Electric Vacuum Pumps (EVPs) offer several advantages, but they also come with significant drawbacks. One notable disadvantage is noise. Moreover, in high-altitude areas where air pressure is lower, EVPs struggle to achieve the same vacuum strength as in lower-altitude regions. This results in reduced vacuum booster assistance and increases the pedal force required for braking.
Two critical shortcomings of EVPs include their limited lifespan and energy inefficiency. Some inexpensive EVP models have lifespans of less than 1,000 hours. Additionally, EVPs do not effectively utilize braking energy recovery systems found in electric vehicles (EVs). During coasting or braking, EVs can use friction to generate current, charging the battery and storing energy. Maximizing this energy recovery can significantly enhance vehicle range and fuel efficiency. In a compact car’s NEDC cycle, full braking energy recovery can save approximately 17% of energy. In urban conditions, where braking energy can account for up to 50% of total energy consumption, improving energy recovery rates becomes crucial for extending vehicle range and improving economy.
EVPs are integrated into the braking system in parallel, meaning the regenerative braking force of the motor is added to the original friction braking force without adjusting it. However, EVPs typically achieve a low energy recovery rate, around 5% of systems like the Bosch iBooster. Furthermore, the coupling and switching between motor regenerative braking and friction braking can lead to discomfort and shock during braking maneuvers.
Despite its drawbacks, EVPs remain widely adopted due to low sales of electric vehicles and limited domestic expertise in designing chassis specifically for electric vehicles, which often results in the use of copied chassis designs.
In scenarios where EVPs are not suitable, Electronic Hydraulic Brake Boosters (EHBs) become necessary. EHBs come in two main types: one equipped with a high-pressure accumulator, known as the “wet” type, and the other where the motor directly activates the master cylinder piston, known as the “dry” type. Hybrid new energy vehicles typically utilize the former type, while notable examples of the latter include the Bosch iBooster.
First, let’s delve into EHBs equipped with high-voltage accumulators, which represent an enhanced iteration of Electronic Stability Program (ESP). ESP itself can be categorized as a form of EHB, offering active braking capabilities.
The left picture is the schematic diagram of a wheel of ESP:
a–control valve N225
b–dynamic control high-pressure valve N227
c–oil inlet valve
d–oil outlet valve
e–brake cylinder
f–return pump
g–active servo
h–low-pressure accumulator
During the boosting stage, the motor and accumulator work together to build up pre-pressure, enabling the return pump to draw brake fluid. This process involves closing N225 and opening N227, while keeping the oil inlet valve open until the wheel achieves the necessary braking force.
The structure and functionality of EHBs have evolved significantly from their origins as adaptations of Electronic Stability Programs (ESP). Unlike the low-pressure accumulators of ESP systems, EHBs utilize high-pressure accumulators capable of accumulating and reusing pressure multiple times. This advancement allows for more efficient braking performance and durability, particularly under demanding conditions.
Technological Challenges and Innovations
The high-pressure accumulator’s ability to withstand and maintain pressure through multiple uses reduces strain on core components like the plunger pump, which is crucial for managing high temperatures and pressures. Continuous and frequent use of ESP’s low-pressure accumulator can lead to premature wear and reduced lifespan of these critical components.
Application and Adoption Across Automotive Brands
Furthermore, the limitations of the low-pressure accumulator restrict ESP to providing a maximum braking force of approximately 0.5g, whereas standard braking forces typically exceed 0.8g. Initially designed for sporadic emergency use—no more than 10 times a year—the ESP system is unsuitable for regular braking demands and is instead employed primarily in auxiliary or emergency situations.
The image above illustrates the high-pressure accumulator used in Toyota’s EBC system, resembling a gas spring in design. Manufacturing high-pressure accumulators presents significant challenges. Initially, Bosch utilized energy storage balls, but nitrogen-based high-pressure accumulators have proven to be the most effective solution.
Toyota pioneered the integration of Electronic Hydraulic Brake (EHB) systems into mass-produced vehicles with the introduction of the first-generation Prius in late 1997, branded as EBC by Toyota. Compared to traditional Electric Vacuum Pumps (EVPs), EHB systems offer substantial improvements in braking energy recovery. This is achieved through decoupling from the pedal and employing a sequential system. Initially, the motor captures energy during braking, which is then used for energy recovery, enhancing overall efficiency.
Challenges and Future Directions
By the end of 2000, Bosch introduced its own Electronic Hydraulic Brake (EHB) system, which Mercedes-Benz adopted for the SL500, branding it as SBC. Initially utilized as an auxiliary system in fuel vehicles, Mercedes-Benz found the SBC system overly complex with numerous pipes. This led to recalls affecting models such as the E-Class, SL-Class, and CLS-Class sedans due to high maintenance costs, with replacements costing over 20,000 yuan each. Mercedes-Benz discontinued the SBC system after 2008.
Undeterred, Bosch continued to refine their EHB technology, eventually transitioning to nitrogen-based high-pressure accumulators. In 2008, Bosch launched HAS-HEV, which gained widespread adoption in hybrid vehicles across Europe and by BYD in China.
Following Bosch’s lead, TRW also entered the EHB market with their own system, known as SCB. Today, many of Ford’s hybrid models incorporate SCB technology, highlighting its ongoing relevance and application in automotive braking systems.
