|Fig. 1: A photo of Flybrid's mechanical KERS implementation (Source: Wikimedia Commons)|
The law of conservation of energy states that energy can neither be created nor destroyed but instead can be continuously converted to different forms. To a Formula 1 car, that means that all the energy that the car possesses while reaching speeds of 200 mph must be transferred to other forms of energy when the car intends to go slower. When the car brakes, this energy is usually transferred to heat and sound energy that, for the purposes of the car and its driver, is lost. The job of KERS is to harvest a portion of this energy and redeploy it into the car as extra horsepower, providing a performance advantage to its driver. 
The initiative for Formula 1 teams to implement KERS into their cars was spearheaded by the president of the FIA, the governing body for Formula One racing. His incentive was to improve the public image of Formula 1's relationship with the environment. The sport has often been associated with a negative contribution to the environment and the introduction of KERS was an attempt to discredit that reputation. At the time of introduction, many road car manufacturers were beginning to offer hybrid cars which used a similar principle to harvest and reuse energy and so the concept of KERS was one that the public could quickly understand and appreciate. 
To the teams competing however, implementing KERS gave the drivers the opportunity of additional acceleration at the times where they need it most. In fact, teams were so convinced of the advantage of KERS that many of the teams competing for the constructors title changed their cars mid-season to stay competitive in the title race.
There are two main implementations of the KERS system and they differ in how the energy is stored. The electrical KERS uses an electromagnet to transfer the kinetic energy to electric potential energy that is eventually converted to chemical energy that is stored in a battery. It then redelivers the stored energy to the drive train by powering a motor. The electric KERS was what many teams started off trying to implement into their cars. However, the battery used to store the energy is very prone to battery fires and can cause electric shocks. After an incident with the BMW Sauber team, where an engineer working on the KERS was burned while testing the system after a practice run, many teams deemed the electric KERS to be unsafe.  Along with other factors such as being heavier than other implementations, the electric KERS implementation is not found inside today's Formula 1 cars.
The mechanical implementation, shown in Fig. 1, was initially developed by Flybrid Systems. To harvest the energy upon braking, the system uses the braking energy to turn a flywheel which acts as the reservoir of this energy. When needed, the redelivery of the energy is similar to that of the electric KERS implementation, the rotating flywheel is connected to the wheels of the car and when called upon provides a power boost. The mechanical implementation of KERS is known to be more efficient than the electric equivalent due to the fewer conversions of the energy that are taking place.
The implementations are similar to that what is used by hybrid passenger cars. The main difference is that in a hybrid car, the redelivered energy replaces the purpose of the engine and powers the car entirely. In Formula 1 this would be infeasible. Instead the energy is used in addition to the current engine. This is not simple and is achieved using by connecting a Continuously Variable Transmission (CVT) to the drivetrain. The CVT subsequently handles the ratio of the torque provided by the motor connected to the engine and the torque from the flywheel.
The obvious benefit of KERS is the boost provided. The KERS boost can provide drivers with an additional 80 bhp for up to 7 seconds a lap. This translates to more powerful acceleration which can make all the difference to a Formula 1 race. Drivers have been using it out of slow corners to help reach their top speed sooner as well as on straights to actually go past what would usually be their top speed. Since its inception KERS has been used in several race changing moments and in 2009, Kimi Raikonnen used KERS to overtake Giancarlo Fisichella in the closing stages of the race to win it.
While KERS could still provide even more power for longer to Formula 1 cars, there are several reasons why the current systems are not reaching their full potential. One of these is the weight of the systems. Formula 1 cars have a maximum weight limit, and the components can weigh up to 45 kilograms.  Bigger flywheels can store and therefore deliver more power but that comes at the price of additional weight.
There have also been concerns about the safety of the systems. If the flywheel, rotating at thousands of revolutions per second, is freed from its constrains, it could inflict heavy damage on anything it collides with. Often that would be the driver. Even when restrained, the flywheel will take time to stop rotating and so engineers and anyone else dealing with the car would need to be careful to stay clear of it. In the event of a crash, this could impede medical staff from providing attention to a driver still in the car.
With regards to motorsport, as Formula 1 continues to strive to achieve a cleaner, greener public image, the sport will implement more rules to promote the use of KERS. The engineering challenge lies in providing teams with the largest possible advantage while still staying within the rules of the governing body.
Some passenger car manufacturers have talked about using a KERS implementation in their cars. Volvo, usually an industry leader in innovation, have already built a development mule of their flagship S60 with a mechanical keys implementation.  The KERS implementation is actually lighter and smaller than the components needed to make a gas-electric hybrid system and tests suggest they would exhibit similar fuel consumptions. Engineers at MIT have also implemented an electric bicycle which makes use of KERS which costs less to produce than a typical electric bicycle and hence could lead to lower costs. 
© Aditya Sarkar. 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.
 S. Holt, "Formula For Success: Kers and DRS," BBC Sport, 26Nov 12.
 P. Marks, "Drama of Formula 1 Receives Technological Boost," New Scientist, 24 Mar 11.
 G. Richards, "Bernie Ecclestone Says Kers Responsible For Fire in Williams Garage," The Guardian, 30 May 12.
 "2015 Formula One Technical Regulations," Fédération de l'Automobile, December 2015.
 S. Evans, "Volvo S60 Flywheel KERS Prototype First Drive," Motor Trend, 29 Jul 13.
 J. Schofield, "A Powered Bicycle Wheel That Learns As It Turns," The Guardian, 15 Dec 09.