JPH06241080A - Air-fuel ratio control device of engine - Google Patents

Abstract

PURPOSE:To provide a means for a lean burn engine equipped with an electric throttle valve, whereby generation of torque shock at the time of switching the zone can be precluded effectively. CONSTITUTION:An engine CE includes a control unit C, which makes zone switching between rich and lean. Therein switching from the lean zone to the rich takes place when the intake negative pressure exceeds the specified boundary value, and the lean zone is widened as much as practicable while the fuel consumption performance and emission performance are enhanced, and zone switch is prevented from taking place near the region where the suction charging efficiency is saturated, and also torque shock at the time of switching the zone is precluded. The switch from the rich zone into the lean is conducted on the basis of the illustrated mean effective pressure, and cycling at switching is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine air-fuel ratio control system.

[0002]

2. Description of the Related Art Generally, in an ordinary spark ignition engine for an automobile, a mixture of fuel (for example, gasoline) and air is compressed by a piston in a combustion chamber and then ignited and burned by a spark plug. It is designed to be used. In such an engine, basically, the air-fuel ratio of the air-fuel mixture (air / fuel ratio A / F) can be arbitrarily set within the combustible range.

When the air-fuel ratio of the air-fuel mixture is changed, when the air-fuel mixture is rich, the engine output is high but the fuel efficiency is poor. On the other hand, when the air-fuel mixture is lean, the engine output decreases, but the fuel efficiency improves. Therefore, conventionally, the rich zone where the air-fuel ratio is set to be relatively rich (for example, the theoretical air-fuel ratio A / F = 14.7) and the air-fuel ratio is set to be relatively lean (for example, A / F = 19 to 24). Set the lean zone to be set, use the rich zone in the operating area where high output is required, use the lean zone in the operating area where high output is not required so much,
A so-called lean burn engine, which is designed to achieve both improvement in engine output and improvement in fuel efficiency, is widely used (see, for example, JP-A-60-230532). In the lean burn engine disclosed in Japanese Patent Laid-Open No. 60-230532, the target air-fuel ratio is set based on the intake negative pressure.

Further, in general, in an engine, the CO and HC generation rates are 2 (A / F is 2 when the air-fuel mixture becomes lean.
Since it becomes lower (as it approaches 0), the lean-burn engine also improves the emission performance from this viewpoint. However, unlike the CO or HC, the NOx generation rate is slightly leaner than the stoichiometric air-fuel ratio.
It has a characteristic that it has a peak at around 16 (A / F).

Therefore, for example, in a lean burn engine in which the air-fuel ratio in the rich zone is set to the theoretical air-fuel ratio (A / F = 14.7), while the air-fuel ratio in the lean zone (A / F) is set to 19-24. If the air-fuel ratio is continuously changed when the zones are switched, the NOx emission rate deteriorates because the NOx generation rate goes through the air-fuel ratio range where the peak occurs every time the zones are switched. So, in general, with a lean burn engine,
The rich / lean zone switching is performed at once (discontinuously) (see, for example, Japanese Patent Laid-Open No. 60-13953).

[0006]

However, when the zones are switched all at once (discontinuously) in this way, the output characteristics of the engine are different between the rich zone and the lean zone, so that there is a problem that torque shock occurs when the zones are switched. is there. Then, for example, JP-A-60-1395
In the conventional lean burn engine disclosed in Japanese Patent No. 3, the engine torque is adjusted by adjusting the ignition timing when the rich / lean zone is switched to suppress the torque shock. However, since the range of engine torque that can be changed by adjusting the ignition timing is naturally limited, there is a problem that the torque shock during zone switching cannot be sufficiently suppressed.

By the way, in recent years, an electric throttle valve which is opened and closed by an electric actuator such as a servomotor according to a signal from the control unit is provided, while the operating state (for example, accelerator opening, engine speed, vehicle speed, brake) is provided by the control unit. The target engine torque (target indicated mean effective pressure) depending on whether the target throttle opening is obtained or not, and the electric throttle valve is set to follow the target throttle opening. There has been proposed a throttle valve control device that opens and closes to control the engine torque.

