JP2002180865A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly release NOX from a NOX absorbent. SOLUTION: The NOX absorbent for absorbing NOX in the case of a lean air/fuel ratio is arranged in an engine exhaust passage and a NOX ammonia sensor 29 is arranged it the engine exhaust passage in the downstream of the NOX absorbent 23. An extra amount of reducing agent for the release of NOX is found from a change in concentration of ammonia detected by the NOX ammonia sensor 29 when a rich air/fuel ratio is established for the release of NOX from the NOX absorbent 23.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

Absorbs NO _X when the air-fuel ratio of the Related Art inflowing exhaust gas is lean, the air-fuel ratio of the inflowing exhaust gas absorbed and becomes rich NO _X released by a reducing agent contained in the exhaust gas the _the NO _X absorbent to reduce placed engine exhaust passage, when the burning fuel under a lean air-fuel ratio is made NO _X in the exhaust gas is absorbed in the NO _X absorbent, NO from _the NO _X absorbent to when releasing the _X it is known internal combustion engines, which the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent rich.

In such an internal combustion engine, when the release of NO _X from the NO _X absorbent is completed, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from rich to lean so that the NO _X absorbent is downstream. the NO _X sensor capable of detecting the concentration of NO _X in the exhaust gas is arranged in the engine exhaust passage, N
The air-fuel ratio of the exhaust gas O _X NO _X concentration detected by the sensor flows into _the NO _X absorbent is regarded as release of the NO _X from _the NO _X absorbent is completed when it becomes constant concentration less from rich 2. Description of the Related Art An internal combustion engine that switches to a lean mode is known (see JP-A-2000-104533).

[0004]

THE INVENTION Problems to be Solved] However NO _X while absorbent from NO _X is released released NO _X
NO from _the NO _X absorbent because it is reduced by the reducing agent
_X is not released, thus while the NO _X from _the NO _X absorbent is released NO _X concentration detected by the NO _X sensor is maintained almost zero. Therefore, it is not possible to determine whether the release of NO _X from the NO _X absorbent has been completed using the NO _X sensor.

[0005] However, in the course of the present inventors, have carried out experiments and studies on _the NO _X absorbent, NO _X the air-fuel ratio of the exhaust gas flowing into the absorbent is absorbed in the NO _X absorbent when the rich NO large amount of reducing agent than the amount required for reducing _X is NO when supplied to the _X absorbent from _the NO _X absorbent in excess that were not used to release and reduce NO _X reducing agent is ammonia it is of has been found to be discharged from _the NO _X absorbent in the form.

Therefore, if the amount of ammonia discharged from the NO _X absorbent is known, the amount of the excess reducing agent is known, and if the amount of the excess reducing agent is known, the NO absorbed by the NO _X absorbent is known.
_The amount of reducing agent required to reduce _X will be known.
Air-fuel ratio of the exhaust gas flowing in this way the NO _X absorbent as may provide the reducing agent amount Knowing the amount of reducing agent necessary for reducing the NO _X that is absorbed in the NO _X absorbent If the rich degree and rich time are set, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent can be switched from rich to lean when the release of NO _X from the NO _X absorbent is completed. Also, notice that _the amount of NO _X when the reducing agent amount is understood _the NO _X absorbent can absorb required for reducing the NO _X, of the NO _X absorbent Knowing the amount of NO _X can absorb _the NO _X absorbent The degree of deterioration can be understood.

When the amount of the excess reducing agent is known, NO
Understand the state of the _X absorbent, also can be appropriately NO _X release control from _the NO _X absorbent.

[0008]

In SUMMARY OF for the] Thus the first aspect, when the air-fuel ratio of the inflowing exhaust gas is lean absorbs NO _X, the air-fuel ratio of the inflowing exhaust gas absorbed and becomes rich NO _X contained in the exhaust gas discharged by the reducing agent is arranged in the reduction to _the NO _X absorbent to the engine exhaust passage, NO _X is NO _X absorbed in the exhaust gas when the combustion under a lean air-fuel ratio has been made Absorbed by the agent, N
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 to be rich when the O _X absorbent should be released NO _X, empty the exhaust gas flowing into the NO _X absorbent NO _X when the fuel ratio is made rich
Sensors excess of the reducing agent from the absorbent to release NO _X was not used to reduce is discharged from the _the NO _X absorbent in the form of ammonia, it can detect ammonia concentration to _the NO _X absorbent in the exhaust passage downstream of And a representative value representing the surplus reducing agent amount is obtained from the change in the ammonia concentration detected by this sensor.

In the second invention, in the first invention,
The integrated value of the ammonia concentration detected by the sensor is used as a representative value. In the third invention, in the first invention,
The maximum value of the ammonia concentration detected by the sensor is set as a representative value. In the fourth invention, in the first invention,
And to reduce the total amount of reducing agent supplied to the NO _X absorbent when the air-fuel ratio of the exhaust gas flowing into the more _the NO _X absorbent representative value increases is rich.

In the fifth invention, in the fourth invention,
And the air-fuel ratio of the exhaust gas flowing into the more _the NO _X absorbent representative value is increased so that short time to rich. In a sixth aspect based on the first aspect, the reference value is set in advance for the representative value, and when the representative value becomes larger than the reference value, the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent becomes rich. when in reducing the total amount of reducing agent supplied to the NO _X absorbent when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent when the representative value is smaller than the reference value is set to the rich and to increase the total amount of reducing agent supplied to the NO _X absorbent.

In the seventh invention, in the sixth invention,
When the representative value becomes larger than the reference value, the time for enriching the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is shortened, and when the representative value becomes smaller than the reference value, NO
_The time for making the air-fuel ratio of the exhaust gas flowing into the _X absorbent rich is lengthened. In 8th aspect of the present invention is the fourth or sixth invention, the sensor is capable of detecting of the NO _X concentration in the exhaust gas in addition to the ammonia concentration in the exhaust gas, it is carried out the combustion under a lean air-fuel ratio air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is changed from lean to rich when the detected NO _X concentration exceeds a predetermined set value by the sensor when being.

In a ninth aspect, in the first aspect,
NO_X NO absorbed by absorbent _X To estimate the amount
NO_X Equipped with absorption amount estimation means, NO_X Estimated amount of absorption
NO estimated by the stage_X NO based on quantity_X Absorbent
To temporarily enrich the air-fuel ratio of exhaust gas flowing into the
The switch time interval is controlled. 10th departure
According to the ninth invention, the NO._X For absorption amount estimation means
NO estimated from_X When the absorption exceeds the allowable value, N
O_X The air-fuel ratio of the exhaust gas flowing into the absorbent changes from lean to
Switch to the switch temporarily.

In the eleventh invention, the tenth invention
And NO_X Absorbent NO_X N for estimating absorption capacity
O_X Equipped with absorption capacity estimation means, NO_X Hand for estimating absorption capacity
NO estimated by the stage_X Acceptable as absorption capacity decreases
The value is reduced. In the twelfth invention, the tenth
In the invention, the sensor measures the ammonia concentration in the exhaust gas.
In addition, NO in exhaust gas_X Concentration can be detected.
NO when combustion is being performed under the air-fuel ratio_X Sucking
NO estimated by yield estimation means_X Absorption amount is acceptable
Detected by the sensor despite not exceeding
NO _X When the concentration exceeds a preset value
Is NO_X Is the air-fuel ratio of the exhaust gas flowing into the absorbent lean?
Is switched to rich.

