JP2004108373A - Emission gas concentration detecting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an emission gas concentration detecting device using a gas sensor capable of accurately measuring a gas concentration for a long period of time. <P>SOLUTION: This device is provided with a NOx occlusion catalyst or a NOx selection reduction catalyst arranged in an exhaust pipe of an internal combustion engine, a NOx sensor attached in the downstream of the NOx occlusion catalyst or the NOx selection reduction catalyst in the exhaust pipe and detecting the concentration of NOx in emission gas, an operating condition establishing means temporarily making an air fuel ratio rich for purifying NOx occluded by the NOx occlusion catalyst, and a means detecting the deterioration condition of the NOx occlusion catalyst based on the change of the detection output of the NOx sensor before and after making the air fuel ratio rich. <P>COPYRIGHT: (C)2004,JPO

Description

(5) The present invention relates to an exhaust gas concentration detection device for measuring the concentration of a harmful gas component contained in gas exhausted from an internal combustion engine.

With the tightening of exhaust gas regulations, there is a strong demand for sensors capable of directly measuring the concentrations of CO, HC and NOx, which are harmful gas components in exhaust gas. Therefore, development of a gas sensor of a limit current type using an oxide semiconductor type or an oxygen ion conductor such as ZrO ₂ has been advanced. An oxide semiconductor type gas sensor is a sensor that utilizes the fact that the change in electric resistance of an oxide semiconductor is proportional to the amount of a predetermined gas component adsorbed on the oxide semiconductor. On the other hand, as a limiting current type gas sensor (NOx sensor), for example, an electrode having NOx dissociation catalytic ability is arranged on an oxygen ion conductor to constitute an oxygen ion pump cell, and the oxygen ion pump cell faces a space facing the oxygen ion pump cell. A sensor that diffuses a gas whose oxygen concentration is controlled, decomposes NOx in the space, pumps out dissociated oxygen ions using the oxygen ion pump cell, and obtains a NOx gas concentration from a current flowing through the oxygen ion pump cell. Proposed.

However, the oxide semiconductor type gas sensor has a problem in its reproducibility. On the other hand, in the limiting current type sensor, although there is no problem in reproducibility and repetition accuracy, there is a problem that the current to be detected in principle (the current flowing in the oxygen ion pump cell) is as small as about several microamps. . In addition, since the decomposition of harmful gas components (for example, NOx) is controlled or suppressed by the catalytic action on the electrode surface, the catalytic activity changes over a long period of use and the zero point (the concentration of the predetermined component is substantially reduced). There is a problem that the detection output of the gas sensor indicating zero is shifted. For this reason, the use environment of any type of gas sensor greatly changes like a sensor attached to an internal combustion engine, particularly an exhaust gas system of a vehicle, and has not been used as a sensor used for a long time.

In view of the above circumstances, an object of the present invention is to provide an exhaust gas concentration detection device using a gas sensor capable of performing highly accurate gas concentration measurement over a long period of time.

According to a first aspect of the present invention, there is provided a NOx storage catalyst disposed in an exhaust pipe of an internal combustion engine, and the exhaust pipe is provided downstream of the NOx storage catalyst in the exhaust pipe. A NOx sensor for detecting the NOx concentration of the fuel cell, operating condition setting means for temporarily setting the air-fuel ratio to a rich atmosphere in order to purify the NOx stored in the NOx storage catalyst, and the NOx sensors before and after the rich atmosphere. Means for detecting a state of deterioration of the NOx storage type catalyst based on a change in the detection output of the NOx storage type catalyst.

According to a second aspect of the present invention, a NOx selective reduction catalyst disposed in an exhaust pipe of an internal combustion engine, and a NOx concentration in exhaust gas which is attached to the exhaust pipe downstream of the NOx selective reduction catalyst is detected. A NOx sensor, means for adding HC to the exhaust gas of the internal combustion engine, and means for detecting a deterioration state of the NOx selective reduction catalyst based on a change in the detection output of the NOx sensor before and after the addition of HC. It is characterized by having.

