JPH08135485A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE: To avoid the temporary change of the air-fuel ratio to the lean and the rich so as to prevent the lowering of transformation efficiency with three way catalyst by correcting the increase quantity in response to a difference between the air-fuel ratio just before the switching and the stoichiometric air-fuel ratio at the time of switching the target air-fuel ratio from the lean to the theoretical value with a change of the engine operation condition. CONSTITUTION: Operation condition whether an engine 1 is operated at the lean air-fuel ratio or the stoichiometric air-fuel ratio is decided on the basis of the detecting signal such as the engine speed detected by a crank angle sensor 8, the cooling water temperature detected by a water temperature sensor 6, the intake air quantity detected by an air flowmeter and a load or the like. A control unit 16 performs the computing on the basis of the program in response to the detecting signal from these sensors. Consequently, at the time of the lean air-fuel ratio, the fuel injection quantity is computed on the basis of the allocation coefficient, and drive is controlled. In the condition except for the lean, a difference between the value detected by the air-fuel ratio sensor 14 and the theoretical value is computed, and the air-fuel ratio is switched from the lean, and increase quantity is corrected, and as a result thereof, after switching an A/F specifying flag, the real air-fuel ratio is quickly condensed, and comes close to the stoichiometric air-fuel ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine such as a gasoline engine for an automobile, and more particularly to an improvement of an air-fuel ratio control device which operates at a lean air-fuel ratio under predetermined operating conditions.

[0002]

2. Description of the Related Art An air-fuel ratio control device which has been designed to operate at a lean air-fuel ratio under predetermined operating conditions has been disclosed in Japanese Patent Laid-Open No. 62-16274.
No. 6, for example. In this type of air-fuel ratio control device, generally, the lean operating region is set with the engine speed and the engine load as parameters so that the vehicle becomes lean in steady operation and slow acceleration operation, etc. The air-fuel ratio control with the stoichiometric air-fuel ratio as the target air-fuel ratio and the air-fuel ratio control with the lean air-fuel ratio as the target air-fuel ratio can be switched according to the number and load. When the control is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio, the fuel injection amount is multiplied by a constant increase correction coefficient for a predetermined time after the switching in order to enhance the response of the enrichment. The injection amount is temporarily increased.

[0003]

However, in the above-mentioned conventional apparatus, when the lean air-fuel ratio is switched to the stoichiometric air-fuel ratio, a constant correction amount is added regardless of the operating state of the engine. There is an excess or deficiency in the correction. In particular, the value of the lean air-fuel ratio is not fixed, but various lean air-fuel ratios are obtained depending on the operating conditions. Therefore, fixed correction cannot be appropriate.
For example, FIG. 4 shows a target air-fuel ratio (specifically, an air-fuel ratio correction coefficient when the theoretical air-fuel ratio is 1 with the basic fuel injection pulse Tp corresponding to the load of the engine and the engine speed as parameters. ) Is shown, but as is clear from this figure, even with the same load, the air-fuel ratio is 17.5 and the rotation speed b In the case of, the air-fuel ratio is 20.7,
There are three or more air-fuel ratio differences. When accelerating from these operating conditions and shifting to control at the theoretical air-fuel ratio,
Conventionally, this difference is not considered at all. Therefore, for example, if a constant correction amount is given so as to be suitable for the transition from the point b to the stoichiometric air-fuel ratio, the transition from the point a to the stoichiometric air-fuel ratio will become temporarily rich and the three-way catalyst The conversion efficiency of HC and CO will decrease. On the contrary, if a constant correction amount is given so as to be suitable for shifting from the point a to the stoichiometric air-fuel ratio, the correction amount becomes insufficient when shifting from the point b to the stoichiometric air-fuel ratio, and λ = 1 ( (λ is an excess air ratio) cannot be reached promptly and becomes lean temporarily, and the conversion efficiency of NOx decreases.

Further, in the above-mentioned conventional structure, the above-mentioned correction is carried out only for a fixed time after switching, and the correction is suddenly set to 0 when the fixed time elapses. However, in such a structure, the actual air-fuel ratio is detected. Due to the synergistic effect with the response delay of the air-fuel ratio feedback control that feedback-controls the fuel injection amount, a large amount of overshoot and undershoot occur at the target point of λ = 1, and then until it converges to the point of λ = 1. The drawback is that it takes time.

