JPH08232646A - Emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE: To contrive to improve accuracy of detecting a NOx release operation completed, by providing a means for judging when a change rate of an output level of an air-fuel ratio sensor, when air-fuel ratio of exhaust gas flowing out from a NOx absorbent is switched from lean ratio to rich ratio, exceeds a predetermined change rate. CONSTITUTION: An exhaust port 8 is connected to a casing 17, built in with a NOx absorbent 18, through an exhaust manifold 15 and an exhaust pipe 16. An O2 sensor 22 is arranged in an exhaust pipe 21 in the downstream of the NOx absorbent 18, and this O2 sensor 22 is connected to an input port 36 through a corresponding AD converter 38. When a change rate per unit time of a current value exceeds a predetermined change rate, the current value is judged to lead to a current value rapid change point, consequently by switching air-fuel ratio of exhaust gas, flowing into the NOx absorbent 18, from the rich ratio to the lean ratio when leading to the current value rapid change point, total NOx absorbed to the NOx absorbent 18 can be released. In this way, completing a release operation can be detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine.

[0002]

2. Description of the Related Art A NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich is arranged in the engine exhaust passage. BACKGROUND ART An internal combustion engine is known in which a lean air-fuel mixture is usually burned and NOx generated at this time is absorbed by a NOx absorbent. In this internal combustion engine, when the amount of NOx absorbed by the NOx absorbent exceeds a certain amount, the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is temporarily made rich in order to release NOx from the NOx absorbent. N
When the air-fuel ratio of the exhaust gas flowing into the Ox absorbent is made rich, the action of releasing NOx from the NOx absorbent is started.

When the air-fuel ratio of exhaust gas is made rich, NO released from the NOx absorbent per unit time.
The amount of x largely fluctuates depending on the temperature of the NOx absorbent and the degree of richness of the air-fuel ratio, so the time required to release all the NOx absorbed from the NOx absorbent is NOx.
The amount of NOx absorbed in the absorbent varies greatly depending on the temperature of the NOx absorbent and the degree of richness of the air-fuel ratio. In this case, if the time during which the air-fuel ratio is made rich is too short, the NOx adsorbed on the NOx is absorbed.
Since x cannot be released sufficiently, the absorption capacity of the NOx absorbent gradually decreases, and eventually NOx cannot be absorbed. On the other hand, if the air-fuel ratio is made rich for too long, Even after all NOx is released from the NOx absorbent, the air-fuel ratio becomes rich, so a large amount of unburned H
C and CO will be released to the atmosphere.

Thus, in order to prevent the NOx absorbent from absorbing NOx or to release a large amount of unburned HC and CO into the atmosphere, the NOx from the NOx absorbent must be removed.
It is necessary to stop the rich control of the air-fuel ratio at the moment when the x release action is completed.
x It is necessary to accurately detect that the release action is complete.

When the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich, the NOx from the NOx absorbent is increased.
While the x-releasing action is being performed, the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent becomes slightly lean, and when the NOx-releasing action of the NOx absorbent is completed, the exhaust gas flowing out of the NOx absorbent is completed. It is known that the air-fuel ratio of will become rich. Therefore, an air-fuel ratio sensor that can detect whether the air-fuel ratio of the exhaust gas is rich or lean is arranged in the engine exhaust passage downstream of the NOx absorbent, and NOx
After changing the air-fuel ratio of the exhaust gas flowing into the NOx absorbent from lean to rich to release NOx from the absorbent, the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent became rich by the air-fuel ratio sensor. NOx when detected
NO when it is judged that the NOx releasing action from the absorbent is completed
x An internal combustion engine is known in which the air-fuel ratio of the exhaust gas flowing into the absorbent is returned to lean again (PCT International Publication W
O94 / 17291).

[0006]

However, the fact that the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent becomes rich means that the action of releasing NOx from the NOx absorbent has already been completed, and therefore As in the above-mentioned internal combustion engine, when the air-fuel ratio of the exhaust gas flowing out from the NOx absorbent becomes rich, it is judged that the NOx releasing action has been completed. You will not be able to judge.

Further, the fact that the air-fuel ratio of the exhaust gas flowing out from the NOx absorbent becomes rich means that a large amount of unburned HC or CO has already been discharged from the NOx absorbent, and therefore the above-mentioned NOx as in an internal combustion engine
When it is determined that the NOx releasing action has been completed when it is detected that the air-fuel ratio of the exhaust gas flowing out from the absorbent has become rich, when it is determined that the NOx releasing action has been completed, a large amount of unreacted There is also a problem that fuel HC and CO are starting to flow out from the NOx absorbent.

[0008]

According to the first aspect of the present invention, in order to solve the above problems, when the air-fuel ratio of the inflowing exhaust gas is lean, NOx is absorbed and the air-fuel ratio of the inflowing exhaust gas is increased. NO that releases absorbed NOx when it becomes rich
x An absorber is placed in the engine exhaust passage, and the air-fuel ratio sensor that produces an output at a level proportional to the air-fuel ratio of the exhaust gas is NO.
It is arranged in the exhaust passage downstream of the x-absorbent, and is N after the air-fuel ratio of the exhaust gas flowing into the NOx-adsorbent in order to release the NOx from the NOx-absorbent is switched from lean to rich.
When the rate of change of the output level of the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing out of the Ox absorbent changes from lean to rich, when the rate of change exceeds a predetermined rate of change, the action of releasing NOx from the NOx absorbent will occur. It is equipped with a judging means for judging that it is completed.