Therefore, in a lean burn engine equipped with such a throttle valve control device, the engine torque is adjusted by controlling the electric throttle valve at the time of zone switching of rich / lean to prevent torque shock at the time of zone switching. What has been done is proposed. Then, in such a lean burn engine, normally, the zone is switched depending on whether or not the target indicated mean effective pressure set based on the accelerator opening degree and the engine speed exceeds a predetermined value (zone switching value). I have to.

However, when the zone switching value (target indicated mean effective pressure) is not appropriate, the following problems occur. That is, if the zone switching value is too small, the lean zone becomes narrower, and there is a problem in that the greatest advantage of the lean burn engine, that is, the fuel consumption improving effect cannot be sufficiently exhibited. On the other hand, if the zone switching value is too large,
At the high load side in the lean zone, the engine torque does not increase even if the accelerator opening is increased, and if the zone is switched from the lean zone to the rich zone in such a state, the engine torque will increase discontinuously. As a result, a problem such as torque shock occurs.

The present invention has been made in order to solve the above-mentioned conventional problems, and is provided with an electric throttle valve so that rich / lean zone switching is performed discontinuously according to an operating state. It is an object of the present invention to provide a means for effectively preventing the occurrence of torque shock at the time of zone switching for an exhausted lean burn engine.

[0011]

In order to achieve the above object, the first invention is a rich zone in which an air-fuel ratio on the rich side is set to a predetermined amount or more than the air-fuel ratio region where the NOx emission ratio is the largest, A lean zone in which an air-fuel ratio on the lean side is set to a predetermined amount or more than the air-fuel ratio region is set, and air-fuel ratio switching means a is provided for switching the air-fuel ratio by switching the zone according to the operating state of the engine. In the engine air-fuel ratio control device, in the lean zone, the throttle opening characteristic setting means b for setting the throttle opening characteristic such that the throttle opening with respect to the accelerator opening becomes larger than in the rich zone, and the intake air with respect to the throttle opening. Boundary intake charging efficiency setting means c for setting the boundary intake charging efficiency corresponding to the point where the rate of change of the charging efficiency drops to a predetermined value is provided. An air-fuel ratio control device for an engine, wherein the air-fuel ratio switching means a is configured to switch from a lean zone to a rich zone when the intake charging efficiency becomes equal to or higher than the boundary intake charging efficiency. To do.

According to a second aspect of the present invention, in the engine air-fuel ratio control apparatus according to the first aspect of the invention, the boundary intake charge efficiency setting means c is adapted to grasp the intake charge efficiency by means of the intake negative pressure, and the throttle Provided is an air-fuel ratio control device for an engine, wherein a boundary intake negative pressure corresponding to a point where a rate of change of intake negative pressure with respect to an opening decreases to a predetermined value is used as a boundary intake charging efficiency.

A third aspect of the invention is an air-fuel ratio control device for an engine according to the first or the second aspect of the invention, wherein air-fuel ratio switching means is provided.
a, switching from the lean zone to the rich zone based on the intake charging efficiency, while switching from the rich zone to the lean zone is performed based on the indicated mean effective pressure. An air-fuel ratio control device is provided.

A fourth aspect of the present invention is the engine air-fuel ratio control apparatus according to the second or third aspect, wherein the boundary intake negative pressure set by the boundary intake charging efficiency setting means c is changed to the intake negative pressure with respect to the throttle opening. An air-fuel ratio control device for an engine is provided, which is provided with a boundary intake negative pressure updating means d that updates according to a change with time of the change rate characteristic.

