JP4518363B2 - 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, and more particularly to an air-fuel ratio control apparatus that performs appropriate feedback control to prevent a shock due to a sudden change in the air-fuel ratio.
[0002]
[Prior art]
In the air-fuel ratio control of the internal combustion engine, it is good by controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine according to the output of the air-fuel ratio sensor arranged in the exhaust system of the internal combustion engine and maintaining the target air-fuel ratio. Exhaust characteristics are ensured.
[0003]
However, if the air-fuel ratio feedback coefficient is changed in response to a sudden change in the output of the air-fuel ratio sensor, a shock (air-fuel ratio shock) due to the sudden change in output may occur and drivability may be reduced. Therefore, a method for preventing an air-fuel ratio shock by providing a limit for the feedback coefficient so that the feedback coefficient does not change beyond the limit value is conventionally known.
[0004]
In Japanese Patent Publication No. 7-117000, in air-fuel ratio feedback control in which vaporized fuel in the fuel tank is supplied during operation of the internal combustion engine, overcorrection of the air-fuel ratio at the time of abnormal output of the exhaust gas concentration detector is prevented. In order to achieve this, a method for expanding the limit of the air-fuel ratio correction coefficient as the fuel tank internal pressure increases is disclosed.
[0005]
[Problems to be solved by the invention]
In the above-described conventional technology, if the fuel injection amount is corrected by updating the feedback coefficient, although the exhaust gas characteristics are expected to improve further, the feedback coefficient is updated from the actual feedback limit due to the demand for drivability. Since it stops in a narrow range (that is, a limit value), further improvement in exhaust gas characteristics cannot be expected. In order to deal with this problem, for example, even if the method of enlarging the limit value disclosed in Japanese Examined Patent Publication No. 7-117000 is applied, the exhaust gas characteristic is improved by simply increasing the limit of the air-fuel ratio correction coefficient, but the exhaust gas characteristic is improved. It is not possible to prevent a fuel ratio shock.
[0006]
Accordingly, it is an object of the present invention to provide an air-fuel ratio control apparatus for an internal combustion engine that can maintain both drivability and improve exhaust characteristics by preventing air-fuel ratio shocks.
[0007]
[Means for Solving the Problems]
In one aspect, the present invention provides an air-fuel ratio feedback control coefficient calculating means for calculating a correction coefficient for feedback control based on an output of an air-fuel ratio sensor provided in an exhaust system of the internal combustion engine, and an air-fuel mixture supplied to the internal combustion engine. And an air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio of the engine based on the correction coefficient, when the correction coefficient reaches a predetermined upper limit threshold or lower limit threshold Further, the present invention provides an air-fuel ratio control apparatus for an internal combustion engine further comprising feedback gain adjusting means for adjusting the feedback gain of the correction coefficient.
[0008]
According to this aspect, in the air-fuel ratio feedback control, even when the feedback correction coefficient reaches the threshold value provided for preventing the air-fuel ratio shock, the feedback gain adjusting means for resetting the feedback gain is provided to provide feedback. Since the air-fuel ratio feedback is continued without stopping the update of the correction coefficient, the exhaust characteristic can be improved and the air-fuel ratio shock can be prevented without deteriorating the followability of the air-fuel ratio.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing an overall configuration of an internal combustion engine (hereinafter referred to as an engine) provided with an air-fuel ratio control apparatus according to an embodiment of the present invention.
[0010]
A throttle valve 3 is arranged in the middle of the intake pipe 2 leading to the engine 1. A throttle valve opening (TH) sensor 4 is connected to the throttle valve 3, and an electric signal corresponding to the opening of the throttle valve 3 is output and supplied to an electronic control unit (hereinafter referred to as ECU) 5. The configuration of the ECU 5 will be described later.
[0011]
A fuel injection valve 6 is provided between the engine 1 of the intake pipe 2 and the throttle valve 3. The fuel injection valve 6 is electrically connected to the ECU 5, and the valve opening time of the fuel injection is controlled by a signal from the ECU 5. The injected fuel is mixed with the air from the intake pipe 2 to become an air-fuel mixture and supplied to the engine 1.
