What is DEF? How diesel exhaust fluid works and why it's essential for fleets

Fleet management comes with a massive host of responsibilities. 

From vehicle maintenance to insurance costs, emissions aren’t always top of mind. But with the ability to reduce nitrogen dioxide emissions by as much as 90 percent, DEF is an important consideration for diesel fleets. 

Continue reading to learn more about what DEF is, how it works and how it impacts the environment and your fleet. 

What is diesel exhaust fluid (DEF)?

DEF is a colorless, nontoxic liquid composed of 32.5 percent high-purity urea and 67.5 percent deionized water. It’s used in diesel vehicles equipped with selective catalytic reduction (SCR) systems. When used for refueling, it can help reduce harmful nitrogen oxides (NOx) emissions. 

What is diesel exhaust fluid used for?

DEF's primary use is to chemically convert NOx into harmless nitrogen gas and water vapor. This process is crucial for meeting stringent environmental regulations and emissions compliance standards, such as EPA 2010 and Euro VI, which aim to significantly lower pollutants from diesel engines.

Beyond environmental benefits, using DEF properly contributes to optimal fuel efficiency and vehicle performance. The SCR system, enabled by DEF, allows the engine to be tuned for greater power output without being constrained by NOx emission limits. This means improved fuel economy and sustained engine performance over the vehicle's lifespan, as the engine can operate in its most efficient range. 

What is DEF made of? 

What is DEF made of? Diesel exhaust fluid is composed of 32.5 percent urea and 67.5 percent deionized water. For those who like trivia, urea is considered to be the first organic compound to be synthesized from inorganic chemicals.

Unlike the safe and fairly inert N2, NOx causes a whole host of issues. When inhaled, it can lead to health problems, and it can both create and destroy ozone, causing a number of environmental issues. So, the less NOx in the atmosphere, particularly around the areas where we live and work, the better. DEF is used with diesel engines to achieve this effect.

Gasoline engines run cooler, so they tend not to produce as much NOx as their diesel counterparts. The result of the lower operating temperature for gasoline engines is less pollution from NOx compounds, and therefore, they do not need to use SCR to reduce the NOx in the exhaust.

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NOx and DEF  

Let’s start at the beginning of the process and discuss how the NOx compounds are created in the first place. Diesel engines run hotter than their gasoline-based counterparts. At temperatures above 2,912°F, which diesel engines can exceed during combustion, nitrogen and oxygen react. The reactions follow the Zeldovich mechanism:

N2 + O → NO + N

N + O2 → NO + O

N + OH → NO + H

(N / N2 = Nitrogen  O / O2 = Oxygen  H = Hydrogen  C = Carbon)

Which can further react to create:

2 NO + O2 → 2NO2

Now that we have created NOx compounds, they need to be dealt with. This is where the DEF and a catalyst come in. The DEF is added to the exhaust gas and then passed through the catalytic chamber. The catalyst itself can be composed of a variety of materials such as metals, metal oxides, perovskites and zeolites.

The urea (CO(NH2)2) from the fluid reacts with the NO by:

4NO + 2CO(NH2)2 + O2 → 4N2 + 4H2O + 2CO2

The ammonia (NH3) can then react with the NOx compounds by:

4NO + 4NH3 + O2 → 4N2 + 6H2O

6NO2 + 8NH3 → 7N2 + 12H2O

As you can see, there are a lot of reactions going on to break down the NOx compounds into N2 (nitrogen), CO2 (carbon dioxide) and H2O (water). So, with a little catalytic chemistry, the bulk of the NOx pollution can be reduced in diesel emissions.

So, why not use ammonia in place of urea when urea produces that much CO2? 

If you were paying attention, you would have noticed that the ammonia can react with both NO and NO2 and does not create any CO2. However, urea releases about 0.75 grams of CO2 per gram of urea used.

