Q. Why SCR?

A. SCR is one of the few methods approved by the EPA to comply with US Regulation 2241. SCR is a cost effective method of meeting NOx requirements, and the infrastructure to support SCR is already in place. DEF, the only solution needed to make SCR work, is available at various retail and whole sale locations. It can be delivered in various quantities and is less expensive than other NOx reduction systems.

Q. How does the SCR system work?

A. SCR is the treatment of diesel exhaust that is implicated before the catalytic converter. An SCR system sprays a fine mist of Diesel Exhaust Fluid (DEF) upstream of the catalytic converter where it gets vaporized and the decomposes to become ammonia and carbon dioxide. The ammonia is the key element to that aides the SCR catalyst in converting the NOx to harmless nitrogen (N2) and water (H2O) bi products.  The reduction efficiency of the SCR system is 90% or more.

Q. What is Diesel Exhaust Fluid (DEF)?

A.  Diesel Exhaust Fluid (DEF) is a precise mixture of urea and de-ionized water that is used in the SCR process.  DEF is 32.5% urea and 67.5% de-ionized water, this mixture must be very precise when blended, and will cause problems if either of the parts is improperly added to the solution.

Q. What is Urea?

A. Urea is a natural form of nitrogen that will turn to ammonia when it gets heated. Currently urea is used in multiple industries outside of DEF; the other most common s industry would be in the agricultural setting where it is used as a fertilizer.

Q. How much does DEF weigh?

A.  DEF weights approximately 9.1 to 9.2 pounds per gallon.

Q. Does DEF smell?

A. DEF may have a light smell similar to ammonia. It is a clear solution, and is harmless at any rate.

Q. Is DEF corrosive?

A. DEF is corrosive against some metals such as Copper and brass. Only approved materials such as High Density Polyethylene (HDPE) may be used in DEF storage, dispensing, or blending equipment. The following is a list of approved and un-approved materials:

Approved DEF Materials Materials to Avoid
Stainless Steel (304, 304L, and 316) Carbon Steels, Zinc coated Carbon Steels, mild iron
High Density Polyethylene Non-ferrous Metals, copper, brass, zinc, lead
Titanium Solder containing lead, silver, zinc, or copper
Polypropylene Aluminum and aluminum alloys
Poly-isobutylene Magnesium and magnesium alloys.
Epoxy Resins
Polytetrafluoroethylene (PTFE) free of additives


Q. Does DEF freeze?

A. DEF will begin to crystallize and freeze at 12 degrees Fahrenheit (-11 degrees Celsius). Both the urea and the de-ionized water will begin to freeze and thaw out at the same temperatures. When DEF freezes it expands roughly 7% its normal volume, but the DEF packages made for DEF are designed to handle this expansion.  As the DEF thaws it will not become diluted or over concentrated.

Q. What are the effects of high heat exposure to the DEF solution?

A. Exposure to high temperatures will not damage DEF, but will shorten the shelf life of the product and possibly let out fumes of ammonia.

Q. Can DEF evaporate?

A.  Yes. DEF is made of 67.5% water and evaporation can occur over time. SCR engines have been in operation in Europe and the Middle East without any evaporation problems for over 5 years. Also field testing in the US in areas with higher temperatures has been conducted and has not suggested that evaporation will cause issues with engine performance. As with regular gas caps on vehicles, it is recommended that the cap on the DEF reservoir on a vehicle be tightly sealed at all times.

Watch what happens when DEF is not properly stored

Q. What is the shelf life of DEF?

A.  The shelf life of DEF can be affected by external forces. Higher temperatures above 90 degrees and prolonged exposure to light will cause shelf life of DEF to drop substantially as the temperatures increases, and light exposure continues. DEF expectations for shelf life, as determined by ISO 22241-3, are minimum expectations for shelf life when stored in cool and constant temperatures.

Q. Are there any special storage requirements or standards for DEF storage?

A.  DEF should ideally be stored in an area that is cool, dry, out of direct sunlight, and well-ventilated. The best temperature for DEF to be stored in would not exceed 77 degrees F (25 degrees C).  Temperature over 90 degrees F will degrade the product and it will NOT return to proper behavior after cooling.

Q. Are there any certifications for the quality of DEF?

A. Yes.  The American Petroleum Institute (API) is a voluntary certification that was launched in March of 2009. This institute certifies the DEF and monitors the market to ensure that the ISO 22241 specifications are DEF are being met.

Q. What is the ISO?

A.  The International Organization for Standardization is a committee that is responsible for making standards for products that must be carried throughout most of the world, at least with all countries that are involved.

Standards That Govern Diesel Exhaust Fluid

ISO 22241-1:2006 specifies the quality characteristics of the NOx reduction agent AUS 32 (aqueous urea solution) which is needed to operate converters with selective catalytic reduction, so-called SCR (Selective Catalytic Reduction) converters, in motor vehicles with diesel engines. SCR converters are particularly suitable for selectively reducing the nitrogen oxide (NOx) emissions of diesel engines.

