Brake energy recuperation strategy systems
Brake energy recuperation strategy systems have evolved throughout the past years, but with the energy crisis looming, these brake systems have lent a great deal of potential to regenerative braking as a mainstream technology. Regenerative braking can be an incredibly complex and confusing subject that might better be left to automotive engineers and experts in the field, yet at the root of regenerative braking is a systematic and logical foundation.
To explain a regenerative brake, in its simplest form it is an avenue to recover energy as an automobile slows down by capturing the kinetic energy and then utilizing it at that moment or storing it for use at a later date. In comparison, the typical braking system in most vehicles today actually wastes the kinetic energy. This happens because the unused energy is turned to heat by friction in the linings of the brake and then lost.
Regenerative braking is pretty much used every day. The most common form of regenerative brake involves using an electric motor as a generator. In electric rail systems the electricity that is generated is fed back into the system, versus a hybrid vehicle, where the energy is stored in a battery or capacitor for later use. Energy may also be stored in hydraulic hybrid lines or in a rotating fly wheel.
Vehicles driven by electric motors use the motor as a generator when using regenerative braking, so it is operated as a generator during braking and its output is supplied to an electrical load, then it transfers the energy to the load provided the braking effect. Regenerative braking is used on hybrid gas or electric automobiles to recover some of the energy lost during the stopping process. The energy is then saved in a storage battery and used later to drive the motor whenever the car is in engaged into electric mode.
The regenerative braking effect drops off at lower speeds, so the friction brake still needs to be utilized in order to bring the vehicle to a complete stop. Manually locking of the rotor is also required to prevent the vehicle from rolling down a hill. The friction brake is a necessary back up in case the regenerative brake fails.
Most road vehicles with regenerative braking only have power on certain wheels, so regenerative braking power only applies to these wheels, therefore in order to provide consistent braking under difficult conditions like rainy or slippery roads, the 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 the 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 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 absorb the surplus energy in order to allow an acceptable emergency braking performance.
Dynamic brakes, as opposed to regenerative brakes, dissipate the electric energy as heat by passing the current through large banks of variable resistors. Vehicles that use dynamic brakes include diesel trains and street car systems, among others. This heat can be used to warm the vehicle interior, or dispensed externally by large radiators to house the resistors.
The main disadvantage of regenerative brakes when comparing them to dynamic brakes is the need to closely match the generated current with the supply characteristics and increased maintenance cost of the lines. With direct current, this requires that the voltage is carefully managed. Only with the development of electronics has this been possible with alternating current supplies, where the supply frequency must also be equaled.
Hybrid vehicles extensively utilize brake energy recuperation strategy systems. Regenerative braking is an integral part of hybrid and electric vehicles. In a micro hybrid, regenerative braking adds more than the basic stop and start system to improve miles per gallon. However, regenerative braking is not limited to hybrids. Other vehicles can take advantage of the kinetic energy captured from deceleration in brake energy recuperation strategy systems.
Using the propulsion motor in hybrid, electric and plug in hybrid electric vehicles to provide regenerative braking is a common design practice. Regenerative braking re-captures and stores part of the kinetic energy that would otherwise be lost to heat during braking. The captured energy is used to recharge the electric batteries reducing the fuel consumption in the hybrid architecture. One of the objectives of brake energy recuperation strategy systems is to keep the battery at a state of charge that allows you to use the electric motors more often.
A typical hybrid with stop or start and regenerative braking can provide up to 7 per cent fuel economy savings over a driving cycle. The contribution of regenerative braking is about half of the total savings. Regenerative braking is part of an almost fifty per cent gain in fuel economy on the Chevrolet Tahoe or GMC Yukon hybrid over their non hybrid equivalents.
In some generic hybrids with standard regenerative braking, manufacturers may not blend the electric and hydraulic brake operation. If it is small enough, they won’t do anything with the brake systems in terms of compensating for decelerator changes. The manufacturer simply adds the regenerator on top of the standard hydraulic brake system. In contrast, the evolving regenerative brake system blends the capabilities of the friction brakes and the regenerative braking from a hybrid’s motor running as a generator.
To avoid higher risk and higher component and system costs, the goal for the next-generation system was to use proven brake components as much as possible. The only new development parts were a vacuum pump and a pedal feel simulator, which are simply a mechanical spring pack and a cut off device on the pedal.
Some of the braking comes from the electric motors in the hybrid power train running as a generator. When the electric motor generates power back into the battery, it puts a load on the driveshaft. The additional braking requirements requested by the driver are provided by the hydraulic portion of the brakes for the brake energy recuperation strategy system.
Although the system can be used on non hybrid vehicles, it does not provide the regenerative portion of brake blending on those vehicles. In these applications, the system provides the driver a simulated pedal touch, computing the driver’s deceleration intent and then applying the boosted hydraulic pressure to the brakes. Future power trains such as gas direct injection with added turbo charging and even diesel engines have significant mechanical losses due to vacuum pumps. As a result, eliminating the use of vacuum is under serious consideration. A hydraulic boosted system is an alternative to a vacuum boosted brake system. These systems optimize the utilization of braking energy in electric and hybrid electric vehicles by controlling the balance between hydraulic braking and regenerative braking to offer greater vehicle range.
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