In the thirteenth invention, the tenth invention
The sensor adds to the concentration of ammonia in the exhaust gas
NO in gas_X Concentration can be detected and lean air-fuel ratio
NO when combustion is taking place under_X Estimated amount of absorption
NO estimated by the stage_X Absorption amount does not exceed allowable value
NO detected by the sensor _X 
When the concentration exceeds the preset value,
It is lowered.

[0015] In the first invention in the 14th invention, it is to detect the degree of deterioration of _the NO _X absorbent based on the representative value. According to a fifteenth aspect, in the fourteenth aspect, as the amount obtained by subtracting the surplus reducing agent amount from the total amount of the reducing agent supplied to the NO _X absorbent decreases, the NO _X
It is determined that the degree of deterioration of the absorbent increases.

[0016] In 14 th invention the 16th invention, the degree of rich is smaller as the degree of deterioration of _the NO _X absorbent increases when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent to a rich .

[0017]

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. FIG.
, 1 is an engine body, 2 is a cylinder block,
3 is a piston reciprocating in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, 5
Denotes a combustion chamber formed between the piston 3 and the cylinder head 4, 6 denotes an intake valve, 7 denotes an intake port, 8 denotes an exhaust valve, and 9 denotes 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 fuel injection valve 11 is provided on the top surface of the piston 3.
Cavity 12 extending from below to below ignition plug 10
Are formed.

The intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch 13, and the surge tank 14 is connected to an intake duct 15 and an air flow meter 16.
Through an air cleaner (shown in SEZ). A throttle valve 18 driven by a step motor 17 is arranged in the intake duct 15. Casing On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19, the exhaust manifold 19 where the _the NO _X absorbent 23 through the catalytic converter 21 and the exhaust pipe 22 an oxidation catalyst or three-way catalyst 20 has been built and visceral 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 30 is composed of a digital computer, and a RAM (random access memory) 32 and a ROM interconnected via a bidirectional bus 31.
(Read only memory) 33, CPU (microprocessor) 34, input port 35 and output port 36. The air flow meter 16 generates an output voltage proportional to the amount of intake air, and the output voltage
7 to the input port 35. 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 35 via a corresponding AD converter 37. Further, a casing 24 in which a built-in _the NO _X absorbent 23
In the exhaust pipe 25 connected to the outlet of the exhaust gas.
A NO _X ammonia sensor 29 capable of detecting both the _X concentration and the ammonia concentration is provided, and an output signal of the NO _X ammonia sensor 29 is input to an input port 35 via a corresponding AD converter 37.

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 35 via a corresponding AD converter 37. Is input to The crank angle sensor 42 generates an output pulse every time the crankshaft rotates 30 degrees, for example, and the output pulse is input to the input port 35. The CPU 34 calculates the engine speed from the output pulse of the crank angle sensor 42. On the other hand, the output port 36 is connected to the ignition plug 10, the fuel injection valve 11, the step motor 17, and the EGR control valve 27 via the corresponding drive circuit 38.

Next, referring to FIG. 2, the NO shown in FIG.
The structure of the sensor section of the _X ammonia sensor 29 will be briefly described. Referring to FIG. 2, NO _X sensor section of the ammonia sensor 29 comprises an oxygen ion conductive solid electrolyte layer such as a six zirconium oxide which are laminated together, these six solid electrolyte layers below, first from top to bottom Layer L
_1, the second layer L _2, the third layer L _3, the fourth layer L _4, the fifth layer L
_5, referred to as the sixth layer L _6.

Referring to FIG. 2, the first layer L ₁ and the third layer L
₃ are arranged, for example a first diffusion-controlling member 50 or the pores of the porous is formed and the second diffusion-controlling member 51 is between, between these diffusion-controlling member 50 and 51 second 1 room 52, the second diffusion-controlling member 51 is between the second layer L ₂ is formed with a second chamber 53. Further, the third layer L ₃ is between the fifth layer L ₅ atmospheric chamber 54 communicating with the outside air are formed. On the other hand, the outer end surface of the first diffusion control member 50 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, is on the inner surface of the first layer L ₁ that faces the first chamber 52 is formed with a cathode-side first pump electrode 55, is on the outer peripheral surface of the first layer L ₁ the anode side First pump electrode 5
6 are formed, and these first pump electrodes 55 and 56 are formed.
During this time, a voltage is applied by the first pump voltage source 57.
When a voltage is applied between the first pump electrodes 55 and 56, the first
Becomes oxygen ions oxygen contained in the exhaust gas in the chamber 52 is in contact with the cathode side first pump electrode 55, the oxygen ions flowing towards the first layer L ₁ to the anode side first pump electrode 56. Therefore, the 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 changed by the first pump voltage source 57. Increases as the voltage of the signal increases.

On the other hand, the 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, since the oxygen concentration in the atmosphere chamber 54 is higher than the oxygen concentration in the first chamber 52, the oxygen in the atmosphere chamber 54 receives a charge by contacting the reference electrode 58 and becomes oxygen ions. , This oxygen ion becomes the third layer L ₃ ,
It moves in the second layer L ₂ and the first layer L ₁ , and discharges electric charges at the first pump electrode 55 on the cathode side. As a result, a voltage V ₀ indicated by reference numeral 59 is generated between the reference electrode 58 and the cathode-side first pump electrode 55. 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. You. 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.

The first pump electrode 55 on the cathode side is NO _X
For materials with low reducibility, for example, gold Au and platinum Pt
Therefore, NO _X contained in the exhaust gas is hardly reduced in the first chamber 52.
Therefore, this NO _X flows into the second chamber 53 through the second diffusion control member 51. On the other hand, the first facing the second chamber 53
_On the inner peripheral surface of the layer L _1, a cathode-side second pump electrode 60 is formed, and a voltage is applied between the cathode-side second pump electrode 60 and the anode-side first pump electrode 556 by a second pump voltage source 61. Is applied. When a voltage is applied between the pump electrodes 60 and 56, oxygen contained in the exhaust gas in the second chamber 53 comes into contact with the cathode-side second pump electrode 60 to become oxygen ions, and the oxygen ions are formed in the first layer. the L ₁ flows toward the anode side first pump electrode 56. Therefore, oxygen contained in the exhaust gas in the second chamber 53 moves in the first layer L ₁ and is pumped out. At this time, the amount of oxygen pumped out is changed by the second pump voltage source 61. Increases as the voltage of the signal 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 flows in the solid electrolyte layer 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
8 and receives oxygen to form oxygen ions. The oxygen ions move in the third layer L ₃ , the second layer L ₂ and the first layer L ₁ , and charge the oxygen at the cathode-side second pump electrode 60. discharge. As a result, a voltage V ₁ indicated by reference numeral 62 is generated between the reference electrode 58 and the cathode-side second pump electrode 60. 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 fed back so that this voltage V ₁ matches the voltage generated when the oxygen concentration in the second chamber 53 is 0.01 ppm. Controlled. That is, 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 5
The oxygen concentration in 3 is maintained at 0.01 ppm.

