JPH07127495A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent the occurrence of the fluctuation of output torque of an engine when NOx is emitted from an NOx absorbent. CONSTITUTION:An NOx absorbent 18 to absorb NOx when an air-fuel ratio of inflow exhaust gas is lean and emit NOx when an air-fuel ratio of inflow exhaust gas is rendered rich is arranged in an engine exhaust passage. A bypass passage 15 bypassing a throttle valve 14 is provided and a bypass control valve 20 is disposed in the bypass passage 15. When NOx is to be emitted form the NOx absorbent 18, fuel-air mixture fed in an engine cylinder is rendered rich and the opening of the bypass control valve 20 is reduced and output torque is decreased by an amount equivalent to the increase of output torque, which is caused by fuel-air mixture being rendered rich.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art A NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich inside the engine exhaust passage. The NOx absorbent absorbs NOx generated when the lean air-fuel mixture is burned, and the theoretical mixture is supplied to the engine cylinder before the NOx absorption capacity of the NOx absorbent is saturated. Internal combustion in which the air-fuel ratio or rich is temporarily set to the stoichiometric air-fuel ratio or rich in the exhaust gas flowing into the NOx absorbent, thereby releasing NOx from the NOx absorbent and reducing the released NOx. The organization is known (see PCT International Publication WO93 / 07363).

[0003]

However, when the air-fuel mixture supplied into the engine cylinder in order to release NOx from the NOx absorbent is switched from lean to stoichiometric air-fuel ratio or rich, the output torque of the engine rapidly increases. There is a problem that a big 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, a NOx absorbent that releases the absorbed NOx when it becomes rich is placed in the engine exhaust passage to
In the internal combustion engine in which the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is switched from lean to stoichiometric air-fuel ratio or rich when NOx should be released from the absorbent, NO
x The intake air required to prevent the engine output torque from fluctuating when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to release NOx from the absorbent is switched from lean to stoichiometric air-fuel ratio or rich. A storage means that stores the reduction amount for each engine operating state, and when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to release NOx from the NOx absorbent is switched from lean to stoichiometric air-fuel ratio or rich. Intake air reducing means for reducing the intake air by the above-described reduction amount according to the operating state of the engine.

[0005]

When the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is switched from lean to stoichiometric air-fuel ratio or rich in order to release NOx from the NOx absorbent, the engine output torque increases. At this time, the intake air amount is reduced and the engine output torque is reduced by an amount corresponding to the increase in the engine output torque.

[0006]

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 16 and an exhaust pipe 17 to a casing 19 containing a NOx absorbent 18.

On the other hand, a bypass passage 15 is branched from the intake duct 12 upstream of the throttle valve 14, and the bypass passage 15 is connected to the surge tank 10. A bypass control valve 20 for controlling the amount of air flowing in the bypass passage 15 is provided in the bypass passage 15. The bypass control valve 20 is driven by a step motor, and the bypass control valve 2 is operated during engine idling operation.
By 0, the engine speed is controlled to the target idling speed.

