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Diesel Exhaust Fluid (DEF), commonly referred to as AdBlue in Europe and standardised as ISO 22241 is an Aqueous Urea Solution made with 32.5% high-purity urea (AUS 32) and 67.5% deionized water. 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) controls the "AdBlue" trademark and uses it to ensure quality standards are maintained in accordance with DIN 70070 and ISO 22241 specifications.
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 (NOx), which are harmful pollutants, from the nitrogen in the air. Selective catalytic reduction is used to reduce the amount of NOx 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 NOx 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.
SCR systems are sensitive to potential chemical impurities in the urea solution, therefore the solvent is demineralized water. The urea solution is clear, non-toxic and safe to handle. However, it can corrode some metals and so must be stored and transported carefully.
DEF is stored in a tank on board the vehicle, and injected into the exhaust stream by a metering system. The injection rate 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 minimises the tank's size (and subsequent obtrusion into vehicle packaging space). An electronic control unit adjusts the addition of fluid in accordance with such parameters as engine operating temperature and speed.
Diesel exhaust fluid is offered to consumers through a variety of quantities by manufacturers ranging from containers of it for single or repeated small usage, up to bulk carriers for consumers that require a large amount of DEF. It is recommended that DEF be stored in a cool, dry, and well-ventilated area that is out of direct sunlight. As of 2013, a number of truck stops are beginning to add DEF pumps, in which diesel exhaust fluid is administered at pumps similarly to diesel, and often located adjacent to fuel pumps such that the vehicle operator can fill up on both without moving the truck.
In the UK, the storage of DEF is now under scrutiny of the Environment Agency who are proposing new controls following an number of spills of DEF into the environment. Although not toxic, DEF can cause the catastrophic de-oxidation of water bodies leading to damage of the aquatic environment.