DEF Diesel Exhaust Fluid: 15 Important FAQs | Fuel Logic

October 10, 2023

What You Need To Know About DEF Diesel Exhaust Fluid


If you run a business in the commercial vehicle or heavy machinery industry, then you're familiar with DEF diesel, an engine additive that helps your engines operate in a more eco-friendly manner.

In most areas across North America, Europe, and other parts of the world, businesses that run diesel equipment are required by law to use Diesel Exhaust Fluid (DEF) to make emissions less harmful.

Most of these regulations are relatively new, so there's still much to be learned about the uses of DEF. In this post, we'll cover everything you need to know about DEF.

15 Important FAQs About Diesel Exhaust Fluid

DEF Diesel

1. What Is Diesel Exhaust Fluid (DEF)?

Diesel Exhaust Fluid is an important solution used to reduce nitrogen oxide emissions from diesel engines. It is a non-flammable, non-toxic, and non-hazardous solution used in selective catalytic reduction (SCR) systems.

DEF is a vital fluid that helps to ensure that machinery and commercial vehicles operate within the emissions standards that help protect the environment.

def fluid

2. What is DEF Made Of?

DEF is a mixture of deionized water and urea. Urea is both pH-neutral and non-toxic and is known for its ability to bind nitrogen, which is why it works so great at capturing the nitrogen compounds in diesel exhaust.

Typically, the ratio of deionized water to urea is 67.5%:32.5%. This ratio is vital to the proper functioning of DEF in the SCR system.

3. What is DEF For in Diesel?

Diesel Exhaust Fuel is used primarily for emission reduction. Diesel engines produce nitrogen oxides during combustion. These emissions are harmful pollutants that have negative effects on the environment. Diesel engines equipped with SCR systems have significantly reduced emissions.

Aside from emission reduction, DEF helps engines to operate more efficiently and perform optimally. The effective management of emissions helps diesel engines to deliver power and fuel efficiently.

4. Is DEF a Harmful Chemical?

No. Diesel Exhaust Fluid is tailored specifically toward the environment's safety and human health. DEF is composed of purified water and urea, a safe compound found naturally in mammal urine. Both these components are non-toxic, so DEF is not considered a harmful chemical.

DEF is also biodegradable, so it's safe for the environment. The quality is regulated by international standards, and it has minimal odor. As such, it is not harmful in any way.

5. How Does Diesel Exhaust Fluid Work?

Diesel Exhaust Fuel works with Selective Catalytic Reduction technology to reduce dangerous emissions from diesel engines. The diesel engine has an SCR system, with the DEF stored separately. When the engine is running, DEF is injected into the exhaust stream in small amounts.

The DEF then mixes with the gases in the exhaust system, where the ammonia from the urea reacts with the nitrogen oxide emissions and breaks them down into nitrogen gas and water vapor, which are safe for the environment.

6. What Are The Storage Requirements For Diesel Exhaust Fluid (DEF)?

DEF is non-toxic, so handling it should be relatively easy. However, DEF is easily contaminated, and contaminated DEF is not only ineffective but could damage your engine. So, what are the storage requirements for DEF?

  • DEF must be stored in a cool and well-ventilated area.
  • DEF must never be stored outdoors, as UV rays and high heat could damage it.
  • Use containers specifically designed for DEF to store it. Repurposing an existing container to store your DEF could contaminate it and ruin the SCR system in your vehicle.
  • Never use DEF as a fuel additive.
  • Any changes in color mean the DEF is contaminated and should be discarded immediately.
  • DEF should only be stored in plastic, rubber, stainless steel, and titanium containers.
def diesel additive

7. How Often Do I Need To Fill Up The DEF Tank?

The frequency of refilling the DEF tank depends on the size of the tank, your vehicle's operating conditions, and the engine's fuel consumption rate. You should regularly check the DEF tank level to ensure you have enough.

Generally, you may need to fill up the DEF tank after a few thousand miles. For a better estimation, refer to your vehicle's monitoring system and documentation.

8. What Happens When I Run Out of DEF?

For most systems, you may not be able to start the engine if the DEF tank is empty. Alternatively, modern diesel engines enter a "reduced power mode" when the DEF levels are critically low. The "reduced power mode" limits the engine's power output and reduces the vehicle's performance.

