JP4033228B2 - Oxygen concentration detector - Google Patents

Description

  The present invention relates to an oxygen concentration detection device including an oxygen concentration detection element that outputs a limit current proportional to the oxygen concentration and a heater that heats the detection element. The present invention relates to an oxygen concentration detection device that can diagnose an abnormality of an oxygen sensor.

  In recent air-fuel ratio control systems, a limit current type oxygen sensor (oxygen concentration detection device) that outputs a limit current proportional to the oxygen concentration in exhaust gas is used, and the microcomputer captures the detection result of the sensor. And the air-fuel ratio is calculated and air-fuel ratio feedback control is performed based on the air-fuel ratio. Thereby, optimum combustion in the internal combustion engine is realized, and harmful components (CO, HC, NOX, etc.) in the exhaust gas are reduced.

On the other hand, in the above air-fuel ratio control system, when the reliability of the detection signal from the oxygen sensor is lowered, the control accuracy is remarkably deteriorated. Therefore, a technique for accurately performing abnormality diagnosis of the oxygen sensor has been demanded. . Thus, for example, in the “air-fuel ratio control device for internal combustion engine” of Patent Document 1 as a conventional technique, a temperature sensor for detecting the temperature of an oxygen sensor (oxygen concentration detection element) is provided, and the temperature detected by the sensor does not rise to a predetermined temperature. In this case, it is described that a heater abnormality is detected. Further, in the “oxygen sensor heater control device” of Patent Document 2, in the device for controlling the heater power supply so that the heater resistance value becomes the target resistance value, the target power is supplied when the heater power supply is outside a predetermined range. It is described that an abnormality in the resistance value is detected.
JP-A-1-232143 Japanese Patent Laid-Open No. 3-189350

  However, the above-described prior art causes the following problems. In the former technique (Patent Document 1) described above, a sensor for detecting the sensor temperature is a requirement, which involves a problem of high cost. In the latter technique (Patent Document 2), it is only determined whether or not the target resistance value is properly set, and it is determined that the abnormality diagnosis process is abnormal during battery replacement or sensor replacement. Limited. Therefore, the reliability of the oxygen sensor could not actually be judged.

  The present invention has been made paying attention to the above-mentioned conventional problems, and its purpose is to propose an abnormality diagnosis technique having a novel configuration, thereby making it possible to accurately and easily detect an abnormality of a limiting current oxygen sensor. An object of the present invention is to provide an oxygen concentration detection device capable of diagnosis.

In order to achieve the above object, the invention described in claim 1 includes a limiting current type oxygen sensor having an oxygen concentration detecting element that outputs a limiting current proportional to the oxygen concentration and a heater that heats the detecting element; An element resistance detecting means for detecting the element resistance of the oxygen sensor, and feedback control of the power supplied to the heater so as to eliminate the deviation between the element resistance detected by the element resistance detecting means and the target element resistance of the oxygen sensor The gist of the present invention is to include heater supply power control means and sensor abnormality diagnosis means for diagnosing abnormality of the oxygen sensor based on whether or not the heater supply power by the heater supply power control means exceeds a predetermined abnormality determination value. In the first aspect of the present invention, there is provided an integrated power value calculation means for calculating an integrated value of heater supply power from the beginning of heater energization, and the sensor abnormality diagnosis means is calculated by the integrated power value calculation means. The abnormality of the oxygen sensor is diagnosed based on whether or not the integrated value of the supplied heater supply power exceeds a predetermined abnormality determination value. Subsequently, in the invention according to claim 2, in the invention according to claim 1, heater initial resistance value detection means for detecting an initial resistance value of the heater at the beginning of heater energization, and heater initial resistance value detection Only when the initial resistance value of the heater detected by the means is within a predetermined range for determining the cold state of the oxygen sensor, the diagnosis permission means permits the abnormality diagnosis by the sensor abnormality diagnosis means.

