JP2010261846A - Signal processor of gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of reductions in acquisition accuracy of an air-fuel ratio based on output signals of A/F sensors 58a and 58b due to in output-signal perturbations of the A/F sensors 58a and 58b caused by exhaust pulsations and the problem of reductions in the controllability of an air-fuel ratio by air-fuel-ratio feedback control. <P>SOLUTION: A cutoff frequency is set as a frequency corresponding to one rotation of a crankshaft 36. A low-pass filter computed on the basis of the cutoff frequency fc is used to perform filter processing on output signals of the A/F sensors 58a and 58b. An average air-fuel ratio based on filter-processed output signals of the A/F sensors 58a and 58b is taken as a control variable of average F/B control. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a signal processing device for a gas sensor that is provided in an exhaust passage of an internal combustion engine and is applied to a gas sensor that detects the concentration of a specific component in exhaust gas in the exhaust passage.

  As this type of gas sensor, an air-fuel ratio sensor (A / A) provided in an exhaust passage of an internal combustion engine to linearly detect an actual air-fuel ratio (intake amount / fuel amount) of an air-fuel mixture supplied to a combustion chamber of the internal combustion engine. F sensor) is known. The A / F sensor is used for feedback control (air-fuel ratio F / B control) of the actual air-fuel ratio to a target value. Here, normally, the target value is set in the vicinity of the air-fuel ratio at which the exhaust purification efficiency of the exhaust purification catalyst provided on the downstream side of the A / F sensor becomes high, so that the exhaust characteristics can be improved. .

  By the way, because exhaust gas is intermittently discharged from the combustion chamber of the internal combustion engine to the exhaust passage, the exhaust pressure fluctuates in the exhaust passage. Therefore, the exhaust pressure applied to the A / F sensor is accompanied by pulsation. It will be. Here, since the output signal of the A / F sensor has pressure dependency due to the structure of the sensor, the sensor output signal may fluctuate due to the exhaust pressure (exhaust pulsation) accompanied by this pulsation. In this case, the controllability of the actual air-fuel ratio may be reduced by using the changed sensor output signal as the detected value of the control amount of the air-fuel ratio F / B control.

  Therefore, conventionally, as can be seen in Patent Document 1 below, exhaust pulsation is obtained by setting the integral value of the sensor output signal over an integral multiple of the exhaust pulsation period as the detected value of the control amount of the air-fuel ratio F / B control. There has also been proposed a technique for suppressing the influence of fluctuations in the output signal of the A / F sensor caused by the above on the controllability of the actual air-fuel ratio.

JP-A-01-206251

  However, when the integrated value is used as the detected value of the control amount of the air-fuel ratio F / B control, the time required to grasp the actual change of the air-fuel ratio as the change of the detected value of the control amount is increased. There is a possibility that the actual air-fuel ratio cannot be grasped with good responsiveness.

  In addition to the A / F sensor described above, for gas sensors that detect the concentration of a specific component in exhaust, the effect of fluctuations in the output signal of the gas sensor due to exhaust pulsation on the determination of the concentration of the specific component in exhaust Such a situation that it is difficult to achieve both suppression and grasping the concentration of the specific component in the actual exhaust gas with high responsiveness is generally common.

  The present invention has been made in order to solve the above-mentioned problems, and its purpose is to suitably suppress the influence of fluctuations in the output signal of the gas sensor due to exhaust pulsation on the grasp of the concentration of specific components in the exhaust. However, an object of the present invention is to provide a signal processing device for a gas sensor that can ascertain the concentration of a specific component in actual exhaust gas as much as possible.

  Hereinafter, means for solving the above-described problems and the operation and effects thereof will be described.

  According to a first aspect of the present invention, there is provided a signal processing apparatus for a gas sensor, which is provided in an exhaust passage of an internal combustion engine and is applied to a gas sensor for detecting a concentration of a specific component in exhaust gas in the exhaust passage. Filter means for removing high-frequency components is provided, and the filter means comprises variable means for variably setting the minimum frequency of the high-frequency components to be removed according to the operating state of the internal combustion engine.

  As the minimum frequency is lowered, the filtered sensor output signal (the output signal of the filter means) is the one in which the fluctuation of the sensor output signal due to the fluctuation of the exhaust pressure applied to the gas sensor is suppressed. Responsiveness to the concentration of specific components in the exhaust gas is reduced. However, the degree of influence of fluctuations in the exhaust pressure applied to the gas sensor differs depending on the operating state of the internal combustion engine, and the degree of priority between the accuracy of grasping the concentration of the specific component in the exhaust gas and the responsiveness tends to change. There is. In the above invention, in view of this point, the minimum frequency is variably set according to the operating state of the internal combustion engine, so that the influence of the fluctuation of the exhaust pressure applied to the gas sensor is suitably suppressed, while the actual exhaust gas is specified. The responsiveness of the output signal of the filter means to the component concentration can be made as high as possible.

  According to a second aspect of the present invention, in the first aspect of the present invention, the variable means increases the minimum frequency of the high-frequency component to be removed as the engine speed of the internal combustion engine increases. To do.

  Since the output signal of the gas sensor has pressure dependency due to the structure of the sensor, it can fluctuate at a frequency (fluctuation frequency) corresponding to the cycle of the exhaust gas discharged to the gas sensor. Here, the higher the engine rotation speed, the higher this fluctuation frequency. For this reason, the fluctuation frequency of the output signal of the gas sensor increases as the engine speed increases. In the above invention, in view of this point, as the engine rotational speed increases, the minimum frequency for removing the fluctuation component of the output signal of the gas sensor due to exhaust pulsation is increased. As a result, fluctuations in the output signal of the gas sensor due to exhaust pulsation can effectively suppress the influence exerted on grasping the concentration of a specific component in the exhaust based on the sensor output signal, while specifying the actual exhaust gas. It is possible to grasp the concentration of the component with high responsiveness as much as possible.

  According to a third aspect of the present invention, in the second aspect of the present invention, the variable means corresponds to the discharge interval of the exhaust discharged from the combustion chamber of the internal combustion engine to the gas sensor. The frequency is lower than the frequency to be used.

