JP3348604B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to an air-fuel ratio control device for an internal combustion engine that changes a target air-fuel ratio in accordance with the amount of rotation fluctuation of the internal combustion engine during so-called lean burn control. About.

[0002]

2. Description of the Related Art Conventionally, when the internal combustion engine is in a steady operation or at a moderate acceleration, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is controlled to be leaner than the stoichiometric air-fuel ratio. BACKGROUND ART An air-fuel ratio control device for an engine is known.

[0003] Such an air-fuel ratio control device for an internal combustion engine, when a predetermined condition (hereinafter, referred to as "lean burn control permission condition") capable of performing lean burn control is satisfied,
While changing the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine from the stoichiometric air-fuel ratio in the lean direction to the predetermined width, the air-fuel ratio is changed to the predetermined lean target air-fuel ratio, and the rotation fluctuation amount of the internal combustion engine is detected. Then, the detected rotation fluctuation amount is compared with a predetermined set value, and when the rotation fluctuation amount is large, control is performed such that the target air-fuel ratio is slightly changed in the rich direction. That is, the engine instability is detected during the lean burn control,
When the detected degree of instability is higher than a preset value corresponding to engine misfire prevention, the air-fuel ratio is increased so that the instability becomes equal to or less than the preset value. No. 6-23553).

[0004]

However, in the above-described conventional air-fuel ratio control apparatus for an internal combustion engine, the air-fuel ratio is controlled using a correction amount of the air-fuel ratio which is enriched in accordance with the amount of rotation fluctuation during lean burn control. Therefore, there arises a problem that the generation amount of NOx increases.

FIG. 19A is an explanatory diagram of a change in the air-fuel ratio due to lean burn control in a conventional air-fuel ratio control device for an internal combustion engine. In FIG. 19A, at the time of the lean burn control (1), the air-fuel ratio is enriched until the combustion fluctuation is stabilized. However, even when the combustion becomes stable, the air-fuel ratio is controlled by the enriched air-fuel ratio. Occurs (2). Further, since the enriched air-fuel ratio is near the combustion limit, the combustion becomes unstable if the leaning is rapidly performed.

[0006] It is an object of the present invention to provide a method for controlling lean burn.
Provided is an air-fuel ratio control device for an internal combustion engine that realizes stable lean burn control and controls a correction amount for making the air-fuel ratio lean when the combustion state is stable, thereby suppressing the generation of NOx. It is in.

[0007]

According to a first aspect of the present invention, there is provided an air-fuel ratio control apparatus for an internal combustion engine which is mounted on an exhaust system of an internal combustion engine and outputs a value proportional to the air-fuel ratio in the exhaust gas. Air-fuel ratio detecting means, feedback control means for feedback-controlling the air-fuel ratio to a predetermined target air-fuel ratio based on the output of the air-fuel ratio detecting means, and the feedback control when the operating state of the internal combustion engine is in a predetermined operating state. Target air-fuel ratio setting means for setting the target air-fuel ratio to an air-fuel ratio leaner than the stoichiometric air-fuel ratio, combustion fluctuation amount detecting means for detecting the combustion fluctuation amount of the internal combustion engine, and detection by the combustion fluctuation amount detecting means. When the combustion fluctuation amount is larger than the first predetermined value, the target air-fuel ratio is corrected to the rich side. When the combustion fluctuation amount is smaller than the second predetermined value which is equal to or less than the first predetermined value, the target air-fuel ratio is reduced. An air-fuel ratio control device for an internal combustion engine having a target air-fuel ratio corrector that corrects the target air-fuel ratio set by the target air-fuel ratio corrector and the target air-fuel ratio corrected by the target air-fuel ratio corrector. and updating means for updating the deviation between the target air-fuel ratio of the actual combustion limit corresponds in the lean-burn control and the set target air-fuel ratio by deviation by the target air-fuel ratio correction means based on the updated deviation by said updating means Correction control means for controlling a correction amount of the target air-fuel ratio to the lean side.

According to the air-fuel ratio control device for an internal combustion engine of the first aspect, the target air-fuel ratio set by the target air-fuel ratio setting means and the target air-fuel ratio corrected by the target air-fuel ratio correction means in accordance with the amount of combustion fluctuation. The target air-fuel ratio corresponding to the actual combustion limit and the set target air-fuel ratio are updated by updating the deviation between the target air-fuel ratio corresponding to the actual combustion limit and the set target air-fuel ratio based on the deviation of the target air-fuel ratio corresponding to the actual combustion limit. The fuel ratio can be detected, and in addition, by correcting and controlling the target air-fuel ratio in accordance with the learned deviation, it is possible to prevent deterioration in drivability due to unstable combustion at the target air-fuel ratio corresponding to the actual combustion limit. it can. As a result, stable lean burn feedback air-fuel ratio control can be realized, and the air-fuel ratio can be made lean when the combustion state is stable, thereby suppressing the generation of NOx.

An air-fuel ratio control device for an internal combustion engine according to claim 2 is
The correction control means, when a deviation between the target air-fuel ratio set by the target air-fuel ratio setting means and the target air-fuel ratio corrected by the target air-fuel ratio correction means is smaller than the deviation updated by the updating means, The amount of correction of the target air-fuel ratio toward the lean side by the target air-fuel ratio correction means is made smaller than when the deviation is larger than the updated deviation.

According to the second aspect of the present invention, the deviation between the target air-fuel ratio set by the target air-fuel ratio setting means and the target air-fuel ratio corrected by the target air-fuel ratio correction means is updated by the updating means. When the deviation is smaller than the updated deviation, the amount of correction of the target air-fuel ratio to the lean side by the target air-fuel ratio correction means is made smaller than when the deviation is larger than the updated deviation. NOx generation amount can be minimized without impairing the operability of the vehicle.

[0011]

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

FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter referred to as an "engine") and a control device thereof according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an intake valve and an exhaust valve (see FIG. (Not shown)) provided in each pair. In the engine 1, the operating characteristics of the intake valve and the exhaust valve (specifically, the valve opening timing and the lift amount, hereinafter referred to as "valve timing") are changed to a high-speed valve timing suitable for a high-speed rotation region of the engine. The valve timing is adapted to be switchable between a valve timing suitable for a medium speed rotation region and a low speed valve timing suitable for a low speed rotation region.

A throttle body 3 is provided in the middle of an intake pipe 2 of the engine 1, and a throttle valve 3 'is provided therein.
Is arranged. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 ′, and outputs an electric signal corresponding to the opening of the throttle valve 3 to be supplied to an electronic control unit (hereinafter referred to as “ECU”) 5. I do.

A fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of an intake valve (not shown) of the intake pipe 2. Each injection valve 6 is connected to a fuel pump (not shown). The ECU 5 is connected to the ECU 5 and is electrically connected to the ECU 5. A signal from the ECU 5 controls a valve opening time of fuel injection.

An output side of the ECU 5 is connected to a solenoid valve 21 for controlling the switching of the valve timing. The opening and closing operation of the solenoid valve 21 is controlled by the ECU 5. The solenoid valve 21 switches the hydraulic pressure of a switching mechanism (not shown) for switching the valve timing between high / medium / low, and the valve timing is adjusted to the high-speed valve timing corresponding to the high / medium / low hydraulic pressure. Is switched between the middle valve timing and the low valve timing. The hydraulic pressure of the switching mechanism is detected by a hydraulic pressure (POIL) sensor 20, and a detection signal is supplied to the ECU 5.

On the other hand, immediately downstream of the throttle valve 3, a pipe 7
An absolute pressure signal (PBA) sensor 8 is provided through the intake pipe, and an absolute pressure signal converted into an electric signal by the absolute pressure sensor 8 is supplied to the ECU 5. Further, an intake air temperature (TA) sensor 9 is attached downstream thereof.
Detects intake air temperature TA and outputs the corresponding electrical signal for EC
Supply to U5.

The engine water temperature (TW) sensor 10 mounted on the main body of the engine 1 is composed of a thermistor or the like, detects the engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5. The crank angle (CRK) sensor 11, TDC sensor 12, and cylinder discrimination (CYL) sensor 22 are mounted around a camshaft (not shown) of the engine 1 or around the crankshaft. The TDC sensor 12 outputs a pulse (hereinafter referred to as a “TDC signal pulse”) at a predetermined crank angle position every time the crankshaft of the engine 1 rotates 180 degrees, and the cylinder discrimination sensor 22 outputs a pulse at a predetermined crank angle position of a specific cylinder. The crank angle sensor 11 outputs a pulse (hereinafter referred to as “C”) at a constant crank angle (for example, 30 degrees) shorter than the cycle of the TDC signal pulse.
RK signal pulse), and these signal pulses are supplied to the ECU 5.

