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Selective Catalytic Reduction (SCR)/Diesel Exhaust Fluid (DEF)

  • Lexicon
  • 4 min read
NOx Sensors Nitrogen Oxide Sensors SCR/DEF

What is SCR? (Selective Catalytic Reduction)


What is SCR/DEF?

Selective Catalytic Reduction (SCR) is a catalyst-assisted method of chemically converting nitrogen oxides (NOx) into Nitrogen (N2) and water (H2O). This is made possible by a reductant, the role of which is played by urea. Ammonia in its pure form is a dangerous substance, and for this reason a 32.5% aqueous urea solution (AdBlue) is used for road transport where the use of urea is regulated.
In North America, this urea solution is called DEF – Diesel Exhaust Fluid.

Selective Catalytic Reduction
Diesel Emission Control System

Although fuel-efficient and technologically powerful, the solution is inexpensive. This system can reduce nitrogen oxide emissions by up to 90%, along with other pollutants. In addition to reducing emissions, the system can also reduce fuel consumption.


The purpose of the SCR system is to reduce the amount of nitrogen oxides in the exhaust gas. The ammonia-based SCR system is used in diesel engines of passenger cars, but also in construction machinery, ships and many solutions for the energy industry.

Construction of the system

In the SCR system, we can distinguish the following components:

  • urea tank (NOx/AdBlue)
  • heater (prevents the fluid from freezing during cold periods)
  • pump
  • injector
  • NOx controller (located at the end part of the SCR system)

When the controller (ECU) detects a failure of the SCR system, it limits engine operation. In trucks, torque is reduced (thus reducing exhaust emissions). The situation is different for construction and agricultural machinery; there are various degrees of SCR failure in such machines. In the worst case, the machine is completely immobilized. The most faulty parts of the SCR system are the pump and NOx controller. To avoid repair or further problems, an SCR or DEF emulator is used.

What is AdBlue/Diesel exhaust fluid

Adblue is an easily accessible urea solution, available at any gas station. There is no risk of flammability or harm to the environment. The AdBlue consumption required by the SCR system is small, amounting to 4-5% of the diesel fuel consumed. Given that the fluid tanks have a capacity of several litres, it does not need to be refilled often.

What are the disadvantages of AdBlue? First of all, it is a highly corrosive fluid, so most of the components that come into contact with it, including the pump and tank, are made of plastic. In addition, AdBlue fluid crystallizes at -11.5 °C, which can cause problems in winter. The precipitated crystals settle to the bottom of the tank and can clog the system, so it’s a good idea to keep your car at a higher temperature.

Is DEF the same as AdBlue?

YES! Adblue is exactly the same as DEF, it is just a brand name. It has become a very popular and widely recognized product that many people use as DEF.

Our solution

SCR is currently the favoured approach for agreeing with European EURO 6 and North American Tier 4 Final emission standards. The goal of these emission standards is to reduce the emission of Nitrogen Oxides into the atmosphere.


The most important aspect of Adblue’s operation is that it bypasses the engine, as the aqueous urea solution is injected directly into the exhaust system. Unlike the particulate filter, it also does not affect the flow of exhaust gases. This means that it has absolutely no effect on the operation and combustion of the fuel-air mixture. AdBlue is relatively hassle-free and very effective way to reduce harmful emissions in your machine.