What is DEF fluid? How does diesel exhaust fluid work?

What is DEF fluid? If you’ve come across the term “DEF” and wondered what it is or had any questions about its use, you’re in the right place. DEF, or diesel exhaust fluid, is a ubiquitous component of almost every type of diesel-powered engine used in the United States.

It’s vital to understand the role it plays in the operation of your diesel equipment. Without that knowledge, you could end up with an engine that does not start up after your next pit stop. Fortunately, modern technologies have made DEF systems pretty user-friendly.

What is DEF Fluid?

DEF is made up of deionized water and urea. These substances help turn nitrous oxide — a harmful type of emission from diesel engines — into water and nitrogen, two everyday substances that make up the air we breathe. Essentially, DEF is a key ingredient in starting the chemical reaction that keeps dangerous gases produced during diesel engine operation out of our environment.

What is DEF fluid

This fluid is an essential part of ensuring that heavy-duty trucks and machinery operate within federal emissions standards set by the Environmental Protection Agency (EPA). Those requirements mean that DEF is a widespread substance used with nearly every diesel engine out there. DEF is the reducing agent in the chemical reaction that occurs in selective catalytic reduction (SCR) systems.

What Is Selective Catalytic Reduction?

Another common acronym you will find surrounding DEF is SCR, or selective catalytic reduction. This emissions control technology is what uses DEF and allows it to interact with the engine’s exhaust. It injects the DEF through a catalyst and into the exhaust stream, where the DEF starts a chemical reaction, turning the nitrogen oxide into nitrogen, water, and some carbon dioxide.

Then, it gets sent out through the tailpipe and into the air, where it is safe to breathe in and does not harm our environment the way nitrogen oxide would. Nitrogen oxide is a pollutant that causes problems such as acid rain, haze, nutrient pollution, and human health issues, particularly respiratory symptoms.

The SCR system sets up an oxidizing atmosphere perfect for a nitrogen oxide reduction reaction. It is an affordable, fuel-efficient, and powerful technology in the fight against high emissions and environmental damage. SCR can reduce nitrogen oxide emissions by up to 90%, along with other pollutants. Aside from emissions reductions, SCR and DEF can also provide fuel efficiency, with many drivers reporting gains of 3-4%.

What Is DEF Made Of?

As mentioned above, deionized water and urea. Urea may sound familiar because it’s eliminated from the human body in urine. The urea used to make DEF, however, is synthetic.

What is DEF fluid

Ordinary water typically contains lots of ions, i.e. atoms and molecules with a positive or negative charge, These particles must be removed to prevent them from interfering in the exhaust purification process. It’s done by by passing the water through a membrane or a collection of charged plastic beads.

Urea is a common component of synthetic fertilizers because of its high nitrogen content. It’s produced by combining ammonia and carbon dioxide. Manufacturing facilities are often close to coal and natural gas refineries that produce ammonia as a byproduct.

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How Does Diesel Exhaust Fluid Work?

The DEF chemical process occurs in three stages:

  • The heat in the catalytic converter breaks down urea into ammonia and isocyanic acid.
  • Isocyanic acid combines with water to produce carbon dioxide and more ammonia.
  • Ammonia and oxygen react with nitrogen oxide and nitrogen dioxide to produce harmless nitrogen gas, water and carbon dioxide.

Carbon dioxide could be eliminated by substituting pure ammonia for urea. The problem is, ammonia has a low boiling point and must be stored under pressure, creating the risk of leaks. Because urea is stable as a liquid at room temperature and nontoxic, so it’s used in the interest of safety, though it burdens the atmosphere with more carbon dioxide.

What Is DEF Used For?

The EPA issued the Diesel Emission Reduction Act (DERA) in 2010, and diesel engines have been using DEF to accommodate emissions restrictions ever since. As we become more conscious of climate change and the need to keep emissions as low as possible, the demand for DEF is likely to increase.

Technologies related to this fluid, such as built-in vehicle monitoring systems, are also likely to become more popular. DEF and SCR systems have been immensely valuable in reducing the significant emissions that come from large semi-trucks that spend all day spitting out dangerous substances.

Of course, DEF is used in much more than diesel trucks — it is also used in many types of machinery and marine and farm equipment. As emissions standards have increased, off-road vehicles, generators, and many other engines began to use DEF. There are even regular consumer vehicles with diesel engines that require DEF.

How Often Do You Need To Fill Up the DEF Tank?

