What is diesel exhaust fluid (DEF)?

Created by Jonathan Fry, Modified on Tue, 1 Apr at 5:13 PM by Jonathan Fry

Key Points

Diesel Exhaust Fluid (DEF) is a solution used in diesel engines to reduce harmful emissions, likely consisting of 32.5% urea and 67.5% deionized water.
DEF works by converting nitrogen oxides (NOx) into nitrogen and water through Selective Catalytic Reduction (SCR), helping meet environmental standards.
An unexpected detail is that running out of DEF can lead to reduced engine power or even prevent the vehicle from starting, affecting vehicle operation.

What is Diesel Exhaust Fluid (DEF)?

Diesel Exhaust Fluid (DEF) is a non-toxic liquid used in diesel engines to lower air pollution. It is typically made of 32.5% urea and 67.5% deionized water, stored separately from the diesel fuel in a tank with a blue cap. DEF is injected into the exhaust system, where it helps reduce harmful nitrogen oxide (NOx) emissions through a process called Selective Catalytic Reduction (SCR). This process converts NOx into harmless nitrogen and water vapor, making diesel vehicles more environmentally friendly.  

Why is DEF Important?

DEF is crucial for modern diesel vehicles, especially those made after 2010, as it helps comply with strict emission regulations set by agencies like the EPA. Without DEF, vehicles might enter a reduced power mode, known as "limp mode," or in some cases, may not start at all, impacting their operation.

Survey Note: Comprehensive Analysis of Diesel Exhaust Fluid (DEF)

This note provides a detailed examination of Diesel Exhaust Fluid (DEF), expanding on its definition, composition, usage, and implications, based on extensive research from reliable sources. The information is organized to offer a professional and thorough understanding, suitable for both general readers and those with technical interest in diesel engine emissions systems.  

Definition and Composition

Diesel Exhaust Fluid (DEF), also known as AUS 32 and sometimes marketed as AdBlue, is a liquid designed to reduce air pollution from diesel engines. Research suggests it is an aqueous urea solution, consistently reported across multiple sources as comprising 32.5% urea and 67.5% deionized water. This composition is critical for its function, as it ensures the fluid can effectively participate in chemical reactions without contaminating the engine. For instance, Wikipedia: Diesel exhaust fluid notes that DEF is referred to as AUS 32 in the ISO 22241 standard, reinforcing its standardized formulation.  

Aspect	Details
Definition	Liquid used to reduce air pollution from diesel engines, lowering NOx via SCR.
Composition	32.5% urea, 67.5% deionized water.
Alternative Names	AUS 32, AdBlue (trademark).
Standards	Must meet ISO 22241-1, as noted by H.O. Penn: Understanding Diesel Exhaust Fluid .

The use of deionized water is particularly important to prevent mineral deposits that could affect the SCR system, and some sources, like Ricochet Fuel Distributors: Diesel Exhaust Fluid Explained, specify "synthetic urea," though this is likely standard urea used in industrial applications.  

Functionality and Usage

DEF is consumed in the Selective Catalytic Reduction (SCR) process, where it is injected into the hot exhaust gas stream. The evidence leans toward DEF facilitating a chemical reaction where water evaporates, and urea decomposes into ammonia and isocyanic acid, which then react with NOx to form nitrogen and water vapor. This is detailed in Geotab: How Diesel Exhaust Fluid Works, which explains that SCR converts NOx into nitrogen, a harmless component comprising about 78% of the atmosphere.  

The injection rate is typically 2–6% of diesel consumption volume, ensuring long refill intervals and minimal tank size, as noted by Wikipedia: Diesel exhaust fluid. This low dosing rate is managed by an electronic control unit that adjusts based on parameters like NOx levels, engine temperature, and speed, according to Diesel Power Products: Diesel Exhaust Fluid (DEF): Everything You Need to Know.  

DEF is not mixed with diesel fuel; it is stored separately and used exclusively in the exhaust system. This separation is crucial, as mixing DEF with diesel can cause costly damage, as highlighted by Fuel Logic: DEF in Diesel Tank. Modern diesel vehicles, particularly those above 74 horsepower and manufactured post-2010, require DEF as part of SCR systems to meet EPA and EU emission standards, as mentioned in EquipmentShare: What Happens if You Run Out of DEF?.  

Importance and Environmental Impact

DEF plays a vital role in reducing NOx emissions, which can irritate the eyes, nose, throat, and lungs at low levels, and cause severe health issues at high levels, such as fluid build-up in the lungs or even death, according to All American CDJR: What is Diesel Exhaust Fluid (DEF) and Why Do I Need It. By reducing NOx by up to 90%, as noted by StartRescue.co.uk: What is AdBlue? Diesel Exhaust Fluid (DEF) - Explained, DEF significantly lowers the environmental and health risks associated with diesel exhaust.  

This reduction is part of broader efforts to make diesel engines cleaner, with technologies like DEF and SCR introduced to meet regulations like Euro 6 (EU, 2015) and EPA standards (U.S., 2010), as seen in CNET: What is diesel exhaust fluid and why is it more important than ever?.  

Operational Implications

Running out of DEF can have significant operational consequences. Research suggests that vehicles may enter a reduced power mode, often called "limp mode," limiting speed to 5 mph, as reported by Azure Chemical: What Happens When You Run Out of Diesel Exhaust Fluid?. Some sources, like HFI Truck Center: What Happens When You Run Out of Diesel Exhaust Fluid?, indicate that the engine might not start at all, though this varies by manufacturer. Diesel technicians, as seen in an X post, confirm that DEF depletion won't harm the engine but will likely trigger power derate, emphasizing the need for regular refills.  

To manage DEF levels, vehicles are equipped with alerts when the tank drops below 2.5%, and tanks are sized proportionately to fuel tanks, ensuring both empty at similar rates, as noted by H.O. Penn: Understanding Diesel Exhaust Fluid. This design helps drivers maintain compliance and avoid operational disruptions.  

Additional Details and Practical Considerations

DEF is non-hazardous but can stain clothes, and it should be stored between 40 and 80 degrees Fahrenheit for up to two years, with no freezing issues in vehicles, according to Ricochet Fuel Distributors: Diesel Exhaust Fluid Explained. It is available in various quantities, from 2.5-gallon jugs to 330-gallon totes, and can be purchased from suppliers like Ricochet Fuel, with pricing varying by quality and quantity.  

An interesting aspect is that DEF is also known by brand names like BlueHDI (PSA Group, including Peugeot and Citroën) and BlueTec (Daimler AG), as mentioned by Wikipedia: Diesel exhaust fluid, indicating its widespread adoption across different manufacturers. Additionally, some sources suggest carrying a gallon of DEF in the truck and using a funnel for easy refills, a practical tip for fleet managers and truck owners.  

Conclusion

In summary, DEF is a critical component for modern diesel engines, enabling compliance with emission standards through SCR technology. Its composition and function are well-documented, with consistent reports across sources confirming its role in reducing NOx emissions and supporting environmental protection. The potential for operational issues when running out underscores the importance of regular maintenance and refills, making DEF an essential consideration for diesel vehicle owners and operators.  

Key Citations