2OBJECTIVES After studying Chapter 7, the reader should be able to: 1 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.
3PRINCIPLES 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
4What Is the Difference Between Mass and Weight 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.
5Reclaiming 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.
6Inertia 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.”
7Regenerative braking has the following advantages: Reduces the drawdown of the battery charge Extends the overall life of the battery pack Reduces fuel consumption
8How much energy is reclaimed depends on a many factors, including weight of the vehicle, speed, and the rate of deceleration.
9The 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).
10Two different regeneration designs include: Series regeneration 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.
12The amount of regenerative braking compared to the amount of friction braking is determined by the electronic brake controller.
13The 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.
14Charging 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.
15There 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.
16REGENERATIVE 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.
17If 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.
18There are four pressure sensors in the braking system and two pressure switches.
20“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.
21HOW 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.
23DECELERATION RATES Typical 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.
24Maximum deceleration rates are usually greater that 0 Maximum deceleration rates are usually greater that 0.8 g and could exceed 1 g in most vehicles.
25Engine 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.
26Electric 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.
27Servicing 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.
28To 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 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.
29SUMMARY 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.
30REVIEW QUESTIONS 1. What is inertia. 2 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.