Regenerative Braking System

Every time we hit the brakes on a moving car, all the energy that the engine has supplied to the wheels goes to waste. In other words, the kinetic energy that is used to power the wheels is converted into heat when brakes are applied. The heat generated is of no use and thus we waste valuable energy.

Engineers have come up with a method to use the braking energy to be converted into electrical energy and recharge the battery. This method of utilizing the braking energy to be converted into electrical energy is known as Regenerative Braking. This system captures most of the kinetic energy and converts into electricity, which can recharge the battery. This system is used more commonly in hybrid and electric cars because it is imperative to recharge the batteries to make the car run longer.

How does braking waste energy?

Any normal guy driving a car would think that he is not wasting much energy in applying the brakes, because he is just using his foot to press the pedal. It doesn’t require a huge amount of force to apply pressure on the brake pedal and bring the car to a stop. So how are we wasting energy while braking?

Look at the above scenario from an engine’s perspective. Engine does all the work to generate power and supply it in the form of kinetic energy to the wheels. A lot of energy in the form of fuel is used to provide the momentum to the wheels. When brakes are applied, the car’s momentum is lost and it either slows down or comes to a halt. All the kinetic energy is wasted in the form of heat due to the friction between the brake pads and the drum. Now to start the car again from the start requires more fuel to provide the momentum. Hence, we are wasting a lot of valuable energy while braking.

Regenerative Braking System:

As already mentioned, regenerative braking technology is used in hybrid and electric cars because the wheels of these cars are powered by electric motors. The electric motors convert the electrical energy from the battery into kinetic energy which drives the wheels. During braking action, instead of using brake pads, the electric motor starts rotating in reverse direction.

When the driver steps on the brake pedal, the electric motor switches to generator mode by reversing its direction of rotation. Through its rotation, the generator converts a portion of the kinetic energy into electrical energy. This electrical energy is then stored in a high voltage battery. As a result of this electricity generation, generative braking torque is produced by the motor which decelerates the wheels. However, the generative braking torque won’t be sufficient to completely halt the vehicle. Therefore, once the vehicle is decelerated to a low speed, conventional friction brakes (disc or drum brake) further assist in stopping the wheels.

In the case of emergency where vehicle has to be stopped immediately, only friction brakes will be used. Regenerative brakes are more useful in stopping a slow moving vehicle.

Advantages of Regenerative Braking:

·         It enables an extended battery range in electric vehicles.

·         Reduces fuel consumption and CO₂ emissions in hybrid vehicles.

         

 Related Topics:

• Disc Brakes

• Drum Brakes

• Anti-lock Braking System (ABS)

• Traction Control System (TCS)

• Electronic Brake force Distribution (EBD) 

• Air Brakes

Air Brakes

Various ways have been found to utilize the air we breathe for our work. Pneumatic drills use air to drill and break the concrete. Heavy loads are supported on air filled rubber tires. Air is also used to provide smooth and comfortable rides in the form of suspension. One such application of air is in the field of brakes, where compressed air is used to arrest the motion of the tires.

Who Invented Air Brake?

George Westinghouse patented a safer air brake on March 5, 1872. These air brakes were first used in the railways. Later, in the early 20^th century, vehicle manufacturers started using air brakes in trucks and other heavy load carrying vehicles. In modern days, you will find air brakes in most of the buses, trucks, trailers and semi-trailers.

Air Brakes Components:

Air brake system uses driver’s control to transmit air pressure to the service brakes. The components of the air brake system are as follows:

·         Air Compressor: It pumps the air to the storage tank or reservoir. The compressor is run by the engine crankshaft through gears or belt drive. The compressor is either air cooled or oil cooled.

·         Governor: The governor controls the pumping of air from the compressor by monitoring the pressure in the air tank. If the air tank reaches the cut-in pressure level (5.85 bar), the governor allows the compressor to pump the air. On the other hand, if the air tank reaches the cut-out pressure (9.3 bar), the governor stops the compressor from pumping air.

