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The true potential of electrohydraulic brake actuation revealed
The brakes of an automobile are unquestionably one of the first lines of safety in motoring and, for this reason, vehicle manufacturers have invested and will certainly continue to dedicate significant investments in order to upgrade vehicle braking systems.
Automotive brakes function using mechanical means such as cables, but actuation has been modernized over time to include hydraulic, pneumatic and electrical methods or a combination of these.
Electrohydraulic brake actuation is considered the first big step towards brake-by-wire systems. It was a mechanism partly developed by Bosch and firstly deployed by Mercedes-Benz nearly at the turn of the millennium.
While conventional hydraulic brakes work purely by using hydraulic fluid lines to transfer pressure from the brake pedal to the callipers, in an electrohydraulic system, sensors measure the amount of pressure applied by the driver on the pedal. These signals are interpreted by an electronic control unit (ECU) and, after taking into consideration other parameters such as the speed of the vehicle, operates the brakes via a hydraulic circuit and solenoid valves. This removes any direct physical connection between the driver and the wheel brakes.
Such a system has immense potential that includes boosting the applied brake pressure in the event of an emergency stop. This enables automakers to integrate autonomous emergency braking (AEB) functionality, wherein the vehicle can monitor the road ahead and apply the brakes if required with no intervention from the driver at all.
Electrohydraulic actuation systems on the market today offer various advantages with respect to conventional hydraulic variants, including:
- Time to lock: this is the time with which the braking system can build up to the desired pressure. In a conventional hydraulic braking system, this is between 300 and 600 milliseconds (ms). In the case of electrohydraulic systems, the time to lock is halved at around 150 ms and some solutions have also reached 120 ms. Offering more braking power quickly particularly benefits some conventional ICE vehicles that require higher braking power such as performance and luxury segment vehicles or pick-up trucks. This is why one of the first models with an electrohydraulic actuation system was an ICE vehicle, the Alfa Romeo Giulia.
- Energy recuperation: the ability to convert kinetic energy from braking into electricity that an alternative propulsion vehicle can use to boost battery range and efficiency. This is why more than 90% of the vehicles that use electrohydraulic systems today are either fully electric (BEV) or hybrid (HEV/PHEV). Recuperation in alternative propulsion vehicles ranges between 15-30%, leading to an increased range for BEVs of about 25-30%.
- Emissions: electrohydraulic braking systems can reduce CO2 emissions by about five grams per kilometer. This reduction is comparable to a 50-kilogram weight reduction strategy on vehicle mass; quite a significant achievement.
- Weight: the weight savings achieved are in the range of 30% especially in the case of one-box electrohydraulic systems that are integrated into one component. One-box solutions currently on the market weigh between four-to-five kilograms, while conventional systems (with separated vacuum booster, master cylinders etc.) are in the six-kilogram range.
Technology growth
Given the above advantages, it is no surprise that the demand
for electrohydraulic brake actuation systems is forecasted to
almost double within the next two years. This is expected to
continue rising to more than 28 million units by the year 2026,
resulting in a growth of market valuation from almost USD3 billion
in 2020 to in excess of USD7 billion in 2026. This implies that
electrohydraulic brake actuation will move from a 13% market share
(out of the total brake actuation system market) in 2021 to around
29% in five years' time.
Several automakers such as General Motors, Tesla and Toyota amongst others, have already adopted this system for their various vehicle platforms - the details of which are outlined below:
- Toyota: is and will remain the OEM with the
most electrohydraulic systems in use due to the high penetration of
alternative propulsion vehicles in its portfolio. When looking to
nameplates such as the Corolla, the installation of
electro-hydraulic actuation systems will almost double in five
years' time.
- General Motors and Ford: electrohydraulic
action system use-cases will be driven not only by electrification
but also by pick-ups like the Ford Everest. A perfect example is
the GM T1XX platform that adopts ZF TRW's variant to achieve
improved handling in high-powered vehicles such as the Chevrolet
Silverado.
- Volkswagen: like other German OEMs, the VW
Group adopted the technology due to its low penetration of electric
and hybrid vehicles in its line-up. Now, with the introduction of
the dedicated electric MEB platform, the units of electro-hydraulic
actuation systems are expected to grow in line with the OEM's
output of electric vehicles.
- Tesla: being a new BEV-based OEM, all Tesla
nameplates have included electrohydraulic brakes, with the Model S
supplied with Bosch's i-booster system.
Over time, electrohydraulic brake actuation is expected to percolate into all vehicle segments, but growth is particularly evident in the C-segment. Although it currently accounts for approximately three million units, IHS Markit data forecasts it to rise to nearly 10 million units by 2026, implying a rise of 230% within a five-year time frame.
The demand from the B-, D- and E- segments are also projected to register steep increases during the same time period at 150% (1.2 million to 3 million units); 120% (4.7 million to 10.4 million units); and 140% (1.8 million to 4.4 million units), respectively. This can be attributed to the fact that as vehicle automation climbs greater heights, this technology is seen as an enabler of autonomous braking functions. As a result, suppliers of this component stand to gain by specifically focusing on electrohydraulic brake actuation and brake-by-wire systems.
Going back to the time-to-lock advantage that the electrohydraulic systems have with respect to conventional hydraulic systems, this reduced time to fully actuate the brakes will be fundamental in L4-L5 autonomous vehicles, in which quick communication amongst ECUs will be fundamental for safety reasons.
Outlook
Based on all the above, one could argue that electrohydraulic systems are seemingly destined to be the new commodity in the chassis area; as EPS has become in steering. However, the reason why technology is unlikely to ever reach these heights is down to its cost.
A conventional hydraulic actuation system, consisting of (in the best solution) a brake booster and ESC (electronic stability control), costs OEMs about 50-60% less than an electro-hydraulic actuation system in the two-box configuration. Significantly, the hydraulic system is 80-90% cheaper when compared with a one-box solution.
Despite the increased penetration across various vehicle segments, this price point is still relatively prohibitive for mass-market OEMs. The system's complexity also means that its cost is unlikely to be reduced by much upon mass volume production. IHS Markit forecasts the price of electrohydraulic systems to go down by about 5-6%, mainly driven by a decrease in ECU costs combined with increased volume output.
Looking to the long term, hydraulic lines are set to become obsolete in a fully electrified and automated future. While electrohydraulic systems can - and likely will - be used in autonomous vehicles, they will be slowly substituted by electromechanical variants that do not include fluid lines at all.
This could potentially further increase the performance and handling of a vehicle, rendering the system more compatible with the artificial intelligence that will 'drive' the cars of the future. Many suppliers are now working on this system and IHS Markit data suggests that the first models on the road equipped with such solutions will appear around 2027.