A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another 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 heat by friction in the brake linings and therefore wasted. The most common form of regenerative brake involves using an electric motor as an electric generator.
In electric railways the generated electricity is fed back into the supply system, whereas in battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use. Energy may also be stored mechanically via pneumatics, hydraulics, or the kinetic energy of a rotating flywheel.
Another form of simple, yet effective regenerative braking is used on the London Underground which is achieved by having small slopes leading up and down from stations. The train is slowed by the climb, and then leaves down a slope, so kinetic energy is converted to gravitational potential energy in the station.
Early examples of this system were the front-wheel drive conversions of horse-drawn cabs by Louis Antoine Krieger (1868–1951). The Krieger electric landaulet had a drive motor in each front wheel with a second set of parallel windings (bifilar coil) for regenerative braking. In England, the Raworth system of ‘regenerative control’ was introduced by tramway operators in the early 1900s, since it offered them economic and operational benefits. Slowing down the speed of the cars or keeping it in hand on descending gradients, the motors worked as generators and braked the vehicles. The tram cars also had wheel brakes and track slipper brakes which could stop the tram should the electric braking systems fail. Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911. Twenty years later, the regenerative braking system was reintroduced.
Regenerative braking has been in extensive use on railways for many decades. The Baku-Tbilisi-Batumi railway in Georgia started utilizing regenerative braking in the early 1930s. This was especially effective on the steep and dangerous Surami Pass. In Scandinavia the Kiruna to Narvik railway carries iron ore from the mines in Kiruna in the north of Sweden down to the port of Narvik in Norway to this day. The rail cars are full of thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts of electricity by their regenerative braking. From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border. Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity.
An Energy Regeneration Brake was developed in 1967 for the AMC Amitron (a compact concept car). It was a completely battery powered urban car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile. Many modern hybrid and electric vehicles use this technique to extend the range of the battery pack. Examples include the Toyota Prius, Honda Insight, the Vectrix electric maxi-scooter, the Tesla Roadster, the Nissan Leaf, and the Chevrolet Volt.
Traditional friction-based braking is used in conjunction with mechanical regenerative braking for the several reasons: The regenerative braking effect drops off at lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. Physical locking of the rotor is also required to prevent vehicles from rolling down hills. The friction brake is a necessary back-up in the event of failure of the regenerative brake. Most road vehicles with regenerative braking only have power on some wheels (as in a two-wheel drive car) and regenerative braking power only applies to such wheels because they are the only wheels linked to the drive motor, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels. Also, the amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery or capacitors. Regenerative braking can only occur if no other electrical component on the same supply system is drawing power and only if the battery or capacitors are not fully charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy.
Under emergency braking it is desirable that the braking force exerted be the maximum allowed by the friction between the wheels and the surface without slipping, over the entire speed range from the vehicle’s maximum speed down to zero. The maximum force available for acceleration is typically much less than this except in the case of extreme high-performance vehicles. Therefore, the power required to be dissipated by the braking system under emergency braking conditions may be many times the maximum power which is delivered under acceleration. Traction motors sized to handle the drive power may not be able to cope with the extra load and the battery may not be able to accept charge at a sufficiently high rate. Friction braking is required to dissipate the surplus energy in order to allow an acceptable emergency braking performance. For these reasons there is typically the need to control the regenerative braking and match the friction and regenerative braking to produce the desired total braking output. The GM EV-1 was the first commercial car to do this. Engineers Abraham Farag and Loren Majersik were issued two patents for this brake-by-wire technology.
Dynamic brakes, unlike regenerative brakes, dissipate the electric energy as heat by passing the current through large banks of variable resistors. Vehicles that use dynamic brakes include forklifts, diesel-electric locomotives, and streetcars. This heat can be used to warm the vehicle interior, or dissipated externally by large radiator-like cowls to house the resistor banks.
Kinetic energy recovery systems (KERS) were used for the motor sport Formula One’s 2009 season, and are under development for road vehicles. As of the 2011 season, 9 teams are using KERS. One of the main reasons that not all cars use KERS is because it adds an extra 25 kilograms of weight, and 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. 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. In 2009, Kimi Räikkönen won the Belgian Grand Prix with his KERS equipped Ferrari. It was the first time that KERS contributed directly to a race victory, with second placed Giancarlo Fisichella claiming ‘Actually, I was quicker than Kimi. He only took me because of KERS at the beginning.’ As of 2014, the power storage of the KERS units will increase from 60 kW to 120 kW. This will be to balance the sport’s move from 2.4 liter V8 engines to 1.6 liter V6 engines.
Automakers including Honda have been testing KERS systems. At the 2008 ‘1,000 km of Silverstone,’ Peugeot Sport unveiled the Peugeot 908 HY, a hybrid electric variant of the diesel 908, with KERS. Peugeot planned to campaign the car in the 2009 Le Mans Series season, although it was not capable of scoring championship points. Vodafone McLaren Mercedes began testing of their KERS at the Jerez test track in preparation for the 2009 F1 season, although at that time it was not yet known if they would be operating an electrical or mechanical system. Toyota has used a supercapacitor for regeneration on Supra HV-R hybrid race car that won the 24 Hours of Tokachi race in 2007.