EBS: Air brakes’ big break

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The electronically controlled braking system (EBS or ECBS), also called electro-pneumatic braking or brake-by-wire, has been waiting in the wings for several years. It’s still back there but, eventually, it promises to provide much needed relief from air-brake balance, timing and compatibility problems.

Many of these problems stem from the fact that air, the medium used to signal and actuate brakes, is compressible. It takes time for air pressure to build up in the lines and act on relay valves, whose job it is to administer actuation air to nearby brake chambers.

The longer the line, the longer the time. And fractions of a second can be critical when you need all brakes available to stop a vehicle ASAP.

Light years ahead
But there is a way to speed up air brakes by getting a signal to the relay valves faster. An air-pressure signal builds, then eventually reaches the speed of sound (about 760 mph at sea level, at 65 degrees F). In contrast, an electronic signal needs no time to build, and travels at the speed of light (186,000 miles per second). So using an electronic signal to tell relay valves to administer air to brake chambers – from a reservoir that’s relatively close to the chambers and valves – is a good way to speed brake-response time. Air still actuates the foundation brakes, but it gets there much faster.

EBSs measure braking demand by means of a sensor at the brake pedal. The brake-pedal signal is sent to an electronic control unit (ECU), which calculates the air pressure needed to fulfill that demand. The ECU then signals relay valves at each axle to provide the needed pressure.

With a fully EBS-equipped rig, during a stop, all brakes apply nearly instantaneously and evenly, which makes for shorter stopping distances and more even brake wear.

“We’ve been testing five Bendix electronic systems for about a year,” says Stanley Christian, fleet maintenance manager for a large, Southwestern private carrier. “The systems have about 150,000 miles on them, and there have been no problems, except for one wheel sensor that backed out of place.

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“Our drivers say they take a little getting used to, but they stop as well, or better than, straight air brakes.”

“We’ve gotten no noticeable comments from drivers, positive or negative,” says Ron Szapacs, maintenance specialist, Air Products and Chemicals, Allentown, Pa., “We’re sure there’s more

Curent EBS: Note electronic control with full, dual-circuit air backup. The result is a more expensive system, but it satisfies FMVSS 121 requirements. Bendix

finite control of the brakes, but our drivers don’t seem to notice.” Air Products has had one Meritor WABCO system, with Arvin Meritor discs, running for two years and 180,000 miles. “Pad wear seems very low,” Szapacs adds, “but it’s hard to know what to expect – we don’t have any non-EBS vehicles with disc brakes for comparison.”

As part of its participation in the Intelligent Vehicle Initiative, U.S. Xpress Enterprises, Tunnel Hill, Ga., has been testing hybrid systems, put together by Volvo, consisting of Bendix controllers and ArvinMeritor disc brakes. “We’ve been running 50 units for about two years,” says Marty Fletcher, director, technology and training.

“So far, we’ve got about 220,000 miles on them, and driver reaction is extremely good. We did have some early data-link problems that caused some of the units to revert back to their air systems but since then, problems have been minimal. We’ve also found that pad wear is extremely low. However, it remains to be seen whether wear or performance characteristics will change as we add mileage.”

“Actually, future systems could allow the EBS ECU to control brake wear in a closed-loop fashion,” speculates Alan Korn, chief engineer, Meritor WABCO Vehicle Control Systems, Troy, Mich. “Then, in non-emergency stops, the ECU could skew some stopping work to the brakes that show the least wear.”

Other benefits derived from EBS, say manufacturers, could include: fewer components and less plumbing, with reduced chances for leakage; the ability to remove air lines from inside the cab; simplified diagnostics; integrated retarder operation; and super-responsive stability control programs. Similar to active-handling options on luxury passenger cars, these systems use individual-wheel braking to correct destabilizing body and trailer motions, reducing the chances of rollovers and other out-of-control situations.

Tractor/trailer compatibility
Trailer EBSs can be signaled from a tractor via the high-speed J1939 or ISO data link, or by an air pressure transducer at the front of the trailer for use with tractors that don’t have EBS. The idea is for EBS to be forward and backward compatible – EBS tractors with non-EBS

Future EBS setup (no pneumatic backup): System measures braking demand by means of a sensor at the brake pedal. The brake-pedal signal is sent to an electronic control unit (ECU), which calculates the air pressure needed to fulfill that demand. The ECU then signals relay valves at each axle to provide the needed pressure. Meritor WABCO

trailers, and non-EBS tractors with EBS trailers. “Compatibility isn’t really that great an issue,” according to Tom Legeza, staff engineer, electronic products, Bendix Commercial Vehicle Systems, Elyria, Ohio “At least, not on the order of mixing disc brakes with drum brakes. ECBS can pretty much stand on its own.”

