At the core of the Bluetec technology is a catalytic converter known by the abbreviation of SCR (Selective Catalytic Reduction)."Selective" refers to the fact that this catalytic converter has a very special task. Its purpose is to selectively convert the nitrogen oxide (NOx) exhaust components to nitrogen and water without generating any undesirable side products. It was developed precisely for this highly specialized task.
The nitrogen oxides are converted utilizing a synthetically produced, water-based urea solution, e.g. AdBlue, which is stored in an auxiliary tank on the Audi Q7 3.0-liter TDI. The substance, consisting of 32.5 percent urea, is continuously sprayed into the exhaust stream before the SCR catalytic converter. It is metered based on the mass flow of the exhaust. Engine management ensures precise regulation, based on information from a NOx sensor located after the SCR catalytic converter.
Atomized into a fine spray by a screen, the urea is transformed in the hot exhaust gas stream upstream of the catalytic converter. In the SCR catalytic converter this reacts with the nitrogen oxides and splits them, as described, into nitrogen and water. The water-based additive AdBlue is non-toxic, odorless and biodegradable.
AdBlue is consumed at an average rate of about 0.1 liter per 100 kilometers. Consumption and tank size are sized so that the AdBlue supply does not need to be replenished until the next scheduled service.
The arrival of the ultra-clean Bluetec diesel engines demonstrates that Audi holds true to its tradition of always being at the very forefront of diesel technology.