DEF (Diesel Exhaust Fluid) is used to reduce harmful nitrogen oxide emissions from diesel engines

DEF (Diesel Exhaust Fluid) is a non-toxic, colorless liquid composed of 32.5% high-purity urea and 67.5% deionized water, used to reduce harmful nitrogen oxide emissions from diesel engines. 

How DEF Works

Selective Catalytic Reduction (SCR): Modern diesel vehicles use an SCR system to inject DEF into the exhaust stream. 

Chemical Reaction: Heat converts the injected fluid into ammonia, which breaks down hazardous nitrogen oxide (\(NO_{x}\)) gases into safe nitrogen and water vapor. 

Emissions Reduction: This process cuts \(NO_{x}\) emissions by up to 90%, helping heavy-duty trucks and diesel vehicles meet environmental regulations.  

Key Operational Facts

Not a Fuel Additive: DEF is stored in a completely separate tank—usually marked with a bright blue cap—and never mixes with diesel fuel. 

Consumption Rate: Vehicles typically consume DEF at a rate of about 2% to 3% of their diesel fuel consumption. 

Freezing Point: DEF freezes at -11°C (12.2°F), but frozen fluid thaws automatically and safely without losing its chemical effectiveness. 

Running Out: Letting the DEF tank run completely empty prevents the engine from restarting or forces the vehicle into a low-speed "limp mode" to ensure compliance.