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1.1 Conventional Braking Systems in Automobiles
The term ‘Braking’ in a moving vehicle means the application of the brakes to reduce its speed or stop its movement, usually by depressing a pedal. The braking distance is the distance between the time the brakes are applied and the time the vehicle comes to a complete stop. In braking systems on conventional vehicles, friction is used to counteract the forward momentum of a moving vehicle. As the brake pads rub against the wheels or a disc that is connected to the axles, excessive heat energy is created. This heat energy dissipates into the air, wasting as much as 30 percent of the vehicle’s generated power. Over time, this cycle of friction and wasted heat energy reduces the vehicle’s fuel efficiency. More energy from the engine is required to replace the energy that was lost by braking.

1.2 Regenerative Braking Systems in Automobiles:
Every time we step on our car’s brakes, we are actually wasting energy. As we know that energy can neither be created nor be destroyed. It can be just converted from one form to another. So when our car slows down, the kinetic energy that was propelling it in the forward direction has to go somewhere. Most of it simply gets released in the form of heat and becomes useless. That energy, which could have been used to do work, is essentially wasted. The solution for this kind of this problem is Regenerative Braking System. This is a new type of braking system that can recollect much of the car’s kinetic energy and convert it into electrical energy or mechanical energy. Regenerative braking is one of the emerging technologies of automotive industry which can prove to be very beneficent. Using regenerative braking system in a vehicle not only results in the recovery of the energy but it also increases the efficiency of vehicle(in case of hybrid vehicles) and saves energy, which is stored in the auxiliary battery.

Driving an automobile involves many braking events, due to which higher energy losses takes place, with greater potential savings. With buses, taxis, delivery vans and so on there is even more potential for economy. As we know that the regenerative braking, the efficiency is improved as it results in an increase in energy output for a given energy input to a vehicle. The amount of work done by the engine of the vehicle is reduced, in turn reducing the amount of energy required to drive the vehicle.

This advancement of technology in braking system controls the speed of the vehicle by converting some amount of the vehicle’s kinetic energy into another useful form of energy. The energy so produced can then be stored as mechanical energy in flywheels, or as, electrical energy in the automobile battery, which can be used again by the vehicle.

A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored by a short term storage system. Energy normally dissipated in the brakes is directed by a power transmission system to the auxiliary battery during deceleration. The stored energy can be converted back into kinetic energy and used whenever the vehicle is to be accelerated. The magnitude of the portion available for energy storage varies according to the type of storage, drive train efficiency, drive cycle and inertia weight. The effect of regenerative brakes is less at lower speeds as compared to that at higher speeds of vehicle. So the friction brakes are needed in a situation of regenerative brake failure, to stop the vehicle completely.

The literature search focused mainly on topics related to electric vehicles (EV), hybrid electric vehicles (HEV) and fuel cell powered hybrid electric vehicles (FCHEV). We reviewed the books related to simulation, critical analysis, and empirical research in detail using available software.

Chetan Kumaar Maini (2005) pointed out in his paper the potential need for the design and development of a globally competitive compact electric concept vehicle for India and concluded that EVs are the best solution to reduce urban pollution and a significant social and economic benefit and will result in the implementation of EVs and HEVs. The report also describes the role of governments and communities around the world in promoting and accelerating EV programs. Marinescu et al., (2010) FISITA F 2010 A-089 presented aspects of a diesel electric hybrid concept car. The diesel powertrain is mounted in a classical position on the front side and the powertrain is mounted on the rear side. Performance tests of prototype vehicles with electric power units and diesel powertrain, a four-wheel drive concept, have not yet been performed in the laboratory.

Carlson et al (2008) conducted dependence studies by evaluating differences in fuel averages for two models of hybrid electric vehicles at previously defined urban driving routes at wide ambient temperatures (-14 ° C to 31 ° C) At ambient temperature. Given the fuel mileage provided by HEV, performance changes for low and high temperature effects on HEV operational efficiency have been investigated for roads in pre-defined cities on the road compared to existing vehicle technology. The results showed that battery power control limits and engine operation vary dramatically with temperature.

In the years following the energy crisis of the early 70’s numerous researchers have studied the feasibility and practicality of implementing hybrid power trains incorporating regenerative braking which have the potential to improve the fuel economy of vehicles operating under urban driving conditions.

Regenerative braking only promises significant gains in town driving since 62.5% of energy is dissipated in the Metropolitan cycle due to frequent braking. If all brake energy could be regenerated with no loss in the regenerative system, fuel consumption would be improved by 33% 1131. Alternative sources state that the addition of regenerative energy storage systems td motor vehicles can achieve theoretical fuel savings of up to 23% in a 1600 kg vehicle on a level road urban driving schedule. This relative saving is reduced as the weight of the vehicle reduces. A 1000 kg vehicle can achieve theoretical savings of 15% 14, 151. Research by Volkswagen has shown that a hybrid drive with both electric drive and ICE offers potential fuel saving of over 20% compared with just 5-646 from purely electric

Fig.4.1 block diagram for regenerative braking system

Fig 5.1 circuit diagram for regenerative braking system
A flywheel is a type of energy storage system which is used to store mechanical energy and then release the stored energy when needed for acceleration. Flywheel is a heavy, high-speed rotating disc that builds up kinetic energy (the force that causes movement) as it spins. The amount of energy stored depends upon how heavier it is and how fast it rotates. Heavier weight and faster rotation results in higher energy storage. We can relate it to a discus thrower in the Olympics. He winds-up, building an increasing store of force and energy as he spins, and then releases the disc and sends it flying through the air.
The method of transmission of energy directly to the vehicle is more efficient rather than first storing it in the battery, as it does not consists of the conversion of energies. As, during the recharging of battery, mechanical energy is converted into electrical energy and during discharging electrical energy is converted into mechanical energy. So, due to these conversions transmission loses occur and the efficiency reduces. As, in the other case, there are no transmission loses since mechanical energy stored in the flywheel is directly transferred to the vehicle in its original form. As the energy is supplied instantly and efficiency is high, these types of systems are used in F-1 cars.

