Kinetic Energy Recovery (KERS) Hybrids in Development at Volvo

Kinetic Energy Recovery Systems, or KERS, were developed for ‘greener’ Formula 1 race cars. Several builders of high performance cars like Ferrari, Maserati, Porsche, and Lotus are now developing KERS for future hybrid sports cars. Volvo is also developing next-generation KERS technology for road-going cars to be sold in much larger numbers.
Unique to the Volvo Flywheel KERS is its installation only on the rear axle. When slowing down, braking energy spins up the flywheel. The engine, which drives the front wheels, is switched off when braking starts. As the car starts moving again the flywheel's rotational energy is transferred to the rear wheels using a specially-designed transmission. The flywheel's energy accelerates the vehicle or provides power at cruising speeds.
Energy stored in the flywheel is sufficient to power the car for short periods. Even so, this has a major impact on fuel consumption, offering up to 20 percent fuel savings. Because the duration of the energy storage – that is, the length of time the flywheel spins – is limited, the system is most effective when there are repeated stops and starts such as in busy urban traffic. This is the case with most hybrid systems.
When flywheel energy is added to the engine's full output, the car gets an extra 80 horsepower. This would allow downsized engines without loss of performance. Indeed, because of the rapid torque build-up from the flywheel, acceleration would be significantly enhanced, allowing a four-cylinder engine to provide the acceleration of a six-cylinder.
Volvo did test a flywheel system in a Volvo 240 back in the 1980s. However, the flywheel was made of steel and had a large diameter, so rotational speed was limited. Now, Volvo will use a 20 centimeter diameter carbon fiber flywheel that weighs about 13 pounds. The Volvo KERS carbon fiber flywheel will spin at up to 60,000 rpm in a vacuum to minimize frictional losses.
Testing of its Flywheel KERS should get under way this year if technical development goes as planned. Volvo says that cars with KERS technology could appear in dealers’ showrooms within a few years.
Volvo also notes that flywheel technology is relatively inexpensive and could be used in much larger number of Volvos. Instead of converting braking kinetic energy into electricity and storing it in an expensive battery pack, energy is stored in a lower cost, high-speed flywheel with power transfer controlled by a compact continuously variable transmission, reportedly to be supplied by Torotrak.