Component description for the NOx sensor with controller unit

B14.40-0047-06

Arrangement

1 NOx sensor control unit

4 NOx sensor

The NOx sensor (4) is fastened to the outside of the SCR catalyst converter outlet chamber.

The control unit of the NOx sensor (1) is fastened to a bracket. The two components are connected by means of an electric wire and together form the NOx sensor assembly with control unit (17B08).

Task

The NOx sensor with control unit (17B08) is used to detect the NOx concentration in the exhaust gas and to monitor the emission limits specified by legislation.

The NOx sensor (4) is actually the measuring probe, while the control unit of the NOx sensor (1) is used to calculate the gross NOx values.

B14.40-0178-71

Structure

1 NOx sensor control unit

2 Connection

4 NOx sensor

17B08 NOx sensor with control unit

The structure of the NOx sensor (4) is similar to that of a wide band lambda probe. It contains the basic elements such as: The so called Nernst cell and the oxygen pump cell.

The front part of the measuring probe that remains in contact with the exhaust gas, comprises a metal housing with orifices and a zirconium dioxide made ceramic body, permeable to gas passage. The ceramic body surface is composed by electrodes lined with a thin layer of platinum on both sides. The measuring probe is in contact with the outside air through an reference air channel.

The metal housing protects the inside ceramic body against mechanical stress and sudden temperature increase.

The NOx sensor (4) has an integrated heating element that is used to quickly reach the foreseen service temperature of 800 °C for the chemical processes that occur.

The electric wire between the sensor (4) and the control unit (1) has an approximate length of 60 mm.

W14.40-1350-76

Operation

3 Measuring probe - ceramic body

5 Chamber

6 Heating element

7 Air channel

8 Chamber

Ip0 Current - main pumping electrode

Ip1 Current - auxiliary pumping electrode

Ip2 Current - measuring electrode

The NOx sensor (4) operates according to the conduction principle of oxygen ions, according to which the wide band probe also operates.

One part of the exhaust gas that flows by the measuring probe (3) pass through a diffusion barrier in the first chamber (8).

In this case, the O2 oxygen concentration is regulated to a value defined beforehand , by means of a voltage applied on the electrode, until the proportion of oxygen is different in both sides of the ceramic body. Due to the special properties of the ceramic body a tension signal is created (Nernst tension) on its limiting surface. This is the measurement for the residual oxygen content of the exhaust gas. The parts of HC, CO and H2 contained in the exhaust gas oxidize on the platinum electrodes.

Next, the gas passes by another diffusion barrier and reach the second chamber (5), where they decay into NO and O2 with the help of an electrode. Besides, the concentration of O2 is regulated to close to zero by means of another electrode. The concentration of nitrogen oxide is deducted as from the size of the pumping current (Ip2).

The control unit of the NOx sensor (1) is used to calculate the necessary pumping tension to regulate the complex system and to calculate the gross NOx values.

The same receives the analogical signals and converts them into digital signals. Next, the control unit of the NOx sensor (1) sends the digital signals via the CAN to the control module of the SCR chassis, that on its turn transmits the same to the engine control module (MR).