Kinetic Energy Recovery Systems (KERS) also known as regenerative braking systems recover the kinetic energy of a decelerating vehicle that is usually dissipated as heat, and store it as useful energy. KERS are used in numerous applications from hybrid road cars such as Toyota's Prius to high level motorsports projects in Formula 1 and sports car racing. In a world with a finite oil supply and increasing demand for the development of efficient vehicles, hybrid technology is extremely valuable to car manufacturers and through road cars and motorsports programs there has been considerable developments in recent history.

     Regenerative braking systems achieve energy recovery and storage in two ways; electric motors and flywheel systems, though both methods have shortcomings. Electric regenerative systems employ the use of electric motors placed somewhere in a vehicles drivetrain. Energy is stored in batteries connected to the electrical motors and can be used to power the vehicle. When the vehicle is decelerating, circuitry reverses the direction of the electric motor and uses the vehicle's momentum to generate energy which is stored in the batteries. Most consumer hybrid vehicles operate this way, however the technology is implemented differently. Consumer hybrid vehicles can be categorized into parallel hybrids and series-parallel hybrids. Combustion engines do not generate a large amount of torque at lower RPM's and are not efficient at accelerating a vehicle from a standstill. Electric motors however produce maximum torque at standstill and are well suited to getting a vehicle off the line efficiently. Series-parallel hybrid vehicles, such as the Toyota Prius, use power split devices to allow mechanical input to come from either the comubustion engine or the electric motor. This is accomplished using a planetary gearset and multiple clutches which not only allows power to come from one source or both, but also changes the gearing to be most effiecient based on where the power is coming from. The power split allows manufacturers to use the electric motor in standing acceleration and low speed situations and can thus develop engines to have a lower power density and less low-RPM torque but are more fuel efficient. As a result the overall efficiency of series-parallel hybrids is high in a variety of driving situations.

     Parallel hybrid technology, used by Honda, connects both the electric motor and the combustion engine to a mechanical transmission that supplies power to the driveline. This is usually done through a differential gearset which allows the supplied torques from each motor to be the same. The downside to parallel hybrids is that the way the two power sources are mechanically coupled means that when one motor is providing the power, the other must supply a large part of the torque even if it is not efficient. As a result, parallel hybrids are more efficient in cruising/highway situations than stop/start. One advantage of the parallel hybrid systems is they can have much smaller batteries, as they do not need to store enough power to run the car on solely the electric motors.