[0015]

EXAMPLES Examples of the present invention will be specifically described below.
As shown in FIG. 2, each cylinder (1
(Only one is shown), basically, when the first and second intake valves 1 and 2 are opened, the first and second independent intake passages are connected via the first and second intake ports 3 and 4. The air-fuel mixture is sucked into the combustion chamber 7 from 5, 6 and is compressed by the piston 8 to be ignited and combusted by the spark plug 9, so that the first and second exhaust valves 10, 11
When the engine is opened, the combustion gas is discharged to the first and second exhaust ports 1
The process of discharging to the first and second independent exhaust passages 14 and 15 via 2 and 13 is repeated. The engine CE has a rich zone in which the air-fuel ratio A / F is set to approximately the theoretical air-fuel ratio (A / F = 14.7, that is, an excess air ratio λ = 1), and the air-fuel ratio is lean (A / F = 1).
9 to 24) is set, and the rich / lean zone switching is performed discontinuously according to the operating state.

By the continuous repetition of such a process, the piston 8 reciprocates in the axial direction in the cylinder bore, and the reciprocating motion of the piston 8 is performed via the connecting rod 16, the crank pin 17, and the crank arm 18. The rotary motion is converted and transmitted to the crankshaft 19. A high voltage is applied to the spark plug 9 from the direct ignition coil 20 at a predetermined timing, and the high voltage causes a spark discharge in the electrode portion of the spark plug 9. The first and second independent exhaust passages 14 and 15 are gathered downstream in one common exhaust passage 41, and a catalytic converter 42 for purifying exhaust gas is provided in the common exhaust passage 41. Common exhaust passage 4
1, an O ₂ sensor 63 that detects the O ₂ concentration in the exhaust gas (that is, the air-fuel ratio) and first and second exhaust temperature sensors 64 and 65 that detect the exhaust gas temperature are provided in advance. In addition, a water temperature sensor 66 that detects the cooling water temperature (engine temperature) is provided in the engine body.

In order to supply air to the combustion chamber 7 of each cylinder, a single common intake passage 21 having an upstream end open to the atmosphere.
The common intake passage 21 is provided with an air cleaner 22 for removing dust in the intake air, an air flow sensor 23 for detecting the intake air amount, and a control unit C in order from the upstream side in the flow direction of the air. An electric throttle valve 25 that is opened and closed by a servo motor 24 according to the applied signal is interposed. The downstream end of the common intake passage 21 is connected to a surge tank 26 that stabilizes the flow of intake air, and the surge tank 26 is provided with the first and second independent intake passages 5 for each cylinder.
The upstream ends of 6 are connected. In addition, this intake system,
A boost sensor 61 that detects an intake negative pressure in the surge tank 26 and an intake temperature sensor 62 that detects an intake air temperature.
And a throttle opening sensor 6 for detecting the throttle opening
7 are provided.

The common intake passage 21 is provided with a bypass intake passage 27 that bypasses the electric throttle valve 25 and allows air to pass therethrough. In the bypass intake passage 27, the flow rate of the air flowing in the bypass intake passage 27 is provided. ISC valve 2 that controls the idle speed by adjusting
8 is installed.

A fuel injection valve 31 for injecting fuel (gasoline) into the air in the first independent intake passage 5 to form a mixture is provided for the first independent intake passage 5. The fuel injection amount of the fuel injection valve 31 is a control unit such that the air-fuel ratio A / F of the air-fuel mixture follows the target air-fuel ratio afw set according to the operating state (engine speed etc.) using the air-fuel ratio map. It is controlled by C.

An on-off valve 32 is provided in the second independent intake passage 6.
The on-off valve 32 is opened and closed by an actuator 33 composed of a negative pressure responsive diaphragm device. Specifically, by the three-way valve 35,
Whether the negative pressure in the surge tank 26 or the atmospheric pressure is introduced into the pressure chamber of the actuator 33 through the negative pressure passage 34 is switched, and when the negative pressure is introduced into the pressure chamber, the actuator 33 opens and closes the valve. When the valve 32 is closed and the atmospheric pressure is introduced into the pressure chamber, the opening / closing valve 32 is opened. The on-off valve 32 is closed in a low load region or the like, at which time air is supplied to the combustion chamber 7 only from the first intake port 3, swirl is generated in the combustion chamber 7, and the air-fuel mixture is stratified in the combustion chamber 7. And the ignitability and combustibility of the air-fuel mixture are improved. The on-off valve 32 is opened in a high load region or the like, and at this time, air is supplied from both intake ports 3 and 4 into the combustion chamber 7 so that intake charging efficiency is improved.

