JP3144183B2 - 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]

2. Description of the Related Art When the air-fuel ratio of inflowing exhaust gas is lean,
NO_xAnd the air-fuel ratio of the exhaust gas
NO absorbed at stoichiometric air-fuel ratio or rich_xEmit
NO _xThe absorbent is placed in the engine exhaust passage, and NO_xabsorption
NO from agent_xTo release NO_xFlow into absorbent
The air-fuel ratio of exhaust gas
Switch for a predetermined period of time, then NO_xabsorption
Return the air-fuel ratio of the exhaust gas flowing into the agent to lean again
Internal combustion engine has already been proposed by the applicant
(See International Publication WO93 / 07363).

[0003]

SUMMARY OF THE INVENTION In this internal combustion engine,
NO_xRelease action is basically NO_xTo some extent in the absorbent
NO_xNO when is absorbed_xAbsorbed by the absorbent
NO_xBased on the idea of releasing all
Therefore, in this internal combustion engine, NO _xabsorption
NO from agent_xTo release NO_xAbsorbed by absorbent
All NO _xN for the time required to release
O_xDetermine the air-fuel ratio of the exhaust gas flowing into the absorbent from lean
The air-fuel ratio or rich is switched.

[0004] However, NO_xNO from absorbent_xRelease
NO when you should_xTotal NO from absorbent_xDo not release
NO if at least partially released_xAbsorbent is good again
NO _xCan be absorbed, and NO_xAbsorbent
NO because it also has a three-way catalyst function_xAll N from absorbent
O_xThe air-fuel ratio is maintained at the stoichiometric air-fuel ratio continuously after is released
NO_xIs NO_xPurify with absorbent
Can be Also, if the mixture is somewhat rich
Almost NO from institutions_xNO because NO is not discharged_xabsorption
Total NO from agent_xKeeps the air-fuel ratio rich after
Almost NO_xWill not be released to the atmosphere
No. NO_xNO_xReleased from absorbent
NO when to do_xTotal NO from absorbent_xTo release
The air-fuel ratio to the stoichiometric air-fuel ratio or rich only for the time required for
Is no longer necessary and therefore NO absorption
NO from collector_xA different perspective on how to release
Will be able to think of another way.

[0005] The present invention is to provide the above mentioned exhaust gas purification apparatus so as to release the NO _x from the NO _x absorbent based on fundamentally different concept from the method employed by the internal combustion engine.

[0006]

That is, according to the present invention, NO _x is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and is absorbed when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio or rich. the _the NO _x absorbent to release the NO _x arranged in the engine exhaust passage, the engine the NO _x discharged from the engine burned lean mixture when the operating condition is a lean combustion region set in advance _the NO _x absorbent allowed absorption, the engine operating state is absorbed burned air-fuel mixture or a rich mixture of the theoretical air-fuel ratio in _the NO _x absorbent when the predetermined stoichiometric air-fuel ratio or a rich burn zone NO to
In an internal combustion engine which is adapted to release _x, and means for estimating the amount of NO _x is absorbed in the NO _x absorbent, NO
and comprising an area changing means for broadening the stoichiometric air-fuel ratio or rich combustion region with narrowing the lean combustion region when the amount of NO _x estimated to be absorbed in the _x absorbent exceeds a predetermined amount.

Furthermore have lean combustion region mentioned above, according to the present invention is defined in the lower load side than the stoichiometric air-fuel ratio or rich burn zone, NO _x amount estimated to be absorbed in _the NO _x absorbent When the amount exceeds a predetermined amount, the above-described region changing means narrows the lean combustion region to a lower load side and widens the stoichiometric air-fuel ratio or the rich combustion region to a lower load side.

