JPH0658080B2 - Air-fuel ratio control method for internal combustion engine - Google Patents

Description

The present invention relates to an air-fuel ratio control method for an internal combustion engine, and more particularly to a theoretical air-fuel ratio control for controlling an air-fuel ratio to a stoichiometric air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio. The present invention relates to an air-fuel ratio control method for an internal combustion engine, which is performed by switching between lean control for side control and open-loop control and feed back control during theoretical air-fuel ratio control and lean control.

[Prior Art] Conventionally, when a voltage is applied, a signal proportional to the residual oxygen concentration in the exhaust gas is output in a region leaner than the stoichiometric air-fuel ratio, and the voltage application is stopped There is known an air-fuel ratio control method in which an oxygen concentration sensor that outputs an inverted signal is attached to the exhaust system and the fuel injection amount TAU is determined according to the following equation to control the air-fuel ratio.

TAU = TP ・ KLEAN ・ FPOWER ・ FAF ・ K
(1) However, TP is the basic fuel injection amount, KLE, which is determined by the engine load (intake air amount or intake pipe pressure) and engine speed.
AN is a lean correction coefficient of less than 1 for controlling the air-fuel ratio to be leaner than the stoichiometric air-fuel ratio, and FPOWER is output by increasing the fuel injection amount when the throttle opening is high (for example, 50 ° or more). Power increase factor, FA to increase
F is a feed back correction coefficient for feed back control of the air-fuel ratio, and K is a correction coefficient during transition.

In such an air-fuel ratio control method, the air-fuel ratio is open-loop controlled to the stoichiometric air-fuel ratio when the engine is cold, etc., and is calculated based on a signal inverted from the theoretical air-fuel ratio from the oxygen concentration sensor after warm-up. The air / fuel ratio is controlled to the stoichiometric air / fuel ratio using the feedback back correction factor FAF, and in the light load region, etc., the air / fuel ratio is open loop controlled leaner than the stoichiometric air / fuel ratio and proportional to the oxygen concentration from the oxygen concentration sensor. Feedback control of the air-fuel ratio to the lean side of the stoichiometric air-fuel ratio is performed by using a lean correction coefficient KLEAN calculated based on the signal. That is, as the control mode, the open loop control and the feed back control with the stoichiometric air-fuel ratio are performed according to the operating state, and the open loop control and the feed back control with a lean region from the stoichiometric air fuel ratio are performed. Also,
When switching between the air-fuel ratio control to the stoichiometric air-fuel ratio and the air-fuel ratio feedback control from the stoichiometric air-fuel ratio to the lean region, and when switching from the stoichiometric air-fuel ratio to the lean region and the air-fuel ratio feedback control to the stoichiometric air-fuel ratio At times, the fuel injection amount is changed by the lean correction coefficient, and at the same time, the oxygen concentration sensor output is switched.

