JP3186393B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

Absorbs NO _x when the air-fuel ratio is lean of the Related Art inflowing exhaust gas, the engine of _the NO _x absorbent air-fuel ratio of the exhaust gas to release the NO _x absorbed when the stoichiometric air-fuel ratio or rich flowing was disposed in the exhaust passage, was estimated with estimating the amount of NO _x absorbed in _the NO _x absorbent N
O _x amount to release NO _x fuel ratio of the exhaust gas flowing into _the NO _x absorbent from _the NO _x absorbent is changed over temporarily to the stoichiometric air-fuel ratio or from lean to rich when it exceeds the allowable value,
The applicant has already proposed an internal combustion engine in which the air-fuel ratio of the exhaust gas flowing into the NO _x absorbent is returned to lean again (International Application No. PCT / JP93 / 0077).
No. 8).

[0003]

However, the fuel and the machine
Seki's lubricating oil contains sulfur, which is
SO_xNO in the form of_xWith NO_xAbsorbed by absorbent.
However, this SO_xIs NO_xIs the temperature of the absorbent high?
One NO_xExhaust gas entering the absorbent is not enriched
And NO_xNot released from the absorbent and therefore NO_xAbsorbent or
NO_xNO to release_xExhaust gas flowing into the absorbent
NO at this time even if the air-fuel ratio of _xAbsorbent temperature
NO if not very high_xOnly released S
O_xWill not be released. Therefore SO_xIs NO_x
SOx gradually accumulates in the absorbent_xWhen the accumulation amount of
NO_xAbsorbent NO_xAbsorption capacity is reduced
Will be.

[0004] mixed for the time required to release the NO _x absorbed in this case consider the case where the air-fuel ratio of for example mixture in order to release _the NO _x absorbent from the NO _x rich You need to be rich. Therefore NO if a small amount of NO _x absorbed in the _x absorbent is the time for the mixture rich must shorter there is actually a limit to say that shorter, thus being absorbed fuel consumption is increased to when the air-fuel mixture rich must longer than necessary rich time when _the amount of nO _x is low. Thus _the NO _x releasing action from _the NO _x absorbent will be preferably performed when the certain amount or more of the NO _x is absorbed. Absorbing NO _x tolerance for the amount of some extent or more NO for performing releasing action of the NO _x in the above-mentioned internal combustion engine
_x amount is set, and thus the above-mentioned internal combustion engine is preferable in this regard.

However, in the above-mentioned internal combustion engine, NO_xSucking
NO for collector_xThe allowable value is determined in consideration of the absorption capacity.
Absent. Therefore NO_xAbsorbent NO_xAbsorption capacity decreased
NO in case_xNO even if the absorption capacity of_xRelease work
Is not performed and thus NO _xCan not be absorbed by absorbent
No_xIs released into the atmosphere. Ma
NO_xIn addition to the judgment that the absorption amount has exceeded the allowable value,
NO when all other predetermined conditions are satisfied_xTo
N to make the air-fuel ratio of the air-fuel mixture rich to release
O_xNO even in the case of a release system_xAbsorbent NO
_xWhen the allowable value is not determined considering the absorption capacity
Is actually NO_xAbsorption is saturated and all other
NO even though the condition is satisfied_xAbsorption is allowed
NO because it is determined that the value has not been exceeded_xRelease work
Is not performed and thus NO_xCan not be absorbed by absorbent
No_xIs released into the atmosphere.

[0006]

According to the present invention, when the air-fuel ratio of the inflowing exhaust gas is lean, NO _x is absorbed, and the air-fuel ratio of the inflowing exhaust gas becomes stoichiometric. in fuel ratio or an internal combustion engine arranged to _the NO _x absorbent to release the absorbed NO _x in the engine exhaust passage when the rich, and _the amount of NO _x estimating means for estimating the amount of NO _x absorbed in _the NO _x absorbent, NO air-fuel ratio to allow temporarily switched that the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent from lean to the stoichiometric air-fuel ratio or rich when the amount of NO _x estimated by _x amount estimating means exceeds the allowable value a switching換許friendly means, gradually lower the absorption of the SO _x in the exhaust gas
NO _x to estimate the absorption of NO _x capacity of the NO _x absorbent to lower
Comprising an absorbent capacity estimating unit, absorption of NO _x capacity is estimated is adapted to reduce the allowable value enough to reduce the absorption of NO _x capacity estimation means.