According to the second aspect of the invention, in order to solve the above-mentioned problems, NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean and absorbed when the air-fuel ratio of the inflowing exhaust gas becomes rich. A NOx absorbent that releases NOx is placed in the engine exhaust passage, and an air-fuel ratio sensor that produces an output at a level proportional to the air-fuel ratio of the exhaust gas is placed in the exhaust passage downstream of the NOx absorbent. After the air-fuel ratio of the exhaust gas flowing into the NOx absorbent to release NOx is switched from lean to rich, the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent is switched from lean to rich A determination means is provided for determining that the NOx releasing action from the NOx absorbent is completed when the rate of change in the output level exceeds a predetermined rate of change.

According to the third aspect of the invention, in the first or second aspect of the invention, the NOx from the NOx absorbent is determined by the determining means.
When it is determined that the release action is completed, the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is switched from rich to lean.

[0011]

[Function] After the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is switched from lean to rich in order to release NOx from the NOx absorbent, the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent is switched from lean to rich. Immediately before the change, the output level of the air-fuel ratio sensor drops sharply. Therefore, in the first aspect of the invention, the sudden decrease in the output level is grasped from the change in the rate of change of the output level, and it is judged that the NOx releasing action from the NOx absorbent is completed at the moment when the output level suddenly decreases.

In the second aspect of the invention, the sudden decrease in the output level is grasped from the change in the change rate of the change rate of the output level, and it is judged that the release action from the NOx absorbent is completed at the moment when the output level suddenly decreases. . When it is determined that the NOx releasing action from the NOx absorbent is completed, the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is switched from rich to lean.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is an exhaust port. Show. The intake port 6 is connected to the surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 17 containing a NOx absorbent 18.

The electronic control unit 30 is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, and a constant power source by a bidirectional bus 31. Connected backup RAM
35, an input port 36 and an output port 37. A pressure sensor 19 that generates an output voltage proportional to the absolute pressure in the surge tank 10 is arranged in the surge tank 10, and the output voltage of the pressure sensor 19 is input to the input port 35 via the corresponding AD converter 38. To be done. An air-fuel ratio sensor 20, which is used when the air-fuel ratio of the air-fuel mixture should be maintained at the stoichiometric air-fuel ratio, is arranged in the exhaust manifold 15.
The air-fuel ratio sensor 20 is input to the input port 36 via the corresponding AD converter 38. Another air-fuel ratio sensor (hereinafter referred to as an O ₂ sensor) 22 is arranged in the exhaust pipe 21 downstream of the NOx absorbent 18, and the O ₂ sensor 22 is connected to the input port 36 via the corresponding AD converter 38. Connected. Further, the input port 36 is connected to a rotation speed sensor 23 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 37 is connected to the spark plug 4 and the fuel injection valve 11 via the corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on the following equation, for example. TAU = TP · K · FAF where TP is the basic fuel injection time, K is the correction coefficient, FAF
Indicates the feedback correction coefficient, respectively. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is previously obtained by an experiment, and is previously RO in the form of a map as shown in FIG. 2 as a function of the absolute pressure PM in the surge tank 10 and the engine speed N.
It is stored in M32. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, when K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and K>
When it becomes 1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The feedback correction coefficient FAF is K = 1.
When 0, that is, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder should be the stoichiometric air-fuel ratio, a coefficient for accurately matching the air-fuel ratio with the theoretical air-fuel ratio based on the output signal of the air-fuel ratio sensor 20. Is. That is, the air-fuel ratio sensor 20 is installed in the combustion chamber 3
When the air-fuel ratio of the air-fuel mixture burned inside is lean, an output voltage of about 0.1 (V) is generated, and when the air-fuel ratio of the air-fuel mixture burned in the combustion chamber 3 is rich, it is 0.9 (V ) Generate an output voltage of the order of. The output voltage (V) of the air-fuel ratio sensor 20 is compared with a reference voltage Vr of about 0.45 (V), and the feedback correction coefficient FAF is decreased when V> Vr and increased when V ≦ Vr. To be At this time, this FAF moves up and down with reference to about 1.0. In addition, K <1.0 or K> 1.0
In case of, FAF is fixed to 1.0.

The target air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder, that is, the value of the correction coefficient K is changed in accordance with the operating state of the engine. In the embodiment of the present invention, it is basically shown in FIG. Thus, it is predetermined as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. That is, as shown in FIG. 3, K <1.0, that is, the air-fuel mixture is lean in the low load operation region on the load lower side than the solid line R, and K in the high load operation region between the solid line R and the solid line S. = 1.
0, that is, the air-fuel ratio of the air-fuel mixture is the stoichiometric air-fuel ratio, and K> 1.0, that is, the air-fuel mixture is rich in the full-load operation region on the higher load side than the solid line S.

FIG. 4 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 4, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer, and is discharged from the combustion chamber 3. The concentration of oxygen O _{2 in} the generated exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in the casing 17
The absorbent 18 uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y is used as a carrier on the carrier. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. The ratio of the air and the fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NOx absorbent 18 is called the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18.
The absorbent 18 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and absorbs and releases the NOx that absorbs the NOx when the oxygen concentration in the inflowing exhaust gas decreases. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx absorbent 18, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3, In this case, the NOx absorbent 18 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is lean, and when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 decreases. It will release NOx.