Next, the invention described in claim 3 is a limiting current type oxygen sensor having an oxygen concentration detection element that outputs a limit current proportional to the oxygen concentration and a heater that heats the detection element, and an element resistance of the oxygen sensor. And a heater supply power control means for feedback-controlling the power supplied to the heater so as to eliminate the deviation between the element resistance detected by the element resistance detection means and the target element resistance of the oxygen sensor. And sensor abnormality diagnosis means for diagnosing abnormality of the oxygen sensor based on whether or not the heater supply power by the heater supply power control means exceeds a predetermined abnormality determination value. According to a third aspect of the present invention, a heater initial resistance value detecting means for detecting an initial resistance value of the heater at the beginning of heater energization, and an initial resistance value of the heater detected by the heater initial resistance value detecting means. Is provided with diagnosis permission means for permitting abnormality diagnosis by the sensor abnormality diagnosis means only when the oxygen sensor is in a predetermined range for determining the cold state of the oxygen sensor. Then, according to the invention described in claim 4, the sensor abnormality diagnosis means sets the abnormality determination value according to the engine speed and the engine load . In this case, an optimal abnormality diagnosis according to the engine operating state can be performed.

  According to the first aspect of the present invention, the heater power supply control means feedback-controls the power supplied to the heater so as to eliminate the deviation between the element resistance detected by the element resistance detection means and the target element resistance of the oxygen sensor. To do. The sensor abnormality diagnosing means diagnoses an abnormality of the oxygen sensor based on whether or not the heater supply power by the heater supply power control means exceeds a predetermined abnormality determination value.

When the heater supply power is feedback controlled by the element resistance of the oxygen sensor as described above, the element resistance (element temperature) is controlled to a desired active region even when the sensor is abnormal such as deterioration. In this case, when a sensor abnormality occurs, an excessive amount of heater power supply is required to keep the element resistance (element temperature) in the active region, and from this, the sensor abnormality can be diagnosed accurately and easily. According to the first aspect of the present invention, the integrated power value calculation means calculates the integrated value of the heater supply power from the beginning of heater energization. The sensor abnormality diagnosis unit diagnoses an abnormality of the oxygen sensor based on whether or not the integrated value of the heater supply power calculated by the integrated power value calculation unit exceeds a predetermined abnormality determination value. That is, by performing abnormality diagnosis based on the integrated value of the heater supply power, the accuracy of the diagnosis data is increased, and accurate abnormality diagnosis is possible. According to the second aspect of the present invention, the heater initial resistance value detecting means detects the initial resistance value of the heater at the beginning of heater energization. The diagnosis permission means permits the abnormality diagnosis by the sensor abnormality diagnosis means only when the initial resistance value of the heater detected by the heater initial resistance value detection means is within a predetermined range for determining the cold state of the oxygen sensor. That is, for example, when heater energization is started at the time of warm-up restart of the engine, the integrated value of the heater supply power is relatively small, and it is not desirable in terms of diagnosis accuracy to diagnose sensor abnormality using the integrated value. Therefore, the abnormality diagnosis is limited to when the sensor is cold, so that a good abnormality diagnosis can always be performed.

According to the third aspect of the present invention, the heater supply power control means feedback-controls the supply power to the heater so as to eliminate the deviation between the element resistance detected by the element resistance detection means and the target element resistance of the oxygen sensor. To do. The sensor abnormality diagnosing means diagnoses an abnormality of the oxygen sensor based on whether or not the heater supply power by the heater supply power control means exceeds a predetermined abnormality determination value. When the heater supply power is feedback controlled by the element resistance of the oxygen sensor as described above, the element resistance (element temperature) is controlled to a desired active region even when the sensor is abnormal such as deterioration. In this case, when a sensor abnormality occurs, an excessive amount of heater power supply is required to keep the element resistance (element temperature) in the active region, and from this, the sensor abnormality can be diagnosed accurately and easily. According to the third aspect of the present invention, the heater initial resistance value detecting means detects the initial resistance value of the heater at the beginning of heater energization. The diagnosis permission means permits the abnormality diagnosis by the sensor abnormality diagnosis means only when the initial resistance value of the heater detected by the heater initial resistance value detection means is within a predetermined range for determining the cold state of the oxygen sensor. That is, for example, when heater energization is started at the time of warm-up restart of the engine, the integrated value of the heater supply power is relatively small, and it is not desirable in terms of diagnosis accuracy to diagnose sensor abnormality using the integrated value. Therefore, the abnormality diagnosis is limited to when the sensor is cold, so that a good abnormality diagnosis can always be performed. According to the fourth aspect of the present invention, the sensor abnormality diagnosis means sets the abnormality determination value according to the operating state of the internal combustion engine. In this case, an optimal abnormality diagnosis according to the engine operating state can be performed.