  The frequency of the change in the concentration of the specific component in the exhaust gas is considered to be largely a frequency (exhaust frequency) determined by the reciprocal of the exhaust interval discharged to the gas sensor. For this reason, in the output signal of the gas sensor, the higher frequency component than the exhaust frequency becomes noise. In the above invention, in view of such a point, by setting the minimum frequency to be variably set to the above setting, the influence exerted by the exhaust pulsation when determining the concentration of the specific component in the exhaust based on the output signal of the gas sensor is minimized. Can be suppressed.

  The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein the gas sensor is an air-fuel ratio sensor, and an air-fuel ratio based on an output signal of the filter means is controlled to a target value. An air-fuel ratio control means for determining the difference between the actual air-fuel ratio and the target value after it is determined that the difference between the actual air-fuel ratio and the target value is large. The minimum frequency to be variably set is increased over a period until it becomes below.

  The output signal of the filter means may cause a response delay with respect to a change in the air-fuel ratio at a frequency equal to or higher than the minimum frequency. In this case, if the difference between the actual air-fuel ratio and the target value of air-fuel ratio (target air-fuel ratio) increases, the change in the output signal of the filter means cannot catch up with the actual air-fuel ratio change, and the actual air-fuel ratio There is a risk that changes cannot be grasped quickly. In this regard, in the above invention, during the period from when it is determined that the difference between the actual air-fuel ratio and the target air-fuel ratio is large, until the difference between the actual air-fuel ratio and the target air-fuel ratio becomes equal to or less than a predetermined value. The response delay of the output signal of the filter means can be suppressed as much as possible by increasing the minimum frequency. As a result, the actual change in the air-fuel ratio can be grasped as quickly as possible from the output signal of the filter means, and as a result, a decrease in controllability of the air-fuel ratio based on the output signal of the filter means can be suppressed as much as possible.

  In the case where the air-fuel ratio control means feedback-controls the air-fuel ratio to the target air-fuel ratio based on the integral calculation value of the value corresponding to the difference between the air-fuel ratio based on the output signal of the filter means and the target air-fuel ratio, When the difference between the air-fuel ratio and the target air-fuel ratio increases, the controllability of the air-fuel ratio based on the output signal of the filter means is likely to be greatly reduced due to an increase in the integral calculation value. large.

  According to a fifth aspect of the present invention, in the fourth aspect of the present invention, when it is determined that the acceleration increase control, the catalyst early warm-up control, or the fuel cut stop control is performed, the filter means is the actual air-fuel ratio. And the target value are determined to be large.

  According to acceleration increase control, early catalyst warm-up control, and fuel cut stop control, the difference between the air-fuel ratio based on the output signal of the filter means and the target air-fuel ratio becomes large. For this reason, according to the said invention, the condition where the difference of the said air fuel ratio and a target air fuel ratio becomes large can be judged appropriately.

  According to a sixth aspect of the present invention, in the invention according to any one of the first to fifth aspects, the gas sensor is an air-fuel ratio sensor, and the filter means is an air-fuel ratio based on an output signal of the filter means. The greater the difference from the stoichiometric air-fuel ratio, the greater the degree of removal of the output signal of the gas sensor having a frequency equal to or higher than the lowest frequency.

  Due to the structure of the air-fuel ratio sensor, if the difference between the actual air-fuel ratio and the theoretical air-fuel ratio is large, the amount of fluctuation of the output signal of the air-fuel ratio sensor due to exhaust pulsation increases. Here, in the above invention, as the difference between the air-fuel ratio based on the output signal of the filter means and the stoichiometric air-fuel ratio increases, the degree of removal of the sensor output signal having a frequency equal to or higher than the lowest frequency increases, thereby causing exhaust pulsation. In the situation where the fluctuation amount of the output signal of the air-fuel ratio sensor to be increased becomes large, this influence can be suitably suppressed.

  The invention according to claim 7 is the invention according to any one of claims 1 to 6, wherein an exhaust turbine of a supercharger is provided on the downstream side of the gas sensor in the exhaust passage. .

  When an exhaust turbine is provided, this becomes an exhaust resistance, and the exhaust pressure in the exhaust passage upstream of the exhaust turbine tends to fluctuate greatly. For this reason, when a gas sensor is arrange | positioned upstream from an exhaust turbine, there exists a possibility that the fluctuation amount of the output signal of a gas sensor may increase. For this reason, in the said invention, the merit provided with the said variable means is large.

  The invention according to claim 8 is the invention according to any one of claims 1 to 7, wherein the internal combustion engine is a multi-cylinder internal combustion engine, and the gas sensor is connected to each cylinder of the internal combustion engine. An air-fuel ratio sensor provided in a collecting portion of the exhaust passage, and operating means for operating a fuel injection valve for injecting and supplying fuel to the internal combustion engine so as to control the air-fuel ratio based on the output signal of the filter means to the target value And a grasping means for grasping a relative air-fuel ratio shift between the cylinders of the internal combustion engine based on an output signal of the gas sensor before being input to the filter means, and a relative air gap between the cylinders. An operation amount correcting means for correcting the operation amount of the fuel injection valve is further provided so that the deviation of the fuel ratio becomes small.

  In order to improve the combustion control accuracy of the internal combustion engine by performing feedback control of the air-fuel ratio for each cylinder, it is required to improve the responsiveness of the output signal of the air-fuel ratio sensor. For this reason, depending on the setting of the minimum frequency in the filter processing, it is difficult to grasp the actual change in the air-fuel ratio with high response based on the output signal of the filter means, and it is difficult to grasp the air-fuel ratio for each cylinder. There is a risk of becoming. In this regard, in the above-described invention, the relative air-fuel ratio deviation between the cylinders of the internal combustion engine is grasped based on the output signal of the air-fuel ratio sensor before being input to the filter means. Then, the operation amount of the fuel injection valve for controlling the air-fuel ratio based on the output signal of the filter means to the target air-fuel ratio is corrected so that this deviation becomes small. Thereby, the influence of the fluctuation of the output signal of the air-fuel ratio sensor due to the exhaust pulsation can be suitably suppressed by the filter process, and the air-fuel ratio feedback control for each cylinder can be performed.

1 is a system configuration diagram according to a first embodiment. FIG. 7 is a flowchart showing a procedure of filtering processing of an A / F sensor output signal according to the embodiment. The figure which shows the frequency spectrum of the A / F sensor output signal before and behind the filter process concerning the embodiment. The time chart which shows the A / F sensor output signal waveform before and behind the filter process concerning the embodiment. The flowchart which shows the procedure of the process which raises the cutoff frequency concerning 2nd Embodiment. The flowchart which shows the procedure of the attenuation coefficient change process concerning 3rd Embodiment.

