What Is DEF? (Diesel Exhaust Fluid)

Last Updated: Oct 24, 2018

What is DEF?

DEF, an acronym for Diesel Exhaust Fluid, is the trucking industry’s response to U.S. Environmental Protection Agency standards requiring all medium and heavy-duty diesel vehicles to significantly reduce engine emissions, particularly nitrous oxide as well as particulate matter. It was mandated to come into use beginning with 2010-manufactured engines.

DEF is a clear, colorless, non-hazardous liquid looking exactly like water but that has a slight odor of ammonia. It is composed of urea and deionized water. The urea component is an organic compound made up of nitrogen, hydrogen, carbon and oxygen.

How DEF Works

DEF is not a fuel additive, and has no influence on the tuning and efficiency of diesel engines. The way it works is that the fluid, an organic compound comprising urea and deionized water, is mixed with the hot gases that follow diesel combustion, breaking DEF down into ammonia and carbon dioxide. These two gases meet with the diesel exhaust and chemically bond with the nitrogen oxides, thus resulting in nitrogen and water exiting the tailpipe. The end result is a much cleaner diesel exhaust.

Video: How DEF Works

Do Not Pee In In Your DEF Tank!

Video: The Chemistry Of DEF

In the early days of DEF use there was talk that because DEF contains urea, and because urine contains urea it was only logical that DEF was made up of horse urine … or cat urine, or pig urine, you get the picture. Some even took it to the "logical" extreme of suggesting that all an enterprising truck driver had to do was pee into his DEF tank. Problem solved, and cheaper.

Not quite.

While urea definitely is a component of both urine and DEF, the manufacture of DEF is done under exacting conditions making sure the urea is pure and free of contamination, and of the right ratio to the deionized water. The water also must be of an exact quality.

"Human urine is an aqueous solution of greater than 95 percent water, with the remaining constituents, in order of decreasing concentration urea 9.3 grams/L, chloride 1.87 grams/L, sodium 1.17 grams/L, potassium 0.750 grams/L, creatinine 0.670 grams/L and other dissolved ions, inorganic and organic compounds."

On the other hand, "Diesel Exhaust Fluid uses automotive grade urea, at 32.5 percent concentration. This urea contains , 0.3 percent Biuret maximum, 0.5 jpercent magnesium max , 0.5 percent calcium max, 0.5 percent phosphate max, 0.5 percent iron max, and 0.5 percent potassium maximum."

That should settle that; don’t pee into your DEF tank unless you would like to expend several thousand dollars in repairs to your vehicle.

How Is DEF Implemented in Diesel Engines?

The trucking industry, particularly the manufacturers of heavy-duty diesel engines, began to meet EPA standards by redesigning engines with a selective catalytic reduction technology. The DEF is sprayed into the exhaust as an after-treatment process; which means that it destroys harmful emissions after combustion, thus giving manufacturers the ability to tune engines to improve fuel efficiency and increase power.

It is said that owners of SCR vehicles experience greater reliability, longer oil change intervals, and cheaper operating costs over the life of the vehicle.

Who Regulates DEF?

The production, handling and transportation of DEF are governed by the Environmental Protection Agency through its ISO 22241 standard. The urea concentration of DEF must by 32.5 percent by weight; which was mandated because that concentration has the lowest freezing temperature of 12 degrees Fahrenheit.

Can DEF Harm Your Engine?

The only harm to an engine would be to the SCR system’s injectors and catalysts if the fluid is contaminated. That’s why there are maximum levels of impurities permitted in DEF.

Is DEF a Hazardous Material?

No. It is not necessary to wear protective clothing when handling DEF. The only concern from getting on clothing, for example, is that DEF might leave a stain. Simply rinse it off with water as soon as possible. If you spill a small amount of DEF, it can be washed away with water or wiped up. If you leave it to dry it will turn into white crystals. These can be washed away with water.

That being said, DEF should not be swallowed. If ingested, a physician should be consulted immediately.

