JP3003447B2 - Exhaust gas purification device for internal combustion engine - Google Patents

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 an internal combustion engine, and more particularly, to an exhaust gas purifying apparatus having a catalyst capable of 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 region (for example, an air-fuel ratio of about 20 to 22) that is significantly leaner than the stoichiometric air-fuel ratio has been developed. A lean NOx catalyst is used to purify NOx in an air-fuel ratio range.

[0003] The lean NOx catalyst is mainly composed of zeolite, and is presumed to temporarily adsorb HC in exhaust gas and reduce NOx by the HC. Therefore, N in the lean NOx catalyst
The presence of HC is indispensable for the Ox purification process, and when the HC / NOx ratio in the exhaust gas decreases, the NOx reduction performance decreases.

[0004] Therefore, lean NO
When the NOx reduction performance of the x catalyst decreases, the H
An exhaust gas purifying apparatus has been proposed in which the C amount is forcibly increased to maintain the NOx reduction performance. As means for forcibly increasing the HC amount in the exhaust gas, the following means A to A are provided. C is adopted. (A). By delaying the fuel injection timing, the amount of fuel flowing into the combustion chamber as a wall flow is increased to suppress the atomization of the fuel, thereby increasing HC in the exhaust gas (see JP-A-3-217640). .

(B). By lowering the temperature of the cooling water, the wall temperature of the combustion chamber is reduced, thereby suppressing the atomization of the fuel and increasing the HC in the exhaust gas (see JP-A-3-229914). (C). Evaporated 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 (JP-A-3-242415).
Reference).

[0006]

SUMMARY OF THE INVENTION
In the means shown in (A) to (C), when the wall flow rate is increased in (A) or the combustion chamber wall temperature is decreased in (B), incomplete combustion increases and combustion becomes unstable. And the drivability could be degraded. In the case of the means (B) for lowering the temperature of the combustion chamber wall, after the HC shortage is determined,
There is a large time loss until the temperature of the cooling water is lowered and the wall temperature of the combustion chamber is actually lowered. During this time, NOx is exhausted without being sufficiently purified, and it is difficult to stably maintain NOx reduction performance. is there.

Further, the means (C) for adding HC to exhaust gas involves adding unused fuel to exhaust gas, which causes a problem of deteriorating fuel efficiency. Also,
In the lean-burn internal combustion engine, when switching the target air-fuel ratio between the output air-fuel ratio region near the stoichiometric air-fuel ratio and the lean air-fuel ratio region, a sharp change in torque does not impair the drivability. The air-fuel ratio is gradually changed. However, as shown in FIG. 4, the amount of NOx emission is highest in the air-fuel ratio range (air-fuel ratio of about 16 to 18) during the switching of the target air-fuel ratio.
As the C / NOx ratio decreases, the NOx of the lean NOx catalyst
Since the reduction performance is reduced, there is a problem that a large amount of NOx is emitted when the target air-fuel ratio is switched.

Here, when the target air-fuel ratio is switched as described above, the fact that the HC amount is insufficient with respect to the NOx amount and the NOx reduction performance is reduced will be compensated for by the HC amount increasing means as described above. Even if the air-fuel ratio falls within the air-fuel ratio range where the NOx concentration peaks, the HC in the exhaust
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 for reduction of NOx emitted in large quantities
In some cases, the amount cannot be secured, and it has been difficult to stably maintain a high NOx reduction ability.

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

[0010]

An exhaust gas purifying apparatus for an internal combustion engine according to the present invention is configured as shown in FIG . In Figure 1, the air-fuel ratio switching means is a means for switching to the large lean range than the output air-fuel ratio range and the stoichiometric air-fuel ratio near the stoichiometric air-fuel ratio according to the target air-fuel ratio of the engine intake mixture in the engine operating condition And the output air-fuel ratio range
The air-fuel ratio is gradually changed between a lean air-fuel ratio range .

An exhaust purification catalyst is provided in an exhaust passage and is
It is a catalyst that has an adsorption capacity and reduces NOx in the presence of HC. The HC concentration increasing means is a 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 is configured to control the output air-fuel ratio range and the lean air-fuel ratio.
During a predetermined period immediately before switching of the air-fuel ratio between the air-fuel ratio range and the fuel-fuel ratio range is started , the H concentration in the exhaust gas is increased by the HC concentration increasing means.
Increase C concentration.

