JP2000110615A - Lean combustion type internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lean combustion type internal combustion engine which incorporates storage type NOX catalyst and which can surely emit NOX from the storage type NOX catalyst storing therein NOX without causing deterioration of drivability and fuel consumption. SOLUTION: A lean combustion type internal combustion engine comprises storage type NOX catalyst 6A provided in an exhaust passage of the internal combustion engine, an NOX storage quantity estimating means 21 for estimating a quantity of NOX stored in the catalyst 6A, an air-fuel ratio setting means 23 for setting an air-fuel ratio of the engine between a lean air-fuel ratio and a rich air-fuel ratio, and an air-fuel ratio correcting means 29 for temporarily correcting an air-fuel ratio set by the air-fuel ratio setting means 23 in accordance with an NOX storage quantity estimated by the NOX storage quantity estimating means 21 when the air-fuel ratio set by the air-fuel ratio comparing means 23 is changed over from a lean air-fuel ratio into a stoichiometric air-fuel ratio or a rich air-fuel ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to NOx in exhaust gas.
The present invention relates to a lean-burn internal combustion engine provided with a storage-type NOx catalyst for purifying NOx.

[0002]

2. Description of the Related Art For example, in order to purify NOx generated at the time of lean combustion of a lean burn internal combustion engine used as an automobile engine, lean NOx can be purified even in an oxygen-excess atmosphere (oxidizing atmosphere) where oxygen in exhaust gas becomes excessive. NO
x catalysts are being developed. One type of lean NOx catalyst has a function of adsorbing NOx in exhaust gas in an oxygen-excess atmosphere (oxidizing atmosphere) and desorbing NOx adsorbed in an atmosphere having a reduced oxygen concentration (reducing atmosphere). There is a NOx catalyst.

Although this storage type NOx catalyst stores NOx in exhaust gas in an oxidizing atmosphere, its storage capacity is limited. When a predetermined amount of NOx is stored on the catalyst, the air-fuel ratio of the engine is reduced. Stoichiometric (stoichiometric air-fuel ratio) or rich air-fuel ratio to actively release and reduce NOx, that is, perform NOx release control.

[0004] Therefore, when the air-fuel ratio is switched from lean to stoichiometric as in Japanese Patent No. 2692530, for example, the N-CO stored on the catalyst is temporarily set to a rich air-fuel ratio.
Techniques for releasing and reducing Ox have been proposed. In this case, since the NOx release control is performed in accordance with the timing at which the driver requests the acceleration operation, the torque shock felt by the driver is smaller than in the case where the NOx release control is performed during steady running, and the drivability is reduced. It is advantageous in terms of.

[0005]

However, in the above-described prior art, since the temporary rich air-fuel ratio when the lean air-fuel ratio is switched to stoichiometric is uniformly set, the fuel is stored on the catalyst. If the NOx storage amount is large, the released NOx is not sufficiently reduced, and the NOx
When the occlusion amount is small, a problem occurs that fuel efficiency is unnecessarily deteriorated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has been developed in such a manner that lean NOx can be reliably released and reduced from a catalyst without deteriorating drivability and fuel efficiency. It is an object to provide an internal combustion engine.

[0007]

Therefore, in the lean burn internal combustion engine of the present invention, the storage NOx is stored in the exhaust passage of the internal combustion engine.
When the air-fuel ratio of the engine is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio by the air-fuel ratio setting device according to the engine operating state, at this time, the air-fuel ratio setting device uses the air-fuel ratio correction device. Set the air-fuel ratio to N
It is configured to temporarily correct in the rich direction according to the NOx storage amount of the storage type NOx catalyst estimated by the Ox storage amount estimation means.

Thus, for example, when the driver requests acceleration and the air-fuel ratio is switched from lean to stoichiometric, the air-fuel ratio is temporarily changed to a rich air-fuel ratio corresponding to the amount of NOx stored in the storage-type NOx catalyst. After the correction, the air-fuel ratio based on the requested engine operating state is obtained, so that the stored NO is reduced without deteriorating drivability and fuel efficiency.
x can be reliably released and reduced from the NOx storage catalyst.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. First, the outline of the configuration of the lean burn internal combustion engine according to one embodiment of the present invention will be described. As shown in FIG. 2, the lean burn internal combustion engine is a four-stroke engine, of a spark ignition type, and It is configured as a direct injection internal combustion engine (direct injection engine) that directly injects fuel into the combustion chamber.

