Autonomous vehicle control systems and battery-electric trucks are poised to put new demands on commercial air brake systems and fast-track further development.
Commercial vehicle air brakes have continuously evolved, especially since 2005, when new federal stopping distance standards pushed OEMs to beef up their performance considerably, says Richard Beyer, vice president of technical sales and vehicle systems at Bendix Commercial Vehicle Systems.
“Since 2005, the industry has seen the addition of significant new features such as roll and yaw stability, an interface to ADAS [advanced drivers assistance systems], and hill-start aid enhancements,” Beyer says. “With that said, there are two mega-trends that are poised to put new demands on the air brake system as we know it, making it clear that an upgraded brake system is needed.”
If truck makers wish to better integrate air brake systems into emerging technologies, such as autonomous vehicle control system and battery-electric trucks, then adding layers of electronic control systems to air brakes to improve their performance seems to be the best path forward, Beyer says.
“In looking at the advanced technology coming at the industry now, it seems clear that current air brake systems will serve as the basic building block for new brake technology that is being developed,” says Beyer’s colleague at Bendix, Alex Augoustidis, who is product group director for the company’s electro-pneumatic brake systems division. “The dual-circuit air brake system will be the mainstay for future brake architecture into the future. “
Basic Brake Building Blocks
Air brakes have been the stopping standard in heavy-duty trucks since Westinghouse first developed them for railroad applications in the late 1800s. By the early 20th century, scaled-down versions of railroad air braking systems were being adapted for the first heavy-duty trucks appearing on roadways.
Hydraulic braking systems depend on hydraulic fluid in a closed, vacuum system, that, if compromised, renders the brakes virtually useless for stopping a vehicle. While robust and reliable enough for smaller vehicles, the systems simply cannot deliver the massive forces required to apply heavy friction pads to a large, commercial vehicle drum or rotor, and hold it in place with enough force to safely stop it in a reasonable distance.
Given those limitations, air brake systems have been the go-to stopping system for large trucks. In addition to being powerful enough to initiate and maintain a braking event on a large truck, air brake systems do not reply on a vacuum system to apply force to pads or drums. This gives them a built-in redundancy that hydraulic brakes do not have. If a crack in a brake line develops, there is no vacuum lost to render the system useless. Because an air brake system uses air to actuate braking force, a leak in an air line simply adds more air to the system. (Assuming the leak is not catastrophic.)
“Today’s air brake systems have many advantages for fleet customers,” Bendix’s Beyer says. “It’s mature, it’s reliable, it’s very capable, and many technicians already know how to troubleshoot and repair it.”
Beyer says that it’s important to understand that the two-circuit braking system is very robust to single-point failures and, in the case of towing combinations, small leaks for single-point failures do not cause the system to completely break down. “That is a critical prerequisite for complex systems as will be needed to support the future vehicle combinations,” he explains. “There are two main uses for pneumatics in commercial vehicles: braking and suspension, and the ease of handling brake control to a towed unit. As long as pneumatics is delivering the total cost of ownership (TCO) and robustness needed for these systems, it will remain a mainstay.”
Disruptive but Complementary Technology
Bendix’s Augoustidis says that as highly automated trucks – which he defines as vehicles with Level 3 autonomous control systems and higher – become more commonplace, the need for a brake system on trucks where every brake application is requested from an electronic control module, acting as a sort of “mission control system,” on the vehicle, will become necessary.
“This approach differs from today’s brake control systems designed primarily for foot pedal-based brake requests,” he explains. “Second, electric vehicles will introduce the ability to simultaneously apply conventional friction braking and motor-based electric braking.”
The current best, new, technology available to meet these demands appears to be electronically controlled braking systems (ECBS or EBS). These are essentially brake-by-wire systems (commonplace in Europe) with the promise to enhance stability controls and better support the regenerative braking systems that feed kinetic energy captured during brake events back into a vehicle’s onboard batteries.
“ECBS is a proven technology. It outperforms pneumatic brake control systems,” says Wolfgang Hahn, ZF’s system innovation leader. “ECBS is expected to be one key enabler for [SAE Level 4 to Level 5] trucking automation because it brings all the brake management together under the stability control.”
According to Hahn, ECBS would offer multiple benefits for heavy trucks, because these systems deliver better trailer stability, reduce delays with the help of electronic brake signals, and better distribute brake forces than air-based systems. The feeling at the pedal could even be configured in the name of comfort.
Depending on the test, Haldex has reported that it has seen EBS equipment reduce stopping distances by as much as 25% when compared to air-braked counterparts. And that’s just the beginning of the potential benefits electronic brakes could bring to Class 8 trucks, says Mark Gregory, chief engineer of applications at Haldex.
