JP4661691B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.

The exhaust gas discharged from the internal combustion engine main body contains components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO _x ). Conventionally, in order to purify these components A three-way catalyst is used. Such a three-way catalyst has a high purification capacity when the air-fuel ratio of the exhaust gas (hereinafter referred to as “exhaust air-fuel ratio”) is substantially the stoichiometric air-fuel ratio. When performing the above, it is necessary to control the amount of fuel supplied to the combustion chamber so that the exhaust air-fuel ratio becomes substantially the stoichiometric air-fuel ratio.

  For this reason, in many internal combustion engines, an air-fuel ratio sensor capable of detecting the exhaust air-fuel ratio is provided in the engine exhaust passage on the exhaust upstream side of the three-way catalyst, and the exhaust air-fuel ratio detected by this air-fuel ratio sensor is almost the same. Feedback (F / B) control is performed to adjust the amount of fuel supplied to the combustion chamber so that the stoichiometric air-fuel ratio is obtained.

  However, on the upstream side of the three-way catalyst, the output of the air-fuel ratio sensor varies due to insufficient exhaust gas mixing, or the air-fuel ratio sensor deteriorates due to the heat of the exhaust gas. In some cases, the air-fuel ratio control accuracy by the F / B control described above is reduced.

  Therefore, an air-fuel ratio sensor capable of detecting the exhaust air-fuel ratio is also provided in the engine exhaust passage on the exhaust downstream side of the three-way catalyst, and the output value of the upstream air-fuel ratio sensor is based on the output of the downstream air-fuel ratio sensor. A double sensor that improves the control accuracy of the air-fuel ratio by performing sub-F / B control that corrects the output value of the upstream air-fuel ratio sensor (resulting in the fuel supply amount) so that it matches the actual exhaust air-fuel ratio The system has already been put into practical use.

  In this double sensor system, a learning value corresponding to a steady deviation between the output value of the upstream air-fuel ratio sensor and the actual exhaust air-fuel ratio is calculated based on the correction amount in the sub F / B control. At the same time, learning control is performed to correct the output value of the upstream air-fuel ratio sensor based on the calculated learning value. Since such a learning value is stored in the RAM of the ECU even when the engine is stopped, for example, the output value of the upstream air-fuel ratio sensor is not sufficiently corrected by the sub F / B control after the internal combustion engine is restarted. However, since the output value of the upstream air-fuel ratio sensor becomes a value in the vicinity of the actual exhaust air-fuel ratio, it is possible to prevent the control accuracy of the air-fuel ratio from being deteriorated, and hence the exhaust emission can be prevented from being deteriorated.

  By the way, for example, when the battery is once removed from the vehicle on which the internal combustion engine is mounted, since the power supply from the battery is interrupted, the learning value stored in the RAM of the ECU is also erased. Therefore, even if a steady deviation occurs between the output value of the upstream air-fuel ratio sensor and the actual exhaust air-fuel ratio when the internal combustion engine is restarted in such a case, such a deviation is compensated by learning control. Therefore, the steady deviation of the output value of the upstream air-fuel ratio sensor must be compensated only by the sub F / B control.

  However, if the steady deviation of the output value of the upstream air-fuel ratio sensor is to be compensated only by the sub F / B control, it takes time until the output value of the upstream air-fuel ratio sensor becomes a value near the actual exhaust air-fuel ratio. Therefore, during this time, the state of low air-fuel ratio control accuracy continues, and therefore the state of poor exhaust emission continues. For this reason, when the internal combustion engine is restarted after power supply from the battery is interrupted, for example, the feedback gain of the sub F / B control (hereinafter referred to as “sub F / B gain”) is increased and the upstream is advanced early. It is necessary to compensate for a steady shift in the output value of the side air-fuel ratio sensor. For this reason, for example, in the air-fuel ratio control device described in Patent Document 1, the sub F / B gain is set to a value larger than the normal sub F / B gain until the learning value in the learning control converges.

JP 2004-316523 A

  Incidentally, in general, the learning value convergence determination in the learning control is performed based on the number of times the exhaust air-fuel ratio is inverted between lean and rich. This is because when the exhaust air-fuel ratio is controlled to be the stoichiometric air-fuel ratio, the exhaust air-fuel ratio is maintained near the theoretical air-fuel ratio as the exhaust air-fuel ratio is reversed between lean and rich. It means that it is.

  On the other hand, the above-described three-way catalyst has an oxygen storage capacity, so that when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is lean, it stores oxygen in the exhaust gas and flows into the three-way catalyst. When the air-fuel ratio of the exhaust gas is rich, the stored oxygen is released and the oxygen and HC and CO in the exhaust gas are oxidized and purified by this oxygen. Such oxygen storage capacity is not always constant from the start of use of the three-way catalyst and gradually deteriorates according to the operation period of the internal combustion engine.

  In such a three-way catalyst, when the oxygen storage capacity of the three-way catalyst is high, that is, when the degree of deterioration of the three-way catalyst is small, even if the exhaust air-fuel ratio is reversed from rich to lean, The air-fuel ratio of the exhaust gas exhausted from the three-way catalyst due to a large amount of oxygen being stored does not reverse immediately, but conversely, even if the exhaust air-fuel ratio is reversed from lean to rich, a large amount is stored in the three-way catalyst. The air-fuel ratio of the exhaust gas discharged from the three-way catalyst is not reversed immediately after the released oxygen is released. For this reason, if the degree of deterioration of the three-way catalyst is small, the period during which the air-fuel ratio of the exhaust gas discharged from the three-way catalyst is reversed between lean and rich becomes long, and therefore the sub F / B gain must be set relatively large. Learning value convergence determination is delayed.

  On the other hand, when the oxygen storage capacity of the three-way catalyst is low, that is, when the degree of deterioration of the three-way catalyst is large, oxygen in the exhaust gas is not stored much in the three-way catalyst, so the exhaust air-fuel ratio is from rich to lean or vice versa. When reversed, the air-fuel ratio of the exhaust gas discharged from the three-way catalyst is also reversed relatively quickly. In this way, if the degree of deterioration of the three-way catalyst is large, the cycle in which the air-fuel ratio of the exhaust gas discharged from the three-way catalyst is reversed between lean and rich becomes short, so the sub F / B gain must be set relatively small. Sub F / B control will diverge.

