A brake is a device that produces artificial resistance to the moving part of a machine to stop or slow down its motion. Any machine or its part that is in motion possesses kinetic energy. As per physical laws, energy cannot be destroyed; its form can only be transformed. The brake system converts the kinetic energy of the moving machine or part into thermal, potential or chemical energy. Brake systems are of various types utilized in different applications in industries, home appliances, vehicles, railway trains, and other sectors. This paper has focused on the brake systems used in vehicles, especially in automobiles.
Evolution of the Break System
The brake system has made impressive developments in last hundred years. Many different methods of braking have been employed since its first use in automobiles. Each new system has taken concepts from earlier designs and improved upon it. However, number one priority in all these developments has been to improve the safety and the efficiency of the car.
The earliest brake system to be used in vehicles consisted of wooden blocks and a single lever. The vehicles had steel-rimmed wheels. The driver stopped the vehicle by pulling the lever, which moved the wooden block to press against the wheel. The system became obsolete by 1890s when rubber tires started replacing steel-rimmed wheels.
In 1899, Gottlieb Daimler first put up the idea of wrapping a cable around a drum and anchoring the cable to the chassis of the vehicle thus creating the concept of a drum brake. French manufacturer Louis Renault is generally credited for developing the mechanical drum brake in 1902 though a year earlier Wilhelm Maybach had designed its simpler version. These brakes were fixed outside the vehicle. Ingression of dust and water made them less effective. The problems created by dust and water were solved by placing the shoes inside the drum brake. Thus, internal expanding shoe drum brake was developed, which was fixed inside the vehicle. These brakes were mechanically operated by employing levers, rods, or cables to operate the drums.
In 1918, Malcolm Loughead proposed the concept of a four-wheel hydraulic brake system. The system used fluid for transfer of force to the brake shoe on pressing of the pedal. By late 1920s, the hydraulic system of braking was in use in most of the cars. As the cars became faster and heavier, higher thermal energy was generated in stopping the cars. This made the hydraulically operated drum brakes less efficient (Patrascu, 2009).
William Lanchester patented the disc brake in 1902 but the system became popular in the 1950s. Chrysler was the first manufacturer to use a disc brake on its Imperial model. In Europe, it was first fitted in Jaguar C-type racing car. The disc brake became popular in the United States only in 1964 with the incorporation of power braking system.
Another major innovation in the brake system was the development of the anti-lock brake system. They were implemented on Ford’s series models in 1969. Further developments in the car brakes have taken place. In the regenerative braking, most of the braking energy is converted into electric energy, which is stored for reuse. Hybrid and electric vehicles employ this method to recharge the battery pack (History of Brakes in Motor Cars/Automobiles).
Basics of Brakes
The brake system performs three functions:
1. Decelerate or stop a moving vehicle.
2. Control vehicle speed on the downhill slope.
3. Keep vehicle stationary on a gradient.
Two basic operations of the brake system are conversion and transfer of energy and generation of torque.
Based on the application of the working principle, brakes can be classified as mechanical brakes, hydraulic brakes, eddy current and electromagnetic brakes.
Mechanical brakes generate a frictional force between the moving part and the stationary part to slow down or stop the moving part. Here the kinetic energy of the moving car is converted into heat. Design of the brake system ensures that
1. Sufficient deceleration is produced to stop the car within shortest possible distance, or as per the expectation of the driver. The driver should not lose control of the car or feel discomfort in applying the brake.
2. The heat energy generated during the braking does not damage the brake system or any other part of the car.
A car weighing 1600 kg is moving at 100 kph. It has kinetic energy = MV2/2G = 618,272 N-M. It will take 3.54 sec. to stop the car at a deceleration of 0.8 G. The deceleration energy is equal to 233 HP. This shows that the brakes have to produce twice the power of the engine. As the Kinetic energy of moving car increases to the square of speed, the speed of the car is more important than the loaded weight of the car for stopping the car quickly (Happian-Smith, 2000).
When a brake is applied to a moving car, there is weight transfer to front wheel i.e. load is increased on front wheels while the load on the rear wheel is reduced. This happens due to inertia associated with the moving car.
Basic mechanism of braking
The following illustration shows the basic working of the mechanical brake. dA is an element of the stationary part of the braking part moving at velocity v. On actuation of the brake, stationary and moving parts come into contact and a normal pressure is created at the contact surface. The elemental contact force dN is equal to the product of the area of contact dA and contact pressure p.
