
Car that runs on air, from the makers of the world's cheapest car - nreece
http://trak.in/tags/business/2008/07/01/tata-motors-air-car-minicat/
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ars
I'm sorry but I don't believe them that it can run for 300km in between
refuels. Compressed air is just not a very dense fuel. Enough fuel to power a
car for that long would be a tank about 1/3 the size of the car.

(My calculation: gasoline stores about 3000 times as much energy as compressed
air per volume. But a gasoline engine is also only 10% efficient, an air
engine would be much more efficient. Assuming 100% efficient, then gasoline
stores 300 times as much. I can go about 300km using 11 or so gallons of gas.
Lets assume a lighter car, so 5 gallons per 300km * 300 = 1500 gallons of air,
which is about 5.5 cubic meters. So you'd need a box 1.75 meters on each side.
Which is about 1/3 of a car.)

Obviously if they can store the air at a higher density then the box can be
smaller, but if they did that it would also have to be stronger and thus
heavier. (Edit: they are running at about 1/3 more pressure than my numbers
assumed.)

And now comes the bigger problem: you waste a tremendous amount of energy by
heating the air when you compress it.

And then you let the air cool, and lose again, because as it expands it gets
_really_ cold, and won't expand as much. On the plus side running the A/C will
be free. But forget about heat in the winter.

Using higher compression makes this much worse.

All in all, I simply don't think this will work for that distance. Maybe as a
30km car it would work fine (and there's a large market for that), but not
300km.

And the heating/cooling losses are really going to hurt efficiency.

And as an aside using vegetable oil is a bad idea. Unless you can seal it away
from air (doubtful) it will oxidize and won't be oil anymore.

Now please tell me if I made any mistakes in my calculations.

~~~
MaysonL
In the winter you compress the air at home and use the heat to keep your house
nice and toasty.

Your calculations are off: energy density of gasoline ~35 MJ/L, compressed air
at 300 bar ~ .15, so per volume is ~230 times, so w 10x efficiency, it's down
to 23X volume required: tank to be used is 340 L : divide by 23 ~= 15 L ~= 4
gal. Much closer to their claim, no?

The compression stations will have heat exchangers, so be able to use at least
some of the wasted energy...

(Note: energy density figures from Wikipedia)

~~~
ars
Man, that's embarrassing. I also got my numbers from wikipedia, but typed .01
instead of .1 in my calculator :(

So yes, now I have to retract what I wrote about the size of the tank. I have
new respect for the power of air.

------
prakash
More info here: <http://www.theaircar.com/acf/air-cars/energy-storage.html>

With all the hype around the Prius, Tesla, etc, I wonder how we never heard of
this till now.

~~~
DaniFong
It's exciting that they're announcing this. I had actually started
specifically working on supporting technologies for this (off-grid sustainable
refueling strategies) about a month ago.

If anyone is interested, email me for details.

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gaius
It doesn't "run on air".

It runs on some traditional power source and stores its energy in the form of
compressed air.

~~~
ovi256
Well, internal combustion engine cars do not run on fossil fuel, but on solar
energy, and they just happen to store it as fossil fuels.

~~~
ntoshev
The difference being that solar energy has already been stored there for you.

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KirinDave
Burning seawater, water "catalysts", cold fusion, and now nothing but
compressed air.

 _Please_ take this story with healthy skepticism. Until we see it in mass
production, it's more likely than not that this story is not what it seems.

~~~
DaniFong
The difference between burning seawater, water catalysts, cold fusion, and
this, is that this is based on science and technologies that have basically
been known for more than a hundred years, and many of the technologies used
have already been quite solidly demonstrated. They simply hadn't been put
together.

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vaksel
isn't compressed air pretty dangerous? i.e. if you end up in an accident

~~~
noonespecial
Few things in the common life of people are as dangerous as 25 gallons of
unleaded.

I'd much rather face any compressed air explosion than be soaked in burning
gas.

~~~
misterbwong
I don't quite buy this argument. Gas is far less volatile than people think.
In the event of an accident, I would much rather face a tank of gasoline,
which most likely wouldn't explode at all, than a tank of compressed air.