(First embodiment)
Hereinafter, a first embodiment in which a signal processing device for a gas sensor according to the present invention is applied to a multi-cylinder gasoline engine system will be described with reference to the drawings.

  FIG. 1 shows a system configuration according to the present embodiment.

  The illustrated engine 10 is a four-stroke engine and is a spark ignition type internal combustion engine. In the present embodiment, a multi-cylinder (in-line 6-cylinder) gasoline engine is assumed as the engine 10, and # 1 to # 6 in the figure indicate the first to sixth cylinders. In the intake passage 11 of the engine 10, in order from the upstream side, an air cleaner 12 that removes foreign matters in the air, an air flow meter 14 that detects the amount of intake air (intake amount), an intercooler 16 that cools intake air, and intake air temperature An intake air temperature sensor 18 for detecting the above and an electronically controlled throttle valve 20 whose opening degree is adjusted by an actuator such as a DC motor are provided. A surge tank 22 is provided on the downstream side of the throttle valve 20, and an intake pressure sensor 24 that detects intake pressure is provided in the surge tank 22. Connected to the surge tank 22 is an intake manifold 30 that introduces intake air into the combustion chamber 28 of each cylinder of the engine 10. An electromagnetically driven fuel injection valve 32 that injects and supplies fuel is provided in the vicinity of the intake port of each cylinder in the intake manifold 30.

  On the other hand, exhaust manifolds 34 a and 34 b are connected to the exhaust ports of the cylinders of the engine 10. Here, in the present embodiment, the exhaust manifold 34a is connected to the exhaust ports corresponding to # 1 to # 3, and the exhaust manifold 34b is connected to the exhaust ports corresponding to # 4 to # 6. On the other hand, each of the intake valve 42 and the exhaust valve 44 is driven by a cam attached to each of the intake side camshaft 38 and the exhaust side camshaft 40 interlocked with the crankshaft 36. According to such a configuration, the air-fuel mixture of intake air and fuel is introduced into the combustion chamber 28 by opening the intake valve 42, and the air-fuel mixture is ignited by the spark discharge of a spark plug (not shown) and used for combustion. The air-fuel mixture subjected to combustion is discharged to the exhaust manifolds 34 a and 34 b as exhaust gas by opening the exhaust valve 44. The compression top dead center of each cylinder is shifted by “120 ° CA”, and appears in the order of # 1, # 5, # 3, # 6, # 2, and # 4.

  The engine 10 includes a crank angle sensor 46 that detects the rotation angle of the crankshaft 36 in the vicinity of the crankshaft 36, a water temperature sensor 48 that detects the temperature of cooling water that cools the engine 10, and the rotation angle of the intake camshaft 38. An intake side cam angle sensor 50 for detecting and an exhaust side cam angle sensor 52 for detecting the rotation angle of the exhaust side cam shaft 40 are provided.

  Exhaust passages 56a and 56b are connected to the gathering portions of the exhaust manifolds 34a and 34b. A / F sensors 58a and 58b are provided in the exhaust passages 56a and 56b. These A / F sensors are sensors that output a linear electric signal in accordance with the oxygen concentration in the exhaust gas and unburned components (CO, HC, H2, etc.), and the actual air-fuel ratio (actual air-fuel ratio) over a wide area. This is a so-called full-range air-fuel ratio sensor that can be detected.

  The downstream sides of the exhaust passages 56a and 56b are connected to a supercharger (turbocharger 60). Thus, in the present embodiment, the A / F sensors 58 a and 58 b are provided on the upstream side of the turbocharger 60. This is primarily intended to improve the responsiveness of the output signals of the A / F sensors 58a and 58b to the actual air-fuel ratio (cylinder-by-cylinder) for each cylinder. Secondly, when the engine 10 is not warmed up at the time of starting the engine, water tends to accumulate due to the fact that exhaust heat does not easily reach the downstream side of the turbocharger 60. This is because the contact frequency of the A / F sensor may increase and the reliability of the A / F sensor may decrease.

  The turbocharger 60 includes an intake compressor 62a provided on the intake passage 11 and an exhaust turbine 62b provided on the downstream side of the exhaust passages 56a and 56b. The exhaust turbine 62b is provided with rotational energy by the exhaust flowing through these exhaust passages, and the intake compressor 62a is driven by this rotational energy. The intake air pressurized by the drive of the intake air compressor 62a is compressed by being cooled by the intercooler 16. Thereby, the charging efficiency of the intake air supplied to the combustion chamber 28 of the engine 10 is improved. Incidentally, the supercharging pressure of the intake air is adjusted by operating an air bypass valve 64 provided in a passage that bypasses between the upstream portion and the downstream portion of the intake compressor 62a on the intake passage 11.

  A second exhaust passage 68 is connected to the downstream side of the turbocharger 60. The second exhaust passage 68 is provided with a first three-way catalyst 70 and a second three-way catalyst 72 that purify harmful components in the exhaust gas as an exhaust aftertreatment system for purifying exhaust gas. The first three-way catalyst 70 and the second three-way catalyst 72 are for purifying NOx, HC and CO in the exhaust. Further, an O2 sensor 74 is provided between the first three-way catalyst 70 and the second three-way catalyst 72 to change the output signal in a binary manner according to the oxygen concentration in the exhaust gas. The O2 sensor 74 detects whether the actual air-fuel ratio is a small value (rich) or a large value (lean) with respect to the theoretical air-fuel ratio (λ = 1) based on the actual oxygen concentration in the exhaust gas. It is.

  The electronic control device (ECU 76) is a control device that operates various actuators necessary for various controls of the engine 10. The ECU 76 detects detection signals from an accelerator sensor 78 that detects the amount of accelerator operation by the user, A / F sensors 58a and 58b, an O2 sensor 74, a crank angle sensor 46, an exhaust side cam angle sensor 52, and the air flow meter 14. Enter sequentially. The ECU 76 performs combustion control of the engine 10 based on these input signals.

