JP4000435B2 - Exhaust gas purification device for in-cylinder injection internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification device for an internal combustion engine, and more particularly to an exhaust gas purification device that can regenerate a NOx catalyst device while suppressing deterioration in fuel consumption and torque fluctuation.
[0002]
[Related technology]
NOx catalysts that purify nitrogen oxides (NOx) in exhaust gas from lean combustion internal combustion engines have been put into practical use. As this type of NOx catalyst, there is one that captures NOx in exhaust gas during a lean combustion operation of an internal combustion engine and reduces the captured NOx during a rich or stoichiometric combustion operation. In the exhaust gas purification apparatus equipped with such a catalyst, when reducing the trapped NOx, for example, as described in Japanese Patent No. 2600492, the air-fuel ratio by main injection is enriched, or as described in Japanese Patent No. 2845103. By performing sub-injection (additional fuel injection) in the expansion stroke or exhaust stroke after the main injection (main fuel injection) is performed, the oxygen concentration in the exhaust gas flowing into the NOx catalyst is lowered to reduce NOx Techniques for promoting reduction are known.
[0003]
[Problems to be solved by the invention]
However, in order to reduce the oxygen concentration in the exhaust gas flowing into the NOx catalyst and promote the reduction of NOx as in the above known technique, the air-fuel ratio in the main injection is enriched or the operation mode is switched for that purpose. Alternatively, it is necessary to inject a large amount of fuel at the time of sub-injection, which causes a problem of torque fluctuation and deterioration of fuel consumption.
[0004]
An object of the present invention is to provide an exhaust purification device for a direct injection internal combustion engine that can regenerate the purification capability of the NOx catalyst device without causing torque fluctuations or fuel consumption deterioration.
[0005]
[Means for Solving the Problems]
  The exhaust emission control device according to the first aspect of the present invention is the main injection under the restriction that when reducing NOx trapped in the NOx catalyst device, the oxygen concentration in the exhaust gas flowing into the NOx catalyst device is hardly reduced. The fuel injection valve is operated by the pulse injection means during the subsequent expansion stroke or exhaust stroke.And the oxygen concentration in the exhaust gas at the inlet of the three-way catalyst device arranged downstream of the NOx catalyst device is zero based on the oxygen sensor output representing the oxygen concentration in the exhaust gas downstream of the NOx catalyst device. Alternatively, the sub-injection amount is set so as to be a value in the vicinity thereof, and the sub-injection is performed by operating the fuel injection valve by the pulse injection meansIt is characterized by that.
[0006]
  According to the first aspect of the present invention, when a decrease in the purification performance (NOx storage capacity) of the NOx catalyst device is determined under appropriate determination conditions by appropriate means, for example, during the expansion stroke after main injection. Alternatively, the fuel injection valve is operated by the pulse injection means during the exhaust stroke. The fuel supplied into the combustion chamber during the expansion stroke or the exhaust stroke burns incompletely, unburned components of the fuel are discharged to the exhaust passage, and the exhaust air-fuel ratio is enriched. As a result, the NOx trapped in the NOx catalyst device is reduced, and the purification performance of the NOx catalyst device is regenerated. In the present invention, the main injection and the sub-injection during the NOx purge operation, in particular, the operation of the fuel injection valve for NOx reduction (sub-injection) hardly restricts the oxygen concentration in the exhaust gas flowing into the NOx catalyst device. Therefore, when the NOx catalyst device is regenerated, enrichment of the air-fuel ratio, switching of the fuel injection mode, or a large amount of sub-injection is performed in the main injection in order to reduce the oxygen concentration in the exhaust gas as in the known art. This is not performed, and torque fluctuation and fuel consumption deterioration during catalyst regeneration are eliminated or reduced.
  Further, the invention according to claim 1 is directed to the oxygen in exhaust gas at the inlet of the three-way catalyst device arranged downstream of the NOx catalyst device based on the oxygen sensor output representing the oxygen concentration in the exhaust gas downstream of the NOx catalyst device. Sub-injection is performed by setting the sub-injection amount so that the concentration becomes zero or a value in the vicinity thereof. By this, the purification performance of the three-way catalyst device is fully exhibited, and purification is performed by the NOx catalyst device. NOx that has not been purified can be purified.