  Accordingly, an object of the present invention is to determine whether the learning value converges as soon as possible within a range in which the sub F / B control does not diverge while considering the degree of deterioration of the three-way catalyst. An object of the present invention is to provide a fuel ratio control device.

  In order to solve the above-described problem, in the first invention, an upstream air-fuel ratio sensor that is disposed on the exhaust upstream side of an exhaust purification catalyst provided in an engine exhaust passage and detects an air-fuel ratio of exhaust gas; A downstream air-fuel ratio sensor disposed on the exhaust downstream side of the purification catalyst and detecting the air-fuel ratio of the exhaust gas, so that the exhaust air-fuel ratio becomes the target air-fuel ratio based on the output value of the upstream air-fuel ratio sensor An air-fuel ratio control apparatus for an internal combustion engine for controlling the fuel supply amount, and when the output value of the upstream air-fuel ratio sensor deviates from the actual exhaust air-fuel ratio, the output value of the downstream air-fuel ratio sensor is Feedback means for correcting the fuel supply amount by feedback to compensate for the deviation between the output value of the upstream air-fuel ratio sensor and the actual exhaust air-fuel ratio using feedback parameters, and the upstream air-fuel ratio A learning value corresponding to a steady deviation between the sensor output value and the actual exhaust air-fuel ratio is calculated based on the correction amount by the feedback means, and the fuel supply amount is corrected based on the calculated learning value. An air-fuel ratio control apparatus for an internal combustion engine in which a feedback parameter value is determined irrespective of the degree of deterioration of the exhaust purification catalyst during normal operation of the internal combustion engine. Convergence determining means for determining convergence of the learning value is further provided, and the value of the feedback parameter is changed according to the degree of deterioration of the exhaust purification catalyst before the convergence determining means determines that the learning value has converged. I did it.

  In the second invention, in the first invention, the feedback parameter includes a feedback gain, and the feedback gain is increased as the deterioration degree of the exhaust purification catalyst is smaller.

  In a third invention, in the first or second invention, the feedback means limits an absolute value of a correction amount by feedback of the fuel supply amount to a predetermined guard value or less, and the feedback parameter includes the guard value. The absolute value of the guard value is increased as the degree of deterioration of the exhaust purification catalyst is smaller.

  In a fourth invention, in any one of the first to third inventions, the convergence determination means includes an air-fuel ratio in which the output value of the downstream air-fuel ratio sensor is richer than the target air-fuel ratio and the target air-fuel ratio. It is determined that the learning value has converged when the number of inversions with respect to the lean air-fuel ratio becomes a predetermined number or more.

  According to the present invention, it is possible to determine the convergence of the learning value as soon as possible within a range in which the sub F / B control does not diverge while considering the degree of deterioration of the three-way catalyst.

  Hereinafter, an air-fuel ratio control apparatus for an internal combustion engine according to the present invention will be described with reference to the drawings. FIG. 1 is an overall view of an internal combustion engine on which a control device of the present invention is mounted. The embodiment shown in FIG. 1 shows the case where the air-fuel ratio control apparatus of the present invention is used in an in-cylinder direct injection type spark ignition internal combustion engine, but other spark ignition type internal combustion engines and compression self-ignition internal combustion engines are shown. It can also be used for institutions.

  Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3 and a cylinder head 4. A combustion chamber formed therebetween, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. As shown in FIG. 1, a spark plug 10 is arranged at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is arranged around the inner wall surface of the cylinder head 4. A cavity 12 extending from the lower side of the fuel injection valve 11 to the lower side of the spark plug 10 is formed on the top surface of the piston 3.

  The intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch pipe 13, and the surge tank 14 is connected to an air cleaner (not shown) via an intake pipe 15. An air flow meter 16 is disposed in the intake pipe 15 and a throttle valve 18 driven by a step motor 17 is disposed. On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19, and this exhaust manifold 19 is connected to a catalytic converter 21 containing a three-way catalyst 20. The outlet of the catalytic converter 21 is connected to the exhaust pipe 22. An air-fuel ratio sensor 23 is disposed in the exhaust manifold 19, that is, the exhaust passage upstream of the exhaust purification catalyst 20, and an oxygen sensor 24 is disposed in the exhaust pipe 22, that is, the exhaust passage downstream of the three-way catalyst 20. The

  The electronic control unit 31 is composed of a digital computer and includes a RAM (random access memory) 33, a ROM (read only memory) 34, a CPU (microprocessor) 35, an input port 36 and An output port 37 is provided. The air flow meter 16 generates an output voltage proportional to the intake air flow rate, and the output voltage is input to the input port 36 via the corresponding AD converter 38. Further, as shown in FIG. 2, the air-fuel ratio sensor 23 generates an output voltage substantially proportional to the air-fuel ratio of the exhaust gas based on the oxygen concentration in the exhaust gas passing through the exhaust manifold 19. On the other hand, as shown in FIG. 3, the oxygen sensor 24, based on the oxygen concentration in the exhaust gas passing through the exhaust pipe 22, that is, the exhaust gas after passing through the three-way catalyst 20, An output voltage that varies greatly depending on whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio (about 14.7). These output voltages are input to the input port 36 via the corresponding AD converter 38.

  A load sensor 41 that generates an output voltage proportional to the amount of depression of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 36 via the corresponding AD converter 38. The For example, the crank angle sensor 42 generates an output pulse every time the crankshaft rotates 30 degrees, and the output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 42. On the other hand, the output port 37 is connected to the spark plug 10, the fuel injection valve 11, and the step motor 17 via a corresponding drive circuit 39.

