How it Works - Selective Catalytic Reduction (SCR)

What is Selective Catalytic Reduction (SCR)?

Selective catalytic reduction, or SCR, is an active emissions control process used to reduce the amount of nitrogen oxide (NOx) emissions from the exhaust of diesel-powered vehicles and equipment.

How Does SCR Work?

SCR works by injecting a reductant, typically a urea compound and usually called a diesel exhaust fluid or DEF, into an exhaust stream, where it mixes with exhaust gas. 


As the gas-reductant mixture passes through the system, it moves through a catalyst, where a chemical reaction converts the NOx in the exhaust into nitrogen (N2), water (H2O) and carbon dioxide (CO2). 


Those compounds are then released via a vehicle’s tailpipe exhaust.


Where are SCR Systems Used?

SCR systems are predominantly used in heavy- and light-duty diesel trucks, but they’re also found in off-highway applications such as mining, construction, and farm equipment, in stationary applications such as power plants, and even in marine applications.  

Why is SCR Important? 

SCR is a proven, cost-effective way of minimizing environmentally harmful NOx emissions from exhaust systems. Vehicle and equipment manufacturers find it extremely useful in balancing emissions requirements with demands for engine performance and fuel efficiency, explaining its widespread use. 

Related Article:

Diesel Engine Emissions Controls