  In the present invention, it is preferable that the NOx catalyst device has an oxidation function (three-way function). In this case, hydrocarbons discharged as unburned fuel components into the exhaust passage are converted into carbon monoxide that promotes reduction of NOx by an oxidation reaction in the NOx catalyst, contributing to NOx purification.
[0007]
In the first aspect of the present invention, there is a restriction that the unburned fuel component discharged to the exhaust passage with the operation of the fuel injection valve by the pulse injection means is hardly oxidized in the exhaust passage on the upstream side of the NOx catalyst device. Below, it is preferable to operate the fuel injection valve by the pulse injection means. In this preferred embodiment, since the unburned components of the fuel injected during the expansion stroke of the internal combustion engine are hardly oxidized in the exhaust passage upstream of the NOx catalyst device, the unburned fuel components are oxidized in the exhaust gas. Oxygen is never consumed. Therefore, coupled with the main feature of the present invention in which the air-fuel ratio is not enriched in the main injection, the oxygen concentration in the exhaust gas flowing into the NOx catalyst device hardly decreases, and a large amount of unburned fuel component exists. The portion flows into the NOx catalyst device and contributes to the promotion of NOx reduction. In this way, the purification capability of the NOx catalyst device is efficiently regenerated without lowering the oxygen concentration in the exhaust gas flowing into the NOx catalyst, causing torque fluctuations, or worsening fuel consumption.
[0008]
In the first aspect of the present invention, it is preferable that the operation of the fuel injection valve by the pulse injection means is performed in the expansion stroke after the main injection performed in the compression stroke. Thus, when the main injection is performed in the compression stroke, for example, the exhaust temperature becomes lower than when the main injection is performed in the intake stroke, and the unburned fuel component is less likely to be oxidized in the exhaust passage upstream of the NOx catalyst device. Therefore, coupled with the main feature of the present invention that the air-fuel ratio is not enriched in the main injection, the oxygen concentration in the exhaust gas flowing into the NOx catalyst device is not lowered, and most of the unburned fuel component Contributes to NOx release and reduction from the NOx catalyst device and promotes regeneration of the NOx catalyst device.
[0010]
In the present invention, it is preferable that the NOx catalyst device has an oxidation function (three-way function). In this case, hydrocarbons discharged as unburned fuel components into the exhaust passage are converted into carbon monoxide that promotes reduction of NOx by an oxidation reaction in the NOx catalyst, contributing to NOx purification.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a lean combustion internal combustion engine equipped with an exhaust emission control device according to an embodiment of the present invention will be described.
The lean combustion internal combustion engine of the present embodiment is an in-cylinder injection spark ignition type in-line four-cylinder gasoline engine that can perform fuel injection in a compression stroke and an expansion stroke as needed in addition to fuel injection in an intake stroke. It is configured. The fuel injection mode in this in-cylinder injection engine changes variously according to changes in the engine operating range, and accordingly, the air-fuel ratio of the air-fuel mixture changes from the super-lean air-fuel ratio to the rich air-fuel ratio. The fuel consumption and the exhaust characteristics are improved while generating the engine output. This type of in-cylinder injection engine is conventionally known, but will be briefly described below.
[0012]
As shown in FIG. 1, an electromagnetic fuel injection valve 6 is attached to the cylinder head 2 of the engine 1 together with a spark plug 4 for each cylinder. The fuel injection valve 6 is connected to a fuel supply device (not shown) having a fuel tank, a low-pressure fuel pump and a high-pressure fuel pump via a fuel pipe, and the fuel in the fuel tank is transferred from the fuel injection valve 6 to the combustion chamber 8. Can be directly injected at a desired fuel pressure.
[0013]
An intake port is formed in the cylinder head 2 in a substantially upright direction for each cylinder, and each intake port communicates with one end of the intake manifold 10. A throttle valve 11 provided on the other end side of the intake manifold 10 is provided with a throttle sensor 11a for detecting a throttle opening θth. Further, an exhaust port is formed in the cylinder head 2 in a substantially horizontal direction for each cylinder, and each exhaust port communicates with one end of the exhaust manifold 12.
[0014]
A muffler (not shown) is connected to the exhaust manifold 12 via an exhaust pipe (exhaust passage) 14, and the exhaust pipe 14 is provided with a high temperature sensor 16 for detecting the exhaust temperature.