  The above-described three-way catalyst 20 has an oxygen storage capacity, so that when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is lean, the three-way catalyst 20 stores oxygen in the exhaust gas and also the three-way catalyst 20. When the air-fuel ratio of the exhaust gas flowing into the exhaust gas is rich, the stored oxygen is released to oxidize and purify HC and CO contained in the exhaust gas.

  In order to effectively utilize the oxygen storage capacity of such a three-way catalyst 20, the three-way catalyst can be purified so that the exhaust gas can be purified even if the air-fuel ratio of the exhaust gas subsequently becomes rich or lean. It is necessary to maintain the amount of oxygen stored in the catalyst 20 at a predetermined amount (for example, half of the maximum oxygen storage amount). If the oxygen storage amount of the three-way catalyst 20 is maintained at the predetermined amount, the three-way catalyst 20 can always exhibit a certain degree of oxygen storage and oxygen release action, and as a result, the three-way catalyst. 20 makes it possible to always oxidize and reduce the components in the exhaust gas. For this reason, in this embodiment, in order to maintain the exhaust purification performance of the three-way catalyst 20, air-fuel ratio control is performed so as to keep the oxygen storage amount of the three-way catalyst constant.

  Therefore, in the present embodiment, the air / fuel ratio sensor (upstream air / fuel ratio sensor) 23 disposed upstream of the three-way catalyst 20 causes the exhaust air / fuel ratio (the exhaust passage upstream of the three-way catalyst 20, the combustion chamber 5, and F / B for the fuel supply amount from the fuel injection valve 11 so that the output value of the air-fuel ratio sensor 23 becomes a value corresponding to the stoichiometric air-fuel ratio. Control is to be performed (hereinafter, this F / B control is referred to as “main F / B control”). As a result, the exhaust air-fuel ratio is maintained in the vicinity of the stoichiometric air-fuel ratio, and as a result, the oxygen storage amount of the three-way catalyst is maintained constant, thereby improving exhaust emission.

Hereinafter, the main F / B control will be specifically described. First, in the present embodiment, a fuel amount (hereinafter referred to as “target fuel supply amount”) Qft (n) to be supplied from the fuel injection valve 11 to each cylinder is calculated by the following equation (1).
Qft (n) = Mc (n) / AFT + DQf (n-1) (1)

  In the above equation (1), n is a value indicating the number of calculations in the ECU 31, and for example, Qft (n) represents the target fuel supply amount calculated by the n-th calculation (that is, at time n). . Mc (n) indicates the amount of air expected to be sucked into the cylinder of each cylinder before the intake valve 6 is closed (hereinafter referred to as “in-cylinder intake air amount”). The in-cylinder intake air amount Mc (n) is a map or calculation formula using, for example, the engine speed Ne and the flow rate of air passing through the intake pipe 15 (hereinafter referred to as “intake pipe passing air flow rate”) mt as arguments. Is obtained experimentally or by calculation, and this map or calculation formula is stored in the ROM 34 of the ECU 31, and the engine speed Ne and the intake pipe passage air flow rate mt are detected during engine operation, and the map is based on these detected values. Or it calculates with a formula. AFT is a target value of the exhaust air-fuel ratio, and in this embodiment, is the stoichiometric air-fuel ratio (14.7). Further, DQf is a fuel correction amount calculated for main F / B control described later. The fuel injection valve 11 injects an amount of fuel corresponding to the target fuel supply amount calculated in this way.

  In the above description, the in-cylinder intake air amount Mc (n) is calculated based on a map or the like using the engine speed Ne and the intake pipe passage air flow rate mt as arguments. You may obtain | require by other methods, such as a calculation formula based on an opening degree, atmospheric pressure, etc.

  FIG. 4 is a flowchart showing a control routine of target fuel supply amount calculation control for calculating the target fuel supply amount Qft (n) from the fuel injection valve 11. The illustrated control routine is performed by interruption at predetermined time intervals.

  First, in step 101, the engine speed Ne and the intake pipe passage air flow rate mt are detected by the crank angle sensor 42 and the air flow meter 16. Next, at step 102, the in-cylinder intake air amount Mc (n) at time n is calculated by a map or a calculation formula based on the engine speed Ne and the intake pipe passage air flow rate mt detected at step 101. Next, at step 103, based on the in-cylinder intake air amount Mc (n) calculated at step 102 and the fuel correction amount DQf (n-1) at time n-1 calculated in main F / B control described later. The target fuel supply amount Qft (n) is calculated by the above equation (1), and the control routine is terminated. The fuel injection valve 11 injects an amount of fuel corresponding to the target fuel supply amount Qft (n) calculated in this way.

Next, main F / B control will be described. In the present embodiment, as the main F / B control, the fuel deviation amount ΔQf between the actual fuel supply amount calculated based on the output of the air-fuel ratio sensor 23 and the target fuel supply amount Qft described above is calculated at each calculation time. The fuel correction amount DQf is calculated so that the fuel deviation amount ΔQf becomes zero. Specifically, the fuel correction amount DQf is calculated by the following equation (2). In the following equation (2), DQf (n−1) is the fuel correction amount in the (n−1) th calculation, that is, the previous calculation, Kmp indicates a proportional gain, and Kmi indicates an integral gain. These proportional gain Kmp and integral gain Kmi may be predetermined constant values or values that change in accordance with the engine operating state.

  FIG. 5 is a flowchart showing a control routine of main F / B control for calculating the fuel correction amount DQf. The illustrated control routine is performed by interruption at predetermined time intervals.

  First, in step 121, it is determined whether or not an execution condition for main F / B control is satisfied. The case where the execution condition of the main F / B control is satisfied means that, for example, the internal combustion engine is not cold started (that is, the engine cooling water temperature is equal to or higher than a certain temperature and the fuel increase at start-up is performed). And the fuel cut control for stopping the fuel injection from the fuel injection valve during the engine operation. If it is determined in step 121 that the main F / B control execution condition is satisfied, the process proceeds to step 122.