The cathode-side second pump electrode 60 is also NO _X
For materials with low reducibility, for example, gold Au and platinum Pt
Is formed of an alloy of, thus NO _X contained in the exhaust gas is hardly reduced even in contact with the cathode side second pump electrode 60. On the other hand, the cathode side pump electrode for NO _X detection on the inner surface of the third layer L ₃ facing the second chamber 53 6
3 are formed. This cathode side pump electrode 63 is NO
A material having a strong reducing property to _X , for example, rhodium R
h and platinum Pt. Therefore, NO _x in the second chamber 53, which is actually the majority of NO, is decomposed into N ₂ and O ₂ on the cathode side pump electrode 63. As shown in FIG. 2, the cathode-side pump electrode 63 and the reference electrode 58
A constant voltage 64 is applied between them, so that O ₂ generated by decomposition on the cathode-side pump electrode 63 becomes oxygen ions and moves in the third layer L ₃ toward the reference electrode 58. At this time, between the cathode side pump electrode 63 and the reference electrode 58, a current I ₁ indicated by reference numeral 65 in proportion to the oxygen ion amount.
Flows.

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

FIG. 3 shows the relationship between the current I ₁ and the concentrations of NO _X and NH ₃ in the exhaust gas. FIG. 3 shows that the current I ₁ is proportional to the NO _x concentration and the NH ₃ concentration in the exhaust gas. On the other hand, the higher the oxygen concentration in the exhaust gas, that is, the leaner the air-fuel ratio, the more the first chamber 52
The amount of oxygen pumped from the outside to the outside increases, and the current I ₂ indicated by reference numeral 66 increases. Therefore it is possible to detect the air-fuel ratio of the exhaust gas from the current I _2.

[0032] Incidentally, the fifth layer L ₅ electric heater 67 is disposed for heating the sensor portion of the NO _X ammonia sensor 29 is provided between the sixth layer L _6, the electric motor 6
Sensor portion of the NO _X ammonia sensor 29 by 7 7
Heated from 00 ° C to 800 ° C. Next, the fuel injection control of the internal combustion engine shown in FIG. 1 will be described with reference to FIG. In FIG. 4A, the vertical axis represents the engine load Q /
N (intake air amount Q / engine speed N), and the horizontal axis represents engine speed N.

FIG. 4 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. 4 (A) the fuel from the fuel injection valve 11 is injected during the intake stroke, uniform mixture combustion is performed at this time. In addition, between the solid line X ₁ and a chain line X ₂ is performed homogeneous mixture combustion under a lean air-fuel ratio, homogeneous mixture combustion line under a stoichiometric air-fuel ratio between the chain line X ₂ and the chain line X ₃ We, 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 shown 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 final fuel injection amount TAUO (= K · TAU) is stored in advance in the ROM 33 and basically multiplied by the correction coefficient K to the basic fuel injection amount TAU.
Is calculated. This correction coefficient K is stored in the ROM 33 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. 4A 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. 4 (a) the original combustion is performed. Further, 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 based on the output signal of the air-fuel ratio sensor 28 so that the theoretical air-fuel ratio feedback Controlled.

[0037] _the NO _X absorbent 23 disposed in the engine exhaust passage, for example alumina as a carrier, with, for example, on the carrier 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.

In any case, the engine intake passage,
NO _X in Toko to refer to the ratio of the combustion chamber 5 and _the NO _X absorbent is supplied to the 23 exhaust passage upstream of the air and fuel (hydrocarbons) and the air-fuel ratio of the exhaust gas flowing into _the NO _X absorbent 23
The absorbent 23 absorbs the NO _X when the air-fuel ratio of the inflowing exhaust gas is lean, the air-fuel ratio of the inflowing exhaust gas is performed to absorbing and releasing action of the NO _X that releases NO _X absorbed and becomes the stoichiometric air-fuel ratio or rich.

If this NO _X absorbent 23 is disposed in the exhaust passage of the engine, 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 a 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 other noble metals, alkali metals, alkaline earths,
The same mechanism is obtained even when rare earth elements are used.

In the internal combustion engine shown in FIG.
When the air-fuel ratio is lean in most operating conditions,
Baking is performed. Combustion with lean air-fuel ratio
When the oxygen concentration in the exhaust gas is high,
At this time, as shown in FIG.
_Two Is 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 ^-or
Is O^2-Reacts with NO _Two (2NO + O_Two → 2NO
_Two ). NO generated next_Two Part of the acid on platinum Pt
Is absorbed into the absorbent while being converted into barium oxide BaO.
While binding, 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 the platinum Pt decreases. When the production amount of NO ₂ decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus nitrate ions NO ₃ ⁻ in the absorbent are released from the absorbent in the form of NO ₂ . At this time, _the NO _X absorbent 23
NO _X released from the large amount of unburned HC contained in the inflowing exhaust gas as shown in FIG. 5 (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 air-fuel ratio from _the NO _X absorbent 23 in a short time when it is in the rich inflowing exhaust gas
_X is released, and since the released NO _X is reduced, NO _X is not discharged into the atmosphere.

In this case, the air-fuel ratio of the inflowing exhaust gas is
NO even at stoichiometric air-fuel ratio_X NO from absorbent 23_X Is released
Is done. However, the air-fuel ratio of the inflowing exhaust gas is
NO if ratio_X NO from absorbent 23_X Gradually
NO because it is only released _X Absorbed by the absorbent 23
All NO_X It takes a slightly longer time to release.

By the way is in the NO _X absorbing capacity of the NO _X absorbent 23 is limited, thus NO _X absorbing capacity of the NO _X absorbent 23 needs to release NO _X from _the NO _X absorbent 23 before saturation . However, _the NO _X absorbent 23 is NO
Among _X absorbing capacity is sufficient to absorb almost all of the NO _X contained in the exhaust gas is no longer E to absorb near Nuisance and part of the NO _X in the limit of the NO _X absorbing capacity, NO and thus
_{When the X} absorbent 23 approaches the limit of NO _X absorption capacity, NO _X
The amount of NO _X discharged from the absorbent 23 starts to increase.

[0044] Therefore NO in some embodiments including the first embodiment according to the present invention that is discharged from _the NO _X absorbent 23
Are after temporarily rich air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 _the NO _X absorbent 23 so as to release the NO _{X when X} amount began to increase. 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, combustion chamber 5
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 air rich, or the air-fuel ratio of the exhaust gas can be made rich by injecting additional fuel at the end of the expansion stroke or during the exhaust stroke. it may also be rich, or can be an air-fuel ratio of the exhaust gas rich by injecting additional fuel to _the NO _X absorbent 23 in the exhaust passage upstream of. In the embodiment according to the present invention, the air-fuel ratio of the exhaust gas is made rich by performing the first method, that is, by performing the homogeneous mixture combustion under the rich air-fuel ratio.

The exhaust gas contains SO._X Contains
Yes, NO_X NO in the absorbent 23_XNot only SO_X 
Is also absorbed. This NO_X SO to absorbent 23_X Absorption of
The mechanism is NO_X Is considered to be the same as the absorption mechanism of
available. That is, NO_X Explained the absorption mechanism of
Carry platinum Pt and barium Ba on the carrier as well
In the case where the flow is made as an example, the flow is
When the air-fuel ratio of the incoming and exhaust gas is lean, oxygen O_Two Is O_Two ^-
Or O^2-Is attached to the surface of platinum Pt in the form of
SO in gas and gas_Two Is O on the surface of platinum Pt_Two ^-Or O ^2-When
React and SO_Three Becomes Then the generated SO_Three Part of
Is further oxidized on platinum Pt and absorbed in the absorbent.
Sulfate ions SO while binding with barium oxide BaO_Four
^2- And diffused into the absorbent in the form of a stable sulfate BaSO_Four
 Generate

However, this sulfate BaSO_Four Is stable
And the air-fuel ratio of the incoming exhaust gas is simply reset.
Sulphate BaSO_Four Is not disassembled
Will remain. Therefore NO_X Time passes in the absorbent 23
The sulfate BaSO _Four Will increase,
Thus, as time passes, NO_X Absorbent 23 absorbs
NO that can be stored_X The amount will be reduced. That is, time
NO as time goes by _X The absorbent 23 will deteriorate.
You.

