Diesel exhaust fluid
Diesel exhaust fluid (DEF) is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. It is standardised as AUS 32 (aqueous urea solution) in ISO 22241. DEF is used as a consumable in selective catalytic reduction (SCR) in order to lower NOx concentration in the diesel exhaust emissions from diesel engines.
The German Association of the Automotive Industry (VDA) registered the trademark AdBlue for AUS 32.
Diesel engines can be run with a lean burn air-to-fuel ratio (overstoichiometric ratio), to ensure the full combustion of soot and to prevent them exhausting unburnt fuel. The excess of air necessarily leads to generation of nitrogen oxides (NO
x), which are harmful pollutants, from the nitrogen in the air. Selective catalytic reduction is used to reduce the amount of NO
x released into the atmosphere. Diesel exhaust fluid (DEF) from a separate tank is injected into the exhaust pipeline, where the aqueous urea vaporizes and decomposes to form ammonia and carbon dioxide. Within the SCR catalyst, the NO
x are catalytically reduced by the ammonia (NH
3) into water ( H2O) and nitrogen (N
2), which are both harmless; and these are then released through the exhaust.
DEF is a 32.5% solution of urea, (NH
2CO. When it is injected into the hot exhaust gas stream, the water evaporates and the urea thermally decomposes to form ammonia and isocyanic acid:
2CO → NH
3 + HNCO
The isocyanic acid hydrolyses to carbon dioxide and ammonia:
- HNCO + H2O → CO2 + NH
Overall, this is
2CO + H
2O → 2 NH
3 + CO2
From this point, ammonia, in the presence of oxygen and a catalyst, will reduce nitrogen oxides:
The overall reduction of NO
x by urea is:
- 2 (NH
2CO + 4 NO + O
2 → 4 N
2 + 4 H
2O + 2 CO2 and
- 2 (NH
2CO + 3 NO
2 → 2 1⁄2 N
2 + 4 H2O + 2 CO2
Operation in winter time
DEF freezes at −11 °C (12 °F). In order for the SCR exhaust cleaning system to function at low temperatures, a sufficient amount of the frozen DEF must be melted in as short time as possible, preferably of the order of minutes. For example, 2010 EPA emissions requirements require full DEF coolant flow within 70 minutes.
Typically, the frozen DEF is melted by heat from the engine, e.g. engine coolant passing through the DEF tank, governed by a thermostatic coolant control valve. This method may take significant time before the SCR exhaust cleaning system is fully operational, often up to an hour.
Another method to thaw DEF (and thus allow for full SCR operation) is to integrate an electrical heater into the DEF tank. This heater must be sized, positioned, and powered adequately to rapidly melt sufficient frozen DEF. It should preferably be self-regulating not to overheat if (part of) the heater is outside of the liquid. It should also preferably be self-regulating to eliminate any complicated sensor and temperature regulating systems. Furthermore, the heater should not exceed 50–60 °C (122–140 °F), as DEF begins to decompose around 60 °C (140 °F). PTC heaters are often used to achieve this.
SCR systems are sensitive to potential chemical impurities in the urea solution, therefore the solvent is demineralized water. The urea solution is clear, non-toxic and safe to handle. However, it can corrode some metals and so must be stored and transported carefully.
DEF is stored in a tank on board the vehicle, and injected into the exhaust stream by a metering system. The injection rate depends on the specific after-treatment system, but is typically 2–6% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimises the tank's size (and subsequent obtrusion into vehicle packaging space). An electronic control unit adjusts the addition of fluid in accordance with such parameters as engine operating temperature and speed.
Diesel exhaust fluid is offered to consumers through a variety of quantities by manufacturers ranging from containers of it for single or repeated small usage, up to bulk carriers for consumers that require a large amount of DEF. It is recommended that DEF be stored in a cool, dry, and well-ventilated area that is out of direct sunlight. As of 2013, a number of truck stops are beginning to add DEF pumps, in which diesel exhaust fluid is administered at pumps similarly to diesel, and often located adjacent to fuel pumps so that the vehicle operator can fill up on both without moving the truck.
- "BlueHDI" tradename used for Group PSA's own selective catalytic reduction system used in Peugeot, Citroen, and DS Automobiles brands.
- "BlueTec" tradename used for Daimler AG's own selective catalytic reduction technology using DEF.
- "Hino Standardized SCR Unit". Hino Motors. Retrieved 30 July 2014.[permanent dead link]
- "The DPR Future" (PDF). Hino Motors. Retrieved 30 July 2014.
- "ISO 22241-4:2009 Diesel engines — NOx reduction agent AUS 32". ISO (International Organization for Standardization). 2009-08-01.
- "What is DEF?" (PDF). Cummins Filtration.
- "How it works". H2Blu.
- EP 2551009, "Diesel exhaust fluid formulation having a high ammonium content and a low freezing point", published 30 January 2013
- Panayi, Adam (2017). "Frequently Asked Questions". discoverdef.com. Retrieved 29 December 2017.
- "cumminsengines.com/def-equipment-delivery-and-storage". DEF Equipment Delivery & Storage - Cummins Engines. Cummins Inc. 2017.
- "The 5 Facts and Fiction of Diesel Exhaust Fluid". Victory Blue - Diesel Exhaust Fluid. 2016. Retrieved 29 December 2017.
- VDA website entry on Adblue
- FactsAboutSCR.com North American SCR Stakeholders Group
- "NOxBLUE Diesel Exhaust Fluid Material Safety Data Sheet" (PDF). November 2010.
- "AdBlue Material Safety Data Sheet" (PDF). September 2014.