The three-way catalyst 14 is disposed in the exhaust pipe 13 of the engine 1 and purifies components such as HC, CO and NOx in the exhaust gas. An oxygen concentration sensor (hereinafter referred to as "LAF sensor") 15 as an air-fuel ratio detecting means is mounted on the exhaust pipe 13 on the upstream side of the three-way catalyst 14, and has an electric signal having a level substantially proportional to the oxygen concentration in the exhaust gas. Output to ECU
5

The ECU 5 further includes an atmospheric pressure (PA) sensor 1.
6. A vehicle speed (VSP) sensor 17, a clutch sensor 18 for detecting engagement / disengagement of a clutch, and a gear position sensor 19 for detecting a shift position of a transmission are connected, and detection signals of these sensors are supplied to the ECU 5.

The ECU 5 shapes input signal waveforms from various sensors, corrects a voltage level to a predetermined level, and converts an analog signal value to a digital signal value. 5b, a storage means 5c for storing various calculation programs executed by the CPU 5b, calculation results, and the like, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, the solenoid valve 21, and the like.

The ECU 5 is provided with a voltage regulator 51. The output voltage of the voltage regulator 51 is A / D converted, and
b. The voltage regulator 51 includes three variable voltage circuits each including a voltage dividing resistor or the like connected to a constant voltage circuit (not shown) in the present embodiment. Adjustable.
The values of the first to third mass-production correction variables KCMPRO1 to 3 used in the program of FIG. 7 described later are determined in accordance with the output voltages of these variable voltage circuits.

The voltage regulator 51 is adjusted so as to compensate for variations in the characteristics of the engine and changes over time at the time of assembling the control device shown in FIG. 1 into the engine or at the time of regular maintenance.

The CPU 5b determines various engine operation states such as a feedback control operation area and an open loop control operation area corresponding to the oxygen concentration in the exhaust gas based on the various engine parameter signals described above, and sets the engine operation state. Accordingly, based on the following equation (1), the TDC
Fuel injection time To of the fuel injection valve 6 synchronized with the signal pulse
ut is calculated.

Tout = Ti × KCMD × KLAF × KEVAP × K
1 + K2 (1) where Ti is a basic fuel amount, specifically, a basic fuel injection time determined according to the engine speed NE and the intake pipe absolute pressure PBA, and is used to determine the Ti value. Ti
The map is stored in the storage unit 5c.

KCMD is a target air-fuel ratio coefficient calculated according to the engine operating state. As is apparent from the above equation (1), if the target air-fuel ratio coefficient KCMD increases, the fuel injection time Tout increases. , KCMD value is a value proportional to the reciprocal of the so-called air-fuel ratio A / F.

KLAF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAF sensor 15 coincides with the target air-fuel ratio during the air-fuel ratio feedback control, and according to the engine operating state during the open-loop control. Is set to a predetermined value.

KEVAP is an evaporation correction coefficient for compensating for the effect of fuel vapor due to purge, and is set to a value of 1.0 when purging is not performed, and 0 to 0 when purging is performed.
It is set between 1.0. The smaller the value of the coefficient KEVAP, the greater the effect of the purge.

K1 and K2 are other correction coefficients and correction variables calculated in accordance with various engine parameter signals, respectively, to optimize various characteristics such as fuel consumption characteristics and engine acceleration characteristics according to the engine operating state. Is set to such a value.

The CPU 5b further outputs a valve timing switching instruction signal according to the engine operating state, and
1 is performed.

The CPU 5b outputs a signal for driving the fuel injection valve 6 and the solenoid valve 21 via the output circuit 5d based on the result calculated and determined as described above.

FIG. 2 is a flowchart of a process for calculating a rotation fluctuation amount DMSSLB used for determining combustion instability, and this process is executed by the CPU 5b.

FIG. 2A shows a CRK process executed every time the CRK signal pulse is generated, in synchronism with the generation of the CRK signal pulse. In step S1, a CRK signal pulse generation time interval is measured. Specifically, as shown in FIG.
Every time it rotates 0 degrees, CRME (n) and CRME (n +
1), CRME (n + 2)... Are measured.

Note that the period during which the crankshaft rotates 180 degrees is divided into every 30 degrees, and each of the periods is # 0STG to # 5STG.
(# 0 stage to # 5 stage).

In step S2, 11 is obtained by the following equation (2).
The latest measured value C from the previous measured value CRME (n-11)
A first total value CR12ME (n) is calculated as the total value of the twelve CRME values up to RME (n).

[0035]

(Equation 1) In this embodiment, the CRK signal pulse has a crankshaft of 30.
Generated every time the motor rotates by the first degree, the first total value CR12M
E (n) is a total value corresponding to one rotation of the crankshaft.
Originally, a value obtained by dividing the first total value CR12ME (n) by 12 is used as CR12ME (n).
In this embodiment, the processing is performed using the total value for simplifying the calculation. By performing such processing, a primary vibration component of one cycle of engine rotation per one rotation of the crankshaft, that is, a mechanical error (manufacturing error, mounting error, etc.) of the pulsar or pickup constituting the crank angle sensor 16 is generated. Noise components can be removed.

The engine speed NE is calculated based on the CR12ME (n) value.

FIG. 2 (b) shows a process executed in # 3STG (# 3 stage, see FIG. 3) which is the same period as the TDC signal pulse generation period. First, step S11
Then, according to the following equation (3), the sum of the six CR12ME values from the calculated value CR12ME (n−5) five times before the first total value CR12ME to the latest calculated value CR12ME (n) is calculated as 2 is calculated as the total value MSME (n).

[0038]

(Equation 2) In the present embodiment, engine 1 is a four-cylinder four-cycle engine, and ignition is performed in one of the cylinders every time the crankshaft rotates 180 degrees. Therefore, the second sum MS
ME (n) is the total value of the first total value CR12ME (n) for each ignition cycle. Originally, the second sum M
The value obtained by dividing SME (n) by 6 is used as MSME (n). However, in this embodiment, the processing is performed using the total value in order to simplify the calculation. By performing such processing, it is possible to remove a secondary vibration component expressed as a torque fluctuation of the engine rotation due to combustion, that is, a vibration component of a crankshaft half rotation cycle.

Next, according to the following equation (4), the rotational fluctuation DM
Calculate SSLB (n).

DMSSLB (n) = | (MSME (n) −MSME
(N-1)) / KDMSSLB | (4) Here, KDMSSLB is set to be inversely proportional to the engine speed NE in order to prevent the control accuracy during the lean burn control from changing according to the engine speed. This is a coefficient that is set so that the rotation fluctuation amount DMSSLB does not change in accordance with the engine speed NE.

FIG. 4 is a main flowchart of a process for calculating the target air-fuel ratio coefficient KCMD and the air-fuel ratio correction coefficient KLAF. This process is executed by the CPU 5b in synchronization with each generation of a TDC signal pulse.

In the figure, first, the engine speed NE
And the intake air absolute pressure PBA, and the rotational fluctuation amount DMSS
The lower threshold value MSLEAN1 and the upper threshold value MSLEAN2 (see FIG. 18) for determining LB combustion instability are set to MSLEAN.
A search is made from the map (step S401).

Next, the output of the LAF sensor 15 is
(Step S40)
2) The LAF feedback area determination processing of FIG. 5 described later is performed (step S403).

Next, in step S404, it is determined whether or not the engine is in the start mode. Specifically, a start mode determination flag FSTMOD indicating “1” indicating that the engine is in the start mode.
Is "1". If FSTMOD = 1, a low water temperature target air-fuel ratio coefficient KTWLAF at the time of low water temperature is searched from the table with reference to the engine water temperature TW (step S405), and the KTWLAF value is set as the KCMD value (step S406). Then, the feedback limit flag FLAFLMT is set to “0” (step S407), and the flag FL indicating that the feedback control of the LAF sensor 16 is being executed is indicated by “1”.
AFFB is set to "0" (step S408), and the air-fuel ratio correction coefficient KLAF is set to "1.0" (step S408).
409), and set the air-fuel ratio correction coefficient integral term KLAFI to "1.
0 ”(step S410), and furthermore, an evaporation purge correction coefficient KE used for compensating for the influence of the fuel vapor.
The VAP is calculated (step S411), and the process ends.

If it is determined in step S404 that the mode is not the start mode, a process of calculating a target air-fuel ratio coefficient KCMD in FIG. 6 described later is performed (step S404).
412) Then, it is determined whether or not the LAF sensor 15 is in the active state (step S413). When the LAF sensor 15 is in the active state, a calculation process of an actual air-fuel ratio (hereinafter, referred to as “detected air-fuel ratio”) KACT is performed based on the output of the LAF sensor 15 (step S414).
The detected air-fuel ratio KACT is made equal to the target air-fuel ratio coefficient KCMD calculated in step S412,
The air-fuel ratio correction coefficient KLAF is calculated (step S41).
5), the evaporative purge correction coefficient K in step S411
The process proceeds to EVAP calculation processing.