The electronic control unit 30 is composed of a digital computer, and has a ROM (read only memory) 32, a RAM (random access memory (RAM) 33, a CPU (microprocessor) 34, and an input which are mutually connected by a bidirectional bus 31. The surge tank 10 includes a port 35 and an output port 36. A pressure sensor 21 that generates an output voltage proportional to an absolute pressure in the surge tank 10 is arranged in the surge tank 10, and the output voltage of the pressure sensor 21 is A.
It is input to the input port 35 via the D converter 37. A throttle switch 22 is attached to the throttle valve 14 for detecting that the throttle valve 14 has reached an idling opening degree and other set opening degrees, and an output signal of the throttle switch 22 is input to an input port 35. A water temperature sensor 23 that generates an output voltage proportional to the engine cooling water temperature is attached to the engine body 1, and the output voltage of the water temperature sensor 23 is input to an input port 35 via an AD converter 38. Further, the input port 35 is connected with a rotation speed sensor 24 for generating an output pulse indicating the engine speed and a vehicle speed sensor 25 for generating an output pulse indicating the vehicle speed. On the other hand, the output port 36 is connected to the step motors of the spark plug 4, the fuel injection valve 11 and the bypass control valve 20 via the corresponding drive circuits 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on the following equation. TAU = K · TP · 14.5 / (A / F) _f where K is a constant, TP is the basic fuel injection time, and (A / F) _f is the target air-fuel ratio. There is. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is previously obtained by an experiment, and the absolute pressure PM in the surge tank 10
And as a function of the engine speed N is stored in advance in the ROM 32 in the form of a map as shown in FIG. The air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is the target air-fuel ratio (A /
F) _f is controlled based on the value of _f , and the target air-fuel ratio (A / F) _f
Is the stoichiometric air-fuel ratio, that is, (A / F) _f = 14.5, the air-fuel mixture supplied to the engine cylinder has the stoichiometric air-fuel ratio. On the other hand, when (A / F) _f > 14.5, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and (A / F) _f
When <14.5, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the internal combustion engine shown in FIG. 1, (A / F) _f > 14.5 is maintained during low-medium-speed / low-medium-load operation, that is, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is lean. Therefore, in the internal combustion engine shown in FIG. 1, the lean air-fuel mixture is burned in most operating conditions.
FIG. 3 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 3, 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 19
The absorbent 18 uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, or yttrium Y is used on the carrier. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. When the ratio of air and fuel 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 NOx absorbent 18 is the air-fuel ratio of the inflow exhaust gas. Is lean, it absorbs NOx, and when the oxygen concentration in the inflowing exhaust gas decreases, the absorbed Nx
It acts to absorb and release NOx that releases Ox. Note that NO
When fuel or air is not supplied into the exhaust passage upstream of the x-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, and therefore NOx absorption in this case. The agent 18 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 decreases. Become.

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, the inflowing exhaust gas becomes considerably lean.
And the oxygen concentration in the inflowing exhaust gas increased drastically.
As shown in (A), these oxygen O₂Is O₂ ^-Or O
^2-It adheres to the surface of platinum Pt in the form of. On the other hand, the inflow exhaust gas
NO in the gas is O on the surface of platinum Pt.₂ ^-Or O^2-React with
And NO₂Becomes (2NO + O₂→ 2 NO₂). Then
NO generated₂Is partially oxidized on platinum Pt.
Is absorbed in the absorbent and does not combine with barium oxide BaO
However, as shown in FIG. 4 (A), nitrate ion NO ₃ ^-
Diffuses into the absorbent in the form of. In this way NOx is N
It is absorbed in the Ox 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. 3, if the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low. Therefore, if the leaning degree of the inflow exhaust gas is low, even if the inflowing exhaust gas is lean. NOx absorbent 18 to NOx
Will be released.

On the other hand, when the air-fuel ratio of the inflowing exhaust gas is made rich by making the air-fuel mixture supplied into the engine cylinder rich, a large amount of unburned HC and CO are discharged from the engine as shown in FIG. , These unburned HC and CO are platinum P
It is oxidized by reacting with oxygen O ₂ ⁻ or O ^{2 −} on t. Further, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, so that NO ₂ is released from the absorbent, and this NO ₂ is unrecovered as shown in FIG. 4 (B). It is reduced by reacting with fuel HC and CO.
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.

Thus, when the air-fuel ratio of the inflowing exhaust gas becomes lean, NOx is absorbed by the NOx absorbent 18, and when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 18 in a short time. It In the internal combustion engine shown in FIG. 1, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is temporarily made rich when the combustion period of the lean air-fuel mixture has passed for a certain period so that NOx is released from the NOx absorbent 18. ing. In this case, even if the air-fuel ratio of the air-fuel mixture supplied to the engine cylinders is changed to the stoichiometric air-fuel ratio, NOx is released from the NOx absorbent 18, but compared to the case where the air-fuel mixture is made rich, all of the NOx absorbent 18 is exhausted. It takes a little longer time to release NOx.

However, when the air-fuel mixture supplied into the engine cylinder is made rich in order to release NOx from the NOx absorbent 18, the output torque of the engine rapidly increases and a large shock is generated. Therefore, in the present invention, when the air-fuel mixture supplied to the engine cylinder is made rich in order to prevent the occurrence of such a large shock, the bypass control valve 20
The intake air amount is reduced by reducing the opening of the engine to reduce the output torque of the engine. By the way, in this case, in order to prevent the occurrence of shock, the torque increase amount due to the rich air-fuel mixture and the torque decrease amount due to the reduction of the intake air amount must be equalized as much as possible. The amount of torque increase due to the rich air changes depending on the engine operating condition when the air-fuel mixture is rich, so the amount of decrease in intake air also changes depending on the engine operating condition when the air-fuel mixture is rich. It will have to be done. Next, this will be described with reference to FIGS.