A frictional force is dF is developed between contacting surfaces. The magnitude of the frictional force is equal to the product of the coefficient of friction and normal force dN.
The moment of the frictional force contributes to braking (Gopinath & Mayuram, “n.d.”).
The most simple braking configuration is band brake where a metal band lined with heat and wear resistant friction material is wrapped around the wheel or braking drum. When the driver applies a force T2 by pressing the brake pedal, tension force T1 is generated due to friction between the drum and the band. T1 is the load force and T2 is the support force. The relationship between the tensions in the band can be expressed through equation:
T1 = T2 eµɵ
Where µ is the coefficient of friction between the band and the brake drum and ɵ is the angle of contact formed between the band the brake drum. ɵ is measured in radians.
In the above figure, the tight side of the band is attached to the support, the slack side is attached to the brake lever, and the drum rotates clockwise towards the slack side. In this configuration, a smaller force P is needed in operation as the friction force assists application of the band. Taking moment, following equation can be written:
P*d = T2 *m2 – T1 *m1 = T2 (m2 – m1 * eµɵ )
When the product of m2 and eµɵ is greater than m2, the brake will grab or it will be self locking. This feature is used for stopping rotation in the reverse direction in the machines. However, in an automobile, self-locking should be avoided (Spotts, 1961).
Brake System Components and Configurations
Brake system components and configurations found generally in the present day automobiles have been described
Foundation Brakes:
It comprises disc brakes and drum brakes or their combination. In case of using only disk brake, a small drum-type parking brake is used.
Drum brake: The drum contains a flat-topped cylinder that is squeezed between the wheel rim and the wheel hub. When the brakes are applied, the brake shoes or pads move into contact with inside surface of the drums to create friction and slow down the rotation of the wheels. Drum brakes are of two categories: leading & trailing and twin leading.
In the leading and trailing drums design, out of the two shoes, one shoe moves in the drum's direction of rotation and the other moves against it. Because of this configuration, the leading and trailing drums operates with equal breaking efficiency for going in the forward direction or in reverse. It is generally used in rear wheels. In the twin leading drum design, both shoes move in the direction of rotation. The configuration provides the best efficiency and is generally front wheels of the vehicle (Patrascu, 2009).
In the first half of the twentieth century, drum brakes were used on all four wheels. Now this has changed. Drum brakes are now usually fitted on the rear wheels while disc brakes are used in the front. The reason behind phasing out of drum brakes is its poor heat dissipation process. This causes high a temperature inside the drum. At high temperature, the coefficient of friction between brake shoe and liner decreases causing brake fade. High temperature also causes deformation, which ultimately leads vibration during braking. The advantage of the drum brakes is lower cost and simple engineering.
Disc Brakes: Disc brakes employ same basic principle as the brakes on a bicycle. It consists of caliper that has two brake pads. Brake pads are made of metal fitted with special lining. On application of brake by the driver, the hydraulic fluid is injected into the caliper. Further, it presses against a piston that in turn, pushes two brake pads against the braking disk or rotor. This clamping action produces friction between the rotor and the pads. The performance of Disc brakes is better than drum brake because of simpler design, lighter weight and better heat dissipation process (Patrascu, 2009). Disc brakes work much better than drum brake in wet conditions, as ingress water is easily removed.
Handbrake: It is also called parking brake or emergency brake. It is a mechanical brake operated by hand. When the vehicle is parked at a place or on slopes, the handbrake is applied by pulling the lever. A cable connects lever to the brake. The brake is attached to rear wheels.
Energy Transmission System
The energy applied by the driver to initiate braking action has to be transmitted to the brakes. Earlier mechanical linkages were used to transmit the energy. Now brakes are actuated by the hydraulic pressure.
Pedal Assembly: Pedal assembly consists of the foot pedal. The driver pushes the pedal to initiate braking action. It utilizes the principle of levers to boost the braking force. With a nominal pedal ratio of 4:1, the pedal force is quadrupled (Gritt, “n.d.”).
