Selective Catalytic Reduction: Notes
In order to comply with stringent EPA guidelines, the new Selective Catalytic Reduction (SCR) system is installed in the new diesel vehicles from BMW. The M57D30T2 engine complies with the EPA Tier 2, Bin 5 requirements. This allows the new diesel vehicles to be sold in all 50 states.
The SCR system is a recently new development in the automotive industry, but this technology has been in use by coal fired power plants for many years.
The term "selective" indicates that the reducing agent prefers to oxidize selectively with the oxygen contained in the nitrogen oxides instead of the oxygen present in the exhaust gas.
The reducing agent is injected into the exhaust system where it is converted to ammonia and carbon dioxide. The resulting ammonia is used within a special catalyst in the exhaust stream.
The resulting reaction converts the unwanted oxides of nitrogen into harmless nitrogen and water.
The preferred reducing agent in an SCR system is ammonia (NH3 ). However, ammonia by itself is toxic and would not be practical or safe to carry in the vehicle. So, an alternative would be a safer "carrier" substance which, in this case, is a urea/water compound.
Urea, (NH2 )2 CO, is commonly used as a fertilizer and is biologically compatible with groundwater and chemically stable for the environment. This allows urea to be used as the reducing agent in the SCR system. The ammonia is then extracted from the urea during an "on-board" chemical reaction which takes place once the urea is injected into the exhaust system.
The official name for the reducing agent is Diesel Exhaust Fluid or DEF. This is the name that will be used in the owner's manual and in this training material.
See note below:
Important note on DEF
In this training material, there are several terms which are in use for DEF. Some of these terms include reductant, reducing agent or urea/water solution.
The technical name used industry wide is AUS32, which is a urea/water solution of which urea comprises 32.5% of the mixture.
Another term which is used is AdBlue, which is the registered trademark for AUS32. However, there are other producers of AUS32. AdBlue is just one of them.
The AdBlue trademark is currently held by the German Association of the Automobile Industry (VDA), who ensure quality standards are maintained in accordance with DIN 70070 specifications.