2. Brake Principle
Vehicle is accelerated
Energy supplied by the engine causes the vehicle speed to increase
Friction
Tractive
resistance
Large amount of
energy still remain
which is called
Kinetic Energy
The existence of kinetic energy can be seen when a vehicle is moving
and neutral gear is selected. The car does not immediately stop;
instead it travels for a considerable distance before become stationary.
Purpose of a Brake
3. Brake Principle 2
In order to drastically reduce the stopping distance of a
vehicle , BRAKE SYSTEM is needed to convert the
energy at a faster rate.
CONVERT KINETIC ENERGY TO HEAT
ENERGY
4. Brake Principle 3
The speed of energy conversion controls the Rate of
Deceleration
Heat
Kinetic Energy
Road force
at driving
wheel
Brake Force
Padel
Heat generation at the brake is obtained by rubbing a fixed pad
or shoe against a rotating object driven by the motion of the
vehicle
5. Brake Principle 4
The brake engineer has two challenges:
1.Create enough deceleration to stop
the car as quickly as the driver wishes,
without exceeding the drivers comfort
level with regard to pedal effort or
pedal travel.
2. Manage the resulting heat energy so
as not to damage the brake system or
the rest of the vehicle.
An interesting observation is that a typical car of this size
has an engine with about 90-105 KW (120-140 HP). This
means that the brakes have to deal with about TWICE
THE POWER of what the engine puts out.
6. Brake Principle 5
Since the kinetic energy of a moving vehicle is a function of the square of
the speed, the speed from which you wish to stop a vehicle is much more
important than the mass of the vehicle. When selecting a brake system
the performance of the powertrain must be taken into consideration.
7. Brake Principle 6
Weight transfer increases the load on the front
wheels while the load on the rear wheels is reduced.
8. Brake Principle 7
When the limit is reached…….the wheel starts to
skid
At that moment extra applied at brake pad
is useless. No benefit is reducing the stopping
distance
Road adhesion is affected by
1. type of road surface
2. Condition of road surface
3. Design of tread & composition of
tread material and depth of tread
On icy surface …………light pressure can
lock-up the brake system
Important information: adhesion between the
tyre & road is the main factor that control the
minimum stopping distance
9. Brake Principle 8
Equation involve:-
2
2
1
mvUk =
FsUw =
The equation for kinetic energy , that
is energy of motion may be given by:-
The work done in bringing the vehicle
to rest is given by :-
When braking a moving vehicle to standstill, the work done
by the brake must be equal to initial kinetic energy
possessed by the vehicle
kw UU =
Where F is an average brake force
s
mv
F
2
2
=
10. Example
A car of mass 800kg is travelling at 36km/h.
Determine the following.
a)The kinetic energy it possesses
b)The average braking force to bring it to rest in 20
meters.
ANSWER : A) 40KJ. B) 2KN
11. Type of Brake
Generally all motors vehicle must be equipped with at least two mutually
independent brake systems.
1.Service Brake System – primarily serve to reduce vehicle speed
2.Emergency brake system – Used to stop a vehicle in the event of a
malfunction
3.Parking brake system – to prevent unintentional rolling of a stationary
vehicle ( Hand Brake)
12. Brake System Requirement
Wheel brake must meet the following
requirements
1.Uniform effectiveness
2.Smooth, graduated response
3.Resistance to contamination & corrosion
4.Extreme reliability
5.Durability
6.Resistance to wear
7.Ease of maintenance
14. Braking Mechanism
When you apply the brakes,
brake fluid is forced under
pressure into the wheel
cylinder, which in turn
pushes the brake shoes
into contact with the
machined surface on the
inside of the drum. When
the pressure is released,
return springs pull the
shoes back to their rest
position. As the brake linings wear, the shoes
must travel a greater distance to reach the drum.
When the distance reaches a certain point, a self-
adjusting mechanism automatically reacts by
adjusting the rest position of the shoes so that they
are closer to the drum.
15. Master Cylinder
To increase safety, most modern car brake systems are broken into two circuits, with two
wheels on each circuit. If a fluid leak occurs in one circuit, only two of the wheels will lose
their brakes and your car will still be able to stop when you press the brake pedal.
The master cylinder supplies pressure to both circuits of the car. It is a remarkable device
that uses two pistons in the same cylinder in a way that makes the cylinder relatively failsafe.
The combination valve warns the driver if there is a problem with the brake system, and also
does a few more things to make your car safer to drive.
16. Master Cylinder operation
When brake pedal is pressedWhen brake pedal is pressed - primary piston closes off the primary compensating
port to develop pressure in primary circuit. Fluid pressure created in front of the
primary piston forces against rear of secondary piston, so that it also moves.
When secondary piston moves, it covers secondary compensating port and pressure
develop in secondary circuit.
As brake pedal continues to be pressed , both piston will moved to displace fluid to
their circuits and apply the brakes.