In step S413, L
When the AF sensor 15 is inactive, the LAF sensor 1
The calculation process of the detected air-fuel ratio KACT using the output of No. 5 is not performed, and the process proceeds to the process of step S407 and thereafter.

FIG. 5 is a flowchart of the LAF feedback area determination processing in step S403.

In the figure, first, the engine speed NE
Is within a predetermined range (step S50).
1, S502). That is, first, the engine speed NE
Is less than or equal to the upper limit rotation speed NLAFH of the predetermined range (step S501). Here, NE> N
When the air-fuel ratio is LAFH, the LAF feedback region determination flag FL indicating that the condition for executing the air-fuel ratio feedback control is satisfied is set to "1" assuming that the condition for executing the air-fuel ratio feedback control is not satisfied.
AFFBZ is set to "0" (step S506), and this processing ends.

In step S501, NE ≦ N
If it is LAFH, it is further determined whether or not the NE value is larger than the lower limit value NLAFL of the predetermined range (step S502). Here, if NE ≦ NLAFL, the condition for executing the air-fuel ratio feedback control is not satisfied, and the LAF feedback region determination flag FLAFFBZ is set to “0” (step S50).
6), end this processing.

In step S502, NE> N
If it is LAFL, it is assumed that the engine speed NE is within the above-mentioned predetermined range, and the routine proceeds to step S503, where the engine 1 performs WOT (throttle valve fully open operation, Wide Ope).
It is determined whether or not the WOT flag FWOT indicating “1” in the (n Throttle) area is “1” (step S503). When FWOT = 1, it is determined that the condition for executing the air-fuel ratio feedback control is not satisfied, and the LAF feedback region determination flag LAFF is determined.
BZ is set to "0" (step S506), and this processing ends. Steps S501 and S502
, The upper limit rotation speed NLAFH and the lower limit rotation speed NL
Hysteresis is set in the AFL to prevent the control from becoming unstable. That is, the upper limit rotation speed NLAHH and the lower limit rotation speed NLAFL are respectively provided with two values, large and small, so that the engine rotation speed NE increases while the lower limit rotation speed NLA increases.
A larger value is applied when the engine speed NE approaches the FL or the upper limit speed NLAFH, and a smaller value is applied when the engine speed NE approaches the lower limit engine speed NLAFL or the upper limit engine speed NLAHH.

In the above step S503, FWOT
If = 0, it is further determined whether or not a flag FFC indicating "1" that the engine 1 is in the fuel cut operation region is "1" (step S504). FFC
If = 1, it is determined that the condition for executing the air-fuel ratio feedback control is not satisfied, the LAF feedback region determination flag FLAFFBZ is set to “0” (step S506), and the process ends.

When FFC = 0 in step S504, it is determined that the engine 1 is not in the fuel cut operation state and the condition for executing the air-fuel ratio feedback control is satisfied, and the LAF feedback region determination flag FLAFFBZ is set to "1". (Step S5
05), this process ends.

Next, the calculation process of the target air-fuel ratio coefficient KCMD in step S412 in FIG. 4 will be described with reference to FIGS.

FIGS. 6 and 7 are flowcharts of the target air-fuel ratio coefficient KCMD calculation processing in step S412 of FIG. This processing is executed in synchronization with the generation of each TDC signal.

In step S612 in FIG. 6, it is determined whether or not a shift change is being performed. This determination is made by detecting whether or not the clutch is connected by the clutch sensor 18. When the answer to step S612 is affirmative (YES), that is, when a shift change is being performed, a predetermined shift change delay time (for example, 500 milliseconds) tmDLYBS is set in a shift change delay timer tmKBS that measures the elapsed time after the end of the shift change. Is started (step S613), and a predetermined F / C delay time tmAF is added to an F / C delay timer tmAFC for measuring the duration of fuel cut.
CDLY (300 milliseconds) is set and started (step S617), and this processing ends.

If the answer to step S612 is negative (N
O), that is, when the shift change is not being performed, the timer value of the shift change delay timer tmKBS is “0”.
Is determined (step S614). This answer is affirmative (YES), that is, a predetermined time tm after the end of the shift change.
If DLYBS has elapsed, the process immediately proceeds to step S618, and if the answer to the determination in step S614 is negative (NO), that is, if the predetermined time tmDLYBS has not elapsed after the end of the shift change, it is determined whether or not the valve timing has been changed. (Step S615). Step S61
If the answer to step 5 is negative (NO), the process proceeds to step S61.
The routine proceeds to step 7, and if affirmative (YES), the shift change delay timer tmKBS is reset to a value of 0 (step S616), and the routine proceeds to step S618.

In step S618, it is determined whether or not fuel cut is being performed. If the answer is affirmative (YES), a predetermined value NTDCX (for example, 6) is set in the TDC counter NFB (step S619), and the F / C delay is performed. It is determined whether the timer value of the timer tmAFC is “0” (step S620). The TDC counter NFB is provided to change the control gain of the air-fuel ratio feedback control according to the number of TDC signal pulses after the end of the fuel cut. If the answer to step S620 is negative (NO),
That is, the fuel cut duration is the predetermined time tmAF.
If the value is less than CDLY, the present process is immediately terminated. If the answer to step S620 is affirmative (YES), that is, if the fuel cut has continued for the predetermined time tmAFCDLY or more, the target air-fuel ratio coefficient KBS for lean burn control is set to a predetermined value substantially equivalent to the stoichiometric air-fuel ratio (A / F = 14.7). KCMDF
C is set (step S621). Next, KCMD
The value is set to the KBS value (step S628), and the process proceeds to step S632 (FIG. 7).

As described above, when the fuel cut duration is short (less than tmAFCDLY), this processing is immediately terminated, and the fuel cut duration is set to tmAFCD.
At the time of CDLY or more, the KBS value is set to the predetermined value KCMDFC corresponding to the stoichiometric air-fuel ratio, so that the supply air-fuel ratio immediately after the end of the fuel cut can be appropriately controlled. That is, when the fuel cut duration is short, the engine operation state hardly changes. Therefore, by starting the feedback control from the value immediately before the fuel cut, a desired supply air-fuel ratio can be quickly obtained. Further, when the fuel cut duration is long, the KBS value is set to substantially the stoichiometric air-fuel ratio value. Therefore, the KBS value set according to the engine operating state after the fuel cut ends is changed to the lean side or the rich side. Irrespective of the value of any of the above, it is possible to quickly follow.

If the answer in step S618 is negative (N
O), that is, when not in fuel cut, KCM
The absolute value of the deviation between the previous value KCMD (n-1) of D and the previously calculated value KACT (n-1) of the detected air-fuel ratio KACT is a predetermined value DKAFC (for example, equivalent to 0.8 in A / F conversion). (Step S62).
3). When the answer is affirmative (YES), that is, when the deviation is equal to or smaller than the predetermined value DKAFC, the count value of the TDC counter NFB is reset to "0" (step S625).
On the other hand, if the determination is negative (NO), the count value of the NFB is decremented by 1 (step S624), and the process proceeds to step S626.

At steps S623 to S625, immediately after the end of the fuel cut, the target air-fuel ratio coefficient KCM
D and the detected air-fuel ratio KACT have a large deviation (DKAFC
In this case, the count value of the TDC counter NFB becomes equal to or more than the value 1, and the control gain of the air-fuel ratio feedback control is set to a smaller value than when NFB = 0.

In step S626, a predetermined time tmAFCDLY is set in the F / C delay timer and started, and then the target air-fuel ratio coefficient KBS for lean burn control is calculated by the processing of FIG. )I do. Next, the KCMD value is changed to KBS
A value is set (step S628), and step S632 is set.
Proceed to (FIG. 7).

In step S632, it is determined whether or not the engine is in an idle state. When the answer is affirmative (YES), the first mass production correction variable KCMPRO1 set in accordance with the output voltage of the voltage regulator 51 is set. It is added to the KCMD value (step S634), and the process proceeds to step S638.
When the answer to step S632 is negative (NO), that is, when the engine is not in the idling state, the lean burn determination threshold value KCMDZL (for example, A) in which the KCMD value is a predetermined value leaner than the stoichiometric air-fuel ratio to check whether the lean burn control is being performed. /
It is determined whether or not the value is smaller than (F = a value corresponding to 18) (step S633). If the answer to step S633 is negative (N
O), that is, when KCMD ≧ KCMDZL, it is determined that the lean burn control is not being performed, and the voltage regulator 51
Second mass production correction variable KC set according to the output voltage of
MPRO2 is added to the KCMD value (step S63)
5) The process proceeds to step S638. When the answer to step S633 is affirmative (YES), that is, when KCMD <KCMDZL and during the so-called lean burn control, the third mass production correction variable KCMPRO3 set according to the output voltage of the voltage regulator 51 is set to the KCMD value. Addition (Step S
636), and proceeds to step S638.