A blow-by gas passage 37 equipped with a PCV valve 36 is provided for introducing blow-by gas generated in the engine (crank chamber) into the surge tank 26.
Is provided. Further, a fresh air introduction passage 38 for supplying a part of the air in the common intake passage 21 into the engine is provided in order to accelerate the discharge of the blow-by gas.

The fuel supply system for supplying fuel (gasoline) to the fuel injection valve 31 will be described below. In this fuel supply system, the fuel in the fuel tank 45 is the fuel filter 4
After being sucked into the fuel pump 47 through 6, the fuel pump 47 is discharged at a predetermined discharge pressure and is supplied to the fuel injection valve 31 through a fuel supply passage 48 in which a filter 49 is interposed. Excess fuel that is not injected by the fuel injection valve 31 is returned to the fuel tank 45 through a fuel return passage 51 in which a pressure regulator 52 that controls the fuel pressure is provided. In order to eliminate the influence of the intake negative pressure, the intake negative pressure in the surge tank 26 is introduced through the negative pressure introducing passage 53.

The air containing the gasoline vapor in the fuel tank 45 is released to the surge tank 26 through the in-tank air release passage 54 in order to prevent the diffusion of the gasoline vapor into the atmosphere. And
In the in-tank air release passage 54, a separator 55 for separating the gasoline mist in the released air, a two-way valve 56, and a gasoline vapor in the released air in order from the upstream side in the flow direction of the air. A canister 57 for adsorbing the toner, an orifice 58, and a duty solenoid valve 59 are interposed.

In the engine CE, by the control unit C equipped with a microcomputer, the intake air amount detected by the air flow sensor 23, the intake negative pressure detected by the boost sensor 61,
Intake air temperature detected by the intake air temperature sensor 62, O ₂
O ₂ concentration (air-fuel ratio) detected by the sensor 63,
The exhaust gas temperature detected by the second exhaust temperature sensors 64, 65, the cooling water temperature (engine temperature) detected by the water temperature sensor 66, the throttle opening detected by the throttle opening sensor 67, the rotation speed sensor (not shown). Engine speed and accelerator opening sensor
Various controls are performed using control information such as the accelerator opening detected by (not shown). However, the general control of the engine CE is not the gist of the present invention, and therefore its explanation is omitted. The gist of the present invention will be described below with reference to the flowcharts shown in FIGS. Only the air-fuel ratio control and the throttle valve control (engine torque control) will be described. The control unit C includes the "air-fuel ratio switching means", "throttle opening characteristic setting means", "boundary intake charging efficiency setting means" and "boundary intake negative pressure updating means" described in the claims. It is a comprehensive control device including.

FIG. 3 is a flowchart of a main routine including air-fuel ratio control and throttle valve control, and FIG. 4 is a throttle valve control subroutine (engine torque control subroutine) called by the main routine.
In the main routine, first, the throttle valve control subroutine is called in step # 1, and the engine torque control (torque control) is performed by the throttle valve control subroutine.

The throttle valve control subroutine called in step # 1 will be described below. When the subroutine is called, first, at step # 20, the throttle opening Tvo, the engine speed Ne and the accelerator opening Ac are read. In step # 21, the target indicated mean effective pressure Pit corresponding to the accelerator opening Ac and the engine speed Ne is calculated by searching the target indicated mean effective pressure map. The target indicated mean effective pressure map is the target indicated effective pressure P.
A map showing it with respect to the accelerator opening Ac and the engine speed Ne, that is, the target indicated mean effective pressure Pit (that is, the most suitable target indicated mean effective pressure Pit that is represented by the accelerator opening Ac and the engine speed Ne). The engine torque is a map that can be read, and its characteristics are set as shown in FIG. 8, for example.

At step # 22, the target indicated mean effective pressure Pit and the target air-fuel ratio afw are read. Here, the target air-fuel ratio afw is determined according to the intake air amount and the engine speed Ne by searching the air-fuel ratio map based on the zone determination result (rich zone / lean zone) in another control routine (not shown). Is set.

