Any tech who works on modern vehicles knows that butterflies-in-the-stomach feeling when we raise the hood of a vehicle and see some components that look so foreign and bizarre to us we shake our head and think, “I hope I never have to work on that!” Inevitably there is a “F.R.E.D.” (Frustrating Ridiculous Electronic Device) controlling the overly complex system making everything even more intimidating. With throttle-by-wire a standard system for several years, we’ve gotten over the butterflies on that system, but the feeling still exists for many techs when it comes to brake-by-wire systems. This article will help you move past your butterflies as the industry moves forward with making yet another ‘drive-by-wire’ subsystem commonplace.
Why brake-by-wire?
Electronically-controlled brake-by-wire is a means to stop the vehicle for:
· Adaptive cruise control
· Self-parking systems
· Autonomous vehicles (self-driving)
Adaptive cruise control systems have been around for around a decade, and the self-parkers are approaching out-of-warranty times. Autonomous vehicles are just getting into your YouTube with early experimentation that has shown to be very promising to help avoid collisions and increase customer convenience. It’s even speculated that unless your planned retirement from auto repair is in the next decade, you might very well see customers’ cars show up at your shop for preventative maintenance services at 2 a.m. (when the vehicle is not being used) via an autonomous trip scheduled by you and your customer. Good luck getting good lube techs to work the graveyard shift!
Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs) were some of the first to sport this new form of brake hydraulic system. Brake-by-wire can take into consideration:
· The need for regenerative (regen) braking requires that conventional friction brakes on the drive wheels be limited in order to maximize the effectiveness of regen braking.
· HEVs extend fuel economy in several ways including Internal Combustion Engine (ICE) auto stop when the ICE shuts off to save fuel when the vehicle stops and on some models while driving. Without the ICE runnin, there is no source of vacuum for conventional vacuum power-assisted braking. EVs do not have an ICE to begin with, so vacuum-assisted anything is out unless you wish to reduce the EV’s range further by using valuable electrical energy to run an electric vacuum pump.
Base brake system mechanical friction components – nothing new!
No matter how much complexity there seems to be in any of the HEV braking systems, the mechanical components that actually stop the vehicle are fairly standard and the ABS/TSC/Stability control sensors and hydraulic controls (solenoids and low pressure pump) are also fairly straightforward.
Regenerative braking
Regenerative braking (Regen) uses the inertia of the vehicle to generate electricity to recharge high voltage batteries. This is accomplished by using the kinetic energy of the vehicle. Kinetic energy is energy of motion. The kinetic energy of a vehicle is what it possesses because it is moving. In a conventional vehicle, the kinetic energy is transferred into heat, through the mechanical braking process into the brake pads and rotors. This heat, which is also energy, is dissipated into the atmosphere and is lost. Regenerative braking captures the kinetic energy to use later in the form of electricity to power various motors. This energy is stored in the high voltage battery pack.
The regenerative braking action starts as soon as you take your foot off the accelerator pedal. Regenerative braking will only start when the computer or electronic control unit (ECU) detects the throttle is not depressed. However, the mechanical brakes do not engage fully during normal braking. This is controlled by a computer system or electronic brake controller (EBC) on most hybrids. This process is called series regeneration. The computer will determine how much the mechanical brakes should engage until a pre-determined speed is attained (typically around 4 to 10 MPH). At this point, the mechanical brakes will be the primary brake. This minimizes the amount of energy lost due to heat/friction in the mechanical braking system and maximizes the amount of energy recaptured and stored in the HV battery in the form of electricity.
Honda – simplicity (kind of)
Honda HEV braking systems are not as ‘brake-by-wire’ as most other HEVs. Going for simplicity, they still utilize a connection between the foot on the brake pedal and the hydraulic brakes. One noticeable variation on a Honda HEV braking system is the presence of a pressurized accumulator similar to those used on conventional non brake-by-wire ABS-equipped vehicles a few years ago. In the late 80s/early 90s, the trend for many OEMs was to scrap the vacuum-power brake booster and conventional master cylinder and throw the function of the hydraulic base brakes into a combination master cylinder and ABS modulator. An electric pump ran to provide power-assisted braking pressure increases from what the driver’s foot could manually muster and a nitrogen-filled pressurized accumulator provided assistance to the power braking functions in case the electric pump could no longer provide power-assisted braking functions. These types of systems were call integral ABS systems. Most HEV/EV braking systems borrow some of these components, and Honda is no exception. However, the business of completely switching from hydraulics brakes to electric regen brakes is not used. In order to get more of a Honda HEV’s braking turned into fuel saving regen braking, the ICE on most Honda HEVs uses a cylinder deactivation system to reduce the effect of engine braking on the vehicle. This provides more stopping opportunity to the regenerative braking system on Hondas, which spells increased fuel economy.
Toyota Prius brake-by-wire operation
Toyota makes up the overwhelming majority of HEVs out there with the Prius being the most popular model in the corporate line. The Prius is classic sophisticated brake-by-wire with an electronic brake system in combination with hydraulics. It uses a pressure transducer and stroke simulator to determine brake pressure. The stroke simulator also gives the driver a normal tactile feel. This system also has traction control and anti-lock brakes. Gen II (2004-2009) and Gen III (2010-current) both have stability controls and BAS, neither of which are unique to HEVs or EVs.
Each wheel is applied the correct amount of braking individually. This is controlled by a brake actuator. It does not have a vacuum power booster.
