JP3352002B2 - Method and apparatus for detecting deterioration in function of nitrogen oxide storage catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a reduction in the function of a nitrogen oxide storage catalyst using a NOx sensor configured to detect the concentration of nitrogen oxides discharged from various combustion equipment such as an internal combustion engine. The present invention relates to a method and an apparatus for detecting a decrease in the function of a nitrogen oxide storage catalyst.

[0002]

2. Description of the Related Art Conventionally, for example, European Patent Application Publication No. 0678740A1, SAE paper
No. 960334 P137-142 1996 and the like, a first measurement chamber communicated with the measured gas side via a first diffusion-controlling layer, and a first diffusion-controlling layer connected to the first measurement chamber via a second diffusion-controlling layer. The communicating second measurement chamber is formed by an oxygen ion conductive solid electrolyte layer, and the first measurement chamber is formed by sandwiching the solid electrolyte layer with a porous electrode.
An oxygen pumping cell and an oxygen concentration measuring cell are formed, and further, in the second measuring chamber, an internal combustion engine is formed by using a NOx sensor in which a solid electrolyte layer is sandwiched between porous electrodes to form a second oxygen pumping cell. 2. Description of the Related Art There is known a NOx concentration measuring device that detects the concentration of nitrogen oxide (NOx) in exhaust gas of an engine or the like.

In this type of NOx concentration measuring device,
By supplying a current to the first oxygen pumping cell so that the output voltage from the oxygen concentration measuring cell becomes a preset constant value, the second oxygen pumping is performed while controlling the oxygen concentration in the first measuring chamber to a constant concentration. Apply a constant voltage to the cell,
Oxygen is pumped out of the second measurement chamber. At this time, the value of N in the gas to be measured is determined based on the current flowing through the second oxygen pumping cell.
Detect Ox concentration.

That is, in the exhaust gas from an internal combustion engine or the like, which is the gas to be measured, other gas components such as oxygen, carbon monoxide, and carbon dioxide exist in addition to NOx. Then, the first oxygen pumping cell
The inside of the measurement chamber is controlled to a low oxygen concentration with very little oxygen, and the oxygen in the second measurement chamber is supplied to the second oxygen pumping cell on the side of the second measurement chamber into which the gas to be measured controlled to the low oxygen concentration flows. By applying a constant voltage in the pumping direction, NOx in the gas to be measured is decomposed into nitrogen and oxygen by the catalytic function of the porous electrode constituting the second oxygen pumping cell, and oxygen is discharged from the second measurement chamber. By sampling and detecting the pump current flowing through the second oxygen pumping cell at that time, the NOx concentration in the gas to be measured can be detected without being affected by other gas components in the gas to be measured.

In this type of NOx concentration measuring apparatus,
In order to accurately detect the NOx concentration by the above-described detection method, it is necessary to heat the sensor to a predetermined activation temperature (for example, 800 ° C. or higher) to activate each cell. Therefore, a heater for heating the sensor is required. Is provided separately.

In recent years, in order to reduce the fuel consumption and increase the efficiency of an internal combustion engine using gasoline as a fuel, an internal combustion engine (lean burn engine or direct injection engine) whose operation is controlled at a lean air-fuel ratio where the amount of air with respect to fuel is large Etc.) have been developed. Then, usually, in an internal combustion engine, unburned components and NOx contained in exhaust gas (HC, CO) and reacted with a three-way catalyst is performed to purify the exhaust gas by reducing NOx into N ₂ However, when the operation was performed at a lean air-fuel ratio, a large amount of oxygen was contained in the exhaust gas, and this oxygen reacted with unburned components, so that NOx could not be purified.

On the other hand, a so-called NOx storage catalyst in which a three-way catalyst is provided with a NOx storage material for accumulating NOx in exhaust gas as nitrate is known. However,
Since the NOx storage amount has a limit, when the NOx storage catalyst is used, before the NOx storage amount reaches the limit,
Temporarily controlling the air-fuel ratio of the fuel-air mixture supplied to the internal combustion engine to a rich air-fuel ratio rich in fuel, exhausting exhaust gas containing a large amount of unburned components from the internal combustion engine,
Control is performed to cause the NOx stored in the Ox storage catalyst to react with the NOx stored in the Ox storage catalyst, thereby refreshing the NOx storage catalyst so that the NOx can be stored again.

[0008] Such a refresh operation is as follows.
It is carried out periodically at fixed time intervals, or by using the NOx concentration measuring device, a NOx sensor is mounted downstream of the NOx storage catalyst in the exhaust passage of the internal combustion engine, and N
The leakage amount of Ox is detected, and the detection is performed as necessary when the leakage amount of NOx becomes equal to or more than a specified amount.

Further, NOx used for such applications
In order to perform accurate control, the sensor is required to have such a performance that the NOx concentration can be identified at least in units of 100 ppm.

[0010]

However, the NOx sensor decomposes the NOx component in the gas to be measured flowing into the first measuring chamber when the first measuring chamber is controlled to have a zero oxygen concentration by the pump current control. Since there is a possibility that the NOx concentration cannot be measured using the second oxygen pumping cell, the oxygen concentration in the first measurement chamber is usually controlled to a state in which a little oxygen remains (for example, a low oxygen concentration of about 1000 ppm). Therefore, the second pump current flowing through the second oxygen pumping cell has an offset under the influence of this residual oxygen. FIG. 8 is a graph schematically showing the relationship between the oxygen concentration in the measured gas and the first pump current, and the relationship between the NOx concentration and the second pump current.

The offset has a dependency on the air-fuel ratio of the gas to be measured, as shown in FIG.
When the air-fuel ratio changes due to a change in the operating state or the like, the air-fuel ratio changes by several to several tens of ppm in response to the change, so that sufficient detection accuracy cannot be obtained. FIG. 9 shows an example (measured for three samples) of the offset of the second pump current when the apparatus was operated using a test gas containing no NOx as the gas to be measured.

For this reason, in order to reliably prevent the amount of NOx leakage from the NOx storage catalyst from increasing beyond the specified value, the refresh operation is started with a considerable margin in the NOx storage capacity in consideration of the offset fluctuation. Therefore, there is a problem that the capacity of the NOx storage catalyst cannot be sufficiently brought out, and this is also the case when the refresh operation is periodically performed.

When the sulfur (S) is present in the exhaust gas, the NOx storage catalyst converts the sulfur into a sulfate to form NOx.
Since the sulfate accumulates in the storage material, and the sulfate is less likely to react with the unburned components than the nitrate, the sulfate cannot be removed even by performing the above-described refreshing operation. Only, the NOx storage capacity decreases.

When the NOx storage capacity is reduced as described above, the NOx purification capacity is reduced when refreshing is performed periodically, and when the NOx concentration is detected and refreshed. However, there has been a problem that frequent refreshing is performed and the fuel efficiency and efficiency of the internal combustion engine are reduced.

It is known that, for example, if the above-mentioned refresh (hereinafter referred to as catalyst burn-off) is performed by raising the temperature of the NOx storage catalyst to a higher temperature, the sulfate can be removed by reacting with the unburned components. Although it is conceivable to remove the sulfate, the burden on the NOx storage catalyst is larger than in the case where the normal nitrate is refreshed, and the NOx storage catalyst may be deteriorated. It was desired to do.

