JPH1182098A - Air-fuel ratio control system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To satisfy a request for high torque when it is made and simultaneously improve the fuel consumption. SOLUTION: An exhaust pipe 3 directed out of an engine 1 is provided with an A/F sensor 16 for outputting a wide and linear air-fuel ratio signal into an ECU 30 in proportion to the oxygen concentration in the exhaust gas. The ECU 30 uses its CPU 31 to set the target air-fuel ratio of mixture supplied to the engine 1 leaner than the stoichiometric air-fuel ratio, based on which lean target ratio lean combustion is executed, and to compute a requested torque for the engine 1 in accordance with the engine speed, intake pressure and the like. A relatively smaller requested torque sets the target air-fuel ratio leaner, while a relatively higher requested torque sets the target air-fuel ratio oscillating between this leaner ratio and a higher ratio at a predetermined cycle. The time interval of this ratio amplitude is set according to the level of the requested torque.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to an air-fuel ratio control device for performing an air-fuel ratio control in an air-fuel ratio lean region, that is, a so-called lean burn control. .

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No.
Japanese Patent Publication No. 208141 discloses an "air-fuel ratio lean control method for an electronically controlled engine", in which feedback control is performed using an air-fuel ratio lean using a lean sensor. Especially in the low and medium load range of the engine,
While the target air-fuel ratio is set to the lean side to improve fuel efficiency, in a high load region such as when the throttle is fully open, the target air-fuel ratio is set to the rich side to improve the driving performance (ensure the required torque). I have to. In this way, by setting the target air-fuel ratio even in a rich region other than the lean region and performing feedback control according to the target air-fuel ratio, it is possible to suppress fluctuations and variations in the air-fuel ratio even when fuel is increased in a high load region.

[0003]

However, in recent years, there has been a tendency that demands for lower fuel consumption have been further intensified, and it is desired to improve the fuel consumption performance even when the vehicle is accelerating or operating under a high load. Under such circumstances, the conventional technology described above cannot improve the fuel efficiency when a high torque is required such as during acceleration or high load operation.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to provide an internal combustion engine capable of improving fuel economy while satisfying a high torque demand. An object of the present invention is to provide an air-fuel ratio control device for an engine.

[0005]

In order to achieve the above object, according to the first aspect of the present invention, when the required torque is at a relatively low level, the control air-fuel ratio is set to a lean air-fuel ratio (first air-fuel ratio). When the required torque is at a relatively high level, the control air-fuel ratio is made to oscillate at a predetermined cycle between the lean air-fuel ratio and the air-fuel ratio richer than the lean air-fuel ratio (second air-fuel ratio instruction device).

According to the above configuration, the fuel-efficient operation of the internal combustion engine can be continued even when a high torque request is received, such as during vehicle acceleration or high load operation. At the same time, it is possible to meet high torque demands. As a result, when high torque is required, it is possible to improve fuel economy while satisfying the torque requirement. In such a case, by inclining the air-fuel ratio in a predetermined cycle, an unintended torque fluctuation does not occur.

[0007] In the first aspect of the present invention, in order to suppress the torque fluctuation and secure good drivability, it is preferable to configure as in the following second or third aspect. That is: In the second aspect of the invention, the time interval for oscillating the air-fuel ratio is set according to the required torque. According to the third aspect of the invention, the amplitude cycle of the air-fuel ratio is gradually changed in accordance with the duration of the high torque request.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) Hereinafter, a first embodiment of the present invention will be described. The air-fuel ratio control system according to the present embodiment sets a target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to a leaner side than the stoichiometric air-fuel ratio, and performs lean combustion based on the target air-fuel ratio. Perform control. As a main configuration of the system, a catalyst (in this embodiment, a lean NOx catalyst) is provided in the exhaust system passage of the internal combustion engine, and a limiting current type air-fuel ratio sensor (A / F) is provided upstream of the catalyst. Sensors). Then, an electronic control unit (hereinafter, referred to as an ECU) mainly including a microcomputer fetches the detection result of the air-fuel ratio sensor, and performs feedback control of the air-fuel ratio based on the sensor detection result.
Hereinafter, the detailed configuration will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an air-fuel ratio control system according to the present embodiment. In FIG. 1, the internal combustion engine is configured as a four-cylinder four-cycle spark ignition engine (hereinafter simply referred to as engine 1).
Is connected to an intake pipe 2 and an exhaust pipe 3. Intake pipe 2
Is provided with a throttle valve 5 linked to the accelerator pedal 4, and the opening of the throttle valve 5 is detected by a throttle opening sensor 6. Further, an intake pressure sensor 8 is provided in the surge tank 7 of the intake pipe 2.

