It runs on the energy stored in the compressed air. So, like hydrogen powered vehicles or fully electric vehicles, the compressed air should not be thought of so much as a fuel source but as a means of storing energy. One would still have to compress the air...And, the ultimate "cleanness" of the entire system would depend on how the electrical or other energy used to compress the air was produced. But, as, they explain on this web page, making the car non-polluting at the place where it is and transferring the issue of pollution back to the source does allow for certain advantages, just as for electric and hydrogen-powered vehicles.