That question specifically depends on miles per gallon and usage of the diesel truck in question. No matter how heavy the load, according to the OE manufacturer, the typical average light duty truck will consume 2-3 gallons of DEF per 800 miles.

However, most new trucks with an average miles per gallon rating of 20+mpg will go roughly 8,000-10,000 miles on a tank full (10 gallons) of DPF. Each truck is different, for example on a Dodge Ram there is a gauge readout of exactly how much DEF is left in the tank, GM has a digital readout and Ford has a simple low DEF light.

Medium Duty and Heavy Duty Fuel models will vary but according to Cummins Filtration DEF consumption will be approximately 2% of the fuel consumed. For every 50 gallons of diesel fuel burned you will use 1 gallon of DEF.

Where Can You Buy DEF?

Don’t be fooled into thinking you can buy DEF just anywhere. DEF is mostly sold at truck stops in big jugs containing multiple gallons of the stuff. Some gas stations will carry DEF but don’t count on it if you are in a pickle. It is important to understand if you don’t refill an empty DEF tank the engine will automatically shut down.

What is DEF fluid

You don’t want to be stranded somewhere with an empty DEF tank because it is not sold everywhere. Common places to buy DEF include TravelCenters of America, Walmart, Target, Love’s Travel Shop, SAPP Brothers, Flying J Truck Stops, Petro Stopping Centers and Pilot Travel Centers, O’Reilly’s, NAPA and Advanced Auto.

What are the Pros and Cons of a DEF Truck?

There are very few cons with DEF as it is a fairly simple procedure to deal with. However when it comes to Diesel Particulate Filter (DPF) and Selective Catalytic Reduction (SCR) there can be many maintenance and repair issues since they prone to clogging. These systems are complicated in design and a simple clogged filter can cause pressure and temperature differentials that can affect the whole performance of the engine.

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The only cons to DEF include the additional upfront cost to the truck, added nominal weight and some additional room to store an extra gallon of the fluid. The pros are better fuel economy, increased horsepower, more optimized combustion, fewer regenerations issues, less wear on the engine and in addition it only releases nitrogen and water vapor into the air.

Benefits Of DEF

Initially, many drivers and heavy vehicle manufacturers complained about DEF mandates. This technology was expensive to implement and added extra steps during the refueling process. However, as the years passed, DEF technology became more affordable and offered many benefits to drivers and the environment.

Benefits for the driver

Exposure to DEF vapors is much less toxic than direct exposure to diesel. Drivers who spend years on the road inhaling diesel may experience many health problems, including cardiovascular disease, asthma, and more.

The switch to DEF prevents unnecessary illnesses and promotes the overall health of drivers and the communities in which they operate. The EPA estimates that this technology will reduce hospital visits, sick days, and deaths. Consequently, the nation will save billions of dollars in the coming years.

Benefits for the vehicle

DEF boosts the efficiency of a truck’s engine. Because it produces low emissions, drivers can set their vehicles to a higher power and reduce the number of oil changes they need between routes. Furthermore, this fluid doesn’t require additional cooling solutions or vehicle installations.

Your engine can last much longer with this highly efficient technology.

Benefits for the environment

DEF primarily benefits the environment by reducing harmful gas emissions into the atmosphere. It also saves taxpayers money on ecological mitigation.

This colorless liquid is stable and safe to store. It will not endanger plants or animals if you handle it properly.

Using DEF significantly reduces your carbon footprint compared to conventional NOx emissions. The product complies with global composition standards and is available for agricultural and chemical industries.

Will DEF Lower My Fuel Mileage?

It is only natural to think that any EPA induced changes to the diesel engine is necessarily a bad one however it is quite the contrary. The major diesel manufacturers discovered it can fine tune the engine anyway it deems fit then allow the SRC and DEF to remove the particulate. The engines are built with performance in mind first and then the SRC, DPF and DEF remove what is needed as an afterthought.

What is DEF fluid

Manufacturers have found that engines containing SCR technology oftentimes get better fuel mileage compared with other smog reduction internal systems. Fuel mixed in with the SCR finds an added source of elements to burn. Fuel mileage can be improved by as much as 5%-7%.

Has This New DEF Technology Ever Been Used Before?

DEF technology has been used for decades in the country in agriculture, industrial and large scale power generation applications. The concept is the same across the board: the urea mixed with heat creates ammonia that causes a chemical reaction that reduces NOx by 70%-95%. In fact 90% of urea production is used as nitrogen-released fertilizer.