A piston 10 reciprocating in the vertical direction in the figure is disposed in a cylinder 9 constituting a cylinder of the engine 1, and the piston 10 is connected to a crankshaft (not shown) via a connecting rod 11. . A combustion chamber 13 defined by a cylinder 9 and a cylinder head 12 is formed above the piston 10. The combustion chamber 13 is connected to the intake pipe 2 and the exhaust pipe 3 via an intake valve 14 and an exhaust valve 15. Communicating.

A limiting current type air-fuel ratio, which outputs a wide-range and linear air-fuel ratio signal in proportion to the oxygen concentration (or the concentration of unburned gas such as carbon monoxide) in the exhaust gas, is provided to the exhaust pipe 3. An A / F sensor 16 composed of a sensor is provided. Further, a NOx catalyst 19 having a NOx purifying function is disposed downstream of the A / F sensor 16 in the exhaust pipe 3. A water temperature sensor 23 for detecting a cooling water temperature is provided in the cylinder 9 (water jacket).

An intake port 17 of the engine 1 is provided with an electromagnetically driven injector 18, and fuel (gasoline) is supplied to the injector 18 from a fuel tank (not shown). In the present embodiment, a multipoint injection (MPI) system having one injector 18 for each branch pipe of the intake manifold is configured. In this case, the fresh air supplied from the upstream of the intake pipe and the fuel injected by the injector 18 are connected to the intake port 1.
The mixture is mixed at 7, and the mixture flows into the combustion chamber 13 (in the cylinder 9) with the opening operation of the intake valve 14.

The ignition plug 27 disposed on the cylinder head 12 is ignited by an ignition voltage from an igniter 28. The igniter 28 is connected to the distributor 20 for distributing the ignition voltage to the ignition plug 27 of each cylinder. The distributor 20 outputs a pulse signal at every 720 ° CA according to the rotation state of the crankshaft. A sensor 21 and a rotation angle sensor 22 that outputs a pulse signal at each finer crank angle (for example, at every 30 ° CA) are provided.

The ECU 30 mainly includes a well-known microcomputer system.
2, a RAM 33, a backup RAM 34, an A / D converter 35, an input / output interface (I / O) 36, and the like. The detection signal of the intake pressure sensor 8 and the A / F sensor 1
6 and the detection signal of the water temperature sensor 23 are A / D
After being input to the converter 35 and A / D converted, the bus 37
Is input to the CPU 31 via the. The detection signal of the throttle opening sensor 6 and the pulse signals of the reference position sensor 21 and the rotation angle sensor 22 are input to the CPU 31 via the input / output interface 36 and the bus 37.

The CPU 31, based on the detection signals of the sensors, controls the engine pressure such as the intake pressure PM, the air-fuel ratio (A / F), the coolant temperature Tw, the throttle opening TH, the reference crank position (G signal), and the engine speed Ne. Detects operating conditions.
Further, the CPU 31 calculates a control signal such as a fuel injection amount and an ignition timing based on an engine operating state, and outputs the control signal to the injector 18 and the igniter 28.

Next, the operation of the air-fuel ratio control system configured as described above will be described. FIG. 2 is a flowchart showing a fuel injection control routine executed by the CPU 31. This routine is executed every fuel injection of each cylinder (in this embodiment, every 180 ° CA).

