JP4272970B2 - Exhaust gas concentration detector - Google Patents

Description

  The present invention relates to an exhaust gas concentration detection device that measures the concentration of harmful gas components contained in gas discharged from an internal combustion engine.

With the tightening of exhaust gas regulations, there is a strong demand for sensors that can directly measure the concentrations of CO, HC, and NOx that are harmful gas components in exhaust gas. Therefore, development of an oxide semiconductor type or a limiting current type gas sensor using an oxygen ion conductor such as ZrO ₂ is underway. An oxide semiconductor type gas sensor is a sensor that utilizes the fact that the change in electrical resistance of an oxide semiconductor is proportional to the amount of adsorption of a predetermined gas component to the oxide semiconductor. On the other hand, as a limiting current type gas sensor (NOx sensor), for example, an oxygen ion pump cell is configured by disposing an electrode having NOx dissociation catalytic ability on an oxygen ion conductor, and the oxygen ion pump cell faces the space. A sensor for diffusing a gas having a controlled oxygen concentration, decomposing NOx in the space, pumping dissociated oxygen ions using the oxygen ion pump cell, and obtaining 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, although there is no problem in reproducibility and repeatability in the limit current type sensor, there is a problem that the current detected in principle (current flowing through the oxygen ion pump cell) is as small as several microamperes. . 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 due to long-term use, and the zero point (the concentration of the predetermined component is substantially reduced). The detection output of the gas sensor indicating zero is shifted. For this reason, any type of gas sensor has not been used as a sensor that is used over a long period of time because the use environment has changed greatly, such as a sensor attached to an internal combustion engine, particularly a vehicle exhaust gas system.

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

  In order to solve the above problems, the present invention provides, in a first aspect, a NOx occlusion type catalyst disposed in an exhaust pipe of an internal combustion engine, and an exhaust gas that is attached downstream of the NOx occlusion type catalyst in the exhaust pipe. A NOx sensor for detecting the NOx concentration of the catalyst, operating condition setting means for temporarily setting the air-fuel ratio to a rich atmosphere to purify NOx stored in the NOx storage catalyst, and the NOx sensor before and after the rich atmosphere is made And a means for detecting the deterioration state of the NOx storage catalyst based on the change in the detected output.

  The method for calibrating the detection output of the gas sensor used in the present invention performs the calibration (calibration) of the gas sensor based on the detection output of the gas sensor under the condition where the concentration of the harmful exhaust gas component can be estimated among the operating conditions of the internal combustion engine. It can be carried out. In general, in an internal combustion engine of a vehicle having an electronically controlled fuel supply device, for example, when no output is required, such as when decelerating, the fuel supply is cut off, so harmful gases such as NOx, HC, CO, etc. in the exhaust gas The concentration of the component is almost the same level as the atmosphere. On the other hand, under normal operating conditions, the concentration of harmful gas components discharged from the internal combustion engine is significantly higher than the atmospheric level. Therefore, by calibrating the gas sensor when the fuel cut works, the concentration of harmful exhaust gas components can be accurately detected even after long-term use. As described above, since the zero point (offset) of the detection output changes due to the long-term use of the gas sensor, this zero point may be corrected. That is, the detection output when the fuel cut is performed may be set to a level indicating, for example, NOx concentration zero.

In addition, when 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 of the zero point) is dependent on the oxygen concentration (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 component to be detected and the oxygen concentration are known. For example, the difference between the initial offset value (OF1) corresponding to the predetermined oxygen concentration stored in the memory and the detected output value (OF2) corresponding to the predetermined oxygen concentration at the time of the fuel cut, “OF1−OF2”. What is necessary is to subtract from the offset value OF [O ₂ ] corresponding to each oxygen concentration stored in the memory and store the value in the memory as a new offset value OF [O ₂ ]. Further, in the sensor capable of measuring the oxygen concentration, when the fuel is cut, a gas having almost the same oxygen concentration as that in the atmosphere, that is, a gas having an oxygen concentration of 20.9% is supplied to the sensor, so that the sensitivity (gain) can be calibrated. For example, the sensitivity of the first oxygen pump current can be calibrated by applying the detection method of the present invention to a NOx sensor, which will be described later, and an accurate oxygen concentration can be measured even after endurance use.

  Further, when the gas sensor is a NOx sensor and this NOx sensor is installed downstream of the NOx storage catalyst, a rich atmosphere spike is introduced to reduce the stored NOx. Since there is almost no NOx emission at this timing, the zero point can be calibrated as described above. Furthermore, in this case, if the detection output of the NOx sensor is compared before and after the spike in the rich atmosphere, the catalyst deterioration (decrease in the NOx occlusion amount) can be detected without considering the change in offset.

