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Compressed air powered Vehicle

 
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It is 2/3/4 wheeled cart with steering mechanism, brakes and a compressed air tank. Highly pressurized backward exhaust jet of air pushes the cart ahead. Speed is controlled by controlling flow of air a the nozzle of tank.

Vehicle has otherwise no moving parts/engine.

VJW, Jan 24 2012

Venturi to improve efficiency http://www.airblast...20air%20blowers.htm
[scad mientist, Jan 25 2012]

[link]






       // Vehicle has otherwise no moving parts/engine. //   

       ... and no pretentions to efficiency, either.   

       Yes, it will work. Just about. Sort of. You're going to need a big, very lightweight air tank (probably titanium).   

       It will be incredibly noisy.   

       Thrust and therefore speed will be difficult to control without a complex valve and some electronics as the feed pressure falls continuously.
8th of 7, Jan 24 2012
  

       My guess is that it should be as efficient as airboat.   

       Noisy ? yes. But that may be a solvable problem. simply covering the nozzle with sound absorbing material may be a good start.   

       About the tank size, : I was thinking same tank size as any other air powered car tank size. May be too optimistic.
VJW, Jan 24 2012
  

       An air motor isn't terribly efficient, and it's still far more effecient than an air jet.
MechE, Jan 24 2012
  

       // Vehicle has otherwise no moving parts //   

       Not even the vehicle itself.   

       The title could use the word "jet", perhaps.   

       // My guess is that it should be as efficient as airboat. //   

       How do you figure that?   

       An airboat has a prop diameter of around six feet. The diameter of an air jet is very much a factor in speed and efficiency--whatever nozzle you rig is going to be much smaller than the air jet created by a prop.   

       Put it this way: Take your compressed air and run it into an airboat engine, and make it an airmotor prop boat, and you will go a lot further (and if you feed the compressed air in, then add gasoline, you'll do even better).   

       Oh, hell. Go down to the toy stores, especially the science-toy stores. You'll find some air-jet balloon vehicles, but they are damned small and silly and run on strings. You'll also find some AirHog airmotor vehicles, with pistons and all, which are bigger and go further..   

       This is one of those things that are fairly easy to think of, but a bit hard to explain why they don't work. I'm just going to say that if it did work, it would already be done.   

       It could be done, but there's no reason to. Well, giggles and fart noises, maybe.
baconbrain, Jan 24 2012
  

       //brakes// ... perhaps not the most necessary accessory imaginable ...
lurch, Jan 24 2012
  

       A variable geometry nozzle would help quite a bit.
FlyingToaster, Jan 24 2012
  

       It's been done better using the compressed air to drive a (rather impressively powerful) motor. Or looking at it another way, hovercraft, areoplanes, sailing boats or any other vehicle driven by wind is in effect, a compressed-air vehicle, it's just the minor details of collection and storage that are different.
zen_tom, Jan 24 2012
  

       [zen_tom] Just for the sake of being über picky, compression of air (in the sense of a change in volume due to a change in pressure) is not required to explain how those vehicles work, and is often omitted from first-order calculations of lift and so forth; obviously it occurs, but it's relatively unimportant, at least at moderately low speeds. Near and above the speed of sound, on the other hand, compression is very important.
spidermother, Jan 24 2012
  

       [spider] But compression can be omitted from the motive calculations on how a compressed air jet works, as well. You need it to determine air-speed out the nozzle, but the actual movement is a simple momentum calculation. Replace air speed out of the nozzle with air speed from the fan, and it's the same calculation that a fan driven vehicle uses.
MechE, Jan 24 2012
  

       Indeed; but compression is vital to the working of a compressed air jet (obviously), whereas it barely enters into the performance of those other vehicles at all.   

       To put it another way, it's possible to predict the aerodynamics of subsonic bodies with great accuracy by testing a model in water rather than air, scaling the velocity appropriately; the vast difference in modulus is not a great source of error, because compression is almost entirely unimportant. The same is (obviously) not the case with a compressed water vs a compressed air jet.   

       To put it another another way, the simple form of Bernoulli's law, which assumes an incompressible fluid, is routinely used to describe subsonic lifting bodies, much as Newton's laws are used for speeds well below light speed - it is an approximation, but a very accurate one.
spidermother, Jan 24 2012
  

       // When you just use a free jet like this most of the gas is expanding after its left the jet, typically in all directions//   

       I think jet has spread of approx 30 degrees. Otherwise sprays would never work.   

       Efficiency of this was a just a wild guess, although I did not add adjective "wild" then.   