If the above-mentioned NOx absorbent 18 is arranged in the exhaust passage of the engine, the NOx absorbent 18 actually acts to absorb and release NOx, but the detailed mechanism of this absorbing and releasing operation is not clear. 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 the case where platinum Pt and barium Ba are supported on the carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, the inflowing exhaust gas becomes considerably lean.
And the concentration of oxygen in the inflowing exhaust gas increased drastically.
As shown in (A), these oxygen O₂Is O₂ ^-Or O
^2-It adheres to the surface of platinum Pt in the form of. On the other hand, inflow exhaust
NO in the gas is O on the surface of platinum Pt.₂ ^-Or O^2-When
Reacts, NO₂Becomes (2NO + O₂→ 2 NO₂). Next
NO generated by₂Is partially oxidized on platinum Pt.
Is absorbed in the absorbent and does not combine with barium oxide BaO
As shown in FIG. 5 (A), nitrate ion NO₃ ^-of
Diffuses into the absorbent in the form. In this way NOx is NO
x Absorbed in the absorbent 18.

NO ₂ is produced on the surface of platinum Pt as long as the oxygen concentration in the inflowing exhaust gas is high, and NO ₂ is absorbed in the absorbent as long as the NO x absorption capacity of the absorbent is not saturated, and nitrate ions NO ₃ ⁻ are produced. Is generated. In contrast the reaction with the amount of NO ₂ oxygen concentration is lowered in the inflowing exhaust gas is lowered backward (NO ₃ ^- → NO ₂₎ proceeds to, thus nitrate ions to the absorber NO ₃ ^- Are released from the absorbent in the form of NO ₂ . That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx absorbent 18. As shown in FIG. 4, when the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low. Therefore, even if the leaning degree of the inflow exhaust gas is low, the air-fuel ratio of the inflowing exhaust gas is lean. Even NOx absorbent 18
NOx will be released from.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, a large amount of unburned H is emitted from the engine as shown in FIG.
C and CO are discharged, and these unburned HC and CO are platinum Pt.
And is oxidized by reacting with oxygen O ₂ ⁻ or O ^{2 −} .
Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas drops extremely, so
O ₂ is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 5 (B). When NO ₂ is no longer present on the surface of platinum Pt in this way, NO ₂ is released one after another from the absorbent. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 18 within a short time.

That is, when the air-fuel ratio of the inflowing exhaust gas is made rich, first, unburned HC and CO immediately react with O ₂ ⁻ or O ^{2 −} on platinum Pt to be oxidized, and then O on platinum Pt is oxidized. _{Even if 2} ⁻ or O ^{2 −} is consumed, if unburned HC and CO still remain, NOx discharged from the absorbent and NOx discharged from the engine are reduced by the unburned HC and CO. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed in the NOx absorbent 18 is released within a short time, and the released NOx is reduced, so that the NOx is released into the atmosphere. Can be prevented.

When the lean air-fuel mixture is burned as described above, NOx is absorbed by the NOx absorbent 18. However, the NOx absorbent capacity of the NOx absorbent 18 is limited, and when the NOx absorbent capacity of the NOx absorbent 18 is saturated, the NOx absorbent 18 can no longer absorb NOx.
Therefore, it is necessary to release NOx from the NOx absorbent 18 before the NOx absorbent capacity of the NOx absorbent 18 becomes saturated.
It is necessary to estimate whether is absorbed. Then this N
A method of estimating the Ox absorption amount will be described.

When the lean air-fuel mixture is burned, the higher the engine load, the more the amount of NOx exhausted from the engine per unit time increases, so the amount of NOx absorbed by the NOx absorbent 18 per unit time increases, Further, as the engine speed increases, the NO emitted from the engine per unit time
Since the amount of x increases, the amount of NOx absorbed by the NOx absorbent 18 per unit time increases. Therefore, NO per unit time
The amount of NOx absorbed by the x absorbent 18 is a function of the engine load and the engine speed. In this case, since the engine load can be represented by the absolute pressure in the surge tank 10, the NOx amount absorbed by the NOx absorbent 18 per unit time is a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Becomes Therefore, in the embodiment according to the present invention, NOx per unit time
Absolute pressure PM of NOx amount NOXA absorbed by the absorbent 18
And as a function of the engine speed N, previously obtained by experiments,
This NOx absorption amount NOXA is stored in advance in the ROM 32 in the form of a map shown in FIG. 6A as a function of PM and N.

On the other hand, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes the stoichiometric air-fuel ratio or rich, NOx is released from the NOx absorbent 18, but the NOx release amount at this time is mainly the exhaust gas amount and the air-fuel ratio. It is affected by the fuel ratio. That is, the amount of NOx released from the NOx absorbent 18 per unit time increases as the amount of exhaust gas increases, and the NOx released from the NOx absorbent 18 per unit time increases as the air-fuel ratio becomes richer.
x quantity increases. In this case, it can be represented by the product of the exhaust gas amount, that is, the intake air amount, the engine speed N, and the absolute pressure PM in the surge tank 10. Therefore, as shown in FIG. The NOx amount NOXD released from the NOx absorbent 18 increases as N · PM increases. Since the air-fuel ratio corresponds to the value of the correction coefficient K, the NOx amount NOXD released from the NOx absorbent 18 per unit time increases as the value of K increases, as shown in FIG. 7B. The NOx amount NOXD released from the NOx absorbent 18 per unit time is previously stored in the ROM 3 in the form of a map shown in FIG. 6B as a function of N · PM and K.
It is stored in 2.