Now, when the routine of FIG. 2 starts,
First, in step 101, the CPU 31 detects the sensor detection result indicating the engine operating state (the engine speed Ne, the intake pressure P
M, cooling water temperature Tw, etc.) and the following step 102
Calculates the basic injection amount Tp according to the engine speed Ne and the intake pressure PM at that time using the basic injection map stored in the ROM 32 in advance. Also, the CPU 31
Determines whether or not the well-known air-fuel ratio F / B condition is satisfied in step 103. Here, the air-fuel ratio F / B condition means that the cooling water temperature Tw is equal to or higher than a predetermined temperature, that the high-
This includes not being in a high load state, and that the A / F sensor 16 is in an active state.

In this case, if the determination in step 103 is negative (if the F / B condition is not satisfied), the CPU 31 proceeds to step 104 and sets the air-fuel ratio correction coefficient FAF to "1.0".
And Setting FAF = 1.0 means that the air-fuel ratio is open-controlled. If the determination in step 103 is affirmative (if the F / B condition is satisfied), the CPU 3
1 proceeds to step 200 to execute a setting process of the target air-fuel ratio λTG. Here, the process of setting the target air-fuel ratio λTG is performed according to the routine of FIG. 3 described later.

Thereafter, at step 105, the CPU 31 sets an air-fuel ratio correction coefficient FAF based on the deviation between the actual air-fuel ratio λ (sensor measurement value) at that time and the target air-fuel ratio λTG. In the present embodiment, the air-fuel ratio F / B control based on the modern control theory is performed. In the F / B control, the air-fuel ratio for matching the detection result of the A / F sensor 16 to the target air-fuel ratio is set. The correction coefficient FAF is calculated by the following (1),
It is calculated using equation (2). The setting procedure of the FAF value is disclosed in detail in Japanese Patent Application Laid-Open No. 1-110853.

FAF = K1 · λ + K2 · FAF1 +... + Kn + 1 FAFn + ZI (1) ZI = ZI1 + Ka · (λTG−λ) (2) In the above equations (1) and (2), λ Is the A / F sensor 16
The converted value of the air-fuel ratio of the limiting current by K1 to Kn + 1 is F /
B represents a constant, ZI represents an integral term, and Ka represents an integral constant. The subscripts 1 to n + 1 are variables indicating the number of times of control since the start of sampling.

After setting the FAF value, the CPU 31 uses the following equation (3) in step 106 to calculate the basic injection amount Tp, the air-fuel ratio correction coefficient FAF, and other correction coefficients FALL (variable correction coefficients such as water temperature, air conditioner load, etc.). )) To calculate the final fuel injection amount TAU.

TAU = Tp · FAF · FALL (3) After calculating the fuel injection amount TAU, the CPU 31 calculates the TAU.
A control signal corresponding to the value is output to the injector 18, and this routine is once ended.

Next, a λTG setting routine corresponding to the processing of step 200 will be described with reference to FIG. In FIG. 3, the CPU 31 firstly executes step 201
Then, based on the engine speed Ne and the intake pressure PM at that time, a normal target air-fuel ratio (hereinafter, referred to as a normal target air-fuel ratio λL) for performing lean burn control is set. The λL value is obtained by searching a target air-fuel ratio map shown in FIG. 4, for example. When the conditions for performing the lean burn control are satisfied, such as during steady-state operation, the normal target air-fuel ratio λL
For example, a value corresponding to A / F = 20 to 23 is set (however, when the lean burn condition is not satisfied, the λL value is set near the stoichiometric value).

Thereafter, the CPU 31 determines in step 202 whether there is a high torque request to the engine 1. Specifically, if it is determined that the engine is in a high speed or high load range based on the engine speed Ne and the intake pressure PM at that time, the torque required for the engine 1 is regarded as a relatively high level, It is determined that "high torque is required". If it is determined that the engine 1 is in the low-medium rotation and low-medium load range, the torque required for the engine 1 is considered to be at a relatively high level, and it is determined that "high torque is not required".

