For the new round of EPA emisisons, urea based diesel exhaust fluid (DEF) and a catalytic converter is used to significantly reduce oxides of nitrogen (NOx) emissions. SCR is the leading technology being used to meet 2010 emission regulations.
Here is a great link to that discusses Cummins’ method and more detail on the fluid.
www.cumminsfiltration.com/pdfs/product_lit/americas_brochures/MB10033.pdf