Furthermore when the air-fuel ratio is lean of the exhaust gas flowing in accordance with the present invention absorbs the NO _x, when the air-fuel ratio of the inflowing exhaust gas of the stoichiometric air-fuel ratio or rich to release the absorbed NO _x NO _x the absorbent was disposed engine exhaust passage, when the lean combustion region the engine operating condition is a predetermined allowed absorb the NO _x discharged from the engine burned lean mixture in _the NO _x absorbent, the engine operating condition in There internal combustion engine so as to release the NO _x absorbed in _the NO _x absorbent by burned air-fuel mixture or a rich mixture of the theoretical air-fuel ratio to the stoichiometric air-fuel ratio or rich combustion region predetermined, NO lean combustion region when it exceeds a means for estimating the amount of NO _x absorbed in the _x absorbent, the amount of _the amount of NO _x is a predetermined estimated to be absorbed in _the NO _x absorbent And a region changing means for widening the stoichiometric air-fuel ratio or the rich combustion region and making the entire combustion region a stoichiometric air-fuel ratio or the rich combustion region.

[0009]

[Action] stoichiometric or rich combustion region with lean burn region is narrowed when it exceeds the amount of amount of NO _x estimated to be absorbed in _the NO _x absorbent in the first aspect is predetermined is NO with so widened opportunity _the NO _x absorbent from the NO _x is released it is made to increase
_The opportunity for NO _{x to} be absorbed by the _x absorbent is reduced.

In the second invention, the lean combustion region is theoretically
Set at a lower load than the air-fuel ratio or rich combustion area
And NO_xNO presumed to be absorbed by the absorbent
_xWhen the amount exceeds a predetermined amount, the lean burn area
The range is narrowed to a lower load side and the stoichiometric air-fuel ratio or
The combustion area of the switch is expanded to the low load side. Stoichiometric air-fuel ratio or
NO when the rich combustion region is expanded to the low load side_xAbsorbent
From NO_xIncreased the chances of release
NO emitted from the engine as the engine load decreases _xAbsolute
Lean combustion area narrows to lower load side due to lower volume
NO _xNO in absorbent_xOpportunities to be absorbed
NO with less_xNO in absorbent_xAs absorbed
The amount is also extremely small.

[0011] is stoichiometric or rich burn zone with eliminating lean burn area spread when the amount of NO _x in the third invention is estimated to be absorbed in _the NO _x absorbent exceeds a predetermined amount is _the NO _x absorbent from the NO _x in the entire range of operation since the entire combustion zone is the stoichiometric air-fuel ratio or rich combustion region is released.

[0012]

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 is connected to a read only memory (ROM) 32, a random access memory (RAM) 33, a CPU (microprocessor) 34, and an input port 35 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 constant, 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-mentioned NO _x absorbent 18 is disposed 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 inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt, and as long as the NO _x absorption capacity of the absorbent is not saturated, NO ₂ is absorbed in the absorbent and nitrate ions NO ₃ ⁻ Is generated. On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the production amount of NO ₂ decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrate ion NO ₃ ⁻ There are released from the absorbent in the form of NO _2. Namely, when the oxygen concentration in the inflowing exhaust gas is released NO _x from the NO _x absorbent 18 when lowered. As shown in FIG. 4, if the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, if the degree of leanness of the inflowing exhaust gas decreases, the air-fuel ratio of the inflowing exhaust gas decreases. Even NO _x absorbent 18
NO _x is to be released from the.

On the other hand, when the air-fuel mixture supplied into the combustion chamber 3 becomes rich at this time 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 emitted 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.

As described above, when the lean mixture is burned, 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.

[0025] 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 absorbent 1 per unit time to increase _x amount
NOx absorbed by the fuel _cell 8 increases. Thus _the amount of NO _x absorbed per unit time per _the NO _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, N
Determined by experiment the amount of NO _x NOXA absorbed in O _x absorbent 18 as a function of the absolute pressure PM and the engine speed N, the map the amount of NO _x NOXA is shown in FIG. 6 (A) as a function of PM and N Is stored in the ROM 32 in advance.