[0005]

An air-fuel ratio control system for an internal combustion engine according to the present invention includes a first control means for controlling a fuel supply amount with a theoretical air-fuel ratio of the internal combustion engine as a target air-fuel ratio, and a lean air-fuel ratio. The second for controlling the fuel supply amount as the target air-fuel ratio
Control means, switching means for switching between the first control means and the second control means in accordance with the engine operating conditions, and the target air-fuel ratio immediately before switching when switching from the second control means to the first control means. Alternatively, it is provided with an air-fuel ratio difference detecting means for detecting a difference between the actual air-fuel ratio and the stoichiometric air-fuel ratio, and an increase correction means for performing an increase correction according to this air-fuel ratio difference at the time of the switching.

According to the invention of claim 2, after the increase correction,
In addition to having a decrease correction means for gradually decreasing the amount corresponding to the increase, a decrease speed setting means for setting the decrease speed according to the magnitude of the increase correction is provided.

According to the invention of claim 3, after the increase correction,
The oxygen concentration sensor provided with upstream of the three-way catalyst when the air-fuel ratio is first inverted from lean to rich after the switching is performed while having a reduction correction means for gradually reducing the amount corresponding to the increase. It is provided with a deceleration speed setting means for setting according to the maximum voltage level.

[0008]

When the second control means for making the lean air-fuel ratio the target air-fuel ratio is switched to the first control means for making the theoretical air-fuel ratio the target air-fuel ratio as the engine operating conditions change, the air-fuel ratio and the theory just before the switching are changed. A difference from the air-fuel ratio is detected, and an increase correction is performed at the time of switching with a correction amount corresponding to this. That is, if the air-fuel ratio difference is large, the increase correction is large, and if the air-fuel ratio difference is small, the increase correction is small.

According to the structure of claim 2, after the increase correction,
The amount corresponding to the increase is gradually reduced. In particular, the deceleration speed corresponds to the magnitude of the increase correction, and the larger the increase correction, the higher the deceleration speed so as to prevent overshoot.

According to the third aspect of the invention, the deceleration rate is determined by the density of the air-fuel ratio at the oxygen concentration sensor position upstream of the three-way catalyst. In other words, when the air-fuel ratio near the sensor is high when the lean-to-rich reverse is first performed, the maximum voltage level of the sensor becomes high, so that the deceleration speed becomes large so as to suppress the overshoot.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a structural explanatory view showing a mechanical structure of an embodiment of an air-fuel ratio control device according to the present invention.

In FIG. 1, reference numeral 1 denotes an internal combustion engine, and a fuel injection valve 2 is arranged for each cylinder toward an intake port of the internal combustion engine 1. A throttle valve 4 is provided in the intake passage 3, and a hot-wire air flow meter 5 for detecting the intake air amount is arranged upstream of the throttle valve 4. Above throttle valve 4
Is provided with a throttle opening sensor 6 for detecting the opening thereof. Further, a water temperature sensor 7 for detecting the cooling water temperature and a crank angle sensor 8 for detecting the engine speed are attached to the internal combustion engine 1.

In the exhaust passage 11 of the internal combustion engine 1, a lean NOx catalyst 12 capable of purifying NOx under a lean air-fuel ratio and a three-way catalyst 13 are arranged in series. Further, a so-called wide area type air-fuel ratio sensor 14 capable of widely detecting the air-fuel ratio is arranged on the upstream side of the lean NOx catalyst 12. Furthermore, the lean NOx catalyst 12 and the three-way catalyst 13
And an oxygen concentration sensor (so-called O ₂ sensor) 15 that shows a stepwise response at the stoichiometric air-fuel ratio.

Reference numeral 16 denotes a control unit to which the detection signals of the respective sensors are input and which controls the fuel injection amount and the like based on these signals. The control unit 16 is composed of a microcomputer system including a CPU, a ROM, a RAM, an I / O port, and the like, and performs various arithmetic processes according to a predetermined program.

Next, the air-fuel ratio control in the above configuration will be described.

