Question: What is DEF?

Answer: Diesel Exhaust Fluid (DEF) is a nontoxic, urea-based chemical reactant necessary for the functionality of Selective Catalytic Reduction (SCR) systems, which reduce oxides of nitrogen (NOx) emissions. Most major engine manufacturers are using SCR systems to meet 2010 emission regulations.

Here’s how it works: DEF is injected into the hot exhaust upstream of the SCR catalyst where it vaporizes to form ammonia and carbon dioxide. The ammonia converts the NOx to harmless nitrogen gas and water. The results are a significant reduction in diesel exhaust emissions.

SCR consumes DEF at a rate equal to about two percent of diesel fuel used; and its price is comparable to a gallon of diesel fuel. The addition of SCR equipment and DEF will add 200-400 pounds to the weight of the vehicle, but the greatest benefit of a SCR system is a 3-5 percent increase in fuel efficiency.

For more information on SCR systems and DEF, check out this Cummins Filtration FAQ.