JP2002266628A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately grasp a state of a maximum NOX absorbing amount of an NOX absorbent. SOLUTION: The NOX absorbent 23 is placed in an engine exhaust passage 22. A sensor 29 capable of detecting an NOX concentration inside exhaust gas is placed in the engine exhaust passage downstream of the NOX absorbent. When an NOX absorbing amount exceeds a determination value and the NOX concentration detected by the sensor is higher than a set value when an air-fuel ratio of the exhaust gas flowing into the NOX absorbent is changed from lean to rich, it is determined that the maximum NOX absorbing amount which can be absorbed by the NOX absorbent has decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.

[0002]

BACKGROUND ART Nitrogen oxides in the exhaust gas discharged from the combustion chamber of an internal combustion engine (NO _X) exhaust purification device arranged NO _X purification catalyst in the engine exhaust passage for purifying are known. As the NO _X purification catalyst absorbs NO _X when the air-fuel ratio of the inflowing exhaust gas is lean, the air-fuel ratio of the inflowing exhaust gas absorbed and becomes rich NO _X
The NO _X absorbent is known to be reduced and released by a reducing agent contained in the exhaust gas.

[0003] There is a limit to the amount of the NO _X which can be absorbed in such _the NO _X absorbent. That is, NO
_{The X} absorbent has a maximum NO _x absorption. The NO _X amount of the absorbent is absorbed NO _X is not able to absorb this maximum NO _X absorbing amount exceeds the _the NO _X absorbent is longer NO _X, to this end _the NO _X absorbent downstream NO _X is leaked. Therefore NO _X before the amount of the NO _X that is absorbed in the NO _X absorbent exceeds the maximum NO _X absorption
The air-fuel ratio of the exhaust gas flowing into the absorbent is switched from lean to rich to release NO _X from _the NO _X absorbent and reduced, and deterioration _the NO _X absorbent when the maximum NO _X absorption amount becomes very small Needs to be determined.

For example, NO_X NO absorbed by absorbent
_X Is the largest NO_X NO before exceeding absorption_X For absorbent
Switching the air-fuel ratio of the incoming exhaust gas from lean to rich
An exhaust gas purifying apparatus is disclosed in Japanese Patent Laid-Open No. 7-139340.
It is disclosed in the gazette. According to the publication, the fuel
NO generated per unit time in the furnace_X Amount of intake
Since it is a function of pressure and engine speed,
NO per unit time based on_X For absorbent
NO absorbed_X And the estimated NO _X Absorption
Is the maximum NO_X NO when the absorption amount is reached_X Flow into absorbent
Switch the air-fuel ratio of exhaust gas from lean to rich
I'm trying.

[0005] The system for determining the deterioration of the NO _X absorbent is known a system disclosed in JP-A-2000-104536. The NO _X sensor capable of detecting the concentration of NO _X in the exhaust gas in _the NO _X storage catalyst downstream arranged according to the publication, when the NO _X concentration detected by the NO _X sensor has reached a predetermined value In a system in which the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst is switched from lean to rich, when the interval for executing the process of switching the air-fuel ratio of the exhaust gas from lean to rich becomes short, the NO _X storage catalyst Is determined to have deteriorated.

[0006]

However, NO_X Absorbent
Maximum NO_X The absorption amount is, for example, NO_X When the absorbent deteriorates
Less. However, the above-mentioned Japanese Patent Application Laid-Open No.
NO_X Maximum N regardless of the state of deterioration of the absorbent
O_X The absorption amount is a constant value. Therefore estimated NO_X 
NO absorption maximum_X NO already when the absorption amount is reached_X 
NO downstream of the absorbent _X May have leaked.

The above-mentioned Japanese Patent Application Laid-Open No. 2000-104536
According to the publication, the presence or absence of deterioration of the NO _X storage catalyst is determined based on the interval at which the process of switching the air-fuel ratio of the exhaust gas from lean to rich is executed. However, the execution interval of the processing for switching the air-fuel ratio of the exhaust gas from lean to rich becomes shorter if the amount of NO _X flowing into the NO _X storage catalyst increases, for example, even if the NO _X storage catalyst has not deteriorated. Therefore, the system disclosed in the publication cannot accurately determine the deterioration of the NO _X storage catalyst.

[0008] The thus or to determine when to switch to rich the air-fuel ratio from the lean exhaust gas so as to reduce and purify NO _X that is absorbed in the NO _X absorbent, or to determine the deterioration of the NO _X absorbent it is extremely important for an exhaust purification apparatus to accurately grasp the state of the maximum NO _X absorption of the NO _X absorbent even purpose provided with _the NO _X absorbent in addition to or. Therefore, an object of the present invention is to provide NO _X
NO _X in the exhaust purification apparatus for an internal combustion engine having an absorbent
It is to accurately grasp the state of the maximum NO _X absorption amount of the absorbent.

[0009]

[MEANS FOR SOLVING THE PROBLEMS]
In the first invention, the air-fuel ratio of the exhaust gas flowing into the engine is lean.
NO if_X Absorbs the exhaust gas flowing into the sky
NO absorbed when fuel ratio becomes rich_X In the exhaust gas
NO released and reduced by the reducing agent_X Absorbent machine
In the exhaust passage, and the NO_X Exhaust flowing into the absorbent
NO in gas_XFrom the amount of NO_X Absorbed by absorbent
NO_X NO to estimate the amount of_X Equipped with absorption amount estimation means,
NO_X NO in exhaust gas flowing out of absorbent_X Check concentration
NO sensor that can output_X In the engine exhaust passage downstream of the absorbent
Place the above NO_X N estimated by the absorption amount estimating means
O_X NO when the absorption amount exceeds the judgment value_X Flow into absorbent
Switch the air-fuel ratio of exhaust gas from lean to rich
In the exhaust gas purifying apparatus for an internal combustion engine,_X absorption
Air-fuel ratio of exhaust gas flowing into the agent from lean to rich
NO detected by the above sensor when changing_X Concentration
NO when concentration is higher than predetermined concentration_X Absorbent
Maximum NO that can be absorbed_X Judge that the absorption amount has decreased
You. That is, according to this, NO_X Absorbed by absorbent
NO_X Two separate hands that can estimate the amount of
Step, ie NO_X Of the absorption amount estimating means and the sensor
One means, NO_X Estimated by absorption amount estimation means
NO _X NO absorption_X Exhaust gas flowing into the absorbent
The timing to switch the air-fuel ratio from lean to rich
At the timing thus determined.
The air-fuel ratio of exhaust gas from lean to rich
NO by the other means, ie, the sensor_X Absorbed by absorbent
NO contained_X I try to estimate the amount.

[0010] In the second invention, in the first invention
Maximum NO_X Above when it is determined that the absorption amount has decreased
Decrease the judgment value. Third to solve the above problems
In the invention of the above, it is assumed that the air-fuel ratio of the exhaust gas flowing in is lean.
NO_X And the air-fuel ratio of the inflowing exhaust gas is reduced.
NO when absorbed_X The return contained in the exhaust gas
NO released and reduced by base agent_X Pass the absorbent through the engine exhaust
Placed in the road,_X In the exhaust gas flowing into the absorbent
NO_XFrom the amount of NO_X NO absorbed by absorbent_X 
NO to estimate the amount of_X Equipped with an absorption amount estimating means;_X Sucking
NO in exhaust gas flowing out of absorbent_X Can detect concentration
NO sensor_X Placed in the engine exhaust passage downstream of the absorbent,
NO detected by the above sensor_X Density exceeds judgment value
NO when_X The air-fuel ratio of the exhaust gas flowing into the absorbent
Of exhaust from an internal combustion engine that switches from lean to rich
NO_X Exhaust gas empties into the absorbent
When the fuel ratio is switched from lean to rich,_X Sucking
NO estimated by yield estimation means_X The absorption amount is predetermined
NO if less than_X Absorbent will absorb
Maximum NO_X It is determined that the absorption amount has decreased. Sand
According to this, NO_X NO absorbed by absorbent_X of
Two separate means by which the quantity can be estimated:
Sensor and NO_X One means of absorption amount estimation means,
That is, NO estimated by the sensor_X NO absorption _X 
Lean to rich air-fuel ratio of exhaust gas flowing into absorbent
To determine when to switch to
The air-fuel ratio of the exhaust gas is changed from lean
When switching to rich, the other means, namely NO_X Sucking
NO by yield estimation means_X NO absorbed by absorbent
_X I try to estimate the amount.