In the above structure, the injection amount for each cycle, that is, the pulse width Ti of the drive pulse signal applied to each fuel injection valve 2 is given by the following equation.

[0018]

## EQU1 ## Ti = Tp × COEF × KRICH × α + TS where Tp is the basic fuel injection amount, and K is calculated by using the intake air amount Q detected by the air flow meter 5, the engine speed N and the constant K. -Required as Q / N. Also COEF
Is composed of various correction coefficients as shown in the following equation.

[0019]

COEF = KABYF + KTW + KAS + KHOT where KTW is a water temperature correction coefficient, KAS is a startup increase correction coefficient, and KHOT is a high temperature correction coefficient. Also, KA
BYF is a fuel-air ratio allocation coefficient, KABYF = KSTOICH under operating conditions that should be the theoretical air-fuel ratio, and KABYF = KLEA under operating conditions that should be a lean air-fuel ratio.
N, which is set according to the operating conditions by referring to the respective predetermined maps.

KRICH is an air-fuel ratio enrichment coefficient, and is a coefficient for realizing the injection amount correction at the time of switching from the lean air-fuel ratio to the stoichiometric air-fuel ratio, which is the subject of the present invention.
The air-fuel ratio enrichment coefficient KRICH will be described later, but KRICH ≧ 1, and when lean, always K
RICH = 1.

Further, α is a feedback correction coefficient calculated based on the detection signal of the oxygen concentration sensor 15, for example, by proportional-plus-integral control. TS is a voltage correction coefficient.

Whether the internal combustion engine 1 is operated at a lean air-fuel ratio or at a stoichiometric air-fuel ratio depends on the engine speed,
It is determined by engine operating conditions such as load and cooling water temperature.
FIG. 2 exemplifies usage patterns of respective air-fuel ratios in a general traveling mode (10.15 mode). Generally, the air-fuel ratio is set to a lean air-fuel ratio under conditions of substantially constant speed traveling and slow acceleration traveling. The theoretical air-fuel ratio is used when idling, which requires stability, and in general acceleration mode.

Next, FIG. 3 is a flowchart showing the flow of the calculation process of the fuel injection amount Ti, which will be described below. The calculation of the injection amount Ti is performed in a fixed cycle that is sufficiently earlier than the injection cycle. First, in step 1, various operating condition signals such as engine speed, intake air amount, cooling water temperature, vehicle speed, etc. are read. Then, in step 2, the state of the A / F regulation flag indicating whether the lean air-fuel ratio or the non-lean air-fuel ratio (theoretical air-fuel ratio and the rich air-fuel ratio) is set is determined. The A / F regulation flag is set based on the engine operating condition by another routine (not shown), and the ignition timing and the like can be switched according to the flag in addition to the fuel injection amount described later. It has become.

If the lean air-fuel ratio is to be determined by this flag determination, the routine proceeds to step 3, where the lean air-fuel ratio allocation coefficient KABYF, that is, the value of KLEAN is determined with reference to the map shown in FIG. . Then, the process proceeds to step 8, and the fuel injection amount Ti is calculated based on the above-mentioned arithmetic expression. The fuel injection valve 2 is drive-controlled by another routine (not shown), and when the predetermined injection timing is reached, the fuel injection valve 2 executes the fuel injection according to the injection amount Ti.

When the flag is in the state of the air-fuel ratio other than lean in step 2, the routine proceeds to step 4, where the air-fuel ratio actually detected by the air-fuel ratio sensor 14 and the theoretical air-fuel ratio (14.7) are set. The difference, that is, the air-fuel ratio difference is calculated. A / F
For the first time after the specified flag switches from lean to other than lean, the actual air-fuel ratio remains the lean air-fuel ratio immediately before switching, and this difference indicates the step between the air-fuel ratio immediately before switching and the stoichiometric air-fuel ratio. . Then, in step 5, this air-fuel ratio difference is compared with a predetermined determination reference value. If the air-fuel ratio difference is larger than the judgment reference value, the routine proceeds to step 6, and the increase correction is started or continued, but if the air-fuel ratio difference is smaller than the judgment reference value or has a negative value, step 9 is executed.
Then, the process proceeds to and the weight reduction correction described later is performed. Instead of detecting the actual air-fuel ratio by the air-fuel ratio sensor 14, the lean air-fuel ratio immediately before switching is estimated from the value of the fuel-air ratio allocation coefficient KLEAN in the map of FIG. 4, and the air-fuel ratio with the theoretical air-fuel ratio is estimated. You can also find the difference.

