JPH06257487A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To adsorb sufficient HC in a catalyst so as to improve NOX treating ability by controlling HC concentration in exhaust to be increased in the predetermined duration right before switching a target air/fuel ratio between an output air/fuel ratio region and a lean air/fuel ratio region. CONSTITUTION:An exhaust emission controlling catalyst for an internal combustion engine is provided with an air/fuel ratio switching means, which switches a target air/fuel ratio of an engine intake mixture between an output air/fuel ratio region around a stoichiometric air/fuel ratio and a lean air/fuel ratio region being larger than the stoichiometric air/fuel ratio according to an engine operating condition. IN this case, an exhaust emission controlling catalyst, which is provided with HC adsorption ability, is arranged in an exhaust passage, so that NOX is reduced in the existence of HC. On the other hand, HC concentration in an exhaust gas upstream of the exhaust emission controlling catalyst is increased by an HC concentration increasing means. The HC concentration increasing means is controlled by an HC concentration controlling means so that HC concentration in the exhaust gas is increased during the predetermined duration right before switching the target air/fuel ratio between the output air/fuel ratio region and the lean air/fuel ratio region by the air/fuel ratio switching means.

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 apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus having a catalyst for adsorbing HC and reducing NOx in the presence of HC in an exhaust passage. The present invention relates to a technique for maintaining the NOx reduction performance of a catalyst.

[0002]

2. Description of the Related Art In recent years, a lean-burn internal combustion engine that burns in an air-fuel ratio range (for example, an air-fuel ratio of about 20 to 22) that is significantly leaner than the theoretical air-fuel ratio has been developed. A lean NOx catalyst is used to purify NOx in the air-fuel ratio range.

The lean NOx catalyst is mainly composed of zeolite, and is presumed to temporarily adsorb HC in exhaust gas and reduce NOx by this HC. Therefore, N in the lean NOx catalyst is
The presence of HC is essential for the purification process of Ox, and when the HC / NOx ratio in the exhaust gas decreases, the NOx reduction performance decreases.

Therefore, due to lack of HC, lean NO
x when the NOx reduction performance of the x catalyst deteriorates
An exhaust gas purification device has been proposed in which the amount of C is forcibly increased so that the NOx reduction performance can be maintained. As a means for forcibly increasing the amount of HC in exhaust gas, the following means A to C is adopted. (A). By delaying the fuel injection timing, the amount of fuel that flows into the combustion chamber as a wall flow is increased and atomization of the fuel is suppressed, thereby increasing HC in the exhaust gas (see Japanese Patent Laid-Open No. 3-217640). .

(B). By lowering the cooling water temperature, the wall temperature of the combustion chamber is lowered, thereby suppressing atomization of fuel and increasing HC in exhaust gas (see Japanese Patent Laid-Open No. 3-229914). (C). Evaporative fuel adsorbed and collected in the canister, or
Blow-by gas is added to the exhaust gas upstream of the lean NOx catalyst to increase HC in the exhaust gas (Japanese Patent Laid-Open No. 3-242415).
(See Japanese Patent Publication).

[0006]

[Problems to be Solved by the Invention]
Among the means shown in (A) to (C), when the wall flow rate of (A) is increased and the combustion chamber wall temperature of (B) is decreased, incomplete combustion increases and combustion becomes unstable. Therefore, there was a fear that the driving performance would deteriorate. Further, in the case of the means for lowering the combustion chamber wall temperature of (B), after it is judged that HC is insufficient,
There is a large time loss until the temperature of the cooling water drops and the wall temperature of the combustion chamber actually drops, and NOx during this time is exhausted without being sufficiently purified, making it difficult to maintain stable NOx reduction performance. is there.

Further, in the means (C) for adding HC to the exhaust gas, since the unused fuel is added to the exhaust gas, there is a problem that the fuel consumption is deteriorated. Also,
In the lean burn internal combustion engine, when switching the target air-fuel ratio between the output air-fuel ratio range near the stoichiometric air-fuel ratio and the lean air-fuel ratio range, so as not to impair drivability due to a sudden change in torque, The air-fuel ratio is gradually changed. However, as shown in FIG. 4, the NOx emission amount becomes the highest in the air-fuel ratio range (the air-fuel ratio of about 16 to 18) during the switching of the target air-fuel ratio.
The C / NOx ratio is reduced, and the NOx of the lean NOx catalyst is reduced.
There is a problem that a large amount of NOx is discharged when the target air-fuel ratio is switched because the reduction performance is deteriorated.

Here, when the target air-fuel ratio is switched as described above, the fact that the HC amount becomes insufficient with respect to the NOx amount and the NOx reduction performance deteriorates will be compensated by the HC amount increasing means as described above. As a result, the air-fuel ratio corresponds to the air-fuel ratio region where the NOx concentration reaches its peak and the HC in the exhaust gas
If the HC amount increasing means is operated based on the result of directly detecting the decrease in the concentration, a response delay occurs and
HC required to reduce large amounts of NOx emitted
In some cases, the amount cannot be secured, and it has been difficult to maintain the NOx reducing ability stably high.

The present invention has been made in view of the above problems, and the increase control of the amount of HC for maintaining the NOx reduction performance of the lean NOx catalyst does not deteriorate engine drivability and is unused. It is an object of the present invention to be realized without using fuel and to be able to stably maintain the NOx reduction performance when switching the target air-fuel ratio in a lean burn internal combustion engine.

[0010]

Therefore, an exhaust gas purification apparatus for an internal combustion engine according to the present invention is constructed as shown in FIGS. 1 and 2. In FIG. 1, the air-fuel ratio switching means switches the target air-fuel ratio of the engine intake air-fuel mixture between an output air-fuel ratio region near the theoretical air-fuel ratio and a lean air-fuel ratio region larger than the theoretical air-fuel ratio according to the engine operating conditions.

