Analysis of Hydraulic Brake Systems
5.2 Boost System Analysis
5.2.3 Hydraulic Brake Booster
In the hydraulic boost system, the energy source is pressurized fluid. In most cases the steering pump is used (Refs. 5.9, 5.10). The brake system remains totally conventional with only the booster and plumbing added. Because two incompatible fluids are used in the two different circuits, extreme care must be taken not to contaminate one circuit with the fluid of the other. If it does occur, all seals must be replaced. Its compact size and high pressure potential allow it to be used in virtually all applications from passenger cars to light- to medium-weight trucks. Although certain details vary between manufacturers, the hydrau-lic booster without accumulator is limited to master cylinder volumes of 33 to 41 cm3 (2 to 2.5 in.3). A schematic of a hydraulic boost system is shown in Fig. 5-6.
The pressure line runs from the steering pump to the brake booster, and from there to the steering gear and back to the reservoir. A spool valve in the brake booster controls the fluid flow from the steering pump. Without any brake ap-plication, the fluid flow is not affected. During braking the fluid flow is restricted, resulting in a corresponding pressure rise in the fluid and pressure application to the booster piston. The spool valve is designed so that the brake and steering operations do not interfere during either apply or release operation.
A reserve pressure accumulator is provided which allows two to three brake applications with the pump failed or engine stalled. In the ‘70s, a spring-loaded accumulator was used, either integral with the booster, or separately mounted in the engine compartment. Later, a gas-charged accumulator was used for energy storage in the event of a pump failure.
For medium- to heavy-vehicle applications, an electrical pump is used as a reserve energy source. In the event that the normal fluid flow from the steering pump is interrupted, the integral flow switch inside the booster closes, which energizes a power relay and provides electric power to the pump. The reserve pump then circulates the fluid throughout the system and builds up pressure as demanded. Master cylinder volumes up to 107 to 115 cm3 (6.5 to 7 in.3) are accommodated by the electric reserve pump design booster.
Figure 5-6. Hydroboost brake system (Bendix).
Because a single steering pump is used to assist power brakes and power steering, the pump must have sufficient flow rate to accommodate both systems in the event of a combined severe braking and steering maneuver to avoid lack of steering assist.
Modifications to the basic hydraulic booster system have been introduced where the steering pump charges a gas-charged accumulator, which, in turn, pressurizes brake fluid. As the driver applies pedal force, the regulated brake fluid pressure is transmitted to the wheel brakes. The advantages of this system include increased reserve capacity in the event of a pump failure, quicker brake torque response time because the brake line pressure does not have to be built up from zero, and sufficient energy source for ABS application and for combined braking and steering maneuvers.
The size of the accumulator is a function of vehicle weight and the number of stops required by one accumulator charge. The schematic of a hydraulic booster is shown in Fig. 5-7. The pressure pB supplied by the accumulator to the booster in addition to the pedal effort by the driver acts on the master cylinder piston, which, in turn, produces the hydraulic brake line pressure to the wheel brakes.
Figure 5-7. Schematic of hydraulic booster.
The effective input force to the booster is determined by the booster area and the pushrod cross-sectional area.
The booster input area ratio AR is given by
AR = (DB / Dp)2 (5-6) where D = booster piston diameter, cm (in.)
D = pushrod d
B
P iiameter, cm (in.)
The booster pressure ratio pR is defined by the ratio of output pressure to input pressure and may be expressed in terms of the diameters as
p = (D / D )R B mc 2 (5-7)
where Dmc = output to master cylinder diameter, cm (in.)
The brake line pressure may be determined for a given booster pressure (or accumulator pressure) once the booster pressure ratio has been computed. With the brake line pressure determined, vehicle deceleration is computed by Eq. (5-3).
The boost circuit fluid volume, i.e., size and operating pressure range of the accumulator, are a function of the maximum accumulator pressure pA and the initial gas charge pressure pG of the gas used for energy storage by the accumulator. The volume ratio VR of the booster is defined by the ratio of the volume displaced at the booster side to the volume displaced at the output side and is determined by
V = (DR B2 − D ) / DP2 mc2 (5-8) A typical booster characteristic is shown in Fig. 5-8, indicating both boosted and no-boost performance.
Figure 5-8. Master cylinder pressure vs. booster input force.
The minimum size of the accumulator required for a safe deceleration of a vehicle in successive stops may be obtained from the accumulator design chart shown in Fig. 5-9.
It is assumed for the preparation of the accumulator design chart that
approximately 67% of the master cylinder volume is required for an emergency stop. The example illustrated in Fig. 5-9 indicates that a vehicle having a master
cylinder volume Vmc = 49 cm3 (3.0 in.3), five emergency stops, a volume ratio VR = 2.4 computed by Eq. (5-8), and a pressure ratio pG/pA = 0.35 requires an accumulator size of approximately 623 cm3 (38 in.3).
If the same energy had to be stored by a vacuum-assist unit, a volume
approximately 40–50 times larger than that associated with a medium-pressure accumulator, or 100–130 times larger than that associated with a high-pressure accumulator would be required.
The energy stored in the accumulator is affected by the ambient temperature.
The fluid volume available for braking at high pressures decreases with decreasing temperature. For example, an accumulator having a volume of 656 cm3 (40 in.3) available between the pressure range of 1448 to 1792 N/cm2 (2100 to 2600 psi) when operating at 353 K (176°F), provides only 246 cm3 (15 in.3) when the temperature is 233 K (–40°F) (Ref. 5.2).
For vehicles that do not have a steering pump, and to avoid the potential problems associated with the use of two different fluids, an integral
accumulator/pump system has been designed. It is operated with brake fluid only. Because brake fluid does not provide adequate lubrication for long periods of operation, an intermittently operating electrical pump is used to charge the accumulator only when needed. The compact design is advantageous for installation in the crowded engine compartment. The basic design has been expanded for application to ABS braking systems.