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 according to the present invention, NO_X Absorbent
23 to SO_X To release NO_X Absorbent 23
NO if temperature is high_X Exhaust gas flowing into the absorbent 23
Richer air-fuel ratio and NO_X Absorbent 23 to SO_X 
To release SO_X To release NO_X Absorbent 2
NO if temperature of 3 is low_X Raise the temperature of the absorbent 23
NO_X Empty exhaust gas flowing into the absorbent 23
The fuel ratio is made rich.

Next, NO_X NO from absorbent 23_X Release
NO to do_X The air-fuel ratio of the exhaust gas flowing into the absorbent 23 is
Amount of reducing agent and NO when enriched_X From absorbent 23
Ammonia NH in discharged exhaust gas_Three Relationship with the concentration of
The staff will be described. First, let's talk about the amount of reducing agent.
NO_X Air-fuel of exhaust gas flowing into absorbent 23
Ratio is too large for the amount of fuel required to achieve the stoichiometric air-fuel ratio.
NO fuel_XUsed for release and reduction of
The amount of this excess fuel is NO_X Used for release and reduction of
According to the amount of reducing agent used. This means NO_X Absorbent
NO from 23_X Should be released in the combustion chamber 5
Even when the air-fuel ratio of the air-fuel mixture is made rich, even at the end of the expansion stroke
Alternatively, even if additional fuel is injected during the exhaust stroke, NO
_X Additional fuel was injected into the exhaust passage upstream of the absorbent 23
Even so.

Incidentally, as in the embodiment of the present invention, N
When NO _X is to be released from the O _X absorbent 23, the combustion chamber 5
When the air-fuel ratio is made rich within the range, assuming that the value of the correction coefficient K for the basic fuel injection amount TAU is K _R , the amount ΔQR of the reducing agent supplied to the NO _X absorbent 23 per fuel injection is It is expressed by the following equation. ΔQR = TAU · (K _R −1.0) Here, TAU is the basic fuel injection amount shown in FIG. 4B, and the correction coefficient K _R is the degree of richness when the air-fuel ratio is set to the rich air-fuel ratio ( (Stoichiometric air-fuel ratio / rich air-fuel ratio). The total amount QR of the fuel injection when the integrated amount ΔQR once per reductant supplied to _the NO _X absorbent 23 reducing agent
Becomes

Next, the concentration of ammonia will be described. When the air-fuel ratio is lean, that is, in an oxidizing atmosphere, almost no ammonia NH ₃ is generated. However, when the air-fuel ratio becomes rich, that is, when the atmosphere becomes a reducing atmosphere, the nitrogen N ₂ in the intake air or the exhaust gas is reduced by the oxidation catalyst or the three-way catalyst 2.
0, reduced by hydrocarbons HC and ammonia N
H ₃ is produced. However NO _X from _the NO _X absorbent 23 when the air-fuel ratio becomes rich is released, the ammonia NH ₃ generated is NO _X is released from _the NO _X absorbent 23 since it is used to reduce the NO _X while in, the ammonia NH ₃ from _the nO _X absorbent 23 is not discharged while the more precisely the supplied reducing agent is used for the release and reduction of nO _X. On the other hand, if the air-fuel ratio is made rich even after the release of NO _X from the NO _X absorbent 23, more precisely,
_If an excess reducing agent is supplied which is not used for releasing and reducing NO _X from the _X absorbent 23, ammonia NH ₃ is no longer consumed for reducing NO _X , and thus at this time, NO 3 Ammonia NH ₃ is discharged from the _X absorbent 23.

[0051] 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, platinum P of _the NO _X absorbent 23 also has a reduced function
t, etc., so that when the air-fuel ratio becomes rich, N
In O _X absorbent 23 could ammonia NH ₃ is produced. However if ammonia NH ₃ as ammonia NH ₃ is generated ammonia NH ₃ is not discharged from _the NO _X absorbent 23 to be used to reduce the NO _X released from _the NO _X absorbent 23 . However, when an excess reducing agent not used for releasing and reducing NO _X from the NO _X absorbent 23 is supplied, ammonia NH ₃ is discharged from the NO _X absorbent 23 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
When an excess reducing agent not used for releasing and reducing NO _X is supplied from the NOx ₃ , the excess reducing agent is discharged from the NO _X absorbent 23 in the form of ammonia NH ₃ , and the amount of ammonia discharged at this time Is proportional to the amount of excess reducing agent. Therefore, the surplus amount of the reducing agent can be determined from the amount of ammonia discharged at this time.

Therefore, in the present invention, a NO _X ammonia sensor 29 capable of detecting the ammonia concentration is disposed in the exhaust passage downstream of the NO _X absorbent 23, and an excess of the ammonia concentration detected by the NO _X ammonia sensor 29 is used. The amount of reducing agent is determined. 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 representing 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. As described above, in the present invention, the surplus reducing agent amount is obtained from the change in the ammonia concentration. More specifically, the representative value representing the surplus reducing agent amount is obtained from the change in the ammonia concentration. ing. This is the basis of the present invention.

When the representative value is obtained, various controls can be performed. Therefore, first, the most basic control of the supply of the reducing agent will be described with reference to FIG. Referring to FIG. 6, .SIGMA.NOX shows the amount of NO _X is absorbed in the NO _X absorbent 23, I ₁ represents the detection current of the NO _X ammonia sensor 29. In FIG. 6, NO
_X and NH ₃ are the NO _X concentration in the exhaust gas and NH ₃
Shows each change of the detected current of the NO _X ammonia sensor 29 due to changes in concentration, both of these detected current is N
Appearing on the detection current I ₁ of the O _X ammonia sensor 29.
A / F indicates the average air-fuel ratio of the air-fuel mixture in the combustion chamber 5, and QR indicates the total amount of the supplied reducing agent.

As shown in FIG._X Absorbent 23
NO absorbed_X The amount ΣNOX increases and NO_X absorption
NO when approaching the absorption capacity limit of agent 23_X From absorbent 23
Is NO_X Begins to emit NO_X Ammonia sensor
29 detected current I₁ Begins to rise. In the example shown in FIG.
Is NO_X NO from absorbent 23_X Begins to emit NO _X 
When the concentration exceeds a predetermined set value, that is, NO_X 
Detection current I of ammonia sensor 29₁ Is predetermined
NO when exceeding set value Is_X NO from absorbent 23_X 
Air-fuel ratio A / F switches from lean to rich to release fuel
Can be Air-fuel ratio A / F is switched from lean to rich
Even if the exhaust gas with rich air-fuel ratio is NO_X Reached absorbent 23
It takes time, so the air-fuel ratio A / F is switched to rich
NO immediately after being obtained_X NO discharged from absorbent 23_X 
Volume continues to increase. Then in the rich air-fuel ratio exhaust gas
NO by the contained reducing agent_X The reduction action of
NO_X NO from the absorbent 23_X Will not be discharged
You. Therefore, when the air-fuel ratio is switched from lean to rich
NO_X Detection current I of ammonia concentration sensor 29₁ Is short
And then drop to zero.