The method for calibrating the detection output of a gas sensor used in the present invention includes calibrating (calibrating) a gas sensor based on a detection output of a gas sensor under conditions in which the concentration of a harmful exhaust gas component can be estimated, among operating conditions of an internal combustion engine. It can be carried out. Generally, in an internal combustion engine of a vehicle having an electronic control type fuel supply device, for example, harmful gases such as NOx, HC, and CO in exhaust gas are used to cut off fuel supply when output becomes unnecessary such as during deceleration. The concentrations of the components are at about the same level as the atmosphere. On the other hand, under normal operating conditions, the concentration of harmful gas components emitted from the internal combustion engine is significantly higher than the level in the atmosphere. Therefore, by calibrating the gas sensor during the fuel cut, the concentration of the harmful exhaust gas component can be accurately detected even after long-term use. Further, as described above, since the zero point (offset) of the detection output changes due to long-term use of the gas sensor, the zero point may be corrected. That is, the detection output when the fuel cut is activated may be set to, for example, a level indicating zero NOx concentration.

If the offset value of the detection output of the gas sensor (the detection output when the concentration of the detected component is zero, the detection output at the zero point) has oxygen concentration dependency (when the offset value changes depending on the oxygen concentration), All offsets (for each oxygen concentration) may be corrected based on the detected output value at the time of the fuel cut in which the concentration of the detected component and the oxygen concentration are known. For example, the difference between the initial value of the offset (OF1) corresponding to the predetermined oxygen concentration stored in the memory and the detection output value (OF2) corresponding to the predetermined oxygen concentration at the time of the fuel cut, "OF1-OF2" What is necessary is just to subtract from the offset value OF [O ₂ ] corresponding to each oxygen concentration stored in the memory and store the value as a new offset value OF [O ₂ ] in the memory. In a sensor capable of measuring the oxygen concentration, a gas having an oxygen concentration almost the same as that in the atmosphere, that is, a gas having an oxygen concentration of 20.9% is supplied to the sensor at the time of fuel cut, so that the sensitivity (gain) can be calibrated. For example, by applying the detection method of the present invention to a NOx sensor described later, the sensitivity of the first oxygen pump current can be calibrated, and the oxygen concentration can be accurately measured even after endurance use.

(4) When the gas sensor is a NOx sensor and this NOx sensor is installed downstream of the NOx storage catalyst, a spike having a rich atmosphere is provided to reduce the stored NOx. At this timing, since there is almost no NOx emission, the zero point can be calibrated in the same manner as described above. Further, in this case, by comparing the detection output of the NOx sensor before and after the spike of the rich atmosphere is applied, it is possible to detect the deterioration of the catalyst (reduction of the NOx storage amount) without having to consider the change in the offset.

According to the present invention, when the NOx sensor for detecting the NOx concentration in the gas exhausted from the internal combustion engine is attached downstream of the NOx storage type catalyst, the air-fuel ratio is set to a rich atmosphere in order to reduce the stored NOx. There is a mode, and the calibration can be performed using this mode, and the deterioration state of the NOx storage type catalyst can be detected. Further, it is also possible to detect the deterioration state of the NOx selective reduction catalyst disposed in the exhaust pipe of a system using a diesel engine that cannot make the air-fuel ratio rich.

Hereinafter, preferred embodiments of the present invention will be described. A gas sensor to which the present invention is preferably applied is a gas sensor capable of detecting the concentration of harmful components in exhaust gas of an internal combustion engine, for example, flammable CO, HC, and NOx components. As the CO sensor, for example, a gas sensor using an oxide semiconductor element such as In ₂ O ₃ is applied. Further, each of the HC sensors includes an oxygen pump element and an oxygen concentration cell element exposed to a test gas, and the oxygen pump element when an electromotive force generated in the oxygen concentration cell element reaches a predetermined value or less. A gas sensor for obtaining a combustible gas component concentration from a current value flowing through the gas sensor is applied. In addition, a gas sensor including an oxygen pump cell, an oxygen sensor cell, and a combustible gas component concentration detector made of an oxide semiconductor is applied. Further, as the NOx sensor, a NOx sensor having two sets of diffusion resistance parts, an oxygen ion pump cell, and a gap is applied. The measurement principle of this NOx sensor is as follows.