The exhaust purification device 30 of the present embodiment stores an NOx in the exhaust gas when the exhaust air-fuel ratio is lean and stores and stores the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich (NOx). (Occlusion catalyst device) 30a and a three-way catalyst 30b arranged downstream thereof. Reference numeral 18 represents an oxygen sensor that is arranged between the NOx catalyst 30a and the three-way catalyst 30b and detects the oxygen concentration in the exhaust gas at the inlet of the three-way catalyst 30b in the exhaust gas flow direction. The NOx sensor 32 arranged downstream of the catalyst 30b and detecting the NOx concentration is shown.
[0015]
The storage-type NOx catalyst 30a is composed of alumina containing a catalyst species composed of noble metals such as platinum (Pt) and rhodium (Rh) and a NOx storage agent composed of alkali metals and alkaline earth metals such as barium (Ba) and potassium (K). As shown in FIG. 2, basically, NOx is once converted to nitrate X-NO in an oxidizing atmosphere as shown in FIG._Three(Where symbol X represents Ba or K, for example) and NOx is reduced to N in a reducing atmosphere mainly containing CO.₂It has the function of reducing to (nitrogen) etc.
[0016]
When the NOx storage capability of the NOx catalyst 30a is saturated and the NOx purification capability is lost, NOx is released into the atmosphere. Therefore, in order to regenerate the NOx storage capability and thus the NOx purification capability of the NOx catalyst 30a, generally, the air-fuel mixture A so-called NOx purge for enriching is periodically performed.
In the NOx purge, the NOx purge technique described in Japanese Patent No. 2600492 makes it necessary to switch the operation mode when shifting to the NOx purge in order to enrich the air-fuel ratio of the air-fuel mixture at the time of main injection related to the NOx purge. A shock is likely to occur in the engine output torque with the mode switching. In addition, torque shock due to operation mode switching can be reduced by reducing the change rate of the air-fuel ratio at the time of main injection by tailing. In this case, however, the air-fuel ratio control procedure becomes complicated and during the air-fuel ratio control. There is a problem of passing through an air-fuel ratio region that causes a shortage of reducing agent.
[0017]
In addition, in the NOx purge technique described in Japanese Patent No. 2845103 or the like, a large amount of reducing agent such as hydrocarbon or carbon monoxide is exhausted by performing sub-injection in the expansion stroke or exhaust stroke after the main injection. The oxygen concentration in the exhaust gas is decreased by causing the reducing agent to oxidize with O 2− etc. on the NOx catalyst to promote NOx release._2-NOx is reduced to nitrogen or the like by the remaining reducing agent that did not contribute to the oxidation reaction. For this reason, it is necessary to sub-inject a large amount of fuel. In addition, during the NOx purge, generally, the air-fuel ratio related to the main injection is enriched or the fuel injection mode is switched, which causes fuel consumption deterioration and torque fluctuation along with a large amount of sub-injection. In this technique, carbon monoxide, which is the main reducing agent that contributes to NOx reduction, is supplied by sub-injection. Carbon monoxide is not only reduced in NOx but also has a considerable amount to promote NOx release. Since it is consumed, the fuel injection amount related to the sub-injection increases.
[0018]
In the exhaust purification system of the present embodiment, focusing on the above-mentioned factors of torque shock occurrence and increase in the sub-injection amount, fuel consumption deteriorates by eliminating the need to enrich the air-fuel ratio at the time of main injection in NOx purge and switching the operation mode therefor. In addition, unburned fuel components (reducing agents) such as hydrocarbons and carbon monoxide that are discharged from the combustion chamber to the exhaust passage by the sub-injection are mostly in the exhaust passage upstream of the NOx catalyst 30a. By performing sub-injection under conditions that do not oxidize, consumption of unburned fuel components due to oxidation reaction is suppressed, and further, by increasing the oxidation function (three-way function) of the NOx catalyst 30a, carbonization that flows into the NOx catalyst. Hydrogen is converted to carbon monoxide by an oxidation reaction on the catalyst to obtain carbon monoxide necessary for NOx reduction, thereby making it possible to reduce the sub-injection amount.