According to steps S632 to S636, the first mass production correction variable KCMDRO1 is applied when the engine is idling, and the value of the target air-fuel ratio coefficient KCMD and the predetermined value KCMD when the engine is not idling.
Since the second or third mass production correction variable KCMPRO2 or KCMPRO3 is applied according to the magnitude relationship with ZL,
Mass production correction suitable for each engine operating state and target air-fuel ratio can be performed without deteriorating exhaust gas characteristics or drivability. In particular, it is advantageous in ensuring the stability of the exhaust gas characteristics in the idle state of the engine and avoiding misfires and fluctuations in engine speed during lean burn control.

In step S638, a KCMD value limit process is performed by the process of FIG. 13 described later. This limit processing is performed so that the difference between the previous value and the current value of KCMD does not exceed an upper limit value set according to the engine operating state, so that the KCMD value is not suddenly changed. However, when the KCMD value is leaner than the stoichiometric air-fuel ratio and the accelerator pedal is suddenly depressed, the KCMD value is immediately increased to a value corresponding to the stoichiometric air-fuel ratio.

Next, referring to FIG. 8, step S in FIG.
Target air-fuel ratio KBS for lean burn control at 627
Will be described.

FIG. 8 is a flowchart of the process of calculating the target air-fuel ratio coefficient KBS for lean burn control in step S627 of FIG.

First, in step S841, the reference value KBSM of the target air-fuel ratio coefficient is calculated by the processing of FIG. 9 described later.

Next, at step S842, a high-load target air-fuel ratio coefficient KWOT to be applied when the engine is in a predetermined high-load operation state is calculated. This KWOT value is read from a KWOT map set in accordance with the engine speed NE and the intake pipe absolute pressure PBA. Step S
At 843, it is determined whether or not a flag FWOT set to "1" is "1" when the engine is in a predetermined high load operation state, and the answer is affirmative (YES), that is, the engine is at a predetermined high load. When the vehicle is in the load operation state, it is determined whether the KWOT value is equal to or larger than the KBSM value (step S84).
4). This answer is affirmative (YES), that is, KWOT ≧ KBS
In the case of M, a timer tm for determining the elapsed time in step S854 to be described later has a predetermined time tm
KCMHLD (for example, 2 seconds) is set and started (step S845), the KBS value is set to the KWOT value (step S846), and the process ends.

If the answer to step S844 is negative (NO), that is, if KWOT <KBSM, the process proceeds to step S84.
5 is performed (step S848), the KBS value is set to the KBSM value (step S856), and the process ends.

If the answer to step S843 is negative (N
O), that is, when the engine is not in the predetermined high load operation state, the reference value KBSM is the predetermined value KBSM corresponding to the stoichiometric air-fuel ratio.
It is determined whether it is smaller than 0 (step S847).
When the answer is negative (NO), that is, when KBSM ≧ KBSM0, the process proceeds to step S848, and the answer is affirmative (YE
S), that is, when KBSM <KBSM0, it is determined whether or not the accelerator pedal is depressed greatly, that is, the amount of change in the opening direction of the throttle valve opening θTH (current detection value−previous detection value) is a predetermined value on the acceleration side (for example, 4) is determined (step S851). When the answer to step S851 is affirmative (YES), the process proceeds to step S845.
Similarly, a predetermined time tmKCMHLD is set in tmKCM and started (step S853).
The S value is set to a predetermined value KBSM0 corresponding to the stoichiometric air-fuel ratio (step S858), and the process ends.

If the answer to step S851 is negative (N
O), that is, when the amount of change in the opening direction of the throttle valve is small, the amount of change in the intake pipe absolute pressure PBA (currently detected value-
The last detected value) DPB is a predetermined value DPBKCR (for example, 80
mmHg) is determined (step S8).
52). This answer is affirmative (YES), that is, DPB> DPB
If KCR, the process proceeds to step S853, and if negative (NO), that is, if DPB ≦ DPBKCR, it is determined whether the timer value of the timer tmKCM is “0” (step S854). When the answer to step S854 is negative (NO), that is, when tmKCM> 0, the process proceeds to step S858, and when the answer is affirmative (YES), the process proceeds to step S856.

Referring now to FIG. 9, step S in FIG.
The calculation process of the reference value KBSM of the target air-fuel ratio coefficient in 841 will be described. FIG. 9 is a flowchart of the process of calculating the reference value KBSM of the target air-fuel ratio coefficient in step S841 of FIG.

First, in step S901, it is determined whether execution of lean burn control based on the evaporation purge correction coefficient KEVAP is permitted. The execution of the lean burn control based on the evaporation purge correction coefficient KEVAP is as follows.
This is permitted when the evaporation purge correction coefficient KEVAP is larger than the predetermined value and the purge amount of the evaporated fuel is small, and is not permitted when the evaporation purge correction coefficient KEVAP is smaller than the predetermined value and the purge amount of the evaporated fuel is large.

If the execution of lean burn control based on the evaporation purge correction coefficient KEVAP is permitted in step S901, it is determined in step S902 whether the execution of lean burn control based on misfire detection is permitted. The execution of the lean burn control based on the misfire detection is performed when the engine is not misfired, and when the engine is misfired, it is determined that the combustion is unstable and the lean burn control is performed. Is not allowed.

If the execution of the lean burn control based on the misfire detection is permitted in step S902, the process proceeds to step S903, where the engine coolant temperature TW is reduced to the low-temperature coolant temperature TWL.
It is determined whether it is higher than EAN5. Step S903
If TW> TWLEAN5 in step S90, step S90
4 and the engine coolant temperature TW is changed to the high-temperature coolant temperature TWLEAN.
Determine if it is lower.

In step S904, TW ≧ TWLEAN
In step S906, the target air-fuel ratio coefficient KTWLAF at the time of low water temperature is initialized to 1.0.

In step S904, TW <TWLEAN
In step S905, the process proceeds to step S905, where it is determined whether the shift position determination value NGR is the fifth speed. Step S
If NGR = 5 at 905, it is determined that the combustion of the engine is stable even if the engine coolant temperature TW is lower than the high-temperature side coolant temperature TWLEAN, and the target air-fuel ratio coefficient KTW at low coolant temperature is determined.
LAF is initialized to 1.0 (step S906).

Next, in step S907, FIG.
The process of searching for a KBSM map is performed by the process of 0, and the process proceeds to step S908.

If the execution of the lean burn control based on the evaporation purge correction coefficient KEVAP is not permitted in step S901, and if the execution of the lean burn control based on the misfire detection is not permitted in step S902, TW ≦ TWLWAN5 in step S903. If there is, or if NGR ≠ 5 in step S905, step S905
909, the target air-fuel ratio coefficient KTWLAF at low water temperature
By searching the table, the target air-fuel ratio coefficient KTW at low water temperature
Determine LAF. Here, in the KTWLAF table, the KTWLAF value is set to decrease as the engine coolant temperature TW increases.

Next, in step S910, it is determined whether the KTWLAF value retrieved in step S909 is smaller than a predetermined value KBSM0 corresponding to the stoichiometric air-fuel ratio.
In step S910, if KTWLA ≧ KBSM0, the reference value KBSM is set to the KTWLAF value (step S911). If KTWLAF <KBS M0 , the reference value KBSM is set to the predetermined value KBSM corresponding to the stoichiometric air-fuel ratio.
It is set to 0 (step S912) and step S908
Proceed to.

In step S908, it is determined whether or not the engine 1 is idling, and if not, the value of a timer tmKBSIDL that counts a predetermined elapsed time since the engine 1 is idling is set. Then (step S912), the present process ends.

If it is determined in step S908 that the engine 1 is in the idle state, the flow advances to step S913 to determine whether or not the engine coolant temperature TW is equal to or higher than the high-temperature side coolant temperature TWLEAN. If TW ≧ TWLEAN in step S913, the process proceeds to step S914, where the timer tmKBSID is set.
It is determined whether or not the value of L is 0. Step S914
When the value of the timer tmKBSIDL is 0, the reference value K
Lean target air-fuel ratio KB in idle state in BSM
SIDL is set (step S915), and the timer tmK is set.
When the value of BSIDL is not 0 and the predetermined elapsed time has not elapsed, the rich target air-fuel ratio KBSIDLR in the idle state is set to the reference value KBSM (step S).
916), this process ends.

Next, the map search processing of the reference value KBSM of the target air-fuel ratio coefficient in step S907 of FIG. 9 will be described with reference to FIG.

FIG. 10 is a flowchart of the map search process for the target air-fuel ratio coefficient reference value KBSM in step S907 in FIG.

First, in step S1001, the reference value KBSM of the target air-fuel ratio coefficient is searched from the KBSM map with reference to the intake pipe absolute pressure PBA and the engine speed NE.

