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What Is DEF? And Other Diesel Exhaust Fluid FAQs

If you operate a fleet of diesel vehicles, drive a diesel-powered personal vehicle, or have offroad equipment with selective catalytic reduction (SCR), you’re already aware of diesel exhaust fluid (DEF). Since Nissan Diesel introduced the first vehicles with SCR back in 2004, increasingly stringent emissions guidelines have caused most diesel manufacturers to adopt these emissions-reducing systems for over-the-road and offroad, as well as installed diesel engines sold in the US.

In conjunction with a diesel particulate filter, a diesel engine with SCR technology runs much cleaner than an otherwise identical engine without these controls. While the filter removes most of the harmful particulate matter the engine produces, the SCR system uses DEF to reduce the amount of dangerous nitrogen oxides (NOx) in the engine’s exhaust.

If you operate diesel engines, you’ve almost certainly purchased DEF to fill up your exhaust fluid tanks. But precisely what is DEF, and how does it help make your diesel engines less harmful to our environment? What is DEF made of? And what happens if you try to operate an SCR-equipped engine without DEF? In this article, we’ll look at the answers to these and other questions about diesel exhaust fluid.

What Is DEF?

Diesel exhaust fluid is a relatively simple product composed of only two ingredients:

  • 5% urea
  • 5% deionized water

Urea

Urea is an organic compound made up of carbon, oxygen, hydrogen, and nitrogen. The livers of most land animals produce urea as part of the process of removing waste products from the body. The urea used in DEF, however, is a fully synthetic product manufactured from ammonia and carbon dioxide.

Deionized Water

Water, in its natural state, is full of different mineral ions, including sodium, calcium, iron, and copper. In order to ensure that the water used in DEF is as contaminant-free as possible, the water is deionized through a process that removes the mineral traces and leaves only pure water.

How Does DEF Work?

Diesel fuel is considered a heavy hydrocarbon, and its combustion tends to generate a lot of soot. To combat the release of soot – excess carbon – from a diesel engine’s exhaust, most diesels run slightly lean, taking in more air than is necessary to provide the oxygen for combustion and helping to burn off the excess carbon.

The unfortunate side effect is that the excess air in the cylinder reacts with combustion byproducts to produce nitrogen oxides (NOx), which are harmful pollutants that can have significant negative impacts on the health of plants, animals, and people nearby.

A selective catalytic reduction system injects a small amount of DEF into the engine’s exhaust stream under carefully controlled conditions. The chemistry behind the reaction is complex, but – in essence – the urea in DEF reacts with the nitrogen oxides in the exhaust and results in three much safer emissions: nitrogen gas (which already makes up more than 70% of our atmosphere), water, and carbon dioxide.

How Does My SCR System Work?

The reactions involved in turning harmful NOx into harmless water, nitrogen, and carbon dioxide are very sensitive to temperature, pressure, and the exact ratio of NO to NO2 in the engine’s exhaust. To ensure that the system is operating at its best, an engine’s SCR system uses sensors to take detailed measurements of the exhaust’s constituents, temperature, and other parameters.

An onboard computer then calculates precisely how much DEF is required to clean up the exhaust, and a pump injects the product into the exhaust at the proper rate.

What Happens If I Try to Run My Engine Without DEF?

You won’t make it very far. Engines equipped with SCR have special controls built in to prevent operators from trying to run their engines without the ecological safeguards provided by the DEF injection.

Most over-the-road engines provide a DEF warning light when the vehicle’s DEF supply is running low. If you don’t stop and fill up with DEF before the supply runs out, your engine will enter what’s known as “reduced power mode.” This mode is exactly what it sounds like – your engine’s performance will be severely limited, and your vehicle may be slowed to a crawl, leaving you limping home at a snail’s pace or calling for a tow.

There have also been rumors circulating that an empty DEF tank can be topped up with water in an emergency. Don’t do it! Adding plain water to an SCR system can absolutely wreck expensive internal components and leave you with a massive repair bill.

Where Can I Buy DEF?

There are three main ways to purchase DEF:

  • In individual, single-use containers of varying sizes at gas stations, auto parts stores, and other retail outlets
  • At the pump at gas stations, truck stops, and cardlocks that offer DEF pumps alongside their diesel pumps
  • In bulk, from a trusted local fuel supplier who will provide you with regular deliveries of fresh DEF and a dispensing system for ease of handling

For those operating only one or two diesel vehicles, the first two options – retail DEF – usually make the most sense. For those operating a fleet of diesel vehicles, offroad equipment, or installed applications, bulk DEF may be a great way to keep engines running clean and save money at the same time.

Running Diesels? You Need DEF! Let the Local Pros at Walthall Help!

Whether you’re running a last-mile distribution service with a handful of box trucks or have a fleet of hundreds of rigs, we have bulk DEF solutions to help you keep your diesels running clean and smooth. In addition, we provide bulk diesel, lubricant, and gasoline supplies for over-the-road, offroad, and installed applications. Give us a call today to find out how Walthall Oil Company can work for you! (478) 781-1234

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