In step # 23, based on the target indicated mean effective pressure Pit and the target air-fuel ratio afw,
The target charging efficiency Cet is calculated.

[Equation 1] Cet = KITPCE ・ Pit ・ (afw / 14.7) ・ fipol (afw, tw) ……………… Equation 1 Cet ………… Target charging efficiency KITPCE ………… Conversion coefficient (constant) Pit ………… Target indicated mean effective pressure afw ………… Target air-fuel ratio tw ……………… Engine temperature (cooling water temperature) fipol (afw, tw)… Fuel efficiency improvement coefficient In equation 1, fipol (afw, tw ) Is a fuel efficiency improvement coefficient showing a characteristic of the fuel efficiency with respect to the target air-fuel ratio, and is a function of the target air-fuel ratio afw and the engine temperature tw (cooling water temperature).

In general, the target charging efficiency Cet is (Cet = KIT
PCE / Pit / afw / 14.7). However, according to such a conventional calculation method, the target air-fuel ratio af
When w is lean, the target charging efficiency Cet becomes excessive,
The engine torque is excessively higher than the proper value. The thermal efficiency also depends on the engine temperature tw. Therefore, in the present embodiment, as shown in Expression 1, the fuel efficiency improvement coefficient fipol (afw, t which is a function of the target air-fuel ratio afw and the engine temperature tw
w) to introduce the target air-fuel ratio afw or engine temperature tw
It is possible to obtain an appropriate target charging efficiency Cet regardless of the above.

At step # 24, the target charging efficiency Cet and the engine speed Ne are read. Step # 25
Then, the target throttle opening Tvot corresponding to the target charging efficiency Cet and the engine speed Ne is calculated by searching the target throttle opening map. The target throttle opening map shows the target throttle opening Tvot as the target charging efficiency Cet.
And the map represented with respect to the engine speed Ne, that is, the most suitable for the momentary operating condition represented by the target charging efficiency Cet and the engine speed Ne, the actual indicated mean effective pressure (engine torque) is ensured. Is a map in which the target throttle opening Tvot that can match the target indicated mean effective pressure Pit (target engine torque) can be read, and its characteristics are set as shown in FIG. 9, for example.

The characteristics of the actual indicated mean effective pressure Pi and the throttle opening Tvo with respect to the accelerator opening Ac are shown in FIG.
The graphs G ₁ and G ₃ are the same. Further, the characteristics of the indicated mean effective pressure Pi and the throttle opening Tvo with respect to the accelerator opening Ac in the conventional control method that does not switch the maps are as shown in graphs G ₂ and G _{4 in} FIG. 5, respectively. In FIG. 5, zone switching is performed when the accelerator opening Ac is α. As is clear from FIG. 5, in the case of the present invention, there is no gap or step in the indicated mean effective pressure Pi, that is, the engine torque even when the zones are switched. Therefore, according to the present invention, torque shock does not occur during zone switching.

The target throttle opening Tvot calculated in step # 25 is sent to another control routine (not shown). In this other control routine, the actual throttle opening Tvo is the target throttle opening Tvot. So that the electric throttle valve 25
Is driven to open and close, which results in the target indicated mean effective pressure Pit
That is, the target engine torque is realized. After step # 25 is executed, the throttle valve control subroutine of this time is ended, and the process returns to step # 2 of the main routine.

The routine following step # 2 of the main routine will be described below. In step # 2, comparison / judgment (zone judgment) as to whether or not the vehicle is operating in a lean zone (lean burn) is performed. In this embodiment, the predetermined high output region is the rich zone (A / F = 14.7, that is, λ = 1), and the other operating region, that is, the low output region is the lean zone (A / F = 19 to 24). It is said that.
Therefore, the engine output is sufficiently increased in the high output range where the air-fuel ratio is rich, while the fuel efficiency and the emission performance are sufficiently improved in the low output range where the air-fuel ratio is lean. Since the air-fuel ratio in the rich zone and the air-fuel ratio in the lean zone are set with a predetermined gap, the NOx emission rate does not pass through the air-fuel ratio area where the NOx generation rate reaches the peak when switching zones, and the emission performance for NOx is extremely high. It will be good.

