JPH07166913A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PURPOSE:To suppress remarkable change in output torque by gradually shifting the air fuel ratio of an air-fuel mixture to a rich side when NOX is to be discharged from an NOX absorbent, in an internal combustion engine, on which the NOX absorbent for absorbing NOX when the air fuel ratio of exhaust gas is lean, is provided on the inside of the exhaust passage. CONSTITUTION:A casing 17 containing an NOX absorbent 18, for which at least one element selected from alkali metal, etc., and a precious metal such as platinum are carried on a carrier such as alumina, is connected to the exhaust port 8 of an engine 1 through an exhaust manifold 15 and an exhaust tube 16. When the air fuel ratio of exhaust gas is lean, NOX is absorbed, and when the oxygen concentration in exhaust gas is reduced, the absorbed NOX is discharged. In this exhaust emission control device, when NOX is to be discharged from the NOX absorbent 18, a fuel injection valve 11 is controlled so that the air fuel ratio of an air-fuel mixture is gradually shifted to a rich side. The output torque of an engine is thus gradually increased, and remarkable change in the output torque is restricted.

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 is the stoichiometric air-fuel ratio or is rich in the engine exhaust passage. When the NOx absorbent should release NOx, the air-fuel ratio of the air-fuel mixture burned in the combustion chamber is switched from lean to rich for a predetermined time, and then the air-fuel ratio of the air-fuel mixture is returned to lean again. A known internal combustion engine is known (see International Publication WO93 / 07363).

[0003]

However, when the air-fuel ratio of the air-fuel mixture is rapidly changed from lean to rich in order to release NOx from the NOx absorbent as in this internal combustion engine, the output torque of the engine changes drastically. Therefore, there is a problem that a shock occurs.

[0004]

In order to solve the above problems, according to the present invention, NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and the air-fuel ratio of the inflowing exhaust gas is the theoretical air-fuel ratio. Alternatively, when rich, a NOx absorbent that releases the absorbed NOx is placed in the engine exhaust passage to
In an internal combustion engine in which the air-fuel ratio of the air-fuel mixture burned in the combustion chamber is made rich when NOx is to be released from the x-absorbent, the air-fuel ratio of the air-fuel mixture is gradually increased when NOx is to be released from the NOx absorbent. It is equipped with air-fuel ratio control means for making it rich.

Further, according to the present invention, in order to solve the above-mentioned problems, N by increasing the air-fuel ratio of the air-fuel mixture.
When the action of releasing NOx from the Ox absorbent is completed, the air-fuel ratio control means gradually increases the air-fuel ratio of the air-fuel mixture.

[0006]

In the first aspect of the invention, the output torque of the engine gradually increases because the air-fuel ratio of the air-fuel mixture is gradually made rich when NOx is to be released from the NOx absorbent. In the second aspect, the air-fuel ratio of the air-fuel mixture is gradually increased when the NOx releasing action from the NOx absorbent by making the air-fuel ratio of the air-fuel mixture rich is completed, so the output torque of the engine slowly decreases. To do.

[0007]

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 comprises a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 which are mutually connected 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 arranged in the surge tank 10, and the output voltage of the pressure sensor 19 is input to the input port 35 via the 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. Further, the input port 35 is connected to a rotation speed sensor 21 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 corresponds to the corresponding drive circuit 3
8 are connected to the spark plug 4 and the fuel injection valve 11, 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 = f · TP · K where f is a constant, TP is a basic fuel injection time, and K is a correction coefficient. 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 obtained in advance by experiments and is stored in advance in the ROM 32 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. 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, if K <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, becomes lean, and if K> 1.0, it is supplied to the engine cylinder. The air-fuel ratio of the air-fuel mixture thus generated becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

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. Further, in the embodiment according to the present invention, during idling operation, K = 1.0, that is, the air-fuel ratio of the air-fuel mixture is set to the theoretical air-fuel ratio.

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, on which potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, barium Ba, an alkaline earth such as calcium Ca, lanthanum La, and yttrium Y are contained. 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 referred to as 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 absorbs 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 engine exhaust passage, the NOx absorbent 18 actually performs the NOx absorption / release operation, but the detailed mechanism of this absorption / release operation is not clear. However, it is considered that this absorbing / releasing action is performed by the mechanism 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, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), the oxygen O ₂ is O ₂ ⁻ or O _2.
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 platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is absorbed on the platinum Pt while being absorbed in the absorbent and combined with barium oxide BaO to be absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. 5 (A). Diffuses in the agent. In this way, NOx is absorbed in the NOx absorbent 18.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is produced on the surface of platinum Pt, and unless the NOx absorption capacity of the absorbent is saturated, NO ₂ is absorbed in the absorbent to generate nitrate ions NO ₃ ^−. 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, when the leaning degree of the inflow exhaust gas is low, for example, the air-fuel ratio of the inflowing exhaust gas is lean. Even NOx absorbent 1
NOx will be released from 8.

