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6 min read.

What Is Diesel Exhaust Fluid & What Does DEF Do?

Sep 15, 2023 4:02:47 PM

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Many businesses that haul goods or heavy machinery rely on medium and heavy-duty trucks to get the job done. These trucks have diesel engines which require the use of not just diesel fuel but also diesel exhaust fluid (DEF). Semi-trucks, buses, and box trucks are just some examples of vehicles that must use diesel exhaust fluid to meet regulatory standards. But what is diesel exhaust fluid? And how does diesel exhaust fluid work? In this article, we'll discuss everything you need to know and explain what is diesel exhaust fluid and why it's important for trucking companies nowadays.

What is DEF fluid?

First, what does DEF mean? DEF, short for diesel exhaust fluid, is a component in diesel engines to reduce harmful emissions and improve environmental sustainability. Diesel exhaust fluid is made from a formulated mixture of urea and deionized water and is injected into the exhaust stream of diesel vehicles that have Selective Catalytic Reduction (SCR) systems. If you operate a diesel truck, then you're required to use DEF fluid in order to meet the Environmental Protection Agency's emission standards for commercial road vehicles.

What is DEF fuel used for?

DEF became a requirement for diesel engines in trucks to help reduce harmful carbon emissions. Diesel fluid emits nitrogen oxides and particulate matter which are considered pollutants. These pollutants are known to harm crops and can directly impact our food supply as well as air quality. To mitigate these harmful effects of burning diesel, the EPA requires medium and heavy-duty vehicles with diesel engines that were manufactured after 2010 to use DEF.

What does DEF do?

What does DEF do inside a diesel engine, and how does DEF work to reduce air pollution? When injected into the exhaust stream of the engine, DEF undergoes a chemical reaction with nitrogen oxides (NOx) present in the exhaust gases. This reaction takes place within the SCR catalyst, breaking down nitrogen oxide molecules into harmless nitrogen gas and water vapor. This chemical reaction helps reduce the emission of nitrogen oxides, which are a significant contributor to air pollution and smog formation. Overall, DEF contributes to cleaner air and reduces harmful environmental pollution by transforming emissions into cleaner components, aligning with regulatory standards, and encouraging more eco-friendly diesel engine operations.

Common Questions About DEF for Trucks

How does DEF work?

When injected into the exhaust stream of the engine, DEF triggers a chemical reaction with nitrogen oxides (NOx) present in the exhaust gases. This reaction takes place within the SCR catalyst, converting nitrogen oxide molecules into nitrogen gas and water vapor.

How often do I have to add diesel exhaust fluid?

The frequency of refilling a vehicle's DEF tank depends on several factors, including the vehicle's DEF tank size, engine efficiency, driving conditions, and load. On average, DEF consumption is around 2-4% of diesel fuel consumption. This means that for every 100 gallons of diesel burned, you might use around 2-4 gallons of DEF. Heavy-duty trucks and commercial vehicles with larger engines tend to use more DEF due to their higher diesel fuel consumption. Smaller vehicles that happen to have diesel engines, including some passenger cars, will need DEF added far less frequently than commercial trucks.

As a general guideline, drivers with DEF tanks can check their dashboards to view their DEF levels. This is usually indicated by a gauge or icon that resembles droplets. The gauge might have markings or segments that show the level of DEF remaining in the DEF tank. If the DEF lamp on your dashboard lights up, here are a few reasons why that might be occurring:

  • Illuminated DEF Lamp: An illuminated DEF lamp means your DEF level is low, and you should refill your DEF tank soon.
  • Flashing DEF Lamp: A flashing DEF lamp indicates critically low levels of DEF that should be resolved as soon as possible to avoid vehicle problems.
  • Flashing DEF Lamp with Red Stop Lamp: If you see a flashing DEF lamp combined with a red stop lamp, then your DEF levels are extremely low, and you are likely to experience power loss.

Diesel Exhaust Fluid (DEF) Lamp

What happens if I run out?

Drivers with DEF tanks should always be aware of their DEF levels to avoid running out. Running out of DEF can lead to several vehicle issues which include:

  • Reduced Engine Power: Many vehicles are designed to go into a low power mode when the DEF tank is empty or extremely low. This is to encourage drivers to refill the DEF tank quickly and prevent the vehicle from operating without proper emission control.
  • Emission Violations: When the vehicle's DEF tank is empty, the SCR system cannot properly reduce nitrogen oxide (NOx) emissions. This can result in the vehicle not meeting emission standards, potentially leading to emission violations, fines, or failing emissions tests.
  • Engine Shutdown: In some cases, if the DEF level becomes critically low or the vehicle enters a reduced power mode due to the absence of DEF, the engine might eventually shut down to prevent further operation without emission control.

Does DEF go bad?

Diesel exhaust fluid has a limited shelf life and can go bad over time. The shelf life of DEF is determined by factors such as temperature, exposure to light, and contaminants. If DEF is not stored properly, it can undergo chemical changes that affect its potency and effectiveness in reducing emissions. To prevent DEF from expiring, it's important to keep it stored in places with a temperature between 14 to 77 degrees Fahrenheit and store it in solid containers if it's been purchased in bulk.

What does DEF fluid do if contaminated?

If DEF has expired or become contaminated, then it can cause corrosion and damage to your vehicle's engine. To prevent filling your tank with expired or contaminated diesel exhaust fluid, make sure to always inspect any changes in color, odor, or crystallization. If you notice cloudiness, discoloration, or visible particles, then the DEF might be contaminated, and you should avoid using it. Other signs that diesel exhaust fluid has expired or become contaminated include reduced vehicle performance, a check engine light coming on, or engine shutdown. If any of these occur, your DEF probably isn't safe to use. Make sure to properly dispose of expired or contaminated DEF before replacing with new fluid.

Where can I find DEF fluid?

Many gas stations and truck stops offer DEF pumps or containers of DEF that can be purchased. Some auto parts stores and service centers might also carry DEF in containers of various sizes, depending on your needs. As the demand for DEF has grown, so too has its availability as a product that can be purchased.

Fleet Fuel Payment Solutions from P-Fleet

Now that you understand what is DEF fluid and why it's important for diesel trucks, you might want to know how your drivers can buy it while on the road. Signing up for a fleet fuel card can help drivers access and purchase DEF, among other fuel products, at various gas stations, truck stops and commercial fueling locations across the U.S. Fleet fuel cards are a great way to enable drivers to purchase the products that they need to operate their truck while also giving management control over those purchases. With mobile apps, drivers can easily identify nearby fueling locations that offer DEF so that they don't run out. Purchase limits can be set for each cardholder. As DEF and diesel purchases are made with a trucking fuel card, fleet managers and accounting teams receive detailed invoices showing when purchases occurred and how much was purchased by each driver, among other details. If you manage a fleet of trucks, then sign up today for a fuel card so that you can realize these benefits.

Learn how to track employee fuel spending without gas receipts →

Topics: Fleet Cards
Kira Odlozil

Written by Kira Odlozil

Based in San Diego, CA, Kira Odlozil is the Digital Content Coordinator at P-Fleet. She writes about fuel management, the trucking industry and business-related topics. When Kira isn’t writing, she’s cooking up new recipes, doing yoga, traveling or all of the above.