[0007]

[Action] NO_xAbsorbent NO_xAs the absorption capacity decreases
NO because tolerance is reduced _xRegardless of absorption capacity
NO_xNO before the absorption capacity is saturated_xTo release
Is issued.

[0008]

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 a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel into the intake port 6 is attached to each branch pipe 9. 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 NO _x absorbent 18.

The electronic control unit 30 is composed of a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and an input port 35 interconnected by a bidirectional bus 31. And an output port 36. A pressure sensor 19 that generates an output voltage proportional to the absolute pressure in the surge tank 10 is disposed in the surge tank 10, and the output voltage of the pressure sensor 19 is input to an input port 35 via an AD converter 37. . A throttle switch 20 that is turned on when the throttle opening reaches an idling opening is attached to the throttle valve 14, and an output signal of the throttle switch 20 is input to an input port 35. An air-fuel ratio sensor 21 is disposed in the exhaust manifold 15, and an output voltage of the air-fuel ratio sensor 21 is input to an input port 35 via an AD converter 38. The input port 35 is connected to a rotation speed sensor 22 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 is connected to the ignition plug 4 and the fuel injection valve 11 via a corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = f · TP · K · FAF where f is a coefficient, TP is a basic fuel injection time, K is a correction coefficient, and FAF is a feedback correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. The basic fuel injection time TP is obtained by an experiment in advance, and is stored in advance in the ROM 32 as a function of the absolute pressure PM in the surge tank 10 and the engine speed N in the form of a map as shown in FIG. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, K <
At 1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and when K> 1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes empty. The fuel ratio 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 to the engine cylinder is to be the stoichiometric air-fuel ratio, a coefficient for making the air-fuel ratio accurately match the stoichiometric air-fuel ratio based on the output signal of the air-fuel ratio sensor 21 It is. This feedback correction coefficient FAF
Moves up and down about 1.0, and this FAF
Decreases when the mixture becomes rich, and increases when the mixture becomes lean. When K <1.0 or K> 1.0, the FAF is fixed at 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 according to the operating state of the engine, and is basically shown in FIG. 3 in the embodiment according to the present invention. As described above, 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 in the low-load operation region on the lower load side than the solid line R, that is, the air-fuel mixture is lean, 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 concentration 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 increases, and the concentration of the exhaust gas from the combustion chamber 3 increases. The concentration of oxygen O ₂ in the exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NO contained in casing 17_x
The absorbent 18 uses, for example, alumina as a carrier, and on this carrier,
For example, potassium K, sodium Na, lithium Li,
Alkali metals such as Cs, barium Ba, cal
Alkaline earths such as calcium Ca, lanthanum La,
At least one selected from rare earths such as thorium Y
And a noble metal such as platinum Pt. organ
Intake passage and NO_xProvided in the exhaust passage upstream of the absorbent 18
The ratio of supplied air and fuel (hydrocarbon) is set to NO _xabsorption
When this is referred to as the air-fuel ratio of the exhaust gas flowing into the agent 18, this NO_x
When the air-fuel ratio of the inflowing exhaust gas is lean, the absorbent 18
NO_xAnd reduce the oxygen concentration in the incoming exhaust gas
And absorbed NO_xReleases NO_xPerform the absorption and release action of
U. Note that NO_xFuel in the exhaust passage upstream of the absorbent 18
(Hydrocarbons) or inflow and exhaust when air is not supplied
The air-fuel ratio of the gas-gas is the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3.
Ratio, and in this case NO_xAbsorbent 18 burns
When the air-fuel ratio of the mixture supplied to the firing chamber 3 is lean
Is NO_xIn the air-fuel mixture supplied into the combustion chamber 3
NO absorbed when oxygen concentration decreases_xTo release
Become.