(1) Exhaust gas flows into the first gap through the first diffusion resistor having diffusion resistance. (2) The first oxygen ion pump cell pumps out the oxygen in the first gap to such an extent that all the NOx is not decomposed (the oxygen partial pressure in the first gap is controlled by the signal output from the oxygen partial pressure detection electrode). Do). (3) The gas (gas whose oxygen concentration is controlled) in the first gap flows into the second gap through the second diffusion resistor. (4) NOx in the second gap is further decomposed into N ₂ gas and O ₂ gas by pumping out oxygen by the second oxygen ion pump cell. (5) At this time, since there is a linear relationship between the second oxygen pump current Ip2 flowing through the second oxygen ion pump cell and the NOx gas concentration, the NOx gas concentration can be detected by detecting Ip2. (6) Further, the oxygen concentration in the exhaust gas can be measured from the first oxygen pump current Ip1 flowing through the first oxygen ion pump cell when the first oxygen ion pump cell pumps out the oxygen in the first gap. Using the measured value of the oxygen concentration, the NOx decomposition rate in the first void portion can be obtained, and the NOx gas concentration can be corrected.

Next, a preferred method of using the NOx sensor in an exhaust gas purification system for a gasoline engine or a diesel engine will be described. Referring to FIG. 7A, in an exhaust gas purification system of a gasoline engine (particularly, a lean burn engine), an oxygen sensor [1] and a NOx storage type three-way catalyst are provided in an exhaust pipe from a gasoline engine to a downstream side. , An oxygen sensor [2] (the above-described NOx sensor that also functions as an oxygen sensor) are sequentially mounted. The oxygen sensor [1] is a sensor for controlling the engine air-fuel ratio, and controls fuel, air, and the like supplied to the engine based on the detection output. On the other hand, the NOx sensor serving also as an oxygen sensor disposed downstream of the NOx storage type three-way catalyst is a sensor for detecting the NOx concentration and determining the operating state of the three-way catalyst and its deterioration. The engine and the like are controlled so that the three-way catalyst operates optimally based on the detected output. This NOx storage type three-way catalyst is obtained by imparting a NOx storage effect to the three-way catalyst. When the excess air ratio λ = 1 (stoichiometric point), the three-way catalyst operates as a normal three-way catalyst, and temporarily stores NOx in a lean state. , And the temporarily stored NOx is purified by periodically adding a rich spike. Generally, the material of the NOx storage type three-way catalyst is Pt to which Ba or the like having a NOx storage effect is added.

(4) The degree of deterioration of the NOx storage catalyst can be detected using the NOx sensor disposed downstream of the NOx storage catalyst. That is, when a spike in a rich atmosphere (preferably, an air-fuel ratio of 14 to 14.5 for about 3 seconds) is introduced to reduce the NOx stored in the NOx storage type catalyst, the detection output of the NOx sensor before and after this spike is applied. Changes. If not deteriorated, the output of the NOx sensor when returning to the lean state after the spike is lower than before the spike. On the other hand, in the case of deterioration, NOx is not purified when returning to the lean state after the spike, and the output of the NOx sensor remains high. Therefore, the deterioration of the catalyst can be determined from the change in the output of the NOx sensor before and after the rich spike. Further, when the spike is applied, almost no NOx is generated, so that the zero point of the NOx sensor can be calibrated.