The exhaust purification catalyst is installed in the exhaust passage and is
It is a catalyst that has the adsorption ability of reducing NOx in the presence of HC. Further, the HC concentration increasing means is means for increasing the HC concentration in the exhaust gas on the upstream side of the exhaust purification catalyst. And
The HC concentration control means causes the HC concentration increasing means to increase the HC concentration in the exhaust gas during a predetermined period immediately before the target air-fuel ratio is switched between the output air-fuel ratio range and the lean air-fuel ratio range by the air-fuel ratio switching means.

Here, it is preferable that the HC concentration control means causes the HC concentration increasing means to increase the HC concentration for a longer period when the engine is running at low speed and under low load. On the other hand, in FIG. 2, the exhaust purification catalyst is a catalyst that is interposed in the exhaust passage and has an HC adsorbing capacity and reduces NOx in the presence of HC. The HC increase request detecting means detects a state in which an increase in the amount of HC in the exhaust is required in order to secure the amount of HC required for the reduction of NOx by the exhaust purification catalyst.

Then, the air-fuel ratio enriching means forcibly sets the air-fuel ratio of the engine intake air-fuel mixture to a target air-fuel ratio for a predetermined period when the HC increase request detecting means detects a state in which an increase in the HC amount is requested. Make it richer than the fuel ratio. Furthermore, when the air-fuel ratio enriching means enriches the air-fuel ratio, the generated torque control means controls the control target other than the air-fuel ratio related to the generated torque in the direction of suppressing the increase in the generated torque due to the enrichment. To control.

Here, when the internal combustion engine is provided with fuel injection means for injecting fuel for each cylinder, the air-fuel ratio enriching means forcibly controls the amount of fuel injected and supplied to a specific cylinder. Only by increasing the air-fuel ratio of the specific cylinder by increasing the correction, the generated torque control means, in order to match the generated torque in the specific cylinder with the generated torque of other cylinders, only for the specific cylinder. The control can be configured to suppress the generated torque.

Further, the air-fuel ratio enriching means forcibly enriches the air-fuel ratios of all the cylinders, and the generated torque control means makes the generated torque for all cylinders to match the generated torque corresponding to the target air-fuel ratio. It is also possible to configure so as to perform the control for suppressing. Further, the generated torque control means may be configured to suppress an increase in generated torque by controlling at least one of the ignition timing and the exhaust gas recirculation amount.

Further, when there is an air-fuel ratio switching means for switching the target air-fuel ratio of the engine intake air-fuel mixture between the output air-fuel ratio region near the theoretical air-fuel ratio and the lean air-fuel ratio region larger than the theoretical air-fuel ratio in accordance with the engine operating conditions. In addition, the HC increase request detection means detects a predetermined period immediately before the target air-fuel ratio is switched between the output air-fuel ratio range and the lean air-fuel ratio range by the air-fuel ratio switching means as the HC amount increase request state. It is better to configure it.

[0017]

According to the exhaust gas purification device having such a configuration, the process for increasing the HC concentration in the exhaust gas on the upstream side of the exhaust gas purification catalyst is performed immediately before the target air-fuel ratio is switched between the output air-fuel ratio region and the lean air-fuel ratio region. Done. Here, H of the exhaust purification catalyst
Since the HC increased by the HC concentration increasing process due to the C adsorption capacity is adsorbed for a certain period of time, even if it may pass through the air-fuel ratio region where the NOx concentration reaches its peak during the air-fuel ratio switching, It is possible to maintain the NOx reduction performance by the HC adsorbed on the.

Here, when the engine is running at low speed and under low load, HC
Since the amount of exhaust of H is small, when increasing the amount of HC immediately before switching the air-fuel ratio, the H
The C amount increasing process is executed. Further, in the present invention, when the amount of HC is required to be increased in order to maintain the NOx reduction performance in the exhaust purification catalyst that has the ability to adsorb HC and reduces NOx in the presence of HC, the exhaust purification catalyst upstream side As a means for increasing the HC concentration of the above, means for forcibly making the air-fuel ratio of the engine intake air-fuel mixture richer than the target air-fuel ratio for a predetermined period is provided. Then, in order to prevent a sudden increase in output due to the enrichment of the air-fuel ratio for increasing the HC concentration, control targets other than the air-fuel ratio are controlled in the output suppressing direction.

The enrichment of the air-fuel ratio may be performed only in a specific cylinder instead of all cylinders. In this case, the enrichment is performed so that the generated torque does not vary among the cylinders. The generated torque of a specific cylinder is made to match the level of another cylinder. Further, the enrichment of the air-fuel ratio may be performed in all cylinders, and in this case, the generated torque is equivalent to the reference target air-fuel ratio,
Control is performed to suppress the generated torque for all cylinders.

In order to suppress the output increase due to the enrichment of the air-fuel ratio, ignition timing retard correction or exhaust gas recirculation amount increase control may be performed. In addition, H due to the enrichment of the air-fuel ratio
If the control for increasing the C amount is executed immediately before switching between the output air-fuel ratio region and the lean air-fuel ratio region, the increase in the NOx emission amount into the atmosphere at the time of switching the air-fuel ratio can be controlled by the engine operability. It can be suppressed without deteriorating.

[0021]

EXAMPLES Examples of the present invention will be described below. In FIG. 3 showing the system configuration of one embodiment, the internal combustion engine 1 is
Air is taken in through the air cleaner 2, the intake duct 3, the throttle chamber 4, and the intake manifold 5.

An air flow meter 6 for detecting the intake air flow rate Q of the engine is installed in the intake duct 3.
A butterfly type throttle valve 7 for adjusting the amount of intake air is installed in the throttle chamber 4, and a potentiometer type throttle sensor 8 for detecting the opening TVO is attached to the throttle valve 7. There is.