[0012]
An intake pipe absolute pressure (PBA) sensor 7 is attached downstream of the throttle valve 3 to detect the absolute pressure and supply it to the ECU 5 as an electrical signal.
[0013]
The engine 1 is provided with an engine coolant temperature (TW) sensor 10. The TW sensor 10 is composed of a thermistor or the like, and is mounted in an engine cylinder peripheral wall filled with cooling water, and supplies a detected water temperature signal to the ECU 5.
[0014]
Further, an engine speed (NE) sensor 11 and a cylinder determination (CYL) sensor 12 are attached to the engine 1. The engine speed sensor 11 outputs a pulse (hereinafter referred to as a TDC pulse signal) at a predetermined crank angle position every 180 degrees rotation of the crankshaft of the engine 1, and the cylinder determination sensor 12 is a predetermined crank angle of a specific cylinder. Output signal pulse at position. Both pulses are supplied to the ECU 5.
[0015]
The catalytic converter 14 is disposed in the exhaust pipe 13 of the engine 1 and purifies components such as HC, CO, and NOx in the exhaust gas. The exhaust pipe upstream of the catalytic converter 14 is provided with an oxygen concentration sensor 15 (hereinafter referred to as an O2 sensor) as an air-fuel ratio sensor. The O2 sensor 15 detects the oxygen concentration in the exhaust gas, outputs an electrical signal corresponding to the detected value, and supplies it to the ECU 5.
[0016]
The ECU 5 is composed of a computer, a ROM for storing programs and data, a RAM for storing programs and data necessary for execution and providing a calculation work area, a CPU for executing programs, and input signals from various sensors. It has an input interface for processing and a drive circuit for sending control signals to valves and the like. Signals from the aforementioned sensors are received by the input interface and processed according to a program stored in the ROM. In FIG. 1, the ECU 5 is shown as a functional block based on such a hardware configuration.
[0017]
The ECU 5 includes functional blocks of an operating state detection unit 21, a feedback correction coefficient calculation unit 22, a correction coefficient determination unit 23, a feedback gain adjustment unit 24, and a feedback control unit 25.
[0018]
The operation state detection unit 21 determines various operation states of the engine 1 based on the output of each sensor signal.
[0019]
Based on various engine parameter signals such as engine water temperature, engine speed, engine load state, and O2 sensor active state, the feedback correction coefficient calculation unit 22 performs various engines such as a feedback control operation region and an open loop control operation region. The operating state is determined, and a feedback correction coefficient KO2 is calculated by a KO2 calculation subroutine which will be described later with reference to FIG.
[0020]
The correction coefficient determination unit 23 determines whether or not the calculated correction coefficient KO2 exceeds a predetermined threshold value. If the threshold value is not exceeded, the correction coefficient KO2 is sent to the feedback control unit 25. When the threshold value is exceeded, control is passed to the feedback gain adjustment unit 24.
[0021]
The feedback gain adjustment unit 24 recalculates the correction coefficient KO2 using a smaller gain than usual as the feedback gain.
[0022]
The feedback control unit 25 receives the correction coefficient KO2 from the correction coefficient determination unit 23 or the feedback gain adjustment unit 24, and calculates the fuel injection time Tout of the fuel injection valve 6 by the following equation in synchronization with the TDC signal pulse.
[0023]
[Expression 1]
Tout = Ti × KO2 × K1 + K2 (1)
[0024]
Here, Ti is the basic fuel injection time, and is determined, for example, by searching a predetermined Ti map according to the engine speed NE and the intake pipe absolute pressure PBA. K1 and K2 are other correction coefficients and correction variables that are calculated according to various engine parameter signals, respectively, and optimize various characteristics such as fuel efficiency characteristics and engine acceleration performance according to engine operating conditions. It is set to such a value. The feedback control unit 25 supplies the fuel injection valve 6 with a drive signal for opening the fuel injection valve 6 based on the calculated TOUT.
[0025]
FIG. 2 shows a flowchart of a subroutine for calculating the feedback correction coefficient KO2. This subroutine is executed in synchronism with the generation of each TDC signal pulse.