Referring to FIG. 7 (b), in the exhaust gas purification system for a diesel engine, a light oil injection valve for injecting light oil as an HC source into the exhaust gas from the diesel engine to the exhaust pipe into the exhaust pipe, An HC sensor (not shown), a NOx selective reduction catalyst, and the above-described NOx sensor are sequentially mounted. The NOx selective reduction catalyst has a function of purifying NOx by decomposing NOx into nitrogen, CO ₂ , and H ₂ O using HC added by light oil injection as a reducing agent. The HC sensor is arranged on the upstream side of the NOx selective reduction catalyst, and performs a function of monitoring the HC concentration in the exhaust gas after light oil injection in order to feedback-control the amount of light oil to be injected into the exhaust gas. is there. Further, the deterioration of the NOx selective reduction catalyst can be detected from the output change of the NOx sensor before and after the addition of HC. That is, according to the catalyst having the NOx purifying ability, the addition of HC reduces the NOx concentration downstream of the catalyst and decreases the NOx sensor output, whereas the catalyst having the deteriorated purifying ability has the HC Does not decrease the NOx concentration, so that the output of the NOx sensor does not decrease.

FIG. 8 is a diagram for explaining a control configuration of the exhaust gas concentration detection system using the NOx sensor according to one embodiment of the present invention. Referring to FIG. 8, the detection system includes an internal combustion engine, a catalyst installed in an exhaust system, a NOx sensor installed downstream of the catalyst, an engine control unit (ECU), and a NOx sensor controller. Be composed. The engine control unit sets the operating conditions (such as the air-fuel ratio) of the internal combustion engine and determines the deterioration of the catalyst. The NOx sensor controller stores means for controlling the NOx sensor, means for detecting and outputting the first and second oxygen pump currents Ip1 and Ip2 flowing through the NOx sensor, and a map showing the relationship between the oxygen concentration and the Ip2 offset value. A memory, an operation unit that reads an offset of Ip2 from the memory according to the oxygen concentration (for example, Ip1), executes a predetermined operation based on this and an output of the detection unit, and outputs an operation result to the memory; A signal indicating operating conditions is input from the control unit, an output signal of the Ip1 and Ip2 detecting means is input, an offset correction signal is output to the memory, and the offset correction signal is output to the memory based on the signal output from the arithmetic unit to the memory. Offset correction means for storing a new offset value.

動作 The operation of this system will be described. The detection means of the NOx sensor controller detects the second oxygen pump current Ip2 of the NOx sensor and outputs it to the calculation unit, which outputs this to the memory. Here, the engine control unit sets an operating condition for setting the NOx concentration in the exhaust gas to substantially zero or substantially the same level as the atmosphere. As an example, this operating condition is set at the time of fuel cut, that is, the NOx concentration is zero and the oxygen concentration is 20.9%. When the signal indicating the condition output from the engine control unit and the Ip1 and Ip2 signals corresponding to the condition output from the Ip1 and Ip2 detecting means are input to the offset correcting means, the offset correcting means stores a predetermined value in the memory. Is output, and Ip2 detected under the above condition is stored in the memory in association with the oxygen concentration of 20.9%. This stored value becomes the calibrated new offset value of the NOx sensor detection output corresponding to the oxygen concentration of 20.9%. Further, the calculation unit reads out each offset value corresponding to each oxygen concentration stored in the memory, and converts each read value, Ip2 under the above-described conditions, and the oxygen concentration 20.9% stored in the memory. A predetermined operation is performed based on the corresponding offset value. Each offset value corresponding to each oxygen concentration is calibrated based on the calculation result and stored in the memory. The calibration of the detection output of the gas sensor is preferably performed periodically during the operation of the internal combustion engine. It is also possible to perform it at idling.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. As the gas sensor, a NOx sensor having the structure shown in FIG. 1 was used. The NOx sensor of FIG. 1 includes a first oxygen ion pump cell 6 including a pair of electrodes 6a and 6b provided with a solid electrolyte layer 5-1 therebetween, and a pair of electrodes provided with a solid electrolyte layer 5-2 therebetween. Oxygen concentration measuring cell 7 provided with oxygen partial pressure detecting electrodes 7a, 7b, second oxygen ion pump cell provided with solid electrolyte layer 5-3, and a pair of electrodes 8a, 8b provided on the surface of solid electrolyte layer 5-4 8 in that order. Insulating layers 11-1, 11-2, and 11-3 are formed between the solid electrolyte layers 5-1, 5-2, 5-3, and 5-4, respectively. Between the first oxygen ion pump cell 6 and the oxygen concentration measurement cell 7, a first measurement chamber (a gap portion) is formed by left and right insulating layers 11-1 and upper and lower solid electrolyte layers 5-1 and 5-2 in the figure. 2), and a second measurement chamber (gap) 4 is similarly defined above the second oxygen ion pump cell 8 by the insulating layer 11-3 and the solid electrolyte layers 5-3 and 5-4. I have. Further, first diffusion holes (diffusion resistance portions) 1 having diffusion resistance are provided on both sides (front and back in FIG. 1) of the sensor in the lateral direction of the sensor in the first measurement chamber 2, respectively. On the other hand, an opening of the second diffusion hole (diffusion resistance portion) 3 is provided separately from the first diffusion hole 1. The second diffusion hole 3 penetrates through the oxygen concentration measurement cell 7 and the solid electrolyte layer 5-3 to communicate the first and second measurement chambers 2, 4 with a diffusion resistance.

