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Diesel Exhaust Fluid / AdBlue

  • Lexicon
  • 4 min read
Diesel Exhaust Fluid AdBlue

What is Diesel Exhaust Fluid? (DEF)

Diesel Exhaust Fluid

Diesel Exhaust Fluid (DEF) is a liquid that was introduced in order to diminish the greenhouse effect caused by diesel engines. Basically it is a solution of 67.5% of deionized water and 32.5% urea. The other name for it is AUS 32.

The solution is injected into the exhaust gas mixture under high pressure through the SCR system in order to reduce the amount of Nitrogen Oxides (NOx) in the exhaust fumes.

What is Urea?

Urea, also known as carbamide due to its diamide structure with carbonic acid, is an organic compound represented by the chemical formula CO(NH₂)₂. This compound contains two amino groups (–NH₂) connected by a carbonyl functional group (–C(=O)–). It stands as the simplest amide derived from carbamic acid.

Urea plays a crucial role in the cellular metabolism of nitrogen-containing compounds in animals and serves as the primary nitrogen-containing substance in mammalian urine. The term “urea” has its origins in Neo-Latin, derived from the French word “urée”, which traces back to the Ancient Greek term “οὖρον” (oûron), meaning “urine”, and ultimately stems from *Proto-Indo-European

This colorless and odorless solid exhibits high solubility in water and is practically non-toxic (with an LD50 of 15 g/kg for rats) . When dissolved in water, it neither displays acidic nor alkaline properties. Within the body, urea participates in various processes, most notably nitrogen excretion.

The liver synthesizes urea by combining two ammonia molecules (NH₃) with a carbon dioxide (CO₂) molecule during the urea cycle. Additionally, urea finds extensive use in fertilizers as a valuable nitrogen source (N) and serves as a critical raw material in the chemical industry.

What is AdBlue?

AdBlue – the trade name under which Verband der Automobilindustrie (VDA) organization has registered an aqueos urea solution with a high (32.5%) purity factor.

In the realm of international standards, DEF (Diesel Exhaust Fluid) is officially denoted as AUS 32—an aqueous urea solution containing 32% urea1. Additionally, DEF is commercially marketed under the trademarked name AdBlue, endorsed by the German Association of the Automotive Industry.

Various automotive manufacturers employ DEF in their Selective Catalytic Reduction (SCR) systems. Notable examples include:

BlueHDI: Utilized in PSA Group vehicles, including Peugeot, Citroën, and DS Automobiles brands.
BlueTec: Adopted by Daimler AG.
FLENDS (Final Low Emission New Diesel System): Integrated into UD Trucks1.

How often do you need to refill AdBlue / Diesel Exhaust Fluid?

Well that depends on what kind of machine you got. If it’s a car you should be good for a long while. However, if it’s a construction or agricultural machine you might not get as lucky. It is estimated that a car with a full tank of Diesel Exhaust Fluid should not need refilling for approximately 600 miles. A full tank in an excavator or a tractor can get you only so far and you should more expect a fraction of that.

So what are the downsides?

The biggest downside of Diesel Exhaust Fluid is that it crystalizes in negative temperature. In theory the DEF pump is supposed to drain the fluid from the system. In reality parts of it stays in the hoses, crystalizes in negative temperatures inevitably leading to problems.

The other downside is that Diesel Exhaust Fluid is corrosive, when injected into the combustion mixture the components of the SCR system become susceptible to failure. The most common elements that fail are DEF injectors and DEF manifold. The SCR/DEF system components are expensive to exchange not to mention it takes a lot of time.


Diesel Exhaust Fluid also branded as AdBlue or AUS32 is a solution of urea and deionized water that’s injected into the combustion mixture in order to reduce the emission of Nitrogen Oxides (NOx) into the atmosphere. Though it seem to be an innovative, quick fix to the greenhouse effect the downsides often overwhelm the idea especially in construction and agricultural business where time is of the essance the best solution to avoid SCR system failures or bypass them without the need for expensive parts exchange is the DEF Emulator