The Environmental Protection Agency (EPA) has very stringent emission standards for diesel engines. To comply, diesel engine builders use a selective catalytic reduction process with diesel exhaust fluid to reduce NOx emissions.
Wondering what diesel exhaust fluid is?
Can your diesel engine run without it?
Stick with us.
We’ve got all the details you’ll need on diesel exhaust fluid.
This Article Contains
- What Is Diesel Exhaust Fluid? (And What Is It For?)
- How Diesel Exhaust Fluid Works
- 6 Essential Details to Know About Diesel Exhaust Fluid
- What Your DEF Warning Light Means
- 4 Diesel Exhaust Fluid FAQs
What Is Diesel Exhaust Fluid? (And What Is It For?)
Diesel exhaust fluid (DEF) is a clear liquid that reduces the harmful emissions from diesel-powered vehicles and heavy-duty machinery through a selective catalytic reduction process. It is a urea solution regulated by the American Petroleum Institute (API), comprising 67.5% deionized water and 32.5% urea.
Here’s a breakdown:
Urea is a refined compound also found in fertilizers, and deionized water is, well, water without ions.
DEF products must be ISO 22241 compliant to be sold, so they’re the same across the board regardless of brand. API certification is optional, acting as additional accreditation to show that these diesel additives meet the necessary purity. Products meeting API standards will display the API certification mark for DEF on their product description.
If you hear terms like AdBlue or BlueDEF — these are DEF brands.
Now that we know what DEF fluid is, let’s see how SCR technology uses it.
How Diesel Exhaust Fluid Works
Today, modern diesel engines offer increased fuel economy and reduced emissions by combining processes like selective catalytic reduction (SCR) and exhaust gas recirculation.
DEF doesn’t go straight into your fuel delivery system (as in, you don’t mix it with your diesel fuel like other diesel additives.) Instead, vehicles with SCR technology have a separate fuel tank and a DEF tank. The DEF tank is insulated, heated, and marked by a blue filler cap, and DEF delivery occurs at the exhaust side of combustion.
Here’s how the SCR system breaks down the harmful NOx emissions (nitrogen oxide) into safer and water vapor:
- First, the engine’s exhaust gas moves through a particulate filter, which captures particulate matter like soot and ash from burning impure fuel.
- A nozzle then sprays diesel exhaust fluid into the exhaust stream of gasses.
- Next, the hot exhaust gas-DEF solution enters your catalytic converter.
- The urea and nitrogen oxide in the exhaust gasses react with various metallic catalysts inside your catalytic converter.
- The heat in the catalytic converter turns DEF’s urea into isocyanic acid and ammonia.
- The resulting isocyanic acid mixes with water (from the DEF solution), producing more ammonia.
- Ammonia and oxygen then react with nitrogen oxide to produce harmless nitrogen gas, water vapor, and carbon dioxide.
- These engine emissions then leave your vehicle’s exhaust and enter the atmosphere — and that’s how NOx emissions are neutralized.
This process is similar to how your gasoline car’s catalytic converter works to meet emission standards. The only difference is that modern diesel engines inject urea into the exhaust stream.
Keep scrolling to learn more about DEF!
6 Essential Details to Know About Diesel Exhaust Fluid
Here are some important aspects of diesel exhaust fluid:
1. Only Use Recommended Materials When Handling DEF
Think of DEF fluid as highly allergic to everything other than titanium, rubber, plastic, and stainless steel. It’s best to stick to ISO-approved containers, possibly of polypropylene or high-density polyethylene plastics. If DEF comes into contact with incompatible compounds, there’s a high risk of contamination.
You must strictly avoid brass, copper, epoxy resins, and nickel for DEF packaging and equipment. Also, avoid reusing DEF containers as they can contaminate the DEF.
2. Freeze/Thaw Cycles Won’t Degrade DEF
Although DEF shelf life is affected by temperature, freezing or thawing cycles won’t degrade your DEF — meaning, it’s OK to use frozen DEF thawed. Remember that DEF is ⅔ water, so it will freeze. Also, the DEF tank is wrapped in heating coils to let DEF thaw rapidly once you start your engine.
Don’t add antifreeze chemicals or additives to keep DEF in liquid form. This will just contaminate it.
3. DEF Likes Cool Shaded Spots for Storage
You should store DEF in a cool, dry, well-ventilated area that’s out of direct sunlight. While freezing will not damage DEF, prolonged exposure to heat and sun will degrade it.
