JPH06264785A - Air-fuel ratio control device of lean-burn engine - Google Patents

Abstract

PURPOSE:To restrain discharge of NOx during the time of changing an air-fuel ratio in most of an intake air flow rate region by changing changeover speed of the air-fuel ratio in accordance with a result of comparison between a computation result of a supplementary air flow rate and the maximum flow rate of a control valve. CONSTITUTION:Change-over speed of an air-fuel ratio at which it is changed over to a target air fuel ratio is enumerated by an enumeration means 33, and from this changeover speed and a drive condition signal, an enumeration means 34 enumerates a basic injection amount. In the meantime, an enumeration means 38 enumerates an increasing amount of the supplementary air flow rate by way of using the changeover speed of the air-fuel ratio and the drive condition signal so that torque becomes same before and after changing over at the time of changing over the target air-fuel ratio, and in accordance with the increasing amount of this supplementary air flow rate, a driving means 39 drives an air control valve. In this case, the changeover speed of the air-fuel ratio is set high until the supplementary air flow rate exceeds the maximum flow rate of the air control valve and low when it exceeds the maximum flow rate by a set setting means 41 in accordance with a judgement result of a judgement means 40.

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 system for a lean burn engine.

[0002]

2. Description of the Related Art The following conditions are used to control fuel supplied to an engine: <a> Cooling water temperature is 80 ° C. or higher, <b> Throttle valve opening is a predetermined value or less, <c> Vehicle speed change is a predetermined value or less When all of the above conditions are satisfied, feedback correction to the stoichiometric air-fuel ratio is stopped and the air-fuel ratio is switched to the target air-fuel ratio on the lean side (see Japanese Patent Laid-Open No. 63-50641). With the lean air-fuel ratio, although the same torque as the theoretical air-fuel ratio is generated, the air flow rate increases and the pumping loss decreases, and the specific heat ratio of the combustion gas increases. It will improve.

In this system, the auxiliary air flow rate is increased at the time of switching to the lean air-fuel ratio so that the torque does not change before and after the switching of the air-fuel ratio, so that the auxiliary air passage bypassing the intake throttle valve 5 as shown in FIG. 21 is provided with a flow rate control valve 22 capable of duty control.
Then, the on-duty Bα0 given to the control valve, Bα0 = AAC _TW + AAC _ABV + AAC _FB , where AAC _TW ; basic characteristic value AAC _ABV according to the cooling water temperature; air increase amount during deceleration AAC _FB ; feedback correction amount Flow path area of control valve 22 ABα0
Further, the air correction rate KBA is calculated from the target air-fuel ratio KMR by referring to the table having the content of FIG. 32, and the required correction area ABα1 of the control valve 22 is calculated from these values as follows: ABα1 = ABα0 + (Aα + ABα0) × KBA ... It is calculated from the flow area of the throttle valve.

Now, with the throttle valve opening equal, the lean side target air-fuel ratio (for example, 2) is changed from the theoretical air-fuel ratio (approximately 15).
When it is switched to 0) (for simplicity, the water temperature before and after switching is almost the same), the difference between the air correction factors before and after switching is multiplied by the total flow passage area (Aα + ABα0). The control valve 22 is opened excessively to prevent the torque from decreasing when the air-fuel ratio is lean.

On the other hand, in the injector 23 located immediately upstream of the throttle valve 5, Qcyl = Qa / N ... Tp = Qcyl / KMR ... where Qa: intake air flow rate measured by an air flow meter N: engine speed Qcyl Intake air flow rate per unit rotation KMR; Basic injection pulse width Tp equivalent to cylinder intake is calculated from the target air-fuel ratio, and from this Ti = Tp × α × K _ACC + Ts ... where α; Air-fuel ratio feedback correction coefficient K _ACC ; The fuel injection pulse width Ti calculated by the transient correction coefficient Ts; invalid pulse width is given.

[0006] Here, the target air-fuel ratio KMR _{expression, KMR = K TW × (NKMR} × n + KMR × (1-n)) ... However, K _TW; water temperature increase correction coefficient n; time constant equivalent value (less than 1 Value) NKMR; calculated by the map value of the target air-fuel ratio obtained from N and Tp, and when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio, the target air-fuel ratio KMR changes with a primary delay as shown in FIG. The purpose of providing the target air-fuel ratio with a first-order delay is to smooth the change in the air-fuel ratio during switching and reduce the torque shock during switching.

However, when the actual air-fuel ratio is switched to the lean air-fuel ratio, the actual air-fuel ratio is further delayed from the target air-fuel ratio KMR indicated by the dashed line in FIG.
When the required correction area ABα1 is calculated from the equation,
The air amount increases too fast, and the difference causes a temporary torque increase before and after the switching. This is because even if the auxiliary air flow rate is increased in response to the target air-fuel ratio with good response, the wall-flow fuel amount does not immediately decrease to the equilibrium value. This is because a mixture richer than the target will be inhaled until the value is settled.

[0008] In order to have a even response delay the increase of the auxiliary air flow so that the phase of the wall flow amount of fuel and the auxiliary air flow supplied fit, | NKBA-KBA _OLD | a criterion L
Comparing _{H, | NKBA-KBA OLD |} > KBA = NKBA × n1 + KBA OLD × (1-n1) when the LH ... However, NKBA; map value of the air correction factor obtained from the KMR KBA _OLD; previous KBA n1 ; the time constant equivalent value (a value smaller than 1), also | NKBA-KBA _OLD | becomes a ≦ LH, KBA = NKBA × n2 + KBA OLD × (1-n2) ... However, n2; time constant equivalent value (n2 <n1 ) Is used to obtain the air correction factor KBA.

When the target air-fuel ratio KMR suddenly changes, the change of the actual air-fuel ratio does not form a delay waveform with a constant time constant, but the change is fast in the initial stage and slow in the latter period. NKBA-KBA _OLD |, depending on the value) by changing the degree of delay, it is possible to suppress the torque step before and after switching as a dashed line in FIG. 33 into smaller ones.

[0010]

By the way, in order to suppress NOx which is frequently generated during the switching of the air-fuel ratio, the switching of the air-fuel ratio must be speeded up. Theoretical air-fuel ratio (≒ 1
In 4.5), NOx emissions are suppressed by the three-way catalyst,
Also, when the lean air-fuel ratio (for example, 22) is obtained, the N
This is because the amount of Ox discharged is small, so that the largest amount of NOx is produced at the air-fuel ratio during switching, such as 17-18.

