View Full Version : An air powered automobile

10-02-2003, 08:45 PM

It looks like this is on the level. A car powered by 2 x 100 litre, 300bar (4351psi) bottles with a compressed air engine.
Supposed to do 110 kph with a range of 300 kilometers on a tank.


How about building it on a motorcycle frame.

10-02-2003, 09:06 PM
How much does it cost to charge two 100 liter bottles of air to 4351 psi?
Consider that a standard gas bottle is around 2000 psi if memory serves me. You will need a pretty big compressor to attain those pressures.

10-03-2003, 12:44 AM
That is too high pressure for someone to blow smoke up your ass. Avoid air car guys, it sounds like they could hurt you.

10-03-2003, 12:59 AM
Scuba Shops can compress air to 3000 easliy enough, if not very fast ( Depending on the cubic feet of the bottles )


10-03-2003, 01:36 AM
What pressure can you get with liquid air?

10-03-2003, 01:44 AM
You lose a huge amount of efficiency first liquifying it and then un-liqufying it. Not cost effective. As an example, the rocket fuel combo liquid oxygen and kerosene does not have nearly as much energy as gaseous oxygen and kerosene. The liquid oxy sucks a lot of heat out of the reaction. But, you can put a lot more LO2 in a tank than O2, so they use LO2.


They don't much worry about cost effectiveness with rockets.

[This message has been edited by Evan (edited 10-03-2003).]

10-03-2003, 02:05 AM
That compressed gas car has been tried many times. When you compress a gas, heat is created, and wasted. You need to have the tank cool in order to fill it. Then when you go to use it, it needs to get all that heat back in order to deliver it's potential of energy. So you need a heavy tank, to store the air, a large heat exchanger to extract heat from surrounding air, at a fast enough rate so the air can deliver power enough to drive the car. And you need an enormous pump, running off energy from somewhere, to run that inefficient cycle. You're better off using that energy at the vehicle itself, in most cases.

10-03-2003, 09:04 AM
my brother was getting his scuba tanks filled by a friend ,they got talking and were stopped by a tremendous bang clang and general explosion related sounds and the sound of high pressure air ,they calmly assesed the situation panickedheaded for the bilges and waited for the debris to settle and some one turn off the compressor,

the burst disk had not popped in the valveon the tank when the pressure went above what it was supposed to, the screw on the yoke of the fill valve failed , the yoke was stretched and the actual thread had sheared right out.
the needle on the pressure gauge read about 1200 pounds and was stuck- it is presumed that it had gone right around the clock and was taking another turn around.
over 5000 psi ?

so you can get highly compressed air ,but allthe cats in the neighbor hood will dissapear for a few weeks .

10-03-2003, 09:18 AM
The rocket noozles are often used as heat exhangers for the O2 & H2 (on the shuttle main engines) to thousands of degrees - this also increases pressure - sort of like turbocharging. As a result an engine the size of a 5 gallon bucket puts out nearly 2 million HP each.

LOX is also far too dangerous to use in domestic equipment (cars, BBQ lighters, etc.) http://bbs.homeshopmachinist.net//wink.gif

10-03-2003, 11:06 AM
Maybe you could use the compressed air in the tires instead?

10-03-2003, 12:11 PM

Yep, and that is where the system loses a lot of power in terms of specific impulse since a lot of heat is used warming up the LOX or H2. However, the main reason that is done is to cool the nozzles. Also, some of the energy picked up is used to run the turbopumps.

Liquified air would not present the same hazard as pure lox but the N2 and O2 have different boiling points so would boil out somewhat seperately (fractional distillation) so you could have a O2 hazard as well.

[This message has been edited by Evan (edited 10-03-2003).]

Forrest Addy
10-03-2003, 12:21 PM
Others things

Energy budget: A 3000 Lb streamlined jelly bean car requires about 30 KwHr to drive 60 MPH for an hour on level ground. If a home conpressor is used to charge automotive air tanks how powerful would it have to be to supply air for an overnight charge to give a 3 hour range in city driving?

