Cars, cars and electric cars

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

  1. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    How do you work that out? Air will pass the fan and exit the rear. Block the hole completely(as it was originally) and it's 100% drag. If some air passes through, you have a working turbine, and less drag.
  2. fillyboy

    fillyboy Screwfix Select

    So have I but I don't use it as a 'rule'.
  3. longboat

    longboat Screwfix Select

    It's all to do with illusion.;)
  4. P J Thompson

    P J Thompson Active Member

    Pretty sure I didn't suggest using the turbine as a brake :D
    I likened it to the system of using braking energy as a source of power. In that you are capturing some of the kinetic energy that you got from the potential energy of your fuel source and converting it back to potential energy.
    Going downhill gravity comes into play and though not free (because you had to use fuel of some sort to get the car up to the summit of the hill) it matters not that you're introducing drag by sticking a turbine in the air flow through HA's tube when you're going downhill. You are effectively turning the force of gravity into potential energy for conversion to kinetic at a later date.
    Yes there would be a small braking effect but that's not to say you're using the turbine as a brake is it :)

    I dunno about your suggested maths but I do concede we aren't talking about a lot of energy here but heck why not? Every little helps :D

    (Eta. oh and I think a screw would be the most efficient way of turning the airflow into rotation, a screw could also operate at very high airflow speeds and forces.)
    Last edited: Jul 29, 2017
  5. chippie244

    chippie244 Super Member

    Just don't put a 3' x 3' steel sheet on top of your car.
    You are really missing out on some very basic knowledge, again.
    btiw2 likes this.
  6. btiw2

    btiw2 Screwfix Select

    On a downhill the car's natural tendency will be to accelerate, the turbine saps some of this energy overcoming (or reducing) this acceleration.
    The turbine is either slowing down the car or reducing its acceleration. i.e. acting like a brake. That's why I called it a brake.
    I didn't think you were suggesting it'd be the only brake. Although emergency stops would be both funny and frightening - "when I slap the dashboard - deploy turbine".
    But, yes, instead of gently braking (potentially wasting the gpe gained) to stop the car accelerating off down the slope, you're slowing the car (or accelerating less quickly) and reclaiming some of that potential energy using the turbine instead of wasting it.

    Nothing wrong with your idea in that it doesn't break any laws of physics.
    So now it's just down to engineering and economics to see if it makes sense.

    I don't know enough about turbines to know what the design would be. Yes, a screw sounds like it makes sense to me (me, who's just admitted he doesn't know much about wind turbine design).

    I didn't realize (until I did the calculations) that the power that a wind turbine generates is proportional to the cube of the wind velocity. Double the wind speed and get eight times the energy.
    The energy of the wind is proportional to the square of its velocity, but velocity factors in again (making it a cube) because if the wind is faster then more of it travels through the turbine every second too.
    That's pretty cool. Ten times the wind velocity gives one thousand times the power.
  7. The best scenario for the car itself in terms of fuel efficiency would be to not have anything on the roof. Anything you place on there from a set of roof rails to a 3' square sheet of steel to a bluebottle who has tenaciously clung on during a 70mph sprint down the A39 will add to the drag, and therefore consume more fuel.

    The same applies to the turbine you propose - it will significantly increase drag. So the question is - will I get more out of the turbine than I put in in extra fuel consumption?

    And the answer - other than when falling over a cliff - is 'no'.

    In true btiw2 fashion, here are the calcs:


    It's as simple as this - if you were to wire that turbine to your electric car's batteries in order to help charge them up, and if you were getting more out in energy than you put in, then your car would run forever.

    And everyone would have a turbine on their car.
    btiw2 likes this.
  8. btiw2

    btiw2 Screwfix Select

    DA's using the laws of thermodynamics[1].

    The laws are paraphrased[2] as:
    • There's this game
    • You can't win
    • You can't break even
    • You can't get out the game

    There's this game (0th law)
    This law gives the definition that are used in thermodynamics i.e. what it means for things to be in thermal equilibrium.

    You can't win (1st law)
    Energy cannot be created or destroyed just converted between forms, and work is energy.

    You can't break even (2nd law)
    Some useful energy is always lost as heat.

    You can't get out the game (3rd law)
    If we try and cheat the heat problem of the 2nd law by getting to absolute zero then this law prevents us by saying we can't get to absolute zero.

    Obviously the real laws are defined mathematically, that's just my paraphrasing[3] of Ginsberg's whimsical theorem.

