Hydro-boost and Electro-Hydraulic power boosters

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Hydro-boost and Electro-Hydraulic power boosters
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Diesel engines

• Diesel engines do not produce manifold vacuum
• Diesel engines do not have a throttle plate to
  restrict air flow at low power levels and as a
  result the air pressure inside the intake
  manifold is always the same as outside air
  pressure.
• In order to use a conventional vacuum power brake
  booster a vacuum pump is required on a diesel
  equipped car.

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Mechanical vacuum pump

• The vacuum pump for the power brakes can be
  driven by
• V-belt or serpentine belt off of the crankshaft
• An eccentric lobe or a slot on the back of a
  camshaft
• Built into the alternator
• Gear driven off of the camshaft or crankshaft

• A vacuum pump adds cost to the vehicle. It also
  adds a parasitic load to the engine

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Low Vacuum in modern cars

• Modern cars may not be able to provide sufficient
  vacuum for the power booster.
• This can be caused by
• Retarded exhaust valve timing when driving at
  cruise speeds VVT.
• Cylinder deactivation strategies for increased
  fuel economy.
• High EGR flow rates.

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Power booster size

• The large volume taken up by the vacuum booster
  chamber makes designing a compact front drive
  vehicles more difficult.
• The power booster needs to be located directly
  forward of the brake pedal assembly, in an area
  where we would like to place the engine and
  transmission.
• Eliminating the bulk of the vacuum booster allows
  more room under the hood for engine and driveline
  components.

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Hydro-boost system

• The hydro-boost system developed by Bendix
  corp. uses hydraulic pressure from the power
  steering pump to operate the brake power assist
  system.
• The power steering pump can supply up to 1200 psi
  of pressure to the hydroboost unit.
• The servo piston in the hydroboost unit has a
  surface area of about 1 ½ which allows the
  hydroboost system to provide up to 2000 lbs. of
  force to the master cylinder.

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Fail safe

• The accumulator provides a reserve of hydraulic
  pressure in the event the engine stalls or the
  power steering pump belt breaks.
• An accumulator provides a reserve of about 2
  ounces of power steering fluid at 900 1200 psi.
• Accumulator fluid can provide only one
  application with full power assist.
• Like all other types of power assist systems if
  the unit fails the brakes can still applied by
  pedal pressure alone.

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Accumulator

• The accumulator is anodized silver or gold and is
  located at the rear of the hydroboost housing.

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Components

• Power steering pump no different that the one
  used on a vehicle with a vacuum power booster.
• Hydroboost unit
• Mounted on the firewall
• An actuator rod connects the hydroboost unit to
  the brake pedal
• Studs on the front of the hydroboost unit for
  mounting the master cylinder
• Accumulator is mounted to the side and below the
  centerline of the unit and can be identified by
  gold or silver anodized finish
• 3 hoses

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Hoses

• High pressure supply line from the power steering
  pump
• High pressure line connects the hydroboost unit
  to the steering gear.
• Low pressure line returns exhausted fluid back to
  the power steering reservoir
• Vehicles built in the 1970s and early 1980s used
  flair nut fittings on the high pressure hoses.
  Vehicles built afterwards use o-ring type
  fittings on the high pressure hoses.
• Low pressure line may use slip on hose fittings
  and hose clamps

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Servo piston

• The pressurized power steering fluid is applied
  to a small piston at the front of the unit.
• The piston is about 1 ¾ OD and is sealed at the
  front by a square cut o-ring
• The piston connects to the master cylinder via a
  non-adjustable pushrod

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Control valve spool valve

• The flow of pressure to the operating cylinder
  and piston is controlled by a spool valve mounted
  at the top of the unit.
• The spool valve has lands and grooves that open
  passages to the operating cylinder or exhaust
  flow back to the pump reservoir.
• The spool valve is connected to the input rod by
  a lever mechanism.
• The spool valve is centered by springs on both
  ends.

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Reaction spring

• The brake pedal connects to the piston via the
  input rod.
• Between the input rod and the piston is a
  reaction spring.
• The reaction spring is attached to one end of the
  control valve lever.

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Operation - brakes not applied

• Spool valve blocks pressure passages to the
  operating cylinder and opens exhaust passage to
  allow any pressure in the operating cylinder to
  return back to the reservoir

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Operation - brake application

• When the input rod connected to the brake pedal
  is pushed forward the resistance of the pistons
  in the master cylinder causes the spring on the
  input rod to compress the reaction spring which
  causes the lever to move the spool valve.
• Movement of the spool valve opens the passage for
  fluid pressure to enter the operating cylinder
• The pressure applied to the rear of the piston
  pushes the master cylinder pistons forward

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Operation - hold

• As the piston moves forward the lever pulls the
  spool valve back into the hold position
• Both pressure and exhaust ports are closed so
  that the pressure applied to the piston remains
  constant

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Operation - brake release

• When pressure on the input rod and reaction
  spring is released the spring pushes the spool
  valve back to the neutral position where the
  pressure port is closed and the exhaust port is
  open

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Reserve application after engine stalls

• The accumulator is connected to the pressure
  inlet side of the spool valve
• When the engine is running power steering fluid
  pressurizes the accumulator.
• A one way check valve is located in the passage
  between the inlet hose fitting and the spool
  valve / accumulator
• If the engine stalls the check valve closes as
  the pressure drops maintain enough pressure and
  volume to the spool valve for one application of
  the brake.

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Service and repair

• Although the hydroboost unit can be rebuilt in
  the field it is normally replaced with a factory
  remanufactured unit as special tools and training
  are required in overhauling the unit.
• R R of the unit is similar to that of a vacuum
  booster with the exception that the three
  hydraulic lines need to be removed.
• Since all three hose fittings are different it is
  not necessary to identify which hose goes where.