·         Air Dryer: Air dryers are installed before the reservoir tank to absorb the moisture from the air. Moisture build up in the tank can lead to brake failures.

·         One-Way Check Valve: It allows the air to flow in one direction only. It is located between the air compressor and the 1^st tank. It prevents the air from going out if the compressor develops a leak.

·         Air Storage Tank: It stores the compressed air pumped by the compressor. The tanks store enough air for braking even if the compressor has stopped working. There is a possibility of moisture and oil entering the tank. Water can cause brake failures, especially in winter where water gets converted into ice. Usually they provide two tanks. First the air is supplied to Wet tank, where the moisture and oil is drained. The moisture free air is then supplied to the Dry tank.

·         Drain Valve: To overcome the problem of moisture and oil getting accumulated in the tank, drain valves are provided at the bottom to drain the water and oil. It usually drains automatically.

·         Safety Valve: Safety valves are imperative in order to make sure that pressure doesn’t build up beyond a point of danger. The valve usually opens if the pressure crosses 10 bar. You would have often heard a loud hissing noise coming out of a truck. The release of pressure through the safety valves causes such noise.

·         Brake Pedal (Foot Valve): Brake pedal or foot valve allows the air pressure from the tank to be applied to the brake chambers. When we press on the brake pedal, the foot valve opens in the storage tank. Higher the force applied on the brake pedal, greater will be the air pressure applied to the brake chambers.

·         Drum Brake: Let’s consider a drum brake in use. When the brake pedal is pressed, the compressed air is supplied to the brake chamber. It is a cylindrical container that pushes the push rod out when pressure is applied. The push rod then moves the slack adjuster, thereby twisting the S-cam. The twisting of S-cam forces the brake shoes to move away from each other and press against the inside of the brake drum.

Alcohol Evaporators:

Some vehicles equipped with air brakes have alcohol evaporators that release alcohol in the storage tank to prevent the formation of ice in the tank.

Emergency Brakes and Parking Brakes:

All vehicles equipped with air brakes must mandatorily have emergency brake and parking brake. Usually spring brakes are used for this purpose. When you are driving, the spring brakes are held back by air pressure. If the air pressure is removed, spring brakes will apply the brakes. When using the parking brake, air pressure is removed off the spring brakes so that spring brakes can apply brake.

The spring brakes will also fully come on if the air pressure falls down due to leakage. Spring brakes will come on if the pressure falls in the range of 1.5 bar to 3 bar.  

Anti-lock Braking System (ABS):

All truck tractors manufactured after March 1, 1997 in US with air brakes are equipped with ABS and also other heavy load vehicles such as trucks, buses have been equipped with ABS after March 1, 1998. ABS is a safety device that prevents the wheels from locking up when heavy brakes are applied.

Related Topics:

• Disc Brakes

• Drum Brakes

• Anti-lock Braking System (ABS)

• Traction Control System (TCS)

• Electronic Brake force Distribution (EBD) 

Disc Brake

These are the most common type of brakes used in modern cars and bikes. Disc brakes use calipers to press brake pads against a disc attached to the wheel. When the brake pads come in contact with the disc, a huge amount of friction is created which ultimately ceases the movement of the wheel.

Typically, the front wheels of a car are installed with disc brakes and the rear wheels are installed with drum brakes. In some cars, all four wheels have disc brakes. Hydraulic disc brakes are the most commonly used disc brakes to help in retarding the motion of a wheel. There are two types in hydraulic disc brakes:

1. Floating Caliper Type (or) Self adjusting Disc brake
2. Fixed Caliper type disc brake

Floating Caliper Type Disc Brake:

In a floating caliper type, the caliper is not fixed in its position around a disc. The caliper slides from left to right through pins and bushings so that the brake pads come in complete contact with the disc and cease the movement of the wheel. It has only one piston to transfer the brake pressure to the brake pads.

