JP3767063B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.
[0002]
[Prior art]
An EGR control valve that controls the amount of EGR gas is disposed in the EGR passage that connects the intake passage and the exhaust passage of the engine, and an air-fuel ratio sensor is installed in the exhaust passage so that the air-fuel ratio becomes the target air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine in which the valve opening degree or the valve opening ratio is feedback-controlled is known (see Japanese Patent Laid-Open No. 63-94061). When the opening degree of the EGR control valve is changed and the EGR gas amount is increased or decreased in the same engine operating state, the fresh air amount is increased or decreased by the corresponding amount. Therefore, the air-fuel ratio can be changed by changing the EGR gas amount. Therefore, in the above-described air-fuel ratio control apparatus, the air-fuel ratio becomes the target air-fuel ratio by controlling the opening degree of the EGR control valve.
[0003]
[Problems to be solved by the invention]
By the way, when the air-fuel ratio is feedback controlled as in the above-described air-fuel ratio control apparatus, the air-fuel ratio fluctuates due to intake pulsation, rotational fluctuation, etc. even if the engine operating state is steady. However, if the opening degree of the EGR control valve is controlled based on the air-fuel ratio that varies in this way, the opening degree of the EGR control valve changes, so that the EGR gas amount fluctuates, and as a result, the air-fuel ratio may diverge from the target air-fuel ratio There is.
[0004]
Therefore, if the opening degree of the EGR control valve is controlled based on the smoothed air-fuel ratio obtained by smoothing the detected air-fuel ratio, fluctuations in the opening degree of the EGR control valve can be reduced, and therefore the air-fuel ratio is diverged. Can be prevented. However, if the opening degree of the EGR control valve is controlled based on the smoothed air-fuel ratio, the responsiveness of the opening degree of the EGR control valve is deteriorated. Therefore, there is a problem that the air-fuel ratio cannot be made to exactly match the target air-fuel ratio simply by controlling the opening degree of the EGR control valve using the smoothed air-fuel ratio.
[0005]
[Means for Solving the Problems]
In order to solve the above problem, according to a first invention, in an internal combustion engine in which an EGR control valve for controlling the amount of EGR gas is disposed in an EGR passage that connects an intake passage and an exhaust passage of the engine, A target air-fuel ratio calculating means for calculating the target air-fuel ratio in response, an air-fuel ratio detecting means for detecting the air-fuel ratio, a smoothing coefficient calculating means for calculating a smoothing coefficient determined according to the opening of the EGR control valve , Smoothed air-fuel ratio calculating means for calculating a smoothed air-fuel ratio by smoothing the detected air-fuel ratio using a smoothing coefficient, and so that the air-fuel ratio becomes a target air-fuel ratio based on the smoothed air-fuel ratio. And an EGR control valve opening degree control means for controlling the opening degree of the EGR control valve , and the smoothing coefficient is such that the degree of smoothing is lower when the opening degree of the EGR control valve is large than when it is small. Stipulated Ratio control system is provided. The influence of the variation of the opening degree of the EGR control valve on the air-fuel ratio varies depending on the opening degree of the EGR control valve. Therefore, in the first invention, the smoothed air-fuel ratio is calculated using a smoothing coefficient determined according to the opening degree of the EGR control valve, and the opening degree of the EGR control valve is controlled based on this smoothed air-fuel ratio. ing. As a result, the air-fuel ratio can be matched to the target air-fuel ratio satisfactorily while ensuring the responsiveness of the opening degree of the EGR control valve.
[0006]
According to the second invention, in the first invention, the opening degree of the EGR control valve is controlled based on the detected air-fuel ratio which is not smoothed during engine transient operation. When the engine transient operation is performed and the target air-fuel ratio is rapidly changed, the opening degree of the EGR control valve needs to be rapidly changed. Accordingly, in the second aspect of the invention, the opening degree of the EGR control valve is controlled based on the detected air-fuel ratio that is not smoothed during engine transient operation, thereby providing a good responsiveness of the opening degree of the EGR control valve. I try to secure it.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Below, the case where this invention is applied to a diesel engine is demonstrated. However, the present invention can also be applied to a spark ignition engine.