If the above-described NO _x absorbent 18 is arranged in the engine exhaust passage, the NO _x absorbent 18 actually performs the absorption and release of NO _x , but the detailed mechanism of the absorption and release is not clear. There is also. 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 the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
These oxygen O ₂ as shown in (A) is O ₂ ^- or O
It adheres to the surface of platinum Pt in the form of ^2- . On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the produced NO ₂ is absorbed in the absorbent while being oxidized on the platinum Pt, and is absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. Diffuses into agent. In this way, NO _x is absorbed in the NO _x absorbent 18.

As long as the oxygen concentration in the incoming exhaust gas is high, platinum
NO on Pt surface_TwoIs generated, and the NO_xAbsorption capacity
NO unless power is saturated_TwoIs absorbed in the absorbent and nitric acid
Ion NO_Three ^-Is generated. In contrast, the inflow exhaust gas
NO in the oxygen concentration in the_TwoWhen the amount of
The reaction is reversed (NO_Three ^-→ NO_Two) And thus suck
Nitrate ion NO in the collector_Three ^-Is NO_TwoFrom the absorbent in the form of
Released. That is, the oxygen concentration in the inflow exhaust gas decreases.
NO_xNO from absorbent 18_xWill be released
You. As shown in FIG. 4, the degree of lean of the incoming exhaust gas
Lower the oxygen concentration in the incoming exhaust gas
If the degree of leanness of the inflow exhaust gas is reduced,
NO even if the air-fuel ratio of the exhaust gas is lean_xAbsorbent 18
From NO _xWill be released.

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, as shown in FIG.
C and CO are discharged, and these unburned HC and CO are converted to platinum Pt.
It reacts with the above oxygen O ₂ ^- or O ^2- to be oxidized.
Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas extremely decreases.
O ₂ is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 5 (B). In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. Therefore NO _x from the NO _x absorbent 18 in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich, that is released.

That is, the air-fuel ratio of the inflowing exhaust gas is made rich.
First, unburned HC and CO are converted to O on platinum Pt._Two ^-or
Is O^2-And immediately oxidized, then platinum
O on Pt_Two ^-Or O^2-Is consumed but unburned HC,
If CO remains, this unburned HC and CO will absorb
NO released from_xAnd NO released from the engine _x
Is reduced. Therefore, the air-fuel ratio of the inflow exhaust gas is reset.
NO in a short time_xAbsorbed by absorbent 18
NO_xIs released, and the released NO
_xNO in the atmosphere due to reduction_xIs discharged
It can be blocked. NO_xAbsorbent
18 has a function of a reduction catalyst, so that
NO even if the air-fuel ratio is stoichiometric_xReleased from absorbent 18
NO_xIs reduced. However, inflow and exhaust
NO if the air-fuel ratio of the gas is the stoichiometric air-fuel ratio_xabsorption
NO from agent 18_xIs released only gradually
_xTotal NO absorbed in absorbent 18_xTo release
Takes a little longer.

When the lean air-fuel mixture is burned as described above, NO _x is absorbed by the NO _x absorbent 18. However there is a limit to the absorption of NO _x capacity of the NO _x absorbent 18, _the NO _x absorbent 18 when saturation absorption of NO _x capacity of the NO _x absorbent 18 is not E longer absorb NO _x.
Therefore NO absorption _of NO _x capacity of the _x absorbent 18 must be released the NO _x from the NO _x absorbent 18 before the saturation, how much to _the NO _x absorbent 18 to the NO _x
Needs to be estimated. Then this N
A method for estimating the O _x absorption amount will be described.