[0026]
First, in step 201, it is determined whether activation of the O2 sensor 15 is completed. Specifically, the internal resistance detection method of the O2 sensor 15 detects whether the output voltage of the O2 sensor 15 has reached the activation start point Vx (for example, 0.6 V), and is activated when it reaches Vx. Is determined. If the determination in step 201 is NO, that is, if the activation of the O2 sensor 15 is not completed, KO2 is set to 1.0 in step 202, and this process ends. If the determination in step 201 is YES, that is, if the activation has been completed, it is determined in step 203 whether or not the engine 1 is operating in the open loop control operation region. This open-loop control operation region refers to a full load region, a low rotational speed region, a high rotational speed region, an air-fuel mixture leaning region, and the like of the internal combustion engine. If the operation is in the open control operation area, the process proceeds to step 202, KO2 is set to 1.0, and this process is terminated.
[0027]
If it is determined in step 203 that the operation state of the engine 1 is in the feedback control operation region, feedback control is executed. In this case, the process proceeds to step 204, and it is determined whether or not the output level of the O2 sensor 15 has been reversed. In the case of YES, that is, when the output level of the O2 sensor is inverted, proportional control (P term control) is performed in the subsequent steps.
[0028]
First, in step 205, it is determined whether or not the output level of the O2 sensor 15 is low level (LOW). When the output level is low level, the NE-tPR table stored in a memory (not shown) in the ECU 5 is searched. Then, a predetermined time tPR corresponding to the engine speed NE is obtained (206). This predetermined time tPR is for keeping the application period of the second correction value PR, which will be described later, constant over the entire engine speed range, and is therefore set to a smaller value as the engine speed NE is larger.
[0029]
Next, it is determined whether or not a predetermined time tPR has elapsed since the previous application of the second correction value PR (207). If the determination in step 207 is YES, that is, if tPR has elapsed, a second correction value PR (208) corresponding to the engine speed NE is obtained from the NE-PR table stored in the memory. When the determination in step 207 is NO, that is, when the predetermined time tPR has not elapsed, the NE-P table stored in the memory is searched separately from the NE-PR table, and the engine speed NE is determined. A first correction value P is obtained (209). The first correction value P is set to a value smaller than the second correction value PR.
[0030]
Next, the proportional coefficient feedback gain Pi, that is, the first correction value P or the second correction value PR is added to the correction coefficient KO2 to calculate the correction coefficient KO2 (210).
[0031]
In step 205, if the output of the O2 sensor 15 is not LOW, the first correction value P corresponding to the engine speed NE is obtained from the NE-P table as in step 209 (211), and the correction value is calculated from the correction coefficient KO2. The correction coefficient KO2 is calculated by subtracting (proportional term feedback gain) P (212).
[0032]
In step 213, it is determined whether or not the calculated correction coefficient KO2 exceeds a predetermined upper limit value KO2PH. If so, KO2 is set to the upper limit (214). If it is less than or equal to the upper limit value, it is subsequently determined whether it is below a predetermined lower limit value KO2PL (215). If so, KO2 is set to the lower limit (216). If it is above the lower limit, KO2 is left as it is. Thus, in proportional control, the correction coefficient KO2 is subjected to a limit check.
[0033]
Subsequently, in step 217, the average value KREF of KO2 is calculated by the following equation using the correction coefficient KO2 calculated in step 210 or 212 or set to the upper and lower limit values in step 214 or 216, respectively. Then, the process is terminated by storing in a memory (not shown). This average value KREF is calculated for each operation region corresponding to the current loop in the feedback control operation region, based on a KREF calculation subroutine (not shown).
[0034]
[Expression 2]
KREFn = Cn x KO2P + (1-Cn) x KREFn '
Here, the value KO2P is the value of KO2 immediately before or immediately after the proportional term (P term) operation, Cn is a variable (where 0 <Cn <1) that is set to an appropriate value by experiment for each operating region, and KREFn 'Is the average value of KO2P obtained up to the previous time in the operation region corresponding to this loop.
