A system used on diesel vehicles that is designed to reduce emissions. A typical SCR system consists of a Diesel Exhaust Fluid (DEF) injection system and a catalytic convertor. They are part of the exhaust system and placed between the engine and the mufflers.
In order to reduce harmful oxides of Nitrogen from diesel exhaust gases, an SCR system first injects DEF into the exhaust gases after they exit the engine. The DEF fluid vaporizes and forms ammonia and carbon dioxide before entering the catalytic convertor. There the catalytic convertor changes the ammonia, carbon dioxide and oxides of nitrogen into harmless nitrogen and water.