As described above, when the lean air-fuel mixture is burned, the NOx absorption amount per unit time is NOXA.
The NOx release amount per unit time is expressed as NOXD when the stoichiometric air-fuel ratio mixture or rich mixture is burned. Therefore, the NOx amount estimated to be absorbed by the NOx absorbent 18 is ΣNOX. Is represented by the following equation.

ΣNOX = ΣNOX + NOXA-NOXD Therefore, in the embodiment according to the present invention, as shown in FIG.
When the NOx amount ΣNOx estimated to be absorbed by the Ox absorbent 18 reaches the maximum allowable value MAX, the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18, the air-fuel ratio of the air-fuel mixture in the embodiment shown in FIG. Is temporarily made rich to release NOx from the NOx absorbent 18.

By the way, in this case, if the time period for making the air-fuel ratio of the inflowing exhaust gas rich is too short, the air-fuel ratio of the inflowing exhaust gas becomes lean again before all the NOx absorbed in the NOx absorbent 18 is released. Will be returned. As a result, N
Since the amount of NOx that continues to be absorbed and held in the Ox absorbent 18 gradually increases, the NOx absorbent 18 cannot absorb NOx at the end, thus causing a problem that NOx is released to the atmosphere. On the other hand, if the time period for making the air-fuel ratio of the inflowing exhaust gas rich is too long, the exhaust gas containing a large amount of unburned HC and CO will reach the NOx absorbent 18 even after all the NOx is released from the NOx absorbent 18. It will flow in. However, in this case, since there is no NOx to be reduced, these unburned HC and CO are discharged from the NOx absorbent 18 as they are, thus causing a problem that a large amount of unburned HC and CO are released to the atmosphere.

To solve these problems, namely N
In order to prevent the release of Ox and unburned HC, CO to the atmosphere, the air-fuel ratio of the inflowing exhaust gas must be returned to lean when the action of releasing NOx from the NOx absorbent 18 is completed, For that purpose, it is necessary to detect the completion of the NOx releasing action from the NOx releasing agent 18. In the present invention, NO from the NOx absorbent 18
Completion of the x releasing action is detected from the air-fuel ratio detected by the air-fuel ratio sensor 22, which will be described below.

That is, when the air-fuel mixture supplied to the combustion chamber 3 becomes rich, exhaust gas containing oxygen O ₂ and unburned HC and CO is discharged from the combustion chamber 3 as shown in FIG. The oxygen O ₂ hardly reacts with the unburned HC and CO, and thus the oxygen O ₂ passes through the NOx absorbent 18 and is discharged from the NOx absorbent 18. On the other hand, when the air-fuel mixture supplied into the combustion chamber 3 becomes rich, N
NOx is released from the Ox absorbent 18. At this time, the unburned HC and CO contained in the exhaust gas are used to reduce the released NOx, so that the NOx absorbent 18 is reduced to N.
While Ox is being released, no unburned HC and CO are emitted from the NOx absorbent 18. Therefore NOx
While NOx is continuously released from the absorbent 18, NOx
The exhaust gas discharged from the absorbent 18 contains oxygen O ₂ but does not contain unburned HC and CO at all. Therefore, during this period, the air-fuel ratio of the exhaust gas discharged from the NOx absorbent 18 is increased. Is a little lean.

Next, when the operation of releasing NOx from the NOx absorbent 18 is completed, unburned HC contained in the exhaust gas,
CO is not used for the reduction of NOx in the NOx absorbent 18, but is directly discharged from the NOx absorbent 18. Therefore, at this time, the air-fuel ratio of the exhaust gas discharged from the NOx absorbent 18 becomes rich. That is, NOx absorbent 1
When NOx release from 8 is completed, NOx absorbent 18
The exhaust gas discharged from the fuel cell changes from lean to rich, and if the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is made lean at this moment, NOx and unburned HC, CO
Will not be released to the atmosphere. Therefore, in the present invention, N
The lean-to-rich change in the air-fuel ratio of the exhaust gas discharged from the Ox absorbent 18 is detected by the air-fuel ratio sensor 22, and at the moment when the air-fuel ratio detected by the air-fuel ratio sensor 22 changes from lean to rich. The air-fuel ratio of the air-fuel mixture supplied into 3 is returned to lean.

As described above, according to the present invention, it is judged whether or not the action of releasing NOx from the NOx absorbent 18 is completed based on the air-fuel ratio detected by the air-fuel ratio sensor 22. This will be described in more detail. To do. The air-fuel ratio sensor 22 shown in FIG. 1 is composed of a cup-shaped tubular body made of zirconia arranged in the exhaust passage, and an anode made of a platinum thin film is provided on the inner surface of the tubular body. Cathodes made of platinum thin films are formed on the outer side surfaces, respectively. The cathode is covered with a porous layer, and a constant voltage is applied between the cathode and the anode. In this air-fuel ratio sensor 22, as shown in FIG. 9, a current I proportional to the air-fuel ratio A / F
(MA) flows between the cathode and the anode. In FIG. 9, I ₀ represents the current value when the air-fuel ratio A / F is the theoretical air-fuel ratio (= 14.6). As can be seen from FIG. 9, the air-fuel ratio A
When / F is lean, the current value I increases as the air-fuel ratio A / F increases in the range of I> I _{0. When} the air-fuel ratio A / F becomes rich below 13.0, the current value I becomes zero. Become.