In the fourth invention, when it is determined in the first or third invention that the maximum NO _X absorption amount has decreased, it is diagnosed that the NO _X absorbent has deteriorated.

[0012]

FIG. 1 shows a case where the present invention is applied to a direct injection type spark ignition engine. However, the present invention can also be applied to a compression ignition type internal combustion engine. Figure 1
1 is an engine body, 2 is a cylinder block, 3
Is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, 5 is a combustion chamber formed between the piston 3 and the cylinder head 4, 6 is an intake valve, 7 is intake air. Port 8 indicates an exhaust valve, and 9 indicates an exhaust port. As shown in FIG. 1, an ignition plug 10 is arranged at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is arranged around the inner wall surface of the cylinder head 4. A cavity 12 is formed on the top surface of the piston 3 and extends from below the fuel injection valve 11 to below the ignition plug 10.

An intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch pipe 13, and the surge tank 14 is connected to an intake duct 15 and an air flow meter 16.
Through an air cleaner (not shown). A throttle valve 18 driven by a step motor 17 is arranged in the intake duct 15. On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19, and the exhaust manifold 19 has a catalyst converter 21 containing an oxidation catalyst or a three-way catalyst 20 and a casing containing a NO _X absorbent 23 via an exhaust pipe 22. 24. The exhaust manifold 19 and the surge tank 14 are connected to each other via a recirculated exhaust gas (hereinafter, referred to as EGR gas) conduit 26, and an EGR gas control valve 27 is disposed in the EGR gas conduit 26.

The electronic control unit 31 is composed of a digital computer, and is connected to a RAM (random access memory) 33 and a ROM via a bidirectional bus 32.
(Read only memory) 34, CPU (microprocessor) 35, input port 36 and output port 37. The air flow meter 16 generates an output voltage proportional to the amount of intake air, and the output voltage
8 to the input port 36. An air-fuel ratio sensor 28 for detecting an air-fuel ratio is attached to the exhaust manifold 19, and an output signal of the air-fuel ratio sensor 28 is input to an input port 36 via a corresponding AD converter 38. The exhaust pipe 25 connected to the outlet of the casing 24 containing the NO _X absorbent 23 contains NO _X in the exhaust gas.
The concentration and ammonia concentration together detectable NO _X ammonia sensor 29 is disposed and the air-fuel ratio sensor 30,
The output signals of the NO _X ammonia sensor 29 and the air-fuel ratio sensor 30 are input to the input port 36 via the corresponding AD converter 38.

A load sensor 41 for generating an output voltage proportional to the amount of depression of the accelerator pedal 40 is connected to the accelerator pedal 40. The output voltage of the load sensor 41 is supplied to an input port 36 via a corresponding AD converter 38. Is entered. The crank angle sensor 42 generates an output pulse every time the crankshaft rotates 30 degrees, for example, and this output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 42. On the other hand, the output port 37 is connected to the corresponding drive circuit 3
9, the ignition plug 10, the fuel injection valve 11, the step motor 17, and the EGR control valve 27 are connected.

Next, the NO _X shown in FIG. 1 with reference to FIG.
The structure of the sensor section of the ammonia sensor 29 will be briefly described. Referring to FIG. 2 NO _X ammonia sensor 2
The sensor section 9 comprises six oxygen ion conductive solid electrolyte layers such as zirconia oxide laminated on each other.
The first solid electrolyte layer is referred to as a first layer L ₁ , a second layer
The layers are referred to as a layer L ₂ , a third layer L ₃ , a fourth layer L ₄ , a fifth layer L ₅ , and a sixth layer L ₆ .

Referring to FIG. 2, the first layer L ₁ and the third layer L ₃
A first diffusion-controlling member 50 and a second diffusion-controlling member 51 having, for example, a porous or fine pore are disposed between the first and second diffusion-controlling members 50 and 51. chamber 52 is formed, the second diffusion-controlling member 51 is provided between the second layer L ₂ is formed with a second chamber 53. Third
Between the layers L ₃ and the fifth layer L ₅ atmospheric chamber 54 communicating with the outside air are formed. On the other hand, the first diffusion-controlling member 5
0 is in contact with the exhaust gas. Therefore, the exhaust gas flows into the first chamber 52 via the first diffusion-controlling member 50, and thus the first chamber 52 is filled with the exhaust gas.

On the other hand, a cathode-side first pump electrode 55 is formed on the inner peripheral surface of the first layer L ₁ facing the first chamber 52,
An anode-side first pump electrode 56 is formed on the outer peripheral surface of the layer L ₁ , and a voltage is applied between the first pump electrodes 55 and 56 by a first pump voltage source 57. When a voltage is applied between the first pump electrodes 55 and 56, oxygen contained in the exhaust gas in the first chamber 52 is changed to the cathode-side first pump electrode 5.
5 and becomes oxygen ions.
The inside layer L ₁ flows toward the anode side first pump electrode 56.
Therefore, oxygen contained in the exhaust gas in the first chamber 52 moves in the first layer L ₁ and is pumped out. At this time, the amount of oxygen pumped out is reduced by the first pump voltage source 57. Increases as the voltage of the signal increases.

Meanwhile, reference electrode 58 is formed on the inner surface of the third layer L ₃ that faces the atmospheric chamber 54. By the way, in the oxygen ion conductive solid electrolyte, if there is a difference in oxygen concentration on both sides of the solid electrolyte layer, oxygen ions move in the solid electrolyte layer from the high oxygen concentration side to the low oxygen concentration side. In the example shown in FIG. 2, the oxygen concentration in the atmosphere chamber 54 is higher than the oxygen concentration in the first chamber 52. And this oxygen ion is in the third layer L
_3, the second layer L ₂ and the first layer L ₁ moves, releasing the charge at the cathode side first pump electrode 55. as a result,
A voltage V ₀ indicated by reference numeral 59 is generated between the reference electrode 58 and the first pump electrode 55 on the cathode side. This voltage V ₀ is proportional to the oxygen concentration difference between the atmospheric pressure chamber 54 and the first chamber 52.

In the example shown in FIG. 2, the voltage of the first pump voltage source 57 is feedback-controlled so that the voltage V ₀ matches the voltage generated when the oxygen concentration in the first chamber 52 is 1 ppm. . That is, oxygen in the first chamber 52 the oxygen concentration in the first chamber 52 is pumped through the first layer L ₁ so that 1P.Pm, oxygen concentration and thereby the first chamber 52 1p Maintained at .pm.

[0021] Note that a material having a low reducibility respect to the cathode side first pump electrode 55 is NO _X, for example, gold Au and are formed of an alloy of platinum Pt, thus NO _X contained in the exhaust gas is first It is hardly reduced in the one room 52. Therefore, this NO _X flows into the second chamber 53 through the second diffusion-controlling member 51. On the other hand, on the inner surface of the first layer L ₁ that faces the second chamber 53 the cathode side second pump electrode 6
0 is formed, and a voltage is applied between the cathode-side second pump electrode 60 and the anode-side first pump electrode 556 by the second pump voltage source 61. These pump electrodes 6
When the voltage between 0,56 is applied oxygen contained in the exhaust gas in the second chamber 53 becomes oxygen ions in contact with the cathode side second pump electrode 60, the oxygen ions first layer L ₁ Flows toward the first pump electrode 56 on the anode side. Therefore, the oxygen contained in the exhaust gas in the second chamber 53 is reduced to the first layer L
_It moves inside ₁ and is pumped out, and the amount of oxygen pumped out at this time increases as the voltage of the second pump voltage source 61 increases.

