How Hydraulic and Electromechanical Brakes Work

What is "brake-by-wire" and why does it matter?

Traditional brake systems haven't changed much in the last century, so the concept of brake-by-wire technology represents a change that automakers and the public have been reluctant to embrace. Brake-by-wire refers to braking systems that control brakes through electrical means.

brake-by-wire
Stefan Weichelt / Getty Images

The Comforting Nature of Hydraulic Brakes

In traditional brake systems, pressing down on the brake pedal generates hydraulic pressure that activates the 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.

Brake-by-wire breaks that connection, which is why the technology is seen by some as more dangerous than electronic throttle control or steer-by-wire.

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 work on the same general principle. Hydraulic or vacuum brake boosters reduce the amount of force the driver has to apply. Technologies like 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 have electrical connections for brake lights and turn signals, it's easy to wire in an electro-hydraulic master cylinder or electric actuators. Similar technologies are available from OEMs, but the safety-critical nature of brakes has resulted in an automotive industry that remains hesitant to adopt brake-by-wire technology. However, with the rise of self-driving and assisted driving systems, brake-by-wire has seen wider use.

Electro-Hydraulic Brakes Stop Short

The current brake-by-wire systems use an electro-hydraulic model that isn't fully electronic. These systems have hydraulic systems, but the driver doesn't directly activate the master cylinder by pressing 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 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 Are 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. Still, it has undergone significant research and testing.

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

Electromechanical brakes include complicated communication networks since each caliper receives multiple data inputs to generate the proper amount of brake force. Due to the safety-critical nature of these systems, there is typically a redundant, secondary bus to deliver raw data to the calipers.

The Sticky Safety Issue of Brake-By-Wire Technology

Hydro-electric and electromechanical brake systems are potentially safer than traditional systems. However, due to the potential for greater integration with ABS, ESC, and similar technologies, safety concerns have held these systems back. Traditional brake systems can and do fail, but only a catastrophic loss of hydraulic pressure will completely prevent the driver from stopping or slowing down. Inherently more complex electromechanical systems have a multitude of potential failure points.

Failover requirements, and other guidelines for the development of safety-critical systems such as 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. At this point, only a couple of OEMs have experimented with electro-hydraulic systems.

Toyota introduced an electro-hydraulic brake system in 2001 for its Estima Hybrid. Variations of its Electronically Controlled Brake (ECB) technology have been available ever since. The technology first appeared in the U.S. 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.