Regenerative brake (KERS Flywheel energy)

Description

A regenerative brake is an energy recovery mechanism which slows a vehicle or object by converting its kinetic energy into a form which can be either used immediately or stored until needed. This contrasts with conventional braking systems, where the excess kinetic energy is converted to unwanted and wasted heat by friction in the brakes. In addition to improving the overall efficiency of the vehicle, regeneration can greatly extend the life of the braking system as its parts do not wear as quickly.

The most common form of regenerative brake involves an electric motor as an electric generator. In electric railways the electricity generated is fed back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored chemically in a battery, electrically in a bank of capacitors, or mechanically in a rotating flywheel. Hydraulic hybrid vehicles use hydraulic motors to store energy in the form of compressed air.

Kinetic energy recovery systems (KERS) were used for the motor sport Formula One’s 2009 season, and are under development for road vehicles. KERS was abandoned for the 2010 Formula One season, but re-introduced for the 2011 season. By 2013, all teams were using KERS with Marussia starting use for the 2013 season. One of the main reasons that not all cars used KERS immediately is because it raises the car’s center of gravity, and reduces the amount of ballast that is available to balance the car so that it is more predictable when turning. FIA rules also limit the exploitation of the system. The concept of transferring the vehicle’s kinetic energy using flywheel energy storage was postulated by physicist Richard Feynman in the 1950s and is exemplified in such systems as the Zytek, Flybrid, Torotrak and Xtrac used in F1. Differential based systems also exist 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 energy of a flywheel can be described by this general energy equation, assuming the flywheel is the system.

Related formulas

Variables

ΔEsystemchange in energy of the flywheel (J)
Einenergy into the flywheel (J)
Eoutenergy out of the flywheel (J)