In the middle of the fire, however, I'm not so sure.

~~~
bprater
Gas vapors are dangerous. Without the air, you can drop a lit match in a gas
tank, without a 'big boom'.

However, in an accident, you'd want to pick the compressed air tank. The tanks
are very strong. Gas tanks in cars are built with very flimsy steel.

Industries like welding has been using high-pressure tanks for decades, and
bottles rarely fail.

~~~
Dobbs
Yes industries like welding have been using high-pressure tanks for decades,
but that's not high pressure tanks that are traveling at 70 miles an hour in a
tight corridor surrounded by hundreds of other cars that likewise are
traveling at 70 mph. Can you say chain reaction?

Compressed air is far more dangerous than gasoline. Gasoline is not a
guaranteed explosion more likely its a fire.

~~~
ars
I would like to see that in action, because it seems to me that allowing gas
to expand that fast is going to drop the temperature of the gas so low it'll
probably freeze CO2 out of the air.

And that process will probably rob a lot of the explosive force.

Can you compress gas so much that if it exploded it would liquefy? With a
quick google I couldn't find an equation relating pressure drop to temperature
in real air (not ideal gas).

~~~
noonespecial
I think its worth mentioning that explosion is not the danger we should be
thinking about when it comes to gasoline.

Gas is a low viscosity, oily (duh) substance held in flimsy metal or plastic
containers that ignites easily and is hard to put out. We are comparing the
danger of being injured by the compressed air explosion with the danger of
being burned severely by sprayed or leaked fuel.

Sign me up for the shrapnel because burns suck huge.

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timcederman
Compressed air engines are fairly common for both cars and other purposes.

I wouldn't call it bunk science - it's certainly nowhere near as bad as the
'runs on seawater' cars.

<http://en.wikipedia.org/wiki/Compressed_air_vehicle>

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illume
oh wow.

A pretty cool idea. I wonder how the compressed air is generated. I know that
there are some < $100 compressed air generators that run off many other types
of power(electricity, petrol etc). I wonder how energy efficient it is in
terms of power. I mean, the energy use has just moved from the car to the
compressed air generation plants.

It seems they have optimized the car for use in urban areas by making it as
lightweight as possible. I wonder if it would pass safety tests in other
countries -- with the car being held together with glue, and relying on
wireless technology (zigbee maybe)?

Great ideas... it seems to have used a whole bunch of new technologies.

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noonespecial
Neat. This also opens the door for more energy dense and pressure stable
compressed gas fuels like nitrogen and CO2.

~~~
ars
I'm sorry, but that sentence doesn't make any sense.

What is pressure stable?

And changing the type of gas has almost zero affect on how much energy you can
store in it by compressing it.

~~~
noonespecial
Sorry, its a stupid paintball reference. It probably is meaningless. What its
_supposed_ to mean is that as a chemical pressurized (to the point of
liquification) boils and creates the pressure in the tank, this pressure
remains relatively stable as long as there is some liquid left to boil.

So most paintball markers use CO2 because you need a simple regulator to get
an even pressure for most of the tank. Some markers use plain air pressure,
they require more complicated regulators because the pressure changes
dramatically during the entire use of the tank.

So the dumb jocks say that the CO2 tanks are "pressure stable", I suppose
because pressure in the tank is constant around 850psi so long as there is any
liquid CO2 in the tank.[1]

I made this comment because this to me seems like a desirable property for a
fuel gas in a compressed gas vehicle to possess. I will attempt to be more
succinct in the future.

[1] And since we are nitpicking, I am in fact aware that that this figure of
850 is variable based on the _temperature_ of the tank.

~~~
ars
Ah! That makes a lot more sense.

The only trouble with using a gas like co2 that liquefies easily is that once
you reach that point (850psi) you pretty much can't go any farther. And they
are running this thing at a much higher pressure than that - about 4500.

I'm sure there are gases that would liquefy at this pressure, but none that
you would want to release into the atmosphere.

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s3graham
(OT: domain extraction doesn't seem to be working with "trak.in")