Fig. 6.1Main components of a typical flywheel
The most common form of regenerative brake involves using an electric motor as an electric generator. The working of the regenerative braking system depends upon the working principle of an electric motor, which is the important component of the system. Electric motor gets activated when some electric current is passed through it. But, when some external force is applied to activate the motor (during the braking), then it behaves as a generator and generates electricity. This means that whenever motor runs in one direction, the electric energy gets converted into mechanical energy, which is then used to accelerate the vehicle and whenever the motor runs in opposite direction, it performs functions of a generator, which then converts mechanical energy into electrical energy, which makes it possible to utilize the rotational force of the driving axle to turn the electric motors, which results in regenerating electric energy for storage in the battery and simultaneously reducing the speed of the car with the regenerative resistance of the electric motors. This electricity is then used for recharging the battery.

The energy efficiency of a conventional car is only about 20 percent, with the remaining 80 percent of its energy being converted to heat through friction. The miraculous thing about regenerative braking is that it may be able to capture as much as half of that wasted energy and put it back to work. This could reduce fuel consumption by 10 to 25 percent. Hydraulic regenerative braking systems could provide even more impressive gains, potentially reducing fuel use by 25 to 45 percent. In a century that may see the end of the vast fossil fuel reserves that have provided us with energy for automotive and other technologies for many years, and in which fears about carbon emissions are coming to a peak, this added efficiency is becoming increasingly important. The added efficiency of regenerative braking also means less pain at the pump, since hybrids with electric motors and regenerative brakes can travel considerably farther on a gallon of gas, some achieving more than 50 miles per gallon at this point. And that’s something that most drivers can really appreciate.

For recovering Kinetic energy of vehicle lost during braking process.

One theoretical application of regenerative braking would be in a manufacturing plant that moves material from one workstation to another on a conveyer system that stops at each point.

Regenerative braking is used in some elevator and crane hoist motors.

Regenerative Braking Systems are also used in electric railway vehicle (London Underground ; Virgin Trains).

Improved Performance.
Improved Fuel Economy- Dependent on duty cycles, power train design, control strategy, and the efficiency of individual components.
Reduction in Engine wears.

Reduction in Brake Wear- Reducing cost of replacement brake linings, cost of labor to install them, and vehicle down time,
Emissions reduction- engine emissions reduced by engine decoupling, reducing total engine revolutions and total time of engine operation.
Operating range is comparable with conventional vehicles- a problem not yet overcome by electric vehicles.

The main limitation of regenerative brakes when compared with dynamic brakes is the need to closely match the electricity generated with the supply. With DC supplies this requires the voltage to be closely controlled and it is only with the development of power electronics that it has been possible with AC supplies where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).
Regenerative braking is necessarily limited when the batteries are fully charged. Because the additional charge from regenerative braking would cause the voltage of a full battery to rise above a safe level, our motor controller will limit regenerative braking torque in this case.
Increases the total weight of vehicle by around 25- 30 Kilograms
Regenerative braking systems require further research to develop a better system that captures more energy and stops faster. As the time passes, designers and engineers will perfect regenerative braking systems, so these systems will become more and more common. All vehicles in motion can benefit from these systems by recapturing energy that would have been lost during braking process. Future technologies in regenerative brakes will include new types of motors which will be more efficient as generators, new drive train designs which will be built with regenerative braking in mind, and electric systems which will be less prone to energy losses.

Of course, problems are expected as any new technology is perfected, but few future technologies have more potential for improving vehicle efficiency than does regenerative braking.

The regenerative braking system used in the vehicles satisfies the purpose of saving a part of the energy lost during braking. Also it can be operated at high temperature range and are efficient as compared to conventional braking system. The results from some of the test conducted show that around 30% of the energy delivered can be recovered by the system. Regenerative braking system has a wide scope for further development and the energy savings. The use of more efficient systems could leads to huge savings in the economy of any country.

M. Ehsani, Modern Electric, Hybrid Electric and Fuel Cell Vehicles: Fundamentals, Theory and Design, CRC Press, 2001
F. J. Thoolen, “Development of an advanced high speed flywheel energy storage system,” Ph.D. Thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2004
R. M. van Druten, “Transmission design of the Zero Inertia powertrain,” Ph.D. Thesis, Technische University Eindhoven, Eindhoven, The Netherlands, 2005
Boretti, “Improvements of Truck Fuel Economy using Mechanical Regenerative Braking,” SAE Technical Paper 2008.

Kaul, S., Gupta, J., Sharma, S., and Kumar, N., “Scope of Regenerative (Magnetic) Braking in the Production of Electricity in Automobiles,” SAE Technical Paper 2010
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M.F. McNitt-Gray, E.M. Hart, N. Wyckoff, J.W. Sayre, J.G. Goldin, D.R. Aberle. Regenerative braking boosts green credentials 205(P):395, 2012
D. Cavouras, P. Prassopoulos, N. Pantelidis. Kinetic Energy Recovery System by means of Flywheel Energy Storage Eur J Radiol 14: 169-172, 2013
J. Toriwaki, A. Fukumura, T. Maruse. Regenerative Braking Efficiency, Trans IEICE Japan, J60-D(12):1101-1108, 2015
S.J.Clegg,”A Review of Regenerative Braking System”, Institute of Transport Studies, University of Leeds, Working paper of 471, 2017