VW keeps on pursuing the “cleanest diesel” idea, with more and more impressive results. The latest announcement is of a concept engine, the newest TDI 2.0-liter common rail diesel that features nitrogen oxide in the catalytic converter.

The key is a filter that absorbs the nitrogen oxide and cleans it, resulting in a 90 percent reduction in emission levels. Larger cars will use a converter with a urea tank and a mesh screen, resulting in lower emissions are well.

The TDI, among other things, meets California’s most stringent emission standards. That should be more than enough for elsewhere in the U.S., where it will make its debut in 2008.