Air Powered Car

[Russianwolf]

http://en.wikipedia.org/wiki/Air_car

What Wiki has on it.

Carbon Fiber Tank to reduce weight.

Possibly a multi level marketing scam.

[Anna]

I calculated based upon the formulaes in Wikipedia for energy storage.
Actually I found another page which took evaluated the constants for 24C and said
the energy per Std Cubic meter (KJ/NM^3) was
= 110 ln(PB/PA)
assuming the air is reduced from 4350 to 14.5 (300 ATM)
then I get 627 KJ/NM^3

and since 300 Atm in 52 Gal = 300*52 Std Gal (at 1 atm)
or 300 x 52 x .003785 M^3

then the stored energy is 37KJ or around 10.3 KW-Hr - the upper theoretical limit.

THis of course assumes that we can hold the temperature constant while we compress the air and then release it back which is in practice extremely difficult to do - it'll want to heat up when compressed and cool off when released both of which will rob the system of energy.

It also assumes you can extract useful work when the pressure differential gets very low, another problem.

Loring, I think the Wikipedia entry is for a closed system (no mass exchange). The car loses air as it leaves the system. The equation also assumes ideal gas, which is not entirely applicable at high pressures (although you can use a correction factor, usually between 0.9 and 1).

I don't have my thermo books anymore and I'll have to rederive all the equations if I'm going to "solve" this. Well, maybe a little more googling will show the equation. It's an unsteady state mass and energy transfer problem.

[Anna]

Maybe I'm the only one in the world that thinks these alternative technologies are worth a look. Who's to say that new ways to make things work, can't happen? Maybe in the not too distant future, we can see cars running on air, and it will be practical. It might not be practical now, but I don't think we should give up on it just yet. No doubt, if it ever looks promising, the lobbyists will have their jobs to do. I think we should look at any possible technology, even if it appears not to have a chance. There was a time when people thought that saving an image (photography) was not possible.

Ed

Ed, I'd love for alternative technologies to be available that will take us off Middle East oil (been driving a hybrid since 2002). But everyone knows that, too, and they've been selling these cars (gas-powered, water-powered, etc) for years to hopeful people like us. One of the common mental exercises we used to do as students was to use the second law of thermo to show that these cars are not possible. We did that for the water-powered cars anyway.

I think there are other more viable technologies that should be developed, probably in combination with each other. It's sad if these air-car guys are scammers because it'll just turn some people into cynics regarding new technologies. Apparently, they've been selling the concept since 1999.


Anna

[Anna]

WHOA NELLIE!!! All I can say is "What the heck is all this?" I musta missed class the day they went over this. Anyway, I learned a long time ago, never argue with a woman.
.

Hey, who's arguing?? This is called a "discussion," where the students pull out their calculators and pen and paper, and start scribbling down their equations, and gesturing ensues. It's really a lot of fun! Haven't you ever read Dilbert?

Okay, I really miss that kind of interaction. You don't meet a lot of engineering types out in the boonies.

Anna

[TheRic]

I agree something doesn't add up. I have filled trailer tires at a gas station that needed 80 lb of pressure, they were down around 50lb. It took me more than 3 minutes to get it full, and they didn't hold no 52 gallons of air.

Even if a high powered compressor is used, how are you going to make a hose flexible enough to fill a car, but hold 5,000 lb of pressure??!! And that it not much of a safety margin, I would think it would have to rated to hold closer to 10,000 lb of pressure!!

I would think that compressing air that much would use more electric / power, than you would be able to get back. Heck why not use the compressed air power to run to compressor motor. Perpetual motion machine anyone?!!!

[LCHIEN]

I agree something doesn't add up. I have filled trailer tires at a gas station that needed 80 lb of pressure, they were down around 50lb. It took me more than 3 minutes to get it full, and they didn't hold no 52 gallons of air.

Even if a high powered compressor is used, how are you going to make a hose flexible enough to fill a car, but hold 5,000 lb of pressure??!! And that it not much of a safety margin, I would think it would have to rated to hold closer to 10,000 lb of pressure!!

I would think that compressing air that much would use more electric / power, than you would be able to get back. Heck why not use the compressed air power to run to compressor motor. Perpetual motion machine anyone?!!!

When I worked seismic boats, we had compressors which could fill 2000 cu inch (about 9 gallons) reservoirs with 2000 psi air in about 15 seconds.
So it would be possible to pump 52 gallons of 4350 psi air in 3-4 minutes using that size compressor. Of course, these compressors were driven by huge supercharged V12 diesels putting out about 3000 HP or something like that. Two of these engine-compressor pairs took up about a 50' x 50' area on the deck of the ship.
These were serious compressors, we had several mechanics on board (these were compressor mechanics in addition to the ships propulsion mechanics who were responsible for engines of approx the same size that drove the boat) whose sole duty it was to keep these babies running. It would be a real major item to have in a gasoline station, replacing gas pumps and all. I would imagine they would recoup this major expense by charging you $10-20-30 to charge up your air tank.

