AIR-FUEL RATIO SENSOR ARRANGEMENT

Abstract: An air-fuel ratio sensor arrangement which includes a sensor having a pair of oxygen ion conductive solid electro­ lytes (10, 12), with each of the electrolytes having electrodes (14, 16, 18, 20) formed on opposite surfaces thereof and being laminated to each other. At least one solid electrolyte (10,12) is made of a gas permeable porous material, and the electrode region between the two solid electrolytes (10, 12) is adapted to be indirectly exposed to exhaust gases. A voltage control device (38) ist provided for controlling an excitation voltage applied between the two electrodes (14, 16) of the first solid electrolyte (10) to maintain a voltage generated between the two electrodes (18, 20) of the second solid electrolyte (12) at a predetermined value in response to a temperature of the solid electrolyte. An oxygen concentra­ tion remaining in the surrounding exhaust gases is detected by measuring a pump current flowing through the first solid electrolyte.

Description: AIfi-vLJE . RATIO SENSOR ARRANGEMENT BACKGROUND OF THE INVENTION1 The present invention relates to a sensor arrangement and, more particularly, to an air-fuel ratio sensor arrangement for an internal combustion engine, with a sensor being adapted to generate an output signal in response to a concentration of oxygen remaining in an exhaust gas of the internal combustion engine. Various types of air-fuel ratio sensors have been proposed which employ oxygen ion conductive solid electrolytes; however, the proposed sensors do not successfully operate at temperatures below about 6000C. in addition to such proposes sensors being strongly influenced by the temperature and failing to provide a quick response. In, for example, United States Patent 4,272,379, a sensor is disclosed which includes a pair of solid electrolytes formed with electrodes on respective sides of a cylindrical member, with a gas diffusion hole End two end faces to which the pair of electrolytes are each attached. A reference chamber is defined by a space surrounding by the solid electrolytes and the cylindrical member. With an oxygen sensor of this type, one solid electrolyte is employed as an oxygen pump cell and the other solid electrolyte is employed as an oxygen sensor cell, with the oxygen concentration in the reference chamber being controlled so as to reduce any temperature influences of a pump current, that is, the output signal. In sensor arrangements, a voltage of an excitation power source for the oxygen pump cell is controlled and an electromotive force of the oxygen sensor cell maintains a constant value, with the pump current flowing through the solid elecirolyte of the oxygen pump cell being measured by means of, for example, an ammeter. However, one disadvantage of the above noted arrangement resides in the fact that it is difficult to detect a concentration of oxygen remaining in the exhaust gases with any high degree of accuracy under conditions in which the temperature of the exhaust gases changes abruptly over a large range. SUMMARY OF THE INVENTION: The aim underlying the present invention essentially resides in providing an air-fuel ratio sensor suitable for an internal combustion engine for a vehicle, which sensor is not influenced even if the temperature of the exhaust gases changes abruptly. . In accordance with advantageous features of the present invention, an air-fuel ratio sensor arrangement is provided which includes a sensor having a pair of oxygen ion conductive solid electrolytes, with each solid electrolyte including electrodes formed on both sides thereof and being laminated. A voltage controller is provided for controlling an excitation voltage applied between the two electrodes of one solid electrolyte to maintain a voltage generated between the two electrodes of the other solid electrolyte to a predetermined value in response to a temperature of the solid electrolyte. A detector may be provided for detecting the oxygen concentration remainin in the surrounding exhaust gases by a measuririg of a pump current flowing through one solid electrolyte. Advantageously, in accordance with further features of the present invention, at least one of the two solid electrolytes is made of a gas permeable porous material, and an electrode region between the two solid electrolytes is indirectly exposed to the surrounding exhaust gases. Alternatively, a separate gas permeable porous member may be sandwiched or interposed between the laminated side electrodes of the two solid electrolytes and the electrodes may be indirectly exposed to the surrounding exhaust gases. Advantageously, in accordance with further features of the present invention, the voltage controller for controlling the excitation voltage applied between the two electrodes of the one solid electrolyte includes means for generating a first signal in accordance with the temperature of the solid electrolyte, with the predetermined value of the voltage controller or adjusting means changing in response to the first signal. It is also possible in accordance with the present invention to provide a means for generating or producing a second signal in response to an electromotive force between electrodes of the other electrolyte, with means being provided for comparing the second signal of an electromotive force detecting means with. the predetermined value signal of the electromotive force setting device, with an excitation voltage controller for changing an output voltage of the electric power source in response to the output signal of the comparing means. Advantageously, the pair of solid electrolytes