Selective Catalytic Reduction (SCR) technology is one of the most cost-effective and fuel-efficient technologies available to help diesel engine emissions. It allows diesel engines to be tuned and optimized toward maximum fuel efficiency, while the SCR systems efficiently treat the engine-out exhaust.

Heavy-duty commercial trucks is the largest sector using SCR technology in the U.S. All heavy-duty diesel truck engines produced after Jan. 1, 2010 must meet the latest EPA emission standards, among the most stringent in the world, reducing particulate matter (PM) and nitrogen oxides (NOx) to near zero levels. SCR can reduce NOx emissions up to 90%, while simultaneously reducing hydrocarbon and carbon monoxide (CO) emissions by 50-90% and PM emissions by 30-50%.

SCR System
Diesel Technology Forum SCR System

How it works

An SCR system injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, or diesel exhaust fluid (DEF). The DEF sets off a chemical reaction that converts nitrogen oxide into nitrogen, water, and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.

SCR technology is designed to permit NOx reduction reactions to take place in an oxidizing atmosphere. It’s called “selective” because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as “reduction” where the DEF is the reducing agent that reacts with NOx to convert pollutants into nitrogen, water, and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can reduce NOx reductions by up to 90%.

Special considerations

One unique aspect of a vehicle with an SCR system is the need for replenishing diesel exhaust fluid (DEF) periodically. DEF is carried in an onboard tank which the operator must replenish. The vehicle operation determines DEF consumption rates. More aggressive driving at higher speeds or while hauling heavy loads will increase DEF consumption. DEF replenishment for heavy-duty vehicles and off-road machines and equipment will vary, depending on the operating conditions, hours used, miles traveled, load factors, and other considerations.

DEF is an integral part of the emissions control system and must be present in the tank at all times to assure continued operation of the vehicle or equipment. Low DEF supply triggers a series of escalating visual and audible indicators to the driver. Once the tank nears empty, the vehicle starting system may be locked out the next time the vehicle is used, preventing it from being started without adequate DEF.

Contributed by the Diesel Technology Forum, a nonprofit organization that raises awareness about the importance of diesel engines, fuel, and technology. Visit