Re: Car powered by compressed air ... is that for real?

From: Doug Jones (
Date: Wed Oct 25 2000 - 20:08:34 MDT

Max M wrote:
> A French company called "ZERO POLLUTION MOTORS Inc." apparently has
> developed a motor that runs on compressed air. A little data:
> range: 200km (120 miles) for only 30 US cents.
> using 300 liters of compressed air (300 bar)
> 10 hours in an urban environment at an average speed of 80km/h.
> refill: 4 hours in a 220 V plug at home, 3 min at servicestation compressor.
> price: $10,000
> weight: 700 kg
> Max Speed: 130km/h (aprox. 85 miles/h)
> This technology sounds a little to good to be true, but I don't know enough
> physics to be able to se through it easily.
> Is it possible to compress enough air and keep it in tanks small enough to
> fit into a car?
> Here is a link from the BBC:
> And the companys own website:
> If something like this was for real it would have a huge socio economical
> influence on the world.

Compressed air can only store about as much energy as vaporizing liquid
air, so an analysis I did of LN2 propulsion a while back applies:

To a first approximation, it's like a noncondensing steam powered
car that needs no fuel for the boiler (but needs lots and lots of
makeup water). The specific heat of nitrogen is far lower than that
of water, but if we assume 1 J/g-K (typical for air at STP), and
warming from 77 to 273 K (call dT=200 to make it easier), that's 200
kJ/kg of LN2. The carnot efficiency is high thanks to the very low
Tlow, 74% theoretically, so with losses call it 100 kJ/kg delivered
usable work.

Good liquid-cooled piston engines are under 0.5 lb/hp-hr (arrgh I
hate slug-feet-furlong units), call it 4E6 ft-lbs/lb, or about 5.3E6
J/kg. This is more than 50 times the energy/mass of LN2, so an LN2
vehicle would get about 1/50th the mileage of a gasoline vehicle.
Somewhere in the range of 1/20 to 1/100, given my broad assumptions

Thus, its range would be poorer than an electric car, and the coal
to rubber on road efficiency would be abysmal- liquefying air fights
thermodynamics all the way.

LN2 is priced at around $.05/lb in bulk, or $.35/gal... about the
same as gasoline before federal and state taxes to pay for
roadways. At 1/20th to 1/100th the mileage of gasoline, LN2 is not
economically or technically feasible as an automobile fuel.

Neat toy, though.

Doug Jones
Rocket Plumber, XCOR Aerospace

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