Diesel Exhaust Fluid (DEF; also known as AUS 32 and marketed as AdBlue) is a liquid used to reduce the amount of air pollution created by a diesel engine. Specifically, DEF is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. DEF is consumed in a selective catalytic reduction (SCR) that lowers the concentration of nitrogen oxides (NOx) in the diesel exhaust emissions from a diesel engine.
Diesel engines are typically operated with a lean burn air-to-fuel ratio (over-stoichiometric ratio) to ensure the full combustion of soot and to prevent them from exhausting unburnt fuel. The excess air leads to the generation of NOx, which are harmful pollutants, from nitrogen in the atmosphere. SCR is used to reduce the amount of NOx released into the atmosphere. DEF from a separate tank is injected into the exhaust pipeline, and the exhaust heat decomposes it to ammonia. Within the SCR catalyst, the NOx are reduced by the ammonia into water and nitrogen, which are both non-polluting. The water and nitrogen are then released into the atmosphere through the exhaust.
- Over-Dosing – too much DEF may not fully hydrolyze, leading to crystalized deposits in the exhaust or injector nozzle
- Contamination – improper storage or handling of DEF can result in dirt entry, rust or tank scale
- Dilution – topping up DEF with water will dilute the solution with the resulting drop in NOx neutralization effectiveness
- Cold Weather – DEF may gel or freeze in colder climates, which may prevent engine operation
- Other Fluids – contamination from other fluids being introduced to the DEF tank (e.g. diesel fuel, coolant, engine oil) can cause damage to the SCR system and engine shutdown
DEF is a 32.5% solution of urea, (NH2)2CO. When it is injected into the hot exhaust gas stream, the water evaporates and the urea thermally decomposes to form ammonia (NH3) and isocyanic acid (HNCO):
(NH2)2CO → NH 3 + HNCO
The isocyanic acid reacts with the water vapor and hydrolyses to carbon dioxide and ammonia:
HNCO + H2O → CO2 + NH3
Overall, thus far:
(NH2)2CO + H2O → 2 NH3 + CO2
Ammonia, in the presence of oxygen and a catalyst, reduces two different nitrogen oxides:
4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O (“standard SCR”) and
6 NO2 + 8 NH3 → 7 N2 + 12 H2O (“NO2 SCR selective catalytic reduction”)
NO + NO2 + 2 NH3 → 2 N2 + 3 H2O (“fast SCR”)
The overall reduction of NOx by urea is then:
2 (NH2)2CO + 4 NO + O2 → 4 N2 + 4 H2O + 2 CO2 and
4 (NH2)2CO + 6 NO2 → 7 N2 + 8 H2O + 4 CO2 and
(NH2)2CO + NO + NO2 → 2 N2 + 2 H2O + CO2
The ratio between NO2 and NO determines which reactions take place and how fast. The highest conversion rates are achieved if equal amounts of NO2 and NO are present, especially at temperatures between 200°C and 350°C. If there is more NO than NO2, fast SCR and standard SCR take place sequentially. If there is more NO2 than NO, fast SCR and NO2 SCR take place sequentially, however, NO2 SCR is slower than standard SCR, and ammonium nitrate can form and temporarily deactivate the catalytic converter.