Diesel Exhaust Fluid

Published by revjust21 on

As the emission norms in automotive sector are becoming strict, Automotive Industries are adopting new technologies to meet the required emission norms. One of the technology being used for Reducing Emissions in diesel engine is Selective Catalytic Reduction which is often termed as SCR. In SCR, Diesel Exhaust Fluid is used to reduce Nitrox Emissions.

Now as we have learnt that DEF is injected into exhaust which results in reduction of Nitrox Emissions. Let us now have look on Diesel Exhaust Fluid.


Diesel Exhaust Fluid (DEF) is often termed as Automotive Urea because it is mixture of 67.5% of Deionized Water and 32.5& of Urea. It is injected into exhaust in form of liquid. When DEF is injected into hot exhaust gases, the water present in it evaporates. The remaining urea is decomposed into ammonia (NH3) and isocyanic acid (HNCO).

(NH2)2CO             →           NH3              +            HNCO

Urea                                   Ammonia               Cyanic Acid

Now the Isocyanic acid is further hydrolyzed forming Ammonia (NH3) and Carbon Dioxide (CO2).

HNCO      →       NH3        +   CO2

Isocyanic Acid     Ammonia         Carbon Dioxide

Now the ammonia thus produced in the presence of oxygen and catalyst reduces Nitrox to Nitrogen, water and carbon dioxide.

4NO        +      4NH3   +    O2  

→     4N2   +    6H2O

  Nitric Oxide         Ammonia       Oxygen       Nitrogen         Water

6NO2       +        8NH3       →      7N2      +       12H2O

    Nitrogen Dioxide       Ammonia          Nitrogen              Water

DEF Freezing

DEF has freezing temperature of -11oc due to which it becomes unfavorable for use in vehicles operating at very low temperatures. To solve the problem of DEF Freezing it is heated by using engine heat or electric heater. The process of melting the DEF must be done in very short time in order to make it viable to be used.

When DEF is melted using Engine Heat, the coolant from engine is circulated into coils present in DEF tank. This process generally takes time as DEF can only be melted when coolant flowing through coils is at sufficient high temperature required to melt DEF. The coolant can only attain that level of temperature when it is in operation for few minutes.

The other method of Melting DPF is by using the electric heater coils inside the DEF tank. The regulation of temperature of heater coils is very important as high temperature which is more than 50-60oc can cause potential harm to chemical composition of DEF.

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-Team Revjust


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