Next, at step S1002, the lean burn control execution permission flag F indicating "0" to permit execution of the lean burn control by the processing of FIG.
The KBSMJG is determined.

Next, in step S1004, it is determined whether or not the flag FKBSMJG determined in step S1002 is "1". If the flag FKBSJG is “0” in step S1004 and the execution of the lean burn control is permitted, the process proceeds to step S1005, and the target air-fuel ratio is set to a predetermined value (KCMDFBH) set in the processing of FIG. Air-fuel ratio enrichment flag FMSFBR indicating that the air-fuel ratio has been enriched
It is determined whether or not ICH is “1”.

If it is determined in step S1005 that the air-fuel ratio enrichment flag FMSBRICH is "1", the flow advances to step S1006 to determine whether or not the shift position exceeds the fourth speed. If the shift position is 4 in step S1006
If the speed is exceeded, or if the flag FM is set in step S1005
If SFBRICH is “0”, step S10
In step 07, it is determined whether or not the shift position exceeds the third speed.

In step S1004, the flag FKBSMJ
If G is "1" and lean burn control is not permitted, if the shift position is 4th speed or less in step S1006, or if the shift position is 3rd speed or less in step S1007, the reference value KBSM is set to The predetermined value KMSM0 corresponding to the stoichiometric air-fuel ratio is set (step S1008),
This processing ends. If the shift position exceeds the third speed in step S1007, this process ends immediately. If the shift position exceeds the fourth speed in step S1006, it is naturally determined in step S1007 that the shift position exceeds the third speed.

According to the processing of FIG. 10, step S100
5, KC during lean feedback control
When the correction amount of the MD value to the rich side is larger than a predetermined value (FMSFBRIC = 1), only when the shift position exceeds the fourth speed (normally, the third speed), the air-fuel ratio lean feedback control is executed. The lean feedback control is executed only at the high shift position where combustion is stable, thereby avoiding unnecessary generation of NOx.

Next, the process of determining the lean burn control execution permission flag FKBSMJG in step S1002 of FIG. 10 will be described with reference to FIG. FIG.
11 is a flowchart of a process for determining a lean burn control execution permission flag FKBSMJG in step S1002 of step S1002.

First, in step S1101, it is determined whether the target air-fuel ratio coefficient KCMD is smaller than a predetermined value KCMD0 corresponding to the stoichiometric air-fuel ratio. KCMD in step S1101
<KCMD0, that is, when the combustion state is the lean state, the process proceeds to step S1102, and the process proceeds to step S1102 described later.
It is determined whether or not the engine load flag FPBKBS set in 1109 and S1110 is “1”.

In step S1101, KCMD ≧ KCMD
If 0, that is, if the combustion state is a rich state, or if the flag FPBKBS is "1" in step S1102, the process proceeds to step S1103, where the absolute pressure PBKBSL in the map shown in FIG. The absolute pressure PBKBSL is retrieved, and the flow advances to step S1108. In step S1102, the flag FP
If BKBS is “0”, the flow advances to step S1104 to search for the high-load-side intake pipe absolute pressure PBKBSH from the intake pipe absolute pressure PBKBSH table shown in FIG.

FIG. 12 is a map for determining the intake pipe absolute pressure PBKBS and the lean burn control execution permission flag FKBSMJG. In FIG. 12, the horizontal axis represents the engine speed NE, and the vertical axis represents the intake pipe absolute pressure PBA indicating the engine load. The engine speed NE is the lower limit engine speed NEKBSL and the upper limit engine speed NEKBS
In the region between H, the region where the intake pipe absolute pressure PBA is equal to or lower than the low load side intake pipe absolute pressure PBKBSL is the lean burn permission region, and the low load side intake pipe absolute pressure PBKBS.
The region between the L value and the high-load-side intake pipe absolute pressure PBKBSH is a hysteresis region. Also, the engine speed N
E is equal to or lower than the lower limit engine speed NEKBSL, the engine speed NE is equal to or higher than the upper limit engine speed NEKBSL, and the absolute pressure PBA in the intake pipe is the absolute pressure PBKBS in the high load side intake pipe.
The region above H is a lean burn control execution prohibition region.
In this map, the low load side intake pipe absolute pressure PBKBSL value and the high load side intake pipe absolute pressure PBKBSH are respectively:
The engine speed NE is set to decrease as the engine speed NE increases. In addition, the lower limit engine speed NEKB
Each of SL and the upper limit engine speed NEKBSH has hysteresis.

After step S1104, step S11
Proceeding to 05, it is determined whether or not the vehicle speed VPLS is lower than a predetermined value VKBSM. This determination has hysteresis. If VPLS <VKBSM in step S1105 and the vehicle speed is low, the process proceeds to step S1106 to determine whether the intake pipe absolute pressure PBKBS is greater than an upper limit value PKBHLMT. PBKBS in step S1106
> PKBHLMT, the absolute pressure PBK in the intake pipe
The upper limit value PKBHLMT is set in the BS (step S1).
107), and proceeds to step S1108.

In step S1105, VPLS ≧ VKB
SM or PBKBS ≦ PKBH in step S1106
If it is LMT, the process skips step S1107 and proceeds to step S1108.

Next, in step S1108, it is determined whether or not the intake pipe absolute pressure PBA is greater than the PBKBS value. If PBA> PBKBS, it is determined by "1" that the intake pipe absolute pressure PBKBS has a high load. The indicated engine load flag FPBKBS is set to “1” (step S
1109) If PBA ≦ PBKBS, the engine load flag FPBKBS is set to “0” (step S1110), and the process proceeds to step S1111.

In step S1111, FIG.
The air-fuel ratio enrichment flag FS set by the process of
It is determined whether or not FBRICH is “1”. In step S1111, the air-fuel ratio enrichment flag FSFB
When RICH is “0”, the process proceeds to step S1113,
The engine speed NE is the lower limit engine speed NEKBSL
Determine if it is greater than. If the air-fuel ratio enrichment flag FSBRRICH is "1" in step S1111, the engine speed NE becomes the lower limit engine speed NEK.
It is determined whether or not the engine speed NEKBSMF (for example, 2500 rpm) in the lean burn permission region, which is set to a value slightly higher than BSL (for example, 1700 rpm), is higher than the lean burn permission area (step S1112). move on.

In step S1113, NE> NEKBS
If it is L, the process proceeds to step S1114, and it is determined whether or not the engine speed NE is smaller than the upper limit engine speed NEKBSH. In step S1114, NE <N
If it is EKBSH, the process advances to step S1115 to determine whether or not the engine load flag FPBKBS set in steps S1109 and S1110 is "1". Here, step S1112, step S111
3. The determination in step S1114 has hysteresis.

In step S1115, when the engine load flag FPBKBS is "0", the lean burn control execution permission flag FKBSMJG is set to "0".

On the other hand, in step S1112, NE ≦ NEK
If BSMF, NE ≦ NE in step S1113.
If KBSL, NE ≧ NE in step S1114.
If it is KBSH, or if the engine load flag FPBKBS is “1” in step S1115, the lean burn control execution permission flag FKBSMJG is set to “1”.

According to the process of FIG. 11, similarly to step S1005 of FIG. 10, in step S1111 the correction amount of the KCMD value to the rich side during the lean feedback control is larger than the predetermined value (FMSBFBRI).
CH = 1), the air-fuel ratio lean feedback control is executed only when the engine speed NE exceeds a lower threshold value NEKBSMF which is larger than the normal case, and the air-fuel ratio is controlled only in the high rotation range where combustion is stable. Unnecessary NOx by executing lean feedback control
Is to avoid the occurrence of

Hereinafter, the KCMD value limit processing in step S638 of FIG. 7 will be described in detail with reference to FIG. FIG. 13 is a flowchart of the KCMD value limit process in step S638 of FIG.

First, in step S1310, the amount of change DKCMD of the KCMD value is calculated as the difference between the current calculated value KCMD (n) and the previous calculated value KCMD (n-1) (KCMD (n) -KC
MD (n-1)), and in step S1311, the rotation fluctuation amount DMSS
A target air-fuel ratio coefficient correction execution flag FSLB indicating "1" to permit execution of the KCMD correction process corresponding to LB
The FB is determined.