“Air brakes famously have a 10-millisecond lag between depressing the brake pedal and onset of brake force to the wheels,” he says. “Obviously, electronic brake response will be instantaneous and vastly improved over current air systems.”
On top of that, Gregory says that electronic brakes will be vastly quieter than hissing and popping air brake systems, with clear benefits for fleets running in urban applications at night and in early mornings.
And that’s not all. “Going to electronic brakes would deliver a major efficiency gain to the truck’s air compressor,” he says. “That goes along with the opportunity to reduce weight and ease maintenance demands, since electronic brakes tend to have about half as many parts as drum or disc air brake systems do. Although we still might need to retain onboard pneumatic systems for suspensions and other systems.”
The onset of new electric powertrains will be a disruptive technology that will drive a push toward electronic braking systems – simply because the two technologies complement one another, says John Bennett, chief technology officer of Meritor.
“For starters, we’re seeing definite moves toward recovering brake energy on commercial vehicles,” Bennett says. “And I think what we’re seeing is a move away from foundational brakes – which you obviously want to limit use of on an electric vehicle. Sometimes you have to. But the preference is to have the vehicle’s kinetic energy captured and used or stored. So, in time, I think we’ll see a sort of natural optimization of brake systems which largely mirror electric truck duty cycles. And I believe that we will maybe see some sort of electronically actuated pneumatic brake systems as a nice intermediate step toward full-blown electronic brakes.”
Bennett also believes electronic brakes will further aid maintenance operations for fleets. He notes that regenerative brake systems do not typically wear as fast as conventional braking systems. Moreover, he says, electronic brake systems will naturally complement any number of electronic sensors. That will not only give technicians increased alerts when there is a problem with a brake system in the future, but also will precisely pinpoint the area and nature of the problem and allow technicians to get to work without much of the diagnosis and troubleshooting that takes place today.
EBS: The Ultimate Goal
As cutting edge as this may sound to North American ears, Beyer is quick to point out that EBS has been the standard brake spec in Europe for more than 20 years, making the technology the next, logical, evolutionary step as the base system for North American commercial vehicle braking systems.
He says Bendix is currently working on electronic brake and steering integration systems that could be the answer to the redundancy riddle facing engineers.
“These redundant braking and steering systems are needed to allow a vehicle to continue in case of single system failures to the next depot, exit or rest area and/or safely hand back to the human driver in autonomous vehicles,” he says. “As was the case with the growth and change we’ve seen in pneumatically controlled brake systems, EBS will also evolve over time. But to start, EBS will utilize electronic signals from the brake pedal to the electro-pneumatic modules (EPMs) that replace today’s relay and traction valves. In addition, some components will be updated that will bring better performance or durability.”
This approach, Augoustidis adds, will improve overall braking accuracy through more precise pressure control to the wheel ends. Also, durability will be improved because the system is designed to activate every time the driver steps on the brakes, compared to an ABS application which only activates when the wheels lock up. And, finally, it will enable the redundancy required for highly automated driving system, as previously mentioned. The new EBS brakes will be highly integrated into the vehicle, similar to the strong integration in today’s air-brake system.
Importantly, Augoustidis says, base ECBS, with a dual-circuit pneumatic system as a basis and for Level 1 and Level 2 automated control systems, has sufficient redundancy for safety should the ECBS fail, as the backup is always the driver.
“However,” he adds, “as Level 3 and higher automated driving systems evolve, there will be a need for new vehicle architectures on the control side for redundant power, plus the braking and steering systems.”
“Lots of systems can start to come together electronically, even with ECBS,” ZF’s Hahn says. “Once you have electronic controls on the braking system, adding functionality to lift axles, tag axles and traction control systems becomes quite easy to do. And, if you wish to integrate powertrain controllability with the braking system, doing so is only possible with electronic brakes.”
For these reasons, Hahn says he believes electronic brakes will begin on tractors, and eventually extend back to the trailer.
“There are several obstacles to this evolution, though,” he says. “We will need to develop some sort of redundant air brake control line, as well as a redundant CAN brake control line. But once we solve those issues, we can introduce brake-blending between the tractor and trailer for coordinated, enhanced, stopping performance. And we’ll be able to transfer captured kinetic energy back and forth between tractors and trailers to extend range for electric trucks.”
The challenges to bringing electronic braking systems online in North America are daunting. However, there is no doubt that given current technology trends, it is a logical – and perhaps even critical step – for enhanced freight efficiency in the near future.
Given the drives for both electric trucks and autonomous trucks, it seems that electronic brakes are not so much an “if” proposition for North America, but a “when will it happen?” scenario.