  In particular, the ECU 76 energizes the fuel injection valve 32 so as to feedback control the actual air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 28 of each cylinder of the engine 10 to a target value (target air-fuel ratio). More specifically, the air-fuel ratio is feedback-controlled for each cylinder (cylinder-specific air-fuel ratio F / B control) in order to suppress variations in the actual air-fuel ratio among the cylinders. In the cylinder-by-cylinder air-fuel ratio F / B control, the injection characteristics of the fuel injection valve 32 of each cylinder differ depending on individual differences and changes over time, or the amount of intake air supplied to the combustion chamber 28 of each cylinder differs. This is performed in order to improve the control accuracy of the actual air-fuel ratio and further improve the exhaust characteristics by suppressing the variation in the air-fuel ratio by cylinder caused by the cause.

  The cylinder-by-cylinder air-fuel ratio F / B control includes feedback control (average F / B control) for uniformly setting the average air-fuel ratio of all cylinders to the target air-fuel ratio, and feedback for reducing variation in the cylinder-by-cylinder air-fuel ratio. Control (F / B control for each cylinder). Here, the cylinder-by-cylinder air-fuel ratio F / B control of # 1 to # 3 will be described as an example. Note that the cylinder-by-cylinder air-fuel ratio F / B control of # 4 to # 6 is the same as the control method of # 1 to # 3, and thus the description thereof is omitted.

  In the average F / B control, first, the target air-fuel ratio is set based on the output signal of the O2 sensor 74. Here, the target air-fuel ratio is set in the vicinity of the air-fuel ratio (λ = 1) at which the exhaust purification efficiency of the first three-way catalyst 70 becomes high. Next, the air-fuel ratio calculated based on the output signal of the A / F sensor 58a is set as the average air-fuel ratio, and F / B correction is performed by PI control (proportional differential control) based on the deviation between the average air-fuel ratio and the target air-fuel ratio. Calculate the coefficient. Then, the basic injection amount is calculated from the intake air amount based on the output signal of the air flow meter 14, the engine speed based on the output signal of the crank angle sensor 46, the accelerator operation amount based on the output signal of the accelerator sensor 78, and the like. By multiplying the basic injection amount by the F / B correction coefficient, an operation amount for feedback control of the average air-fuel ratio to the target air-fuel ratio is calculated.

  On the other hand, in the cylinder-by-cylinder F / B control, first, the cylinder-by-cylinder air-fuel ratios # 1 to # 3 are calculated based on the output signal of the A / F sensor 58a. Next, the fuel injection amount correction coefficient corresponding to each of # 1 to # 3 is calculated separately so that the variation in the cylinder-by-cylinder air-fuel ratio between # 1 and # 3 is reduced.

  Then, the final fuel injected from the fuel injection valves 32 corresponding to each of # 1 to # 3 is obtained by multiplying the operation amount for performing feedback control of the average air-fuel ratio to the target air-fuel ratio by the correction coefficient. Calculate the amount.

  By the way, the output signals of the A / F sensors 58a and 58b have pressure dependency due to the structure of the sensors. For this reason, the ECU 76 grasps the actual air-fuel ratio from the sensor output signal by assuming a specific pressure as the exhaust pressure applied to the A / F sensors 58a and 58b. However, the exhaust pressure applied to the A / F sensors 58a and 58b is accompanied by pulsation. This is because exhaust gas is discharged from the combustion chamber 28 of each cylinder of the engine 10 to the A / F sensors 58a and 58b at predetermined crank angle intervals (240 ° CA), thereby being added to the A / F sensors 58a and 58b. This is because the exhaust pressure varies. Here, since the exhaust discharge interval becomes shorter as the engine speed increases, the frequency (fluctuation frequency) at which the exhaust pressure applied to the A / F sensors 58a and 58b fluctuates increases as the engine speed increases. Further, the fluctuation frequency is dominated by the reciprocal value of the exhaust interval from the combustion chamber 28 to the sensor and the integral multiple thereof. When the exhaust pressure applied to the A / F sensors 58a and 58b fluctuates at the fluctuation frequency, the specific pressure assumed by the ECU 76 deviates from the exhaust pressure actually applied to the sensor, so that the sensor output signal becomes the fluctuation frequency. There is a risk that the accuracy of grasping the actual air-fuel ratio by the ECU 76 may be reduced.

  In particular, in the present embodiment, the output signal of the A / F sensors 58a and 58b largely fluctuates with the fluctuation of the exhaust pressure due to the setting for performing the cylinder-by-cylinder air-fuel ratio F / B control. There is a risk that the decrease in the accuracy of grasping the fuel ratio will be significant. That is, as described above, the A / F sensors 58a and 58b are provided on the upstream side of the exhaust turbine 62b. Here, on the upstream side of the exhaust turbine 62b, the exhaust turbine 62b becomes an exhaust resistance, so that the exhaust pressure in the vicinity of the A / F sensors 58a and 58b is likely to fluctuate greatly. When the exhaust pressure fluctuates greatly, the amount of fluctuation of the sensor output signal may increase. Further, by improving the responsiveness of the output signals of the A / F sensors 58a and 58b in order to improve the control accuracy of the cylinder-by-cylinder air-fuel ratio F / B control, the influence of the exhaust pulsation on the sensor output signal becomes significant. There is a possibility that the fluctuation amount of the output signal further increases.

  Here, for the cylinder-by-cylinder F / B control, the air-fuel ratio of each cylinder is measured at the same timing when the exhaust pressure applied to the A / F sensors 58a and 58b becomes the same, thereby eliminating the influence of fluctuations in the sensor output signal. be able to. That is, according to such setting, sensor output fluctuations caused by exhaust pulsation are the same among the sensor output signals measured for each cylinder, so that the A / F sensors 58a and 58b for each cylinder at the above timings. Based on the output signal, the relative air-fuel ratio shift between the cylinders can be grasped with high accuracy.

  On the other hand, since the absolute value of the average air-fuel ratio becomes a problem, it is required to accurately grasp the absolute value of the average air-fuel ratio based on the output signals of the A / F sensors 58a and 58b.

  Therefore, in this embodiment, the output signals of the A / F sensors 58a and 58b input to the ECU 76 are filtered to remove the sensor output fluctuation component caused by exhaust pulsation from the output signals of the A / F sensors 58a and 58b. To do. This suppresses a decrease in the accuracy of grasping the average air-fuel ratio due to fluctuations in the sensor output signal due to exhaust pulsation.