Next, in step S1312, it is determined whether a target air-fuel ratio coefficient correction execution flag FSLFBB is "1". In step S1312, the flag FSLBF
If B is "0", it is determined that the combustion is stable, and in step S1322, the previously calculated value KCMD (n-1) of the target air-fuel ratio coefficient is set to the predetermined value KCM corresponding to the stoichiometric air-fuel ratio.
It is determined whether it is smaller than D0. The answer is affirmative (YE
S), that is, when KCMD (n−1) <KCMD0 and the KCMD value is leaner than the stoichiometric air-fuel ratio, the previously calculated value KCMD (n−1) is equal to the lean predetermined value KCMD.
It is determined whether it is larger than CMDX (for example, A / F = 17) (step S1325). This answer is affirmative (Y
ES), that is, when KCMD (n-1)> KCMDX, the decrease variable DKC2 corresponding to the change speed of the target air-fuel ratio in the lean direction is set to the first lean decrease predetermined value DKC.
2L (for example, A / F = 0.3 equivalent) is set (step S1326), and the process proceeds to step S1328.
This is applied to the formula for calculating the current value KCMD (n) of the MD value, and K
This is to reduce the CMD value. Step S1325
Is negative (NO), that is, KCMD (n-1) ≦ K
In the case of CMDX, the decrease variable DKC2 is set to a second lean-side decrease predetermined value DKC2M (for example, A / F = 0.1 equivalent) smaller than the first lean-side decrease predetermined value (step S1327), and step S1327 is performed. Proceed to S1328.

In step S1328, it is determined whether or not the engine is idle, and the answer is affirmative (YES).
In the case of, the increasing variable DKC1 corresponding to the changing speed of the target air-fuel ratio in the rich direction corresponds to the changing speed for idle.
Rua idle for increasing a predetermined value DKC1IDL (e.g. A / F
= 2.0) (step S1332),
Proceed to step S1343. The increase variable DKC1 is the present value K of the KCMD value in step S1345 described later.
This is applied to the calculation formula of CMD (n) to increase the KCMD value. If the answer to step S1328 is negative (N
O), that is, when the engine is not in the idle state, it is determined whether or not the engine speed NE is lower than a predetermined speed NKCMD (for example, 1800 rpm) (step S1329). When this answer is affirmative (YES),
The increase variable DKC1 is set to the predetermined idle increase value DKC1.
Low rotation increase predetermined value DKC1M1H smaller than IDL
(Eg, A / F = 1.0) (step S1330) On the other hand, when the answer is negative (NO), the increase variable DKC1 is set to the predetermined increase value DKC for low rotation.
Increase predetermined value DKC1M1L for high rotation smaller than 1M1H
(For example, A / F = 0.05 equivalent) (step S1331), and the process proceeds to step S1343.

In step S 1343, it is determined whether or not the change amount DKCMD of the KCMD value is a negative value. When the answer is affirmative (YES), that is, when the KCMD value changes in a decreasing direction, the absolute value of the deviation DKCMD is determined. It is determined whether the value is smaller than the decrease variable DKC2 (step S1346). If the answer to step S1346 is negative (N
O), that is, when | DKCMD | ≧ DKC2,
This time value KCMD (n) is calculated as (KCMD (n-1) -DKC
2) (Step S1347) On the other hand, if the answer to Step S1346 is affirmative (YES), the flow immediately proceeds to Step S1348.

If the answer in step S1343 is negative (N
O), that is, when DKCMD ≧ 0 and the KCMD value changes in the increasing direction, it is determined whether or not the absolute value of the amount of change DKCMD is smaller than the increasing variable DKC1 (step S1344). When the answer to step S1344 is negative (NO), that is, when | DKCMD | ≧ DKC1, the current value KCMD (n) is changed to (KCMD (n−1) +
DKC1) (step S1345). On the other hand, when the answer to step S1344 is affirmative (YES), the process immediately proceeds to step S1348.

According to steps S1343 to S1347, if the absolute value of the change amount DKCMD of the KCMD value is larger than the increase variable DKC1 or the decrease variable DKC2, the current value KCMD (n) is changed to the DKC1 value or DKC2 value and the previous value KCMD (n By changing to the value calculated by -1), it is possible to prevent a sudden change in the KCMD value and a deterioration in drivability.

On the other hand, when the answer to the above step S1322 is negative (NO), that is, when KCMD (n-1) ≧ KCMD0 and the KCMD value is a value corresponding to the stoichiometric air-fuel ratio or on the rich side, steps S1333-S134 are executed.
In 2, the decrease variable DKC2 or the increase variable DKC1 is set, and the flow advances to step S1343.

First, in step S1333, it is determined whether or not the WOT flag FWOT has a value of 1. If the answer is negative (NO), the increase variable DKC1 is set to a normal increase predetermined value DKC1M2 (for example, A / F = 0). .3 equivalent value)
(Step S1339), and Step S1342
Proceed to. When the answer to step S1333 is affirmative (YES), that is, when FWOT = 1 and the engine is in the predetermined high load operation state, the previous value KCMD (n-1) is set to the low water temperature target used when the engine water temperature TW is low. It is determined whether it is larger than the air-fuel ratio coefficient KTWLAF (step S1334). When the answer is negative (NO), the process proceeds to step S1339, and when the answer is affirmative (YES), it is determined whether a failure of a system such as a sensor connected to the ECU 5 is detected (step S1335). If the answer to step S1335 is affirmative (YES), that is, if some failure is detected, the increase variable DKC1 is set to the high water temperature increase predetermined value DKC1H larger than the normal increase predetermined value DKC1M2 (for example, A / F = 0.8 equivalent). ) (Step S1340), and step S13
Proceed to 42.

If the answer in step S1335 is negative (N
O), that is, when a failure is not detected, the high water temperature rich flag FXWO which is set to “1” when the engine is in the predetermined high load operation state and the engine water temperature TW is high.
It is determined whether or not T is “1” (step S133)
6). When the answer is affirmative (YES), the process proceeds to the step S1340, and when the answer is negative (NO), it is determined whether or not the high-speed valve timing is selected (step S1337). If the answer to step S1337 is negative (N
O), that is, when the low-speed valve timing is selected, it is determined whether or not the throttle valve fully open flag set to “1” when the throttle valve is substantially fully open is FTHWOT is “1” (step S133).
8). When the answer to step S1337 or S1338 is affirmative (YES), that is, when the high-speed valve timing or the low-speed valve timing is selected and the throttle valve is substantially fully open, the process proceeds to step S1339. If both the answers in steps S1337 and S1338 are negative (NO), that is, when the low-speed valve timing is selected and the throttle valve is not substantially fully opened, the increase variable DKC1 is set to the high load increase predetermined value smaller than the normal increase predetermined value DKC1M2. The value DKC1L (for example, A / F =
(Equivalent to 0.05) (step S1341),
Proceed to step S1342.

In step S1342, the decreasing variable DKC2
To the rich-side decrease predetermined value DKC2H (for example, A / F = 0.
4 (a value corresponding to 4) (step S1341), and the process proceeds to step S1343.

At step S1312, flag FSLBF
If B is “1”, KCMD correction processing by DMSSLB is performed by the processing of FIG. 16 described later (step S1356), and the process proceeds to step S1348.

In steps S1348 to S1351, K
Perform CMD value limit check. That is, KCMD
The value is compared with predetermined upper and lower limits KCMMLH, KCMML (steps S1348, S1349). If the KCMD value is larger than the upper limit KCMMLMH, the KCMD value is set to the upper limit (step S1351), and the KCM is set.
When the D value is smaller than the lower limit value KCMMLML, KCM
The D value is set to its lower limit (step S135)
0).

Next, in step S1352, a process of calculating the average value DMSBAVE of the rotation fluctuation is performed by the process of FIG. 16 described later, and this process ends.

Next, the process for determining the KCMD correction execution flag FSLBFB in step S1311 in FIG. 13 will be described with reference to FIG. FIG. 15 is a flowchart showing step S1 in FIG.
It is a flowchart of the determination processing of the KCMD correction execution flag FSLBFB of 311.

First, referring to the absolute pressure PBA in the intake pipe,
The combustion instability determination coefficient α is retrieved from the combustion instability determination coefficient table SLBALPH (step S1501). Here, the coefficient α is set to decrease as the load increases. The combustion unstable side determination coefficient α is a threshold value for setting a correction speed when the KCMD value is corrected to the rich side in accordance with the combustion fluctuation detection characteristic peculiar to the engine by the processing of FIG.

Next, it is determined whether or not the DKCMD value set in step S1310 of FIG. 13 is positive (step S1502). DKCMD> 0 in step S1502
If so, the target air-fuel ratio coefficient KCMD has changed to the rich side, and the DMSBAVE update flag FAVEHLD indicating that the update of the DMSBAVE value is permitted is indicated by "1".
Is set to “0” (step S1503), the target air-fuel ratio coefficient correction execution flag FSLFBB is set to “0” (step S1504), and the process ends.

If DKCMD ≦ 0 in step S1502, the flow advances to step S1505 to determine whether the target air-fuel ratio coefficient correction execution flag FSLBFB is “1”. In the first time, FSLBFB = 0, and in this case, it is further determined whether or not the previous value KCMD (n-1) of the target air-fuel ratio coefficient KCMD is smaller than the lean burn determination threshold value KCMDZL (step S1506).