[0019]
Specifically, in the exhaust purification system of the present embodiment, the main injection is performed while maintaining the air-fuel ratio in the main injection related to the NOx purge operation at the same value (for example, 30) as the air-fuel ratio in the lean combustion operation prior to the NOx purge operation. Sub-injection is carried out in the middle or later stage of the expansion stroke, and at this time, the total air-fuel ratio of the main injection and the sub-injection is a predetermined value (for example, slightly richer than the theoretical air-fuel ratio (14.7)). 14), the air-fuel ratio in the sub-injection is set. This total air-fuel ratio is a leaner value than the air-fuel ratio (for example, 12) according to the conventional NOx purge operation.
[0020]
FIG. 3 shows the relationship between the total air-fuel ratio, the NOx purification efficiency, and the oxygen concentration. In FIG. 3, the ◯ mark, Δ mark, and ● mark indicate the NOx purification efficiency with respect to the total air-fuel ratio, the oxygen concentration at the NOx catalyst inlet, and the NOx. The change in oxygen concentration at the catalyst outlet is shown. FIG. 3 shows that when the total air-fuel ratio is in the vicinity of the value 14, the NOx purification efficiency is high and the oxygen concentration at the NOx catalyst outlet, that is, the three-way catalyst inlet, is in the vicinity of the value 0.
[0021]
In the present embodiment, the NOx purification action of the three-way catalyst 30b disposed downstream of the NOx catalyst 30a is sufficiently exerted on the NOx catalyst 30a based on the changes in the NOx purification efficiency and the oxygen concentration shown in FIG. In order to be able to purify the NOx that was not present, the air-fuel ratio, the injection time, and the sub-injection amount (the initial value is fixed) according to the sub-injection according to the output of the oxygen sensor 18 that detects the oxygen concentration at the inlet of the three-way catalyst 30b. Stipulated value) is variably controlled.
[0022]
In the present embodiment, the sub-injection for the NOx purge is performed when the lean combustion operation in the compression stroke injection mode is performed prior to the NOx purge operation. In this case, the main injection in the NOx purge operation is also performed by the compression stroke injection, and the exhaust temperature is lower than in the case of performing the main injection by the intake stroke injection, and in the exhaust passage on the upstream side of the NOx catalyst 30a. Unburned fuel components are less likely to be oxidized. As a result, the oxygen concentration in the exhaust gas flowing into the NOx catalyst 30a does not decrease, and most of the unburned fuel component contributes to NOx release and reduction in the NOx catalyst 30a.
[0023]
Further, in the present embodiment, the front catalyst is not disposed on the upstream side of the NOx catalyst 30a, and the unburned fuel component is favorably supplied to the NOx catalyst 30a. However, a front catalyst may be provided as long as it does not interfere with the supply of unburned fuel components to the NOx catalyst 30a.
In addition, in order to enhance the oxidation function (three-way function) of the NOx catalyst 30a, the NOx catalyst 30a of the present embodiment contains a large amount of, for example, Pt and Rh, and functions to convert hydrocarbons to carbon monoxide (CO generation ability). The required amount of carbon monoxide can be secured without supplying carbon monoxide required for NOx purification by sub-injection.
[0024]
According to the exhaust emission control device having the above-described configuration, when NOx is occluded by the NOx catalyst 30a during the lean combustion operation of the engine 1 and the NOx occlusion capacity is saturated, the fuel injection valve 6 is operated during the expansion stroke after the main injection. By performing the valve opening operation by the pulse injection means, the secondary injection for the NOx purge is performed. In the present embodiment, an electronic control unit (ECU) 40 that controls the operation of the engine 1 has the function of the pulse injection means. The ECU 40 includes an input / output device, a storage device (ROM, RAM, non-volatile RAM, etc.), a central processing unit (CPU), a timer counter, etc., and a throttle sensor 11a, a crank angle sensor 13, and a high temperature sensor 16 on its input side. Various sensors such as the oxygen sensor 18 and the NOx sensor 32 are connected, and an ignition plug 4 and a fuel injection valve 6 are connected to the output side.
[0025]
In connection with engine operation control, the ECU 40 selects a fuel injection mode based on detection information input from various sensors and calculates a fuel injection amount, an ignition timing, and the like. For example, the target in-cylinder pressure (target average effective pressure Pe) corresponding to the engine load based on the throttle opening degree information θth from the throttle sensor 11a and the engine rotational speed information Ne detected based on the crank angle information from the crank angle sensor 13. ) Is determined, and the fuel injection mode is set according to the target average effective pressure Pe and the engine rotational speed information Ne. Then, the fuel injection amount is determined based on the target air-fuel ratio (target A / F) set from the target average effective pressure Pe and the engine speed Ne.
