Selective catalytic reduction (SCR) is an advanced emission-control technology that can help light-, medium-, and heavy-duty diesel vehicles meet stringent regulations on nitrogen oxides (NOx) emissions. In an SCR system, a liquid reducing agent composed of urea and water, known as Diesel Exhaust Fluid (DEF), is combined with engine exhaust in the presence of a catalyst to convert smog-forming NOx into harmless nitrogen and water vapor.
The process starts with ultra-low sulfur diesel fuel combusted in an optimized diesel engine. Hot exhaust from the engine flows through a diesel particulate filter (which removes particulate matter) toward the SCR catalyst. DEF from a storage tank is injected into the exhaust stream, and the exhaust and finely atomized DEF enter the SCR catalyst chamber together. In the presence of the SCR catalyst, the exhaust and DEF react to convert NOx into nitrogen (N2) and water vapor.
One important requirement of an SCR system is consistently refilling the DEF storage tank. This occurs at approximately the interval of recommended oil changes for light-duty vehicles. The interval varies based on application for medium- and heavy-duty vehicles. A DEF distribution system is being established to meet refilling needs.
Selective catalytic reduction has been used for decades in marine and large stationary diesel engines. The technology is used extensively with heavy-duty highway vehicles in Europe. The use of SCR in highway and non-road engines has been demonstrated in the United States, and several auto manufacturers are incorporating SCR into their U.S. diesel products. Tests have shown that SCR can reduce NOx emissions by 75 percent to 90 percent.