Kinetic Energy Recovery Systems

A kinetic energy recovery system (often known simply as KERS, or kers) is an automotive system for recovering a moving vehicle's kinetic energy under braking. The recovered energy is stored in a reservoir (for example a flywheel or high voltage batteries) for later use under acceleration. Examples include complex high end systems such as the Zytek, Flybrid, Torotrak and Xtrac used in Formula One racing and simple, easily manufactured and integrated differential based systems such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS).

Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating a continuously variable transmission (CVT). The CPC-KERS is similar as it also forms part of the driveline assembly. However, the whole mechanism including the flywheel sits entirely in the vehicle’s hub (looking like a drum brake). In the CPC-KERS, a differential replaces the CVT and transfers torque between the flywheel, drive wheel and road wheel.

The first of these systems to be revealed was the Flybrid. This system weighs 24 kg (53 lbs) and has an energy capacity of 400 kJ after allowing for internal losses. A maximum power boost of 60 kW (81.6 PS, 80.4 HP) for 6.67 seconds is available. The 240 mm (9.4") diameter flywheel weighs 5.0 kg (11 lbs) and revolves at up to 64,500 rpm. Maximum torque at the flywheel is 18 Nm (13.3 ftlbs), and the torque at the gearbox connection is correspondingly higher for the change in speed. The system occupies a volume of 13 litres. Already in 2006, a first KERS system based on supercapacitors has been studied at EPFL (Ecole Polytechnique Fédérale de Lausanne) in the framework of the development of the "Formula S2000". A 180kJ system has been developed in collaboration with other institutes (Multiphysic Modeling of a Hybrid Propulsion System for a Racecar Application, A. Rufer, P. Barrade, M. Correvon and J.-F. Weber, iamf EET-2008 : European Ele-Drive Conference, International Advanced Mobility Forum, Geneva, Switzerland, 11-13 March 2008).

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