JP2005337139A - Air fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To surely prevent deterioration of emissions by appropriately suppressing abnormal air fuel ratios caused by a downstream sensor, in sensors detecting air fuel ratios in an upstream and an downstream exhaust emission control catalyst. <P>SOLUTION: It is detected based on a value of frequency of appearance Lr that output voltage of an O2 sensor existing in a downstream of a catalyst converter slips to a lean side, a control width B is established according to the frequency of appearance Lr (S208) and a plus side guard value dVgrd(+) is established with adding to a learning value DVG of correction amount (S210, S218). Degree of the abnormal condition of the O2 sensor can be reflected on the correction amount by the control width B, and variation of characteristics of an air fuel ratio sensor in the upstream side can be reflected to the plus side guard value sVgrd(+) since the learning value DVG is referred. Consequently, the correction value can be limited in an appropriate range where the degree of abnormal condition of the O2 sensor and variation of characteristics of the air fuel ratio sensor can be absorbed. A target is achieved thereby. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention includes sensors for detecting an air-fuel ratio state from exhaust components on the upstream side and the downstream side of an exhaust purification catalyst provided in an exhaust system of an internal combustion engine, and based on the output value of the upstream sensor. The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that executes control and executes correction for the air-fuel ratio control based on an output value of a downstream sensor.

  Conventionally, in order to effectively function an exhaust purification catalyst of an internal combustion engine, a sensor (for example, a sensor whose output value gradually changes in accordance with the air-fuel ratio) is provided upstream of the exhaust purification catalyst, and the theoretical air-fuel ratio is the center. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is controlled within an extremely narrow range. Further, by providing another sensor (for example, an O2 sensor) on the downstream side of the exhaust purification catalyst so that exhaust purification by the exhaust purification catalyst is always properly performed under such air-fuel ratio controlled conditions, exhaust gas is provided. There is a system for correcting the air-fuel ratio control based on the oxygen concentration on the downstream side of the purification catalyst (see, for example, Patent Document 1).

In addition, there is an abnormality diagnosis system that detects element cracks in an exhaust purification catalyst downstream sensor from an output frequency distribution (see, for example, Patent Document 2).
JP-A-7-197837 (page 6-8, FIG. 3) JP 2003-14683 (page 6-7, FIG. 7)

  When the occurrence of cracks in the downstream sensor is detected by the abnormality diagnosis system as shown in Patent Document 2, the oxygen concentration in the exhaust gas is detected to be considerably higher than the actual due to the output deviation of the downstream sensor. Yes. In such a case, in the system of Patent Document 1, the correction for lowering the air-fuel ratio by increasing the fuel concentration is performed excessively. For this reason, when an abnormality is diagnosed by the abnormality diagnosis system, there is a possibility that the emission has already deteriorated considerably from the air-fuel ratio suitable for exhaust gas purification, and further, even in the retreat travel until the downstream sensor is replaced, The correction for excessively reducing the air-fuel ratio is continued. Therefore, the techniques disclosed in Patent Documents 1 and 2 cannot cope with the deterioration of emissions during retreat travel.

  In order to cope with such emission deterioration, it is conceivable that the correction amount for the air-fuel ratio control is limited based on the output deviation of the downstream side sensor until the abnormality is clearly detected or during the retreat travel. However, such restrictions based on the output deviation of the downstream sensor may not be able to absorb variations in upstream sensor characteristics. For this reason, there is a possibility that prevention of emission deterioration is insufficient.

  An object of the present invention is to reliably prevent the deterioration of the emission when the downstream sensor is abnormal by appropriately suppressing the abnormality of the air-fuel ratio caused by the output of the downstream sensor of the exhaust purification catalyst.

In the following, means for achieving the above object and its effects are described.
The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 includes sensors for detecting an air-fuel ratio state from exhaust components on an upstream side and a downstream side of an exhaust purification catalyst provided in an exhaust system of the internal combustion engine, An air-fuel ratio control apparatus for an internal combustion engine that executes air-fuel ratio control based on an output value of an upstream sensor and performs correction for the air-fuel ratio control by a correction amount set based on an output value of a downstream sensor. And a downstream sensor abnormal state detection unit that detects an abnormal state of the downstream sensor, and a correction that holds the value of the correction amount as a learning value when the downstream sensor abnormal state detection unit does not detect the abnormal state. Correction for the air-fuel ratio control based on the learning value held by the correction amount learning means when the amount learning means and the downstream sensor abnormal state detection means detect an abnormal state Characterized in that a correction amount adjusting means for setting the malfunction-time limitations.

  The correction amount adjusting means uses the learning value held by the correction amount learning means, and when the downstream sensor abnormal state detecting means detects an abnormal state, the correction amount for the air-fuel ratio control based on this learning value. The limit for abnormal time is set in.

  For this reason, first, the abnormal state of the downstream side sensor can be reflected in the air-fuel ratio control as the abnormal limit of the correction amount. Since the value of the correction amount is learned and held in the learning value that is the reference for the abnormal limit, when the abnormal state is not detected, the upstream sensor characteristic varies in the abnormal limit. Is reflected. Therefore, the correction amount can be limited to an appropriate range in which variations in upstream sensor characteristics can be absorbed.

  In this way, it is possible to appropriately suppress the air-fuel ratio abnormality caused by the output of the downstream side sensor of the exhaust purification catalyst, and it is possible to reliably prevent the deterioration of the emission when the downstream side sensor is abnormal.

  The air-fuel ratio control apparatus for an internal combustion engine according to claim 2, wherein the correction amount adjusting means is configured to detect an abnormality limit width of the correction amount for the air-fuel ratio control based on the downstream sensor abnormal state detection. It is set according to the degree of the abnormal state detected by the means.

  The width of the limit for abnormality in the correction amount may be set according to the degree of the abnormal state. In this way, by setting a limit width corresponding to the degree of the abnormal state to the correction amount, it is possible to execute a restriction corresponding to the degree of the abnormal state on the correction amount. It becomes difficult to influence. In this case as well, the range for the abnormal time limit is set with the learning value as a reference, so that the correction amount can be limited to an appropriate range in which variations in upstream sensor characteristics can be absorbed.

