DEF is used in a process called selective catalytic reduction (SCR). SCR reduces nitrogen oxide emissions from the exhaust of diesel engines. DEF is a 32.5% solution of high-purity urea in reverse osmosis water that is clear, non-toxic and is safe to handle. However, it can be corrosive for some metals, and must be stored and transported using the correct materials.
DEF is carried onboard SCR-equipped vehicles in specially designed tanks, and is dosed into the SCR system at a rate equivalent to 2–5% of diesel consumption. This low dosing rate ensures long refill periods and minimizes the tank's impact on chassis space. DEF is used in Diesel engines for on-highway trucks, off-highway vehicles such as farm equipment, mining equipment, diesel generators for such entities as airports and hospitals and is expanding into the marine sector.
On-highway SCR systems are currently in use in many parts of the world, including Europe, Canada, Mexico, Japan, Australia, Hong Kong, Taiwan, Korea, New Zealand and Singapore.
The United States Environmental Protection Agency’s (US EPA) 2010 initial Greenhouse Gas (GHG) legislation limits NOx to levels that requires North American trucks to be equipped with SCR post-2010. Additional phases of the legislation continue to support SCR to further reduce GHG emissions and improve fuel economy.
EPA and the National Highway Traffic Safety Administration (NHTSA) are taking coordinated steps to enable the production of a new generation of clean vehicles, through reduced greenhouse gas (GHG) emissions and improved fuel use from onroad vehicles and engines.
In August 2011, in response to a Presidential Memorandum issued in May 2010. EPA in coordination with NHSTA issued greenhouse gas emissions and fuel economy standards for medium and heavy duty trucks manufactured in model years 2014-2018. The agencies estimate that the combined standards will reduce CO2 emissions by about 270 million metric tons and save about 530 million barrels of oil over the ed fuel use alone will enable $50 billion in fuel savings to accrue to vehicle owners, or $42 billion in net savings.