Introduction:
The NOx sensor is mounted directly after the Selective Catalytic Reduction (SCR-catalyst) in the exhaust. The NOx sensor checks whether there is still NOx present in the exhaust gases. If this is the case, the harmful substances in NOx are not sufficiently converted into the harmless nitrogen (N2) and water (H2O). In that case, more AdBlue will be injected.
The NOx sensor is not only used in systems with an SCR-catalyst and AdBlue but also in systems with just a NOx storage catalyst. In the latter system, the storage catalyst will be regenerated by temporarily using a richer mixture; extra fuel is then injected.
Function of the NOx Sensor:
The image shows the components of the NOx sensor and the control unit. The NOx sensor consists of two chambers, two pump cells, and a heating element. The sensor element is made of zirconium dioxide; this material has the property that when an electrical voltage is applied, the negative oxygen ions flow from the negative electrode to the positive electrode.
In the first chamber, a portion of the exhaust gas is collected. The oxygen concentration is lowered there, allowing the nitrogen oxide content in the exhaust gas to be measured. Based on various oxygen levels in the exhaust gas and the reference cell, an electrical voltage can be measured at the electrodes. The control unit of the NOx sensor maintains this voltage at a constant value. This value corresponds to an air/fuel ratio of lambda = 1. To achieve this, oxygen is pumped in or out of the first chamber by the pump cell, and the oxygen concentration in the first chamber can be regulated to a specific value.
The exhaust gas flows from the first to the second chamber (see image). The NOx molecules in the exhaust gas are split at a special electrode into N2 and O2. Because the voltage at both the inner and outer electrodes constantly amounts to 450mV, the oxygen ions move from the inner to the outer electrode. Here, the current flowing through the oxygen pump is indicative of the nitrogen content in the second chamber. Since the current through the oxygen pump equals the nitrogen oxide content in the exhaust gas, the amount of nitrogen oxides can be determined based on this.
The NOx sensor can only function when the temperature of the exhaust gases is high enough. At that moment, no condensed water will be present, and the measurement will not be affected. The measured signal is sent by the control unit of the NOx sensor to the engine control unit.