  In this way, it is possible to appropriately suppress the air-fuel ratio abnormality caused by the output of the downstream side sensor of the exhaust purification catalyst, and it is possible to reliably prevent the deterioration of the emission when the downstream side sensor is abnormal.

  The air-fuel ratio control apparatus for an internal combustion engine according to claim 3, wherein the downstream side sensor abnormal state detecting means detects an abnormal state according to an appearance frequency distribution of output values of the downstream side sensor. It is characterized by doing.

  When an element crack or the like occurs in the downstream sensor, the output value of the downstream sensor does not correspond to the air-fuel ratio realized by the air-fuel ratio control, and the output value shifts. Since the degree of the deviation can be determined by looking at the appearance frequency distribution of the output value of the downstream sensor, an abnormal state can be detected according to the appearance frequency distribution.

  In the air-fuel ratio control apparatus for an internal combustion engine according to claim 4, in any one of claims 1 to 3, the downstream-side sensor abnormal state detecting means has an appearance frequency distribution of output values of the downstream-side sensor on the lean side. It is characterized by determining that the degree of an abnormal state is high as it becomes large.

  The output value of the downstream side sensor deviates further from the actual air-fuel ratio due to cracking. This is the same in both the O2 sensor in which the output value changes abruptly between the rich side and the lean side around the theoretical air-fuel ratio, and also in the air-fuel ratio sensor in which the output value changes gradually according to the air-fuel ratio. The degree of deviation toward the lean side corresponds to the degree of progress of cracking. From this, it can be determined that the degree of the abnormal state is higher as the appearance frequency distribution of the output value of the downstream sensor becomes larger on the lean side.

  Therefore, the correction amount adjusting means sets the abnormal limit for the correction amount as described above according to the appearance frequency distribution on the lean side, thereby determining the degree of abnormal state of the downstream sensor and the variation in the upstream sensor characteristics. Can be reflected in the correction amount of the air-fuel ratio control.

  In this way, it is possible to appropriately suppress the air-fuel ratio abnormality caused by the output of the downstream side sensor of the exhaust purification catalyst, and it is possible to reliably prevent the deterioration of the emission when the downstream side sensor is abnormal.

  The air-fuel ratio control apparatus for an internal combustion engine according to claim 5, wherein the downstream sensor abnormal condition detecting means provides a reference output value on the lean side, and outputs the downstream sensor. Among them, the appearance frequency of the output on the lean side with respect to the reference output value is detected as the degree of the abnormal state of the downstream sensor.

  As a method for detecting the deviation of the output value on the lean side due to element cracking, a reference output value is provided on the lean side, and the appearance frequency of the output value on the lean side further than this reference output value is determined in the abnormal state of the downstream sensor. You may detect as a grade. By providing a reference output value in this way, and counting the frequency of whether the output value is leaner or richer than the reference output value and calculating the frequency, the degree of abnormal state of the downstream sensor can be detected. Therefore, it is possible to easily detect the abnormal state of the downstream sensor.

  In the air-fuel ratio control apparatus for an internal combustion engine according to claim 6, in any one of claims 1 to 5, the correction amount adjusting means has a degree of abnormal state detected by the downstream sensor abnormal condition detecting means. It is characterized in that, as the value is higher, the range for the abnormal time limit is set in a direction to suppress the richness of the air-fuel ratio.

The correction amount adjusting means may set the range for the abnormal time limit so that the richness of the air-fuel ratio is suppressed as the degree of the abnormal state of the downstream sensor is higher.
The air-fuel ratio control apparatus for an internal combustion engine according to claim 7, wherein the downstream side sensor has an output value that changes abruptly between the rich side and the lean side around the theoretical air-fuel ratio. It is characterized by being an O2 sensor.

  Here, the O2 sensor can be used as the downstream sensor. The output value of the O2 sensor changes abruptly around the theoretical air-fuel ratio, but even with such an output, correction for air-fuel ratio control can be executed based on the output value. Since the degree of abnormal state of the O2 sensor appears in the appearance frequency distribution of the output value of the O2 sensor, it becomes easy to set the abnormal limit of the correction amount reflecting the abnormal state of the downstream sensor.

  In this way, it is possible to appropriately suppress the air-fuel ratio abnormality caused by the output of the downstream side sensor of the exhaust purification catalyst, and it is possible to reliably prevent the deterioration of the emission when the downstream side sensor is abnormal.

  The air-fuel ratio control device for an internal combustion engine according to claim 8 is the air-fuel ratio control device according to any one of claims 1 to 7, wherein the upstream side sensor is an air-fuel ratio sensor whose output value gradually changes according to the air-fuel ratio, or a theoretical sky It is an O2 sensor whose output value changes abruptly between the rich side and the lean side around the fuel ratio.

  As the upstream sensor, the air-fuel ratio sensor or the O2 sensor can be used. When the air-fuel ratio control is performed using such an upstream sensor, the abnormal state of the downstream sensor is corrected with respect to the correction amount for correcting the output of the upstream sensor or the air-fuel ratio control parameter calculated from the output. Set the limit for abnormal conditions that reflects.

  In this way, it is possible to appropriately suppress the air-fuel ratio abnormality caused by the output of the downstream side sensor of the exhaust purification catalyst, and it is possible to reliably prevent the deterioration of the emission when the downstream side sensor is abnormal.

  The air-fuel ratio control apparatus for an internal combustion engine according to claim 9, wherein the air-fuel ratio control is based on a difference between an output value of the upstream sensor and a theoretical air-fuel ratio equivalent output value. And a fuel injection amount calculation process is executed, and a correction by the correction amount calculated based on the difference between the output value of the downstream sensor and the theoretical air-fuel ratio equivalent output value is added in the calculation process. To do.

  By adding the correction based on this correction amount, the fuel injection amount is calculated so as to obtain an appropriate air-fuel ratio, and exhaust purification by the exhaust purification catalyst is always executed appropriately. Then, the abnormal limit is set to this correction amount.