On the other hand, as described above, in the oxygen ion conductive solid electrolyte, if there is a difference in the oxygen concentration on both sides of the solid electrolyte layer, the oxygen ion in the solid electrolyte layer moves from the high oxygen concentration side to the low oxygen concentration side. Moves. In the example shown in FIG. 2, the oxygen concentration in the atmosphere chamber 54 is higher than the oxygen concentration in the second chamber 53, so that the oxygen in the atmosphere chamber 54 contacts the reference electrode 58 to receive a charge and generate oxygen ions. The oxygen ions move in the third layer L ₃ , the second layer L _2, and the first layer L ₁ , and discharge charges at the cathode-side second pump electrode 60. As a result, the voltage V indicated by reference numeral 62 is applied between the reference electrode 58 and the cathode-side second pump electrode 60.
₁ occurs. This voltage V ₁ is proportional to the oxygen concentration difference between the atmospheric pressure chamber 54 and the second chamber 53.

In the example shown in FIG. 2, the voltage of the second pump voltage source 61 is controlled by feedback so that the voltage V ₁ matches the voltage generated when the oxygen concentration in the second chamber 53 is 0.01 ppm. Is done. That oxygen in the second chamber 53 the oxygen concentration in the second chamber 53 is pumped through the first layer L ₁ so that 0.01P.Pm, whereby the second chamber 53
The oxygen concentration inside is maintained at 0.01 ppm.

The cathode side second pump electrode 60 is also NO_X To
On the other hand, materials having low reducibility such as gold Au and platinum Pt
Alloys and therefore contained in exhaust gas
NO_X Is in contact with the cathode-side second pump electrode 60
Hardly reduced. On the other hand, the third facing the second chamber 53
Layer L_Three NO on the inner circumference_X Cathode side pump electrode for detection
63 are formed. This cathode side pump electrode 63 is N
O_X Materials that have a strong reducibility against, for example, rhodium
It is made of Rh or platinum Pt. Therefore the second room
NO in 53_X In fact, most of the NO is on the cathode side
N on the pump electrode 63_Two And O_Two And is decomposed into
As shown in FIG. 2, the cathode side pump electrode 63 and the reference
A constant voltage 64 is applied between the pole 58 and
As a result, it was decomposed and generated on the cathode side pump electrode 63.
O_Two Is oxygen ions and the third layer L_Three Inside reference electrode 58
Move towards. At this time, the cathode side pump electrode 63 and the base
A sign proportional to the amount of oxygen ions between the quasi-electrode 58
Current I shown at 65 ₁ Flows.

As described above, NO _X is hardly reduced in the first chamber 52, and oxygen is hardly present in the second chamber 53. Thus current I ₁ is proportional to the NO _X concentration in the exhaust gas, it becomes possible to detect the concentration of NO _X in the exhaust gas from the current I ₁ and thus. On the other hand, ammonia NH ₃ contained in exhaust gas is decomposed into NO and H ₂ O in the first chamber 52 (4NH
₃ + 5O ₂ → 4NO + 6H ₂ O), this decomposed NO
Flows into the second chamber 53 through the second diffusion-controlling member 51. This NO is N ₂ on the cathode side pump electrode 63.
And is decomposed into O _2, O ₂ produced decomposition moves toward the reference electrode 58 of the third layer L ₃ becomes oxygen ions. Also at this time, the current I ₁ is reduced to the NH contained in the exhaust gas.
₃ proportional to the concentration and thus the current I ₁
The H ₃ concentration can be detected.

FIG. 3 shows the current I₁ And NO in exhaust gas_X concentration
And NH_Three The relationship with the concentration is shown. From Figure 3 the current
I₁ Is NO in exhaust gas_X Concentration and NH_Three Proportional to concentration
You can see that it is doing. On the other hand, the oxygen concentration in the exhaust gas
The higher the air-fuel ratio, the leaner the air-fuel ratio,
The amount of oxygen pumped out from 2 increases, and
The current I indicated by _Two Increase. Therefore, this current I_Two 
Thus, the air-fuel ratio of the exhaust gas can be detected.

Note that N is provided between the fifth layer L ₅ and the sixth layer L _6.
O _X ammonia and an electric heater 67 for heating the sensor portion of the sensor 29 is disposed, the electric heater 67
Sensor portion of the NO _X ammonia sensor 29 by 70
Heated from 0 ° C to 800 ° C. Figure 4 is _the NO _X absorbent 2
3 shows the output voltage E (V) of the air-fuel ratio sensor 30 disposed in the exhaust pipe 25 downstream of the exhaust pipe 25, that is, the output signal level of the air-fuel ratio detection means in a general expression. As can be seen from FIG. 4, the air-fuel ratio sensor 30 generates an output voltage of about 0.9 (V) when the air-fuel ratio of the exhaust gas is rich,
When the air-fuel ratio of the exhaust gas is lean, an output voltage of about 0.1 (V) is generated. That is, in the example shown in FIG. 4, the output signal level indicating rich is 0.9 (V), and the output signal level indicating lean is 0.1 (V).
(V).

On the other hand, it is also possible to use a NO _X from the current I ₂ of the ammonia sensor 29 can detect the air-fuel ratio of the exhaust gas, thus NO _X ammonia sensor 29 as an air-fuel ratio detecting means as described above. In this case, there is no need to provide the air-fuel ratio sensor 30. Next, FIG.
The fuel injection control of the internal combustion engine shown in FIG. 1 will be described with reference to FIG. In FIG. 5A, the vertical axis represents engine load Q / N (intake air amount Q / engine speed N).
The horizontal axis represents the engine speed N.

[0029] Figure 5 in the operating region of lower load than the solid line X ₁ in (A) has a stratified charge combustion is performed. That is, at this time, as shown in FIG.
The fuel F is injected from 1 into the cavity 12.
This fuel is guided by the inner peripheral surface of the cavity 12 to form an air-fuel mixture around the ignition plug 10.
0 causes ignition combustion. At this time, the average air-fuel ratio in the combustion chamber 5 is lean.

On the other hand, in the region of the high load side than the solid line X ₁ in FIG. 5 (A) the fuel from the fuel injection valve 11 is injected during the intake stroke, uniform mixture combustion is performed at this time. In still between solid X ₁ and a chain line X ₂ is performed homogeneous mixture combustion under a lean air-fuel ratio, the uniform mixture combustion under a stoichiometric air-fuel ratio between the chain line X ₂ and the chain line X ₃ Done,
Uniform mixture combustion under a rich air-fuel ratio in the high load side is performed than the chain line X _3.

In the present invention, the basic fuel injection amount TAU required for setting the air-fuel ratio to the stoichiometric air-fuel ratio is formed as a function of the engine load Q / N and the engine speed N as shown in FIG. The basic fuel injection amount TAU is basically multiplied by the correction coefficient K to obtain the final fuel injection amount TAUO (= K · TA).
U) is calculated. The correction coefficient K is stored in the ROM 34 in advance in the form of a map as a function of the engine load Q / N and the engine speed N as shown in FIG.

The value of the correction coefficient K is smaller than 1.0 in an operation region on the lower load side than the dashed line X _{2 in} FIG. 5A in which combustion is performed under a lean air-fuel ratio. It is larger than 1.0 even in the operating range of the high load side than the chain line X ₃ shown in FIG. 5 (a) where combustion takes place under. The correction coefficient K is in the operating region between the chain line X ₂ and a chain line X ₃ is 1.0, the air-fuel ratio at this time is feedback controlled based on the output signal of the air-fuel ratio sensor 28 so that the theoretical air-fuel ratio Is done.

[0033] The engine exhaust passage _the NO _X absorbent 23, for example alumina placed in a carrier, the carrier on, for example potassium K, sodium Na, lithium Li, cesium Cs, barium Ba, calcium Ca And at least one selected from the group consisting of alkaline earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt. In this case, to place the particulate filter consisting of a casing 24 in the example cordierite, alumina on the particulate filter can be supported to _the NO _X absorbent 23 to a carrier.

[0034] Any engine intake passage even when the, the ratio of the amount of air to the amount of fuel supplied to the combustion chamber 5 and _the NO _X absorbent 23 in the exhaust passage upstream of (hydrocarbon) N
O _X absorbent Toko called air-fuel ratio of the exhaust gas flowing into 23 NO _X absorbent 23 absorbs NO _X when the air-fuel ratio of the inflowing exhaust gas is lean, the air-fuel ratio of the inflowing exhaust gas or the stoichiometric air-fuel ratio carry out the absorption and release action of NO _X to release the NO _X absorbed and become rich.

