Diesel exhaust fluid

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Diesel exhaust fluid for sale in small quantities

Diesel exhaust fluid (DEF), commonly referred to as AdBlue in Europe and standardised as ISO 22241[1] is an aqueous urea solution made with 32.5% high-purity urea (AUS 32) and 67.5% deionized water. DEF is used as a consumable in selective catalytic reduction (SCR) in order to lower NOx concentration in the diesel exhaust emissions from diesel engines.[2]

DEF is used in Daimler AG's BlueTec system that has been built into several diesel trucks and cars. The German Association of the Automotive Industry (VDA) controls the "AdBlue" trademark and uses it to ensure quality standards are maintained in accordance with DIN 70070 and ISO 22241 specifications.


Diesel engines can be run with a lean burn air-to-fuel ratio (overstoichiometric ratio), to ensure the full combustion of soot and to prevent the exhaust of unburnt fuel. The excess of oxygen necessarily leads to generation of nitrogen oxides (NOx), which are harmful pollutants, from the nitrogen in the air. Selective catalytic reduction is used to reduce the amount of NOx released into the atmosphere. Diesel exhaust fluid (from a separate DEF tank) is injected into the exhaust pipeline, the aqueous urea vaporizes and decomposes to form ammonia and carbon dioxide. Within the SCR catalyst, the NOx are catalytically reduced by the ammonia (NH3) into water (H2O) and nitrogen (N2), which are both harmless; and these are then released through the exhaust.[3]

Diesel Exhaust Fluid (DEF) is an emissions control liquid required by modern diesel engines. It is injected into the exhaust stream. DEF is never added to diesel fuel. It is a non-hazardous solution of 32.5% urea in 67.5% de-ionized water. DEF is clear and colorless, and looks exactly like water. It has a slight smell of ammonia, similar to some home cleaning agents. DEF is used in by Selective Catalytic Reduction (SCR) technology to remove harmful NOx emissions from diesel engines.

In January 2010, the U.S. Environmental Protection Agency (EPA) brought in new emissions standards requiring medium- and heavy-duty vehicles to significantly reduce engine emissions, particularly NOx and particulate matter (PM). Vehicle manufacturers use SCR to meet these standards. DEF is sprayed into the exhaust, breaking down NOx gases into nitrogen and water using an advanced catalyst system. As a result most new diesel trucks, pickups, SUVs, and vans are now fitted with SCR technology and have a DEF tank that must be regularly refilled.

EPA set the emissions standards to improve air quality. NOx and PM emissions are associated with a wide range of health problems including respiratory and cardiovascular diseases, aggravation of asthma, acute respiratory symptoms, chronic bronchitis and decreased lung function. The EPA estimates that the emission standards will prevent 8,300 premature deaths, more than 9,500 hospitalizations and 1.5 million work days lost due to illness, saving approximately $70.3 billion by 2030. But yet there is still the cost of buying the Diesel Exhaust Fluid.

SCR is a so-called "aftertreatment" technology, which means that it destroys harmful emissions after combustion. This gives manufacturers greater scope to tune engines to improve fuel efficiency and increase power. Owners of SCR vehicles enjoy greater reliability and longer oil change intervals, which add up to impressive operating cost savings over the life of the vehicle

DEF (Diesel Engine Fluid) is a 32.5% solution of Urea (NH2)2CO. When the urea solution is injected into the hot exhaust gas steam the water evaporates. The urea thermally decomposes to form ammonia and isocyanic acid:

(NH2)2CO → NH3 + HNCO

The Isocyanic acid hydrolyses to carbon dioxide and ammonia:

HNCO + H2O → CO2 + NH3

The overall reduction of NOx by urea is:

2(NH2)2CO + 4NO + O2 → 4N2 + 4H20 + 2CO2


SCR systems are sensitive to potential chemical impurities in the urea solution, therefore the solvent is demineralized water. The urea solution is clear, non-toxic and safe to handle. However, it can corrode some metals and so must be stored and transported carefully.

DEF is stored in a tank on board the vehicle, and injected into the exhaust stream by a metering system. The injection rate depends on the specific after-treatment system, but is typically 2-6% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimises the tank's size (and subsequent obtrusion into vehicle packaging space). An electronic control unit adjusts the addition of fluid in accordance with such parameters as engine operating temperature and speed.

Diesel exhaust fluid is offered to consumers through a variety of quantities by manufacturers ranging from containers of it for single or repeated small usage, up to bulk carriers for consumers that require a large amount of DEF. It is recommended that DEF be stored in a cool, dry, and well-ventilated area that is out of direct sunlight. As of 2013, a number of truck stops are beginning to add DEF pumps, in which diesel exhaust fluid is administered at pumps similarly to diesel, and often located adjacent to fuel pumps such that the vehicle operator can fill up on both without moving the truck.

In the UK, the storage of DEF is now under scrutiny of the Environment Agency who are proposing new controls following a number of spills of DEF into the environment. Although not toxic, DEF can cause the catastrophic de-oxidation of water bodies leading to damage of the aquatic environment.[4]


  1. ^ "ISO 22241 Specification". ISO. Retrieved 15 March 2012. 
  2. ^ "What is DEF?". Cummins Filtration. 
  3. ^ "How it works". H2Blu. 
  4. ^ [dead link]"Large spills into water bodies". 

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