AdBlue is an exhaust fluid, not a fuel additive. It’s stored in a seperate tank and is topped up via usually filled up either next to your fuel filler, in the boot or under the bonnet. It’s a trade name registered by the German car manufacturers association, bu is the most recognised form of Diesel Exhaust Fluid (DEF) Tiny amounts of AdBlue are injected into the flow of exhaust gases. At high temperatures AdBlue turns to ammonia and carbon dioxide. Inside the SCR (Selective Catalytic Reduction) catalyst, harmful nitrogen oxide in the exhaust reacts with the ammonia and is transformed to harmless nitrogen and water.