A water/urea based additive, is stored in a tank (like diesel fuel), however, unlike fuel it is not injected into the engine, but directly into a specially modified part of the vehicle’s exhaust. This begins a chemical reaction which removes the polluting oxides of nitrogen (NOx), converting them into harmless oxygen and nitrogen in a process known as Selective Catalytic Reduction, or SCR for short. SCR is only applicable to diesel engines and the more economically you drive, typically, the less AdBlue you will use.