ISO 22241-2:2006 specifies test methods required for determination of the quality characteristics of the NOx reduction agent AUS 32 (aqueous urea solution) specified in ISO 22241-1.

ISO 22241-3:2008 describes best practice recommendations and requirements for handling, transportation and storing of NOx reduction agent AUS 32 (aqueous urea solution), specified in ISO 22241-1. These recommendations and requirements are necessary to preserve the specified quality of AUS 32 from any point of production to the point where it is filled into the onboard tank of the vehicle, in order to ensure the proper function of the selective catalytic reduction (SCR) converter systems.

ISO 22241-4:2009 specifies the refilling interface for the NOx reduction agent AUS 32 in compliance with ISO 22241-1, which is needed to operate converters with a selective catalytic reduction (SCR) exhaust treatment system.

ISO 22241-4:2009 specifies the minimum functional and geometric requirements of an open refilling system, in order to ensure compatibility between the on-board refilling system and the off-board refilling system.

ISO 22241-4:2009 applies to commercial vehicles and buses as defined in ISO 3833 and having a gross vehicle mass of more than 3,5 t, designed to use stationary off-board refilling systems. ISO 22241-4:2009 also applies to the nozzle of stationary off-board refilling systems.

Reference: ISO Website

Q. Is there a way to know if the DEF I purchased will work?

A.  The DEF you purchase should have a variation of certifications listed on the label of the bottle, or if you are receiving it in bulk quantities the delivery agency should have that information available. The following standards should be printed on the bottle: German Institute of Standardization DIN707000, the ISO-22241-1, AUS-32 specifications. These standards all meet the purity of 32.5% urea content in the DEF solution. For more information on these specifications you can visit www.iso.org

Q. How can I determine when the DEF I purchased was manufactured?

A.  Every DEF package that is sold should have a date code located somewhere on the labeling or laser engraved on the container itself. Different capacity DEF containers will have them at different locations, but every package should have the date available for the consumer.

Q. What does SCR do to the fuel economy of my vehicle?

A.  The SCR system will help a diesel engine run more efficiently. SCR will deteriorate the need for particulate filter regeneration, something that consumes extra fuel, and in turn allow the engine to run cooler. This will give a vehicle not only better economics in relation to fuel, but also will suppress the need for consistent maintenance.

Q. How do I know how much DEF I will need?

A.  DEF consumption should be an average of 2% (50:1 ratio). This will also depend on the type of truck, load being carried, altitude of travel, and other various factors.

Q. How often will the DEF tank on my vehicle need to be refilled?

A.  Assuming that your vehicle will consume at the average rate of DEF consumption, the following calculations can be applied. For a vehicle that gets 6mpg on average one gallon of DEF will deliver the vehicle 300 miles of NOx reduced traveling.

Average miles traveled by a delivery vehicle per year= 100,000mi

Average MPG for Diesel heavy duty vehicle= 6MPG

100,000mi / 6mpg = approximately 16,666.gallons of fuel per year

DEF usage equals 2% of fuel consumption = 333 gallons of DEF per year

Average DEF tank size = 10 gallons

Annual DEF fill-ups = 33 times

Q. What would happen if diesel fuel were to enter the DEF reservoir on a vehicle?

A.  To help prevent diesel from entering the DEF container, the standard nozzle of a DEF dispenser has been made approximately 2mm smaller than the standard diameter of a diesel fuel dispenser. The diesel dispenser will not fit into the DEF port. However, if somehow a non-DEF substance were to enter the DEF container on a vehicle, the SCR system would notice this and notify the driver of contamination. Depending on the severity of contamination, the vehicle may have to be taken in to a servicing center.

Q. What would happen if DEF got into the diesel fuel tank?

A.  Additional to the different size fuel nozzles for DEF and diesel fuel the DEF tank has a blue cap on it to help the driver distinguish between the two. If DEF were to enter the diesel fuel tank, the first few quarts should be handled by a water fuel separator.

Q. Is there a possibility of DEF freezing while on the vehicle?

A.  If DEF were to freeze while the vehicle was not in operation the initial start of the engine would not be disabled. The SCR system has been equipped with technology to quickly thaw out DEF that is frozen and the unfreezing process of DEF will not negatively impact the operation of the vehicle nor will it cause degradation of the DEF.  While in operation the same technology of the SCR system will keep the DEF from freezing onboard an actively running vehicle.

Q.  What would happen if a vehicle ran out of DEF?

A.  Vehicles that use DEF will have fluid level indicator similar to that of a gas gauge. This should be located on the dash board as well, and will have a low level warning light that comes on to warn the driver when DEF levels have become critically low. Once the vehicle is completely out of DEF it will still operate, but will have severely reduced power levels (reduce speed to 5 mph) to encourage the operator to refill the DEF supply.  After the DEF system has been refilled the vehicle will continue to operate as normal.