If this NO _X absorbent 23 is disposed in the engine exhaust passage, the NO _X absorbent 23 actually performs the absorption and release of NO _X , but the detailed mechanism of this absorption and release is not clear in some parts. . However, it is considered that this absorption / release action is performed by the mechanism as shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

In the internal combustion engine shown in FIG.
Combustion with lean air-fuel ratio in most operating conditions
Is performed. In this way, combustion occurs when the air-fuel ratio is lean.
If this is done, the oxygen concentration in the exhaust gas is high,
At this time, as shown in FIG._Two But
O_Two ^-Or O^2-On the surface of platinum Pt. one
On the other hand, NO in the inflowing exhaust gas becomes O on the surface of platinum Pt._Two ^-Ma
Or O^2-Reacts with NO _Two (2NO + O_Two → 2N
O_Two ). NO generated next_Two Part of is on platinum Pt
Barium oxide BaO being absorbed in the absorbent while being oxidized
6A, the nitrate ion N is combined as shown in FIG.
O_Three ^-Diffuses into the absorbent in the form of NO in this way_X 
Is NO_X It is absorbed in the absorbent 23. In the incoming exhaust gas
NO on the surface of platinum Pt as long as oxygen concentration is high_Two Is generated
NO_X NO unless absorption capacity is saturated_Two But
Nitrate ion NO absorbed in the absorbent_Three ^-Is generated.

On the other hand, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas decreases, and as a result,
The amount of NO ₂ generated on the surface of platinum Pt decreases. NO ₂
And the amount of production is reduced reaction backward (NO ₃ ^- → NO ₂₎
The process proceeds, thus nitrate ions to the absorber NO ₃ ^- is NO ₂
Released from the absorbent in the form of At this time, _the NO _X absorbent 2
NO _X released from 3 large amount of unburned HC contained in the inflowing exhaust gas as shown in FIG. 6 (B), is caused to reduction by reaction with CO. In this way, when NO ₂ is no longer present on the surface of platinum Pt, N ₂ is successively removed from the absorbent.
O ₂ is released. Therefore NO _X from _the NO _X absorbent 23 in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich is released, yet NO _X is discharged into the atmosphere to the released NO _X is reduced It will not be done.

In this case, the air-fuel ratio of the inflowing exhaust gas is controlled.
NO even if stoichiometric_X NO from absorbent 23_X Is released
It is. However, the air-fuel ratio of the incoming exhaust gas is
NO if set to_X NO from absorbent 23_X Gradually
NO because it is not released_XAbsorbed by absorbent 23
All NO_X It takes a slightly longer time to release. When
NO at the time_X NO of absorbent 23_X The absorption capacity is limited
And therefore NO _X NO of absorbent 23_X I'm tired of absorption
NO before summing_X NO from absorbent 23_X Must be released
It is necessary. But NO_X Absorbent 23 is NO_X Absorption capacity
Is sufficient, almost all N contained in exhaust gas
O_X Absorbs NO_X Partial when approaching the limit of absorption capacity
NO_X Can no longer be absorbed, and thus NO_X Absorbent 2
3 is NO_X NO when approaching the limit of absorption capacity_X Absorbent 23
NO that flows downstream from_X The volume starts to increase.

[0039] Therefore, in the first embodiment of the present invention estimates the NO _X absorption of the total, which is absorbed in the NO _X absorbent 23, NO _X when the NO _X absorption amount close to the maximum NO _X absorption It is from _the NO _X absorbent 23 so as to release the NO _X fuel ratio of the exhaust gas flowing into the absorbent 23 temporarily rich. In this case, the method of the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 rich There are various ways. For example, the air-fuel ratio of the exhaust gas can be made rich by making the average air-fuel ratio of the air-fuel mixture in the combustion chamber 5 rich, and the exhaust gas can be made rich by injecting additional fuel at the end of the expansion stroke or during the exhaust stroke. the air-fuel ratio can either be rich or air-fuel ratio of the exhaust gas by injecting additional fuel to _the NO _X absorbent 23 in the exhaust passage upstream of it is also possible to rich. In the embodiment of the present invention, the first method, that is, the air-fuel ratio of the exhaust gas is made rich by performing uniform mixture combustion under the rich air-fuel ratio.

By the way, the exhaust gas contains SO _X , and the NO _X absorbent 23 contains not only NO _X but also SO _X
Is also absorbed. It is considered that the mechanism of absorbing SO _X into the NO _X absorbent 23 is the same as the mechanism of absorbing NO _X. That is, the case where platinum Pt and barium Ba are carried on the carrier as in the description of the NO _X absorption mechanism will be described as an example. As described above, when the air-fuel ratio of the inflowing exhaust gas is lean, oxygen O ₂ becomes O _{2. 2} ^-
Or O ²⁻ is attached to the surface of platinum Pt in the form of O ²⁻ , and SO ₂ in the inflowing exhaust gas is converted to O ₂ ⁻ or O ₂ on the surface of platinum Pt.
Reacts with ^2- to form SO ₃ . Next, a part of the generated SO ₃ is further oxidized on the platinum Pt, absorbed in the absorbent and combined with barium oxide BaO, and diffused into the absorbent in the form of sulfate ion SO ₄ ²⁻ , thereby stabilizing. Sulfate Ba
Generate SO ₄ .

However, the sulfate BaSO ₄ is stable and difficult to decompose, and the sulfate BaSO ₄ remains without being decomposed simply by making the air-fuel ratio of the inflowing exhaust gas rich. _The NO _X absorbent 23 as thus NO _X in the absorbent 23 hours will be sulfate BaSO ₄ increases as elapses, thus to time has elapsed
Will decrease the amount of NO _X that can be absorbed. That will be _the NO _X absorbent 23 deteriorates over time.

However, in this case, NO_X Absorbent 23 temperature
NO when the temperature exceeds a certain temperature, for example, 600 ° C._X absorption
Sulfate BaSO in agent 23_Four Decomposes, at this time
NO _X Rich air-fuel ratio of exhaust gas flowing into absorbent 23
NO_X Absorbent 23 to SO_X Releasing
Can be. Therefore, in the embodiment of the present invention, NO_X Absorbent 23
To SO_X To release NO_X Temperature of absorbent 23
NO if high_XOf the exhaust gas flowing into the absorbent 23
NO with rich air-fuel ratio_X Absorbent 23 to SO_X Release
Let out, SO_X To release NO_X Absorbent 23
NO if temperature is low_X Raise the temperature of the absorbent 23
And NO_X Air-fuel ratio of exhaust gas flowing into absorbent 23
Is made rich.

[0043] Then outflow from the amount and _the NO _X absorbent 23 of the reducing agent when the air-fuel ratio of the exhaust gas flowing from _the NO _X absorbent 23 in the NO _X absorbent 23 to be released NO _X rich downstream The relationship with the concentration of ammonia NH ₃ in the exhaust gas will be described. First, the amount of the reducing agent will be described. Excess fuel to the fuel quantity required for the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 to the stoichiometric air-fuel ratio of the excess fuel because it is used for release and reduction of the NO _X the amount is equal to the amount of reducing agent used in the release and reduction of NO _X. This is _the NO _X absorbent 2
3 from the air-fuel ratio of the mixture in the combustion chamber 5 when releasing the NO _X even when the rich, even when injecting additional fuel during the expansion stroke end or exhaust stroke, NO
_This is true even when additional fuel is injected into the exhaust passage upstream of the _X absorbent 23.

Next, the concentration of ammonia will be described.
When the air-fuel ratio is lean, that is, in an oxidizing atmosphere
Ammonia NH_Three Rarely occurs. However, air fuel
Inhalation when the ratio becomes rich, that is, when the atmosphere becomes a reducing atmosphere
Nitrogen N in air or exhaust gas_Two Is an oxidation catalyst or three
Reduced by the hydrocarbon HC in the primary catalyst 20,
Monia NH_Three Is generated. However, the air-fuel ratio
NO_X NO from absorbent 23_X Is released and raw
Ammonia NH formed_Three Is this NO_X To reduce
NO because it is used for_X NO from absorbent 23_X Is released
While the supplied reducing agent is NO_X Release
And NO while used for reduction_X Absorbent 2
Ammonia NH downstream from 3_Three Does not leak. Against this
NO _X NO from absorbent 23_X After the release of
If the air-fuel ratio is made rich, more precisely, N
O_X NO from absorbent 23_X Used to release and reduce
If excess reducing agent not supplied is supplied, ammonia NH_Three 
Is no longer_X Will not be consumed for the reduction of
Therefore, at this time NO_X Downstream from absorbent 23
Ammonia NH_Three Will be leaked.