[Problems to be Solved by the Invention] However, it takes a predetermined time from the injection of fuel until the exhaust gas from the injected fuel reaches the detection portion of the oxygen concentration sensor. If the output of the oxygen concentration sensor is switched at the same time, the exhaust air-fuel ratio before the specified time is detected by the oxygen concentration sensor when switching to feedback control, and the air-fuel ratio feedback correction coefficient is erroneously corrected. There was a problem that the emission worsened. That is, as shown in FIG. 2 (A), considering the case where fuel is injected into each cylinder of a four-cylinder engine and the control mode is switched from feedback control to stoichiometric air-fuel ratio to feedback control to lean, Then, when the control mode is switched, the lean correction coefficient KLEAN is reduced to reduce the fuel injection amount TAU, and at the same time, the oxygen concentration sensor is switched to be used as an O ₂ sensor that outputs a signal inverted at the stoichiometric air-fuel ratio. Therefore, it has been switched to use as a lean sensor that outputs a signal proportional to the oxygen concentration in the exhaust gas in a lean range from the theoretical air-fuel ratio. At this time, the exhaust gas due to the fuel reduced according to the lean correction coefficient KLEAN for the specific cylinder has to undergo the four steps of intake, compression, explosion and exhaust in order to reach the detection site of the sensor. There is a delay of about 720 ° C. Therefore, the sensor detects the exhaust air-fuel ratio of about 720 ° C before, that is, the exhaust air-fuel ratio when the feedback control is performed to the stoichiometric air-fuel ratio, and the sensor output and the target voltage determined according to the lean correction coefficient KLEAN are As a result of comparison, the air-fuel ratio feedback correction coefficient FAF is made smaller, whereby fuel in the next cylinder is injected. Therefore, the air-fuel ratio feedback correction coefficient FAF gradually decreases, and the lean correction coefficient KLEA
The fuel injection amount reduced by N is further reduced by the air-fuel ratio feedback back correction coefficient FAF, and the air-fuel ratio deviates to the lean side from the engine required air-fuel ratio as shown by the broken line, and the driver's viability deteriorates and the HC emission amount increases. To do.
On the contrary, as shown in FIG. 2 (B), when the control mode is switched from the lean feedback control to the stoichiometric air feedback control, the air-fuel ratio feedback correction coefficient is changed so as to gradually increase. As indicated by the broken line, the air-fuel ratio deviates to the latch side from the engine-requested air-fuel ratio, the driver viability deteriorates, and the HC and CO emissions increase.

The present invention is made to solve the above problems, and when the air-fuel ratio control to the stoichiometric air-fuel ratio is switched to the feed back control to lean and when the feed back to the stoichiometric air-fuel ratio from the air-fuel ratio control to lean. An object of the present invention is to provide an air-fuel ratio control method for an internal combustion engine in which the driver fuelability and exhaust emission are improved by preventing the air-fuel ratio from deviating from the required air-fuel ratio when switching to control.

[Means for Solving Problems] In order to achieve the above object, the present invention provides a stoichiometric air-fuel ratio control for controlling an air-fuel ratio to a stoichiometric air-fuel ratio and a lean control for controlling an air-fuel ratio leaner than the stoichiometric air-fuel ratio. Is switched by changing the fuel injection amount according to the operating state, and the signal inverted at the stoichiometric air-fuel ratio and the signal proportional to the residual oxygen concentration in the exhaust gas in the lean region from the theoretical air-fuel ratio are changed according to the operating state. Operates the feedback control that corrects the fuel injection amount using the correction coefficient and the feedback loop control that does not use the correction coefficient. In an air-fuel ratio control method for an internal combustion engine, which is switched depending on the state, the control modes are from open-loop control of stoichiometric air-fuel ratio control to feedback control of lean control and lean control. When control is switched from open-loop control to stoichiometric air-fuel ratio control feed-back control, the feed-back control is started after a lapse of a predetermined period after changing the fuel injection amount, and the control mode changes from stoichiometric air-fuel ratio control feed-back control to lean control. It is characterized in that when the feedback control of the feedback control and the lean control is switched to the control of the feedback control of the stoichiometric air-fuel ratio control, the signal is switched after a lapse of a predetermined period after changing the fuel injection amount.

