What is diesel exhaust fluid?

DEF is a liquid that helps reduce harmful emissions from your diesel engine
An undated image illustrating a diesel exhaust fluid (DEF) tank. — Underhoodservice
An undated image illustrating a diesel exhaust fluid (DEF) tank. — Underhoodservice

If you own or drive a diesel vehicle, you may have heard of diesel exhaust fluid (DEF) and wondered what it is and why it is important. 

DEF is a liquid that helps reduce the harmful emissions from your diesel engine and improve its performance. 

What is diesel exhaust fluid?

DEF is a clear, colourless, and non-toxic solution of 32.5% urea and 67.5% deionised water. Urea is a compound found in urine, but the urea used in DEF is synthetically produced and very pure. 

Diesel exhaust fluid is also known as AUS 32 (aqueous urea solution 32%) or AdBlue, which is also a registered trademark of the German Association of the Automotive Industry.

How does DEF work?

DEF is used in conjunction with a technology called selective catalytic reduction (SCR) that lowers the concentration of nitrogen oxides (NOx) in diesel exhaust emissions. NOx are harmful pollutants that contribute to smog, acid rain, and respiratory problems.

SCR works by injecting DEF into the exhaust stream of the diesel engine. The heat from the exhaust converts the urea in DEF into ammonia and carbon dioxide. The ammonia then reacts with the NOx in the presence of a catalyst and converts them into nitrogen and water, which are harmless gases that are released into the atmosphere.

By using DEF and SCR, diesel vehicles can meet the stringent emission standards set by the Environmental Protection Agency (EPA) and other regulatory bodies around the world. SCR also helps improve the fuel efficiency and performance of diesel engines by allowing them to operate at optimal conditions without compromising on power or torque.

How to use DEF properly?

If your diesel vehicle is equipped with SCR, you must fill up a separate tank with DEF periodically. The amount and frequency of DEF consumption will depend on various factors such as your driving style, vehicle load, ambient temperature, and engine condition. Typically, you will use about 2-3% of DEF for every gallon of diesel fuel consumed.

You can find DEF at most gas stations, truck stops, or auto parts stores. You can also buy it online or in bulk from various suppliers. When buying DEF, make sure it meets the quality standards specified by ISO 22241 or the API Diesel Exhaust Fluid Certification Programme.

To fill up your DEF tank, simply locate the blue cap under your hood or near your fuel filler cap and pour in the fluid using a funnel or a nozzle. 

Do not overfill or spill the fluid as it may cause damage to your vehicle or the environment. If you run out of DEF, your vehicle will warn you with a dashboard indicator or an audible alert. If you ignore the warning and continue to drive without DEF, your vehicle will eventually limit your speed or power until you refill the tank.