  In this way, it is possible to appropriately suppress the air-fuel ratio abnormality caused by the output of the downstream side sensor of the exhaust purification catalyst, and it is possible to reliably prevent the deterioration of the emission when the downstream side sensor is abnormal.

[Embodiment 1]
FIG. 1 is a schematic configuration diagram of a gasoline engine (hereinafter abbreviated as “engine”) 2 as an internal combustion engine mounted on a vehicle and an electronic control unit (hereinafter referred to as “ECU”) 4 as an air-fuel ratio control device. Is shown. The engine 2 is a four-cylinder engine here, but only one cylinder is shown in a longitudinal sectional view in FIG. The number of cylinders may be another number of cylinders, for example, 3 cylinders, 6 cylinders, or 8 cylinders. In the drawing, one intake valve 2a and one exhaust valve 2b are shown for each cylinder, but a 4-valve engine or a 5-valve engine may be used.

  The output of the engine 2 is finally transmitted as traveling driving force to the wheels via the transmission. The engine 2 is provided with a spark plug 14 that ignites the air-fuel mixture in the combustion chamber 10. The combustion chamber 10 is provided with an intake port 16 that is opened and closed by an intake valve 2 a, and a fuel injection valve 12 that injects fuel toward the intake port 16 in the middle of each intake passage 20 connected to the intake port 16. Is provided for each cylinder. The intake passage 20 is connected to a surge tank 22, and a throttle valve 26 whose opening degree is adjusted by a motor 24 is provided upstream of the surge tank 22. The intake air amount GA is adjusted by the opening of the throttle valve 26 (throttle opening TA). The throttle opening degree TA is detected by the throttle opening degree sensor 28 and read into the ECU 4. The intake air amount GA is detected by an intake air amount sensor 30 provided upstream of the throttle valve 26 and read into the ECU 4. The fuel injection valve 12 may be an in-cylinder injection type gasoline engine that directly injects fuel into the combustion chamber 10.

  Further, the combustion chamber 10 is provided with an exhaust port 32 that is opened and closed by an exhaust valve 2 b, and a catalytic converter 38 is disposed in the middle of an exhaust passage 36 connected to the exhaust port 32. A three-way catalyst as an exhaust purification catalyst is disposed in the catalytic converter 38. Although one catalytic converter 38 is shown in the drawing, two catalytic converters 38 may be provided upstream and downstream of the exhaust. In this case, a three-way catalyst as a start catalyst is arranged in the upstream catalytic converter in a relatively small amount so that the catalyst can be warmed up in a short time when the engine is started. In the downstream catalytic converter, a three-way catalyst as a main catalyst is disposed in a sufficient amount as a main catalyst.

  An air-fuel ratio sensor 40 that outputs a voltage Vaf signal that gradually changes corresponding to the air-fuel ratio of the exhaust is disposed in the exhaust passage 36 upstream of the catalytic converter 38 as shown in FIG. Here, “3.3 V” corresponds to a theoretical air-fuel ratio equivalent output value. Further, in the exhaust passage 36 on the downstream side of the catalytic converter 38, as shown in FIG. 3, an O2 sensor 44 that outputs a voltage Vo2 signal that changes abruptly around the theoretical air-fuel ratio corresponding to the air-fuel ratio of the exhaust is disposed. Yes. Here, “0.45 V” corresponds to the theoretical air-fuel ratio equivalent output value.

  The ECU 4 is an engine control circuit configured mainly with a digital computer. In addition to the throttle opening sensor 28, the intake air amount sensor 30, the air-fuel ratio sensor 40, and the O2 sensor 44, the ECU 4 inputs signals from sensors that detect the operating state of the engine 2. That is, an accelerator opening sensor 48 that detects the amount of depression of the accelerator pedal 46 (accelerator opening ACCP), an engine speed sensor 50 that detects the engine speed NE from the rotation of the crankshaft, and a reference crank based on the rotation phase of the intake camshaft. A signal is input from the reference crank angle sensor 52 that determines the angle. Further, a signal is also input from a coolant temperature sensor 54 that detects the engine coolant temperature THW. In addition to such sensors, various sensors are provided as necessary.

  The ECU 4 controls the fuel injection timing, the fuel injection amount, the ignition timing, and the throttle opening of the engine 2 by a control signal for the fuel injection valve 12, the ignition plug 14, or the throttle valve motor 24 based on the detection contents from each sensor described above. Control the degree TA etc. The fuel injection amount is feedback-controlled by the outputs of the air-fuel ratio sensor 40 and the O2 sensor 44 so as to achieve the target air-fuel ratio, here the stoichiometric air-fuel ratio.

  Next, the air-fuel ratio control process among the controls executed by the ECU 4 will be described. A flowchart of this processing is shown in FIG. This processing is repeatedly executed at every constant rotation of the crankshaft (every 180 ° for a four-cylinder engine). The steps in the flowchart corresponding to the individual processing contents are represented by “S˜”.

  When the air-fuel ratio control process is started, it is first determined whether or not a condition (sub air-fuel ratio feedback condition) for correcting the air-fuel ratio control by the O2 sensor 44 is satisfied (S102). Examples of the sub air-fuel ratio feedback condition include the following (1) to (5). (1) The engine coolant temperature THW is equal to or higher than a specified value. (2) The engine 2 has been started. (3) Fuel increase such as increase after start-up, warm-up increase, power increase, OTP increase to prevent overheating of the catalyst is not being executed, and the specified time has elapsed since such increase. (4) The fuel cut is not being executed, and the specified time has elapsed since the fuel cut was completed. (5) After the engine 2 is started, the output of the O2 sensor 44 is reversed at least once (determined that the lean output has changed to the rich output or vice versa, that is, the O2 sensor 44 has been activated). And so on. Only when all these conditions are satisfied, it is determined that the sub air-fuel ratio feedback condition is satisfied (“YES” in S102).

  If the sub air-fuel ratio feedback condition is satisfied (“YES” in S102), then the reference voltage Vos (here, 0.45 V output at the theoretical air-fuel ratio) and the O2 sensor 44 are The O2 sensor output voltage deviation ΔVo2 is calculated from the difference from the output voltage Vo2 being output (S104).