When brake pedal is pressed, hydraulic fluid from the master cylinder is supplied to the caliper. The brake fluid applies the brake pressure on the inner side of the piston. The piston presses inner brake pad against the disc. Since, the disc is fixed, the caliper slides from left to right, therefore bringing the outer brake pads in contact with the disc. This produces a greater friction, thereby stopping the disc from rotating.

Floating type disc brakes are lighter, cheaper and more reliable. It is used in most of the vehicles.

Fixed Caliper Type Disc Brake:

In a fixed caliper type, the caliper is solidly mounted on the caliper bracket around the disc. Unlike the floating type, it has a minimum of 2 pistons, one on each side. The number of pistons can go up to 6 (3 on each side). The pistons push the brake pads against the disc, therefore retarding the motion of the disc.

The advantage of multiple pistons is that the brake pressure is distributed evenly on both sides and there is more squeezing power on the pistons.

It is more expensive than the floating type and usually preferred in luxury cars. For ABS application, both disc brake types are compatible.

Most of the vehicles are equipped with ventilated discs. The vents help in dissipating the heat, therefore cooling it down.

Anti lock Braking System (ABS)

What is ABS?

ABS is the short form for Anti lock Braking System which is a safety device installed in vehicles.

Why is ABS used?

ABS is used to prevent wheel locking while applying brakes to the wheels. In certain scenarios owing to the surrounding conditions (such as wet roads, dry surface, snowy surface, loose gravel, etc.), wheels might skid when brakes are applied suddenly.

In other words, wheels might get locked which increases the stopping distance of the vehicle and also defies the driver the control on the steering, leading to fatal injuries and in certain cases can lead to death. ABS helps in maintaining the traction between the wheels and the surface, thereby preventing the wheels from locking. It also allows the driver to steer the vehicle and avoid obstacles.

When was ABS introduced?

ABS was developed in the year 1929 by Gabriel Voisin, a French national who designed it only for aircraft. It was in the year 1958 that ABS was experimented on a Royal Enfield Super Meteor Motorcycle and it greatly reduced the stopping distance of the motorcycle. The system was however not used for commercial purpose.

Later in the year 1970, Ford added an electronic ABS system called 'sure-track' on the rear wheels of the Lincoln Continental model as optional for the customers. A year later, Ford made it standard in Lincoln models. Chrysler, along with the Bendix corporation, developed a computerized three channel, four sensor all wheels ABS called 'Sure Brake' for its Imperial model launched in 1971.

In the year 1978, Mercedes Benz introduced the second generation ABS with the help of Bosch. Mercedes Benz was the first company to introduce commercial cars with ABS and also ensured large scale production of ABS. This was all possible due to the revolution in electronics. Integrated circuits and computers could be built which with the help of computer codes could detect the speed of the wheels and actuating the valves in no time and made the process simpler and accurate.

ABS Components:

ABS has 4 main components:

1.Speed sensors: ABS has to know when the wheel is about to lock up. So sensors are located over the wheels to provide this information.

2. Valves: Valves are located in the brake line of each brake which can be controlled by the ABS. Valve has 3 positions in some systems:

• In position one, the valve is open and the pressure from the master cylinder is passed right through it to the brake.
• In position two, the valve is closed, thereby blocking the pressure from the master cylinder to the brake.
• In position three, the valve releases some amount of pressure from the brake. The controller signals the valve to release the pressure once the sensor senses the wheel is about to slip.

3. Pump: Pump is used to restore the pressure that was released from the brakes. The controller will control the pump and makes it pump the required amount of pressure to the brake.

4. Controller: Controller is the brain of the ABS, also called the Electronic Control Unit (ECU) which receives the signals from the wheel speed sensors and modulates the ABS system by controlling the braking valves. If the vehicle is about to slip, controller will release the pressure from the brakes through the valve.

ABS Working:

The controller (ECU) continuously monitors the speed of each wheel with the help of speed sensors. If the ECU detects that one wheel is rotating slower (for a 4 wheeled vehicle) than the other wheels, then it actuates the valve to reduce the braking pressure on that particular wheel so that it can start rotating faster and match the speed of the other wheels.

