Diesel Exhaust Fluid? What is DEF, and why do we use it?

Small, medium and large businesses considering diesel exhaust fluid (DEF) for their commercial diesel vehicles generally have the same seven questions about DEF (also called AdBlue).
Here are answers to help you make informed decisions. And there’s more good news: You don’t need an advanced degree in chemistry to comply with federal emissions regulations.
1. What is Diesel Exhaust Fluid (DEF)?
Diesel exhaust fluid isn’t a fuel, although it’s transported and stored like fuel in separate, dedicated tanks.
The nontoxic, nonflammable and biodegradable fluid, when injected into vehicle exhaust produced by normal diesel combustion, works with a technology called selective catalytic reduction (CSR) to reduce poisonous, highly reactive nitrogen oxide (NOX) emissions.
Because NOX contributes to air pollution and smog, the Environmental Protection Agency (EPA) requires most diesel engine heavy-duty trucks manufactured after 2010 to include CSR.
DEF helps vehicles with CSR technology meet EPA requirements for diesel exhaust emissions.
2. What is Diesel Exhaust Fluid Made Of?
Diesel exhaust fluid ingredients are typically 32.5% synthetic, vehicle-grade urea (ammonia) and 67.5% deionized water.
DEF must meet a purity standard set by the International Standards Organization (ISO).
The ISO defines technically pure urea as an “industrially produced grade of urea (CAS Number 57-13-6) with traces of biuret, ammonia and water only, free of aldehydes or other substances such as anticaking agent, and free of contaminants such as sulphur (sic) and its compounds, chloride, nitrate or other compounds.”
The ISO defines pure water as “water very low in inorganic, organic or colloidal contaminants, produced, for example, by single distillation, by deionization, by ultra-filtration or by reverse osmosis.”
3. What Does Diesel Exhaust Fluid Do?
DEF converts NOX emissions produced during diesel engine combustion into harmless gases — nitrogen and water vapor — through a chemical change.
Vehicles with CSR technology have a tank for diesel fuel and another tank for DEF. (Diesel fuel and DEF are not meant to mix. Do not combine them. DEF injection happens outside the engine after diesel combustion.) Two-tank systems ensure the proper diesel-to-DEF ratio, which is about 50-to-1 for most diesel vehicle manufacturers.
When a CSR-equipped engine is running, the CSR system injects a small amount of DEF into the diesel exhaust stream. DEF mixes with the heated exhaust gases, and the DEF breaks down into carbon dioxide and ammonia. Then, the NOX in the SCR catalyst reacts with the ammonia and breaks down into safe nitrogen gas and water vapor.
DEF has several benefits. Beyond cleaner air, DEF contributes to better engine performance and fuel economy. Because injection occurs outside the engine, diesel engine manufacturers have boosted power output and achieved a cleaner burn and more efficient combustion.
4. Does DEF go bad?
Yes, DEF can go bad because it contains no preservatives. It naturally degrades over time the longer it is stored and longevity very much depends on storage conditions. DEF goes bad more quickly in temperatures higher than 65°F but when stored at lower temperatures, shelf life is extended.
Most DEF producers recommend use within one to two years of production dates. Many diesel engine manufacturers recommend using DEF certified by the American Petroleum Institute (API).


Water contamination is also a major contributor to causing DEF to go bad. Water in a DEF tank should be avoided at all costs because it will contaminate the product and potentially damage any vehicle run off that product.
5. Does Diesel Exhaust Fluid Freeze?
Yes, diesel exhaust fluid freezes at 11 F, and when it does, its volume expands up to 7%. For commercial vehicles with built in DEF tanks we recommend leaving enough room for thermal expansion to prevent cracked tanks.
If DEF freezes, its composition will not change. Do not add anything, including water, to DEF to help it melt. Frozen DEF in a vehicle tank will melt when the engine is running. Water in DEF tanks will contaminate the DEF fuel tank and can ruin a vehicle.
6. What Color is a Diesel Exhaust Fuel Cap?
Most diesel exhaust fuel caps are blue to differentiate from diesel fuel caps, which are green. The color coding helps prevent inadvertent contamination of both diesel and DEF in tanks. (The actual color of DEF, however, is clear.)

7. How Do You Store DEF?
Many businesses store DEF in bulk DEF tanks and DEF transfer tanks at their business premises to ensure their vehicles have access to DEF on demand. These are stainless steel storage tanks and fittings designed and constructed specifically to accommodate the corrosive characteristics of DEF. This corrosive nature demands the need for DEF products to be transported, stored and pumped with the appropriate equipment.


Some commercial and consumer vehicle manufacturers build small DEF tanks into their diesel vehicles during the assembly process. This ensures the DEF tank always travels with the vehicle and can be filled up at fuel stations for convenience.

When a DEF tank on a vehicle runs dry, the vehicle likely will go into “limp mode” with a noticeable decrease in performance and eventual shutdown.
Petroleum industry experts, market analysts and trade associations, including the Petroleum Equipment Institute (PEI), expect the demand for DEF will increase and require increasingly larger aboveground and underground storage tanks.
“Most people associated with the petroleum marketing and equipment industries know that aboveground tanks (ASTs) storing ‘oil’ are regulated by EPA under the Spill Prevention, Control and Countermeasure (SPCC) rule,” wrote Rick Long, former executive vice president of PEI, in a blog. “Since DEF is composed of aqueous urea which does not meet the SPCC definition of ‘oil,’ ASTs containing DEF (including drums, totes, IBCs and tanks) are not regulated by EPA.
“However, a word of caution is necessary here: Some states develop and enforce regulations that are more stringent than the federal rules.”
Long, who also is an attorney, recommends checking your state’s aboveground storage tank regulations to ensure DEF storage compliance.
More Resources
Recommended Practices for the Storage and Dispensing of Diesel Exhaust Fluid (DEF)
Need Help? Ask the Experts
For help in choosing the right DEF storage tank, it makes sense to ask the experts. Unity Fuel Solutions is North America’s leader in double walled storage tanks. For advice on the tank and system that’s right for you, call the Unity team at 800-234-1689.