ON THE test track at Berlin's now-defunct Tempelhof airport, a black audi a3 swings into view. It looks and sounds much like any other audi apart from a logo on the door bearing the letters TCNG.  This is the German car giant's answer to the future of green motoring, entered in a competition run by the tyre company Michelin.  The lettering indicates that the car's turbocharged engine runs on compressed natural gas (CNG), the same stuff that is piped to our homes.  Using gas as a motor fuel is nothing new - several manufacturers offer CNG cars in continental Europe and taxis and buses in Beijing run on it. although "cleaner" than petrol or diesel, it does not contribute much to lowering carbon emissions.  With audi, however, the twist is that the gas has been created, indirectly, by wind turbines in the North Sea. audi is ploughing tens of millions of euros into the scheme. If all goes to plan, the first of its a3 TCNG vehicles will be on sale in 2013.  audi calls its fuel e-gas. It is, in fact, methane, chemically identical to natural gas, and will be fed into the public gas network in Germany.  That's the clever part. By running cars on conventional gas, there is no need to set up a new pipe network or complex delivery system - the infrastructure is already in place.  Michael Dick, audi's chief of technical d