Here, it is preferable that the HC concentration control means is configured to increase the HC concentration by the HC concentration increasing means for a longer period of time when the engine speed is low and the load is low.
No.

[0013] Further , the HC concentration increasing means includes an engine suction port.
The air-fuel ratio of the mixture is forcibly made richer than the target air-fuel ratio.
The concentration of HC in the exhaust gas on the upstream side of the exhaust purification catalyst
Can be configured to increase the degree of
As shown in FIG. 2, when the air-fuel ratio is enriched by the HC concentration increasing means , the air-fuel ratio other than the air-fuel ratio related to the generated torque is controlled so as to suppress the increase in the generated torque accompanying the enrichment. Generated torque control means for controlling the control target
Preferably, it is provided.

In the case where the internal combustion engine is provided with fuel injection means for injecting and supplying fuel for each cylinder, the HC concentration increasing means includes a fuel injection means for injecting and supplying fuel to a specific cylinder. Only the air-fuel ratio of the specific cylinder is enriched by forcibly increasing the amount, and the generated torque control means adjusts the generated torque of the specific cylinder to match the generated torque of another cylinder. Can be configured to perform control to suppress the generated torque only for.

Further, the HC concentration increasing means forcibly enriches the air-fuel ratio of all cylinders, and the generated torque control means adjusts the generated torque to all cylinders so as to match the generated torque corresponding to the target air-fuel ratio. It is also possible to perform a control to suppress. Further, it is preferable that the generated torque control means controls at least one of the ignition timing and the exhaust gas recirculation amount to suppress an increase in the generated torque.

[0016]

[0017]

According to the exhaust purification device of the acting Such configuration, switching of the air-fuel ratio between the output air-fuel ratio zone and the lean air-fuel ratio region is started
Just before it is, processing is performed to increase the exhaust HC concentration upstream of the exhaust purification catalyst. Here, since the HC increased by the HC concentration increasing process is adsorbed for a certain period of time by the HC adsorbing ability of the exhaust purification catalyst, even if the NOx concentration may pass through the air-fuel ratio range where the NOx concentration peaks during the air-fuel ratio switching. The NOx reduction performance can be maintained by the HC previously adsorbed as described above.

Here, when the engine speed is low and the load is low, HC
In order to increase the amount of HC immediately before switching the air-fuel ratio, the lower the rotation speed and the lower the load, the longer the H
The C amount increasing process is executed. Further, according to the present invention, the means for increasing the HC concentration on the upstream side of the exhaust purification catalyst is provided.
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 of time , and the fact that the air-fuel ratio is made richer to increase the HC concentration causes a sudden increase in output. In order to suppress, control targets other than the air-fuel ratio are controlled in the output suppression direction.

The air-fuel ratio can be enriched only in a specific cylinder, not in all cylinders. In this case, the air-fuel ratio is enriched 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. The enrichment of the air-fuel ratio may be performed in all cylinders. In this case, the generated torque is equivalent to the reference target air-fuel ratio.
Control to suppress the generated torque is applied to all cylinders.

[0020] The To suppress the output increase due to the enrichment of the air-fuel ratio, has good by performing the increase control of the retard correction and exhaust gas recirculation amount of the ignition timing.

[0021]

Embodiments of the present invention will be described below. In FIG. 3 showing a system configuration of an embodiment, an internal combustion engine 1 includes:
Air is sucked 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 interposed in the intake duct 3.
The throttle chamber 4 is provided with a butterfly type throttle valve 7 for adjusting the intake air amount, and the throttle valve 7 is provided with a potentiometer type throttle sensor 8 for detecting the opening TVO. I have.

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, a fast opening / closing on / off in response to 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 cooling water temperature are provided. On the other hand, each branch of the intake manifold 5 is provided with an electromagnetic injector 12 for intermittently injecting fuel supplied from a fuel tank (not shown) and adjusted to a predetermined pressure by a pressure regulator to the engine 1. The fuel supply amount, and hence the air-fuel ratio of the engine intake air-fuel mixture, can be controlled via the valve opening time of the injector 12.

Further, the spark plugs 13 face the combustion chambers of the respective cylinders.
A high voltage generated by an ignition coil 14 in response to an ignition timing control signal is distributed and supplied to the ignition plug 13 via a distributor 15. The distributor 15 has a built-in crank angle sensor 16.
A detection signal is output at every predetermined crank angle. The engine speed is calculated based on the detection signal from the crank angle sensor 16.

