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What is Diesel Exhaust Fluid (DEF)?

DEF tank   Diesel Exhaust Fluid (DEF) is a non-hazardous solution, which is 32.5% urea and 67.5% de-ionized water. DEF is sprayed into the exhaust stream of diesel vehicles to break down dangerous NOx emissions into harmless nitrogen and water. This system is called Selective Catalytic Reduction (SCR) and can be found on 2010 and later model year trucks. DEF is not a fuel additive and never comes into contact with diesel. It is stored in a separate tank, typically with a blue filler cap.

 

What is Selective Catalytic Reduction (SCR)?

   SCR technology uses a catalyst system to break down dangerous NOx emissions produced by diesel engines into nitrogen and water. The chemical reactions used in SCR systems require a constant feed of ammonia gas. In automotive applications SCR delivers ammonia using a urea solution called Diesel Exhaust Fluid (DEF). DEF is sprayed into the exhaust stream by an advanced injection system and then converted into ammonia on a catalyst. The ammonia breaks down dangerous NOx emissions produced by diesel engines into nitrogen and water.

 

How much Diesel Exhaust Fluid (DEF) will my truck consume?

   Diesel Exhaust Fluid (DEF) consumption is measured as a ratio of diesel fuel use, normally termed the "dosing rate" or "treat rate". Medium- and heavy-duty vehicles have a dosing rate of 2-3%. This means that if your truck has a fuel efficiency of six miles per gallon and a dosing rate of 3% it will use approximately 1 gallon of DEF every 200 miles.

 Feedback from a number of fleets suggests that the dosing rate is slightly lower than originally predicted by manufacturers, at around 2.0-2.5%.

 

What will happen if my truck runs out of Diesel Exhaust Fluid (DEF)?

   The U.S. Environmental Protection Agency requires vehicle manufacturers to put measures in place to ensure that vehicles cannot run without Diesel Exhaust Fluid (DEF). Before a truck's DEF tank runs empty the driver is given a series of alerts on their dashboard displays (much the same way as if they were running low on diesel). Generally speaking, when the DEF tank level drops below 10% an amber warning lamp will come on, at 5% this lamp starts flashing and below 2.5% a solid amber warning light is displayed.

   If the truck is allowed to run out of DEF the engine's power is reduced, a solid red warning will be displayed and the vehicle speed will be limited to 5 mph until the DEF tank is refilled.

 

What happens if DEF freezes? 

DEF service

Vehicle operation is not affected. During vehicle operation, SCR systems provide heat to the DEF tank and lines. The system thaws DEF rapidly and keeps it flowing to the aftertreatment system regardless of the outside temperature.

   Diesel Exhaust Fluid is formulated with 32.5 percent urea and 67.5 percent deionized water. In storage or when the engine is not in use, DEF will freeze at 12°F (-11°C). At this concentration level, the urea and the water freeze and thaw at the same rate – ensuring that you always have the correct amount of each. Engines with SCR are specifically programmed for optimal performance at this ratio, which is why it is important to make sure you get a quality brand of DEF that meets ISO specifications.

   Cummins Aftertreatment Systems are designed with heated lines and DEF tanks. In extremely cold conditions, they will rapidly thaw the DEF solution. There will be no delay in engine startup.

    There is one operational difference to note: Like any water-based fluid, DEF expands when it is frozen (by approximately 7 percent). In cold weather, when the operator shuts the vehicle down, waiting 60 seconds before turning off the battery will allow the fluid to drain back out of the hoses and into the DEF tank.

 

Is there any danger of filling the Diesel Exhaust Fluid (DEF) tank with diesel and what problems will it cause?

   There are a number of safeguards put in place to prevent confusion between diesel and Diesel Exhaust Fluid (DEF) filler necks. The standard nozzle diameter for DEF is 0.75 inches (19mm), compared to 0.87 inches (22 mm) for diesel, preventing the diesel nozzle from ever being inserted into the DEF tank. In addition, the filling cap for the DEF tank is blue and will be clearly marked 'Diesel Exhaust Fluid' with the accompanying ISO standard number.

   However, a small number of cases have been reported where diesel has been put in the DEF tank. Diesel is less dense than DEF and will float on top of the DEF in the tank, but even small amounts of diesel can damage your SCR system and we recommend that you contact your dealer immediately and do not drive the vehicle.

   If DEF is filled into the diesel tank, the engine will stop running almost immediately, and the vehicle will need to be taken to a service center for repair.

 

If I switch Diesel Exhaust Fluid (DEF) suppliers, what should I do with the fluid which remains in bulk storage tanks and do I need to have them cleaned out?

   No, simply run the tank as empty as possible before refilling. Diesel Exhaust Fluid (DEF) is a pure chemical, so your bulk storage tank does not need to be cleaned out unless it is contaminated. DEF from different API-certified suppliers can be mixed.

 

Is Diesel Exhaust Fluid (DEF) corrosive?

Def   Diesel Exhaust Fluid (DEF) is not toxic, harmful or dangerous. In fact, of all the fluids used in a truck, such as diesel, engine oil, brake fluid, antifreeze and windscreen wash, DEF is the least hazardous. However, DEF is corrosive for some metals such as carbon steel, aluminum, copper and zinc, and DEF should not be stored in containers made of these materials. Your DEF supplier can advise you further. ISO22241 provides a list of materials that are recommended and not recommended, but makes it clear that neither list is exhaustive.

 

Do I need to wear protective clothing when filling up the DEF tank?

   Wearing protective clothing is not necessary when handling DEF, however, Diesel Exhaust Fluid (DEF) can stain clothes. If you do spill any DEF on your clothing, rinse it off with water.

More info about DEF here Service and Cleaning of DEF System