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On newer diesels Adblue is used to reduce NOx emissions. It's mainly used to meet Euro 6 standards.
Details from the AA:
> AdBlue is a colourless, non-toxic mixture of urea and de-ionised water. 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 in\to harmless nitrogen and water.