[0021] is _the amount of NO _x absorbed per unit time per _the NO _x absorbent 18 to _the amount of NO _x discharged from the higher unit time per engine becomes higher the engine load increases when the lean air-fuel mixture is burned Increases and the higher the engine speed, the more NO is discharged from the engine per unit time.
NO _{x to x} amount is absorbed per unit time _the NO _x absorbent 18 in order to increase is increased. Therefore, NO per unit time
amount of NO _x absorbed in the _x absorbent 18 becomes a function of the engine load and the engine speed. In this case, the engine load is absolute pressure PM and the engine speed N of the absolute amount of NO _x that have at pressure representative absorbed in unit time per _the NO _x absorbent 18 since it is the surge tank 10 in the surge tank 10 Is a function of Therefore, in the embodiment according to the present invention, NO _x per unit time
Absolute pressure PM of the amount of NO _x NOXA that is absorbed by the absorbent 18
And as a function of the engine speed N obtained by experiments in advance,
Figure this amount of NO _x NOXA as a function of PM and N 6
It is stored in the ROM 32 in advance in the form of a map shown in FIG.

On the other hand, although the air-fuel ratio of the mixture fed into the engine cylinder NO _x is released from the NO _x absorbent 18 becomes the stoichiometric air-fuel ratio or rich _the NO _x releasing amount at this time primarily exhaust gas Affected by volume and air-fuel ratio. That is, NO of _the amount of NO _x amount exhaust gas is discharged from the higher per unit time _the NO _x absorbent 18 increases increases, the air-fuel ratio is discharged from the higher per unit time _the NO _x absorbent 18 becomes rich
_x amount increases. In this case, the amount of exhaust gas, that is, the amount of intake air, can be represented by the product of the engine speed N and the absolute pressure PM in the surge tank 10, and therefore, as shown in FIG. _the amount of NO _x NOXD released from _the NO _x absorbent 18 increases as N · PM increases. Further, since the air-fuel ratio corresponds to a value of the correction coefficient K NO _x amount NOXD released from per unit time _the NO _x absorbent 18 as shown in FIG. 6 (C) increases as the value of K increases I do. _The amount of NO _x NOXD released from the unit time per _the NO _x absorbent 18 in advance in the form of a map shown in FIG. 7 (A) as a function of N · PM and K ROM 3
2 is stored.

Further, _the NO _x absorbent 18 nitrate ions NO ₃ and the absorbent temperature is high in ^- that _the NO _x releasing rate from _the NO _x absorbent 18 is increased so easily decomposed. In this case, since the temperature of the NO _x absorbent 18 is substantially proportional to the exhaust gas, the NO _x release rate Kf as shown in FIG.
Increases as the exhaust gas temperature T increases. Therefore NO
_{When the x} release rate Kf is taken into consideration, NO per unit time
amount of NO _x released from the _x absorbent 18 will be represented by the product of the NOXD and _the NO _x releasing factor Kf shown in FIG. 7 (A). In the embodiment according to the present invention, the exhaust gas temperature T is determined in advance as a function of the absolute pressure PM in the surge tank 10 and the engine speed N in the form of a map shown in FIG.
It is stored in M32.

As described above, the lean mixture burns.
NO per unit time _xNOXA absorption
And a mixture of stoichiometric air-fuel ratio or rich mixture
NO per unit time when burned_xThe amount released
NO because it is expressed by Kf · NOXD_xAbsorbed by absorbent 18
NO presumed to be collected_xThe quantity ΣNOX is expressed by the following equation.
Will be forgotten.

ΣNOX = ΣNOX + NOXA−Kf · NOXD As described above, in the embodiment according to the present invention, basically, FIG.
, The value of the correction coefficient K distinguished by the solid lines R and S
The air-fuel ratio is controlled in accordance with Therefore, from the solid line R in FIG.
The lean air-fuel mixture (K <1.0)
NO because it can be burned_xIs NO_xAbsorbed by absorbent 18
In a region on the higher load side than the solid line R in FIG.
(K = 1.0) or a rich mixture (K> 1.
0) is burned, so NO from the NO absorbent 18_x
Will be released. Therefore, the solid line R in FIG.
NO if low load operation and high load operation are alternately repeated_x
NO of absorbent 18_xThe absorption capacity will not be saturated,
In practice, there are many opportunities for low load operation, and therefore NO_xSucking
NO for collector 18_xThe absorption capacity will be saturated
You. Therefore, in this case, NO_xNO of absorbent 18_xabsorption
NO before capacity saturates _xNO from absorbent 18_xRelease
It is necessary to make it.

