NOx and NH3 Sensors

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Automotive NOx sensors are primarily of the amperometric type, with two or three electrochemical cells in adjacent chambers. The first cell electrochemically pumps O2 out of the sample so it does not interfere with the NOx measurement in the second cell. Commercial sensors, available from several suppliers, are used for the control of NOx adsorber and SCR aftertreatment. NH3 sensors have been also developed for use in SCR systems.

NOx Sensors

The development of exhaust NOx sensors started in the 1990s. Commercial sensors were first introduced in 2002 for lean-burn gasoline passenger cars with NOx adsorbers. Since 2005, their application was extended into diesel SCR systems. The most common in-situ NOx measurement technology relies on Y-stabilized ZrO2 electrochemical sensors [984], similar in construction and operating principle to the wide band oxygen sensors. Figure 1 shows the NOx sensor known as UniNOx or Smart NOx sensor, developed and manufactured in cooperation between NGK Insulators (ceramic sensing element) and Continental (electronic control unit). Sensors are available from other suppliers such as NGK Spark Plug [3737] and Bosch [3740] while others such as Denso, have sensor development programs [3739][3738].

[photo]
Figure 1. Continental VDO UniNOx sensor

(Source: VDO)

The performance parameters of NOx sensors have improved as the technology matured. In the early 2000s, engine manufacturers were calling on sensor suppliers to provide NOx sensors with a sensitivity of 40-20 ppm NOx [622]. Within a decade, NOx sensor technologies under development had sensitivity targets of ±5 ppm NOx and ±5 ppm NH3 [2571].

Commercial NOx sensors for automotive applications are primarily of the amperometric type. Figure 2 illustrates the basic operating principle. The sensor uses two or three electrochemical cells in adjacent chambers. The first cell electrochemically pumps O2 out of the sample so it does not interfere with the NOx measurement in the second cell. The need to remove O2 allows this type of NOx sensor to serve a dual purpose; it can also detect exhaust O2 level. The O2 in the first cell is reduced and the resulting O ions are pumped through the zirconia electrolyte by applying a bias of approximately -200 mV to -400 mV. The pumping current is proportional to the O2 concentration. The remaining gases diffuse into the second cell where a reducing catalyst causes NOx to decompose into N2 and O2. As with the first cell, a bias of -400 mV applied to the electrode dissociates the resulting O2 which is then pumped out of the cell; the pumping current of the second cell is proportional to the amount of oxygen from the NOx decomposition. An additional electrochemical cell can be used as a Nernstian lambda sensor to help control the NOx sensing cell. All HC and CO in the exhaust gas should be oxidized before the NOx sensing cell to avoid interference. Also, any NO2 in the sample should be converted to NO prior to NOx sensing to ensure the sensor output is proportional to the amount of NOx [3741].

[schematic]
Figure 2. Schematic representation of an amperometric NOx sensor

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