Fortunately, alerts and warning lights will indicate that DEF levels are low before they get to that point. Monitor your vehicles' DEF levels and refill the tank in time to avoid operational inconveniences.

def diesel fuel

9. Can I Substitute Water For DEF?

No. Water is not a suitable substitute for DEF. Most systems will identify water as low-quality DEF, and the engine won't start. Water does not contain the components needed to convert nitrous oxides to harmless compounds. Essentially, you wouldn't be compliant, and you would be harming the environment.

10. How Long Does Diesel Exhaust Fluid Last?

DEF's shelf life varies depending on the storage conditions. The effectiveness of DEF can be affected by exposure to light, temperature, and contact with contaminants. If stored under the right conditions and in temperatures not exceeding 65 degrees, DEF could last up to 2 years.

11. Can A Diesel Engine Run Without DEF?

Some diesel engines can technically run without DEF, but you will notice a reduced performance as many diesel engines are designed to enter "limp mode" when DEF levels are extremely low.

In some cases, the engine will not run at all if there is no DEF. To avoid dealing with operational inefficiency and costly delays, take DEF-level warnings seriously.

diesel exhaust fluid

12. How Much Does Diesel Exhaust Fluid Cost?

DEF costs roughly $6 to $10 per gallon, depending on factors such as brand, supplier, container size, and region. DEF is mainly priced based on volume, so buying larger quantities may offer better prices.

The prices vary over time due to supply and demand, market fluctuations, and regulation changes, so it's important to compare prices from different suppliers to find the best deal for you.

13. Where Can I Buy DEF?

You have several options when it comes to buying DEF. You can find DEF at your local auto parts store or a fueling station. These options are mainly for when you're in a pinch and need DEF urgently.

It's more convenient to purchase DEF from a wholesale supplier. This way, you're guaranteed cost-effective and high-quality DEF. This is especially important for businesses with a fleet of vehicles or diesel machinery.

You can order DEF from Fuel Logic for quick delivery wherever you are in the United States.

def diesel truck

14. Do All Diesel Engines Require DEF?

Diesel Exhaustive Fuel is used by vehicles with the Selective Catalytic Reduction system, so not all diesel engines require DEF. Older diesel engines might not have SCR systems, so they wouldn't need DEF. DEF is mostly required by diesel engines in vehicles that are required to meet stringent emissions regulations.

15. Why Do I Need DEF?

You need DEF if your vehicle's diesel engine is equipped with SCR systems that help reduce dangerous emissions produced by diesel engines. DEF is especially important for heavy-duty vehicles like commercial trucks. You need DEF to comply with the EPA's standards on emissions.

Best Practices and Tips for Handling Diesel Exhaust Fluid (DEF)

def vs diesel

The safe and proper handling of DEF is important for optimum performance and efficiency. Here are some tips and best practices to help you buy and store DEF properly.

Purchase High-Quality DEF

Ensure you only buy the highest-quality DEF. For DEF to work effectively, it must be in its purest and highest quality. Contaminated DEF is not only ineffective at reducing emissions but could also damage your engines and necessitate costly repairs and replacements.

Never Mix DEF With Fuel

On most vehicles, the inlet for the fuel tank and the inlet for the DEF tank are in the same area, so it's possible to get them mixed up. Mixing DEF with fuel has grave consequences.

Putting DEF in your fuel tank may lead to the engine stopping and cause severe damage. On the other hand, fuel in the DEF tank could damage the SCR system, leading to costly repairs. It's important always to double-check to avoid costly mistakes.

Never Add Water

Since water is one of the components of DEF, you may be tempted to add water to extend your DEF's mileage. This is extremely dangerous for a couple of reasons. 

For one, adding water changes the urea concentration, compromising the DEF's efficiency. Secondly, adding water introduces impurities that may clog and degrade the SCR system, resulting in unplanned downtime and costly maintenance charges.

Only Use Trusted Suppliers

It's crucial to buy DEF that meets the quality standards. Failing to use certified DEF could result in many consequences, like compromising your engines and SCR systems. As such, it is essential only to buy DEF from trusted suppliers.

At Fuel Logic, we offer various fueling services, including DEF supply and delivery. Don't Delay, Order Your DEF Today!

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suite 100, Midlothian, Texas, 76065
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