Mercedes-Benz OM 471 Introduction Into Service Manual page 215

System components
Function
2 Ammonia blocking catalytic converter
3 SCR catalytic converter
A Exhaust (from DPF)
B Hydrolysis segment
C AdBlue"
D Exhaust gas (end product)
The exhaust (A), which has been pre>cleaned by the diesel
oxidation catalytic converter (DOC) and the diesel particulate
filter (DPF), continues flowing through the mixing tube, the so>
called hydrolysis segment (B). This is where the quantity of
AdBlue" that has been calculated by the exhaust aftertreatment
control unit (ACM) (A60) on the basis of the sensor information is
injected in.
The AdBlue" first converts itself into ammonia (NH
then continues flowing in the direction of the SCR catalytic
converter (3).
The second stage of the reduction process takes place in the
honeycomb structure that it contains. The nitrous oxide molecules
collide with the ammonia molecules (NH
in the form of heat. Only nitrogen (N
left over as products of this chemical reaction, which are not
harmful to the environment
212
) here, and
3
), and energy is released
3
) and water vapor (H O) are
2 2
– This printout will not be recorded by the update service. Status: 09 / 2011 –
The SCR catalytic converter (3) must be at a certain operating
temperature for this process to take place (known as selective
catalytic reduction). This is 250 ∞C for this purpose.
The exhaust, which has had most of the NOx removed by the SCR
catalytic converter (3) passes the ammonia blocking catalytic
converter (2) before it enters the atmosphere.
Here, the platinum on the surface of the ammonia blocking
catalytic converter makes the unused NH
the O molecules.
2
Some of the few NOx molecules that were not reduced in the
previous stage of the process are also reduced to NO and H
W14.40>1588>05
molecules oxidize with
3
O.
2