[0026]
In connection with the NOx purge, the ECU 40 executes a NOx purge control routine shown in FIG. In the NOx purge control routine, it is determined whether or not a NOx purge condition is satisfied (step S1). Although the NOx purge condition can be set in various ways, in the present embodiment, when the lean combustion operation in the compression stroke injection mode is currently performed and the lean combustion operation is performed for a predetermined time (for example, 30 seconds). It is determined whether the NOx purge condition is satisfied, the current injection mode is other than the compression stroke injection mode, the total execution time of the lean combustion operation is less than a predetermined time, or the NOx purge operation is performed for a predetermined time (for example, When the NOx purge operation is performed immediately after the completion of the NOx purge operation, it is determined that the NOx purge condition is not satisfied.
[0027]
When it is determined in step S1 that the NOx purge condition is satisfied, the NOx purge mode is set and the timer for measuring the elapsed time from the start of the NOx purge operation is reset and started. Then, the NOx purge operation is performed according to the setting of the NOx purge mode (step S2).
Specifically, the fuel injection time (hereinafter referred to as the main injection time) related to the main injection in the NOx purge operation is determined. Here, the main injection time is set to a fuel injection time for achieving the same target air-fuel ratio as the target air-fuel ratio in the lean combustion operation in the current compression stroke injection mode. Next, the fuel injection time related to the sub-injection (hereinafter referred to as sub-injection time) has an air-fuel ratio (for example, a value 14 that is a rich air-fuel ratio in the vicinity of the theoretical air-fuel ratio) suitable for NOx purge by the main injection and the sub-injection. The fuel injection time is set as achieved.
[0028]
Next, the main injection start timing and end timing for performing main injection over the main injection time are determined, and the ignition timing is determined. Further, the sub-injection start timing and end timing for performing the sub-injection over the sub-injection time are determined.
When any of the cylinders of the engine 1 reaches the main injection start time, the fuel injection valve 6 corresponding to the cylinder is opened, and when the main injection end time comes, the fuel injection valve 6 is closed. When the ignition timing comes, the spark plug 4 is activated. Here, the main injection is performed during the compression stroke in the cylinder. In the expansion stroke after the main injection, the fuel injection valve 6 is opened when the sub-injection start time comes, and the fuel injection valve 6 is closed when the sub-injection end time comes.
[0029]
As described above, in step S2, main injection and sub-injection are performed in each cylinder of the engine 1. When the sub-injection ends, the oxygen concentration in the exhaust gas at the inlet of the three-way catalyst 30b is detected based on the output of the oxygen sensor 18, and it is determined whether or not this oxygen concentration is below a specified value (for example, 0.5%). A determination is made (step S3). If the oxygen concentration is less than or equal to the specified value, the sub-injection time, that is, the sub-injection time is corrected so as to decrease the fuel injection amount in the sub-injection by a predetermined value (step S4). On the other hand, if the oxygen concentration is not less than the specified value, the sub injection time is corrected to increase so that the sub injection time increases by a predetermined value (step S5).
[0030]
When the correction of the sub-injection time in step S4 or S5 is completed, the process returns to step S1 and it is determined whether or not the NOx purge mode is still established. If the NOx purge mode is established, the above control procedure is repeated. On the other hand, if the NOx purge mode is not established due to the NOx purge operation being performed for a predetermined time, the NOx purge operation is terminated.
[0031]
As a result of performing the NOx purge control of FIG. 4, for example, if there is a possibility that the lean combustion operation continues for 30 seconds and the purification efficiency of the NOx catalyst 30a decreases, the NOx purge operation is performed for 2 to 3 seconds.
FIG. 5 shows changes with time of the air-fuel ratio and exhaust gas component concentration during the NOx purge operation. In the NOx purge operation, the main injection is performed in the same compression injection mode as the fuel injection mode before shifting to the NOx purge operation, and the air-fuel ratio in the main injection is the air-fuel ratio in the compression lean operation before shifting to the NOx purge. Is maintained at the same value, for example, value 30, and fuel consumption deterioration and torque shock are prevented. The total air-fuel ratio of the main injection and the sub-injection is set to a slightly rich value 14 for example.
