How Does Selective Catalytic Reduction (SCR) Work?

If your diesel car requires you to dose AdBlue periodically, it will be fitted with an SCR catalyst. Should this system cease working, the consequences can be even worse than an illuminated engine management light, or an MOT failure. After a mileage countdown, the engine will not start.

Why is SCR necessary?

The importance of Euro VI exhaust emission compliance is recognised by many British people as the standard that their car must achieve to enter urban low emissions zones for zero charge.

One of the main pollutants involves NOx, formed by a group of gases that can cause respiratory problems when inhaled.

Manufacturers use several ways to reduce NOx, one of which is exhaust gas recirculation. Yet, EGR alone tends not to be able to reduce NOx enough to meet the Euro VI limits. The solution is Selective Catalyst Reduction (SCR).

How does SCR work?

Unlike EGR, SCR treats NOx exhaust gases after they have been formed. As exhaust gases leave the engine, they enter the exhaust system into the oxidising catalytic converter, where some gases are converted.

They then pass into the diesel particulate filter (DPF), which removes the soot particles. Finally, they go into the SCR catalytic converter, which turns the NOx into harness nitrogen gas and water vapour.

This order can vary. For instance, on certain Citroën/Peugeot BlueHDi vehicles, the gases pass through the SCR catalyst before entering the DPF, not afterwards.

Unlike the oxidising catalytic converter, the SCR catalyst needs urea to work, which is expended during the chemical reaction. Diesel Emission Fluid, known by the brand name, AdBlue, provides a very pure urea in a safe, water-soluble form.

AdBlue is stored within a separate tank and is injected into the exhaust system under certain conditions, once the gases reach between 150-200 degrees Celsius. The system considers how the vehicle is driven and how much urea is held by the SCR catalyst.

The engine's electronic management system determines when and how much AdBlue to inject, using NOx and gas temperature sensors that are fitted to the exhaust system.

After passing through its injector, AdBlue passes through a mixer within the exhaust system, where its water content evaporates, causing the urea to be deposited on the SCR catalyst, where it is spent in the chemical reaction.

What goes wrong?

The AdBlue level must be topped-up occasionally, the frequency of which varies considerably between makes and models. A faster driving style also increases consumption. While AdBlue is harmless to health and can be rinsed away with water, it corrodes certain metals. Therefore, avoid spilling it.

Overfilling the AdBlue reservoir can also cause system faults, because an air space is necessary at the top of the tank to accommodate fluid expansion, should the AdBlue freeze.

Apart from the SCR catalyst failing, the system's sensors go wrong, too. Naturally, other related components can also deteriorate, such as wiring.

The AdBlue injector possesses several spray holes that atomise the fluid before it enters the mixing plate and the SCR catalyst. Apart from failing electrically, they can leak, causing white urea crystals to build on the tip. This affects the spray pattern and can over-saturate the SCR catalyst with AdBlue, causing clogging.

Perhaps the best advice is one that you can take from the cosy confines of the driver's seat. Do not ignore dashboard warnings about the SCR system and take any engine non-start mileage countdowns seriously, by consulting a garage at the earliest opportunity.

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