NOx is reduced at the cathode and the pumped oxygen ion current is a measure for the amount of NOx [5]. The current designs of such NOx sensors are using two pumping cells and chambers. At the first pumping cell, free oxygen is eliminated from the gas by using an oxygen selective electrode. In the second adjacent chamber, NOx is dissociated at a highly catalytic second electrode. The very low amount of NOx (typically < 100 ppm) downstream of a catalyst competes with a high amount of free oxygen. This necessitates careful removal of oxygen from the first chamber without dissociation of NOx at the same time. With high sophisticated electrode materials and a controlled pumping voltage, it is possible to linearly measure 50 ppm NOx in air.
To decrease both fuel consumption and carbon dioxide production and thus contribute to reducing the greenhouse effect, new engines with an excess of air versus the stoichiometric ratio have been developed. As TWC does not operate efficiently when the emission mixture departs from stoichiometry, different solutions have been proposed by car manufacturers [6]. They include either a continuous catalytic reduction of NOx or a chemical trap with periodic regeneration times. Application of a NOx sensor would control the catalyst’s operation and monitor the combustion efficiency of the engine. The requirements for such sensors are similar to those of lambda oxygen sensors presently mounted in stoichiometric engines showed that they function reliably, withstand vibrations, are economical, operate at high temperatures, possess low detection limits and are able to operate in harsh environments, such as the corrosive environment within the engine, containing oxygen with water vapor in the range 3 �C 8 %.
Current NOx sensor research Brefeldin_A and development is focused on either optical or electronic methods for detection. NOx optical sensor technology is among the fastest growing for mechatronic applications, as a result of its versatility, ease of use, high speed, accuracy, and capability for integration in high performance automated inspection systems. A wide range of optical sensors based on different operating principles does in fact already exist. Semiconductor laser based sensors are characterized by important properties such as high sensitivity, reliability, possibility of miniaturization, and fabrication that is compatible with mass production. Further development of optical sensors will begin a new era for online inspection of production processes providing the potential for increased productivity and quality. Some key factors still need to be improved in order to reach a wide market, e.g., beam quality, power, wall plug efficiency, wavelength range, tunability, and maximum operating temperature.