[0045] This is caused even when the oxidation catalyst or three-way catalyst 20 upstream of _the NO _X absorbent 23 is not provided. That is, since the NO _X absorbent 23 also includes a catalyst such as platinum Pt having a reducing function, ammonia NH ₃ may be generated in the NO _X absorbent 23 when the air-fuel ratio becomes rich. However, even if ammonia NH
_{Even if 3} is produced, this ammonia NH ₃ is NO _X
Since it is used to reduce NO _X released from the absorbent 23, ammonia NH ₃ does not flow downstream from the NO _X absorbent 23. N However, from _the NO _X absorbent 23
O ammonia NH _{3 X} from _the NO _X absorbent 23 as excess of the reducing agent that is not used has been described above as being provided to release reduced to downstream will flow out.

As described above, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 is made rich, the NO _X absorbent 2
3 from the excess of the reducing agent that is not used to release and reduce NO _X is supplied the excess of the reducing agent flows out from _the NO _X absorbent 23 downstream in the form of ammonia NH _3, ammonia flowing out at this time The amount is proportional to the amount of excess reducing agent. Therefore, the amount of excess reducing agent can be determined from the amount of ammonia flowing out at this time. The amount of ammonia is capable of detecting the ammonia concentration NO _X ammonia sensor 29
Is detected by In this case, the integrated value of the ammonia concentration is considered to represent the surplus reducing agent amount, and therefore, it can be said that the integrated value of the ammonia concentration is a representative value indicating the surplus reducing agent amount. Further, it can be considered that the maximum value of the ammonia concentration represents the surplus reducing agent amount, and therefore, it can be said that the maximum value of the ammonia concentration is a representative value representing the surplus reducing agent amount.

Next, a first embodiment of the supply control of the reducing agent will be described with reference to FIG. Referring to FIG. 7, ΣNO
X is the total being absorbed in the NO _X absorbent 23 NO
_X indicates the _X amount, and I ₁ is the NO _X ammonia sensor 29
3 shows the detection current. Note NO _X and NH ₃ in FIG. 7 shows each change in the detected current of the NO _X ammonia sensor 29 due to the change of the NO _X concentration and the NH ₃ concentration in the exhaust gas, both NO _X ammonia sensor of the detection current It appears in the 29 detected currents I ₁ . Also E
Represents the output voltage of the air-fuel ratio sensor 30, and A / F represents the average air-fuel ratio of the air-fuel mixture in the combustion chamber 5.

[0048] In the first embodiment the total N to _the NO _X absorbent 23
O _X absorption to estimate, controlling based rich time interval air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 to be re-rich after the rich to the estimated value of the total NO _X absorption, It is to be modified further based on the rich time interval detected current I _1. That this first embodiment is provided with a total NO _X absorption amount estimation means for estimating an amount of NO _X in total that are absorbed in the NO _X absorbent 23, the total NO _X as shown in FIG. 7 the total NO _X absorption amount ΣNOX allowable value N, which is estimated by the absorption amount estimation means
When OXmax is exceeded, the air-fuel ratio is temporarily switched from lean to rich.

The amount of NO _X discharged from the engine Sadamari substantially the determined operating state of the engine, therefore _the NO _X absorbent 2
In No. 3, the amount of NO _X absorbed at each time is also substantially determined when the operating state of the engine is determined. Accordingly advance determined by experiment the NO _X absorption NA to _the NO _X absorbent 23 per unit time in accordance with the engine operating state in this first embodiment, the NO _X absorption NA is the engine load Q / N and Engine speed N
ROM as a function of the map as shown in FIG.
34.

Therefore, according to the total NO _X absorption amount estimating means of this embodiment, the NO _X absorption amount NA according to the engine operating state shown in FIG.
Total NO _X estimated to be absorbed by the O _X absorbent 23
An absorption amount ΣNOX is calculated. However the air-fuel ratio is the value of NA becomes negative in this operating region because NO _X is released from _the NO _X absorbent 23 is in the operating region becomes the stoichiometric air-fuel ratio or a rich air-fuel ratio.

On the other hand, the allowable value NOXmax is NO_X Absorbent 2
Total SO absorbed in 3 _X As the amount increases
Th, that is, NO_X As the absorption capacity of the absorbent 23 decreases,
It will be made smaller. By the way, it depends on the fuel in the injected fuel.
Contains almost a fixed percentage of sulfur,
NO_X SO absorbed at any time by the absorbent 23_X Amount is jet
It is proportional to the integrated value ΣTAU of the injected fuel amount TAU. Accordingly
In the first embodiment of the leverage, as shown in FIG.
As the integrated value ΣTAU increases, the allowable value NOXmax gradually increases
Is reduced to

[0052] In the first embodiment is basically an air-fuel ratio is temporarily switched from lean to rich when the total NO _X absorption amount ΣNOX as described above exceeds the allowable value NOXmax. In this case, during the operation of the engine, the allowable value NOXmax gradually decreases as shown in FIG. Therefore, it can be understood that the rich time interval becomes gradually shorter when almost the same operation state continues. The total NO _X when this first embodiment tolerance NOXmax is to begin NO _X from _the NO _X absorbent 23 downstream flows out during lean operation
The value is set to a value smaller than the absorption amount. Hence the air-fuel ratio before the start NO _X from _the NO _X absorbent 23 downstream flows out during lean operation is switched from lean to rich in the first embodiment.

By the way, the calculated total NO_X Absorption amountΣNO
X is the actual total NO_X If there is a deviation from the absorption amount
In this case, ΣNOX <NOXmax
_X NO downstream from absorbent 23_X May begin to leak
is there. FIG. 7 shows NO at this time._X Ammonia sensor 29
Detection current I₁ Is shown. That is, NO _X 
The air-fuel ratio of the exhaust gas flowing into the absorbent 23 changes from lean to
Switch to the detection current I₁ Is the set value I_S 
Has been reached. This means that ΣNOX <NOXmax
Nevertheless NO_X Is NO_X Flow downstream from absorbent 23
It means that it started to put out.

Therefore, in the first embodiment, when the total NO _X absorption amount ΣNOX estimated by the total NO _X absorption amount estimating means exceeds the allowable value NOXmax, that is, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 is reduced. detection current I of the NO _X ammonia sensor 29 when it is switched from lean to rich
_{When 1} is larger than the set value I _S , that is, when the NO _X concentration in the exhaust gas downstream of the NO _X absorbent 23 is higher than a predetermined concentration, the NO _X absorbent 23 can absorb the maximum. the amount of NO _X (hereinafter, referred to as the maximum NO _X absorption.) is determined to have decreased.

In the first embodiment, when it is determined that the maximum NO _X absorption amount has decreased, the allowable value NOXmax is decreased by a predetermined amount B, and when the decrease in the maximum NO _X absorption amount is relatively large. For example, the allowable value NOXma
It determines that x is _the NO _X absorbent 23 is deteriorated when it becomes smaller than the value NOXmin predetermined. Σ
NOX <When NO _X from despite _the NO _X absorbent 23 is NOXmax downstream began to flow out, ie the detected current I ₁ is set value I of the NO _X ammonia sensor 29
The air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 when exceeding the _S rich temporarily switched from lean, may be reduce by an amount B which is defined a tolerance NOXmax advance.

[0056] Incidentally excess fuel air-fuel ratio is switched from lean to rich, i.e. reducing agent an air-fuel ratio of the exhaust gas flowing out from _the NO _X absorbent 23 because they are consumed for the reduction of NO _X downstream almost It becomes the stoichiometric air-fuel ratio. In this case, the reason why this is so becomes the air-fuel ratio is only slightly lean of the exhaust gas is not clear when the NO _X absorbent 23 has not deteriorated flowing out from _the NO _X absorbent 23 to downstream, _the NO _X absorbent NO if 23 deteriorates
The air-fuel ratio of the exhaust gas flowing downstream from the _X absorbent 23 tends to be slightly rich. However, in any case, when the releasing operation of NO _X from the NO _X absorbent 23 is completed, the air-fuel ratio of the exhaust gas flowing downstream from the NO _X absorbent 23 decreases.