  FIG. 2 shows a procedure for filtering the output signals of the A / F sensors 58a and 58b according to the present embodiment. This process is repeatedly executed by the ECU 76, for example, at a predetermined cycle.

  In this series of processing, first, in step S10, the measured output signals of the A / F sensors 58a and 58b are subjected to low-pass filter (LPF) processing, so that sensor output fluctuation components caused by exhaust pulsation are detected from the sensor output signals. Remove. In the present embodiment, the filtering process is performed using a secondary LPF. The transfer function G (s) of the LPF is represented by the following expression (1), and the amplitude characteristic and the phase characteristic of the transfer function G (s) are represented by the following expressions (2) and (3), respectively. Here, ζ is an attenuation coefficient, ωn is a natural angular frequency, fn is a natural frequency, f is a frequency of an input signal, and there is a relationship of ωn = 2πfn.

本実施形態では、まずＬＰＦのカットオフ周波数ｆｃ（ＬＰＦの入力信号の振幅に対する出力信号の振幅の比が１／√２となる周波数）を、クランク軸３６の１回転（３６０°ＣＡ）に対応する周波数とする。すなわち、エンジン回転速度Ｎｅ（ｒｐｍ）を用いて、カットオフ周波数ｆｃを、「Ｎｅ／６０（Ｈｚ）」に設定する。これにより、カットオフ周波数ｆｃは、エンジン回転速度が高くなるほど高くなる。これは、フィルタ処理されたＡ／Ｆセンサ５８ａ、５８ｂの出力信号（フィルタ出力信号）の応答性を目標空燃比が変化する過渡時等における要求に対して過度に低くならないようにしつつも、フィルタ出力信号の精度（平均空燃比の絶対値を示す精度）を極力高く保つための設定である。ここでは、フィルタ出力信号に、平均空燃比を高精度に表現することが要求されているものの、気筒別の空燃比の変動についてはこれを表現する要求がないことに鑑み、カットオフ周波数ｆｃを、センサへと排気が排出される周期の逆数である上記変動周波数よりも低周波とした。なお、上記の式（１）に示した固有周波数ｆｎ及び減衰係数ζは、カットオフ周波数ｆｃに基づき設定することができる。

In this embodiment, first, the LPF cutoff frequency fc (the frequency at which the ratio of the amplitude of the output signal to the amplitude of the input signal of the LPF is 1 / √2) corresponds to one rotation (360 ° CA) of the crankshaft 36. Frequency. That is, the cut-off frequency fc is set to “Ne / 60 (Hz)” using the engine rotation speed Ne (rpm). Thereby, the cut-off frequency fc increases as the engine speed increases. This is because the responsiveness of the output signals (filter output signals) of the filtered A / F sensors 58a and 58b is not excessively lowered with respect to the request at the time of a transient when the target air-fuel ratio changes, etc. This is a setting for keeping the accuracy of the output signal (accuracy indicating the absolute value of the average air-fuel ratio) as high as possible. Here, the filter output signal is required to express the average air-fuel ratio with high accuracy, but in view of the fact that there is no requirement to express the fluctuation of the air-fuel ratio for each cylinder, the cutoff frequency fc is set. The frequency is lower than the fluctuation frequency, which is the reciprocal of the cycle in which exhaust is discharged to the sensor. The natural frequency fn and the attenuation coefficient ζ shown in the above equation (1) can be set based on the cut-off frequency fc.

  Specifically, in view of the fact that the ECU 76 is a digital arithmetic processing means, in the present embodiment, as the LPF, a digital filter represented by the following expression (4) corresponding to the transfer function represented by the expression (1) ( Fx filter) (x [n]: input data (output signals of A / F sensors 58a and 58b before filtering), y [n]: output data (sensor output signals after filtering), h [k ]: Filter coefficient). Here, since the filter coefficient h [k] is determined by the cutoff frequency fc, the filter coefficient h [k] is variably set according to the engine speed.

図３に、上記手法で算出されたフィルタを用いた場合におけるセンサ出力信号の変動成分の除去の効果を示す。詳しくは、図３（ａ）に、フィルタ処理前におけるＡ／Ｆセンサ５８ａ、５８ｂの出力信号のスペクトルを示し、図３（ｂ）に、フィルタ処理後におけるセンサ出力信号のスペクトルを示す。なお、図３では、エンジン回転速度が２０００ｒｐｍの場合において、カットオフ周波数ｆｃをクランク軸３６の１回転に対応する周波数である３３Ｈｚとして設定したフィルタの効果を示す。図示されるように、フィルタ処理前においては、排気バルブ４４の開閉に起因したＡ／Ｆセンサ５８ａ，５８ｂに加わる排気圧力の変動周波数である５０Ｈｚ及びその高調波の周波数である１００Ｈｚ、１５０Ｈｚの周波数成分が大きくなっている。一方、フィルタ処理後においては、カットオフ周波数ｆｃである３３Ｈｚ以上の周波数成分が除去されている。

FIG. 3 shows the effect of removing the fluctuation component of the sensor output signal when the filter calculated by the above method is used. Specifically, FIG. 3 (a) shows the spectrum of the output signals of the A / F sensors 58a and 58b before the filtering process, and FIG. 3 (b) shows the spectrum of the sensor output signal after the filtering process. FIG. 3 shows the effect of a filter in which the cutoff frequency fc is set to 33 Hz, which is a frequency corresponding to one rotation of the crankshaft 36, when the engine rotation speed is 2000 rpm. As shown in the figure, before filtering, the frequency of 50 Hz which is the fluctuation frequency of the exhaust pressure applied to the A / F sensors 58a and 58b due to the opening and closing of the exhaust valve 44 and the frequency of 100 Hz and 150 Hz which are harmonics thereof. Ingredients are getting bigger. On the other hand, after the filtering process, the frequency component of 33 Hz or higher which is the cut-off frequency fc is removed.

  FIG. 4 shows output signal waveforms of the A / F sensors 58a and 58b before and after the filter processing. As shown in the drawing, fluctuation components of the sensor output signal due to exhaust pulsation are mixed in the output signals of the A / F sensors 58a and 58b before the filter processing. On the other hand, the sensor output signal after the filtering process has the fluctuation component removed.

  Returning to the description of FIG. 2, in the subsequent step S12, the above-described average F / B control is performed based on the filter output signal.