Accordingly, the present invention provides an NO
An object of the present invention is to make it possible to accurately detect a decrease in the function of the x storage catalyst.

[0017]

According to a first aspect of the present invention, there is provided a first oxygen pumping cell comprising an oxygen ion conductive solid electrolyte layer sandwiched between porous electrodes. A first measurement chamber having an oxygen concentration measurement cell and communicated with the measured gas side via a first diffusion-controlled layer;
A second oxygen pumping cell comprising an oxygen ion conductive solid electrolyte layer sandwiched between porous electrodes, comprising a second measurement chamber communicated with the first measurement chamber via a second diffusion-controlled layer. In addition, a NOx sensor having a heater for heating each of the cells to a predetermined activation temperature is disposed downstream of a nitrogen oxide storage catalyst attached to an exhaust pipe of an internal combustion engine, and the function of the nitrogen oxide storage catalyst is A detection method for detecting a decrease, wherein a first pump current is passed through the first oxygen pumping cell so that an output voltage of the oxygen concentration measurement cell becomes a constant value, and an oxygen concentration in the first measurement chamber is kept constant. And a constant voltage is applied to the second oxygen pumping cell in a direction of pumping oxygen from the second measurement chamber, whereby the second oxygen pumping cell is controlled according to the concentration of nitrogen oxides in the gas to be measured. When the flowing second pump current is detected, and after the operation control of the internal combustion engine at the lean air-fuel ratio in which the ratio of oxygen to fuel is large is started, the increase in the second pump current is detected by a predetermined fixed value. And determining that the storage capacity of the nitrogen oxide storage catalyst is reduced.

That is, in the detection method according to the first aspect, the NOx sensor is driven in the same driving direction as when measuring the NOx concentration using the NOx sensor. Thus, the second pump current flowing through the second oxygen pumping cell is detected.

At the time of operation control at a lean air-fuel ratio in which exhaust gas containing a large amount of NOx is exhausted from the internal combustion engine,
NOx in the exhaust gas is accumulated in the NOx storage catalyst in the form of nitrate. When the accumulated nitrate increases, the storage capacity of the NOx storage catalyst gradually decreases. Since the leaked NOx (that is, the NOx concentration in the gas to be measured) increases, the second pump current also increases.

Incidentally, as described above, the offset of the second pump current is affected by the oxygen concentration in the first measurement chamber, and the oxygen concentration in the first measurement chamber is the oxygen concentration in the gas to be measured, ie, Since the offset current is affected by the air-fuel ratio, the offset current has a dependency on the air-fuel ratio. However, in the detection method of the present invention, instead of using the absolute value of the second pump current, after starting the operation control with the lean air-fuel ratio, Whether the second pump current has increased by a fixed value, that is, the relative value of the second pump current is used to determine the deterioration of the function of the NOx storage catalyst.

Therefore, according to the present invention, if the air-fuel ratio of the gas to be measured does not fluctuate greatly due to a sudden change in the operating state of the internal combustion engine during detection, the influence of this offset can be reliably removed. Deterioration of the function of the NOx storage catalyst can be accurately determined. Then, based on this determination result, for example, a refresh operation for removing nitrate can be performed only when necessary, so that the fuel efficiency and efficiency of the internal combustion engine can be improved.

Next, a second aspect of the present invention is the first aspect.
In the detection method according to the above, when the time required for increasing the second pump current by the fixed value is smaller than a preset time threshold value, the storage capacity of the nitrogen oxide storage catalyst is abnormal. Is determined.

That is, as described above, as NOx is accumulated in the NOx storage catalyst, the amount of NOx leaking from the NOx storage catalyst increases, and accordingly, the second pump current flowing to the second oxygen pumping cell also increases. . And N
When an abnormality such as accumulation of sulfate or separation of the NOx storage material occurs in the Ox storage catalyst, the ability to store nitrate is reduced by that amount, and the rate of increase in the amount of NOx leakage from the NOx storage catalyst increases. That is, since the rate of increase of the second pump current is increased, if the rate of increase of the second pump current is examined based on the time required for the second pump current to increase by a fixed value, etc. An abnormality can be determined.

Therefore, if a refresh operation for removing sulfates is performed based on the result of this determination, the refresh operation can be performed only when it is really necessary, and an unnecessary burden is imposed on the NOx storage catalyst. Since it is not performed, the durability of the device can be improved. Further, for example, if the abnormality is not solved even if the refresh operation for removing the sulfate is performed, it is possible to know that an abnormality that cannot be eliminated by refreshing, such as peeling of the NOx storage material, has occurred.

On the other hand, the invention according to claim 3 comprises a first oxygen pumping cell and an oxygen concentration measuring cell in which an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes, And a second oxygen pumping cell in which a solid electrolyte layer having oxygen ion conductivity is interposed between porous electrodes, and a first measurement chamber communicated with the gas to be measured through the second diffusion-controlling layer. A NOx sensor provided with a heater for heating each cell to a predetermined activation temperature, and a NOx sensor attached to an exhaust pipe of an internal combustion engine. A detection device arranged downstream of the storage catalyst to detect a decrease in the function of the nitrogen oxide storage catalyst, wherein the first oxygen pumping cell is provided with a first oxygen pumping cell so that an output voltage of the oxygen concentration measurement cell becomes a constant value. Pass 1 pump current, First pump current control means for controlling the oxygen concentration in the first measurement chamber to be constant, and constant voltage application means for applying a constant voltage to the second oxygen pumping cell in a direction to pump oxygen from the second measurement chamber. A second pump current detecting means for detecting a second pump current flowing through the second oxygen pumping cell in accordance with a nitrogen oxide concentration in the gas to be measured, and an internal combustion engine with a lean air-fuel ratio having a large ratio of oxygen to fuel. After the operation control is started, when an increase in the second pump current is detected by a preset fixed value, a first determination unit that determines that the storage capacity of the nitrogen oxide storage catalyst has decreased, It is characterized by having.

That is, the detection device of the present invention is characterized in that
First, the first pump current control means passes the first pump current to the first oxygen pumping cell so that the output voltage of the oxygen concentration measurement cell becomes a constant value, The oxygen concentration in the first measurement chamber is controlled to be constant, and the constant voltage applying means applies a constant voltage to the second oxygen pumping cell in a direction to pump oxygen from the second measurement chamber. Then, the second pump current detecting means detects the second pump current flowing through the second oxygen pumping cell, and the first determining means starts operation control of the internal combustion engine based on the lean air-fuel ratio in which the ratio of oxygen to fuel is large. After that, when an increase in the second pump current is detected by a predetermined fixed value, it is determined that the storage capacity of the nitrogen oxide storage catalyst has decreased.

Therefore, according to the detection device of the third aspect, the detection method of the first aspect is realized to easily and accurately reduce the function of the NOx storage catalyst from the relative value of the second pump current. The detection can be performed well, and the fuel consumption and efficiency of the internal combustion engine can be improved.

Normally, operation control at a lean air-fuel ratio is performed during a stable operation state, such as during operation at a constant speed. However, the operation state is controlled by a driver's accelerator operation or a change in the state of a traveling road. If the oxygen concentration (air-fuel ratio) in the gas to be measured temporarily fluctuates due to the change, the offset of the second pump current changes because it has a dependency on the air-fuel ratio, and the offset of the second pump current changes. Even if the value is obtained, the influence of the offset current cannot be completely removed, and the determination accuracy of the first determination unit deteriorates.