An intake passage 2 and an exhaust passage 3 are connected to the combustion chamber 1 so that they can communicate with each other. The communication between the intake passage 2 and the combustion chamber 1 is controlled by an intake valve 4.
The communication between the exhaust passage 3 and the combustion chamber 1 is controlled by an exhaust valve 5. The intake passage 2 is provided with an air cleaner and a throttle valve (not shown).
The exhaust passage 3 is provided with an exhaust purification device 6 and a muffler (muffler) not shown.

An ignition plug 7 is provided at the upper center of the combustion chamber 1, and an injector (fuel injection valve) 8 is provided at an upper side edge of the combustion chamber 1. The injector 8 is arranged so that its opening faces the combustion chamber 1 so as to directly inject fuel toward the combustion chamber 1 in the cylinder. With such a configuration, the air drawn in according to the opening of the throttle valve (not shown) is drawn into the combustion chamber 1 by opening the intake valve 4, and the electronic control unit (ECU) 2
On the basis of the signal from 0, the fuel is mixed with the fuel directly injected from the injector 8. Then, the fuel is burned by ignition of the spark plug 7 at an appropriate timing to generate an engine torque, and then discharged from the combustion chamber 1 to the exhaust passage 3 as exhaust gas. After purifying the three harmful components of CO, HC, and NOx, the muffler silences and desorbs to the atmosphere.

The lean burn internal combustion engine is an in-cylinder injection engine, and the timing of fuel injection can be freely set irrespective of the opening and closing of the intake valve. Therefore, the fuel injection is performed by the above-described lean super-lean combustion. A late injection mode (late lean operation mode or late lean mode) in which fuel is injected during the compression stroke (particularly in the latter half of the compression stroke) to achieve combustion operation (hereinafter referred to as lean operation) and improve fuel efficiency, and premixing Realizes lean operation by combustion,
The first injection mode (first lean operation mode or the first lean mode), in which fuel is injected during the intake stroke (particularly the first half of the intake stroke) to obtain output by gentle acceleration, and the stoichiometric operation (stoichiometric air-fuel ratio operation) by premixed combustion. The stoichiometric mode (stoichiometric operation mode), in which fuel is injected during the intake stroke to improve the output compared to the previous injection mode, and the rich operation (air-fuel ratio smaller than the stoichiometric air-fuel ratio) by premixed combustion are realized. Enrich mode (open loop mode) that improves output over stoichio operation mode
And a mode according to the operating state of the engine (the state of the load and the number of revolutions) is selected.

By the way, during lean operation such as the latter lean operation mode or the earlier lean operation mode, NOx in exhaust gas cannot be sufficiently purified only by a normal three-way catalyst, and therefore, as shown in FIG. 6 is
The storage type lean NOx catalyst (hereinafter referred to as lean NOx catalyst or simply NOx catalyst) 6A is combined with a three-way catalyst 6B.

That is, when the air-fuel ratio is lean, the NOx concentration sharply increases while little CO and HC are contained in the exhaust gas, but this NOx functions in an oxidizing atmosphere (that is, an oxygen-excess atmosphere). The NOx catalyst 6A stores the CO, HC and NOx in the exhaust gas by the three-way function of the three-way catalyst 6B under the stoichiometric air-fuel ratio.

Since the NOx catalyst 6A is a storage type, NO
If it continues to occlude x, it will eventually reach saturation and occlude
NOx that cannot be released is released into the atmosphere
become. Therefore, the NOx catalyst 6A does not reach the saturated state.
It is necessary to release the stored NOx
However, this release of NOx depends on the atmosphere around the NOx catalyst 6A.
Is stored in a reducing atmosphere (ie, oxygen-deficient state).
NOx _Two(This is NOx
Supply of HC and CO (reducing agent)
NO_TwoTo reduce N_TwoBy discharging as
It is supposed to do it.

That is, under the above-described lean operation, since the surroundings of the NOx catalyst 6A are in an oxidizing atmosphere, the NOx catalyst 6A has NOx generated by lean combustion.
Is stored, and the NOx thus stored is released and decomposed in a reducing atmosphere. Therefore, in the present lean-burn internal combustion engine, the exhaust passage is configured to have a reducing atmosphere so that the NOx stored in the NOx catalyst 6A can be released.