The exhaust gas from the engine 1 is supplied to the exhaust manifold 1
7, the exhaust duct 18, the lean NOx catalyst 19 (exhaust gas purifying catalyst), and the muffler 20 are discharged into the atmosphere. The exhaust duct 18 is provided with 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. Further, an exhaust gas temperature sensor 22 for detecting the exhaust gas temperature is provided downstream of the lean NOx catalyst 19.

The lean NOx catalyst 19 is made of a zeolite supporting a transition metal or a noble metal, has a HC adsorption capacity (HC storage effect), and has a function of reducing NOx in the presence of HC in an oxidizing atmosphere. It is. A control unit 23 having a built-in microcomputer receives a detection signal from a water temperature sensor 24 that detects a cooling water temperature of the engine, a knock sensor 25 that detects a 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 of the ignition plug 13, and the amount of auxiliary air adjusted by the idle control valve 9.

In FIG. 3, reference numeral 26 denotes a canister, which adsorbs and collects the evaporated fuel in a fuel tank (not shown), and desorbs and supplies the adsorbed and collected evaporated fuel to an engine intake system. The control unit 23 sets the target air-fuel ratio of the engine intake air-fuel mixture to an output air-fuel ratio range near the stoichiometric air-fuel ratio and a combustion stability limit larger than the stoichiometric air-fuel ratio in accordance with the operating conditions detected by the various sensors. Switching to a nearby lean air-fuel ratio region (air-fuel ratio of about 20 to 22) is performed, and the fuel injection amount is controlled in accordance with the target air-fuel ratio set by the switching. Therefore, the engine 1 of the present embodiment is a so-called lean burn engine.

Here, as described above, the lean NOx catalyst reduces NOx due to the presence of HC, so if the amount of HC is insufficient, the NOx reduction performance is reduced. In particular, when the air-fuel ratio is changed between the output air-fuel ratio range and the lean air-fuel ratio range as described above, as shown in FIG. 4, the air-fuel ratio range where the NOx emission peaks (the air-fuel ratio
16-18) and at this time HC / NOx
The NOx ratio in the lean NOx catalyst 19 decreases.
Therefore, a large amount of NOx may be discharged into the atmosphere when the air-fuel ratio is switched.

Therefore, in the present embodiment, the amount of HC is ensured at the time of switching the air-fuel ratio as described below so that the NOx reduction performance can be maintained. The flowchart of FIG. 5 shows the control for switching the target air-fuel ratio between the output air-fuel ratio range (substantially the stoichiometric air-fuel ratio) and the lean air-fuel ratio range. In this embodiment, the function as the air-fuel ratio switching means is provided by software in the control unit 23 as shown in the flowchart of FIG. In this embodiment, the engine 1 is an in-line four-cylinder engine.

The routine shown in the flowchart of FIG.
This is executed every cycle. First, information such as the engine load, the rotation speed, and the coolant temperature is read (S1).
1). The engine load is calculated by another routine based on an intake air flow rate Q detected by the air flow meter 6 and an engine rotation speed Ne calculated based on a detection signal from the crank angle sensor 16. Injection amount Tp (←
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 condition is included in a preset lean operating region. By determining whether or not the condition is such that the vehicle is operable, it is determined whether or not the lean operation is possible (S12). Here, if it is determined that the lean operation is possible, 1 is set to a lean determination flag FLEAN indicating a setting switching state of the target air-fuel ratio (S13), and the lean air-fuel ratio is not allowed 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 time of the previous execution of this program (S15). Here, when the currently set lean determination flag FLEAN 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 switching from the output air-fuel ratio range to the lean air-fuel ratio range. When the switching is performed, 1 is set to the enrichment flag FRICH (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, for the determination in S15 at the next execution of this program, the lean determination flag FLEAN set this time is set to the previous value FL-OLD (S
17). Next, in response to the result of the determination as to whether or not the lean operation is possible, an actual target air-fuel ratio is set (S18).

Details of the setting of the target air-fuel ratio in S18 are shown in the flowcharts of FIGS. still,
In this embodiment, the HC concentration increasing means and the HC concentration control means are used.
Stage, the functions as occurs torque control means, as shown in the flowchart of FIG. 6 and FIG. 7, the control unit 23 is provided by 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 judging the initial state of the control for enriching the air-fuel ratio is judged (S202).