However, when switching to the target air-fuel ratio on the lean side is performed in a region where the intake air flow rate is high, even if the differential pressure across the control valve 22 is small or the auxiliary air flow rate is increased, Since the amount cannot be increased above the maximum flow rate of the control valve 22, even if the switching speed of the target air-fuel ratio is made high, a torque step actually occurs due to insufficient increase of the auxiliary air flow rate.

In order to eliminate the influence on the drivability due to this torque step, it suffices to slow down the air-fuel ratio switching speed. However, if this happens, NOx emission during the air-fuel ratio switching cannot be suppressed.

Although it is conceivable to increase the maximum flow rate of the control valve 22 in order to speed up the switching of the air-fuel ratio,
When the control valve flow rate is set to a large value, on the other hand, when the control valve 22 is commonly used for the idle speed control, it becomes difficult to finely adjust the control valve flow rate in the idle speed control. Therefore, the control valve flow rate is simply increased. Doing so is not an appropriate measure.

Therefore, according to the present invention, most of the intake air is increased by increasing the switching speed of the air-fuel ratio until the calculation result of the auxiliary air flow rate exceeds the maximum flow rate of the control valve, and slowing the switching speed when it exceeds the maximum flow rate. The purpose is to suppress the emission of NOx during the switching of the air-fuel ratio in the flow rate range.

[0015]

The first invention, as shown in FIG. 1, is a means 31 for judging whether or not the operating condition signal is a lean condition, and a lean condition according to this condition according to the result of this judgment. Means 32 for calculating the target air-fuel ratio according to the operating condition signal when the target air-fuel ratio is other than the lean condition and the target air-fuel ratio according to the condition other than the lean condition. Means 33 for calculating the switching speed, means 34 for calculating the basic injection amount from the switching speed of the target air-fuel ratio and the operating condition signal, and a device 35 for supplying the fuel calculated based on the basic injection amount to the intake pipe. And an air control valve 37 for adjusting an auxiliary air flow rate that bypasses the intake throttle valve 36, and a target air-fuel ratio when switching the target air-fuel ratio (that is, a target air-fuel ratio according to a lean condition and a condition other than a lean condition). Means 38 for calculating the increase amount of the auxiliary air flow rate by using the switching speed of the target air-fuel ratio and the operating condition signal so that the torque becomes the same before and after the switching, and the auxiliary air flow rate A means 39 for driving the air control valve 37 in accordance with the amount of increase, a means 40 for determining whether the auxiliary air flow rate exceeds the maximum flow rate of the air control valve 37, and an auxiliary air flow rate based on the result of the determination. Means 41 are provided for setting the switching speed of the target air-fuel ratio fast until the maximum flow rate of the control valve 37 is exceeded, and slowing the switching speed of the target air-fuel ratio when the maximum flow rate is exceeded.

The second aspect of the present invention, as shown in FIG. 34, means 3 for determining whether or not the operating condition signal is a lean condition.
1 and means for calculating a target air-fuel ratio corresponding to this condition under the lean condition and a target air-fuel ratio corresponding to the condition other than the lean condition according to the operating condition signal from the result of this determination 32, means 33 for calculating the switching speed of the air-fuel ratio until switching to the target air-fuel ratio, means 34 for calculating the basic injection amount from the switching speed of the target air-fuel ratio and the operating condition signal, and the basic injection amount. A device 35 for supplying the fuel calculated on the basis to the intake pipe, an air control valve 51 that throttles the intake air according to the control amount without mechanically interlocking with the accelerator pedal, and before and after the switching when the target air-fuel ratio is switched. Means 52 for calculating the amount of change in the intake air flow rate using the target air-fuel ratio switching speed and the operating condition signal so that the torque is the same, and the air control according to the amount of change in the intake air flow rate. A means 53 for driving 51, a means 54 for judging whether the intake air flow rate exceeds the maximum flow rate of the air control valve 51, and a result of this judgment that the intake air flow rate exceeds the maximum flow rate of the air control valve 51. Up to the target air-fuel ratio, and means 55 for setting the target air-fuel ratio switching speed slower when the maximum flow rate is exceeded.

[0017]

[Operation] When switching to the target air-fuel ratio on the lean side is performed in the high intake air flow rate region, the differential pressure across the air control valve is small, and even if it is desired to increase the auxiliary air flow rate, the maximum air control valve Since the amount cannot be increased more than the flow rate, even if the switching speed of the target air-fuel ratio is uniformly increased, a torque step may actually occur due to insufficient increase of the auxiliary air flow rate.

On the other hand, in the first invention, it is judged whether or not the auxiliary air flow rate exceeds the maximum flow rate of the air control valve.
If the target air-fuel ratio switching speed is slowed only when the maximum flow rate is exceeded and the target air-fuel ratio switching speed is increased at other times, even when the air-fuel ratio is switched in the high intake air flow rate range, the target air-fuel ratio will continue to be at a high speed halfway. Will be switched, which will shorten the time in the air-fuel ratio during the switching, and NOx
Emissions of are reduced. The second aspect of the present invention is provided with an air control valve that is not mechanically interlocked with the accelerator pedal and is provided with an air control valve that throttles intake air according to a control amount, instead of the intake throttle valve mechanically interlocked with the accelerator pedal. As in the first aspect of the invention, NOx emission during the switching of the target air-fuel ratio can be suppressed in this aspect of the invention as well.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 2, the air sucked from an air cleaner 11 is temporarily stored in a collector portion 12a having a constant volume and then flows into a cylinder of each cylinder through a branch pipe. An injector 3 is provided in the intake port 12b of each cylinder, and fuel is intermittently injected from the injector 3 in synchronization with engine rotation.

When a certain condition is satisfied, the target value of the air-fuel ratio is switched from the stoichiometric air-fuel ratio to the lean side air-fuel ratio. At this switching, the auxiliary air flow rate is increased and corrected (when changing to the stoichiometric air-fuel ratio, the correction is reduced. By doing so, the torque control is performed so that the torque becomes the same before and after the switching.

Therefore, the auxiliary air passage 21 bypassing the intake throttle valve 5 is provided with the flow control valve 22 having a relatively large flow rate. The control valve 22 is of a proportional solenoid type, and as the on-duty (ON time ratio of a constant cycle) from the control unit 2 increases, the flow rate of auxiliary air flowing through the passage 21 increases.

The reason why the control valve 22 has a relatively large flow rate is to allow the torque control at the time of switching the fuel-air ratio to be performed with certainty and reliably. However, when the control valve flow rate is increased, a torque more than the driver's request may be generated due to a malfunction of the control valve 22, so a fail safe function is provided as described later. The control valve 22 is commonly used for idle speed control as well.

