AdBlue

AdBlue is the trade name for a clear, non-toxic fluid used to reduce the harmful emissions of modern diesel engines. It is a precisely controlled solution of high-purity synthetic urea in demineralised water, and it is stored in a dedicated tank separate from the diesel fuel itself. Carried in tens of millions of cars, vans and lorries, it has become an unremarkable part of diesel ownership, yet it is central to the after-treatment that allows diesels to meet stringent air-quality limits.

The fluid is a 32.5 per cent urea solution, a composition standardised internationally under the name AUS 32 and the specification ISO 22241, with AdBlue being the most widely recognised brand. That exact concentration is chosen because it has the lowest freezing point of any urea-water mix, around minus eleven degrees Celsius, and because it gives the optimum amount of reactant for the chemistry that follows. Purity is tightly controlled, since contaminants would poison the catalyst the system relies upon.

AdBlue does its work within the selective catalytic reduction, or SCR, system in the exhaust. A metering injector sprays a fine mist of the fluid into the hot exhaust stream, where the heat breaks the urea down into ammonia. As the gases pass over a specially coated SCR catalyst, the ammonia reacts with the oxides of nitrogen produced by combustion and converts them into ordinary nitrogen and water vapour, both harmless and abundant in the air we breathe. By targeting NOx specifically, the system tackles the pollutant most associated with diesel's contribution to smog and respiratory harm.

This after-treatment is what enables current diesels to satisfy the Euro 6 emissions standard, which sets NOx limits so low that engine tuning alone cannot reach them. Consumption is modest but real, typically of the order of one to two litres for every thousand kilometres, varying with engine size and driving style. Tanks are sized so that a refill is needed every several thousand kilometres, often coinciding roughly with a service, and the fluid can be bought in containers at filling stations and accessory shops or dispensed at the pump.

Motorists do need to keep the tank topped up, and the consequences of neglect are deliberately severe. The car warns the driver well in advance with dashboard messages and a countdown of remaining range; if the AdBlue runs out entirely, emissions legislation requires that the engine, once switched off, will not restart until the fluid is replenished, preventing the vehicle being driven with its emissions control disabled. AdBlue should not be confused with a fuel additive, nor with the diesel particulate filter, which is a separate device that traps soot. Together with the SCR catalyst and the particulate filter, AdBlue forms part of the layered system that allows a modern diesel to run cleanly within Euro 6 limits.