The recall of 680,000 Mercedes-Benz and Maybach models is a very public final nail in the coffin of electro-hydraulic brakes. The technology has been a failure, reports

Electro-hydraulic brakes (EHB) use an electronic accumulator tied to sensors to translate driver pedal pressure into braking force via a standard hydraulic fluid system. If the electronics fail, the hydraulics function in the traditional way, through direct pedal pressure.

But with two potentially defective systems per thousand vehicles, the electronic part of the system is in operation with a 2,000ppm failure rate. That’s just too high.

Systems costs too high
Joint teams from Bosch and Mercedes-Benz started work on electro-hydraulic brakes in 1996.

The product was introduced as Sensotronic Brake Control in the 2001 Mercedes-Benz SL, and had its first volume application with the 2002 Mercedes-Benz E-Class.

But the technology did not take off as expected. The estimated additional benefits the system offered did not correspond to the additional cost of up to €200 per car, the company says. The extra cost is due to the complex electronics needed to manipulate a system still based entirely on hydraulic pressure.

EHB systems weigh less, require a smaller installation space and do not need a brake booster. But brake system engineers and executives say that most of the benefits promised by the system can be met by upgraded conventional hydraulic systems, such as ESP/ABS systems.

“Nearly all functions can be fulfilled with proven hydraulic systems,” a Bosch spokesman said – without the risks and cost involved in new technologies.

Bosch, with a brake systems business worth over €5bn, invested over €150m and several years of effort in electro-hydraulic brakes – a technology that now looks like a dead end.

The failure of Bosch and DaimlerChrysler has discouraged other OEMs from working on EHB systems. If Bosch and Mercedes-Benz – traditional technology leaders – cannot get the concept to work for consumers, other players are doubtful of any potential. PSA had planned to fit EHB to the mid-cycle facelift of its 607 model later this year, but has reverted to a traditional hydraulic system.

Only Toyota also offers electro-hydraulic brakes, in its Prius hybrid-electric car; the system is supplied by Advics.

Failure casts shadow over X-by-wire
The failure will have a knock-on effect on the planned move to brake-by-wire systems, also known as electro-mechanical brake systems. By-wire systems dispense with hydraulics altogether, using wires instead of fluid lines and electrical actuators instead of pistons.

Electro-hydraulic brakes were intended as a stepping-stone to the more advanced technology.

But Bosch has also turned down the candle on its work on full by-wire systems.
And with no customers in sight for the technology, other suppliers have also scaled back advanced brake development.

“The biggest setback is that now everyone is scared and there is a large pendulum swing away from new technology. I can tell you that brake-by-wire is now not less than ten years away in light of these developments,” said Josef Pickenhahn, vice president, brake engineering at TRW.

By-wire systems were not in any case expected until 2010 prior to this development. In part that is because of the repeated postponement of 42-volt electrical systems, another failed innovation initiative.

By-wire requires a “fail-operational” safety level, not just a fail-safe system – the brakes must still function in the event that the primary electrical network fails.
This requires expensive redundant circuits, which current thinking says can only be provided through high voltage systems.

This in turn is hindering the development of more closely integrated safety and performance assistance systems -part of the “roadmap” of leading tier 1 suppliers such as Bosch, Continental and Delphi.
The plan was to integrate chassis, brakes, and steering systems with surround sensors to strengthen vehicle’s driver assistance capabilities.

But the delay in moving to electrical braking systems is moving the goal still further into the future.

Prior to the Sensotronic recall Bosch has enjoyed a good record on faults – it was involved in only one of the 271 recalls recorded by the Kraftfahrt-Bundesamt in Germany in 2002 and 2003, according to the company.