“SCR” stands for Selective Catalytic Reduction. It’s the name of a post-combustion technology treatment for diesel engines, which allows them to reduce their harmful emissions of NOx (nitrogen oxides). This technology was first introduced in the 80’s; power plants, cement , waste incinerators, glass factories and refineries have been utilizing an ammonia based reagent in a stationary SCR system to reduce NOx emissions in the atmosphere. During the years the size of the SCR system has been reduced and adapted to diesel vehicles. Moreover, instead of ammonia (dangerous and not easy to handle), the system now utilize Urea, ammonia safe carrier, diluted in de-ionized water. Almost all of the main engine manufacturers use this technology to meet the requirements of new legislation on NOx (Nitrogen oxides) emissions.
- SCR catalyst
- DEF injection unit
- DEF tank
- DEF injection control unit.
DEF is injected from the vehicle’s dedicated DEF storage tank into the exhaust pipe, which is just before the SCR catalyst, but downstream of the engine. As it is heated in the exhaust, DEF releases ammonia (NH3) and carbon dioxide (CO2). When the nitrogen oxide (NOx) gases from the exhaust pipe react inside the catalyst with ammonia, the harmful NOx molecules in the exhaust are converted into harmless nitrogen and water, which is released into the atmosphere as steam.
SCR system installed in the vehicle works automatically. DEF is automatically injected into the exhaust depending on your vehicle engine’s load. The dosage of DEF injected depends on the level of NOx measured by the sensor placed at the exit of the exhaust pipe.