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 air-fuel 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 air-fuel ratio is controlled according to the value of the correction coefficient K distinguished by the solid lines R and S. Therefore, in the region on the lower load side than the solid line R in FIG. 3, the lean mixture (K <1.0) is burned, so that NO _x is absorbed by the NO _x absorbent 18 and the load is higher than the solid line R in FIG. In the region on the side, a mixture having a stoichiometric air-fuel ratio (K = 1.0) or a rich mixture (K> 1.
0) is burned, so that NO _x
Will be released. Thus by the solid line R in FIG. 3 in Sakai low-load operation and high-load operation are alternately repeated when the NO _x
When the NO _x absorption capacity of the absorbent 18 does not saturate and thus the lean mixture is burned, the NO _x
NO _x absorption agent 18 is to be well absorbed.

[0030] However for example the operating state continues as indicated by the point Y in FIG. 3, therefore absorption of NO _x capacity of the lean mixture combustion is continued _the NO _x absorbent 18 is saturated. N in the embodiment according to the present invention therefore being absorbed in the NO _x absorbent 18 is performed continuously lean mixture combustion
When the amount of O _x ΣNOX exceeds a predetermined amount, a region (K = 1.
The region where the lean air-fuel mixture is burned (region where K <1.0) is narrowed to a region on the lower load side than the broken line T.
Therefore, when the operating state indicated by the point Y in FIG. 3 continues is switched to the stoichiometric air-fuel ratio from a lean mixture when more than the amount that _the amount of NO _x ΣNOX is predetermined, therefore NO
_The action of releasing NO _x from the _x absorbent 18 is started.

On the other hand, even if the operating region (region where K = 1.0) where the air-fuel ratio is the stoichiometric air-fuel ratio is widened as indicated by the broken line T, the region where the lean air-fuel mixture should be burned (K <
1.0) because there remains the lean air-fuel mixture in this region (K <1.0) are burned _the NO _x absorbent 1
8 is performed to absorb NO _x . However the NO _x absorbing capability of the NO so _x amount is very small even when the operating region of the low load side is performed than the broken line T of Figure 3 NO _x absorbent 18 to be discharged from the engine is saturated when the low-load operation without first, nO _x amount of absorption of nO _x capacity of the nO _x absorbent 18 is even released into the atmosphere even if saturation does not occur particularly large problem because very small. In this case, NO _x
In order to completely prevent the NO _x absorption capacity of the absorbent 18 from saturating, when the NO _x amount ＸNO _x exceeds a predetermined amount, the entire region on the lower load side than the solid line S in FIG. What is necessary is just to let it be the combustion region (K = 1.0) of the air-fuel ratio mixture.

The switching of the air-fuel ratio between the solid line R and the broken line T shown in FIG. 3 is performed by a map. That is, the value of the correction coefficient K, which is divided by the solid line S and the solid line R in FIG. 3, is stored in advance in the ROM 32 in the form of a map I shown in FIG.
The value of the correction coefficient K, which is divided by the solid line S and the dashed line T in FIG. 3, is a function of the absolute pressure PM in the surge tank 10 and the engine speed N in the map II shown in FIG. Is stored in the ROM 32 in advance.
Therefore usually determined correction coefficient K based on the map I shown in FIG. 8 (A), NO _x amount ΣNOX is corrected based on the map II shown in FIG. 8 (B) exceeds a predetermined amount coefficient K Is determined.

Next, the air-fuel ratio control will be described in more detail with reference to FIG. For example _the amount of NO _x ΣNO operating conditions have continued combustion of the lean air-fuel mixture to be absorbed in _the NO _x absorbent 18 when the being performed indicated at a point Y in FIG. 3
X gradually increases. Next, at t ₁ , the amount of NO _x ΣN
When OX reaches a predetermined first upper limit value MAX1, the map for determining the correction coefficient K is switched from map I in FIG. 8A to map II in FIG. 8B. As a result, the air-fuel ratio becomes the stoichiometric air-fuel ratio from the lean, NO _x amount Σ and thus
NOX gradually decreases. Correction coefficient (t ₂ in FIG. 9) when then amount of NO _x ΣNOX is less than the second predetermined upper limit value MAX2 (<MAX1), and for example, the throttle valve is made to closed to the idling opening degree The map defining K is switched from map II to map I. This is repeated below.

