DEF

Diesel exhaust fluid is a clear, colorless liquid composed of 32.5 percent automotive-grade urea and 67.5 percent deionized water. It is injected into the exhaust stream of diesel engines equipped with selective catalytic reduction (SCR) technology to convert nitrogen oxides (NOx) into harmless nitrogen and water vapor. DEF does not burn; instead, it reacts chemically with hot exhaust gases to neutralize pollutants before they exit the tailpipe.

DEF became mandatory in the United States in 2010 for heavy-duty diesel trucks, and later for pickup trucks and smaller commercial vehicles, to meet EPA Tier 2 Bin 5 emission standards. A typical heavy-duty truck carries 10 to 15 gallons in a separate tank and consumes approximately 2 to 6 percent of fuel volume depending on engine load and duty cycle. Light-duty diesel vehicles use proportionally less.

DEF is corrosive to copper, zinc, and some aluminum alloys, so storage tanks and lines must be constructed from stainless steel, mild steel, or suitable plastics such as polyethylene. The fluid crystallizes at temperatures below minus 11 degrees Celsius and must be thawed before use; most vehicles include tank heaters. Contamination with dirt, metal particles, or incompatible fluids will damage the SCR catalyst and trigger warning lights, requiring expensive repairs.

Quality matters: DEF must meet ISO 22241 standards for purity and composition. Off-spec fluid or counterfeit products are a common source of SCR system failures. The shelf life is typically one year when stored in sealed containers at moderate temperatures.

The term DEF is now industry standard across manufacturers. It is sold at truck stops, service stations, and automotive retailers, typically in 2.5-gallon jugs or bulk dispensers. Drivers of vehicles equipped with SCR systems must monitor DEF levels alongside fuel and oil, as the system will limit engine power if the tank runs dry.