However, the exhaust gas contains SO _x , and the NO _x absorbent 18 contains not only NO _x but also SO _x
Is also absorbed. Absorption mechanism of the SO _x to the _the NO _x absorbent 18 is considered to be the same as the mechanism of absorption of NO _x. That is, assuming that platinum Pt and barium Ba are supported on the carrier as in the case of describing the NO _x absorption mechanism, as an example, when the air-fuel ratio of the inflowing exhaust gas is lean as described above, oxygen O ₂ Is O ₂
^- is attached to the surface of the platinum Pt in the form of inflow exhaust SO ₂ in the gas is O ₂ on the surface of the platinum Pt ^- reacts with SO ₃
Becomes Then the SO ₃ generated while bonding with the barium oxide BaO is absorbed into the absorbent while being further oxidized on the platinum Pt, is diffused in the absorbent in the sulfate ions SO ₄ ^2-form. Next, this sulfate ion SO ₄ ^2- is combined with barium ion Ba ²⁺ to form sulfate BaSO ₄ . As the amount of sulfate BaSO ₄ in the absorbent increases, N
The NO _x absorption capacity of the O _x absorbent 18 decreases. Therefore N
Before the NO _x absorption capacity of the O _x absorbent 18 is saturated, the NO _x
To release NO _x from the absorbent 18 needs to estimate S O _x absorption of the NO _x absorbent 18.

By the way _the amount of SO _x contained in exhaust gas discharged from the engine per unit time is increased becomes accompanied therewith when increased fuel injection amount TAU, increased becomes accompanied therewith higher the engine speed N I do. Therefore, the NO _x absorbent 18
The absorbed SO _x amount SOXA absorbed per unit time is expressed by the following equation. _{SOXA = k 1 · TAU · N} (k 1 is a constant) On the other hand, _the NO _x absorbent 18 fixed temperature T ₀ of the temperature determined by _the NO _x absorbent 18 becomes the decomposed sulfate BaSO ₄ higher than, for example, 500 ° C. , NO _x absorbent 1 this time
Air-fuel ratio of the exhaust gas flowing into 8 becomes rich when NO _x
SO _x is released from the absorbent 18. In this case, when the degree of the richness of the air-fuel ratio of the exhaust gas is kept constant, the SO _x release rate f (T) increases as the temperature T of the NO _x absorbent 18 increases, as shown in FIG. And the SO _x release rate f when the temperature of the NO _x absorbent 18 is kept constant.
(K) increases as the correction coefficient K increases, that is, as the air-fuel ratio of the exhaust gas becomes richer. Thus released per unit time from _the NO _x absorbent 18 SO _x release amount SOX
D is represented by the following equation.

SOXD = k ₂ · f (T) · f ( K ) Accordingly, the SO _x amount ΣSOX estimated to be absorbed by the NO _x absorbent 18 is expressed by the following equation. ΣSOX = ΣSOX + SOXA-SOXD Incidentally N of the NO _x absorbent 18 in this embodiment of the present invention
Before the O _x absorption capacity is saturated, the NO _x absorbent 18
NO _x when the of the NO _x absorption capacity has exceeded the allowable value MAX
Release action is performed. _The NO _x absorbent 18 in the SO _x is _the NO _x absorbent 18 when _the NO _x absorbent 18 of the NO _x absorbing ability when the MAX ₀ SO _x amount ΣSOX of the SO _x absorption when not at all absorbed The NO _x absorption capacity is (MAX ₀ −ΣSOX). Absorption of NO _x capacity of the NO _x absorbent 18 in this embodiment of the present invention is absorption of NO _x capacity (MAX ₀ -ΣSOX) by a predetermined value α than low NO _x
It is defined that the absorption value is the allowable value MAX, and therefore the allowable value MAX is represented by the following equation.

