1. Brake Energy Recuperation Strategy
Brake energy recuperation strategy systems have evolved throughout the past several
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.
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2. 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.
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3. 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.
Using a specially designed alternator as the electrical machine for generating the
energy, the system implements a unique charging system strategy. Instead of
constantly running the alternator to maintain a high level of charge in the battery, the
system only charges to eighty per cent of its capacity using power from the engine. This
relieves the power requirements to drive the alternator from the engine. When the
driver applies the brakes, the intelligent alternator control is activated to provide
additional energy to the battery. The alternator is also engaged in overrun conditions.
The system maintains a reserve charge adequate for the power requirements while the
car is idling and sufficient to start the engine under all circumstances. From the braking
side, the vehicle's conventional brakes contribute whatever additional braking is needed
to meet the driver's commands.
Formula one racing has also embraced the brake energy recuperation strategy system,
referred to as the kinetic energy recovery system. The device recovers the kinetic
energy that is present in the waste heat created by the car’s braking process. It stores
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4. that energy and converts it into power that can be called upon to boost acceleration.
There are principally two types of system, battery or electrical, and flywheel or
mechanical. Electrical systems use a motor-generator incorporated in the car’s
transmission which converts mechanical energy into electrical energy and vice versa.
Once the energy has been captured, it is stored in a battery and released when it is
required.
Another utilization of brake energy recuperation strategy systems are trains. The energy
put into accelerating a train, as an example, and into moving it uphill is stored in the
train as kinetic and potential energy. In vehicles with electric traction motors, a great
part of this energy can be reconverted into electric energy by using the motors as
generators when braking. The electric energy is transmitted backwards along the
conversion chain. This is known as regenerative braking and is widely used in railways.
However, the use of dynamic braking does not necessarily mean that the recovered
energy is used to save energy. Diesel electric trains will often have dynamic brakes to
save the braking pads and the recovered energy is just dissipated in brake resistors.
Energy recovery is especially powerful on local and regional lines with frequent stops.
Nevertheless, even on high speed traffic regenerative braking offers potential for energy
efficiency. Although regenerative braking is in wide spread use in many countries, there
is still a great potential for increasing the share of recovered energy. Energy recovery is
only an option whenever another train in the system can use the energy at the same
time. The probability for this depends on train density and possible transmission
distance.
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