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Old 03-30-2011, 07:05 PM   #1 (permalink)
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The Aircar thread, a true unicorn or just very inefficient short range transport.

(my first well thought out air car thread dissappeared and never posted ah well, this one gets the point I guess but it simply doesn't sound as wimsical or funny)

The aircar BS has always amused me and the prototypes were always reviewed and hyped like any true unicorn but unlike most unicorns there is a kernal of truth, you could drive this unicorn 20 miles at 25mph or in the case of the air bike 18mph for 20miles. (real homebuilt vehicles)

In either event to bust this unicorn we need the energy capacity of compressed air in a given volume versus pressure and reasons why the air car was only 5% efficient on a good day.

That said the possibility exists for the air car to be usable for some people if we could overcome its dismal efficiency (similar to a model T with flat tires and a stuck carb)

The reasons the air car is only 5% efficient is the pump is only 10-20% efficient on a good day and the motor very similar to the above, add to that the electric motor running the pump is typically only 80%; very sad story but could we do better with scale? I think on the compression side YES, heres why the existing pumps are so inefficient.

1. High pressure means very high tolerances and very high friction
2. High pressure means any small gap between the one way valve and the piston results in a heavy loss of compressed air (which is energy)
3. High pressure air seals always leak a bit and have losses while compressing.

How could we over come this? If it were me I would think using a very tall depth of a liquid to compress the air with a low fluid resistance conveyor to bring the air to the bottom with minimal fluid friction could easily increase this efficiency (with scale) to well over 80%, with a good motor design perhaps 90%, all the air that would be compressed would be deposited into the vessel at the bottom, expansion losses would be minimal, frictional losses could be made minimal with the proper shape, size and speed of the vertical conveyor (ideally you wouldn't want the conveyor to really displace any water while it moved, proper surface shaping could reduce the fluid resistance).

Anyone see any other ways we could make the air car efficient enough (albeat still very limited) to be usuable and reasonable on short distance vehicles? If a large scale system of efficiently compressing air were developed that could move the air car up to 10-20% efficiency (if some minor improvements could be made to the motor) with some limited regenability. Oddly that is the typical trip efficiency of most late model over the road cars.

Anyone think using a fluid to compress air would be very inefficient? I don't see why it would be; bypasses almost ever negative that a normal compressor experiences (and the negatives on a normal compressor grow as pressures grow, whereas with a fluid it would not)

Feel free to add the other strangeness discussed previously into this topic and any new strangeness you want, make my unicorn fly

Cheers
Ryan

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Old 03-30-2011, 07:28 PM   #2 (permalink)
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How about an air hybrid? You'd use a small diesel engine fitted with a Jake brake. Instead of venting the compressed air, you'd store it, and use it later for acceleration.
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Old 03-30-2011, 07:55 PM   #3 (permalink)
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Quote:
Originally Posted by jamesqf View Post
How about an air hybrid? You'd use a small diesel engine fitted with a Jake brake. Instead of venting the compressed air, you'd store it, and use it later for acceleration.
Don't laugh, Actually this would work for regen, but only at about 10% efficiency which is still better than absolutely nothing. Main problems.
1. Initially regen braking power would be very small and grow as the pressure increased.
2. Once the pressure started increasing you could actually get to a high enough pressure that no more would be stored or possibly cause damage. (I lean toward no more energy would be stored but you would still have brakes)

The limit to the air pressure would be whatever the motors compression ratio happens to be.

This could maybe be used as a cheap low wear instant start stop solution for big motors or a method of reducing the pumping losses from the turbo during the initial takeoff. (it would require variable or controllable timing on the valves though)

Actually, the more I think about it adding start stop to semis would be a great idea and being able to boost enough right after startup that they would never want to smoke wouldn't be a bad thing either.

Not to mention cheap, any existing semi with a jake could be retrofitted in this way. Why add a pump when you already have an excellent 8 piston pump?

Keep the unicorns coming.
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Old 03-30-2011, 08:14 PM   #4 (permalink)
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How much do high pressure air tanks weigh, I wonder? And, if they fail, they will explode -- they don't fail gracefully.
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Old 03-30-2011, 08:31 PM   #5 (permalink)
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How much do high pressure air tanks weigh, I wonder? And, if they fail, they will explode -- they don't fail gracefully.
Not really, the local guys blew out an acetylene tank, it made a lot of noise and was a fire hazard but didn't do much else (it got wheeled outside), air tanks are usually designed similarly and they aren't "that" heavy at least in comparison to a semi, especially if they were sized mainly to start the semi and add boost initially.

The pressure from a semi motor certainly wouldn't get into the massively dangerous PSI area maybe 270psi at most which is less than spectacular if it went. Could easily be designed with shell and relief to keep it safe, not to mention an onboard tank like that could be used to suppliment air brakes, AC, onboard air tools LoL

Oh and did I mention most semis already have 120psi air tanks onboard anyway? Afterall how do they release the parking brake?
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Old 03-30-2011, 11:20 PM   #6 (permalink)
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Quote:
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How much do high pressure air tanks weigh, I wonder? And, if they fail, they will explode -- they don't fail gracefully.
They weigh quite a lot, as a steel tank used for compressed air or gas needs to be substantial enough not to rupture.

No, tanks don't explode. Within their rated standard they are very safe. At worst if they do fail it is "gracefully" from rust and end up with a small leak - often in the form of pinhole corrosion, due to condensed moisture present from the compressed air in the tank.
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Old 03-30-2011, 11:54 PM   #7 (permalink)
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Originally Posted by Thymeclock View Post
They weigh quite a lot, as a steel tank used for compressed air or gas needs to be substantial enough not to rupture.

No, tanks don't explode. Within their rated standard they are very safe. At worst if they do fail it is "gracefully" from rust and end up with a small leak - often in the form of pinhole corrosion, due to condensed moisture present from the compressed air in the tank.
I saw a video of a car that ran on 100% hydrogen where the tank was deliberately breached in a staged accident. It didn't explode. Instead, the hydrogen shot out like a blowtorch, maybe 1 foot long if I remember correctly. If the breach was in your direction, you would be toast. Otherwise the flame would burn itself out ASAP.

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Old 03-31-2011, 12:35 AM   #8 (permalink)
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The question does not seem to be if they work but how well, I don't think they work well because you are using electricity to compress the air and storing that energy until it's used to power the car, there is a power company that is doing something like this to store off peek electricity but they are freezing it to a liquid state then running it in a modern steam engine, more or less what an air car does with the compressed air, thing is, to make this set up work they need a way to keep the air engine warm and that is why it's not done here but they do play around with it in places like India that stay warm and everyone wants air conditioning, gas engines produce heat as a waste, air engines produce cold.
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Old 03-31-2011, 09:04 AM   #9 (permalink)
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How much do high pressure air tanks weigh, I wonder? And, if they fail, they will explode -- they don't fail gracefully.
High pressure (3000+psi) aluminum or steel SCUBA tanks that hold 80 to 100 cubic feet of air, are 35-45 pounds.
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Old 03-31-2011, 09:07 AM   #10 (permalink)
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Somebody started this a while back. Seems Tata bought the idea. Tata Motors air-powered City Cat car readies for production run - Newlaunches.com
IIRC the issue was getting the onboard tanks approved by the transportation safety departments.

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