Diesel Basics - AdBlue and SCRs

Diesel cars emit mono-nitrous-oxide gases (NOx) which contribute to smog and acid rain, and, together with sunlight, the generation of tropospheric ozone (which can irritate the respiratory system).  

Modern diesel engines are equipped with SCR systems (SCR = Selective Catalytic Reduction) featuring an AdBlue system to clean up emissions, reducing harmful nitrous oxides into diatomic nitrogen and water.  

This process is aided by a catalyst.  Catalysts tend to be oxides (for example of vanadium, molybdenum and tungsten). Porous ceramic materials are used as supports e.g. titanium oxide.

AdBlue is the brand name for a Diesel Exhaust Fluid. This combination of demi water (demineralized water) and urea is added to the exhaust gases to clean up NOx.

This multistage cleaning process operates as follows:

1. Exhaust fumes created

2. Fumes passed thru DPF taking out soot particles (cause particulate pollution)

3. NOx sensor measures the mono-nitrous-oxides (NOx)

4. Based on NOx reading, AdBlue control unit determines amount of fluid needed to clean up NOx

5. Fumes enter catalyst, injector sprays DEF cleaning up NOx from fumes

6. Another NOx sensor measures post catalyst NOx. Remainder of NOx, with nitrogen and water, leaves the exhaust pipe.

This process is required by emissions standards such as Euro 6. It does not stop the generation of the generation of pollutants but rather is an aftertreatment system to post process the pollutants into less harmful chemical products.