Regenerative brake

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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.

Contents

[edit] 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.
  • The friction brake is a necessary back-up in the event of failure of the regenerative brake.
  • 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 or if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy.
  • 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]

[edit] The motor as a brake

Regenerative braking utilizes the fact that an electric motor can also act as a generator. The vehicle's electric traction motor is reconnected as a generator during braking and its output is connected to an electrical load. It is this load on the motor that 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]

[edit] 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.

When rail operator c2c's began using regenerative braking with a fleet of Bombardier Class 357 EMUs, monitoring over the first two weeks showed an immediate energy saving of 15%. Savings of 17% are claimed for Virgin Trains Pendolinos.[4] There is also less wear on friction braking components.

[edit] 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.

[edit] Use in motor sport

Max Mosley of the FIA has announced that all cars will become hybrid by 2013, along with other changes to the vehicles. The governing body of international motor sport, the FIA, has allowed the use of 60 kW "Kinetic Energy Recovery Systems" (KERS), in the regulations for the 2009 Formula One season.[5][6]

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."[7]

The hybrid system that will be phased in is known as KERS, which stands for Kinetic Energy Recovery System. KERS does not store as much energy as a traditional hybrid system, but it only weighs 55 pounds and the limited energy storage capacity is well suited for Formula-style racing.

The first of these systems to be revealed was the Flybrid[8] which appeared in an article in Racecar Engineering magazine.

The biggest difference between KERS and a regular battery-electric hybrid is that KERS stores recovered waste energy in a rotating flywheel. Instead of converting waste energy into electricity and then back into useful energy again with an electric motor, KERS simply transfers the kinetic energy to a flywheel via the F1 car’s transmission during deceleration. When the driver presses a “boost” button, the gear ratio on the output side of the flywheel is changed so as to reduce its speed and transfer kinetic energy back to the car, resulting in acceleration.

The Flybrid F1 KERS System 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 20-cm 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.

The Honda Racing F1 Team became the first team to introduce and test a development F1 KERS system on track in May of 2008.

[edit] Use in compressed air cars

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

[edit] See also

[edit] 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. "Regenerative braking boosts green credentials", Railway Gazette International, July 2, 2007. Retrieved on 2008-03-21. 
  5. ^ 2009 Formula One Technical Regulations (PDF). FIA (December 22, 2006). Retrieved on 2006-12-22.
  6. ^ Chris Ellis (December 26, 2006). Formula One: 'Braking' New Ground. EVWorld. Retrieved on 2008-03-21.
  7. ^ 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 on 2008-01-20.
  8. ^ Charles Armstrong-Wilson (February 21, 2008). F1 KERS: Flybrid. Racecar Engineering. Retrieved on 2008-03-21.