Meanwhile, the total amount Q of the reducing agent an air-fuel ratio is supplied to _the NO _X absorbent 23 is switched from lean to rich
R gradually increases, and accordingly, the NO _x amount ΣNO _x absorbed by the NO _x absorbent 23 gradually decreases. Meanwhile has to switch to a lean air-fuel ratio from the rich when the total amount QR of the reducing agent has reached the target value QR _S in the example shown in FIG. 6, the NO _X absorbent 23 in the case shown in FIG. 6 After the absorbed NO _X amount becomes zero, the air-fuel ratio is switched from rich to lean.

[0057] from _the NO _X absorbent 23 is in this case NO _X
Since the excess reducing agent not used for releasing and reducing the oxygen is supplied, and the ammonia NH ₃ is discharged from the NO _x absorbent 23 at this time, the detection by the NO _x ammonia sensor 29 is performed as shown in FIG. current I ₁ is increased. In this case, the integrated value ΔI of the detection current I _{1 and} the maximum value I of the detection current I ₁ indicated by hatching in FIG.
max represents the amount of excess reducing agent. Therefore, in this embodiment used to lower the amount of reducing agent to be supplied to the next of the NO _X emission only excess of the reducing agent calculated based on these integrated value ΣI or the maximum value Imax. Hence it is at the next of the NO _X release amount of reducing agent required to reduce releases NO _X that is absorbed in the NO _X absorbent 23 is supplied.

Meanwhile, again when the air-fuel ratio for NO _X absorbing capacity of the SO _X which is absorbed in the NO _X absorbent 23 comes increased _the NO _X absorbent 23 is lowered is switched from lean to rich ammonia is discharged from _the NO _X absorbent 23. Integrated value of the amount of reducing agent to be supplied when again the next of the NO _X emission in this case is detected current I ₁ .SIGMA.I or maximum value I
The surplus reducing agent calculated based on max is reduced. _The NO _X absorbent 23 in this way is made in this embodiment
The air-fuel ratio is switched from rich to lean when the release of NO _X from the NOx is completed, and the supply of the reducing agent to the NO _X absorbent 23 can be stopped.

Incidentally, the target value QR of the reducing agent to be supplied_S 
Is NO_X NO that can be absorbed by the absorbent 23_X Represents the quantity
You. Therefore, in this embodiment, the target value QR_S Is predetermined
NO when the set value is smaller than the set value SS_X Absorbent 2
3 to SO_X Is to be released. Also, over time
NO due to change_X Target value even when absorbent 23 has deteriorated
QR_S Decreases, and therefore this target value QR_S From NO_X Sucking
The degree of deterioration of the absorbent 23 is understood. Note that NO_X Absorbent 2
NO when 3 is not deteriorated_X Is in the form of nitrate ions
NO _X NO because it diffuses deep into the absorbent 23_X absorption
Nitrate is formed deep inside the agent 23. In this case, NO
_X NO from absorbent 23_X Release the air-fuel ratio.
H, that is, the correction coefficient K_R It is preferable to increase the value of
Good. In contrast, NO_X Absorbent 23 deteriorates
And NO_X Is NO in the form of nitrate ions_X Deep inside the absorbent 23
NO at this time_X From absorbent 23
NO_X In order to release the air-fuel ratio,
Positive coefficient K_R Need not be so large. Therefore
In the embodiment according to the present invention, the supplement for making the air-fuel ratio rich is
Positive coefficient K _R Is the target value QR as shown in FIG._S But
The larger, the higher.

FIG. 8 shows the first embodiment described with reference to FIG.
2 shows a routine for execution. Referring to FIG.
First, in step 100, as shown in FIG.
The basic fuel injection amount TAU is calculated from the map. Next
NO in step 101_X NO from absorbent 23_X Release
NO indicating what should be issued_X The release flag is set
Is determined. NO_X Release flag is set
If not, proceed to step 102 and NO_X Ammo
Detection current I of the near sensor 29₁ Is the set value I_S Has exceeded
It is determined whether it is not. I₁ ≤I_S , That is, NO_X 
NO of absorbent 23 _X When there is still room for absorption capacity
Jump to step 105.

In step 105, the correction coefficient K is calculated from the map shown in FIG. Then step 106
Now, by multiplying the basic fuel injection amount TAU by the correction coefficient K, the final fuel injection amount TAUO (= K · TAU)
Is calculated, and fuel injection is performed with the injection amount TAUO. Next, at step 107, the target value QR of the reducing agent
_It is determined whether _S has become smaller than the set value SS for SO _X release, and when QR _S ≧ SS, the processing cycle is completed.

On the other hand, in step 102, I₁ > I_S 
Is determined, that is, NO _X Absorbent 23 to N
O_X Begins to be discharged and proceeds to step 103, NO_X release
The flag is set and then the process proceeds to step 104 where N
H_Three The detection flag is set. Then step 105
Proceed to. NO_X When the release flag is set, the next processing
In cycle, go from step 101 to step 108
From the relationship shown in FIG._R Is calculated. Then
In step 109, the correction coefficient K is added to the basic fuel injection amount TAU.
_R Is multiplied by the final fuel injection amount TAUO
(= K_R ・ TAU) is calculated, and this injection amount TAUO
Thus, fuel injection is performed. At this time, the lean air-fuel ratio
Homogeneous mixture under stratified combustion or lean air-fuel ratio
Switching from combustion to homogeneous air-fuel mixture combustion under rich air-fuel ratio
Yes, so NO_X NO from absorbent 23_X of
The release action is started.

Next, at step 110, the amount ΔQR of the reducing agent supplied to the NO _X absorbent 23 per one fuel injection is calculated based on the following equation. AQr = TAU · total amount QR of the (K _R -1.0) then step 111 the reducing agent supplied to _the NO _X absorbent 23 by adding the reducing agent amount AQr the QR is calculated. Then the total amount QR of the reducing agent in step 112 is determined whether or not exceeds the target value QR _S is, jumps to step 107 when the QR ≦ QR _S. NO _X releasing flag proceeds to step 113 becomes a QR> QR _S contrast is reset, then the total amount QR of the reducing agent in step 114 is cleared. Next, the routine proceeds to step 107. If NO _X release flag is reset the air-fuel ratio is switched from rich to lean.

On the other hand, in step 107, QR _S <S
Process for releasing SO _X from _the NO _X absorbent 23 proceeds to step 115 when a determination is made that the S is performed. That is, the air-fuel ratio is made rich while maintaining the temperature of the NO _X absorbent 23 almost 600 ° C. or higher. When the release operation of SO _X from the NO _X absorbent 23 is completed, step 11 is performed.
Proceeding to 6, the predetermined maximum total reducing agent amount QRma
x is the target value QR _S.

FIG. 9 shows a routine for calculating the target value QR _S. Referring to FIG. 9, first, at step 200, it is determined whether or not the NH ₃ detection flag is set. This NH ₃ detection flag is set when I ₁ > I _S in step 102 of FIG. When the NH ₃ detection flag is set, the routine proceeds to step 201, where it is determined whether or not the operation region of the engine is a preset operation region. This set operation region is a narrow operation region determined by the engine load Q / N and the engine speed N. When the operation region of the engine is within the set operation region, the process proceeds to step 202.

In step 202, it is determined whether or not the elapsed time t since the setting of the NH ₃ detection flag has exceeded a predetermined time t ₁ . This fixed time t ₁ is a time from when the air-fuel ratio is made rich to lean, until the detection current I ₁ of the NO _X ammonia sensor 29 ends to drop to zero.
t> whether becomes to t ₁ NH ₃ detection flag goes to step 203 the elapsed time t after the set exceeds a predetermined time t ₂ is determined. This fixed time t ₂ is NO _X
NO _X ammonia sensor 29 whatever the amount of ammonia when ammonia is discharged from the absorbent 23
Is sufficient time to detect ammonia concentration.
When t ≦ t _2, the process proceeds to step 204.