FIG. 1 is a functional block diagram illustrating the fuel injection control for the NOx catalyst 6A and the three-way catalyst 6B. As shown in FIG. 1, the ECU 20 of the lean-burn internal combustion engine according to the present embodiment includes an operation mode selection unit 24, a target air-fuel ratio setting unit 23, and a fuel injection control unit 25.
And an ignition control means 30. These processing means 24, 23, 25, 30 include an O2 sensor 9, a NOx sensor (storage state detecting means) 10, a NOx catalyst temperature sensor which are input to the ECU _20. 11, timer 12, engine speed sensor 13, accelerator position sensor 1
The fuel injection control and the ignition control are performed based on a signal from a sensor 4 or another sensor (for example, an air flow sensor) 15 or the like.

The ECU 20 calculates an effective effective pressure Pe from each information of the engine speed Ne detected by the engine speed sensor 13 and the accelerator opening θ detected by the accelerator position sensor (APS) 14. The pressure calculating means 28 is provided, and the average effective pressure Pe calculated by the effective pressure calculating means 28 is used together with the engine speed Ne as the operating state of the engine for engine control such as fuel injection control. ing.

The operation mode selecting means 24 selects one of the above operation modes according to the engine speed Ne and the average effective pressure Pe.
Further, the target air-fuel ratio setting means 23 selects a target air-fuel ratio map corresponding to the operation mode selected by the operation mode selection means 24, and uses the selected target air-fuel ratio map to set the engine speed Ne and the average. An optimum target air-fuel ratio is set in each operation mode according to the effective pressure Pe.

The fuel injection control means 25 is based on the target air-fuel ratio AFa set by the target air-fuel ratio setting means 23 and the intake air flow (intake air amount) detected by an air flow sensor (other sensor 15) not shown. In addition to setting the fuel injection amount (injector drive time), the fuel injection timing map corresponding to the operation mode selected by the operation mode selection means 24 is selected, and the engine is selected by using the selected fuel injection timing map. The fuel injection end timing for performing combustion is set according to the rotation speed Ne and the average effective pressure Pe. Then, a fuel injection start timing and a fuel injection end timing are calculated based on a value obtained by appropriately correcting the set value, and the fuel injection valve (injector) 8 is controlled based on the calculated value. Has become.

The ignition control means 30 selects a target air-fuel ratio map corresponding to the operation mode selected by the operation mode selection means 24, and uses the selected target air-fuel ratio map to determine the engine speed Ne and the average effective speed. In accordance with the pressure Pe, an optimal ignition timing is set in each operation mode,
The operation (ignition timing) of the ignition plug 7 is controlled based on the set timing.

The target air-fuel ratio setting means 23 is provided with an air-fuel ratio correction means 29. During the stoichiometric feedback operation, the target air-fuel ratio is determined based on the O ₂ concentration in the exhaust gas detected by the O ₂ sensor 9. By performing the feedback control of the fuel ratio, the stoichiometric operation is appropriately performed while repeating the rich and lean operations near the stoichiometric air-fuel ratio, and the exhaust gas purifying function of the three-way catalyst 6B is surely exhibited.

That is, in the stoichiometric mode, the target air-fuel ratio setting means 23 first sets the target air-fuel ratio AFa to the stoichiometric air-fuel ratio (for example, 14.7), and thereafter sets the target air-fuel ratio AFa to O _2.
The target air-fuel ratio AFa is finely corrected by the air-fuel ratio correcting means 29 according to the output from the sensor 9. If the output from the O ₂ sensor 9 indicates an excess air state (ie, the air-fuel ratio is lean), the target air-fuel ratio setting means 2
3, as in the following equation (1), the previous target air-fuel ratio AFa
The current target air-fuel ratio AFa (n) is calculated by multiplying (n-1) by a feedback gain α1 (α1 ≒ 1 and α1 <1) slightly smaller than 1.
(N) is slightly corrected to a richer side than the previous time.

AFa (n) = AFa (n−1) · α1 (1) On the contrary, the output from the O ₂ sensor 9 is in the air shortage state (ie,
If the air-fuel ratio indicates a rich air-fuel ratio, the target air-fuel ratio setting means 23 sets the feedback gain α2 slightly larger than 1 in the previous target air-fuel ratio AFa (n−1) as in the following equation (2). By multiplying (α2ＡＦ1 and α2> 1), the current target air-fuel ratio AFa (n) is calculated, and the target air-fuel ratio AFa (n) is slightly corrected to be leaner than the previous time.