Further, in order to control the amount of auxiliary air supplied to the engine by bypassing the throttle valve 7, an idle control valve 9 whose opening is controlled, and a fast opening / closing in accordance with an air conditioner signal. An idle control valve (FICD) 10 and an air regulator 11 for adjusting the amount of auxiliary air according to the temperature of the cooling water are provided. On the other hand, each branch portion of the intake manifold 5 is provided with an electromagnetic injector 12 for intermittently injecting and supplying to the engine 1 fuel that is pressure-fed from a fuel tank (not shown) and adjusted to a predetermined pressure by a pressure regulator. It is possible to control the fuel supply amount, and hence the air-fuel ratio of the engine intake air-fuel mixture, through the valve opening time of the injector 12.

Also, the spark plug 13 is exposed to the combustion chamber of each cylinder.
Is provided, and the high voltage generated in the ignition coil 14 is distributed and supplied to the spark plug 13 via the distributor 15 in response to the ignition timing control signal. The distributor 15 has a built-in crank angle sensor 16, and the crank angle sensor 16 is
A detection signal is output for each predetermined crank angle. The engine speed is calculated based on the detection signal from the crank angle sensor 16.

Exhaust gas from the engine 1 is exhausted from the exhaust manifold 1
7, the exhaust duct 18, the lean NOx catalyst 19 (exhaust gas purification catalyst), and the muffler 20 to be discharged into the atmosphere. An oxygen sensor 21 for detecting the oxygen concentration in the exhaust gas, which is closely related to the air-fuel ratio of the engine intake air-fuel mixture, is installed in the exhaust duct 18. An exhaust gas temperature sensor 22 that detects the exhaust gas temperature is provided downstream of the lean NOx catalyst 19.

The lean NOx catalyst 19 is composed of a zeolite supporting a transition metal or a noble metal, has a HC adsorbing capacity (HC storage effect), and has a function of reducing NOx in the presence of HC in an oxidizing atmosphere. Is. The control unit 23 having a built-in microcomputer inputs the detection signals from the water temperature sensor 24 that detects the cooling water temperature of the engine, the knock sensor 25 that detects the knocking vibration of the engine, and the like in addition to the various sensors described above. It has a function of controlling the fuel injection amount, the ignition timing by the spark plug 13, and the auxiliary air amount adjusted by the idle control valve 9.

In FIG. 3, a canister 26 adsorbs and collects the evaporated fuel in a fuel tank (not shown), and desorbs and supplies the evaporated fuel to the engine intake system. The control unit 23, the target air-fuel ratio of the engine intake air-fuel mixture, according to the operating conditions detected based on the various sensors, the output air-fuel ratio region near the stoichiometric air-fuel ratio, combustion stability limit larger than the theoretical air-fuel ratio A lean air-fuel ratio range (air-fuel ratio of about 20 to 22) is switched to the vicinity, and the fuel injection amount is controlled according to the target air-fuel ratio that is set to be switched. Therefore, the engine 1 of this embodiment is a so-called lean burn engine.

Since the lean NOx catalyst reduces NOx due to the presence of HC as described above, if the amount of HC is insufficient, the NOx reduction performance deteriorates. Particularly, when changing the air-fuel ratio between the output air-fuel ratio range and the lean air-fuel ratio range as described above, as shown in FIG.
16-18)), and at this time, HC / NOx
As the ratio becomes smaller, NOx in the lean NOx catalyst 19
Since it is no longer possible to expect a reduction process of NOx, a large amount of NOx may be emitted into the atmosphere when the air-fuel ratio is switched.

Therefore, in this embodiment, the amount of HC is secured in the following manner when the air-fuel ratio is switched so that the NOx reducing performance can be maintained. The flowchart of FIG. 5 shows the control of switching the target air-fuel ratio between the output air-fuel ratio range (substantially the theoretical air-fuel ratio) and the lean air-fuel ratio range. Incidentally, the function as the air-fuel ratio switching means in this embodiment is provided by the control unit 23 as software, as shown in the flowchart of FIG. In addition, in this embodiment, the engine 1 is an in-line four-cylinder engine.

The routine shown in the flow chart of FIG.
It is executed every cycle. First, information such as engine load, rotation speed, cooling water temperature, etc. is read (S1
1). The engine load is a basic fuel calculated in another routine based on the intake air flow rate Q detected by the air flow meter 6 and the engine rotation speed Ne calculated based on the detection signal from the crank angle sensor 16. Injection amount Tp (←
It can be represented by K × Q / Ne).

Then, based on the information on the engine load and the rotational speed, it is determined whether or not the current operating conditions are included in a preset lean operating region, and further, based on the information on the cooling water temperature, the lean operation is performed. By determining whether or not the driving is possible, it is determined whether or not lean operation is possible (S12). If it is determined that the lean operation is possible, the lean determination flag FLEAN indicating the setting switching state of the target air-fuel ratio is set to 1 (S13), and the lean operation is impossible and the output air-fuel ratio range ( When it is necessary to perform combustion (stoichiometric operation) at the stoichiometric air-fuel ratio, the lean determination flag FLEAN is set to 0 (S1).
Four).

Next, the lean determination flag FLEAN set this time is compared with the lean determination flag FLEAN (= FL-OLD) set at the previous execution of this program (S15). Here, when the lean determination flag FLEAN set this time is different from the previous value, in other words, switching from the lean air-fuel ratio range to the output air-fuel ratio range, or from the output air-fuel ratio range to the lean air-fuel ratio range When the switching is performed, the enrichment flag FRICH is set to 1 (S16).