On the other hand, at this time, if 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.
It is oxidized by reacting with the oxygen O ₂ ⁻ or O ^{2 −} above.
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, the unburned HC and CO immediately react with O ₂ ⁻ or O ^{2 −} on the platinum Pt to be oxidized, and then the O on the platinum Pt is oxidized. ₂ ^- or O ^2- is still unburned HC be consumed,
If CO remains, NOx discharged from the absorbent by the unburned HC and CO and NOx discharged from the engine.
Is reduced. 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 released.
Since x is reduced, NOx can be prevented from being discharged into the atmosphere. Further, since the NOx absorbent 18 has the function of a reduction catalyst, the NOx released from the NOx absorbent 18 can be reduced even if the air-fuel ratio of the inflowing exhaust gas is changed to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NOx is gradually released from the NOx absorbent 18 and therefore NO.
It takes a little longer time to release all the NOx absorbed in the x absorbent 18.

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 absorbing capacity of the NOx absorbent 18 becomes saturated.
It is necessary to estimate whether is absorbed. Then this N
A method for estimating the Ox absorption amount will be described.

When the lean air-fuel mixture is being burned, the higher the engine load, the more the amount of NOx discharged 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 x amount increases, the 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 amount NOXA as a function of PM and N is shown in FIG.
It is stored in advance in the ROM 32 in the form of the map shown in FIG.

On the other hand, when the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio or becomes 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 empty amount. 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 exhaust gas amount increases, and the NOx released from the NOx absorbent 18 per unit time increases as the air-fuel ratio becomes richer.
The amount of x 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 per unit time as shown in FIG. 6 (B). The NOx amount NOXD released from the NOx absorbent 18 increases as N · PM increases. Further, 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. 6 (C). 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. 7A as a function of N · PM and K.
It is stored in 2.

Further, when the temperature of the NOx absorbent 18 becomes high, the nitrate ion NO ₃ ^{− in the} absorbent is easily decomposed, so that the NOx release rate from the NOx absorbent 18 increases. In this case, since the temperature of the NOx absorbent 18 is almost proportional to the exhaust gas, as shown in FIG. 7B, the NOx release rate Kf
Becomes larger as the exhaust gas temperature T becomes higher. Therefore NO
When the x emission rate Kf is taken into consideration, NO per unit time
The NOx amount released from the x absorbent 18 is represented by the product of NOXD and NOx release rate Kf shown in FIG. 7 (A). In the embodiment according to the present invention, the exhaust gas temperature T
Is the absolute pressure PM in the surge tank 10 and the engine speed N
ROM in advance in the form of the map shown in FIG.
It is stored in 32.

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

ΣNOX = ΣNOX + NOXA−Kf · NOXD As described above, in the embodiment according to the present invention, basically, FIG.
At, 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, since the lean air-fuel mixture (K <1.0) is burned, NOx is absorbed by the NOx absorbent 18, and the higher load side than the solid line R in FIG. In the region, a stoichiometric air-fuel ratio mixture (K = 1.0) or a rich mixture (K> 1.
0) is burned, so NO from the NOx absorbent 18
x will be emitted. Therefore, if the low load operation and the high load operation are alternately repeated with the solid line R in FIG.
Although the NOx absorbing capacity of the x absorbent 18 does not saturate, there are many opportunities to actually operate on the lower load side than the solid line R. Therefore, in reality, the NOx absorbent 18 is forcibly forced from the NOx absorbent 18 during engine operation. Need to be released.

In the embodiment according to the present invention, when the engine deceleration operation is stopped, the fuel supply is stopped, and then the fuel supply is restarted to shift to the idling operation, so that the NOx absorbent 18 absorbs a certain amount or more of NOx. If it is estimated that the air-fuel ratio is high, the air-fuel ratio of the air-fuel mixture is temporarily made rich. FIG. 8 shows changes in the air-fuel ratio of the air-fuel mixture at this time, that is, changes in the correction coefficient K. That is, as shown in FIG. 8, the fuel supply is stopped during the engine deceleration operation (fuel cut),
Next, when the fuel supply is restarted (return), the estimated NOx amount ΣNOX absorbed by the NOx absorbent 18 is the first
When the maximum value MAX1 is exceeded, the air-fuel ratio of the air-fuel mixture is gradually increased from lean (K <1.0) to rich (K> 1.0). At this time, when the air-fuel ratio of the air-fuel mixture becomes smaller than the theoretical air-fuel ratio (K = 1.0), the action of releasing NOx from the NOx absorbent 18 is started, so that NOx is released.
The amount ΣNOX gradually decreases. Next, the air-fuel ratio of the air-fuel mixture is maintained at a constant rich degree (K = KK) for a while,
Next, it is gradually decreased to the stoichiometric air-fuel ratio (K = 1.0).