FIG. 5 is a diagram for conceptually understanding the present invention and shows the relationship between the intake air amount Q and the engine output torque. Note that each curve in FIG. 5 shows the case of different air-fuel ratios indicated by numerical values. For example, a lean mixture with an air-fuel ratio of 20.0 is being burned,
If the air-fuel ratio is made rich from this state to 13.5 in order to release NOx from the NOx absorbent 18, the output torque of the engine will increase by ΔT in FIG. However, at this time, if the intake air amount Q is reduced by ΔQ in FIG. 5, the output torque of the engine will not change. Therefore, in the present invention, the NOx absorbent 18
In order to release x, the air-fuel ratio of the air-fuel mixture is changed from 20.0 to 1
When it is set to 3.0, the intake air amount Q is decreased by ΔQ. This is the basic idea of the present invention.

By the way, in the embodiment shown in FIG. 1, when the NOx absorbent 18 should release NOx, the intake air amount Q is reduced by reducing the opening degree of the bypass control valve 20. The bypass control valve 20 is driven by the step motor as described above, and the opening degree of the bypass control valve 20 increases as the step position ST of the step motor increases. Therefore, in the embodiment shown in FIG. 1, when NOx should be released from the NOx absorbent 18, the step position ST of the step motor ST is decreased by the step amount ΔST required to reduce the intake air amount Q by ΔQ.
Is reduced.

As can be seen from FIG. 5, when the air-fuel ratio of the air-fuel mixture is switched from lean to rich (= 13.5), the intake air reduction amount ΔQ that does not cause a change in the output torque of the engine is the lean value before switching. The amount Q of intake air, that is, the step amount ΔST to be reduced, must be changed depending on the air-fuel ratio of the lean air-fuel mixture before switching. On the other hand, in the embodiment according to the present invention, the lean air-fuel ratio when the lean air-fuel mixture is to be burned is the absolute pressure PM in the surge tank 10.
And a function of the engine speed N. Therefore, the step amount ΔST to be reduced must be determined based on the absolute pressure PM in the surge tank 10 and the engine speed N immediately before switching to the rich mixture.

Further, FIG. 5 is a conceptual diagram as described above, and the engine output torque is not actually a function of only the intake air amount Q, but both the absolute pressure PM in the surge tank 10 and the engine speed N. It becomes a function. Therefore, the change amount ΔT of the output torque indicated by ΔT in FIG. 5 is a function of the absolute pressure PM and the engine speed N in the surge tank 10 immediately before switching to the rich air-fuel ratio, and therefore the step amount ΔST to be reduced is the rich air-fuel ratio. It is a function of the absolute pressure PM and the engine speed N in the surge tank 10 immediately before switching to the fuel ratio.

When the opening of the throttle valve 14 increases, that is, when the absolute pressure PM in the surge tank 10 increases, the differential pressure across the throttle valve 14 decreases, so the step position of the step motor is decreased by a certain amount. At this time, the reduction rate of the intake air amount becomes small. Therefore, as shown in FIG. 6, the reduction rate of the engine output torque when the step position of the step motor is reduced by a certain amount decreases as the absolute pressure PM in the surge tank 10 increases.

When all of the above are summed up, after all, even if the air-fuel ratio of the air-fuel mixture is switched from lean to rich in order to release NOx from the NOx absorbent 18, the step amount ΔST that should be reduced so that the output torque of the engine does not change. Is determined by the absolute pressure PM in the surge tank 10 and the engine speed N before switching to the rich air-fuel ratio. Actually, the step amount ΔST to be reduced is experimentally obtained as a function of the absolute pressure PM in the surge tank 10 and the engine speed N, and the step amount ΔST is previously set in the form of a map as shown in FIG. It is stored in the ROM 32.

Next, the control method of the air-fuel ratio and the bypass control valve 20 will be described with reference to FIG. As shown in FIG. 8, in the embodiment of the present invention, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio during engine idling operation. At this time, the bypass control valve 2
The engine speed is controlled to the target idling speed by 0, and at this time, the step position ST of the step motor ST
The average value ST _{0 of the above} is calculated. Next, when the throttle opening is increased, the air-fuel ratio is made lean, and the opening of the bypass control valve 20 is increased, that is, the step position ST of the step motor is increased. The step position ST at this time is different from the average step position ST ₀ during idling operation with respect to the surge tank 10 at that time.
Based on the internal absolute pressure PM and the engine speed N, the step position is increased by a step amount ΔST determined from FIG. 7.