[0057] Figure 7 shows the case where the air-fuel ratio of the exhaust gas flowing out from _the NO _X absorbent 23 when the air-fuel ratio is switched from lean to rich to downstream and has a just slightly lean, NO _X absorption When the release action of NO _X from the agent 23 is completed, that is, the total NO _X absorption amount ΣNO
It can be seen that when X approaches zero, the output voltage E of the air-fuel ratio sensor 30 changes toward the output signal level indicating that it is rich, that is, rises. This change in the output signal level E has a good response, so that the output signal level E
It can be switched air-fuel ratio when the releasing action of the NO _X from _the NO _X absorbent 23 be switched to the air-fuel ratio from rich to lean has been completed from rich to lean based on the change.

Therefore, in the embodiment shown in FIG. 7, a reference voltage E _S is set for the output voltage E of the air-fuel ratio sensor 30. That is, using a general expression, the output signal level E of the air-fuel ratio detecting means is obtained. and the air-fuel ratio from rich to switch to a lean when previously set the reference level E _S, the output signal level E exceeds the reference level E _S against.

The output voltage E of the air-fuel ratio sensor 30 changes with a good response to the completion of the NO _X releasing operation. However, the output voltage E varies due to variations in the performance of the air-fuel ratio sensor 30 and the NO _X absorbent 23 or changes over time. E changes in various ways. Therefore, if the reference level E _S not be switched air-fuel ratio from rich to lean upon completion of release of the NO _X it is fixed to a constant value arises.

On the other hand, if an excess reducing agent not used for releasing and reducing NO _X is supplied from the NO _X absorbent 23 when the air-fuel ratio is switched from lean to rich, at this time, the NO _X absorbent 23 since ammonia NH ₃ flows out to the downstream from the detection current I ₁ of the NO _X ammonia sensor 29 as shown in FIG. 7 is increased. In this case, the maximum value Imax of the integrated value ΣI and detected current I ₁ of the detected current I ₁ indicated by hatching in FIG. 7 represents the excess of reducing agent amount.

Although the detection current I ₁ of the NO _X ammonia sensor 29 has a response delay with respect to the completion of the NO _X release, the surplus reducing agent amount can be accurately obtained from the detection current I ₁ . Therefore, in the present invention, the detection current I ₁ of the NO _X ammonia sensor 29 is used.
The reference level E _S is changed based on the change in the exhaust gas, that is, the air-fuel ratio of the exhaust gas is switched from rich to lean when the release of NO _X from the NO _X absorbent 23 is completed based on the change in the ammonia concentration. I try to make it.

More specifically, the integrated value of the detected current I ₁ Σ
I, or set in advance the target value of the smaller value relative to the maximum value Imax of detected current I _1, when ΣI or Imax becomes greater than the target value, that is, when the amount of reducing agent excess is large from rich The reference level E _S is lowered so that the air-fuel ratio switching time to lean is advanced so that the surplus reducing agent amount becomes _small , that is, the reference level E _S is changed to the output signal level side indicating lean, and ΔI
Alternatively, when Imax is smaller than the target value, that is, when the surplus reducing agent amount is zero or close to zero, the switching timing of the air-fuel ratio from rich to lean is delayed so that the surplus reducing agent amount becomes small. E _S is increased, that is, the reference level E _S is changed to an output signal level indicating richness.

FIGS. 10 and 11 show a routine for executing the first embodiment. Referring to FIG. 10 and FIG. 11, first, in step 100, FIG.
The basic fuel injection amount TAU is calculated from the map shown in (B). Then NO _X release flag indicating that it should release the NO _X from step 101, _the NO _X absorbent 23 is whether it is set or not. When the NO _X release flag is not set, the routine proceeds to step 102, where the NO _X absorption amount N per unit time is obtained from the map shown in FIG.
A is calculated. Next, at step 103, this NO _X
By adding the absorption amount NA to ΣNOX, the total NO _X absorption amount ΣNOX estimated to be absorbed by the NO _X absorbent 23 is calculated.

Next, at step 104, the final injection amount TAUO is added to ΣTAU to calculate the integrated value 噴射 TAU of the injected fuel amount. Then step 105
Then, the allowable value NOXmax is calculated from the relationship shown in FIG. 9 based on the integrated value ΣTAU. Then step 10
At 6, the allowable value NOXmax is reduced by the correction amount ΔX. Next, at step 108, the total NO _X absorption amountΣNO
It is determined whether X has exceeded the allowable value NOXmax. Σ
When NOX ≦ NOXmax, i.e. _the NO _X absorbent 23
Step 1 when there is still room in of the NO _X absorbing capacity
Jump to 09.

In step 109, the correction coefficient K is calculated from the map shown in FIG. Then step 11
At 0, the basic fuel injection amount TAU is multiplied by the correction coefficient K to obtain the final fuel injection amount TAUO (= K · TA
U) is calculated, and fuel injection is performed with this injection amount TAUO. Next, at step 111 NO _X absorbent 2
It is determined whether or not to perform SO _X release processing for releasing SO _X from Step 3. When it is not necessary to perform the SO _X release processing, the processing cycle is completed. On the other hand, when a determination is made that the ΣNOX> NOXmax in step 108 NO _X release flag is set the routine proceeds to step 112, and then NH ₃ detection flag is set the routine proceeds to step 113. Next, the routine proceeds to step 109.

When the NO _X release flag is set, the routine proceeds from step 101 to step 115 in the next processing cycle, where the rich correction coefficient K _R is calculated. Next, at step 116, the rich correction coefficient K _R is added to the basic fuel injection amount TAU.
Is multiplied by the final fuel injection amount TAUO
(= K _R · TAU) is calculated, and fuel injection is performed with this injection amount TAUO. At this time is switched from the homogeneous mixture combustion under stratified combustion or a lean air-fuel ratio at a lean air-fuel ratio uniform mixture combustion under a rich air-fuel ratio, then the _the NO _X absorbent 23 NO _X
The release action of is started.

Next, at step 117, the air-fuel ratio sensor 3
0 is the reference voltage E_S Is determined whether or not
It is. E ≦ E_S If so, the process proceeds to step 111. this
For E> E_S Then, the routine proceeds to step 118, where ΣN
OX is set to zero, NO_X The release flag is reset.
NO_X Is the air-fuel ratio rich when the release flag is reset?
Is switched to lean. On the other hand, in step 111
SO_X If it is determined that release treatment should be performed,
Go to step 119 and NO_X Absorbent 23 to SO _X Release
Is performed. That is, NO_X Absorbent 23 temperature
The air-fuel ratio is rich while maintaining the
It is. Then NO_X SO from absorbent 23_X Release action of
Is completed, ΣTAU is made zero.

FIG. 12 shows a routine for correcting the allowable value NOXmax. Referring to FIG. 12, first, at step 120, it is determined whether or not the total NO _X absorption amount ＸNOX is larger than the allowable value NOXmax (ΣNOX> NOXmax). In step 120, ΣNOX ≦ NO
Although the routine ends when it is judged that Xmax detected current I ₁ of the NO _X ammonia sensor 29 is larger than the set value I _S proceeds to step 121 when it is judged that ΣNOX> NOXmax (I _{1> IS} ) is determined.

When it is determined in step 121 that I ₁ ≤I _S , the routine is terminated, but I ₁ > I _S
When it is determined that the allowable value NO is satisfied, the process proceeds to step 122, and the allowable value NO used in step 106 of FIG.
The correction amount ΔX for Xmax is increased by the value B. That is, in step 120, ΣNOX> NOX
determines that the maximum NO _X absorption of the NO _X absorbent 23 when a I _1> I _S in is and step 121 a max is decreased, the routine thereafter in FIG. 10 since the correction amount ΔX is made to increase in step 122 Is executed, the NOXmax is further reduced in step 106, and the determination in step 108 is executed based on the reduced NOXmax. This allows NO _X
The amount of NO _X flowing downstream of the absorbent 23 can be kept low.

