Mercedes-Benz OM 471 Introduction Into Service Manual page 213

System components
Function
2 Ammonia slip catalytic converter
3 SCR catalytic converter
A Exhaust (from electric motor)
B Hydrolysis segment
C AdBlue"
D Exhaust gas (end product)
The exhaust (A) flows through the inlet
tube of the exhaust aftertreatment unit
into the mixing tube, the so>called
hydrolysis segment (B).
Here, the amount of AdBlue" computed by
the exhaust aftertreatment (ACM) control
unit (A60) based on the sensor information
is injected.
The AdBlue" is converted here in a first process step into ammonia
(NH ) and then continues to flow in the direction of the SCR
3
catalytic converter (3).
In the honeycomb bodies contained in it, the second stage of the
reduction process takes place: The nitrogen oxide molecules meet
the ammonia molecules (NH ) - energy in the form of heat is
3
released. Only nitrogen (N ) and water vapor (H
2
as products of this chemical reaction, which are not harmful to the
environment
For this procedure, which is known as selective catalytic reduction,
a certain operating temperature is necessary for the SCR catalytic
converter (3). This is 250 ∞C for this purpose.
210
O) are left over
2
– This printout will not be recorded by the update service. Status: 09 / 2011 –
The exhaust that has already been cleaned from NOx by the SCR
catalytic converter (3) for the most part also flows through the
ammonia slip catalytic converter (2) before it is emitted into the
environment.
Here, the NH molecules that have not been used up are oxidized
3
by the platinum on the surface of the ammonia slip catalytic
converter (2) together with the O
Some of the few NOx molecules that haven not been reduced
during the previous processing step are also reduced to NO and
H O.
2
W14.40>1588>05
molecules.
2