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Diesel exhaust fluid (DEF) is an aqueous urea solution made with 32.5% urea 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.

CR 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.

Working Principles
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 (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 (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 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.

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

The overall reduction of NOx by urea is:

2 (NH2)2CO + 4 NO + O2 → 4 N2 + 4 H2O + 2 CO2 and
2 (NH2)2CO + 3 NO2 → 2 1⁄2 N2 + 4 H2O + 2 CO2

Executive Standard: GB 29518-2013