How Hydraulic and Electromechanical Brakes Work

The idea behind brake-by-wire technology is to do away with the reliance on hydraulic pressure to activate brakes. Stefan Weichelt / E+ / Getty

Traditional brake systems haven't changed a whole lot in the last century, so the concept of brake-by-wire technology represents a sea change that automakers and the public at large have both been reluctant to embrace. While traditional hydraulic systems have their issues, there is something reassuring in having a direct, physical connection between your foot and the brake pads or shoes located at the four corners of your vehicle. Brake-by-wire breaks that connection, which is why the technology is seen as inherently more dangerous than electronic throttle control or even steer-by-wire.

The Comforting Nature of Hydraulic Brakes

The way that traditional brake systems have worked for decades is that pressing down on the brake pedal generates hydraulic pressure that is then used to activate brake shoes or pads. In older systems, the pedal acts directly on a hydraulic component known as a master cylinder. In modern systems, a brake booster, usually powered by vacuum, magnifies the force of the pedal and makes it easier to brake.

When the master cylinder is activated, it generates hydraulic pressure in the brake lines. That pressure subsequently acts on the slave cylinders present in each wheel, which either pinch a rotor between brake pads or press brake shoes outwards into a drum.

Modern hydraulic brake systems are more complicated than that, but they still work on the same general principle. Hydraulic or vacuum brake boosters reduce the amount of force the driver has to apply, and technologies such as anti-lock brakes and traction control systems are capable of automatically activating or releasing the brakes.

Electric and electro-hydraulic brakes have traditionally been used only on trailers. Since trailers already have electrical connections for brake lights and turn signals, it’s a simple matter to wire in an electro-hydraulic master cylinder or electric actuators. Similar technologies are available from a couple of OEMs, but the safety-critical nature of brakes has resulted in an automotive industry that remains hesitant to adopt brake-by-wire technology in any real capacity.

Electro-Hydraulic Brakes Stop Short

The current crop of brake-by-wire systems use an electro-hydraulic model that isn’t fully electronic. These systems still have hydraulic systems, but the driver doesn’t directly activate the master cylinder by pressing on the brake pedal. Instead, the master cylinder is activated by an electric motor or pump that is regulated by a control unit.

When the brake pedal is pressed in an electro-hydraulic system, the control unit uses information from a number of sensors to determine how much braking force each wheel needs. The system can then apply the necessary amount of hydraulic pressure to each caliper.

The other main difference between electro-hydraulic and traditional hydraulic brake systems is how much pressure is involved. Electro-hydraulic brake systems typically operate under much higher pressures than traditional systems. Hydraulic brakes operate at around 800 PSI under normal driving conditions, while Sensotronic electro-hydraulic systems maintain pressures between 2,000 and 2,300 PSI.

Electromechanical Systems Truly Brake-by-Wire

While production models still use electro-hydraulic systems, true brake-by-wire technology does away with hydraulics entirely. This technology hasn’t shown up in any production models due to the safety-critical nature of brake systems, but it has undergone significant research and testing.

Unlike electro-hydraulic brakes, all of the components in an electro-mechanical system are electronic. The calipers have electronic actuators instead of hydraulic slave cylinders, and everything is governed directly by a control unit instead of a high-pressure master cylinder. These systems also require a number of additional hardware, including temperature, clamp force, and actuator position sensors in each caliper.

Electromechanical brakes also include complicated communication networks since each caliper has to receive multiple data inputs in order to generate the proper amount of brake force. And due to the safety-critical nature of these systems, there typically has to be a redundant, secondary bus to deliver raw data to the calipers.

The Sticky Safety Issue of Brake-by-Wire Technology

While hydro-electric and electromechanical brake systems are potentially safer than traditional systems, due to the potential for greater integration with ABS, ESC, and other similar technologies, safety concerns have held them back. Traditional brake systems can and do fail, but only a catastrophic loss of hydraulic pressure will completely rob the driver of the ability to stop or slow down, while inherently more complex electromechanical systems have a multitude of potential failure points.

Failover requirements, and other guidelines for the development of safety-critical systems like brake-by-wire, are governed by functional safety standards like ISO 26262

Who Offers Brake-by-Wire Technology?

Redundancy and systems that are capable of working with a reduced amount of data will eventually make electromechanical brake-by-wire technology safe enough for widespread adoption, but at this point only a couple of OEMs have experimented with electro-hydraulic systems.

Toyota first introduced an electro-hydraulic brake system in 2001 for its Estima Hybrid, and variations of its Electronically Controlled Brake (ECB) technology have been available ever since. The technology first appeared in the US for the 2005 model year with the Lexus RX 400h.

An example where brake-by-wire technology suffered from a failure to launch was when Mercedes-Benz pulled its Sensotronic Brake Control (SBC) system, which had also been introduced for the 2001 model year. The system was officially pulled in 2006 after a costly recall in 2004, with Mercedes claiming that it would offer the same functionality of its SBC system via a traditional hydraulic brake system.