JP2003232771A - Apparatus for measuring concentration of gas and apparatus for cleaning exhaust gas of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To implement both a function for measuring the concentration of NOx and a function as an O<SB>2</SB>sensor for indicating Z characteristics in an apparatus for measuring the concentration of gases and for measuring the concentration of NOx in exhaust gases. <P>SOLUTION: The apparatus for measuring the concentration of gases is provided with a pump cell 58 having both the characteristic of making a current flow corresponding to the amount of discharge of oxygen while discharging the oxygen from a chamber 4 for the gases to be measured by receiving the impression of a voltage and the characteristic of suddenly changing an electromotive force according to the presence or absence of the oxygen in the chamber 48 for the gases to be measured. A sensor cell 66 for making a current flow corresponding to the concentration of NOx in the gases to be measured by receiving the impression of a voltage is provided downstream of the pump cell 58. In the case of a request for detecting the concentration of NOx, a switch circuit 88 is switched to a grounding side to impress the voltage on the pump cell 58. In the case of a request for Z characteristics, the switch circuit 88 is switched to the side of an A/D converter 86 to drive the pump cell 58 in electromotive force type mode. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring device and an exhaust gas purifying device for an internal combustion engine, and particularly to a gas concentration measuring device suitable for measuring NOx concentration in exhaust gas, and the device. Exhaust gas purification device

[0002]

2. Description of the Related Art Conventionally, for example, Japanese Patent Laid-Open No. 2000-21413.
Japanese Unexamined Patent Publication 0 discloses a gas concentration sensor that detects the NOx concentration contained in exhaust gas. This gas concentration sensor includes a pump cell and a sensor cell. The pump cell has a characteristic that oxygen is discharged (pumped) from the gas chamber to be measured when a predetermined voltage is applied, and a current corresponding to the discharged amount is passed. Further, the sensor cell decomposes NOx existing in the measured gas chamber into nitrogen and oxygen, and further, under the condition that a predetermined voltage is applied, discharges oxygen in the measured gas chamber, and according to the discharged amount. It has the property of passing an electric current.

The conventional gas concentration sensor described above includes a pump cell on the upstream side of the gas chamber to be measured and a sensor cell on the downstream side thereof. According to this structure, first, oxygen remaining in the gas is removed from the exhaust gas that has flowed into the measured gas chamber. When the exhaust gas from which oxygen has been removed reaches the sensor cell, NOx in the exhaust gas is decomposed into nitrogen and oxygen here. Then, the sensor cell generates a current according to the resulting amount of oxygen, that is, the amount of NOx in the exhaust gas. Therefore, the conventional gas concentration sensor described above can measure the NOx concentration in the exhaust gas based on the value of the current flowing through the sensor cell.

By the way, in the above-mentioned conventional gas concentration sensor, the pump cell, as described above, discharges oxygen contained in the exhaust gas under the condition that the predetermined voltage is applied, and the current corresponding to the discharged amount. Has the property of circulating. That is, in the above conventional gas concentration sensor, the value of the current passed by the pump cell under application of a predetermined voltage is
It corresponds to the oxygen concentration in the exhaust gas. Therefore, according to the conventional gas concentration sensor, the oxygen concentration in the exhaust gas can be measured in addition to the NOx concentration in the exhaust gas.

[0005]

[Problems to be Solved by the Invention] To control an internal combustion engine,
Conventionally, O ₂ sensors have been widely used. This O ₂ sensor has a characteristic that the output is remarkably changed depending on whether oxygen remains in the gas to be detected. Such characteristics, for example, when it is necessary to determine whether the exhaust air-fuel ratio is lean or rich, a characteristic that shows a linear change with respect to the oxygen concentration in the gas to be detected, that is, ,
It is more useful than the characteristics of the pump cell in the conventional gas concentration sensor described above.

Therefore, in the gas concentration sensor for NOx measurement,
If the output characteristic similar to that of the O ₂ sensor can be provided together with the characteristic of generating an output according to the NOx concentration in the gas to be detected, the usefulness of the gas concentration sensor can be further enhanced. The present invention has been made in view of the above points,
A first object of the present invention is to provide a gas concentration sensor having both the function of measuring the NOx concentration in the gas to be detected and the function of significantly changing the output depending on the presence or absence of oxygen in the gas to be detected. The present invention also provides an exhaust gas purifying apparatus for an internal combustion engine, which uses the above gas concentration sensor to execute a process for maintaining the exhaust gas purifying ability.
The purpose of.

[0007]

The invention according to claim 1 is
In order to achieve the above-mentioned first object, a gas concentration measuring device for measuring the NOx concentration in a measurement gas introduced into a measurement gas chamber, the measurement target being subjected to a predetermined voltage application. A pump cell having a characteristic of flowing an electric current according to the amount of the discharged oxygen while discharging oxygen from the gas chamber, and a characteristic of rapidly changing the electromotive force according to whether oxygen is present in the gas chamber to be measured. , By applying a predetermined voltage, targeting the gas to be measured after oxygen is removed by the pump cell, a sensor cell for passing a current according to the concentration of NOx contained therein, and the pump cell A pump cell power supply for applying the predetermined voltage,
Pump cell current measuring means for measuring the current flowing through the pump cell, a sensor cell power supply for applying the predetermined voltage to the sensor cell, a sensor cell current measuring means for measuring the current flowing through the sensor cell, and the pump cell is A switching unit that switches between a limiting current mode mode in which a predetermined voltage is applied and an electromotive force mode in which the voltage is not applied, and an electromotive force of the pump cell when the electromotive force mode is realized. And an electromotive force measuring means for measuring.

The invention according to claim 2 is the gas concentration measuring device according to claim 1, wherein the pump cell decomposes NO ₂ contained in the gas to be measured into NO, and a result of the decomposition. Further having a characteristic of discharging the generated oxygen from the measured gas chamber, the sensor cell decomposes NOx in the measured gas into nitrogen and oxygen, by receiving the predetermined voltage application, the decomposition of The resultant oxygen is discharged from the measurement gas chamber, and a characteristic of flowing an electric current according to the discharged amount is characterized.

According to a third aspect of the present invention, in the gas concentration measuring apparatus according to the second aspect, the pump cell includes one of a zirconia layer and one of the zirconia layer so as to be exposed in the measured gas chamber. Pt-Au alloy electrode disposed on the surface, and a Pt electrode disposed on the other surface of the zirconia layer so as to be exposed to the atmosphere, the sensor cell, a zirconia layer, and the measured gas chamber and respectively. Pt electrodes arranged on both surfaces of the zirconia layer so as to be exposed to the atmosphere.