AFa (n) = AFa (n−1) + α2 (2) On the other hand, late lean mode, early lean mode,
In the case of the enrichment mode, the target air-fuel ratio setting means 23 sets an open loop for each control cycle in correspondence with the engine speed Ne and the engine load (here, the average effective pressure Pe is adopted). For example, a map is provided in which the target air-fuel ratio AFa is associated with the engine speed Ne and the engine load Pe for each operation mode,
The target air-fuel ratio AFa is set based on the engine speed Ne and the engine load Pe using a map corresponding to each operation mode.

However, the air-fuel ratio correction means 29 of the lean burn internal combustion engine has a correction function unique to the engine in addition to the air-fuel ratio correction during the stoichiometric feedback operation. That is, the air-fuel ratio correction unit 29 switches from lean operation (operation in the late lean operation mode or early lean operation mode) to stoichiometric feedback operation (operation in the stoichiometric mode) or rich operation (operation in the enrich mode).
Is switched to the target air-fuel ratio AFa according to the engine rotation state Ne and the load state Pe for a predetermined time T1.
However, it is configured to temporarily reduce (enrich) correction.

The reduction correction (enrichment correction) of the target air-fuel ratio AFa is performed by setting the surroundings of the NOx catalyst 6A to a reducing atmosphere (ie, a state of insufficient oxygen), thereby reducing the NOx catalyst 6A.
It is intended to promote the release of NOx from the air (NOx purge). That is, even in the stoichiometric feedback operation or the rich operation, the NOx from the NOx catalyst 6A is normally
At this time, the target air-fuel ratio AFa is further reduced (enriched) to obtain NOx.
The purpose is to increase the temperature of the atmosphere of the catalyst 6A more quickly to a predetermined temperature range and make the atmosphere of the NOx catalyst 6A a stronger reducing atmosphere to promote the desorption of NOx from the NOx catalyst 6A.

At the time of this enrichment correction, even in the stoichiometric mode, the target air-fuel ratio AFa is set in an open loop and fuel injection control is performed. As described above, the target air-fuel ratio AFa at the time of the enrichment correction is, as described above, a target air-fuel ratio AFa set based on the engine speed Ne and the engine load Pe using the map corresponding to each operation mode. As shown in equation (3), the correction gain (enrichment correction coefficient) β
Is calculated by multiplying

AFa = AFa × β (3) Note that the correction gain β is slightly smaller than 1 (ie,
β ≒ 1 and β <1), the correction gain β is
The value is set to a value corresponding to the amount of NOx stored by the NOx catalyst 6A. Here, as shown in the following equation (4), the correction gain β is calculated using the integrated value TL of the operation time in the lean mode (the latter lean operation mode or the earlier lean operation mode) as a parameter indicating the NOx storage amount.

Β = A · TL where A: coefficient (4) The integrated value TL of the lean mode operation time is reset to 0 when the NOx purging of the NOx catalyst 6A is completed.
If the NOx purge ends in the middle, it is decremented according to the degree of the NOx purge. That is,
If the mode is switched from the stoichiometric mode or the enriched mode to the lean mode before the air-fuel ratio enrichment correction is performed for the predetermined time T1, the air-fuel ratio enrichment correction is temporarily stopped. In the fuel ratio enrichment correction, it is considered that the NOx purge of the NOx catalyst 6A is not completed. Therefore, here, the time TT when the air-fuel ratio enrichment correction is actually performed and the predetermined time T1
Based on the ratio (TT / T1), the integrated value TL of the lean mode operation time is corrected by the following equation (5).

TL = TL (1−TT / T1) (5) However, this air-fuel ratio enrichment correction is performed by the NOx catalyst 6A.
Ambient temperature to a predetermined temperature range and the NOx catalyst 6
By reducing the atmosphere of A to NOx, the desorption of NOx from the NOx catalyst 6A is promoted. From the start of the air-fuel ratio enrichment correction, until the ambient temperature of the NOx catalyst 6A rises to a predetermined level. Takes a certain amount of time. Therefore, when the additional fuel injection is continued for a predetermined time T1,
During the time T1-t1 obtained by subtracting the time t1 required until the ambient temperature of the NOx catalyst 6A rises to the predetermined level from the predetermined time T1, the surroundings of the NOx catalyst 6A are NO.
The ambient temperature will be x desorbable. In consideration of such a point, the above equation (5) may be changed to the following equation (6).

TL = TL [1− (TT−t1) / (T1−t1)] (6) Note that the time T1, the time t1, and the coefficient A in the equation (4) are characteristics of the engine. Or based on the characteristics of the NOx catalyst 6A. The ignition control means 30 of the lean burn internal combustion engine includes a function (ignition timing correction means) 31 for correcting the ignition timing.
Is provided. The ignition timing correction means 31 controls the ignition timing in synchronization with the air-fuel ratio enrichment correction for the NOx purge in order to suppress the increase in engine output accompanying the air-fuel ratio enrichment correction for the NOx purge. The ignition timing is corrected by, for example, retarding the ignition timing.