If it is determined that there is no request for a high torque,
U31 proceeds to step 203, and clears a period counter for counting the number of fuel injections at the time of rich / lean amplitude to "0". In the following step 204, C
The PU 31 sets the λL value searched for in the map to the target air-fuel ratio
It is set as TG, and then returns to the original routine of FIG.
The λTG value in step 204 becomes the final value of the target air-fuel ratio, and is used for calculating the FAF value in step 105 of FIG.

On the other hand, if it is determined that a high torque request is present, the CPU 31 determines in step 205 and the subsequent steps that the normal target air-fuel ratio λL and the target air-fuel ratio set to a richer side than this (hereinafter, for convenience, Rich target air-fuel ratio λ
R) to make the target air-fuel ratio λTG amplitude. Hereinafter, the details will be described.

That is, in step 205, the CPU 31 sets the rich target air-fuel ratio λR based on the engine speed Ne and the intake pressure PM, for example, by searching a map. The rich target air-fuel ratio λR is set such that the richer the engine speed Ne or the higher the intake pressure PM, the higher the richness. Incidentally, the λR value is not necessarily set in the rich region, but may be any value set on the rich side with respect to the λL value.

On the condition that the period counter at that time is "0" (step 206: YES), the CPU 3
1 proceeds to step 207, and based on the engine speed Ne and the intake pressure PM, the lean time TL and the rich time TR
Set. If step 206 is NO (if the cycle counter is $ 0), the processing of step 207 is skipped.

The lean time TL and the rich time TR are set such that the higher the engine speed Ne or the higher the intake pressure PM, the shorter the time width. At this time, the rich time TR is obtained by a map search, whereas the lean time TL is obtained from the rich time TR and the coefficient α as TL = TR · α.

The coefficient α is obtained, for example, using the characteristic diagram of FIG. 5. If the intake air amount Qa retrieved from the map from the engine speed Ne and the intake pressure PM is small, α> 1 and the intake air amount Qa is large. In this case, α <1. That is, when α> 1, TL> TR, and the fuel efficiency is prioritized over the output torque. When α <1, TL <TR, and the output torque is prioritized over the fuel efficiency. Incidentally, it is desirable to set TL and TR to a considerable time of about 10 injections at the maximum.

Thereafter, the CPU 31 increments the cycle counter by "1" at step 208. Thereafter, the CPU 31 determines in step 209 whether or not the value of the cycle counter has reached a value corresponding to the set rich time TR. If the period counter <TR and the negative determination is made in step 209, the CPU 31 proceeds to step 2
10, the obtained λR value is added to the final target air-fuel ratio λT
G is set, and then the process returns to the original routine of FIG. In other words, during the period of time elapsed from the setting of the lean time TL and the rich time TR (step 207) is “0 to TR”, the FAF value is set at the rich target air-fuel ratio λR (see FIG. 2). Step 105), this FAF
The value controls the air-fuel ratio.

On the other hand, if the period counter is equal to or greater than TR and the result of step 209 is affirmative, the CPU 31 proceeds to step 211 to change the obtained λL value to the final target air-fuel ratio λ.
Set as TG. Thereafter, the CPU 31 determines in step 212 that the value of the cycle counter is equal to the set lean time T.
It is determined whether or not a value corresponding to the total time “TL + TR” of L and the rich time TR has been reached, and the cycle counter <TL +
If it is TR and a negative determination is made in step 212, the process returns to the original routine of FIG. That is, the lean time TL
During the period from the setting of the rich time TR (step 207) to the period of time “TR to TL + TR”, the FAF value is set at the normal target air-fuel ratio λL (step 105 in FIG. 2). The value controls the air-fuel ratio.

On the other hand, when the period counter ≧ TL + TR and the affirmative determination is made in step 212, the CPU 31
In step 213, the cycle counter is cleared to "0", and thereafter, the process returns to the original routine of FIG. In the next process following the clearing of the cycle counter, the determination in step 206 is affirmative, and the lean time TL and the rich time TR are newly set. Then, based on the TL and TR, the air-fuel ratio is amplitude-controlled again between the normal target air-fuel ratio λL and the rich target air-fuel ratio λR.

