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Regenerative braking

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A regenerative brake is a mechanism that reduces vehicle speed by converting some of its kinetic energy into another useful form of energy. This captured energy is then stored for future use or fed back into a power system for use by other vehicles.

For example, electrical regenerative brakes in electric railway vehicles feed the generated electricity back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use. Other forms of energy storage which may be used include compressed air and flywheels.

Regenerative braking should not be confused with dynamic braking, which dissipates the electrical energy as heat and thus is less energy efficient.

Limitations

Traditional friction-based braking is still used with electrical regenerative braking for the following reasons:

  • The regenerative braking effect rapidly reduces at lower speeds, therefore the friction brake is still required in order to bring the vehicle to a complete halt, although malfunction of a dynamo can still provide resistance for a while.
  • 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 2WD car) and regenerative braking power only applies to such wheels, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels.
  • 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. No regenerative braking effect can occur if another electrical component on the same supply system is not currently drawing power and if the battery or capacitors are already 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 is 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 absorb 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 2 patents for this 'Brake by Wire' technology.[1][2]

The motor as a Generator

Regenerative braking utilizes the fact that an electric motor can also act as a generator. The vehicle's electric traction motor is operated as a generator during braking and its output is supplied to an electrical load. It is the transfer of energy to the load which provides the braking effect.

An early example of this system was the Energy Regeneration Brake, developed in 1967 for the Amitron. This was a completely battery powered urban concept car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile.[3]

Electric railway vehicle operation

During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors (dynamic braking) or back into the supply (regenerative braking)
For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction.

Braking effort is proportional to the product of the magnetic strength of the field windings, times that of the armature windings.

Savings of 17% are claimed for Virgin Trains Pendolinos.[4] There is also less wear on friction braking components. The Delhi Metro prevented around 90000 tonnes of Carbon Dioxide from being released into the atmosphere by regenerating 112,500 Megawatt hours of electricity the use of the regenerative braking system during the period 2004-2007. It is expected that the Delhi Metro will prevent over 1,00,000 tons of Carbon Dioxide from being emitted per year once its phase II is complete through the use of regenerative braking.[5]

Comparison of dynamic and regenerative brakes

Main article: Dynamic brake

Dynamic brakes ("rheostatic brakes" in the UK), 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. If designed appropriately, this heat can be used to warm the vehicle interior. If dissipated externally, large radiator-like cowls are employed to house the resistor banks.

The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics. With DC supplies, this requires that the voltage be closely controlled. Only with the development of power electronics has this been possible with AC supplies, where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).

A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors. This results in a near constant speed for all trains as the motors rotate with the supply frequency both when motoring and braking.

Kinetic Energy Recovery Systems

Kinetic Energy Recovery Systems (KERS) are currently under development both for F1 motor sport and road vehicles. 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 complex high end systems such as the Zytek, Flybrid[6], Torotrak[7][8] and Xtrac used in F1 and simple, easily manufactured and integrated differential based systems such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS)[9]

Xtrac & 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.

Use in motor sport

FIA

F1 teams began testing Kinetic Energy Recovery Systems, or KERS, in January 2009.

Teams have said they must respond in a responsible way to the world's environmental challenges.[10]

The FIA, allowed the use of 60 kW KERS, in the regulations for the 2009 Formula One season.[11]

Energy can either be stored as mechanical energy (as in a flywheel) or can be stored as electrical energy (as in a battery or supercapacitor).[12]

Motorcycles

KTM racing boss Harald Bartol has revealed that the factory raced with a secret Kinetic Energy Recovery System (KERS) fitted to Tommy Koyama's motorcycle during the season-ending 125cc Valencian Grand Prix. [13]

History

The first of these systems to be revealed was the Flybrid[14] which appeared in an article in Racecar Engineering magazine.

Flybrid Systems F1 KERS weighs 24 kg and has an energy capacity of 400 kJ after allowing for internal losses. A maximum power boost of 60 kW (81.6 PS) for 6.67 sec is available. The 240mm diameter flywheel weighs 5.0 kg and revolves at up to 64,500 rpm. Maximum torque is 18 Nm. The system occupies a volume of 13 liters. It may not be used by all of the F1 teams but a few, such as Williams F1 are going to use it, if not at the first race, at one point during the season.