According to a fourth aspect of the present invention, in the detection device of the third aspect, an oxygen concentration detecting apparatus detects an oxygen concentration in a gas to be measured from a first pump current flowing through the first oxygen pumping cell. Means for correcting the offset of the second pump current included in the detection result by correcting the detection result of the second pump current detection means in accordance with the oxygen concentration detected by the oxygen concentration detection means. And a first correction means for compensating for the minute.

In the detection apparatus of the present invention thus configured, the oxygen concentration detecting means detects the oxygen concentration in the gas to be measured from the first pump current flowing through the first oxygen pumping cell, and the first correction means By correcting the detection result by the second pump current detection means according to the detected oxygen concentration, the offset fluctuation included in the detection result is compensated.

The pump current control for controlling the current flowing in the first pumping cell to keep the oxygen concentration in the first measurement chamber constant is performed by controlling the pumping cell and the oxygen concentration in the measurement chamber where diffusion of the gas to be measured is limited. The operation is the same as the operation when measuring the oxygen concentration in the gas to be measured using a well-known all-area air-fuel ratio sensor provided with a measurement cell. The pump current flowing through the first pumping cell is Since it is proportional to the concentration, the oxygen concentration can be measured from the current value.

Therefore, according to the present invention, even if the oxygen concentration of the gas to be measured changes due to a change in the operating state or the like and the offset of the second pump current fluctuates accordingly,
Since the fluctuation is compensated, it is possible to determine with a high degree of accuracy the function of the nitrogen oxide storage catalyst.

Further, in the present invention, since the NOx sensor itself is used for detecting the oxygen concentration without using another sensor, a change in the environment affecting the second pump current can be accurately detected. , The second pump current can be accurately compensated. Here, as described above, the offset of the second pump current has a dependency on the oxygen concentration (air-fuel ratio) of the gas to be measured, but also has a dependency on temperature (temperature characteristic). As shown in FIG. 11, when the element temperature of the NOx sensor deviates from the target temperature to be controlled, the oxygen concentration dependence of the offset increases. FIG. 11 is a graph showing the results of measuring the temperature characteristics of the offset of the second pump current while changing the oxygen concentration.

Therefore, according to a fifth aspect of the present invention, in the detecting device of the third aspect, a temperature detecting means for detecting a temperature of the NOx sensor, and a temperature detecting means for detecting the temperature of the NOx sensor detected by the temperature detecting means. A second correction unit is provided, which corrects the detection result of the second pump current detection unit in accordance with the temperature to compensate for the temperature of the detection result.

That is, in the present invention, the temperature detecting means is set to NO.
The temperature of the x sensor is detected, and the second correction unit corrects the detection result of the second pump current detection unit in accordance with the temperature of the NOx sensor detected by the temperature detection unit.
This detection result is temperature compensated.

Therefore, according to the present invention, even if the temperature of the NOx sensor temporarily changes due to a change in the operation state and the offset of the second pump current changes,
Since the fluctuation can be compensated for, it is possible to more accurately determine the deterioration of the function of the nitrogen oxide storage catalyst.

When the oxygen concentration or the temperature of the NOx sensor greatly fluctuates and exceeds the range where accurate correction can be performed, the determination by the first determination means is stopped. In this case, erroneous determination can be reliably prevented. Next, according to a sixth aspect of the present invention, in the detecting device according to any one of the third to fifth aspects, a time required for detecting an increase of the second pump current by the fixed value is measured. Current increase time measuring means to perform, and when the detection result by the current increase time measuring means is smaller than a preset time threshold value, it is determined that the storage capacity of the nitrogen oxide storage catalyst is abnormal. 2 determination means.

That is, the detection device of the present invention is characterized in that
Wherein the current increasing time measuring means measures a time required for detecting an increase of the second pump current by a fixed value, and the second determining means determines a current increasing time. When the result of the measurement by the measuring means is smaller than a preset time threshold value, it is determined that the storage capacity of the nitrogen oxide storage catalyst is abnormal.

Therefore, according to the present invention, the detection method according to the second aspect is realized, and the change in the relative value of the second pump current with time indicates the accumulation of sulfate and the separation of the NOx storage material.
The abnormality of the Ox storage catalyst can be detected simply and accurately, and the fuel economy and efficiency of the internal combustion engine and the durability of the device can be improved.

The time threshold value is determined based on the NOx storage capacity of the NOx storage catalyst and the NOx concentration in the exhaust gas discharged from the internal combustion engine.
The amount of leakage may be estimated and set. By the way, NOx
The higher the concentration of NOx in the exhaust gas flowing into the storage catalyst, and even if the concentration of NOx is the same, the larger the flow rate of the inflowing exhaust gas, the larger the amount of NOx to be stored in the NOx storage medium. The temporal change of the current value detected by the second pump current detecting means also changes according to the state of the inflowing exhaust gas.

Therefore, according to a seventh aspect of the present invention, in the detection device of the sixth aspect, the flow rate of the exhaust gas flowing into the nitrogen oxide storage catalyst and the concentration of the nitrogen oxide in the exhaust gas are detected. A time threshold for setting the time threshold to a smaller value as the flow rate is higher and the nitrogen oxide concentration is higher, based on the detection result of the inflow gas state detection means and the inflow gas state detection means. A setting means is provided.

As described above, if the optimum time threshold is set according to the flow rate of the exhaust gas flowing into the NOx storage catalyst and the NOx concentration of the exhaust gas, the abnormality of the NOx storage catalyst can be reduced. Detection can be performed even more accurately. And
The NOx concentration in the inflowing exhaust gas and the flow rate of the inflowing exhaust gas are highly related to the operating state of the internal combustion engine, such as the engine speed and the negative pressure of the intake pipe. Item 7. The detection device according to Item 7, wherein the inflow gas state detection means may be configured to estimate a flow rate of the exhaust gas and a nitrogen oxide concentration in the exhaust gas from an operation state of the internal combustion engine.

In this case, it is not necessary to provide new means for measuring the state of the exhaust gas flowing into the NOx storage catalyst, and various parameters used for controlling the internal combustion engine can be used. The accuracy of the detection device can be improved.

[0044]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] FIG. 1 is a schematic configuration diagram showing the entire configuration of a device for detecting a decrease in the function of a nitrogen oxide (NOx) storage catalyst according to a first embodiment of the present invention, and FIG. FIG. 3 is an exploded perspective view of the NOx sensor 2 showing a mounting position of the NOx sensor 2.