According to the above series of processes, the air-fuel ratio changes as shown in the time chart of FIG. 6 when the vehicle is accelerating or when the vehicle is running under a high load. That is, when a high torque request is input during the periods T1 and T2 in the figure due to the acceleration command of the vehicle or high load operation, the air-fuel ratio that has been controlled in the lean region up to the predetermined time interval (T1 and T2) in each of the periods T1 and T2. Rich time TR,
In accordance with the lean time TL), the air-fuel ratio is switched between a lean region and a region richer than the lean region, and the air-fuel ratio oscillates. In this case, it is possible to cope with a high torque request in each of the periods T1 and T2 while the lean burn control is continued.

Incidentally, in the air-fuel ratio control system according to the present embodiment, the step 202 in FIG. 3 corresponds to the required torque determining means. Step 204 in FIG. 3 corresponds to the first air-fuel ratio instructing means.
0,211 corresponds to the second air-fuel ratio command means.

According to the present embodiment described in detail above, the following effects can be obtained. (A) In this embodiment, when the required torque is at a relatively low level, the control air-fuel ratio is set to a lean air-fuel ratio, and when the required torque is at a relatively high level, the control air-fuel ratio is set to a lean air-fuel ratio and richer than the lean air-fuel ratio. And the air-fuel ratio on the side at a predetermined cycle. According to the above configuration, when high torque is required, it is possible to improve fuel economy while satisfying the torque requirement. In such a case, by inclining the air-fuel ratio in a predetermined cycle, an unintended torque fluctuation does not occur.

(B) Further, time intervals (lean time TL, rich time T) in which the air-fuel ratio is oscillated according to the degree of the required torque, ie, the engine speed Ne and the intake pressure PM.
R) was set. Specifically, the lean time TL and the rich time TR are set such that the time width becomes shorter as the engine speed Ne becomes higher or the intake pressure PM becomes higher. In this case, it is possible to suppress the torque fluctuation and secure good drivability.

FIG. 7 is experimental data showing the relationship between the total time of the lean time TL and the rich time TR and the torque fluctuation at that time in the amplitude control of the air-fuel ratio. According to the figure, it is understood that the torque fluctuation is suppressed as “lean time + rich time” is smaller, that is, as the air-fuel ratio is made to oscillate in a shorter cycle.

(C) By oscillating the air-fuel ratio between the lean and rich states as described above, the NOx purifying ability of the NOx catalyst 19 can be restored each time, and it can contribute to the emission improvement.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.
However, in the configuration of the second embodiment, the first
The same reference numerals are given to the same components in the drawings and the description is simplified. The following description focuses on differences from the first embodiment.

That is, in the first embodiment, the target air-fuel ratio λTG for the air-fuel ratio control is set using the engine speed Ne and the intake pressure PM as parameters, and when setting the λTG value when a high torque is required, Ne is used. , PM according to PM
The TG value was made to swing between lean and rich. On the other hand, in the present embodiment, the target air-fuel ratio λTG is set using the throttle opening TH and a unit time change amount of the throttle opening TH (hereinafter referred to as a throttle opening difference ΔTH) as a parameter, or when a high torque is required. Is performed.

Here, FIG. 8 shows λ in this embodiment.
4 is a flowchart illustrating a TG setting routine, which replaces the routine of FIG. 3 in the first embodiment. The details will be described below.

In FIG. 8, the CPU 31 first obtains a normal target air-fuel ratio λL by searching a target air-fuel ratio map shown in FIG. 9, for example, based on the throttle opening TH at that time in step 301. In step 302, it is determined whether or not the engine 1 is operating under a high load based on the throttle opening TH. In step 303, it is determined whether or not the engine 1 is in the middle of acceleration based on the throttle opening difference ΔTH. If a negative determination is made in both steps 302 and 303, the CPU 31 proceeds to step 304 and clears the cycle counter to "0". In the following step 305,
The CPU 31 sets the λL value searched for in the map as the target air-fuel ratio λTG, and then returns to the original routine of FIG.

