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 presence of a catalyst to convert smog-forming nitrogen oxides (NOx) into harmless nitrogen and water vapor.
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.
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.