[0067] detected current I ₁ of the step 204 in NO _X ammonia sensor 29 is calculated. Then step 2
05 In the integrated value ΣI of detected current by adding the detected current I ₁ in ΣI is calculated. Next, when it is determined at step 203 that t> t ₂ , the routine proceeds to step 206, where the multiplication result obtained by multiplying the integrated value ΣI of the detected current by the proportionality constant C ₁ is a surplus reducing agent amount QRR (=
It is a C ₁ · ΣI). Next, at step 207, the target value QR _S is updated by subtracting the surplus reducing agent amount QRR from the current target value QR _S.

Next, at step 208, ΔI is cleared, and at the same time, the NH ₃ detection flag is reset. Next, at step 209, it is determined whether or not the updated target value QR _S is equal to or less than a predetermined limit value QRmin.
When QR _S <QRmin, the routine proceeds to step 210, where N
Deterioration flag indicating that O _X absorbent 23 is deteriorated is set. When the deterioration flag is set, for example, a warning lamp is turned on.

[0069] Figure 10 shows another example of the routine for calculating the target value QR _S. Referring to FIG. 10, first, at step 300, it is determined whether or not the NH ₃ detection flag is set. This NH ₃ detection flag is set when I ₁ > I _S in step 102 of FIG. Whether the set operation region operating region of the engine was previously the routine proceeds to step 301, it is determined when the NH ₃ detection flag has been set.
The set operation range includes the engine load Q / N and the engine speed N.
It is a narrow operating area determined from the following. When the operation region of the engine is within the set operation region, the process proceeds to step 302.

In step 302, it is determined whether or not the elapsed time t from when the NH ₃ detection flag is set exceeds a predetermined time t ₁ . This fixed time t ₁ is a time from when the air-fuel ratio is made rich to lean as described above, until the detection current I ₁ of the NO _X ammonia sensor 29 ends to drop to zero. t> whether becomes to t ₁ NH ₃ detection flag goes to step 403 the elapsed time t after the set exceeds a predetermined time t ₂ is determined. This fixed time t ₂ is a time sufficient for the NO _x ammonia sensor 29 to detect the ammonia concentration regardless of the amount of ammonia when the ammonia is discharged from the NO _x absorbent 23 as described above. is there. When t ≦ t _2, the process proceeds to step 304.

[0071] detected current I ₁ of the step 304 NO _X ammonia sensor 29 is calculated. Then step 3
In 05 the detected current I ₁ whether large is determined than Imax. When I ₁ > Imax, step 306 is executed.
Willing I ₁ is the maximum value Imax of detected current. Next, when it is determined in step 303 that t> t ₂ , the routine proceeds to step 307, where the maximum value I of the detected current is calculated.
multiplication result obtained by multiplying the proportional constant C ₂ to max is the excess reducing agent amount _{QRR (= C 2 · Imax)} . Then excess reducing agent amount QR from step 308 current target value QR _S
The target value QR _S is updated by subtracting R.

Next, at step 309, Imax is cleared, and at the same time, the NH ₃ detection flag is reset. Next, at step 310, it is determined whether or not the updated target value QR _S is equal to or less than a predetermined limit value QRmin. If QR _S <QRmin, the routine proceeds to step 311, where a deterioration flag indicating that the NO _X absorbent 23 has deteriorated is set. When the deterioration flag is set, for example, a warning lamp is turned on.

Next, a second embodiment will be described with reference to FIG. In this embodiment, as shown in FIG. 11 (A), a reference value is set in advance for a representative value representing the surplus reducing agent amount. More specifically, in the first example, the reference value Sr is set in advance for the integrated value ΔI of the detection current of the NO _X ammonia sensor 29, and as shown in FIG. The integrated value 検 出 I of the detected current is the reference value Sr
Air-fuel ratio when it is larger than the total amount of reducing agent supplied to _the NO _X absorbent 23 when it is rich is made to decrease, the representative value as shown in FIG. 11 (C), i.e. the detection current When the integrated value ΔI becomes smaller than the reference value Sr, the total amount of the reducing agent supplied to the NO _X absorbent 23 when the air-fuel ratio is made rich is increased. That is, the supply amount of the reducing agent is controlled such that the integrated value ΔI of the detected current becomes the reference value Sr.

[0074] Moreover, the reference value Imax is set in advance for the maximum value Imax of detected current of the NO _X ammonia sensor 29 as in the second example shown in FIG. 11 (A),
As shown in FIG. 11B, when the representative value, that is, the maximum value Imax of the detected current is larger than the reference value Imaxr, the reducing agent supplied to the NO _X absorbent 23 when the air-fuel ratio is made rich. Is reduced, and FIG. 11 (C)
As shown in the figure, the representative value, that is, the maximum value Imax of the detection current
There total amount of reducing agent which air is supplied to _the NO _X absorbent 23 when it is rich is made to increase when it becomes smaller than the reference value Imaxr. That is, the supply amount of the reducing agent is controlled such that the maximum value Imax of the detected current becomes the reference value Imaxr.

Unlike the first embodiment, the second embodiment has an advantage that the supply amount of the reducing agent can be increased when the supply amount of the reducing agent is excessively reduced. Figure 12 shows a calculation routine of the target value QR _S for performing a first example of the second embodiment. The operation control routine shown in FIG. 8 is used as the operation control routine also in the second embodiment.

Referring to FIG. 12, first, at step 400, it is determined whether or not the NH ₃ detection flag is set. This NH ₃ detection flag is set when I ₁ > I _S in step 102 of FIG. Whether the set operation region operating region of the engine was previously the routine proceeds to step 401, it is determined when the NH ₃ detection flag has been set. This set operation region is a narrow operation region determined by the engine load Q / N and the engine speed N. When the operation region of the engine is within the set operation region, the routine proceeds to step 402.

In step 402, it is determined whether or not the elapsed time t since the setting of the NH ₃ detection flag has exceeded a predetermined time t ₁ . This fixed time t ₁ is a time from when the air-fuel ratio is made rich to lean as described above, until the detection current I ₁ of the NO _X ammonia sensor 29 ends to drop to zero. t> whether becomes to t ₁ NH ₃ detection flag goes to step 403 the elapsed time t after the set exceeds a predetermined time t ₂ is determined. This fixed time t ₂ is a time sufficient for the NO _x ammonia sensor 29 to detect the ammonia concentration regardless of the amount of ammonia when the ammonia is discharged from the NO _x absorbent 23 as described above. is there. When t ≦ t _2, the process proceeds to step 404.

[0078] detected current I ₁ of the step 404 NO _X ammonia sensor 29 is calculated. Then step 4
05 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 403 that t> t ₂ , the routine proceeds to step 406, where it is determined whether the integrated value ΔI of the detected current is larger than the reference value Sr. If ΣI> Sr, the routine proceeds to step 407, where the target value QR _S is reduced by a predetermined set value α, and then step 4
Go to 09. On the other hand, when ΔI ≦ Sr, the routine proceeds to step 408, where the target value QR _S is increased by a predetermined set value α, and then the routine proceeds to step 409.

At step 409, ΔI is cleared, and at the same time, the NH ₃ detection flag is reset. Next, at step 410, it is determined whether or not the updated target value QR _S is equal to or less than a predetermined limit value QRmin. QR _S
<When the QRmin deterioration flag indicating that _the NO _X absorbent 23 is deteriorated proceeds to step 411 is set. When the deterioration flag is set, for example, a warning lamp is turned on.

