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How does the SCR process work?

The NOX reduction process starts with an engine burning clean Ultra Low Sulfur Diesel and producing exhaust that is already much cleaner due to leaner and more complete combustion.

Under the control of the vehicle's onboard computer, precisely metered spray patterns of DEF are injected into the exhaust stream ahead of the SCR converter allowing the exhaust gases and atomized mist of DEF to enter the converter simultaneously. Together with the catalyst inside the converter, the mixture undergoes a chemical reaction that converts the nitrogen oxides to elemental nitrogen (N2) and water (H2O).

Exhaust gases are monitored via a sensor as they leave the SCR catalyst. Feedback is supplied to the main computer that adjusts the DEF flow to keep NOX levels within acceptable parameters.


  • DEF is a clear, non-toxic, non-flammable, and nonhazardous organic compound consisting of 32.5% urea and 67.5% de-ionized water that requires no special handling.
  • To achieve the nitrogen oxide (NOX) levels required by the EPA 2010 standard, most engine manufacturers are utilizing Selective Catalytic Reduction (SCR) systems that require the addition of DEF.
  • DEF can be stored at temperatures of between 12°F (-11°C) and 86°F (30°C) for a maximum of 12 months. Shelf life deteriorates to 6 months when temperatures are held at a constant 95°F (35°C) or higher.
  • Consumption rates vary but are usually around 5% of diesel fuel consumed. It can be expected that 5 litres of DEF fluid will be used for every 100 litres of diesel fuel.