On the other hand, if the determination in step 302 is affirmative (in the case of high-load operation),
If the result of the determination in step 03 is affirmative (during acceleration), the CPU
In the process 31 and subsequent steps, the target air-fuel ratio λTG is made to oscillate by the normal target air-fuel ratio λL and the rich-side target air-fuel ratio λR.

That is, if the determination in step 302 is affirmative, the CPU 31 determines the rich target air-fuel ratio λR in step 306 based on the throttle opening TH, for example, by searching a map. The rich target air-fuel ratio λR is set such that the rich degree increases as the throttle opening TH increases. Also, on condition that the period counter at that time is “0” (step 307: YES), the CPU
31 proceeds to step 308 to set the lean time TL and the rich time TR based on the throttle opening TH. If step 307 is NO (if the cycle counter is $ 0), the processing of step 308 is skipped. The lean time TL and the rich time TR are set such that the greater the throttle opening TH, the shorter the time width.

If the determination in step 303 is affirmative, the CPU 31 determines a rich target air-fuel ratio λR in step 309, for example, by searching a map based on the throttle opening difference ΔTH. The rich target air-fuel ratio λR is
It is set so that the richer the degree of richness becomes, the larger the throttle opening difference ΔTH is. Further, on condition that the period counter at that time is “0” (Step 310
ES), the CPU 31 proceeds to step 311, and sets the lean time TL and the rich time TR based on the throttle opening difference ΔTH. If step 310 is NO (if the cycle counter is $ 0), the processing of step 311 is skipped. The lean time TL and the rich time TR are set such that the greater the throttle opening difference ΔTH, the shorter the time width.

Thereafter, the CPU 31 increments the cycle counter by "1" at step 312. Thereafter, the CPU 31 determines in step 313 whether or not the value of the cycle counter has reached a value corresponding to the set rich time TR. If cycle counter <TR and step 313 is negative, the CPU 31 proceeds to step 3
14, the obtained λR value is converted to the final target air-fuel ratio λT
G is set, and then the process returns to the original routine of FIG. That is, the setting of the lean time TL and the rich time TR (the time elapsed from step 308 or 311 is "0 to T
In the period “R”, the air-fuel ratio is controlled at the rich target air-fuel ratio λR.

On the other hand, if the cycle counter ≧ TR and the affirmative determination is made in step 313, the CPU 31 proceeds to step 315, and sets the obtained λL value to the final target air-fuel ratio λ.
Set as TG. Thereafter, the CPU 31 determines in step 316 that the value of the cycle counter is equal to the set lean time T.
It is determined whether or not a value corresponding to the total time “TL + TR” of L and the rich time TR has been reached, and the cycle counter <TL +
If it is TR and a negative determination is made in step 316, the process returns to the original routine of FIG. That is, the setting of the lean time TL and the rich time TR (Step 308 or 3)
During the period of time elapsed from 11) “TR to TL + TR”, the air-fuel ratio is controlled at the normal target air-fuel ratio λL.

On the other hand, if the period counter ≧ TL + TR and step 316 is affirmatively determined, the CPU 31
In step 317, the cycle counter is cleared to "0", and thereafter, the process returns to the original routine of FIG. In the next process following the clearing of the cycle counter, step 307 or step 310 is affirmatively determined, and the lean time TL and the rich time TR are newly set. Then, based on the TL and TR, the air-fuel ratio is amplitude-controlled again between the normal target air-fuel ratio λL and the rich target air-fuel ratio λR.

In the second embodiment, steps 302 and 303 in FIG. 8 correspond to the required torque determining means. Step 305 in FIG. 8 corresponds to first air-fuel ratio command means, and steps 314 and 315
Corresponds to the second air-fuel ratio command means.

According to the second embodiment, as in the first embodiment, when high torque is required, it is possible to improve fuel economy while satisfying the torque requirement.

