Generally, in most of the cars, conventional friction brakes are used to stop or control the speed of cars while it is in motion. During the running condition, the car having lots of kinetic energy, when the brake is applied, this kinetic energy is losses by heat energy due to the friction between the brake shoe and brake plate. Thus this extra power loss reduces the overall efficiency of the vehicle. This type of heat losses of energy can be limit by use of regenerative breaking system known as RBS.
Generally, the main objective of regenerative breaking system is to recover as much as possible torque while brakes are being applied. The kinetic energy of vehicle can be stored in another form of energy. In regenerative braking system, energy can be converted by means of generator, dynamometer, alternator, pump, compressor or flywheel or in other storage devices.
Now a day the regenerative breaking system is applied to many hybrid vehicles. But due to the complexity of design it can be applied to limited cars only. In this case, by using alternator or dynamo, the kinetic energy of the wheel, can be converted into the electric energy which is wasted while applying brakes.
It goes without saying that brakes are one of the most important control components of vehicle. They are required to stop the vehicle within the smallest possible distance and this is done by converting the kinetic energy of the vehicle into the heat energy which is dissipated into the atmosphere.
1.1.2 Requirements of braking
Following are the main requirements of braking
It must be strong enough to stop the vehicle within a minimum distance in an emergency. But this should also be consistent with safety. The driver must have proper control over the vehicle during emergency braking and the vehicle must not skid.
The brakes must have good anti-fade characteristics i.e. their effectiveness should not decrease with constant prolonged application for example, while descending hills. These requirements demand that cooling of brakes should be very efficient.
1.2 Types of brakes
Following types of brakes used in automobile.
There are mainly four types of brakes.
Pneumatic or Air brakes
1.2.1 Mechanical brakes
As shown in figure the disc brake consists of a cast iron disc bolted to the wheel hub and the stationary housing called caliper. The caliper is connected to some stationary part of the vehicle, like the axle casing or the hub axle and is cast in two parts containing a piston.
(a) (b )
Figure 1: Disc Brake
When the brakes are applied, hydraulically actuated pistons move the friction pads into contact with disc, applying equal and opposite forces on the later. On releasing the brakes, the rubber sealing rings act as return springs and retract the pistons and the friction pads away from the disc.
For a brake of this type,
T = 2µpaR
Where, µ = co efficient of friction p
= fluid pressure
a = cross-sectional area of one piston
R = distance of the longitudinal axis of the piston from the wheel
2. Drum Brake
Figure 2: Drum Brake
A brake drum is attached concentric to the axle hub whereas in the axle casing is mounted a back plate. In case of front axle, the brakes plate is bolted to the steering knuckle.
The back plate is made of pressed steel sheet and is ribbed to increase rigidity and to provide support to expander, anchor, and brake shoes. It also protects the drum and shoe assembly from mud and dust. Moreover, it absorbs the complete torque reaction of the shoes due to which reason it is sometimes also called „Torque plate?.
Friction linings are mounted on the brake shoes. One or two reactor springs are used which serve to keep the brakes are not applied. The brake shoes are anchored at one end, whereas on the other ends force F is applied by means of some brake actuating mechanism, which forces the brake shoe against the revolving drum, thereby applying the
1.2.2 Hydraulic Brakes
Figure 3: Hydraulic Brake System
Most of the cars today used hydraulically operated foot brakes on all the four wheels with which an additional hand brake mechanically operated on the rear wheels. An outline of the hydraulic braking system is shown in figure. The main component in this is the master cylinder, which contains reservoir for the brake fluid. Master cylinder is operated by the brake pedal and is further connected to the wheel cylinders in each wheel through steel pipe lines, unions and flexible hoses.
The system is so designed that even when the brakes are in the release position, a small pressure of about 50 kPa is maintained in the pipe lines to ensure that the cups of the wheel cylinder are kept expanded.
1.2.3 Pneumatic Brakes
The operation of air brake is similar to the hydraulic brake except that in their case compressed air is used to apply the brakes instead of hydraulic pressure. Air brakes are commonly used on heavy vehicles, like trucks, buses, etc.
The complete layout circuit is shown in figure. Compressor takes air from the atmosphere through the filter and compressed air is sent to the reservoir through the unloader valve, which gets lifted at a pre-determined reservoir pressure (about 900 kPa) and relieves the compressor of the load.
From the reservoir air goes to the brake chamber also called the diaphragm units at each wheel, through the brake valve. When the brakes are applied, the air pressure in the reservoir decreases when the pressure drops to approximately 700 kPa, the governor again cuts in the compressor to raise system pressure. In case the air system pressure falls to about 400 kPa, a warning usually in the form of buzzer is sounded.
1.2.4 Electromagnetic Brakes
These brakes are mostly used where an electric motor is already part of the automobile vehicle. Many hybrid gasoline or electric vehicles use the electric motor as a generator to charge electric batteries and also used as a regenerative brake in this type of vehicle used electromagnetic brakes.
Some diesel or electric railroad locomotives use the electric motors to generate electricity which is given to a resistor bank and dissipated as heat. Some vehicles, such as some transit buses, do not already have an electric motor but use an auxiliary “retarder” brake that is effectively a generator with an internal short-circuit.
Similar types of such a brake are eddy current brakes, and electro-mechanical brakes (which actually are magnetically driven friction brakes, but nowadays are often just called
“electromagnetic brakes” as well.