DEF (Diesel Exhaust Fluid) is, at a chemical Level, a combination of deionized water and urea that transform harmful nitrous oxide emissions into harmless water and gas. DEF is the registered trademark of VDA for AUS32, or Aqueous Urea Solution 32.5%
SCR engines use a diesel exhaust fluid (DEF) such as DEF. The fluid is stored in a separate tank in the system and injected into the exhaust stream. This injection causes a chemical reaction in the SCR catalytic converter which turns oxides of nitrogen (NOx) into environmentally friendly components, diatomic nitrogen (N2) and water. As a result, selective catalytic reduction systems are being used as a sustainable and effective way of protecting the environment.