[0032]
When sub-injection is performed in the expansion stroke, the fuel is incompletely burned in the combustion chamber, and unburned fuel components such as unburned HC (hydrocarbon) are discharged from the combustion chamber to the exhaust passage 14. For this reason, as shown in FIG. 5, the unburned HC concentration in the exhaust gas at the NOx catalyst inlet rapidly increases when the NOx purge operation is started, and rapidly decreases when the NOx purge operation ends.
[0033]
Since the main injection in the lean combustion operation and the NOx purge operation prior to the NOx purge operation is performed in the compression stroke injection mode, the exhaust temperature is relatively low, and almost no unburned HC exists in the exhaust passage 14 upstream of the NOx catalyst 30a. As shown in FIG. 5, the oxygen concentration in the exhaust gas at the inlet of the NOx catalyst hardly decreases during the NOx purge operation.
A part of the unburned HC undergoes an oxidation reaction in the NOx catalyst 30a having an oxidation function (ternary function), and is converted into carbon monoxide that contributes to the reduction of NOx. Unburned HC and carbon monoxide form a reducing atmosphere around the NOx catalyst 30a, NOx stored in the form of nitrate in the NOx catalyst 30a is released, and then the nitrate is carbonated under the action of carbon monoxide. NOx is reduced by conversion to NOx.
[0034]
In this way, unburned HC is consumed for NOx release and carbon monoxide generation, so the unburned HC concentration at the NOx catalyst outlet is smaller than the value at the NOx catalyst inlet (FIG. 5). Further, in the NOx catalyst 30a, conversion from unburned HC to carbon monoxide or conversion from HC or carbon monoxide to nitrogen gas, carbon dioxide gas or water is performed while consuming oxygen. The oxygen concentration is smaller than the value at the NOx catalyst inlet (FIG. 5). Note that the carbon monoxide concentration in the exhaust gas at the NOx catalyst inlet in the present embodiment is smaller than the value in a conventionally known exhaust purification device (FIG. 5).
[0035]
By the release and reduction of NOx, the NOx storage capacity of the NOx catalyst 30a and thus the purification efficiency are regenerated. That is, as can be seen from the NOx concentration in the exhaust gas at the NOx catalyst inlet and outlet and the three-way catalyst outlet shown in FIG. 5, NOx purification is performed by the NOx catalyst 30a, and NOx that has not been purified by the NOx catalyst 30a is further reduced to three. The original catalyst 30b is purified.
[0036]
The present invention is not limited to the above embodiment, and can be variously modified.
For example, in the embodiment, the first requirement that the lean combustion operation in the compression stroke injection mode is currently performed (that is, the main injection is performed in the compression stroke injection mode) and the lean combustion operation is performed for a predetermined time. However, the NOx purge condition in the present invention is not limited to this.
[0037]
Among the NOx purge conditions in the embodiment, the first requirement is that the unburned fuel component hardly oxidizes in the exhaust passage on the upstream side of the NOx catalyst (in a broad sense, the oxygen concentration in the exhaust gas flowing into the NOx catalyst is almost zero). The second requirement represents a typical example of the second condition that the purification efficiency of the NOx catalyst may be lowered. On the other hand, in the present invention, the establishment of the NOx purge condition may be determined when at least the second condition is satisfied. Since the NOx purification efficiency can be determined from, for example, the estimated value of the NOx adhesion amount on the NOx catalyst, in the present invention, for example, when the estimated amount of the nitrate X-NO3 adhesion amount exceeds the determination reference value, the NOx purge condition The establishment can be determined. In other words, the above first requirement represents that the main injection is performed in the compression stroke injection mode to lower the exhaust temperature, while the exhaust temperature can be lowered by ignition timing control or EGR control. Therefore, it is possible to control the ignition timing and the EGR amount so that the exhaust temperature decreases during the NOx purge operation.