In order to achieve the second object, the invention according to claim 4 is an exhaust gas purifying apparatus for an internal combustion engine,
The NOx storage catalyst arranged in the exhaust passage, the gas concentration measuring device according to any one of claims 1 to 3 arranged downstream of the NOx storage catalyst, and the pump cell driven in the limiting current mode. In the case of, based on the value of the current flowing through the sensor cell, NOx in the measured gas
NOx concentration detection means for detecting the concentration, when the NOx concentration is less than a predetermined judgment value, lean control execution means for executing lean control for controlling the air-fuel ratio of the air-fuel mixture to a value biased to lean, and the NOx Is equal to or more than the determination value, rich spike control execution means for performing rich spike control to make the air-fuel ratio of the air-fuel mixture rich, and after the NOx is equal to or more than the determination value, the drive mode of the pump cell A switching execution means for switching from the limiting current method mode to the electromotive force mode, and when the pump cell is driven in the electromotive force mode, the electromotive force of the pump cell becomes rich from a value corresponding to lean. Rich inversion determination means for determining whether or not the value has been inverted to a corresponding value,
After the rich spike control is started, when the electromotive force of the pump cell changes to a value corresponding to rich, while switching the drive mode of the pump cell to the limiting current method mode, the rich spike control is terminated,
Lean control resuming means for resuming the lean control.

According to a fifth aspect of the present invention, there is provided the exhaust gas purifying apparatus for an internal combustion engine according to the fourth aspect, wherein the deterioration state determining means for determining whether or not the NOx storage catalyst has deteriorated, and the pump cell. While driving in the electromotive force mode, based on the electromotive force of the pump cell, catalyst regeneration control execution means for performing catalyst regeneration control so that the air-fuel ratio of the air-fuel mixture becomes a predetermined value biased to rich, When it is determined that the NOx storage catalyst has deteriorated, the catalyst regeneration control start means for starting the catalyst regeneration control, and when the termination condition of the catalyst regeneration control is satisfied, the drive mode of the pump cell is set to the limiting current method mode. And a catalyst regeneration control terminating unit that terminates the catalyst regeneration control and restarts the lean control.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. It should be noted that elements common to each drawing are denoted by the same reference numerals, and redundant description will be omitted.

Embodiment 1. FIG. 1 is a diagram for explaining the configuration of the first embodiment of the present invention. As shown in FIG. 1, the internal combustion engine 10 includes an intake passage 12 and an exhaust passage 14.
Are in communication. The intake passage 12 has an air filter 16 at the upstream end. The air filter 16 includes an intake air temperature sensor 1 for detecting the intake air temperature THA (that is, the outside air temperature).
8 is assembled.

An air flow meter 20 is arranged downstream of the air filter 16. The air flow meter 20 is a sensor that detects the intake air amount Ga flowing through the intake passage 12. A throttle valve 22 is provided downstream of the air flow meter 20. A throttle sensor 24 for detecting the throttle opening TA is provided near the throttle valve 22.
And an idle switch 26 that is turned on when the throttle valve 22 is fully closed.

A surge tank 28 is provided downstream of the throttle valve 22. Further, a fuel injection valve 30 for injecting fuel into an intake port of the internal combustion engine 10 is arranged further downstream of the surge tank.

In the exhaust passage 14, a starting catalyst 32 and a NOx storage catalyst 34 are arranged in series. Starting catalyst 32
Is a device that purifies the exhaust gas by utilizing an oxidation-reduction reaction, and is provided mainly for purifying the exhaust gas immediately after the internal combustion engine 10 is started. The NOx storage catalyst 34 is
As described below, the device purifies exhaust gas by storing NOx contained in the exhaust gas.

That is, in the internal combustion engine 10 of this embodiment, lean control is executed to control the air-fuel ratio of the air-fuel mixture to be leaner than the stoichiometric air-fuel ratio. Exhaust gas tends to contain unburned components such as HC and CO as the air-fuel ratio becomes richer, while NOx tends to be contained as the air-fuel ratio becomes leaner. Therefore, during execution of the lean control, the exhaust gas containing a large amount of NOx is discharged from the internal combustion engine 10.

The NOx storage catalyst 34 can store NOx within the range of its own storage capacity, and when exhaust gas containing HC and CO is discharged while NOx is stored, While releasing the stored NOx, HC and CO can be reduced by using oxygen contained in NOx. In the internal combustion engine 10 of the present embodiment, as will be described later, the NOx storage catalyst 3
The lean control is executed while the storage capacity of 4 has a margin. During this period, NOx in the exhaust gas is stored in the NOx storage catalyst 34, so that the exhaust gas is purified.

Further, the system of the present embodiment executes the rich spike control for enriching the air-fuel ratio for a short period (for example, 5 seconds) when the NOx storage catalyst 34 has no storage capacity. While executing rich spike control, HC and
Exhaust gas containing CO flows into the NOx storage catalyst 34. During this period, the NOx storage catalyst 34 purifies HC and CO while recovering the storage capacity by releasing NOx. In the system of the present embodiment, by repeating such processing, good exhaust emission characteristics are always maintained.

In the exhaust passage 14, a gas concentration measuring device 36 is arranged downstream of the NOx storage catalyst 34. The gas concentration measuring device 36 is a main part of the present embodiment, and has a function of measuring the NOx concentration in the exhaust gas flowing out to the downstream of the NOx storage catalyst 34, and whether the exhaust gas contains oxygen. It has a function of issuing an output depending on whether or not it is. The configuration of the gas concentration measuring device 36 will be described later in detail with reference to FIG.

The internal combustion engine 10 is also equipped with various sensors such as a water temperature sensor 38 for measuring the cooling water temperature THW. The gas concentration measuring device 36 is connected to an ECU (Electronic Control Unit) 40 together with those sensors. The ECU 40 is a control unit of the system of the present embodiment, and controls the fuel injection valve 30 and the like based on the outputs of the various sensors described above.

FIG. 2 is a diagram for explaining the configuration of the gas concentration measuring device 36 used in this embodiment and the configuration of the drive circuit of the gas concentration measuring device 36 included in the ECU 40.

As shown in FIG. 2, the gas concentration measuring device 36
Includes zirconia layers 42 and 44 and an insulating layer 46. A measured gas chamber 48 is provided between the two zirconia layers 42 and 44. Further, at positions adjacent to the zirconia layers 42 and 44, atmospheric holes 50 and 52 isolated from the measured gas chamber 48 by the layers are formed.