    The problem with a turbine (whether it's in a hole or not) is the 1st law.
    The energy that the turbine gains is either given up by the car (slowing it down), or extra work (by the battery) has to be done to provide the energy sapped by the turbine.
    The second law then kicks us in the nuts by saying this will never be 100% efficient and we'll lose useful energy through heat (friction).

    PJ's suggestion is to make use of energy that you were going to want to waste anyway. i.e. if the car is going downhill then it'd want to accelerate due to gravity. It's then you use this turbine to collect the energy to bleed off that acceleration.

    [3] Because it's my paraphrasing (and I'm certainly not a physicist) then it may be wrong.
    If you really care, you should probably look it up and get real definitions from somewhere other than "BTIW2 on the screwfix forum" - because that guy's an idiot.
  9. btiw2

    btiw2 Screwfix Select

    Possibly the funniest comment I will read today.
  10. Aren't there some 'super'-cars which do use air braking? A pop-up flap at t'back employed during heavy braking from speed? Or is that just to keep the tail down when needed?

    Either way, they could install wee turbines in there. And the added 'whheeeeeeeeee' sound - like a vintage siren ring - would be a fun bonus.

    But no way on this planet, with its laws of energy and thermowhatsits, would the cost of this installation be justified in terms of energy gain.

    Again, here are the calcs:
  11. P J Thompson

    P J Thompson Active Member

    I'm not sure that last bit is true, DA.

    None of us have looked into the economics of installing turbines that would work as I suggested (I'm discounting the idea of them always being 'on' because that's pre school day dreaming).
    It may not actually be that high a cost if the system is designed in from the off.
    Of course, if you live in the Netherlands my version of the idea would be pretty useless :D
    So whilst it can't reasonably suggested that the idea is anything more than an aid to extending charge range....for some situations, it also can't reasonably be said that the idea is a complete non starter.

    A lot of great developments start with a pretty barmy first idea that then gets expanded upon and honed.

    Deleted member 33931 likes this.
  12. PJ, the new Elon Musk.:)

    But, hey, you are right in principle... :(
  13. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Don't be stupid.
  14. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    You are missing the point. The car will be moving forward, regardless.
    Wasted energy is that of a solid part at the front of a car.
    Anything less than solid(which allows air through) gains energy back.
    A tube with a turbine in it will allow more air through than a solid piece of bodywork.
    No extra energy is required.

    And for those still confused, I put the sheet of metal on the roof of the car to create drag, then took some drag away by inserting a fan instead, AS AN EXAMPLE. Not a design consideration. pffffft.
    P J Thompson likes this.
  15. P J Thompson

    P J Thompson Active Member

    But apart from busses, lorries etc, most vehicles are designed with aerodynamics/ drag in mind nowadays. They're designed to slip through the air aren't they. Not be flat faced slabs.

    Or have I just been googling too many Astons? :D
    longboat likes this.
  16. longboat

    longboat Screwfix Select

    I don't think anyone is disputing the fact that such a system would generate power.
    The problem lies with the amount generated, not that it can in theory be done.

    Strapping a generator onto the output side of the motors would be more efficient, but still pointless if it were to run all the time.
  17. chippie244

    chippie244 Super Member

    You started it ;);)
  18. chippie244

    chippie244 Super Member

  19. The very best thing to do would be to not add anything at all that would contribute to drag. Once you add anything - even a 'useful' turbine - the amount of energy used by the vehicle to overcome that extra drag will exceed the energy gain from the turbine.The only time that wouldn't be the case is if you were going down a hill. But even then the car would be held back more than if there wasn't a turbine on it's roof or body, so you'd likely find yourself having to ease on the throttle even on gentle descents.

    Perhaps you imagine that the turbine blades are easy to turn in the wind and therefore have little drag? That's not the case - if you try and harness the electricity being generated, you make these blades harder to turn. If you try and capture a useful amount of leccy, the blades will be ery reluctant to turn.

    I'm sure you've once owned a car, Mr Ha, that produced a loud squeal from the fan belt when it was first fired up? And that squeal continued for a short while before mysteriously going away? That was due to the alternator being heavily called on to replenish the battery after it was used to turn the starter motor. Why does the fan belt slip? Because the alternator becomes very hard to turn when a heavy load is asked of it.

    You drive off down t'road and turn on the lights, wipers and heated rear screen - and it starts to squeal again...
  20. fillyboy

    fillyboy Screwfix Select

    Only on an old banger with a worn belt in damp weather. Never heard a belt slip in any other circumstances. Maybe it's time for you to trade in the Anglia.

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