Febi Explains SCR Systems

Febi shares everything you need to know about SCR systems.

Diesel engines of heavy-duty vehicles contribute excess oxides of nitrogen NOx and their related health problems in almost all regions. The mitigation of environmental pollutants like NOx from the diesel engine exhaust is a secondary NOx removal technique by aftertreatment. Selective catalytic reduction (SCR) is the latest technology adopted by many automobile manufacturers to mitigate NOx, wherein ammonia NH3 is used as a reducing agent.

SCR systems in modern vehicles are operated with a solution of 32.5 per cent synthetic urea and 67.5 per cent demineralised water, known under the brand name of AdBlue. The trademarks for AdBlue are held by the German association of the Automotive Industry (VDA). It is not a hazardous substance as it is biodegradable, non-toxic, colourless and water soluble.

Combined with SCR catalytic converter, the Diesel exhaust fluid AdBlue converts nitrogen oxides NOx emitted by the diesel engine into nitrogen N2 and water vapor H2O, nonpolluting components already present in the atmosphere:

• AdBlue is injected by the dosage module into the exhaust pipe upstream of the SCR catalyst.
• Under the effect of heat, urea decomposes into ammonia NH3.
• By chemical reaction with ammonia in the SCR catalyst, nitrogen oxides from the engine are converted into nitrogen N2 (a harmless gas, the main component of the air we breathe) and water H2O.

Keep SCR systems running

Every SCR system is equipped with an urea filter, also known as AdBlue filter. It ensures that no impurities enter the metering unit with the AdBlue liquid to guarantee a long service life of the sensitive SCR system components. The fineness and change intervals are comparable to those of a diesel fuel filter. Some AdBlue filters are equipped with compensating elements. These flexible elements avoid damage to the system and the filter by reducing the free volume in the housing and absorbing pressure forces when the urea water solution expands at temperatures below freezing.

Some AdBlue filters are equipped with compensating elements. These flexible elements avoid damage to the system and the filter by reducing the free volume in the housing and absorbing pressure forces when the urea water solution expands at temperatures below freezing.

Timely maintenance avoids problems

If the urea filter is clogged by impurities or the SCR system no longer functions due to neglected maintenance or damaged components, the engine control unit switches to an emergency program. Engine speed and power will be reduced depending on vehicle type. If the fault in the system is not rectified, the drive to the closest garage will happen at walking speed in the worst case.

Only completion of the required maintenance/repair of the SCR system by a specialist workshop will make it possible to end the emergency program. Benefits of febi urea filters at a glance:

• More than 20 Urea filters and highest coverage of 97 per cent for the European Big 7 and further Asian vehicles in the IAM.
• Patented compensation elements prevent damage due to freezing.
• High filter capacity to protect the sensitive SCR system, for long replacement intervals and low downtimes.
• Dynamically growing product range for SCR components.

Further SCR system components

Another evitable component of the SCR system are NOx sensors. They monitor the function and effectiveness of the SCR system by measuring the nitrogen and oxygen concentration in the exhaust gas. The data from the NOx sensors is also used to calculate the amount of AdBlue required for an optimal reduction process.

NOx sensors have a limited lifetime. Due to ageing as a result of oxidation of the electrolyte material Zirconium Dioxide in the sensor, the reliability of the measurements decreases over the time. Also, the extreme temperature changes will shorten the lifetime due to the extremely porous and hard ceramic material and its temperature sensitivity.

As a result the driver receives an error message after a certain time when the measures getting more and more implausible. The lifetime depends on many factors so it is not predictable at all. In addition to NOx sensors, AdBlue fluid and the corresponding filters, febi also offers many other spare parts for SCR systems, such as tank caps, metering modules and exhaust gas temperature sensors.

Rely on tested OE matching quality replacement parts from febi. The febi brand is part of the bilstein group, the umbrella organisation for several other strong brands.