How Does Diesel Exhaust Fluid Work?

Diesel-powered vehicles and equipment rely on selective catalytic reduction (SCR) technology to reduce harmful emissions. Diesel exhaust fluid (DEF) is a lynchpin in that chemical process. Without it, modern diesel engines cannot meet federal emissions standards or operate efficiently.

Businesses and operators can explore what DEF does, how it works, and why it’s required to make informed decisions about compliance, performance, and long-term cost savings.

What Is Diesel Exhaust Fluid?

Diesel exhaust fluid is a precise mixture of 32.5% urea and 67.5% deionized water. This combination is injected into the exhaust stream of diesel engines to break down harmful nitrogen oxide (NOx) emissions into harmless nitrogen and water. DEF is not a fuel additive — it never enters the engine itself — but instead works within the SCR system to reduce pollution.

The chemical composition of DEF is carefully regulated to ensure effectiveness. High-quality DEF is essential for keeping diesel engines compliant with federal emissions laws and avoiding costly fines or mechanical issues caused by contaminated fluid.

What Does Diesel Exhaust Fluid Do?

If you’re new to buying DEF for your diesel equipment, you might wonder, what is diesel exhaust fluid used for? DEF’s primary function is to neutralize nitrogen oxides, which are some of the most harmful emissions from diesel engines. These gases contribute to smog, acid rain, and respiratory issues. By using DEF in conjunction with an SCR system, diesel vehicles can reduce NOx emissions by up to 90% — a significant improvement by any measure.

Without DEF, post-2010 diesel engines can’t function properly. If a vehicle’s DEF tank runs dry, most systems automatically limit power output or prevent the engine from starting until it is refilled. This ensures that all diesel-powered equipment stays within emissions compliance.

When Is DEF Required?

The EPA’s 2010 Diesel Emissions Reduction Act mandated that all new medium- and heavy-duty diesel engines use SCR systems to comply with stricter NOx emissions standards. As a result, DEF became a requirement for nearly all commercial diesel vehicles, including:

• Freight trucks and over-the-road semis
• Construction equipment and heavy machinery
• Farm and agricultural vehicles
• Emergency generators and backup power systems

Using DEF is not optional for diesel vehicles manufactured after 2010. Running out of DEF or using contaminated fluid can result in engine malfunctions, increased emissions, and potential fines.

How Does DEF Work in an SCR System?

DEF is injected into the exhaust stream after combustion but before emissions exit the vehicle. 

It’s a bit of a chemistry lesson, but it essentially works like this: When exposed to high temperatures inside the SCR catalyst, DEF decomposes into ammonia (NH₃) and carbon dioxide (CO₂). The ammonia then reacts with harmful NOx emissions and breaks them down into harmless nitrogen (N₂) and water vapor (H₂O). 

To be clear, here it is again, summed up in bullet form:

• Urea breaks down into ammonia and carbon dioxide.
• Ammonia reacts with NOx in the SCR catalyst.
• Final emissions are nitrogen and water vapor — both naturally occurring components of clean air.

This method dramatically reduces harmful pollutants without affecting engine performance. In fact, it probably enhances fuel efficiency and fuel economy.

Dr. Frank Cook, the senior vice president of new product development with Peak Commercial & Industrial (the maker of BlueDEF), shared the following interesting tidbit only a few years after the 2010 law went into effect. While discussing their August 2013 online survey of the trucking industry, Cook explained that for every dollar spent on DEF, it is about 40-50 cents less spent on diesel fuel — and he supplied numbers to break it down and demonstrate how he got there. "With the use of DEF with SCR technology,” Cook said, “the engine can be tuned to run at optimal conditions, which gives a more complete fuel burn and better fuel economy.”

How Should DEF Be Stored and Handled?

Without proper storage and handling, DEF won’t maintain its effectiveness. Temperature fluctuations and contamination can degrade the fluid and lower its ability to neutralize emissions. If you need some safe DEF storage best practices, here are the big ones to preserve its quality and integrity:

• Keep DEF in sealed, dedicated containers to prevent contamination.
• Store it at temperatures between 30°F and 77°F to maintain stability.
• Avoid exposure to direct sunlight or high humidity.

DEF has a shelf life of up to two years if stored properly, but high temperatures can shorten its usability. For businesses using large quantities, bulk DEF storage solutions make for a consistent supply without the risk of degradation.

Where to Buy DEF in Georgia and Alabama

Businesses and operators in Georgia and Alabama should source high-quality DEF from a reliable local supplier to guarantee compliance, performance, and cost savings. At Whatley Oil, we provide bulk DEF deliveries and on-site storage solutions so that your business can maintain operations without unexpected interruptions or costly downtime.

Whatley Oil supports trucking fleets, construction companies, and agricultural operations across the region with strategic delivery options and high-purity DEF. Call us up to learn more about DEF storage, delivery, and compliance solutions for your business.