(First embodiment)
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment in which an oxygen concentration detection device according to the present invention is embodied in an air-fuel ratio control device for an automobile internal combustion engine will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram showing an outline of an air-fuel ratio control apparatus for an internal combustion engine in the present embodiment. In FIG. 1, an intake pipe 2 and an exhaust pipe 3 are connected to a 4-cylinder spark ignition gasoline internal combustion engine (hereinafter referred to as an engine) 1. An air cleaner 4 is provided in the uppermost stream portion of the intake pipe 2, and a surge tank 5 is provided in the middle of the intake pipe 2. On the upstream side of the surge tank 5, a throttle valve 17 that is linked to a depression operation of an accelerator pedal (not shown) is disposed. An ISC valve (idle speed control valve) 19 is provided in the bypass passage 18 that bypasses the throttle valve 17.

  In addition, an injector 6 is disposed in an intake pipe (intake port) 2 for each cylinder in the engine 1. The fuel in the fuel tank 7 is sucked up by the fuel pump 8 and supplied to the pressure regulator 10 through the fuel filter 9. The pressure regulator 10 supplies the fuel adjusted to a constant pressure to the injector 6 and returns surplus fuel to the fuel tank 7. The injector 6 is opened by power supply from the battery 15 and injects fuel. The fuel injected by the injector 6 is mixed with the intake air to become an air-fuel mixture, which is supplied to the combustion chamber 12 of each cylinder via the intake valve 11.

  An intake air temperature sensor 20 is provided in the vicinity of the air cleaner 4, and the intake air temperature is detected by the sensor 20. The surge tank 5 is provided with an intake pipe internal pressure sensor 22, which detects the intake pipe internal pressure (intake negative pressure). The cylinder block of the engine 1 is provided with a water temperature sensor 23 for detecting the temperature of engine cooling water.

  A spark plug 13 is disposed in the combustion chamber 12 of each cylinder. In the igniter 14, a high voltage is generated from the voltage of the battery 15, and the high voltage is distributed to each spark plug 13 by the distributor 16. A cylinder discriminating sensor 24 and a crank angle sensor 25 are disposed in the distributor 16. The crank angle sensor 25 generates a crank angle signal at every predetermined crank angle (for example, every 30 ° CA) accompanying the rotation of the crankshaft of the engine 1. The cylinder discrimination sensor 24 generates a cylinder discrimination signal at a specific position (for example, compression TDC of the first cylinder) of the specific cylinder accompanying the rotation of the crankshaft of the engine 1.

  The exhaust pipe 3 of the engine 1 is provided with a limiting current type oxygen sensor 26, and this oxygen sensor 26 outputs a linear detection signal in proportion to the oxygen concentration in the exhaust gas. A catalytic converter (not shown) is disposed downstream of the oxygen sensor 26 so that exhaust gas is purified by the converter.

  Detection signals from the sensors are input to an electronic control unit (hereinafter referred to as ECU) 40. The ECU 40 operates using the battery 15 as a power source, starts the engine 1 by an ON signal of the ignition switch 28, and corrects the air-fuel ratio by increasing / decreasing the air-fuel ratio correction coefficient based on the output signal of the oxygen sensor 26 during engine operation. Is controlled in the vicinity of the target air-fuel ratio (for example, the theoretical air-fuel ratio). Further, the ECU 40 executes sensor abnormality diagnosis processing described later to diagnose the presence or absence of abnormality of the oxygen sensor 26, and when an abnormality occurs, the warning lamp 29 is lit to warn the driver of the occurrence of the abnormality.

  FIG. 2 is a diagram showing a schematic cross section of the oxygen sensor 26 and an electrical configuration of the ECU 40 connected to the oxygen sensor 26. In FIG. 2, the oxygen sensor 26 protrudes toward the inside of the exhaust pipe 3, and the sensor 26 is roughly divided into a cover 31, a sensor body 32, and a heater 33. The cover 31 has a U-shaped cross section, and a plurality of small holes 31a communicating with the inside and outside of the cover are formed on the peripheral wall. The sensor body 32 generates a limiting current corresponding to the oxygen concentration in the air-fuel ratio lean region or the carbon monoxide (CO) concentration in the air-fuel ratio rich region.