[Formula 1] ΔVo2 ← Vos − Vo2
Next, an integral value SUM and a smoothed integral value ASUM are calculated based on the O2 sensor output voltage deviation ΔVo2 (S106).

The integral value SUM is calculated as shown in Equation 2. The integral value SUM on the right side is the value at the previous calculation.
[Formula 2] SUM ← SUM + ΔVo2
The annealed integration value ASUM indicates an average value that suppresses short-cycle fluctuations of the integration value SUM, and is obtained as a weighted average value of the integration value SUM as shown in Equation 3. The smoothed integration value ASUM on the right side is the value at the previous calculation.

[Formula 3] ASUM <-{(n-1) .ASUM + SUM} / n
Here, the numerical value n is set to a value exceeding 1, for example. Note that processing other than the weighted average value may be performed, and it is only necessary to obtain the smoothed integrated value ASUM as an average value in which fluctuations in the integrated value SUM are reduced.

When the integral value SUM and the smoothed integral value ASUM are calculated (S106), the change amount dΔVo2 of the O2 sensor output voltage deviation ΔVo2 is calculated as shown in Equation 4 (S108).
[Formula 4] dΔVo2 ← ΔVo2−ΔVo2old
Here, the previous value ΔVo2old on the right side is the value of the O2 sensor output voltage deviation ΔVo2 at the time of the previous calculation.

  Next, a correction amount dVaf for correcting the output voltage Vaf of the air-fuel ratio sensor 40 is calculated based on the O2 sensor output voltage deviation ΔVo2, the integral value SUM, and the change amount dΔVo2 calculated as described above. (S110).

[Formula 5] dVaf ←
KP ・ ΔVo2 + KI ・ SUM + KD ・ dΔVo2
Here, as the coefficients KP, KI, and KD, values determined in advance through experiments are used.

  If the sub air-fuel ratio feedback condition is not satisfied (“NO” in S102), the correction amount dVaf is based on the latest smoothed integration value ASUM calculated when the previous sub air-fuel ratio feedback condition is satisfied. It is calculated as shown in Equation 6 (S112).

[Formula 6] dVaf ← KI · ASUM
When the correction amount dVaf is calculated in step S110 or step S112, a learning process (S113) of the correction amount dVaf is performed next. The correction amount learning process is shown in the flowchart of FIG.

  In the correction amount learning process (FIG. 5), first, a weighted average value dVafave of the correction amount dVaf is calculated by Equation 7 (S132). Here, the weighted average value dVafave on the right side is a value at the time of the previous calculation.

[Formula 7] dVafave ←
{(M−1) · dVafave + dVaf} / m
Here, the numerical value m is set to a value exceeding 1, for example. Note that processing other than the weighted average value may be performed, and it is only necessary to obtain an average value in which fluctuations in the correction amount dVaf are reduced.

  Next, it is determined whether or not a learning condition is satisfied (S134). Examples of the learning conditions include the following (1) and (2). (1) The O2 sensor abnormality determination flag Xdss described later is OFF (a state that is not abnormal). (2) The calculation of Expression 7 was performed continuously for the reference number of times that the weighted average value dVafave is appropriately obtained.

  If all of the conditions (1) and (2) are satisfied (YES in S134), the weighted average value dVafave calculated in step S132 is stored in the ECU 4 as the learning value DVG of the correction amount dVaf. It is stored in the backup RAM provided (S136).

  If one of the conditions (1) and (2) is not satisfied (NO in S134), the present process is temporarily terminated. Therefore, for example, when the O2 sensor abnormality determination flag Xdss is switched from OFF to ON, the learning value DVG holds the value calculated immediately before in the period of Xdss = OFF.

Returning to the description of the air-fuel ratio control process (FIG. 4), when the correction amount learning process (S113) is completed, the guard process (S114) of the correction amount dVaf is performed next.
The guard process for the correction amount dVaf is shown in the flowchart of FIG.

  First, it is determined whether or not the correction amount dVaf is greater than or equal to “0 (V)” (S152). If dVaf ≧ 0 (“YES” in S152), it is determined whether dVaf ≦ dVgrd (+) (S154). Here, the plus side guard value dVgrd (+) is an upper limit value set in a guard value setting process, which will be described later, and represents a limit set in the correction amount dVaf based on a learning value, which will be described later.

  If dVaf ≦ dVgrd (+) (“YES” in S154), the guard process is temporarily terminated without changing the correction amount dVaf. However, if dVaf> dVgrd (+) (“NO” in S154), the value of the positive guard value dVgrd (+) is set to the value of the correction amount dVaf (S156). Thus, the value of the correction amount dVaf is limited with the plus side guard value dVgrd (+) as the upper limit. Thus, the guard process is temporarily terminated.

  On the other hand, if dVaf <0 (“NO” in S152), it is determined whether dVaf ≧ dVgrd (−) (S158). The minus side guard value dVgrd (−) is also a lower limit value set in a guard value setting process, which will be described later, and represents a limit set in the correction amount dVaf based on a learning value, which will be described later.

  If dVaf ≧ dVgrd (−) (“YES” in S158), the guard process is temporarily terminated without changing the correction amount dVaf. However, if dVaf <dVgrd (−) (“NO” in S158), the negative guard value dVgrd (−) is set as the value of the correction amount dVaf (S160). Thus, the value of the correction amount dVaf is limited with the negative guard value dVgrd (−) as the lower limit. Thus, the guard process is temporarily terminated.

  Returning to the description of FIG. 4, when the above-described guard process (S114) is completed, the output voltage Vaf of the air-fuel ratio sensor 40 is corrected by the correction amount dVaf as shown in Expression 8, and the control voltage value Vafc is calculated (S116). ).

[Formula 8] Vafc ← Vaf + dVaf
Then, based on the control voltage value Vafc, the fuel injection amount finj at which the target air-fuel ratio is achieved with respect to the intake air amount GA detected from the intake air amount sensor 30 is determined by feedback calculation to open the fuel injection valve 12. It is calculated as a valve period (S118). The value of the fuel injection amount finj represented by this time is set in the drive circuit of the fuel injection valve 12 (S120). As a result, the air-fuel ratio of the air-fuel mixture sucked into the combustion chamber 10 is feedback controlled.