If it is determined in step # 2 that the vehicle is operating in the lean zone (YES), the lean zone routine of steps # 3 to # 11 is executed. In this lean zone routine, basically, the lean zone is switched to the rich zone when the rate of change of the intake charge efficiency with respect to the throttle opening decreases to a predetermined value (boundary intake charge efficiency). There is. Here, the boundary intake charging efficiency is not a constant value as in the conventional case, but is set according to the operating state of the engine CE every moment.

As shown in FIG. 6, when the throttle opening Tvo is increased in the lean zone, the intake charging efficiency Ce initially increases with it, but when the throttle opening Tvo reaches a certain level or more, the intake charging is increased. The increase in efficiency Ce is saturated. And thus, the intake charging efficiency Ce
When the increase in the fuel pressure approaches the saturation point, the intake charge efficiency Ce does not increase easily even when the accelerator pedal is depressed, and the engine torque cannot be adjusted with the accelerator pedal as it is. Therefore, when the rate of change of the intake charging efficiency Ce with respect to the throttle opening Tvo decreases to a predetermined value (that is, the boundary intake charging efficiency), the rich zone is switched to,
It is designed to obtain sufficient engine output. In this way, switching from lean zone to rich zone
Since the intake charging efficiency is set based on the operating state, the control accuracy of the torque control can be improved. It should be noted that the boundary intake air charging efficiency is updated over time in order to respond to changes in engine characteristics over time. Note that, in FIG. 6, a graph G ₅ shows a state at low rotation, and a graph G ₆ shows a state at high rotation.

However, since the intake charging efficiency cannot be detected directly, the intake charging efficiency is grasped by the intake negative pressure in this embodiment. For example, the boundary intake charging efficiency is represented by the intake negative pressure and is set as the boundary intake negative pressure. Here, since the intake negative pressure accurately corresponds to the intake charging efficiency, the control accuracy of such air-fuel ratio control is enhanced.

FIG. 7 shows the switching from the lean zone to the rich zone when the indicated average effective pressure is a constant value (zone switching value).
The characteristics of the indicated mean effective pressure Pi and the throttle opening Tvo with respect to the accelerator opening Ac in the case of being performed depending on whether or not the value exceeds When the zone switching value is set to a relatively small value β ₁ (graphs G ₇ and G ₉ ), since the intake charging efficiency Ce is far from the saturation point at the time of zone switching, the indicated average effective pressure Pi (engine torque) There are no gaps or steps. In this case, of course, if the zone switching value is too small, the lean zone becomes narrower, resulting in deterioration of fuel efficiency. On the other hand, when the zone switching value is set to a relatively large value β ₂ (graphs G ₈ and G ₁₀ ), since the intake charging efficiency Ce has reached the saturation point during zone switching, there is a gap in the indicated mean effective pressure Pi. Or, a step will be generated and a torque shock will occur.

Specifically, first at step # 3, the engine speed Ne and the intake negative pressure boost are read, and then at step # 4, the boundary intake negative pressure Boost is read from the boundary intake negative pressure table. The boundary intake negative pressure Boost is the zone determination intake negative pressure, and when the intake negative pressure boost is equal to or higher than the boundary intake negative pressure Boost (when the charging efficiency becomes high, that is, the throttle opening becomes large). At that time), the lean zone is switched to the rich zone.

Here, the boundary intake negative pressure Boost is basically a point at which the rate of change of the intake negative pressure boost with respect to the throttle opening Tvo decreases to a predetermined value, that is, the throttle opening T.
It is set to a point where the rate of change of the intake charging efficiency Ce with respect to vo drops to a predetermined value. Therefore, the lean zone can be expanded as much as possible without bringing the intake charge efficiency Ce close to saturation. That is, at the most appropriate time according to the operating state of the engine CE, without causing a decrease in fuel efficiency,
In addition, the lean zone is switched to the rich zone without causing torque shock.