In order to suppress CO and HC that increase due to unstable combustion in the lean air-fuel ratio range, the intake port 12b is provided so that swirl is given to the intake air flowing into the combustion chamber.
A swirl control valve 13 having a notch (not shown) in a part is provided in the vicinity of. In the lean air-fuel ratio region, the swirl control valve 13 is fully closed to throttle the intake air to increase the flow velocity of the intake air and generate swirl in the combustion chamber. Exhaust pipe 1 in the stoichiometric air-fuel ratio range
NOx is purified by the three-way catalyst 19 provided in No. 8.

In order to control the amount of fuel supplied from the injector 3 and the flow rate of auxiliary air flowing through the flow rate control valve 22, the control unit 2 receives signals from various sensors for detecting engine operating conditions necessary for control. Has been entered. Reference numeral 4 is a hot-wire type air flow meter for detecting the flow rate of air taken in from the air cleaner 11, 6 is a throttle sensor for detecting the opening of the intake throttle valve 5, 7 is a signal for each unit crank angle and ref signal (reference of crank angle). A signal for each position), 8 is a water temperature sensor, and 9 is a wide range air-fuel ratio sensor capable of widely detecting the actual air-fuel ratio from the stoichiometric air-fuel ratio to the lean side air-fuel ratio.

By the way, when the lean side target air-fuel ratio is switched in the high intake air flow rate region, the control valve 22 may have a small differential pressure across the control valve 22 or the auxiliary air flow rate may be increased. Since it cannot be increased beyond the maximum flow rate of
Even if the switching speed of the target air-fuel ratio is increased, a torque step actually occurs due to insufficient increase of the auxiliary air flow rate. In order to eliminate the influence on the drivability due to this torque step, it suffices to slow down the air-fuel ratio switching speed.
It is not possible to suppress the emission of NOx during the switching of the air-fuel ratio.

In order to deal with this, in this example, the switching speed of the target air-fuel ratio is changed until the calculation result of the control valve passage area which determines the increase amount of the auxiliary air flow rate exceeds the control valve passage area at the maximum flow rate. The target air-fuel ratio switching speed is set slower if the control valve flow passage area at the maximum flow rate is exceeded.
Before explaining this, the overall control will be performed by using the flowcharts shown in FIGS. 3 to 29 and the characteristic diagram showing the contents of the tables and maps used for this control.
Flow control, <2> target air-fuel ratio setting, and <3> injection amount control.

Considering that the fuel control is performed aiming at the target air-fuel ratio, and finally the supplied fuel amount is obtained from the detected value of the air flow rate, (air flow rate) × (fuel air ratio) = (supply The fuel-air ratio is easier to handle than the air-fuel ratio because the relationship of the fuel amount) is established. Therefore, the fuel-air ratio is used for some numerical values below.

<1> Flow Control of Control Valve 22 <1-1> Relationship with Idle Speed Control As shown in FIG. 15, the torque control duty Tcvdty is set separately from the on-duty ISCONP for idle speed control. Calculate in advance (Step 7 in FIG. 15)
2) When the closed condition of the idle speed control is no longer satisfied, the torque control duty Tcv is replaced with ISCONP.
dty is output (steps 73 and 74 in FIG. 15).

Here, the on-duty ISCONP for idle speed control is, for example, ISCONP = Areg + ISCi + ISCp + ISCtr + ISCat + ISCa + ISCrfn (1) where Areg: warm-up duty (corresponding to an air regulator) ISCi; idle feedback correction integral component ISCp; idle feedback Differentiation of correction ISCtr; Air increase during deceleration (equivalent to dashpot) ISCat; A / T vehicle N ← → D range correction (large in D range) ISCa; Correction when air conditioner is on ISCrfn; Radiator fan on It is calculated by the correction amount of (step 71 in FIG. 15).

Warm-up duty Ar of formula (1)
For the first time after the engine is started, the value of “eg” is used by directly inserting the value (table value) obtained from the table reference according to the cooling water temperature at that time into Areg as a variable, and thereafter, at a constant cycle (eg, every 1 second). While comparing the table value according to the cooling water temperature with the previous value, increase / decrease the Areg value (for example, table value> previous Areg
g = Areg + 1, table value <Ar at previous Areg
eg = Areg-1) A value that works until the warm-up is completed. For this reason, no air regulator is provided in the intake pipe. A
Values other than reg are the same as conventional ones.

<1-2> Torque Control Duty Tcvdty This is calculated by a subroutine as shown in FIGS.

First, referring to the table having the contents shown in FIG. 18 from the throttle valve opening TVO, the throttle valve passage area Atvo is referred to, and from the basic duty Iscdt given to the control valve 22, the table having the contents shown in FIG. Control valve channel area A
isc0 is obtained, and the sum of these is entered into the variable Aa0 as the basic flow path area (steps 81 and 82 in FIG. 16). It should be noted that since all table references (also for map references) have interpolation calculation, they are simply referred to as table references (map references) below.

Here, the basic duty Iscdt is Iscdt = (Iscdty−Tcvofs) × Tcvgin (2) where Isddty: reduction basic duty Tcvofs; control valve rising duty Tcvgin; duty correction rate.

Reduction basic duty Iscdt of equation (2)
y is the feedback correction amount ISCcl (= ISCi + I) held at the end of the previous feedback correction condition.
SCp) is subjected to weight reduction correction, and Iscdty = ISCcl × Gistv (21) where Gistv: weight reduction correction rate (value of 0 or more and 1 or less). This reduction correction reduces the maximum flow rate of the control valve 22 by expanding the range in which the control valve 22 can be moved for torque control when switching the air-fuel ratio, and is a feedback correction that performs a minute flow rate control aiming at a target value. This is to prevent the valve precision from being reduced under the conditions.

Control valve rising duty Tc of equation (2)
The vofs and the duty correction rate Tcvgin are corrections when the battery voltage drops and will be described later.

From the basic flow passage area Aa0, the equilibrium required area Tatcvh of the control valve is expressed as Tatcvh = Aa0 × Kqh0 × (1 / (Dml × LTCGIN #)-1) (3) where Kqh0; differential pressure correction factor LTCGIN # Torque control gain Dml; target fuel-air ratio ramp response value (step 84 in FIG. 16).

In order to make the expression (3) easy to understand, Tatcvh = Aa0 × (1 / Tdml-1) (3-1) However, if Tdml is a map value of the target fuel-air ratio, this expression is a conventional example. It is an equation similar to the second term of the equation.