The Electronic Hydraulic Brake (EHB) system has faced significant challenges, including complexity, vulnerability to vibration in high-voltage accumulators, reliability concerns, large size, high costs, and questionable service life, resulting in substantial maintenance expenses.
In 2010, Hitachi introduced a groundbreaking innovation with the world’s first dry EHB system, named E-ACT. This pioneering technology represented a significant advancement in EHB capabilities. The development of E-ACT spanned a rigorous 7-year R&D cycle, including nearly 5 years of intensive reliability testing.
Bosch entered the field in 2013 with the launch of the first-generation iBooster, followed by the second-generation iBooster in 2016. The second-generation iBooster achieved a level of quality comparable to Hitachi’s E-ACT, marking a milestone where Japanese advancements in EHB technology surpassed those of their German counterparts.
?The above picture shows the structure of E-ACT
The Rise of Dry EHB Technology
The dry Electronic Hydraulic Brake (EHB) system directly propels the push rod using a motor, which then actuates the master cylinder piston. This transformation of rotational force into linear motion force is facilitated by a roller screw mechanism, such as in Hitachi’s E-ACT. Additionally, a ball screw acts as a reducer, slowing down the motor’s speed to enhance torque for pushing the master cylinder piston.
The concept behind dry EHB systems is straightforward, but its delayed adoption stemmed from the stringent reliability requirements of automotive braking systems. These systems necessitate ample performance redundancy and reliability. Challenges include designing a motor that is compact yet capable of high speeds (exceeding 10,000 revolutions per minute), generating substantial torque, and effectively dissipating heat. Precision machining is crucial for the reducer, and optimizing system integration with the master cylinder hydraulic system adds further complexity.
As a result, the development and implementation of dry EHB technology occurred relatively late in the automotive industry.
?The picture above shows the internal structure of the first-generation iBooster.
The first-generation iBooster employs a worm gear for two-stage deceleration, significantly amplifying linear motion torque. This design choice has been adopted broadly across Tesla’s entire lineup, as well as Volkswagen’s new energy vehicles and the Porsche 918. General Motors also integrates the first-generation iBooster in models like the Cadillac CT6 and Chevrolet Bolt EV.
Renowned for its efficiency, the first-generation iBooster reportedly converts up to 95% of regenerative braking energy into electricity. This capability greatly extends the cruising range of electric vehicles. Moreover, its response time is notably quicker, about 75% faster compared to wet EHB systems utilizing high-pressure accumulators.
The image on the right displays our product, Part# EHB-HBS001 Electric Hydraulic Brake Booster, identical to the left image. The left assembly showcases the second-generation iBooster, which incorporates a dual-stage worm gear leading to a primary ball screw for deceleration. This design innovation significantly reduces its overall size while enhancing control accuracy.
Our product line features four series with boosters ranging from 4.5kN to 8kN, suitable even for 9-seat small passenger cars. This versatility underscores our commitment to providing high-performance braking solutions tailored to diverse automotive needs.
In 2018, the IBC system will debut on GM’s K2XX platform, primarily used for their pickup series, designed for fuel vehicles, yet adaptable to electric vehicles.
The complexity of designing and controlling hydraulic systems necessitates extensive experience and precision machining capabilities, an area where China has historically lagged. The reliance on foreign expertise has led to a near-complete monopoly by joint ventures and international firms in designing and manufacturing hydraulic brake systems, including EHB. The integration and overall design of EHB systems must align closely with hydraulic components, reinforcing foreign dominance in this critical automotive technology sector.
In contrast to EHB, there’s the promising Electronic Mechanical Brake (EMB) system, theoretically nearly flawless. It eliminates hydraulic components, offering cost efficiency and a rapid response time of just 90 milliseconds, significantly faster than iBooster. However, EMB has several drawbacks: Firstly, it lacks a backup system, demanding unparalleled reliability in power and bus communication systems. Secondly, its limited braking force poses a challenge due to motor size constraints, especially for small motors unable to generate the required 1-2 kW typical of conventional cars. Thirdly, operating in high-temperature environments near brake pads strains semiconductor components and permanent magnet motors, which are prone to demagnetization. Finally, developing EMB for diverse vehicle chassis is complex and costly, inhibiting modular design and driving up development expenses.
Addressing the insufficient braking force of EMB remains a hurdle, given the physical limitations of permanent magnets and the demanding operational environment. Nevertheless, EMB has found practical applications such as in electronic parking systems (EPB) and rear-wheel installations like the Audi R8 E-TRON, where high braking force isn’t critical.
?The front wheel of the Audi R8 E-TRON is still a traditional hydraulic design, and the rear wheel is an EMB.
?The picture above shows the EMB system of the R8 E-TRON.
The motor diameter for EMB systems is typically comparable to the size of a little finger. Major brake system manufacturers like NTN, Shuguang Industry, Brembo, NSK, Wanxiang, Wanan, Haldex, and Wabco are all actively engaged in advancing EMB technology. Established leaders such as Bosch, Continental, and ZF TRW are also making significant strides in this area. Despite these efforts, it’s unlikely that EMB will completely supplant hydraulic braking systems.