The embodiment of the present invention can be realized in the following modes other than the above. When the target air-fuel ratio λTG is made to swing between lean and rich when a high torque is required, the rich time TR and the lean time TL are variably set according to the duration of the high torque request. For example, as shown in the time chart of FIG. 10, at the beginning of generation of a high torque request, relatively long rich time TR and lean time TL are set, and the rich time TR and lean time TL are gradually shortened as time elapses. . In order to realize this, the coefficient β is obtained using the characteristic diagram of FIG. 11, and the coefficient β may be multiplied by the rich time TR and the lean time TL. Conversely, at the beginning of the generation of a high torque request, the rich time TR and the lean time TL are set relatively short, and the rich time TR
It is also possible to gradually increase the lean time TL.

The setting method of the rich time TR and the lean time TL is changed. That is, in the above-described embodiment, the rich time TR is searched for on the map, and the lean time TL is calculated as “TL = TR · α”. Conversely, the lean time TL may be searched on the map. Good. Further, both TR and TL may be set by a map search, or TR and TL may be fixed values independent of the engine operating state.

Further, the rich time TR and the lean time T
When setting L, TR and TL may be guarded by a predetermined value so that a torque fluctuation exceeding a predetermined allowable level does not occur due to the amplitude of the air-fuel ratio.

In the air-fuel ratio control system in each of the above embodiments, the air-fuel ratio F / B control is performed so as to eliminate the deviation between the actual air-fuel ratio (sensor measured value) and the target air-fuel ratio λTG. May be changed. For example, the present invention can be embodied in a system for performing air-fuel ratio open control, and in such a case, both improvement in fuel efficiency and securing of torque can be achieved.

As a parameter for determining the torque request, the engine speed information, the intake pressure information, and the throttle opening information are appropriately combined and used to construct an air-fuel ratio control system for obtaining the above-mentioned effects. May be.

In the air-fuel ratio control system according to the above embodiment, a lean NOx catalyst is provided in the engine exhaust pipe. However, this configuration may be modified to embody a system using a three-way catalyst, for example.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an outline of an air-fuel ratio control system for an engine according to an embodiment of the invention.

FIG. 2 is a flowchart showing a fuel injection control routine.

FIG. 3 is a flowchart illustrating a λTG setting routine.

FIG. 4 is a map for setting a lean target air-fuel ratio according to an engine speed and an intake pressure.

FIG. 5 is a characteristic diagram for obtaining a coefficient α according to an intake air amount.

FIG. 6 is a time chart for explaining an operation in the embodiment.

FIG. 7 is a graph for confirming an effect in the embodiment.

FIG. 8 is a flowchart illustrating a λTG setting routine according to the second embodiment.

FIG. 9 is a map for setting a lean target air-fuel ratio according to a throttle opening.

FIG. 10 is a time chart for explaining an operation in another embodiment.

FIG. 11 is a characteristic diagram for obtaining a coefficient β according to an elapsed time after a high torque request is input.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 16 ... A / F sensor (limit current type air-fuel ratio sensor), 19 ... NOx catalyst, 30 ... EC
U (electronic control device), 31... Required torque determination means, first
Of the air-fuel ratio command means and the second air-fuel ratio command means C
PU.

Claims (3)

[Claims] An air-fuel ratio control device for an internal combustion engine that sets a target air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine leaner than a stoichiometric air-fuel ratio and performs lean combustion based on the target air-fuel ratio. Request torque determination means for determining the degree of torque required for the internal combustion engine; first air-fuel ratio command means for setting the control air-fuel ratio to a lean air-fuel ratio when the required torque is relatively low; An air-fuel ratio control device for an internal combustion engine, comprising: a second air-fuel ratio command means for causing a control air-fuel ratio to oscillate in a predetermined cycle between a lean air-fuel ratio and an air-fuel ratio richer than the lean air-fuel ratio. . 2. The air-fuel ratio of an internal combustion engine according to claim 1, wherein said second air-fuel ratio command means includes means for setting a time interval for oscillating an air-fuel ratio in accordance with the degree of the required torque to be determined. Fuel ratio control device. 3. An air-fuel ratio control device for an internal combustion engine according to claim 1, wherein said second air-fuel ratio command means gradually changes the amplitude cycle of the air-fuel ratio in accordance with the duration of a high torque request. .