In step 6, the above-mentioned air-fuel ratio enrichment coefficient KRICH is set in accordance with the above-mentioned air-fuel ratio difference along the characteristics shown in FIG. Therefore, when the lean air-fuel ratio is switched to the stoichiometric air-fuel ratio, the larger the step of the air-fuel ratio, the larger the correction amount is, and the point of λ = 1 is reached quickly. On the contrary, when the step of the air-fuel ratio is small, the correction amount becomes small, and the enrichment due to excessive correction can be avoided. In this step 6, the value immediately after the A / F regulation flag is switched from lean to other than lean is always held as the air-fuel ratio difference.

Next, at step 7, the stoichiometric air-fuel ratio or the fuel-air ratio allocation coefficient KABYF at the time of rich, that is, KSTO.
The value of ICH is determined with reference to a map (not shown). The value of KSTOICH is 1 or more, for example, 1
It is a value of ~ 1.5. Then, the process proceeds to step 8, and the fuel injection amount Ti is calculated based on the above-mentioned arithmetic expression. When the fuel injection valve 2 reaches a predetermined injection timing, the fuel injection valve 2 executes fuel injection according to the injection amount Ti. Therefore, as shown in FIG. 6, when the A / F regulation flag is switched from lean to the stoichiometric air-fuel ratio, the fuel injection amount Ti increases to a level corresponding to the stoichiometric air-fuel ratio, and in addition to this, Correction by the air-fuel ratio enrichment coefficient KRICH is added.

On the other hand, as a result of this increase correction, the A / F
After the specified flag switches from lean to non-lean, the actual air-fuel ratio becomes relatively thick and approaches the stoichiometric air-fuel ratio relatively quickly. Therefore, the air-fuel ratio difference obtained in step 4 becomes equal to or smaller than the determination reference value in step 5, and step 5
To go to step 9. Thereby, thereafter, the air-fuel ratio enrichment coefficient KRICH is gradually reduced. Specifically, in step 9, the weight reduction correction coefficient KDEC
By determining R and multiplying this reduction correction coefficient KDECR by the air-fuel ratio enrichment coefficient KRICH, the coefficient KR
Decrease ICH. That is, if the previous air-fuel ratio enrichment coefficient is KRICH _(OLD) , the new coefficient KRICH
Is calculated as KRICH = KRICH _(OLD) × KDECR. Therefore, the fuel injection amount Ti gradually decreases as shown in FIG. Here, the value of the reduction correction coefficient KDECR that determines the speed of this reduction is determined in a form corresponding to the value of the air-fuel ratio enrichment coefficient KRICH in the initial stage, that is, immediately after switching, with reference to the map shown in FIG. . Specifically, if the initial air-fuel ratio enrichment coefficient KRICH is large, the deceleration rate increases accordingly. As a result, overshoot due to the synergistic effect with the air-fuel ratio feedback control, and hence undershoot due to its reaction, can be suppressed,
The actual air-fuel ratio can be quickly converged near the point of = 1. The air-fuel ratio enrichment coefficient KRICH is KRIC
Since H ≧ 1, the weight reduction correction is gradually performed until the value reaches 1. FIG. 9 shows an example of the reduction rate depending on the magnitude of the reduction correction coefficient KDECR.

As mentioned above, the air-fuel ratio enrichment coefficient K
In addition to the method of determining the deceleration rate according to the value of RICH, the oxygen concentration sensor 15 located immediately before the three-way catalyst 13
There is also a method of determining the deceleration speed according to the magnitude of the output voltage when the lean-to-rich first inversion is performed after the air-fuel ratio is switched. FIG. 8 shows the weight reduction correction coefficient KDECR in this case.
Of the above output voltage, the higher the output voltage at the first inversion, the greater the deceleration rate. In this method, the actual shading of the air-fuel ratio at the position immediately before the three-way catalyst 13 is detected, and the deceleration rate is defined accordingly, so that the N
It is the best in terms of Ox conversion efficiency.