  In step 122, the output value VAF (n) of the air-fuel ratio sensor 23 at the time of the nth calculation is detected. Next, at step 123, the output correction value efsfb (n) of the air-fuel ratio sensor 23 calculated by the control routine of sub-F / B control described later and the sub-F / B learning value efgsfs described later are output detected at step 122. By adding to the value VAF (n), the output value of the air-fuel ratio sensor 23 is corrected, and the corrected output value VAF ′ (n) at the time of the n-th calculation is calculated (VAF ′ (n) = VAF ( n) + efsfb (n) + efgsfsb (n)).

  Next, at step 124, based on the corrected output value VAF '(n) calculated at step 123, the actual air-fuel ratio AFR (n) at time n is calculated using the map shown in FIG. The actual air-fuel ratio AFR (n) calculated in this way is a value that substantially matches the actual air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 at the time of the n-th calculation.

Next, at step 125, the fuel deviation amount ΔQf between the fuel supply amount calculated based on the output of the air-fuel ratio sensor 23 and the target fuel supply amount Qft is calculated by the following equation (3). In the following formula (3), values in the nth calculation are used for the cylinder intake air amount Mc and the target fuel supply amount Qft, but values before the nth calculation are used. May be used.
ΔQf (n) = Mc (n) / AFR (n) −Qft (n) (3)

  In step 126, the fuel correction amount DQf (n) at time n is calculated from the above equation (2), and the control routine is terminated. The calculated fuel correction amount DQf (n) is used in step 103 of the control routine shown in FIG. On the other hand, if it is determined in step 121 that the main F / B control execution condition is not satisfied, the control routine is terminated without updating the fuel correction amount DQf (n).

  By the way, the output of the air-fuel ratio sensor 23 may be shifted due to the deterioration of the air-fuel ratio sensor 23 due to the heat of the exhaust gas. In such a case, the air-fuel ratio sensor 23 that originally generates an output value as shown by a solid line in FIG. 2 generates an output value as shown by a broken line in FIG. 2, for example. When a deviation occurs in the output value of the air-fuel ratio sensor 23 as described above, the air-fuel ratio sensor 23 generates, for example, an output voltage that is generated when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio rather than the stoichiometric air-fuel ratio. It is generated when it is lean. Therefore, in the present embodiment, the sub-F / B control using the oxygen sensor (downstream air-fuel ratio sensor) 24 compensates for the deviation generated in the output value of the air-fuel ratio sensor 23, and the output value of the air-fuel ratio sensor 23 is A value corresponding to the actual exhaust air-fuel ratio is set.

  That is, as shown in FIG. 3, the oxygen sensor 24 can detect whether the exhaust air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio, and whether it is richer than the stoichiometric air-fuel ratio or lean. There is almost no deviation in the determination of whether there is any. For this reason, when the actual exhaust air-fuel ratio is lean, the output voltage of the oxygen sensor 24 has a low value, and when the actual exhaust air-fuel ratio is rich, the output voltage of the oxygen sensor 24 has a high value. It has become. Therefore, when the actual exhaust air-fuel ratio is substantially the stoichiometric air-fuel ratio, that is, when the up-and-down is repeated near the stoichiometric air-fuel ratio, the output voltage of the oxygen sensor 24 repeatedly reverses between a high value and a low value. . From this point of view, in this embodiment, the output value of the air-fuel ratio sensor 23 is corrected so that the output voltage of the oxygen sensor 24 repeats inversion between a high value and a low value.

  FIG. 6 is a time chart of the actual exhaust air-fuel ratio, the output value of the oxygen sensor, the output correction value efsfb of the air-fuel ratio sensor 23, and the sub F / B learning value efgsfs. In the time chart of FIG. 6, although the actual exhaust air-fuel ratio is controlled so as to become the stoichiometric air-fuel ratio, a deviation occurs in the air-fuel ratio sensor 23 and the actual exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio. In this case, the deviation occurring in the air-fuel ratio sensor 23 is compensated.

In the example shown in FIG. 6, at the time t ₀ , the actual exhaust air-fuel ratio is not the stoichiometric air-fuel ratio, but is leaner than the stoichiometric air-fuel ratio. This is because when the air-fuel ratio sensor 23 is deviated and the actual exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio sensor 23 outputs an output value corresponding to the stoichiometric air-fuel ratio. This is because it is output. At this time, the output value of the oxygen sensor 24 is a low value.

  As described above, the output correction value efsfb of the air-fuel ratio sensor 23 is added to the output value VAF (n) in order to calculate the correction output value VAF ′ (n) in step 123 of FIG. Therefore, when the output correction value efsfb is a positive value, the output value of the air-fuel ratio sensor 23 is corrected to the lean side, and when the output correction value efsfb is a negative value, the output value of the air-fuel ratio sensor 23 is It is corrected to the rich side. As the absolute value of the output correction value efsfb increases, the output value of the air-fuel ratio sensor 23 is corrected larger.

  If the output value of the oxygen sensor 24 is low even though the output value of the air-fuel ratio sensor 23 is almost the stoichiometric air-fuel ratio, the output value of the air-fuel ratio sensor 23 is shifted to the rich side. means. Therefore, in the present embodiment, when the output value of the oxygen sensor 24 is a low value, as shown in FIG. 6, the output correction value efsfb is increased and the output value of the air-fuel ratio sensor 23 is made lean. It is going to be corrected to the side. On the other hand, when the output value of the oxygen sensor 24 is high even though the output value of the air-fuel ratio sensor 23 is substantially the stoichiometric air-fuel ratio, the value of the output correction value efsfb is decreased to reduce the air-fuel ratio. The output value of the sensor 23 is corrected to the rich side.

Specifically, the value of the output correction value efsfb is calculated by the following equation (4). In the following equation (4), efsfb (n−1) is an output correction value at the (n−1) th time, that is, the previous calculation, Ksp indicates a proportional gain, and Ksi indicates an integral gain. ΔVO (n) represents an output deviation between the output value of the oxygen sensor 24 and the target output value (a value corresponding to the theoretical air-fuel ratio in the present embodiment) at the time of the n-th calculation.