The configuration of the sensor body 32 will be described in detail. In the sensor body 32, an exhaust gas side electrode layer 36 is fixed to the outer surface of the solid electrolyte layer 34 formed in a cup shape in cross section, and an atmosphere side electrode layer 37 is fixed to the inner surface. A diffusion resistance layer 35 is formed outside the exhaust gas side electrode layer 36 by plasma spraying or the like. The solid electrolyte layer 34 is made of an oxygen ion conductive oxide that is formed by dissolving CaO, MgO, Y ₂ O ₃ , Yb ₂ O ₃ or the like as a stabilizer in ZrO ₂ , HfO ₂ , ThO ₂ , Bi ₂ O ₃ or the like. The diffusion resistance layer 35 is made of a heat-resistant inorganic substance such as alumina, magnesia, siliceous material, spinel, mullite or the like. Both the exhaust gas side electrode layer 36 and the atmosphere side electrode layer 37 are made of a noble metal with high catalytic activity such as platinum, and the surface thereof is subjected to porous chemical plating or the like. The area and thickness of the exhaust gas side electrode layer 36 are about 10 to 100 mm ² and 0.5 to 2.0 μm, while the area and thickness of the atmosphere side electrode layer 37 are 10 mm ² or more. And about 0.5 to 2.0 μm. The solid electrolyte layer 34 corresponds to an oxygen concentration detection element.

  The heater 33 is accommodated in the atmosphere-side electrode layer 37, and heats the sensor body 32 (the atmosphere-side electrode layer 37, the solid electrode layer 34, the exhaust gas-side electrode layer 36, and the diffusion resistance layer 35) by the generated heat energy. . The heater 33 has a heat generation capacity sufficient to activate the sensor body 32.

  In the oxygen sensor 26 configured as described above, the sensor main body 32 generates a concentration electromotive force at the theoretical air-fuel ratio point, and generates a limit current corresponding to the oxygen concentration in the lean region from the theoretical air-fuel ratio point. In this case, the limit current corresponding to the oxygen concentration is determined by the area of the exhaust gas side electrode layer 36, the thickness of the diffusion resistance layer 35, the porosity, and the average pore diameter. The sensor main body 32 can detect the oxygen concentration with a linear characteristic, but a high temperature of about 650 ° C. or higher is required to activate the sensor main body 32, and Since the active temperature range is narrow, the active region cannot be controlled by heating only with the exhaust gas of the engine 1. Therefore, in this embodiment, the temperature control of the sensor main body 32 is performed by the heating control of the heater 33 described later. In the region richer than the stoichiometric air-fuel ratio, the concentration of carbon monoxide (CO), which is an unburned gas, changes almost linearly with respect to the air-fuel ratio, and the sensor body 32 has a limit current corresponding to the CO concentration. appear.

  The voltage-current characteristics of the sensor body 32 will be described with reference to FIG. As shown in FIG. 3, the current-voltage characteristics are as follows: the inflow current to the solid electrolyte layer 34 of the sensor body 32 that is proportional to the detected oxygen concentration (air-fuel ratio) of the oxygen sensor 26, and the applied voltage to the solid electrolyte layer 34. Indicates that the relationship is linear. When the sensor main body 32 is in the active state at the temperature T = T1, the stable state is shown by the characteristic line L1, as shown by the solid line in FIG. In such a case, the straight line portion parallel to the voltage axis V of the characteristic line L1 specifies the limit current of the sensor body 32. The increase / decrease of the limit current corresponds to the increase / decrease of the air / fuel ratio (that is, lean / rich). The limit current increases as the air / fuel ratio becomes leaner, and the limit current decreases as the air / fuel ratio becomes richer.

  In this voltage-current characteristic, a voltage range smaller than the straight line portion parallel to the voltage axis V is a resistance dominant region, and the slope of the characteristic line L1 in the resistance dominant region is the solid electrolyte layer 34 in the sensor body 32. Specified internal resistance (hereinafter referred to as element resistance). Since the element resistance changes with a change in temperature, when the temperature of the sensor main body 32 decreases, the inclination decreases due to the increase in element resistance. That is, when the temperature T of the sensor main body 32 is at T2 lower than T1, the current-voltage characteristic is specified by the characteristic line L2 as shown by the broken line in FIG. In this case, the straight line portion parallel to the voltage axis V of the characteristic line L2 specifies the limit current of the sensor body 32 at T = T2, and this limit current substantially coincides with the limit current based on the characteristic line L1.

  If a positive applied voltage Vpos is applied to the solid electrolyte layer 34 of the sensor body 32 in the characteristic line L1, the current flowing through the sensor body 32 becomes the limit current Ipos (see point Pa in FIG. 3). Further, if a negative applied voltage Vneg is applied to the solid electrolyte layer 34 of the sensor body 32, the current flowing through the sensor body 32 becomes a negative temperature current Ineg proportional to only the temperature without depending on the oxygen concentration (FIG. 3). (See point Pb).