The enrichment flag FRICH is a flag indicating a state in which combustion is performed at an air-fuel ratio richer than the air-fuel ratio before switching for a predetermined period before actually switching the air-fuel ratio (see FIG. 9). On the other hand, when the lean determination flag FLEAN is the same as the previous time, the process proceeds without setting the enrichment flag FRICH.

Then, the lean determination flag FLEAN set this time is set to the previous value FL-OLD for the determination in S15 at the next execution of this program (S).
17). Next, the actual target air-fuel ratio is set based on the result of the determination as to whether or not the lean operation is possible (S18).

Details of the target air-fuel ratio setting in S18 are shown in the flow charts of FIGS. 6 and 7. still,
In this embodiment, the functions of the HC concentration increasing means, the HC concentration controlling means, the HC increasing demand detecting means, the air-fuel ratio enriching means, and the generated torque controlling means are controlled as shown in the flow charts of FIGS. 6 and 7. The unit 23 is provided as software.

In the flowcharts of FIGS. 6 and 7, first, the enrichment flag FRICH is determined (S201). When the enrichment flag FRICH is 1, the flag FR-OLD for determining the initial state of the control for enriching the air-fuel ratio is determined (S202).

Here, if it is determined that the flag FR-OLD is 0, and it is in the initial state of the enrichment control, after the flag FR-OLD is set to 1 (S20).
3) As shown in FIG. 8, the period TR for forcibly enriching the air-fuel ratio before the actual switching of the air-fuel ratio is read from the map previously assigned by the engine load and the engine rotation speed (S204). Then, the timer t _R for measuring the enrichment period TR is counted up by a predetermined value Δt (S205), and the measurement of the enrichment period is started.

That is, even if the switching of the air-fuel ratio between the output air-fuel ratio region and the lean air-fuel ratio region is set, the switching is not started immediately, but the start of switching the air-fuel ratio is delayed by the enrichment period TR. Then, during the delay period, a large amount of HC is discharged by burning with an air-fuel ratio richer than the air-fuel ratio before switching, and a lean N
The maximum amount of HC within the possible range is adsorbed to the Ox catalyst 19 (see FIG. 9).

In the lean NOx catalyst 19, the total amount MO of HC that can be adsorbed is determined by the amount of the catalyst carried. Therefore, the amount of HC emission from the engine is determined from the operating conditions and the air-fuel ratio setting, and from this value, the amount of HC emission is increased. The time until the adsorption amount reaches the adsorbable amount MO is obtained, and this time is defined as the enrichment period TR. Since the HC discharge amount increases as the load and rotation speed increase, the enrichment period TR becomes shorter as the load and rotation speed of the engine become shorter, in other words, as the load and rotation speed of the engine become longer. Is set. Therefore, if the predetermined enrichment process is performed only during the enrichment period TR, it is predicted that the maximum amount of HC that can be adsorbed to the lean NOx catalyst 19 will be adsorbed.

Next, the map of the air-fuel ratio and the ignition timing for the normal operation, which is assigned by the engine load and the engine speed, is referred to on the basis of the lean determination flag FLEAN, and these reference values are respectively MKMR and MADV. (S206). As shown in FIG. 10, two types of air-fuel ratio / ignition timing maps for normal operation are prepared in advance, one for the output air-fuel ratio range (for stoichiometry) and the other for the lean air-fuel ratio range. One of the maps is selected based on the flag FLEAN, and the target lean air-fuel ratio and ignition timing are referred to from the selected map based on the engine load and the rotation at that time.

Then, the reference values MKMR, MADV are assigned to each cylinder as KMR (i), ADV (i).
It is stored in (i = 1 to 4) (S207). Here, the lean determination flag FLEAN is inverted with respect to the previous time, and normally, if the lean determination flag FLEAN is 1, the lean air-fuel ratio range (lean operation) map is referred to, and the lean determination is also performed. If the flag FLEAN is 0, the map for the output air-fuel ratio range (stoichiometric operation) will be referred to.

However, in the present embodiment, the air-fuel ratio switching is delayed during the enrichment period TR (while the flag FRICH is 1), and the enrichment period TR has elapsed since the lean determination flag FLEAN was inverted. Only after that, the air-fuel ratio is switched (see FIG. 9). Therefore, as described above, in S206, which is the enrichment period TR in which the enrichment flag FRICH is determined to be 1, in switching from the lean air-fuel ratio region where the lean determination flag FLEAN is 0 to the output air-fuel ratio region, Refer to the map for the lean air-fuel ratio range before switching, and check the lean determination flag FL.
When switching from the output air-fuel ratio range (theoretical air-fuel ratio) where EAN is 1 to the lean air-fuel ratio range, the map for the output air-fuel ratio range before switching shall be referred to.

In the above, the normal data corresponding to the air-fuel ratio setting before switching is set as the air-fuel ratio / ignition timing of all cylinders. However, in the following S208 and S209, only the air-fuel ratio of the specific cylinder is made rich. Therefore, a process of rewriting the air-fuel ratio / ignition timing setting of the specific cylinder is performed. That is,
When the air-fuel ratio switching between the output air-fuel ratio range and the lean air-fuel ratio range is instructed, the start of actual switching control is simply delayed for cylinders other than the specific cylinder, but for the specific cylinder, the delay period In order to forcibly discharge a large amount of HC inside, at least it is made richer than other cylinders.

In the control unit 23, an air-fuel ratio / ignition timing map for enriching the specific cylinder is set in advance from the output air-fuel ratio range (stoichiometric) as shown in FIGS. 10 (c) and 10 (d). A map used when switching to the air-fuel ratio range and a map used when switching to the opposite direction are provided.For example, during the enrichment period when switching from the output air-fuel ratio range to the lean air-fuel ratio range, other cylinders The map of FIG. 10 (a) is used, while the map of FIG. 10 (c) is used for the specific cylinder.