[0029] MAX = MAX ₀ -ΣSOX-α Figure 9 shows the relationship between the change and _the NO _x releasing action of the tolerance MAX. As shown in FIG. 9, when the lean mixture (K <1.0) is being burned, NO _x
NO _x amount ΣN estimated to be absorbed by the absorbent 18
OX and SO _x amount {SOX rises together, SO _x amount _}
As the SOX increases, the allowable value MAX is decreased. When the NO _x amount ΣNOX exceeds the allowable value MAX, the air-fuel mixture is temporarily made rich (K> 1.0).
The NO _x amount ΣNO _x sharply decreases due to the release of O _x . On the other hand, at this time, the temperature T of the NO _x absorbent 18, that is, the temperature T of the exhaust gas does not increase so much (T
<T ₀ ) No release action of SOX is performed. Temperature T of then for example the engine full load operation (K> 1.0) is performed _the NO _x absorbent 18, that is, the exhaust gas temperature T rises (T> T
₀ ) The release action of SO _x is performed together with the release action of NO _x , and thus the SO _x amount ΣSOX decreases rapidly.

FIGS. 10 to 12 show a routine for controlling the air-fuel ratio. This routine is executed by interruption every predetermined time. Referring to FIGS. 10 to 12, first, at step 100, the basic fuel injection time TP is calculated from the map shown in FIG. Then whether _the amount of NO _x ΣNOX is estimated to be absorbed in _the NO _x absorbent 18 at step 101 is greater than the allowable value MAX or not. When the .SIGMA.NOX ≦ MAX whether _the NO _x releasing flag showing that should be released NO _x proceeds to step 103 has been set or not. If the NO _x release flag has not been set, the routine proceeds to step 104, where the correction coefficient K is calculated from the relationship shown in FIG.

Next, at step 105, it is determined whether or not the correction coefficient K is smaller than 1.0. When K <1.0, i.e. absorption of NO _x amount NOXA per unit time from the map shown in FIG. 6 (A) proceeds to step 106 when the operating state to combust a lean air-fuel mixture is calculated. Then _the NO _x releasing amount NOXD step 107 is made zero, then the feedback correction coefficient FAF in step 108 is fixed to 1.0. Then step 11
At 7, the fuel injection time TAU is calculated based on the following equation.

[0032] TAU = f · TP · K · FAF Meanwhile, willing FIG 7 (A) from the map shown in per unit time _the NO _x releasing in step 112 when it is judged that K ≧ 1.0 at step 105 The quantity NOXD is calculated. Then _the NO _x releasing factor Kf is calculated from the map shown in relation to FIG. 7 (C) shown in step 113 FIG. 7 (B), the then N per unit step 114 Time
O _x absorption amount NOXA is made zero. Then step 11
In 5, it is determined whether the correction coefficient K is larger than 1.0.
BR> When K> 1.0, that is, when the operation state is such that the rich air-fuel mixture is to be burned, the process proceeds to step 117 via step 108.

On the other hand, when K = 1.0, that is, when the stoichiometric air-fuel mixture is to be burned, the routine proceeds to step 116, where the feedback correction coefficient FAF is calculated based on the output signal of the air-fuel ratio sensor 21. Proceed to step 117. In step 116, when the air-fuel ratio sensor 21 detects that the air-fuel ratio has become rich, the FAF is decreased, and when it is detected that the air-fuel ratio has become lean, the FAF is increased. The air-fuel ratio will be maintained.

[0034] Based on the step 118 following equation continues to step 117 is absorbed in _the NO _x absorbent 18 NO _x
The quantity ΣNOX is calculated. ΣNOX = ΣNOX + NOXA−Kf · NOXD Next, at step 119, it is determined whether or not the NO _x amount ΣNOX has become negative. In step 121, it is determined whether the correction coefficient K is larger than 1.0. When K> 1.0, that is, when the rich air-fuel mixture is to be burned, the routine proceeds to step 122, where the exhaust gas temperature T obtained from the map shown in FIG. 7C is a constant value T ₀ (FIG. 8A). Is determined. When T> T ₀ , step 123
Then, f (T) is calculated from the relationship shown in FIG. 8A, and then the process proceeds to step 124, where f (K) is calculated from the relationship shown in FIG. 8B. Next, at step 125, the SO _x release amount SOXD per unit time (= k ₂ · f)
(T) · f (K)) is calculated, and then step 126
In this case, the SO _x absorption amount SOXA is set to zero. Next, the routine proceeds to step 129.