  In FIG. 2, a bias control circuit 41 is connected to the exhaust gas side electrode layer 36 of the sensor body 32, and the bias control circuit 41 is connected to the atmosphere side electrode layer of the sensor body 32 via a sensor current detection circuit 45. 37 is connected. The bias control circuit 41 includes a positive bias DC power source 42, a negative bias DC power source 43, and a changeover switch circuit 44. Both the negative electrode of the positive bias DC power source 42 and the positive electrode of the negative bias DC power source 43 are connected to the exhaust gas side electrode layer 36.

  The changeover switch circuit 44 connects only the positive side electrode of the positive bias DC power source 42 to the sensor current detection circuit 45 in the first switching state and the negative side electrode of the negative bias DC power source 43 in the second switching state. Are connected to the sensor current detection circuit 45. That is, when the changeover switch circuit 44 is in the first switching state, the positive bias DC power supply 42 positively biases the solid electrolyte layer 34 of the sensor body 32, and a positive current flows through the solid electrolyte layer 34. On the other hand, when the changeover switch circuit 44 is in the second switching state, the negative bias DC power supply 43 negatively biases the solid electrolyte layer 34, and a negative current flows through the solid electrolyte layer 34. In such a case, the terminal voltages of the DC power sources 42 and 43 correspond to the above-described applied voltages Vpos and Vneg, respectively.

  The sensor current detection circuit 45 detects a current flowing from the atmosphere-side electrode layer 37 of the sensor body 32 to the changeover switch circuit 44 or a current flowing in the opposite direction, that is, a current flowing through the solid electrolyte layer 34. Further, the heater control circuit 46 performs duty control on the power supplied from the battery power source VB to the heater 33 in accordance with the element temperature of the oxygen sensor 26 and the heater temperature, and controls the heating of the heater 33. A current flowing through the heater 33 (hereinafter referred to as heater current Ih) is detected by the current detection resistor 50.

  The A / D converter 47 converts the current detected by the sensor current detection circuit 45 (Ipos, Ineg in FIG. 3), the heater current Ih, and the voltage applied to the heater 33 (hereinafter referred to as heater voltage Vh) into digital signals. The data is output to the microcomputer 48. The microcomputer 48 includes a CPU 48a that executes various arithmetic processes, a memory 48b that includes a ROM and a RAM, and the like, and a bias control circuit 41, a heater control circuit 46, and a fuel injection control device (hereinafter referred to as EFI) according to a predetermined computer program. 49 is controlled. The EFI 49 receives the various sensor signals as engine information and detects the intake air temperature Tam, the intake negative pressure Pm, the cooling water temperature Thw, the engine speed NE, the vehicle speed Vs, and the like. Then, fuel injection by the injector 6 is controlled based on these engine information. In this embodiment, the CPU 48a in the microcomputer 48 constitutes heater control means, element resistance detection means, sensor abnormality diagnosis means, and heater supply power estimation means.

  Next, the operation of this embodiment will be described along various control programs executed by the CPU 48a in the microcomputer 48. FIG. Hereinafter, heater energization control, air-fuel ratio detection processing, and sensor abnormality diagnosis processing will be described in this order.

  The flowchart of FIG. 4 shows a heater energization control routine that is started in accordance with the ON operation of the ignition switch 28 and is executed by the CPU 48a at a predetermined time period. In FIG. 4, the CPU 48 a determines heater control flags F <b> 1 and F <b> 2 indicating the control state of the heater 33 in step 101. That is, in the first embodiment, according to the ON operation of the ignition switch 28, the heater control is shifted in the order of 100% duty control → first heater energization control → second heater energization control. F1 = 1 indicates that the first heater energization control is being executed, and the heater control flag F2 = 1 indicates that the second heater energization control is being executed.

  In this case, at the beginning of the heater energization control, the heater control flags F1 and F2 are both cleared to “0” (initial value), and the CPU 48a proceeds to step 102 and performs the duty 100% control. That is, the heater control circuit 46 of FIG. 2 is controlled with 100% duty to fix the power supplied to the heater 33 to the maximum value, and the heater 33 is rapidly heated. Further, the CPU 48a reads the heater resistance RH calculated at step 103 based on the heater voltage Vh and the heater current Ih (RH = Vh / Ih), and at the next step 104, whether the heater resistance RH is 2Ω or more (RH ≧≧). 2Ω). If RH <2Ω, the CPU 48a ends this routine as it is. In this case, 100% duty control is continued.