In this sensor, electrodes 8a and 8b of a porous metal (Pt, Rh alloy, etc.) are formed together on the same surface of the solid electrolyte 5-4 constituting the second oxygen ion pump cell 8. Although the electrodes 8a and 8b are separated from each other by the insulating layer 11-3, oxygen ions are conducted through the solid electrolyte layer 5-4, and the second oxygen pump current Ip2 flows. The electrode 8b is prevented from being in direct contact with the outside air of the sensor by the solid electrolyte layer 5-4, the insulating layer 11-3, and the lead portion 8d, and via the porous lead portion 8d having diffusion resistance. The oxygen pumped by the second oxygen ion pump cell 8 can be led out. Further, lead portions (lines) 8c (see FIG. 2) and 8d are electrically connected to the electrodes 8a and 8b, respectively, and the lead portion 8d electrically connected to the outer electrode 8b of the second measurement chamber 4 is porous. It is capable of diffusing oxygen ions. Therefore, oxygen decomposed from the NOx gas and pumped to the electrodes 8a to 8b by the second oxygen ion pump cell 8 is released through the lead 8d. FIG. 2 corresponds to a plan cross-sectional view indicated by the arrow A line in FIG. Referring to FIG. 2, it can be seen that the lead portion 8d is in contact with the outside air (in the atmosphere or the atmosphere of the gas to be measured), and communicates the outside air with the electrode 8b via the diffusion resistance.

The measurement principle of the NOx sensor shown in FIG. 1 is as described above in the section of the embodiment, and the terminal of the controller is electrically connected to each electrode of the NOx sensor via a lead portion, and the first diffusion is performed. An electromotive force corresponding to the oxygen concentration in the exhaust gas introduced into the first measurement chamber 2 through the hole 1 is generated between the pair of electrodes 7a and 7b of the oxygen concentration measurement cell 7, and the voltage due to the electromotive force is constant. The voltage applied to the first oxygen ion pump cell 6 is controlled such that the control is performed by digital control using a microcomputer or by analog control. Excess oxygen is pumped from the first measurement chamber 2 by the first oxygen ion pump cell 6, and the gas to be measured whose oxygen concentration is controlled to a certain level is diffused into the second measurement chamber 4 through the second diffusion holes 3. Then, a voltage is applied to the pair of electrodes 8a and 8b of the second oxygen ion pump cell 8, and the remaining oxygen is further pumped out, and the catalytic action of the Pt alloy (or Rh alloy) electrode causes NOx is decomposed into N ₂ and O _2, by the O ₂ conducts the solid electrolyte layer 5-4 of the second oxygen ion pump cell 8 as ions, the provided 4 out second measurement chamber A second oxygen pump current Ip2 flows between the pair of electrodes 8a and 8b of the two-oxygen ion pump cell 8 according to the amount of NOx gas decomposed. By measuring this Ip2, the NOx gas concentration can be measured.