As shown in FIGS. 1 and 2, the detection device of the present embodiment includes an exhaust pipe S attached to an internal combustion engine S1 of a vehicle.
2, a NOx sensor 2 attached downstream of the NOx storage catalyst S3, a first oxygen pumping cell (hereinafter, referred to as a first pump cell) 4 and an oxygen concentration measuring cell (hereinafter, referred to as a Vs cell) 6, which constitute the NOx sensor 2. And the current flowing through the first pump cell 4 (hereinafter, referred to as
Drive circuit 40 for detecting IP1 (referred to as first pump current)
And a current (hereinafter, referred to as a second pump current) I flowing when a constant voltage is applied to a second oxygen pumping cell (hereinafter, referred to as a second pump cell) 8 constituting the NOx sensor 2.
The detection circuit 42 for detecting P2 and a pair of heaters 12 and 14 provided in the NOx sensor 2 are energized so that the cells 4, 6, and
Heater heating circuit 44 for heating heater 8 and NOx sensor 2
A temperature sensor 46 for detecting a nearby temperature TH, a pressure sensor 47 for measuring a negative pressure Pb of an intake pipe S4 attached to the internal combustion engine S1, and a rotation speed of an output shaft of the internal combustion engine S1 (hereinafter referred to as an engine rotation speed). 3.) The rotation sensor 48 for detecting Ne, the drive circuit 40 and the heater energizing circuit 44 are controlled, and the detection signals VIP1 and VIP2 from the drive circuit 40 and the detection circuit 42, and the detection signal TH from each of the sensors 46, 47 and 48. , Pb, Ne, the NOx storage catalyst S3
And an electronic control circuit (hereinafter referred to as an ECU) 50 composed of a microcomputer for detecting a decrease in the function of the microcomputer.

Further, the detecting device of this embodiment controls the operating state of the internal combustion engine S1 so that the lean air-fuel ratio becomes a lean air-fuel ratio when the operating state is stable, such as at a constant speed, and a stoichiometric air-fuel ratio otherwise. The operation control information indicating that the operation control at the lean air-fuel ratio is being performed is input from the engine control device 52 that performs the operation.

Here, of the exhaust gas flowing through the exhaust pipe S2, the one exhausted from the internal combustion engine S1 and flowing into the NOx storage catalyst S3 is referred to as exhaust gas,
What flows out of the x storage catalyst S3 is called a gas to be measured.
Next, as shown in FIG.
The pump cell 4 forms rectangular porous electrodes 4b and 4c and lead portions 4bl and 4cl thereof on both sides of a solid electrolyte layer 4a formed in a plate shape, respectively.
A round hole is formed in the solid electrolyte layer 4a so as to penetrate the center portion of the solid electrolyte layer 4c, and a porous filler is filled in the round hole to form a diffusion-controlling layer 4d.

The Vs cell 6 is provided on both sides of a solid electrolyte layer 6a having the same shape as the solid electrolyte layer 4a of the first pump cell 4.
Circular porous electrodes 6b, 6c and their lead portions 6bl, 6cl are formed, respectively, and further, a round hole is formed in the solid electrolyte layer 6a so as to penetrate through the center of the porous electrodes 6b, 6c. By filling the round hole with a porous filler,
This is one in which a diffusion-controlling layer 6d is formed.

The porous electrode 6 of the Vs cell 6
b, 6c and the porous electrodes 4b, 4c of the first pump cell 4 have substantially the same center positions on the solid electrolyte layers 4a, 6a, and when the Vs cell 6 and the first pump cell 4 are stacked, each diffusion The rate limiting layers 6d and 4d are configured to face each other. The circular porous electrodes 6b and 6c formed in the Vs cell 6 are smaller than the rectangular porous electrodes 4b and 4c formed in the first pump cell 4. Also,
On the front and back surfaces of the Vs cell 6, an insulating film made of alumina or the like is formed so as to cover the lead portions 6bl and 6cl from the outside in order to prevent current leakage from the lead portions 6bl and 6cl. Between the portions 6bl and 6cl, there is formed a leakage resistance portion 6f that allows a part of oxygen pumped into the porous electrode 6c to leak to the porous electrode 6b by the energization control described later.

The first pump cell 4 and the Vs cell 6 formed in this manner are stacked via a solid electrolyte layer 18 having the same shape as the solid electrolyte layers 4a and 6a. A rectangular hole larger than the porous electrode 4c is formed in the solid electrolyte layer 18 at a position facing each of the porous electrodes 4c and 6b, and the hole functions as the first measurement chamber 20. I do.

The Vs cell 6 also has a porous electrode 6c side.
The solid electrolyte layers 22 having the same shape as the solid electrolyte layers 4a and 6a are stacked. The solid electrolyte layer 22 is provided with a round hole of the same size at the same position as the diffusion controlling layer 6d of the Vs cell 6, and the porous hole is filled in the round hole to form a diffusion controlling layer. 22d are formed.

On the other hand, similarly to the first pump cell 4, the second pump cell 8 has rectangular porous electrodes 8b and 8c and lead portions 8bl and 8cl on both sides of a solid electrolyte layer 8a formed in a plate shape. Is formed. The second pump cell 8 is stacked on the solid electrolyte layer 22 via a solid electrolyte layer 24 formed in exactly the same manner as the solid electrolyte layer 18. As a result, the rectangular hole formed in the solid electrolyte layer 24 functions as the second measurement chamber 26.

The NOx sensor 2 excluding the heaters 12 and 14 is manufactured by sintering at a predetermined temperature after laminating and integrating the above components. Also, both sides of the stacked body of the first pump cell 4, the Vs cell 6, and the second pump cell 8 stacked in this manner, that is, the first pump cell 4 and the second
Outside the pump cell 8, heaters 12 and 14 are stacked at predetermined intervals by spacers 28 and 29, respectively.

The heaters 12 and 14 have the same shape as the solid electrolyte layers 4a, 6a and 8a.
4a and the cells 4 of the heater substrates 12a and 14a.
Heater wirings 12b and 14b formed on the surface facing surface 8
And the lead portions 12bl and 14bl, and the spacers 28 and 29 form the first heater wires 12b and 14b.
The porous electrodes 4b and 8c of the pump cell 4 and the second pump cell 8 are opposed to each other with a gap therebetween.
The heater wires 12b and 14b are arranged on the lead portions 12bl and 14bl side.

The heater substrates 12a and 14a are made of alumina, and the heater wiring is formed by screen-printing and firing a paste made by mixing alumina with platinum powder on an alumina sheet. The alumina sheet becomes the heater substrates 12a and 14a and the spacers 28 and 29 by firing. The heaters 12 and 14 are joined from both surfaces of the first pump cell 4 and the second pump cell 8 which have already been fired, using a ceramic adhesive, and become a complete NOx sensor 2.

Here, each of the solid electrolyte layers 4a, 6a,
As a solid electrolyte material constituting 8a, a solid solution of zirconia and yttria or a solid solution of zirconia and calcia are typical, but a solid solution of hafnia, a perovskite-type oxide solid solution, a trivalent metal oxide solid solution and the like are also available. Can be used. For the porous electrodes provided on the surfaces of the solid electrolyte layers 4a, 6a, 8a, it is preferable to use platinum, rhodium, or an alloy thereof having a catalytic function. And, as a forming method, for example, a mixture of platinum powder and the same material powder as the solid electrolyte layer is made into a paste,
A method of forming a thick film by screen printing on a solid electrolyte layer and then sintering, and a method of forming a film by vapor deposition are known. Further, it is preferable that the diffusion controlling layers 4d, 6d, and 22d use ceramics having a small through hole or porous ceramics.

