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What is the SCR system?

  • SCR

253 million. That’s the number of cars on European roads today1. Impressive, isn’t it? It really shows how important cars are to our modern lifestyle.
Of these 253 million, 3% (or 7.59 million) were new cars sold in 2015 and 49% of them were diesels.

Most of today’s big cities are taking a hard line on diesel cars, considering them to be out of place in urban areas and a technology that should disappear from our roads as soon as possible.
But why are they so eager to ban diesel cars? One of the main problems with diesel cars is that they emit a particle named NOx. This molecule has a negative effect on health (like all fine particles) and in a densely populated area such as a city, it is understandable that an authority’s foremost concern is protection.

HOWEVER, what our politicians omit to say is that thanks to ever more stringent EU standards, the car industry has made great progress in reducing these emissions to almost nothing. Today, the 2 leading technologies available on the market are known by their abbreviations EGR and SCR. A recent study conducted by the European Federation of Transport & Environment suggests the SCR system is much more efficient than the EGR system. So what is this SCR system? How is it made and what does it do to your car?


What is an SCR system and where is it located on your car?

Selective Catalytic Reduction, or SCR for short, is a technology for converting nitrogen oxide (NOx) into water and nitrogen (without the oxides).



As shown in the diagram, the AdBlue® solution (32.5% UREA + 67.5% purified water) is injected into the SCR catalyst. A reaction takes place in the catalyst and transforms the NOx into H2O and N2.


 How does SCR work?


To put it simply: numerous reactions take place in the SCR catalyst. Liquid AdBlue is injected into the catalyst and reacts with the elements of the catalyst and the NOx. This leads to a 90% reduction in NOx being discharged from the exhaust line. What remains after the reaction is nitrogen, water and carbonate dioxide (N2, H2O and some CO2).
To ensure optimal efficiency, the gas entering the SCR catalyst must be at a certain temperature. In other words, the exhaust must be warm enough! So, the position of the catalyst in the exhaust line is not chosen at random…

Some car manufacturers, such as PSA Peugeot Citroën, deploy their SCR catalyst upstream of the DOC catalyst. This is the ideal position for reducing the heating time of the SCR catalyst.


SCR is not as new as many people think


SCR catalysts were developed in 1957 by an American company, the Engelhard Corporation (now BASF). Since then the industry has used SCR technology to reduce the NOx emitted by industrial plants, boats, trains and many other sectors. Learn more about these sectors on our website:

SCR technology has been used for many years in the transport industry. Such expertise helped manufacturers develop leading solutions for reducing NOx emissions. In a previous article we mentioned that, thanks to this technology, a truck manufactured in 2016 emits less NOx than a car manufactured in the year 2000….



Future of SCR technology


Over the past few years SCR technology has been approved and enhanced. Recent studies made by the European commission tend to prove that this solution, used in the right conditions, is a very solid tool for fighting NOx.

Despite the negative media attention paid to diesel cars, such vehicles fitted with an efficient SCR system and AdBlue now produce very low emissions. They emit less CO2 and consume less fuel than petrol cars.

Alternative solutions have been marketed, but nothing as yet provides as good results as AdBlue combined with SCR technology…






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