According to the present invention, when the NOx sensor for detecting the NOx concentration in the gas discharged from the internal combustion engine is attached downstream of the NOx occlusion type catalyst, the air-fuel ratio is made rich to reduce the occluded NOx. Although there is a mode, the calibration can be executed using this mode, and the deterioration state of the NOx occlusion catalyst can also be detected .

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

(1) The exhaust gas flows into the first gap portion through the first diffusion resistance portion having diffusion resistance. (2) The first oxygen ion pump cell pumps out oxygen in the first gap to such an extent that all NOx is not decomposed (the oxygen partial pressure in the first gap is controlled by a signal output from the oxygen partial pressure detection electrode) To do). (3) The gas in the first gap portion (gas whose oxygen concentration is controlled) flows into the second gap portion through the second diffusion resistance portion. (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 that flows to the first oxygen ion pump cell when the first oxygen ion pump cell pumps out oxygen in the first gap. The measured value of the oxygen concentration can be used to determine the NOx decomposition rate in the first gap, and the NOx gas concentration can be corrected.

  Next, a preferred method of using the NOx sensor in an exhaust gas purification system of a gasoline engine or a diesel engine will be described. Referring to FIG. 7 (a), in an exhaust gas purification system for a gasoline engine (particularly a lean burn engine), an oxygen sensor [1], a NOx occlusion type three-way catalyst is provided in the exhaust pipe from the gasoline engine toward the downstream side. The oxygen sensor [2] (the NOx sensor described in detail above, which also serves as an oxygen sensor) is attached in order. The oxygen sensor [1] is a sensor for engine air-fuel ratio control, and fuel, air, and the like supplied to the engine are controlled based on the detection output. On the other hand, the NOx sensor combined with the oxygen sensor disposed downstream of the NOx occlusion 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 detection output. This NOx occlusion type three-way catalyst is obtained by adding a NOx occlusion effect to a three-way catalyst, and operates as a normal three-way catalyst at an excess air ratio λ = 1 (stoichiometric point), and temporarily stores NOx in a lean state. The NOx temporarily stored is purified by periodically storing rich spikes. Generally, the material of the NOx occlusion type three-way catalyst is obtained by adding Ba or the like having NOx occlusion effect to Pt.

  In addition, the degree of deterioration of the NOx storage catalyst can be detected using the NOx sensor arranged downstream of the NOx storage catalyst. That is, when a rich atmosphere spike (preferably, about 3 seconds, air-fuel ratio 14 to 14.5) is input in order to reduce NOx stored in the NOx storage catalyst, the detection output of the NOx sensor before and after this spike. 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, when it is deteriorated, NOx is not purified when returning to the lean state after the spike, and the output of the NOx sensor remains high. Therefore, it is possible to determine the deterioration of the catalyst from the change in the output of the NOx sensor before and after the rich spike. Further, since almost no NOx is generated when the spike is inserted, the zero point of the NOx sensor can be calibrated.

With reference to FIG.7 (b), in the exhaust gas purification system of the diesel engine which concerns on a reference example, it is for injecting light oil as HC source in exhaust gas in exhaust gas toward the downstream from a diesel engine. A light oil injection valve, an HC sensor (not shown), a NOx selective reduction catalyst, and the NOx sensor detailed above are attached in this order. The NOx selective reduction catalyst serves to purify NOx by decomposing it into nitrogen, CO ₂ , and H ₂ O using HC added by light oil injection as a reducing agent. The HC sensor is disposed upstream of the NOx selective reduction catalyst, and serves to monitor the HC concentration in the exhaust gas after the light oil injection in order to feedback control the amount of light oil to be injected into the exhaust gas. is there. Furthermore, the deterioration of the NOx selective reduction catalyst can be detected from the change in the output of the NOx sensor before and after the addition of HC. That is, according to the catalyst having NOx purification ability, the addition of HC decreases the NOx concentration downstream of the catalyst and the NOx sensor output decreases. Since NOx concentration does not decrease even when NO is added, the output of the NOx sensor does not decrease.

  FIG. 8 is a diagram for explaining the 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. Composed. The engine control unit sets operating conditions (such as an air-fuel ratio) of the internal combustion engine and determines 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, Ip2 flowing through the NOx sensor, and a map showing the relationship between the oxygen concentration and the Ip2 offset value. A memory, a calculation unit that reads an offset of Ip2 from the memory according to an oxygen concentration (for example, Ip1), executes a predetermined calculation based on the offset and an output of the detection unit, and outputs a calculation result to the memory; an engine A signal indicating an operating condition is input from the control unit, an output signal of the Ip1, Ip2 detecting means is input, an offset correction signal is output to the memory, and the memory is based on the signal output from the arithmetic unit to the memory. Offset correction means for storing a new offset value.