Well, ammonia has a few problems that make it difficult to use in vehicles:

• Pure ammonia boils at -91.4°F, so to store it at room temperature as a liquid, you would need to keep it at high pressure.
• This type of storage has the potential to leak, particularly in the event of a road accident.
• It does not take a significant quantity of ammonia fumes to create an experience that is unpleasant to downright dangerous.
• In liquid form, ammonia is highly corrosive.

Contrast this to a urea solution, which:

• Can be stored at room temperature and pressure
• Has low reactivity
• Does not evaporate quickly
• Will corrode some metals, which is why it is often stored in plastic
• Is also nontoxic, with urea being used in other applications such as fertilizer, animal feed and even some cosmetic products

With these advantages over ammonia, it’s understandable why urea is the material of choice. As always, there is room for improvement, and we can look forward to seeing this replaced by even better technologies.

How does diesel exhaust fluid work? 

In the SCR system, the hot exhaust gases leave the engine, and a precise amount of DEF is injected into the exhaust stream. The heat from the exhaust causes the DEF to decompose into ammonia (NH3) and carbon dioxide (CO2). This ammonia then travels into the SCR catalytic converter.

Inside the SCR catalyst, the ammonia reacts chemically with the NOx present in the exhaust gas. This reaction converts the hazardous NOx into harmless nitrogen gas (N2) and water vapor (H2O), which are then safely released into the atmosphere.

What happens if you run out of DEF?

Before a vehicle runs out of DEF, there will be warnings and the vehicle will go into what’s called “limp mode” until it’s refilled. Limp mode reduces speed and performance. To ensure optimal performance and compliance with emission standards, modern diesel vehicles are equipped with onboard DEF monitoring systems.

These systems continuously track the DEF level, purity and injection rate, alerting the driver through dashboard lights or messages if the fluid is low or if there's a system malfunction, often initiating limp mode to encourage refilling and prevent excessive emissions.

How often do you need to refill DEF? 

Generally, light-duty diesel vehicles may need a DEF refill every 5,000-7,000 miles, or roughly every three months (often coinciding with oil changes). For a vehicle with a 5-gallon DEF tank, this could mean refilling every third or fourth time you fill up your diesel tank.

The frequency with which you need to refill DEF varies significantly depending on several factors, including:

• Vehicle type and size: Heavy-duty trucks and equipment generally consume more DEF than light-duty diesel vehicles due to their larger engines and more demanding operating conditions.
• Engine size and load: Bigger engines working harder (e.g., towing heavy loads, driving uphill, frequent stop-and-go traffic) will produce more NOx and thus require more DEF.
• Driving conditions and style: Aggressive driving with rapid acceleration, harsh braking and high speeds increases DEF consumption. Smoother, steadier driving can help conserve DEF.
• DEF tank size: Obviously, a larger DEF tank will allow longer intervals between refills.

Does DEF expire? 

DEF does expire and has a limited shelf life. While it's a stable chemical solution, the urea concentrate within it can decompose and degrade over time, especially if not stored under optimal conditions. Generally, DEF has an expected shelf life from the manufacture date of 12 to 18 months when stored properly. 

How to store and handle DEF

Exposure to high temperatures, particularly above 90°F (32°C), can significantly accelerate DEF’s degradation, reducing its shelf life to as little as six months. The ideal storage conditions are:

• In a cool, dry and well-ventilated place
• Out of direct sunlight
• Temperatures ideally between 12°F (-11°C) and 86°F (30°C)
• In plastic to avoid corrosion 

While not classified as hazardous, DEF should be handled with care to prevent contamination and spills. Use dedicated, clean equipment for dispensing, and wear gloves to protect sensitive skin, as prolonged contact can cause mild irritation.

Keep your fleet running clean with Geotab 

Beyond its use in reducing emissions, diesel exhaust fluid is an important component of fleet optimization, keeping diesel fleets running smoothly and efficiently. 

Learn more about fleet optimization with Geotab’s range of fleet management solutions.