According to this NOx sensor, since the electrode 8b, which is the opposite electrode to the electrode 8a of the second oxygen ion pump cell 8 in the second measurement chamber 4, is installed inside the element (between the stacked solid electrolytes), The electrolyte layer 5-4 and the insulating layer 11-3 serve as protection means for the electrode 8b, and the lead portion 8d serves as diffusion resistance means, so that the electrode 8b is cut off from the atmosphere of the gas to be measured (exhaust gas) and directly contacts the outside air. Oxygen pumped around the electrode 8b is pooled, the oxygen concentration around (near) the electrode 8b is stabilized, and the pair of electrodes 8a, The electromotive force generated between 8b is stabilized. Further, since the generated electromotive force is stabilized, the effective pump voltage (Vp2−electromotive force) of the pump voltage Vp2 applied to the second oxygen ion pump cell 8 is stabilized, and the NOx gas concentration measurement depends on the oxygen concentration. Sex is reduced.

[Production example]
Next, an example of manufacturing the NOx sensor shown in FIG. 1 will be described. FIG. 3 is a layout diagram of the NOx sensor shown in FIG. Although the sheet and paste shown in FIG. 3 are in a green state, the same reference numerals are given to the reference numerals assigned to the NOx sensor shown in FIG. In FIG. 3, a ZrO ₂ green sheet, an electrode paste, and the like are laminated in order from the upper left to the lower left, and from the upper right to the lower right, dried, and fired to produce an integrated sensor. A paste material such as an insulating coat and an electrode is laminated and formed by screen printing on a predetermined ZrO ₂ green sheet. Next, an example of manufacturing each component such as a ZrO ₂ green sheet will be described.

[Forming of ZrO ₂ sheet (5-1 layer to 5-4 layer)]: ZrO ₂ powder was calcined in an atmospheric furnace at 600 ° C for 2 hours. 30 kg of calcined ZrO ₂ powder, 150 g of a dispersant, and 10 kg of an organic solvent are placed in a trommel together with 60 kg of cobblestone, mixed and dispersed for about 50 hours, and a solution obtained by dissolving 4 kg of an organic binder in 10 kg of an organic solvent is added. For 20 hours to obtain a slurry having a viscosity of about 10 Pa · s. A ZrO ₂ green sheet having a thickness of about 0.4 mm was prepared from this slurry by a doctor blade method, and dried at 100 ° C. for 1 hour.

[Printing paste]
(1) For the first oxygen ion pump electrode 6a, oxygen partial pressure detection electrode (oxygen reference electrode) 7b, and second oxygen ion pump electrodes 8a and 8b: 20 g of Pt powder, 2.8 g of ZrO ₂ powder, and an appropriate amount of an organic solvent Put in a grinder (or pot mill), mix for 4 hours, disperse, add 2 g of organic binder dissolved in 20 g of organic solvent, add 5 g of viscosity modifier and mix for 4 hours A paste having a viscosity of about 150 Pa · s was produced.

(2) For the first oxygen ion pump electrode 6b, oxygen partial pressure detection electrode (oxygen reference electrode) 7a: 19.8 g of Pt powder, 2.8 g of ZrO ₂ powder, 0.2 g of Au powder, and an appropriate amount of organic solvent Or a pot mill) and mixed for 4 hours to disperse. A solution prepared by dissolving 2 g of an organic binder in 20 g of an organic solvent is added thereto, and 5 g of a viscosity modifier is further added. The mixture is mixed for 4 hours to obtain a viscosity of 150 Pa · s. A paste of the degree was prepared.