  In the subsequent step S14, cylinder-by-cylinder F / B control is performed. Here, first, the timing at which exhaust is discharged from the cylinder for which the air-fuel ratio is detected by each of the A / F sensors 58a and 58b (opening timing of the exhaust valve 44) is used as a reference timing, and the exhaust is A / F. The output signal (raw value) of the A / F sensor 58a (58b) is measured at the timing when the specified time that is assumed to be required to reach the sensor 58a (58b) has elapsed. Then, in order to feedback control each measurement value to the average value of the three cylinders # 1 to # 3 (# 4 to # 6), a fuel injection amount correction coefficient corresponding to each cylinder is calculated separately. Here, each measured value is not necessarily a value indicating the air-fuel ratio of the corresponding cylinder with high accuracy. However, variations in measured values among the cylinders have a strong correlation with variations in the air-fuel ratio among the cylinders. This is because if the operating state of the engine 10 is the same, the exhaust pressure applied to the A / F sensors 58a, 58b is considered to be equal to each other at the timing when the specified time has elapsed from the reference timing. For this reason, the correction coefficient for controlling each measured value to the average value is an operation amount for suppressing variation in the air-fuel ratio for each cylinder.

  In addition, when the process of step S14 is completed, this series of processes is once complete | finished.

  According to the embodiment described in detail above, the following effects can be obtained.

  (1) The cut-off frequency fc is set as a frequency corresponding to one rotation of the crankshaft 36, and the output signals of the A / F sensors 58a and 58b are filtered using the LPF calculated based on the cut-off frequency fc. . Thereby, the fluctuation component of the sensor output signal due to the exhaust pulsation can be removed from the average air-fuel ratio which is the control amount of the average F / B control, and the influence of the fluctuation of the sensor output signal on the average F / B control. It can suppress suitably.

  (2) The output of the A / F sensor 58a (58b) at the timing when the specified time that is assumed to be required until the exhaust reaches the A / F sensor 58a (58b) from the opening timing of the exhaust valve 44. The signal was measured, and the fuel injection amount was manipulated for each cylinder so as to reduce the variation between these measured values. Thereby, the dispersion | variation in the air-fuel ratio according to each cylinder between each cylinder can be reduced easily, without raising the grasping precision of the air-fuel ratio according to cylinder. Further, in cooperation with the average F / B control, the air-fuel ratio of each cylinder can be controlled to the target air-fuel ratio with high accuracy.

  (3) The exhaust turbine 62b is provided downstream of the A / F sensors 58a and 58b. In this case, the exhaust pressure in the vicinity of the A / F sensors 58a and 58b on the upstream side of the exhaust turbine 62b is likely to fluctuate greatly, and the fluctuation amount of the sensor output signal is likely to increase. For this reason, this embodiment in which the A / F sensors 58a and 58b are provided on the upstream side of the exhaust turbine 62b has a high utility value of the filter processing.

(Second Embodiment)
Hereinafter, the second embodiment will be described with reference to the drawings with a focus on differences from the first embodiment.

  In the present embodiment, after it is determined that the difference between the average air-fuel ratio based on the filter output signal and the target air-fuel ratio is large, the difference between the average air-fuel ratio and the target air-fuel ratio becomes a predetermined value or less. A process of increasing the cut-off frequency fc is performed over a period. This is because the average air-fuel ratio based on the filter output signal cannot react to a change in the actual air-fuel ratio having a frequency equal to or higher than the cutoff frequency fc. In other words, when the difference between the average air-fuel ratio and the target air-fuel ratio becomes large, a response delay occurs in the filter output signal with respect to the change in the actual air-fuel ratio, so that the change in the actual air-fuel ratio is grasped as the change in the average air-fuel ratio. There is a risk that a time difference will occur and the change in the actual air-fuel ratio cannot be quickly grasped. In this case, the controllability of the average air-fuel ratio is reduced, such as overshoot or undershoot due to an increase in the integral term based on the deviation between the average air-fuel ratio and the target air-fuel ratio in feedback control, and consequently the exhaust characteristics. May get worse. Therefore, in this embodiment, the difference between the average air-fuel ratio based on the filter output signal and the target air-fuel ratio becomes large, and the cutoff frequency fc is set in the transition period, which is a period until the average air-fuel ratio converges to the target air-fuel ratio. By increasing it, the response delay of the filter output signal is suppressed as much as possible, and the decrease in controllability of the average air-fuel ratio is suppressed as much as possible.

  FIG. 5 shows a processing procedure for increasing the cut-off frequency fc of the LPF according to the present embodiment. This process is repeatedly executed by the ECU 76, for example, at a predetermined cycle. In FIG. 5, the same steps as those shown in FIG. 2 are given the same step numbers for the sake of convenience.

  In this series of processes, first, in step S16, it is determined whether or not the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is large. In this embodiment, when it is determined that any one of the following controls (a) to (c) is performed, it is determined that the difference is in a large state.

  (A) Acceleration increase control: This control is for setting the target air-fuel ratio to the rich side (λ = 1.0 → 0.9) in order to improve the acceleration response of the engine 10. Therefore, when it is determined that the acceleration increase control is performed, it is determined that the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is large.

  (A) Early catalyst warm-up control: This control is for setting the target air-fuel ratio to the lean side so as to suppress the deterioration of exhaust characteristics immediately after the engine 10 is started. For this reason, when it is determined that the early catalyst warm-up control is performed, it is determined that the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is large. The catalyst early warm-up control means that when the temperature of the first three-way catalyst 70 and the second three-way catalyst 72 is low, the exhaust temperature is raised so that the temperature of these catalysts reaches the activation temperature. Control.

  (C) Stop control of fuel cut: In fuel cut control, the air-fuel ratio F / B control is usually stopped by performing fuel cut start control, and the air-fuel ratio F / B is controlled by performing fuel cut stop control. Start control. Here, by performing the fuel cut start control, the actual air-fuel ratio becomes “intake amount / 0” (very large value). Therefore, the fuel cut stop control is performed thereafter, so that the difference between the target air-fuel ratio λa and the average air-fuel ratio λm at the time when the air-fuel ratio F / B control is started tends to be large. Therefore, when it is determined that the fuel cut stop control is performed, it is determined that the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is large.

