Diesel Exhaust Fluid
Diesel Exhaust Fluid (DEF)—sometimes known simply by the name of its active component, urea—is a key component of selective catalytic reduction (SCR) systems, which help diesel vehicles meet stringent emission regulations. DEF is a liquid reducing agent that reacts with engine exhaust in the present of a catalyst to convert smog-forming nitrogen oxides (NOx) into harmless nitrogen and water vapor.
Composition
Current DEF formulations are a nontoxic, colorless, and odorless mixture of the chemical urea and purified water. The use of alternative reducing agents—such as diesel fuel—is also being explored.
Urea is a nitrogen-containing compound that transforms into ammonia when heated. It occurs naturally or is synthesized from natural gas and is used in various industries, including as a fertilizer in agriculture. A urea-based DEF is used widely in Europe under the brand name AdBlue.
Use in Diesel Vehicles
Diesel Exhaust Fluid is carried onboard a vehicle in a tank separate from the fuel tank. The vehicle's DEF tank must be refilled periodically. Experience in Europe indicates that average DEF consumption is about 5% of diesel fuel consumption. Refilling the DEF tank 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.
Other than refilling the tank, no actions are required by the driver for an SCR/DEF system to function. A warning light alerts the driver when the DEF level is approaching a low level. If the DEF were allowed to run out completely, the vehicle would experience reduced performance or would cease to operate. Cold weather is another consideration. Because DEF freezes at 12 degrees Fahrenheit, it must be protected from extreme cold during storage.
Distribution
A coalition of industries—including truck manufacturers, truck stops, retail fueling stations, fuel distributors, and urea providers—is developing a network of bulk DEF filling stations at truck stops and diesel service stations across North America. DEF also will be available at participating auto dealers, auto parts stores, and other retail locations. It likely will be distributed in individual containers designed for filling one vehicle as well as in bulk tanks for fleet or retail fueling station applications. Some distributors may also recycle DEF.