I get this idea from Planet Mechanics show of National Geographic Channel . I have embedded a clip of that show below. This project uses a similar concept as of air cars because both of them use compressed air as a fuel.

Concept :
In this project we compress gas into a cylinder. Compress air behaves like a spring,. more it get compressed, more force it generates. We can use this force for doing any mechanical work and most of the pneumatic projects use this concept.

How much it cost ?

It would not cost much if you have old bicycle and one cylinder in a scrap. All the other items such as engine (Which can be unrepairable motor pump  ) , pipe , valve are cheap and easily available in market.

How its work ?

Its upon you , how you want to use compressed air energy , but I suggest you to use old piston method which was used in steam engines, only difference is, here we use compressed air to push the piston downwards as shown in Video :

Related Projects :

Automobile Projects