

First viable compressed air car: get 200-300km per tank of air. - gscott
http://www.businessweek.com/autos/content/mar2007/bw20070319_949435.htm?chan=rss_topDiscussed_ssi_5

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smanek
Compressed air has great power density, but awful energy density. I.e., you
can unload power very quickly from it but can't store much at all.

Looking at this
[http://www.popularmechanics.com/automotive/new_cars/4217016....](http://www.popularmechanics.com/automotive/new_cars/4217016.html)
popular mechanics article, this engine uses 340 Liters of air (under STP)
compressed at 4350 PSI.

If my math is right, that only provides a theoretical maximum of about 55 MJ
of energy, which is about the same as about 1.5 liters of gasoline.

A combustion engine runs at about 20% efficiency and, if memory serves,
compressed air engines at around 30%. But let's give MDI the benefit of the
doubt and assume their engines are 50% efficient.

That means this car claims to be able to go 200-300KM (125-185 miles) on the
equivalent of about 4 Liters (or 1 gallon) of gasoline.

Now, getting 150 miles/gallon isn't physically impossible, but it seems pretty
difficuly and does raise some red flags to me. And that's assuming they have a
50% efficient engine, which _is_ extraordinarily unlikely. Assuming a 30%
efficient engine, this car is doing the equivalent of 250 miles/gallon.

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elad
Maybe part of the explanation is that it's very light-weight - the car in the
photo looks tiny, it says it's made of fiberglass, and my guess is that the
engine must also weigh much less than a comparable internal combustion engine
(you don't need as much metal to contain the combustion...).

Of course, that probably comes with a heavy price to passenger safety.
Fiberglass won't give you any protection in a crush. That may be the reason
why Tata is involved and not some European manufacturer. The Euro market has
some serious safety regulations.

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mynameishere
_Fiberglass won't give you any protection in a crush_

Damn, just put the tank of air in front. Any wreck will cause a mighty
explosion balancing out the various vectors involved. I haven't done (and
couldn't do, really) the math, but this seems perfectly reasonable.

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olefoo
Or use the compressed air to fill a whole vehicle airbag that triggers on
items on a collision path.

This would mean that your safety systems are highly dependent on software and
effective sensors, but hey, cheap car.

~~~
elad
It would also mean that your level of protection depends on the amount of air
in your tank.

As someone who chronically forgets to fill-up, I wouldn't like that idea...

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softwarejim
"tubular chassis that is glued not welded"

Oh yeah I feel safe already. I wonder what its crash rating will be?

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dmix
That felt like reading a press release.

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tx
Ok, we won't be burning gasoline in a vehicle, we'll be burning it to compress
air when refueling.

For similar reasons I don't get the fuss about electric "zero-emission" cars.
Zero? Like electricity just magically comes out of nowhere...

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asp742
It's like software. If you cleanly separate the front-end and back-end it's
very easy to tune and swap out the back-end without the user noticing.

~~~
tx
Like your analogy! :) The bigger picture is that we're approaching an energy
crisis. That's why gas is getting more expensive, and that is why there is
even an interest in these cars to begin with: the message isn't to "separate
front end and back end" but to appeal to people who want to _save money on
energy costs_ , i.e. gasoline. But it won't work, because gasoline is just the
medium, the real beef here is energy, and it's not going to get cheaper
because we need more and more of it. It's not only gasoline that's getting
expensive, my gas and electric bills are ahead of inflation as well.

"Solutions" covered by the press aren't really solving the problem, they're
just shifting it around. "Wow! We have an air-powered car!" - without an
explanation where the energy to compress that air is going to come from...

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jcl
If you assume the problem is "Where will we find energy to sustain our current
consumption rate?", then you're right: air-cars aren't a solution. Such a
solution might not even exist.

But if you assume the problem is "How can we relieve market pressure on our
most constrained resource?", air and electric cars _are_ a solution. The hope
is that by diversifying the sources of energy used for transportation, we can
continue to use oil for other purposes that we haven't found cost-effective
replacements for yet -- like plastics and jet fuel.

So while the cars won't solve the energy crisis by themselves, maybe they'll
buy us enough time to reach the "then a miracle occurs" discovery that does
solve it. Or, failing that, maybe they'll help civilization make a slow
transition to reduced consumption, rather than a sharp crash.

~~~
khafra
Materials science needs to get on the ball and invent Adamantium, from Marvel
Comics. Then we can have compressed air-powered airliners.