Since the lean burn internal combustion engine as one embodiment of the present invention is configured as described above, the air-fuel ratio setting means 23 controls the fuel injection control (injector control) and the ignition as shown in FIG. Control (spark plug control)
Is performed. That is, first, the ECU 20 reads the detection information of each sensor such as the engine speed Ne detected by the engine speed sensor 13 and the accelerator opening θ _AC detected by the accelerator position sensor 14 (step S10). At the start of control, flags FC and FL described later are both initialized to 0.

Then, the effective effective pressure calculating means 28 calculates an average effective pressure Pe from the engine speed Ne and the accelerator opening θ _AC (step S12). Further, the operating mode selecting means 24 selects one operating mode from the latter lean operating mode, the former lean operating mode, the stoichiometric mode, and the enriched mode according to the engine speed Ne and the average effective pressure Pe (step). S2
0).

The target air-fuel ratio setting means 23 sets the target air-fuel ratio AFa on the basis of the selected operation mode, the engine speed Ne and the average effective pressure Pe, for example, using a previously stored map or the like. When the stoichiometric mode is selected, the stoichiometric air-fuel ratio (for example, 1) is set as the target air-fuel ratio AFa.
4.7) is set (step S30). In the ignition control means 30, the selected operation mode and the engine speed Ne are determined.
The ignition timing is set based on the average effective pressure Pe (step S40).

Then, it is determined whether or not the flag FC is 1 (step S50). This flag FC is set to 1 when the air-fuel ratio enrichment correction for NOx purge is being performed.
It is set to 0 when the air-fuel ratio enrichment correction is not performed. Since the flag FC is initially 0, the process proceeds to step S60, and it is determined whether the flag FL is 1 or not. This flag FL is set to 1 in the lean mode (that is, in the late lean operation mode and the first lean operation mode), and is set to 0 in the other operation modes (that is, in the stoichiometric mode and the enrich mode). . Flag FL
Since the initial value is zero, the process initially proceeds to step S70, where it is determined whether or not the current mode is the lean mode.

Here, if the engine is in the lean operation, the process proceeds from step S70 to step S170, where the flag FC is set to 0 (here, the flag FC = 0 is held), and the flag FL is set to 1. In step S130, it is determined whether the flag FC is 0 (that is, the air-fuel ratio enrichment correction for NOx purging is not performed) and the stoichiometric mode is set. In this case, the stoichiometric mode is set. Since there is not, the process proceeds from step S130 to step S140.

In step S140, the fuel injection amount (injector amount) is determined by the fuel injection control unit 25 based on the target air-fuel ratio AFa set by the target air-fuel ratio setting unit 23 and the intake amount (intake air amount) detected by the air flow sensor. Driving time), and the fuel injection end timing is set according to the engine speed Ne and the average effective pressure Pe using the fuel injection timing map corresponding to the selected operation mode.

Then, the process proceeds to step S150, where the set values are appropriately corrected, and the fuel injection start timing and the fuel injection end timing are calculated. Based on the calculated values, the fuel injection valve (injector) 8 is controlled. Control the drive. Further, the process proceeds to step S160, in which the drive of the ignition plug 7 is controlled at a timing corresponding to the ignition timing set in step S40.

On the other hand, when the flag FC is 0 (that is, the air-fuel ratio enrichment correction for NOx purge is not performed in the previous control cycle) and the flag FL is 1 (that is, in the lean mode in the previous control cycle). When the operation mode is switched from the lean mode to another operation mode in the state of
10, S12, S20, S30, S40, S50, S6
0, the process proceeds from step S70 to step S80,
The flag FC is set to 1 (here, the flag FC = 0 is held), and the flag FL is set to 0.

