|Fig. 1: Electric Ford Focus. (Source: Wikimedia Commons)|
As has been described by Matthew Brown, a huge amount of energy is wasted in the transportation industry.  Today's focus on the environment had brought about the innovation of various fields. One of the most affected fields has been the automotive track. With the push to make technology more environmentally friendly, a lot of attention has been paid to electric vehicles.  The high fossil fuel consumption in vehicles has prompted research in ways to improve the consumption rate. One of the technologies that has come out of this search for improvement is regenerative braking.
The brake system in a traditional combustion engine is based on hydraulic braking technology.  However, this technology has various downsides - mainly a large waste of energy.  A moving car has an immense amount of kinetic energy. When the driver steps on the brakes, there is a lot of energy conversion going on. The kinetic energy of the car turns into heat energy as the car slows down. Because cars are so heavy, large amounts of heat energy is produced. Since the produced heat energy is not captured in a hydraulic braking system, there is a large amount of wasted energy that could be used for other vehicle related tasks. On top of this, the heat causes the brakes of the car to wear down and become weaker and weaker. 
The use of regenerative braking systems in electric vehicles has been able to overcome many of the disadvantages of the traditional hydraulics braking system. In urban settings,regenerative braking recycles about half of the total brake energy. There are various other positives about the technology.  In fact, the benefits are becoming more and more beneficial to society that cars like the one in Fig. 1 are becoming more and more popular on the road.
To begin to understand regenerative braking, its important to first form a foundation for how an electric car works to begin with. Whereas traditional gasoline powered vehicles rely on the combustion of gas for the motor to function, in an electric vehicle the motor is powered by taking currents from the battery. The motor takes in the electrical energy from the battery and converts it into the mechanical rotation of the wheels. 
Regenerative braking can be thought of as an energy recovery mechanism.  When the motor detects braking, it switches into regenerative mode and acts like a generator. Instead of letting kinetic energy convert to heat, the motor converts the kinetic energy to electrical energy used to restore the batteries or capacitors. During this process, the brake controller system monitors the speed of the wheels and determines the torque required to brake and the excessive energy that can be converted into electricity and put back into the battery.  This energy stored in the battery and capacitor is then used for vehicle re-acceleration.  Proper use of a regenerative braking system brings many improvements compared to the traditional hydraulic braking system. These improvements include energy conservation, wear reduction, better fuel consumption, and more efficient braking. 
Wear on brakes is reduced due to the inclusion of a flywheel. A flywheel is an electromechanical battery. Its used to absorb mechanical energy and keep that energy in reserve. The flywheel has a floating magnet that converts electric energy kinetic energy and vice versa. Simply put, the flywheel is stores excess energy. Wear is reduced because the flywheel smoothens out variations in shaft speed by taking into account changes in torque while braking. 
In stop-and-go traffic, regenerative braking can provide most of the braking force. This leads to more fuel efficient braking, overall travel, and better fuel consumption. In higher speeds, with less stopping involved, regenerative braking has improved fuel consumption by up to 20%. 
One of the reasons that electric vehicles are not as popular as gas fueled vehicles is their low range. However, regenerative braking is extending electric vehicle ranges by as much as 10- 25%. This is due to the recovery of energy that hydraulic braking loses through heat. 
A heavy commuter truck fitted with regenerative braking operates at near maximum engine efficiency. The percentage of energy lost from breaking is at a low 2%. Braking only consumes 5% of the fuel. Emissions are decreased as mentioned earlier, the brake controller system closely monitors the amount of torque needed and so engine operation decreases to prevent waste. On the other hand, a similarly heavy truck without the benefits of regenerative braking wastes as much as 60-65% of energy by using its brakes. Imagine the benefits of fitting large commuter trucks with innovative technologies such as regenerative braking. 
|Fig. 2: Cars are traveling at high speeds and require quick deceleration on highways. (Source: Wikimedia Commons)|
Despite the amount of research and development that has been placed into regenerative braking there are still various issues to tackle. For one, regenerative braking is limited the the physical capabilities of various parts of the braking system. For example, if the drive motor is used to regenerate power, then it is still restricted by the same limits as if it were acting as a motor.  Another issue is it that it is virtually impossible and actually impractical to recover all braking energy at high speeds or high deceleration rates. Imagine cars driving on a highway such as the one shown by Fig. 2. When these cars need to brake quickly, high amounts of energy is required to get the cars to safely stop. In turn, some of the braking energy must be applied directly to the brakes and not into the regenerative system. 
A third issue is that issue of variable control. The amount of energy recouped from regenerative braking is highly dependent on various variables such as wind, motor torque variations, temperature changes, vehicle age, terrain, weight transfer, rolling resistance, etc. Motor torque variations cause another issue for regenerative braking. Drivers tend to vary the amount of braking. For example, driver might increase their pressure on the brake as they slow down. This variation in torque causes inefficiencies since the regenerative braking must take into account the variations in torque as it calculates the amount of energy to recoup. 
A more serious issue arises when regenerative braking is applied to a two-wheel-drive brake system. When regenerative brakes kick in, negative torque is applied to the drive wheels. Therefore, in a two-wheel- drive vehicle, negative torque is applied to the wheels with brakes while the wheels with no brakes are able to spin freely. This uneven distribution of negative torque cases uneven tire and brake wear. This could potentially cause skidding on the wheels with brakes. 
Driving is an extremely wasteful process. Large amounts of kinetic energy is turned into heat and gone from the car. Thankfully, a regenerative braking system makes use of this heat to regenerate the batteries of the electric vehicle. Despite the amount of research and development that has gone into regenerative braking, there is still significant room for improvement. Regenerative braking is still very limited and dependent on uncontrollable variables. Also, danger can arise if regenerative braking is applied to two-wheel-drive brake systems. However, regenerative braking does have various benefits. A proper implementation of regenerative braking system extends driving range, improves braking efficiency, reduces brake wear, and improves energy conservation.
© Juan Leis-Pretto. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. Brown, "Studies of Regenerative Braking in Electric Vehicle," Physics 240, Stanford University, Fall 2016.
 X. Nian, et al., "Regenerative Braking System of Electric Vehicle Driven by Brushless DC Motor," IEEE 6712129, IEEE Trans. Ind. Electron. 61, 5789 (2014).
 M. K. Yoong, et al., "Studies of Regenerative Braking in Electric Vehicle", IEEE 5686984, 20 Nov 10.
 N. D. Lakshmi et al., "Energy Efficient Electric Vehicle Using Regenerative Braking System," International Journal of Advance Research, Ideas and Innovations in Technology 3, 155 (2017).
 L. Li et al., "Analysis of Downshift's Improvement to Energy Efficiency of an Electric Vehicle During Regenerative Braking," Appl. Energy 176, 125 (2016).
 J. L.Oliver, "Regeneration and Brake Management System," U.S. Patent 7647997, 19 Jul 10.