[0034] After the amount of NO _x NOX when the operating state at the point Y in FIG. 3 continues maps is larger than the first upper limit value MAX1 is switched from the map I on the map II, NO _x the amount NOX is will first immediately _the amount of NO _x NOX again map switch the map II on the map I when it is smaller than the upper limit value MAX1 exceeds the first upper limit value MAX1, lean and theory was thus The air-fuel ratio is repeatedly switched in a short cycle. After the map in the embodiment according to the present invention is switched from the map I on the map II in order to prevent in this way that the lean and the stoichiometric air-fuel ratio is repeatedly switched in a short period, NO _x amount ΣNOX
Is not switched from the map II to the map I unless the value is less than or equal to the second upper limit value MAX2.

FIGS. 10 and 11 show a routine for controlling the air-fuel ratio, and this routine is executed by interruption every predetermined time. 10 and is absorbed in _the NO _x absorbent 18, first, at step 50 and reference to FIG. 11 NO _x amount ΣNOX has an upper limit value MAX
It is determined whether or not it is greater than This upper limit value MAX is set to the first value as an initial value during the initialization processing at the time of engine start.
Is the maximum value MAX1. When ΣNOX ≦ MAX, the routine proceeds to step 51, where it is determined whether the flag F indicating that the map II (FIG. 8B) should be used is set. If the flag F has not been set, the routine jumps to step 54, where the map I shown in FIG.
Is calculated from the correction coefficient K. Next, at step 55, the maximum value MAX is set to the first maximum value MAX1, and then the routine proceeds to step 56.

In step 56, the basic fuel injection time TP is calculated from the map shown in FIG. Then step 57
In, 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 58 when the operating state to combust a lean air-fuel mixture is calculated. Next, at step 59 NO _x emissions NO
XD is made zero, and then in step 60, the feedback correction coefficient FAF is fixed at 1.0. Next, at step 61, the fuel injection time TAU is calculated based on the following equation.

[0037] TAU = f · TP · K · FAF other hand, release NO _x per unit time from the map shown in FIG. 7 (A) proceeds to step 62 when it is judged that K ≧ 1.0 in step 57 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 63 FIG. 7 (B), the then absorption of NO _x amount NOXA per unit step 64 time is made zero. Next, at step 65, it is determined whether or not the correction coefficient K is larger than 1.0. K>
When the value is 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 61 via step 60.

On the other hand, when K = 1.0, that is, when the air-fuel mixture having the stoichiometric air-fuel ratio is to be burned, the routine proceeds to step 66, where the feedback correction coefficient FAF is calculated based on the output signal of the air-fuel ratio sensor 21. Proceed to step 61. In step 66, 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, FF is detected.
Since the AF is increased, the air-fuel ratio is maintained at the stoichiometric air-fuel ratio.

The amount NO _x absorbed in _the NO _x absorbent 18 on the basis of the following equation: At step 67 subsequent to step 61 sigma
NOX is calculated. .SIGMA.NOX = .SIGMA.NOX + whether NOXA-Kf · NOXD then _the amount of NO _x .SIGMA.NOX step 68 becomes negative is discriminated, are .SIGMA.NOX is zero proceeds to step 69 when it is .SIGMA.NOX <0.

On the other hand, in step 50 ΣNOX> M
If it is determined that AX (= MAX1), step 70 is executed.
, The flag F is set, and then the routine proceeds to step 71, where the correction coefficient K is calculated from the map II shown in FIG. That is, the map for determining the correction coefficient K is switched from map I to map II. Then step 72
MAX is set to the second maximum value MAX2, and step 5
Proceed to 6. When MAX = MAX2, ΣNOX ≦ MA
Steps 70, 71, 72 until X (= MAX2)
To step 56 via NO ≦ MAX (= MAX
When it becomes 2), the process proceeds to step 51. At this time, since the flag F is set, the routine proceeds to step 52, where it is determined whether or not the throttle valve 14 has been closed to the idling opening degree based on the output signal of the throttle switch 20, that is, whether or not the deceleration operation has been performed. Is done. When the throttle valve 14 is kept open, the routine proceeds to step 71.

