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Thread: Compressed Air car

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    *Flashaholic* Illum's Avatar
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    Default Re: Compressed Air car

    a dampener can compensate for noisy, but this technology might actually work. Imagine a compressor that puts stack gas from refinety and power plants in tubes then use the same gas to power the car

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    Flashaholic* Thujone's Avatar
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    Default Re: Compressed Air car

    I had read some on this in the past. There are stateside companies working on it as well. UPS and the Military were listed as investors. And the article I read also mentioned regenerative braking that would recharge the air tanks. Very promising tech in my opinion.
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    00101100000011010000101001100001011011100110010000 10000001110100011010000110010100100000011101110110 1111011100100110110001100100
    00100000011010010111001100100000011101000110100001 11001001100101011000010111010001100101011011100110 1001011011100110011100101110


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    Flashaholic* Fallingwater's Avatar
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    Default Re: Compressed Air car


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    Default Re: Compressed Air car

    I can't see compressing air being any more efficient than charging batteries or converting water to Hydrogen but I guess it could be cheaper, The idea of using a Hybrid version and having a combustion engine to compress air does not seem like it would be any more efficient than using the gas engine to power the car. But maybe someone can correct me
    I need to find a cheaper hobby

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    Default Re: Compressed Air car

    And I can see compressed air storage as being much less efficient than battery storage... Ever play with an air compressor? There is quite a bit of heat generated. Part of the energy goes into compressing the air (raising the air pressure)--the other part goes into heating the air.

    IIRC, even the picture of the guy using an air compressor to re-charge his car had a big fan/cooler on it. That waste heat is wasted energy.

    At best, the Air Car has more similarities with the Ultra Capacitor car than a battery car. The capacitor voltage varies with charge--The Air Tank energy varies with pressure. A battery, more or less, is constant voltage because of the chemical reactions.

    And for design, it is easier to deal with a fixed voltage than a constantly changing voltage. Remember, P=I*V... We need power to turn the wheels, so if that super capacitor is useful over a 4:1 range of voltage, that is also a 1:4 range of current... So when the capacitor is near low voltage--the wires have to handle 4x the current. And at full charge, the electronics would have to manage 4x the voltage. Possible, but expensive to build such a controller (a battery controller would be much cheaper to build the power electronics).

    For an "air motor"--there is something missing from their design discussion... Imagine you have a full tank and want to use your "slightly modified" piston motor to run on the air. You can't just hit that motor with 3,000 PSI straight from the tank--you would get the drag-racer tire burn-out of the century.

    So, you try and pass that 3,000 PSI through a valve (or regulator). 3,000 PSI in, say 200 psi out. And your engine starts moving the car in a controlled manner.

    However, you just dropped that pressure, effectively, as through a resistor. There was a lot of energy wasted (notice, that the air has now been cooled as it went through the valve).

    I am not a pneumatics engineer--but one should want to recover that wasted energy as the pressure/volume is modulated for normal driving. One method would be to send the air through a turbine (variable nozzle/flow geometry) and, I guess, you could now send that turbine energy to a generator or hydraulic (or something else) for something useful (add torque to the wheels?).

    But if you are already using a turbine, junk the piston engine and just use a turbine.

    But, in any case, I don't see how a "compressed air" car would be good for much more than use in regenerative braking or some special use like powering a city bus between stops (and overhead wires to run a compressor for energy to the next stop).

    -Bill

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    Default Re: Compressed Air car

    But what do I know... MDI also has a new Business Week writeup too.
    Most importantly, it is incredibly cost-efficient to run – according to the designers, it costs less than one Euro per 100Km (about a tenth that of a petrol car). Its mileage is about double that of the most advanced electric car (200 to 300 km or 10 hours of driving), a factor which makes a perfect choice in cities where the 80% of motorists drive at less than 60Km. The car has a top speed of 68 mph.

    Refilling the car will, once the market develops, take place at adapted petrol stations to administer compressed air. In two or three minutes, and at a cost of approximately 1.5 Euros, the car will be ready to go another 200-300 kilometres.

    As a viable alternative, the car carries a small compressor which can be connected to the mains (220V or 380V) and refill the tank in 3-4 hours.
    ...
    90m3 of compressed air is stored in fibre tanks. The expansion of this air pushes the pistons and creates movement. The atmospheric temperature is used to re-heat the engine and increase the road coverage. The air conditioning system makes use of the expelled cold air. Due to the absence of combustion and the fact there is no pollution, the oil change is only necessary every 31.000 miles.
    -Bill

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    Default Re: Compressed Air car

    Quote Originally Posted by BB
    And I can see compressed air storage as being much less efficient than battery storage... Ever play with an air compressor? There is quite a bit of heat generated. Part of the energy goes into compressing the air (raising the air pressure)--the other part goes into heating the air.

    IIRC, even the picture of the guy using an air compressor to re-charge his car had a big fan/cooler on it. That waste heat is wasted energy.
    True, but you're competing with internal combustion. Not exactly a high-efficiency process. How much energy would be spent building a whole car full of batteries and large motors? How much energy is lost in power charging conversion? I'll bet that any switching supply big enough to charge a car would also have a fan. So, I'm not sure it is as clear cut as you make it sound.

    Quote Originally Posted by BB
    So, you try and pass that 3,000 PSI through a valve (or regulator). 3,000 PSI in, say 200 psi out. And your engine starts moving the car in a controlled manner.