If you store DEF between 10 and 90℉, it could last up to a year. However, if the DEF maximum temperature doesn’t exceed 75℉, its shelf life could extend to approximately two years.
Always follow the product information and safety data sheet for storage and shelf life.
4. Always Dispense Directly from the DEF Container
Take extra care when dispensing DEF. Ensure the DEF container nozzle and the DEF tank fill-point is clean. Avoid using funnels or other apparatus to transfer DEF — even if these appear clean, they could still have contaminants that could transfer into the DEF tank.
5. Color Change Indicates Contaminated DEF
Anything other than a clear liquid means that you have a contaminated fluid. If it’s colored or cloudy, it’s contaminated. For example, when DEF comes into contact with brass or copper, it’ll appear light blue; when in contact with steel or galvanized steel, it will turn a rusty color.
6. Contaminated DEF Can’t Be Restored
Once DEF gets contaminated, you can no longer use it. You must replace it to ensure your diesel vehicle meets the emission requirements and doesn’t experience further damage.
Since you now know the basics of diesel exhaust fluid, let’s learn more about your DEF dashboard light.
What Your DEF Warning Light Means
By now, you should have a good understanding of your SCR system. Any abnormality in the SCR system triggers your DEF light. An SCR system malfunction can affect your engine’s performance, so you must diagnose the issue ASAP.
Here are some common issues that trigger your DEF light:
- A low DEF level
- A malfunctioning DEF sensor
- A faulty DEF injector
Your best bet for resolving the DEF dashboard light issue is to call a mechanic. They can run diagnostics, make the proper repairs, and ensure that your vehicle is neutralizing harmful NOx emissions as it should.
Still have some unanswered questions about diesel exhaust fluid?
Let’s dive right in.
4 Diesel Exhaust Fluid FAQs
Here are the answers to some frequently asked questions regarding exhaust fluid:
1. How Often Should I Fill My DEF Tank?
For standard diesel-powered vehicles, you should fill your DEF tank once every 3 to 4 times that you pump diesel fuel. The frequency can vary with operating conditions and your fuel consumption.
For a light-duty diesel truck, it depends on miles per gallon and its usage. According to some OEMs, a light-duty truck will use around 2-3 gallons of DEF every 800 miles.
However, trucks with an average miles per gallon rating of 20+ can go around 8,000 to 10,000 miles on a tank full (10 gallons) of DEF.
2. Can My Diesel Engine Run Without DEF?
Well, yes and no. Here’s why:
The Environmental Protection Agency (EPA) ensures that all diesel engine builders integrate a warning system to notify drivers of the status of their SCR system.
If you ignore this warning, your diesel vehicle will cease to work. Some diesel engines will operate on reduced power (or limp) mode. This precaution limits the number of times you can turn the engine over.
Treat your DEF tank like your fuel tank — top up a low DEF level to prevent being stranded because of an empty DEF tank.
3. Can I Substitute Water for DEF?
Absolutely NOT!
Just like gear oil would not be used as a substitute for engine oil, plain water cannot be used as a substitute for DEF.
Please avoid pouring water into your DEF reservoir, as your diesel vehicle will identify it as poor quality and won’t start. Even if your car starts — water can’t convert harmful emissions into harmless ones. So, you’ll be putting your car and the environment at risk.
4. Where Can I Buy DEF?
You can’t find a DEF supply just anywhere. It’s mostly sold at truck stops in big jugs containing multiple gallons.
Some commercial fuel stations have DEF, but don’t count on it in emergencies since not all fuel stations will stock it. Instead, refill your DEF tank when you can or carry an extra DEF supply in your diesel vehicle for emergencies.
You can also get DEF delivery from online suppliers or buy DEF products directly from retailers like Walmart or Target.
It’s worth noting that DEF brands can mix since they all have to meet API standards. So, if your tank runs out of AdBlue, you can top it up with BlueDEF.
Wrapping Up
Diesel exhaust fluid is essential in diesel engines, especially if you want to comply with emission regulations and keep your vehicle running smoothly. If you need help with your DEF tank, require diesel engine repair, or some other maintenance like an engine oil change — call AutoNation Mobile Service.
AutoNation Mobile Service’s expert mobile mechanics come straight to you!
Whether you’re driving a diesel pickup or heavy-duty vehicle, all our repairs come with a 12-month/12,000-mile warranty.
Contact us for a consultation using our online booking service.