  As described above, in the example shown in FIG. 6, as the output correction value efsfb of the air-fuel ratio sensor 23 increases, the deviation generated in the output value of the air-fuel ratio sensor 23 is corrected, and the actual exhaust air-fuel ratio is reduced. Gradually approach the stoichiometric air-fuel ratio.

  Here, if the state in which the output value of the air-fuel ratio sensor 23 deviates from the value corresponding to the actual air-fuel ratio is continued, the output value of the air-fuel ratio sensor 23 is excessively corrected by the sub F / B control. In this case, the air-fuel ratio controllability is hindered. For this reason, in the present embodiment, a guard value for limiting the output correction value efsfb is set, and when the output correction value efsfb reaches the guard value, the output correction value efsfb is limited by the guard value, thereby the air-fuel ratio sensor. A technique for preventing over-correction of 23 is known.

In the present embodiment, an upper limit guard value that is the upper limit value Grdh that can be taken by the output correction value efsfb and a lower limit guard value Grdl that is a lower limit value that can be taken by the output correction value efsfb are set in advance as such guard values. In the example shown in FIG. 6, the output correction value efsfb reaches the upper limit guard value Grdh at time t ₁ , and thus the output correction value is low even though the output value of the oxygen sensor 24 is low. The value of efsfb is not increased and remains at the upper guard value Grdh.

  In this way, the output value of the air-fuel ratio sensor 23 is appropriately corrected by the sub F / B control. However, the sub F / B control is interrupted when the internal combustion engine is stopped or the fuel cut control is performed, for example. As a result, the value of the output correction value efsfb is reset to zero. Thereafter, when the internal combustion engine is started again or when the fuel cut control is terminated, the sub F / B control is resumed. However, since the output correction value efsfb is reset to zero, the air-fuel ratio sensor 23 It takes time to correct the output value to an appropriate value again.

  Therefore, in the present embodiment, the sub F / B learning value efgsfs corresponding to the steady deviation generated between the output value of the air / fuel ratio sensor 23 and the value corresponding to the actual exhaust air / fuel ratio is set to the sub F / B. While calculating based on the output correction value efsfb in the B control, the output value VAF of the air-fuel ratio sensor 23 is corrected based on the calculated sub F / B learning value efgfsb. The sub F / B learning value efgsfs calculated in this way is not reset to zero even if the internal combustion engine is stopped or the fuel cut control is performed, for example. Even after the above, the output value of the air-fuel ratio sensor 23 can be corrected again to an appropriate value relatively early by the sub F / B control.

  Specifically, when the output correction value efsfb when the predetermined time ΔT has elapsed from the previous learning time is a positive value, the sub F / B learning value efgsfsb is increased and the output correction value efsfb is negative. If it is a value, the sub F / B learning value efgsfsb is decreased. Further, the amount of increase or decrease of the sub F / B learning value efgfsb is set to increase as the absolute value of the output correction value efsfb increases.

In particular, in the present embodiment, the output correction value efsfb and the sub F / B learning value efgsfs when the predetermined time ΔT has elapsed from the previous learning time are updated by the following equations (5) and (6), respectively. In the following formulas (5) and (6), α is an annealing rate, and is a positive value that is equal to or less than 1 (0 <α ≦ 1). Therefore, in the example shown in FIG. 6, since the output correction value efsfb is a positive value at time t ₂ , the output correction value efsfb is reduced by the following equations (5) and (6) and the sub F / F B learning value efgfsb is made to increase, since Similarly, the output correction value even at the time t ₃ efsfb is a positive value, the sub with an output correction value efsfb is caused to decrease by the following equation (5) and (6) The F / B learning value efgfsb is increased.
efsfb = efsfb− (efsfb−efgfsb) · α (5)
efgfsb = efgfsb + (efsfb−efgfsb) · α (6)

  7 and 8 are flowcharts showing a control routine of sub F / B control for calculating the output correction value efsfb. The illustrated control routine is performed by interruption at predetermined time intervals.

  First, in step 141, it is determined whether or not an execution condition for sub F / B control is satisfied. The case where the execution condition of the sub F / B control is satisfied includes the fact that the internal combustion engine is not being cold started and the fuel cut control is not being executed, as in the execution condition of the main F / B control. When it is determined that the sub F / B control execution condition is not satisfied, the control routine is terminated.

  On the other hand, if it is determined in step 141 that the sub F / B control execution condition is satisfied, the routine proceeds to step 142. In step 142, a value obtained by adding 1 to the time counter count is set as a new value of the time counter count (count = count + 1). The time counter count is a counter that represents an elapsed time since the previous learning was performed, that is, the previous sub F / B learning value efgsfsb was updated.

Next, at step 143, an output deviation ΔVO (n) between the output value of the oxygen sensor 24 and the target output value at time n is calculated. In step 144, based on the output deviation ΔVO calculated in step 143, the provisional output correction value efsfb ′ (n) is calculated using the following equation (7) similar to the above equation (4).

  Next, at step 145 and step 146, whether or not the provisional output correction value efsfb ′ (n) calculated at step 144 is equal to or lower than the upper guard value Grdh, and the provisional output correction value efsfb ′ (n) is the lower guard value Grdl. It is determined whether or not this is the case. If it is determined in steps 145 and 146 that the provisional output correction value efsfb ′ (n) is larger than the upper guard value Grdh (efsfb ′ (n)> Grdh), the process proceeds to step 147. In step 147, the upper limit guard value Grdh is set to the output correction value efsfb (n) at the time of the nth calculation (efsfb (n) = Grdh). On the other hand, if it is determined in steps 145 and 146 that the provisional output correction value efsfb ′ (n) is smaller than the lower limit guard value Grdl (efsfb ′ (n) <Grdl), the process proceeds to step 148. In step 148, the lower limit guard value Grdl is set as the output correction value efsfb (n) at the time of the n-th calculation (efsfb (n) = Grdl).

  If it is determined in steps 145 and 146 that the provisional output correction value efsfb ′ (n) is equal to or lower than the upper guard value and equal to or higher than the lower guard value Grdl (Grdl ≦ efsfb ′ (n) ≦ Grdh), step 149 Proceed to In step 149, the provisional output correction value efsfb '(n) is set to the output correction value efsfb (n) at the time of the nth calculation (efsfb (n) = efsfb' (n)).