Thermodynamics: Remember the Otto cycle! Direct air power is the mose inefficient of all schemes to store and transport energy. You waste 30% of the air's available energy in compressing it to 130 PSI and another 30% expanding it through an air tool to atmospheric. How much energy is wasted compressing and expanding air to 4300 PSI? What would the air motor look like? Whould there be some kind of heat recovery and air post heat?

Tankage: Would 200 liters of 4300 PSI air be enough?

Safety: 200 liters of air compressed to 4300 PSI in a collision?

Practical experience: WW II torpedos were powered by an elaborate compressed 1500 PSI air/alcohol burner/turbine drive that would develop about 100 HP for 6 minutes. Wartime engineers strugged for 4 years to make a quantum improvement in endurace but failed. Later improvement in torpedo range required entirely new modes of power generation.

Convenience: How do you charge the air bottles without a service infra-structure?

There sure to be stuff I missed but as a practical suggestion direct air motor propulsion has too many drawbacks. Air/fuel has some promise but its inefficient and endurance is low.

Weston Bye
10-03-2003, 02:19 PM
Seemed like a good idea when I was a kid. Wanted to put an air motor and tank on my bicycle, travel from gas station to gas station where (back then) they had "free air". Never got it together though.

10-03-2003, 02:21 PM
there are flow losses in compressed air systems, and throttling losses.

Compression must be "isothermal" not "adiabatic" or the tank would get too hot to be safe.

These conditions and associated losses make it very inefficient.

To avoid these problems you would have to leave the heat in the air as you compress it.

You would have to have some means to instantly expose the entire piston area to the tank when the piston is at TDC, and instantly close it off after it had moved a certain distance (so that the air could expand nearly all the way to atmospheric).
That would theoretically avoid most of the losses of throttling and so forth.

But it is nearly physically unrealizable. You would have to insulate the tank and engine. And you would have to devise an instant valve, which isn't possible, so throttling losses would still occur......problems, problems.....

I have no idea how these folks are avoiding the unavoidable losses and making it appear so perfect....

Sounds like it is mostly a trade-off of inefficiency and extra pollution "somewhere else" so that there is less in the city.

One could consider that a sort of urban arrogance, "you get our pollution, we don't want it".

It is true, no matter if the eco-nuts do say it, "there is no 'away' " ....

[This message has been edited by Oso (edited 10-03-2003).]

Rich Carlstedt
10-03-2003, 04:16 PM
If you have a chance, read the article..

Sorry guys,but it is Pie-In-The-Sky technowledgy ...better known as "Pipe Dreams"
AKA --Media Hype for the Green Folk
Now I know some of you guys will disagree, but here is what I scanned from the article.
The tanks are almost 7 feet long and 9 inches in diameter, and there are 4 of them, not two !.
The tanks are made from Fibre glass !! (at 4500 PSI !)
The Compressor is built into the engine and takes 28KW of electrical power to recharge the tanks.
A range of 180 miles at 35 MPH means .155 KW consumed per mile at 100 % Efficiency
(Using Forrest's Jelly bean numbers, it takes 1 KW to move 100 pounds at 60 MPH, So 35 MPH would be half, or .5 KW )
BUT .....
That means the car and occupants have to weigh .155/.5= 76 # TOTAL !

But before you get your knickers-in-a-twist..read carefully and you will see the small print..the engine "also" runs on "petroleum fuel"

Now under facts, the following is quoted when the inquirer asked about heat for occupants in winter operation...and this is the quote
"The heating works using the energy produced by the considerable temperature difference between the head of the piston at 400ºC and the cold air expelled between 0ºC and -30ºC. When a new vehicle is made, it is shown to the media and then to the ordinary citizens."

I have never seen an air appliance get hot when consuming compressed air ...its against Boyle's Law and nature..
unless you add a little gasoline !

Looks like GPBS!

