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The Compressed air car

  1. Sep 12, 2007 #1
    The compressed air car is a much desired development in the search for an alternative cleaner, non-polluting form of motivating power that can be used in transportation. For the past 150 years the IC Piston engine, working on the Otto cycle and first invented in 1866 by Nicholaus Otto, has been the mainstay of personal and commercial road transport. Yet with all the billions of dollars spent on trying to improve the performance of the IC Piston engine, every working sequence of the engine being examined and tested with the latest computer technology and simulations, by millions of people around the world, it is amply clear that if a solution is to be found for a more efficient engine, it wonbe based on IC piston technology. Even with all the 21st. Century innovations and improvements, such as MPFI, double overhead cams, multiple exhaust and inlet valves etc., the IC piston engine efficiency remains at 20%!
    Think of it, an amazing and unacceptable 80% of the energy in the fuel used is wasted, by comparison, rockets and jets have an efficiency of 70% and better or are more than two and a half times as efficient as piston engine cars, they are also, relatively, pollution free. Unfortunately, both turbines and rockets require or have required until this date, continuous combustion to deliver this order of efficiency. When used in road transport they are therefore very fuel inefficient using about 8
     
    Last edited: Sep 12, 2007
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  3. Sep 12, 2007 #2

    mgb_phys

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    Diesel engines are more than 50% efficient.
    I'm not sure that rockets are that efficent in terms of explosive energy -> forward kinetic energy, especially if you have to keep the rocket around and refill it.
    Rockets are also highly poluting, they create the same CO2 NOx as any other air burning process. Solid fuel motors add lots of exotic hydrocarbons and corrosives from the oxidiser and things like aluminium from the rate moderators.

    Even pure H2 + O2 engines create polution by producing O3 and H2O2.
     
  4. Sep 12, 2007 #3

    LURCH

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    I think straight electric cars, running on motors, will be ready for the road before compressed air. I also think it would be more efficient, if electricity were used to compress the air.
     
  5. Sep 12, 2007 #4

    russ_watters

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    Regular IC engines are more like 30-35% efficient. But that's besides the point:

    What makes you think an air powered car is going to be efficient?

    Btw, this looks like a cut and paste....
     
  6. Sep 12, 2007 #5

    Kurdt

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    Electric cars are looking the best bet. The Tesla is excellent and theres another sporty model based on the design of the Ariel Atom. Both can do well over 100 miles and the tesla is capable of over 200 miles. Good enough for a commute and city driving. There is a company near my home town that creates electric delivery vehicles that can carry up to 3 tons of cargo a range of 100 miles which many companies are now using for short range transport. Electric motors are highly efficient (~80% - 90%) as well. The only problem comes with disposing of batteries which are no longer usable. All we need now is to develop renewable electricity generation.
     
  7. Sep 12, 2007 #6

    rcgldr

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    How effiecient are electrical cars when all aspects are taken into account, generation, storage, "fueling of cars", storage in cars, usage in cars?
     
  8. Sep 12, 2007 #7

    Kurdt

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  9. Sep 12, 2007 #8

    russ_watters

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    Slightly OT, but we have all the technology we need to cleanly power every electric dream we have for the next thousand years or so. The trick is fixing the politics to make it happen. Until that happens, electric cars (or air cars, even if they were possible/viable) don't help the pollution situation, they hurt it.
     
  10. Sep 12, 2007 #9

    russ_watters

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    Total energy conversion efficiency from the power outlet to the wheels is probably on the order of 80% or so, after the power is converted several times. Not bad - and that doesn't include regenerative braking, which would be a no-brainer standard feature on electric cars.
     
  11. Sep 13, 2007 #10

    rcgldr

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    Still aren't relatively high pollution sources like coal one of the cheapest current methods of generating electricity? Nuclear power would seem like the best but there needs to be a means of being harness the nuclear energy of what are currently called waste products which have a shorter half life continuing the fission process until the half lifes are so short that the final waste products are virtually non-radioactive.
     
  12. Sep 13, 2007 #11
    What is the efficiency of energy transformation from mining coal, say, to actual electrical automotion utilizing all practical energy conserving features?
     
  13. Sep 13, 2007 #12

    russ_watters

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    The energy required to mine it is mostly a smoke-screen put up by "environmentalists". It isn't a significant factor. Anyway, if you are just making power, the thermodynamic efficiency is somewhere around 45%. For cogen, maybe 85%.