  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, and the calculation unit outputs this to the memory. Here, the engine control unit sets an operating condition that makes the NOx concentration in the exhaust gas 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 detection means are input to the offset correction means, the offset correction means stores a predetermined value in the memory. The offset correction signal 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 new offset value calibrated for the NOx sensor detection output corresponding to the oxygen concentration of 20.9%. In addition, the calculation unit reads each offset value corresponding to each oxygen concentration stored in the memory, and calculates each read value, Ip2 in the above condition, and oxygen concentration 20.9% stored in the memory. A predetermined calculation is performed based on the corresponding offset value. Based on the calculation result, each offset value corresponding to each oxygen concentration is calibrated and stored in the memory. Such calibration of the detection output of the gas sensor is preferably performed periodically during operation of the internal combustion engine. It can also be done at idling.

  An embodiment of the present invention will be described below 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 pair of electrodes 6a and 6b provided with a pair of electrodes 6a and 6b provided with a solid electrolyte layer 5-1 interposed therebetween, and a pair of electrodes provided with a solid electrolyte layer 5-2 interposed therebetween. Oxygen concentration measuring cell 7 provided with oxygen partial pressure detecting electrodes 7a, 7b, second electrolyte ion pump cell provided with a pair of electrodes 8a, 8b provided on the surface of solid electrolyte layer 5-3, solid electrolyte layer 5-4 8 is laminated in 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, the first measurement chamber (gap portion) is formed by the left and right insulating layers 11-1 and the upper and lower solid electrolyte layers 5-1 and 5-2. ) 2 is defined, and similarly, the second measurement chamber (gap) 4 is 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. Yes. Furthermore, first diffusion holes (diffusion resistance portions) 1 having diffusion resistance are respectively provided on both sides of the sensor in the lateral direction of the sensor (front and back in FIG. 1) of the first measurement chamber 2. On the other side, the opening of the second diffusion hole (diffusion resistance part) 3 is provided apart from the first diffusion hole 1. The second diffusion hole 3 penetrates the oxygen concentration measurement cell 7 and the solid electrolyte layer 5-3 and communicates the first and second measurement chambers 2 and 4 with diffusion resistance.

  In this sensor, electrodes 8a and 8b of porous metal (Pt, Rh alloy, etc.) are formed on the same surface of the solid electrolyte 5-4 constituting the second oxygen ion pump cell 8. The electrodes 8a and 8b are separated from each other by the insulating layer 11-3, but oxygen ions are conducted through the solid electrolyte layer 5-4, and a second oxygen pump current Ip2 thereby flows. The electrode 8b is prevented from direct contact with the sensor outside air 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 out by the second oxygen ion pump cell 8 can be led out to the outside. Furthermore, lead portions (wires) 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 possible to diffuse oxygen ions. Accordingly, the 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 portion 8d. FIG. 2 corresponds to a cross-sectional plan view taken along line A 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 gas atmosphere to be measured), and the outside air is communicated with the electrode 8b through 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 controller terminal is electrically connected to each electrode of the NOx sensor via a lead portion, so that 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 measuring cell 7, and the voltage due to this electromotive force is constant. The voltage applied to the first oxygen ion pump cell 6 is controlled so that (the control by the controller may be digital control using a microcomputer or analog control). Then, surplus oxygen is pumped out from the first measurement chamber 2 by the first oxygen ion pump cell 6, and the gas under measurement controlled to a constant oxygen concentration diffuses into the second measurement chamber 4 through the second diffusion hole 3. Then, a voltage is applied to the pair of electrodes 8a and 8b of the second oxygen ion pump cell 8 to further pump out the remaining oxygen, and by the catalytic action of the electrode made of Pt alloy (or Rh alloy, etc.), NOx is decomposed into N ₂ and O ₂ , and this O ₂ becomes ions and conducts through the solid electrolyte layer 5-4 of the second oxygen ion pump cell 8, thereby providing the second inside and outside of the second measurement chamber 4. A second oxygen pump current Ip2 corresponding to the amount of NOx gas decomposed between the pair of electrodes 8a and 8b of the two oxygen ion pump cell 8 flows. By measuring this Ip2, the NOx gas concentration can be measured.

  According to this NOx sensor, 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 placed 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. The oxygen pumped out around the electrode 8b is pooled, the oxygen concentration around (near) the electrode 8b is stabilized, and the pair of electrodes 8a of the second oxygen ion pump cell 8 are The electromotive force generated between 8b is stabilized. Further, by stabilizing the generated electromotive force, 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, a manufacturing example of 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 as those attached to the NOx sensor shown in FIG. In FIG. 3, a ZrO ₂ green sheet, electrode paste, and the like are laminated in the order from the upper left to the lower left and from the upper right to the lower right, and dried and fired to produce an integrated sensor. Paste materials such as insulating coatings and electrodes are laminated 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.