(3) For insulation coating and protection coating: (1) 50 g of alumina powder and an appropriate amount of an organic solvent are put into a grinder (or pot mill), mixed and dissolved for 12 hours, and further added with 20 g of a viscosity modifier and mixed for 3 hours. Thus, a paste having a viscosity of about 100 Pa · s was prepared.

(4) For Pt-containing porous material (for lead wire): {10 g of alumina powder, 1.5 g of Pt powder, 2.5 g of organic binder, and 20 g of organic solvent are put in a grinder (or pot mill) and mixed for 4 hours. Further, 10 g of a viscosity modifier was added and mixed for 4 hours to prepare a paste having a viscosity of about 100 Pa · s.

(5) For the first diffusion hole: {10 g of alumina powder having an average particle size of about 2 μm, 2 g of an organic binder, and 20 g of an organic solvent are put in a grinder (or pot mill), mixed, dispersed, and further, 10 g of a viscosity modifier is added. The mixture was added and mixed for 4 hours to prepare a paste having a viscosity of about 400 pa · s.

(6) For carbon coating: 4 g of carbon powder, 2 g of organic binder, and 40 g of organic solvent are put in a grinder (or pot mill), mixed and dispersed, and 5 g of a viscosity modifier is added, followed by mixing for 4 hours. A paste was made. In addition, by forming a carbon coat by printing, for example, contact between the first oxygen ion pump electrode 6b and the oxygen reference electrode 7a is prevented. The carbon coat is used for forming the first measurement chamber 2 and the second measurement chamber 4. Since carbon is burned off during firing, the carbon coat layer does not exist in the fired body.

[Pellets]
(7) For second diffusion hole: 20 g of alumina powder having an average particle size of about several μm, 8 g of an organic binder, and 20 g of an organic solvent are put into a grinder (or a pot mill), mixed for 1 hour, granulated, and then molded. A pressure of about 2 t / cm ² was applied by a press to produce a cylindrical press-formed body (green state) having a diameter of 1.3 mm and a thickness of 0.8 mm. This green pressed body is inserted into predetermined portions of the second and third layers of ZrO ₂ green sheets, integrated by pressing, and fired to form second diffusion holes 3 in the sensor.

[ZrO ₂ lamination method]
After the second and third layers are pressed, a portion (diameter: 1.3 mm) through which the second diffusion hole 3 penetrates is punched. After the punching, a green columnar molded body serving as the second diffusion hole 3 is embedded, and one to four layers of ZrO ₂ green sheets are pressed under a pressure of 5 kg / cm ² and a pressing time of 1 minute.

[Binder removal and firing]
The pressed compact is debindered at 400 ° C. × 2 hours and baked at 1500 ° C. × 1 hour.

[Example of use]
The thus manufactured NOx sensor having the structure shown in FIG. 1 was applied to a real machine and a 500-hour durability test was performed. The configuration of the NOx concentration detecting device is the same as that shown in FIG. 8, and the mounting position of the NOx sensor is the same as that shown in FIG. Further, the controller that controls the NOx sensor determines the gain value ((standard NOx concentration−0) / (current generation amount) of the detection output (second oxygen pump current) of the NOx sensor set using the model gas evaluation device described later. Offset)) and the offset value are stored in the memory. In particular, offset values corresponding to oxygen concentrations from 0% to 20.9% (21%) are stored in a memory for the purpose of canceling the oxygen concentration dependency of the offset. A predetermined offset value is read out based on the oxygen concentration obtained from the oxygen pump current, and the offset value used for calculating the NOx gas concentration is optimally set.

Two sets of the controller and the NOx sensor are prepared, and the initial characteristics of the NOx sensors are measured by a model gas evaluation device. When the analyzer output indicates that the NOx concentration is zero, the controller based on the second oxygen pump current is used. Each controller was adjusted so that the detection output of was zero. Next, these NOx sensors were attached to the exhaust pipe of a gasoline engine with a displacement of 3000 cc, respectively, and a durability test was performed for 500 hours in the mode shown in FIG. 4 while controlling the NOx sensors with the respective controllers (rotation in the figure). The number after the number indicates the relative accelerator opening). One controller executed the method of using the gas sensor according to one embodiment of the present invention, and calibrated the offset (zero point) at the time of fuel cut in the endurance mode. The other controller did not calibrate the zero point. The calibration method by one controller is as follows.

