There are 2 competing air-powered engines: one from France that uses an engine similar to the internal combustion type we're all familiar with, but powered by air; and another developed in Australia that uses a unique rotary design (the "di Pietro Engine").
Part 1: France
Part 2: Australia
I'm more partial to the Australian design. Critics claim air is only a energy medium and filling up with air still takes an energy source, therefore, not solving any energy problems. While (perhaps) this is true, it also produces no emissions.