I see in the air car article on Wikipedia that they link to a MDI web page stating that so far they've achieved 7.2 km (about 5 mi) of city driving on a tank of air. They extrapolated given a larger tank and higher pressure and better tires, lighter chassis, moving the engine, improving the engine etc.,etc., etc that they could hit 125 miles... A lot of if's; they claim all of them are plausible or proven, just haven't combined all into one package.

I note that the Wiki air car article estimated the maximum yield of 300 Bar in 300 liters (79 gal) tank as 12 KW-Hr or the equivalent of 1.4 liters of gasoline. Not to far from my computation of 10.3 KW-Hr for 52 gal at 300 Bar.

I'm a little excited (in a bad way) about a carbon fiber lightweight (light enough not to bog down a tiny lightweight car anyway) tank holding 52 gallows or 300 or 400 liters (seen all these figures kicked around) of 300 or 400 or 450 bar pressure. Its far from my field of expertise but the risk of failure is catastrophic in my opinion.

[Stytooner]

Gasoline can be explosive as well. Look how many years it to0k them to figure out that putting the fuel tank outside the frame was a really poor idea. Many of the gas tanks now aren't made of steel any longer either. Even the small cans you fill up for a lawn mower are now mostly all plastics.
Plastics and composites have come a long way in a short time.

I used to drive a van around for one of the jobs I had. In the right rear corner of this van was an oxy acetelene torch. It was strapped into the corner of course. I was about 22 years old then and this was in Germany. I saw another van that had just gotten creamed in the rear by a big Mercedes truck. The back of that van was now right behind the drivers seat. It dawned on me instantly and I diverted back to the shop and stored my torches there permanently. I wasn't so worried about the tanks, but the valves getting damaged.
Even if they can figure out how to contain that much pressure in a CF tank, the weak link will still be the valves.

Another thing to think about is that a screw type AC makes air a lot faster than a standard piston type does.
One medium sized screw compressor took the place of three large piston type compressors in one of the factories I set up. The tools were often running the air out with the piston types, but never even came close with the screw type.
We are only talking 120 PSI though.

[LCHIEN]

more observations

Gasoline can be explosive as well. Look how many years it to0k them to figure out that putting the fuel tank outside the frame was a really poor idea. Many of the gas tanks now aren't made of steel any longer either. Even the small cans you fill up for a lawn mower are now mostly all plastics.
Plastics and composites have come a long way in a short time.

One of the things about gasoline and gasoline powered cars, gasoline has some very favorable characteristics. One it has great chemical energy stored for its weight and volume - enough that a gallon of it will take a car some 30-40 miles. Yes, its flammable, especially when its allowed to exist in vapor form, but in a closed tank, you rarely see a automobile explode completely (outside of movies) from the gas tank going up in a collision. When it does, its spectacular, but its not that frequent an occurrence given the frequency of crashes.

OTOH, a compressed air car would have a lot of mechanical energy stored up which requires a mechanically strong physical container which, if puncture or damaged in the least way will allow the mechanical energy to be released in a totally explosive manner. The physical size is also not favorable compared to gasoline - 52 gallons of compressed air will take you only 125 miles (or so they hope) or a little over 2 miles per gallon of space taken in the car (compared to 30-40 miles for gasoline).

A gallon of gasoline weighs maybe 6 pounds. 300 gallons of air compressed to fit into 1 gallon (thats how we get 300 atm) will weigh about 3.4 pounds. So comparitively speaking, on the basis of vehicle weight, 3.4 pounds of air will take you 2 miles and 6 pounds of gasoline will take you 30 miles. So a much bigger percentage of vehicle weight will be taken by air for a compressed air car than by gasoline in a gaoline driven car for any reasonable range. (BTW, the high energy per volume of gasoline is why totally electric cars are rare, and hybrids are in - the portability of gasoline energy is key).

Not to mention the disparity in the weight of the container, a 18 gallon tank of gasoline being made of plastic compared to a 52 gallon tank of air, carbon fiber, yes, but thick enough to withstand 300 Atm. The air tank will almost certainly be a ball-shape, because it gives the maximum strength per wall thickness/volume etc. but it will be very awkward to fit in the car, given its size (3 times the volume of a gas car's tank) and shape.

[TheRic]

When I worked seismic boats, we had compressors which could fill 2000 cu inch (about 9 gallons) reservoirs with 2000 psi air in about 15 seconds. ......