FIG. 10 shows changes in the air-fuel ratio (A / F) in of the exhaust gas flowing into the NOx absorbent 18 and the air-fuel ratio sensor 22.
2 shows the change in the current I flowing between the cathode and the anode and the change in the air-fuel ratio (A / F) out of the exhaust gas flowing out from the NOx absorbent 18. As shown in FIG. 10, the air-fuel ratio (A / F) in of the exhaust gas flowing into the NOx absorbent 18 is switched from lean to rich and the Nx from the NOx absorbent 18 is changed.
When the Ox releasing action is started, the air-fuel ratio (A / F) out of the exhaust gas flowing out from the NOx absorbent 18 rapidly decreases to near the stoichiometric air-fuel ratio, so that the current value I rapidly decreases to near I _0. . Then NOx from NOx absorbent 18
While the releasing action is being performed, the air-fuel ratio (A / F) out of the exhaust gas flowing out from the NOx absorbent 18 is held in a slightly lean state, so that the current value I is slightly larger than I _0. Held in.

Next, when the NOx releasing action from the NOx absorbent 18 is completed, the air-fuel ratio (A / F) out of the exhaust gas flowing out from the NOx absorbent 18 rapidly becomes small and becomes rich, so that the current value I rapidly. Descend to zero. When the NOx releasing action from the NOx absorbent 18 is completed in this way, the current value I suddenly changes to the current value I immediately before the air-fuel ratio (A / F) out changes from lean to rich. Be seen. As shown in FIG. 10, the air-fuel ratio (A / F) out is not yet rich at the current value sudden change point P. Therefore, when the current value I reaches the current value sudden change point P, the NOx absorbent If the air-fuel ratio (A / F) in of the exhaust gas flowing into 18 is switched from rich to lean, the air-fuel ratio (A / F) out of the exhaust gas flowing out of the NOx absorbent 18
It is possible to release all the NOx absorbed in the NOx absorbent 18 without making the NOx rich at all. That is, when the current value I reaches the current value sudden change point P, the NOx absorbent 1
If the air-fuel ratio (A / F) in of the exhaust gas flowing into 8 is switched from rich to lean, the total absorption N from the NOx absorbent 18
Ox emission can be completed and NOx
Air-fuel ratio (A / F) ou of the exhaust gas flowing out from the absorbent 18
Since t does not become rich, a large amount of unburned HC and CO
Will be released into the atmosphere.

When the NOx absorbent 18 deteriorates, N
The amount of NOx that can be absorbed by the Ox absorbent 18 gradually decreases. FIG. 11 shows changes in the current value I when the NOx absorbent 18 is deteriorated, and each numerical value represents the amount of NOx that the NOx absorbent 18 can absorb. As can be seen from FIG. 11, the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18 (A
/ F) in is switched from lean to rich, the time until the current value I reaches the current value sudden change point P becomes shorter as the NOx absorbent 18 deteriorates, but it depends on the degree of deterioration of the NOx absorbent 18. Regardless, the current value sudden change point P is reached when the current value I is slightly larger than I ₀ . Therefore, when the current value I reaches the current value sudden change point P, the NOx absorbent 18
If the air-fuel ratio (A / F) in of the exhaust gas flowing into the engine is switched from rich to lean, the air-fuel ratio (A / F) of the exhaust gas flowing out of the NOx absorbent 18 will be irrespective of the degree of deterioration of the NOx absorbent 18. ) NO without making out rich at all
All NOx absorbed in the x absorbent 18 can be released.

Whether or not the current value I has reached the current value sudden change point P can be detected from the fact that the rate of change ΔI of the current value I per unit time becomes large. Therefore, in the first embodiment of the present invention, the current value I When the rate of change ΔI of the value I per unit time exceeds a predetermined rate of change, it is determined when the current value I reaches the current value sudden change point P. The air-fuel ratio (A /
F) The rate of change ΔI of the current value I per unit time also increases in the section Z in which the current value I sharply drops due to the change of lean from rich to the current value I, so that this section Z is the sudden change point P of the current value. To prevent erroneous determination, in the first embodiment, during the time t _{1 in} FIG. 11, it is not determined whether or not the current value sudden change point P is reached.

Further, as shown in FIG. 11, a zone Z in which the current value I drops sharply is a preset current value Is.
This is a region where the current value I is larger than that. Therefore, in the second embodiment according to the present invention, when the current value I is larger than the set current value Is, it is not judged whether or not it is the current value sudden change point P. On the other hand, when the current value I reaches the current value sudden change point P, the change rate Δ (ΔI) of the rate of change ΔI of the current value I per unit time becomes large. Therefore, in the third embodiment of the present invention, the current value I When the rate of change Δ of I per unit time ΔI (ΔI) exceeds the rate of change of a predetermined rate of change, it is determined that the current value I has reached the current value sudden change point P. ing. Note that the rate of change Δ (ΔI) of the rate of change ΔI becomes large in Q in FIG. 11 except for the point P at which the current value suddenly changes.
Only ₁ point and Q ₂ points. In this case, the sign of the rate of change Δ (ΔI) of the rate of change ΔI at the point Q ₂ is opposite in sign to the sign of the rate of change Δ (ΔI) of the rate of change ΔI at the sudden change point P of the current value. It is possible to determine whether there are _two points or the current value sudden change point P.