  Next, at step 150, it is determined whether or not the time counter count is equal to or greater than a predetermined value count0 (count ≧ count0). The predetermined value count0 is set to a time longer than the time required for the sub F / B control to converge to some extent from the previous update of the sub F / B learning value. If it is determined in step 150 that the time counter count is smaller than the predetermined value count0 (count <count0), a sufficient time has not elapsed since the last update of the sub F / B learning value. The control routine is terminated without updating the B learning value. On the other hand, if it is determined in step 150 that the time counter count is equal to or greater than the predetermined value count0 (count ≧ count0), a sufficient time has elapsed since the last update of the sub F / B learning value. In step 151, the output correction value efsfb is corrected, and in step 152, the sub F / B learning value efgsfs is updated. Next, in step 153, the time counter count is reset to zero.

  By the way, the sub F / B learning value efgsfsb is stored in the RAM 33 of the ECU 31. However, if the battery is removed, power supply from the battery is interrupted, or the RAM 33 is destroyed, the sub F / B learning value efgsfsb is stored. The B learning value efgsfsb is deleted. As described above, if the sub F / B learning value efgsfsb is reset to zero, the output value of the air-fuel ratio sensor 23 is again set to an appropriate value at an early stage by the sub F / B control after the stop of the internal combustion engine, the fuel cut control, or the like. It becomes impossible to correct even the value. For this reason, after the power supply from the battery is interrupted, the sub F / B learning value efgfsb needs to be set to an appropriate value at an early stage.

  Thus, in the present embodiment, the F / B gain (proportional gain Ksp and integral gain Ksi) in the sub F / B control is sub-F / B learning until the sub F / B learning value efgsfsb converges to a certain value. The value efgfsb is set to a value larger than a normal value after convergence near a certain value. As a result, the change in the output correction value efsfb calculated in the sub F / B control can be increased. Therefore, the sub F / B learning value efgsfsb can be brought close to an appropriate value at an early stage, and learning can be performed at an early stage. It can be converged.

  By the way, the three-way catalyst 20 has an oxygen storage capacity, so that when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is lean, it stores oxygen in the exhaust gas and flows into the three-way catalyst. When the air-fuel ratio of the exhaust gas is rich, the stored oxygen is released and the oxygen and HC and CO in the exhaust gas are oxidized and purified by this oxygen. Such oxygen storage capacity is not always constant from the start of use of the three-way catalyst 20, and gradually decreases according to the operation period of the internal combustion engine or the like (that is, the three-way catalyst 20 deteriorates).

  In the state where the oxygen storage capacity of the three-way catalyst 20 is high, even if the actual air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is reversed from lean to rich, the oxygen stored in the three-way catalyst 20 is exhaust gas. The air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 does not become rich until almost no oxygen is stored in the three-way catalyst 20. Conversely, even if the actual air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is reversed from rich to lean, excess oxygen in the exhaust gas flowing into the three-way catalyst 20 is stored in the three-way catalyst 20. Therefore, the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 does not become rich until the limit of the oxygen storage capacity of the three-way catalyst 20 is reached. That is, in a state where the oxygen storage capacity of the three-way catalyst 20 is high, the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 is equal even if the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is reversed between rich and lean. Immediately, there is no inversion between rich and lean, and therefore the inversion period between the rich and lean air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 tends to be longer.

  On the other hand, in the state where the oxygen storage capacity of the three-way catalyst 20 is low, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is reversed from lean to rich, the three-way catalyst 20 hardly stores oxygen. The air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 is immediately reversed to rich. Conversely, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is reversed from rich to lean, excess oxygen in the exhaust gas flowing into the three-way catalyst 20 is hardly occluded by the three-way catalyst 20. The air-fuel ratio of the exhaust gas flowing out from the catalyst 20 is immediately reversed to lean. That is, when the oxygen storage capacity of the three-way catalyst 20 is low, if the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is reversed between rich and lean, the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 is also immediately rich. The inversion cycle between the rich and lean air-fuel ratios of the exhaust gas that reverses between leans and flows out of the three-way catalyst 20 tends to be short.

  Here, whether or not the sub F / B learning value efgsfsb after interruption of power supply from the battery has converged to a certain value is determined by reversing the output value of the oxygen sensor 24 between rich and lean. It is performed according to the number of times, or according to the fluctuation range of the integrated value of the deviation between the output value of the oxygen sensor 24 and the target output value (in this embodiment, the value corresponding to the theoretical air-fuel ratio). Therefore, when the oxygen storage capacity of the three-way catalyst 20 is high, that is, when the degree of deterioration of the three-way catalyst 20 is low, the inversion cycle between the rich and lean air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 20 is reduced. Since it tends to be long, the convergence determination of the sub F / B learning value efgfsb is often delayed, and the fluctuation range of the integrated value of the deviation between the output value of the oxygen sensor 24 and the target output value tends to be large. . Therefore, in order to converge the sub F / B learning value efgsfsb at an early stage, the F / B gain of the sub F / B control is increased to make the output value of the oxygen sensor 24 easily swing between rich and lean. At the same time, the sub F / B learning value may be greatly updated by one learning so that the absolute values of the guard values Grdh and Grdl can be increased and the output correction value efsfb can take a larger absolute value. It needs to be possible.

  Conversely, when the three-way catalyst 20 has a low oxygen storage capacity, that is, when the degree of deterioration of the three-way catalyst 20 is high, the inversion cycle between the rich and lean air-fuel ratios of the exhaust gas flowing out from the three-way catalyst 20. Therefore, when the F / B gain of the sub F / B control is increased, the output correction value efsfb often diverges. For this reason, in order to suppress the divergence of the output correction value efsfb, the F / B gain of the sub F / B control is reduced to make the output value of the oxygen sensor 24 difficult to swing between rich and lean, It is necessary to reduce the absolute values of the values Grdh and Grdl so that the output correction value efsfb does not easily shake.