     Flywheel regenerative braking systems achieve energy recovery and storage in a very differnt way. Originally postulated in 1950 by American physicist Richard Feynman, the concept of transferring a vehicles kinetic energy through flywheel energy storage is not new. However the advances of modern technology has allowed engineers to overcome many of the difficulties originally faced in automobile applications. Flywheels can hold a significant amount of rotational energy, however this dependant on a few factors. To calculate the amount of rotational energy stored within the flywheel we use the equation: T = 1/2 • Iω² where T=Rotational Energy, I=Moment of Inertia, and ω=Angular Velocity in Radians/Second. To calculate the moment of inertia, the equation I = cMr² is used where c=Inertial Constant, M=Mass in kg, and r=Radius in m. Effectively the rotational energy T = 1/2 • cMr²ω². This means that in order to achieve a useful amount of energy storage, for example 400kJ, the flywheel needs to have a large mass, a large radius, or a high angular velocity. In the case of an automobile a large mass would be detrimental to efficiency and a large flywheel radius would be hard to package, so the angular velocity of the flywheel needs to be extremely high. Using the equation T = 1/2 • cMr²ω² where T=400kJ, c=1/2 (Inertial constant for a solid cylinder around its center), M=6kg, and r=.1 m, we solve for ω and get 5657 radians/second. This means that the flywheel must spin at a speed upward of 54,000 RPM to hold enough energy. In the past such speeds were unobtainable, but modern technology has allowed engineers to obtain such speeds. Flybrid Technologies, a company at the forefront of flywheel KERS, package their flywheels in a vacuum sealed enclosures to reduce aerodynamig drag and windage effects and use magnetic bearings to reduce friction, allowing the flywheel to spin faster, reach operational speed more quickly, and spin for longer. The other main hurdle with achieving sufficient rotational speed for energy storage applications is finding a material with adequate tensile stregnth to withstand the rotational forces without warping. KERS flywheels are made of composite materials such as carbon fiber which have extremely high tensile stregnth. The main limitation for flywheel energy storage is tensile stregnth of the material used to construct the flywheel. Generally speaking, the higher the tensile stregnth, the faster the wheel can be spun and the more energy can be stored. In the event the rotational force overcome the tensile stregnth of the flywheel, the flywheel will shatter realeasing all of its stored energy at once. This is a large safety concern with the use of flywheel energy storage, however composite materials tend to dissintegrate quickly into a red-hot powder when shattered rather than chunks of flying shrapnel that the shattering of a steel flywheel would produce. Flybrid Technologies is also very confident of their containment systems stating that when subjected to crash tests involving peak deceleration of over 20g, there were no failures and the flywheel continued spinning at high speeds. Obviously there is no need for the flywheel to still be spinning in the event of a crash so they have developed an dissipation system using patent pending energy dissipation rings to bring the flywheel from 60,000 RPM to rest in 5.9 seconds. The company said that there was "so little noise and vibration that we were not at all sure [the energy] had all gone" and that the damage was contained to just the flywheel with the housings able to be reused. Another potential downside to the use of a flywheel is the adverse gyroscopic effects a spinning disc would have on a vehicle's handling performance. The gyroscopic processional forces are proportional to rotational speed and inertia, so while the roatational speed of flywheel systems is very high, the mass is very low, causing the resulting gyroscopic forces to be very low. The forces are still there however, but when approached differently, if the flywheel was mounted to a set of gimbals and allowed a range of motion, the flywheel could be used to increase stability while cornering.

     Flywheels recover energy through the use of a continuously variable transmission (CVT) which allows an infinte variation of gear ratios between a maximum and minimum. The flywheel system is connected to the driveline of the vehicle through a CVT, and through manipulation of the CVT, gains and releases power. When the CVT is moved towards a gear ratio that speed the flywheel up it is storing energy. When the CVT is moved towards a ratio that slows the flywheel down, by way of conservation of energy, the rotational energy is returned to the drivetrain. In motorsports applications, the manipulation of the CVT is controlled electronically through a button pressed by the driver and is used to aid in acceleration rather than to increase efficiency and mileage.

     Motorsport has been a significant driving force in the development of kinetic energy recovery systems in recent years. The 2009 Formula 1 regulations included KERS, allowing teams to use regenerative braking to provide 60kW of additional power over a single lap. While the power output stipulated by the regulations was deemed to be too small to allow sigificant innovation in terms of technology, the presence of KERS in Formula 1 has seen inovation in terms of packaging and beneficial over questions of safety. Weight and packaging are significant when it comes to the competiveness of a racing car, and as a result Formula 1 teams began experimenting with the use of supercapacitators which are smaller, lighter, and more environmentally friendly to produce than traditional batteries. The addition of KERS in motorsport cas also allowed for a critical look at the safety of such systems as racing cars travel at much higher speeds than road cars and thus are subject to higher forces in crashes.

http://www.racecar-engineering.com/articles/f1/182014/f1-kers-flybrid.html

http://www.ret-monitor.com/articles/723/flywheel-energy-storage/

http://www.flybridsystems.com/index.html

http://en.wikipedia.org/wiki/Regenerative_brake

http://en.wikipedia.org/wiki/Moment_of_inertia

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/regenerative-braking.htm

http://www.eki-gmbh.com/innovationen/KERS_Info_Version_080819.pdf

http://en.wikipedia.org/wiki/Hybrid_car