However, the sign of the change rate Δ (ΔI) of the change rate ΔI at the point Q ₁ and the sign of the change rate Δ (ΔI) of the change rate ΔI at the sudden change point P of the current value have the same sign. Therefore, it is not possible to determine whether it is the Q ₁ point or the current value sudden change point P. Therefore, in the third embodiment, time t in FIG.
_During the period of ₂ , the determination as to whether or not the current value sudden change point P is made is not made.

The current I flowing between the cathode and the anode of the air-fuel ratio sensor 22 is converted into a voltage and input into the input port 36. In the electronic control unit 30, this voltage is converted into a corresponding current value I again and this is converted into a corresponding current value I. The air-fuel ratio is controlled based on the current value I. 12 and 13 show an injection control routine for executing the first embodiment, and this routine is executed by interruption at regular time intervals.

Referring to FIGS. 12 and 13, first, at step 100, the basic fuel injection time TP is calculated from the relationship shown in FIG. Next, at step 101, it is judged if the NOx release flag, which is set when NOx should be released from the NOx absorbent 18, is set. When the NOx releasing flag is not set, the routine jumps to step 107 and the correction coefficient K is calculated based on FIG. Next, at step 108, it is judged if the correction coefficient K is 1.0. When K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture should be the stoichiometric air-fuel ratio, the routine proceeds to step 109, where the feedback correction coefficient FAF is calculated based on the output signal of the air-fuel ratio sensor 20. Then, it proceeds to step 113. On the other hand, the correction coefficient K is 1.0
If not, proceed to step 110 and set FAF to 1.0.
Is fixed to step 113, and then the process proceeds to step 113. Step 1
In 13, the fuel injection time TAU is calculated based on the following equation.

TAU = TP.K.FAF Next, at step 114, it is judged if the correction coefficient K is smaller than 1.0. When K <1.0, that is, when the lean air-fuel mixture should be burned, the routine proceeds to step 115, where the NOx absorption amount NOXA is calculated from FIG. 6 (A). Next, at step 116, the NOx release amount NOXD is made zero, and then the routine proceeds to step 119. On the other hand, when it is determined in step 114 that K ≧ 1.0,
That is, when a stoichiometric air-fuel ratio mixture or a rich mixture should be burned, the routine proceeds to step 117, where NOx from FIG.
The release amount NOXD is calculated. Next, at step 118, the NOx absorption amount NOXA is made zero, and then step 1
Proceed to 19. In step 119, NOx is calculated based on the following equation.
NOx amount ΣN estimated to be absorbed by the absorbent 18
OX is calculated.

ΣNOX = ΣNOX + NOXA-NOXD Next, at step 120, it is judged if ΣNOX becomes negative. If ΣNOX <0, then step 1
In step 21, ΣNOX is made zero. Then step 1
At 22, it is determined whether or not ΣNOX exceeds the allowable maximum value MAX (FIG. 8). When ΣNOX> MAX, the routine proceeds to step 123, where the NOx releasing flag is set.

When the NOx releasing flag is set, in the next processing cycle, the routine proceeds from step 101 to step 102, where it is judged if the fixed time t ₁ shown in FIG. 11 has elapsed. When the constant time t ₁ has not elapsed, the routine proceeds to step 111, where the correction coefficient K is set to a constant value KK of about 1.1 to 1.2. Next, in step 112, FAF is fixed to 1.0, and then the process proceeds to step 113. At this time, the air-fuel ratio of the air-fuel mixture is switched from lean to rich, and the action of releasing NOx from the NOx absorbent 18 is started. When the fixed time t ₁ has elapsed, the routine proceeds from step 102 to step 103, where the current value I ₁ of the air-fuel ratio sensor 22 is subtracted from the current value I ₁ of the air-fuel ratio sensor 22 at the time of the previous interruption to obtain the current value I The change rate ΔI (= I ₁ −I) is calculated. Next, at step 104, it is determined whether or not the rate of change ΔI of the current value I is larger than a predetermined rate of change X, that is, whether or not the current value I reaches the current value sudden change point P (FIGS. 10 and 11). Is determined. When ΔI ≦ X, that is, when the current value I does not reach the current value sudden change point P, the routine proceeds to step 111, where the air-fuel ratio of the air-fuel mixture is maintained rich.

When ΔI> X, that is, when the current value I reaches the current value sudden change point P, step 104 to step 105
Then, the NOx releasing flag is reset. Next, at step 106, ΣNOX is made zero, and then the routine proceeds to step 107. Therefore, at this time, the air-fuel ratio of the air-fuel mixture is switched to the air-fuel ratio determined by the correction coefficient K, usually the lean air-fuel ratio.

14 and 15 show an injection control routine for executing the second embodiment, and this routine is executed by interruption at regular time intervals. This routine is different from the routines shown in FIGS. 12 and 13 only in step 202, and the other steps are exactly the same. That is, referring to FIGS. 14 and 15, first, at step 200, the basic fuel injection time TP is calculated from the relationship shown in FIG. Next, at step 201, it is judged if the NOx releasing flag is set. When the NOx releasing flag is not set, the routine proceeds to step 207 and the correction coefficient K is calculated based on FIG. Then step 208
Then, it is determined whether or not the correction coefficient K is 1.0. K
= 1.0, that is, when the air-fuel ratio of the air-fuel mixture should be the stoichiometric air-fuel ratio, the routine proceeds to step 209, where the air-fuel ratio sensor 20
The feedback correction coefficient FAF is calculated based on the output signal of Then, it proceeds to step 213. On the other hand, when the correction coefficient K is not 1.0, the routine proceeds to step 210, where FA
F is fixed at 1.0, and then the process proceeds to step 213.
In step 213, the fuel injection time TAU is calculated based on the following equation.
Is calculated.

