What do you think of this?

[DanielM]

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It would work, but compressed air doesn't store enough energy for the space it takes up to be practical. MythBuster's fans may remember the episode where they powered a boat with compressed air. It didn't go very far, or very fast.

Gasoline has a very high energy density for the volume, that's why it's the current technology.

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I correct myself. Tata Motors (a big Indian car company) has a compressed air prototype, the MiniCAT.

http://www.greencarcongress.com/2007...otors_sig.html

I still don't think it's practical. You'll see from the posts on that site that it is less efficient then current hybrid regenerative braking.