*Diesel exhaust fluid (DEF; also known as AUS 32 and marketed as AdBlue) is a liquid used to reduce the amount of air pollution created by a diesel engine. Specifically, DEF is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. DEF is consumed in selective catalytic reduction (SCR) that lowers the concentration of nitrogen oxides (NOx) in the diesel exhaust emissions from a diesel engine.
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 their exhausting unburnt fuel. The excess air necessarily leads to generation of NOx, which are harmful pollutants, from the nitrogen in the air. SCR is used to reduce the amount of NOx released into the atmosphere. DEF from a separate tank is injected into the exhaust pipeline. Within the SCR catalyst, the NOx are reduced by the ammonia into water and nitrogen, which are both non-polluting. The water and nitrogen are then released into the atmosphere through the exhaust.

The injection rate of DEF into the exhaust depends on the specific after-treatment system, but is typically 2–6% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimizes the tank's size (and intrusion into vehicle packaging space). An electronic control unit adjusts the addition of fluid in accordance with such parameters as engine operating temperature and speed.

AdBlue makes its impact with E6 engines, less so with E5 and little for older engines.

*Source Wikipedia