TAU = TP.K.FAF Next, at step 214, it is judged if the correction coefficient K is smaller than 1.0. When K <1.0, that is, when the lean air-fuel mixture should be burned, the routine proceeds to step 215, where the NOx absorption amount NOXA is calculated from FIG. 6 (A). Next, at step 216, the NOx release amount NOXD is made zero, and then the routine proceeds to step 219. On the other hand, when it is determined in step 214 that K ≧ 1.0,
That is, when the stoichiometric air-fuel ratio air-fuel mixture or rich air-fuel mixture should be burned, the routine proceeds to step 217, where NOx from FIG.
The release amount NOXD is calculated. Next, at step 218, the NOx absorption amount NOXA is made zero, and then at step 2
Proceed to 19. In step 219, NOx is calculated based on the following equation.
NOx amount ΣN estimated to be absorbed by the absorbent 18
OX is calculated.

ΣNOX = ΣNOX + NOXA-NOXD Next, at step 220, it is judged if ΣNOX becomes negative, and if ΣNOX <0, then step 2
In step 21, ΣNOX is made zero. Then step 2
At 22, it is determined whether or not ΣNOX exceeds the allowable maximum value MAX (FIG. 8). When ΣNOX> MAX, the routine proceeds to step 223, where the NOx releasing flag is set.

When the NOx release flag is set, in the next processing cycle, the routine proceeds from step 201 to step 202, where it is determined whether the current value I of the air-fuel ratio sensor 22 becomes smaller than the predetermined current value Is (FIG. 11). Is determined. When I ≧ Is, the routine proceeds to step 221, where the correction coefficient K is set to a constant value KK of about 1.1 to 1.2. Next, FAF is fixed to 1.0 in step 212, and then the process proceeds to step 213. At this time, the air-fuel ratio of the air-fuel mixture is switched from lean to rich, and the NOx absorbent 1
The release action of NOx from 8 is started.

Next, when I <Is, the routine proceeds from step 202 to step 203, where the current value I ₁ of the air-fuel ratio sensor 22 at the time of the previous interruption is changed to the current air-fuel ratio sensor 22.
Of the current value I by subtracting the current value I of
I (= I ₁ −I) is calculated. Then step 204
Then, it is determined whether or not the rate of change ΔI of the current value I has become larger than a predetermined rate of change X, that is, whether or not the current value I has reached the current value sudden change point P (FIGS. 10 and 11). To be done. When ΔI ≦ X, that is, when the current value I has not reached the current value sudden change point P, the routine proceeds to step 211, where the air-fuel ratio of the air-fuel mixture is maintained rich.

When ΔI> X, that is, when the current value I reaches the current value sudden change point P, step 204 to step 205
Then, the NOx releasing flag is reset. Next, at step 206, ΣNOX is made zero, and then the routine proceeds to step 207. Therefore, at this time, the air-fuel ratio of the air-fuel mixture is switched to the air-fuel ratio determined by the correction coefficient K, usually the lean air-fuel ratio.

16 and 17 show an injection control routine for executing the third embodiment, and this routine is executed by interruption at regular time intervals. 16 and 17, first, at step 300, the basic fuel injection time TP is calculated from the relationship shown in FIG. Next, at step 301, it is judged if the NOx releasing flag is set. When the NOx releasing flag is not set, the routine jumps to step 308 and the correction coefficient K is calculated based on FIG. Next, at step 309, it is judged if the correction coefficient K is 1.0. When K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture should be the stoichiometric air-fuel ratio, the routine proceeds to step 310, where the feedback correction coefficient FAF is calculated based on the output signal of the air-fuel ratio sensor 20. Then, it proceeds to step 314. On the other hand, when the correction coefficient K is not 1.0, step 311
Go to and fix FAF to 1.0, then step 3
Proceed to 14. In step 314, the fuel injection time TAU is calculated based on the following equation.

TAU = TP.K.FAF Next, at step 315, it is judged if the correction coefficient K is smaller than 1.0. When K <1.0, that is, when the lean air-fuel mixture is to be burned, the routine proceeds to step 316, where the NOx absorption amount NOXA is calculated from FIG. 6 (A). Next, at step 317, the NOx release amount NOXD is made zero, and then the routine proceeds to step 320. On the other hand, when it is determined in step 315 that K ≧ 1.0,
That is, when the stoichiometric air-fuel ratio air-fuel mixture or rich air-fuel mixture should be burned, the routine proceeds to step 318, where NOx is determined from FIG. 6 (B).
The release amount NOXD is calculated. Next, at step 319, the NOx absorption amount NOXA is made zero, and then step 3
Go to 20. In step 320, NOx is calculated based on the following equation.
NOx amount ΣN estimated to be absorbed by the absorbent 18
OX is calculated.

ΣNOX = ΣNOX + NOXA-NOXD Next, at step 321, it is judged if ΣNOX becomes negative. If ΣNOX <0, then step 3
Proceeding to 22, ΣNOX is made zero. Then step 3
At 23, it is determined whether or not ΣNOX exceeds the allowable maximum value MAX (FIG. 8). When ΣNOX> MAX, the routine proceeds to step 324, where the NOx releasing flag is set.