are made of a gas permeable and porous material and, preferably, the solid electrolytes include stablized zircon. It is also possible in accordance with the present invention for either of the pair of electrolytes to be made of a gas impermeable material. Accordingly, it is an object of the present invention to provide an air-fuel ratio arrangement for an internal combustion engine of a vehicle which is not adversely influenced by abrupt temperature changes of the exhaust gases. Another object of the present invention resides in providing an airfuel ratio sensor which functions realiably at a low temperature such as, for example, less than 5000C. A still further object of the present invention resides in providing an air-fuel ratio sensor arrangement which enables a rapid response time. A still further object of the present invention resides in providing an air-fuel ratio sensor which is simple in construction and therefore relatively inexpensive to manufacture. These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purposes of illustration only, several embodiments in accordance with the present invention BRIEF DESCRIPTION OF THE DRAWINGS: : Figure 1 is a partially schematic block diagram of an air-fuel ratio sensor arrangement constructed in accordance with the present invention; Figure 2 is a graphical illustration of the relationship between measured values of a most appropriate or optimum electromotive force corresponding to a temperature of a solid electrolyte; Figure 3 is a graphical illustration of a relationship between temperature and resistivity of a solid electrolyte; Figure 4 is a graphical illustration of a relationship between oxygen concentration and pump current when a voltage is maintained at a predetermined value; Figure 5 is a graphical illustration of a relationship between oxygen concentration and a pump current when a voltage is changed in response to a temperature of the solid electrolyte; ; Figure 6 is a cross-sectional view of an oxygen sensor constructed in accordance with the present invention; Figure 7 is a cross-sectional view of another embodiment of an oxygen sensor constructed in accordance with the present invention; and Figure 8 is a cross-sectional view taken along the line VIII-VIII in Figure 7. DETAILED DESCRIPTION: Referring now to the drawings wherein like reference numerals are used through the various views to designate like parts and, more particularly, to Figure 1, according to this figure, an oxygen sensor arrangement includes an oxygen sensor having a first and second solid electroly t 10, 12, with each solid electrolyte 10, 12 being gas permeable and porous and being made for example, of zirconium stabilized with an eiernent such as. for example, yttrium. The first and second solid electrolytes 10, 12 are formed, on both sides thereof, with porous electrodes 14. 16 and 18, 20 made of. for example, platinum, with the porous electrodes 16, 18 being laminated together in the mannet illustrated in Figure 1. The first solid electrolyte 10 is employed as an oxyen pump cell and the second solid electrolyte 12 is used as an oxygen sensor cell. The lamination and adhesive process of the solid electrolytes 10, 12 is carried out by forming the electrodes on both sides of zirconium sheets. theremally contact-bonding the zirconium sheets and subsequently, sintering the zirconium sheets at a high temperature. The porosity of the solid electrolytes 10, 12 may be changed as required by adjusting the particle size of the zirconium in the sheet state and the intering temperature after the thermal contact-bonding process. The electrodes 16, 18 are buried between or encompassed by the solid electrolBes 10, 12 and the electrodes 14, 20 are exposed to an exhaust gas environment. The electrodes 16, 18 are exposed to the exhaust gas indirectly through a plurality of very fine pores of the first and second solid electrolytes 10, 12. An internal capacity of the electrodes 16, 18 is determined by the porosity thereof, and the electrodes 16, 18 have a very small value because the electrodes 16, 18 have a thickness of several m to several tens of m. As a result, even if the porosities of the first and second solid electrolytes 10, 12 are reduced to restrict the flow rate of oxygen diffusing into the region of the electrodes from the surrounding exhaust gases through the fine pores in the solid electrolytes 10, 12, the oxygen sensor maintains a rapid response of, for example, several tens of a millisecond. Additionally, a pump current Ip flowing through the first solid electrolyte 10 can be reduced by reducing a flow rate of oxygen diffusing into the region of the electrodes 16, 18 so that it is possible to improve the low temperature operability caused by the temperature dependency of the condctivity of oxygen ions of the solid electrolyte 10. The sensor arrangement further includes a variable excitation power source 22 having a cathode 24, connected to the electrodes 16, 18, and an anode 26 connected to the electrode 14. Oxygen, in the region of the electrodes 16, 18. is reduced into oxygen ions, and the ions migrate through the first solid electrolyte 10 to the electrode 14. The ions are then oxidized back into an oxygen gas and discharged into the exhaust gas. At the same time, the ilow rate of the oxygen ions migrating through the first solid electrolyte 16. i.e.. the pump current, Ip, is measured by an ammeter 28. The pump current I? corresponds to the flow rate of.the oxygen ions diffusing from the exhaust gas into the region of the electrodes 16, 18. A reference oxygen concentration in a region of the electrodes 16, 18 is