  FIG. 7 shows the guard value setting process. This process is a process that is repeatedly executed at regular time intervals. When this process is started, the output voltage Vo2 of the O2 sensor 44 is first read (S182).

  Next, it is determined whether or not the monitor condition is satisfied (S184). Here, the monitor condition is a condition for determining whether the output abnormality of the O2 sensor 44 itself can be determined from the output voltage Vo2 of the O2 sensor 44. For example, “(1) Activation of the O 2 sensor 44 is completed, (2) Sub air-fuel ratio feedback control is being executed (Steps S104 to S110 in FIG. 4 are being executed), and (3) A specified time has elapsed after returning from the fuel cut. (4) The intake air amount GA is larger than a specified value, and (5) The engine is not in an idle state ”. Note that (3) is for waiting until the influence of the fuel cut is eliminated after returning from the fuel cut, and (4) and (5) clearly indicate that the O2 sensor 44 has a crack in the output voltage. In order to make it appear in Vo2, it is a condition to sufficiently increase the exhaust back pressure.

  If the monitor condition is satisfied (“YES” in S184), then the monitor time Mt is counted up (S186). The monitoring time Mt is set to “0” in the initial setting when the ECU 4 starts up, and is a timer counter for counting the total elapsed time when the monitoring condition is satisfied.

  Next, it is determined whether or not the output voltage Vo2 is lower than 0.05V (S188). Here, the appearance frequency distribution of the output voltage Vo2 when the O2 sensor 44 is normal is shown in FIG. The bar graph shows the appearance frequency at intervals of 0.05V.

  As described above, when the O2 sensor 44 is normal, the sub-air-fuel ratio feedback control appears at approximately the same frequency on the low voltage side and the high voltage side centering on 0.45V, and 0V ≦ Vo2 <0.05V. In the extremely lean region, the frequency of occurrence is very small.

  FIG. 8B shows a state where the exhaust gas slightly leaks to the atmosphere side of the O2 sensor 44 due to the initial element cracking. Due to the slight leakage of the exhaust gas, the output voltage Vo2 of the O2 sensor 44 is inclined to the lean side, and the frequency of occurrence is rapidly increased in the region of 0V ≦ Vo2 <0.05V.

  FIG. 8C shows a state in which element cracking has progressed and more exhaust gas is leaking to the atmosphere side of the O2 sensor 44 than in the case of FIG. For this reason, the output voltage Vo2 of the O2 sensor 44 is an output only on the lean side, and the frequency of occurrence is extremely high in the region of 0V ≦ Vo2 <0.05V.

  As described above, the influence of the element crack clearly appears as the appearance frequency of the output voltage Vo2 of the O2 sensor 44 in the region of 0V ≦ Vo2 <0.05V. The determination of whether or not Vo2 <0.05 V in step S188 is a determination for obtaining the appearance frequency for this region.

  If Vo2 <0.05 V (“YES” in S188), the excessive lean time Lt is counted up (S190). The excessive lean time Lt is set to “0” in the initial setting when the ECU 4 starts up, and is a timer counter for counting the total elapsed time when 0V ≦ Vo2 <0.05V.

  After step S190 or after determining that Vo2 ≧ 0.05 V (“NO” in S188), it is determined whether or not the monitor time Mt is equal to or greater than the monitor reference time Jt (S192). If Mt <Jt (“NO” in S192), the present process is temporarily terminated as it is.

  If the monitoring time Mt ≧ Jt is satisfied by repeating the above-described processing (“YES” in S192), as shown in Expression 9, the appearance frequency Lr (0V ≦ Vo2 <0.05V within the monitoring time Mt ( %) Is calculated (S194).

[Formula 9] Lr ← 100 · Lt / Mt
And using this appearance frequency Lr, the guard value calculation process (S196) for setting the guard values dVgrd (+) and dVgrd (−) described above is executed.

The guard value calculation process is shown in FIG. In this process, first, it is determined whether or not the O2 sensor abnormality determination flag Xdss is ON (S202).
The O2 sensor abnormality determination flag Xdss is set in an O2 sensor abnormality determination process (FIG. 10) described later.

  If Xdss = OFF is set here (NO in S202), then a preset upper guard value Gp is set as the positive guard value dVgrd (+) (S204). Then, a preset lower limit guard value Gm is set as the negative guard value dVgrd (−) (S220). In the present embodiment, “0.3 V” is set as the upper limit guard value Gp, and “−0.3 V” is set as the lower limit guard value Gm.

  That is, as shown in FIGS. 11 to 13, in the region where Xdss = OFF, dVgrd (+) = Gp and dVgrd (−) = Gm, which are fixed to constant values. 12 shows a case where the learning value DVG is larger than the case of FIG. 11, and FIG. 13 shows a case where the learning value DVG is on the negative side.

  If Xdss = ON is set (YES in S202), the first limit value A is the appearance frequency Lr obtained in step S194 of the guard value setting process (FIG. 7) from the function (or map) fa. (S206).

Here, the function fa (Lr) is expressed as in Expression 10.
[Formula 10] A <-Gp-Ka (Lr-Lim)
Here, the coefficient Ka is related to the sensitivity that reflects the change in the appearance frequency Lr in the first limit value A, and is set to a relatively large sensitivity. The abnormality determination value Lim is a reference value for determining an abnormal state of the appearance frequency Lr in an O2 sensor abnormality determination process (FIG. 10) described later. For example, the abnormality determination value Lim = 30% is set.

Next, the limit width B is calculated from the function (or map) fb based on the appearance frequency Lr (S208).
Here, the function fb (Lr) is expressed as in Expression 11.

[Formula 11] B ← −Kb · Lr + C
Here, the coefficient Kb is related to the sensitivity that reflects the change in the appearance frequency Lr in the limit width B, and is set to a relatively small sensitivity (Ka> Kb). The constant C is set to “C> Kb · 100”.

