Mercedes-Benz OM 471 Introduction Into Service Manual page 114

Function
3 Measuring probe (ceramic body)
5 Chamber
6 Heating element
7 Reference air duct
8 Chamber
Ip0 Pump current (main pump electrode)
Ip1 Pump current (auxiliary pump
electrode)
Ip2 Pump current (measuring electrode)
The EATU output NOx sensor (A57 b1) functions according to the
principle of the so>called oxygen ions line according to which the
wide>band oxygen sensor also functions.
Part of the exhaust flowing past the measuring probe (3) passes
through a diffusion barrier into the first chamber (8).
In this case, the O concentration is regulated at a defined value
2
by means of the so>called pump voltage applied to an electrode
until the oxygen content differs on either side of the ceramic
body.
The special properties of the ceramic body create the signal
voltage (Nernst voltage) at its friction planes. This is the measure
for the residual oxygen content in the exhaust.
The traces of HC, CO and H in the exhaust oxygenate at the
2
electrode made out of platinum. The gas then passes over a
further diffusion barrier and arrives in the second chamber (5)
where it is broken down with the aid of a second NOx electrode
into NO and O .
2
i Introduction of engine OM 471 and exhaust aftertreatment > 09/2011 >
– This printout will not be recorded by the update service. Status: 09 / 2011 –
System components
The O concentration is also regulated to close to zero at the same
2
time with the aid of a further electrode. The nitrous oxide
concentration is deduced from the size of the pump current (Ip2)
required.
The EATU output NOx sensor control unit (A57) is used to
compute the required pump voltages, to regulate the complex
system and to compute the NOx raw values. It receives the analog
signals from the EATU output NOx sensor (A57 b1) and digitizes
them. It then forwards them as digital CAN messages to the
exhaust aftertreatment (ACM) control unit (A60) at a defined
transfer rate, which then evaluates them.
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