Hydraulic braking system: Earlier mechanical levers or linkages were used to transmit force from one point to another. Modern cars are powerful and thousand of pound of pressure have to be exerted on each of four brakes to stop the cars. Hydraulic fluid as transmission medium has advantages. Mechanical links are not required. Force multiplication can be easily achieved by applying Pascal’s law. The system consists of the master cylinder that is directly connected to the lever of the brake pedal. On pressing the brake pedal, the primary piston of the master cylinder is pushed. As the piston moves, the pressure increases in the primary cylinder, which in turn pushes the secondary piston to compress the fluid in its circuit. Two circuits are used as a safety so that in the case of a leak in the primary circuit, secondary circuit takes over. This way master cylinder converts mechanical pressure to hydraulic pressure. The hydraulic pressure is transmitted by brake pipes and brake hoses to the slave cylinders located at each wheel. Brake shoes or pads are pushed by the piston of slave cylinder to initiate braking action.
Brake system layout: To ensure safety, all modern cars have dual systems with two brakes
operated by each subsystem. In case one subsystem fails to operate, the other subsystem can provide sufficient braking power. The layouts of subsystems have been shown on above diagram. They can be front-rear hydraulic split or diagonal split (Gritt, “n.d.”).
Proportionating Valves: Proportionating valves are fixed after master cylinder in the hydraulic system. It reduces the pressure of rear brakes. In the diagonal split system, two valves are required. This ensures a proper balance of the vehicle.
Brake lines: Brake lines are made of double wall steel tube of 3/16 inch O.D. Its construction is very tough to eliminate of its breaking (Gritt, “n.d.”).
Brake Fluids: Brake fluid used in the hydraulic system are glycol based or silicon based. by Society of Automotive Engineers (SAE) Standard J1703 and Federal Motor Vehicle Safety Standard (FMVSS) 116 defines the specifications for all automotive brake fluids. Brake fluids should have a high boiling point to avoid the formation of vapor. Some of the other characteristics are low freezing point, unreactive to metal or rubber, ability to absorb moisture that enters the hydraulic system (Introduction to Braking System, 2012, p 24) .
Vacuum booster: The vacuuum booster is also force multipliers generally used with disc brake. Its one end is connected to the pedal linkage and the other to the master cylinder’s piston. It has a diaphragm, a clever valve and a check valve. It requires a source to create vacuum. Differential pressure created between vacuum and atmospheric pressure is utilized to boost the pedal pressure.
Air Brake and Vacuum Brake System: Compressed air operated brakes are used in trucks with trailers, big trucks, and railways. Air pressure is used for the operation of the brake. An air compressor linked to an engine is used for compressing air. Compressed air has an advantage over hydraulic fluid. Small air leakage is easily made up by the compressor and performance of compressed air operated system does not decline whereas small leakage makes hydraulic system ineffective.
Vacuum brake system was used in railways of many countries. It has been replaced by air brake system.
Braking Performance
Most of the time,the car brake are applied gently. However, in the case of an emergency, full brakes are applied to stop the car in shortest possible distance avoiding wheel locking. If both front wheels are locked, the vehicle can not be steered, it goes straight and can hit something. In the case of rear wheel locking, there is every chance of the rear wheels spinning around front. The front wheel will lock before the rear wheel, if there is more front brake torque than dynamic front weight and rear brake will lock before the front if there is more rear brake torque than dynamic rear weight. Optimum braking is achieved when brake torque distribution and dynamic weight distribution are matched. Distance traveled by the vehicle after applying brake depends on tire road friction, vehicle balance and reaction time of the driver and system to actuate the brake (Guiggiani, 2014).
Car suspension, besides absorbing vibration , is designed to keep the tires in contact with road surface. Tires work in conjugation with suspension geometry and weight transfer dynamics to provide grip.
Tire-road friction during slip depends on adhesion i.e. intermolecular bonding between rubber and surfaces and hysteresis i.e. energy loss due to rubber deformation. The coefficient of friction (µ) of the rubber compound and the tire’s construction increases up to 20% slip but starts decreasing above the value and system become unstable. The coefficient of friction, µ is function of characterisitics of tire and road surface and road condition. µ for a dry clean road varies between 0.8 to 1.0 whereas it drops between 0.2 to 0.65 for a wet surface contaminated with dirt (Muller, Uchanski, & Hedrick, 2003).