The brakes are also operated by hydraulics when vehicle is less than 4 MPH and the ABS system is activated. Although the main hydraulic unit (called a brake actuator, and it is mounted behind the inverter) has a great deal of wiring going to it – it is NOT the module with the primary logic for braking. Containing the usual electric pump motor, solenoids and a pressure accumulator, it does carry out hydraulic work for ABS/TSC/VSC as well as brake-by-wire brake apply. The Hybrid ECU, which controls HV battery charging and motor operation, monitors the battery control/monitor module and communicates with the skid control ECU.
Toyota skid control ECU – the real ‘brains’
The skid control ECU module contains the inputs for wheel speed sensors, steering angle sensors and lateral/yaw inputs and then determines the correct amount of hydraulic friction brake application based on vehicle speed, ABS status, the pedal stroke sensor and HV battery pack state of charge. The skid control ECU module then commands the ABS hydraulic unit (brake actuator) to activate the pump and solenoids in order to apply a traction control like apply of the mechanical friction brakes. The Hybrid ECU, being part of this communications network (CAN), provides a command to the inverter/electric motor control module in order to provide a progressive apply of field current into the motor generator’s stator windings. The more the current applied to the stator windings the more regen braking occurs. Because the friction braking system is so dependent on 12 volt power to the various modules and the brake hydraulic unit, a back-up 12-volt power supply (power supply containing 22 capacitors) is located in the rear compartment next to the 12-volt auxiliary battery to short-term supply power in the event there is a loss of power from the 12-volt auxiliary battery.
Preventive brake maintenance for hybrids
On any older vehicle, who hasn’t applied a parking brake only to find that you’re most likely the first one to operate it in several years? It sticks because of rust and corrosion in the cable, and you end up spending time getting it freed up and lubricated. With hybrid brake-by-wire, the regeneration function is so effective you’ll likely see the brake pads last well over 150,000 miles. So what is there to do? Remember the parking brake that’s seldom used and sticks? Think of HEV/EV service brakes that way. Instead of brake pad/shoe R&R along with rotor machining/replacement, you have the sticky/rusty component syndrome. The piston can stick in the caliper, the caliper’s bushings/O rings get dry, etc. So everything you would do on a thorough brake job (lube bushings, push back caliper piston) would be a good idea to limber up the seldom-used service brake components on an HEV/EV. Ensure free movement of the caliper pistons and other hardware at least yearly. This is especially true in areas where road salt is commonly used. This is a very common problem with some Toyota Prius models because of the steel pistons used in early models.
Brake rotors are prone to become rusty as well and should have the rust/glaze removed with some Scotch Brite or equivalent to prevent brake noise.
It is not recommended to use vacuum bleeding on hybrid vehicle. Pressure bleeding, however, is recommended with most manufacturers.
Safety tip: Keeping the hands clear!
Whether the pads are finally worn out or you are just doing a little PM on the front calipers, on many brake-by-wire vehicles including Ford HEVs you’ll want to remember to deactivate the control of the system when working around the calipers. On some HEVs such as the Ford Escape, when the driver’s door is opened, the self-check system is activated. On a normal ABS system, we’ve heard this self-check occur with a pump motor activation/solenoid cycling. OEMs have managed to quiet this function down from when we used to hear it at about 15 MPH years ago. On HEVs with brake-by-wire, OEMs don’t want to wait until the vehicle is going 15 MPH before checking to see if the brake-by-wire system is working. As the door opens, chimes go off and other distracting noises to cover the sound of the pump turning on and solenoids being activated along with the front calipers being pressurized with a couple of thousand PSI. This self-test feature works great, unless you are in the process of removing a brake bad from a dismounted caliper and the service advisor opens the door to get the odometer reading. In that case, your hand will be in a brake-by-wire vice and you’ll be spending the rest of your day having hand surgery for the bones that were broken. As with any new system, read all you can, get all the training you can and practice all the safety procedures published by the manufacturer because ‘F.R.E.D.’ is still taking more ground as we move closer and closer to the total drive-by-wire vehicle!
Under the hood of this Gen II Prius space is tight with the brake hydraulic actuator (looks like an ABS module containing solenoids, an electric pump and accumulator inaccessible until you remove the top cover of the inverter. CAUTION: ALWAYS obtain proper training and wear the correct personal safety equipment (Class 0 / 1,000 volt gloves and safety glasses) to prevent exposure to high voltage circuits. Failure to do so could result in injury or even death. Even when the vehicle is powered down and the hybrid battery pack service plug is removed, the high voltage capacitors under that inverter cover should be treated as if they might not have drained off in less than 1 second as they are designed to do. The “Cut Valve” ensures that the hydraulic system for the friction brakes still provides emergency braking pressure when the brake pedal is depressed even if the brake-by-wire system is inoperative. This feature was not working when author’s Prius experienced a total braking loss.
On Lexus (pictured) and Toyota a professional level subscription gives you not only OEM service info and training but also allows any J2534 universal pass through programming to operate with a laptop computer as the factory scan tool allowing access to any PID, bi-directional command and configuration in addition to module programming. In the above data PID graph the top (green) trace is engine RPMs, the 2nd trace down (magenta) master cylinder torque, (foot’s power on pedal) the 2nd from bo5om (purple) trace regenerative brake torque (MG 2 stator winding field strength) and the bo5om trace (blue) Vehicle Speed. Notice the master cylinder torque matching regenerative brake torque. This vehicle’s drive wheels (85 % of braking power of the total vehicle) are doing 100 percent regen braking.
