Cars, cars and electric cars

Discussion in 'Just Talk' started by joinerjohn1, Jul 26, 2017.

  1. chippie244

    chippie244 Super Member

    You could just make a smaller car.
    btiw2 likes this.
  2. fillyboy

    fillyboy Screwfix Select

  3. You are right - up to a point.

    If you were to alter a car body by making a hole in it so that it reduced drag, then that would be a fuel-efficiency improvement. If you then installed a turbine in that hole to garner power from that air flow, you would revert the car's drag back to a point that was worse than before, or at least any gain in electrical power from that turbine would be more than offset by an increase in drag.

    The more power you try to extract from the flowing wind - larger turbine, more powerful generator etc - the greater the drag. In all events, whatever you obtain from that electrical generator would be more than offset by increased fuel consumption. Ie - overall it would be a net loss of energy.

    You don't get something for nothing. Taking power from the self-generated 'wind' of a moving car would consume more power than you would get out of it.

    Another example - imagine you got yourself a tiny wee hand-held generator with small 2" blades and held it out the car window and used it to charge your mobile phone. Imagine further that your hand or open window didn't add to the drag - only the teeny tiny wee generator that barely produces the 5V required to charge your phone is actually causing any drag, a teeny tiny amount.

    Ok, you wouldn't notice any practical effect of that tiny bit of increased 'drag' on your car, and you wouldn't notice any decrease in mpg since there are much greater factors involved here such as route, temp, speed etc.

    BUT, you WOULD have used up MORE fuel in your car as a result of that wee turbine that you got out of it in electricity.

    Or, in other words, if you could improve - reduce - the drag of your car by making holes in it, then that would be the thing to do, and don't stick any turbines in it.

    Did I do well, btiw2?
    Last edited by a moderator: Jul 28, 2017
  4. P J Thompson

    P J Thompson Active Member

    It is of course true that the laws of physics prevent you getting something for nothing in this context and that if you tried to use HAs idea 'under power' you'd be shooting yourself in the foot.
    However you could use HA's idea to gain something back on the downhills, a bit like regenerative braking tech. Flip the turbine screw into action on the downhills and you could gain some of the energy back that got you up the hill.
    So it's not a completely daft idea and we may see something like it developed.

    You could use a simple inclinometer to trigger a relay that put the screw/impeller into the duct on the downhills.
    You'd need to do fairly extensive testing of the effects of having holes in the vehicle on aerodynamics and handling of course
    Last edited: Jul 28, 2017
  5. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    Try this. Take a solid sheet of metal 3ft x 3ft and weld it upright to the car roof. That is gonna produce a lot of drag.
    Now cut the biggest hole in it that you can, and put fan blades in it. The drag MUST be less than the solid sheet of metal, and the fan will spin.
  6. P J Thompson

    P J Thompson Active Member

    Still more drag than not having the sheet there at all though eh :)
  7. longboat

    longboat Screwfix Select

    I've got a big 'twelve incher', I love it and the missus is quite fond of it too.
    However, it's mainly used for display purposes, more ornamental, if you will, although it does see some occasional action when things get hot.

    The problem is that it's just too big for everyday practical usage. It's got the thrust of a spitfire taking off when it's on full pelt and it makes the earth-move, never mind the ambience.

    You'd think such a monster would drain ones blood, but no, a measly 110W.
    That's all.

    So, put that in reverse and what could you get out even with the most efficient generator going?
    200W, perhaps?
    That's /hour, bear in mind.

    Not a lot, is it?

    Well, not considering you've got a foot wide rod violating you're cockpit.
    Deleted member 33931 likes this.
  8. facilities

    facilities Guest

    IMG_0292.JPG How about this
  9. facilities

    facilities Guest

    longboat likes this.
  10. longboat

    longboat Screwfix Select

  11. longboat

    longboat Screwfix Select

    btiw2 likes this.
  12. facilities

    facilities Guest

    It was at rest in the photo when the car is moving the post would rise, a bit like that aero thingy does on the back of a Porsche
    longboat likes this.
  13. btiw2

    btiw2 Screwfix Select

    Very good.

    Isn't it weird how Screwfix has turned into a work-energy theorem/conservation of energy discussion?
    Meanwhile, on the sciences forums physicists are arguing whether you should use compo or expanding foam to repoint a wall.

    So using a turbine as a brake. Yeah. That could work.

    We can put some best case numbers on it.
    Let's say our turbine was so good that it stopped the air dead calm after going through.
    Best case is all the energy the air loses our batteries collect.
    Air has a density of, roughly, 1kg/m^3.
    We can start to see the problem though. The hypothetical car weighed one tonne (although Chippies suggestion of just using a smaller car is obviously a good idea if we want to save energy) and cubic metre of air is one thousand times lighter.

    But let's carry on anyway.
    We need to decide on the cross section of our turbine. If we say, what? 50cm diameter (I know that sounds big but I'm trying to help)?
    So roughly 0.2 m^2 cross section.
    Let's say the air travels into the turbine at 30m/s (which sounds like a frightening speed for our big turbine. Don't they turn wind turbines off when the wind is too fast? But whatever).
    So every second we get 6 m^3 of air.
    So let's consider one second and get the power. 1/2 x 6 x 30 x 30 = 2.7 kW.
    ie. roughly 3kJ per second.
    However, we got 450 kJ from the brakes.
    Those are very different sizes of number.

    Maybe it's slightly easier to store energy from a turbine than braking? I don't know, but I don't think this is going to help much.

    I could be wrong on my numbers or calculations, and again, I've assumed perfect efficiency (can someone stop the engineers' laughing?).
  14. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Yeh, just using as an example. If you got a flat solid part of the car, it's 100% drag. Hole it, put a fan in it, gotta be less than 100% drag, surely.
  15. facilities

    facilities Guest

    Don't call me Shirley
  16. facilities

    facilities Guest

    Regenerative braking may be the answer, also plan your journey so as it's all downhill problem solved :)
  17. That is true.

    But the drag caused by the big fan would consume more car fuel than the fan itself produced in electricity. So still not worth doing.
  18. May I suggest a cliff? :rolleyes:
    facilities likes this.
  19. I'm pretty sure that's another excellent post.
  20. Did you add the scratches on the car's front from the slightly too-close blade?

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