Next, a third embodiment will be described with reference to FIGS. _The NO _X absorbent 2 In this example
Estimates the NO _X absorption amount to 3, controlled based on the estimated NO _X absorption rich time interval between re-rich air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 23 after the rich, further The rich time interval is corrected based on the detected current I ₁ , and the rich time is calculated based on the integrated value of the detected current Σ
The control is performed based on a representative value such as I or the maximum value Imax of the detected current.

That is, in the third embodiment, NO_X Absorbent 2
NO absorbed in 3_X NO for estimating the amount_X Sucking
It has a means for estimating the yield, as shown in FIG.
NO _X NO estimated by the absorption amount estimation means_X Absorption Σ
When NOX exceeds the allowable value NOXmax, the air-fuel ratio is
To switch temporarily from rich to rich. organ
NO emitted from_X Once the engine operating condition is determined,
Settled and therefore NO_X NO absorbed by the absorbent 23_X 
The amount is almost determined when the operating condition of the engine is determined. So this
In the third embodiment, the per unit time according to the operating state of the engine is
NO_X NO to absorbent 23_X The absorption amount NA is determined in advance by experiments.
This NO_X Absorption amount NA is engine load Q / N
And as a function of the engine speed N as shown in FIG.
It is stored in the ROM 33 in advance in the form of a tip.

In this embodiment, the NO _X absorption amount NA according to the engine operation state shown in FIG. 14 is integrated during the operation of the engine, and the NO _X amount ＸNOX which is estimated to be absorbed by the NO _X absorbent 23 thereby. Is calculated. However, the value of NA becomes negative because the air-fuel ratio 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 in such operating range.

[0083] On the other hand, the allowable value NOXmax is _the NO _X absorbent 2
As the amount of SO _X absorbed in No. 3 increases, that is, NO
_The smaller the absorption capacity of the _X absorbent 23 is, the smaller the absorption capacity is.
By the way, the injected fuel contains sulfur at a substantially constant rate determined by the fuel. Therefore, the amount of SO _X absorbed by the NO _X absorbent 23 is calculated as the integrated value of the injected fuel amount TAUΣTAU
Is proportional to Therefore, in the third embodiment, as shown in FIG. 15, as the integrated value ΣTAU of the injected fuel amount increases, the allowable value NOXmax is gradually reduced.

[0084] In the third embodiment is basically NO _X absorption amount ΣNOX as the aforementioned air-fuel ratio temporarily switched from lean to rich when it 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. Further, the allowable value NOXmax In this third embodiment is set to a value smaller than the NO _X absorption amount when NO _X from _the NO _X absorbent 23 during the lean operation starts to release. Therefore the air-fuel ratio becomes the switched from lean to rich before the start NO _X is released from _the NO _X absorbent 23 during lean operation in this third embodiment.

However, the calculated NO _x absorption amountΣN
If OX deviates from the actual amount of absorbed NO _x , even though ΣNO x <NO xmax, N
NO _X may start to be released from the O _X absorbent 23. So when the NO _X from despite _the NO _X absorbent 23 is ΣNOX <NOXmax began to release in the third embodiment, i.e., NO _X ammonia sensor 29
Of the detected current I ₁ is rich temporarily switched air-fuel ratio when it exceeds the set value I _S from lean, tolerance NOXmax
Is reduced by a predetermined amount B.
That is, in this third embodiment so that to correct the allowable value NOXmax by detection current I _1.

FIGS. 16 and 17 show a routine for executing the third embodiment. Referring to FIGS. 16 and 17, first at step 500, FIG.
The basic fuel injection amount TAU is calculated from the map shown in FIG. Then NO _X release flag indicating that it should release the NO _X from step 501 in _the NO _X absorbent 23 is whether it is set or not. If the NO _X release flag has not been set, the routine proceeds to step 502, where the NO _X absorption amount NA per unit time is obtained from the map shown in FIG.
Is calculated. Then _the amount of NO _X ΣNOX is estimated to be absorbed in _the NO _X absorbent 23 by adding in step 503 the NO _X absorption NA to ΣNOX
Is calculated.

Next, at step 504, an integrated value ΣTAU of the injected fuel is calculated by adding the final injection amount TAUO to ΣTAU. Next, at step 505, the allowable value NOXmax is calculated from the relationship shown in FIG. 15 based on the integrated value ΣTAU. Then step 506
Then, the allowable value NOXmax is reduced by the correction amount ΔX. Then whether or not the detected current I ₁ of the NO _X ammonia sensor 29 in step 507 exceeds the set value I _S is determined. Whether NO _X absorption amount ΣNOX proceeds to step 508 when I ₁ ≦ I _S has exceeded the allowable value NOXmax is determined. When ΣNOX ≦ NOXmax, ie, N
If there is still room in the NO _X absorption capacity of the O _X absorbent 23, the routine jumps to step 509.

In step 509, the correction coefficient K is calculated from the map shown in FIG. Then step 510
Now, by multiplying the basic fuel injection amount TAU by the correction coefficient K, the final fuel injection amount TAUO (= K · TAU)
Is calculated, and fuel injection is performed with the injection amount TAUO. Next, at step 511, the target value QR of the reducing agent
_It is determined whether _S has become smaller than the set value SS for SO _X release, and when QR _S ≧ SS, the processing cycle is completed.

On the other hand, in step 508, {NOX>
When it is determined that NOXmax has been reached, step 51
NO _X releasing flag proceeds to 2 is set, then NH ₃ detection flag is set the routine proceeds to step 513.
Next, the routine proceeds to step 509. Further, when it is determined that becomes I _1> I _S in step 507 before whether ΣNOX> NOXmax in step 508 is determined, i.e. from _the NO _X absorbent 23 when starts to discharge the NO _X 514 The preset value B is added to the correction amount ΔX. Then NO _X release flag is set the routine proceeds to step 512. Accordingly, at this time, the allowable value NOXmax is reduced by the set value B.

When the NO _X release flag is set, the routine proceeds from step 501 to step 515 in the next processing cycle, and the correction coefficient K _R is calculated from the relationship shown in FIG. Next, at step 516, the final fuel injection amount TAU is multiplied by the correction coefficient K _R to the basic fuel injection amount TAU.
UO (= K _R · TAU) is calculated, and this injection amount TAU
Fuel injection is performed with O. 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
The release action of _X is started.

Next, at step 517, the amount ΔQR of the reducing agent supplied to the NO _X absorbent 23 per one fuel injection is calculated based on the following equation. AQr = TAU · total amount QR of the (K _R -1.0) then step 518 the reducing agent supplied to _the NO _X absorbent 23 by adding the reducing agent amount AQr the QR is calculated. Then the total amount QR of the reducing agent in step 519 it is determined whether or not exceeds the target value QR _S is, jumps to step 511 when the QR ≦ QR _S. NO _X releasing flag proceeds to step 520 becomes a QR> QR _S contrast is reset, then the total amount QR of the NO _X absorption ΣNOX and a reducing agent in step 521 is cleared. Next, the routine proceeds to step 511. If NO _X release flag is reset the air-fuel ratio is switched from rich to lean.