Two minor incidents have been reported during testing of KERS systems in 2008. The first occurred when the Red Bull Racing team tested their KERS battery for the first time in July, it malfunctioned and caused a fire scare, resulting in the team's factory being evacuated.[15] The second was less than a week later when a BMW Sauber mechanic was given an electric shock when he touched Christian Klien's KERS-equipped car during a test at the Jerez circuit.[16]

Races

Automobile Club de l'Ouest, the organizer behind the annual 24 Hours of Le Mans event and the Le Mans Series is currently "studying specific rules for LMP1 which will be equipped with a kinetic energy recovery system."[17] Peugeot was the first manufacturer to unveil a fully functioning LMP-1 car in the form of the 908 HY at the 2008 Autosport 1000km race at Silverstone.[18]

Autopart makers

Bosch Motorsport Service (part of the subsidiary Bosch Engineering GmbH) is developing a KERS for use in motor racing. Hybrid systems by Bosch Motorsport comprise an electricity storage system (a lithium-ion battery with scalable capacity or a flywheel), the electric motor (weigh between four and eight kilograms with a maximum power level of 60 kW) and the KERS controller, containing the power electronic, battery management, and management system for hybrid and engine functions . The Bosch Group offers a range of electric hybrid systems for commercial and light-duty applications [19].

Carmakers

BMW and Honda are testing it.[20]. At the 2008 1000 km of Silverstone, Peugeot Sport unveiled the Peugeot 908 HY, a hybrid electric variant of the diesel 908, with a KERS system. Peugeot plans to campaign the car in the 2009 Le Mans Series season, although it will not be capable of scoring championship points.[21]

Vodafone McLaren Mercedes have recently begun testing of their KERS system at the Jerez test track in preparation for the 2009 F1 season, although it is not yet known if they will be operating an electrical or mechanical system.[[22]]. In November 2008 it was announced that Freescale Semiconductor will collaborate with McLaren Electronic Systems to further develop its KERS system for McLaren's Formula 1 car from 2010 onwards. Both parties believe this collaboration will improve McLaren's KERS system and help the system filter down to road car technology [23] .

Toyota has used a supercapacitor for regeneration on Supra HV-R hybrid race car that won the 24 Hours of Tokachi race in July 2007.[24]

Use in compressed air cars

Regenerative brakes are being used in compressed air cars to refuel the tank during braking.

See also

References

  1. ^ GM patent 5775467 – Floating electromagnetic brake system.
  2. ^ GM patent 5603217 – Compliant master cylinder.
  3. ^ Time Magazine, Business Section, Next: the Voltswagon?, December 22, 1967.
  4. ^ Roger Ford (July 2, 2007). "Regenerative braking boosts green credentials". Railway Gazette International. Retrieved 2008-03-21. {{cite news}}: Check date values in: |date= (help)
  5. ^ "Delhi Metro prevents 90,000 tons of CO2".
  6. ^ Flybrid Systems
  7. ^ Torotrak
  8. ^ Torotrak, Xtrac & CVT pdf
  9. ^ CPC-KERS
  10. ^ "Teams Comment on F1's Environmental Future". FIA. October 08, 2008. Retrieved 2009-01-14. {{cite web}}: Check date values in: |date= (help)
  11. ^ "2009 Formula One Technical Regulations" (PDF). FIA. December 22, 2006. Retrieved 2006-12-22. {{cite web}}: Check date values in: |date= (help)
  12. ^ FIA management (December 22, 2006). "2009 FORMULA ONE TECHNICAL REGULATIONS" (PDF). FIA. Retrieved 2008-07-08. {{cite web}}: Check date values in: |date= (help)
  13. ^ http://www.crash.net/MotoGP/News/142605/1/ktm_beats_f1_with_secret_kers_debut.html
  14. ^ http://www.racecar-engineering.com/articles/f1/182017/f1-kers-flybrid.html
  15. ^ "KERS failure caused Red Bull fire scare". autosport.com. 2008-07-17. Retrieved 2008-07-22. {{cite news}}: Check date values in: |date= (help)
  16. ^ "BMW mechanic escapes KERS scare". autosport.com. 2008-07-22. Retrieved 2008-07-22. {{cite news}}: Check date values in: |date= (help)
  17. ^ "ACO Technical Regulations 2008 for Prototype "LM"P1 and "LM"P2 classes, page 3" (PDF). Automobile Club de l'Ouest (ACO). 2007-12-20. Retrieved 2008-01-20.
  18. ^ http://www.racecar-engineering.com/news/people/273697/peugeot-reveal-hybrid-racer-for-2009.html
  19. ^ http://www.greencarcongress.com/2008/11/bosch-developin.html
  20. ^ http://www.carmondo.de/blog/2008/07/03/honda-und-bmw-mit-formel-1-hybriden/ Template:De
  21. ^ "Peugeot Sport Hybrid". Racecar Engineering. 2008-09-13. Retrieved 2008-09-13.
  22. ^ http://www.racecar-engineering.com/news/people/274178/mclaren-on-track-with-kers.html
  23. ^ McLaren to work with Freescale on KERS November 12, 2008
  24. ^ Green Car Congress: Toyota Hybrid Race Car Wins Tokachi 24-Hour Race; In-Wheel Motors and Supercapacitors
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