Here, the map of FIG. 10A and the map of FIG.
Map, when compared to the respective maps and 10 maps the (d) of FIG. 10 (b), control using the data S _ij and S _'ij or L _ij and L' _ij corresponding to the same load and rotation When it does, the same generated torque is obtained. During the enrichment period TR, only specific cylinders are enriched. Therefore, if only the setting of the air-fuel ratio is made different among the cylinders, the generated torque becomes large in the specific cylinders, and the generated torque between the cylinders becomes unbalanced. It becomes a balance. Therefore, even if the air-fuel ratio is made rich, a map in which the ignition timing is corrected is adjusted so that the generated torque does not become larger than that of other cylinders due to the enrichment. (See FIG. 11) so that the generated torque of the specific cylinder to be enriched is equal to the generated torque of the other cylinder that is not enriched.

That is, as shown in FIG. 11, the air-fuel ratio of a specific cylinder is set to 9 for the other cylinders that are burned at a predetermined lean air-fuel ratio.
If it is made substantially rich, the generated torque of the enriched cylinder only increases with the enrichment. Therefore, the air-fuel ratio / ignition timing data that can offset the increase in the generated torque due to the enrichment by the ignition timing retard setting (retard) is obtained in advance by an experiment, and this is set as the enrichment map. Then, the generated torque in the cylinder controlled using the normal map and the generated torque in the cylinder controlled using the enrichment map are
Under the same load and rotation conditions, they are made approximately equal.

In S208, the enrichment air-fuel ratio / ignition timing map set as described above is referred to according to the switching direction of the air-fuel ratio, and the reference values of the air-fuel ratio / ignition timing depending on the engine load / rotation. Are set to KMR (1) and ADV (1). The control values for normal operation are set in the above-mentioned steps for KMR (1) and ADV (1).
It is replaced with the reference value of the enrichment map.

In this embodiment, as shown in FIG. 9, the air-fuel ratio is controlled to about 14 to 16 in the output air-fuel ratio range, and the air-fuel ratio is controlled to about 20 to 22 in the lean air-fuel ratio range. As opposed to
When switching from the output air-fuel ratio range to the lean air-fuel ratio range,
The actual switching is started after the air-fuel ratio of the specific cylinder is reduced to 10-12 (rich) for a predetermined period, and
When switching from the lean air-fuel ratio range to the output air-fuel ratio range,
The actual switching is started after the air-fuel ratio of the specific cylinder is reduced (riched) to about 8 to 10 for a predetermined period.

In the predetermined period TR immediately before the air-fuel ratio is switched as described above, only the air-fuel ratio of the specific cylinder is increased with respect to the target air-fuel ratio before the switching (the air-fuel ratio of the other cylinder) without increasing the torque. Air-fuel ratio to make rich
When the ignition timing is set, then the previous value KMR-O of the set air-fuel ratio / ignition timing for the specific cylinder (first cylinder)
As the LD (1) and ADV-OLD (1), those referred to the enrichment map are set instead of those referred to the normal map (S209).

Next, it is determined whether or not a predetermined enrichment period TR has elapsed based on the time measured by the timer t _R (S210). Then, if the enrichment period TR has not elapsed, the process directly proceeds to the setting step of the target air-fuel ratio after S214, but if it is determined that the enrichment period TR has elapsed, the flags FRICH and FR-O are set.
The LD and the timer t _R are reset to zero (S211), and the switching of the air-fuel ratio delayed by the enrichment period TR is started.

When the enrichment flag FRICH is zero-lit, the lean determination flag FLEAN is started from the next time.
Corresponding to, the normal map for the output air-fuel ratio region is referred to when the flag FLEAN is 0 (Fig. 10 (a)), and the normal map for the lean air-fuel ratio region is indicated when the flag FLEAN is 1 (Fig. 10 (b)), the common air-fuel ratio / ignition timing of each cylinder is set (S212, S213).
).

In steps S214 to S222, when the air-fuel ratio is switched between the output air-fuel ratio range and the lean air-fuel ratio range, the air-fuel ratio before the switching is gradually changed to the air-fuel ratio after the switching, and the air-fuel ratio is changed. This is a step for setting the target air-fuel ratio so that a sudden output change due to switching does not occur.
First, the air-fuel ratio KMR set with reference to the map this time
(I) and the previous air-fuel ratio KMR-OLD (i) are compared to determine whether or not the air-fuel ratio increase setting has been made (S).
214).

When it is determined that the set air-fuel ratio is increasing, the data obtained by adding the predetermined step amount ΔRMR to the previous value KMR-OLD (i) and the set air-fuel ratio this time are set. KMR (i) is compared and the smaller data is set as the final target air-fuel ratio TMR (i).
Therefore, when the output air-fuel ratio range is shifted to the lean air-fuel ratio range, the step amount ΔRMR is gradually changed not from the stoichiometric air-fuel ratio to the final lean air-fuel ratio at once.

On the other hand, when the set air-fuel ratio obtained by referring to the map is not changing or is decreasing, the data is obtained by subtracting the predetermined step amount ΔLMR from the previous value KMR-OLD (i). , This time set air-fuel ratio KMR
(I) is compared, and the larger data is set as the final target air-fuel ratio TMR (i). Therefore, when shifting from the lean air-fuel ratio region to the output air-fuel ratio region, the lean air-fuel ratio does not change to near the stoichiometric air-fuel ratio at once, but gradually changes by the step amount ΔLMR.

Further, when the set air-fuel ratio obtained by referring to the map has not changed, that is, in the stable stoichiometric air-fuel ratio or lean air-fuel ratio operation state, or during the enrichment period TR, the map reference value is obtained. Is directly set as the target air-fuel ratio TMR (i). S217 ~
In S219, similarly to the setting of the target air-fuel ratio TMR (i), the ignition timing TADV (i) is gradually changed.