17. Master Cylinder operation
When pedal is released:When pedal is released:
Recuperation – action of the piston to return to its position that creates lowRecuperation – action of the piston to return to its position that creates low
pressure in front of the piston so that momentarily pressure in reservoir ispressure in front of the piston so that momentarily pressure in reservoir is
higher than the pressure in cylinder. Thus cause small amount of fluid tohigher than the pressure in cylinder. Thus cause small amount of fluid to
flow from reservoir to the front part of cylinder.flow from reservoir to the front part of cylinder.
This keep the cylinder full of fluid ready for next brake application.This keep the cylinder full of fluid ready for next brake application.
18. Master Cylinder Mechanism
When the first circuit leaks, the pressure
between the primary and secondary
cylinders is lost. This causes the primary
cylinder to contact the secondary cylinder.
Now the master cylinder behaves as if it has
only one piston. The second circuit will
function normally, but the driver will have to
press the pedal further to activate it. Since
only two wheels have pressure, the braking
power will be severely reduced.
Loss of brake fluid will increaseLoss of brake fluid will increase
pedal travel and brakes willpedal travel and brakes will
operate on two wheels onlyoperate on two wheels only
20. Drum Brake 2
Figure shows that as the brake shoes contact the drum, there is a kind of wedging
action, which has the effect of pressing the shoes into the drum with more force.
The extra braking force provided by the wedging action allows drum brakes to use a
smaller piston than disc brakes. But, because of the wedging action, the shoes must be
pulled away from the drum when the brakes are released. This is the reason for some of
the springs. Other springs help hold the brake shoes in place and return the adjuster
arm after it actuates.
21. Drum Brake 3
In the figure, you can see that as the pad wears down, more space will form between the
shoe and the drum. Each time the car stops, the shoe is pulled tight against the drum.
When the gap gets big enough, the adjusting lever rocks enough to advance the
adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it
unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes
wear a little more, the adjuster can advance again, so it always keeps the shoes close to
the drum.
23. Disk Brake 1
A disc brake system consists of a brake
rotor, a brake caliper and brake pads.
When the brake pedal is applied,
pressurized hydraulic fluid squeezes the
brake pad friction material against the
surface of the rotating brake disc.
The result of this contact produces
friction which enables the vehicle to slow
down or stop.
The disc brake is a lot like the brakes on a
bicycle. Bicycle brakes have a caliper,
which squeezes the brake pads against
the wheel. In a disc brake, the brake pads
squeeze the rotor instead of the wheel,
and the force is transmitted hydraulically
instead of through a cable. Friction
between the pads and the disc slows the
disc down.
24. Disk Brake 2
Vented disc brakes have a set of
vanes, between the two sides of the
disc, that pumps air through the
disc to provide cooling
The single-piston floating-caliper disc brake is self-centering
and self-adjusting. The caliper is able to slide from side to side
so it will move to the center each time the brakes are applied.
Also, since there is no spring to pull the pads away from the
disc, the pads always stay in light contact with the rotor (the
rubber piston seal and any wobble in the rotor may actually
pull the pads a small distance away from the rotor).
In cars with disc brakes on all four wheels, an emergency
brake has to be actuated by a separate mechanism than the
primary brakes in case of a total primary brake failure. Most
cars use a cable to actuate the emergency brake.
25. Valves for Brake System
The valve does the job of three separate devices:
•The metering valve
•The pressure differential switch
•The proportioning valve
The metering valve section of the combination valve is required on cars that have disc brakes on
the front wheels and drum brakes on the rear wheels. You know that the disc brake pad is normally
in contact with the disc, while the drum brake shoes are normally pulled away from the drum.
Because of this, the disc brakes are in a position to engage before the drum brakes when you push
the brake pedal down.
The metering valve compensates for this, making the drum brakes engage just before the disc
brakes. The metering valve does not allow any pressure to the disc brakes until a threshold
pressure has been reached. The threshold pressure is low compared to the maximum pressure in
the braking system, so the drum brakes just barely engage before the disc brakes kick in.
Having the rear brakes engage before the front brakes provides a lot more stability during braking.
Applying the rear brakes first helps keep the car in a straight line, much like the rudder helps a
plane fly in straight line.
Metering Valve
26. Valves for Brake System
Pressure Differential Switch
The pressure differential valve is the device that alerts you if you have a leak in one
of your brake circuits. The valve contains a specially shaped piston in the middle of
a cylinder. Each side of the piston is exposed to the pressure in one of the two
brake circuits. As long as the pressure in both circuits is the same, the piston will
stay centered in its cylinder. But if one side develops a leak, the pressure will drop
in that circuit, forcing the piston off-center. This closes a switch, which turns on a
light in the instrument panel of the car. The wires for this switch are visible in the
picture shown.
27. Valves for Brake System
Proportioning Valve
The proportioning valve reduces the pressure to the rear brakes. Regardless of
what type of brakes a car has, the rear brakes require less force than the front
brakes.
