AdBlue
Diesel exhaust fluid (DEF) is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. It is standardised as AUS 32 (aqueous urea solution) in ISO 22241. DEF is used as a consumable in selective catalytic reduction (SCR) in order to lower NOx concentration in the diesel exhaust emissions from diesel engines.
The German Association of the Automotive Industry (VDA) registered the trademark AdBlue for AUS 32.
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 them exhausting unburnt fuel. The excess of air necessarily leads to generation of nitrogen oxides (NO
x), 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 (DEF) from a separate tank is injected into the exhaust pipeline, where 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 (NH
3) into water ( H2O) and nitrogen (N
2), which are both harmless; and these are then released through the exhaust.
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