[0035]
The ratio of KO2P to KREFn at the time of each P-term operation changes depending on the value of variable Cn, that is, the calculation speed of average value KREFn changes, so this Cn value depends on the specifications of the target air-fuel ratio feedback device, engine, etc. An optimal KREFn can be obtained by setting to an appropriate value.
[0036]
Referring to FIG. 2 again, if the result of determination in step 204 is NO, that is, if the output level of the O2 sensor 15 is not inverted, integral control (I-term control) is executed in step 218 and thereafter. First, as in step 205, it is determined whether or not the output level of the O2 sensor 15 is at a low level (LOW) (218). If the determination in step 218 is YES, that is, if the output level of the O2 sensor 15 is low, the TDC signal pulse is counted (219), and it is determined whether or not the count number NIL has reached a predetermined value NI. (220).
[0037]
If NO in step 220, the correction coefficient KO2 is held at the immediately preceding value (221). In the case of YES, a new correction coefficient KO2 is calculated by adding a predetermined integral term feedback gain Δk to the correction coefficient KO2 (222).
[0038]
In step 223, the feedback control coefficient KO2 is set to the integral control upper limit threshold value KO2IH. This threshold value is provided to prevent a shock due to a sudden change in the air-fuel ratio, and is set to a value smaller than the actual feedback limit. This KO2IH may be a preset value or may be obtained by searching a value corresponding to the driving state from a map.
[0039]
Subsequently, it is determined whether or not the correction coefficient KO2 calculated in step 222 is larger than the upper threshold value KO2IH (224). If the determination in step 224 is NO, that is, if it is equal to or less than the upper threshold value, the correction coefficient KO2 is left as it is, and the process proceeds to step 226. If the determination in step 224 is YES, that is, if it is larger than the upper threshold value, a gain slower than normal is used instead of the integral term feedback gain Δk. Specifically, the feedback correction coefficient KO2 is recalculated by the following equation (225).
[0040]
[Equation 3]
KO2 ＝ KO2 + Δk × KIH
[0041]
Here, KIH is a coefficient for reducing the integral term feedback gain, and is preset in the range of 0 <KIH <1. Or you may set KIH by searching the value according to the driving | running state from the map.
[0042]
When the value of the correction coefficient KO2 is finally determined, the count number NIL is reset to 0 (226). In this way, when the output of the O2 sensor 19 continues to be at a lean level, every time the TDC signal pulse is generated NI times in the direction of correcting this (that is, every time the count number NIL reaches NI), the correction coefficient KO2 is set. Integration control is executed by adding Δk or Δk × KIH.
[0043]
If the answer in step 218 is NO, the number of TDC signal pulses is counted (227), and it is determined whether the count number NIH has reached a predetermined value NI (228). If the determination in step 228 is NO, the correction coefficient KO2 is held at the value immediately before (234). If the determination in step 228 is YES, a predetermined value Δk is subtracted from the correction coefficient KO2 (229).
[0044]
In step 230, a lower limit threshold value KO2IL for integral control of the feedback correction coefficient KO2 is set. Similar to the upper limit threshold value KO2IH described above, KO2IL may be a preset value or may be obtained by searching a value corresponding to the operating state from a map.
[0045]
Subsequently, it is determined whether or not the feedback correction coefficient KO2 calculated in step 229 is smaller than the lower threshold KO2IL (231). If the determination in step 231 is NO, that is, if it is greater than or equal to the lower threshold value, the value of KO2 is left as it is, and the process proceeds to step 233. If the determination in step 231 is YES, that is, if it is smaller than the lower threshold value, a gain slower than normal is used instead of the integral term feedback gain Δk. Specifically, the feedback correction coefficient KO2 is recalculated by the following equation (232).
[0046]
[Expression 4]
KO2 ＝ KO2−Δk × KIL
[0047]
KIL is a coefficient for reducing the feedback gain, and is set in advance in the range of 0 <KIL <1, or is set by searching a value corresponding to the driving state from the map, like KIH.
[0048]
When the value of the correction coefficient KO2 is determined, the count number NIH is reset to 0 (233). In this way, when the output of the O2 sensor 19 remains at the rich level, every time the TDC signal pulse is generated NI times in the direction of correcting the output (that is, every time the count number NIH reaches the predetermined value NI), the correction coefficient. Integral control is executed by subtracting Δk or Δk × KIL from KO2.