Here, it is determined that the flag FR-OLD is 0, and when the enrichment control is in the initial state, the flag FR-OLD is set to 1 (S20).
3) As shown in FIG. 8, a period TR for forcibly enriching the air-fuel ratio before actually switching the air-fuel ratio is read from a map previously allocated based on 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 range and the lean air-fuel ratio range is set, the switching is not immediately started but the switching start of the air-fuel ratio is delayed by the enrichment period TR. During the delay period, a large amount of HC is discharged by burning at an air-fuel ratio richer than the air-fuel ratio before switching, and lean N
The maximum amount of HC within a possible range is adsorbed on the Ox catalyst 19 (see FIG. 9).

In the lean NOx catalyst 19, the total amount MO of adsorbable HC is determined by the supported amount of the catalyst. Therefore, the amount of HC discharged from the engine is obtained from the operating conditions and the air-fuel ratio setting, and the HC amount is further determined from this value. The time required for the amount of adsorption to reach the adsorbable amount MO is determined, and this time is defined as the enrichment period TR. Since the amount of HC emission increases as the load becomes higher and the engine speed increases, the enrichment period TR is set to a shorter period as the engine load and the engine speed increase, in other words, a longer period as the engine load and the engine speed decrease. Is set to be 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, a map of the air-fuel ratio / ignition timing for normal operation, which is assigned based on the engine load and the engine speed, is referred to based on the lean determination flag FLEAN, and the reference values are referred to as MKMR and MADV, respectively. (S206). As shown in FIG. 10, two types of air-fuel ratio / ignition timing maps for the normal operation are prepared in advance, one for the output air-fuel ratio range (for stoichiometric) 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 KMR (i), ADV (i) assigned to each cylinder.
(I = 1 to 4) (S207). Here, the lean determination flag FLEAN is inverted with respect to the previous time, and if the lean determination flag FLEAN is 1, the lean determination flag FLEAN is normally referred to a map for a lean air-fuel ratio range (lean operation). If the flag FLEAN is 0, a map for the output air-fuel ratio range (stoichiometric operation) is referred to.

However, in the present embodiment, the switching of the air-fuel ratio is delayed during the enrichment period TR (while the flag FRICH is 1), and the enrichment period TR elapses after the lean determination flag FLEAN is inverted. The air-fuel ratio is switched for the first time later (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, when switching from the lean air-fuel ratio range in which the lean determination flag FLEAN is 0 to the output air-fuel ratio range, Referring to the map for the lean air-fuel ratio range before the switching, the lean determination flag FL
When switching from the output air-fuel ratio range (the stoichiometric air-fuel ratio) in which EAN is 1 to the lean air-fuel ratio range, a map for the output air-fuel ratio range before switching is referred to.

In the above description, the normal data corresponding to the air-fuel ratio setting before switching is set as the air-fuel ratio and ignition timing of all cylinders. However, in the next S208 and S209, only the air-fuel ratio of the specific cylinder is enriched. Therefore, a process of rewriting the air-fuel ratio / ignition timing setting of the specific cylinder is performed. That is,
When an instruction is given to switch the air-fuel ratio between the output air-fuel ratio range and the lean air-fuel ratio range, the start of the actual switching control is simply delayed for the cylinders other than the specific cylinder. In order to forcibly discharge a large amount of HC within the cylinders, at least the other cylinders are made richer.

As shown in FIGS. 10 (c) and 10 (d), the control unit 23 stores the air-fuel ratio and ignition timing map for enriching the specific cylinder in advance from the output air-fuel ratio range (stoichiometric). 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, in the enrichment period when switching from the output air-fuel ratio range to the lean air-fuel ratio range, other maps are used. For the specific cylinder, the map of FIG. 10C is used.

Here, the map shown in FIG. 10A and the map shown in FIG.
10B, the map of FIG. 10B and the map of FIG. 10D are compared with each other. Control using the data S _ij and S ′ _ij or L _ij and L ′ _ij corresponding to the same load / rotation is performed. Then, the same generated torque can be obtained. In the enrichment period TR, only the specific cylinder is enriched. Therefore, if only the setting of the air-fuel ratio is made different between the cylinders, the generated torque increases in the specific cylinder and the generated torque between the cylinders becomes It becomes a balance. Therefore, even if the air-fuel ratio is enriched, a map in which the ignition timing is corrected is set to an air-fuel ratio / ignition timing map for enrichment 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 cylinders that are not enriched.