As to inhaling DEF, under normal conditions of use, inhalation is not expected. However, when pumping DEF in a closed area it is possible that inhalation of a small quantity of ammonia fumes from DEF might occur. If you do inhale these fumes from DEF, move to an area of fresh air. A physician should be consulted if you experience any continued symptoms, such as irritation to nose or throat. DEF does sometimes have a slight ammonia smell (similar to home cleaning agents), but these are not harmful.

How Do I Store DEF?

DEF requires care in storage. For example, DEF is corrosive to many materials, including carbon steel, copper, nickel, epoxy resins and aluminum. The main issue with incorrect storage is that the DEF may become contaminated by leaching from the materials and cause failure with the SCR system, meaning the whole batch of improperly stored DEF would be useless.

Temperature is also a concern with storage of DEF; which has a limited shelf life depending on the temperature. Storing DEF at 80 degrees will reduce its shelf life to just 12 months while keeping it at 60 degrees can extend its usability to 36 months!

Recommended handling and storage containers for DEF include titanium, rubber, plastic, and stainless steel. It is recommended to only use chemical equipment when handling DEF, as petroleum equipment can contain unknown residue. Also, DEF requires dedicated equipment. A tank that has previously held any other product will contaminate DEF.

How Will I Know If DEF Has Gone Bad?

Since DEF should always remain the same color, basically like clear water, any change from this should alert a driver that the fluid in his DEF tank has become contaminated. And, once contaminated, the DEF cannot be restored or reconstituted. Importantly, DEF is not susceptible to freezing and thawing. Within the parameters of the temperature ranges for storage longevity, freezing DEF does no harm to it.

What Does DEF Cost?

Like most truck expenses, the price of DEF depends on location, time of year, and availability. At the truck center fuel island, DEF will run cheaper than the diesel fuel price. As of this writing, October 2018, DEF is running about $2.799 to $2.859 per gallon, compared to diesel fuel’s $3.459 to $3.899 price.

Truck terminals can purchase their DEF in bulk volume in drums or totes and save about 30 percent over the truck center prices.

How Much DEF Will a Truck Consume?

DEF consumption is expected to be approximately 2 percent of fuel consumption, depending on vehicle operation, duty cycle, geography, load ratings, etc. Since DEF consumption is directly related to fuel consumption, a truck averaging 6 MPG can expect to go approximately 300 miles on one gallon of DEF.

DEF weighs about 9 pounds per gallon.

Can I Make My Own DEF?

It is not recommend that drivers make DEF themselves. DEF has strict requirements for maintaining concentration and purity of ingredients that is critical to the proper functioning and longevity of the SCR system.

Cummins and other OEMs require that DEF used with their SCR systems meet all ISO22241 specifications as well as API certification requirements. It is recommended that end users purchase certified DEF and avoid blending it themselves.

For more information about the quality standards, refer to ISO 22241 which details specifications for DEF quality, handling, testing, transportation storage, and refilling.

What If My Truck Runs Out Of DEF?

The EPA has taken into consideration the possibility that a driver of a DEF-equipped truck might choose to run without having to be encumbered by the extra expense and trouble of having to keep his DEF tank filled. Vehicle manufactures have built-in DEF level warnings with the added incentive that the vehicle will fail to operate properly without DEF.

Basically, before a truck's DEF tank runs empty drivers are 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 percent an amber warning lamp will come on, at 5 percent this lamp starts flashing and below 2.5 percent 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.

Are There Any Other Dangers Associated With DEF?

One concern expressed by some is the possibility of inadvertently putting DEF into the diesel tank, or vice versa. A number of safeguards are in place to prevent that happening.

The standard nozzle diameter for DEF is 0.75 inches, compared to 0.87 inches 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 is clearly marked "Diesel Exhaust Fluid" with the accompanying ISO standard number.

However, mixups have happened. In a small number of cases diesel has been put into 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 the SCR system and it is recommended to 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.

Terminal:

A facility where trucking companies operate out of, or their "home base" if you will. A lot of major companies have multiple terminals around the country which usually consist of the main office building, a drop lot for trailers, and sometimes a repair shop and wash facilities.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OOS:

When a violation by either a driver or company is confirmed, an out-of-service order removes either the driver or the vehicle from the roadway until the violation is corrected.

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