Exhaust Gas Sensors Research Articles

Exhaust gas sensors for automotive emission control

This report gives an overview on ZrO 2 automotive exhaust gas sensors, the development steps of thimble-type oxygen sensors and the technology and design of planar-type oxygen sensors. Furthermore, advanced exhaust gas sensor systems, catalyst monitoring sensors and future developments are described.

Investigations on selected gallium and strontium doped lanthanum chromites as electrode materials for HC detection

Perovskite-type mixed oxides with the formula LaCr1−yGayO3−δ and La0.95Sr0.02Cr1−yGayO3−δ with y=0.1; 0.2; 0.3 were investigated in view of their usability as electrode materials in exhaust gas sensors for hydrocarbon (HC) detection. The electrode materials were characterized regarding their catalytic activity for propylene and propane oxidation and their potentiometric behavior in oxygen containing gaseous mixtures of these combustibles. The potentiometric measurements with propane indicate higher sensitivities of the strontium doped materials than of the materials without strontium. The comparison of these results with those of the catalytic investigations using propane show, that a lower catalytic activity comes along with a higher sensitivity. The electrodes show non-Nernstian behavior.

Determining the concentration of limited gaseous noxious components in the exhaust gas of internal combustion engines by full stream measurement using titania gas sensors

The prototype of an exhaust gas sensor presented in this article is capable of performing full stream measurements within the exhaust pipe of internal combustion engines. The measurement system consists of a metal oxide gas sensor array and a control unit for sensor temperature control, sensor signal recording and data analysis. Extensive experimental work was carried out with gasoline and diesel engines on engine test benches. The project, supported by the State of Saxony, was performed by GPP Chemnitz mbH, ZfM at the Free TU Chemnitz, ZAFT e.V. at HTW Dresden and ZMD AG Dresden.

Selective ammonia exhaust gas sensor for automotive applications

In order to meet future NOx emission standards for commercial vehicles, so called urea selective catalytic reduction (SCR) systems will be introduced. In the SCR converter ammonia serves as a reducing agent for selective NOx reduction. An ammonia exhaust gas sensor is required to optimize the injected amount of urea and to assure that no ammonia emissions occur. This paper reports on the development of a selective thick film ammonia sensor that can meet the requirements for applications in automotive exhaust gas. Main topic is the development of the functional gas sensitive film. Zeolites were successfully investigated as selective functional materials for this kind of sensors. It is shown, that the type of the zeolite, its module, the working temperature and the working frequency determine sensitivity and selectivity of the sensor. From the most promising zeolite material—zeolite H-ZSM5 with a module of 140, sensors were manufactured and tested in engine test benches. It could be shown, that this kind of sensors is a useful tool to control an ammonia-SCR system and to detect ammonia slip.

Composites of Ce 0.8Gd 0.2O 1.9 and Gd 0.7Ca 0.3CoO 3− δ as oxygen permeable membranes for exhaust gas sensors

The transport and catalytic properties of mixed conducting composites prepared from Gd 0.7Ca 0.3CoO 3− δ (GCC) and Ce 0.8Gd 0.2O 1.9 (CGO) were determined. In total, three compositions with 43, 60 and 75 vol.% CGO have been prepared. The composites have potential applications as oxygen permeable membrane in an amperometric sensor for NO x detection in exhaust gases. At all compositions, three different phases were found in the annealed composite as a result of solid state reactions during annealing. The gadolinium content of the CGO phase was increased. The GCC was transformed into a phase with K 2NiF 4-structure and an additional CoO phase appeared. Electron conductivities were determined in the range 100 to 750 °C. The oxygen ion conductivities were measured by a microelectrode technique between 650 and 750 °C. Permeation measurements were carried out between 850 and 1000 °C. Effective ionic conductivities were calculated from the permeation results. The latter agreed well with the microelectrode data extrapolated to higher temperatures. The composition dependence of the conductivities was explainable by a statistical percolation model. The investigated composites exhibit good oxygen ion conductivities. Typical ionic conductivity values for a composite with 75 vol.% CGO increased from 4×10 −4 (Ω cm) −1 at 650 °C to 4.2×10 −2 (Ω cm) −1 at 1000 °C. The heterogeneous catalysis of surface gas reactions was tested by exposing powdered samples to various gaseous mixtures with O 2, N 2, CO, CO 2, H 2O, SO 2 and propene at temperatures up to 800 °C. No conversion of NO to oxygen and nitrogen as well as no extensive formation of N 2O was detected. It is concluded that the composite membranes are suitable for the intended applications.