Next, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder in order to release NOx from the NOx absorbent 18 is switched from lean to rich, while the air-fuel ratio is being made rich, the steps of the bypass control valve 20 are performed. The step position ST of the motor is set to the average step position ST ₀ during idling operation. That is, while the air-fuel ratio is rich, the step position ST of the step motor is equal to the step amount Δ.
It can be reduced by S. This reduced step amount ΔS is a step amount determined from the absolute pressure PM and the engine speed N in the surge tank 10 immediately before the air-fuel ratio is switched from lean to rich, and this step amount ΔS is from lean to rich. The step amount is predetermined so that the engine output torque does not fluctuate when switched to. Therefore, even if the air-fuel ratio is switched from lean to rich, the output torque of the engine does not change. When the air-fuel ratio is switched from rich to lean, the step position S of the step motor is again controlled to the step position obtained by adding the step amount ΔS to the average step position ST ₀ during idling operation.

FIG. 9 shows a routine for performing idling control of the engine, and this routine is performed by interruption at regular time intervals. Referring to FIG. 9, first, at step 50, it is judged if the engine is idling or not. When it is determined from the output signal of the throttle switch 22 that the throttle valve 14 is at the idling opening degree and from the output signal of the vehicle speed sensor 25 that the vehicle speed is substantially zero, it is determined that the idling operation is being performed. When the engine is not idling, the processing cycle is completed, and when the engine is idling, the routine proceeds to step 51.

In step 51, the basic fuel injection time TP is calculated from the map shown in FIG. Then step 52
Then, the fuel injection time TAU is calculated by multiplying the basic fuel injection time TP by a constant K. Therefore, at this time, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio. Next, at step 53, the lean time count value TL indicating the continuous combustion time of the lean mixture is set to zero.

Next, at step 54, the engine speed N is checked.
Standard idle speed N_oAdd a small constant value ΔN to
Value (N_OIt is determined whether it is higher than + ΔN). N
> N ₀When + ΔN, go to step 55 and step
The step position ST of the motor is decreased by 1,
Thus, the engine speed N is reduced. Then step
Proceed to p58. On the other hand, in step 54, N ≦ N₀+
If it is determined to be ΔN, proceed to step 56.
The engine speed N is (N₀-ΔN) is determined whether it is lower than
To be done. N <N₀-If ΔN, proceed to step 57
The step position ST of the step motor is increased by 1.
Thus, the engine speed N is increased. Next
Then proceed to step 58. On the other hand, in step 56
N ≧ N₀If it is determined to be −ΔN, step 5
Jump to 8. Therefore, when idling, the engine
Rotation speed N is N₀+ ΔN> N> N₀-Maintained within the range of ΔN
Will be done. In step 58, the idling operation
Average value ST of step position ST when being performed₀But
It is calculated.

FIG. 10 and FIG. 11 show a routine for controlling the air-fuel ratio etc. other than during idling operation, and this routine is executed by interruption at regular time intervals. Referring to FIGS. 10 and 11, first, at step 60, it is judged if the engine is idling or not. The processing cycle is completed during the idling operation, and the routine proceeds to step 61 when not during the idling operation. In step 61, the basic fuel injection time TP is calculated from the map shown in FIG. Next, at step 62, it is judged from the output signal of the temperature sensor 23 whether or not the engine cooling water temperature Tw is higher than a predetermined constant value, for example, 80 ° C. When Tw ≧ 80 ° C., the routine proceeds to step 63, where the engine speed N is a predetermined constant value, for example 4000.
It is determined whether it is lower than rpm. N ≦ 4000r.
When it is pm, the routine proceeds to step 64, where it is judged from the output signal of the throttle switch 22 whether or not the opening degree θ of the throttle valve 14 is smaller than a predetermined constant value, for example, 70 °. When θ ≦ 70 °, the process proceeds to step 65.

In step 65, the target lean air-fuel ratio (A / F) ₀ is calculated from the absolute pressure PM in the surge tank 10 and the engine speed N. Next, at step 66, it is judged if the final target air-fuel ratio (A / F) _f is 14.5, that is, smaller than the theoretical air-fuel ratio. Normally, the final target air-fuel ratio (A / F) _f is larger than 14.5 when the lean air-fuel mixture is to be burned.
Proceed to 7. In step 67, the step amount ΔST is calculated from the map shown in FIG. Next, at step 68, the lean time count value TL is incremented by 1. Next, at step 69, the lean time count value TL
Has exceeded a predetermined start time TC of NOx release. When TL <TC, step 70
Proceed to.

