Autonomous vehicles and battery-electric trucks are no longer figments of engineers’ imaginations. They are poised to reshape almost everything we know about building and operating heavy trucks — including traditional braking systems.

Truck air brake systems have evolved continually since the early years of the 20th century, when the Westinghouse Air Brake Company (later Wabco), introduced scaled-down versions of the braking systems first pioneered for trains in the 1800s. Performance has improved, and we’ve built in several layers of redundancy and added some electronic trickery in the form of antilock systems and stability control systems that can apply the brakes at specific wheel-ends.

“Since 2005, the industry has seen the addition of significant new features such as roll and yaw stability, an interface to ADAS [advanced driver assistance systems], and hill-start aid enhancements,” says Richard Beyer, vice president of technical sales and vehicle systems at Bendix Commercial Vehicle Systems.

The next decade will see more changes wrought on our not-so-humble braking systems than in the previous century. The systems must evolve to keep up with the twin mega-trends of automation and electrification.

“They 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,” Beyer adds.

Obviously better integration with various electronic control systems is required, without necessarily adding layers of electronic control systems to air brakes to improve their performance. It’s probably better to start with a more or less clean sheet, 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

The industry knows and understands the current system. It’s robust and reliable, and the two-circuit design already offers the level of redundancy that safety advocates, regulators, and truck engineers are looking for. The system can tolerate a complete breakdown of one circuit while the other continues functioning, albeit in a slightly limited way. Among the questions engineers are grappling with is how to bring that level of reliability and redundancy to an electronically controlled system.

“That is a critical prerequisite for complex systems as will be needed to support the future vehicle combinations,” Beyer explains.

Part of the challenge lies in layering new technology on top of existing technology, before the existing technology can completely replace the legacy systems. Electronic controls can replace much of the signal side of the brake system, but it will be some years before air reservoirs and pneumatic brake actuators are replaced by electromagnetic brake actuators.

Without going too deep into the weeds, in current brake systems, when the driver presses down on the foot valve, the valve sends air, a signal, to the relay valves located near the wheel-end-mounted brake actuators. Those valves are also plumbed to the air reservoirs. The signal from the foot valve opens the relay valve, allowing air to flow from the reservoirs through the ABS modulator to the actuators, causing the brakes to apply.

The time that elapses between the opening of the foot valve and the brakes applying is called brake lag. Certain design requirements demand the brakes apply within a certain amount of time, and those requirements are becoming more demanding. Electronic signals from the foot valve to the relay valves at the rear of the chassis would be instantaneous, virtually eliminating brake lag. Electronic brake controls also would eliminate some of the tubing and valving currently in use.

As trucks become more highly automated, they will require electronic brake controls.

“This approach differs from today’s brake control systems designed primarily for foot-pedal-based brake requests,” Augoustidis explains. “Second, electric vehicles will introduce the ability to simultaneously apply conventional friction braking and motor-based electric braking [also called regenerative braking].”

Disruptive but Complementary Technology

Electronically controlled braking systems (ECBS or EBC), sometimes called brake-by-wire, are already in service in some applications in Europe. In addition to simplifying the physical layout of the pneumatic side of the braking system and speeding up brake application timing, they enhance stability control functionality.

“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 [system].”

According to Hahn, ECBS would offer multiple benefits for tractor-trailer units, because these systems could deliver better trailer stability, reduce brake lag at the trailer wheels with the help of electronic brake signals. And as we are already familiar with, in stability-control events, brake application pressures can be distributed to individual wheels more precisely than with current pneumatic valves and actuators.

In various test scenarios, Haldex reports EBS-equipped trucks have shortened stopping distances by as much as 25% compared to fully air-braked trucks. And that’s just the beginning of the potential benefits electronic brakes could bring to Class 8 trucks, says Mark Gregory, chief application engineer 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.”

Further improvements to application timing and application pressure precision are possible with electro-mechanical brake actuators. These devices would replace the pneumatic brake chambers now found at every wheel-end.

Fully electronic braking would allow precise control of application pressure and timing at individual wheels and would be fully integrated into what will evolve from the current antilock braking system (ABS) and electronic stability control (EBS) systems. For example, depending on the weight and load configuration of the truck, wheels on more heavily loaded axles may receive a higher brake torque request than the lighter axles to prevent lock-up on slippery roads.

The challenge facing developers at this point is the millions of legacy trailers that will be on the roads for decades to come. Tractors with full electric brake systems would likely not be immediately compatible with pneumatically braked trailers.

Looking further out, electrifying the brake system could allow the downsizing of the air compressor, if the need to supply brakes is eliminated, leaving just the suspension.

“Going to electronic brakes would deliver a major efficiency gain to the truck’s air compressor,” Gregory 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.”

Regen or split-system braking

With electric vehicles, whether battery or fuel-cell powered, braking is a significant source of energy for replenishing batteries, and engineers will want to optimize vehicle deceleration events for maximum energy recovery. This will involve balancing the braking effort across the electric motor or motors and the foundation brakes at the wheels.

ECBS supports “split-system” braking, where the job of stopping the truck is shared between the foundation brakes and the regenerative braking system. The goal would be to optimize the charging opportunities for electric trucks by adjusting the level each braking system contributes to the stopping effort.

“We’re seeing definite moves toward recovering brake energy on commercial vehicles,” says John Bennett, chief technology officer of Meritor. “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. 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.”

For an intermediate step to full-blown electronic brakes, Bennett believes we may see some sort of electronically actuated pneumatic brake systems.

He also believes electronic control of vehicle deceleration will help lower maintenance costs for fleets. More regenerative braking means brake wear will be reduced. Less-frequent brake jobs mean labor and parts savings.

Moreover, he says, fully electronic brake systems will naturally complement any number of electronic sensors. That will not only give technicians more 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 diagnostics and troubleshooting that takes place today.

EBS: The Ultimate Goal

As cutting-edge as all 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 a logical next 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.

For autonomous vehicles, he says, “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.”

In trucks with today’s Level 1 and Level 2 automated control systems, Augoustidis says, an electronically controlled braking system with a dual-circuit pneumatic system has enough redundancy for safety if the ECBS fails, because the driver is the backup.

“However, 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,” he says.

And there’s more. “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.”

For these reasons, Hahn says he believes electronic brakes will begin on tractors, and eventually extend back to the trailer. But he sees a few obstacles to this evolution.

“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, enhance 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 future.

Given the rapid development of both electric trucks and autonomous technologies, it seems that electronic brakes are not so much an “if” proposition for North America, but “when.”

Editor's note: As of Dec. 29, 2021, this article has been updated from the original October 2021 web version to clarify some technical aspects of brake operation.This corrected version also appeared in the December 2021 issue of HDT magazine.