Next, at step 123 in FIG. 12, an allowable value NOXmax is calculated from the relationship shown in FIG.
Next, at step 124, the calculated NOXma
x is reduced by the correction amount ΔX, and the routine proceeds to step 125. In step 125, the corrected NOXmax is smaller than the allowable minimum value NOXmin (NOXmax <NOX
min)). N in step 125
OXmax ≧ when it is determined that NOXmin ends the routine but when it is judged that NOXmax <NOXmin NO _X absorbent 2 proceeds to step 126
3 is deteriorated, and the fact is displayed.

FIG. 13 shows the target level E._S To calculate
Shows a routine. Referring first to FIG.
In step 200, NH_Three The detection flag is set
Is determined. This NH_Three Figure shows detection flag
In step 108 of step 11, ΣNOX> NOXmax
Set when it becomes NH_Three Detection flag set
If so, the routine proceeds to step 201 where NH_Three detection
The elapsed time t since the flag was set is a certain time t₁ 
Is determined. This fixed time t ₁ Is air fuel
NO after ratio is made rich from lean_X Ammonia
Current I of the sensor 29₁ Time until the temperature has dropped to zero
Between. t> t₁ , The process proceeds to step 202 and N
H_Three The elapsed time t since the detection flag was set is constant
Time t_TwoIs determined. This fixed time t_Two
 Is NO_X Ammonia leaked downstream from absorbent 23
NO matter what amount of ammonia_X Ammo
Enough for near sensor 29 to detect ammonia concentration
Time. t ≦ t_Two Proceeds to step 203
No.

[0072] detected current I ₁ of the step 203 NO _X ammonia sensor 29 is calculated. Then step 2
04 In the integrated value ΣI of detected current by adding the detected current I ₁ in ΣI is calculated. Next, when it is determined in step 202 that t> t ₂ , the routine proceeds to step 205, where it is determined whether the integrated value ΔI of the detected current is larger than the target value Sr. When ΣI> Sr, the routine proceeds to step 206, where the target level E _S is decreased by a predetermined set value α, and then the routine proceeds to step 208. On the other hand, when ΔI ≦ Sr, the routine proceeds to step 207, where the target level E _S is increased by a predetermined set value α, and then the routine proceeds to step 208. At step 208, ΔI is cleared and the NH ₃ detection flag is reset.

FIG. 14 shows another example of a routine for calculating the target level E _S. Referring to FIG. 14, first, at step 300, it is determined whether or not the NH ₃ detection flag is set. This NH ₃ detection flag is set in step 108 of FIG.
Set when Xmax is reached. When the NH ₃ detection flag is set, the routine proceeds to step 301, where N 3
H ₃ elapsed time t from the detection flag is set whether exceeds a predetermined time t ₁ is determined. This fixed time t
₁ is a time until the detected current I ₁ of the NO _X ammonia sensor 29 after the fuel-air ratio is from lean to rich as described above finishes reduced to zero. t> whether becomes to t ₁ NH ₃ detection flag goes to step 302 the elapsed time t after the set exceeds a predetermined time t ₂ is determined. As described above, the fixed time t ₂ is the same as the NO _x ammonia sensor 29 regardless of the amount of ammonia when the ammonia flows downstream from the NO _x absorbent 23.
Is sufficient time to detect ammonia concentration.
When t ≦ t _2, the process proceeds to step 303.

[0074] detected current I ₁ at step 303, Imax
It is determined whether or not it is greater than I _1> maximum value I of I ₁ is detected current proceeds to step 304 when the Imax
max. Next, at step 302, t> t ₂
When it is determined that the current value has reached, the routine proceeds to step 305, where it is determined whether the maximum value Imax of the detected current is greater than the target value Imaxr. When Imax> Imaxr, the routine proceeds to step 306, where the target level E _S is decreased by a predetermined set value α, and then the routine proceeds to step 308.
On the other hand, when Imax.ltoreq.Imaxr, step 307 is executed.
Then, the target level E _S is increased by a predetermined set value α, and then the routine proceeds to step 308. In step 308 Imax is cleared, NH ₃ detection flag is reset.

Next, a second embodiment will be described. FIG.
NO as shown in_X Toe absorbed by absorbent 23
Tal NO_X Absorption amount ΣNOX increases and NO_X Absorbent 2
NO when approaching the absorption capacity limit of 3_X Downstream from absorbent 23
NO_X Begins to leak NO_X Ammonia ammonia
Detection current I₁ Begins to rise. As shown in FIG.
NO in the example_X NO downstream from absorbent 23_X Is leaked
NO_X If the concentration exceeds a preset value
I.e. NO_X Detection current I of ammonia sensor 29
₁ Is a predetermined set value I_S NO when exceeding_X Sucking
NO from collector 23 _X Air / fuel ratio A / F is lean to release
Is switched to rich. Air-fuel ratio A / F from lean
Exhaust gas with rich air-fuel ratio is NO even if switched to rich
_X Since it takes time to reach the absorbent 23, the air-fuel ratio A
NO immediately after / F is switched to rich_X Absorbent 23
NO that flows downstream_X Volume continues to increase. Next
NO due to the reducing agent contained in the exhaust gas with air-fuel ratio_X of
NO for starting reduction action_X Downstream from absorbent 23
NO_X Will not leak. Therefore, the air-fuel ratio is
NO when switching from rich to rich_X Ammonia sensor
29 detected current I₁ Rises briefly and then drops to zero
You. On the other hand, when the air-fuel ratio is switched from lean to rich
NO_X NO from absorbent 23_X Release action of
Thus NO_XNO absorbed by absorbent 23_X Quantity
NO_X Gradually decreases.

In the second embodiment, when the detection current I ₁ of the NO _X ammonia sensor 29 exceeds the set value I _S , that is, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 is switched from lean to rich. Total NO when
_{The X} absorption amount ΣNOX should be near the allowable value NOXmax. However the maximum NO _X absorption when _the NO _X absorbent 23 is deteriorated is reduced. Therefore, at this time, I ₁
> I total NO _X absorption amount ΣNOX be a _S permissible value NO
It becomes lower than Xmax.

Therefore, in the second embodiment, NO_X Ammonia
Current I of the sensor 29₁ Is the set value I _S Total
NO_X Absorption amount ΣNOX is smaller than allowable value NOXmax
Sometimes NO_X Maximum NO of absorbent 23_X The absorption is reduced
It is determined that_X Absorption amount ΣNOX is allowable minimum value ΣN
NO when smaller than OXmin_X Absorbent 23 deteriorates
It is determined that it has been performed.

FIG. 16 shows a routine for executing the second embodiment. Referring to FIG. 16, first, at step 300, the basic fuel injection amount TAU is calculated from the map shown in FIG. Then step 301
In NO _X release flag showing that from _the NO _X absorbent 23 should be released NO _X is whether it is set or not. NO _X releasing flag when not set detected current I ₁ of the NO _X ammonia sensor 29 proceeds to step 302 it is determined whether or not exceeds the set value I _S is. When I ₁ ≦ I _S , that is, the NO _X absorbent 23
Step 3 when there is still room in of the NO _X absorbing capacity
Jump to 05.

In step 305, the correction coefficient K is calculated from the map shown in FIG. Then step 30
In step 6, the basic fuel injection amount TAU is multiplied by the correction coefficient K to obtain the final fuel injection amount TAUO (= K · TA
U) is calculated, and fuel injection is performed with this injection amount TAUO. Next, at step 307 NO _X absorbent 2
3 whether or not to perform the SO _X release process for releasing SO _X is determined from the. When it is not necessary to perform the SO _X release processing, the processing cycle is completed.

On the other hand, in step 302, I ₁ > I _S
If it is determined that becomes, that is, the NO _X begins to flow out from _the NO _X absorbent 23 proceeds to step 103 N
O _X release flag is set, then NH ₃ detection flag is set the routine proceeds to step 304. Next, the routine proceeds to step 305. When the NO _X release flag is set, in the next processing cycle, steps 301 to 308 are performed.
Then, the rich correction coefficient K _R (> 1.0) is calculated. Then the final fuel injection amount TAUO (= K _R · TAU) is calculated by multiplying the rich correction factor K _R to step 309 in the basic fuel injection quantity TAU, the fuel injection is performed by the the injection amount TAUO. At this time is switched from the homogeneous mixture combustion under stratified combustion or a lean air-fuel ratio at a lean air-fuel ratio uniform mixture combustion under a rich air-fuel ratio, then the _the NO _X absorbent 23 the action of releasing of the NO _X is started.