Investigations towards the use of Gd 0.7Ca 0.3CoO x as membrane in an exhaust gas sensor for NO x

Application of a material with the nominal composition Gd 0.7Ca 0.3CoO x is considered for use as membrane material in an exhaust gas sensor for NO x . SEM–EDX and XRD measurements revealed that after sintering at 1200°C in air, three co-existing phases are present: (Gd 0.6Ca 0.4) 2CoO x (60 vol.%), GdCoO 3 (26 vol.%) and CoO. Catalytic activities were investigated using temperature-programmed reaction (TPR). Whereas the composite Gd 0.7Ca 0.3CoO x shows no catalytic activity towards NO x reduction, as required in the targeted application, the single phase components (Gd 0.6Ca 0.4) 2CoO x and GdCoO 3, were found both to be active. The permeation rates exhibited by the composite material, at 1000°C, are considered too low for real application of this material. Oxygen permeation rates through perovskite-structured GdCoO 3− δ and (Gd 0.6Ca 0.4) 2CoO 4− δ , having the K 2NiF 4-structure, were found to be below the range of detection (<10 −11 mol/cm 2 s).

Investigations on selected gallium doped lanthanum chromites as electrode materials for HC detection

Perovskite-type mixed oxides with the formula La1−xGa0.3Cr0.7O3−δ (x=0.2; 0.1; 0.05; 0) were investigated in view of their usability as electrode materials in exhaust gas sensors for HC detection. The electrode materials were characterised regarding their structure, catalytic activity for propene oxidation and potentiometric behaviour in combustible gaseous mixtures. XRD-measurements indicate, that the solid oxide method does not permit lanthanum deficiencies x ≥ 0.05. The electrodes show non Nernstian behaviour.

Physico-chemical investigations on selected gallium doped lanthanum chromites

Investigations on lanthanum gallium chromium mixed oxides of the compositions La1.0Ga0.2Cr0.8O3−d and La0.9Ga0.2Cr0.8O3-d are presented regarding their structure, redox stability, conductivity and catalytic activity for the oxidation of propene. The mixed oxide has perovskite-type structure, high redox stability, an electronic p-type conductivity and low catalytic activity. If the perovskite-type compound has a deficiency of lanthanum, the electronic conductivity, surface area and catalytic activity are significantly higher. The catalytic activity is likely comparable to that of gold. Similar to gold electrodes of solid electrolytes, oxygen electrodes formed with gallium doped lanthanum chromite show a relatively high sensitivity to hydrocarbons such as propene at temperatures of about 700 °C. The mixed oxide is possibly suitable as electrode material for exhaust gas sensors using oxide-ion conducting solid electrolytes.

Improvement on Functions and Performances of Exhaust Gas Sensor and its Electronic Module

This improvement on functions and performances of exhaust gas sensor should be indispensable to qualify the emission regulation of ULEV. The tubular zirconia cell with limited diffusion layer laminated platinum foil heater on it have been prepared for sensor. The electronic module mainly consists of an ASIC and a single-chip micro-computer. By time shared (4msec) feed back control for constant electromotive force (0.5V) and constant cell temperature (700°C), it have been achieved frequency response (20Hz), warm-up time (5sec) and required on board durability.