Conversely, for a wheel rotating faster than the other wheels, the controller actuates the valve to increase the braking pressure on that particular wheel to slow it down and match other wheels' speed.

The driver can feel the ABS working through his or her foot through the brake pedals which will be pulsating. In some cases, ABS can release or apply the brake pressure 15 times per second.

Types of ABS:

ABS can be classified into various types based on the number of speed sensors and valves:

1.        Three channel, three sensor ABS: This system has a speed sensor and a valve for each of the front wheels, and a common speed sensor and a valve for both the rear wheels. For rear wheels, it is located on the rear axle. This system provides maximum efficiency for the front wheels. In the case of rear wheels, both the wheels have to lock up for the ABS to function. There is always a possibility of one of the rear wheels to lock up, reducing the effectiveness of ABS.

2.        One channel, one sensor ABS: This system is commonly used only for rear wheels which will have a common valve and a speed sensor. Both the rear wheels have to lock up so that the ABS can work. This is a rather ineffective system.

3.        Three Channel, four sensor ABS: This system employs separate speed sensors for each wheel and separate valves for each of the front wheels. A common valve is used for the rear wheels. If the controller detects lock up of one of the rear wheels, then brake pressure will be applied on both the rear wheels.

4.        Four channel, four sensor ABS: Separate speed sensors and valves are used for each wheel. This system provides the maximum brake effectiveness.

5.        Two channel, two sensor ABS: Separate speed sensors are used on each wheel and separate valves are used for both front and rear axle. If there is a wheel lock up on any wheel (say front left wheel), then braking pressure will be applied to both the front wheels.

Drum Brakes (Self-Actuating Brakes)

Drum brakes work with the help of brake shoes or brake pads that use frictional force between the pads and the drum to stop the wheel. The brake pads are pressed against the inner surface of the drum to create the frictional force.

The drum brake working principle was first patented by Louis Renault, a French industrialist, in the year 1902. However, the first drum brakes were used by Maybach in the year 1900.

Design:



A drum brake consists of various components:

• A pair of brake pads with linings made of materials having high frictional coefficient. The brake pads are hinged either at a single point or independent locations not far from each other. The other ends remain in contact with the pistons in the wheel cylinder.

• Wheel drum which rotates along with the wheels. It is usually made of cast iron.

• A wheel cylinder which has a pair of pistons for each brake pad. When brake pedal is depressed, oil pressure increases and acts on the pistons. The pistons force the brake pads to press against the wheel drum.

• Brake return springs allowing the brake pads to return to their original positions once the brake pedal is released.

• Drum brakes also have an emergency brake system which can be operated through a lever by a hand brake.

• Self adjusting system to automatically re-adjust the distance required for the brake pads to travel to press against the wheel drum when brakes are applied. Brake pads start wearing with time, so self adjusting mechanism will help in changing the distance. This is the reason why they are also called self actuating brakes.

Working:

When the brake pedal is depressed by the driver, the master cylinder pressurises the hydraulic oil and sends this hydraulic pressure to the wheel cylinder. This hydraulic pressure acts on the double-acting pistons which push the brake pads outside. The brake pads press against the inner surface of the wheel drum and create friction. This friction between the brake pads and the drum brings the wheels to rest.

Once the brake pedals are released, the return springs pull the brake pads back to the original position.

Self-Actuating Mechanism:

Drum brakes are also known as self-actuating brakes. As the brake pads wears out, they have to  travel a greater distance to press against the wheel drum. Therefore, self adjusting mechanism is used to automatically re-position the resting point of the hinges on which the brake pads are hinged, so as to get them closer to the drum.

Advantage of using a drum brake:

There is only one major advantage of drum brakes, it is cheap.

Disadvantages of a Drum brake:

• Complicated design, use of several components compared to a disc brake.


• Requires regular service, especially the brake pads which wear out quickly and needs to be replaced.

