AdBlue is the German brand name for a clear, non-toxic—though slightly corrosive to some metals—aqueous urea solution used to treat exhaust on modern clean diesel engines. The generic name for a chemically equivalent solution used in the non-European market (predominantly North America) is Diesel Emissions Fluid (DEF).

The primary use of AdBlue and similar DEFs is to be used in conjunction with a Selective Catalytic Reduction (SCR) converter to control oxides of nitrogen (NOx) diesel emissions. On average, NOx emissions are reduced by approximately 80 percent because of this process.

How DEFs Work

The AdBlue solution is comprised of 32.5 percent high purity urea diluted in distilled water and carried onboard the diesel vehicle in a special independent tank. Under the direction of the onboard computer and a NOx sensor, the fluid is pumped into the exhaust stream at the rate of 2 to 4 ounces to a gallon of ultra-low sulfur diesel fuel (ULSD) consumed. There, in the hot exhaust stack, the urea solution is converted into ammonia (NH3) which reacts with NOx in the exhaust. The resulting chemical breakdown and re-bonding of the constituent elements of each reactant produce plain nitrogen and water vapor instead of harmful oxides of nitrogen.

Standardized as Aqueous Urea Solution (AU) 32, the AdBlue solution is trademarked to German company the German Association of the Automotive Industry (VDA), but there is a variety of other DEFs available on the American market including BlueTec by German automotive corporation Daimler AG and the Canadian version H2Blu.

How and Where Is AdBlue Replenished?

Refilling the AdBlue tank is not a do-it-yourself task. Although it is possible to purchase the solution at the retail level, it is generally available only through a dealership or service shop. The systems are designed with a capacity of several gallons (seven to ten) which translates into several thousands of miles. Under normal vehicle operating conditions, the DEF tank needs to be refilled only during regularly scheduled maintenance.

However, as of 2013, trucks and diesel engine cars have been created to allow users to refill their own DEF tanks. As a result, a number of truck stops and gas stations have begun offering a DEF pump next to the diesel fuel pump. You may even purchase small quantities—or order large containers for commercial use—to keep at home.

Although safe to handle and non-toxic, AdBlue can eat through some metals. It is recommended that DEFs be stored at cool temperatures away from direct sunlight and moisture in a well-ventilated area. According to a Cummins Filtration report on the standard, AdBlue freezes at 12 degrees Fahrenheit, but the process of freezing and thawing does not degrade the product as the water in the urea solution will freeze and thaw as the fluid does.