On the other hand, in step 511, QR _S <S
When it is determined that S has been reached, the routine proceeds to step 522, where processing for releasing SO _X from the NO _X absorbent 23 is performed. That is, the air-fuel ratio is made rich while maintaining the temperature of the NO _X absorbent 23 almost 600 ° C. or higher. When the release operation of SO _X from the NO _X absorbent 23 is completed, step 52 is performed.
Proceeding to 3, the predetermined maximum reducing agent total amount QRma
x is a target value QR _S, is further ΣTAU is zero.

In the third embodiment, the target value QR _S is calculated by the routine shown in FIG. 9, FIG. 10, or FIG.

[0094]

As described above, it is possible to appropriately control the release of NO _X from the NO _X absorbent.

[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 a basic fuel injection amount, a correction coefficient, and the like.

FIG. 5 is a diagram for explaining the NO _X absorbing / releasing action of the NO _X absorbent.

FIG. 6 is a time chart showing a detection current and the like of a NO _X ammonia sensor.

FIG. 7 is a diagram illustrating a correction coefficient when a rich air-fuel ratio is set.

FIG. 8 is a flowchart for controlling operation of the engine.

9 is a flow chart for calculating a target value QR _S.

10 is a flow chart for calculating a target value QR _S.

FIG. 11 is a time chart showing a detection current and the like of a NO _X ammonia sensor.

12 is a flow chart for calculating a target value QR _S.

FIG. 13 is a time chart showing changes in the NO _X absorption amount and the air-fuel ratio.

FIG. 14 is a view showing a map of an NO _X absorption amount.

FIG. 15 is a diagram showing allowable values.

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

FIG. 17 is a flowchart for controlling operation of the engine.

[Explanation of symbols]

11 ... Fuel injection valve 23 ... NO _X absorbent 29 ... NO _X ammonia sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/08 F01N 3/18 C 3/18 3/24 R 3/28 301C 3/24 B01D 53/34 129A 3/28 301 (72) Inventor Junichi Kako 1 Toyota Town, Toyota City, Aichi Prefecture F-term in Toyota Motor Corporation (reference) 3G091 AA02 AA11 AA12 AA17 AA24 AA28 AB02 AB03 AB06 AB13 BA11 BA14 BA15 BA19 BA20 BA33 CA18 CB02 CB03 CB07 DA01 DA02 DA03 DA04 DA10 DB06 DB07 DB08 DB10 DB11 DB15 DC01 EA01 EA05 EA07 EA30 EA31 EA33 EA34 FA11 FB10 FB11 FB12 FC02 GA06 GB01X GB02W GB02Y NE0303 NE23 PA01Z PD01Z PD03Z PE01Z PE03Z PF03Z 4D002 AA12 AC10 BA03 BA04 DA01 DA04 DA25 DA46 GA03 GB02 GB08

Claims (16)

[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 placed engine exhaust passage, when the burning fuel under a lean air-fuel ratio is made NO _X in the exhaust gas is absorbed in the NO _X absorbent, NO from _the NO _X absorbent 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 to be rich when should be released _X, NO _X
NO _X in excess of the form of the reducing agent is ammonia which has not been used to release and reduce NO _X from _the NO _X absorbent when the air-fuel ratio of the exhaust gas flowing into the absorbent is made rich
It is discharged from the absorber, finding a representative value representing the amount of reducing agent the surplus by placing sensors capable of detecting the ammonia concentration to the NO _X absorbent in the exhaust passage downstream of the change in the ammonia concentration detected by the sensor Exhaust gas purifying apparatus for an internal combustion engine. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein an integrated value of the ammonia concentration detected by the sensor is set as the representative value. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein a maximum value of the ammonia concentration detected by the sensor is set as the representative value. Wherein N when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent as the representative value increases is rich
O _X exhaust purification system of an internal combustion engine according to claim 1 which is adapted to reduce the total amount of reducing agent supplied to the absorber. 5. The exhaust gas control apparatus according to claim 4 which is adapted to shorten the time for the air-fuel ratio of the exhaust gas flowing into the more _the NO _X absorbent the representative value increases rich. 6.] A reference value for the representative values is set in advance, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is made rich when the representative value is greater than the reference value reducing the total amount of reducing agent supplied to _the NO _X absorbent, N represents when the representative value is smaller than the reference value
O _X fuel ratio of the exhaust gas flowing into the absorbent exhaust purification system of an internal combustion engine according to claim 1 which is adapted to increase the total amount of reducing agent supplied to the NO _X absorbent when it is rich. 7. When the representative value becomes larger than the reference value, the time for enriching the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is shortened, and when the representative value becomes smaller than the reference value, the NO _X absorption is reduced. 7. The exhaust gas purifying apparatus for an internal combustion engine according to claim 6, wherein the time for making the air-fuel ratio of the exhaust gas flowing into the agent rich is lengthened. 8. The sensor is capable of detecting concentration of NO _X in the exhaust gas in addition to the ammonia concentration in the exhaust gas, detected by the sensor when burning fuel under a lean air-fuel ratio has been made 7. The exhaust gas purifying apparatus for an internal combustion engine according to claim 4, wherein the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent is switched from lean to rich when the determined NO _X concentration exceeds a predetermined set value. . 9. comprising a NO _X absorption amount estimation means for estimating an amount of NO _X is absorbed in the NO _X absorbent, the N
The rich time interval for temporarily making the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent rich based on the NO _X amount estimated by the O _X absorption amount estimating means is controlled. Exhaust purification device for internal combustion engine. It is temporarily switched to a rich air-fuel ratio of the inflowing exhaust gas from lean to _the NO _X absorbent when the 10. NO _X absorption amount estimated by the NO _X absorption amount estimation means exceeds the allowable value An exhaust gas purifying apparatus for an internal combustion engine according to claim 9. 11. comprising a NO _X absorbing capacity estimation means for estimating the NO _X absorbing capacity of the NO _X absorbent, the NO _X
An exhaust purification system of an internal combustion engine according to claim 10 which is estimated by the absorption capacity estimation means the NO _X absorption capacity is made to decrease the allowable value as reduced. 12. The sensor is capable of detecting concentration of NO _X in the exhaust gas in addition to the ammonia concentration in the exhaust gas, the NO _X absorbent when burning fuel under a lean air-fuel ratio has been made When the NO _X concentration detected by the sensor exceeds a predetermined set value even though the NO _X absorption amount estimated by the amount estimating means does not exceed the allowable value, the NO _X absorbent is The exhaust gas purifying apparatus for an internal combustion engine according to claim 10, wherein the air-fuel ratio of the inflowing exhaust gas is switched from lean to rich. 13. The sensor is capable of detecting concentration of NO _X in the exhaust gas in addition to the ammonia concentration in the exhaust gas, the NO _X absorbent when burning fuel under a lean air-fuel ratio has been made the allowable value decreases when the nO _X absorbed amount estimated by the amount estimation means exceeds a set value of nO _X concentration predetermined detected by the sensor in spite of not exceed the allowable value The exhaust gas purifying apparatus for an internal combustion engine according to claim 10, wherein the exhaust gas purifying apparatus is used. 14. An exhaust purification system of an internal combustion engine according to claim 1 which is adapted to detect the degree of deterioration of _the NO _X absorbent based on the representative value. 15. wherein the total amount of reducing agent supplied to _the NO _X absorbent to claim 14, the degree of deterioration of _the NO _X absorbent is determined to be larger as the amount obtained by subtracting the amount of reducing agent the excess is reduced Exhaust purification device for internal combustion engine. 16. A method according to claim degree of rich is smaller as the degree of deterioration of _the NO _X absorbent increases when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent to a rich 1
An exhaust gas purifying apparatus for an internal combustion engine according to claim 4.