  When the heater resistance RH ≧ 2Ω is satisfied in step 104, the CPU 48a proceeds to step 105, sets “1” to the heater control flag F1, and executes the first heater energization control in subsequent step 106. Here, in the first heater energization control, the control duty of the heater 33 is obtained from a first map including the engine load (for example, intake negative pressure Pm) and the engine speed NE. Here, the map is set so that the element temperature of the oxygen sensor 26 becomes a predetermined activation temperature. For example, a large duty is set because the amount of heat of exhaust gas is small in a low load or low rotation range. It has become so. After the flag F1 is set, the CPU 48a directly proceeds from step 101 to step 106, and performs the first heater energization control.

  Then, when the processing proceeds from step 106 to step 107, the CPU 48a reads the element resistance (internal resistance of the solid electrolyte layer 34) Zdc of the oxygen sensor 26. The element resistance Zdc can be calculated from the element application voltage Vneg (negative application voltage) and the negative current Ineg detected by the sensor current detection circuit 45 (Zdc = Vneg / Ineg). In step 108, the CPU 48a determines whether or not the element resistance Zdc has become 90Ω or less (whether Zdc ≦ 90Ω). If Zdc> 90Ω, the CPU 48a directly ends this routine. In this case, the first heater energization control is continued. FIG. 5 shows the relationship between element temperature and element resistance Zdc.

  When Zdc ≦ 90Ω is satisfied in step 108, the CPU 48a proceeds to step 109 to operate the heater control flag F1 to “0” and the heater control flag F2 to “1”. In the subsequent step 110, the second heater energization is performed. Implement control. Here, in the second heater energization control, a second map different from the first map is used, and the control duty of the heater 33 is determined according to the engine load (for example, intake negative pressure Pm) and the engine speed NE. Is calculated (however, the characteristics are almost the same). Then, after setting the flag F2 = 1, the CPU 48a directly proceeds from step 101 to step 110, and performs the second heater energization control. As described above, in this embodiment, the energization of the heater 33 is open-controlled by the 100% duty at the beginning of the control and the subsequent first and second heater energization controls.

  On the other hand, the flowchart of FIG. 6 shows an air-fuel ratio detection routine that is started in accordance with the ON operation of the ignition switch 28 and executed by the CPU 48a every 8 msec, for example.

  In FIG. 6, the CPU 48a first executes sensor activity determination processing in steps 201-204. That is, the CPU 48a applies a predetermined voltage Vm in the element resistance detection region of FIG. 7 in step 201 (for example, Vm = −1 volt), and in step 202, the current Im detected by the sensor current detection circuit 45 of FIG. (See FIG. 7). In step 203, the CPU 48a calculates the element resistance Zdc from the applied voltage Vm and the detection current Im (Zdc = Vm / Im).

  Further, in step 204, the CPU 48a determines whether the oxygen sensor 26 is active based on whether or not the element resistance Zdc is within a predetermined activity determination range (KREL to KREH) (for example, KREL = 10Ω, KREH = 90Ω). That is, if KREL ≦ Zdc ≦ KREH and step 204 is affirmatively determined, the CPU 48a considers that sensor activation has been completed, and proceeds to step 205. On the other hand, if a negative determination is made in step 204, the CPU 48a repeatedly executes steps 201 to 204 until an activation determination is made.

  In step 205, the CPU 48a applies “0.4 volts” to the oxygen sensor 26 as an initial value of the applied voltage Vp in the A / F detection region of FIG. Further, the CPU 48a reads the limit current Ip (n) detected by the sensor current detection circuit 45 of FIG. 2 in step 206, and in the subsequent step 207, converts the limit current Ip (n) into an air-fuel ratio (A / F). To do. In step 208, the CPU 48a calculates an applied voltage Vp (n + 1) for detecting the next air-fuel ratio, and {Vp (n + 1) = f (Ip)}, and applies the applied voltage Vp (n + 1) to the oxygen sensor 26. Apply. That is, in FIG. 7, when the air-fuel ratio (A / F) at (n) is “16” and the air-fuel ratio (A / F) at (n + 1) is “15”, Ip (n) is applied by applying Vp (n). n) is detected, and Ip (n + 1) is detected by applying Vp (n + 1).