Then, the process proceeds to a step S90, wherein the timer 1
Start counting 2. That is, the timer count value T
(Initial value 0) is added by the time t corresponding to the control cycle. Further, it is determined whether or not the timer count value T is equal to or less than a predetermined time T1 (step S100). If the timer count value T has not reached the predetermined time T1, step S1
From 00, the process proceeds to step S110, and the target air-fuel ratio correction unit 29 corrects the target air-fuel ratio (enrichment of the air-fuel mixture).
Perform That is, as in the above equation (3), in step S30, the target air-fuel ratio AFa set based on the operation mode, the engine speed Ne, and the engine load Pe is multiplied by the enrichment correction coefficient β. The target air-fuel ratio AFa is corrected. Note that the enrichment correction coefficient β is N
Although it is set according to the NOx storage amount by the Ox catalyst 6A,
Here, the integrated value TL of the operation time in the lean mode is considered to be a parameter corresponding to the NOx occlusion amount, and the equation (4)
Is calculated based on the integrated value TL of the lean mode operation time.

Next, the routine proceeds to step S120, where the ignition timing correction means 31 increases the output of the engine with the air-fuel ratio enrichment correction for NOx purging in step S110 with respect to the ignition timing set in step S40. The ignition timing is corrected by, for example, retarding the ignition timing to suppress the ignition timing. Further, the process proceeds to step S130,
It is determined whether the flag FC is 0 and the mode is the stoichiometric mode. In this case, since the flag FC is 1, the process proceeds from step S130 to step S140.

Then, the process proceeds to step S140, in which the fuel injection control means 25 uses the target air-fuel ratio AFa corrected by the target air-fuel ratio correction means 29 in step S110 and the intake air amount (intake air amount) detected by the air flow sensor. To set the fuel injection amount (injector drive time)
The fuel injection end timing is set according to the engine speed Ne and the average effective pressure Pe using the fuel injection timing map corresponding to the selected operation mode.

Further, the process proceeds to step S150, where the fuel injection start timing and the fuel injection end timing are calculated after appropriately correcting the set values, and the fuel injection valve (injector) 8 of the fuel injection valve (injector) 8 is calculated based on the calculated values. Control the drive. Further, the process proceeds to step S160, in which the drive of the ignition plug 7 is controlled at a timing corresponding to the ignition timing corrected by the ignition timing correction means 31 in step S120.

As described above, in a certain control cycle, the flag FC
Is set to 1 (step S80) (that is, when the air-fuel ratio enrichment correction for NOx purge is performed), from the next control cycle, steps S10, S20, S
30 and S40, from step S50 to step S
Go to 190. In this step S190, it is determined whether or not the operation mode is the lean mode, and the state is not the lean mode (ie, the stoichiometric mode or the enriched mode).
Continues, the process proceeds to step S90, and as described above, the timer count value T (initial value 0) is added by the control cycle time t, and it is determined whether or not the timer count value T is equal to or less than the predetermined time T1 ( Step S100), timer count value T
If does not reach the predetermined time T1, the routine proceeds to step S110, where the target air-fuel ratio correction (enrichment correction of the air-fuel mixture) is performed.

Then, the routine proceeds to step S120, in which the ignition timing is corrected.
If C is not 0 and is in the stoichiometric mode "state, the process proceeds to step S140, where the fuel injection amount and the fuel injection end timing are set based on the corrected target air-fuel ratio and the like, and the process proceeds to step S150. The drive of the fuel injection valve 8 is controlled in accordance with the fuel injection start timing and the fuel injection end timing based on the above, and the process proceeds to step S160 to control the drive of the ignition plug 7 at a timing corresponding to the corrected ignition timing.

If the state other than the lean mode continues, until the timer count value T exceeds the predetermined time T1, the above-described processing, that is, the air-fuel ratio enrichment correction for NOx purge and the increase in engine output accordingly. Is performed to suppress the ignition timing. When the timer count value T exceeds the predetermined time T1, the timer count value T is changed to the predetermined time T1.
Is exceeded, steps S100 to S180
Then, the flag FC and the timer T are both reset to 0, and the process proceeds to step S130.

At this time, if it is the stoichiometric mode, it is determined in step S130 that "the flag FC is 0 and the stoichiometric mode", so the process proceeds to step S210 to perform stoichiometric feedback control regarding fuel injection. . That is, if the output from the O ₂ sensor 9 indicates an excess air state (air-fuel ratio is lean), the previous target air-fuel ratio AFa (n−1) is larger than 1 as in the above equation (1). The current target air-fuel ratio AFa (n) is calculated by multiplying by a slightly smaller feedback gain α1, and the target air-fuel ratio AFa (n) is slightly corrected to be richer than the previous time. Conversely, if the output from the O ₂ sensor 9 indicates an air shortage condition (air-fuel ratio is rich), the previous target air-fuel ratio AFa (n−1) is increased from 1 as in the above equation (2). Is multiplied by a slightly larger feedback gain α2 (α2 ≒ 1 and α2> 1) to obtain the current target air-fuel ratio AFa.
(N) is calculated, and the target air-fuel ratio AFa (n) is slightly corrected to be leaner than the previous time.