A new method for on-vehicle detection of catalyst malfunction based upon measurement of non-equilibrated gas mixtures

Legislation is shortly to be enacted that will require the on-vehicle detection of catalyst malfunction. This is not straightforward for several reasons, such as uncertainty over which parameters would give the best indication of catalyst condition, the unavailability of suitable sensors, the passive nature of the catalyst and the need to avoid false fault detection. It is proposed here that measurement of the degree of non-equilibration in the exhaust gases as they exit a faulty catalyst could be used as a suitable criterion for the assessment of catalyst condition. In order to demonstrate the concept, we have prepared several resistive-type SnO 2 thin-film sensors with differing amounts of surface Pt doping. The sensors are exposed to CO/O 2/CO 2 gas mixtures at varying levels of non-equilibration. The highest sensitivities to the level of non-equilibration are shown by the undoped sensors. This is an accordance with our theory that increasing the catalytic activity of the sensor itself decreases the available signal. This is in opposition to all currently available exhaust gas sensors, whose high catalytic activity is necessary for unambiguous indication of the stoichiometric air/fuel ratio, which also discards the non-equilibrated parameter.

Lambda detection with thin-film metal oxides using synthetic exhaust gas mixtures

New exhaust gas sensors based on the gas dependence of the electric conductivity of metal oxide thin films have been developed to realize a cylinder-selective lambda measurement in internal combustion engines. To understand these results, the lambda characteristics of the sensors were measured in synthetic gas mixtures with concentrations similar to the exhaust of an internal combustion engine. The typical lambda characteristics of the sensors show a strong change in resistance near the stoichiometric point, and a slight dependence on the lambda value in the rich (lambda <1) and lean (lambda > 1) regions. The lambda dependence is similar to that of the computed oxygen partial pressure of the used gas mixtures in equilibrium. This allows us to assume that on the sensor surface the thermodynamic equilibrium is established between the gases and the metal oxide determined by the concentration of oxygen vacancies. From a comparison of the computed with the measured lambda characteristic of the sensor materials, it can be assumed that at temperatures of about 1000 °C the resistance of thin-film metal oxides is determined by the concentration of oxygen vacancies as a function of the oxygen partial pressure. A dispersion of catalytically active platinum can enhance the reaction rate to establish the equilibrium even at temperatures lower than 1000 °C.

A proportional air-fuel ratio zirconia oxygen sensor

A novel time-sharing control circuit is used to convert a linear lean exhaust gas oxygen sensor into a proportional extended range (lean or rich exhaust) sensor. The time-sharing oscillator allows a single electrochemical cell to act as both the voltage sensor and as an oxygen pump, thus reducing the complexity of sensor geometry and fabrication compared to the universal exhaust gas oxygen (UEGO) sensor design. The control circuit has been demonstrated in the laboratory using two forms of commercially available ZrO 2 cells: (1) a design with a small chamber that opens to the exhaust by a small hole; (2) a design that provides for a limited diffusion region by use of a porous ceramic layer over the outer electrode. The porous-layer design (or a ceramic coating with one or more holes) appears to be a potential candidate for a linear exhaust gas sensor where interest is primarily in a linear response around stoichiometry.

New generation of automotive sensors to fulfil the requirements of fuel economy and emission control

The new passenger car regulations for future emission levels and the increasing demands for fuel economy have led to strong efforts to develop new engine-management systems, exhaust-treatment technologies and new automotive sensors. In this paper a new generation of automotive sensors that help to fuffil the requirements for fuel economy and emission control is presented. The demands on these new sensors include the following improvements: (i) better resolution, higher accuracy and better long-term stability, especially under severe environmental conditions; (ii) good performance at high temperature (e.g., up to 150 °C for rotational speed sensors and 1100 °C for exhaust gas sensors); (iii) short response times for dynamic cylinder-selective measurements; (iv) more efficient compensation methods by measuring different physical parameters. This talk gives an overview on the physical principle, the design, the technology and the application of a new generation of automotive sensors. Representing a potential ‘high-volume’ sensor, we report on an active rotational speed sensor to measure the angular position of the crankshaft. This sensor is based on a newly developed differential Hall IC. The high angular resolution allows more accurate ignition timing as well as ‘misfiring’ detection to be achieved. For the precise control of combustion in future engine-management systems, an ‘air flow sensor’ for cylinder-specific carburation, a ‘combustion pressure sensor’ for thermodynamic calculation of the combustion and a ‘fast exhaust gas sensor’ for cylinderselective exhaust-gas analysis can be used. This report will focus on the development of a fast oxygen sensor to measure the oxygen partial pressure of the exhaust gas. As an example of a smart sensor, an ‘alcohol fuel sensor’ for alternative fuel concepts is introduced. This sensor is a key component of a flexible fuel system which can be run on arbitrary mixtures of gasoline and methanol. This sensor measures three physical parameters: the capacitance, the conductivity and the temperature, in order to determine the methanol/gasoline ratio in the fuel rail with high accuracy.