  Therefore, in the present embodiment, after the sub F / B learning value efgsfs stored in the RAM 33 is erased due to interruption of power supply from the battery, etc., until the sub F / B learning value efgsfs converges, the three-way catalyst 20 The magnitude of the F / B gain of the sub F / B control is changed in accordance with the oxygen storage capacity, and the absolute values of the guard values Grdh and Grdl are changed.

  FIG. 9 shows the maximum amount of oxygen that can be stored by the three-way catalyst 20 (hereinafter referred to as “maximum oxygen storage amount”) OSCMAX and the F / B gain (proportional gain Ksp, integral gain Ksi) of sub F / B control. FIG. As can be seen from FIG. 9, the larger the maximum oxygen storage amount OSCMAX is, the larger the proportional gain Ksp is (FIG. 9 (a)), and the larger the maximum oxygen storage amount OSCMAX is, the larger the integral gain Ksi is ( FIG. 9B). In the figure, Ksp1 and Ksi1 indicate the proportional gain Ksp and integral gain Ksi after the sub F / B learning value efgfsb converges, respectively. The proportional gain Ksp and integral before the sub F / B learning value efgfsb converges. Regardless of the maximum oxygen storage amount OSCMAX, the gain Ksi is larger than the proportional gain Ksp1 and the integral gain Ksi1 after convergence of the sub F / B learning value.

  Thus, by increasing the F / B gain of the sub F / B control when the oxygen storage capacity is high, the inversion period between the rich and lean output of the oxygen sensor 24 is shortened, and the oxygen sensor The fluctuation range of the integrated value of the deviation between the output value of 24 and the target output value (in this embodiment, a value corresponding to the theoretical air-fuel ratio) is also reduced. For this reason, it is possible to prevent a delay in determining the convergence of the sub F / B learning value based on the number of output inversions of the oxygen sensor 24 and the integrated value of the output deviation. On the other hand, by reducing the sub F / B gain when the oxygen storage capacity is low, it is possible to suppress the sub F / B control from divergence, thereby reducing exhaust emission and drivability. Deterioration can be suppressed.

  FIG. 10 is a diagram illustrating the relationship between the maximum oxygen storage amount OSCMAX and the guard values (upper guard value Grdh, lower guard value Grdl) for sub F / B control. As can be seen from FIG. 10, the larger the maximum oxygen storage amount OSCMAX, the larger the upper limit guard value Grdh (FIG. 10 (a)), and the larger the maximum oxygen storage amount OSCMAX, the smaller the lower limit guard value Grdl. (FIG. 10B) The absolute value of the upper limit guard value Grdh and the lower limit guard value Grdl increases as the maximum oxygen storage amount OSCMAX increases.

  Thus, by increasing the absolute value of the guard value of the sub F / B control when the oxygen storage capacity is high, the output correction value efsfb can take a larger value, and therefore the output of the oxygen sensor 24 is increased. The inversion period between the rich and lean directions becomes shorter, and the fluctuation range of the integrated value of the deviation between the output value of the oxygen sensor 24 and the target output value becomes smaller. For this reason, it is possible to prevent a delay in determining the convergence of the sub F / B learning value based on the number of output inversions of the oxygen sensor 24 and the integrated value of the output deviation. On the other hand, by reducing the absolute value of the guard value of the sub F / B control when the oxygen storage capacity is low, it is possible to suppress the sub F / B control from being diffused, and thus exhaust emission. It is possible to suppress the deterioration of driving performance and the deterioration of drivability.

  For example, the maximum oxygen storage amount OSCMAX controls the air-fuel ratio of the exhaust gas so that the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 20 is alternately switched between rich and lean, and the air-fuel ratio of the inflowing exhaust gas is It is calculated based on the time until the air-fuel ratio of the exhaust gas discharged from the three-way catalyst 20 switches between rich and lean until it switches between rich and lean.

  FIG. 11 is a flowchart showing a control routine of control for setting the F / B gain and the guard value of the sub F / B control. The illustrated control routine is performed by interruption at predetermined time intervals.

  First, in step 161, it is determined whether or not the convergence flag Xst is zero. The convergence flag Xst is a flag that is set to 1 when the sub F / B learning value efgfsb is converged, and is set to 0 otherwise. After the sub F / B learning value efgfsb is erased and the sub F / B learning value efgfsb has not converged, the convergence flag Xst is 0, so the routine proceeds to step 162. In step 162, it is determined whether or not the output value of the oxygen sensor 24 is inverted between rich and lean. If it is determined that the output value of the oxygen sensor 24 has been inverted, a value obtained by adding 1 to the inversion number COX in step 163 is set as a new inversion number COX (COX = COX + 1), and the process proceeds to step 164. On the other hand, if it is determined that the output value of the oxygen sensor 24 is not inverted, the process proceeds to step 164 without going through step 163.

  Next, in step 164, it is determined whether or not the number of inversions COX is equal to or greater than the predetermined number of inversions COX1, that is, whether or not the sub F / B learning value efgsfs has converged. If it is determined in step 164 that the number of inversions COX is smaller than the predetermined number of inversions COX1, the process proceeds to step 165. In step 165, the maximum oxygen storage amount OSCMAX is calculated based on the output of the oxygen sensor 24. Next, at step 166, based on the maximum oxygen storage amount OSCMAX calculated at step 165, the proportional gain Ksp, integral gain Ksi, upper limit guard value Grdh, and lower limit guard value Grdl are calculated using the maps shown in FIGS. The

  On the other hand, if it is determined in step 164 that the number of inversions COX is equal to or greater than the predetermined number of inversions COX1, the process proceeds to step 167. In step 167, regardless of the maximum oxygen storage amount, the proportional gain Ksp, the integral gain Ksi, the upper guard value Grdh, and the lower guard value Grdl are set to Ksp1, Ksi1, Grdh1, and Grd11. Next, at step 168, the convergence flag Xst is set to 1.