10-03-2003, 04:20 PM
It is not intuitive but the more you compress the air AND then let it cool to it's starting temperature, the less energy it contains. As per the Ideal Gas Law the temperature of a gas increases as the volume it occupies is decreased. If this heat is removed from the compressed gas the energy content of the gas is reduced. In other words, you lose the energy you put in compressing it unless as OSO says (and Forest implies) you could super insulate the tank.

BTW, the smartass answer to how much pressure can you get with liquid air is "none". It's like asking how much pressure can you get with water. You must boil it (either one) and that requires an energy input. You might as well use that energy source to run the prime mover directly.

I ran the math. If you could compress one molar volume (22.14L) of an ideal gas at STP to 5000 psi with no heat loss it would occupy .065 L and the temp would be 91,000C (that's not a mistake!). Pretty hard to put that in a tank and keep it there.

[This message has been edited by Evan (edited 10-03-2003).]

10-03-2003, 06:49 PM
It uses compressed air, and a fuel. Ok. The air is reheated with the waste heat from combustion. There's a way to get some of the energy back into the compressed air, and some of the heat out of the exhaust. Ok. Initial filling of the tanks will still be very inefficient, and the weight of four tanks still means too much weight for the car. That thing must be a compact little lead slug potential bomb. So far a couple of pluses, and a couple of minuses. Not an improvement in transportation overall. I do believe there could be an improvement in air motor technology involved, though it wouldn't ammount to a breakthrough, just an adaptation to use compressed air and a fuel simultaneously. If you ignore the hassle of getting air tanks filled to that pressure, the weight issue, and the danger potential, the rest is a good thing.

10-03-2003, 09:40 PM
Darryl: I think you touched on the hardest part in your first post to this thread. There must be a heat exchanger to liquefy the liquid to a gas. Even for the powers under discussion the heat exchanger is going to be large (and due to pressures involved) heavy. One problem with steam cars and locomotives was the lack of a heat exchanger of reasonable size. I suspect that little car NEEDs its engine to burn gas to heat the air that pushes the pistons in the air engine.

Sure would not want my seat belt fastened to the air tank in any manner-JATO!!

10-03-2003, 10:08 PM
JATO haha. I'm being accused of setting up my backpack for that, as it has tanks that make it look like it's ready to launch.
I have a friend who worked in a garage at one time. They were sitting around, when a tank managed to fall over. Broke the valve off somehow (safety cap not screwed on?) and took off like a rocket, around the shop, then headed off across the street, the rest I don't recall. No one was injured, but that just goes to show that even in a non-mobile situation, the danger of high pressure gas tanks is still there. If that one had gone on fire, well-

10-03-2003, 11:45 PM
On the local PBS station, they ran a segment on a guy who built a three wheeler powered by compressed air. You sat on top of the twin air tanks and the single rear wheel was powered by a twin oscillating cyl engine. It could take two people for a ride around the guy's field. Only part I didn't like was having to sit on the compressed air tanks.

10-04-2003, 12:39 PM
Darryl: I've told this story before so if any one recognizes it-I am not GETTING senile.

in mid 1950's I repaired elevated water tanks. A tank at New Iberia La had a jail house built between the four legs. That was a laid back town- the doors were stove bolted together (maybe 1/4 inch, i think even smaller) part of the job was to tack the nuts because the locals would remove the door, continue their drunk and return to jail. No attendant,I digress, but point is it was not a maximum security jail.

We were hoisting a oxygen jug to the cat walk- maybe a little over 100 feet. The boys rigged off the cap (just a hook in the slots) cap came off went though the mans roof, broke off the neck and destroyed the interior walls (there were four cells I think). Took out at least one commode and other plumbing as well as bending the walls. Truely awe inspiring- even the sheriff (maybe chief of police) was impressed. That near 3000 psi got that bottle moving, and it was slow to stop. A practical application of Newtons law? My list of expended materials drew comment (company paid for all material excepting rods, gas for welder). I paid forlabor and remainder wasmy profit. I don't remember if the expense was alloed or not. I think company paid. I wish i could remember what my "official" story said.

Prior to that time I had heard stories of Sea bees riding jugs in WWll. After that I doubted any one could hold on to a fully charged jug.