The gas concentration measuring device 36 is provided with an introduction hole 54 communicating with the measured gas chamber 48. Introduction hole 54
Is the measured gas chamber 48 for the exhaust gas flowing through the exhaust passage 14.
To the exhaust passage 14 through the diffusion resistance layer 56. The diffusion resistance layer 56 is a porous material that allows the exhaust gas to flow from the exhaust passage 14 into the introduction hole 54.

The exhaust gas flowing into the introduction hole 54 travels inside the measured gas chamber 48 along a predetermined flow path.
In this distribution route, the most upstream part is
A pump cell 58 is provided. The pump cell 58 is
The zirconia layer 42 and the Pt-Au electrode 60 and the Pt electrode 62 arranged on both sides of the zirconia layer 42. The Pt-Au electrode 60 is provided so as to be exposed in the measured gas chamber 48. Further, the Pt electrode 62 is provided so as to be exposed to the atmosphere hole 50.

The sensor cell 6 is provided downstream of the pump cell 58.
6 is provided. The sensor cell 66 includes a zirconia layer 44 and two Pt electrodes 72 and 74 arranged on both sides thereof.
It consists of and. The Pt electrodes 72 and 74 are provided such that one electrode 72 is exposed to the gas chamber 48 to be measured and the other electrode 74 is exposed to the atmosphere hole 52.

Pt-Au electrode 60 and Pt electrodes 62, 72, 7
When 4 reaches a predetermined activation temperature, oxygen in the exhaust gas is ionized, NO ₂ in the exhaust gas is decomposed into NO,
Further, it exhibits the property of decomposing NO in the exhaust gas into nitrogen and oxygen ions. The gas concentration measuring device 36 uses an insulating layer 46 in order to raise the electrodes to an activation temperature.
Is equipped with a heater 97.

The drive circuit of the gas concentration measuring device 36 is a CPU
Equipped with 80. A D / A converter 82 for controlling the voltage applied to the pump cell 58 is connected to the CPU 80. The D / A converter 82 is connected to the Pt electrode 62 of the pump cell 58 via the current measuring circuit 84. The current measuring circuit 84 detects a current flowing through the pump cell 58, that is, a current flowing from the Pt electrode 62 through the zirconia layer 42 to the Pt-Au electrode 60. The current value detected by the current measuring circuit 84 is supplied to the CPU 80 as digital data.

An A / D converter 86 for detecting electromotive force is also connected to the CPU 80. A / D converter 86
Pt-Au of the pump cell 58 via the switching circuit 88.
It is connected to the electrode 60. The switching circuit 88 is a CPU
This circuit is controlled by 80 to selectively realize a state in which the Pt-Au electrode 60 is connected to the A / D converter 86 and a state in which the electrode 60 is grounded.

Further, the drive circuit of the gas concentration measuring device 36 supplies a power supply 94 for applying a predetermined DC voltage between the Pt electrodes 72, 74 of the sensor cell 66, and a current flowing through the sensor cell 66 under the voltage application. A current measuring circuit 96 for measuring is provided. The current value measured by the current measuring circuit 96 is supplied to the CPU 80 as digital data, like the detection value of the current measuring circuit 84 described above.

In the drive circuit having the above structure, the CPU
When it is necessary to detect the NOx concentration in the exhaust gas, 80 switches the switch circuit 88 so that the Pt-Au electrode 60 of the pump cell 58 is grounded. Hereinafter, driving in this state will be referred to as “driving in the limiting current mode mode”. Further, when it is necessary for the CPU 80 to determine whether or not oxygen remains in the exhaust gas flowing downstream of the NOx storage catalyst 34, in other words, the exhaust air-fuel ratio downstream of the NOx storage catalyst 34. When it is necessary to determine whether the fuel cell is rich or lean, the Pt-Au electrode 60 of the pump cell 58 is A
Switch circuit 88 to be connected to the / D converter 86
Switch. Hereinafter, driving in this state is referred to as "driving in electromotive force mode".

FIG. 3 shows the Pt-Au electrode 60 of the pump cell 58.
Shows the equivalent circuit in the case where is grounded, that is, the pump cell 58 is driven in the limiting current mode. As shown in FIG. 3, in this case, a state equivalent to that in which the current measuring circuit 84 and the variable DC power source 98 are connected in series is formed between the Pt electrode 62 and the Pt-Au electrode 60.

When the Pt electrode 62 reaches the above activation temperature, it decomposes NO ₂ in the measured gas chamber 48 into NO and oxygen ions, and also decomposes oxygen into oxygen ions. Furthermore, the pump cell 58, that is, the configuration in which the Pt electrode 62 and the Pt-Au electrode 60 sandwich the zirconia layer 42, pumps oxygen ions generated in the measured gas chamber 48 when driven in the limiting current mode. The characteristics of discharging to the air holes 50 are shown. Therefore, when the pump cell 58 is driven in the limiting current mode, most of the oxygen existing in the form of molecules in the gas chamber 48 to be detected is discharged to the atmosphere hole 50 and NOx contained in the exhaust gas. A phenomenon occurs in which (NO ₂ and NO) is monogasified into NO. That is, in this case, almost all of the oxygen is removed and the exhaust gas in the state where NOx is converted into a single gas NO reaches the position of the sensor cell 66.

When the pump cell 58 is driven in the limiting current mode, the pump cell 58 circulates a current according to the amount of oxygen discharged from the gas chamber 48 to be detected to the atmosphere hole 50. It can be considered that the amount of oxygen discharged in this state substantially corresponds to the oxygen concentration in the exhaust gas. Therefore, the CPU 80 of the drive circuit detects the oxygen concentration in the exhaust gas based on the current value measured by the current measurement circuit 84 when the pump cell 58 is driven in the drive limit current mode mode. You can

The sensor cell 66 is provided on the measured gas chamber 48 side.
The Pt electrode 72 is provided. The Pt electrode has higher activity than the Pt-Au electrode. Therefore, the above-mentioned Pt-Au
The electrode 60 stops decomposing NO ₂ in the exhaust gas, whereas the Pt electrode 72 can decompose the NO remaining in the exhaust gas in a single gas state into nitrogen and oxygen. . Then, the sensor cell 66, under the voltage application from the power supply 96, generates oxygen ions generated by the decomposition of slightly remaining oxygen molecules in the exhaust gas and oxygen ions generated by the decomposition of NO. By pumping, an electric current corresponding to the amount of those oxygen ions is passed. Here, since the number of oxygen molecules remaining in the exhaust gas is extremely small, the current generated by the decomposition thereof can be ignored with respect to the current generated by the decomposition of NO.
Therefore, the CPU 80 detects the value of the current flowing due to NO, that is, the current value corresponding to the number of oxygen ions generated by the decomposition of NO, by the current measuring circuit 96 connected to the sensor cell 66. can do.