[Operation] Next, the operation of the present invention will be described. In the present invention, the open-loop control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio without using the feed-back correction coefficient (theoretical air-fuel ratio control), and the feed-back control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio using the feed-back correction coefficient, Open loop control that controls the air-fuel ratio leaner than the theoretical air-fuel ratio (lean control) without using the feed-back correction coefficient and feed-back control that controls the air-fuel ratio leaner than the stoichiometric air-fuel ratio using the feedback back-correction coefficient. The air-fuel ratio is controlled in four control modes. When the control mode is switched from open-loop control that controls the air-fuel ratio to the stoichiometric air-fuel ratio to feedback control that controls the air-fuel ratio to the lean side of the stoichiometric air-fuel ratio and when the air-fuel ratio is controlled to the lean side of the stoichiometric air-fuel ratio When the control is switched to the feed back control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio, the feed back control is started after a predetermined period has elapsed since the fuel injection amount was changed. As a result, when the open loop control is switched to the feed back control and the control is switched between the lean control and the stoichiometric air-fuel ratio control, the exhaust gas due to the changed fuel reaches the exhaust system by changing the fuel injection amount. After the time point, feedback control can be started. For this reason,
It is possible to prevent the air-fuel ratio from being erroneously corrected by detecting the exhaust gas from the fuel for open loop control by starting the feed back control from the time when the exhaust gas from the fuel for feedback control reaches the exhaust system. Also, when the control mode is switched between the feed back control that controls the air-fuel ratio to the stoichiometric air-fuel ratio and the feed back control that controls the air-fuel ratio to a lean side of the stoichiometric air-fuel ratio, the fuel injection amount changes for a predetermined period of time. After a lapse of time, signal switching is performed. As a result, when the control is switched between the stoichiometric air-fuel ratio control and the lean control during the feedback control, the oxygen concentration sensor is detected after the exhaust gas due to the changed fuel injection amount and the changed fuel reaches the exhaust system. Signal is switched. Therefore, after the time when the exhaust gas from the lean control fuel reaches the exhaust system, the signal can be switched for the lean control feed back control to perform the feed back control, and the exhaust gas from the fuel for the stoichiometric air-fuel control can be exhausted. After reaching the system, the signal can be switched for the feedback control of the stoichiometric air-fuel ratio control, and the air-fuel ratio is prevented from being erroneously corrected when switching between the lean control and the stoichiometric air-fuel ratio control during the feedback control. It

The above-mentioned predetermined period is preferably equal to the period until the exhaust gas due to the injected fuel reaches the exhaust system.

[Effects of the Invention] As described above, according to the present invention, when the fuel control is switched between the lean control and the stoichiometric air-fuel ratio control, the feedback control is started after a predetermined period after changing the fuel injection amount. Or, since the signal is switched to the feedback control signal, the correction coefficient for the feedback control is erroneously corrected when the control mode is switched, so that the air-fuel ratio is prevented from shifting, which deteriorates the exhaust emission and driver viability. The effect of being able to prevent is obtained.

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 3 shows an internal combustion engine (engine) to which the present invention can be applied. A throttle valve 8 is disposed downstream of an air cleaner (not shown), and the throttle valve 8 is in a fully closed state. A slot switch 10 having an idle contact that is turned on at an idle position) and a power contact that is turned on at a slot opening of 50 ° or more is attached, and a surge tank 12 is provided downstream of the slot valve 8. A diaphragm type pressure sensor 6 is attached to the surge tank 12. Also, the throttle valve 8
Bypass path 14 so that the surge tank 12 on the upstream side of the throttle valve and the surge tank 12 on the downstream side of the throttle valve are communicated with each other.
Is provided. The bypass passage 14 has an idle speed control (ISC) valve 16 whose opening is adjusted by a pulse motor 16A having a 4-pole stator.
B is attached. The surge tank 12 communicates with the combustion chamber of the engine 20 via the intake manifold 18 and the intake port 22. A fuel injection valve 24 is attached to each cylinder or each group of cylinders so as to project into the intake manifold 18.

The combustion chamber of the engine 20 is connected to a catalyst device (not shown) filled with a three-way catalyst via an exhaust port 26 and an exhaust manifold 28. An oxygen concentration sensor 30 having a resistor R having one end connected to the vehicle battery and the other end connected to the collector of the transistor Tr is attached to the exhaust manifold 28.
The oxygen concentration sensor 30 outputs an air-fuel ratio signal proportional to the residual oxygen concentration in the exhaust gas in the air-fuel ratio region leaner than the stoichiometric air-fuel ratio by turning on the transistor Tr and applying a voltage to the resistor R. Acts as a lean sensor. Further, by turning off the transistor Tr and stopping the application of the voltage to the resistor R, the transistor Tr functions as an O ₂ sensor that outputs a signal inverted at the stoichiometric air-fuel ratio. A cooling water temperature sensor 34 is attached to the engine block 32 so as to penetrate the engine block 32 and project into the water jacket. This cooling water temperature sensor 34
Detects the engine cooling water temperature and outputs a water temperature signal.