detected in terms of an electromotive force e5 between the two electrodes 18, 20 on both sides of the second solid electrolyte 12. Thus, the first solid electrolyte 10 functions as the oxygen pump cell and the second solid electrolyte 12 functions as an oxygen sensor cell. The electromotive force e, generated by the oxygen sensor cell is detected by means of an electromotive force detector 30, with an electromotive force setting device 32 being operable to set the most appropriate or optimum electromotive force eO according to the temperature of the second solid electrolyte 12. The electromotive force setting device 32 is adapted to generate a voltage having a value dependent upon temperature data T of the solid e1ectrolyte i2 so as to improve ihe temperature influence and low temperalure operation, which are caused by the conductivity cf oxygen ions of the solid electrolyte 12. The .e.=DerRT-re data T of he sod electrolyte 12 can be obtained easily by, for example, a temperature sensor 34 operatively connected to the solid electrolyte 12. As shown in Figure 2, a relationship between the electromotive force e5 of a solid electrolyte and temperature varies in accordance with an effectiveness of the electrode material as a catalyst. Figure 2 provides an illustration of results obtained between measured values when, for example, a platinum sputtered electrode is used to promote the catalyzing effect. As shown in Figure 3, the temperature can readily be determined if the rrsisdvity is known. Therefore, the resistivity of the solid electrolyte 12 is measured to detect the temperature of the solid electrolyte 12 by way ol the graphical illustration of Figure 3 so that a voltage eO having the same magnitude of that of the electromotive force es, corresponding to the temperature shown in Figure 2, may be generated by the electromotive force setting device 32. The electromotive force e5 detected by the electromotive force detector 30 and the electromotive force eO set by the electromotive force setting device 32 are compared by a comparator 36 so that an excitation voltage controller 38 is operated by an output corresponding to the difference, whereby a voltage E of the variable excitation power source 22 is controlled by the excitation voltage controller 38 so that the electromotive force es has a predetermined value equal to that of the electromotive force eO set by the electromotive force setting device 32. In the above described sensor arrangement, the oxygen concentration in the region of the electrode 16, 18 can be controlled to a lower level at a constant ratio than the level in the exhaust gas environment, thereby providing a pump current Ip which is at a level sufficient to respond tothe oxygen concentration in the exhaust gas even at low temperatures. Furthermore, the electromotive force e5 of the oxygen sensor cell is set at a constant value that is dependent upon temperature, and it is; possible to provide an air-fuel ratio sensor which is not very much influenced by temperature. An internal capacity of the region corresponding to the referenced chamber is small so that it is possible to provide an air-fuel ratio sensor which has a rapid response. As shown in Figure 4, illustrating a relationship between oxygen concentration and pump current for different temperature parameters, in a sensor arrangement wherein e5 = 10mV, the sensor retains a sufficient sensitivity to oxygen concentration .even at a temperature or 5000C so that the low temperature operability of the sensor of the present invention is considerably improved. Moreover, as evident from Fig. 4, when the electromotive force yes is constant at l0mV independent of the temperature, the pump current I is extremely dependent upon the temperature. p Figure 5 provides an example wherein the voltage E of the variable excitation power source 22 is varied so that the electromotive force es is 10mV at a temperature of 8000C, 14mV at 7000C, 18mV at 6000C, and 25mV at 5000C. As shown in Figure 5, in these situations, the influence of the pump current I upon temperature is remarkably reduced and, consequently, p even if the temperature of the exhaust gas varies over a wide range, the concentration of oxygen remaining in the exhaust gases can be detected with a higher accuracy if the excitation voltage E is so varied that the voltage e5 has predetermined different values in accordance with variations of the temperature of the exhaust gases. While the oxygen sensor in the sensor arrangement of Figure 1 includes solid electrolytes 10, 12 fashioned of a gas permeable material, it is also possible for one of the solid electrolytes to be formed with a gas impermeable material. In this connection, as shown in Figure 6. solid electrolytes 10, 12 are provided which are fashioned as gas impermeable solid electrolytes, with a gas permeable porous ceramic material 40 being sandwiched or interposed between the electrodes 16, 18 and the respective solid electroyltes 10, 12. By virtue of this arrangement, the electrodes 16 and 18 are indirectly exposed to the surrounding exhaust gases through the ceramic member 40. It is also possible as shown most clearly in Figures 7 and 8 to provide an oxygen sensor wherein holes 42 are formed in the electrodes 16, 18, and the electrodes 16, 18 are indirectly exposed to the surrounding exhaust gases through gas permeable solid electrolytes 44, 46. While We have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to one having ordinary skill in the art and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such modifications as are encompassed by the scope of the appended claims.