At the position where the sensor cell 66 is arranged, NOx in the exhaust gas is converted to NO as a single gas. For this reason,
The number of oxygen ions flowing through the sensor cell 66 was decomposed.
It is the same as the number of NO. Further, the NO concentration existing at the position where the sensor cell 66 is arranged is the same as the NOx concentration before the single gasification of NO. Therefore, the CPU 80 uses the sensor cell 66.
The NOx concentration in the exhaust gas can be accurately detected based on the current value measured by the current measuring circuit 96 of FIG.

As described above, according to the gas concentration measuring apparatus of the present embodiment, the pump cell 58 is driven in the mode of the limiting current method, that is, the Pt-Au electrode 60 is grounded and is suitable for the Pt electrode 62. By applying such a voltage, the NOx concentration in the exhaust gas can be accurately detected.

Next, the relationship between the voltage to be applied to the Pt electrode 62 when the pump cell 58 is driven in the limiting current mode and the air-fuel ratio of the exhaust gas will be described with reference to FIG. FIG. 4 is a diagram showing the relationship between the voltage applied to the Pt electrode 62 of the pump cell 58 and the current flowing through the pump cell 58 (hereinafter referred to as “pump cell current”) using the air-fuel ratio of the exhaust gas as a parameter.

The curves drawn together with the symbols of A / F16 or A / F18 in FIG. 4 are the voltage-current characteristics when the air-fuel ratio (A / F) of the exhaust gas is 16 or 18, respectively. Indicates. The curve drawn with the symbol of Air in FIG. 4 is the voltage when the gas to be detected is pure air.
The current characteristics are shown. Each of these curves shows that the pump cell current has a certain convergence value with increasing applied voltage.

This convergent value corresponds to a current value when almost all oxygen existing in the exhaust gas is ionized and discharged. Therefore, in order for the pump cell 58 to efficiently remove oxygen in the exhaust gas, a voltage such that the pump cell current reaches a converged value should be appropriately applied to the Pt electrode 62 according to the air-fuel ratio of the exhaust gas. is necessary.

Further, in the area shown as the NOx decomposition area in FIG. 4, the NOx contained in the exhaust gas is, that is, Nx.
It shows the region where not only O ₂ but also NO is decomposed into nitrogen and oxygen. Gas concentration measuring device 3 of the present embodiment
In 6, in order for the sensor cell 66 to pass a current according to the NOx concentration, the pump cell 58 needs to decompose only NO ₂ and reach the sensor cell 66 with NO as it is. Therefore, the Pt electrode 62 of the pump cell 58
The voltage applied to must be below the NOx decomposition range.

That is, in the present embodiment, in order to accurately detect the NOx concentration by the gas concentration measuring device,
It is necessary to control the voltage applied to the Pt electrode 62 of the pump cell 58 to an appropriate value capable of converging the pump cell current according to the air-fuel ratio of the exhaust gas and not reaching the NOx decomposition region. The straight line shown as the control center in FIG. 4 is a set of applied voltages satisfying those requirements.

In the present embodiment, the ECU 40 detects the oxygen concentration in the exhaust gas based on the measurement value of the current measuring circuit 84 when the pump cell 58 is driven in the limiting current mode as described above. That is, the exhaust air-fuel ratio can be detected. The ECU 40 is shown in FIG.
A map defining the value of the applied voltage shown as the center of control in relation to the exhaust air-fuel ratio is stored. And ECU4
0 determines the voltage applied to the pump cell 58 by referring to the map based on the exhaust air-fuel ratio detected based on the measurement value of the current measuring circuit 84.

As described above, in the system of this embodiment, the ECU 40 can apply an appropriate voltage to the Pt electrode 62 when driving the pump cell 58 in the limiting current mode. Therefore, the pump cell 5
8 is driven in the mode of the limiting current method,
Oxygen in the measured gas chamber 48 can be efficiently removed. Since the effect of residual oxygen is superimposed on the current flowing through the sensor cell 66, it is desirable that the effect of residual oxygen be small in measuring the NOx concentration based on the current. According to the system of this embodiment, the pump cell 58
As a result, oxygen is efficiently removed, and as a result, the influence of residual oxygen is suppressed to a sufficiently small level, so that the NOx concentration can be calculated extremely accurately based on the current flowing through the sensor cell 66.

Next, the case where the pump cell 58 is driven in the electromotive force mode will be described with reference to FIG.
As described above, the CPU 80 of the drive circuit is configured so that the NOx storage catalyst 34
When it is necessary to determine whether the exhaust air-fuel ratio is rich or lean in the downstream of, the pump cell 58 is driven in the electromotive force mode. At this time, Pt-Au of the pump cell 58
The electrode 60 is connected to the A / D converter 86 via the switch circuit 88.

FIG. 5 shows the Pt-Au electrode 60 of the pump cell 58.
Shows an equivalent circuit in a state of being connected to the A / D converter 86. In this state, the A / D converter 86 and the CPU 80 connected thereto function as a voltmeter that measures the electromotive force generated in the Pt-Au electrode 60. Therefore, that state can be represented as the voltmeter 100 is connected between the Pt electrode 62 and the Pt-Au electrode 60, as shown in FIG.

The pump cell 58 having a structure in which the zirconia layer 42 is sandwiched between the Pt electrode 62 and the Pt-Au electrode 60 is provided depending on whether the exhaust air-fuel ratio in the measured gas chamber 48 is lean or rich, that is, The electromotive force is greatly changed depending on whether or not oxygen is contained in the exhaust gas guided to the measured gas chamber 48.