In step S1506, the KCM
When D (n-1) ≧ KCMDZL, the lean burn is not in progress, and the process proceeds to the step S1503 and thereafter. When KCMD (n-1) <KCMDZL, it is determined that the lean burn is in progress. Step S1507
The absolute value of the deviation DKCMD is calculated as shown in FIG.
It is determined whether or not the value is greater than the DKCMDX value calculated in step S1619 of No. 6.

In the above step S1507, | DK
When CMD | ≦ DKCMDX, the target air-fuel ratio coefficient KCMD (n−1) is a value close to the combustion limit. Therefore, in order to start monitoring the combustion fluctuation, the target air-fuel ratio coefficient correction execution flag FSLFBB is set to “1”. (Step S150
8), end this processing. In step S1507, when | DKCMD |> DKCMDX, the process proceeds to step S1509, and it is determined whether or not the absolute value of the deviation DKCMD is greater than a combustion variation determination prohibition threshold DKCMDX2. This combustion fluctuation determination prohibition threshold value DKCMDX
2 is set to a value larger than the DKCMDX value, and functions as a threshold for prohibiting the correction of the target air-fuel ratio coefficient to the rich side according to the combustion instability at the start of the lean feedback control of the air-fuel ratio.

In the above step S1509, | DK
If CMD |> DKCMDX2, the above step S1
Proceeding to step 503, if | DKCMD | ≦ DKCMDX2, the second combustion in which the rotation fluctuation amount DMSSLB is further multiplied by the combustion instability determination coefficient α and the rotation fluctuation average value DMSBAVE calculated in the processing of FIG. It is determined whether or not the value is larger than the instability determination upper threshold α × DMSBAVE (step S1510).

In step S1510, the DMS
If SLB> α × DMSBAVE, it is determined that combustion is unstable, and the process proceeds to step S1508, where DM
If SSLB ≦ α × DMSBAVE, DMSS
The LB value is equal to the first combustion instability determination upper threshold value MSLEA.
It is determined whether it is larger than N2 (step S151).
1).

In step S1511, the DMS
If SLB> MSLEAN2, it is determined that combustion is unstable, and the target air-fuel ratio coefficient correction execution flag FSLBF
B is set to “1” (step S1508), and DMSS
If LB ≦ MSLEAN2, it is determined that the combustion is stable, and steps S1503 and S1504 are executed, and this process ends.

[0126] As described above in detail, according to the process of FIG. 15, as shown in FIG. 18, D KCMD value D KCMDx
When the value shifts to the lean side from the binary value (NO in step S1509 of FIG. 15), the monitoring of the DMSSLB value is started (steps S1510 and S1511), and the DMSSLB is started.
If the value exceeds the MSLEAN2 value or α × DMSBAVE value, the target air-fuel ratio coefficient correction execution flag FSLFBFB = 1
(The same step S1508), the KCMD value correction process is started. However, the absolute value of the DKCMD value is DKC
When it is smaller than the MDX value (step S150
7 and NO ), the KCMD correction execution flag FSLBFB is set to "1" regardless of the DMSSLB value (step S1508).

In step S1509, the absolute value of the deviation DKCMD is set to the combustion fluctuation determination prohibition threshold DKCMD.
It is determined whether or not it is larger than X2, and | DKCMD |> D
If KCMDX2, the target air-fuel ratio coefficient correction execution flag FSLBFB is set to "0" (step S1504). In this case, the target air-fuel ratio coefficient KCMD value is corrected to the rich side due to unstable combustion. Not done. Therefore, it is not possible to erroneously determine that the torque change when the target air-fuel ratio shifts from the stoichiometric air-fuel ratio to the lean target air-fuel ratio is combustion unstable, and erroneously correct the target air-fuel ratio to the rich side. Therefore, the target air-fuel ratio can be smoothly shifted to the lean target air-fuel ratio.

The KCMD correction processing by DMSSLB in step S1356 in FIG. 13 will be described below with reference to FIG. This process is executed in synchronization with each generation of the CRK signal pulse. FIG. 16 is a flowchart of the KCMD correction process by DMSSLB in step S1356 in FIG.

First, in step S1601, DMSSL
It is determined whether or not the B value is greater than the MSLEAN2 value.
When MSSLB> MSLEAN2, it is determined that combustion is unstable, and the KCMD value is corrected to the rich side. In this case, the rich side correction amount DKCR added to the KCMD value is set to a relatively large value. First correction amount DKCR1
(For example, A / F = 0.02) (step S1602). The MSLEAN2 value is a threshold value for determining combustion instability that is set according to the operating state regardless of the individual difference of the engine.

In the above step S1601, DMSSLB
If ≦ MSLEAN2, it is further determined whether or not the DMSSLB value is larger than α × DMSBAVE value (step S1603). DMSSLB> α × DMSB
If AVE, the KCMD value is corrected to the rich side,
In this case, the rich-side correction amount DKCR is set to a relatively small second correction amount DKCR2 (for example, A / F = 0.
(DKCR1> DKCR2) (step S1604). α × DMSBAVE value is an average value DM of rotation fluctuation detected when combustion is stable.
This is a threshold value for determining combustion instability calculated by multiplying the SBAVE value by a coefficient α set according to the operating state. When it is determined that combustion is unstable based on this threshold value,
The target air-fuel ratio coefficient KCM is determined by the relatively small DKCR2 value.
Is corrected to D, as possible out to fine-tune the optimal point of the target air-fuel ratio coefficient KCMD is harmony driver Byi utility and fuel economy is obtained in accordance with the individual difference of each engine.

Thus, DKCR1 or DKC
The rich-side correction amount DKCR set in R2 is calculated by KCMD
Value is added to the previous value KCMD (n-1), and KCMD
(N) is determined (step S1605), and then, in step S1606, it is determined whether the KCMD value is larger than an upper limit value KCMDFBH during lean burn control (for example, A / F = 19).

In step S1606, KCMD (n)>
Oh lever in KCMDFBH, the upper limit value K the KCMD (n) value
Set CMDFBH (step S1607), the air-fuel ratio enrichment flag F MSFBRICH to "1" (step S1608), sets the DMSBAVE update flag FAVEHLD to "1" (step S16
09). In step S1606, KCMD (n) ≦ KC
If it is MDFBH, the process proceeds to step S1609.

In step S1603, the DMS
If SLB ≦ α × DMSBAVE, step S
Proceed to 1610, and the DMSSLB value is set to the lower threshold value MSLEA.
It is determined whether it is greater than N1. Step S1610
If DMSSLB> MSLEAN1, KCMD
Of the previous value KCMD (n-1)
(N) (step S1611), and the DMSBAVE update determination flag FAVEHLD is set to “0” (step S1612).

In step S1610, the DMS
If SLB ≦ MSLEAN1, it is determined that the combustion is stable, and the flow advances to step S1613 to execute KCMD.
In order to further correct the value to the lean side, the deviation DK described above is used.
It is determined whether or not the absolute value of the CMD is larger than the deviation DKCMDX between the target air-fuel ratio coefficient corresponding to the actual combustion limit during the lean burn control and the set target air-fuel ratio coefficient. If | DKCMD |> DKCMDX in step S1613, the lean-side correction amount DKCL is set to a relatively large correction amount DKCL1 (for example, A / F = 0.01 equivalent) (step S1614), and | DKCMD is set. | ≦ DKC
If it is MDX, the lean correction amount DKCL is set to a relatively small correction amount DKCL2 (step S1).
615).

As described above, by changing the correction speed to the lean side depending on whether or not the absolute value of the deviation DKCMD is larger than the DKCMDX value, it is possible to avoid excessive correction to the lean side.

When the setting of the lean correction amount DKCL is completed, it is determined in step S1616 whether or not the absolute value of the deviation DKCMD is smaller than the set DKCL. If | DKCMD | <DKCL, the KCMD value Is used as the current value KCMD (n) as it is, and the process proceeds to step S1612. If DKCMD ≧ DKCL, DKDK is calculated from the previous value KCMD (n−1) of the KCMD value.
The current value KCMD (n) of the target air-fuel ratio coefficient KCMD is calculated by subtracting the CL value (step S161).
7), the process proceeds to step S1612.

Next, when the setting processing of the flag FAVEHLD in steps S1609 and S1612 is completed, in step S1618, the KBSM value retrieved in the routine of FIG.
A / F = 21.4 ) is determined to be smaller than KBS
When M <KBSDKCX, the DKCMDX calculation process for updating the DKCMDX value is performed by the following equation (5).
(Step S1619), and the obtained DK
The CMDX value is stored in the backed-up storage unit of the storage unit 5c, and the process ends.

DKCMDX (n) = c × | DKCMD | + (1−c)
× DKCMDX (n-1) (5) where c is a constant of 0 to 1.0.

In step S1618, KBSM ≧ KBS
If it is DKCX, the process ends without performing the DKCMDX value calculation process.

