Exhaust gas sensors

Air is normally the reference gas used in the exhaust gas sensor. If the oxygen partial pressure in the engine exhaust gas is known as a function of the engine air/fuel ratio, the theoretical galvanic potential of the sensor is easily determined by the Nernst equation. 

Eddy, David S. Physical Principles of the Zircoiiia Exhaust Gas Sensor, IEEE Trans.Vehicniar Technol. VT-23 (1974), pp. 125-128. 

Titanium and tin are important in terms of photocatalytic and sensor appHcations. There is insufficient space to continue this description here, but the Hterature on these topics can be found elsewhere [222-232]. Tungstated zirconia, for example, is a promising candidate for an ammonia exhaust gas sensor required to control a Urea-SCR (Selective Catalytic Reduction) system in diesel-engined cars [233, 234]. 

Moos R, Muller R, Plog C, Knezevic A, Leye H, Irion E, Braun T, Marquardt KJ, and Binder K. Selective ammonia exhaust gas sensor for automotive applications. Sens Actuactors B 2002 83(1-3) 181-189. 

Logothetis, E.M. 1980. Resistive-type exhaust gas sensors. Ceramic Engineering Science Proceedings 1, 281-301. 

Moos, R. (2005) A brief overview on automotive exhaust gas sensors based on electroceramics. Int. J. Appl. Ceram. Tech., 2 (5), 401-13. 

Riegel, J., Neumann, H. and Wiedenmann, H.-M. (2002) Exhaust gas sensors for automotive emission control. Solid State Ionics, 152—153, 783-800. 

FIGURE 3.3 Demands of the automotive industry for exhaust gas sensors. 

The development of new NO, sensors has been driven by the strong demand of the automotive and combustion industries worldwide. Figure 3.3 shows market trends of the automotive exhaust gas sensors since 2003. The worldwide production of vehicles will presumably reach saturation by 2010 [5,8], in contrast, the automotive exhaust gas sensors maiket is stiU predicted to grow to over 100 million sensors per year in the next few years. Furthermore, approximately one half of all NO, emissions into the environment are currently due to power plants and industrial boilers [6]. Although it has been more than two decades since the commercial use of X-sensors began, the deployment of NO electrochanical sensors in combustion control has been lacking. The requirements for NO sensors to be used in a commercial combustion application are summarized in [7, 8], and they are as follows ... 

A control system capable of optimizing both fuel economy and exhaust gas emissions would be the ideal solution. The structure of such a system was generally shown in [23], where emissions have been considered on the basis of a neural network emission model. The more appropriate control seems to be based upon an exhaust gas sensor guided concept. This would not only allow the best compromise between driver demands, vehicle response, fuel consumption and emissions to be found, but also keep the vehicle s emission level at the lowest possible deterioration versus life time. 

Pollution legislation continues driving further reductions in engine emissions. The dominant exhaust-gas sensors today and in the near future are oxygen partial pressure sensors - also called lambda sensors. Due to the high temperature of exhaust gas, these sensors are made by ceramics technology in combination with thick-film processing. 

Today s market for exhaust-gas oxygen sensors is about 90 million units, growing to more than 110 million units and to a market value of about 1.5 billion by 2009. The new types of exhaust-gas sensors mentioned above are too new to allow estimating their market penetration in the next generations of systems with any certainty - nevertheless, these new sensor types will finally generate their market. 

This section gives an overview of the materials and technologies used in current and future exhaust-gas sensors. Because of the dominant role of ZrOz-based oxygen sensors, this sensor type is used as the main example for introducing the various material and technology aspects in more detail. However, we want to cover briefly most other sensor types and alternative materials as well. 

To lead into this broad subject of what materials and technologies are suitable for exhaust-gas sensors, two main questions should be considered first what is valuable to be measured or what sensor type is needed in an exhaust gas and what are the environmental conditions ... 

With multilayer technology being used in exhaust-gas sensors, the complexity is increased by using the third dimension. 

Electrodes and catalytic materials in front of the electrode (pre-catalyst) play an important role in the design of specific functions or characteristics of exhaust gas sensors. For excellent 02 detection in nonequilibrium raw emissions, platinum has become established as the dominant electrode material, because it efficiently supports the ionic-atomic transfer reaction of oxygen. Additionally, it ensures that the raw gas quickly achieves thermodynamic equilibrium, to enable an accurate lambda measurement. Sometimes it can be useful to add additional metals such as Rh, Pd, or Au to improve or prevent NOx reduction or to add ceramic additives such as zeolites or Ce02 to adsorb oxygen or other species [14—16]. 

The exhaust gas sensor business is dominated by two substrate materials Zr02, with its excellent thermal and mechanical properties, and A1203. Although the thermal and mechanical behavior of Al203 is slightly inferior to that of zirconia... 

Electrical heating of the sensor plays an important role in exhaust-gas sensors because almost all sensor principles work only at high temperatures, and the efficiency of the heater determines the time from engine start to control readiness (light-off time). Two types of heaters are used in oxygen sensors. 

H. Weyl, Exhaust Gas Sensors in Automotive Electronics Handbook, chap.6, 2nd ed (R. K. Jurgen, ed.) McGraw-Hill, New York, NY, USA 1999. 

Assembly and Mechanical Design of Automotive Exhaust-Gas Sensors... 

Several stress factors affect an exhaust-gas sensor during its lifetime (Fig. 17.15.12, Tab. 7.15.2). Inside the exhaust pipe at gas velocities up to 80 m/s, high temperature, catalyst poisons, particles, and water are assaulting the sensor. From outside thermal, mechanical, and chemical impacts attack the housing, cable outlet, wire harness, and connector depending on the installation position... 

Tab. 7.15.2 Overview of environmental stress factors for exhaust-gas sensors...

Lampe U., Gerbhnger J., and Meixner H., Comparison of transient response of exhaust gas sensors based on thin films of selected metal oxides. Sens. Actuators B, 7, 787-791, 1992. 

Moos, R. (2005), A Brief Overview on Automotive Exhaust Gas Sensors Based on Electroceramics. International Journal of Applied Ceramic Technology, 2, 401-13. 

---