The above target air-fuel ratios TMR (i) and TAD
V (i) is performed for each cylinder,
Once the series of processing in S214 to S219 is completed, the cylinder number counter i (initial value = 1) is incremented by 1. And
Until the counter i exceeds 4, in other words, the target air-fuel ratios TMR (i) and TAD for all cylinders.
Until the setting of V (i) is completed, the processes of S214 to S219 are repeated.

Target air-fuel ratio TMR (i) and TADV
When the setting of (i) is completed for all the cylinders, the counter i is reset to 0 which is an initial value (S222), and then the target air-fuel ratios TMR (i) and TADV (i) are not shown in the figure. It is passed to a control routine to control the injection amount and TADV corresponding to the target air-fuel ratio TMR (i).
Ignition timing control corresponding to (i) is performed.

The control shown in the flow charts of FIGS. 6 and 7 will be briefly described. When switching between the output air-fuel ratio range (stoichiometric operation) and the lean air-fuel ratio range (lean operation), the switching is performed. Postponed for a predetermined period TR,
Only the air-fuel ratio of the specific cylinder is made rich during the switching postponement period. In the enrichment of the air-fuel ratio, the ignition timing is retarded compared to the other cylinders in order to suppress the increase in the generated torque due to the enrichment, and the ignition timing is set to the same level as the generated torque of the other cylinders.

By enriching the specific cylinder as described above, the amount of HC contained in the exhaust gas from the specific cylinder can be forcibly increased, and the amount of HC discharged from the specific cylinder can be increased. By setting the period TR, it is possible to adsorb the maximum amount of HC that can be adsorbed to the lean NOx catalyst 19 within the air-fuel ratio switching delay period. Then, the rich period TR elapses, and the lean NOx catalyst
After the state in which it is estimated that the maximum amount of HC has been adsorbed in 19 is started, the actual air-fuel ratio switching is started.

Here, the NOx emission amount from the engine 1 in the air-fuel ratio range (air-fuel ratio 16-18) between the output air-fuel ratio range near the stoichiometric air-fuel ratio and the lean air-fuel ratio range (air-fuel ratio 20-22). However, if a large amount of HC is positively adsorbed to the lean NOx catalyst 19 immediately before the air-fuel ratio switching is started as described above, a large amount of NOx is actually discharged from the engine. When the fuel ratio is reached, the presence of the adsorbed HC makes it possible to maintain the NOx reducing performance.

Therefore, according to this embodiment, when the target air-fuel ratio is switched between the output air-fuel ratio range (theoretical air-fuel ratio) and the lean air-fuel ratio range, the output is gradually changed so as to avoid a sudden change. Even with a configuration in which the air-fuel ratio is changed, it is possible to favorably reduce NOx that is discharged in large amounts in the air-fuel ratio region during switching, and it is possible to avoid an increase in the NOx emission amount while maintaining operability.

In the predetermined period immediately before the air-fuel ratio switching, the injection timing is delayed, the combustion chamber wall temperature is lowered, and further, the blow-by gas and the evaporated fuel desorbed from the canister 23 are added to the exhaust system. As a result, the lean NOx catalyst 19 may be made to adsorb HC that can be used for processing a large amount of NOx discharged during the air-fuel ratio switching.

However, if the air-fuel ratio of the engine intake air-fuel mixture is made rich as in the present embodiment, incomplete combustion will be greatly increased to secure the HC amount,
Since the fuel that is not used for combustion is not used to secure the HC amount and the HC amount can be increased in a responsive manner, NOx stability can be stabilized without significantly deteriorating the drivability and fuel consumption of the engine. It is possible to achieve effective purification.

Immediately before switching the air-fuel ratio, the torque generated by the cylinder that is made rich to adsorb the maximum amount of HC to the lean NOx catalyst 19 is controlled by the other cylinders (normally controlled by the air-fuel ratio before switching). In order to make the torque equal to the torque generated by the activated cylinder), the air-fuel ratio and the ignition timing may be set so that the indicated mean effective pressures Pe are equal to each other. However, even if the indicated mean effective pressure Pe is made equal, dPe
Since / dθ cannot be made equal to each other, the axial excitation force becomes different between the enriched cylinder and the other cylinders, and the axial excitation force in the enriched cylinder becomes larger.

On the other hand, in the case of an in-line four-cylinder engine, the most significant effect on engine vibration is the axial excitation force of the first cylinder farthest from the flywheel. Also, the axial excitation force of the 4th cylinder is
Although it is directly applied to the crank main bearing, the main bearing is the most severe in terms of strength due to the excitation force of the flywheel. Therefore, in the above-described embodiment, the specific cylinder to be enriched is the first cylinder for simplification of description, but the second cylinder and the third cylinder are enriched in order to reduce engine vibration and improve durability. It is desirable to use a specific cylinder.

Further, as shown in the flow chart of FIG. 12, it is possible to sequentially switch the cylinders to be enriched with respect to the cylinders normally controlled in the predetermined period immediately before the switching of the air-fuel ratio. The flowchart of FIG. 12 corresponds to steps S208 and S20 in the flowchart of FIG. 6 described above.
It is executed instead of the 9th part.

First, the enriched cylinder counter CYL-N
The cylinder designated by (initial value = 1) is set as the enriched cylinder, and the air-fuel ratio KMR and ignition timing ADV for this enriched cylinder are determined with reference to the map (S31). Note that the air-fuel ratio KMR and the ignition timing ADV are rich in air-fuel ratio but equivalent in generated torque to a cylinder that is not controlled to be rich but is normally controlled according to the air-fuel ratio setting before switching. It will be.