Referring to FIG. 1, 1 is a cylinder block, 2 is a piston, 3 is a cylinder head, 4 is a combustion chamber, 5 is an intake port, 6 is an intake valve, 7 is an exhaust port, 8 is an exhaust valve, and 9 is a combustion chamber 4. A fuel injection valve 10 for directly injecting fuel into the fuel injection valve 10 and an engine drive type fuel pump for pumping fuel to the fuel injection valve 9 are shown. The intake port 5 of each cylinder is connected to a common surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to an air flow meter 14 and an air cleaner 15 via an intake duct 13. An intake throttle valve 16 driven by a negative pressure or electromagnetic actuator or a wire directly connected to the accelerator pedal 39 is disposed in the intake duct 13. On the other hand, the exhaust port 7 of each cylinder is connected to a common exhaust manifold 17. The fuel pump 10 is controlled based on an output signal from the electronic control unit 30.
[0008]
An EGR control valve 19 for controlling the amount of EGR gas flowing in the EGR passage 18 is disposed in the EGR passage 18 that connects the exhaust manifold 17 and the intake duct 13 downstream of the intake throttle valve 16. The valve body of the EGR control valve 19 is fixed to the diaphragm 20, and the opening degree of the EGR control valve 19 is controlled according to the pressure in the negative pressure chamber 21 formed on one side of the diaphragm 20. A negative pressure formed by an engine-driven vacuum pump 23 or the atmosphere is selectively introduced into the negative pressure chamber 21 via an electromagnetic three-way valve 22. The three-way valve 22 is controlled based on an output signal from the electronic control unit 30. The three-way valve 22 is controlled with a duty ratio DUTY, which is the ratio of the time during which the negative pressure chamber 21 should be connected to the vacuum pump 23 with respect to the cycle time. Therefore, the opening degree of the EGR control valve 19 is increased as the duty ratio DUTY is increased.
[0009]
An electronic control unit (ECU) 30 comprises a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, and an input connected to each other via a bidirectional bus 31. A port 35 and an output port 36 are provided. The air flow meter 14 generates an output voltage proportional to the intake air amount Ga, and the output voltage of the air flow meter 14 is input to the input port 35 via the AD converter 37. The input port 35 is connected to a crank angle sensor 38 that generates an output pulse every time the crankshaft rotates, for example, 30 degrees. The CPU 34 calculates the engine speed N based on this output pulse. Further, the output voltage of the depression amount sensor 40 that generates an output voltage proportional to the depression amount DEP of the accelerator pedal 39 is input to the input port 35 via the AD converter 41. On the other hand, the output ports 36 are respectively connected to the fuel pump 10 and the three-way valve 22 via corresponding drive circuits 42.
[0010]
In the internal combustion engine of FIG. 1, the excess air ratio is controlled by controlling the opening of the EGR control valve 19, that is, the duty ratio DUTY. In this case, if the duty ratio DUTY is controlled based on the detected excess air ratio LACT, the duty ratio DUTY may fluctuate due to fluctuations in the excess air ratio, and thus the excess air ratio may diverge. Therefore, in the internal combustion engine of FIG. 1, the duty ratio DUTY is controlled based on the smoothed excess air ratio LSM obtained by smoothing the detected excess air ratio LACT. This smoothed excess air ratio LSM is calculated based on the following equation.
[0011]
LSM = LSM + K ・ (LACT-LSM)
Here, K is a smoothing coefficient. By controlling the duty ratio DUTY using the smoothed excess air ratio LSM in this way, the influence of fluctuations in the excess air ratio can be reduced, and therefore, the excess air ratio can be prevented from diverging.
However, when the duty ratio DUTY is controlled using the smoothed excess air ratio LSM, the responsiveness of the duty ratio DUTY deteriorates. On the other hand, the influence of the variation of the duty ratio DUTY on the excess air ratio varies depending on, for example, the engine load and the duty ratio DUTY. That is, when the engine load is high, the influence of the fluctuation of the excess air ratio becomes smaller than when the engine load is low, and when the duty ratio DUTY is large, the influence of the fluctuation of the excess air ratio becomes smaller than when the duty ratio DUTY is small. Thus, when the influence of the fluctuation of the excess air ratio is small, it is preferable to ensure the responsiveness of the duty ratio DUTY rather than reducing the influence of the fluctuation. Therefore, in this embodiment, when the fuel injection amount Q representing the engine load is large, the degree of smoothing is made lower than when it is small, and when the duty ratio DUTY is large, the degree of smoothing is made lower than when it is small. As a result, good responsiveness of the duty ratio DUTY can be ensured while reliably preventing the excess air ratio from diverging.