Oxygen gas sensors based on CeO 2 thick and thin films

Ceria is a promising material for resistive-type oxygen exhaust gas sensors. This paper describes the preparation of ceria thick and thin films and the development of prototype sensors with an additionally integrated platinum temperature sensor and a sensor heater. The CeO 2 films exhibit very good adhesion, high thermal shock resistance and no apparent aging under changing gas compositions and at temperatures up to 1000 °C. The electrical conductivity of CeO 2 films as a function of the oxygen partial pressure and the temperature behaves analogously to that of bulk material. In a comparison of model calculations with measured data, it is shown that, depending on the material composition, CeO 2 sensors are suitable for closed-loop combustion control of stoichiometric or lean air/fuel mixtures. Response times from 5 to 10 ms can be realized with thin- or thick-film CeO 2 sensors.

Platinum wire wedge bonding: A new IC and microsensor interconnect

Ultrasonic ball or wedge bonding of Au or Al has been the traditional method for mak-ing electrical interconnects between die and chip header for most Integrated Circuit (IC) and microsensor devices. Electrical interconnections made from these materials may, however, be unsuitable for some device applications. Some microsensors (e.g., oxygen exhaust gas sensors) can be subject to temperatures as high as 900° C in both oxidizing and reducing atmospheres, conditions for which the properties of Au or Al are unsuit-able. In this report we describe a new means of making electrical connections between die and chip header. Interconnects are made by using a conventional wire bonder to ultrasonically wedge bond 0.032 mm diameter Pt wires to both Pt and Al thin films. Interconnects made in this manner are remarkably strong, as compared with Au wire bonds, with pull strengths of ∼13 g. Electrical measurements show contact resistances of ≲0.05 Ω. Annealing tests show that bonds made to thin metal films of Pt on Ti on sapphire show no appreciable signs of electrical or mechanical degradation after an-neals at 900° C for 1 h in pure oxygen.

Silicon operating system standardizes software: C. McMinn, R. Markowitz, J. Wharton and W. Grundmann. Electronics 135 (8 September 1981)

This chapter discusses methods involving the use of oxygen or air/fuel ratio exhaust gas sensors to determine the oxygen storage capacity of a catalytic converter by Ford Motor Co. Since 1992, Ford has presented a significant number of methods for diagnosing the deterioration of catalytic converters. Most of these methods have been applied to an engine layout. A controller is presented in the chapter as a conventional microcomputer including a microprocessor unit, input ports, output ports, read-only memory (ROM) for storing the control program, random-access memory (RAM) for temporary data storage, which may also be used for counters or timers, keep-alive memory for storing learned values, and a conventional data bus. The controller receives various signals from sensors coupled to the engine, including the measurement of inducted mass air flow from a mass air-flow sensor, engine-coolant temperature from a temperature sensor, and the indication of engine speed (rpm) from a tachometer.

Physical principles of the zirconia exhaust gas sensor

A brief description of the zirconia exhaust gas sensor is given. The electrochemical principles of the sensor are reviewed. Starting from the Nernst equation, theoretical relations are derived which relate the galvanic potential of the sensor to the composition of the exhaust gas from an internal combustion engine. Theoretical curves of galvanic potential versus the inlet air-fuel ratio are shown and discussed. The utility of the zirconia exhaust gas sensor is defined in applications such as closed-loop engine control systems.