In order to satisfactorily release NOx from the NOx absorbent 18, it is preferable to slow the flow rate of the exhaust gas and lengthen the contact time between the exhaust gas and the NOx absorbent 18. However, when the supply of fuel is restarted at the time of deceleration of the engine, the engine speed has decreased considerably, and therefore the flow velocity of exhaust gas has decreased considerably. Therefore NO
In order to satisfactorily release NOx from the x absorbent 18, in the embodiment according to the present invention, the air-fuel ratio of the air-fuel mixture is made rich when the fuel supply is restarted.

Further, as shown in FIG. 8, the air-fuel ratio of the air-fuel mixture at the moment when the fuel supply is restarted is, for example, K = 0. A lean air-fuel ratio of about 7 is used. Next, the air-fuel ratio of the air-fuel mixture is set to a predetermined rich degree (K = K
K), but the air-fuel ratio of the air-fuel mixture is gradually decreased during this period, so the output torque of the engine increases slowly. Therefore, at this time, there is a risk that a shock will occur due to a sudden change in the output torque of the engine. Absent. Further, while the air-fuel ratio of the air-fuel mixture is increased from the predetermined rich degree (K = KK) toward the stoichiometric air-fuel ratio (K = 1.0), the air-fuel ratio of the air-fuel mixture is slowly increased. Since the output torque of the engine decreases slowly, there is no danger of shock occurring at this time as well.

If the deceleration operation is performed at a certain frequency, NOx is temporarily increased by temporarily increasing the air-fuel ratio of the air-fuel mixture when the fuel supply is restarted, as shown in FIG.
It is possible to avoid saturation of the NOx absorption capacity of the absorbent 18. However, there is a risk that the NOx absorption capacity of the NOx absorbent 18 will be saturated when the vehicle runs at a constant speed with a lean air-fuel mixture for a long time.
Therefore, in this case, the air-fuel ratio of the air-fuel mixture must be temporarily made rich while the lean air-fuel mixture is being combusted. FIG. 9 shows changes in the air-fuel ratio in this case.

That is, referring to FIG. 9, the NOx amount ΣNOX estimated to be absorbed by the NOx absorbent 18 when the lean air-fuel mixture is burning is the second maximum value MA.
When X2 (> MAX1) is exceeded, the air-fuel ratio of the air-fuel mixture is gradually reduced toward a predetermined rich degree (K = KK). Next, the air-fuel ratio of the air-fuel mixture is maintained at a predetermined rich degree (K = KK) for a while, and then gradually increased toward the original lean air-fuel ratio. Therefore, also in this case, the output torque of the engine changes slowly, and thus it is possible to prevent a shock from occurring.

As shown in FIG. 8, when the fuel supply is restarted, the air-fuel ratio has a predetermined rich degree (K = K).
The time maintained at K) is shorter than that shown in FIG. In the case shown in FIG. 8, this is because the NOx releasing action from the NOx absorbent 18 is performed even during the idling operation performed thereafter, so that all the NOx absorbed in the NOx absorbent 18 while the air-fuel ratio is made rich. This is because there is no need to release

Next, the method of controlling the air-fuel ratio according to the present invention will be described with reference to FIGS. FIG. 10 shows a routine for processing flags related to stop and return of fuel supply, and this routine is executed by interruption at regular time intervals. Referring to FIG. 10, first, at step 100, it is judged if the cut flag indicating that the fuel supply should be stopped is set. If the cut flag is not set, the routine proceeds to step 101, where it is judged if the cut condition for stopping the fuel supply is satisfied. Throttle valve 1
4 is in the idling position and the engine speed is, for example, 1
When it is higher than 200 rpm, it is judged that the cutting condition is satisfied. When the cutting condition is not satisfied, the processing cycle is completed. On the other hand, when the cut condition is satisfied, the routine proceeds to step 102, where the cut flag is set (FIG. 8). When the cut flag is set, the fuel supply is stopped.

