Many diesel engines use what's known as a 'Selective Catalytic Reduction' system to reduce harmful nitrogen oxide emissions. This type of system, used to help the engine meet increasingly strict environmental regulations, is extremely efficient and can reduce NOx emissions by up to 95 per cent.
For SCR to work, however, it requires a 'reducing agent' to be fed into the catalyst. This reducing agent enables the required reactions to take place within the catalytic assembly, allowing it to break down the unhealthy substances in the exhaust gas into harmless by-products.
The reducing agent used is known as 'Diesel Exhaust Fluid' - DEF - or, more commonly, 'AdBlue' - which is the trademark used by the German Association of the Automotive industry; this organisation represents companies including Volkswagen and BMW, so AdBlue is a commonly cited name among those manufacturers' diesel line-ups.
DEF injection systems are also used in industrial applications in order to reduce NOx emissions. The concept, as is often the case, is not a new one; SCR itself was patented in 1957 by the Engelhard Corporation - which was among the first companies to produce automotive catalytic converters - and was later employed in reducing the emissions of chemical production facilities and power plants.
Nissan began using SCR technology in its diesel truck engines in 2004 and, in early 2005, Mercedes-Benz also introduced an SCR-based emissions control system for its trucks. Then, in 2006, the company unveiled the first automotive market application of SCR in the form of its 'BlueTEC' set-up for the E-Class and GL-Class. Before long, AdBlue or DEF-based SCR systems were commonplace.
What does AdBlue do?
The primary purpose of an SCR system is to cut the engine's harmful NOx emissions - as these unwanted combustion by-products can cause health issues and contribute to the greenhouse gas effect. As a result, NOx emissions can have a particularly significant impact in urban areas where a large number of diesel engines operate at low speeds and in heavy traffic.
An SCR is similar in construction to a conventional catalytic converter, featuring an internal honeycomb core formed from ceramics containing - or covered in - catalytic materials. To allow the SCR system to clean up the harmful exhaust emissions, however, it requires a reducing agent to be injected.
In automotive applications, AdBlue - or any type of DEF - is injected into the exhaust stream ahead of the SCR assembly. This DEF is formed of 32.5 per cent synthetic urea and 67.5 per cent deionised water; consequently, it is biodegradable, colourless, non-hazardous and soluble in water. This particular ratio is chosen as, when the temperature drops below -11 degrees Celcius, both freeze at the same rate - preventing the mixture changing wildly once it begins to thaw.
When the AdBlue hits the hot exhaust gas, it decomposes to form carbon dioxide and ammonia. This flows into the SCR core and, when the ammonia encounters the catalytic elements inside, it reacts with the NOx in the exhaust stream. The reactions that take place convert the NOx into water and nitrogen, which is harmless.
The AdBlue injection system itself is relatively straightforward and typically consists of a standalone tank with a level sensor, a dosing valve and a metering module; injection is continuous but varies depending on the engine temperature, load and speed so that the optimum amount of DEF is injected.
Many manufacturers, in order to meet the new rules introduced in 2014, have elected to fit SCR systems in an effort to minimise NOx emissions. It has also been standard on trucks since 2006, helping minimise their environmental impact.
What's the catch?
AdBlue consumption often ranges from 1.5-5.0 per cent of the fuel consumption of the vehicle in question. Some engines can, however, burn through more AdBlue than that. As a result, owners may find themselves being warned about the DEF tank level and having to refill it more often than they may like; many, though, will find refills at every service sufficient.
If the AdBlue tank runs dry, some engines may limit their output while running - while others may not allow you to restart the engine after you have stopped, until you refill the tank. Land Rovers, for example, will not start once the AdBlue tank has depleted and will require at least 3.6 litres of AdBlue to permit a restart.
It is not overly expensive, though, often costing around £10 for ten litres. Consequently, given the benefits, the drawbacks are relatively moot for most private owners - although, for truck fleets, the additional cost of AdBlue may prove a significant consideration.