Claims: WE CLAIM: 1. An air-fuel ratio sensor arrangement comprising: an air sensor means including a pair of oxygen ion conductive solid electrolytes (10, 12), electrode means (14, 16, 18, 20) formed on the respective sides of each of the solid electrolytes (10, 12), said solid electrolytes (10, 12) being arranged to form a laminated structure wherein one electrode means is disposed along an outer surface of each of the solid electrolytes and two electrode means are interposed between opposed inner surfaces of the solid electrolytes (10, 12), means are provided for enabling a power source to be applied between two electrode means of one of the solid electrolytes, and means for enabling a connection of the two electrode means of the other solid electrolyte with an oxygen concentration measuring means. 2. An air-fuel ratio sensor according to claim 1, wherein said solid electrolytes are formed ofa gas permeable and porous material. 3. An air-fuel ratio sensor arrangement according to claim 2, wherein said material is stabilized zirconium material. 4. An air-fuel ratio sensor arrangement according to claim 1, wherein said solid electrolytes (10, 12) are formed of gas impermeable material, and wherein a gas permeable porous material (40) is arranged between the two electrode means (16, 18) interposed between opposed inner surfaces of the solid electrolytes (10, 12). 5. An air-fuel ratio sensor arrangement according to claim 1, wherein said solid electrolytes (10, 12) are formed of a gas impermeable material, the two electrode means (16, 18) interposed between opposed inner surfaces of the solid electrolytes (10, 12) are laminated to each other, and wherein a further solid electrolyte (40) is disposed at each axial end of the first mentioned solid electrolytes (10, 12), said further solid electrolyte (40) being formed of a gas permeable porous material. 6. An air-fuel ratio sensor arrangement according to claim 5, wherein means are formed in each of the two electrode means (16,18) interposed between opposed inner surfaces of the first mentioned solid electrolytes (14 12) for enabling an indirect exposing of said last-mentioned electrode means (12) to exhaust gases. 7. An air-fuel ratio sensor arrangement according to claim 1, further comprising an electric power source means connected to the two electrode means of one solid electrolyte, means for adjusting an excitation voltage of said electric power source means applied between the two electrode means to maintain a voltage generated between the two electrode means of the other solid electrolyte at a predetermined value, and means for measuring the oxygen concentration remaining in surrounding exhaust gases by measuring a pump current flowing through one solid electrolyte. 8. An air-fuel ratio sensor arrangement according to claim 7, wherein the predetermined value of the voltage adjusting means changes in response to a temperature of the associated solid electrolyte. 9. An air-fuel ratio sensor arrangement according to claim 7, wherein said voltage adjusting means includes means for generating a first signal in accordance with a temperature of one of the solid electrolytes (10, 12) means for producing a signal corresponding to the predetermined value in response to the first signal, means for producing a second signal in response to an electromotive force between the electrode means of. the other solid electrolyte, means for comparing the second signal corresponding to the predetermined value, excitation voltage controller means for changing an input voltage of the electric power source means in response to an output signal of said comparing means. 10. An air-fuel ratio sensor arrangement according to claim 7, wherein said pair of solid electrolytes (10, 12) are made of a gas permeable and porous. material. 11. An air-fuel ratio sensor arrangement according to claim 7, wherein said pair of solid electrolytes (10, 12) includes stabilized zirconium. 12. An air-fuel ratio sensor arrangement according to claim 7, wherein at least one of the pair of electrolytes (10, 12) is made of a gas impermeable material. 13. An air-fuel ratio sensor arrangement comprising a first electrode means formed on one surface of a solid electrolyte adapted to be exposed to an exhaust gas, a porous material surrounding at least an outer peripheral portion of the second electrode, means formed in said second electrode for enabling a commurcaton with the porous material, whereby oxygen ions supplied to said second electrode from said first electrode are returned to an exhaust gas atmosphere through said means for enabling a communication and the porous material as oxygen gas. 14. An air-fuel ratio sensor arrangement comprising a first solid electrolyte (10) provided with a first and second electrode means ( 14, 16) on opposite surfaces thereof, a second solid electrolyte/provided with third (18) and fourth (20) electrode means on opposite surfaces thereof, said first and second solid electrolyte (10, 12) being laminated so as to provide a contact between the second electrode means and the third electrode means, said solid electrolytes being surrounded by a porous material with means being formed between the second electrode and third electrode for enabling a communication with the porous material. 15. An air-fuel ratio sensor arrangement according to claim 14, wherein said means for enabling a communication is a hole formed between the second electrode means and third electrode means.

Inventors: HARADA, KOHJI, MIKI, MASAYUKI, SASAYAMA, TAKAO, SUZUKI, SEIKOU
Applicants: HITACHI, LTD.
Application year: 1983
Main classification: G01N27
IPC codes: G01N27