The most recent area of NOx sensor application are urea-SCR systems for light- and heavy-duty diesel engines. To satisfy various OBD (on-board diagnostics) requirements, SCR systems typically use a NOx sensor downstream of the SCR catalyst. If excessive NOx or ammonia concentrations exist at the SCR outlet, an OBD malfunction will be triggered, as NOx sensors are sensitive to both gases. Depending on the SCR control strategy, another NOx sensor may be installed in front of the SCR catalytic converter. If two sensors are installed, the conversion rate of the SCR catalytic converter can be easily determined.

  Further development of NOx sensors is driven by future heavy-duty engine emission standards such as those being proposed by CARB and the US EPA for 2027. The NOx limits may be lowered to values as low as 0.015 g/bhp-hr, while the durability and useful life requirements could be extended up to 850,000 miles (1,360,000 km) and 18 years. Improved sensor performance would not only be required for potential changes to OBD thresholds but also for in-use emissions monitoring that is being proposed as an alternative to the more conventional durability demonstrations. NOx sensor technology would need to develop further to be able to monitor emissions at low NOx levels, over the whole duty cycle of heavy-duty vehicle operations, and over their entire useful life.

  The most common in-situ NOx measurement technology relies on yttrium-stabilized ZrO2 (YSZ) electrochemical sensors , similar in construction and operating principle to broadband oxygen sensors. Commercial sensors are available from Continental/NGK  and Bosch , while others such as Denso have sensor development programs . The YSZ sensors are discussed in detail in the following sections.

The two final sections of this article cover, respectively, new NOx sensor developments and ammonia sensors. The latter technology, based on the same YSZ electrochemical system, has been commercialized in some SCR applications, but its use remains limited.

   Commercial NOx sensors for automotive applications are primarily YSZ electrochemical sensors of the amperometric type. Figure 1 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.

Figure 1. Schematic representation of an amperometric NOx sensor

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