Why "Diesel Exhaust Fluid?"

Yesterday I wrote about our new Ford diesel pickup truck needing DEF (diesel exhaust fluid) in addition to diesel fuel, and as an aside, mentioned I didn't know what it did. Now I know, courtesy of Wikipedia, and their explanation of the chemistry involved is below:

Diesel engines can be run with a lean burn air-to-fuel ratio (overstoichiometric ratio), to ensure the full combustion of soot and to prevent the exhaust of unburnt fuel. The excess of oxygen necessarily leads to generation of nitrogen oxides, which are harmful pollutants, from the nitrogen in the air. Selective catalytic reduction is used to reduce the amount of NO_x released into the atmosphere. Diesel exhaust fluid (from a separate DEF tank) is injected into the exhaust pipeline, the aqueous urea vaporizes and decomposes to form ammonia and carbon dioxide. Within the SCR catalyst, the NO_x are catalytically reduced by the ammonia (NH₃) into water (H₂O) and nitrogen (N₂), which are both harmless; and these are then released through the exhaust.

DEF (Diesel Exhaust Fluid) is a 32.5% solution of Urea (NH₂)₂CO. When the urea solution is injected into the hot exhaust gas stream the water evaporates. The urea thermally decomposes to form ammonia and isocyanic acid:

(NH₂)₂CO → NH₃ + HNCO

The Isocyanic acid hydrolyses to carbon dioxide and ammonia:

HNCO + H₂O → CO₂ + NH₃

The overall reduction of NOx by urea is:

2(NH₂)₂CO + 4NO + O₂ → 4N₂ + 4H₂O + 2CO₂

Now you know the "how" and the "why."