20 Facts You Need To Know About Diesel Exhaust Fluid (DEF)

The Environmental Protection Agency was created in the footsteps of the Clean Air Act of 1970. For 46 years the EPA has been constantly evolving and enacting laws to address the environmental needs of this country. For many years owners of three-quarter and one-ton light duty pickup trucks enjoyed no additional smog equipment on their vehicles. All of that changed in 2008 with the EPA required the use of diesel particulate filters on all three-quarter-ton and larger trucks as well as required biannual smog tests which included a visual inspection of the vehicle to make sure the DPF parts were still on the truck. In 2010 the regulations got even tighter.



Many thought that the era of big power and torque were over and vowed to never purchase a new truck ever again. However, something spectacular happened and the complete opposite thing occurred. It turns out that Americans truly do adapt and overcome. Every one of the manufacturers figured out a way to cut down on the NOx levels all the while making more horsepower and torque than ever before. Innovation is bread out of strife.



The engineering breakthrough came through the use of the selective catalytic reduction. The vast majority of these systems use diesel exhaust fluid (mixture of urea and deionized water) sprayed into the exhaust system to break down the generated NOx into harmless nitrogen and water molecules. Since the DEF (Diesel Exhaust Fluid) is introduced in the exhaust, also called after-treatment technology, the manufacturer is free to build as much power as they want. The DEF is stored in a separate tank which is insulated and heated and is marked by a blue filler cap.



Still there are two factions of diesel guys out there; those who have accepted to the EPA changes and those who are still vehemently against any regulations whatsoever despite the work around technological advancements. For those not willing to accept the changes there has been a shift to older used diesel engines or remanufactured diesel engines that have been grandfathered in. This article aims to go over the cold hard facts about DEF and educate the populace into making smarter diesel operator decisions.

1. What exactly is Diesel Exhaust Fluid (DEF)?

From a strictly chemical disposition DEF is a mixture of 67.5% deionized water and 32.5% urea. Urea is a compound in Nitrogen that turns to ammonia when heated and is used in a variety of industries. Urea is technically derived from a byproduct of urine but for mass production purposes it is synthetically made. Most DEF products are regulated by the American Petroleum Institute. Lets take a look at the science behind DEF when mixed with exhaust. Chemically, DEF is firstly comprised of (NH2)2CO; and when injected into the hot exhaust gas the water evaporates leaving ammonia and isocyanic acid.



STEP 1: DEF Becomes Ammonia and Isocyanic Acid: (NH2)2CO → NH3 + HNCO



STEP 2: The Isocyanic Acid chemically breaks down with water into Carbon Dioxide and Ammonia:

HNCO + H2O → CO2 + NH3 overall which is this: (NH2)2CO + H2O → 2NH3 + CO2



STEP 3: At this point during the chemical reaction Ammonia will, in the presence of oxygen and a catalyst, will reduce nitrogen oxides:

2NO + 2NH3 + ½O2 -> 2N2 + 3H2O and 3NO2 + 4NH3 -> 7/2N2 + 6H2O



STEP 4: The overall reduction of NOx by urea is:

2(NH2)2CO + 4NO + O2 → 4N2 + 4H2O + 2CO2 and 2(NH2) 2CO + 3NO2 -> 7/2N2 + 4H2O + 2CO2

2. 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. Here are some Medium and Heavy Duty Consumption projections by our friends over at Cummins Filtration:





3. 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. 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. We have also compiled a list of the most popular manufactures of DEF here.

4. 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.



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.

5. Is emissions production really an important issue?





Whether it is a big deal or not is not really not up for debate considering all 2008 light-medium diesel and up have to comply with the EPA Regulations. NOx has been blamed for smog, a rise in greenhouse gasses and acid rain. The DEF as part of the Selective Catalytic Reduction system (SCR) turns NOx into pure nitrogen and water vapor. Climate change is a heated debate but we can all agree that spewing more gasses of any kind into the environment isn’t something we need more of.