A spark plug 38 is attached to each cylinder so as to penetrate the cylinder head 36 of the engine 20 and project into the combustion chamber. The ignition plug 38 is connected via a distributor 40 and an igniter 42 to an electronic control circuit 44 composed of a microcomputer or the like.
Inside the distributor 40, a cylinder discriminating sensor 46 and a rotation angle sensor 48, each of which is composed of a signal rotor fixed to the distributor and a pick-up fixed to the distributor housing, are mounted. In the case of a four-cylinder engine, the cylinder discrimination sensor 46 outputs a cylinder discrimination signal every 720 ° CA,
The rotation angle sensor 48 outputs an engine speed signal, for example, every 30 ° CA.

As shown in FIG. 4, the electronic control circuit 44 includes a central processing unit (MPU) 60, a read only memory (ROM) 6
2. Random access memory (RAM) 64, backup RAM (BU-RAM) 66, input / output port 6
8, input port 70, output ports 71, 72, 74, 7
6 and a bus 78 such as a data bus or a control bus that connects them. I / O port 6
8 is an analog-to-digital (A / D) converter 78,
The pressure sensor 6 and the cooling water temperature sensor 34 are connected via the multiplexer 80 and the buffers 82 and 84.
The MPU 60 controls the multiplexer 80 and the A / D converter 78 to output the pressure sensor 6 and the water temperature sensor 3.
The four outputs are sequentially converted into digital signals and stored in the RAM 64. The oxygen concentration sensor 30 is connected to the input port 70 via the A / D converter 88A and the comparator 86A, and the oxygen concentration sensor 30 is connected via the A / D converter 88B and the current-voltage converter 86B. The cylinder discrimination sensor 46 and the rotation angle sensor 48 are connected via the waveform shaping circuit 90, and the slot switch 10 is connected. The output port 71 is connected to the base of the transistor Tr, and the output port 72 is the drive circuit 9.
2 is connected to the igniter 42 via the output port 74
Is connected to the fuel injection valve 24 via a drive circuit 94, and the output port 76 is connected to a pulse motor 16A of the ISC valve via a drive circuit 96. Note that 98
Is a clock and 100 is a timer. The ROM 62 stores in advance programs and the like for control routines described below.

Next, a control routine of the first embodiment in which the present invention is applied to the above engine will be described.

FIG. 5 shows a part of the main routine of this embodiment, which is calculated on the basis of the current intake pipe pressure PM and the rotation angle sensor output which are A / D converted in step 100 and stored in the RAM. The current engine speed NE stored in RAM is taken in, and in step 102, the intake pipe pressure PM and engine speed NE are changed as in the conventional case.
The basic fuel injection amount TP is calculated based on It should be noted that in step 104, a correction coefficient K at the time of transition for increasing the fuel injection amount during acceleration and decreasing the fuel injection amount during deceleration.
Is calculated.