That is, the Pt-Au electrode 60 and the Pt electrode 62 arranged on the surface of the zirconia layer 42 generate a predetermined electromotive force when oxygen is present in the gas they are in contact with, and the gas It has a characteristic of rapidly losing electromotive force when oxygen is not present therein. Therefore, the pump cell 5
In the configuration of No. 8, when oxygen is present in the exhaust gas (that is, when the exhaust air-fuel ratio is lean), the Pt-Au electrode 60 and the Pt electrode 62 generate electromotive force almost equally, and as a result, The potential difference between both electrodes (electromotive force of the pump cell 58) has a small value. On the other hand, when oxygen does not exist in the exhaust gas (that is, when the exhaust air-fuel ratio is rich), the Pt electrode 6
Since only 2 emits an electromotive force and the Pt-Au electrode 60 does not emit an electromotive force, the potential difference between them (electromotive force of the pump cell 58) becomes a large value.

FIG. 6 is a diagram showing the change in the electromotive force in relation to the exhaust air-fuel ratio. As shown in this figure, the electromotive force of the pump cell 58 is, specifically, the measured gas chamber 48.
When the exhaust air-fuel ratio is rich, the value becomes large, while when the exhaust air-fuel ratio is lean, the value becomes small.

According to the configuration shown in FIG. 2, the potential input to the A / D converter 86 is substantially equal to the potential of the Pt electrode 62 when the exhaust air-fuel ratio is lean. Then, when the exhaust air-fuel ratio is rich, a lower potential than the potential of the Pt electrode 62 is input to the A / D converter 86. That is, when the potential of the Pt electrode 62 is used as a reference value, the potential input to the A / D converter 86 is almost the reference value when the exhaust air-fuel ratio is lean, and is negative when the exhaust air-fuel ratio is rich. Becomes the value of. Therefore, FIG.
Strictly speaking, the voltage characteristic shown in (1) corresponds to the characteristic obtained when the potential input to the A / D converter 86 is inverted.

As described above, when the pump cell 58 is used in the electromotive force mode, that is, when it is used in the state where no voltage is applied, it depends on whether the exhaust air-fuel ratio is lean or rich. Shows a characteristic of rapidly changing the electromotive force, that is, a so-called Z characteristic. Therefore, the ECU 40 drives the pump cell 58 in the electromotive force mode to accurately determine whether the exhaust air-fuel ratio is lean or rich based on the voltage value input to the A / D converter 86. Therefore, the exhaust air-fuel ratio can be detected extremely accurately in the region near the stoichiometric air-fuel ratio.

Next, with reference to FIGS. 7 and 8, the rich spike control executed by the ECU 40 in order to maintain a good exhaust emission characteristic in the system of this embodiment will be described. As described above, in the system of the present embodiment, lean control is executed to control the air-fuel ratio of the air-fuel mixture to be leaner than the stoichiometric air-fuel ratio. Then, in order to release the NOx accumulated in the NOx storage catalyst during the execution of the lean control, the rich spike control is appropriately executed.

FIG. 7 is a timing chart for explaining the content of the rich spike control executed in this embodiment. More specifically, FIG. 7A shows the waveform of the air-fuel ratio control target (hereinafter, referred to as “control A / F”) during and before and after the execution of the rich spike control.
Further, FIG. 7B is a waveform of the NOx concentration in the exhaust gas measured by the gas concentration measuring device 36. Furthermore, FIG.
(C) is a waveform of the exhaust air-fuel ratio (hereinafter referred to as “detection A / F”) detected by the gas concentration measuring device 36.

The timing chart shown in FIG. 7 shows the NOx concentration detected by the gas concentration measuring device 36, that is, N
An example is shown in which the NOx concentration downstream of the Ox storage catalyst reaches a predetermined determination value α at time t1. In this example, lean control is performed until the time t1 in which the air-fuel ratio of the air-fuel mixture is lean. At this time, while the NOx storage catalyst 34 has a sufficient NOx storage capacity, the NOx contained in the exhaust gas is NOx.
It is captured by the storage catalyst 34. Therefore, as shown in FIG. 7B, the NOx concentration downstream of the NOx storage catalyst 34 is maintained at a low value.

If the lean control is continuously executed, the NOx storage catalyst 34 will soon have no room for its storage capacity, and the NOx concentration downstream thereof will start to rise. Then, when the NOx storage catalyst 34 becomes a state in which it cannot sufficiently absorb NOx, and when it is no longer possible to maintain good emission characteristics, the NOx concentration in the downstream of the NOx storage catalyst 34 becomes
The judgment value α is reached.

In the present embodiment, the ECU 40 controls the pump cell 5 of the gas concentration measuring device 36 during the lean control.
8 is driven in the limiting current mode. Therefore, during execution of the lean control, the NOx concentration can be accurately detected by the gas concentration measuring device 36 (see FIG. 7 (B)).

When the NOx concentration detected by the gas concentration measuring device 36 reaches the judgment value α, the ECU 40 starts lean spike control instead of lean control and drives the pump cell 58 at that time (time t1). The mode is switched from the limiting current type mode to the electromotive force type mode.

The rich spike control is control for maintaining the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine 10 at a value sufficiently rich as compared with the stoichiometric air-fuel ratio. Therefore, in the example shown in FIG. 7, the control A / F that was lean until time t1 is changed to a significantly rich value after time t1 (see FIG. 7A).

During the rich spike control, the air-fuel ratio of the air-fuel mixture is controlled to be rich. Therefore, the exhaust gas containing unburned components such as CO and HC flows into the NOx storage catalyst 34. In this case, the NOx storage catalyst 34 releases NOx stored therein and oxidizes CO and HC using oxygen contained in NOx. As a result, exhaust gas that has been appropriately purified and has a slightly lean air-fuel ratio flows downstream of the NOx storage catalyst 34.

When the rich spike control is continued, eventually, almost all of the NOx stored in the NOx storage catalyst 34 is released, and the NOx storage catalyst 34 cannot properly oxidize CO and HC. When this state is reached, the NOx storage catalyst 34
Exhaust gas containing CO and HC, that is, exhaust gas rich in air-fuel ratio, flows out downstream of the exhaust gas.

While the rich spike control is being executed, the pump cell 58 is driven in the electromotive force mode as described above. Therefore, as shown in FIG.
The change in the exhaust air-fuel ratio in the downstream of the NOx storage catalyst 34 is accurately reflected in the electromotive force of Eq.
Therefore, the ECU 40 determines, based on the electromotive force, the timing at which the rich exhaust gas starts to flow downstream of the NOx storage catalyst 34, that is, the NOx stored in the NOx storage catalyst 34.
It is possible to detect the timing when almost all of the

In the timing chart shown in FIG. 7, after the rich spike control is started, the detection A /
The example shows that F has changed from lean to rich. In the present embodiment, when such a change appears in the detection A / F, the ECU 40 ends the rich spike control and restarts the lean control at the same time, and sets the drive mode of the pump cell 58 to the electromotive force type. Switch from the mode to the limiting current mode.