Then, as shown in Expression 12, the learning value DVG stored in the backup RAM is added to the limit width B to set the second limit value GB (S210).
[Formula 12] GB ← B + DVG
That is, as shown in the region of Xdss = ON as shown in FIGS. 11 to 13, two limit value lines of the first limit value A line and the second limit value GB line are set. Become.

  Next, it is determined whether or not the first limit value A is larger than the second limit value GB (S212). If A> GB here (YES in S212), the first limit value A is set to the positive guard value dVgrd (+) (S214).

  On the other hand, if A ≦ GB (NO in S212), it is determined whether or not GB <Gp (S216). If GB <Gp (YES in S216), the second limit value GB is set to the positive guard value dVgrd (+) (S218). If GB ≧ Gp (NO in S216), the preset upper guard value Gp is set as the positive guard value dVgrd (+) (S204).

  Then, after the positive guard value dVgrd (+) is set in step S214, S218, or S204, the preset lower limit guard value Gm is set to the negative guard value dVgrd (−). (S220). In this way, this process is once completed.

  In particular, except for a special case where GB ≧ Gp for some reason, in the region where Xdss = ON as shown in FIGS. 11 to 13, the first limit value A and the second limit value GB as shown by the solid line. The larger of the values is set as the positive guard value dVgrd (+).

  When the second limit value GB is set to the plus side guard value dVgrd (+), the plus side guard value dVgrd (+) is set higher by the limit width B with reference to the learning value DVG. Become.

Next, the O2 sensor abnormality determination process (FIG. 10) will be described. This process is repeatedly executed at regular time intervals.
When this process is started, it is first determined whether or not a determination condition is satisfied (S242). Here, the determination condition is that the monitoring condition of the guard value setting process (FIG. 7) is satisfied. Therefore, even if the monitor condition is not satisfied (NO in S242), no substantial process is performed in this process.

  If the monitoring condition is satisfied (YES in S242), the determination elapsed time counter CNT is then incremented (S244). The determination elapsed time counter CNT is a counter that is initially set to CNT = 0 when the ECU 4 is started.

  Next, it is determined whether or not the appearance frequency Lr obtained in step S194 of the guard value setting process (FIG. 7) is equal to or higher than the abnormality determination value Lim (S246). If Lr ≧ Lim (YES in S246), the abnormal accumulation counter CSNG is incremented (S248). The abnormal accumulation counter CSNG is a counter that is initially set to CSNG = 0 when the ECU 4 is started.

If Lr <Lim (NO in S246), the abnormal accumulation counter CSNG is not incremented.
Then, after step S248 or when it is determined NO in step S246, it is determined whether or not the determination elapsed time counter CNT is equal to or greater than the determinable time value T1 (S250). If CNT <T1 (NO in S250), it is determined that a sufficient time has not elapsed since the determination condition is satisfied, and the process is temporarily terminated as it is.

  On the other hand, if CNT ≧ T1 (YES in S250), it is then determined whether or not the abnormality accumulation counter CSNG is equal to or greater than the abnormality state accumulation time value T2 (S252). This abnormal state accumulated time value T2 represents a state where Lr ≧ Lim, that is, an accumulated time during which it can be reliably determined that the state is abnormal.

  If CSNG ≧ T2 (YES in S252), it is determined that the abnormal state is surely set, and the O2 sensor abnormality determination flag Xdss is set to ON (S254). If CSNG <T2 (NO in S252), it is determined that there is no abnormal state, and the O2 sensor abnormality determination flag Xdss is set to OFF (S256).

  When the O2 sensor abnormality determination flag Xdss is set in step S254 or S256, the value of the determination elapsed time counter CNT is cleared (S258), and further the value of the abnormality accumulation counter CSNG is cleared (S260). Is temporarily terminated.

  An example of the control according to the present embodiment is shown in the timing chart of FIG. Here, before time t0, the appearance frequency Lr of “0V ≦ Vo2 <0.05V” is less than Lim, plus side guard value dVgrd (+) = “0.3V”, minus side guard value dVgrd (−) = “ −0.3V ”. Therefore, the correction amount dVaf is restricted to a range of “−0.3 V to 0.3 V” that is a normal range.

  When the appearance frequency Lr becomes equal to or higher than Lim and Xdss = ON (t0), after passing through the intermediate region based on the first limit value A, the limit width B decreases as the appearance frequency Lr increases. Since the limit width B is a width from the learning value DVG, the plus side guard value dVgrd (+) is set from the position based on the learning value DVG to the position of the limit width B.

  The correspondence relationship with the claims in the configuration described above corresponds to the air-fuel ratio sensor 40 corresponding to the upstream sensor and the O2 sensor 44 corresponding to the downstream sensor. The processes in steps S186 to S194 of the guard value setting process (FIG. 7) and the O2 sensor abnormality determination process (FIG. 10) correspond to the process as the downstream sensor abnormality state detecting means. The correction amount learning process (FIG. 5) corresponds to a process as correction amount learning means. The guard value calculation process (FIG. 9) corresponds to a process as a correction amount adjusting unit. The second limit value GB is an abnormal limit, and the limit width B corresponds to the abnormal limit width.

According to the first embodiment described above, the following effects can be obtained.
(I). If the output voltage Vo2 of the O2 sensor 44 existing on the downstream side of the catalytic converter 38 deviates from the standard sensor output voltage due to some cause, in this case, due to element cracking, the appearance frequency distribution becomes as shown in FIG. Appear clearly. For example, in the case of element cracking, the standard appearance frequency distribution shifts to the lean side.

  In the present embodiment, the state of the output frequency distribution is detected from the value of the appearance frequency Lr satisfying 0V ≦ Vo2 <0.05V, the limit width B is set according to the value of the appearance frequency Lr, and the correction amount In addition to the learning value DVG of dVaf, a positive guard value dVgrd (+) is set.

  Therefore, first, the extent of the abnormal state of the O2 sensor 44 can be reflected in the correction amount dVaf by the limit width B. Then, by setting the limit width B with the learning value DVG as a reference, the plus-side guard value dVgrd (+) also reflects the characteristic variation of the air-fuel ratio sensor 40. Therefore, the correction amount dVaf can be limited to an appropriate range in which the variation in characteristics of the air-fuel ratio sensor 40 can be absorbed together with the degree of abnormal state of the O2 sensor 44.