That is, as shown in FIG. 11, the air-fuel ratio of a specific cylinder is set to 9 with respect to the other cylinders burned at a predetermined lean air-fuel ratio.
If the enrichment is made as large as possible, only the generated torque of the enriched cylinder increases with the enrichment. Therefore, data of the air-fuel ratio and the ignition timing that can offset the increase in the torque generated by the enrichment by setting the retard of the ignition timing (retard) is obtained by an experiment in advance, and this is set as a map for enrichment. 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:
They are made substantially equal under the same load and rotation conditions.

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

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 is controlled to about 20 to 22 in the lean air-fuel ratio range. Against
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 (enriched) to about 10 to 12 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 (enriched) 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 generated with respect to the target air-fuel ratio before switching (the air-fuel ratio of another cylinder) without increasing the generated torque. Air-fuel ratio to be enriched
After setting the ignition timing, the previous value KMR-O of the set air-fuel ratio / ignition timing for the specific cylinder (the first cylinder) is set.
As the LD (1) and the ADV-OLD (1), those that refer to the enrichment map are set instead of those that refer 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). If the enrichment period TR has not elapsed, the process directly proceeds to the target air-fuel ratio setting step after S214. If it is determined that the enrichment period TR has elapsed, the flags FRICH and FR-O are determined.
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 reset to zero, the lean determination flag FLEAN will be used next time.
When the flag FLEAN is 0, the normal map for the output air-fuel ratio range (FIG. 10A) is referred to, and when the flag FLEAN is 1, the normal map for the lean air-fuel ratio range (FIG. Referring to 10 (b)), a common air-fuel ratio and ignition timing for 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 is changed gradually from the air-fuel ratio before the switching to the air-fuel ratio after the switching. This is a step for setting the target air-fuel ratio so as not to cause a sudden output change accompanying the switching.
First, the air-fuel ratio KMR set with reference to the map this time
(I) is compared with the previous air-fuel ratio KMR-OLD (i) to determine whether the air-fuel ratio is set to be increased (S).
214).

If it is determined that the set air-fuel ratio is increasing, the data obtained by adding a predetermined step amount ΔRMR to the previous value KMR-OLD (i) and the current set air-fuel ratio KMR (i) is compared, and the smaller data is set as the final target air-fuel ratio TMR (i).
Accordingly, when shifting from the output air-fuel ratio range to the lean air-fuel ratio range, the air-fuel ratio does not change from near the stoichiometric air-fuel ratio to the final lean air-fuel ratio at once, but gradually changes by the step amount ΔRMR.

On the other hand, if the set air-fuel ratio obtained by referring to the map has not changed or has decreased, the data obtained by subtracting a predetermined step amount ΔLMR from the previous value KMR-OLD (i) is used. , The current 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 air-fuel ratio does not change from the lean air-fuel ratio to near the stoichiometric air-fuel ratio at once, but gradually changes by the step amount ΔLMR.

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 operating state, or in the enrichment period TR, the map reference value is used. Is directly set as the target air-fuel ratio TMR (i). S217 ~
In S219, a process of gradually changing the ignition timing TADV (i) is performed in the same manner as the setting of the target air-fuel ratio TMR (i).

The above target air-fuel ratio TMR (i) and TAD
V (i) is performed for each cylinder,
Once the series of processing from S214 to S219 is completed, the cylinder number counter i (initial value = 1) is incremented by one. 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 processing of S214 to S219 is 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 the target air-fuel ratio TMR (i) and TADV (i) are set to an air-fuel ratio / ignition timing (not shown). To the control routine to control the injection amount and TADV corresponding to the target air-fuel ratio TMR (i).
The ignition timing control corresponding to (i) is performed.

The control shown in the flowcharts of FIGS. 6 and 7 will be described briefly. When switching between the output air-fuel ratio range (stoichiometric operation) and the lean air-fuel ratio range (lean operation), the switching is performed. For a predetermined period TR,
In the switching delay period, only the air-fuel ratio of the specific cylinder is enriched. When the air-fuel ratio is enriched, the ignition timing is set to be retarded as compared with the other cylinders in order to suppress an increase in the generated torque due to the enrichment, and is set to the same level as the generated torque of the other cylinder.