  If it is determined in step S16 that the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is large, the process proceeds to step S18, and the cutoff frequency fc is changed to normal filtering (step S10 in FIG. 2). From the cut-off frequency fL set in step (5), a process of setting a frequency fH (> fL) higher than the frequency fL is performed. This process is for suppressing the delay in the response of the filter output signal to the change in the actual air-fuel ratio as much as possible by increasing the cutoff frequency fc of the LPF. If the cut-off frequency fH is too high, the fluctuation components of the output signals of the A / F sensors 58a and 58b due to exhaust pulsation may not be properly removed. For this reason, both the influence of the response delay of the filter output signal on the controllability of the average air-fuel ratio λm and the influence of the fluctuation of the sensor output signal caused by the exhaust pulsation on the controllability can be minimized. It is desirable to determine the cut-off frequency fH. Incidentally, considering that the air-fuel ratio update cycle is the interval between fuel injections in each cylinder, the cut-off frequency fH is the exhaust emission between the cylinders to be detected by the A / F sensors 58a and 58b. It is desirable that the value be equal to or less than the reciprocal of the interval.

  In subsequent step S20, it is determined whether or not the absolute value of the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is equal to or smaller than a predetermined value Δ. This process is for determining whether or not the average air-fuel ratio λm has converged to the target air-fuel ratio λa.

  If it is determined in step S18 that the average air-fuel ratio λm converges, the process proceeds to step S10, and normal filter processing is performed by changing the cut-off frequency fc from fH to fL.

  In addition, when the process of step S10 is completed, this series of processes is once complete | finished.

  As described above, in the present embodiment, the difference between the average air-fuel ratio λm and the target air-fuel ratio λa is predetermined after it is determined that the difference between the average air-fuel ratio λm based on the filter output signal and the target air-fuel ratio λa is large. By increasing the cut-off frequency fc over the period until the value Δ or less, the response delay of the average air-fuel ratio λm can be suppressed as much as possible, and the change in the actual air-fuel ratio is regarded as the change in the average air-fuel ratio λm. It is possible to grasp as quickly as possible. As a result, an increase in integral term in the cylinder-by-cylinder air-fuel ratio F / B control can be suppressed as much as possible, and a decrease in controllability of the average air-fuel ratio λm can be suppressed as much as possible. As a result, the generated torque and exhaust characteristics of the engine 10 can be quickly reduced. Can be as required.

(Third embodiment)
Hereinafter, the third embodiment will be described with reference to the drawings with a focus on differences from the first embodiment.

  In the present embodiment, a process of increasing the secondary LPF attenuation coefficient ζ is performed as the difference between the average air-fuel ratio and the stoichiometric air-fuel ratio increases. This is because the fluctuation amount of the sensor output signal due to the exhaust pulsation increases when the difference between the actual air fuel ratio and the theoretical air fuel ratio is large due to the structure of the A / F sensors 58a and 58b. is there. That is, when acceleration increase control, early catalyst warm-up control, or fuel cut stop control is performed, the difference between the actual air-fuel ratio and the stoichiometric air-fuel ratio increases, and the amount of fluctuation in sensor output signal due to exhaust pulsation increases. To do. For this reason, when the difference between the actual air-fuel ratio and the stoichiometric air-fuel ratio is large, the LPF amplitude characteristic near the fluctuation frequency of the exhaust pressure is reduced by increasing the LPF damping coefficient ζ, and the A / F sensors 58a, 58b. The degree of removal of the fluctuation component of the sensor output signal due to the exhaust pulsation is increased from the output signal.

  FIG. 6 shows a procedure of LPF attenuation coefficient changing processing according to this embodiment. This process is repeatedly executed by the ECU 76, for example, at a predetermined cycle.

  In this series of processing, first, in step S22, a deviation between the average air-fuel ratio and the theoretical air-fuel ratio based on the filter output is detected. This process is for ascertaining whether or not fluctuations in the output signals of the A / F sensors 58a and 58b increase due to exhaust pulsation.

  In the subsequent step S24, processing for changing the damping coefficient ζ is performed in accordance with the difference between the detected average air-fuel ratio and theoretical air-fuel ratio. Specifically, the damping coefficient ζ is increased as the deviation (absolute value) increases. On the other hand, when the deviation is 0 or near 0, the attenuation coefficient ζ is not changed. If the attenuation coefficient ζ is excessively large, the degree of removal of the fluctuation component of the sensor output signal due to the exhaust pulsation increases, but the phase delay of the LPF phase characteristic increases, so that the response delay of the filter output signal is reduced. May grow. For this reason, the influence of the delay in the response of the filter output signal on the controllability of the average air-fuel ratio and the influence of fluctuations in the sensor output signal caused by exhaust pulsation on the controllability are attenuated so as to be minimized. It is desirable to increase the coefficient ζ.

  In addition, when the process of step S24 is completed, this series of processes is once complete | finished.

  As described above, according to the present embodiment, as the difference between the average air-fuel ratio and the stoichiometric air-fuel ratio increases, the LPF damping coefficient ζ is increased, so that the output signals of the A / F sensors 58a and 58b caused by exhaust pulsation are reduced. In a situation where the fluctuation becomes large, the fluctuation component of the sensor output signal due to the exhaust pulsation can be suitably removed from the sensor output signal, and the error of the average air-fuel ratio can be further suitably suppressed.

(Other embodiments)
Each of the above embodiments may be modified as follows.

  The means for feedback control of the air-fuel ratio is not limited to the one having an integral term, and may be one not having this.

  In the first embodiment, the A / F sensors 58a and 58b are filtered using a secondary LPF, but the present invention is not limited to this. For example, filtering may be performed using a higher-order or first-order LPF. Furthermore, a band pass filter may be used. In this case, it is desirable to transmit a frequency that is equal to or greater than the reciprocal of the time corresponding to one combustion cycle and equal to or less than the cut-off frequency fc.