FIG. 6 shows an interrupt routine for each 180 ° C. executed by an interrupt signal for each 180 ° A formed by the output of the cylinder discrimination sensor and the rotation angle sensor, and is stored in a predetermined address of the RAM in step 106. The present control mode MO, that is, the control mode determined in the previous interrupt processing (before 180 ° C) is stored in the address storing the control mode M180 before 180 ° C, and the control mode M180 is stored before 360 ° C. The control mode M360 is stored in the memory address, and the control mode M360 is stored at 540 ° C.
The previous control mode M540 is stored in the address where the control mode M540 is stored, and the control mode M540 is stored in the address where the control mode M720 before 720 ° C. is stored. The previous control mode M7
20 is erased. Next, at step 108, the intake pipe pressure PM, the engine speed NE, and the engine cooling water temperature TH.
The control mode MO is determined based on the current operating condition defined by W etc., and stored in the above-mentioned predetermined address of the RAM. As a result, the control mode 720 ° C before the current time is 180
It will be stored in the RAM every ° C. In the present embodiment as the control mode described above, the open-loop control for controlling the air-fuel ratio to be higher than the stoichiometric air-fuel ratio, the open-loop control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio without using the air-fuel ratio feed back correction coefficient FAF, and the air-fuel ratio feed back Feed back control for controlling the air-fuel ratio to a logical air-fuel ratio using the correction coefficient FAF, open-loop control for controlling the air-fuel ratio to a target air-fuel ratio leaner than the stoichiometric air-fuel ratio without using the air-fuel ratio feed back correction coefficient FAF Five control modes of the feed back control for controlling the air-fuel ratio to the target air-fuel ratio on the lean side of the stoichiometric air-fuel ratio are defined using the feed-back correction coefficient FAF.

In the next step 110, it is judged which of the above control modes the current control mode MO belongs to, and when the air-fuel ratio is controlled from the stoichiometric air-fuel ratio to the latch, that is, the power contact of the slot switch 10. When is turned on, in step 112, the lean correction coefficient KLEAN is set to 1.0.
And the power increase coefficient FPOWER is set to a predetermined value exceeding 1 (for example, 1.15), and in the control mode in which the air-fuel ratio is controlled to the stoichiometric air-fuel ratio, the lean correction coefficient KLEAN is set to 1.0 in step 114. In the control mode in which the power increase coefficient FPOWER is set to 1.0 and the air-fuel ratio is controlled leaner than the stoichiometric air-fuel ratio, step 116 is performed as shown in the map of FIG. 7 (1), (2), (3). A lean correction coefficient KLEAN, which is three-dimensionally determined according to the intake pipe pressure PM, the engine speed NE, and the engine cooling water temperature THW, is calculated based on the map, and the power increase coefficient FPOWER is set to 1.0.
Go to. At step 118, the fuel injection amount TAU is calculated based on the above equation (1). Then, the air-fuel ratio is controlled by opening and closing the fuel injection valve and injecting an amount of fuel corresponding to the fuel injection amount TAU in a fuel injection routine (not shown).

FIG. 1 shows a calculation routine of the air-fuel ratio feedback back correction coefficient FAF executed by interruption every 12 msec. In step 120, the control mode M720, that is, the control mode 720 ° C. before the present time is judged, and the open loop control is performed. If so, in step 122, the air-fuel ratio feedback back correction coefficient FAF is set to 1.0, and the process returns to the main routine. As a result, the lean correction coefficient KLEAN is set to 1.0 in step 112 of FIG. 6 in the case of open loop control for controlling the air-fuel ratio to be higher than the stoichiometric air-fuel ratio, so the power increase coefficient FPOWER set in step 112 is set to the power increase coefficient FPOWER. On the basis of the above equation (1), the fuel injection amount TAU is increased from the basic fuel injection amount TP, and the air-fuel ratio is controlled to the rich state. Further, in the case of open loop control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio, the lean correction coefficient KLEAN and the power increase coefficient FPOWER are set to 1.0 in step 114 of FIG. An amount of fuel corresponding to the basic fuel injection amount TP is injected according to the above equation (1). If the open-loop control is performed to control the air-fuel ratio to the lean side of the stoichiometric air-fuel ratio, the power increase coefficient FP is set in step 116 of FIG.
Since the OWER is set to 1.0, the above is based on the lean correction coefficient KLEAN calculated in step 116.
According to the equation (1), the fuel injection amount TAU is the basic fuel injection amount TP
The air-fuel ratio is controlled leaner by further reducing the amount.