In step # 5, it is determined whether or not the intake negative pressure boost is smaller than the boundary intake negative pressure Boost, and ab
If oost ≧ Boost (NO), switching to the rich zone (λ = 1) is performed in step # 10. Subsequently, in step # 11, the boundary indicated average effective pressure Pi that serves as a reference when switching from the rich zone to the lean zone is updated. Specifically, this target indicated mean effective pressure P
A value obtained by subtracting the hysteresis component Pih from it is newly set as the boundary indicated mean effective pressure Pi. In this way, since the boundary indicated mean effective pressure Pi is updated according to the operating state, it becomes possible to cope with the change over time of the engine characteristics. Further, since the amount of hysteresis Pih is subtracted, cycling does not occur when switching zones. Then, the process returns to step # 1.

On the other hand, if it is determined in step # 5 that boost <Boost (YES), the current lean zone is maintained in step # 6 and zone switching is not performed. After this, in step # 7, the throttle opening Tvo of this time
Whether or not (t) is larger than the previous throttle opening Tvo (t-1), that is, whether or not the throttle opening Tvo has increased is compared and determined. Then, in step # 8, the intake negative pressure ab
It is compared / determined whether oost (t) is larger than the previous intake negative pressure boost (t-1), that is, whether the intake negative pressure boost has changed.

Here, when the intake negative pressure boost is not changing even though the throttle opening Tvo is increasing (YES in step # 7 and N in step # 8).
O), after the intake negative pressure boost of this time is newly set to the boundary intake negative pressure Boost in step # 9, the process returns to step # 1. That is, the boundary intake negative pressure Boost is updated according to the operating state. As a result, it is possible to always obtain an appropriate boundary intake negative pressure Boost according to changes in engine characteristics over time.
The accuracy of torque control is improved. In other cases, steps # 8 and # 9 are skipped and the process returns to step # 1.

If it is determined in step # 2 that the vehicle is not in the lean zone (NO), that is, if the vehicle is operating in the rich zone, the target indicated mean effective pressure Pit is the boundary indicated average in step # 12. It is compared / determined whether it is smaller than the effective pressure Pi. In this embodiment, switching from the rich zone to the lean zone is performed based on the indicated mean effective pressure.

That is, in order to improve the control accuracy when switching from the lean zone to the rich zone (during acceleration),
Zone switching is performed based on the intake negative pressure. However, at the time of switching from the rich zone to the lean zone (during deceleration), the intake charge amount increases with the zone switching because of the transition from the rich air-fuel ratio state to the lean state. Therefore, the intake negative pressure increases (the charging efficiency increases), and there is a possibility that the change from the lean zone to the rich zone may be performed again due to the change in the intake negative pressure. When such a phenomenon occurs, cycling for zone switching occurs,
In this embodiment, in order to prevent the occurrence of such a problem, switching from the rich zone to the lean zone is performed based on the indicated mean effective pressure.

If it is determined in step # 12 that Pit <Pi (YES), the process is switched to the lean zone in step # 13, and if it is determined that Pit ≧ Pi (NO), step S13. In # 14, the current rich zone is maintained and zone switching is not performed. Then, the process returns to step # 1.

As described above, according to the air-fuel ratio control and the throttle valve control, the lean zone is expanded as much as possible, the fuel consumption performance and the emission performance are enhanced, and the zone switching is performed in the vicinity of the region where the intake charging efficiency is saturated. This is prevented, and torque shock at the time of zone switching is prevented. Further, since the charging efficiency is grasped by the intake negative pressure, the intake charging efficiency can be grasped accurately and the control accuracy of the air-fuel ratio control can be enhanced. Furthermore, since switching from the rich zone to the lean zone is performed based on the indicated mean effective pressure, cycling does not occur at the time of zone switching. Moreover, since the boundary intake negative pressure is updated according to the change in the engine characteristic, it is possible to cope with the change over time in the engine characteristic, and the control accuracy of the torque control is further enhanced.