That is, referring to FIG. 5, when a fuel cut signal output from an ECU (engine control unit) of a gasoline engine is input, one controller detects a detection output of the controller proportional to the second oxygen pump current. The value is stored as an offset value (OF2) corresponding to O ₂ = 20.9%. Next, the offset value (OF1) corresponding to O ₂ = 20.9% stored in the memory is read, and the difference between OF1 and OF2 is calculated. This “OF1-OF2” is subtracted from the offset value OF [O ₂ ] corresponding to each oxygen concentration stored in the memory, and the value (OF [O ₂ ] − (OF1-OF2)) is calibrated. The new offset value OF [O ₂ ] is stored in the memory.

Fig. 6 shows the results of the durability test. Referring to the results of the durability test shown in FIG. 6, according to the system according to the example in which the calibration was performed, the NOx concentration detection output of the controller hardly changed even after the 500-hour durability test. In contrast, the output of the system of the comparative example in which the calibration was not performed increased by about 400 ppm.

FIG. 2 is a schematic diagram for explaining a structure of a NOx sensor used in one embodiment of the present invention. FIG. 2 is a diagram for explaining a plane cross section indicated by an arrow A line in FIG. FIG. 2 is a diagram for explaining a layout of the NOx sensor shown in FIG. 1. FIG. 2 is a diagram for explaining a durability test mode performed using the NOx sensor shown in FIG. 1. FIG. 4 is a diagram for explaining a method of calibrating the detection output of the NOx sensor according to one embodiment of the present invention. It is a figure for demonstrating the result of an endurance test, and the square plot in the figure shows an Example and the triangle plot shows the data of a comparative example. (A) And (b) is a figure for demonstrating the exhaust gas concentration detection apparatus using the NOx sensor which concerns on one Embodiment of this invention, (a) is a gasoline engine (especially lean burn engine). FIG. 1B is a diagram for explaining an exhaust gas concentration detection device applied to an exhaust gas purification system of a diesel engine. It is a figure for explaining the exhaust gas concentration detection system using the NOx sensor concerning one embodiment of the present invention.

Explanation of reference numerals

1: 1st diffusion hole (diffusion resistance part)
2: First measurement chamber (gap)
3: Second diffusion hole (diffusion resistance part)
4: Second measurement chamber (gap)
5-1,..., 5-4: solid electrolyte layer 6: first oxygen ion pump cell 6a, 6b: electrode 6c: lead portion 7: oxygen concentration measurement cell 7a, 7b: electrode 8: second oxygen ion pump cell 8a , 8b: electrodes 8c, 8d: lead portions 11-1,..., 11-3: insulating layer

Claims (2)

A NOx storage catalyst disposed in an exhaust pipe of an internal combustion engine, a NOx sensor attached to the exhaust pipe downstream of the NOx storage catalyst to detect a NOx concentration in exhaust gas, and a NOx storage catalyst stored in the NOx storage catalyst. Operating condition setting means for temporarily setting the air-fuel ratio to a rich atmosphere in order to purify the NOx, and detecting a deterioration state of the NOx storage type catalyst based on a change in the detection output of the NOx sensor before and after the rich atmosphere is set. And an exhaust gas concentration detecting device. A NOx selective reduction catalyst disposed in an exhaust pipe of the internal combustion engine; a NOx sensor attached to the exhaust pipe downstream of the NOx selective reduction catalyst to detect a NOx concentration in exhaust gas; and an exhaust gas of the internal combustion engine. A means for adding HC into the gas, and means for detecting a deterioration state of the NOx selective reduction catalyst based on a change in the detection output of the NOx sensor before and after the addition of HC. Gas concentration detector.

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