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Anti-lock Braking System(ABS)
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1. GENERAL INFORMATION :
A. NECCESSITY OF ABS WITH EBD :
Whenever the brakes are applied, the road
wheels transmit this braking force to the road
surface. To achieve optimal deceleration each
wheel should be braked to the point of imminent
lock. In this manner maximum possible braking
force would be imparted to the road surface.
However if a wheel is braked beyond this level it
will lock which can result in the vehicle skidding
and the driver loosing control.
EBD :
During normal braking there is always a level of
load transfer from the rear axle to the front. Due to
this occurrence it is necessary to ensure that the
rear axle wheels do not reach the point of lock
before the front axle wheels.
On vehicles without ABS and EBD this is
achieved through the use of a pressure conscious
regulating valve (PCRV) which limits the pressure
to the rear axle to give a front first locking
sequence. The PCRV allows only a
predetermined pressure to the rear wheels to
avoid locking of the rear axle wheels before the
front axle wheels. As a mechanical fixed ratio
component it cannot adapt to the condition of the
vehicle brakes, tyres, road or the vehicle load
conditions, and hence it cannot achieve ideal
braking at the rear axle
To enable optimum braking at the rear axle while
maintaining vehicle stability during braking, EBD
is provided. It enables the distribution of brake
force between the front and rear axle brakes to be
adjusted at any point of time, depending on the
condition of the vehicle brakes, tyres; the road, or
vehicle load conditions.
In the case of EBD failure the driver will be alerted
.It is unsafe to drive the vehicle with EBD not
working.
ABS:
In emergency braking conditions and when driving
on wet or slippery road surfaces like snow and
Ice, gravel, steel plates and bridges etc.., it is
possible that the braking forces at the wheel
exceed those possible to transmit to the road
surface. This will result in wheel locking (stopping
rotating). If one or more wheels lock under braking
the vehicle can go into a skid and no longer be
controlled by the driver as he will have lost
steerability.
To avoid the wheels from locking and prevent the
vehicle from skidding ABS is provided. It prevents
the wheels from locking and maintains the
steerability of the vehicle thereby helping the
driver to maintain stability of the vehicle while
bringing it to a stop in the safest possible way.
In case of ABS failures the driver will be alerted
and the conventional braking circuit will still be
operational.
ENGINE DRAG CONTROL (EDC) :
Engine braking is an act of using engine
compression as braking force instead of service
brakes. This is done when prolonged and
continuous brakes have to be applied. The
efficiency of the service brakes reduces in such
applications as they are subjected to high
temperatures. In such situations engine braking is
preferred as engine has a system to dissipate the
heat generated.
During such braking the vehicle will be running in
gear such that the wheels are connected to the
engine. Hence there are chances that the engine
may get stalled if the speed is very less resulting
in the drive wheels getting locked. EDC avoids
such situation by slightly increasing the engine
torque to avoid engine stalling and thus avoids the
wheel slipping.
In case of EDC failure the ABS will still be
working.
Cornering Stability Control (CSC):
CSC Stabilizes the vehicle in bends by pressure
reduction at the inner wheels during partial
braking, Countermeasure to ensure stability while
braking in curves.
NOTE:
ABS ECU in vehicle calculates the Vehicle
Speed and Odometer signals. If ABS ECU
fails or CAN bus fails vehicle speed and
odometer will not function.
ABS ECU interacts and controls the TOD
functionality. If the ABS communication with
TOD ECU is lost, standard ABS functions
shall continue operating normally unless there
is no other fault.