An electro-hydraulic braking system is one that effectively does away with the direct connection between the brake pedal and the car's braking system. These set-ups, which are the first step towards a complete automotive brake-by-wire system, are primarily lighter and smaller than conventional servo-assisted hydraulic systems. This is because electro-hydraulic brake systems use hydraulic pumps to generate the required brake pressure, whereas a conventional system would typically require a bulky vacuum booster.
EHB systems can also improve the braking capabilities of the car, allowing for precise control of the braking pressure at each wheel, while boosting refinement. For example, in a conventional servo-hydraulic braking set-up, triggering the ABS would cause the brake pedal to pulse as the system operated. Electro-hydraulic systems, due to their design, do not suffer from this trait.
Because EHB systems do not require a vacuum to function properly, they are often employed in electric and hybrid cars - such as the Toyota Prius. Their ease of integration with other functions, such as regenerative braking and adaptive cruise control, further boosts their appeal.
How do EHB systems work?
In a conventional servo-hydraulic braking system, the pressure exerted by the driver on the brake pedal is transmitted to a piston in the car's brake master cylinder. The movement of this piston, during braking, increases the pressure of the brake fluid in the car's brake lines. This pressurised brake fluid, in turn, acts on the pistons in the calipers - causing the pads to clamp the discs and slow the car.
Unassisted brakes can require a significant amount of pedal pressure to function effectively, which is why most cars built since the late 1960s feature a vacuum servo. This device, sometimes referred to as a brake booster, greatly reduces the pedal effort required when braking.
This amplification of the driver's inputs makes it easier to apply higher levels of braking force, which can cut braking distances. Vacuum boosters are large and heavy assemblies, however, which obvious implications aside, also means that they are tricky to package in an engine bay.
In an electro-hydraulic set-up, the required brake pressure is instead typically generated by a hydraulic pump and accumulator assembly. The distribution of brake pressure around the car is controlled by a hydraulic unit, fed with pressurised fluid from the accumulator and motor, while a brake ECU interprets various inputs and generates the appropriate control signals.
When the driver steps on the pedal, sensors monitor the pressure applied and the travel of the pedal itself. The ECU interprets these signals, along with other key inputs such as vehicle speed and steering angle, and generates command signals for the hydraulic control unit.
Pressurised brake fluid is then discharged from the accumulator, travelling through solenoid-operated valves in the hydraulic control unit to the individual brakes, slowing the car. This approach has many advantages, such as the system being able to continue to increase braking pressure if an emergency situation is detected - whereas a driver might otherwise let off slightly.
These systems do still feature what is effectively a conventional master cylinder, to which the brake pedal is connected. In EPB systems the master cylinder will feature a pressure simulator which generates increased pressure as the pedal is depressed - in order to ensure suitable brake pedal feel. The master cylinder is also hydraulically linked to the braking system, in some cases just the front wheels, in order to provide emergency stopping power should an element of the EPB fail.
These configurations do not require high-voltage electrical systems, either, unlike the true brake-by-wire systems undergoing development. This helps keep costs down while permitting improved functionality.
Electro-hydraulic braking in automotive applications
Hybrid and electric cars often utilise EHB systems, in order to avoid the need for a vacuum pump to generate vacuum for a conventional brake booster - as their intermittently operating engines may not otherwise generate sufficient vacuum for braking duties. Many Toyota and Lexus hybrid models use the company's 'Electronically Controlled Brake' system, for example - including the likes of the LFA. This set-up was introduced in June 2001, in the hybrid version of the Toyota Estima.
The first mass-produced electro-hydraulic braking system for conventional cars arrived in the R230 generation of Mercedes-Benz SL-Class, which was launched in late 2001. The system, which was called 'Sensotronic Brake Control', was co-developed with Bosch and also used in the E-Class and SLR McLaren.
There were many issues, however, including failures that often resulted in drivers having to apply far more pedal pressure than expected - and several recalls were issued as a result. Many also disliked the feel and actuation of the SBC. This, in conjunction with the system's failures, rattled customers' confidence in electro-hydraulic braking.
Unfortunately, for Mercedes, conventional servo-hydraulic systems quickly developed to the point that they could offer many of the features offered by SBC. Consequently, in June 2006, SBC was discontinued from mainstream models such as the E-Class. The low-volume models, such as the SL, retained it until the end of their respective generations due to the excessive cost of converting them back to conventional servo-hydraulic set-ups.
Bosch promptly wound down its electro-hydraulic research, and curtailed its work on brake-by-wire systems, due to a lack of interest, support and confidence. It has, however, recently developed an 'iBooster' which uses an electric motor to assist the driver's braking action - removing the need for a vacuum booster.
Other applications of electro-hydraulic braking systems include the new Alfa Romeo Giulia. It uses a Continental-developed system, called the MK C1 HAD, which combines the ABS, ESC, parking brake, booster functions, driver assistance and autonomous braking features into a single unit. This set-up, like others, is claimed to be smaller, lighter, quieter, more efficient and quicker to respond.
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