We recently got a chance to play with a diesel-powered Audi A6 from Europe (click here for photos), and were impressed by its performance. At the 2008 Detroit auto show, Audi displayed the system that will clean up the diesel A6's exhaust when it's imported to the U.S. It is a complex system, and uses the AddBlue technology also embraced by Mercedes-Benz. The exhaust first goes through a particulate filter to remove the heavier bits, then the AddBlue solution breaks down the nitrous-oxide. A filler cap for the AddBlue tank sits right next to the gas cap. We also noticed a rather large tank at the end of the system, just before the exhaust tips. This display shows that Audi is preparing the U.S. public for the debut of its diesel cars, but we were surprised that we didn't see a diesel Audi A6 on the show floor.