Then, the process proceeds to step S140, in which the fuel injection amount and the fuel injection end timing are set based on the target air-fuel ratio and the like corrected in step S210, and the process proceeds to step S150. The drive of the fuel injection valve 8 is controlled in accordance with the injection end timing, and the process proceeds to step S160, in which the drive of the spark plug 7 is controlled at a timing corresponding to the ignition timing set in step S40.

Thereafter, if the state other than the lean mode continues, FC = 0 and FL = 0 are continued.
After steps S10, S20, S30, S40, and S50, the process proceeds from step S60 to step S130. If the stoichiometric mode is set, the target air-fuel ratio is feedback-corrected in step S210, and based on the result of the correction, otherwise. In the case of the enrichment mode, the fuel injection amount and the fuel injection end timing are set based on the target air-fuel ratio and the like set in step S30, and the fuel injection start timing and the fuel injection end timing based on these settings are set. The drive of the valve 8 is controlled (step S150), and the drive of the spark plug 7 is controlled at a timing corresponding to the ignition timing set in step S40 (step S160).

The timer count value T is equal to a predetermined time T1.
Before returning to the lean mode, the process proceeds from step S190 to step S200 via steps S10, S20, S30, S40 and S50, and F
It is assumed that C = 0 and FL = 1. Then, the process proceeds to step S130. In this case, since the mode is not the stoichiometric mode, the process proceeds from step S130 to step S140, and sets the fuel injection amount and the fuel injection end timing based on the target air-fuel ratio and the like set in step S30. The driving of the fuel injection valve 8 is controlled in accordance with the fuel injection start timing and the fuel injection end timing based on this setting (step S150), and step S4 is performed.
The drive of the spark plug 7 is controlled at a timing according to the ignition timing set at 0 (step S160).

It should be noted that the timer count value T is equal to a predetermined time T1.
If the air-fuel ratio enrichment correction for NOx purging is continued until the time exceeds NO, the NOx storage amount of the NOx catalyst 6A is reset to 0, but before the timer count value T reaches the predetermined time T1, NOx When the air-fuel ratio enrichment correction for purging is completed, the ratio (TT) between the time TT during which the air-fuel ratio enrichment correction was actually performed and the predetermined time T1 (TT
/ T1), the NOx storage amount of the NOx catalyst 6A is reduced to (1-TT / T1) or [1- (TT-
t1) / (T1−t1)] times, the integrated value TL of the lean mode operation time is corrected by the above equation (5) or (6), and the enrichment correction coefficient β is calculated based on the integrated value TL.
Set.

As described above, in the present lean-burn internal combustion engine, when the mode is switched from the lean mode to another mode (specifically, the stoichiometric mode or the enriched mode), the NOx storage of the NOx catalyst 6A is performed for the predetermined time T1. Since the correction is made to the target air-fuel ratio based on the amount, the temperature of the atmosphere of the NOx catalyst 6A is quickly raised to a predetermined temperature range, and the atmosphere of the NOx catalyst 6A is changed to a stronger reducing atmosphere, so that NOx desorption from the NOx catalyst 6A is promoted. The NOx desorption (N) from the NOx catalyst 6A is reliably or sufficiently performed within the predetermined time T1.
Ox purge) is performed.

In particular, from the lean mode to other modes (specifically, stoichiometric mode and enriched mode)
When the lean-burn internal combustion engine is applied as an automobile engine, the engine output is increased when the vehicle accelerates or when the vehicle speed is approaching uphill to maintain the vehicle speed. At that time,
Since the engine output torque changes, even if the air-fuel ratio is corrected to be rich according to the change, the change in the engine output torque due to the correction is not conspicuous. In other words, the driver and the occupant do not easily feel the torque shock due to the air-fuel ratio enrichment correction, and even if they feel the torque shock, they do not feel it uncomfortable when accelerating or climbing a hill.

Therefore, the NOx storage capacity of the storage NOx catalyst can be restored without causing unpleasant torque shock. When the air-fuel ratio enrichment correction for NOx purge is completed before the timer count value T reaches the predetermined time T1, the time TT when the air-fuel ratio enrichment correction is actually performed and the predetermined time T1 (TT / T1), the integrated value TL of the lean mode operation time (ie, the NOx of the NOx catalyst 6A)
The amount of occlusion is estimated and the enrichment correction coefficient β is set, so this is reflected at the next air-fuel ratio enrichment correction,
The NOx purge of the NOx catalyst 6A can be reliably performed.