When the specific cylinder is enriched as described above, the amount of HC contained in the exhaust gas from the specific cylinder can be forcibly increased. By setting the period TR, the maximum amount of HC that can be adsorbed to the lean NOx catalyst 19 can be adsorbed within the air-fuel ratio switching delay period. Then, after the enrichment period TR has elapsed, the lean NOx catalyst
After the state where it is estimated that the maximum amount of HC is adsorbed in 19, the actual air-fuel ratio switching is started.

Here, in the air-fuel ratio range (air-fuel ratio 16 to 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 to 22), the NOx emission from the engine 1 is determined. However, if a large amount of HC is positively adsorbed on the lean NOx catalyst 19 immediately before the start of the air-fuel ratio switching as described above, a large amount of NOx is actually discharged from the engine. At the time of the fuel ratio state, the NOx reduction performance can be maintained by the presence of the adsorbed HC.

Therefore, according to the present embodiment, when the target air-fuel ratio is switched between the output air-fuel ratio range (the stoichiometric air-fuel ratio) and the lean air-fuel ratio range, the output is gradually reduced to avoid a rapid change in the output. Even with a configuration in which the air-fuel ratio is changed, a large amount of NOx discharged in the air-fuel ratio range in the middle of switching can be favorably reduced, and an increase in NOx emission can be avoided while maintaining drivability.

In a predetermined period immediately before the air-fuel ratio switching, the injection timing is delayed, the temperature of the combustion chamber wall is reduced, and the blow-by gas and the evaporated fuel desorbed from the canister 23 are added to the exhaust system. By doing so, HC that can be used for the treatment of NOx discharged in large quantities during the switching of the air-fuel ratio may be adsorbed to the lean NOx catalyst 19.

However, if the air-fuel ratio of the engine intake air-fuel mixture is made rich as in the present embodiment, incomplete combustion is greatly increased in order to secure the HC amount.
The fuel that is not used for combustion is not used at all to secure the HC amount, and the HC amount can be increased with good response. Therefore, the stability of NOx can be maintained without significantly deteriorating the operability and fuel efficiency of the engine. It is possible to achieve effective purification.

By the way, immediately before the switching of the air-fuel ratio, the torque generated by the cylinder to be enriched in order to adsorb the maximum amount of HC to the lean NOx catalyst 19 is controlled by another cylinder (normally at the air-fuel ratio before switching). In order to make the torque equal to the torque generated by the cylinder, the air-fuel ratio / ignition timing at which the indicated average effective pressure Pe becomes equal to each other may be used. However, even if the indicated mean effective pressure Pe is equal, dPe
Since / dθ cannot be made equal, the shaft exciting force differs between the enriched cylinder and the other cylinders, and the shaft exciting force in the rich cylinder becomes larger.

On the other hand, in the case of an in-line four-cylinder engine, the effect on the engine vibration is such that the shaft exciting force of the first cylinder farthest from the flywheel is the largest. In addition, the shaft exciting force of the fourth cylinder is
It is directly applied to the crank main bearing, but the main bearing is subjected to the exciting force of the flywheel and has the strictest strength. Therefore, in the above embodiment, the specific cylinder to be enriched is designated as the first cylinder for the sake of simplicity, but the second and third cylinders are enriched for the purpose of reducing engine vibration and improving durability. It is desirable to use a specific cylinder.

Further, as shown in the flowchart of FIG. 12, it is also possible to sequentially switch the cylinders to be enriched with respect to the cylinders normally controlled in a 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.
It is performed instead of part 9.

First, the enriched cylinder counter CYL-N
With the cylinder designated by (initial value = 1) as the enriched cylinder, the air-fuel ratio KMR and the ignition timing ADV for this enriched cylinder are determined with reference to a map (S31). It should be noted that the air-fuel ratio KMR and the ignition timing ADV are richer as the air-fuel ratio but are equal to the generated torque for a cylinder that is controlled according to the air-fuel ratio setting before switching normally without being enriched. It is what becomes.

Next, the set air-fuel ratio KMR and ignition timing ADV are set to KMR-OLD and ADV-OLD as the previous values for the enriched cylinder (S3).
2). Next, the enriched cylinder counter CYL-N is set to 1
If the value of the enriched cylinder counter that has been incremented by one exceeds 4 (S34), a process of resetting it to the initial value of 1 is performed (S35).