FIG. 10 shows an example of the state of convergence to the point of λ = 1 when the air-fuel ratio is switched by the above-described correction of the present invention. In addition, the NOx emission amount (at the three-way catalyst 13 outlet side) Value) is shown. Further, the conventional example is based on a system in which a fixed correction is added only for a fixed period as described above, and particularly shows characteristics when the correction amount is slightly insufficient. As shown in FIG. 10, according to the present invention, NOx can be reduced by appropriate correction, and λ
It will quickly converge to the point of = 1.

[0031]

As is apparent from the above description, according to the air-fuel ratio control system for an internal combustion engine of the present invention, when the target air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio, the air-fuel ratio immediately before the switching is changed. By performing the increase correction according to the air-fuel ratio difference between the fuel ratio and the stoichiometric air-fuel ratio, it is possible to avoid the temporary leaning or richening immediately after the switching, and the point of λ = 1 is quickly reached. Therefore, it is possible to prevent deterioration of HC, CO, or NOx immediately after switching.

Further, according to the second and third aspects, the overshoot and the undershoot with respect to the point of λ = 1 due to the synergistic effect with the air-fuel ratio feedback control can be reduced, and the point of λ = 1 can be promptly changed after the switching. Can be converged to.

[Brief description of drawings]

FIG. 1 is a structural explanatory view showing an embodiment of an air-fuel ratio control device according to the present invention.

FIG. 2 is an explanatory diagram showing an example of a lean air-fuel ratio region and a stoichiometric air-fuel ratio region.

FIG. 3 is a flowchart showing a flow of injection amount control in this embodiment.

FIG. 4 is a characteristic diagram showing a map of a fuel-air ratio allocation coefficient KLEAN when lean.

FIG. 5: Air-fuel ratio difference and air-fuel ratio enrichment coefficient KRI during switching
The characteristic view which shows the relationship with CH.

FIG. 6 is a time chart showing an example of changes in vehicle speed, air-fuel ratio, and fuel injection pulse width Ti.

FIG. 7 is a characteristic diagram showing a relationship between an air-fuel ratio enrichment coefficient KRICH and a reduction correction coefficient KDECR at the time of switching.

FIG. 8: Output voltage of oxygen concentration sensor and reduction correction coefficient KD
The characteristic view which shows the relationship with ECR.

FIG. 9 is a characteristic diagram showing an example of a deceleration rate.

FIG. 10 is a time chart showing changes in the actual air-fuel ratio and changes in the NOx emission amount at the time of switching.

[Explanation of symbols]

 2 ... Fuel injection valve 12 ... Lean NOx catalyst 13 ... Three-way catalyst 14 ... Air-fuel ratio sensor 15 ... Oxygen concentration sensor

Claims (3)

[Claims] 1. A first control means for controlling a fuel supply amount with a theoretical air-fuel ratio of an internal combustion engine as a target air-fuel ratio, and a second control means for controlling a fuel supply amount with a lean air-fuel ratio as a target air-fuel ratio. Switching means for switching between the first control means and the second control means according to the engine operating conditions, and a target air-fuel ratio or an actual air-fuel ratio immediately before switching when switching from the second control means to the first control means. An air-fuel ratio control device for an internal combustion engine, comprising: an air-fuel ratio difference detection means for detecting a difference from a stoichiometric air-fuel ratio; and an increase correction means for performing an increase correction according to the air-fuel ratio difference at the time of switching. 2. After the increase correction, there is provided a decrease correction means for gradually decreasing the amount corresponding to the increase, and the decrease speed is
The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, further comprising a deceleration speed setting means that is set according to the magnitude of the increase correction. 3. A decrease correction means for gradually reducing the amount corresponding to the increase after the increase correction, and the decrease speed thereof
After the above switching, it is characterized by including a deceleration speed setting means for setting according to the maximum voltage level of the oxygen concentration sensor arranged upstream of the three-way catalyst when the air-fuel ratio first reverses from lean to rich. The air-fuel ratio control device for an internal combustion engine according to claim 1.