In the equation (3-1), (1 / Tdml-
Since 1) corresponds to the air-fuel ratio difference from the theoretical air-fuel ratio, the value obtained by multiplying it by Aa0 as the total flow passage area represents the increased area (the decreased area when switching to the theoretical air-fuel ratio). Of.

For example, at the theoretical air-fuel ratio (14.5), the target fuel-air ratio map value Tdml is 1, and the air-fuel ratio is 20 on the lean side.
Therefore, Tdml has a value of about 0.66. In addition, T
The value of dml is 1 or 0.66, because the fuel-air ratio (target fuel-air ratio) is not the reciprocal of the air-fuel ratio itself, but a relative value with the theoretical fuel-air ratio being 1, as will be described later. This is because

Here, when 1 is put in Tdml of the equation (3-1), Tatcvh = 0, and when 0.66 is put in Tdml, Tatcvh = 1 / 0.66-1≈0.52, and (0. 52-0) × Aa0 is the increased area when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio.

Returning to equation (3), the differential pressure correction factor Kqh0 is
Even with the same flow path area, the higher the load, the more throttle valve 5 and control valve 2
This is a correction for making the flow rate the same even when the differential pressure between the front and rear is different because the differential pressure between the front and rear becomes smaller and the flow rate becomes smaller.
Therefore, the difference from Qh0 (which is a known linearized flow rate, which is determined from the throttle valve opening TVO, the engine speed N, and the cylinder volume V) as a load is referred to by referring to the table having the content of FIG. Pressure correction factor Kqh
0 is obtained (step 83 in FIG. 16). The torque control gain LTCGIN # in the equation (3) is a value required for matching.

The upper limit of the equilibrium required area Tatcvh is a value (TC) obtained by subtracting the control valve passage area Aisc0 from the passage area TCVMAX # at the maximum flow rate of the control valve 22.
VMAX # -Aisc0) (step 85 in FIG. 16). This is because the value of Aisc0 is a value that has already been used in idle speed control, and the remainder after subtracting this is the range in which the control valve 22 can be moved for torque control during air-fuel ratio switching.

Tatcvh> TCVMAX # -Aisc
When it reaches 0 (that is, when the upper limit is reached), FA
ACOF = 1 is set (steps 86 and 88 in FIG. 16).
This flag FAACOF is a flag for varying the target air-fuel ratio switching speed, and will be described later.

Tatcv0 = Tatcvo + (Tatcvh-Tatcvo) * Tcvtvc (4) where Tatcvo; previous value of Tatcvh; advance compensation time constant corresponding value (value of 1 or more) Then, the compensation area is calculated (step 91 in FIG. 16). When the volume of the intake pipe downstream of the control valve 22 in the MPI system is large, the delay of the air in the intake pipe is relatively larger than the delay of the fuel, so that the air can be advanced in accordance with the fuel with good response. , The phases of the air flow rate and the fuel supply to the cylinder are matched.

When the volume of the intake pipe downstream of the control valve is small in the SPI system, the fuel flows into the cylinder later than the air, so that the flow of air into the cylinder is delayed in accordance with the fuel, so that Tatcv0 = Tatcv0 _n-1 + (Tatcvh-Tatcv0 _n-1 ) × Tcvtc (5) where Tatcv0 _n-1 ; previous value of Tatcv0 Tcvtc; delay compensation time constant equivalent value (value less than 1) By calculating the delay compensation area (step 92 in FIG. 16), the phases of air and fuel supply to the cylinders are matched.

The flow charts of FIGS. 16 and 17 are based on MP
In order to be able to use both of the two types of the engine, the intake system having a large intake pipe volume and the SPI system having a small intake pipe volume, whether Tcvtc ≧ 1.0, Tcvtc ≧ 1. When it is 0, it is judged that the engine has a large intake pipe volume, and the above equation (4) is changed to
If Tcvtc <1.0, the equation (5) is adopted (steps 90, 91 and 9 in FIG. 16).
0.92).

The lead compensation or delay compensation time constant equivalent value Tcvtc in the equations (4) and (5) is obtained from the engine speed N by referring to the table having the contents shown in FIG. 21 (step 89 in FIG. 16). Although FIG. 21 shows characteristics for both a large intake pipe volume and a small intake pipe volume, the engine shown in FIG. 2 does not require the characteristics for a small intake pipe volume. Lead compensation or delay compensation area Tat obtained in this way
The required correction area Tatcv is calculated from cv0 by Tatcv = Tatcv0 _n-DLYIS + Aisc0 + Aokuri (6) where Aokuri; advance feed amount (step 93 in FIG. 17).

Here, the required correction area Tatcv has a lower limit of 0 and an upper limit of the flow path area TCVMAX # at the maximum flow rate of the control valve.
And a value added with MXOS #, which is an allowance for compensation (TC
VMAX # + MXOS #). Due to this limitation, these limit values are required correction area T of equation (6).
atcv may overflow. In order to reflect this protruding amount to the next time (after 10 ms), if Tatcv <0, the value of the underflowed Tatcv is put in the variable Aokuri as the advance amount (step 94 in FIG. 17,
95), also when Tatcv> TCVMAX # + MXOS #, the overflow value of Tatcv- (TCVMAX # + MXOS #) is put in the variable Aokuri as the advance amount (steps 96 and 97 in FIG. 17). This Aoku
Next, the value of ri will be calculated as the required correction area Ta by using the equation (6).
It is used when calculating tcv.

Tatcv0 _{n -DLYIS in the} equation (6) is a value before the lead-compensation or delay-compensation area Tatcv0 by a predetermined number (for example, DLYIS #). This takes into account the dead time from when the valve opening signal is sent to the injector 3 until the injector 3 actually starts opening.

The required correction area Tatcv is converted into the on-duty Dtytc by referring to the table having the contents of FIG. 22 (step 99 of FIG. 17), and the torque control duty Tcvdty is Tcvdty = Dtytc / Tcvgin + Tcvofs (7) where Tcvgin ; Duty correction factor Tcvofs; Calculated by the control valve rising duty (steps 100 and 101 in FIG. 17).
This equation is equivalent to the equation (2) obtained for Isddty.

Control valve rising duty Tc of equation (7)
Until the on-duty reaches a certain value, vofs is the amount by which the control valve 22 does not substantially operate as shown in FIG. 25, and is obtained from the battery voltage Vb by referring to the table having the content of FIG. As shown in FIG. 25, in the proportional solenoid type control valve 22, the control valve rising duty Tcvofs increases as the battery voltage Vb decreases.

The duty correction rate Tcvgin is shown in FIG.
The table is obtained by referring to the table. This is shown in Figure 2.
In the flow rate characteristic of the control valve 22 of No. 5, since the slope of the straight line that rises diagonally becomes smaller as the battery voltage Vb decreases, this is a correction for making the control valve flow rate the same even if the battery voltage Vb decreases.

