Diesel Exhaust Fluid (DEF) is a compound of urea and deionized water. It comprises 32.5% urea and 67.5% water. DEF is a critical component in the Selective Catalytic Reduction (SCR) system used to reduce harmful emissions in diesel engines. This helps reduce the air pollution from diesel engines by breaking the nitrogen oxide into harmless nitrogen gas and water vapor.
DEF came into existence due to a requirement by the Environmental Protection Agency
for diesel engines to operate within federal emission standards. To cut down the emissions, the manufacturers added DEF as a reducing agent in the SCR.
The technology works by injecting the fluid into the exhaust over a catalyst. The chemical reaction which breaks down the NOx into the water, carbon (iv) oxide, and nitrogen occur here.