AdBlue

AdBlue is a brand name for a diesel exhaust fluid (DEF) used in select diesel engines to reduce nitrogen oxide (NOx) emissions. The solution consists of 32.5% urea by weight dissolved in 67.5% demineralized water. It is used in many modern diesel vehicles, including trucks, buses, and passenger cars, to help meet emission standards such as Euro 4, 5, and 6.

In the exhaust system, AdBlue is injected into the flow where it thermally decomposes to ammonia. The ammonia then participates in a selective catalytic reduction (SCR) reaction with NOx over a catalyst, forming nitrogen and water. AdBlue is not burned as fuel and is separate from the engine’s fuel and oil systems.

AdBlue is a registered trademark of the Verband der Automobilindustrie (VDA) in Europe. In other regions the product is commonly referred to as Diesel Exhaust Fluid (DEF). It is stored in a dedicated tank in the vehicle and is not a fuel additive; contamination or improper use can damage the SCR system.

Storage and handling considerations include a freezing point around −11 °C, so it should be kept in appropriate containers and protected from extreme cold and heat. If the AdBlue reservoir runs dry, many vehicles will issue warnings or enter a reduced-power mode until refilled. AdBlue is non-toxic but should not be ingested, and spills should be cleaned promptly to avoid staining or contamination.