[0047]

According to the first aspect of the present invention, the lean zone is switched to the rich zone when the rate of change of the intake charge efficiency with respect to the throttle opening decreases to a predetermined value. The fuel consumption performance and the emission performance are enhanced as much as possible, and zone switching is prevented from being performed in the vicinity of a region where the intake charging efficiency is saturated, and torque shock during zone switching is prevented.

According to the second invention, basically, the same operation and effect as those of the first invention can be obtained. Further, since the charging efficiency is grasped by the intake negative pressure, the intake charging efficiency is accurately grasped and the control accuracy of the air-fuel ratio control is enhanced.

According to the third invention, basically, the same operation and effect as those of the first or second invention can be obtained. Furthermore, since switching from the rich zone to the lean zone is performed based on the indicated mean effective pressure, cycling does not occur when switching from the rich zone to the lean zone.

According to the fourth invention, basically, the same action and effect as those of the second or third invention can be obtained. Further, since the boundary intake negative pressure is updated according to the change in the engine characteristic, it is possible to cope with the change over time in the engine characteristic, and the control accuracy of the torque control is further enhanced.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of first to fourth inventions corresponding to claims 1 to 4.

FIG. 2 is a system configuration diagram of an engine including an air-fuel ratio control device according to the present invention.

FIG. 3 is a flowchart showing a control method of air-fuel ratio control and throttle valve control.

FIG. 4 is a flowchart of a throttle valve control subroutine.

FIG. 5 is a diagram showing characteristics of indicated average effective pressure and throttle opening with respect to accelerator opening.

FIG. 6 is a diagram showing a characteristic of charging efficiency with respect to an accelerator opening.

FIG. 7 is a diagram similar to FIG. 5 in the case where zone switching is performed with a constant value.

FIG. 8 is a diagram showing characteristics of a target indicated mean effective pressure map.

FIG. 9 is a diagram showing characteristics of a target throttle opening map.

[Explanation of symbols]

 CE ... Engine C ... Control unit 24 ... Servo motor 25 ... Electric throttle valve 31 ... Fuel injection valve 61 ... Boost sensor

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Takagi 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Motor Corporation

Claims (4)

[Claims] 1. A rich zone in which an air-fuel ratio richer than a predetermined amount than the air-fuel ratio region where the NOx emission ratio is the largest is set, and an air-fuel ratio leaner than a predetermined amount in the air-fuel ratio region is set. In the air-fuel ratio control device of the engine provided with the air-fuel ratio switching means for switching the air-fuel ratio by switching the zone according to the operating state of the engine. Throttle opening characteristic setting means for setting the throttle opening characteristic such that the throttle opening becomes large with respect to the opening, and the boundary intake filling efficiency corresponding to the point where the rate of change of the intake filling efficiency with respect to the throttle opening falls to a predetermined value. And a boundary intake air charging efficiency setting means for setting the intake air charging efficiency is set. That it is adapted to perform switching to the rich zone from the lean zone when it becomes an air-fuel ratio control system for an engine according to claim. 2. The air-fuel ratio control system for an engine according to claim 1, wherein the boundary intake charging efficiency setting means is adapted to grasp the intake charging efficiency by the intake negative pressure, and intake air with respect to the throttle opening degree. An air-fuel ratio control device for an engine, wherein a boundary intake negative pressure corresponding to a point where the rate of change of negative pressure decreases to a predetermined value is used as boundary intake charging efficiency. 3. The air-fuel ratio control device for an engine according to claim 1 or 2, wherein the air-fuel ratio switching means performs switching from the lean zone to the rich zone based on the intake charging efficiency, while the rich zone. The air-fuel ratio control device for an engine, wherein switching from a lean zone to a lean zone is performed based on an indicated mean effective pressure. 4. The engine air-fuel ratio control apparatus according to claim 2 or claim 3, wherein the boundary intake negative pressure set by the boundary intake charging efficiency setting means is a rate of change of the intake negative pressure with respect to the throttle opening. An air-fuel ratio control system for an engine, characterized in that boundary air intake negative pressure updating means for updating the characteristics according to changes over time is provided.