[0012]
In the present embodiment, the smoothing coefficient K representing the degree of smoothing is calculated as the product of the smoothing coefficient KQ based on the fuel injection amount Q and the smoothing coefficient KEGR based on the duty ratio DUTY (K = KQ · KEGR). . As shown in FIG. 2, the smoothing coefficient KQ is made smaller than when the fuel injection amount Q is small, that is, when the fuel injection amount Q is small as can be seen from the above-described calculation formula for the smoothed excess air ratio LSM. The degree of smoothing is increased compared to when there are many. The smoothing coefficient KQ is stored in advance in the ROM 32 in the form of a map as shown in FIG. On the other hand, as shown in FIG. 3, the smoothing coefficient KEGR is made smaller when the duty ratio DUTY is small than when it is large, that is, when the duty ratio DUTY is small, the degree of smoothing is made larger than when it is large. The smoothing coefficient KEGR is stored in advance in the ROM 32 in the form of a map as shown in FIG. Note that the smoothing coefficients KQ and KEGR are determined between 0 and 1, respectively. Further, the fuel injection amount Q can be expressed by a pump command value, an injector energization time, a pump pressure feed value, and the like.
[0013]
However, when the engine transient operation is performed and the target excess air ratio is rapidly changed, the duty ratio DUTY needs to be rapidly changed. That is, it is necessary to ensure the responsiveness of the duty ratio DUTY. Therefore, in this embodiment, the duty ratio DUTY is calculated based on the detected excess air ratio LACT that is not smoothed during engine transient operation, and based on the smoothed excess air ratio LSM when not during engine transient operation. The duty ratio DUTY is calculated. In this way, it is possible to maintain good responsiveness of the duty ratio DUTY during transient operation.
[0014]
FIG. 4 shows a routine for executing the above-described excess air ratio control method. The excess air ratio control routine shown in FIG. 4 is executed by interruption every predetermined time.
Referring to FIG. 4, first, at step 40, the fuel injection amount Q is read. This fuel injection amount Q is calculated as a product of the basic fuel injection amount QB and the correction coefficient in a routine (not shown), for example. The basic fuel injection amount QB is an injection amount required to make the engine output torque a required torque, and is stored in advance in the ROM 32 as a function of the depression amount DEP of the accelerator pedal 39 and the engine speed N. In the following step 41, the target excess air ratio LTGT is calculated. The target excess air ratio LTGT is a best excess air ratio to reduce the NO _x exhausted from the engine while preventing the smoke discharged from the engine, it is determined by experiment. This target excess air ratio LTGT is stored in advance in the ROM 32 in the form of a map shown in FIG. 5 as a function of the engine operating state, that is, for example, the fuel injection amount Q and the engine speed N. In the subsequent step 42, the actual excess air ratio LACT is calculated. In the internal combustion engine of FIG. 1, the actual excess air ratio LACT is calculated using the intake air amount Ga detected by the air flow meter 14 and the fuel injection amount Q. An excess air ratio sensor or an air / fuel ratio sensor that generates an output voltage corresponding to the excess air ratio or the air / fuel ratio may be arranged in the engine exhaust passage, and the excess air ratio LACT may be detected by this sensor.
[0015]
In the following step 43, it is determined whether or not the engine is currently in transient operation. In the internal combustion engine of FIG. 1, it is possible to determine whether or not the engine is in transient operation. However, in the internal combustion engine of FIG. 1, changes in the depression amount DEP of the accelerator pedal 39, the intake air amount Ga, the fuel injection amount Q, the engine speed N When the rate is greater than a predetermined set change rate, it is determined that the operation is in transient operation. When the change rate is less than the set change rate, it is determined that the operation is not during transient operation. If it is determined that it is not during transient operation, then the routine proceeds to step 44.