After that, the ECU 40 uses the gas concentration measuring device 36.
Based on the NOx concentration and the detected A / F detected by
The switching between the lean control and the rich spike control and the switching of the drive mode of the pump cell 58 are repeatedly executed by the above method. As a result, in the system according to the present embodiment, the rich spike control is started at an appropriate timing and is continued for an appropriate period without excess or deficiency. Therefore, according to the system of the present embodiment, it is possible to maintain good exhaust emission characteristics at all times during operation of the internal combustion engine 10.

FIG. 8 shows a flow chart of a control routine executed by the ECU 40 in order to realize the above-described rich spike control. It should be noted that the routine shown in FIG. 8 is assumed to be started under a situation where the fuel injection amount is controlled by lean control and the pump cell 58 is driven in the limit current mode mode.

In the routine shown in FIG. 8, first, the NOx concentration in the exhaust gas is detected based on the detected value of the gas concentration measuring device 36, specifically, the value of the current flowing through the sensor cell 66 (step 200). ).

Next, it is judged whether or not the NOx concentration detected in the above step 200 exceeds the judgment value α (step 202). As a result, when it is determined that the NOx concentration does not exceed α, it can be determined that there is still a margin left in the storage capacity of the NOx storage catalyst 34. In this case, in order to continue the lean control, the current processing cycle is ended without any processing thereafter.

On the other hand, when it is determined that the NOx concentration exceeds the determination value α, it can be determined that the NOx storage catalyst 34 has already stored NOx having the full capacity. In this case, after the drive mode of the pump cell 58 is switched from the limiting current mode mode to the electromotive force mode (step 204),
The rich spike control is started (step 206).

Next, the air-fuel ratio of the exhaust gas detected by the gas concentration measuring device 36, that is, the detection A / F is detected based on the electromotive force of the pump cell 58 (step 20).
8). Next, it is judged whether or not the detected A / F is inverted from lean to rich (step 210).

The processes of steps 208 and 210 are repeatedly executed until the condition of step 210 is satisfied. Then, when it is determined in step 210 that the detected A / F is inverted from lean to rich, the rich spike control is ended (step 212), and the drive mode of the pump cell 58 is further switched to the limit current mode mode. (Step 214). According to the series of processes described above, as described with reference to FIG. 7, the rich spike control can be appropriately executed at an appropriate time only during a period without excess or deficiency, and the exhaust emission characteristic of the internal combustion engine 10 can be obtained. , Can always be kept good.

In the system of this embodiment, the NOx storage catalyst 34 is poisoned by the sulfur component as it is used as the exhaust gas purifying device. NOx of NOx storage catalyst 34
The storage capacity decreases as the poisoning progresses. Therefore, in order for the NOx storage catalyst 34 to exhibit a sufficient purification capacity, it is necessary to perform a regeneration process for eliminating the poisoning state when the poisoning progresses to some extent.

FIG. 9 is a flowchart of a poisoning regeneration control routine executed by the ECU 40 to eliminate the poisoning state of the NOx storage catalyst 34. In the routine shown in FIG. 9, first,
It is determined whether or not the reproduction flag is set (step 220). When the regeneration flag is determined to require regeneration of the NOx storage catalyst 34, more specifically,
This is a flag that is set when a significant deterioration of the NOx storage catalyst 34 is recognized. In the system of this embodiment, NO
NOx that can be stored inside the x-storage catalyst 34 due to deterioration
As the amount decreases, the execution time of rich spike control,
That is, the time until the rich exhaust gas flows out to the downstream of the NOx storage catalyst 34 after the rich spike control is started is reduced. Therefore, in this embodiment, EC
U40 sets the reproduction flag when the execution time of the rich spike control does not reach the predetermined determination time.

When it is determined in step 220 that the regeneration flag is not set, it can be determined that the poisoning regeneration control need not be executed. Therefore, if such a determination is made, the current processing cycle is ended without any further processing thereafter. on the other hand,
If it is determined that the regeneration flag is set, then after the poisoning regeneration control is started, a predetermined time (for example, 1
It is determined whether or not 0 minutes has elapsed (step 2)
22). It should be noted that the above predetermined time was poisoned by sulfur
The time is preset as the time for continuing the poisoning regeneration control in order to regenerate the NOx storage catalyst 34.

When it is determined in step 222 that the predetermined time has not elapsed, the processing for eliminating the poisoning state of the NOx storage catalyst 34 is executed thereafter.
Specifically, first, the electromotive force mode is selected as the drive mode of the pump cell 58 (step 22).
4). Therefore, after that, the gas concentration measuring device 36
It functions as a sensor that rapidly changes the output depending on whether the exhaust air-fuel ratio downstream of the storage catalyst 34 is lean or rich, that is, as an O ₂ sensor that exhibits the Z characteristic.

In the routine shown in FIG. 9, next, poisoning regeneration control is executed (step 226). NOx storage catalyst 3
The poisoning state of No. 4 can be eliminated by raising the exhaust temperature and continuously maintaining the temperature of the NOx storage catalyst 34 high. The exhaust temperature required to eliminate the poisoning state can be obtained by maintaining the air-fuel ratio of the air-fuel mixture at a value that is slightly richer than the stoichiometric air-fuel ratio. Therefore, in the present embodiment, the control for making the air-fuel ratio of the air-fuel mixture slightly richer than the stoichiometric air-fuel ratio is executed as the poisoning regeneration control.

In the poisoning regeneration control described above, specifically, the fuel injection amount is set so that the exhaust air-fuel ratio of the internal combustion engine 10 becomes a desired value (a value slightly richer than the stoichiometric air-fuel ratio). It is realized by controlling. In the system of the present embodiment, the air-fuel ratio of the air-fuel mixture is continuously controlled to be rich while the poisoning regeneration control is being executed. Therefore, after the control is started and an appropriate time has elapsed, the starting catalyst 32 Becomes a state in which all the stored oxygen is released. When the starting catalyst 32 is in such a state, the starting catalyst 32 does not exert any purification action, and therefore the exhaust air-fuel ratio becomes substantially the same value before and after that.