On the other hand, when the throttle valve 14 is closed to the idling opening degree, the routine proceeds to step 53, where the flag F is reset. Then, the routine proceeds to step 54, where the correction coefficient is obtained from the map I shown in FIG. K is calculated. That is, the map for determining the correction coefficient K is switched from the map II to the map I. Then step 55
And MAX is again set to the first maximum value MAX1.

[0042]

[Effect of the Invention] first of the NO _x absorbing capability of the NO _x absorbent so opportunities absorption of the NO _x is made to decrease with opportunities NO _x emissions can be made to increase the present invention that can be inhibited from saturated it can. Amount of NO _x released into the atmosphere even if the absorption of NO _x capacity of the NO _x absorbent is saturated with absorption of NO _x capacity of the NO _x absorbent in the second invention it is possible to further suppress the saturation Can be made very small.

The absorption of NO _x capacity of the NO _x absorbent in the third invention can be completely prevented from saturation.

[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 in exhaust gas discharged from the engine. C
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 _the NO _x releasing amount NOXD like.

FIG. 8 is a diagram showing a map of a correction coefficient K;

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.

[Explanation of symbols]

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

Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI F01N 3/28 301 F01N 3/28 301C ZAB ZAB F02D 41/04 301 F02D 41/04 301C (56) References JP-A-1-163432 (JP) , A) JP-A-58-72631 (JP, A) JP-A-63-189641 (JP, A) International publication 93/12863 (WO, A1) International publication 93/8383 (WO, A1) International publication 93/7363 (WO, A1) (58) Field surveyed (Int. Cl. ⁷ , DB name) F02D 41/14 310 F02D 41/04 301

Claims (3)

(57) [Claims] When the air-fuel ratio of the inflowing exhaust gas is lean,
NO_xAnd the air-fuel ratio of the exhaust gas
NO absorbed at stoichiometric air-fuel ratio or rich _xEmit
NO_xThe absorbent is placed in the engine exhaust passage, and the engine
When the condition is in the predetermined lean combustion region, the lean mixture
NO released from the engine after burning Aiki_xNO_x
Absorbed by the absorbent, and the engine operating condition
Stoichiometric air-fuel ratio or stoichiometric air-fuel ratio in the rich combustion region
Combustion of Aiki or rich mixture to NO_xAbsorbed by absorbent
NO contained_xInternal combustion engine
Then, NO_xNO absorbed by absorbent_xEstimate quantity
Means to do and NO_xPresumed to be absorbed by the absorbent
NO_xWhen the amount exceeds a predetermined amount,
Stoichiometric air-fuel ratio or rich
An internal combustion engine having an area changing means for expanding a combustion area.
Exhaust gas purification device. Wherein is the lean combustion region is defined on the lower load side than the stoichiometric air-fuel ratio or rich burn zone, NO _x
Stoichiometric or rich combustion region with lean burn region is narrowed to lower the load side by the area changing means low when the amount of NO _x estimated to be absorbed in the absorbent exceeds a predetermined amount The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is extended to a load side. 3. When the air-fuel ratio of the inflowing exhaust gas is lean.
NO_xAnd the air-fuel ratio of the exhaust gas
NO absorbed at stoichiometric air-fuel ratio or rich _xEmit
NO_xThe absorbent is placed in the engine exhaust passage, and the engine
When the condition is in the predetermined lean combustion region, the lean mixture
NO released from the engine after burning Aiki_xNO_x
Absorbed by the absorbent, and the engine operating condition
Stoichiometric air-fuel ratio or stoichiometric air-fuel ratio in the rich combustion region
Combustion of Aiki or rich mixture to NO_xAbsorbed by absorbent
NO contained_xInternal combustion engine
Then, NO_xNO absorbed by absorbent_xEstimate quantity
Means to do and NO_xPresumed to be absorbed by the absorbent
NO_xWhen the amount exceeds a predetermined amount,
Stoichiometric air-fuel ratio or rich
Expand the combustion area to make the entire combustion area stoichiometric or rich.
Exhaust of an internal combustion engine provided with a region changing means to be a burning region
Purification device.