When the cut flag is set, step 1
From 00 to step 103, it is judged if the return condition for restarting the fuel supply is satisfied. For example, when the engine speed is 900 rpm or less, it is determined that the return condition is satisfied. When the return condition is not satisfied, the processing cycle is completed. On the other hand, when the return condition is satisfied, the routine proceeds to step 104, where the cut flag is reset. When the cut flag is reset, the fuel supply is restarted. Next, at step 105, NO
NOx amount Σ estimated to be absorbed by the x absorbent 18
It is determined whether or not NOX is larger than the first maximum value MAX1 (FIG. 8). When ΣNOX> MAX1, the routine proceeds to step 106, where the NOx release flag 1 is set (FIG. 8), then the routine proceeds to step 107, where the correction coefficient K is set to a constant value smaller than 1.0, for example, 0.7.

11 to 14 show an air-fuel ratio control routine, and this routine is executed by interruption at regular time intervals. Referring to FIG. 11, first, at step 200, it is judged if the cut flag is set or not. When the cut flag is not set, the routine proceeds to step 201, where the basic fuel injection time TP is calculated from the map shown in FIG. Then step 202
NO that indicates that NOx should be released
It is determined whether or not the x emission flag 1 is set. When the NOx release flag 1 is not set, the routine proceeds to step 203, where it is judged if the NOx release flag 2 indicating that the NOx releasing action should be performed is set or not. When the NOx release flag 2 is not set, the routine proceeds to step 204, where the correction coefficient K is calculated from the relationship shown in FIG.

Next, at step 205, 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 in an operating state, the routine proceeds to step 206, where the NOx absorption amount NOXA per unit time is calculated from the map shown in FIG. 6 (A). Next, at step 207, the NOx release amount NOXD is made zero, and then at step 211, the fuel injection time TAU is calculated based on the following equation.

On the other hand, when it is judged in step 205 that K ≧ 1.0, the routine proceeds to step 208, where the NOx amount NOXD released per unit time is calculated from the map shown in FIG. 7 (A). It is calculated. Next, at step 209, the NOx release rate Kf is calculated from the relationship shown in FIG. 7 (B) and the map shown in FIG. 7 (C), and then at step 210, Nx per unit time is calculated.
The Ox absorption amount NOXA is set to zero. Then step 21
Go to 1.

In step 212 following step 211, the NO absorbed in the NOx absorbent 18 is calculated based on the following equation.
The x amount ΣNOX is calculated. ΣNOX = ΣNOX + NOXA−Kf · NOXD Next, at step 213, it is judged if the NOx amount ΣNOX becomes negative. If ΣNOX <0, the routine proceeds to step 214, where ΣNOX is made zero. Next, at step 215, the NOx amount ΣNOX is the second maximum value MA.
It is determined whether or not it has become larger than X2 (FIG. 9).
When ΣNOX> MAX2, the routine proceeds to step 216, where the NOx release flag 2 is set, and then at step 217, the current correction coefficient K is stored as K ₀ . ΣNOX increases when the lean air-fuel mixture is being burned, and therefore it is determined that ΣNOX> MAX2 is when the lean air-fuel mixture is burning, so the value of K ₀ is 1.0. Will be a smaller value.

On the other hand, the NOx releasing flag 1 is set (see FIG. 8) from step 202 to step 21 in FIG.
In step 8, it is determined whether or not the KK flag is set. After the NOx release flag 1 is set, when the process proceeds to step 218 for the first time, the KK flag is not set, so the process proceeds to step 219, and the constant value ΔK is added to the correction coefficient K. Next, at step 220, it is judged if the correction coefficient K has become larger than a constant value KK of about 1.2 to 1.3. If K <KK, the process proceeds to step 205 in FIG. Therefore, the air-fuel ratio of the air-fuel mixture gradually decreases from the lean air-fuel ratio (K = 0.7) toward the rich air-fuel ratio (K = KK). Next, when K ≧ KK, the routine proceeds to step 221, where the KK flag is set, then the routine proceeds to step 222, where the target count value C ₁ is calculated. As shown in FIG. 15, this target count value C ₁ becomes larger as the NOx amount ΣNOX becomes larger.

When the KK flag is set, step 21
From 8 to step 223, the count value C is incremented by 1. Next, at step 224, it is judged if the count value C has become larger than the target count value C ₁ . When C <C ₁ , the routine proceeds to step 205, and during this time, the air-fuel ratio of the air-fuel mixture is the rich air-fuel ratio (K
= KK). As shown in FIG. 15, as ΣNOX increases, C ₁ increases. As the absorbed NOx amount ΣNOX increases, the time during which the air-fuel ratio is maintained at the rich air-fuel ratio (K = KK) increases. NO
This is to increase the release amount of x.