Wasn't doubting a compressor COULD fill up a tank that quick, and at that pressure. The article says "The second way involves hooking the car up to a compressed air station, such as the type found at gas stations."

It makes it sound like there already are air compressors / pumps that could fill a tank that quick at that pressure. Maybe over in Europe, but I have never seen one here in the US that could do that amount of pressure, not to mention in that amount of time.

.....Even if they can figure out how to contain that much pressure in a CF tank, the weak link will still be the valves.....

I agree the valves, the lines would all have to be much stronger. Any minor wreck could weaken the lines, valves, tank, etc. Then when you least expect it, you could have shrapnel flying.

[momo44]

Just for reference:

A 15HP 5000 psi compressor (huge for gas stations - normally 1HP) will make 18.6 cfm of air at 5000psi. A 52 gallon tank containing 4350psi air has 2057.1cf of air at atmospheric pressure. To compress the tank to 4350psi would take the 15HP compressor 1 hour 50 minutes and 35 seconds. The gas station could have a total of 13 customers in one day, while the compressor never shut off.
The 15HP compressor puts out 16.6HP actual, so the customer uses 22.82kWh of electricity. At a cost of $.09/kWh, and a motor efficiency of 92%, it costs $2.23 to fill the tank, assuming no other losses. $29.02 per day and $10593 per year for the station owner just for electricity! That does not include compressor cost ($15,000 at least) and maintenance.

Not a good deal for a gas station...........

[scorrpio]

I did not see the mention of this in the thread, but any SCUBA diver out there is routinely strapping on a tank with 3500-4000psi of air. And any diving shop has a compressor that can fill 20-30 of such tanks in a fairly short order. Any live-aboard dive boat will have a 5000psi compressor on board.

[ExYankee]

D*mn! We got some serious engineers here!

I did a quick and dirty CAD model to try and guess the gallon volume of a standard dive tank @ 7 ¼” x 21” assuming a ½” wall gave me 2.4 gallons. So this tank is 21 times as large as a SCUBA tanks at 50% more pressure!

[momo44]

I did not see the mention of this in the thread, but any SCUBA diver out there is routinely strapping on a tank with 3500-4000psi of air. And any diving shop has a compressor that can fill 20-30 of such tanks in a fairly short order. Any live-aboard dive boat will have a 5000psi compressor on board.

Scorrpio,

A scuba tank holds 50 - 150 cf of air, a 52 gallon tank contains 2057 cf of air at 4250psi, so it is the equivalent of filling ~40 scuba tanks.

[momo44]

Just thinking of efficiency losses here.....

A car traveling ~50mph has about 10HP(7.5kW) worth of drag on it. If this car will travel for 2 hours (100 miles) it will need 15kWh of power just to overcome drag. The best efficiency for this air motor is probably no more than 50%, so this is 30kWh of air power needed. The best theoretical compressor will produce air at 15% efficiency, so or requires electrical input becomes 200kWh or in other words, a 15HP (11 kW) 4350psi compressor running for 17.87 hours! This produces 7979 cf of 4350psi air which takes a 202 gallon tank. To run for 100 miles on a 52 gallon tank would require the car only go maybe 20 mph or so with no acceleration or deceleration.

[LCHIEN]

Wasn't doubting a compressor COULD fill up a tank that quick, and at that pressure. The article says "The second way involves hooking the car up to a compressed air station, such as the type found at gas stations."

It makes it sound like there already are air compressors / pumps that could fill a tank that quick at that pressure. Maybe over in Europe, but I have never seen one here in the US that could do that amount of pressure, not to mention in that amount of time.



I agree the valves, the lines would all have to be much stronger. Any minor wreck could weaken the lines, valves, tank, etc. Then when you least expect it, you could have shrapnel flying.

Ric, Yes, the quoted article made it sound like there were compressors now at gas stations that could do it. But in fairness I pointed out that the MDI web page says that it is ANTICIPATED that gas stations would have these (assuming air cars became popular). So the article author is at fault, not the company pushing the technology.

Momo, I pointed out that there are large (Stytooner they were compound screw type) industrial compressors that can fill the target tanks in a few minutes but they would be huge and expensive (on a scale about 10,000 times the kind of compressors most of you see). This is understandable as we are basically transferring enough energy to propel a car 125 miles, in a matter of a two-3 minutes. So for a commercial "refueling" station for air cars, you'd want to charge the tank in 2-3 minutes, you're going to need a compressor costing several hundred thousand dollars. Won't run off normal electrical outlet either - probably diesel (so we're losing part of the green appeal). I suggested a charge of $20-30 for a charge, I still think that would be reasonable to pay for one, the energy used to charge the tank, and two, the amortized cost of the expensive facility.