can we convert the linear motion into rotational motion in cars using a fly wheel under the car ?
Certainly. It would be simpler to use the regular wheels. Look up regenerative braking.
ok but will it replace the brakers ,its impossible as how would they convert the great linear kinetic energy of the car into rotational energy in very short time ? would the fly wheels replace fuel and brakes ?
KERS could in theory be used to replace brakes but I believe it's hard to make a KERS system work efficiently over a wide speed range and there is the safety issue - what happens if the KERS system fails?
KERS would save fuel in a road car but you have to work out if the extra weight of the KERS system is going to effect performance in other ways. I believe many electric cars have regenerative braking/KERS as they already have most of what's needed to add KERS anyway.
Instead of saving fuel the F1 motor racing rules allow the recovered energy to be used to provide a short burst of extra power to make overtaking easier.
These days all F1 cars use an electrical system where the energy is stored in batteries, however the Williams team did develop a flywheel based system which is still used but not in F1..
It works the other way with motor cars, where the rotational energy of the engine is converted to linear Kinetic Energy.
Also, there are 'flywheel powered buses' which are 'charged up' by spinning a flywheel at the terminus and they make it round their route. A clutch is used to engage and disengage the drive for the wheels. I don't think regen braking is used, though.
It is difficult to do away with brakes completely because it's difficult to produce the level of negative acceleration that braking requires. You need around -1g for breaks whereas an engine only needs to give say g/5, for passenger comfort.
The transmission system would need to be very robust for a flywheel to do this and you would also need a very flexible gear system. Doing it electrically is probably the best way. (You couldn't do without brakes!)
The flywheel replaces neither fuel nor brakes, although it supplements both.
It can't completely replace the fuel because you need some way of getting the car and the flywheel moving when the flywheel is completely spun down, because a practical flywheel cannot store nearly as much energy as an equivalent weight of fuel, and because a flywheel large enough to completely replace the fuel tank would take much longer to spin up to speed than it takes to refill the fuel pump.
It can't completely replace the brakes, both because of the possibility of mechanical failure and because (as you surmise) it's not practical to convert all the kinetic energy of a moving vehicle in the time needed to do a full controlled panic stop. However, most driving doesn't involve panic stops, and recovering the kinetic energy in those cases instead of wasting heating up the brake pads, is advantageous.
You might want to google for "Porsche 911 GT3 R", see what you find.
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