Next, the set air-fuel ratio KMR and ignition timing ADV are set in KMR-OLD and ADV-OLD as previous values in the enriched cylinder (S3).
2). Next, the enriched cylinder counter CYL-N is set to 1
When the enriched cylinder counter is incremented by 1 (S33) and exceeds 1 (S34), the initial value 1 is reset (S35).

Therefore, the enriched cylinder counter CYL
-N changes 1 → 2 → 3 → 4 → 1 for each cycle, and the cylinders that are enriched accordingly are # 1 → for each cycle.
It will change in the order of # 2 → # 3 → # 4 → # 1. When the air-fuel ratio is made rich, the combustion chamber wall temperature of the cylinder decreases, but if the configuration is such that the enriched cylinders are sequentially changed as described above, only the wall temperature of one cylinder decreases and the thermal stress of the cylinder decreases. Does not become an imbalance.

Further, in the above embodiment, one of the four cylinders is used.
Although only the cylinders are forcibly enriched in order to secure the HC amount, two or three cylinders or all the cylinders may be enriched within a predetermined period before switching the air-fuel ratio. For example, when switching from the lean operation to the output air-fuel ratio (theoretical air-fuel ratio) for sudden acceleration, it may be necessary to switch the air-fuel ratio within a relatively short time. If it is attempted to temporarily shorten the period for enrichment (enrichment period TR) as the air-fuel ratio is switched rapidly, it is necessary to increase the HC concentration during the enrichment period. Therefore, it may be considered to increase the number of cylinders to be temporarily enriched, and it is possible to temporarily enrich all the cylinders depending on the required HC concentration.

The flow chart of FIG. 13 is a flow chart showing the control for enriching not only one cylinder but all the cylinders when the engine 1 is rapidly accelerated, and is additionally executed in the flow chart of FIG. 6 described above. ing. That is, when the timer t _R for measuring the enrichment period TR immediately before the air-fuel ratio switching is counted up (S205), the opening TVO of the throttle valve 7 detected by the throttle sensor 8 is read (S1001).

Then, the difference ΔTVO between the opening TVO read this time and the previous value TVO-OLD, that is, the change amount of the opening TVO during one cycle is calculated (S1002), and the difference ΔTVO is calculated.
Based on TVO, it is determined whether or not the throttle valve 7 is being operated to open at a predetermined rate or more (rapid acceleration) (S1003). Here, if it is not in the rapid acceleration state, S20
Proceeding to step 6, the process for enriching only one specific cylinder is performed as described above.

On the other hand, if it is determined that the acceleration is rapid, the process of adsorbing the required amount of HC to the lean NOx catalyst 19 needs to be ended immediately, so that the richening map is set to enrich all cylinders. The air-fuel ratio and ignition timing of all cylinders are set by referring to (S1004, S1005). Here, if the configuration is such that all cylinders are made rich at the same time as described above, there will be no variation in the generated torque between the cylinders, but if only the process of making the air-fuel ratio rich is performed, the generated torque will be increased simultaneously with the enrichment. It will increase rapidly. However, although the enrichment map is enriched with respect to the air-fuel ratio, it is possible to obtain substantially the same generated torque as when the control is performed using the normal air-fuel ratio / ignition timing map by setting the ignition timing retard angle. Since it is set, the generated torque does not suddenly increase even if richening is performed for all cylinders.

Although omitted in the flowchart of FIG. 13, when the all-cylinder enrichment process is performed in response to the rapid acceleration determination, the enrichment period TR is shortened as compared with the case where only one cylinder is enriched. Good to do. If all cylinders are made rich as described above, there will be no imbalance in the axial excitation force between the cylinders, so
It is advantageous in terms of durability, and since the maximum amount of HC can be adsorbed to the lean NOx catalyst 19 within a short time, the enrichment period can be shortened.

Further, in the above, in the process of enriching the air-fuel ratio in the predetermined period before starting the air-fuel ratio switching, all the cylinders are enriched only during the rapid acceleration, and one cylinder is not activated during the rapid acceleration. However, it is also possible to have a configuration in which all the cylinders are enriched even when the vehicle is not rapidly accelerated. Alternatively, the acceleration may be determined to be a rapid acceleration or a gentle acceleration, and for example, only one cylinder may be enriched except during acceleration, two cylinders may be enriched during gentle acceleration, and all cylinders may be enriched during rapid acceleration. Further, when enriching the two cylinders or the three cylinders, the cylinder group to be enriched may be sequentially changed.

In the above embodiment, the engine 1 having the lean NOx catalyst 19 containing zeolite as a main component in the exhaust passage is described. However, the catalyst has an HC adsorbing ability and reduces NOx in the presence of HC. In this case, by increasing the amount of HC on the upstream side of the catalyst immediately before switching the air-fuel ratio as described above, it is possible to adsorb a sufficient amount of HC for NOx treatment to the catalyst before switching is started. As a result, the NOx treatment capacity during the switching can be enhanced, and therefore, not only the lean NOx catalyst but also a three-way catalyst may be used.

Further, in the above embodiment, the increase in the generated torque due to the enrichment of the air-fuel ratio is offset by the retard setting of the ignition timing. However, particularly in the configuration in which all the cylinders are enriched, the exhaust gas recirculation amount is increased. It is also possible to suppress the increase in the generated torque due to the enrichment by the control described above. Furthermore, the combination of the ignition timing and the exhaust gas recirculation amount may be used to suppress an increase in the generated torque.

The HC adsorption capacity of the lean NOx catalyst is determined by the exhaust gas temperature (catalyst temperature) as well as the HC concentration and gas flow rate.
Since it also changes depending on the exhaust temperature, the rich period TR may be changed according to the exhaust temperature detected by the exhaust temperature sensor 22.