On the other hand, the heater wiring 12 of the heaters 12 and 14
b and 14b are made of a composite material of ceramics and platinum or a platinum alloy, and the lead portions 12bl and 14bl are preferably made of platinum or a platinum alloy in order to reduce the resistance value and reduce the electric loss in the lead portion. preferable. Also, the heater substrate 1
Alumina is used for 2a, 14a and spacers 28, 29.
Spinel, forsterite, steatite, zirconia and the like can be used.

Next, as shown in FIG. 1, the first pump cell 4 of the NOx sensor 2 and the porous electrodes 4c, 6b of the Vs cell 6 on the first measuring chamber 20 side are grounded via a resistor R1. And the other porous electrodes 4b and 6c
Connected to 0. The drive circuit 40 has a constant voltage V at one end.
CP is applied, the other end is connected to a resistor R2 connected to the porous electrode 6c of the Vs cell 6, and an open / close switch S is connected to the negative input terminal.
The porous electrode 6c of the Vs cell 6 is connected via W1,
The differential amplifier AMP has a reference voltage VCO applied to the + input terminal and an output terminal connected to the porous electrode 4b of the first pump cell 4 via the resistor R0.

The driving circuit 40 operates as follows. That is, first, a certain minute current iCP is supplied to the Vs cell 6 via the resistor R2, so that oxygen in the first measurement chamber 20 is pumped into the porous electrode 6c side of the Vs cell 6. The porous electrode 6c is closed by the solid electrolyte layer 22 and communicates with the porous electrode 6b through the leakage resistance portion 6f. The space has a constant oxygen concentration and functions as an internal oxygen reference source.

As described above, the porous electrode 6 of the Vs cell 6
When the c-side functions as the internal oxygen reference source, an electromotive force is generated in the Vs cell 6 according to the ratio of the oxygen concentration in the first measurement chamber 20 to the oxygen concentration on the internal oxygen reference source side, and the porous electrode 6
The c-side voltage Vs is a voltage corresponding to the oxygen concentration in the first measurement chamber 20. Since this voltage is input to the differential amplifier AMP, the differential amplifier AMP outputs a voltage corresponding to a deviation (VCO−input voltage) between the reference voltage VCO and the input voltage. Is applied to the porous electrode 4b of the first pump cell 4 via the resistor R0.

As a result, the first pump current IP1 flows through the first pump cell 4, and the first pump current IP1 causes
Control is performed so that the electromotive force generated in the Vs cell 6 becomes a constant voltage (in other words, the oxygen concentration in the first measurement chamber 20 becomes a constant concentration). That is, the driving circuit 40
Functions as a pump current control means, and the diffusion-controlling layer 4d
When the gas to be measured flows into the first measurement chamber 20 via the, the oxygen concentration in the first measurement chamber 20 is controlled such that the oxygen concentration in the first measurement chamber 20 becomes constant.

The first measuring chamber 2 controlled as described above
The oxygen concentration within 0 is set to a low oxygen concentration (for example, 1000 ppm) in which a small amount of oxygen is present so that the NOx component in the gas to be measured in the first measurement chamber 20 is not decomposed by the application of the first pump current IP1. ), And the reference voltage VCO for determining the oxygen concentration is 10
A value of about 0 mV to 200 mV is set. A resistor R0 provided between the output of the differential amplifier AMP and the porous electrode 4b is for detecting the first pump current IP1.
The detection signal of P1 is input to the ECU 50.

On the other hand, the second pump cell 8 of the NOx sensor 2
A constant voltage VP2 is applied between the porous electrodes 8b and 8c via a resistor R3 as a constant voltage applying means constituting the detection circuit 42. The application direction of this constant voltage VP2 is
In the second pump cell 8, the porous electrode 8c side is a positive electrode and the porous electrode 8 is so configured that an electric current flows from the porous electrode 8c to the 8b side and oxygen in the second measurement chamber 26 is pumped out.
The b side is set to be the negative electrode. The constant voltage VP2 is supplied from the first measurement chamber 20 to the diffusion-controlling layers 6d and 22d.
The voltage is set to a voltage that can decompose the NOx component in the gas to be measured in the second measurement chamber flowing through d and pump out the oxygen component, for example, 450 mV.

The resistor R3 applies a second pump current I flowing through the second pump cell 8 by applying the constant voltage VP2.
This is for converting P2 into a voltage VIP2 and inputting it to the ECU 50 as a detection signal of the second pump current IP2. In the function deterioration detecting device for the NOx storage catalyst S3 of the present embodiment thus configured, the operation of the drive circuit 40 causes the gas to be measured to flow through the diffusion-controlled layer (first diffusion-controlled layer) 4d. The oxygen concentration in the first measurement chamber 20 is controlled to a constant oxygen concentration, and the gas to be measured in the first measurement chamber 20 controlled to the constant oxygen concentration passes through the diffusion control layers (second diffusion control layers) 6d and 22d. As a result, the first pump current IP1 flowing through the first pump cell 4 changes in accordance with the oxygen concentration in the gas to be measured, and the second pump current IP2 flowing through the second pump cell 8 changes into the second pump chamber 4. NOx in gas
The ECU 50 reads the detection signals VIP1 and VIP2 representing the currents IP1 and IP2 on the ECU 50 side and executes predetermined arithmetic processing to determine the oxygen concentration and the NOx concentration in the gas to be measured. Can be measured.

It is necessary to control the temperature of the NOx sensor 2 to be constant in order to ensure the measurement accuracy of each of these concentrations. For this reason, the temperature TH detected by the temperature sensor 46 is required.
Is controlled so as to reach the target temperature.

Here, FIG. 4 shows that the engine control unit 52 controls the operation of the internal combustion engine S1 at the stoichiometric air-fuel ratio (hereinafter, referred to as operation control).
Operation control with lean air-fuel ratio (hereinafter referred to as normal control)
7 is a graph showing a measurement result of a second pump current IP2 detected by a NOx sensor 2 attached downstream of the NOx storage catalyst S3 when the control is switched to lean control.

As shown in FIGS. 4A and 4B, the internal combustion engine S
When control 1 switches from normal control (A / F (air-fuel ratio) $ 14) to lean control (A / F ??), NOx
Since the NOx storage capacity of the storage catalyst S3 has room,
NOx hardly leaks out of the x storage catalyst S3.
Thereafter, as the amount of NOx stored as nitrate on the NOx storage catalyst S3 increases and the NOx storage capacity of the NOx storage catalyst S3 decreases, the amount of NOx leaking downstream of the NOx storage catalyst S3, that is, And the second pump current IP2 accordingly increases.
When the NOx storage catalyst S3 finally becomes unable to store NOx, the NOx concentration of the gas to be measured becomes NOx.
The value becomes substantially the same as the NOx concentration of the exhaust gas flowing into the storage catalyst S3.

FIG. 4 (c) shows the measurement results when sulfur is contained in the NOx storage catalyst S3 when the fuel contains sulfur, and the NOx storage capacity is reduced. , The slope when the second pump current IP2 increases is large. Hereinafter, such a NOx storage catalyst S
The function deterioration detection process executed by the ECU 50 to detect the function deterioration 3 will be described with reference to the flowchart shown in FIG.

This process is repeatedly executed after the NOx sensor 2 is activated by energizing the heaters 12 and 14. As shown in FIG. 5, in this processing, first, at S110
In (S represents a step), the timer used in the present process is reset, and in S120, lean control by the engine control device 52 is started based on the operation control information input from the engine control device 52. It is determined whether the lean control has been started.
Move to 30.