[0049]
As explained above, in the integral control, unlike the proportional control, when the correction coefficient KO2 is larger than the upper threshold value or smaller than the lower threshold value, the feedback gain is made smaller than usual. Re-set and continue updating the feedback correction coefficient.
[0050]
FIG. 3 is a diagram comparing the control according to the conventional method and the control according to the present embodiment. In FIG. 3, (1) shows a case where the feedback control according to the conventional method for limiting the correction coefficient is applied, and (2) shows a case where the feedback control according to the present embodiment shown in FIG. 2 is applied. In each of (1) and (2), (a) shows the transition of the feedback correction coefficient, and (b) shows the actual air-fuel ratio of the air-fuel mixture supplied to the engine 1.
[0051]
In the control (1) of the conventional method, when the feedback correction coefficient KO2 reaches the lower threshold KO2L at time A, it is equal to the lower threshold until the calculated KO2 exceeds the lower threshold (B). As a result, the air-fuel ratio temporarily fluctuates greatly toward the rich side, which deteriorates the exhaust characteristics.
[0052]
On the other hand, in the control (2) according to the present embodiment, even when the feedback correction coefficient KO2 reaches the lower limit threshold KO2L at the time point A, the correction coefficient KO2 is continuously updated although the feedback gain is reset to a smaller value. Therefore, it can be seen that the air-fuel ratio rich amount is greatly reduced compared with the conventional method of (1), and the deterioration of the exhaust characteristics can be suppressed.
[0053]
【The invention's effect】
According to the present invention, even when the feedback correction coefficient reaches the threshold value provided for preventing the air-fuel ratio shock, the air-fuel ratio is maintained without stopping the update of the feedback correction coefficient by using a smaller gain as the feedback gain. Since the feedback is continued, the exhaust characteristic can be improved without deteriorating the followability of the air-fuel ratio, and an air-fuel ratio shock can be prevented by suppressing a sudden change in the feedback correction coefficient.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an internal combustion engine including an air-fuel ratio control apparatus according to an embodiment of the present invention.
FIG. 2 is a flowchart of a subroutine for calculating a feedback correction coefficient KO2.
FIG. 3 is a diagram showing transitions of a feedback correction coefficient (a) and an actual air-fuel ratio (b) in conventional feedback control, and (2) is a feedback correction coefficient (a in feedback control according to the present invention). ) And the actual air-fuel ratio (b).
[Explanation of symbols]
1 Internal combustion engine
5 Electronic control unit (ECU)
6 Fuel injection valve
11 Engine speed sensor
15 Air-fuel ratio sensor
21 Operating state detector
22 Feedback correction coefficient calculator
24 Feedback gain adjuster
25 Feedback control unit

Claims (2)

Air-fuel ratio feedback control coefficient calculating means for calculating a correction coefficient for air-fuel ratio feedback control based on the output of an air-fuel ratio sensor provided in the exhaust system of the internal combustion engine;
An air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine based on the correction coefficient,
Determination means for determining whether or not the output level of the air-fuel ratio sensor has changed from a rich level to a lean level, or from a lean level to a rich level;
The air-fuel ratio feedback control coefficient calculation means calculates the correction coefficient based on a predetermined feedback gain at every predetermined timing when the output level of the air-fuel ratio sensor is not reversed.
When the correction coefficient is greater than a predetermined upper threshold value, the feedback gain of the correction coefficient is decreased by a predetermined percentage and when the correction coefficient is smaller than the predetermined upper threshold value, An internal combustion engine further comprising feedback gain adjusting means for reducing a feedback gain of the correction coefficient by a predetermined percentage when the correction coefficient is smaller than a lower threshold value than when the correction coefficient is larger than a predetermined lower threshold value. Air-fuel ratio control device. The feedback gain adjusting means compares the correction coefficient with the predetermined upper threshold value or the predetermined lower threshold value at each predetermined timing when the output level of the air-fuel ratio sensor is not reversed. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein:

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