Diesel exhaust fluid – AdBlue

Diesel exhaust fluid (commonly known as AdBlue®) is an aqueous solution of urea sprayed in a controlled manner into the exhaust gas flow through a Selective Catalytic Reduction (SRC) catalyst fitted to diesel vehicles to help reduce emissions of nitrogen oxides (NOx).

Increasing consumer demand for improved fuel efficiency has encouraged more and more car buyers to invest in diesel engine vehicles. Commercial vehicles have long used diesel engines because their greater torque or ‘pulling-power’ is more suitable for pulling heavy loads. Now, more than 60% of passenger cars on European roads run on diesel.

With regulatory pressure to improve air quality, vehicle manufacturers are contributing by lowering the quantity and improving the quality of vehicle emissions. Heavy-duty commercial vehicles have been using SCR and AdBlue for some time, and this technology is now being brought to passenger cars with the introduction of the new Euro 6 pollutant emissions standard.

Diesel engines must run ‘lean’ (ie with a greater proportion of oxygen than is theoretically required for complete combustion of the fuel) in order to prevent the formation of soot and other particles. This excess of oxygen leads to the generation of oxides of nitrogen (NOx) which contributes to ambient NO2 emissions.

Unlike petrol engines where the normal three-way catalyst reduces CO, HC and NOx, diesel NOx emissions can be reduced by a SCR catalyst which involves a controlled dosing of AdBlue (32.5% high purity urea and 67.5% de-ionised water) into the SCR catalyst. Once dosed into the catalyst, AdBlue decomposes into ammonia (NH3) and CO2. The SCR catalyst and the ammonia reduce NOx into water and nitrogen, which are both harmless.

AdBlue is stored in a tank onboard the vehicle, and is used at a rate between 2-6% of diesel consumption volume. The quantity is dosed automatically depending on the engine conditions, and the low usage rate ensures that tanks need only be topped up infrequently.

At the major filling stations on motorways, most truck diesel pumps will have an AdBlue dosing system nearby, so that diesel and AdBlue can be refilled conveniently.

Diesel passenger cars are a different matter. Those passenger cars already available with SCR systems may require refilling of the AdBlue tank only at every oil-change or service (so there is no need for the customer to refill the AdBlue tank) or it may be possible for the customer to refill the AdBlue tank via a container available from service centres or garages. The auto industry is working on the roll-out of a convenient AdBlue refilling infrastructure that would be adjacent to the normal diesel filling pump – and we trust that the fuel suppliers will agree on the need for this infrastructure for the benefit of all our customers.


Content type Fact
Vehicle types All vehicles
back to topback to top