When the NOx releasing flag is set, in the next processing cycle, the routine proceeds from step 301 to step 302, where it is judged if the fixed time t ₂ shown in FIG. 11 has elapsed. When the constant time t ₂ has not elapsed, the routine proceeds to step 312, where the correction coefficient K is set to a constant value KK of about 1.1 to 1.2. Next, in step 313, FAF is fixed to 1.0, and then the process proceeds to step 314. At this time, the air-fuel ratio of the air-fuel mixture is switched from lean to rich, and the action of releasing NOx from the NOx absorbent 18 is started.

When a certain time t ₂ has passed, the routine proceeds from step 302 to step 303, and the current value I ₁ of the air-fuel ratio sensor 22 is subtracted from the current value I ₁ of the air-fuel ratio sensor 22 at the time of the previous interruption to obtain the current. The rate of change ΔI (= I ₁ −I) of the value I is calculated. Next, at step 304, the change rate Δ (ΔI) of the change rate of the current value I is calculated by subtracting the change rate ΔI ₁ of the current value I at the time of the previous interruption from the change rate ΔI of the current current value I. To be done. Next, at step 305, it is determined whether or not the rate of change Δ (ΔI) of the rate of change of the current value I has become larger than the predetermined rate of change Y of the rate of change, that is, the current value I has a sudden change point P of the current value.
It is determined whether or not (FIGS. 10 and 11) has been reached.
When Δ (ΔI) ≦ Y, that is, the current value I is the current value sudden change point P
If not, the routine proceeds to step 312, where the air-fuel ratio of the air-fuel mixture is maintained rich.

When Δ (ΔI)> Y, that is, when the current value I reaches the current value sudden change point P, the routine proceeds from step 305 to step 306, and the NOx releasing flag is reset. Next, at step 307, ΣNOX is made zero, and then the routine proceeds to step 308. Therefore, at this time, the air-fuel ratio of the air-fuel mixture is switched to the air-fuel ratio determined by the correction coefficient K, usually the lean air-fuel ratio.

[0059]

Industrial Applicability The completion of the action of releasing NOx from the NOx absorbent can be accurately detected.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a map of a basic fuel injection time.

FIG. 3 is a diagram illustrating a correction coefficient K;

FIG. 4 Unburned HC and C in exhaust gas discharged from the engine
It is a diagram which shows the concentration of O and oxygen roughly.

FIG. 5 is a diagram for explaining a NOx absorption / release effect.

FIG. 6 NOx absorption amount NOXA and NOx release amount NO
It is a figure which shows XD.

FIG. 7 is a diagram showing a NOx release amount NOXD.

FIG. 8 is a time chart of air-fuel ratio control.

FIG. 9 is a diagram showing a current value flowing between an anode and a cathode of an O ₂ sensor.

FIG. 10 is a diagram showing changes in the current value flowing between the anode and the cathode of the O ₂ sensor.

FIG. 11 is a diagram showing a change in a value of a current flowing between an anode and a cathode of an O ₂ sensor.

FIG. 12 is a flowchart showing a first embodiment for controlling fuel injection.

FIG. 13 is a flowchart showing a first embodiment for controlling fuel injection.

FIG. 14 is a flowchart showing a second embodiment for controlling fuel injection.

FIG. 15 is a flowchart showing a second embodiment for controlling fuel injection.

FIG. 16 is a flowchart showing a third embodiment for controlling fuel injection.

FIG. 17 is a flowchart showing a third embodiment for controlling fuel injection.

[Explanation of Codes] 11 ... Fuel injection valve 15 ... Exhaust manifold 18 ... NOx absorbent 20, 22 ... Air-fuel ratio sensor

Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location F02D 41/04 305 F02D 41/04 305Z (72) Inventor Satoshi Iguchi 1 Toyota-cho, Toyota-shi, Aichi Toyota Motor Vehicle Within the corporation

Claims (3)

[Claims] A NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich is arranged in the engine exhaust passage. , An air-fuel ratio sensor that produces an output at a level proportional to the air-fuel ratio of the exhaust gas is placed in the exhaust passage downstream of the NOx absorbent, and the exhaust gas that flows into the NOx absorbent in order to release NOx from the NOx absorbent is exhausted. The change rate of the output level of the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent changes from lean to rich after the fuel ratio has been changed from lean to rich has exceeded a predetermined change rate. Sometimes NO from NOx absorbent
x An exhaust emission control device for an internal combustion engine, comprising a determination means for determining that the release action has been completed. 2. An NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich is arranged in the engine exhaust passage. , An air-fuel ratio sensor that produces an output at a level proportional to the air-fuel ratio of the exhaust gas is placed in the exhaust passage downstream of the NOx absorbent, and the exhaust gas that flows into the NOx absorbent in order to release NOx from the NOx absorbent is exhausted. The rate of change of the output level change rate of the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing out of the NOx absorbent changes from lean to rich after the fuel ratio is changed from lean to rich NOx when the rate of change is exceeded
An exhaust emission control device for an internal combustion engine, comprising a determination means for determining that the action of releasing NOx from the absorbent has been completed. 3. The air-fuel ratio of the exhaust gas flowing into the NOx absorbent is switched from rich to lean when it is judged by the judging means that the NOx releasing operation from the NOx absorbent has been completed. An exhaust gas purification device for an internal combustion engine as described.