When it is determined in step 120 that the control mode M720 is the feed back control to the stoichiometric air-fuel ratio,
In step 124, the voltage application to the resistor R housed in the oxygen concentration sensor 30 is stopped to act as an O ₂ sensor that outputs a signal inverted at the stoichiometric air-fuel ratio, and in step 126 the A / D converter. 88A is operated to perform A / D conversion of the sensor output, and in step 128 a target voltage corresponding to the stoichiometric air-fuel ratio is set. At this time, the operation of the A / D converter 88B is stopped. In the next step 130, the A / D converted value of the sensor output is compared with the target voltage,
If the A / D converted value is larger than the target voltage, that is, if the current air-fuel ratio is higher than the theoretical air-fuel ratio, step 1
At 32, set the air-fuel ratio feedback correction coefficient FAF to a predetermined value α.
If the A / D conversion value is equal to or less than the target voltage (for example, 0.002), the air-fuel ratio feedback correction coefficient FAF is increased by a predetermined value α in step 134. As a result, the air-fuel ratio feedback correction coefficient FAF changes with respect to the sensor output voltage as shown in FIG. 9, and the lean correction coefficient KLEAN and the power increase coefficient FPOWER are set to 1.0 in step 114 of FIG. Therefore, (1) above
According to the formula, the fuel injection amount TAU is determined by the basic fuel injection amount TP and the air-fuel ratio feedback back correction coefficient FAF,
The air-fuel ratio is controlled to a value near the stoichiometric air-fuel ratio.

When the control mode M720 is determined to be the feedback control to the lean side of the stoichiometric air-fuel ratio in step 120, the transistor Tr is turned on via the output port 71 in step 136, and the oxygen concentration sensor 30 is stored. A voltage is applied to the resistor R being operated to act as a lean sensor, and in step 138 the A / D converter 88B is operated to perform A / D conversion of the sensor output,
At step 140, the target voltage corresponding to the lean correction coefficient KLEAN is calculated from the map shown in FIG. At this time, the operation of the A / D converter 88A is stopped. In the next step 142, the A / D converted value of the sensor output is compared with the target voltage calculated in step 140, and if the A / D converted value is larger than the target voltage, that is, the current air-fuel ratio is higher than the theoretical air-fuel ratio. If it is leaner than the target air-fuel ratio on the lean side, at step 144, the air-fuel ratio feedback correction coefficient F
If AF is increased by a predetermined value α (for example, 0.002) and the A / D conversion value is equal to or lower than the target voltage, the air-fuel ratio feedback correction coefficient FAF is decreased by a predetermined value α in step 146.
As a result, the air-fuel ratio feedback correction coefficient FAF changes with respect to the sensor output voltage as shown in FIG. 10, and the lean correction coefficient KLEAN is changed in step 116 of FIG.
Is set to a value less than 1.0 and the power increase coefficient FPOWER
Is set to 1.0, the fuel injection amount TAU is determined by the basic fuel injection amount TP, the lean correction coefficient KLEAN, and the air-fuel ratio feedback back correction coefficient FAF according to the above formula (1), and the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. It is controlled to a value near the target air-fuel ratio.

When the open loop control is switched to the feed back control, the control is switched from the time when the control mode is switched to 72.
Since the calculation of the air-fuel ratio feedback back correction coefficient FAF is started after 0 ° C. A and the fuel injection amount is corrected, 720
The feed back control is started after the temperature of .degree.

