What Is Diesel Exhaust Fluid?

Updated: Jul. 24, 2023

Diesel exhaust fluid, or DEF, is a solution of deionized water and urea. It eliminates noxious nitrogen oxides from diesel engine exhausts.

It wasn’t that long ago when people reviled cars, trucks and buses with diesel engines for their thick, black exhaust fumes. Where I grew up, city buses left commuters gasping for air as they pulled away from the stop. Long-haul trucks were even worse, though thankfully their exhaust pipes pointed up, and not at the vehicle behind them.

Today, you hardly ever see that sooty black smoke anymore, filled with pollutants like nitrogen oxide and nitrogen dioxide — emissions you’d expect from a coal-fired power plant. Credit that to requirements put in place in 2010 by the Environmental Protection Agency.

Vehicles with diesel engines now must have selective catalyst reduction (SCR). A filter removes most of the soot from exhaust fumes, which are then treated with diesel exhaust fluid (DEF). From there, the exhaust passes into a catalytic converter, where the DEF reacts with the exhaust gas to turn it into a harmless combination of nitrogen and water.

Modern diesel vehicles have a DEF reservoir much like the oil reservoir, which must be kept full. As a fail-safe, the vehicle won’t start if the reservoir runs dry.

What Is Diesel Exhaust Fluid?

Diesel exhaust fluid consists of 67.5% deionized water and 32.5% urea. It’s readily available in the United States under several brand names, including Peak Blue DEF and Valvoline Diesel Emissions Fluid,

DEF is sold mostly at truck stops. You can also find it at car parts outlets and some big box stores, like Walmart and Target. It costs about $30 per gallon. One gallon will treat 50 to 200 gallons of fuel, depending on the vehicle. The DEF reservoir in diesel vehicles usually has a blue cap.

What Does Diesel Exhaust Fluid Do?

DEF significantly reduces nitrogen oxide and nitrogen dioxide emissions, which together form a class of pollutants called nitrogen oxides. These pollutants form a highly reactive, brownish gas that turns the air yellow in cities with lots of vehicular photochemical smog.

Nitrogen oxides also react with other volatile organic compounds in the atmosphere to produce ozone, contributing to health issues like coughing, wheezing, reduced lung function and asthma. They’re also a component of acid rain.

What Is DEF Made Of?

As mentioned above, deionized water and urea. Urea may sound familiar because it’s eliminated from the human body in urine. The urea used to make DEF, however, is synthetic.

Ordinary water typically contains lots of ions, i.e. atoms and molecules with a positive or negative charge, These particles must be removed to prevent them from interfering in the exhaust purification process. It’s done by by passing the water through a membrane or a collection of charged plastic beads.

Urea is a common component of synthetic fertilizers because of its high nitrogen content. It’s produced by combining ammonia and carbon dioxide. Manufacturing facilities are often close to coal and natural gas refineries that produce ammonia as a byproduct.

How Does Diesel Exhaust Fluid Work?

The DEF chemical process occurs in three stages:

  • The heat in the catalytic converter breaks down urea into ammonia and isocyanic acid.
  • Isocyanic acid combines with water to produce carbon dioxide and more ammonia.
  • Ammonia and oxygen react with nitrogen oxide and nitrogen dioxide to produce harmless nitrogen gas, water and carbon dioxide.

Carbon dioxide could be eliminated by substituting pure ammonia for urea. The problem is, ammonia has a low boiling point and must be stored under pressure, creating the risk of leaks. Because urea is stable as a liquid at room temperature and nontoxic, so it’s used in the interest of safety, though it burdens the atmosphere with more carbon dioxide.