OBJECTIVES After studying Chapter 7, the reader should be able to: 1. Describe how regenerative braking works. 2. Explain the principles involved in regenerative braking. 3. Discuss the parts and components involved in regenerative braking

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OBJECTIVESAfter studying Chapter 7, the reader
should be able to1. Describe how regenerative
braking works.2. Explain the principles
involved in regenerative braking.3. Discuss the
parts and components involved in regenerative
braking systems.4. Describe the service
procedures involved with regenerative brakes.
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PRINCIPLES OF REGENERATIVE BRAKINGIf a moving
object has a mass, it has inertia. Inertia is
the resistance an object to a change in its state
of motion. In other words, an object in motion
tends to stay in motion and an object at rest
tends to stay at rest unless acted on by an
outside force. A hybrid vehicle reclaims energy
by converting the energy of a moving object,
called kinetic energy, into electrical energy.
According to basic physicsA force applied to
move an object results in the equationF
maF being the forcem being the massa
being the acceleration
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What Is the Difference Between Mass and
Weight?Mass is the amount of matter in an
object. One of properties of mass is inertia.
Mass is a fixed property of an object, and weight
is a force that is dependent on gravity.
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Reclaiming Energy in a HybridOn a hybrid
vehicle that has regenerative brakes, the kinetic
energy of a moving vehicle that would normally be
lost due to braking can be reclaimed.
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Inertia is the fundamental property of physics
that is used to reclaim energy from the vehicle.
Instead of using 100 friction brakes (base
brakes), the braking torque is transferred from
the wheels back into the motor shaft.This
process is called regeneration or simply
reclaiming energy.
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Regenerative braking has the following
advantages Reduces the drawdown of the battery
charge Extends the overall life of the battery
pack Reduces fuel consumption
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How much energy is reclaimed depends on a many
factors, including weight of the vehicle, speed,
and the rate of deceleration.
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The amount of kinetic energy in a moving vehicle
increases with the square of the speed. This
means that at 60 mph, the kinetic energy is four
times the energy of 30 mph. The speed is doubled
(times 2) and the kinetic energy is squared (2
2 4).
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Two different regeneration designs include
Series regeneration.All series regenerative
braking systems use an electro-hydraulic brake
(EHB) system, which includes the hydraulic
control unit that manages the brake cylinder
pressures, as well as the front-rear axle brake
balance. Most hybrid vehicles use this type of
regenerative braking system.Parallel
regeneration.
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The amount of regenerative braking compared to
the amount of friction braking is determined by
the electronic brake controller.
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The controller of the drive motors uses a varying
frequency to control power and speed. The
controller can quickly change the frequency, and
can therefore change the operation of a typical
AC synchronous motor from propelling the vehicle
(called motoring) to a generator.
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Charging BatteriesKinetic energy can be
converted into electrical energy with a
generator, and it can be returned to the
high-voltage batteries and stored for later use.
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There are some limitations that will always
affect even the best regenerative braking systems
including It only acts on the driven wheels.
The system has to be designed to allow for
proper use of the antilock braking system.
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REGENERATIVE BRAKINGThe Toyota Prius is
equipped with a center dash LCD that shows how
many watt-hours of regeneration have occurred
each 5 minutes. These are indicated by small
suns that appear on the display, and each sun
indicate 50 watt-hours. When a sun appears,
enough power has been put back into the battery
to run a 50-watt light bulb for an hour.
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If the driver is pushing only gently, the master
cylinder piston displacement will be small and
the hydraulic brakes will be only gently
applied.As master cylinder pressure increases,
so does the amount of regeneration that can
automatically be applied.
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There are four pressure sensors in the braking
system and two pressure switches.
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B Means BrakingAll Toyota hybrid vehicles
have a shift position marked B. This position
increases engine braking (increases engine RPM)
when descending long steep grades to help prevent
the battery from being charged at too great a
rate.
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HOW THE REGENERATION SYSTEM WORKSTo keep hybrid
electric vehicles feeling as much like other
vehicles as possible, the hybrids from Toyota and
Honda have both the regeneration and conventional
brakes controlled by one brake pedal. In the
first part of its travel, the brake pedal
operates the regenerative brakes alone, and then
as further pressure is placed on the pedal, the
friction brakes come into play as well.The wear
of the hydraulic brakes and pads will also be
greatly reduced.When the brake pedal is pushed,
the ECU then checks the difference between the
starting value and the brake pedal on value and
sets the regeneration value according to this
difference.
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DECELERATION RATESTypical deceleration rates
include the followingComfortable deceleration
is about 8.5 ft./sec2 (3 m/s2).Loose items in
the vehicle will fly above 11 ft./sec2 (3.5
m/s2).Maximum deceleration rates for most
vehicles and light trucks range from 16 to 32
ft./sec2 (5 to 10 m/s2).One g is the
acceleration of gravity, which is 32 feet per
second per second.
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Maximum deceleration rates are usually greater
that 0.8 g and could exceed 1 g in most vehicles.
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Engine Design Changes Related to Regenerative
BrakingSome hybrid vehicles, such as the
second-generation Honda Civic and Accord, use a
variation of the VTEC valve actuation system to
close all of the valves in three cylinders in
both the V-6 and the inline four-cylinder engines
during deceleration.
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Electric Motor Becomes a GeneratorWhen a motor
is used for regenerative braking, it acts as a
generator and produces an alternating current
(AC). The AC current needs to be rectified
(converted) to DC current to go into the
batteries. Each of the three main power wires
coming out of the motor needs two large diodes.
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Servicing Regenerative Braking SystemsOn the
Ford Escape hybrid system, the regenerative
braking system checks the integrity of the brake
system as a self-test. After a certain amount of
time, the brake controller will energize the
hydraulic control unit and check that pressure
can be developed in the system.This is performed
when a door is opened as part of the wake-up
feature of the system.The ignition key does not
have to be in the ignition for this self-test to
be performed.This is done by developing brake
pressure for short periods of time.
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To prevent physical harm or causing damage to the
vehicle when servicing the braking system, the
technician should do the following1. In order
to change the brake pads, it is necessary to
enter the Pad Service Mode on a scan tool and
disable the self-test. This will prevent brake
pressure from being applied. 2. Disconnect the
wiring harness at the hydraulic control unit.3.
Check service information regarding how to enter
pad service mode, which would allow the brakes
to be serviced without the possibility of causing
harm.
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SUMMARY1. All moving objects that have mass
(weight) have kinetic energy.2. The
regenerative braking system captures most of the
kinetic energy from the moving vehicle and
returns this energy to high-voltage batteries to
be used later to help propel the vehicle.3. The
two types of regenerative braking include
parallel and series.4. Brushless DC and AC
induction motors are used in hybrid electric
vehicles to help propel the vehicle and to
generate electrical energy back to the batteries
during braking.5. Most hybrid electric vehicles
use an electro-hydraulic braking system that
includes pressure sensors to detect the pressures
in the system.6. The controller is used to
control the motors and turn them into the
generators as needed to provide regenerative
braking.
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REVIEW QUESTIONS1. What is inertia?2. What
is the difference between series and parallel
regenerative braking systems?3. What happens in
the regenerative braking system when the
high-voltage batteries are fully charged?4.
Describe what occurs when the driver first
releases the accelerator pedal and then starts to
brake on a hybrid electric vehicle equipped with
regenerative braking.