It is important to note that automotive-grade urea is of a much higher grade purity than fertilizer urea. If a lower grade fertilizer urea is used in automotive engines you risk disintegration of the SCR and possible ruin the engine. It may even trigger ECM sensors to incorrectly prompt a DEF Tank Empty warning.

Does DEF Evaporate After A Period of Disuse?

The answer is yes and no. The temperature at which DEF combines with NOx exhaust right out of the cylinder head with the valves wide open is between 1400-1600 F. The chemical reaction takes place at much hotter temperatures than on a hot sunny day. For example it would take over two years a constant rate of 125 degrees F for the DPF to turn into ammonia and evaporate.

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However any temperature over 86 F you risk some evaporation due to DEF being almost 2/3 water. You don’t have to worry about a gallon or two of the stuff turning bad or evaporating from disuse unless exposed to consistent hot climates.

Is DEF a Toxic and Harmful Chemical?

The active ingredient in DEF, urea, has been chemically synthesized since 1828 first by German scientist Friedrich Wöhler after treating silver cyanate with ammonium chloride.

Urea was first discovered in urine by Dutch chemist Herman Boerhaave in 1727. Urea is mostly used in fertilizer for agricultural but also found the chemical industry, explosives, lotions, skin creams, hair removers, plastics, dish soaps and power fuel cells. Urea and consequently DEF is not overly toxic to humans. Urea can be irritating to eyes, skin and the respiratory tract but not life threatening.

High concentrations in the blood can be damaging to humans however ingestion of low concentrations of urea ,given an adequate water ingestion of water, are not harmful. In nature urea can cause algal booms which when decomposes above its heating or melting point can cause toxic gases. Mixed with certain oxidants, chlorides, nitrites can cause fire or even explosions.

Does DEF Have a Low Freeze Point?

The standard 32.5% solution of DEF will begin go crystallize and freeze at 12 F. The Urea and water in DEF, when mixed, will freeze at the same rate. This is beneficial to the user because when the fluid thaws the DEF solution does not become diluted or overly concentrated.

What is DEF fluid

Freezing and thawing cycles has no impact on the grade of the product. DEF expands when frozen as much as 6.5% – 7% by volume. The packaging allows for leeway for freezing periods.

What is The Best Method to Keep DEF from freezing?

sitting in the back of your truck and given an extended period of hot days the fluid can become unstable and decompose but at a very minimal rate. A diluted DEF without the mixture of 32.5% urea can be damaging to DEF and SCR but cases of that happening are rare.

Consequently DEF will freeze in the DEF tank on extremely cold days below 12 F. That is perfectly normal and will not hurt the engine. The SCR systems are designed to provide heat to the DEF tank which will quickly thaw the tank and supporting supply lines.

How Can You Determine The Age of a Container of DEF?

Every single DEF package has a manufacturers date located somewhere on the product. Most likely it is on the front of the package near the bottom. This date code will tell the exact date the batch was produced and subsequently the age of the bottle of DEF.

One gallon containers have a laser code imprinted on the bottle. Larger 2-5 gallon tanks usually have a small date code imprinted on the label of the product. Larger drums of DEF fluid (55 gallons +) and totes (275-355 gallons) will have a larger label applied to the side or top of the drum.

Reading a manufactures code can be a bit tricky and each one is a little bit different. Usually the first digit of the date code represents the batch number and the next 6 digits reflect the date the batch was filled at the factory.


What are the different delivery modes of DEF?

DEF can be purchased in several ways. There are varying sizes of jugs/containers a driver can purchase. This requires the driver to transfer the DEF into the vehicle manually. DEF can also be dispensed into the vehicle through a fueling dispenser when equipped.

Is DEF the Same as AdBlue?

AdBlue is simply a brand name of DEF registered in Germany. It has become a very popular and widely recognized product that many people use interchangeably with DEF. Despite the name, AdBlue is not actually blue and is colorless.

Can You Substitute Water in for DEF?

No, you cannot put water in the DEF reservoir. The system will identify it as poor quality DEF and will not start up. Even if it did, water does not contain the ingredients needed to convert harmful gases to non-harmful ones. You would be doing a lot of harm to the environment and any efforts toward compliance.

Above is information about What is DEF fluid? that we have compiled. Hopefully, through the above content, you have a more detailed understanding of How does diesel exhaust fluid work? Thank you for reading our post.

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