    Remember, though, that coal is almost all carbon, so the exhaust is almost all carbon dioxide. Contrast that with methane, which exhausts 2/3 water and 1/3 CO2 (and a higher energy content).
     
  14. Sep 13, 2007 #13

    russ_watters

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    Yes.
    Without reprocessing, we may be able to last a few hundred years on nuclear power. With reprocessing (and the technology already exists), we can go for thousands of years.
     
  15. Sep 13, 2007 #14
    I recall from a graduate class I took in the early 1980's that - even then - some coal powered plants were pushing 92% efficiency. I also remember that the definition of efficiency was a matter of contentious debate.
     
  16. Sep 14, 2007 #15
  17. Sep 14, 2007 #16

    russ_watters

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    To engineers, "efficiency" is not a matter of debate, it has a specific meaning. To get 92% efficiency, you just need a really really good cogen plant. Ie, most of the energy goes to heat. A high end residential furnace is 96% efficient.
     
  18. Sep 16, 2007 #17
    mqb phs
    I think around 40% efficiency for the diesel under ideal running conditions is more accurate. As for the comparison between piston engines and rockets, there seems to be a long standing myth that expansion processes are more efficient than reaction processes. They are not. Look at the super gun, invented by Gerald Bull for example. It weighed something in the region of 2,100 tons, was strectehed out over 510 ft, had the explosives placed in stages along the barrel and still could not get enough energy to propel its relatively small 600 Kg payload into orbit, the range was 1000kms. By contrast a relatively small rocket using half the 'fuel" and weighing a fraction of the supergun, is easily able to achieve escape velocity and place much bigger payloads in orbit. How can you still say that the IC piston engine is more efficient or powerfu than a rocket that uses reaction forces? It doesn't make sense. As I said at the beginning of this thread the only reason rockets have not been used conventionally is because they burn fuel continuously and so are fuel intensive motors. If they could be fired intermittently as has been suggested in the Rotary Pulse Jet they would be both fuel efficient and give equivalent or greater power than a piston engine with equivalent size combustion chambers.
     
    Last edited: Sep 16, 2007
  19. Sep 17, 2007 #18

    mgb_phys

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    The 50% figure was for the best marine diesel engines - car engines are less because fixed losses are a larger problem in smaller engines.
    The reason the super gun can't put payloads into orbit is that it must deliver the maximum velocity at the point of greatest atmospheric drag - just at the end of the barrel, whereas rockets have the lowest velocity at ground level and accelerate as the air thins.

    An explosive rocket fuel fully combusting has very high efficency in terms of potential chemical energy into heat. To have high efficency in converting the heat into motion the exhaoust gasses would have to be cool ( thermodynamic efficency is prop. to delta T), since the exhaust is so hot there is extra energy wasted in this unused heat.

    As a way of storing energy to push an object forward very quickly it is fantastic, however if there was a cable elavator to orbit - using a diesel powered winch would be better.
     
  20. Sep 17, 2007 #19
    OK, forget about space, let's talk about range! The scud, which was a pretty inefficient type of missile, was just twenty or thirty feet in length, used a fract5ion of the fuel equivalent of the scud and could carry a 450Kg payload for 500Kms or so. How does this compare with the huge 510 ft, 2,100 ton super gun ! What about recoilless guns, that gave an almost equivalent perfomrance to normal guns even though there was no sealing of the breech and hence no build up of pressure ?
     
  21. Sep 18, 2007 #20

    mgb_phys

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    I'm not sure what you are arguing.
    A rocket is an easier way to carry a payload a large distance - it has practical advantages in that the peak accelaration is lower so a complicated payload is easier to build but is inefficent because you have to carry the weight of fuel with you.

    A supergun means you only have to launch the payload - like an electric train you can leave the fuel stationary behind you. It is large and complicated to build but this is amortized over many cheaper launches.
    The practical difficulties are the very high peak acceleration the payload is subjected to and the stationary and rather obvious "sitting duck" nature of the weapon.
    Although in strictly efficency terms this can be an advantage - the German V3 supergun near Calais was never likely to be able to lauch a realistic attack on london but a huge effort in RAF bombers was expended over a number of years trying to destroy it.
    It has been suggested that considered as a deliberate honey-pot target it was quite sucessful.
     
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