<2> Setting of target fuel-air ratio The target fuel-air ratio is map value Tdml → lamp response value Dml →
The damper value Kmr is calculated in this order.

<2-1> Target Fuel-Air Ratio Map Value Tdml As shown in FIG. 6, under lean conditions, the target fuel-air ratio MDMLL is referred to by referring to the lean map having the contents of FIG.
If it is not a lean condition, the target fuel-air ratio MDMLS is obtained by referring to the non-lean map of FIG. Place in Tdml (step 34 in FIG. 6).

Here, the MDM that becomes the target fuel-air ratio map value
The values of LL and MDMLS are not the values of the fuel-air ratio itself as shown in FIGS. 7 and 8, but the relative values that make the theoretical fuel-air ratio 1.0.

<2-2> Ramp Response Value Dml of Target Fuel-Air Ratio As the name implies, the waveform of the ramp response value Dml is, as shown in FIG. 10, the ramp response (speed) with respect to the step change map value Tdml. If the change rate of the fuel-air ratio in the lean direction is Dmll and the change rate of the fuel-air ratio in the rich direction is Dmlr, as shown in FIG. 9, Dmlold (Previous Dml) and T
Since it is possible to know in which direction the change is due to the comparison of dml, if Dmlold <Tdml, it is determined that the change to the rich direction is made, and Tdml and (Dmlold + D
Put the smaller one of mlr) into Dml, and vice versa.
When old ≧ Tdml, it is assumed that the switching is in the lean direction, and the larger of Tdml and (Dmlold-Dmll) is put into Dml (steps 44 and 4 in FIG. 9).
5, steps 44 and 46), the ramp response value (speed variable response value) is obtained. Considering NOx, the degree of activity of the catalyst is better at the time of switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, so that Dmll can be made smaller than Dmlr.

On the other hand, if any of the following conditions <a> start switch is ON <b> Tdml ≧ upper limit value TDMLR # is satisfied, Dml = Tdml (steps 41 and 42 in FIG. 9). , 43).

The lamp response value Dml is obtained in synchronization with the engine rotation (in synchronization with the ref signal). The exhaust performance is determined in synchronization with the engine rotation because the exhaust performance changes in synchronization with the engine rotation.

<2-3> Damper value Kmr of target fuel-air ratio As shown in FIG. 12, for the lamp response value Dml, Dmlo = Dml × Fbyatc + Dmlon _-1 × (1-Fbyatc) (9) , Fbyatc; a first-order delay is added by a value corresponding to the delay time constant (value less than 1) (step 65 in FIG. 12).

This is because when the intake pipe volume downstream of the control valve is large in the MPI system, fuel is supplied by the formula (9) in accordance with the increase amount of the auxiliary air flow rate which has a delay until it reaches the cylinder from the control valve 22. By delaying (if the target air-fuel ratio is delayed by the formula (9), fuel supply will eventually be delayed), the phases of fuel and air supply to the cylinders will be matched when the air-fuel ratio is switched. Therefore,
The expression (9) is equivalent to the expression of the conventional example in that it is a smoothing process (the expression of the conventional example corresponds to FIGS. 9 and 10 of the present invention for the purpose of smoothing the torque change). .

As the damper value Dmlo of the equation (9), a value up to three times before is stored and stored a predetermined number of times before (eg, DLYF).
The value of #BA before) is put into the variable Kmr (step 66 in FIG. 12). The value before DLYFBA # is shown in FIG.
As shown in 4, delay of control valve 22 (dead time)
Is taken into consideration.

However, when any of the following conditions <a> the start switch is ON, <a>Dml> 1.0, <c> | ΔTVO | ≧ predetermined value DTVOTR # is satisfied. Does not perform delay processing (steps 61, 62, 63, 67 in FIG. 12).

Fbyatc corresponding to the delay time constant of equation (9)
Is obtained by referring to the large intake pipe volume table in the table having the contents shown in FIG. 13 (step 64 in FIG. 12). The delay in the increase / decrease in the air flow rate at the time of switching the air-fuel ratio increases as the engine speed N decreases.
The value of Fbyatc is set as shown in 3 (only for a low engine speed range for an engine with a small intake pipe volume).

FIG. 13 also shows the characteristics for small intake pipe volume downstream of the control valve in a superimposed manner. In an engine of the SPI system in which the intake pipe volume downstream of the control valve is small, air responds better than fuel. Since it is good, there is no need to delay the fuel, so Fbyatc = 1.0. That is, FIG.
1 becomes unnecessary according to the size of the intake pipe volume downstream of the control valve of the engine in which the table having the contents is actually mounted.
By deleting one of the characteristics of No. 3, the flowchart of FIG. 12 can be shared with the SPI method.

When the intake pipe volume downstream of the control valve in the MPI system is large, in order to make the supply phases of both the increased amount of the auxiliary air flow rate and the fuel amount match when switching the air-fuel ratio,
There are two examples of (a) delaying the fuel amount in accordance with the increase amount of the auxiliary air flow amount, or (b) advancing the increase amount of the auxiliary air flow amount in accordance with the fuel amount amount. The flowchart of FIG. 3 incorporates two embodiments (a total of four embodiments including the components for the SPI method). Therefore, since one of them is selected at the embodiment level, the case of FIG.
6. Steps 90, 91 and 92 of 6 must be deleted, and in the case of another embodiment for (b), steps 64 and 65 of FIG. 12 must be deleted.

<2-4> Target fuel-air ratio Tfbya This is as shown in FIG. 6. Tfbya = Kmr + Kas + Ktw (8) where Kmr: target fuel-air ratio damper value Kas; start-up increase correction coefficient Ktw; water temperature increase The correction coefficient is calculated (step 38 in FIG. 6).

Here, the post-starting amount increase correction coefficient Kas is a value that is determined according to the cooling water temperature during cranking, and gradually decreases with time immediately after the engine is started. The water temperature increase correction coefficient Ktw is calculated from the cooling water temperature in the table. Is a known value (steps 37 and 36 in FIG. 6), both of which are known.

<3> Injection amount control <3-1> Fail safe of control valve 22 As shown in FIG. 5, the output voltage Q of the air flow meter 4
a is A / D converted and then converted into air flow rate units by referring to a table (steps 21 and 22 in FIG. 5), but the air flow rate Q obtained by this conversion is limited to the upper limit value Fqmax (step 23 in FIG. 5). , 24). This is to prevent the malfunction of the control valve 22 having a relatively large flow rate, because the auxiliary air having a large flow rate flows into the cylinder and a torque larger than the driver's request is generated.