Further, the NOx storage catalyst 34 exerts almost no purification action on exhaust gas having a rich air-fuel ratio.
Therefore, during execution of the poisoning regeneration control, the NOx storage catalyst 34
Before and after, there is no substantial change in the exhaust air-fuel ratio. That is, during execution of the poisoning regeneration control, there is substantially no change in the air-fuel ratio in the process in which the gas discharged from the internal combustion engine 10 reaches the downstream of the NOx storage catalyst 34. Therefore, in the system of the present embodiment, poisoning regeneration control can be realized by detecting the exhaust air-fuel ratio in the gas concentration detection device 36 and controlling the fuel injection amount based on the detected value.

During execution of the poisoning regeneration control, it is necessary to control the exhaust air-fuel ratio to a value slightly richer than the stoichiometric air-fuel ratio as described above. In order to perform such control with high accuracy, it is desirable that the sensor that detects the exhaust air-fuel ratio has high sensitivity in the vicinity of the theoretical air-fuel ratio. In the present embodiment, the poisoning regeneration control is performed while driving the pump cell 58 in the electromotive force mode. In this case, the pump cell 58 functions as an O ₂ sensor exhibiting the Z characteristic, and thus exhibits high sensitivity to changes in the air-fuel ratio near the stoichiometric air-fuel ratio. Therefore, according to the system of the present embodiment, the air-fuel ratio control is performed with high accuracy during execution of the poisoning regeneration control, as compared with the case where an air-fuel ratio sensor that exhibits a linear output change with respect to the air-fuel ratio is used. be able to.

When a preset predetermined time (for example, 10 minutes) elapses after the poisoning regeneration control is started, FIG.
In the routine shown in (4), it is determined that the condition of step 222 is satisfied. In this case, thereafter, the poisoning regeneration control is ended (step 228), the regeneration flag is reset (step 230), and the drive mode of the pump cell 58 is further switched to the limit current mode mode (step 232). As a result, the gas concentration measuring device 36
The state in which the concentration can be detected is restored, and the control of the internal combustion engine 10 by the lean control is restarted.

As described above, according to the routine shown in FIG. 9, the pump cell 58 can be made to function as an O ₂ sensor showing the Z characteristic when execution of poisoning regeneration control is required. Then, in that state, the pump sensor 5
By utilizing the high sensitivity shown by 8, it is possible to control the air-fuel ratio of the air-fuel mixture to a desired air-fuel ratio with extremely high accuracy during execution of the poisoning regeneration control. Therefore, according to the system of the present embodiment, it is possible to effectively prevent the air-fuel ratio from becoming rough during the execution of the poisoning regeneration control.

By the way, in the first embodiment described above, the starting catalyst 32 is provided in the exhaust passage 14 together with the NOx storage catalyst 34, but the starting catalyst 32 is not always necessary. That is, in the exhaust passage 14,
It is also possible to provide only the NOx storage catalyst 34 and omit the starting catalyst 32.

In the first embodiment described above, D /
The A converter 82 is the “pump cell power supply” of claim 1, the current measuring circuit 84 is the “pump cell current measuring means” of the claim 1, and the power supply 94 is the “sensor cell power supply” of the claim 1. The current measuring circuit 96 is the “sensor cell current measuring means” described in claim 1, the switch circuit 88 is the “switching means” described in claim 1, and the A / D
The converter 86 and the CPU 80 correspond to the "electromotive force measuring means" described in claim 1, respectively.

Further, in the above-described first embodiment,
The ECU 40 executes the process of the step 200, whereby the "NOx concentration detecting means" of claim 4 executes the lean control when the condition of the step 202 is not satisfied, and thus the "NOx concentration detecting means" of the claim 4 executes the lean control. The "lean control execution means" executes the processing of the step 206, and the "rich spike control execution means" according to claim 4 executes the processing of the step 204. The switching execution means "executes the processing of the step 210, and the switching execution means"
The described “rich inversion determination means” is the same as the above step 212.
5. The method according to claim 4, wherein the processes of steps 214 and 214 are executed.
The described "lean control restarting means" is realized.

Further, in the above described first embodiment,
The "deterioration state determination means" according to claim 5 causes the ECU 40 to execute the processing of the step 220 or to set the regeneration flag used in the step 220, whereby the steps 224 and 226. The "catalyst regeneration control executing means" and the "catalyst regeneration control starting means" according to claim 5 by performing the processing of step 5 and the catalyst of claim 5 by performing the processing of steps 228 and 232. The “reproduction control ending means” is realized.

[0084]

Since the present invention is constructed as described above, it has the following effects. Claim 1
According to the above-described invention, by driving the pump cell in the limiting current mode, the pump cell can be made to perform the oxygen discharge process. Therefore, in this case, the NOx concentration can be measured based on the output of the sensor cell. Further, according to the present invention, by driving the pump cell in the electromotive force mode, the electromotive force of the pump cell can be rapidly changed based on whether or not oxygen is present in the measured gas chamber. Therefore, according to the present invention, the function of measuring the NOx concentration and the function equivalent to that of the O ₂ sensor can be selectively realized.

According to the second aspect of the present invention, the pump cell has a characteristic of decomposing NO ₂ existing in the gas chamber to be measured into NO and discharging oxygen generated as a result. Therefore, according to the present invention, the NOx in the gas to be detected can be
Can be gasified to NO. If NO ₂ and NO are mixed in the NOx decomposed by the sensor cell, the NOx concentration cannot be directly obtained from the current passed through the sensor cell. On the other hand, in the present invention, since NOx decomposed in the sensor cell is converted into a single gas of NO, the NOx concentration in the gas to be detected can be accurately detected based on the current.

According to the invention described in claim 3, it is possible to realize the pump cell and the sensor cell having the characteristics required in claim 1 or 2.

According to the invention described in claim 4, the pump cell is driven in the limiting current mode when the NOx concentration needs to be detected. When the NOx concentration obtained as a result is low, it is determined that the NOx storage catalyst has a sufficient storage capacity, and the lean control is continued. Then, when it is determined that the NOx concentration has risen and the margin has been exhausted, the rich spike control is started at that time and the drive mode of the pump cell is switched to the electromotive force mode. As a result, during execution of the rich spike control, pump cell O ₂
Functions like a sensor. When the NOx stored in the NOx storage catalyst is consumed along with the execution of the rich spike control, rich gas begins to flow out near the gas concentration measuring device, and the electromotive force of the pump cell reverses to a value corresponding to rich. To do. Then, by this inversion, the drive mode of the pump cell is returned to the limiting current method mode, and the lean control is restarted. By repeating the above processing, according to the present invention, rich spike control can be executed with proper timing and period without excess or deficiency, and NOx and rich components (CO and HC) can be provided downstream of the NOx storage catalyst. Can be effectively prevented from being discharged.