  In this manner, the air-fuel ratio abnormality caused by the output of the O2 sensor 44 downstream of the catalytic converter 38 can be appropriately suppressed, and the deterioration of emissions when the O2 sensor 44 is abnormal can be reliably prevented.

  (B). As a method for detecting that the output value shifts to the lean side due to element cracking in the O2 sensor 44, a reference output value (0.05 V in the present embodiment) is provided on the lean side, and the lean side further than this reference output value. The appearance frequency Lr of the output value is detected as the degree of the abnormal state of the O 2 sensor 44. By calculating the appearance frequency Lr in this way, the degree of abnormal state of the O2 sensor 44 can be easily detected.

[Other embodiments]
(A). In the above-described embodiment, the plus side guard value dVgrd (+) is changed as shown in FIGS. 11 to 13, but the plus side guard value dVgrd (+) may be changed in other patterns. For example, it may be changed as shown in FIGS.

In the case of FIG. 15, in the case where the second limit value GB is set for the plus side guard value dVgrd (+), the limit width B is kept constant without being changed according to the appearance frequency Lr.
In the case of FIG. 16, the first limit value A is not provided, and the second limit value GB is set to the plus-side guard value dVgrd (+) in the region where Xdss = ON.

In the case of FIG. 17, the first limit value A is not provided, and the limit width B is constant without changing according to the appearance frequency Lr.
In the case of FIG. 18, the first limit value A exists, but B = 0 and is constant.

In the example of FIG. 17, B = 0 may be set.
In any of the cases of FIGS. 15 to 18, the positive guard value dVgrd (+) is set based on the learning value DVG when the second limit value GB is set.

  (B). In the above-described embodiment, the appearance frequency of the output voltage Vo2 in the O2 sensor 44 is determined on the lean side with “0.05V” as a boundary (reference output value). The appropriate reference output value differs depending on the type of sensor or the type of O2 sensor. Therefore, the reference output value is not limited to “0.05 V”, and the effect of the first embodiment is produced by obtaining and setting an appropriate reference output value for determining an abnormality of the O 2 sensor by experiment. Can be made.

  (C). In the above embodiment, the limit width B is changed depending on the frequency of appearance of the output voltage Vo2 of the O2 sensor, but the following may be performed in addition to this. That is, the ratio or difference between the appearance frequency peak value on the lean side and the appearance frequency peak value on the rich side is calculated with respect to the reference voltage Vos, and when the appearance frequency peak value on the lean side is larger than a certain level, the ratio Alternatively, the limit width B may be set to be small according to the difference.

  (D). In the above embodiment, as shown in the air-fuel ratio control process (FIG. 4), the correction amount dVaf is obtained based on the value of the output voltage Vo2 of the O2 sensor to correct the output voltage Vaf of the air-fuel ratio sensor 40. The output voltage Vo2 of the O2 sensor may be reflected in the air-fuel ratio control by other methods.

  For example, the calculation of Expression 8 is not executed in step S116 of the air-fuel ratio control process (FIG. 4), and the fuel injection correction amount or the fuel injection amount corresponding to the correction amount dVaf is calculated when calculating the fuel injection amount finj in step S118. The correction coefficient is added to, subtracted from, or multiplied by the fuel injection amount finj. As a result, the output voltage Vo2 of the O2 sensor can be reflected in the air-fuel ratio control. A fuel correction amount guard value corresponding to the guard values dVgrd (+) and dVgrd (−) is set and limited with respect to the fuel injection correction amount or the fuel injection amount correction coefficient. In the case of correction for increasing the fuel injection amount finj under this restriction, the same effect as that of the above embodiment is achieved by setting the plus side fuel correction amount guard value as shown in FIGS. 11 to 13 and 15 to 18. Can be generated.

  (E). In each of the above embodiments, the sensor provided on the upstream side of the catalytic converter is an air-fuel ratio sensor that outputs a voltage Vaf signal that gradually changes in accordance with the air-fuel ratio as shown in FIG. As shown, an O2 sensor whose output changes abruptly around the theoretical air-fuel ratio may be used as the upstream sensor.

  Also in this case, the output voltage value of the upstream O2 sensor may be corrected by the correction amount dVaf obtained from the value of the output voltage Vo2 of the downstream O2 sensor. Alternatively, the fuel injection amount obtained from the output voltage value of the upstream O2 sensor may be corrected as described in (d). By limiting the correction as described above, it is possible to produce the same effect as that of the above embodiment.

  As a method for correcting the air-fuel ratio in this case, a skip fuel correction amount for increasing or decreasing the fuel stepwise when the upstream O2 sensor changes between lean (<Vos) and rich (> Vos) is provided. In this case, the skip fuel correction amount may be corrected according to the value of the output voltage Vo2 of the downstream O2 sensor. Then, the increase-side skip fuel correction amount when it changes from lean (<Vos) to rich (> Vos) may be limited based on the appearance frequency distribution of the output voltage Vo2 of the O2 sensor.

  (F). In the above embodiment, an air-fuel ratio sensor that outputs a voltage signal that gradually changes corresponding to the air-fuel ratio as shown in FIG. 2 may be used instead of the O2 sensor on the downstream side of the catalytic converter.

  Also in this case, the output voltage value of the upstream sensor is corrected by the correction amount dVaf obtained based on the output voltage value of the downstream air-fuel ratio sensor. Alternatively, the fuel injection amount is corrected as shown in (d). Also in such an air-fuel ratio sensor, the appearance frequency distribution of the output value is shifted to the lean side (in this case, shifted to the high voltage side) due to element cracking or the like. Therefore, by limiting the correction amount dVaf and the fuel injection correction amount as described above according to the change degree of the appearance frequency distribution, it is possible to produce the same effect as in the above embodiment.