Therefore, the enriched cylinder counter CYL
−N changes from 1 → 2 → 3 → 4 → 1 every cycle, and the enriched cylinders become # 1 → cycle every cycle.
It changes sequentially from # 2 → # 3 → # 4 → # 1. When the air-fuel ratio is enriched, the combustion chamber wall temperature of the cylinder decreases. However, if the enriched cylinder is sequentially changed as described above, only the wall temperature of one cylinder decreases and the thermal stress of the cylinder decreases. Is not unbalanced.

Further, in the above embodiment, one of the four cylinders
Although only the cylinders are forcibly enriched in order to secure the amount of HC, a configuration may be adopted in which two or three cylinders or all cylinders are 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 (stoichiometric air-fuel ratio) for rapid acceleration, the air-fuel ratio may need to be switched within a relatively short time. In order to shorten the period for enriching temporarily (enrichment period TR) as the air-fuel ratio is rapidly switched, it is necessary to increase the HC concentration in the enrichment period. Therefore, it is conceivable that the number of cylinders to be temporarily enriched is increased to cope with it. Depending on the required HC concentration, it is possible to temporarily enrich all cylinders.

The flowchart of FIG. 13 is a flowchart showing control for enriching not only one cylinder but all cylinders when the engine 1 is rapidly accelerated, and is additionally executed in the flowchart of FIG. ing. That is, when the timer t _R for measuring the enrichment period TR immediately before switching of the air-fuel ratio 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 currently read opening TVO and the previous value TVO-OLD, that is, the amount of change in the opening TVO during one cycle is calculated (S1002).
Based on the TVO, it is determined whether or not the throttle valve 7 is being opened at a predetermined rate or more (rapid acceleration) (S1003). Here, if the vehicle is not in the sudden acceleration state, S20
Proceeding to 6, the process for enriching only one specific cylinder is performed as described above.

On the other hand, if it is determined that the vehicle is accelerating rapidly, the process of adsorbing the required amount of HC to the lean NOx catalyst 19 needs to be completed immediately. Therefore, the enrichment map is used to enrich all the cylinders. , The air-fuel ratio and ignition timing of all cylinders are set (S1004, S1005). Here, if the configuration is such that all cylinders are enriched at the same time as described above, the generated torque does not vary among the cylinders. However, if only the process of enriching the air-fuel ratio is performed, the generated torque is simultaneously increased. It will increase rapidly. However, the enrichment map enriches the air-fuel ratio. However, by setting the ignition timing to be retarded, 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. Since it is set, even if the enrichment is performed for all cylinders, the generated torque does not suddenly increase.

Although not shown in the flowchart of FIG. 13, when the all-cylinder enrichment process is performed in response to the sudden acceleration determination, the enrichment period TR is shortened compared to the case where only one cylinder is enriched. Good. With the configuration in which all the cylinders are enriched as described above, there is no imbalance in the shaft excitation force between the cylinders.
This is advantageous in terms of durability, and the maximum enrichment period can be shortened because the maximum amount of HC can be adsorbed on the lean NOx catalyst 19 in a short time.

In the above description, in the process for enriching the air-fuel ratio for a predetermined period before the start of the air-fuel ratio switching, all cylinders are enriched only at the time of rapid acceleration, and one cylinder is used except during rapid acceleration. Only the enrichment is performed, but the configuration may be such that all the cylinders are enriched even at times other than the time of rapid acceleration. Further, the acceleration may be determined to be rapid acceleration or moderate 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 or three cylinders, the cylinder group to be enriched may be sequentially changed.

In the above-described embodiment, the engine 1 having the lean NOx catalyst 19 containing zeolite as a main component in the exhaust passage has been described. However, the catalyst has the ability to adsorb HC and reduces NOx in the presence of HC. Then, by increasing the HC amount on the upstream side of the catalyst immediately before the switching of the air-fuel ratio as described above, it is possible to cause a sufficient amount of HC to be adsorbed on the catalyst for NOx treatment before the switching is started. Thus, the NOx processing capacity during the switching can be increased, so that a three-way catalyst may be used instead of the lean NOx catalyst.

In the above-described embodiment, the increase in the torque generated by enriching the air-fuel ratio is offset by setting the retard of the ignition timing. In particular, in the configuration in which all cylinders are enriched, the exhaust gas recirculation amount is reduced. , The increase of the generated torque accompanying the enrichment can be suppressed. Furthermore, a configuration in which an increase in generated torque is suppressed by a combination of the ignition timing and the exhaust gas recirculation amount may be adopted.