[ZrO ₂ sheet (5-1 to 5-4 layer) molding]: 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 dispersant and 10 kg of organic solvent are placed in Trommel with 60 kg of cobblestone, mixed and dispersed for about 50 hours, and 4 kg of organic binder dissolved in 10 kg of organic solvent is added thereto. 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.

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

(2) For first oxygen ion pump electrode 6b, oxygen partial pressure detecting electrode (oxygen reference electrode) 7a: Pt powder 19.8g, ZrO ₂ powder 2.8g, Au powder 0.2g, an appropriate amount of organic solvent, Or in a pot mill), mixed and dispersed for 4 hours, added with 2 g of an organic binder dissolved in 20 g of an organic solvent, added with 5 g of a viscosity modifier, and mixed for 4 hours to a viscosity of 150 Pa · s. About a paste was prepared.

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

  (4) Pt-containing porous material (for lead wires): 10 g of alumina powder, 1.5 g of Pt powder, 2.5 g of organic binder, and 20 g of organic solvent are placed in a raking machine (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 diameter of about 2 μm, 2 g of organic binder, and 20 g of organic solvent are placed in a raking machine (or pot mill), mixed and dispersed, and further 10 g of viscosity modifier is added. It 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 raking machine (or pot mill), mixed and dispersed, and further 5 g of viscosity modifier is added and mixed for 4 hours. A paste was prepared. Note that by forming the 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 to form the first measurement chamber 2 and the second measurement chamber 4. Since carbon burns away 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 placed in a raking machine (or pot mill), mixed for 1 hour, granulated, and molded. A cylindrical press-molded body (green state) having a diameter of 1.3 mm and a thickness of 0.8 mm was produced by applying a pressure of about 2 t / cm ² with a press. The green state press-molded body is inserted into a predetermined portion of the _second and third layers of ZrO ₂ green sheets, pressed and integrated, and then baked to form second diffusion holes 3 in the sensor.

[ZrO ₂ lamination method]
After the second and third layer pressure bonding, a portion (diameter 1.3 mm) through which the second diffusion hole 3 passes is punched out. After punching, a green cylindrical molded body to be the second diffusion hole 3 is embedded, and 1 to 4 layers of ZrO ₂ green sheets are pressure-bonded at a pressing force of 5 kg / cm ² and a pressing time of 1 minute.

[Binder removal and firing]
The pressure-bonded molded body is debindered at 400 ° C. for 2 hours and fired at 1500 ° C. for 1 hour.

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

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

That is, referring to FIG. 5, when one of the controllers receives the fuel cut signal output from the ECU (engine control unit) of the gasoline engine, the controller outputs the detected 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 taken. 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. With reference to the results of the endurance 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 endurance test for 500 hours. On the other hand, the output of the comparative system that did not execute calibration increased by about 400 ppm.

It is a schematic diagram for demonstrating the structure of the NOx sensor used in one Example of this invention. FIG. 2 is a view for explaining a plane cross section indicated by an arrow A line in FIG. It is a figure for demonstrating the layout of the NOx sensor shown in FIG. It is a figure for demonstrating the endurance test mode performed using the NOx sensor shown in FIG. It is a figure for demonstrating the calibration method of the detection output of the NOx sensor which concerns on one Example of this invention. It is a figure for demonstrating the result of an endurance test, the square plot in the figure shows the data of 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 and reference example of this invention in order , (a) is a gasoline engine (especially FIG. 2B is a diagram for explaining an exhaust gas concentration detection apparatus applied to an exhaust gas purification system of a lean burn engine, and FIG. It is a figure for demonstrating the exhaust gas concentration detection system using the NOx sensor which concerns on one Embodiment of this invention.

Explanation of symbols

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, solid electrolyte layer 6: first oxygen ion pump cell 6a, 6b: electrode 6c: lead part 7: oxygen concentration measuring cell 7a, 7b: electrode 8: second oxygen ion pump cell 8a , 8b: Electrodes 8c, 8d: Lead portions 11-1, ..., 11-3: Insulating layers

Claims (1)

  A NOx occlusion type catalyst disposed in the exhaust pipe of the internal combustion engine, a NOx sensor which is attached downstream of the NOx occlusion type catalyst in the exhaust pipe and detects the NOx concentration in the exhaust gas, and is occluded by the NOx occlusion type catalyst. In order to purify the NOx, the operating condition setting means for temporarily setting the air-fuel ratio to a rich atmosphere, and the deterioration state of the NOx occlusion type catalyst are detected based on the change in the detection output of the NOx sensor before and after the rich atmosphere. And an exhaust gas concentration detecting device.

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