According to the fifth aspect of the invention, when it is determined that the NOx storage catalyst has deteriorated, the drive system of the pump cell is switched to the electromotive force mode. As a result, the pump cell is in a state of functioning as a sensor having high sensitivity before and after the stoichiometric air-fuel ratio. In the present invention, by utilizing the high sensitivity, the air-fuel ratio of the air-fuel mixture is well controlled and controlled to the air-fuel ratio effective for regeneration of the NOx storage catalyst (the air-fuel ratio slightly richer than the theoretical air-fuel ratio). . Then, the state is continued until the termination condition of the catalyst regeneration control is satisfied. Therefore, according to the present invention, when the NOx storage catalyst deteriorates, the state can be returned to an appropriate state.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a system configuration according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining a configuration of a gas concentration measuring device used in the first embodiment of the present invention and a configuration of a drive circuit thereof.

FIG. 3 is an equivalent circuit showing a state in which the gas concentration measuring device shown in FIG. 2 is driven in a limiting current method mode.

FIG. 4 is a diagram for explaining a relationship between a voltage applied to a pump cell and a pump cell current.

5 is an equivalent circuit showing a state in which the gas concentration measuring device shown in FIG. 2 is driven in an electromotive force mode.

6 is a diagram for explaining an output characteristic (Z characteristic) exhibited by a pump cell in the gas concentration measuring device shown in FIG.

FIG. 7 is a diagram for explaining the content of rich spike control executed in the system according to the first embodiment of the present invention.

FIG. 8 is a flowchart of a rich spike control routine executed by the ECU in the first embodiment of the present invention.

FIG. 9 is a flowchart of a poisoning regeneration control routine executed by the ECU in the first embodiment of the present invention.

[Explanation of symbols]

10 Internal combustion engine 14 Exhaust passage 32 Starting catalyst 34 NOx storage catalyst 36 Gas concentration measuring device 40 ECU (Electronic Control Unit) 42,44 Zirconia layer 48 Measured gas chamber 50,52 Atmosphere 58 Pump cell 60 Pt-Au electrode 62, 72, 74 Pt electrodes 66 sensor cells 82 D / A converter 84,96 Current measurement circuit 86 A / D converter 88 switch circuit 94 power supply

Claims (5)

[Claims] 1. A measuring gas introduced into a measuring gas chamber,
A gas concentration measuring device for measuring NOx concentration, wherein when a predetermined voltage is applied, oxygen is discharged from the gas chamber to be measured while flowing a current according to the discharged amount, A pump cell having the characteristic of rapidly changing the electromotive force depending on whether oxygen is present in the measurement gas chamber, and the gas to be measured after the oxygen is removed by the pump cell by receiving a predetermined voltage application. Targeting
A sensor cell for passing a current according to the concentration of NOx contained therein, a pump cell power source for applying the predetermined voltage to the pump cell, and a pump cell current measuring unit for measuring a current passed by the pump cell, and A sensor cell power supply that applies the predetermined voltage to the sensor cell, a sensor cell current measuring unit that measures a current that the sensor cell circulates, a limit current mode mode in which the pump cell receives the application of the predetermined voltage, and the voltage A switching means for switching between an electromotive force type mode in which no voltage is applied; and an electromotive force measuring means for measuring an electromotive force of the pump cell when the electromotive force type mode is realized. Concentration measuring device. 2. The pump cell further has a characteristic of decomposing NO ₂ contained in the gas to be measured into NO, and discharging oxygen generated as a result of the decomposition from the gas chamber to be measured, By decomposing NOx in the gas to be measured into nitrogen and oxygen, and applying the predetermined voltage,
2. The gas concentration measuring device according to claim 1, wherein the gas concentration measuring device has a characteristic that while discharging oxygen generated as a result of the decomposition from the measurement gas chamber, a current corresponding to the discharged amount is passed. 3. The pump cell comprises a zirconia layer, a Pt—Au alloy electrode disposed on one surface of the zirconia layer so as to be exposed to the gas chamber to be measured, and the zirconia layer so as to be exposed to the atmosphere. Of the Pt electrode disposed on the other surface of the zirconia layer, and the sensor cell includes a zirconia layer and Pt electrodes disposed on both surfaces of the zirconia layer so as to be exposed to the measured gas chamber and the atmosphere, respectively. The gas concentration measuring device according to claim 2, wherein 4. An exhaust gas purifying device for an internal combustion engine, wherein the NOx storage catalyst is arranged in an exhaust passage, and the gas according to claim 1, which is arranged downstream of the NOx storage catalyst. Concentration measuring device, when the pump cell is driven in the limiting current method mode, based on the value of the current passed by the sensor cell, NOx concentration detection means for detecting the NOx concentration in the measured gas, When the NOx concentration is lower than a predetermined judgment value, lean control execution means for executing lean control for controlling the air-fuel ratio of the air-fuel mixture to a value biased to lean, and when the NOx is equal to or higher than the judgment value. A rich spike control executing means for executing a rich spike control for making the air-fuel ratio of the air-fuel mixture rich, and a driving mode of the pump cell after the NOx becomes the judgment value or more, the limiting current method mode Switching execution means for switching from the electromotive force mode to the electromotive force mode, and when the pump cell is driven in the electromotive force mode, whether the electromotive force of the pump cell is inverted from a value corresponding to lean to a value corresponding to rich Rich inversion determination means for determining whether or not, after the rich spike control is started, when the electromotive force of the pump cell changes to a value corresponding to rich, the drive mode of the pump cell to the limiting current method mode Along with switching, ending the rich spike control,
An exhaust gas purifying apparatus for an internal combustion engine, comprising: lean control restarting means for restarting the lean control. 5. A deterioration state determination means for determining whether or not the NOx storage catalyst has deteriorated, and a mixture of air-fuel mixture based on an electromotive force of the pump cell while driving the pump cell in the electromotive force mode. Catalyst regeneration control executing means for executing catalyst regeneration control so that the fuel ratio becomes a predetermined value biased to rich, and catalyst regeneration control start to start the catalyst regeneration control when it is determined that the NOx storage catalyst has deteriorated And a catalyst regeneration control terminating means for terminating the catalyst regeneration control and resuming the lean control while switching the drive mode of the pump cell to the limiting current mode mode when the catalyst regeneration control termination condition is satisfied. The exhaust gas purifying apparatus for an internal combustion engine according to claim 4, further comprising: