JP2002364412A - Exhaust emission control device for engine with turbo supercharger - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrict a lowering of exhaust gas emission control effect by supplying a NOx reducing agent when emitting NOx, giving consideration to reduction of the NOx reducing agent due to the agitation by a turbo supercharger. SOLUTION: When setting an air-fuel ratio at a rich ratio, a waste gate is opened to control the exhaust gas to bypass a turbine. The waste gate is opened when the NOx trap quantity reaches an allowable trap quantity D to be decided on the basis of the NOx trap characteristic of the NOx trap catalyst after a lag time from a start of a change of the air-fuel ratio to a rich ratio till the supercharge pressure is lowered, and a change in the air-fuel ratio to a rich ratio is started in response to a change in the supercharge pressure.

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 a turbocharged engine provided with a trap catalyst for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage.

[0002]

2. Description of the Related Art Japanese Patent No. 2,586,739 discloses trapping NOx when the inflowing air-fuel ratio is lean,
In an exhaust gas purifying apparatus for an internal combustion engine having a NOx trap catalyst that releases trapped NOx when the inflowing air-fuel ratio has a low oxygen concentration, the amount of NOx trapped by the catalyst is estimated, and the NOx amount is determined in advance. A configuration is disclosed in which, when a predetermined allowable amount is exceeded, the concentration of oxygen in exhaust gas flowing into the catalyst is reduced to release trapped NOx.

[0003]

In contrast to the above prior art, when considering a system for purifying exhaust gas by mounting the above conventional NOx trap catalyst on a direct injection type engine equipped with a turbocharger, the air-fuel ratio is reduced. When switching from lean to rich and releasing NOx, rich exhaust gas and lean exhaust gas are agitated by the turbocharger and unburned components of the fuel are re-burned. There is a problem that HC serving as a NOx reducing agent decreases, and the exhaust gas purification effect decreases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to supply a NOx reducing agent at the time of NOx release in consideration of a decrease in the NOx reducing agent due to agitation of a turbocharger, thereby improving the exhaust gas purification effect. An object of the present invention is to provide an exhaust gas purifying apparatus for an engine with a turbocharger, which suppresses the decrease.

[0005]

In order to achieve the above object, an exhaust gas purifying apparatus for a turbocharged engine according to the present invention is provided with a trap for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage. In the exhaust purification device of the turbocharged engine equipped with a catalyst, a bypass passage for bypassing the turbocharger, and a bypass valve for opening and closing the bypass passage, the amount of NOx trapped by the trap catalyst is estimated. If it is determined that the NOx amount has reached the threshold value, the air-fuel ratio is set to be rich by the air-fuel ratio setting means for making the air-fuel ratio of the exhaust gas passing through the trap catalyst rich. A valve control means for opening the bypass valve.

[0006] Preferably, the valve control means opens the bypass valve when the amount of exhaust gas exceeds a predetermined amount in a predetermined operation region of the engine.

Preferably, when the engine is in the predetermined operation range, the air-fuel ratio setting means corrects at least one of a rich degree of the air-fuel ratio, a rich period, and the threshold value.

An exhaust gas purifying apparatus for an engine with a turbocharger according to the present invention is an exhaust gas purifying apparatus for an engine with a turbocharger provided with a trap catalyst for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage. An air-fuel ratio setting means for estimating the amount of NOx trapped by the trap catalyst and enriching the air-fuel ratio of exhaust gas passing through the trap catalyst if it is determined that the amount of NOx has reached a threshold value; Correction means is provided for increasing the degree of richness of the air-fuel ratio set by the air-fuel ratio setting means in accordance with the amount of unburned components in the exhaust gas reburned by stirring of the turbocharger.

An exhaust purification system for an engine with a turbocharger according to the present invention is an exhaust purification system for an engine with a turbocharger having a trap catalyst for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage. An air-fuel ratio setting means for estimating the amount of NOx trapped by the trap catalyst and enriching the air-fuel ratio of exhaust gas passing through the trap catalyst if it is determined that the amount of NOx has reached a threshold value; Correction means is provided for lengthening the rich period of the air-fuel ratio set by the air-fuel ratio setting means in accordance with the amount of unburned components in the exhaust gas reburned by stirring of the turbocharger.

An exhaust emission control device for an engine with a turbocharger according to the present invention is an exhaust emission control device for an engine with a turbocharger provided with a trap catalyst for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage. An air-fuel ratio setting means for estimating the amount of NOx trapped by the trap catalyst and enriching the air-fuel ratio of exhaust gas passing through the trap catalyst if it is determined that the amount of NOx has reached a threshold value; In correcting the threshold value, the unburned component in the exhaust gas is smaller than the permissible trap amount depending on the trap performance of the trap catalyst by an amount of NOx that is increased by recombustion due to the stirring of the turbocharger. Correction means for correcting to a value.

[0011]

As described above, according to the first aspect of the present invention, the amount of NOx trapped by the trap catalyst is estimated, and if it is determined that the NOx amount has reached the threshold, the trap catalyst passes through the trap catalyst. When the air-fuel ratio of the exhaust gas to be enriched is increased and the air-fuel ratio is set to be rich, the bypass valve is opened to release the NOx in consideration of the reduction of the NOx reducing agent due to the agitation of the turbocharger. Sometimes NO
By supplying the x reducing agent, it is possible to suppress a decrease in the purification effect of the exhaust gas.

According to the second aspect of the present invention, by opening the bypass valve when the amount of exhaust gas exceeds the predetermined amount in the predetermined operation region of the engine, the amount of exhaust gas passing through the bypass passage is large. The effect of supplying HC can be enhanced by suppressing the stirring effect of the turbocharger due to the opening of the bypass valve.

According to the third aspect of the present invention, when the engine is in the predetermined operating range, the air-fuel ratio rich degree, the rich period,
Alternatively, by correcting at least one of the threshold values, the HC supply effect can be enhanced by other control parameters.

According to the present invention, the amount of NOx trapped by the trap catalyst is estimated, and if it is determined that the amount of NOx has reached the threshold value, the air-fuel ratio of the exhaust gas passing through the trap catalyst is increased. At the same time, by increasing the richness of the air-fuel ratio in accordance with the amount of unburned components in the exhaust gas reburning due to the stirring of the turbocharger, the amount of HC reduction due to the stirring effect of the turbocharger is reduced. It is possible to sufficiently secure HC as a NOx reducing agent in anticipation.

According to the fifth aspect of the present invention, the amount of NOx trapped by the trap catalyst is estimated, and if it is determined that the amount of NOx has reached the threshold, the air-fuel ratio of the exhaust gas passing through the trap catalyst is increased. At the same time, by extending the rich period of the air-fuel ratio in accordance with the amount of the unburned components in the exhaust gas reburning by the stirring of the turbocharger, the amount of HC reduction due to the stirring effect of the turbocharger is reduced. NO in anticipation
HC as x reducing agent can be sufficiently secured.

According to the present invention, the amount of NOx trapped by the trap catalyst is estimated, and if it is determined that the amount of NOx has reached the threshold value, the air-fuel ratio of the exhaust gas passing through the trap catalyst is increased. In addition, when correcting the threshold value, the unburned component in the exhaust gas is increased due to the re-combustion due to the agitation of the turbocharger, which is larger than the allowable trap amount depending on the trap performance of the trap catalyst.
By correcting to a value smaller by the amount of x, HC as the NOx reducing agent can be sufficiently secured in anticipation of the amount of HC reduction due to the stirring effect of the turbocharger.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an exhaust gas purifying apparatus for a turbocharged engine according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The embodiment described below is an example of a means for realizing the present invention, and the present invention is a modification or modification of the following embodiment without departing from the gist thereof, for example, a cylinder. The present invention can be applied not only to an injection engine but also to a port injection engine and an internal combustion engine that requires purification of gasoline, diesel, and other exhaust gases. [Overall Configuration] As shown in FIG. 1, an engine with a turbocharger according to the present embodiment is a spark-ignition in-cylinder injection engine 1.
(Hereinafter referred to as “engine 1”), and when the intake valve 2 is opened, air for fuel combustion is sucked into the combustion chamber 4 from the intake passage 3. Then, the fuel injection valve 5 is injected into the air in the combustion chamber 4 at a predetermined timing.
Fuel (gasoline) is directly injected from the (fuel injection device) to form an air-fuel mixture.

This air-fuel mixture is compressed by the piston 6, and at a predetermined timing, a spark plug 7 (spark ignition device)
It is ignited by and burns. The combustion gas, that is, the exhaust gas, is discharged to the exhaust passage 9 when the exhaust valve 8 is opened.

Fuel is supplied to the fuel injection valve 5 via a fuel supply passage 10 by a high-pressure fuel pump 11.
As described above, since the high-pressure fuel pump 11 is used, fuel injection can be performed without any problem even in the latter half of the compression stroke in which the pressure in the combustion chamber 4 becomes high. The fuel injection valve 5 is a swirl type injector, and is arranged such that the fuel injection hole directly faces the combustion chamber 4. Further, the fuel injection valve 5 is arranged such that when the piston 6 is located near the top dead center position, fuel can be injected into a cavity 6 a formed at the top of the piston 6. Thereby, the fuel injected from the fuel injection valve 5 in the latter half of the compression stroke is
It is repelled by the cavity 6a and is stratified (stratified) around the spark plug 7. In this way, the fuel or air-fuel mixture is stratified and its ignitability is enhanced,
The air-fuel ratio can be made significantly leaner, and fuel efficiency can be improved.

In the intake passage 3, an electric throttle valve 12 for restricting the air and a surge tank 2 for stabilizing the air flow are arranged in order from the upstream side when viewed in the flow direction of the air (intake air).
1 and a gas flow control valve 13 for adjusting the inflow direction of air into the combustion chamber 4 in order to generate swirl. Here, the electric throttle valve 12 is driven by an electric actuator 12 a that operates according to a control signal output from the control unit 20 (ECU), and adjusts the amount of air flowing into the combustion chamber 4. . Although not shown, an air cleaner (not shown) that removes dust and the like in the air and an air flow that detects the air flow rate are arranged in the intake passage 3 upstream of the electric throttle valve 12 in order from the upstream side. A sensor, a blower (pump) of the turbocharger 15, which will be described later, and an intercooler for cooling air heated by the blower and having a high temperature are provided.

A linear O ₂ sensor 14 for detecting the oxygen concentration in the exhaust gas and, consequently, the air-fuel ratio (a normal λO whose output reverses near λ = 1) ₂ ), a turbine 15a of the turbocharger 15, an upstream catalytic converter 16,
A downstream catalytic converter 17 is provided.

Although not shown in detail, the upstream catalytic converter 16 is of a two-bed type, and a three-way catalyst for purifying NOx, HC, CO, etc. is loaded on the upstream bed thereof, The bed is mainly loaded with a NOx purification catalyst for purifying NOx. The upstream catalytic converter 16 may be of a one-bed type, and may be equipped with only a three-way catalyst.

The downstream catalytic converter 17 is of a one-bed type, traps NOx in exhaust gas at a lean air-fuel ratio, and releases NOx when the air-fuel ratio becomes rich (λ <1). A NOx trap catalyst for purifying NOx by reacting with HC in the exhaust gas and reducing it to H ₂ , N ₂ , and CO ₂ is mounted. It should be noted that all of the catalysts exhibit sufficient purifying power when their temperature is equal to or higher than the activation temperature. However, when their temperatures are lower than the activation temperature, sufficient purifying power cannot be obtained.

The upstream and downstream catalytic converters 16,
17 is provided with catalyst temperature sensors 16a and 17a for directly detecting the temperature of each catalyst. In addition, both catalysts 16, 1
The temperature of 7 can also be indirectly estimated from operating conditions such as engine speed, engine water temperature, elapsed time after starting, exhaust gas temperature, air-fuel ratio, and the like.

A wastegate 31 that bypasses the turbocharger 15 is provided in the exhaust passage 9 of the engine 1. The waste gate 31 is opened and closed by a waste gate valve 32, and generally suppresses an excessive rise in the supercharging pressure.

Further, an EGR passage 18 for recirculating a part of the exhaust gas in the exhaust passage 9 to the intake passage 3 is provided in the engine 1.
The EGR passage 18 is provided with an EGR valve 19 for controlling an EGR gas flow rate. The valve opening period and the opening / closing timing of the intake valve 2 can be changed by the variable valve timing mechanism 22.

The control unit 20 includes the engine 1
Which performs various engine controls based on various types of control information. Specifically, various control information such as an intake air amount, a throttle opening, a crank angle, an engine speed, an engine water temperature (engine temperature), and an air-fuel ratio are input to the control unit 20. The control unit 20 controls the fuel injection amount of the fuel injection valve 5 (air-fuel ratio control), the injection timing control, and the ignition plug 7 based on the control information.
Control of the ignition timing (ignition timing control), control of the opening of the electric throttle valve 12, control of the opening of the EGR valve 19,
The control of the opening degree of the gas flow control valve 13 and the control of the valve opening period and the opening / closing timing of the intake valve 2 are performed.

However, the control unit 20
The normal control of the engine 1 by
Further, since such normal control is not the gist of the present invention, the description thereof is omitted, and the control relating to the exhaust gas purification which is the gist of the present invention will be described below. [Exhaust Gas Purification Control] FIG. 2 is an air-fuel ratio map for the engine 1 described above. In this map, the operating range of the engine defined by the engine speed and the engine load is
The air-fuel ratio is divided into a stratified combustion region A having a lean air-fuel ratio, a rich uniform combustion region B, and a fuel cut region C.

The stratified combustion region A is set on the low to medium rotation and low to medium load side where the operation frequency is high. In this region A, the air-fuel ratio is made larger than the stoichiometric air-fuel ratio (λ> 1).
During the lean operation in this region A, the fuel is injected during the compression stroke (second-stage injection), and the fuel is unevenly distributed near the ignition plug 5 to perform stratified combustion. During this lean operation, NOx in the exhaust gas is trapped by the NOx trap catalyst 17, and both the fuel economy performance and the exhaust performance are improved.

The uniform combustion area B is set on the high rotation speed and high load side, which is an operation area for high-speed operation and acceleration.
In this region B, the air-fuel ratio is set to be smaller than the stoichiometric air-fuel ratio (λ <1) or the stoichiometric air-fuel ratio (λ =
1). During the rich operation in this region B, fuel is injected during the intake stroke (pre-stage injection), and the fuel is sufficiently vaporized and atomized in the combustion chamber 4. During the rich operation, when the air-fuel ratio is made smaller than the stoichiometric air-fuel ratio, NOx trapped in the NOx trap catalyst 17 is released, and oxidation and reduction reaction with CO and HC produces a good torque and exhaust gas. Performance is improved. On the other hand, when the air-fuel ratio is set to the stoichiometric air-fuel ratio, CO, HC, and NOx in the exhaust gas are simultaneously purified by the three-way catalyst 16.

The fuel cut area C is set on the medium to high speed and low load side. In this region C, the fuel injection into the combustion chamber 4 is stopped.

In FIG. 2, the boundary between the stratified combustion region A and the uniform combustion region B is indicated by the symbol "L". Also, EGR
Are indicated by hatching. In the engine 1, in the operating range of low to medium rotation and low to medium load, EG
R control is performed. As a result, EGR control is performed in substantially the entire stratified combustion region A and a part of the uniform combustion region B. In the EGR control, for example, the opening degree of the EGR valve 19 is feedback-controlled so that the air-fuel ratio in the combustion chamber 4 or the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio according to each of the regions A and B. This is control for converging the EGR gas amount to the target EGR gas amount.

In the engine control, during the cold start (catalyst activation period), the fuel injection by the fuel injection valve 5 is performed in the latter half of the compression stroke before the ignition timing, and the fuel injection is performed in the ignition stage. It is performed separately from the latter stage fuel injection executed in the first half of the expansion stroke after the timing. The first-stage fuel injection may be performed in the first half of the intake stroke or the compression stroke. Here, the fuel injection amount in the first stage fuel injection is set to be equal to or larger than the fuel injection amount in the second stage fuel injection. Thereby, the exhaust gas temperature is effectively increased by the pre-combustion (combustion of the fuel injected by the pre-stage fuel injection) and the post-combustion (combustion of the fuel injected by the post-stage fuel injection), and more than necessary in the post-combustion. Is prevented from being discharged.

Here, the fuel injection start timing of the latter-stage fuel injection is set in the range of ATDC 30 to 90 ° CA. As a result, the exhaust gas temperature is more effectively raised by the pre-combustion and the post-combustion, and more than necessary unburned HC is more effectively prevented from being discharged in the post-combustion.

The ignition timing of the ignition plug 7 is set before the compression top dead center. As a result, it is possible to effectively promote the temperature rise or activation of the exhaust gas purifying catalyst in both the catalysts 16 and 17 while suppressing the deterioration of the fuel economy performance.

Further, in the engine 1, EGR is performed in the stratified combustion region. During this EGR control, a rich spike operation for switching the air-fuel ratio to rich and releasing NOx is also performed in parallel. In this case, the oxygen concentration in the combustion chamber 4 is lower during the rich operation and the unburned oxygen concentration contained in the EGR gas is lower than during the lean operation. If it is maintained at about the same level as above, the oxygen concentration in the combustion chamber 4 will be extremely reduced, causing a problem of deterioration in combustibility.

Therefore, in the EGR control of the engine 1, the EGR amount is reduced during the rich spike operation as compared with the normal lean operation. As a result, the amount of fresh air is increased by that amount, and the problem of deterioration in combustibility is suppressed. In this case, if the EGR amount is reduced to zero, the effect of EGR cannot be obtained, and a large amount of NOx component is generated in the exhaust gas, and the reducing agent (CO, HC) purifies the generated NOx component. Is consumed in a larger amount, and the amount that can be used for purifying the original trap NOx becomes smaller.
In the EGR control of the engine 1, the EGR amount is not reduced to zero even during the rich spike operation, and the EGR amount is reduced to a predetermined amount larger than zero, that is, the EGR control is continued.
The generation amount of the NOx component is suppressed.

The engine 1 controlled as shown in the map of FIG. 2 has the following features (i) to (iii). That is, (i) the turbocharger 15 absorbs the heat of the exhaust gas and the temperature of the exhaust gas drops to about 100 ° C.

(Ii) In response to the restriction in the stratified combustion region due to the air-fuel ratio in the vicinity of the spark plug 7 becoming too rich, increasing the air charge by the turbocharger 15 increases the air load (high load). (& High vehicle speed side).

(Iii) The unburned fuel additionally injected in the expansion stroke and oxygen in the exhaust gas are stirred by the turbine 15a (stirring effect), and the unburned fuel is reburned in the exhaust passage 9 and the exhaust gas is exhausted. The temperature rises and the HC in the exhaust gas decreases.

However, when an exhaust gas is purified by mounting a NOx trap catalyst on the engine 1 having the above characteristics, the following problems (iv) and (v) occur. (Iv) When the low load operation is continued, the temperature of the NOx trap catalyst decreases due to the decrease in the exhaust gas temperature (the temperature at which the optimum purification characteristics can be obtained is about 350 ° C.), and the NO
x's trapping ability is reduced.

(V) The unburned fuel is recombusted by the agitation effect of the turbine 15a, and HC, which is a NOx reducing agent, is reduced, and the ability to purify the released NOx is reduced.

Therefore, in this embodiment, the following control (1) is performed as a countermeasure against the above-mentioned adverse effect (iv). That is,
(1) If the NOx trap amount is estimated and reaches a predetermined NOx threshold value, when the temperature of the NOx trap catalyst is equal to or lower than the predetermined temperature, fuel is additionally injected in the compression stroke while the operation in the stratified combustion region is performed and the fuel is emptied. The fuel ratio is set to rich (λ <1).
When the temperature of the NOx trap catalyst is equal to or higher than a predetermined temperature,
It is set to rich by reducing the amount of air in the uniform combustion region.

On the other hand, in the present embodiment, the following controls (2) to (4) are performed as countermeasures against the adverse effect (v).
That is, (2) NOx is reduced by an amount corresponding to the reduction of the NOx releasing effect.
When a predetermined trap amount smaller than the allowable trap amount determined from the NOx trap characteristic of the trap catalyst is reached, the air-fuel ratio is set to rich.

(3) As much as the NOx releasing effect is reduced,
The air-fuel ratio is made larger than the rich degree determined from the NOx releasing characteristics of the NOx trap catalyst, or the rich period is made longer.

(4) When the air-fuel ratio is set to be rich, the waste gate valve 32 is opened to control the exhaust gas to bypass the turbine 15a. More specifically, the control of the waste gate valve 32 reaches the allowable trap amount determined from the NOx trap characteristic of the NOx trap catalyst after a delay time from the start of enrichment of the air-fuel ratio to the reduction of the supercharging pressure. The engine is opened at that time, and the enrichment of the air-fuel ratio is started in accordance with the change in the supercharging pressure.

By the above control, the time from the start of enrichment of the air-fuel ratio to the enrichment of the air-fuel ratio can be shortened, and NOx emission can be executed without loss. When returning to the lean state, first, the waste gate valve 32 is closed, and the supercharging pressure is increased before the lean state. With this configuration, it is possible to prevent a misfire or the like due to insufficient boost pressure under a stratified combustion region in an atmosphere with a rich air-fuel ratio. [Control Mode of Air-Fuel Ratio] FIG. 3 is a flowchart illustrating a control mode of the air-fuel ratio of the present embodiment.

As shown in FIG. 3, in step S1, an estimated trapping amount X of NOx trapped from the last end of NOx release to the present in a stratified combustion region having a lean air-fuel ratio is calculated.

In the next step S3, the NOx release delay time A due to the stirring effect of the turbine 15a is calculated.

In step S5, the amount of NOx increase B trapped during the NOx release delay time A is estimated and calculated.

In step S7, the integrated trap amount X
Is the sum of NOx increase amount B and NOx of the NOx trap catalyst.
It is determined whether or not the allowable trap amount X0 determined by the trap characteristic has been exceeded. If the allowable trap amount X0 is less than the allowable trap amount X0 (NO in step S7), the process returns to step S1 and repeats the above processing.

On the other hand, in step S7, the allowable trap amount X0
Is reached (YES in step S7),
The program proceeds to 9, where the air-fuel ratio is set to rich (rich spike) and NOx is released.

In step S11, the calculation for estimating the accumulated trap amount X is continued.

In step S13, the calculation of estimating the integrated trap amount X and the estimation are continued until the NOx release delay time A elapses from the start of NOx release in step S9, and if the time A has elapsed (step S13). YES), the NOx release amount Y is calculated in step S15.

In step S17, the residual NOx trap amount XY is calculated, and in step S19, the calculation of the NOx release amount Y and the residual NOx trap amount XY is continued until the residual NOx trap amount XY becomes zero.

In step S19, the remaining NOx trap amount X
When Y becomes zero (YES in step S19), the air-fuel ratio is made lean in step S21, and the NOx emission is terminated. [Calculation of NOx integrated trap amount X] Step S1
The estimation of the integrated trap amount X of NOx is performed according to the flowchart shown in FIG. 4, and the basic concept of this estimation will be described first with reference to FIGS.

As shown in FIG. 5, during the lean operation, the NOx trappable amount Xd that can be trapped per unit time by the NOx trap catalyst 17 decreases with time, and the NOx trap catalyst 17 passes without being trapped. The NOx passing amount Xf increases. First from the combustion chamber 4 to the exhaust passage 9
The amount of NOx in the exhaust gas discharged to the NOx is referred to as an NOx initial discharge amount (Raw · NOx amount) Xb. The NOx trap catalyst 17 is a selective reduction catalyst, and reduces and purifies a certain amount of NOx components even during lean operation. Assuming that the selective reduction purification rate is “α” and the selective reduction purification amount is “Xn”, N
NOx supply amount Xc supplied to Ox trap catalyst 17
Is a value obtained by subtracting the purification amount Xn from the NOx initial discharge amount Xb. The remaining value obtained by subtracting the NOx trappable amount Xd from the NOx supply amount Xc is the NOx passage amount Xf.

The curves a and b shown in FIG.
Represents the change over time. The solid line a indicates the case where the exhaust gas temperature is low, and the broken line b indicates the case where the exhaust gas temperature is high. The accumulated trap amount X is represented by an area surrounded by the curve a or b up to that point and the supply amount Xc. In FIG. 5, a case where the exhaust gas temperature is low is shown as a hatched portion by taking a as an example.

When the exhaust gas temperature is low, the instantaneous NOx trappable amount Xd is large over a long period of time, and therefore the NOx passage amount Xf is small over a long period of time, as compared with b when the exhaust gas temperature is high. That is, the purification ability of the NOx trap catalyst 17 is maintained at a high level for a long period of time. Similarly, when the exhaust gas temperature is low, the accumulated trap amount X increases earlier in comparison with b when the exhaust gas temperature is high. However, as described above, the purification ability of the NOx trap catalyst 17 is high. Therefore, when the start of the NOx release process is determined only by the accumulated trap amount X, the NOx release process is executed promptly for the catalyst with high purification capability, while the NOx release process is easily performed for the catalyst with low purification capability. It is unreasonable not to run.

Therefore, not only when the accumulated trap amount X exceeds the predetermined allowable amount, but also irrespective of such determination conditions, the instantaneous NOx trappable amount Xd becomes smaller than the predetermined amount, and the NOx passing amount becomes smaller. It is preferable to start the NOx release process also when Xf has exceeded a predetermined amount. This makes it difficult to reliably increase the amount of NOx released through the NOx trap catalyst 17 to the atmosphere.

Further, the instantaneous NOx trappable amount Xd is
It decreases as the accumulated trap amount X increases. That is, as the value of the accumulated trap amount X itself increases, the amount of increase per unit time decreases. Therefore, the estimation accuracy is improved by estimating the integrated trap amount X in consideration of this tendency.

FIG. 6 shows a change in the instantaneous NOx trappable amount Xd with respect to a change in the integrated trap amount X. As described above, the instantaneous NOx trappable amount Xd
Decreases as the integrated trap amount X increases.
What is important here is that the instantaneous NOx trapping amount Xd is not always the instantaneous NOx trapping amount Xe. The integrated trap amount X is substantially a value obtained by integrating the instantaneous NOx trap amount Xe.

When the instantaneous NOx trapping amount Xd is greater than the instantaneous NOx supply amount Xc, the instantaneous NOx supply amount X
c is trapped, the instantaneous NOx trap amount X
The instantaneous NOx supply amount Xc is adopted as the value of e.
Conversely, when the instantaneous NOx supply amount Xc is larger than the instantaneous NOx trappable amount Xd, the instantaneous NOx supply amount X
c is not trapped in its entirety, but passes through a part thereof. Therefore, the value of the instantaneous NOx trap amount Xe is
The x trappable amount Xd is employed.

Therefore, the instantaneous NOx supply amount Xc and the instantaneous NOx
The instantaneous NOx trap amount Xe is rationally estimated by comparing the smaller value with the Ox trappable amount Xd and setting the smaller value as the instantaneous NOx trap amount Xe. As a result, the estimation accuracy of the integrated trap amount X is improved. Will be done. Then, the instantaneous NOx trappable amount Xd is
When adopted as the value of the x trap amount Xe, the instantaneous NOx trap amount Xe decreases as the integrated trap amount X increases.

The estimation of the accumulated trap amount X starts by calculating the engine load based on the engine speed and the accelerator opening in step S31 of the flowchart of FIG. Next, in step S32, the NOx initial emission amount Xb is set based on the engine load and the engine speed. Here, the NOx initial emission amount Xb is set to a larger value as the engine load is larger and the engine speed is higher. Next, in step S33, the selective reduction purification rate α of the NOx trap catalyst 17 is set based on the exhaust gas temperature and the NOx initial exhaust concentration Cn. The selective reduction purification rate α is set to a smaller value as the exhaust gas temperature is higher, and is set to a larger value as the NOx initial exhaust concentration Cn is higher.

Next, in step S34, the NOx supply amount Xc is set based on the selective reduction purification rate α and the NOx initial discharge amount Xb. The NOx supply amount Xc is obtained, for example, according to the following equation.

Xc = Xb × (1−α) Here, the selective reduction purification rate α is a numerical value from 0 to 1 (0 ≦ α ≦ 1). (Xb × α) is the selective reduction purification amount Xn.

Next, in step S35, the NOx supply amount Xc, the accumulated trap amount X, and the NOx
The trappable amount Xd is set. The NOx trappable amount Xd is set to a larger value as the NOx supply amount Xc is larger, and is set to a smaller value as the integrated trap amount X is larger.
The value is set to a smaller value as the exhaust gas temperature is higher.

Next, in step S36, the smaller value of the NOx supply amount Xc and the NOx trappable amount Xd is set as the current value of the NOx trap amount, that is, the instantaneous NOx trap amount Xe. Then, in step S37, the accumulated trap amount X is updated by adding the instantaneous NOx trap amount Xe to the accumulated trap amount X, and the current value of the accumulated trap amount X is obtained. [Calculation of residual NOx trap amount XY] Step S1
The estimation of the remaining NOx trap amount XY in FIG. 7 is performed according to the flowchart shown in FIG.
At 41, the current value of the NOx release amount, that is, the instantaneous NOx release amount Ye is set based on the air-fuel ratio, the exhaust gas flow rate, the remaining NOx trap amount XY, and the exhaust gas temperature. Here, the instantaneous NOx release amount Ye is set to a larger value as the air-fuel ratio becomes richer, set to a larger value as the exhaust gas flow rate increases, and set to a larger value as the residual NOx trap amount XY increases. The higher the temperature, the larger the value.

Then, in step S42, this instant NO
By subtracting the x release amount Ye from the remaining NOx trap amount XY, the remaining NOx trap amount XY is updated, and the current value of the remaining NOx trap amount XY is obtained. [Control Mode of Wastegate Valve] FIG. 8 is a flowchart for explaining a control mode of the wastegate valve of the present embodiment. FIG. 9 is a time chart showing changes in the air-fuel ratio and the NOx trap amount, the supercharging pressure, and the operation timing of the wastegate valve when the flow of FIG. 8 is implemented.

As shown in FIGS. 8 and 9, step S5
3, in the stratified operation region where the waste gate valve 32 is closed and the air-fuel ratio is lean, as in step S1 in FIG.
Estimate calculation of the integrated trap amount X of x.

In the next step S53, a supercharging pressure reduction time C from when the waste gate 32 is opened until the supercharging pressure actually starts to decrease is calculated.

In step S55, the amount of NOx increase D trapped during the boost pressure drop time C is estimated and calculated. This is a process for considering the NOx increase amount D that increases during the delay time until the boost pressure actually starts to decrease.

In step S57, the sum of the accumulated trap amount X and the NOx increase amount D is the NO of the NOx trap catalyst.
It is determined whether or not the allowable trap amount X0 determined from the x trap characteristic has been exceeded. If the allowable trap amount X0 is less than the allowable trap amount X0 (NO in step S57), the process returns to step S51 and repeats the above processing.

On the other hand, in step S57, the allowable trap amount X
If it has reached 0 (YES in step S57), the process proceeds to step S59, and the wastegate valve 32 is opened.

If the supercharging pressure is lower than the predetermined pressure in step S61 (YES in step S61), step S6 is performed.
The program proceeds to 3, where the air-fuel ratio is set to rich (rich spike) and NOx emission is executed.

In step S65, the calculation of estimating the accumulated trap amount X is continued.

In step S67, a supercharging pressure rising time E from when the wastegate valve 32 is closed until the supercharging pressure actually starts to increase is calculated.

In step S69, the amount of NOx released F during the boost pressure rising time E is estimated and calculated. This is a process for considering the NOx release amount F released during the delay time until the boost pressure actually starts increasing.

In step S71, step S1 in FIG.
Similar to 7, the NOx release amount Y is calculated.

In step S73, the residual NOx trap amount XY is calculated, and in step S75, the calculation of the NOx release amount Y and the residual NOx trap amount XY is continued until the residual NOx trap amount XY becomes zero.

In step S75, the remaining NOx trap amount X
If Y has become zero (YES in step S75), the flow advances to step S77 to close the wastegate valve 32.

If the supercharging pressure rises above the predetermined pressure in step S79 (YES in step S79), step S8
The program proceeds to 1, and the air-fuel ratio is made lean to release NOx.

The calculation of the integrated trap amount in step S51 and the remaining NOx trap amount XY in step S73 are as described with reference to FIGS. [Fuel Injection Control Based on EGR Control] FIG. 10 is a flowchart illustrating a fuel injection control based on EGR control according to the present embodiment.

As shown in FIG. 9, in step S91,
As in step S1 of FIG. 3, the estimated trapping amount X of NOx from the end of the previous NOx release to the present is estimated and calculated.

In the next step S93, it is determined whether or not the accumulated trap amount X has exceeded the allowable trap amount X0 determined by the NOx trap characteristic of the NOx trap catalyst. If it is smaller than the allowable trap amount X0 (N in step S93)
O), the process returns to step S91 to repeatedly execute the above processing.

On the other hand, in step S93, the allowable trap amount X
If it has reached 0 (YES in step S93), the process proceeds to step S95, in which the air-fuel ratio is set to rich (rich spike) and NOx emission is executed.

In step S97, the calculation for estimating the accumulated trap amount X is continued.

In step S99, it is determined whether or not the temperature of the NOx trap catalyst 17 has exceeded a predetermined temperature. If the temperature has not exceeded the predetermined temperature (NO in step S99).
In step S101, fuel is additionally injected in the expansion stroke to set the air-fuel ratio to rich.

If the temperature exceeds the predetermined temperature in step S99 (YES in step S99), step S1
In step 03, the air-fuel ratio is set to be rich by reducing the intake air amount in the uniform operation region.

In step S105, if the EGR valve 19 is open (YES in step S105), the process proceeds to step S107, and the fuel injection amount in the compression stroke is corrected in accordance with the degree of opening of the EGR valve 19. This prevents the unburned fuel injected in the expansion stroke from being recirculated to the intake system and the air-fuel ratio in the next compression stroke from becoming too rich.

If the EGR valve 19 has not been opened in step S105 (NO in step S105),
The process proceeds to step S109 without performing the fuel injection amount decrease correction.

In step S109, the NOx release amount Y is calculated.

In step S111, step S in FIG.
Similarly to step 17, the residual NOx trap amount XY is calculated, and the processing from step S99 is continued until the residual NOx trap amount XY becomes zero in step S113.

If the residual NOx trap amount XY becomes zero in step S113 (YE in step S113)
S), the process proceeds to step S115, the air-fuel ratio is made lean, and the NOx emission is terminated.

The control modes described with reference to FIGS. 3 and 8 may be executed by switching according to the engine load. More specifically, in the low load operation state where the engine speed is lower than the predetermined vehicle speed and the exhaust gas amount (<predetermined amount) is small in the stratified combustion region, the process of FIG. 3 is executed, while the exhaust gas amount is higher than the predetermined vehicle speed. In the high-load operation state where the (> predetermined amount) is large, one or both of the processes in FIG. 3 and FIG. 8 are executed. The reason for such control is that if the amount of exhaust gas is small, most of the exhaust gas eventually flows to the turbine 15a and the amount of exhaust gas passing through the wastegate is small. This is because the effect of supplying HC while suppressing the stirring effect is reduced.

In the low load operation region where the amount of exhaust gas is small, the air-fuel ratio may be controlled by changing any of the rich degree, the rich period, and the allowable trap amount, or a combination thereof.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a control system of an engine according to an embodiment.

FIG. 2 is a diagram showing an air-fuel ratio map for the engine of the embodiment.

FIG. 3 is a flowchart illustrating a control mode of an air-fuel ratio according to the present embodiment.

FIG. 4 is a flowchart illustrating a procedure for estimating an integrated trap amount of NOx in FIG. 3;

FIG. 5 is a diagram for explaining the theory of estimating the integrated trap amount of NOx.

FIG. 6 is a diagram for explaining the theory of estimating the integrated trap amount of NOx.

FIG. 7 is a flowchart showing a procedure for estimating a residual NOx trap amount in FIG. 3;

FIG. 8 is a flowchart illustrating a control mode of the wastegate valve according to the embodiment.

FIG. 9 shows the air-fuel ratio and NOx when the flow of FIG. 8 is implemented.
5 is a time chart showing a change in a trap amount, a supercharging pressure, and an operation timing of a wastegate valve.

FIG. 10 is a flowchart illustrating a fuel injection control mode based on EGR control according to the present embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine 2 Intake valve 3 Intake passage 4 Combustion chamber 5 Fuel injection valve 6 Piston 7 Spark plug 8 Exhaust valve 9 Exhaust passage 10 Fuel supply passage 11 High-pressure fuel pump 12 Electric throttle valve 12a Electric actuator 13 Gas flow control valve 14 Linear O ₂ Sensor 15 Turbocharger 15a Turbine 16 Upstream catalytic converter (three-way catalyst) 17 Downstream catalytic converter (NOx trap catalyst) 18 EGR passage 19 EGR valve 20 Control unit 21 Surge tank 22 Variable valve timing mechanism 31 Westgate 32 Westgate valve

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02B 37/18 F02D 23/00 E F02D 23/00 43/00 301E 43/00 301 301R 301T F02B 37/12 301A F-term (reference) 3G005 EA16 FA35 GB28 GD11 GD16 HA04 HA12 HA18 HA19 JA02 JA36 JA39 JA45 3G084 AA03 BA08 BA09 DA10 EB02 FA07 FA10 FA20 FA27 FA29 FA33 FA38 3G091 AA02 AA10 AA11 AA13 AA18 EA01 EA01 EA01 EA01 FB12 3G092 AA06 AB02 BA01 BA03 BA07 BB01 DB03 DC03 DC09 EA05 FA17 HA01Z HA06Z HA16X HB01X HD02Z HD05Z HD07Z HE01Z HE03Z HE08Z 3G301 HA04 HA11 HA16 HA18 JA25 LA01 LA03 LB04 MA01 MA11 MA18 NE01 NE13 PA01Z PA11Z12

Claims (6)

[Claims] 1. A turbocharger comprising: a trap catalyst for trapping NOx in exhaust gas downstream of a turbocharger in an exhaust passage; a bypass passage for bypassing the turbocharger; and a bypass valve for opening and closing the bypass passage. In the exhaust purification device of the supercharged engine, the amount of NOx trapped by the trap catalyst is estimated, and if it is determined that the amount of NOx has reached the threshold, the air-fuel ratio of the exhaust gas passing through the trap catalyst is rich. An engine with a turbocharger, comprising: an air-fuel ratio setting unit that sets the air-fuel ratio to be rich by the air-fuel ratio setting unit; and a valve control unit that opens the bypass valve. Exhaust purification equipment. 2. The turbocharger according to claim 1, wherein the valve control means opens the bypass valve when the amount of exhaust gas exceeds a predetermined amount in a predetermined operation region of the engine. Exhaust gas purification device for engine. 3. The air-fuel ratio setting means according to claim 1, wherein when the engine is in the predetermined operation region, the air-fuel ratio setting means corrects at least one of a rich degree of the air-fuel ratio, a rich period, and the threshold value. Exhaust gas purification device for engines with turbochargers. 4. An exhaust purification device for a turbocharged engine equipped with a trap catalyst for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage, wherein an amount of NOx trapped by the trap catalyst is estimated. If it is determined that the NOx amount has reached the threshold value, the air-fuel ratio setting means for making the air-fuel ratio of the exhaust gas passing through the trap catalyst rich, and the air-fuel ratio rich degree set by the air-fuel ratio setting means, Correction means for increasing the amount of unburned components in the exhaust gas in accordance with the amount of re-combustion by stirring of the turbocharger. 5. An exhaust purification device for a turbocharged engine equipped with a trap catalyst for trapping NOx in exhaust gas downstream of the turbocharger in an exhaust passage, wherein an amount of NOx trapped by the trap catalyst is estimated, If it is determined that the NOx amount has reached the threshold value, the air-fuel ratio setting means for making the air-fuel ratio of the exhaust gas passing through the trap catalyst rich, and the air-fuel ratio rich period set by the air-fuel ratio setting means, An exhaust purification device for an engine with a turbocharger, comprising: a correction means for increasing the amount of unburned components in the exhaust gas in accordance with the amount of reburning by stirring of the turbocharger. 6. An exhaust purification system for a turbocharged engine equipped with a trap catalyst for trapping NOx in exhaust gas downstream of a turbocharger in an exhaust passage, wherein an amount of NOx trapped by the trap catalyst is estimated. Air-fuel ratio setting means for enriching the air-fuel ratio of the exhaust gas passing through the trap catalyst if it is determined that the NOx amount has reached the threshold value; and, depending on the trap performance of the trap catalyst, in correcting the threshold value And a correcting means for correcting the unburned component in the exhaust gas to a value smaller than the allowable trap amount by an amount of NOx which increases by recombustion of the exhaust gas by stirring the turbocharger. Exhaust purification system for turbocharged engines.