Traction Control System (TCS)

What is TCS?

TCS stands for Traction Control System and is a safety feature used in cars to maintain the traction between the road and the driven wheels. In the past, driver had to remove his feet odd the accelerator pedal on slippery roads to avoid slipping of the wheels. Today with the help of electronic traction control system, the power from the engine delivered to the wheels can be limited.

When was TCS introduced?

Before TCS, rear wheel drive cars were installed with limited slip differential, a mechanical system that delivers more torque to the non slipping wheel, while still allowing some wheel spin to occur.

In the year 1971, Buick introduced MaxTrac system to detect the rear wheel spin with the help of a computer system. MaxTrac system modulated the engine power to those slipping wheels to provide more traction. Cadillac came up with Traction Monitoring System (TMS) in 1979 on its Eldorado model.

How TCS works?

Electronic Traction Control system makes use of the same wheel speed sensors used for Anti-lock Braking System (ABS). When the TCS system detects that one wheel is rotating slower than the other wheels, it modulates the brake valves to reduce the brake pressure on that particular wheel and avoid slipping. In addition to this, TCS also regulates the engine power delivered to the slipping wheels.

Traction Control System does not increase traction, but only prevents the wheels from slipping by maintaining traction. A vehicle might still get stuck or lose control even with TCS on it. For better protection, a car should feature both TCS and ABS.

The powertrain can reduce the torque supplied to the slipping wheels by cutting off the fuel supply, limiting throttle action, retarding spark ignition, shutting down some cylinders, and many other methods.

Traction Control can also help in cornering. If too much throttle is supplied, then the wheels might lose traction and slip sideways. TCS limits the power supplied to those wheels and thus help in increasing the grip of the tires.

Electronic Brakeforce Distribution (EBD)

What is EBD?

EBD stands for Electronic Brakeforce Distribution, an automotive technology used as a safety feature.

Why is EBD used?

EBD is a automotive brake technology used to vary the brake force on each wheel of a vehicle automatically based on the road conditions, speed, load, etc. It always works with the help of ABS (Anti lock Braking System).

The weight of a vehicle may not be evenly distributed among all the wheels, so in that case, the brake force required on each wheel would also vary. EBD automatically senses the variation in loads and distributes the brake pressure to wheels based on the requirements. This helps in stopping the vehicle without losing control over it.

How EBD works?

Friction is the most important aspect in making a car move in the right direction. Whether we accelerate or apply brakes, friction between the tires and road helps in making the car move forward and also stop it without losing control. But in some conditions (for e.g. icy roads), there is not enough friction to stop the vehicle and also to steer the vehicle.

It is important to drive carefully to avoid losing control of a vehicle when driving on roads providing less friction. When one suddenly applies brake, there is always a chance of wheel lock, which can lead to skidding. ABS helps in preventing wheel lock by sensing the slip ratio ( slip ratio is the difference of the speed at which a vehicle is moving to the rotating speed of a tire ). ABS can calculate the slip ratio of individual tires and vary the brake force applied to each tire, therefore avoid skidding.

EBD is also useful when a car is negotiating a turn. While turning, the outer wheels of a vehicle rotate faster than the inner wheels. There might be a scenario where too much brake force is applied to the inner wheels, therefore leading to over steering. EBD can sense the slipping of inner wheels and reduce the brake force applied to the inner wheels and help the vehicle negotiating the curve safely.

EBD components:

• Speed sensors: To sense the speed at which each wheel rotates and also to sense the speed at which a vehicle is moving. There is no specific sensor used to sense the speed of a vehicle. Average speed of all wheels rotating is considered as the vehicle speed.


• Brake force modulator: It is used to vary the brake force supplied to each wheel hydraulically. The ECU actuates the valves to vary the brake force.


• Electronic Control Unit (ECU): It receives input from the speed sensors, calculates the slip ratio of each wheel and actuates the brake force modulator to vary the brake pressure on each wheel to avoid skidding.