For Universal Specialty Vehicles, Inc and their diesel chassis customers these are terms they need to be familiar with.
On January 1, 2010 engine manufacturers had to meet the US Environmental Protection Agency’s more stringent diesel emissions standards. These regulations dramatically decrease discharges of particulate matter (soot and ash) and nitrogen oxide, almost completely eliminating these emissions from on-road diesel engines. Most engine manufacturers have chosen the Selective Catalyst Reduction (SCR) system. The SCR is a system that uses a urea based diesel exhaust fluid (DEF) and a catalytic converter to significantly reduce oxides and nitrogen emissions. According to Cummins, SCR technology is the leading technology being used to meet 2010 emissions regulations.
SCR is the aftertreatment technology that treats exhaust gas downstream of the engine. Small quantities of diesel exhaust fluid (DEF) are injected into the exhaust upstream of a catalyst, where it vaporizes and decomposes to form ammonia and carbon dioxide. The ammonia is the desired product which in conjunction with the SCR catalyst, converts the oxides of nitrogen (NOx) to harmless nitrogen and water.