  In the control routine after the convergence flag Xst is set to 1, it is determined in step 161 that the convergence flag Xst is not 0, and the process proceeds to step 169. In step 169, it is determined whether a condition for erasing the sub F / B learning value efgfsb stored in the RAM 33 of the ECU 31 is satisfied. Examples of the condition for deleting the sub F / B learning value include interruption of power supply from the battery. If it is determined in step 169 that the condition for erasing the sub F / B learning value is not satisfied, the control routine is terminated. On the other hand, if it is determined that the condition for deleting the sub F / B learning value is satisfied, the routine proceeds to step 170. In step 170, the number of inversions is reset to zero, and then in step 171 the convergence flag Xst is reset to zero.

  In the above embodiment, the air-fuel ratio sensor provided upstream of the three-way catalyst is an air-fuel ratio sensor that reacts linearly. However, the present invention is also applied to a so-called double oxygen sensor system in which the upstream air-fuel ratio sensor is also an oxygen sensor. It is also possible to apply. However, in this case, the output of the oxygen sensor is not proportional to the amount of residual oxygen in the exhaust gas, and the fuel supply amount is not directly proportional to the upstream oxygen sensor output. The oxygen balance of the original catalyst cannot be estimated. Therefore, an integral value of the deviation between the air-fuel ratio correction coefficient and the air-fuel ratio sub F / B learning value is used as means for estimating the oxygen balance of the three-way catalyst.

In the above embodiment, the three-way catalyst is used as the exhaust purification catalyst. However, the exhaust purification catalyst is not limited to this, and any exhaust purification catalyst having an oxygen storage capability may be used. For example, NO _X A NO _x storage-reduction catalyst or the like having the storage ability may be used.

  Further, in the above embodiment, the main F / B control is executed so that the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio, and the sub-F / B control is executed so that the output value of the oxygen sensor becomes near the stoichiometric air-fuel ratio. However, the target air-fuel ratio of the F / B control may not be the stoichiometric air-fuel ratio, and may be an air-fuel ratio richer or leaner than the stoichiometric air-fuel ratio.

  Furthermore, in the above embodiment, the output value of the air-fuel ratio sensor 23 is corrected by the sub F / B control. For example, the target air-fuel ratio in the main F / B control is corrected by the sub F / B control. Also good.

1 is an overall view of an internal combustion engine to which an air-fuel ratio control apparatus for an internal combustion engine of the present invention is applied. It is the figure which showed the relationship between an exhaust air fuel ratio and the output voltage of an air fuel ratio sensor. It is the figure which showed the relationship between an exhaust air fuel ratio and the output voltage of an oxygen sensor. It is a flowchart which shows the control routine of the target fuel supply amount calculation control which calculates a target fuel supply amount. It is a flowchart which shows the control routine of the main F / B control which calculates a fuel correction amount. 4 is a time chart of an actual exhaust air-fuel ratio, an output value of an oxygen sensor, an output correction value of an air-fuel ratio sensor, and a sub F / B learning value. It is a part of flowchart which shows the control routine of sub F / B control which calculates an output correction value. It is a part of flowchart which shows the control routine of sub F / B control which calculates an output correction value. It is a figure showing the relationship between the maximum oxygen storage amount and the F / B gain of sub F / B control. It is a figure showing the relationship between the maximum oxygen storage amount and the F / B gain of sub F / B control. It is a flowchart which shows the control routine of the control which sets the F / B gain and guard value of sub F / B control.

Explanation of symbols

1 Engine Body 3 Piston 5 Combustion Chamber 6 Intake Valve 8 Exhaust Valve 10 Spark Plug 11 Fuel Injection Valve 31 ECU
41 Load sensor 42 Crank angle sensor

Claims (4)

An upstream air-fuel ratio sensor disposed on the exhaust upstream side of the exhaust purification catalyst provided in the engine exhaust passage and detecting the air-fuel ratio of the exhaust gas; and an exhaust air exhaust gas downstream of the exhaust purification catalyst disposed on the exhaust downstream side of the exhaust purification catalyst. An air-fuel ratio control apparatus for an internal combustion engine, comprising a downstream air-fuel ratio sensor for detecting a fuel ratio, and controlling a fuel supply amount so that an exhaust air-fuel ratio becomes a target air-fuel ratio based on an output value of the upstream air-fuel ratio sensor Because
When the output value of the upstream air-fuel ratio sensor deviates from the actual exhaust air-fuel ratio, the output value of the upstream air-fuel ratio sensor and the actual value are measured using feedback parameters based on the output value of the downstream air-fuel ratio sensor. Feedback means for correcting the fuel supply amount by feedback to compensate for the deviation from the exhaust air-fuel ratio of
A learning value corresponding to a steady deviation between the output value of the upstream air-fuel ratio sensor and the actual exhaust air-fuel ratio is calculated based on the correction amount by the feedback means, and based on the calculated learning value An air-fuel ratio control apparatus for an internal combustion engine, comprising a learning means for correcting a fuel supply amount, wherein a value of a feedback parameter is determined regardless of a degree of deterioration of the exhaust purification catalyst during normal operation of the internal combustion engine.
A convergence determining means for determining convergence of the learning value after the learning value is reset;
An air-fuel ratio control apparatus for an internal combustion engine, wherein a value of a feedback parameter is changed in accordance with a degree of deterioration of the exhaust purification catalyst before the learning value is determined to have converged by the convergence determining means.   The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the feedback parameter includes a feedback gain, and the feedback gain is increased as the degree of deterioration of the exhaust purification catalyst is smaller.   The feedback means limits the absolute value of the correction amount by feedback of the fuel supply amount to a predetermined guard value or less. The feedback parameter includes the guard value, and the deterioration degree of the exhaust purification catalyst is smaller. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2, wherein an absolute value of the guard value is increased.   When the number of times the output value of the downstream air-fuel ratio sensor is inverted between the air-fuel ratio richer than the target air-fuel ratio and the air-fuel ratio leaner than the target air-fuel ratio becomes a predetermined number or more The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the learning value is determined to have converged to the initial value.

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