FIG. 26 is a flow chart for obtaining the upper limit value Fqmax. 26, the throttle valve passage area Fatvo is referred to the table having the contents of FIG. 27, and the predicted value Aisc of the control valve passage area when the control valve 22 operates normally is shown in FIG. Each table is obtained (step 111 in FIG. 26,
112).

In FIG. 27, Tvoabs on the horizontal axis is a value obtained by subtracting TVO when the throttle valve opening TVO is fully closed, or AO.
FST # is an offset amount for making the throttle valve opening TVO correspond to the actual air flow rate. The horizontal axis of Aacdt in FIG. 28
y is a value of either ISCONP (on-duty for idle speed control) or Tcvdty (torque control duty).

The total flow passage area (Fatvo + A) of the throttle valve flow passage area Fatvo and the predicted value Aisc of the control valve flow passage area
isc) is Pqmax = (Fatvo + Aisc) × KAQGIN # (10) where KAQGIN #; is converted into an air flow rate unit by a constant (step 11 in FIG. 26).
3). Pqmax in the equation (10) is a predicted value of the total intake air flow rate when it is assumed that the control valve 22 operates normally. From this predicted value Pqmax, the upper limit value Fqmax is obtained by Fqmax = Pqmax × Qmxg (11), where Qmxg; gain (step 115 in FIG. 26).

The gain Qmxg is the ratio (Q / Pqma) between the air flow rate Q obtained from the air flow meter 4 and the predicted value Pqmax.
x) is obtained by referring to the table having the contents of FIG. 29 (step 114 in FIG. 26). In FIG. 29, Q / P
The value of the gain Qmxg is constant in the area where qmax is small, but the gain Qmxg is large in the area where Q / Pqmax is large.
The value of mxg is increased. This is to prevent the lean misfire because, for example, when the control valve 22 is fully opened and stuck in a region where the throttle valve opening TVO is small, the fuel-air ratio excessively shifts to the lean side and lean misfire occurs and the rotation speed drops too much. is there. Since Q / Pqmax represents the degree of malfunction of the control valve 22, when this degree is large, the upper limit value Fqma with respect to the predicted value Pqmax is set so as to prevent lean misfire.
The ratio of x is increased.

<3-2> Basic Injection Pulse Width Tp Corresponding to Cylinder Intake In FIG. 5, when Q> Fqmax, the upper limit value Fqma
The basic injection pulse width Tp corresponding to the cylinder intake air is calculated from x (= Q), or Q as it is, by a known equation Tp0 = (Q / Ne) × KCONST # × Ktrm (12) Tp = Tp0 × Fload + Tp × (1−Fload) (13) However, Tp0; basic injection pulse width equivalent to the throttle valve section KCONST #; constant giving a base air-fuel ratio Ktrm; trimming coefficient Fload; intake pipe air delay coefficient (FIG. 5) Step 25).

The equation (13) takes into consideration the response delay of the air in the intake pipe at the time of transition (related to the change of the operating condition and not related to the switching of the air-fuel ratio).

<3-3> Fuel Injection Pulse Width Ti FIG. 3 is a flow chart for calculating the fuel injection pulse width Ti to the injector 3 provided in the intake port 12b, which is expressed as Ti = Tp × Tfbya × (α + αm ) × Ktr + Ts (14) However, α: air-fuel ratio feedback correction coefficient αm; air-fuel ratio learning control coefficient Ktr; transient correction coefficient Ts: calculated by the invalid pulse width according to the battery voltage (step 2 in FIG. 3), and Is output at the injection timing as shown in FIG. 4 (step 11 in FIG. 4).

The transient correction coefficient Ktr in the equation (14) corrects the transportation delay of the fuel in the intake pipe, and has the same value as the K _ACC in the conventional equation. For example, the initial value is a value that is determined according to | ΔTVO | at the time when the absolute value | ΔTVO | of the throttle valve opening change amount exceeds a predetermined value (that is, when acceleration or deceleration is determined), and decreases with time. .

This concludes the outline. Now, when switching to the lean air-fuel ratio is performed by moving the control valve 22 and increasing the auxiliary air flow rate, the faster the increasing amount of the auxiliary air flow rate, the faster the switching speed of the target air-fuel ratio. The maximum flow rate of 22 cannot be increased.

Therefore, by comparing the equilibrium required area Tatcvh of the control valve 22 which determines the increase amount of the auxiliary air flow rate with the control valve flow passage area TCVMAX # at the maximum flow rate, the auxiliary air flow rate is equal to the maximum flow rate of the control valve 22. You can know if you have exceeded it.

However, of the control valve passage area, the portion used for idle speed control (Aisc0) cannot be used for torque control when switching the air-fuel ratio, so the control valve passage at the maximum flow rate. If the area is TCVMAX #,
A value obtained by subtracting the control valve passage area Aisc0 from this (T
CVMAX # -Aisc0) is the equilibrium required area Tatcv
It becomes the upper limit value for h (step 85 in FIG. 16).

Therefore, Tatcvh ≦ TCVMAX #
-At Aisc0, FAACOF = 0, Tatcvh> TC
When VMAX # -Aisc0 is reached (that is, when the upper limit is reached), FAACOF = 1 is set (steps 86 and 87, steps 86 and 88 in FIG. 16). FAACOF
Is a flag for varying the speed of the ramp response value of the target air-fuel ratio.

Using the value of this flag FAACOF, as shown in FIG. 11, when FAACOF = 0, the predetermined values DDMLLH # and DDMLRH # having large values are set to the variable Dml.
l, put in Dmlr (steps 51 and 52 in FIG. 11),
When FAACOF = 1, DDMLLLH #, DDML
A value smaller than RH # is put in the variables Dmll and Dmlr.

As shown in FIG. 10, the variables Dmll and Dmlr determine the rate of change of the lamp response value Dml (the amount of change in the fuel-air ratio per control).
If a large value is entered in, the lamp response value Dml changes at a high speed when the air-fuel ratio is switched, and the variables Dmll, Dml
By setting a small value for r, the lamp response value Dml
Changes at a slow rate.

