Well that stands for Selective Catalytic Reduction and it’s an advanced emissions control system that reduces the amount of nitrogen oxide into the atmosphere.
It works by injecting a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel-powered vehicle or machine.
It’s usually an automotive-grade urea with purified water and sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), which are natural components of the air we breathe. These are then expelled through the vehicle exhaust. SCR technology can achieve nitrogen oxide reductions up to 90%.
All heavy-duty diesel truck engines manufactured after 1 January 2010 must meet the EPA emissions standards, one of the toughest standards in the world, reducing particulate matter and nitrogen oxides close to zero levels.
(The Environmental Protection Agency (EPA) was established in December 1970 by an executive order of United States President Richard Nixon. The EPA is an agency of the United States federal government whose mission is to protect human and environmental health).
SCR systems can also be combined with a diesel particulate filter to achieve even greater emission reductions for particulate matter. SCR systems are now seen in a growing number of diesel passenger vehicles.
There are some special considerations for using SCR in diesel vehicles as there is a need to replenish the Diesel Exhaust Fluid (DEF) on a regular basis.
This fluid is carried in an on-board tank which must be replenished often at the time of a service or oil change.
DEF is an key part of the emissions control system and must be present in the tank at all times to assure continued operation of the vehicle.
On-board tanks to store DEF are typically located in the spare tire area for passenger vehicles, while tractor trailers typically have a DEF tank alongside the diesel fuel saddle tank.