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Please note: This summary is provided to help you understand the regulations. Consult the references provided for links to the full text of the regulations.

Diesel Exhaust Fluid (DEF) for Selective Catalytic Reduction (SCR)

Road Rail Air Water

This page contains information on rules applying to diesel exhaust fluid (DEF) for highway engines. EPA has also set emissions standards for non-road vehicles including locomotives and marine engines that are prompting engine manufacturers to adopt similar technology for these applications. For more information, see the TERC pages on Emissions from Marine Engines and Engine Emissions for Locomotives.


As required by EPA's 2007 Heavy-Duty Highway Rule, diesel engine manufacturers are reducing the amount of nitrogen oxides (NOx) their engines emit. The NOx standards for heavy-duty truck engines were phased-in from 2007 to 2010. The 2010 NOx standard is 0.20 grams per brake-horsepower-hour (g/bhp-hr). To reach this level, the majority of new engine manufacturers employ selective catalytic reduction (SCR). Exhaust Gas Recirculation (EGR), is an alternative technology offered by a limited number of manufacturers to meet the EPA 2010 emissions requirements.

Selective catalytic reduction treats exhaust gas downstream of the engine. It works by converting nitrogen oxides into nitrogen (N2) and water (H2O). Both of these are safe for the environment when released. The process for making this conversion is to combine the engine exhaust stream containing NOx with a reductant, ammonia gas (NH3), that is supplied by a urea-based solution termed diesel exhaust fluid (DEF). The combined exhaust stream then comes in contact with a catalytic converter to produce the reaction that transforms the NOx into the N2 and H2O.

DEF is a solution made up of purified water and 32.5% automotive-grade urea. DEF usage is about 2% of diesel fuel usage; for every 50 gallons of diesel fuel burned, you will use about 1 gallon of DEF. For heavy-duty trucks, this translates to about one gallon of DEF for every 300 miles driven. DEF on-board tanks range in size from 6 to 23 gallons depending on the truck's application. Long-haul trucks, with 23 gallon DEF tanks, can go nearly 7,000 miles between DEF fill-ups. DEF is available in various packaged volumes at many locations including truck stops, truck dealerships, and engine distributors. It is also available for bulk dispensing on the fuel island at an increasing number of truck stops. It can also be stored and dispensed at fleet locations, using bulk tanks.

What is the purpose of the regulations

Older diesel engines produce relatively high levels of gaseous emissions of nitrogen oxides (NOx). NOx is a significant concern because it mixes with other compounds to create smog, which is a harmful form of air pollution. Engine manufacturer regulations specify the maximum amount of pollutants allowed in the exhaust gases from a diesel engine (i.e., tailpipe emission standards). Once sold, the responsibility of the engine shifts from the manufacturer to the owner of the truck. A separate set of regulations, referred to as anti-tampering rules assure that truck owners correctly maintain their engines so that they will continue to meet the tailpipe emission standards. These rules also contain engine maintenance recordkeeping requirements so that inspectors can assess if the maintenance standards are being followed or not.


The U.S. Environmental Protection Agency (EPA) has published rules that apply to maintaining or rebuilding heavy-duty highway diesel engines. These rules prohibit disabling an emission control system component, including SCRs, for model year 2004 or later. There are also recordkeeping requirements associated with the standards. These rules are discussed elsewhere on TERC, see Engine Maintenance/Rebuilding and Anti-Tampering Rules.

Best Practices

Vehicles that use DEF have a gauge similar to a fuel gauge that will indicate the level of DEF. Usually, there is also a DEF low level warning lamp that will illuminate when DEF is low. Drivers should plan ahead to be certain that their supply of DEF is not depleted. If the vehicle is operated such that one would run completely out of DEF, vehicle power will be reduced enough to encourage the operator to refill the DEF tank. Once the tank has been refilled the engine will resume normal power levels.

DEF freezes to a "slush" type consistency at 12°F, not to a solid, so just as with diesel, stored DEF needs to be protected from extended periods of severe cold.

EPA Resources

U.S. EPA -- Office of Transportation and Air Quality Contacts. This contact list contains the names and telephone numbers of U.S. EPA employees who can answer your specific regulatory questions regarding selective catalytic reduction and diesel exhaust fluid.

More Resources

Emission Standards for Heavy Duty Trucks. A detailed summary of historical and current standards prepared by DieselNet.