Further, when FAACOF = 1, the variable Dm
The values to be entered in 11 and Dmlr are not fixed values, and values having a magnitude corresponding to the absolute value | ΔTVO | of the throttle valve opening change amount are selected and entered (steps 51 and 53 in FIG. 11). For example, if one variable Dmll is represented, then | ΔTVO
| <DTVO1 #, the predetermined value DDMLL0 # is changed to DTVO
1 # ≦ | ΔTVO | <DTVO2 # and predetermined value DDMLL
1 # is DTVO2 # ≦ | ΔTVO | <DTVO3 # and a predetermined value DDMLL2 # is DTVO3 # ≦ | ΔTVO |
The predetermined value DDMLL3 # is selected with. However,
DDMLL0 # <DDMLL1 # <DDMLL2 # <D
DMLL3 # <DDMLLLH #.

Here, the operation of this example in the high intake air flow rate range will be described with reference to FIG. 30. The left half of the figure is the operation waveform at the time of switching to the lean air-fuel ratio, and the auxiliary air flow rate is increased. The equilibrium required area Tatcvh that determines the amount is the upper limit value (T
After reaching CVMAX # -Aisc0), the speed of the target fuel-air ratio ramp response value Dml is switched from a rapid value to a slow value. Even during the switching of the target air-fuel ratio, the rate of change of the lamp response value Dml is reduced only when the equilibrium required area Tatcvh reaches the upper limit value. Therefore, even when switching the air-fuel ratio in the high intake air flow rate range as shown in FIG. Since the target air-fuel ratio can be switched at high speed up to, the time during which the air-fuel ratio is in the middle of switching can be shortened, so that the NOx emission amount can be reduced.

Similarly, at the time of switching to the stoichiometric air-fuel ratio shown in the right half, the rate of change of the lamp response value Dml increases during the switching, thereby suppressing the NOx emission amount.

On the other hand, when the characteristic of NOx when the target air-fuel ratio is slowly switched from the beginning is shown in FIG. 30 by overlapping with a broken line, the time in the middle air-fuel ratio becomes long.
The amount of NOx emissions is increasing.

As described above, in this example, the equilibrium required area Tatcvh of the control valve 22 which determines the increase amount of the auxiliary air flow rate is directly observed, and only when it exceeds the upper limit value, the change speed of the target fuel-air ratio ramp response value Dml is slowed down. By doing so, the switching speed of the target air-fuel ratio can be increased in most intake air flow rate regions. In areas other than the high intake air flow rate range, the equilibrium required area T
In most cases, atcvh does not reach the upper limit value. At this time, the target fuel-air ratio ramp response value changes at a high speed until the end of the switching, so that NOx emission during the air-fuel ratio switching can be suppressed. .

Further, the rate of change of the lamp response value after the equilibrium required area Tatcvh reaches the upper limit value is not a constant speed, but a value corresponding to the absolute value | ΔTVO | Select and put. | ΔTVO | is a value for observing the degree of change in operating conditions, and during rapid acceleration / deceleration, even if there is a large change in torque when switching the target air-fuel ratio, it is less likely to be detected as a shock by the driver as a shock as in the steady state. By increasing the rate of change of the lamp response value Dml as the degree of acceleration / deceleration increases, NOx emission during the air-fuel ratio switching can be suppressed to the maximum while maintaining a balance with the drivability.

In the embodiment, the case where the control valve 22 is commonly used for idle speed control has been described. However, when the control valve 22 is not commonly used for idle speed control, the upper limit value (TCV
Instead of MAX # -Aisc0), the flow passage area TCVMAX # at the maximum flow rate of the control valve 22 will be compared. In the embodiment, the flow area of the control valve is compared (Tatcvh and TC
Although VMAX # -Aisc0 is compared), the flow rate of the control valve itself may be compared (control valve flow rate obtained from Tatcvh and control valve flow rate obtained from TCVMAX # -Aisc0).

In the embodiment, the case where the auxiliary air flow rate is increased has been described. However, the air control valve, which is not mechanically interlocked with the accelerator pedal and throttles intake air according to the control amount, is mechanically connected to the accelerator pedal. The present invention can also be applied to what is provided instead of the intake throttle valve that is interlocked with.

[0091]

According to the first aspect of the present invention, it is determined whether or not the operating condition signal is the lean condition. From the result of this determination, when the lean condition is set, the target air-fuel ratio corresponding to this condition is set, and when the condition other than the lean condition is set, The target air-fuel ratio according to conditions other than this lean condition is calculated according to the operating condition signal, the air-fuel ratio switching speed until switching to the target air-fuel ratio is calculated, and the target air-fuel ratio switching speed and operating condition signal are calculated. The basic injection amount is calculated from the above, and while the fuel calculated based on this basic injection amount is supplied to the intake pipe, an air control valve that adjusts the auxiliary air flow rate that bypasses the intake throttle valve is provided, and the target air-fuel ratio At the time of switching, the increase amount of the auxiliary air flow rate is calculated using the switching speed of the target air-fuel ratio and the operating condition signal so that the torque is the same before and after the switching, and the air control is performed according to the increase amount of the auxiliary air flow rate. It is determined whether or not the auxiliary air flow rate exceeds the maximum flow rate of the air control valve, and from the result of the determination, the switching speed of the target air-fuel ratio until the auxiliary air flow rate exceeds the maximum flow rate of the air control valve. Is configured to be fast, and the target air-fuel ratio switching speed is set to be slow when the maximum flow rate is exceeded. Therefore, the target air-fuel ratio switching speed can be increased in most intake air flow rate regions, and NOx emission during switching is suppressed. be able to.

A second aspect of the present invention judges whether the operating condition signal is a lean condition or not, and based on the result of this judgment, the target air-fuel ratio according to this condition is obtained under the lean condition, and when the condition other than the lean condition is reached, the lean air condition is reached. The target air-fuel ratio according to conditions other than the conditions is calculated according to the operating condition signal, the switching speed of the air-fuel ratio until switching to the target air-fuel ratio is calculated, and the basic speed is calculated from the target air-fuel ratio switching speed and the operating condition signal. The injection amount is calculated, and the fuel calculated based on this basic injection amount is supplied to the intake pipe, while an air control valve that throttles the intake according to the control amount without mechanically interlocking with the accelerator pedal is provided. When the target air-fuel ratio is switched, the amount of change in the intake air flow rate is calculated using the target air-fuel ratio switching speed and the operating condition signal so that the torque is the same before and after the change, and the amount of change in the intake air flow rate is calculated in accordance with this amount of change. While driving the air control valve, it is determined whether or not the intake air flow rate exceeds the maximum flow rate of the air control valve. From the result of this determination, the target is maintained until the intake air flow rate exceeds the maximum flow rate of the air control valve. Since the air-fuel ratio switching speed is fast, and the target air-fuel ratio switching speed is set to be slow when the maximum flow rate is exceeded,
The same effect as that of the first invention is obtained.