    However, you just dropped that pressure, effectively, as through a resistor. There was a lot of energy wasted (notice, that the air has now been cooled as it went through the valve).
    I'm not a pneumatics engineer either, but I think is a false analogy. The pressure regulator is more like a switching regulator than a linear regulator. If I take a flow of 10 liters per minute at 300 bar and drop it to 30 bar I should end up with 100 liters per minute. Flow is like current and pressure is like voltage. The ideal gas law says that PV=nRT, and you can assume the number of gas atoms is constant when you decompress. You can also assume the temperature is constant. (I know gas cools when you expand it, but you could heat it back to the same temperature as the gas in the cylinder by just running the decompressed air through a radiator.) So, you have a constant right-hand side, so then, pressure and volume are linearly related.

    I believe the "energy loss" is ultimately due to reduced efficiency of an engine running at reduced pressure. The efficiency of an air engine should be related to the difference between the highest and lowest pressure in the cylinder during the power stroke. The low pressure point is limited by atmospheric pressure so the delta at 30 bar is only about 29 bar instead of 299 bar.

    Food for thought. I'd love to hear other people's takes on the matter.
    Last edited by ckthorp; 05-20-2007 at 12:26 AM. Reason: add bit about energy loss on the bottom

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    Default Re: Compressed Air car

    Quote Originally Posted by jason9987
    I can't see compressing air being any more efficient than charging batteries or converting water to Hydrogen but I guess it could be cheaper, The idea of using a Hybrid version and having a combustion engine to compress air does not seem like it would be any more efficient than using the gas engine to power the car. But maybe someone can correct me
    Go to a local dive shop and watch them fill a SCUBA tank to 3000psi. Better yet, find a fire station that has a unit that will fill the "new" SCBA tanks to 4500 psi.
    The compressors are about the size of a car engine and the electric motor that drives them is, well, about the size of a small engine. Then you need multiple storage tanks to get maximum use of the gas at it's working pressure.
    Then you need an engine to actually do something.
    The higher the working pressure, the smaller the cylinders can be (F = PA) but anything between 0 psi and the working pressure is wasted gas and wastes the energy it took to get there. So a high pressue engine wastes energy more energy than a large one but a low pressure system will have to be very large.
    The ideal gas law gives an idea of what goes on (But this is a real gas and the actual equations are far more complex.) and there is a loss in pressure as the gas cools. It is enough to make SCUBA divers angry when they get a "hot fill"

    Anyone who has ever partially opened a bottle of air and "run out" also knows the ideal gas law does not explain the problem.

    A steam engine is probably more practical and efficient.

  10. #10

    Default Re: Compressed Air car

    I thought dayyyum when they said the range, but at 4'45" he said it uses gas to compress the air as it goes...

  11. #11

    Default Re: Compressed Air car

    Quote Originally Posted by Handlobraesing
    I thought dayyyum when they said the range, but at 4'45" he said it uses gas to compress the air as it goes...
    TANSTAAFL
    Ignoring heat, you need about 200 cubic feet of air at atmospheric pressure = 1 bar = 15 psia to get one cubic foot of air at 3000 psi That's about 3 standard, in the USA, "80" Aluminium tanks, which hold about 77 cubic feet each. It takes a *BIG* compressor to fill one of these in two or three minutes.

    He'd be better off with the propellor driven by the wind of the car generating electricity to run the car...

  12. #12

    Default Re: Compressed Air car

    On efficiency, temperature is never constant. And this thing is going to involve pretty high rates of conversion from potential to kinetic energy, which always impacts effciency. One time I sketched out a pneumatic system for an offroad vehicle winch system; while pneumatic motors have a fantastic power/weight ratio, air tanks have a poor energy/weight ratio.

    One of the common factors behind many previous wundercars is that they use a lot of expensive materials, hand-fab, and terribly crash-intolerant structural designs. I looked at the website, http://www.theaircar.com, and found no mention of the curb weight of the prototype. That's a serious red flag. If the thing weighs 1000 lb, its simply not going to offer the crash protection Americans expect these days.

    Of course, increasing the road fatality rate will help reduce the population, and that's a good thing for the environment...

    And consider what happens with high pressure cylinders when the valve lets go. Its pretty exciting.

    I'd suggest that pneumatic power could make great sense in a factory environment where you've got shop air available at many manifolds around the factory, and you could use regulated air taps from the cart to power pneumatic hand tools. As a replacement for IC engines, I'm very skeptical. Things that seem to work great in prototypes don't look so great when you shoehorn them into a real car like a Honda Accord.

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    Flashaholic* Trashman's Avatar
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    Default Re: Compressed Air car

    Taken from this month's Popular Mechanics:

    "India's largest automaker is set to start producing the world's first commercial air-powered vehicle. The Air Car, developed by ex-Formula One engineer Guy Negre for Luxembourg-based MDI, uses compressed air, as opposed to gas-and-oxygen explosions of internal-combustion models, to push its engine's pistons. Some 6000 zero-emissions Air Cars are scheduled to hit Indian streets in August of 2008. Barring any last-minute design changes on the way to production, the Air Car should be surprisingly practical. The $12,700 CityCAT, one of a handful of planned Air Car models, can hit 68 mph and has a range of 125 miles. It will take only a few minutes for the CityCAT to refuel at gas stations equipped with custom air compressor units; MDI says it should cost around $2 to fill the car's carbon-fiber tanks with 340 liters of air at 4350 psi. Drivers also will be able to plug into the electrical grid and use the car's build-in compressor to refill the tanks in about 4 hours. Of course, the Air Car will likely never hit
    American shores, especially considering its all-glue construction. But that doesn't mean the major automakers can write it off as a bizarre Indian experiment--MDI has signed deals to bring its design to 12 ore countries, including Germany, Israel and South Africa. -- MATT SULLIVAN"


    What exactly is an "all-glue" construction? Is it's chassis epoxied together? Panels still bolted on, or do they just slap some Elmer's on there and clamp 'em till they dry?

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