In S130, the timer reset in S110 is started, and in S140, it is determined whether the standby time Tw has elapsed. This is because immediately after switching from the normal control to the lean control, the oxygen concentration in the exhaust gas becomes unstable in accordance with the switching of the control, and the offset of the second pump current IP2 also fluctuates to perform accurate detection. Because you can't. Therefore, the standby time Tw is set to such a length that the fluctuation of the second pump current IP2 based on the control switching sufficiently converges.

When it is determined in S140 that the standby time Tw has elapsed, the flow shifts to S150, where the detection signal V
By reading IP2, the second pump current IP2 is detected, and the processing as the second pump current detecting means is executed.
Next, in S160, the temperature sensor 46, the pressure sensor 47,
And the rotation sensor 48 perform processing as temperature detecting means for detecting the temperature TH of the NOx sensor 2, the intake pipe negative pressure Pb, and the engine speed Ne, and inflow gas state detecting means. The allowable fluctuation range of the negative pressure Pb and the engine speed Ne is calculated.

After the timer is started in S130, the average value is calculated from the intake pipe negative pressure Pb and the engine speed Ne which are repeatedly detected in S160. A predetermined range around the average value (for example, ± 10%) is set as the set value.

The allowable fluctuation range is determined by the NOx storage catalyst S3
Flow rate of exhaust gas flowing into the exhaust gas, NOx concentration in exhaust gas,
This is a reference value for detecting a sudden change in the air-fuel ratio, etc.
The state of the exhaust gas is not directly detected, but indirectly detected from the operating state of the internal combustion engine S1 such as the engine speed Ne and the intake pipe negative pressure Pb, which are factors that determine the state of the exhaust gas.

That is, in this embodiment, as will be described later,
The second pump current IP2 detected at 150 is used as the sensor temperature T
Although the temperature is compensated based on H, if the operating state changes significantly, sufficient accuracy may not be ensured even if the temperature is compensated, so the range in which sufficient accuracy can be secured by temperature compensation is limited. It is doing.

At S180, the intake pipe negative pressure Pb and the engine speed Ne detected at S160 are calculated as follows.
It is determined whether or not it is within the range of the allowable fluctuation range set in S170.
90, the timer is stopped, and at S200, N
After outputting the refresh request for the Ox storage catalyst S3 to the engine control device 52, the present process is terminated.

On the other hand, if it is determined in S180 that both the intake pipe negative pressure Pb and the engine speed Ne are within the allowable fluctuation range, the process proceeds to S210 and proceeds to S160.
Based on the sensor temperature TH detected at step S150, a process as a second correction means for correcting the second pump current IP2 detected at step S150 is executed.

That is, in this embodiment, the heater energizing circuit 4
4 is controlled so that the temperature detected by the temperature sensor 46 is constant. However, when the internal combustion engine S1 is accelerated, the exhaust gas temperature temporarily decreases as the intake air amount increases, If the exhaust gas temperature rises temporarily as the intake air amount decreases during the deceleration of the engine S1, the NOx sensor 2 is affected by the temperature change, and both the first pump current IP1 and the second pump current IP2 change. In particular, the second pump current IP2 requires a relatively long time (about one minute) to return to a stable state. This is because the first pump current IP1 is affected by the exhaust gas temperature, so that the first measurement chamber 2
Once the oxygen concentration within 0 deviates from the target concentration,
This is because it takes time to return the oxygen concentration to the target concentration, and the offset of the second pump current IP2 changes based on the change in the oxygen concentration.

Therefore, in the present embodiment, even if the exhaust gas temperature changes abruptly, the temperature sensor 46 detects NO so that the fluctuation of the offset of the second pump current IP2 can be accurately canceled.
The temperature of the x sensor 2 is obtained, the temperature correction amount is obtained using a map for calculating the temperature correction amount as shown in FIG. 7, for example, and the second pump current IP2 is corrected.

Then, in S220, using the corrected second pump current IP2, a relative value ΔIP2 (= IP2) with the second pump current IP2o (time t1 in FIG. 4) detected first after the timer is started. -IP2o), and the following S23
0, the relative value △ IP2 is equal to a predetermined fixed value I
c, the processing is performed as first determining means for determining whether or not the NOx storage catalyst S
It is assumed that the storage capacity of No. 3 still has room (not deteriorated), and the process returns to S150.

Note that S350 to S380, S410
While the processing of S430 to S430 is repeated, NOx in the exhaust gas is accumulated as nitrate in the NOx storage catalyst S3.
As time passes and the amount of accumulated nitrate increases, NO
Since the storage capacity of x decreases and the NOx concentration in the gas to be measured downstream of the NOx storage catalyst S3 increases, the second pump current IP2 and, consequently, the relative value ΔIP2 gradually increase.

As a result, if the relative value △ IP2 becomes equal to or larger than the fixed value Ic, and the determination in S230 is affirmative, the process proceeds to S240 to stop the timer and store the current timer value as the measured value TO. A process as an increase time measuring means is executed, and in S250, the NOx storage catalyst S3
A process as time threshold setting means for calculating a time threshold Tth for detecting an abnormality in the occlusion capacity is executed.

The time threshold value Tth corresponds to the exhaust gas flow rate and the NOx concentration estimated from the average values of the intake pipe negative pressure Pb and the engine speed Ne repeatedly detected in S160 while the timer is running. Based on the N in the measured gas
The estimated time required for the second pump current IP2 for detecting the Ox concentration to exceed the fixed value Ic is set as a set value. Note that the time threshold value Tth may be set in advance by using a map in which each average value of the intake pipe negative pressure Pb and the engine speed Ne is used as a parameter.

At S260, a process as a second judging means for judging whether or not the measured value TO stored at S240 is smaller than the time threshold Tth is executed. The process proceeds to S200 assuming that the storage catalyst S3 has deteriorated in function due to the accumulation of nitrate, and outputs a refresh request for the NOx storage catalyst S3 to the engine control device 52.

On the other hand, if an affirmative determination is made in S260, it is determined that the NOx storage catalyst S3 has caused an abnormality such as accumulation of sulfate or separation of the NOx storage material, and a catalyst burnout request is output to the engine control device 52. Thereafter, the present process ends. When a refresh request is input from the ECU 50, the engine control device 52 temporarily controls the air-fuel ratio to become rich, discharges unburned gas from the internal combustion engine S1, and outputs the unburned gas and the NOx storage catalyst. The NOx storage catalyst S3 is refreshed by reacting the nitrate accumulated in S3. Further, when a catalyst burn-off request is input from the ECU 50, a state in which the sulfate accumulated in the NOx storage catalyst S3 reacts and is reduced is temporarily created to refresh (burn-off processing) the NOx storage catalyst S3. Execute.

As described above, in the NOx storage catalyst function deterioration detecting device of this embodiment, the specific time t1 after the start of the lean control for controlling the operation of the internal combustion engine S1 at the lean air-fuel ratio is started. When the relative value ΔIP2 of the detected second pump current IP2 increases by a fixed value Ic,
That is, when NOx leaking from the NOx storage catalyst S3 increases by a predetermined amount, it is determined that the storage capacity of the NOx storage catalyst S3 has decreased, and the relative value △ IP2 is set to the fixed value Ic.
Is increased by the time threshold Tth.
If it is smaller, it is determined that there is an abnormality in the storage capacity of the NOx storage catalyst S3.