6. 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. 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%

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.

8. 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. 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.

9. 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.

10. What happens to the engine if the DEF Tank is empty?

All diesel engine manufacturersare now required by the EPA to integrate some tiered warning system (internal gauges on the dash) to let the driver know exactly how close to empty the DEF tank is. If you ignore the DEF warning the truck will cease to work. Some diesel engine manufacturers allow the engine to go into reduced power mode so the truck can “limp home” or limit the number of times you can turn the engine over. At some point though the diesel engine will not start. Treat the DEF tank just like you would the fuel tank; you don’t want to end up stranded somewhere because you didn’t refill the DEF tank.

11. 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. 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.

12. What is the best method to keep DEF from freezing?

It is perfectly safe to keep a gallon or two of DEF in your vehicle however it is not advisable. DEF will start to decompose at 86 F. It is very easy to forget about the DEF 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.

13. Can I add anti-freezing solution to the DEF mixture to keep it from freezing?

DEF is a very specific formula of 32.5% Urea and water however the solution contains other compounds in minute quantities to stabilize the product. An additive to the mixture would upset the very precise chemical makeup and thus reduce the NOx reducing properties. Further blending of the DEF mixture will compromise its ability to work properly as will cause harm to the SCR system.

14. How much does DEF Weigh and will it load down my tuck?

DEF weighs about 9 lbs per gallon. A typical light duty/medium duty truck will have a tank roughly 3-5 gallons.

15. How is the production of DEF regulated and can I make my own?

It is not recommended that the direct consumer produce their own DEF. DEF is closely regulated and has precise requirements for maintaining chemical purity and contains ingredients that are crucial to working in conjunction with the SCR system. Caterpillar, Cummins, Detroit Diesel among others require that the DEF to be used with the SCR systems and meet all ISO guidelines and API requirements. API certification is a completely voluntary program established by the American Petroleum Institute (API) which certifies the chemical pureness of DEF and that manufacturers meet ISO specifications. All major brands of DEF available to the consumer market meet API Certification.

16. What is the shelf life of DEF?

If the DEF is stored at ambient temperatures of 75 F with no major periods of exposure to heat over 86 F then the batch of DEF will last roughly two years. If a package of DEF is exposed to periods of heating the fluid will last approximately one year.

17. Who are the major manufacturers of DEF Fluid?

There are many manufacturers of DEF Fluid. The website, “Oilmen Truck Tanks” has compiled a list of 13 major manufactures. DEF is found at most major truck stops, auto parts stores and convenience stores for roughly $2-$3 per gallon.

18. 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.

19. How do you identify a DEF Filling Pump Vs. a Diesel Fuel Pump?

There have been multiple measures implemented to prevent diesel engine fuelfrom being pumped into the DEF tank and vice versa. “Green” is the international color of diesel filling stations and pumps. “Blue” has been adopted as the color of choice to represent DEF fluid. A standard nozzle diameter of 19 mm has been designed to dispense DEF; a standard diesel fuel nozzle is 22 mm. The tank cap on pretty much all trucks should also be a "brightly colored blue" as a last preventative visual measure to stop diesel from going into the DEF tank.

20. What should I do if I accidentally dispense diesel fuel into the DEF?

First off, do not panic. Secondly, do not under any circumstances start the engine. Simply dumping diesel fuel in the DEF and vice versa (DEF in the fuel tank) will not harm your engine if you do not start the engine. The SCR should recognize that there is a solution other than DEF in the tank and notify the driver via the ECM readout on the dashboard. It is also important not to move the vehicle. Moving the vehicle can causing the spread of the fuel into the lines and into the SCR. The best thing to do is drain the tank with the vehicle in its original position. If the engine is started for even a short time diesel fuel will ruin the SCR catalyst which is very costly to replace and will be out of warranty. If DEF enters the fuel system and spread throughout the engine it will ruin the diesel engine eventually. The fuel system lines are not compatible with DEF and slowly corrode and over time.