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PHOTO CAPTION: Close up view of KERS flywheel in Porsche 918 hybrid race car.

Will Flywheels Finally Find Their Way Into Hybrids?

Flywheels are gradually finding their way into high performance racers, but will they someday also be available in more conventional vehicles?

Published: 29-Dec-2011

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.

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