Anti-Lock Braking System
The primary purpose of anti-lock braking system (ABS) is to improve stability and control of the vehicle during braking. The system comprises of a wheel speed sensor, a hydraulic modulator and an Electronic Control Unit (ECU). It is a closed-loop control device. It detects the wheels’ angular velocities and accelerations using wheel speed sensor. EBS uses this information to determine vehicle velocity, calculate the onset of wheel lockup due to high braking force and modulate brake pressure. Braking force is reapplied and again on the onset of wheel lock up, it reduces the brake force. The cyclic operation ensures that the brakes operate near their most efficient point. It also enables a driver to maintain steering ability by maintaining a substantial portion of side force capability. It also shortens the stoppage distance (Snyder, Jones, Grygier, & Garrott, 2005, p-2). There are different configurations of ABS. Most advanced has four channel, four sensor system to control each wheel speed separately. Another system controls each of front wheels and a single channel and valve to prevent lock-up of both rear wheels. The most basic type has single-channel, single sensor system for both rear wheels (Burton, Delaney, Newstead, & Fildes, 2004, p-3).
Brake Assist System
It has been observed that the drivers in emergency hesitate slightly in pushing the brake. In many occasions after slamming the brake, he decreases the pedal pressure. The delay may be very small but avoidance of this delay may avert an accident. A brake assist system is a safety feature that helps in avoiding an accident by decreasing time delay in application of the brake. A brake assist system contains electronic components to monitor the speeds with which the brake is applied. A small computer records the driver’s braking pattern. From analyzing the data, it can recognize the critical situation. The system has an electric pump for storing brake fluid at high pressure. When the driver pushes the pedal faster than normal, the computer interprets emergency and automatically triggers the brake assist system. The brake assist system within milliseconds releases the high-pressure oil to stop the car quickly. On releasing the brake pedal by the driver, the brake assist system goes back to standby mode. The brake assist system, which was originally installed in high-end cars, are required to be installed on all new passenger cars since 2004 in the EU and since 2011 in the United States.
Legislation and Testing Procedure
The National Highway Traffic Safety Administration (NHTSA) was set up by the Highway Safety Act of 1970 to carry out safety administration of vehicles with the aim to save lives, and prevent injuries. It achieves these aims by carrying out research, establishing safety standards and enforcement activities for vehicles. It establishes and validates tests to compliance of the standards. All vehicles sold in the United States should meet Federal Motor Vehicle Safety Standards (FMVSS) (This is NHTSA. 2006).
FMVSS codifies design, construction, performance, and durability requirements for motor vehicles and regulates safety-related components. Standard No. 105 specifies standards of hydraulic and Electric Brake Systems in passenger cars. Standard 166 specifies characteristic of fluids in hydraulic systems of motor vehicles (Federal Motor Vehicle Safety Standards And Regulations, 1998).
The paper “NHTSA Light Vehicle ABS Performance Test Development” presents the testing performance of the anti-lock braking system on different light vehicles studied by developing a methodology. The project helped in developing suitable minimum performance criteria for the safe operation of antilock brake systems (Snyder, Jones, Grygier, & Garrott, 2005, p-iii). Based on these tests and feedback from different stakeholders, FMVSS standard for ABS was developed.
Recent Advances and environment
Conventional vehicles run on gasoline and are the main source of air pollution, Carbon dioxide emission, and environment degradation. Hybrid vehicles and electric vehicles use electric energy for propulsion. However, fossil fuel is also burnt in the generation of electricity. It has been reported that vehicles moving in a metropolis waste 62.5% energy due to frequent braking (Clegg, 1996). In a conventional vehicle, the kinetic energy of the vehicle is lost as heat generated by friction between the brake pads and wheels. The regenerative braking system stores part of the kinetic energy. The stored energy is converted back into kinetic energy by electric motor and used to accelerate the vehicle. The regenerative braking system is used in hybrid or electric vehicles. If all the brake energy could be regenerated, there could be 33% savings in fuel consumption. Thus, regenerative braking system improves fuel efficiency in vehicles and is environmental friendly.
An internal combustion engine and an electrical motor are used for driving a hybrid vehicle. Electric motor when used in reverse function works as generators and convert kinetic energy into electrical energy. Electric motors of hybrid vehicles are used as generators when using regenerative braking, transforming kinetic energy from wheels into electric energy. The electric energy thus recharges the storage batteries.
The regenerative braking system has certain disadvantages. At low speed, its effectiveness decreases and it cannot completely stop a vehicle quickly. It cannot be used as a brake for a stationary vehicle. Hence, it has to be used in conjunction with the conventional braking system. This makes designing a regenerative braking system in the hybrid vehicles a complex exercise since the distribution of the required braking force between two braking systems should be made in such a way so as to maximize kinetic energy recovery (Sangtarash et el., 2007).
References
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