FIG. 11 shows the above embodiment in the case where the control mode is switched from the air-fuel ratio feedback control to the stoichiometric air-fuel ratio to the air-fuel ratio feedback control to the lean side from the stoichiometric air-fuel ratio in the state where the engine speed NE is 1200 rpm. Timing is shown. When the current control mode MO is switched from the stoichiometric air-fuel ratio feedback control to the lean air-fuel ratio feedback control, the lean correction coefficient K
The fuel injection amount TAU is reduced by LEAN. At this time, the oxygen concentration sensor acts as an O ₂ sensor that outputs a signal inverted at the stoichiometric air-fuel ratio, and the air-fuel ratio feedback correction coefficient FAF is calculated based on this signal to correct the fuel injection amount. When the period from when the fuel is injected until the exhaust gas due to the injected fuel reaches the detection site of the sensor, that is, when about 720 ° C. elapses, the control mode M720 is switched to the control mode MO, which causes the oxygen concentration sensor to theoretically operate. It is switched to act as a lean sensor that outputs a signal proportional to the residual oxygen concentration in the exhaust gas on the lean side of the air-fuel ratio, and the air-fuel ratio feedback correction coefficient is calculated based on this signal to correct the fuel injection amount. . The change in the exhaust gas air-fuel ratio and the feed back correction coefficient FAF at this time is shown by the solid line in FIG. 2 (A). Further, changes in the exhaust gas air-fuel ratio, the feedback correction coefficient FAF and the like when the feedback control of the lean control is switched to the feedback control of the stoichiometric air-fuel ratio control are shown by solid lines in FIG. 2 (B).

As described above, in this embodiment, the air-fuel ratio feedback correction coefficient is always calculated based on the control mode before the period (about 720 ° C. A) until the exhaust gas due to the injected fuel reaches the detection portion of the sensor. Since the fuel injection amount is corrected by the fuel injection amount, the feedback control (start of feedback control or switching of signals) is performed by a new control mode from the time when the exhaust gas due to the fuel changed by switching the control mode reaches the sensor detection area. It is possible to prevent erroneous correction of the air-fuel ratio when the fuel injection amount changes.

Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, in addition to the control of the first embodiment, when the lean correction coefficient is switched and the fuel injection amount is changed according to the operating state during the air-fuel ratio feedback control to the lean side of the theoretical air-fuel ratio, By delaying the temperature by 720 ° C. and changing the target voltage, deviation of the air-fuel ratio due to the change of the fuel injection amount during the feedback control of the lean control is prevented. Since the main routine and the interrupt routine every 12 msec in this embodiment are substantially the same as those in FIGS. 1 and 5, the description thereof is omitted, and the interrupt routine at every 180 ° C. shown in FIG. 12 is as shown in FIG. The same parts are designated by the same reference numerals and the description thereof will be omitted.

When the control mode is determined to be lean control in step 110, in step 150, the current lean correction coefficient KLEANO stored in the predetermined address of the RAM is stored in the lean correction coefficient KLEAN18 of 180 ° C. before. The value of the lean correction coefficient KLEAN18 is stored in the address where the lean correction coefficient KLEAN36 before 360 ° C. A is stored, and the value of the lean correction coefficient KLEAN36 is stored in the address where the lean correction coefficient KLEAN54 before 540 ° C. is stored. Then, the value of the lean correction coefficient KLEAN54 is stored in the address where the lean correction coefficient KLEAN72 before 720 ° C. A is stored. The previous lean correction coefficient KL
The EAN 72 is erased. Next, at step 116, the seventh
As shown in FIGS. (1) to (3), the operating state, that is, the map of the lean correction coefficient KLEAM determined according to the intake pipe pressure PM, the engine speed NE and the engine cooling water temperature THW is changed to the current operating state. Corresponding lean correction factor KLEAN
O is calculated and stored in RAM.

Further, the current lean correction coefficient KLEANO is set to 1.0 in steps 112 and 114, and the fuel injection amount TAU is calculated in step 118 using the current lean correction coefficient KLEANO. In this embodiment, 1
Step 140 of interrupt routine (Fig. 1) every 2 msec
Then, the target voltage is calculated from the map of FIG. 8 based on the lean correction coefficient KLEAN72 before 720 ° C.