As described above, in the present embodiment, the relative value ΔIP2 which offsets the offset of the second pump current IP2,
Since the deterioration of the function of the NOx storage catalyst S3 is detected using the temporal change (gradient) of the pump current IP2 without using the absolute value of the second pump current IP2, the second pump current I2
Accurate detection can be performed without being affected by the offset of P2.

Further, in this embodiment, the temperature fluctuation of the offset of the second pump current IP2 is compensated based on the sensor temperature TH detected by the temperature sensor 46.
Even if the sensor temperature fluctuates during detection, it is not affected by this. In this embodiment, the internal combustion engine S
The parameters (intake pipe negative pressure Pb, engine speed Ne) representing the operating state of No. 1 are sequentially detected, and if the detected values are out of the allowable fluctuation range, the operating state is assumed to have changed abruptly, and the function is performed. The determination of the drop is stopped, and a refresh request is output immediately.

Accordingly, it is possible to reliably prevent an erroneous determination that the catalyst burn-out request that places a burden on the apparatus is output in vain, thereby improving the reliability and durability of the apparatus. [Second Embodiment] Next, a second embodiment will be described.

This embodiment is different from the first embodiment in that the ECU 50
Only the part of the function deterioration detection processing performed by the present embodiment is different, so only the different part of this processing will be described. Here, FIG. 6 is a flowchart showing the function deterioration detection processing in this embodiment. In steps S310 to S350, the timer is reset, and the operation control of the internal combustion engine S1 is switched to the lean control, just like the first embodiment. When the standby time Tw elapses, the second pump current I
Start detecting P2.

Then, in S360, the temperature sensor 46,
The sensor temperature T is determined by the pressure sensor 47 and the rotation sensor 48.
H, the intake pipe negative pressure Pb and the engine speed Ne are detected, and a process as oxygen concentration detecting means for detecting the first pump current IP1 by reading the detection signal VIP1 is executed.

At S370, the allowable fluctuation range of the first pump current IP1 is calculated. The permissible fluctuation range is calculated by calculating an average value of the first pump current IP1 repeatedly detected in S360, and a predetermined range (for example, ± 10
%) As the set value. Then, in S380, it is determined whether or not the first pump current IP1 detected in S360 is within the range of the allowable variation set in S370. Example S190,
The processing of S390 and S400 is executed exactly as in S200, that is, the timer is stopped, a refresh request for the NOx storage catalyst S3 is output to the engine control device 52, and this processing ends.

On the other hand, if an affirmative determination is made in S380, the flow shifts to S410, and in S350 based on the sensor temperature TH detected in S160 and the first pump current IP1.
The processing as the first and second correction means for correcting the second pump current IP2 detected in the step (1) is executed.

That is, in this embodiment, in addition to the temperature compensation performed in the first embodiment, a change in the oxygen concentration (air-fuel ratio) of the gas to be measured is directly detected to more precisely correct the second pump current IP2. are doing. Specifically, in order to make the second pump current IP2 correspond only to the NOx concentration in the gas to be measured, the gas to be measured containing no NOx is measured as described above (see FIG. 10).
The offset value of the second pump current IP2 corresponding to the oxygen concentration obtained at the time of the above) is stored in a map in advance, and the oxygen concentration in the measured gas detected from the first pump current IP1 is used as a parameter, The detected second pump current IP2 is corrected by the offset value read from the map.

Then, in subsequent S420 to S470, the first
In exactly the same manner as in S220 to S270 of the embodiment, based on the corrected second pump current IP2, the relative value ΔIP2 with the second pump current IP2o first detected after the timer starts.
(S420), and the relative value △ IP2 is a fixed value Ic
It is determined whether or not this is the case (S430). If the relative value △ IP2 is smaller than the fixed value, the process returns to S350. On the other hand, if the relative value △ IP2 is equal to or larger than the fixed value, the timer is stopped and the value at that time is stored as the measured value TO (S440).
The time threshold value Tth is calculated based on the average value of the intake pipe negative pressure Pb and the engine speed Ne repeatedly detected in (S450), and the measured value TO stored in S440 is used as the time threshold value Tth. It is determined whether it is smaller than (S4
60). As a result, if the measured value TO is equal to or longer than the time threshold value Tth, a refresh request is output to the engine control device 52 (S400). If the measured value TO is smaller than the time threshold value Tth, the engine control device 52 is output. (S470), and terminates the present process.

As described above, according to the NOx storage catalyst function deterioration detecting device of the present embodiment, the relative value ΔIP2 of the second pump current IP2 and the second pump current Using the temporal change (gradient), the second pump current I
Since the function deterioration of the NOx storage catalyst S3 is detected without using the absolute value of P2, accurate detection can be performed without being affected by the offset of the second pump current IP2.

Further, in this embodiment, the second pump current IP2 is corrected not only based on the sensor temperature TH but also based on the oxygen concentration of the gas to be measured detected based on the first pump current IP1. Therefore, even if the sensor temperature TH or the oxygen concentration (air-fuel ratio) of the gas to be measured fluctuates during the detection, accurate detection can be performed without being affected by the fluctuation.

Further, since the NOx sensor 2 detects the oxygen concentration of the gas to be measured without providing a new sensor, the environmental change affecting the second pump current IP2 can be detected more accurately. And the second pump current I
P2 can be accurately corrected.

In the above embodiment, the first detection value IP2o, which is a reference value for calculating the relative value △ IP2 of the second pump current, is detected when the NOx storage capacity of the NOx storage catalyst S3 has a sufficient margin. Is assumed to be
When the storage capacity of the NOx storage catalyst S3 is significantly reduced due to a large amount of separation of the NOx storage material or the like, as shown in FIG. 8, immediately after switching to the lean control, the second pump current IP2 rapidly increases. . In this case, at the time point t1 when the first detection is performed after the elapse of the standby time Tw, the current value has already risen greatly, and therefore, the relative value of the second pump current と し てAfter calculating IP2,
It is possible that the relative value △ IP2 does not exceed the fixed value Ic and the control becomes impossible.

Therefore, in order to prevent such a situation, for example, an upper limit of the timer value is determined, and when a time-out occurs, a catalyst burn-off request is output assuming that the storage capacity of the NOx storage catalyst S3 is abnormal. May be configured. Further, a plurality of fixed values to be compared with the second pump current IP2 are set in a stepwise manner, and the slopes of the second pump current IP2 during the period from one fixed value to the next fixed value are respectively obtained. Any abnormal situation may be estimated.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments, and can be implemented in various modes. For example, in the above embodiment, the temperature TH of the NOx sensor 2 is obtained from the temperature sensor 46.
As described in JP-A-296676, the resistance value of the Vs cell 6 is detected, and the NOx sensor 2 is detected based on the detected resistance value.
May be determined.