The lean burn internal combustion engine has been created in response to an increase in the NOx storage capacity due to the recent improvement in the performance of the storage NOx catalyst. During normal driving, it is unlikely that only lean operation will be continued, and even when driving almost steady, it is often switched from lean operation to stoichio operation or rich operation in order to accelerate or climb uphill due to road conditions. Therefore, when the operation is switched from lean operation to other operation, NOx
By performing NOx purging of the catalyst 6A, the NOx catalyst 6
It is based on the idea that the NOx saturation state of A does not occur.

However, the NOx storage amount of the NOx catalyst 6A is estimated (for example, the integrated value TL of the lean mode operation time).
From the NOx catalyst 6A If it becomes saturated, the air-fuel ratio can be enriched for a short period of time at an appropriate timing during lean mode operation,
In particular, in a direct injection internal combustion engine, apart from fuel injection for main combustion (combustion for generating engine output), timing not consumed in main combustion (timing that does not affect engine output), For example, the fuel injection may be performed after the middle stage of the expansion stroke or during the exhaust stroke to promote the NOx purge.

As described above, NOx during the lean mode operation
Even if the purging is also used, the NOx purge is performed in synchronization with the timing of switching from the lean operation to the stoichiometric operation or the rich operation unless the NOx catalyst 6A becomes saturated with NOx during the lean mode operation.
Ox purging can be performed without causing unpleasant torque shock.

When the air-fuel ratio of the main combustion is enriched and the NOx purge is performed during the lean mode operation, as in the present embodiment, the ignition timing control is performed so as to reduce the increase in the engine output. May be performed. In this embodiment, the correction of the air-fuel ratio is performed by multiplying the correction coefficients (correction amounts) α1, α2, and β. However, the correction amounts corresponding to the correction coefficients α1, α2, and β are set. Alternatively, the air-fuel ratio may be corrected by adding or subtracting the correction amount.

[0060]

As described above in detail, according to the lean-burn internal combustion engine of the present invention, the stored NOx can be removed from the NOx storage catalyst without deteriorating drivability and fuel efficiency. This has the effect that it can be reliably released and reduced.

[Brief description of the drawings]

FIG. 1 is a control block diagram showing a main configuration of a lean burn internal combustion engine as one embodiment of the present invention.

FIG. 2 is a schematic diagram showing a configuration of a lean burn internal combustion engine as one embodiment of the present invention.

FIG. 3 is a flowchart showing control by a lean burn internal combustion engine as one embodiment of the present invention.

[Explanation of symbols]

3 exhaust passage 6 the exhaust purification device 6A NOx catalyst 6B three-way catalyst 7 spark plug 8 injector (fuel injection valve) 9 O ₂ sensor 10 NOx sensor (storage state detecting means) 20 ECU 23 air-fuel ratio setting means 24 Operation mode selection means 25 Fuel injection control means 29 air-fuel ratio correction means 30 ignition control means 31 ignition timing correction means

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F01N 3/28 301C F02D 41/14 310 F02D 41/14 310L (72) Inventor Doga Takashi Hara 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Kazuhide Tsugai 3-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Hiromitsu Ando 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation F-term (reference) 3G091 AA12 AA24 AB03 AB05 AB09 BA01 CA18 CB02 CB03 CB05 DA02 DB06 DB10 DC03 EA01 EA02 EA05 EA07 EA12 EA18 EA33 FB10 FB11 FB10 FB11 HA36 HA37 HA38 HA42

Claims (1)

[Claims] 1. A storage NOx catalyst provided in an exhaust passage of an internal combustion engine for storing NOx in exhaust gas in an oxidizing atmosphere and releasing or reducing the stored NOx in a reducing atmosphere, and the storage NOx catalyst. NOx occlusion amount estimating means for estimating the NOx occlusion amount occluded in the engine; air-fuel ratio setting means for setting an air-fuel ratio of the engine between a lean air-fuel ratio and a rich air-fuel ratio in accordance with the engine operating state; When the air-fuel ratio set by the air-fuel ratio setting means was switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio, the air-fuel ratio set by the air-fuel ratio setting means was estimated by the NOx storage amount estimating means. A lean-burn internal combustion engine comprising: air-fuel ratio correction means for temporarily correcting in the rich direction according to the NOx storage amount.