A nitrogen oxide sensor or 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.
Continental Automotive Systems / 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 exists at lower temperatures and N2O5 for example. Although, owing to lower combustion temperatures, diesel engines produce lower Engine-out NOx emissions than do spark-ignition gasoline engines, the inexistent NOx aftertreatment causes diesel engines to emit significantly more NOx at the tailpipe compared to a typical gasoline engine with 3-way catalyst. In addition the diesel oxydation catalyst significantly increases the fraction of NO2 in “NOx” by oxidizing the NO to over 50 % using the excess oxygen in the diesel exhaust gases.