       I think this scheme might work on Go-karts or mopeds but not on heavier vehicles.
VJW, Jan 24 2012
  

       I've made little steam-rocket boats on this principle. Nothing more than a used CO2 cannister, part filled with water, suspended over a candle, all on a floating base. Steam shoots out the back, boat goes forwards. It works, but, yes, it's slow and inefficient.
spidermother, Jan 24 2012
  

       My dad at one time added a 3-foot propellor to a 1.5 hp Briggs and Straton engine, which he set on the back of one of his Quadracycles (4-wheel recumbent bike) for the fun of it. It did about 40 MPH.   

       For slower vehicles than jets, for air-propelling it's simply much more efficient to move a lot of air more slowly than a small amount of air very quickly, hence the AirHog's (and my father's) implementation of a piston-powered propellor.
RayfordSteele, Jan 24 2012
  

       The Mythbusters built a boat powered by two compressed air cylinders. The two giant cylinders propelled the boat forward about 20 feet at a whopping 5kts.
DIYMatt, Jan 24 2012
  

       Using a large industrial CO2 bottle (70# or so), this scheme would move a go-cart (with passenger) about ten feet. Adapting it for use on a moped would be a very interesting way to hurt yourself.   

       Compressed gases alone do not make for effective (forget efficient) rocket propulsion in atmosphere. If they did, we'd send the Space Shuttle into orbit with air instead of A-P.
Alterother, Jan 24 2012
  

       Yeah, compressed air is one of the less-efficient means of storing power, even disregarding heat losses during compression. And tankage is a bugger.   

       // Noisy ? yes. But that may be a solvable problem. simply covering the nozzle with sound absorbing material may be a good start. //   

       Um, no. It's the air swirling in air that makes the noise.
baconbrain, Jan 24 2012
  

       I think he meant covering the nozzle so that the air doesn't swirl out?
pocmloc, Jan 24 2012
  

       If you closed off the nozzle completely, that would solve the noise problem without greatly reducing the efficiency.
MaxwellBuchanan, Jan 24 2012
  

       An ideal nozzle design producing a laminar stream would both minimize noise and maximize efficiency. It's still going to be far less efficient than the simplest air motor driving the axle.
MechE, Jan 24 2012
  

       Good one, [MaxB].   

       I once rigged an air motor as a pre-rotator on a gyroplane. It could spin the heavy rotor up to skull-cracking speeds. When we wanted to empty the air tank, we'd just yank the hose and let the air jet out--nothing ever went anywhere.   

       I've used many an air hose and nozzle as a duster, and never had any troubles with reaction. And that was about as nice a jet nozzle as is easily available. If you want to build augmenters and silencers, you might as well switch over to pistons.
baconbrain, Jan 25 2012
  

       I think you could improve the efficiency without adding moving part is you used a venturi. This converts the small volume flow of high pressure air to a much larger volume, lower pressure stream. Does anyone know if this could approach the efficiency of an air motor attached to a fan?   

       See link. At the end of the "air mover" .pdf file, there is a warning: "Secure the air blower in place prior to turning on the air supply or it will tend to move from its intended position". I was hoping for a warning that was a little more dire. Personally I'd prefer a vehicle that did more than "tend to move".
scad mientist, Jan 25 2012
  

       It's more accurate to say that a venturi device converts high velocity, low flow rate into low velocity, high flow rate. Pressure potential has already been converted into kinetic energy by the primary nozzle.   

       A group who made pulse jet engines and powered go-carts with them found that a venturi cowl increased the performance, especially at low speeds, but they emphasised that the overall efficiency was still lower than that of a continuous jet engine, let alone a propellor. So I think the answer to your question is no.
spidermother, Jan 25 2012
  

       You need more reaction mass. Something heavy to lob backwards...   

       A prop works great as it grabs loads of air and pushes it backwards. Also, it's not limited to the mass it carries. Whereas the air cylinder idea is.   

       Compressed air can be used to push other fluids out of a nozel and hence provide more reaction mass. Water is great as it's much heavier than air. By extension mercury is much better still.
saedi, Jan 26 2012
  

       Powered wheels are even better, as the reaction mass is effectively infinite.
spidermother, Jan 26 2012
  

       Limited to the mass of the earth, surely?
pocmloc, Jan 26 2012
  

       [pocmloc] No, actually. Imagine driving on a tiny asteroid. The asteroid rotates a bit, but that simply pulls the destination (elsewhere on the asteroid) towards you, so you don't get there any later. Mutatis mutandis, ceteris paribus, increasing the mass of the Earth does not increase your acceleration relative to the surface of the earth. Therefore, the earth's mass is _effectively_ infinite in this context, Q.E.D.
spidermother, Jan 26 2012
  

       Clever.
pocmloc, Jan 26 2012
  

       //By extension mercury is much better still.//   

       I second that.
not_morrison_rm, Jan 26 2012
  

       [spidermother], thanks for that description. I will remember it next time I am poling my canoe up a river.
baconbrain, Jan 26 2012
  
      
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