Diesel Exhaust Fluid

What is Diesel Exhaust Fluid?

Diesel Exhaust Fluid (DEF) is an emissions control liquid required by modern diesel engines. It is injected into the exhaust stream. DEF is never added to diesel fuel. It is a non-hazardous solution of 32.5% urea in 67.5% de-ionized water. DEF is clear and colorless, and looks exactly like water. It has a slight smell of ammonia, similar to some home cleaning agents. DEF is used in by Selective Catalytic Reduction (SCR) technology to remove harmful NOx emissions from diesel engines.

In January 2010, the U.S. Environmental Protection Agency (EPA) brought in new emissions standards requiring medium- and heavy-duty vehicles to significantly reduce engine emissions, particularly NOx and particulate matter (PM). Vehicle manufacturers use SCR to meet these standards. DEF is sprayed into the exhaust, breaking down NOx gases into nitrogen and water using an advanced catalyst system. As a result most new diesel trucks, pickups, SUVs, and vans are now fitted with SCR technology and have a DEF tank that must be regularly refilled.

SCR is a so-called “aftertreatment” technology, which means that it destroys harmful emissions after combustion. This gives manufacturers greater scope to tune engines to improve fuel efficiency and increase power. Owners of SCR vehicles enjoy greater reliability and longer oil change intervals, which add up to impressive operating cost savings over the life of the vehicle.