The Selective Catalytic Reduction (SCR) system is the final phase of the exhaust aftertreatment processes to remove emissions from the engine exhaust.

It follows the Diesel Particulate Filter (DPF) and the Diesel Exhaust Fluid (DEF)/Urea injection systems. The SCR system uses the DEF fluid to initiate the chemical reaction of the ammonia in the Urea and Nitrogen Oxides (NOx) to produce harmless nitrogen (N2) and water (H20).

Exhaust system components can become hot enough during operation and testing to cause burns or ignite and melt combustible materials. The exhaust and exhaust components can remain hot after the vehicle stops moving and has been shut down. To avoid the risk of fire, property damage, burns or other serious personal injury, allow the exhaust system to cool before beginning repairs or service. Make sure that no combustible materials are located where they are likely to come in contact with hot exhaust or exhaust components.

Components

The final aftertreatment system components are immediately downstream of the DPF and are comprised of the decomposition tube, the SCR, and the Ammonia Slip Catalyst (ASC).

Decomposition tube

• The decomposition tube is designed to help atomize and mix the Diesel Exhaust Fluid (DEF) that is sprayed into the exhaust stream.
• “Decomposition” refers to the chemical reaction of the DEF (water and urea) with heat that result in carbon dioxide and ammonia.

SCR catalyst (3 sections)

• Two SCR catalyst sections: The SCR catalyst is where ammonia reacts with NOx by passing over the catalyst and converting it to nitrogen and water.
• ASC catalyst section: The ASC catalyst is where the final trace amounts of ammonia are removed from the exhaust.

There are three sensors to monitor system performance:

• SCR intake temperature sensor: The SCR intake temperature sensor is located between the first and second SCR catalysts. This sensor measures exhaust gas temperature after the first SCR catalyst to help determine SCR efficiency.
• SCR outlet temperature sensor: The SCR outlet temperature sensor is located at the end of the aftertreatment system, after the ASC. This temperature sensor measures the temperature of exhaust gases after all aftertreatment has occurred. The ECM uses this temperature measurement and other temperature sensors throughout the aftertreatment system to determine if aftertreatment is effective.
• SCR outlet NOx sensor: The SCR outlet NOx sensor is located at the back of the aftertreatment system, downstream of the ASC. The outlet NOx sensor measures the NOx quantity that remains after all aftertreatment has occurred. This sensor also includes a heater to bring the sensor up to operating temperature quickly. The ECM compares the inlet and outlet NOx sensor values to determine if the aftertreatment system is efficiently removing NOx. If NOx levels remain too high at the outlet NOx sensor, the ECM may command additional EGR or additional DEF dosing to lower these levels.

Operation

Decomposition Tube – In the decomposition tube, a complex three-step chemical reaction takes place.

Step One: Evaporation – DEF is sprayed into the hot exhaust. Water evaporates and what remains in the decomposition tube is solid Urea in the form of Urea crystals and water vapor.

Step Two: Then thermolysis – the decomposition of compounds by heat – breaks the Urea crystals down into an ammonia molecule and an isocyanic acid molecule.

Step Three: Finally, hydrolysis – a reaction with water where the bond of a molecule is broken into two parts by the addition of a molecule of water. The isocyanic acid molecule hydrolyzes (combines with the water vapor) to release another ammonia molecule and one carbon dioxide molecule. The ammonia, carbon dioxide, and NOx then move into the SCR catalyst.

SCR Catalyst – In the SCR catalyst, the ammonia vapor reacts with the NOx, breaking the NOx into nitrogen and water vapor.

ASC Catalyst – Finally, any trace amounts of ammonia remaining are broken down by the ASC catalyst into nitrogen.