Next, at step 310, the air-fuel ratio sensor 3
0 of the output voltage E is whether exceeds the reference voltage E _S is determined. Proceeds to step 307 when the E ≦ E _S. On the other hand, if E> E _S , the process proceeds to step 111 and NO
_{The X} release flag is reset. When the NO _X release flag is reset, the air-fuel ratio is switched from rich to lean.

On the other hand, if it is determined in step 307 that the SO _X release process should be performed, the process proceeds to step 112, where the process of releasing SO _X from the NO _X absorbent 23 is performed. That is, the temperature of the NO _X absorbent 23 almost 60
The air-fuel ratio is made rich while maintaining the temperature at 0 ° C. or higher. In the second embodiment, the target level E _S is set to the value shown in FIG.
Is calculated by the routine shown in FIG.

[0083] Figure 17 is decreased maximum NO _X absorption of the NO _X absorbent, furthermore a routine for determining that _the NO _X absorbent is deteriorated. In FIG. 17, first, at step 400, it is determined whether or not the detection current I ₁ of the NO _X ammonia sensor 29 is larger than the set value I _S (I ₁ > I _S ). When it is determined in step 400 that I ₁ ≦ I _S , the routine ends, but I ₁ > I _S
Is smaller than the total NO _X absorption .SIGMA.NOX allowed values predetermined value γ value smaller than NOXmax in step 401 when it is judged that the (.SIGMA.NOX <N
OXmax−γ).

In step 401, ΣNOX ≧ ΣNO
The routine ends when it is judged that xmax-gamma but determines the maximum NO _X absorption of .SIGMA.NOX <When it is determined that the ΣNOXmax -γ NO _X absorbent 23 is decreased, the total NO in step 402 _X absorption amountΣNOX
Is smaller than the allowable minimum value ΣNOXmin (ΣNOX <Σ
NOXmin) is determined. When it is judged that ΣNOX ≧ ΣNOXmin in step 402 to end the routine is a diagnosis of _the NO _X absorbent 23 is deteriorated in Step 403 when it is judged that ΣNOX <ΣNOXmin, so indicate.

[0085]

Separate the two means can estimate the amount of the NO _X that is absorbed in the NO _X absorbent according to the present invention, i.e. NO _X absorbed amount estimating means and the sensor and one of the using the air-fuel ratio of the exhaust gas flowing into the NO _X absorption amount estimated by means in the NO _X absorbent from lean to determine when to switch to a rich air-fuel ratio of the exhaust gas at were determined thus timing Estimated by the other means when switching from lean to rich
_The maximum NO _X absorption amount is determined from the _X absorption amount. Thus the maximum NO _X absorption of the state of the NO _X absorbent by associating separate two means can estimate the NO _X absorption amount can be accurately grasped.

[Brief description of the drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a diagram showing a structure of a sensor section of the NO _X ammonia sensor.

FIG. 3 is a diagram showing a current detected by a NO _X ammonia sensor.

FIG. 4 is a diagram showing an output voltage of an air-fuel ratio sensor.

FIG. 5 is a diagram showing a basic fuel injection amount, a correction coefficient, and the like.

FIG. 6 is a diagram for explaining the NO _X absorbing / releasing action of the NO _X absorbent.

FIG. 7 is a time chart showing an output voltage of an air-fuel ratio sensor, a detection current of a NO _X ammonia sensor, and the like.

FIG. 8 is a view showing a map of an NO _X absorption amount.

FIG. 9 is a diagram showing allowable values.

FIG. 10 is a flowchart for controlling the operation of the engine.

FIG. 11 is a flowchart for controlling operation of the engine.

FIG. 12 is a flowchart for correcting NOXmax.

13 is a flow chart for calculating a target level E _S.

14 is a flow chart for calculating a target level E _S.

FIG. 15 is a time chart showing an output voltage of the air-fuel ratio sensor, a detection current of the NO _X ammonia sensor, and the like.

FIG. 16 is a flowchart for controlling operation of the engine.

FIG. 17 is a flowchart for diagnosing deterioration of the NO _X absorbent.

[Explanation of symbols]

11 ... Fuel injection valve 23 ... NO _X absorbent 29 ... NO _X ammonia sensor 30 ... air-fuel ratio sensor

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) F01N 3/28 301 F02D 41/04 305A F02D 41/04 305 41/14 310E 41/14 310 43/00 301E 43/00 301 301T 45/00 314Z 45/00 314 B01D 53/36 101A 101B F-term (reference) 3G084 AA04 BA09 BA13 BA20 BA24 DA10 DA12 DA19 DA22 DA27 EB12 FA07 FA10 FA26 FA28 FA29 FA38 3G091 AA12 AA17 AA24 AB17 BA17 BA17 CB02 DA02 DB11 DC01 EA01 EA07 EA33 EA34 FB10 FC01 GA06 GB02W GB03W GB04W GB05W GB06W HA10 HA14 HA36 HA37 HB05 3G301 HA01 HA04 HA13 HA16 JA15 JA25 JA33 LB04 LC01 MA11 NA08 ND01 NE01 NE13 PA01Z PA11Z PA17Z01 PD01Z03A01 BA33Y BC01 BC05 CD06 DA01 DA02 DA08 DA20 EA04

Claims (4)

[Claims] 1. A absorbs NO _X when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the reducing agent an air-fuel ratio of the inflowing exhaust gas contained in the exhaust gas absorbed NO _X and becomes rich the _the NO _X absorbent to reduce disposed engine exhaust passage, and is absorbed from the amount of the NO _X in the exhaust gas flowing into the _the NO _X absorbent to the _the NO _X absorbent N
Comprising a NO _X absorption amount estimation means for estimating the amount of O _X, NO
A sensor capable of detecting concentration of NO _X in the exhaust gas flowing out from the _X absorbent disposed on _the NO _X absorbent downstream of the engine exhaust passage, NO _X is estimated by the NO _X absorption amount estimation means
In the exhaust purification apparatus for an internal combustion engine was switched to the rich air-fuel ratio of the exhaust gas flowing into the NO _X absorbent from lean when the absorption amount exceeds the judgment value, empty the exhaust gas flowing into the NO _X absorbent ratio of as NO _X concentration detected by the sensor when switched from lean to rich when it is determined that when higher than predetermined concentration has decreased a maximum NO _X absorption of _the NO _X absorbent can absorb Exhaust purification device for internal combustion engine. 2. The method according to claim 1, wherein the determination value is reduced when it is determined that the maximum NO _X absorption amount has decreased.
An exhaust gas purifying apparatus for an internal combustion engine according to claim 1. Wherein absorbing the NO _X when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the reducing agent an air-fuel ratio of the inflowing exhaust gas contained in the exhaust gas absorbed NO _X and becomes rich the _the NO _X absorbent to reduce disposed engine exhaust passage, and is absorbed from the amount of the NO _X in the exhaust gas flowing into the _the NO _X absorbent to the _the NO _X absorbent N
Comprising a NO _X absorption amount estimation means for estimating the amount of O _X, NO
_A sensor capable of detecting the NO _X concentration in the exhaust gas flowing out of the _X absorbent is disposed in the engine exhaust passage downstream of the NO _X absorbent, and when the NO _X concentration detected by the sensor exceeds a determination value. in the exhaust purification system of an internal combustion engine in which the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent from lean to switch to rich, when switched to a rich air-fuel ratio of the exhaust gas flowing into the NO _X absorbent from lean NO above
Exhaust purification for an internal combustion engine which is adapted when NO _X absorption amount estimated by _X absorption amount estimation means is less than a predetermined amount is determined to have become less maximum NO _X absorption of _the NO _X absorbent can absorb apparatus. 4. Maximum NO_X Judge that the absorption amount has decreased
NO when done _X Diagnose that the absorbent has deteriorated
An exhaust purification system for an internal combustion engine according to claim 1 or 3, wherein
Place.