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What is SCR?

Selective Catalytic Reduction (SCR) is an active emissions control technology system used in many advanced diesel engines.

What is Selective Catalytic Reduction?

Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that reduces tailpipe emissions of nitrogen oxides (NOx) down to near-zero levels in newer generation diesel-powered vehicles and equipment. The SCR system involves several components packaged together with other parts of the emissions control system. Each manufacturer has its own variations of the type and sequencing of different components in the system.

SCR system

*Schematic is not representative of all manufacturers' approach to achieve near zero emissions.

How does it work?

SCR is an active emissions control system. Hot exhaust gases flow out of the engine and into the SCR system where aqueous urea (known as Diesel Exhaust Fluid, or DEF) is sprayed onto a special catalyst. The DEF sets off a chemical reaction in the exhaust on a special catalyst that converts nitrogen oxides into nitrogen, water, and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe. The exhaust also passes through a particulate filter somewhere in the system and then is then expelled through the vehicle tailpipe.

The design of SCR technology is such that it permits nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is 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 the pollutants into nitrogen, water, and tiny amounts of CO2. The DEF is rapidly broken down to produce the oxidizing ammonia in the exhaust stream.

Why is SCR important?

SCR technology is one of the most cost-effective and fuel-efficient technologies available to help virtually eliminate emissions from diesel engines. Since 2011, all heavy-duty diesel truck engines utilize SCR technology to comply with the latest EPA emissions standards. 

These standards require reducing particulate matter (PM) and nitrogen oxides (NOx) to near zero levels. 

SCR can reduce NOx emissions up to 90% while simultaneously reducing HC and CO emissions by 50-90%, and PM emissions by 30-50%. SCR systems can also be combined with a diesel particulate filter to achieve even greater emission reductions for PM.

The high efficiency of SCR systems in reducing NOx emissions allows manufacturers to balance engine performance and maximize fuel economy, while still achieving near zero emissions. Some SCR-equipped heavy-duty commercial truck operators report fuel economy gains of more than 4%.

Benefits of SCR

Where is SCR used?

SCR has been used for decades as a primary control strategy for reducing industrial stationary source emissions. Beyond on-highway commercial trucks, some off-road engines and equipment like those used in construction and farming, marine, rail, and power generation also utilize SCR systems to comply with EPA’s fourth generation emissions standards, known as “Tier 4.” These standards require similar reductions in NOx, PM, and other pollutants as from on-highway vehicles. With its superior return in both economic and environmental benefits, SCR is also found in diesel powered pick-up trucks, vans, and SUVs.

What are the special considerations of using SCR?

SCR systems are active systems. Compared to the passive catalytic converters on gasoline vehicles, SCR systems require replenishing Diesel Exhaust Fluid (DEF) on a periodic basis to ensure emissions system performance. The need to refill DEF  is directly related to vehicle fuel consumption. Failure to refill DEF tanks can result in immobilization of the vehicle or machine and a requirement for service.

What is DEF?

Diesel Exhaust Fluid (DEF) is a non-toxic fluid composed of 32% automotive grade aqueous urea and purified water. DEF is available with a variety of storage and dispensing methods. Storage options consist of various size containers such as bulk, totes and bottles or jugs. The American Petroleum Institute rigorously tests DEF to ensure that it meets industry-wide quality standards. DEF is sometimes referred to as AdBlue in Europe and in the US.

A nationwide DEF distribution infrastructure is in place making DEF is readily available in bulk, at retail stores, online and nearly all fueling stations that carry diesel fuel. On-board tanks to store DEF typically range in size from 5 to 22 gallons and are easily identified by a blue cap and/or labeling “DEF ONLY.” For pick-up trucks and SUVs, the DEF filler port is typically located alongside the fuel filler nozzle area. (SEE IMAGE)  For heavy duty tractor trailers, the DEF tank is typically alongside the diesel fuel tank on the side of the vehicle near the driver’s door. Due to the diverse nature of off-road engines and equipment, the location of the DEF tank and filler port is variable.

SCR lid in fuel

The DEF tank fill opening is designed to accommodate a DEF fill nozzle only to ensure only DEF is put into the tank. Diesel fuel should never be put in DEF tank and vice-versa. To protect against this misfuelling, a diesel fuel nozzle will not fit into the DEF tank opening. In addition, the DEF tank has a blue lid to differentiate it from the diesel tank which may have a yellow or green fuel cap.

For light-duty vehicles, DEF refill intervals typically occur around the time of a recommended oil change, while 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.Typically, DEF consumption is around 3% of fuel consumption: (example:100-gal fuel consumption would consume about 3 gallons of DEF).

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 warning indicators to the driver or operator. If the DEF is not replenished, the series of operator inducements progresses and eventually can lead to derating the engine and limiting vehicle speed, and ultimately locking out the starting system.

Proper storage of DEF is required to prevent the liquid from freezing at temperatures below 12 degrees Fahrenheit. Most vehicle DEF tanks and dispensing systems have warming devices.

What is the Future for SCR Technology?

The U.S. Environmental Protection Agency (EPA) recently announced efforts to develop more stringent tailpipe emission standards for commercial vehicles close to zero through the “Clean Trucks Initiative.” Refinements to SCR systems including dosing strategies, catalyst packaging and location are all likely to be considered as part of future emissions compliance strategies.

Need DEF? Check out the DEF Locator below to find it near you!

DEF is widely available wherever automotive supplies are sold including service stations, convenience stations, major retailers and is also distributed via bulk tanker to individual fleet locations. To find a DEF dispensing location, visit Discover DEF.


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