1.8.
Power Brake
Power brakes (also
called "power assisted" brakes) are designed to use
the power of the engine and/or battery to enhance braking power.
Most all modern
vehicles are used power assisted brakes.
Power brakes are
assisted by vacuum, air, hydraulic and or electricity.
Any mechanism which
adds to the driver's effort in applying the brakes is called a power or servo mechanism,
although that effort remains a considerable part of the total braking effort
required.
As
the weight of the vehicle goes on increasing,
more braking effort is needed
to stop the vehicle
when a limit is reached above
which it is not possible for an ordinary driver to apply
the effort needed conveniently.
This limit is normally reached at about three tonnes, the weight of the vehicle. Beyond this, the help of the 'servo action' or self energization of the power brakes is taken.
This limit is normally reached at about three tonnes, the weight of the vehicle. Beyond this, the help of the 'servo action' or self energization of the power brakes is taken.
The four most common
types of power brakes are: vacuum suspended; air suspended; hydraulic
booster, and electro-hydraulic booster.
1.8.1. Construction of Power Brake (vacuum assisted type):
The power brake system is shown in
Fig. 1.17 & 1.18. Vacuum reservoir is connected - through a non -return
valve to the inlet manifold between the carburettor and the engine.
Vacuum reservoir is
further connected to the servo cylinder on both sides of the piston; on left
side the connection is through the control unit, while on the right side it is
direct connection.
Control unit also contains a piston to which two valves are attached. The upper valve controls the connection between the atmosphere and left side of the piston in the servo cylinder.
The lower valve controls the connection between the vacuum reservoir and left side of the servo cylinder piston. The piston in the control unit itself is actuated by the brake pedal through the master cylinder as shown in the Fig. 1.17.
Control unit also contains a piston to which two valves are attached. The upper valve controls the connection between the atmosphere and left side of the piston in the servo cylinder.
The lower valve controls the connection between the vacuum reservoir and left side of the servo cylinder piston. The piston in the control unit itself is actuated by the brake pedal through the master cylinder as shown in the Fig. 1.17.
Fig. 1.17 shows the
layout of a vacuum operated master vac Power brake (servo) system using
drum type brakes on the rear wheels and disc brakes on the front wheels.
1.7.1. Working of Power Brake:
Fig 1.18 illustrates the working operation of vacuum assisted power brake with servo system. When the brake pedal is free, upper valve in the control unit is closed and the lower one is open.
Thus both sides of the piston in the servo cylinder are exposed to the engine vacuum. However, when the brake pedal is pressed to apply the brakes, the pressure of the brake fluid pushes the piston in the control unit up, thereby closing the lower valve there and opening the upper valve.
Thus left side of the servo piston is exposed to atmospheric pressure, whereas vacuum acts on the right side. This causes the servo piston to move to the right.
This movement is utilized to apply the brakes in the wheels through some linkage which may be mechanical or hydraulic.
Thus the force to be exerted by the driver for applying the brakes is considerably reduced and practically the whole of the breaking effort is supplied by the engine vacuum.
Thus both sides of the piston in the servo cylinder are exposed to the engine vacuum. However, when the brake pedal is pressed to apply the brakes, the pressure of the brake fluid pushes the piston in the control unit up, thereby closing the lower valve there and opening the upper valve.
Thus left side of the servo piston is exposed to atmospheric pressure, whereas vacuum acts on the right side. This causes the servo piston to move to the right.
This movement is utilized to apply the brakes in the wheels through some linkage which may be mechanical or hydraulic.
Thus the force to be exerted by the driver for applying the brakes is considerably reduced and practically the whole of the breaking effort is supplied by the engine vacuum.
When the brake pedal is
applied, the movement opens an air valve which lets in atmospheric pressure air
to one chamber of the booster.
Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure.
The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed.
Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure.
The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed.
This force, in addition
to the driver's foot force, pushes on the master cylinder piston. The diaphragm
will stop moving when the forces on both sides of the chamber reach
equilibrium.
1.7.2. Power Brake Booster:
It is the main component or heart of power brakes. It
increases power throughout the system. Fig 1.19 illustrates the vacuum
booster or vacuum servo is used in most modern power
brake systems.
The vacuum/ power
booster is attached between the master cylinder and the brake pedal and
multiplies the braking force applied by the driver.
These units consist of
a hollow housing with a movable rubber diaphragm across the center, creating
two chambers.
When attached to the low-pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered.
A return spring keeps the diaphragm in the starting position until the brake pedal is applied.
When attached to the low-pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered.
A return spring keeps the diaphragm in the starting position until the brake pedal is applied.
A more commonly used
sequence on modern automobiles is to have the servo cylinder, called vacuum
-operated booster operated directly by the brake pedal through leverage.
The booster unit actuates the push rod to operate the master cylinder which is placed in front of the booster unit.
The booster unit actuates the push rod to operate the master cylinder which is placed in front of the booster unit.
Most cars use power
booster unit, which employ a vacuum-powered booster device to provide added
thrust to the foot pressure applied.
The brake booster
usually uses vacuum from the engine intake to boost the force applied by the
pedal on to the master cylinder, or may employ an extra vacuum pump to enable
it.
Without the engine running the brake pedal feels very hard and ineffective on the braking capability.
Without the engine running the brake pedal feels very hard and ineffective on the braking capability.
1.7.3.
Working of Power Booster:
As brakes are applied
atmospheric pressure is allowed to enter the rear of the booster through an air
control valve increasing pressure on the master cylinder.
When the brake pedal is
depressed the air valve moves forward allowing atmospheric pressure to enter
the rear of the brake booster assembly.
The higher pressure in
the rear forces the diaphragm to move forward increasing the pressure applied
to the master cylinder.
A spring in the front
chamber forces the diaphragm rearward when the brake pedal is released.
Vacuum in the intake
manifold is applied to a rubber diaphragm on the vacuum side of the booster
chamber (red speckled area).
The piston rod 7
(See Fig 1.18.) is pushed inside when the brakes are applied and
vacuum is supply to the brake booster assembly rear section through the bell
valve.
When the engine is
running and the brake are not depressed there is vacuum on both sides the
booster diaphragm (vacuum Suspended).
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| Fig 1.17.a: Power brake system arrangement |
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