When an O2 sensor is used on the upstream side, the skip correction amount on the increase side may be limited as described in (e).
(G). In the above embodiment, when the O2 sensor abnormality determination flag Xdss = ON, only the plus side guard value dVgrd (+) is set in the limit width B with the learning value DVG as a reference. In addition, with respect to the negative guard value dVgrd (−), when Xdss = ON, the limit width = “− 0.3V” (“0.3V to the negative side”) is made constant with the learning value DVG as a reference. May be set.

1 is a schematic configuration diagram of an engine and an ECU according to a first embodiment. The graph which shows the relationship between the exhaust air-fuel ratio by the air-fuel ratio sensor upstream of a catalytic converter, and the output voltage Vaf. The graph which shows the relationship between the exhaust air fuel ratio by the O2 sensor downstream of a catalytic converter, and the output voltage Vo2. 3 is a flowchart of air-fuel ratio control processing executed by the ECU according to the first embodiment. The flowchart of a correction amount learning process similarly. The flowchart of the guard process of correction amount dVaf similarly. The flowchart of a guard value setting process similarly. The graph which shows the grade of the normal state and abnormal state in the appearance frequency distribution of the output voltage Vo2 of an O2 sensor. 4 is a flowchart of guard value calculation processing executed by the ECU according to the first embodiment. The flowchart of an O2 sensor abnormality determination process similarly. FIG. 6 is a set state explanatory diagram of a plus side guard value dVgrd (+) and a minus side guard value dVgrd (−) in the first embodiment. Similarly, a setting state explanatory diagram of the plus side guard value dVgrd (+) and the minus side guard value dVgrd (−). Similarly, a setting state explanatory diagram of the plus side guard value dVgrd (+) and the minus side guard value dVgrd (−). 4 is a timing chart illustrating an example of processing according to the first embodiment. FIG. 12 is an explanatory diagram of setting states of a plus side guard value dVgrd (+) and a minus side guard value dVgrd (−) in another embodiment. FIG. 10 is an explanatory diagram of setting states of a plus side guard value dVgrd (+) and a minus side guard value dVgrd (−) in another embodiment. FIG. 12 is an explanatory diagram of setting states of a plus side guard value dVgrd (+) and a minus side guard value dVgrd (−) in another embodiment. FIG. 12 is an explanatory diagram of setting states of a plus side guard value dVgrd (+) and a minus side guard value dVgrd (−) in another embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 2 ... Engine, 2a ... Intake valve, 2b ... Exhaust valve, 4 ... ECU, 10 ... Combustion chamber, 12 ... Fuel injection valve, 14 ... Spark plug, 16 ... Intake port, 20 ... Intake passage, 22 ... Surge tank, 24 ...... Throttle valve motor, 26 ... throttle valve, 28 ... throttle opening sensor, 30 ... intake air amount sensor, 32 ... exhaust port, 36 ... exhaust passage, 38 ... catalytic converter, 40 ... air-fuel ratio sensor, 44 ... sensor, 46 ... accelerator pedal, 48 ... accelerator opening sensor, 50 ... engine speed sensor, 52 ... reference crank angle sensor, 54 ... cooling water temperature sensor.

Claims (9)

Sensors for detecting an air-fuel ratio state from exhaust components are provided upstream and downstream of an exhaust purification catalyst provided in an exhaust system of the internal combustion engine, and air-fuel ratio control is executed based on the output value of the upstream sensor. And an air-fuel ratio control apparatus for an internal combustion engine that executes correction for the air-fuel ratio control by a correction amount set based on an output value of a downstream sensor,
A downstream sensor abnormal state detecting means for detecting an abnormal state of the downstream sensor;
Correction amount learning means for holding the value of the correction amount as a learning value when the downstream sensor abnormal state detection means does not detect an abnormal state;
Correction for setting an abnormal time limit to the correction amount for the air-fuel ratio control based on the learning value held by the correction amount learning unit when the downstream sensor abnormal state detection unit detects an abnormal state A quantity adjusting means;
An air-fuel ratio control apparatus for an internal combustion engine, comprising: 2. The correction amount adjusting unit according to claim 1, wherein the correction amount abnormality limit width for the air-fuel ratio control is set according to the degree of the abnormal state detected by the downstream sensor abnormal state detecting unit. An air-fuel ratio control apparatus for an internal combustion engine characterized by the above. 3. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the downstream sensor abnormal state detecting means detects an abnormal state according to an appearance frequency distribution of output values of the downstream sensor. 4. The downstream sensor abnormal state detecting means according to claim 1, wherein the downstream sensor abnormal state detecting means determines that the degree of abnormal state is higher as the appearance frequency distribution of the output values of the downstream sensor is larger on the lean side. An air-fuel ratio control apparatus for an internal combustion engine. 5. The downstream sensor abnormal state detection means according to claim 1, wherein a reference output value is provided on the lean side, and an output of a lean side from the reference output value among the outputs of the downstream sensor appears. An air-fuel ratio control apparatus for an internal combustion engine, wherein the frequency is detected as a degree of an abnormal state of the downstream sensor. 6. The correction amount adjustment unit according to claim 1, wherein the abnormality amount is controlled in a direction in which the richness of the air-fuel ratio is suppressed as the degree of the abnormal state detected by the downstream sensor abnormal state detection unit is higher. An air-fuel ratio control apparatus for an internal combustion engine, characterized by setting a time limit width. 7. The air-fuel ratio of an internal combustion engine according to claim 1, wherein the downstream sensor is an O2 sensor whose output value changes abruptly between the rich side and the lean side with respect to the theoretical air-fuel ratio. Control device. The upstream sensor according to any one of claims 1 to 7, wherein the upstream side sensor is an air-fuel ratio sensor whose output value gradually changes in accordance with the air-fuel ratio, or an output value abruptly on the rich side and the lean side centering on the theoretical air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine, characterized in that the O2 sensor changes. 9. The air-fuel ratio control according to claim 1, wherein the air-fuel ratio control performs a fuel injection amount calculation process based on a difference between an output value of the upstream sensor and a theoretical air-fuel ratio equivalent output value. An air-fuel ratio control apparatus for an internal combustion engine, wherein a correction based on the correction amount calculated based on a difference between an output value of the downstream sensor and an output value corresponding to a theoretical air-fuel ratio is added.

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