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What is a SCR system?

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253 million… This is the number of cars on the European roads today1. Quite impressive isn’t it? This really shows how important the car is for in our today’s life.
3% of these 253 million (i.e. 7.59 million) are new cars sold in 2015 and out of these, 49% were diesel cars.

As you probably know, most of today’s big cities are fighting against diesel cars, explaining that this kind of car has nothing to do in a city and must disappear from our roads as soon as possible.
Why are they so eager to exit diesel cars from the city? Mostly because one of the main problems of our diesel cars is that they emit a particle named NOx. This molecule is quite bad for our health (like all fine particles) and when you have such a high density of people in one place, we can understand that mayors want to protect them.

However, what our politicians omit to say is that thanks to stronger and stronger EURO emission levels, the car industry made great effort to reduce them to almost nothing. Today, there are 2 main systems available on the market. The EGR and the SCR system. Recently a study conducted by the European Association of Transport & Environment explained that the SCR system is much more efficient than the EGR system. So what is this SCR system? How is it made and what is it doing to your car?

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

The SCR “Selective catalytic reduction” system is a mean of converting the nitrogen oxides (NOx)  into water and nitrogen (without the oxides).



As presented in the diagram, the AdBlue® solution (32.5% of UREA + 67.5% of pure water) is injected in the SCR catalyst. In the catalyst, a reaction takes place and transforms the NOx into H2O and N2.

 How Does the SCR works?

To make it simple, let’s say that a number of reactions are taking place in the SCR catalyst. AdBlue is injected and the reaction of this liquid with the elements of the catalyst and the NOx leads to a reduction of 90% of the NOx at the end of the exhaust line. After this reaction, you will only find some nitrogen, water and carbonate dioxide (N2, H2O and some CO2).
To work at an optimum efficiency, the gas coming in the SCR catalyst must be at a certain temperature. This means that the exhaust must be warm enough! So the position of the catalyst in the exhaust line is not random…

Some car manufacturers, such as PSA Peugeot Citroën, decided to implement their SCR catalyst before the DOC catalyst. The reason was that this position is ideal to reduce the heating time of the SCR catalyst.

SCR is not something as new as you can imagine

SCR catalyst has been developed in 1957 by an American company, the Engelhard Corporation (today BASF). The SCR technology is used since then in the industry, to reduce the NOx emissions of industrial plants, boats, trains, and many other sectors. You can see the sectors on our website:

SCR technology has been used for many years in the transport industry. This expertise helped manufacturers to develop a leading solution in NOx reduction. In a previous article, we mentioned that thanks to this technology, a truck manufactured in 2016 emit less NOx than a car manufactured in 2000….



Future of SCR technology

The SCR technology has been approved and enhanced over the past few years and the recent studies made by the European commission tend to prove that this solution, used in the right condition, is a very solid tool to fight the NOx.

Despite the harrow made on Diesel cars, combined with an efficient SCR system and AdBlue, DIESEL vehicles have now very low emission levels. They emit less CO2 and consume less fuel than petrol cars.

Some other solution tried to come in the market, but nothing provided as good results as AdBlue with SCR technology…


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