[0079]

As described above, according to the present invention,
In an exhaust gas purification device that treats NOx in exhaust gas with a catalyst that has an adsorption capacity for HC and reduces NOx in the presence of HC, an output air-fuel ratio region near the stoichiometric air-fuel ratio and a lean air-fuel ratio region that is larger than the stoichiometric air-fuel ratio. When the target air-fuel ratio is switched between and, the amount of HC on the upstream side of the catalyst is increased to adsorb sufficient HC to the catalyst during a predetermined period immediately before the switching.
There is an effect that the Ox processing capacity can be enhanced.

Since the air-fuel ratio of the engine intake air-fuel mixture is made rich as a means for increasing the amount of HC on the upstream side of the catalyst, the incomplete combustion is not significantly increased, and the fuel consumption is significantly increased. There is an effect that the HC amount can be increased responsively without causing any serious deterioration, and the drivability can be kept good without the output fluctuating due to the enrichment.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention.

FIG. 2 is a block diagram showing the basic configuration of the present invention.

FIG. 3 is a schematic diagram showing a system configuration of an embodiment.

FIG. 4 is a diagram showing how the emission amounts of NOx and HC change with respect to the air-fuel ratio.

FIG. 5 is a flowchart showing air-fuel ratio switching control.

FIG. 6 is a flowchart showing HC increase control when switching the air-fuel ratio.

FIG. 7 is a flowchart showing HC increase control when switching the air-fuel ratio.

FIG. 8 is a diagram showing a map of an HC increase control (enrichment) period.

FIG. 9 is a time chart showing how the air-fuel ratio changes in the example.

FIG. 10 is a diagram showing an air-fuel ratio / ignition timing map in the embodiment.

FIG. 11 is a diagram showing the relationship between the air-fuel ratio / ignition timing and the generated torque.

FIG. 12 is a flowchart showing an example of changing the enriched cylinder.

FIG. 13 is a flowchart showing an embodiment in which all cylinders are made rich.

[Explanation of symbols]

 1 Internal Combustion Engine 6 Air Flow Meter 12 Injector 16 Crank Angle Sensor 22 Lean NOx Catalyst 23 Control Unit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location F02D 21/08 301 C 7049-3G 41/14 310 A 8011-3G 41/36 B 8011-3G 43 / 00 301 B 7536-3G H 7536-3G N 7536-3G 45/00 301 G 7536-3G F02P 5/15 B

Claims (7)

[Claims] 1. An air-fuel ratio switching means for switching a target air-fuel ratio of an engine intake air-fuel mixture between an output air-fuel ratio region near the theoretical air-fuel ratio and a lean air-fuel ratio region larger than the theoretical air-fuel ratio in accordance with engine operating conditions. An exhaust gas purification apparatus for an internal combustion engine, comprising: an exhaust gas purification catalyst which is interposed in an exhaust passage and has an ability to adsorb HC and reduces NOx in the presence of HC; and an exhaust gas HC concentration upstream of the exhaust gas purification catalyst. H concentration in the exhaust gas by the HC concentration increasing device during a predetermined period immediately before the target air-fuel ratio is switched between the output air-fuel ratio region and the lean air-fuel ratio region by the air-fuel ratio switching device.
An exhaust gas purification apparatus for an internal combustion engine, comprising: an HC concentration control means for increasing the C concentration. 2. The internal combustion engine according to claim 1, wherein the HC concentration control means causes the HC concentration increasing means to increase the HC concentration for a longer period when the engine is running at low speed and under low load. Exhaust purification device. 3. An exhaust purification catalyst which is interposed in an exhaust passage and has the ability to adsorb HC and reduces NOx in the presence of HC, and the HC required for the reduction of NOx by the exhaust purification catalyst.
In order to secure a sufficient amount, an HC increase request detecting means for detecting a state in which an increase in the amount of HC in the exhaust gas is required, and when the state in which an increase in the HC amount is required is detected by the HC increase request detecting means, Air-fuel ratio enriching means for forcibly enriching the air-fuel ratio of the engine intake air-fuel mixture for a predetermined period above the target air-fuel ratio; and when the air-fuel ratio enriching means enriches the air-fuel ratio, the enrichment is performed. An exhaust emission control device for an internal combustion engine, comprising: a generated torque control means for controlling a control target other than an air-fuel ratio related to the generated torque in a direction of suppressing an increase in the generated torque. 4. An internal combustion engine comprising fuel injection means for injecting fuel for each cylinder, wherein the air-fuel ratio enriching means forcibly corrects the amount of fuel injected and supplied to a specific cylinder. In order to make only the air-fuel ratio of the specific cylinder rich, and the generated torque control means match the generated torque of the specific cylinder with the generated torque of other cylinders, the generated torque is suppressed only for the specific cylinder. The exhaust emission control device for an internal combustion engine according to claim 3, wherein the control is performed. 5. The air-fuel ratio enriching means forcibly enriches the air-fuel ratios of all the cylinders, and the generated torque control means makes the generated torque for all cylinders equal to the generated torque corresponding to the target air-fuel ratio. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the exhaust gas purifying apparatus is configured to suppress the exhaust gas. 6. The generated torque control means suppresses an increase in generated torque by controlling at least one of an ignition timing and an exhaust gas recirculation amount.
Or an exhaust emission control device for an internal combustion engine according to any one of 5). 7. An air-fuel ratio switching means for switching the target air-fuel ratio of the engine intake air-fuel mixture between an output air-fuel ratio region near the theoretical air-fuel ratio and a lean air-fuel ratio region larger than the theoretical air-fuel ratio in accordance with engine operating conditions. The HC increase request detecting means detects a predetermined period immediately before the target air-fuel ratio is switched between the output air-fuel ratio range and the lean air-fuel ratio range by the air-fuel ratio switching means as the HC amount increase request state. The exhaust emission control device for an internal combustion engine according to claim 3, 4, 5, or 6.