On the other hand, in step 121, K ≦ 1.0
Is determined, or in step 122, T ≦ T
When it is determined to be ₀ , the process proceeds to step 127, where the SO _x absorption amount SOXA (= k ₁ · TA) absorbed per unit time.
U · N) is calculated. Release of SO _x amount SOXD in step 128 and then is made zero, then step 12
Go to 9. In step 129, the SO _x amount い_て is calculated based on the following equation.
SOX is calculated.

ΣSOX = ΣSOX + SOXA-SOXD Next, at step 130, it is determined whether or not the SO _x amount ΣSOX has become negative.
Go to 32. In step 132, the allowable value M is calculated based on the following equation.
AX is calculated.

Meanwhile MAX = MAX ₀ -ΣSOX-α, when it is judged that .SIGMA.NOX> MAX in step 101 is set _the NO _x releasing flag proceeds to step 102. Next, the routine proceeds from step 103 to step 109, where the correction coefficient K is set to a constant value KK. This constant value KK is a value of about 1.1 to 1.3 at which the air-fuel ratio of the mixture becomes a rich mixture of about 11 to 13. Then whether step 110 NO _x amount ΣNOX has become zero or negative is determined. When ΣNOX> 0, the routine jumps to step 112. On the other hand, ΣN
When OX ≦ 0, the routine proceeds to step 111, where the NO _x release flag is reset, and then proceeds to step 112. Therefore, when ΣNOX> MAX, the mixture is made rich (K = KK) until ΣNOX ≦ 0.

It should be noted that in the embodiment described so far, ΣNOX
> So that it is to perform the action of releasing immediately NO _x When a MAX issue switching換許friendly fuel ratio for _the NO _x releasing when all met becomes and other predetermined conditions .SIGMA.NOX> MAX It may be.

[0039]

Issues a switching換許friendly fuel ratio to release the NO _x from the NO _x absorbent before _the NO _x absorbent is saturated even when the absorption of NO _x capacity of the NO _x absorbent according to the present invention was reduced be able to.

[Brief description of the 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 showing a correction coefficient K;

FIG. 4 shows unburned HC and C in exhaust gas discharged from the engine.
FIG. 3 is a diagram schematically showing the concentrations of O and oxygen.

FIG. 5 is a diagram for explaining a NO _x absorption / release effect.

6 is a diagram showing the absorption of NO _x amount NOXA like.

7 is a diagram showing the absorption of NO _x amount NOXD like.

FIG. 8 is a diagram showing an SO _x release rate.

FIG. 9 is a time chart of air-fuel ratio control.

FIG. 10 is a flowchart showing air-fuel ratio control.

FIG. 11 is a flowchart showing air-fuel ratio control.

FIG. 12 is a flowchart showing air-fuel ratio control.

[Explanation of symbols]

16 ... exhaust pipe 18 ... NO _x absorbent

────────────────────────────────────────────────── (5) References JP-A-5-302508 (JP, A) WO 93/25806 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) F01N 3/08 F02D 41/02 330 F01N 3/24

Claims (1)

(57) [Claims] 1. A absorbs NO _x when the air-fuel ratio of the inflowing exhaust gas is lean, the engine and _the NO _x absorbent air-fuel ratio of the exhaust gas to release the NO _x absorbed when the stoichiometric air-fuel ratio or rich flowing in an internal combustion engine disposed in the exhaust passage, and _the amount of NO _x estimating means for estimating the amount of NO _x absorbed in _the NO _x absorbent, _the amount of NO _x estimated by the amount of NO _x estimating means exceeds the allowable value to decrease gradually and the air-fuel ratio switching換許friendly means for permitting the temporarily switched that the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent from lean to stoichiometric or rich, the absorption of the sO _x in the exhaust gas when
Comprising the absorption of NO _x capacity estimation means for estimating the NO _x absorption of NO _x capacity of the absorbent that, as the absorption of NO _x capacity estimated by the absorption of NO _x capacity estimation means to reduce the allowable value enough to reduce Exhaust purification device for an internal combustion engine.