there are four pressure sensors in the braking system and two pressure switches

Hybrid and Alternative Fuel VehiclesBy James D Halderman and Tony Martin

© 2009 Pearson Education, Inc.Pearson Prentice Hall

Upper Saddle River, NJ 07458




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 systems.4. Describe the service procedures involved with regenerative

brakes.




PRINCIPLES OF REGENERATIVE BRAKING

If 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 physics:A force applied to move an object results in the equation:

F = ma“F” being the force“m” being the mass

“a” being the acceleration




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.




Reclaiming Energy in a Hybrid

On 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.




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.”




Regenerative braking has the following advantages: Reduces the drawdown of the battery charge

Extends the overall life of the battery pack Reduces fuel consumption




How much energy is reclaimed depends on a many factors, including weight of the vehicle, speed, and the rate of

deceleration.




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).




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.




The amount of regenerative braking compared to the amount of friction braking is determined by the

electronic brake controller.




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.




Charging Batteries

Kinetic 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.




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.




REGENERATIVE BRAKING

The 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.




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.




There are four pressure sensors in the braking system and two pressure switches.




“B” Means Braking

All 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.




HOW THE REGENERATION SYSTEM WORKS

To 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.




DECELERATION RATESTypical deceleration rates include the following:

Comfortable 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.




Maximum deceleration rates are usually greater that 0.8 g and could exceed 1 g in most vehicles.




Engine Design Changes Related to Regenerative Braking

Some 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.




Electric Motor Becomes a Generator

When 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.




Servicing Regenerative Braking Systems

On 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.




To prevent physical harm or causing damage to the vehicle when servicing the braking system, the

technician should do the following:1. 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.




SUMMARY

1. 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.




REVIEW QUESTIONS

1. 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.

there are four pressure sensors in the braking system and two pressure switches

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