[sooyeah]

I was wondering if it would be possible to use the rear axel in a car to generate electricity.
Most cars are front wheel drive, so why could you not use the rear axel to generate the electricity needed to drive it?
Don't forget you can use as much space at the back as you like, and use the main rear axel, to drive other magnets and coils.

It would power itself, you would just need a battery to start, or you could combine with other fuels, but surely it is possible to generate the power needed from coils alone?
You could have two batteries and the charge one while using the other.

How many coils/magnets and what size would they need to be, to generate the power needed?

Could it be unsafe as they may generate too much power?

So Mr and Mrs scientist, my question is this, how feasible is this idea?

[lightarrow]

I was wondering if it would be possible to use the rear axel in a car to generate electricity.
Most cars are front wheel drive, so why could you not use the rear axel to generate the electricity needed to drive it?
Don't forget you can use as much space at the back as you like, and use the main rear axel, to drive other magnets and coils.

It would power itself, you would just need a battery to start, or you could combine with other fuels, but surely it is possible to generate the power needed from coils alone?
You could have two batteries and the charge one while using the other.

How many coils/magnets and what size would they need to be, to generate the power needed?

Could it be unsafe as they may generate too much power?

So Mr and Mrs scientist, my question is this, how feasible is this idea?

What do you think would give energy to that dynamo? Think about it.

[eric l]

This system is - or has been - used in a type of brake for buses and trucks.  A dynamo in the rear axle is switched on and deliver current to a battery, e.g. when cruising down a slope.  Just let me search for more details. 
I know this system is currently in use in those bicycles that have a small electrical auxiliary motor :  you simply switch to "DRIVE" in order to have the motor assisting your effort, or switch to "BRAKE" when going down a slope.

[Soul Surfer]

What you arre thinking of is a perpetual motion machine and perpetual motion machines do not work becas although you could generate power from the rotating back wheels of a car you could never generate more than the power you are using to pull the car along Assuming the road was level.  It might be possible to recover a little energy byb using a generator as a brake when going downhill but it is almst certainly not worth the extra weight.

The one place I think that it might work is for a power assisted bicycle where at the price of a little more effort going along on ther level you could get assistance when going uphill but I have not seen any poert assisted bikes with electromagnetic breaking and power gathering form going downhill.

[eric l]

but I have not seen any poert assisted bikes with electromagnetic breaking and power gathering form going downhill.

By way of example :  http://www.bionx.ca/en/main/default/35.shtml
(but there are dozens of them, although you'll find more links in other languages than English)

[sooyeah]

I was suggesting a twin system. How much electricity could a huge rear dynamo generate?

If the dynamo/dynamos were combined with solar, how much electricity could they generate together?

The car would not be a 'perpetual motion machine', as the solar would hopfully make up the difference; But if that still wasn't enough then you could charge the car from home.
The dynamo and solar panels would work to reduce the amount of electricity needed from each charge. 

What if you had two batteries one charged from home, the other charged by the motion of the car and solar panels, so when one became low the other would take over? Eventually the car would run out of electricity, but you should get a big mileage increase.

  

[another_someone]

I was suggesting a twin system. How much electricity could a huge rear dynamo generate?

The issue is not the size of the dynamo, but how much energy you want to extract from the system (how much you wish to slow the vehicle down), and how efficient the dynamo is.

If you had a dynamo and electric motor that were 100% efficient, and no frictional effects (including no air resistance, no rolling resistance, etc.), then you could extract 100% of the energy of motion of the vehicle, which would bring the vehicle to a standstill, and then use that same energy to then bring the vehicle back up to speed.

Reality kicks in.  There are frictional effects, and there are inefficiencies in the dynamo, the electric motor, the electrical transmission systems, the electrical storage mechanisms, etc., and only a small fraction of the kinetic energy of the vehicle can be recovered by the dynamo and reused (still useful, and regenerative braking, as Eric has pointed out, is used, but most of that energy is still dissipated as unwanted heat).

If the dynamo/dynamos were combined with solar, how much electricity could they generate together?

You cannot just look at the dynamo and solar panels themselves - you have to look at the entire system, including how much energy is lost in the battery used to store the energy, in the wires used to carry the current, and in the efficiency of the electric motors used - plus all of the usual issues with any vehicle regarding aerodynamic efficiency, efficiency of the tyres, etc.

Looking at the solar panels alone, the following is one commercial solar panel (don't claim it is the best, nor the worst, just the first I happened to find, and probably typical of what is around):

http://www.yachtbits.com/sunware/sunware_high_power_compact_solar_panels.php

39 Watt Panel
Peak Output - 39watts
Max Current - 2.83amps @ 13.8volts
Open circuit volts - 22.0volts
Qty of cells - 39
Dimensions - 740x440mm
Weight - 4.5kg
Termination Point - Left or Right hand options

48 Watt Panel
Peak Output - 48watts
Max Current - 3.48amps @ 13.8volts
Open circuit volts - 24.0volts
Qty of cells - 39
Dimensions - 780x460mm
Termination Point - Right hand

69 Watt Panel
Peak Output - 69watts
Max Current - 5.0 amps @ 13.8volts
Open circuit volts - 24.0volts
Qty of cells - 39
Dimensions - 600x890mm
Termination Point - Right hand

Note that the power output quoted is peak (assuming you have ideal conditions - which will rarely happen), and it still will cover a substantial part of the area of the roof of a conventional  saloon car simply to deliver about 0.1% of the required power (car engines are typically 30Kw to well over 100Kw, and these panels only produce 70W in ideal conditions).

The car would not be a 'perpetual motion machine', as the solar would hopfully make up the difference; But if that still wasn't enough then you could charge the car from home.
The dynamo and solar panels would work to reduce the amount of electricity needed from each charge.

 

Home charging (if you are talking about taking power from the grid) may reduce local pollution, but is actually the least efficient (and actually very expensive) way of running a vehicle.

Although power stations are relatively efficient at generating electricity, the power losses in sending the power over the wires is substantial, and generally enough to offset any improvement in efficiency in generating the power in a centralised location.  The losses in the batteries (self discharge, internal resistance, etc.) then also have to be added to the overall efficiency calculations of the system.

Electrical power is clean (at least at the point of use - and that is a key qualification to remember), but generally not very efficient.  No doubt that many of the inefficiencies will be addressed over time, but looking at where we are at present, those inefficiencies are there.