[Brief description of drawings]

FIG. 1 is a diagram corresponding to a claim of the first invention.

FIG. 2 is a system diagram of an embodiment.

FIG. 3 is a flow chart for explaining calculation of a fuel injection pulse width Ti.

FIG. 4 is a flow chart for explaining an output of a fuel injection pulse width Ti.

FIG. 5 is a flowchart for explaining calculation of a basic injection pulse width Tp equivalent to cylinder intake.

FIG. 6 is a flowchart for explaining calculation of a target fuel-air ratio Tfbya.

FIG. 7 is a characteristic diagram for explaining the contents of the lean map.

FIG. 8 is a characteristic diagram for explaining the content of a non-lean map.

FIG. 9 is a flow chart for explaining calculation of a ramp response value Dml of a target fuel-air ratio.

FIG. 10 is a waveform diagram of a lamp response value Dml of a target fuel-air ratio.

FIG. 11 is a flowchart for explaining calculation of change rates Ddmlr and Ddmll.

FIG. 12 is a flowchart for explaining calculation of a damper value Kmr of a target fuel-air ratio.

FIG. 13 is a delay time constant equivalent value Fbyat of two examples.
It is a characteristic view which piles up the table content of c.

FIG. 14 is a waveform diagram for explaining the dead time of the control valve 22.

FIG. 15 is a flowchart for explaining calculation of on-duty to the control valve 22.

FIG. 16 is a flowchart for explaining calculation of a torque control duty Tcvdty that is common to two embodiments other than the two embodiments.

FIG. 17 is a flowchart for explaining calculation of a torque control duty Tcvdty.

FIG. 18 is a characteristic diagram showing the table contents of the throttle valve flow passage area Atvo.

FIG. 19 is a characteristic diagram showing the table contents of the control valve passage area Aisc0.

FIG. 20 is a characteristic diagram showing the table contents of the differential pressure correction rate Kgh0.

FIG. 21 is a characteristic diagram showing the table contents of the delay-advance compensation time constant equivalent value Tcvtc of two examples other than the two examples in an overlapping manner.

FIG. 22 is a characteristic diagram showing table contents of a basic duty Dtytc.

FIG. 23 is a characteristic diagram showing the table contents of a duty correction rate Tcvgin.

FIG. 24 is a characteristic diagram showing the table contents of the control valve rising duty Tcvofs.

FIG. 25 is a flow rate characteristic diagram of the control valve 22.

FIG. 26 is a flowchart for explaining calculation of an upper limit value Fqmax.

FIG. 27 is a characteristic diagram showing the table contents of the throttle valve flow passage area Fatvo.

FIG. 28 is a characteristic diagram showing table contents of a predicted value Aisc of the control valve flow passage area.

FIG. 29 is a characteristic diagram showing table contents of a gain Qmxg.

FIG. 30 is a waveform diagram for explaining the operation when switching the target fuel-air ratio in the high intake air flow rate range.

FIG. 31 is a system diagram of a conventional example.

FIG. 32 is a characteristic diagram showing the table contents of the air correction factor KBA of the conventional example.

FIG. 33 is a waveform diagram for explaining the operation of the conventional example.

FIG. 34 is a diagram corresponding to the claim of the second invention.

[Explanation of symbols]

 2 Control unit 3 Injector (fuel supply device) 4 Air flow meter 5 Intake throttle valve 6 Throttle sensor 7 Crank angle sensor (rotation speed sensor) 12a Collector part 12b Intake port 21 Auxiliary air passage 22 Flow control valve (air control valve) 31 Lean Condition determination means 32 Target air-fuel ratio calculation means 33 Switching speed calculation means 34 Basic injection amount calculation means 35 Fuel supply device 36 Intake throttle valve 37 Air control valve 38 Auxiliary air flow rate increase amount calculation means 39 Driving means 40 Judging means 41 Switching speed setting Means 51 Air control valve 52 Intake air flow rate change amount calculating means 53 Driving means 54 Judging means 55 Switching speed setting means

Claims (2)

[Claims] 1. A means for determining whether or not an operating condition signal is a lean condition, and from the result of this determination, a target air-fuel ratio according to this condition is provided under the lean condition, and a condition other than the lean condition is provided under conditions other than the lean condition. Means for calculating the target air-fuel ratio according to the operating condition signal, means for calculating the switching speed of the air-fuel ratio until the target air-fuel ratio is switched, and the target air-fuel ratio switching speed and the operating condition signal Means for calculating the basic injection amount from the device, a device for supplying the fuel calculated based on this basic injection amount to the intake pipe, an air control valve for adjusting the auxiliary air flow rate bypassing the intake throttle valve, and the target air amount. A means for calculating the increase amount of the auxiliary air flow rate using the target air-fuel ratio switching speed and the operating condition signal so that the torque becomes the same before and after the change of the fuel ratio, and the auxiliary air flow amount increase amount. According to the above, a means for driving the air control valve, a means for determining whether the auxiliary air flow rate exceeds the maximum flow rate of the air control valve, and the auxiliary air flow rate from the determination result determines the maximum flow rate of the air control valve. An air-fuel ratio control device for a lean burn engine, comprising: means for setting the switching speed of the target air-fuel ratio fast until the speed exceeds the maximum flow rate and slowing the switching speed of the target air-fuel ratio when the maximum flow rate is exceeded. 2. A means for determining whether or not the operating condition signal is a lean condition, and from the result of this determination, in the lean condition, a target air-fuel ratio according to this condition is obtained, and in the case of a condition other than the lean condition, other than this lean condition. Means for calculating the target air-fuel ratio according to the operating condition signal, means for calculating the switching speed of the air-fuel ratio until the target air-fuel ratio is switched, and the target air-fuel ratio switching speed and the operating condition signal Means for calculating the basic injection amount from the device, a device for supplying fuel calculated based on this basic injection amount to the intake pipe, and air control that throttles the intake air according to the control amount without mechanically interlocking with the accelerator pedal A valve, a means for calculating the amount of change in the intake air flow rate using the target air-fuel ratio switching speed and the operating condition signal so that the torque will be the same before and after switching when the target air-fuel ratio is switched, Means for driving the air control valve in accordance with the amount of change in the incoming air flow rate, means for determining whether or not the intake air flow rate exceeds the maximum flow rate of the air control valve, and the intake air flow rate is determined from this determination result. A lean burn engine, characterized in that a means for setting the switching speed of the target air-fuel ratio high until the maximum flow rate of the air control valve is exceeded and setting the switching speed of the target air-fuel ratio slow when the maximum flow rate is exceeded is provided. Air-fuel ratio controller.