--- In evworld@yahoogroups.com,
"Alan Zelicoff" <zalan8587@q...> included:

> The notion of a compressed-air powered engine
> for automotive transport is fascinating.
> But, unless I am missing something important in the
> description MDI "air car" (http://www.theaircar.com/)
> something doesn't work out in the energetics.

Here's the BBC's report on MDI
from 46 months ago:
http://news.bbc.co.uk/1/hi/world/americas/992431.stm

Note the constant-volume combustion chamber.
Ordinary motor fuel is what the thing would really run on,
if it were destined ever to run.
Note the Mexico City factory that was supposed
to start up three-plus ago.

>
>
> ... The specifications of the car state that
> it carries 90 liters of air at 300 atmospheres (bar) pressure,
> approximately.
> 2. Thus, the total energy storage on board is
> 27,000 liter-atmospheres (which is, in fact an energy unit
> odd as it may seem to some readers).
> 3. ONE liter atmosphere is a little bit more
> than 100 joules at room temperature;
> thus, 27,000 liter-atmospheres is 2.7 million joules.
> 4. While this sounds like a lot, 2.7 million joules
> is the same as 2.7 million watt seconds
> or 2.7 thousand KILO watt seconds or about 0.8
> kilowatt hours.

A round three-quarters of a kWh.

> Note that a gallon of gasoline has about 30 kw-hrs of
> energy equivalent and that most battery powered cars
> (on the drawing boards or in production) have
> at least 15 kw-hrs (most have closer to
> 30 kw-hrs).

Yes. Here is what I get for a comparison of
various propulsion energy reservoirs.
Per 300 driveshaft kWh at the driveshaft,

108 L 96 kg · Gasoline
448 L 516 kg · Hydrogen, -253°C liquid
972 L 1,222 kg · Hydrogen, 10-kpsi gas
666 L 1,533 kg · Zinc pellets
245 L 331 kg · Boron pellets
208 L 323 kg · Aluminum pellets
W/C L 7,880 kg · GM EV1 NiMH battery pack
W/C L 52,000 kg · Paintball air tanks

The masses include estimates for empty tank mass,
and for the combustible metals and metalloids,
they include extra onboard energy for the oxygen
separator that makes these work nicely.

My estimate for compressed air tank mass are from
a web page, apparently no longer there, by
Michael Patrick Johnson. He used paintball air tanks,
glass-fibre-wrapped aluminum with air at 3,000 psi, ~200 bar,
as his example.
Air mass is negligible, so maybe at 300 bar the mass
would be only 35 tonnes.

> 5. Yet, the MDI is advertised as being able to travel
> about 200 kilometers on an air-charge.

I don't believe anyone has seen an air car go 10 km.

> Given that it is no more aerodynamic than
> battery powered cars (although it is less weighty)
> and that electric motors are,
> for all intents and purposes nearly 100% efficient ...

75 percent is a fair figure, I think,
and is the basis of my EV1 figures above.

> ...in converting on-board energy into movement,
> this suggests that the MDI is
> 20 to 30 times more efficient yet.
> That just doesn't make sense.

I'm glad you posted. I hated the prospect of
either letting the innumerate, but insincere
assertions-of-possibility I've been seeing
on these boards go by, or responding to them.
No, the lightning in Florida can't power the
whole world, and I'm not a bad guy for saying so.
I'm a nice guy. The people uttering the falsehoods
are the bad guys.


--- Graham Cowan
http://www.eagle.ca/~gcowan/Paper_for_11th_CHC.doc --
How individual mobility gains nuclear cachet