The amount of brake force that can be applied to a wheel without locking it
depends on the amount of weight on the wheel. More weight means more brake
force can be applied. If you have ever slammed on your brakes, you know that an
abrupt stop makes your car lean forward. The front gets lower and the back gets
higher. This is because a lot of weight is transferred to the front of the car when
you stop. Also, most cars have more weight over the front wheels to start with
because that is where the engine is located.
If equal braking force were applied at all four wheels during a stop, the rear wheels
would lock up before the front wheels. The proportioning valve only lets a certain
portion of the pressure through to the rear wheels so that the front wheels apply
more braking force. If the proportioning valve were set to 70 percent and the brake
pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear
brakes would get 700 psi.
28. Vacuum Booster
Back in the day, when most cars had drum
brakes, power brakes were not really necessary
-- drum brakes naturally provide some of their
own power assist. Since most cars today have
disk brakes, at least on the front wheels, they
need power brakes. Without this device, a lot of
drivers would have very tired legs.
The vacuum booster is a metal canister that
contains a clever valve and a diaphragm. A rod
going through the center of the canister
connects to the master cylinder’s piston on one
side and to the pedal linkage on the other.
29. Vacuum Booster 2
The photo above shows the check valve, which is a one-way valve that only allows air
to be sucked out of the vacuum booster. If the engine is turned off, or if a leak forms in
a vacuum hose, the check valve makes sure that air does not enter the vacuum
booster. This is important because the vacuum booster has to be able to provide
enough boost for a driver to make several stops in the event that the engine stops
running -- you certainly don't want to lose brake function if you run out of gas on the
highway.
31. ABS 1
The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact
patch is sliding relative to the road) has less traction than a non-skidding wheel. If you
have been stuck on ice, you know that if your wheels are spinning you have no
traction. This is because the contact patch is sliding relative to the ice. By keeping the
wheels from skidding while you slow down, anti-lock brakes benefit you in two ways:
You'll stop faster, and you'll be able to steer while you stop.
There are four main components to an ABS system:
•Speed sensors
•Pump
•Valves
•Controller
32. ABS 2
Speed Sensors
The anti-lock braking system needs some way of knowing when a wheel is about to lock up.
The speed sensors, which are located at each wheel, or in some cases in the differential,
provide this information.
Valves
There is a valve in the brake line of each brake controlled by the ABS. On some systems, the
valve has three positions:
•In position one, the valve is open; pressure from the master cylinder is passed right through
to the brake.
•In position two, the valve blocks the line, isolating that brake from the master cylinder. This
prevents the pressure from rising further should the driver push the brake pedal harder.
•In position three, the valve releases some of the pressure from the brake.
Pump
Since the valve is able to release pressure from the brakes, there has to be some way to put
that pressure back. That is what the pump does; when a valve reduces the pressure in a line,
the pump is there to get the pressure back up.
Controller
The controller is a computer in the car. It watches the speed sensors and controls the
valves.
33. ABS 3
ABS at Work
The controller monitors the speed sensors at all times. It is looking for decelerations in the
wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid
deceleration. If left unchecked, the wheel would stop much more quickly than any car could.
It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a
wheel that locks up could stop spinning in less than a second.
The ABS controller knows that such a rapid deceleration is impossible, so it reduces the
pressure to that brake until it sees an acceleration, then it increases the pressure until it
sees the deceleration again. It can do this very quickly, before the tire can actually
significantly change speed. The result is that the tire slows down at the same rate as the car,
with the brakes keeping the tires very near the point at which they will start to lock up. This
gives the system maximum braking power.
When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes
from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15
times per second.
34. EBD and ESC
Electronic brakeforce distribution or EBD or EBFD is an automobile brake technology that
automatically varies the amount of force applied to each of a vehicle's brakes, based on road
conditions, speed, loading, etc. Always coupled with anti-lock braking systems, EBD can
apply more or less braking pressure to each wheel in order to maximize stopping power
whilst maintaining vehicular control.
EBD may work in conjunction with ABS and Electronic Stability Control ("ESC") or Vehicle
Stability Control (VSC) to minimize yaw accelerations during turns. ESC compares steering
wheel angle to vehicle turning rate using a yaw rate sensor. "Yaw" is the vehicle's rotation
around its vertical center of gravity (turning left or right). If the yaw sensor detects more/less
yaw than the steering wheel angle should create, the car is understeering or oversteering
and ESC activates one of the front or rear brakes to rotate the car back into its intended
course. For example, if a car is making a left turn and begins to understeer (the car plows
forward to the outside of the turn) ESC activates the left rear brake, which will help turn the
car left. The sensors are so sensitive, and the actuation is so quick that the system may
correct direction before the driver reacts. ABS helps prevent wheel lock-up and EBD helps
apply appropriate brake force to make ESC work effectively.
