Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF). The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.
SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called "selective" because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as "reduction" where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream.SCR technology alone can achieve NOx reductions up to 90 percent.
SCR technology does not change the design or operation of the engine but does allow manufacturers to tune engines to boost performance, increase engine reliability and achieve fuel savings.
Engines equipped with SCR are able to function at optimal combustion temperatures, which increases fuel efficiency and contributes to the production of engine power. One of the greatest benefits fleet managers recognize with SCR technology is the cost savings associated with increased fuel efficiency. Post 2016 heavy-duty trucks by Tata Motors, Ashok Leyland, Bharat Benz and Mahindra can achieve fuel savings of around 5% compared to pre 2016 models with similar engine specifications; off-road machines with SCR also report fuel savings of 5% and higher.
SCR also results in longer overall engine life, as there is a reduced dependency on exhaust gas recirculation (EGR), less heat rejection and greater component reliability.