Further, in the above embodiment, the time threshold value is set based on the intake pipe negative pressure Pb and the engine speed Ne. However, as long as the time threshold value affects the flow rate of the exhaust gas and the NOx concentration, Parameter may be used. Further, in the second embodiment, the oxygen concentration is detected by the NOx sensor 2, but the oxygen concentration may be detected by using another oxygen sensor.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating a configuration of an entire device for detecting a decrease in the function of a nitrogen oxide storage catalyst according to an embodiment.

FIG. 2 is an explanatory diagram illustrating a mounting position of a NOx sensor according to the embodiment.

FIG. 3 is an exploded perspective view illustrating a configuration of a NOx sensor according to the embodiment.

FIG. 4 is a waveform diagram of a second pump current output from the NOx sensor of the embodiment.

FIG. 5 is a flowchart illustrating a function deterioration detection process repeatedly executed in the ECU of the embodiment.

FIG. 6 is a flowchart illustrating a function deterioration detection process according to the second embodiment.

FIG. 7 is a graph showing an example of a map used for correcting the temperature of a second pump current.

FIG. 8 is a waveform diagram of a second pump current detected when the NOx storage capacity of the NOx storage catalyst is extremely reduced.

FIG. 9 is a graph schematically showing a relationship between an oxygen concentration and a first pump current and a relationship between a nitrogen oxide concentration and a second pump current.

FIG. 10 is a graph showing a relationship between an oxygen concentration (air-fuel ratio) of a gas to be measured containing no NOx and a second pump current.

FIG. 11 is a graph showing a result of measuring a temperature characteristic of an offset of a second pump current.

[Explanation of symbols]

2 NOx sensor 4 First pump cell 6 Oxygen concentration measurement cell 8 Second pump cell 12 Heaters 18 and 22
24 solid electrolyte layer 20 first measurement chamber 26 second measurement chamber 28 spacer 40 drive circuit 42 detection circuit 44 heater energization circuit 46 temperature sensor 47 pressure sensor 48 rotation sensor 50 ECU 52 Engine control device S1
... internal combustion engine S2 ... exhaust pipe S3 ... NOx storage catalyst S4
… Intake pipe

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI G01N 27/416 G01N 27/46 327N 27/419 331 (56) References JP-A-7-63096 (JP, A) JP-A Heisei 7-208151 (JP, A) JP 10-71325 (JP, A) JP 10-212933 (JP, A) JP 10-259714 (JP, A) JP 11-148910 (JP, A) A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 27/26 391 F01N 3/08 F01N 3/20 F02D 41/14 310 F02D 45/00 368 G01N 27/416 G01N 27/419

Claims (8)

(57) [Claims] A first oxygen pumping cell and an oxygen concentration measuring cell in which an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes, and the first oxygen pumping cell and the oxygen concentration measuring cell communicate with the gas to be measured via the first diffusion-controlling layer. And a second oxygen pumping cell in which an oxygen ion-conductive solid electrolyte layer is sandwiched between porous electrodes, and is communicated with the first measurement chamber via a second diffusion-controlling layer. NOx having a second measurement chamber and a heater for heating each of the cells to a predetermined activation temperature.
A detection method in which a sensor is disposed downstream of a nitrogen oxide storage catalyst attached to an exhaust pipe of an internal combustion engine to detect a decrease in the function of the nitrogen oxide storage catalyst, comprising: an output voltage of the oxygen concentration measurement cell. A first pump current is supplied to the first oxygen pumping cell so that the constant value becomes a constant value, and the oxygen concentration in the first measurement chamber is controlled to be constant. By applying a constant voltage in the direction of pumping oxygen,
A second pump current flowing through the second oxygen pumping cell is detected in accordance with the nitrogen oxide concentration in the gas to be measured, and after the operation control of the internal combustion engine is started at a lean air-fuel ratio in which the ratio of oxygen to fuel is large. Detecting a decrease in the storage capacity of the nitrogen oxide storage catalyst when the increase in the second pump current is detected by a preset fixed value, and determining that the storage capacity of the nitrogen oxide storage catalyst is reduced. Detection method. 2. The nitrogen oxide storage according to claim 1, wherein the time required for increasing the second pump current by the fixed value is smaller than a preset time threshold. A method for detecting a decrease in the function of a nitrogen oxide storage catalyst, comprising determining that the storage capacity of the catalyst is abnormal. 3. A first oxygen pumping cell and an oxygen concentration measuring cell in which an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes, and are connected to the gas to be measured via the first diffusion-controlling layer. And a second oxygen pumping cell in which an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes, and is connected to the first measurement chamber via a second diffusion-controlling layer. NOx having a second measuring chamber and a heater for heating each of the cells to a predetermined activation temperature.
A detection device for disposing a sensor downstream of a nitrogen oxide storage catalyst attached to an exhaust pipe of an internal combustion engine to detect a decrease in the function of the nitrogen oxide storage catalyst, the output voltage of the oxygen concentration measurement cell A first pump current is passed through the first oxygen pumping cell so that the oxygen concentration in the first measurement chamber becomes constant.
Pump current control means; constant voltage applying means for applying a constant voltage to the second oxygen pumping cell in a direction to pump oxygen from the second measurement chamber; and (2) a second pump current detecting means for detecting a second pump current flowing through the oxygen pumping cell; and a predetermined fixed value after starting operation control of the internal combustion engine with a lean air-fuel ratio having a large ratio of oxygen to fuel. A first determination unit that determines that the storage capacity of the nitrogen oxide storage catalyst has decreased when the increase in the second pump current is detected. Detection device. 4. An oxygen concentration detecting means for detecting an oxygen concentration in a gas to be measured from a first pump current flowing through the first oxygen pumping cell, and according to an oxygen concentration detected by the oxygen concentration detecting means,
And a first correction unit that corrects a detection result of the second pump current detection unit to compensate for a variation in an offset of the second pump current included in the detection result. Item 4. An apparatus for detecting a decrease in the function of a nitrogen oxide storage catalyst according to Item 3. 5. A temperature detecting means for detecting a temperature of said NOx sensor, and a detection result of said second pump current detecting means is corrected according to a temperature of said NOx sensor detected by said temperature detecting means. 5. The apparatus for detecting a decrease in the function of a nitrogen oxide storage catalyst according to claim 3, further comprising: a second correction unit configured to perform temperature compensation on the detection result. 4. 6. A current increase time measuring means for measuring a time required for detecting an increase of the second pump current by the fixed value by the first determination means, and And a second determination unit configured to determine that the storage capacity of the nitrogen oxide storage catalyst is abnormal when the detection result is smaller than a preset time threshold value. An apparatus for detecting a decrease in the function of a nitrogen oxide storage catalyst according to claim 5. 7. An inflow gas state detection means for detecting a flow rate of exhaust gas flowing into the nitrogen oxide storage catalyst and a nitrogen oxide concentration in the exhaust gas, and according to a detection result of the inflow gas state detection means. 7. The nitrogen oxide storage catalyst according to claim 6, further comprising a time threshold value setting means for setting the time threshold value to a smaller value as the flow rate increases and the nitrogen oxide concentration increases. Function degradation detection device. 8. The nitrogen oxidation apparatus according to claim 7, wherein the inflow gas state detection means estimates a flow rate of the exhaust gas and a nitrogen oxide concentration in the exhaust gas from an operation state of the internal combustion engine. Detecting device for deterioration of function of occlusion catalyst.