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Discharging the accumulator

• Prior to replacement or service the accumulator
  pressure needs to be released.
• Depressing and then releasing the brake pedal 5
  to 10 times should be sufficient to release all
  pressure in the system.

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Fluid leaks

• Brake fluid accumulating around the outside of
  the hydroboost unit is an indication of a failure
  of the rear piston seal in the master cylinder
  and is not a fault of the hydroboost system.
• Power steering fluid leaks can usually be
  observed by cleaning the outside of the unit with
  BrakeKleen and then starting the engine.
• A black light use with dye may make detection of
  the leak easier.

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System testing

• Before doing any system testing make sure the
  brake fluid and power steering fluid is correct
  and check the power steering belt tension.
• With the engine shut off depress and release the
  brake pedal several times to release the
  accumulator pressure.
• Depress the brake pedal slightly and start the
  engine.
• The pedal shut drop slightly and then push back
  up slightly.
• If this does not occur the booster unit is the
  likely fault.

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Electro-Hydraulic power boosters
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Two types of electro-hydraulic power boosters

• Stand alone systems are electro-hydraulic system
  that are not integrated with an ABS system.
• If the vehicle has ABS the abs system is
  completely independent of the booster system
• Integrated ABS / booster systems
• The ABS system and the power booster system work
  together
• This type of system is common on late model cars

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Powermaster system

• Manufactured by AC Delco for use on GM cars in
  the 1980s and 1990s
• Used extensively with turbocharged vehicles.
• Used brake fluid as a hydraulic fluid.
• An electric fluid pump supplied brake fluid under
  high pressure to the booster chamber.
• The booster piston and cylinder were integrated
  with the master cylinder

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Construction
Switch

• Fluid reservoir with 3 chambers
• Hydraulic pump electrically driven
• Master cylinder design
• Integral boost cylinder and piston
• Control valve inside master cylinder supplies
  pressure to operating chamber as needed
• Pump relay and control circuit
• Pressure switch mounted on master cylinder body
• Accumulator attached to the side of the master
  cylinder body

Reservoir
Accumulator
Pump Motor
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Accumulator
Warning !

• The accumulator holds a reserve of several ounces
  of brake fluid under very high pressure.
• You must follow the manufactures service
  procedures to avoid injury and property
  destruction.

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Operation

• When accumulator pressure drops below 600 psi
  pump will run until pressure exceeds 720 psi.
• Check valve in accumulator holds pressure inside
  accumulator when pump is not running
• Control valve operates in the same manner as a
  hydroboost unit
• Control valve opens the pressure passage to the
  operating chamber when pedal is depressed and
  closes it under hold and release conditions
• Exhaust port of control valve allows pressure in
  operating chamber to be exhausted back to brake
  fluid reservoir when in released condition and
  closes when in applied or hold mode.

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Fail safe mode

• Accumulator holds sufficient volume for several
  applications of the brake in the event of an
  electrical or pump failure.
• Fluid pressure switch will activate a warning
  light on the dash if pressure is below minimum
  level while the ignition is on.

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Fluid level

• The reservoir is divided into 3 chambers 2 for
  the brake circuits and the third for the booster.
  
• The fluids do not intermix when replacing the
  brake fluid the booster fluid must be replace
  separately from the regular brake fluid.
• All three chambers use DOT 3 fluid
• Before checking the fluid levels the brake pedal
  should by applied and released 20 times with the
  ignition key off.

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Service

• Before replacing any components or removing the
  unit discharge the accumulator by pumping the
  brake pedal 20 times with the key off.
• Serviceable components
• Pressure switch
• Accumulator
• Hydraulic pump

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Testing the system

• With the key off depress and release the brake
  pedal 6 times.
• With your foot on the brake pedal turn the
  ignition switch to the run position but dont
  start the engine.
• You should here the electric pump run and feel
  the pedal drop slightly and then rise slightly
  against you foot. You also may see the indicator
  light flash momentarily.

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Integrated ABS and electro-hydraulic brakes

• The addition of an accumulator in a brake system
  opens up the possibility of adding a number of
  automated system to the vehicle that can improve
  performance and safety.
• With a reserve of pressurized brake fluid we can
  add a Traction Control System TCS that uses the
  ABS wheel speed sensors to detect when a wheel is
  slipping on snow or ice. The system can then
  reduce engine torque and apply the brake on the
  slipping wheel so that the torque is sent to the
  wheels that are not slipping.
• A similar strategy is used in an Automatic
  Stability Control system ASC. During corning
  the inside rear brake can be applied slightly to
  improve steering response and flatten the turn.

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Teves MK 2 Integrated Booster
Power booster section
Pressure Switch
Three way valve
Hydraulic Solenoid unit
Electric Motor Pump
Accumulator
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Integrated ABS Hydraulic booster systems

• Integrated ABS-Boosters function similarly to the
  Powermaster stand alone unit.
• A power asset cylinder piston is located at the
  rear of the master cylinder.
• A spool valve and reaction spring opens and
  closes pressure and exhaust ports in proportion
  to the pressure applied by the brake pedal rod.
• An electric pump and accumulator supply brake
  fluid under high pressure to the spool valve.

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Brake by wire

• Brake-by-wire systems have been introduced by
  several manufacturers.
• A brake by wire system uses accumulator pressure
  for power brake assist.
• There is no power booster piston between the
  brake pedal and master cylinder.
• Accumulator pressure from the electrically driven
  pump is added to the pressure developed by the
  master cylinder inside the ABS hydraulic unit.
• If there is an electrical failure the fluid from
  the master cylinder passes straight through the
  ABS unit in fail safe mode.