In step 70, the target lean air-fuel ratio (A / F) ₀ is made the final target air-fuel ratio (A / F) _f . Next, at step 71, the step position ST of the step motor is calculated by adding the step amount ΔST to the average step position ST ₀ during idling operation. Next, at step 72, the fuel injection time TAU is calculated based on the following equation.

TAU = K · TP · 14.5 / (A / F) _f Therefore, at this time, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes the target lean air-fuel ratio (A / F) _0. . Next, at step 73, the step motor is driven so that the step position of the step motor becomes the step position ST obtained at step 71.

When the lean air-fuel mixture is continuously burned and it is judged at step 69 that TL ≧ TC, the routine proceeds to step 76, where the lean time count value TL is made zero, and then at step 77, the final time The target air-fuel ratio (A / F) _f is the target rich air-fuel ratio (A /
F) _R , for example 13.5. Then in step 78
Then, the step position ST of the step motor is set to the average step position ST ₀ during the idling operation. When the final target air-fuel ratio (A / F) _f becomes the rich air-fuel ratio (A / F) _R , in the next processing cycle, steps 66 to 7 are performed.
In step 4, the rich count value TR is incremented by 1. Next, at step 75, the rich count value T
For example, it is determined whether R has passed 5 seconds. TR <
When it is 5 seconds, the routine proceeds to step 76, and when TR ≧ 5 seconds, the routine proceeds to step 79, where the rich count value TR is made zero. Therefore, when TL ≧ TC, the air-fuel mixture supplied into the engine cylinder is set to the target rich air-fuel ratio (A / F) _R for 5 seconds, during which the step position ST of the step motor is the average step position during idling operation. ST
_Will be maintained at ₀ .

On the other hand, in step 62, Tw <80 ° C.
Or N> 4000r. In step 63.
pm or θ> 70 ° in step 64
When it is determined that the lean time count value TL is zero, the routine proceeds to step 80. Next, at step 81, the final target air-fuel ratio (A / F) _f is set to, for example, 14.5, and then the routine proceeds to step 78. Therefore, at this time, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio, and the step position ST of the step motor is set to the average step position ST ₀ during idling operation.

In the above-described embodiments, the bypass control valve 20 controls the reduction of the intake air when the NOx absorbent 18 should release NOx. However, the intake air amount may be reduced by driving the throttle valve 14 by a motor so that the throttle valve 14 opens and closes according to the depression amount of the accelerator pedal, and closing the throttle valve 14 at the time of releasing NOx.

[0036]

EFFECTS OF THE INVENTION In any engine operating condition for releasing NOx from the NOx absorbent, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is changed from lean to rich, and in some cases even if the air-fuel ratio is switched to the stoichiometric air-fuel ratio, It is possible to prevent the output torque from changing.

[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] 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. 4 is a diagram for explaining the action of absorbing and releasing NOx.

FIG. 5 is a diagram showing a relationship between an output torque and an intake air amount Q.

FIG. 6 is a diagram showing a relationship between a change rate of output torque and an absolute pressure PM in a surge tank.

FIG. 7 is a diagram showing a map of a step amount ΔST.

FIG. 8 is a time chart showing the control of the air-fuel ratio and the bypass control valve.

FIG. 9 is a flowchart showing idling control.

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

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

[Explanation of symbols]

 14 ... Throttle valve 15 ... Bypass passage 18 ... NOx absorbent 20 ... Bypass control valve

Claims (1)

[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 becomes the stoichiometric air-fuel ratio or rich inside the engine exhaust passage. In an internal combustion engine in which the NOx absorbent is to be released and the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is switched from lean to stoichiometric air-fuel ratio or rich, NOx is released from the NOx absorbent. In order to prevent the output torque of the engine from changing when the air-fuel ratio of the air-fuel mixture supplied to the engine cylinders is changed from lean to stoichiometric air-fuel ratio or rich, the reduction amount of intake air required for each engine operating state And the storage means stored for the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to release NOx from the NOx absorbent from lean to the theoretical air-fuel ratio or An exhaust gas purification apparatus for an internal combustion engine, comprising: intake air reducing means for reducing the intake air by the above-described reduction amount according to the operating state of the engine when switching to rich.