Selective Catalytic Reduction Systems (SCR Systems) have been specifically designed as a result of changing legislation to operate within diesel engines. They have been designed to increase fuel efficiency and DEF (diesel exhaust fluid) is used as a reducing agent within this system.

An SCR system converts NOx (nitrogen oxides) in nitrogen and water (commonly found in the environment) and in doing so, it acts as an advanced active emissions control technology. The SCR system works by injecting a liquid reducing agent (DEF) through a specialist catalyst into the exhaust stream of a diesel engine.

SCR technology has been designed to allow nitrogen oxide reduction reactions to take place in an oxidising atmosphere. In layman’s terms this means that when the exhaust fumes leave the engine of a diesel vehicle, they pass through a diesel filter, trapping the diesel particles. As these fumes pass through, DEF is injected into the exhaust and this sets off a chemical reaction, known as 'reduction'. The DEF acting as the reducing agent reacts with the nitrogen oxides, converting the pollutants into nitrogen, water and tiny amounts of CO2 - all of which are natural elements commonly found in the air we breathe every day. SCR technology alone can achieve NOx reductions of up to 90%.