What is the percentage of energy recovery in Kinetic Energy Recovery Systems(KERS) in cars? Kinetic Energy Recovery Systems (KERS) use flywheels to recover energy from the kinetic motion of cars. They use a rotating flywheel that generates energy as it rotates- this  generates the electric energy for recovery. Do these systems use the flywheel on braking - so that when the car brakes the rotating flywheel generates electricity? Or, energy recovery is done even when the car is in motion, when the flywheel rotates with the cars' axles? Also, what is the percentage of energy recovery (ie, energy recovered from the flywheel vs the energy needed to rotate the flywheel - if this is a good measure!) of these mechanisms?   
 A: KERS is also known as regenerative braking. It is a system found on the latest hybrid cars.
When a vehicle is stopping,  the brakes are applied to the discs to generate friction to slow it down. This generates heat which is energy that is lost or wasted.
Regenerative braking creates friction in addition to the brakes, however, some of the energy is converted to electrical potential instead of heat.
Using the flywheel at all times during the journey/race would result in lower performance. It is only used when braking, however, in KERS the energy is used as a boost in the straights in a race for example.
The efficiency would depend on the generator you were using. More coils, better efficiency however more coils means more weight. You need to find the balance.
A: KERS systems (as used in F1 and leMans cars) rely on a flywheel to store kinetic energy. When braking, the transmition is clutched trough a 1:n gearbox to the flywheel. That is, one full revolution of the car's wheels will make the flywheel turn n revolutions. The flywheel system will then store the energy until it is needed. Since a flywheel system rotates on ball-bearings and inside a vacuum, the losses are minimum. When the energy is needed on acceleration, there are two ways to use it:
a) Clutch the flywheel to the transmition whith a reduction gearbox n:1 and recover the stored kinetic energy directly
b) Store the flywheel energy as electricity in a  battery, by means of connecting the flyWheel to an alternator. That energy is then transformed back into kinectic by electric motors.
As for efficiency, of course the a) method is vastly more efficient, as there are no transformations. just losses as heat and drag through the gearboxes.
The b) method is as efficient as the alternator, the battery and the electric motor are. And also has the gearboxes in between.
In practical terms, for a racecar, that is either accelerating or braking hard, the a) method is preferred as more efficient and direct. 
However, for a street car, it is different. As it is always stopping, the long term storage option of a battery is much more useful. 
Nowadays, even racecars are forced, by rules, to rely exclusively on method b), as a means to further develop the technology for everyday use.