As a result, the lean correction coefficient is changed when the operating state is changed, and the target voltage is changed when 720 ° C. A has passed thereafter. Therefore, when the operating state changes during the feedback control to the lean, the fuel is injected in an amount corresponding to the changed lean correction coefficient and the feedback correction coefficient determined based on the target voltage of 720 ° C. before. , When the exhaust gas from the injected fuel reaches the detection portion of the lean sensor, the amount of fuel injected according to the changed lean correction coefficient and the feed back correction coefficient determined based on the changed target voltage is injected. To be done. Therefore, the air-fuel ratio does not deviate from the required air-fuel ratio due to the erroneous correction of the feed back correction coefficient.

In addition, although the engine which determines the basic fuel injection amount by the intake pipe pressure and the engine speed has been described above, the present invention is not limited to this, and the basic fuel injection amount is determined by the intake air amount and the engine speed. It can also be applied to a prescribed engine.

Further, in the above description, an example in which the sensor output is switched or the feed back control is started when a predetermined rank angle (720 ° C.A in the above example) has elapsed when the control mode is changed has been described. The target value may be changed after a delay corresponding to the time required to reach the detection portion of the lean sensor. Further, this delay period may be changed according to the operating state (for example, the time is shortened as the engine speed increases). Further, in the above example is described using one of the oxygen concentration sensor may be used two of O ₂ sensors and lean sensor.

[Brief description of drawings]

FIG. 1 is a flow chart showing an interrupt routine interrupted every 12 msec of the first embodiment of the present invention, FIG. 2 is a diagram showing a comparison between the conventional characteristic and the characteristic of the above embodiment, and FIG. FIG. 4 is a schematic diagram of an internal combustion engine to which the present invention is applicable, FIG. 4 is a block diagram showing details of the control circuit of FIG. 3, FIG. 5 is a flow chart showing a main routine of the above embodiment, and FIG. Of 18
FIG. 7 is a flowchart of an interrupt routine executed every 0 ° C.
(1) to (3) are diagrams showing the map of the lean correction coefficient, FIG. 8 is a diagram showing the map of the target voltage with respect to the lean correction coefficient, and FIG. 9 is the air-fuel ratio feedback correction coefficient at the stoichiometric air-fuel ratio control. FIG. 10 shows a change in the air-fuel ratio feedback correction coefficient during lean control, and FIG.
FIG. 1 is a diagram showing an example of control timing of the above embodiment,
FIG. 12 is a flow chart showing an interrupt routine executed every 180 ° C. of the second embodiment of the present invention. 6 ... Pressure sensor, 10 ... Slot switch, 24 ... Fuel injection valve, 30 ... Oxygen concentration sensor.

Claims (2)

[Claims] 1. A stoichiometric air-fuel ratio control for controlling the air-fuel ratio to the stoichiometric air-fuel ratio and a lean control for controlling the air-fuel ratio to a lean side of the stoichiometric air-fuel ratio are switched by changing the fuel injection amount according to the operating state, and A signal that is inverted with the stoichiometric air-fuel ratio as a boundary and a signal that is proportional to the residual oxygen concentration in the exhaust gas in a lean region from the stoichiometric air-fuel ratio are switched and output according to the operating state, and the air-fuel ratio is fed back control based on the output signal. In the air-fuel ratio control method of the internal combustion engine, which determines the correction coefficient for controlling the fuel injection amount by using the correction coefficient and the open-loop control that does not use the correction coefficient by switching according to the operating state. The mode is stoichiometric air-fuel ratio control open-loop control to lean control feedback control and lean control open-loop control to stoichiometric air-fuel ratio control feedback control. When the fuel injection amount is changed to the feedback control, the feedback control is started after a lapse of a predetermined period, and the control mode is changed from the theoretical air-fuel ratio control feedback control to the lean control feedback control and the lean control feedback control. An air-fuel ratio control method for an internal combustion engine, characterized in that the signal is switched after a lapse of a predetermined period after the fuel injection amount is changed when the fuel ratio control is switched to the feed back control. 2. The air-fuel ratio control method for an internal combustion engine according to claim 1, wherein the predetermined period is equal to the period until the injected fuel reaches the exhaust system.