[0016]
Steps 44 to 46 are parts for calculating the smoothing coefficient K. First, at step 44, a smoothing coefficient KQ based on the fuel injection amount is calculated from the map of FIG. In the following step 45, the opening degree of the EGR control valve 19, that is, the smoothing coefficient KEGR based on the duty ratio DUTY is calculated from the map of FIG. In the following step 46, the smoothing coefficient K is calculated as the product (KQ · KEGR) of the smoothing coefficient KQ and the smoothing coefficient KEGR. In the following step 47, a smoothed excess air ratio LSM obtained by smoothing the detected excess air ratio LACT is calculated based on the following equation.
[0017]
LSM = LSM + K ・ (LACT-LSM)
In the following step 48, LSM is set to L and then jumps to step 50.
On the other hand, when it is determined in step 43 that the engine is in a transient operation, the routine proceeds to step 49 where the detected excess air ratio LACT that is not smoothed is set to L. Next, the routine proceeds to step 50.
[0018]
Subsequent steps 50 to 53 are parts for calculating the duty ratio DUTY. First, at step 50, the proportional term P is calculated based on the following equation.
P = KP · (LTGT-L)
Here, KP is a proportional gain constant. In the following step 51, the integral term I is calculated based on the following equation.
[0019]
I = I + KI · (LTGT-L)
Here, KI is an integral gain constant. In the following step 52, the differential term D is calculated based on the following equation.
D = KD ・ (L-LOLD)
Here, KD is a differential gain constant, and LOLD is L in the previous processing routine. In the subsequent step 53, the duty ratio DUTY is calculated as the sum (P + I + D) of the proportional term P, the integral term I, and the derivative term D. The three-way valve 22 is driven with this duty ratio DUTY. In the next step 54, L is set to LOLD.
[0020]
Thus, in this embodiment, the influence of fluctuations in the excess air ratio can be reduced, and the excess air ratio can be stabilized. As a result, the gain constants KP, KI, and KD can be set relatively large, and therefore the initial response during transient operation can be further enhanced.
In this embodiment, the gain constants KP, KI, and KD are constant values. However, when the duty ratio DUTY is small, the gain constants KP, KI, and KD are made smaller than when the duty ratio DUTY is large. When it is small, the proportional term P, the integral term I, and the differential term D may be smaller than when it is large.
[0021]
【The invention's effect】
The air-fuel ratio can be accurately matched with the target air-fuel ratio while ensuring the responsiveness of the opening degree of the EGR control valve.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine.
FIG. 2 is a diagram showing a smoothing coefficient based on a fuel injection amount.
FIG. 3 is a diagram showing a smoothing coefficient based on a duty ratio.
FIG. 4 is a flowchart for executing excess air ratio control.
FIG. 5 is a diagram showing a target excess air ratio.
[Explanation of symbols]
4 ... Combustion chamber 9 ... Fuel injection valve 13 ... Intake duct 14 ... Air flow meter 17 ... Exhaust manifold 18 ... EGR passage 19 ... EGR control valve

Claims (2)

In an internal combustion engine in which an EGR control valve that controls the amount of EGR gas is arranged in an EGR passage that connects an intake passage and an exhaust passage of the engine, target air-fuel ratio calculating means that calculates a target air-fuel ratio according to the engine operating state; and air-fuel ratio detecting means for detecting an air-fuel ratio, smoothing and smoothing coefficient calculation means for calculating a smoothing coefficient determined in accordance with the opening degree of the EGR control valve, the air-fuel ratio issued該検 using the smoothing coefficient And a smoothed air-fuel ratio calculating means for calculating a smoothed air-fuel ratio, and an EGR control valve opening for controlling the opening of the EGR control valve so that the air-fuel ratio becomes the target air-fuel ratio based on the smoothed air-fuel ratio An air-fuel ratio control apparatus , wherein the smoothing coefficient is determined such that when the opening degree of the EGR control valve is large, the degree of smoothing is lower than when it is small .   The air-fuel ratio control apparatus according to claim 1, wherein the opening degree of the EGR control valve is controlled so that the air-fuel ratio becomes a target air-fuel ratio based on the detected air-fuel ratio that has not been smoothed during engine transient operation.

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