-2000s, the FIA proposed that Formula One should incorporate kinetic energy recovery from 2009 so, from 2006 onwards, motor racing companies were working on finding ways to harvest energy under braking, store it until at was required and then deploy it when needed, either at strategic points to decrease lap time or just to keep the opposition at bay.

All sorts of schemes were considered, from conventional electric hybrids to flywheels and pneumatic systems. Eventually, everyone opted to run an electric system, although more than one team seriously investigated alternatives.

High-speed flywheels often have high energy densities, but have to run in a vacuum if frictional losses aren’t to be too great. The problem comes in transferring the energy out of the vacuum chamber and to a device such as a constantly variable transmission, which allows the energy to be smoothly deployed for propulsion. There are a few alternatives to this. The two commercially successful options, both initially developed for motorsport, are now both finding a wide range of applications including racing, advanced prototypes for mainstream automotive companies, and notably on public transport.

One solution is a special type of shaft seal, developed by a British company based at the Silverstone race circuit. Although it develops hybrid systems, it might be argued that its ingenious seal is its real treasure. It is simple in principle and allows the vacuum flywheel chamber to suffer very little leakage, so the vacuum is easily maintained with a very small pump that needs to run only rarely.

The second option, which has been used very successfully in endurance racing, is to run the flywheel as an integral part of an electric motor. Energy is stored and recovered electrically, therefore a high-speed rotating shaft does not need to cross the boundary between the vacuum chamber and the outside world.

There are other, less well-developed options that have yet to be successfully deployed in racing. One of these is magnetic gearing, where there is no need for a physical coupling between the shaft in the vacuum chamber and the output shaft, which runs in ambient conditions.

There is one definite advantages to flywheel hybrid systems, whether they are fully mechanical or electro-mechanical, and that is the fact that the storage medium is not a battery. The flywheels used in motor racing and for automotive hybrid development are made from simple materials that are well understood, namely carbon fibre-reinforced polymer composites and steel.

What is more, they are not prey to the various maladies that batteries are known to suffer from, such as finite lifetime, an inability to work over the full charge-discharge range without suffering shortened life or the ‘thermal runaway’ events that sometimes make the news when a lithium-ion battery goes wrong. On the other hand, with electrical systems nobody needs a patent to exploit them, and the technology is well understood by many companies.

Written by Wayne Ward