When it is determined in step 224 that C> C ₁ , the routine proceeds to step 225, where the constant value ΔK is subtracted from the correction coefficient K. Next, at step 226, it is judged if the correction coefficient K has become smaller than 1.0. While K> 1.0, the routine proceeds to step 205. At this time, therefore, the air-fuel ratio of the air-fuel mixture is the rich air-fuel ratio (K = KK).
Becomes gradually larger toward the stoichiometric air-fuel ratio (K = 1.0). When K ≦ 1.0, the routine proceeds to step 227, where NOx
The emission flag 1 is reset. Then step 228
The KK flag is reset, then step 2
At 29, the count value C is set to zero.

On the other hand, when the NOx releasing flag 2 is set (see FIG. 9), the routine proceeds from step 203 in FIG. 11 to step 230 in FIG. 14 and it is determined whether or not the KK flag is set. After the NOx release flag 2 is set, when the process proceeds to step 230 for the first time, the KK flag is not set, so the process proceeds to step 231, and the constant value ΔK is added to the correction coefficient K. Then step 23
In 2, the correction coefficient K is a constant value KK of about 1.2 to 1.3
It is determined whether or not it has become larger than. If K <KK, the process proceeds to step 205 in FIG. Therefore, the air-fuel ratio of the air-fuel mixture gradually decreases from the lean air-fuel ratio toward the rich air-fuel ratio (K = KK). Next, when K ≧ KK, the routine proceeds to step 232, where the KK flag is set, then the routine proceeds to step 234, where the target count value C ₂ is calculated.
This target count value C ₂ is NO as shown in FIG.
The larger the x amount ΣNOX, the larger the value, and C ₁
It is a considerably larger value than.

When the KK flag is set, step 23
The routine proceeds from 0 to step 235 where the count value C is incremented by 1. Next, at step 236, it is judged if the count value C has become larger than the target count value C ₂ . While C <C ₂ , the routine proceeds to step 205, during which the air-fuel ratio of the air-fuel mixture is the rich air-fuel ratio (K
= KK). As shown in FIG. 15, C ₂ increases as ΣNOX increases. As described above, the air-fuel ratio is maintained at the rich air-fuel ratio (K = KK) as the absorbed NOx amount ΣNOX increases. This is because the time is lengthened and the amount of NOx released is increased.

When it is judged in step 236 that C> C ₂ , the routine proceeds to step 237, where the constant value ΔK is subtracted from the correction coefficient K. Next, at step 238, it is judged if the correction coefficient K has become smaller than K ₀ (the original lean air-fuel ratio). Step 2 while K> K ₀
Therefore, the air-fuel ratio of the air-fuel mixture gradually increases from the rich air-fuel ratio (K = KK) toward the lean air-fuel ratio (K = K ₀ ). When K ≦ K ₀ , step 239
Then, the NOx releasing flag 2 is reset. Next, at step 240, the KK flag is reset, then at step 241, the count value C is made zero.

[0042]

The air-fuel ratio of the air-fuel mixture is gradually changed when the air-fuel ratio of the air-fuel mixture is temporarily made rich in order to release NOx from the NOx absorbent, so a shock occurs due to a sudden change in the engine output torque. Can be prevented.

[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 showing 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 the action of absorbing and releasing NOx.

FIG. 6 is a diagram showing a NOx absorption amount NOXA and the like.

FIG. 7 is a diagram showing a NOx release amount NOXD and the like.

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

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

FIG. 10 is a flowchart for processing a flag.

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

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

FIG. 13 is a flowchart showing air-fuel ratio control.

FIG. 14 is a flowchart showing air-fuel ratio control.

FIG. 15 is a diagram showing a target count value.

[Explanation of symbols]

 16 ... Exhaust pipe 18 ... NOx absorbent

Claims (2)

[Claims] 1. 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 is the stoichiometric air-fuel ratio or rich in the engine exhaust passage. In the internal combustion engine in which the air-fuel ratio of the air-fuel mixture burned in the combustion chamber is made rich when NOx is to be released from the NOx absorbent, the air-fuel mixture is exhausted when NOx is to be released from the NOx absorbent. An exhaust gas purification apparatus for an internal combustion engine, comprising an air-fuel ratio control means for gradually increasing the fuel ratio. 2. The air-fuel ratio control means gradually increases the air-fuel ratio of the air-fuel mixture when the NOx releasing action from the NOx absorbent by making the air-fuel ratio of the air-fuel mixture rich is completed. Exhaust gas purification device for internal combustion engine.