The HC adsorbing capacity of the lean NOx catalyst depends on not only the HC concentration and gas flow rate but also the exhaust temperature (catalyst temperature).
The enrichment period TR may be changed according to the exhaust gas temperature detected by the exhaust gas temperature sensor 22.

[0079]

As described above, according to the present invention,
In an exhaust gas purifying apparatus that treats NOx in exhaust gas with a catalyst that has an ability to adsorb HC and reduce 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 larger than the stoichiometric air-fuel ratio The target air-fuel ratio gradually turns off between
When the switching is performed, the HC amount on the upstream side of the catalyst is increased for a predetermined period immediately before the start of the switching, so that sufficient HC is adsorbed to the catalyst. There is an effect that can be.

As means for increasing the HC amount on the upstream side of the catalyst, the air-fuel ratio of the engine intake air-fuel mixture is made rich, so that incomplete combustion is not greatly increased and the fuel efficiency is significantly increased. Although the HC amount can be increased with good response without causing any serious deterioration, there is an effect that the drivability can be maintained satisfactorily without the output fluctuating due to the enrichment.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing a 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 a state of a change in the emission amount of NOx and HC with respect to an air-fuel ratio.

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

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

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

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

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

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

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

FIG. 12 is a flowchart illustrating an example in which the enrichment cylinder is changed.

FIG. 13 is a flowchart illustrating an embodiment for enriching all cylinders.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 6 Air flow meter 12 Injector 16 Crank angle sensor 19 Lean NOx catalyst 23 Control unit

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI F02D 21/08 301 F02D 21/08 301C 41/14 310 41/14 310A 41/36 41/36 B 43/00 301 43/00 301B 301H 301N 45/00 301 45/00 301G F02P 5/15 F02P 5/15 B (58) Fields investigated (Int. Cl. ⁷ , DB name) F02D 41/04 330 F01N 3/24 ZAB F02D 41/14 310 F02D 41/36 F02D 43/00 301

Claims (6)

(57) [Claims] 1. A means for switching a target air-fuel ratio of an engine intake air-fuel mixture between an output air-fuel ratio region near a stoichiometric air-fuel ratio and a lean air-fuel ratio region larger than the stoichiometric air-fuel ratio in accordance with engine operating conditions.
Between the output air-fuel ratio range and the lean air-fuel ratio range.
An exhaust purification device for an internal combustion engine having an air-fuel ratio switching means for gradually changing the air-fuel ratio, the exhaust purification catalyst being interposed in an exhaust passage, having an ability to adsorb HC, and reducing NOx in the presence of HC. HC concentration increasing means for increasing the HC concentration in the exhaust gas on the upstream side of the exhaust purification catalyst; and the output air-fuel ratio range by the air-fuel ratio switching means.
Switching of the air-fuel ratio between the lean air-fuel ratio range and
And HC concentration control means for increasing the HC concentration in the exhaust gas by the HC concentration increasing means during a predetermined period immediately before the exhaust gas purification apparatus. 2. The internal combustion engine according to claim 1, wherein the HC concentration control means causes the HC concentration increase means to increase the HC concentration for a longer period of time when the engine speed is low and the load is low. Exhaust purification equipment. 3. An engine intake air-fuel mixture according to claim 1, wherein
Forcibly make the air-fuel ratio richer than the target air-fuel ratio
With this, the concentration of HC in the exhaust gas on the upstream side of the exhaust purification catalyst is increased.
The air-fuel ratio is enriched by the HC concentration increasing means.
The increase in the torque generated due to the enrichment
Control other than the air-fuel ratio related to the generated torque.
Control means for controlling the generated torque.
3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1 or 2. 4. An internal combustion engine comprising fuel injection means for injecting and supplying fuel for each cylinder, wherein said HC concentration increasing means is provided.
However, by forcibly increasing and correcting the amount of fuel injected and supplied to the specific cylinder, only the air-fuel ratio of the specific cylinder is enriched, and the generated torque control means reduces the generated torque of the specific cylinder to that of another cylinder. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein control is performed to suppress the generated torque only for the specific cylinder so as to match the generated torque. 5. The HC concentration increasing means forcibly enriches the air-fuel ratio of all cylinders, and the generated torque control means adjusts the generated torque to all cylinders so as to match 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 control for suppressing the exhaust gas is performed. Wherein said generated torque control means according to claim 3, characterized in that to suppress an increase in the generated torque by controlling one least also the ignition timing and exhaust gas recirculation amount,
The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 4 and 5.