Kansas City, Mo.-based Haldex Brake Systems’ European parent company released trailer-only EBS in the U.K. almost two years ago, and has some 2,100 units in service. “The system has performed very well,” according to John McKinley, Haldex’s vice president, product planning. “It’s quite capable of working on its own – you don’t need to use it with an EBS-equipped tractor.” The company plans to bring the system to North America next year. A tractor system, says McKinley, is in the preliminary development stages.

Is progress decelerating?
While there’s no argument among manufacturers and fleet users and testers that EBS is a superior system, it still hasn’t made its way into the mainstream. “Our system is available from Freightliner,” says Meritor WABCO’s Korn. “But it’s very rarely spec’d.”

There are two reasons for this, he explains, and both revolve around cost. Federal Motor Vehicle Safety Standard (FMVSS) 121 requires a redundant braking system in case of system failure. While today’s dual-circuit air-brake systems satisfy that requirement, an electronic system needs a full pneumatic backup in case of a total electrical failure. So customers who spec EBS are essentially paying for two brake systems.

The second reason is a kind of chicken-or-egg thing. EBS is a low-volume item, with no economy of scale, which makes it expensive to build…which makes it expensive to buy, which keeps it a low-volume item. Meritor WABCO’s tractor system lists for around $3,500, and their trailer system is expected to go for less than $1,000.

“You have to consider development, parts and OEM installation costs,” says Kevin Romanchok, Bendix’s product line director, electronic braking. Although Bendix’s EBS – developed by Bosch and Knorr Bremse – isn’t yet available in North America, except for testing purposes, “It’s safe to say that you’ll be looking at about two to three times the cost of an antilock system (ABS). For that reason, we envision a transition, wherein we’ll start adding advanced features in an ABS platform – things like stability control and enhanced brake diagnostics. But we’ll be ready for ECBS if and when the market demands.”

Braking the cycle
Something has to give if EBS is going to catch on, and one answer may well come from across the pond. “EBS is common in Europe,” notes Meritor WABCO’s Korn. “Almost every new heavy truck is equipped with it. Our next-generation product will likely be based on our European system. That will give us some parts commonality and economy of scale. That should allow us to reduce the price and build some volume.

“Also, in the U.S., it’s ‘Show me.’ I think suppliers need to do a better job of proving that the benefits will cost-justify the expense.”

“There’s a different value perception in Europe,” agrees Bendix’ Romanchok. “Here, we see a gradual evolution of brake systems.”

Another catalyst for widespread use of EBS could appear within the next few years. If, after examining test results, the National Highway Traffic Safety Administration (NHTSA) rewrites FMVSS 121 to require a less complex and less expensive pneumatic backup system, EBS cost could be reduced considerably.

“We’re doing some EBS testing now, which is funded by the Federal Highway Administration (FHWA) through the Society of Automotive Engineers (SAE),” reveals Dick Radlinski, president of braking-system consultants Radlinski & Associates, East Liberty, Ohio, and former chief of NHTSA’s Crash Avoidance Research Branch. “We’re looking at system performance and safety in virtually all combinations of EBS and non-EBS-equipped vehicles.

“We’re also looking at failure modes with a simpler, single-circuit pneumatic (1P) backup system,” Radlinski says. “We want to determine whether that arrangement is any less safe than today’s all-pneumatic systems.

“It’s significant that, for these tests, we’re using North American-type equipment, with drum brakes,” Radlinski says. “Up until now, virtually all EBS testing has been done with European-type vehicles, with disc brakes. Since discs are inherently better than drums, that makes it difficult to isolate the benefits attributable solely to EBS.”

FHWA has also announced an EBS field test, in which fleets and EBS manufacturers will participate, to be completed in the first quarter of 2004. “After NHTSA examines all the test results, there’s a chance it will rewrite 121 to allow a 1P backup system,” says Radlinski. “It really wouldn’t require that much modification. But I don’t think it will happen before 2005-2006.”

NHTSA might also allow a partial pneumatic backup, adds Meritor WABCO’s Korn. “Perhaps on the steer and trailer axles, but not the drive axles. It depends on whether it needs to perform as well as if everything were working.

“Even though EBS is extremely reliable, I think there will always have to be some sort of backup system,” Korn concludes. “But, ultimately, it will be up to NHTSA to define that system’s stopping performance requirements.”