[0038]
In the embodiment, the air-fuel ratio related to the main injection in the NOx purge operation is maintained at the same value as the air-fuel ratio in the lean combustion operation prior to the NOx purge operation. It can be set to a value on the lean side. In this case, it is possible to purge NOx under a state in which the oxygen concentration in the exhaust gas flowing into the NOx catalyst is increased. In this case as well, the total air-fuel ratio of the main injection and the sub-injection is set to the NOx release, A slightly rich value suitable for reduction is preferable.
[0039]
  AlsoIn the embodiment, the fuel injection valve is operated during the expansion stroke after the main injection. However, the fuel injection valve may be operated during the exhaust stroke or during both the expansion stroke and the exhaust stroke. good.
[0040]
Further, in the above embodiment, as the NOx catalyst, the NOx in the exhaust gas is stored when the exhaust air-fuel ratio is lean, and the stored NOx is released and reduced when the exhaust air-fuel ratio is stoichiometric or rich. Although the catalyst is used, the present invention is not limited to this. NOx in the exhaust gas is adsorbed when the exhaust air-fuel ratio is lean, and the adsorbed NOx is separated from the unburned fuel component when the exhaust air-fuel ratio is stoichiometric or rich. You may use the NOx catalyst of the system reduced by contacting and making it react.
[0041]
【The invention's effect】
  The exhaust emission control device according to the present invention reduces the NOx trapped by the NOx catalyst device during the expansion stroke after the main injection under the restriction that the oxygen concentration in the exhaust gas flowing into the NOx catalyst device hardly decreases. Alternatively, since the fuel injection valve is operated during the exhaust stroke, it is possible to regenerate the purification capacity of the NOx catalyst device without causing torque fluctuations or fuel consumption deterioration.
  Further, based on the oxygen sensor output representing the oxygen concentration in the exhaust gas downstream of the NOx catalyst device, the oxygen concentration in the exhaust gas at the inlet of the three-way catalyst device arranged downstream of the NOx catalyst device becomes zero or a value in the vicinity thereof. Since the sub-injection amount is set so that the sub-injection is performed, the purification performance of the three-way catalyst device can be sufficiently exerted, and NOx that has not been purified by the NOx catalyst device can be purified.
[Brief description of the drawings]
FIG. 1 is a schematic view of an internal combustion engine equipped with an exhaust emission control device according to an embodiment of the present invention.
2 is a diagram showing NOx occlusion, release, and reduction actions of the NOx catalyst shown in FIG. 1. FIG.
FIG. 3 is a graph showing the relationship between the total air-fuel ratio of main injection and sub-injection in NOx purge operation, NOx purification efficiency, and oxygen concentration at the NOx catalyst inlet / outlet.
FIG. 4 is a flowchart of a NOx purge control routine executed by the electronic control unit shown in FIG.
FIG. 5 is a diagram showing changes with time of the air-fuel ratio and exhaust gas component concentration during NOx purge operation.
[Explanation of symbols]
1 Engine (Cylinder injection type internal combustion engine)
6 Fuel injection valve
14 Exhaust pipe (exhaust passage)
30 Exhaust purification catalyst device
30a NOx catalyst (NOx storage catalyst device)
30b Three-way catalyst
40 Electronic control unit (pulse injection means)

Claims (1)

In an exhaust purification apparatus for a cylinder injection internal combustion engine having a fuel injection valve for directly injecting fuel into a combustion chamber,
A NOx catalyst device that is provided in the exhaust passage of the internal combustion engine and captures NOx in the exhaust gas when the exhaust air-fuel ratio is a lean air-fuel ratio and reduces the captured NOx when the exhaust air-fuel ratio is the stoichiometric or rich air-fuel ratio When,
When the trapped NOx is reduced, the fuel injection valve is operated during the expansion stroke or the exhaust stroke after the main injection under the constraint that the oxygen concentration in the exhaust gas flowing into the NOx catalyst device is hardly reduced. Pulse injection means for operating and performing secondary injection ;
An oxygen sensor disposed downstream of the NOx catalyst device in the exhaust passage;
A three-way catalyst device disposed downstream of the NOx catalyst device in the exhaust passage,
Based on the output of the oxygen sensor, the sub-injection amount is set so that the oxygen concentration in the exhaust gas at the inlet of the three-way catalyst device becomes zero or a value close thereto, and the operation of the fuel injection valve by the pulse injection means An exhaust purification device for a cylinder injection internal combustion engine, characterized in that the sub-injection is performed by the above .

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