Diesel Exhaust Fluid (DEF) is an aqueous urea solution used in Selective Catalytic Reduction (SCR) to lowerNOx concentration in the exhaust emissions from diesel engines. The solution may also be referred to asAUS32 , shorthand for Aqueous Urea Solution, 32.5%, or as AdBlue, a trademark held by the German Association of the Automobile Industry (VDA), which ensure quality standards are maintained in accord with ISO 22241 specifications. Initially the specification was described in DIN 70070. In 2006 the worldwide ISO 22241 standard was introduced, which also stipulates analytical test protocols to verify adherence to specifications, as well as requirements for storage, transport and handling of the fluid.
SCR systems are sensitive to potential chemical impurities in the urea solution, therefore the solvent isdemineralized water. The solution is clear, non-toxic and safe to handle. However, it can corrode some metals and so must be stored and transported in vessels made of materials not so affected. DEF is stored in a tank onboard the vehicle, and injected into the exhaust stream by a metering system at a rate of 3–5% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimises the tank's obtrusion into vehicle packaging space. An Electronic control unit adjusts the addition of fluid in accord with such parameters as engineoperating temperature and speed.