The compressed air car seems the most likely, modern light weight air tanks would remove the biggest problem that people investigating that technology have always run into.

No, that's not the problem. The problem is there's not enough power density in compressed air to move a practical vehicle.

Consider a simple pneumatic drill. [link|http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=200305273&R=200305273|This] drill is driven by compressed air. It turns at 700 rpm and uses 4 CFM of air (probably at 90 psi). That's a lot of air. It's the equivalent of a ~ 1/4 HP electric drill. Consider that's just to turn some gears and a chuck and a drill bit at 700 rpm.

Now consider what it would take to scale that up to drive a 2000 - 3000 pound automobile.

Pneumatic tools are good because the power is supplied by a remote device - each tool doesn't need to be sized to generate its own power. But compressed air has very little power density compared to gasoline.

[link|http://news.bbc.co.uk/1/hi/world/europe/2281011.stm|Here] is a BBC News story about it from 2002.

The air is compressed at pressure about 150 times the rate you would put into car tyres or your bicycle.

An earlier version of the car that we drove was noisy and slow, and a tiny bit cumbersome.

But then this vehicle will not be competing with a Ferrari or Rolls Royce. And the manufacturers are not seeking to develop a Formula One version of the vehicle.

What the company is aiming at is the urban motorist: delivery vehicles, taxi drivers, and people who just use their car to nip out to the shops.

The latest vehicle is said to have come on leaps and bounds from the early model we drove.

It is said to be much quieter, a top speed of 110 km/h (65 mph), and a range of around 200 km before you need to fill the tanks up with air.

Filling up

The car comes fitted with its own compressor so you can fill up at home. But that would take four hours.

The company has developed the technology to refill the vehicle in three minutes, although there are no service station forecourts with the compressed air machines to do that yet.

Let's do some rough calculations:

200 km is 124 miles.

Bicycle tires are pumped up to ~ 100 psi, so the tank on the car is at 15,000 psi. The pressure in the combustion chamber in a gasoline spark-ignition car is about [link|http://www-mat.ee.tu-berlin.de/files/research/sic_sens/sic_sen3.htm|50 atmospheres] = 725 psi. Note that that's far higher than the pressure required by a pneumatic drill (that requires 4 CFM at 90 psi). A 1 liter engine running at 1500 rpm draws in 1500 l/minute = 53 CFM.

Such an engine, even a tiny 1 liter engine, would need about 53 CFM of air at 725 psi to run one minute. Even a tank at 15,000 psi would need to be very large to supply such a huge amount of air. P1V1 = P2V2 so 53x725 /15000 = 2.5 cubic feet tank. For 1 minute of operation. For 2 hours of runtime? You'd need a 300 cubic foot air tank (a cube nearly 7 feet on a side). At 15,000 psi.

Even taking the fact that these are very rough calculations (enhancements of factors of ~ 2 are possible), the car is snake oil.

Cheers,
Scott.