As described above, in the process of FIG. 16, when the detected rotation fluctuation DMSSLB is larger than the fixed threshold value MSLEAN2, the target air-fuel ratio KCMD is corrected to the rich side with a relatively large correction amount DKCR1. DMSSLB ≦ MSLEAN2 and DMSSLB>
When α × DMSBAVE, relatively small DK
Since the target air-fuel ratio coefficient KCMD is corrected by the CR2 value, the target air-fuel ratio coefficient KCM is corrected according to the individual difference of each engine.
D can be fine-tuned to the optimal point where harmony between driver viability and fuel economy can be obtained.

Referring now to FIG. 17, the average value DMSBAVE of the rotation fluctuation in step S1352 in FIG.
The calculation process will be described. This process is executed in synchronization with each generation of the TDC signal pulse. FIG. 17 is a flowchart of the calculation process of the average value DMSBAVE of the rotation fluctuation.

First, in step S1701, it is determined whether or not a fuel cut determination flag FFC indicating "1" indicating that the fuel is being cut is "1" (step S1701). Since the cutting is being performed, the present process ends without updating the DMSBAVE value. If FFC = 0 in step S1701, the process advances to step S1702 to determine whether or not the DMSBAVE update determination flag FAVEHLD is "1". In step S1702, if FAVEHLD = 1, the process ends without updating the DMSBAVE value, and if FAVEHLD = 0, the DMSBAVE value calculation process is executed according to the following equation (6), and the DMSBAV value is calculated.
The E value is updated (step S1703), and the process ends.

DMSBAVE (n) = c × DMSSLB + (1-c)
× DMSBAVE (n-1) (6) where c is a constant of 0 to 1.0.

According to the present embodiment, steps S1601, S1603, and S1610 in FIG.
In, the amount of rotation fluctuation of the engine 1 is detected, and the target air-fuel ratio coefficient KCMD is corrected according to the detected amount of rotation fluctuation of the engine 1, so that stable lean-burn feedback air-fuel ratio control can be realized ( FIG.
(2) of (b)).

The deviation DKC between the target air-fuel ratio coefficient corresponding to the actual combustion limit and the set target air-fuel ratio coefficient during the lean burn control calculated and stored in step S1619 of FIG.
Steps S1613 to S1613 based on MDX
By the process of 1617, the shift speed of the actual control air-fuel ratio to the lean side during the lean feedback is faster when the DKCMDX value is richer than the DKCMDX value (YES in step S1613 in FIG. 16), and when it is leaner (the same in step S161
3 (NO in FIG. 19), the amount of NOx generated can be minimized without impairing the operability during lean burn due to unstable combustion (see (4) in FIG. 19B).

[0146]

As described above in detail, according to the air-fuel ratio control apparatus for an internal combustion engine of the first aspect, the target air-fuel ratio set by the target air-fuel ratio setting means and the combustion fluctuation amount by the target air-fuel ratio correction means. The target air-fuel ratio corresponding to the actual combustion limit and the set target air-fuel ratio are updated by updating the deviation between the target air-fuel ratio corresponding to the actual combustion limit and the set target air-fuel ratio based on the deviation from the target air-fuel ratio corrected according to By learning, the target air-fuel ratio corresponding to the actual combustion limit can be detected, and in addition, by controlling the correction of the target air-fuel ratio in accordance with the learned deviation, unstable combustion at the target air-fuel ratio corresponding to the actual combustion limit is achieved. The driving performance can be prevented from deteriorating. As a result, stable lean burn feedback air-fuel ratio control can be realized, and the air-fuel ratio can be made lean when the combustion state is stable, thereby suppressing the generation of NOx.

According to the air-fuel ratio control apparatus for an internal combustion engine of the second aspect, the deviation between the target air-fuel ratio set by the target air-fuel ratio setting means and the target air-fuel ratio corrected by the target air-fuel ratio correction means is updated by the updating means. When the deviation is smaller than the updated deviation, the amount of correction of the target air-fuel ratio to the lean side by the target air-fuel ratio correction means is made smaller than when the deviation is larger than the updated deviation. NOx generation amount can be minimized without impairing the operability of the vehicle.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an internal combustion engine and a control device thereof according to an embodiment of the present invention.

FIG. 2 is a flowchart of a process for calculating an engine rotation fluctuation amount DMSSLB.

FIG. 3 is a diagram for explaining a relationship between measurement of a parameter representing a rotation speed of an engine and a rotation speed of a crankshaft.

FIG. 4 shows a target air-fuel ratio coefficient KCMD and an air-fuel ratio correction coefficient K
It is a flowchart of the main flowchart of LAF calculation processing.

FIG. 5 is a flowchart of a LAF feedback area determination process.

FIG. 6 is a flowchart of a program for calculating the target air-fuel ratio coefficient KCMD in step S412 in FIG.

FIG. 7 is a continuation of the flowchart of FIG. 6;

[8] the target air-fuel ratio coefficient in step S626 of FIG. 6
This is a program for calculating a numerical reference value KBSM.

FIG. 9 is a flowchart of a process for calculating a reference value KBSM of a target air-fuel ratio coefficient in step S841 of FIG. 8;

FIG. 10 is a flowchart of a process of calculating a reference value KBSM of a target air-fuel ratio coefficient in step S907 of FIG. 9;

11 is a flowchart of a process for determining a lean burn control execution permission flag FKBSMJG in step S1002 in FIG.

FIG. 12 is a map for determining an intake pipe absolute pressure PBKBS and a lean burn control execution permission flag FKBSMJG.

FIG. 13 is a flowchart of a program for performing a KCMD value limit process in step S638 of FIG. 7;

FIG. 14 is a flowchart that follows the flowchart of FIG. 13;

FIG. 15 is a flowchart of a process for determining a combustion instability monitoring determination flag FSLBFB in step S1311 of FIG. 13;

FIG. 16 shows DMSSLB in step S1356 of FIG.
It is a flowchart of a feedback process.

FIG. 17 shows an average value of rotation fluctuation DMSBAVE.
It is a flowchart of a calculation process of.

FIG. 18 is an explanatory diagram of a KCMD correction process corresponding to DMSSLB.

FIG. 19A is a diagram illustrating a change in the air-fuel ratio due to lean burn control in a conventional air-fuel ratio control device for an internal combustion engine, and FIG. 19B is a diagram illustrating the air-fuel ratio of the internal combustion engine according to the embodiment of the present invention; FIG. 6 is an explanatory diagram of a change in an air-fuel ratio due to lean burn control in the fuel ratio control device.

[Explanation of symbols]

 Reference Signs List 1 internal combustion engine 5 electronic control unit 6 fuel injection valve 15 exhaust concentration sensor

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor: Yuichiro Tanabe 1-4-1, Chuo, Wako-shi, Saitama Pref.

Claims (2)

(57) [Claims] 1. An air-fuel ratio detecting means which is attached to an exhaust system of an internal combustion engine and outputs a value proportional to an air-fuel ratio in exhaust gas, and sets an air-fuel ratio to a predetermined target air-fuel ratio based on an output of the air-fuel ratio detecting means. Feedback control means for performing feedback control; target air-fuel ratio setting means for setting a target air-fuel ratio of the feedback control to an air-fuel ratio leaner than a stoichiometric air-fuel ratio when an operation state of the internal combustion engine is in a predetermined operation state; A combustion fluctuation amount detecting means for detecting a combustion fluctuation amount of the engine; and, if the combustion fluctuation amount detected by the combustion fluctuation amount detecting means is larger than a first predetermined value, the target air-fuel ratio is corrected to a rich side to correct the first air-fuel ratio. An air-fuel ratio control device for an internal combustion engine having target air-fuel ratio correction means for correcting the target air-fuel ratio to a lean side when the second air-fuel ratio is smaller than a second predetermined value that is not more than a predetermined value. Target air-fuel ratio and the set target air-fuel ratio of the actual combustion limit corresponds in the lean-burn controlled by the deviation between the target air-fuel ratio is corrected to the target air-fuel ratio the target air-fuel ratio set by the setting means by the target air-fuel ratio correction means characterized in that it has and the update means for updating the deviation, and a correction control means for controlling the amount of correction to the lean side of the target air-fuel ratio according to the target air-fuel ratio correction means based on the updated deviation by said updating means An air-fuel ratio control device for an internal combustion engine. 2. The correction control means according to claim 1, wherein the deviation between the target air-fuel ratio set by said target air-fuel ratio setting means and the target air-fuel ratio corrected by said target air-fuel ratio correction means is updated by said update means. 2. The internal combustion engine according to claim 1, wherein, when the difference is smaller, the correction amount of the target air-fuel ratio toward the lean side by the target air-fuel ratio correction means is smaller than when the difference is larger than the updated difference. Air-fuel ratio control device.