Shown above is Audi’s clean diesel engine and ultra-low emissions system, which is a good example of how all clean diesel systems operate. Diesel exhaust is first treated by an oxidation catalyst to convert carbon monoxide and unburned hydrocarbons into carbon dioxide and water vapor. A particulate filter then traps and burns off polluting particulate matter. Some systems also use urea injection to create ammonia for a reaction (in the final catalyst) that removes nitrogen oxide. The final result is a car that meets strict emissions standards.