NOx sensor

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A NOx sensor is typically a high temperature device built to detect nitrogen oxides in combustion environments such as an automobile or truck tailpipe or a smokestack.

1 Availability
2 Motivating factors
3 Difficulties
- 3.1 Harsh environment
- 3.2 High sensitivity and durability required
4 Conclusion
5 Links

[edit] Availability

Siemens VDO / NGK are in production of a NOx sensor for automotive and truck applications. Several automobile and related companies such as Delphi, Ford, Chrysler, and Toyota have also put extensive research into development of NOx sensors. Many academic and government labs are pushing to develop the sensors as well. The term NOx actually represents several forms of nitrogen oxides such as NO (nitric oxide), NO₂ (nitrogen dioxide) and N₂O (nitrous oxide aka laughing gas). In a gasoline engine NO is the most common form of NOx being around 93% while NO₂ is around 5% and the rest is N₂O. There are other forms of NOx such as N₂O₄ (the dimer of NO₂) but it only exisits at lower temperatures and N₂O₅ for example. Owing to higher combustion temperatures, diesel engines produce even greater quantities of NOx than do spark-ignition gasoline engines.

[edit] Motivating factors

The drive to develop a NOx sensor is pushed by environmental factors. The NOx gases can cause various problems such as smog and acid rain. So many governments around the world have passed laws to limit their emissions (along with other combustion gases such as SOx (oxides of sulfur) , CO (carbon monoxide) and CO₂ (carbon dioxide, CO2 sensor) and hydrocarbons) Companies have realized that a way to minimize NOx is to first detect them and then employ some sort of feedback loop in the combustion process to minimize emissions by means such as combustion optimization or by the regeneration of NOx traps.

[edit] Difficulties

[edit] Harsh environment

Due to the high temperature of the combustion environment only certain types of material can operate in situ. The majority of NOx sensors developed have been made out of ceramic type metal oxides with the most common being yttria stabilized zirconia (YSZ), which is currently used in the decades old oxygen sensor. The YSZ is compacted into a dense ceramic and actually conducts oxygen ions (O^2-) at the high temperatures of a tailpipe such at 400 °C and above. To get a signal from the sensor a pair of high temperature electrodes such as noble metals (platinum, gold, or palladium) or other metal oxides are placed onto the surface and an electrical signal such as the change in voltage or current is measured as a function of NOx concentration.

[edit] High sensitivity and durability required

The levels of NO are around 100-2000 ppm (parts per million) and NO₂ 20-200 ppm in a range of 1-10% O₂. The sensor has to be very sensitive to pick up these levels.

The main problems that have limited the development of a successful NOx sensor (which are typical of many sensors) are: Selectivity, Sensitivity, Stability, Reproducibilty, Response time, Limit of Detection and Cost. In addition due to the harsh environment of combustion the high gas flow rate can cool the sensor which alters the signal or it can delaminate the eletrodes over time and soot particles can degrade the materials.

[edit] Conclusion

The NOx sensing element mentioned above is only a part of the whole device, which also include the packaging and electronics. The development of a very good NOx sensor is very highly desirable and has the potential to serve a large market thus there is a large push to make one work. It is often the most sought combustion gas sensor.