  The method for setting the cut-off frequency fc is not limited to a frequency corresponding to one rotation (360 ° CA) of the crankshaft 36, but the accuracy of grasping the average air-fuel ratio based on the filter output signal, the average air-fuel ratio What is necessary is just to set suitably according to the request | requirement about this controllability. That is, for example, when priority is given to suppressing fluctuations in the output signals of the A / F sensors 58a and 58b caused by exhaust pulsation, the cutoff frequency fc is lower than the frequency corresponding to one rotation of the crankshaft 36. It is good also as a frequency. Here, it is desirable that the cutoff frequency fc be equal to or greater than the reciprocal of the time corresponding to one combustion cycle. For example, when priority is given to the responsiveness of the output signals of the A / F sensors 58a and 58b, the frequency may be higher than the frequency corresponding to one rotation of the crankshaft 36. Here, the cut-off frequency fc is preferably set to a frequency equal to or lower than the frequency corresponding to the valve opening cycle (240 ° CA) of the exhaust valve 44 in the air-fuel ratio detection target cylinder of each of the A / F sensors 58a and 58b. . In particular, if the cutoff frequency fc is set to a frequency corresponding to the valve opening cycle of the exhaust valve 44 of the cylinder for which the air-fuel ratio is to be detected, the air-fuel ratio for each cylinder can be grasped based on the filter output signal.

  Incidentally, in actual filter processing, the intensity of a signal having a frequency lower than this is reduced in the vicinity of the cutoff frequency fc. For this reason, for example, when it is desired to suppress the cutoff frequency fc to a frequency component that is larger than the frequency corresponding to the valve opening cycle of the exhaust valve 44 in the detection target cylinder of each of the A / F sensors 58a and 58b. Alternatively, the cut-off frequency fc may be set slightly higher than this frequency.

  In the second embodiment, when it is determined that acceleration increase control, catalyst early warm-up control, or fuel cut stop control is performed, the difference between the average air-fuel ratio λm and the target air-fuel ratio λa increases. Although it was judged that there was, it is not restricted to this. For example, when the absolute value of the difference between the average air-fuel ratio λm and the target air-fuel ratio λa based on the filter output signal is detected and it is determined that the absolute value is larger than a predetermined threshold, the difference is large. You may judge.

  In the third embodiment, by changing (increasing) the LPF attenuation coefficient ζ according to the difference between the average air-fuel ratio and the stoichiometric air-fuel ratio, the amplitude characteristics of the LPF near the fluctuation frequency of the exhaust pressure are reduced. However, it is not limited to this. For example, the amplitude characteristic may be reduced by lowering the cutoff frequency fc of the LPF.

  The engine system is not limited to the one having the two exhaust passages 56a and 56b illustrated in FIG. 1 of the first embodiment. And the turbocharger 60 may be connected by a single exhaust passage. Even in this case, if the A / F sensor is provided on the upstream side of the turbocharger 60, the exhaust pressure in the vicinity of the A / F sensor is likely to fluctuate greatly, and the fluctuation amount of the sensor output tends to increase. Is effective.

  The gas sensor for detecting the concentration of a specific component in the exhaust gas is not limited to the A / F sensor. For example, a NOx sensor for detecting the NOx concentration in the exhaust gas, an HC sensor for detecting the HC concentration in the exhaust gas, or an O2 sensor. There may be. If there is a possibility that the output of these sensors will fluctuate due to exhaust pulsation, the application of the present invention is effective.

  -The internal combustion engine is not limited to a spark ignition type internal combustion engine such as a gasoline engine. For example, it may be a compression ignition type internal combustion engine such as a diesel engine.

  DESCRIPTION OF SYMBOLS 10 ... Engine, 14 ... Air flow meter, 44 ... Exhaust valve, 46 ... Crank angle sensor, 52 ... Exhaust side cam angle sensor, 56a, 56b ... Exhaust passage, 58a, 58b ... A / F sensor, 60 ... Turbocharger, 62b ... exhaust turbine, 76 ... ECU (one embodiment of signal processing device for gas sensor).

Claims (8)

In a signal processing device for a gas sensor that is provided in an exhaust passage of an internal combustion engine and is applied to a gas sensor that detects a concentration of a specific component in exhaust in the exhaust passage.
Filter means for removing high frequency components of the output signal of the gas sensor,
The filter means comprises a variable means for variably setting a minimum frequency of the high-frequency component to be removed according to an operating state of the internal combustion engine.   2. The signal processing apparatus for a gas sensor according to claim 1, wherein the variable means increases the minimum frequency of the high-frequency component to be removed as the engine speed of the internal combustion engine increases.   The variable means is characterized in that the variably set minimum frequency is a frequency equal to or lower than a frequency corresponding to an exhaust interval of exhaust exhausted from a combustion chamber of the internal combustion engine to the gas sensor. Item 3. A signal processing device for a gas sensor according to Item 2. The gas sensor is an air-fuel ratio sensor,
Air-fuel ratio control means for controlling the air-fuel ratio based on the output signal of the filter means to its target value;
The filter means is in a period from when it is determined that the difference between the actual air-fuel ratio and the target value is large until the difference between the actual air-fuel ratio and the target value becomes a predetermined value or less. 4. The gas sensor signal processing device according to claim 1, wherein the variably set minimum frequency is increased.   The filter means determines that the difference between the actual air-fuel ratio and the target value is large when it is determined that acceleration increase control, catalyst early warm-up control, or fuel cut stop control is performed. 5. The signal processing apparatus for a gas sensor according to claim 4, wherein the signal processing apparatus is one. The gas sensor is an air-fuel ratio sensor,
The filter means increases the degree of removal of the output signal of the gas sensor having a frequency equal to or higher than the lowest frequency as the difference between the air-fuel ratio based on the output signal of the filter means and the stoichiometric air-fuel ratio increases. The signal processing apparatus for a gas sensor according to any one of claims 1 to 5.   The signal processing apparatus for a gas sensor according to claim 1, wherein an exhaust turbine of a supercharger is provided on the downstream side of the gas sensor in the exhaust passage. The internal combustion engine is a multi-cylinder internal combustion engine;
The gas sensor is an air-fuel ratio sensor provided in a collecting portion of an exhaust passage connected to each cylinder of the internal combustion engine,
Operating means for operating a fuel injection valve for injecting and supplying fuel to the internal combustion engine in order to control the air-fuel ratio based on the output signal of the filter means to the target value;
Grasping means for grasping a relative air-fuel ratio shift between the cylinders of the internal combustion engine based on an output signal of the gas sensor before being input to the filter means;
The operation amount correcting means for correcting the operation amount of the fuel injection valve so as to reduce a relative air-fuel ratio shift between the cylinders. The signal processing device for a gas sensor according to item.

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