  Thereafter, the CPU 48a determines whether or not a predetermined time has elapsed since the start of air-fuel ratio detection in step 209. In this case, if the predetermined time has not elapsed, the CPU 48a repeatedly executes the above-described steps 206 to 209, and proceeds to step 210 if the predetermined time has elapsed. The CPU 48a performs the sensor activity determination process in steps 210 to 213 similar to the steps 201 to 204 described above.

  That is, the CPU 48a determines whether or not the element resistance Zdc calculated in Steps 210 to 212 is within a predetermined activation determination range (KREL to KREH). If KREL ≦ Zdc <KREH, the CPU 48a considers that the sensor is activated, and proceeds to step 206 to execute the above-described air-fuel ratio detection process. On the other hand, if the determination in step 213 is negative, the CPU 48a repeatedly executes steps 210 to 213.

  Next, a sensor abnormality diagnosis routine will be described with reference to FIG. The flowchart of FIG. 8 is executed by the CPU 48a every predetermined time (for example, every 32 msec). In FIG. 8, the CPU 48a first determines preconditions for sensor abnormality diagnosis in steps 301-307. Specifically, in step 301, it is determined whether or not the intake air temperature Tam is equal to or higher than a predetermined determination value KTA (for example, 5 ° C.). In step 302, the cooling water temperature Thw is equal to or higher than a predetermined determination value KTW (for example, 5 ° C.). It is determined whether or not. In step 303, it is determined whether or not the engine speed NE is equal to or higher than a predetermined determination value KNE (for example, 500 rpm). In step 304, the vehicle speed Vs is less than the predetermined determination value KSPD (for example, 100 km / h). It is determined whether or not there is. Further, in step 305, it is determined whether or not the elapsed time CAST after engine start is equal to or longer than a predetermined determination value KCAST (for example, 20 seconds). In step 306, the battery voltage VB is determined to be a predetermined determination value KVB (for example, 13V). ) It is determined whether or not it is above. In step 307, it is determined whether or not the fuel cut flag XFC indicating that the fuel cut is being executed is cleared to "0", that is, whether or not the fuel cut is not performed.

  In the present embodiment, of the above preconditions, the integrated value of the heater supply power is estimated from the elapsed time CAST after the engine start and the battery voltage VB, and when these values are equal to or greater than the predetermined value, the integrated value of the heater supply power is estimated. It is determined that the value has reached a predetermined value or more. That is, if these conditions are satisfied, it is considered that the sensor is in an activated state or a state that should naturally be activated, and abnormality diagnosis is permitted. By this condition setting, abnormality diagnosis can be performed more accurately.

  If any of the above steps 301 to 307 is negatively determined, the CPU 48a ends this routine as it is, and if all of steps 301 to 307 are positively determined, the process proceeds to the subsequent step 308 and the element resistance Zdc of the oxygen sensor 26 is set. Based on the sensor abnormality diagnosis. Here, the element resistance Zdc of the oxygen sensor 26 is calculated as in steps 201 to 203 of FIG.

  In step 308, the CPU 48a determines whether or not the element resistance Zdc is smaller than the first resistance determination value KREL (10Ω in this embodiment). If Zdc <KREL, the CPU 48a proceeds to step 309. If the element resistance Zdc is smaller than the first resistance determination value KREL, it means that the element temperature has increased too much, and the CPU 48a determines in step 309 that the oxygen sensor 26 has an element high temperature abnormality. This element high temperature abnormality includes the following modes. That is, it includes a mode in which the heater resistance of the oxygen sensor 26 is smaller and the current flows too much, and a mode in which the wire harness on the ground side of the heater 33 is always short-circuited and current control cannot be performed.

  If Zdc ≧ KREL, the CPU 48a determines in step 310 whether or not the element resistance Zdc is greater than or equal to a second resistance determination value KREH (90Ω in the present embodiment). If Zdc ≧ KREH, Proceed to 311. If the element resistance Zdc is equal to or higher than the second resistance determination value KREH, it means that the element temperature remains low without increasing, and the CPU 48a determines in step 311 that the oxygen sensor 26 is abnormal in element low temperature. To do. The element low temperature abnormality includes the following modes. In other words, the heater resistance of the oxygen sensor 26 varies in a larger direction, the current decreases, the heater 33 deteriorates and the resistance increases, the current decreases, and the current due to the disconnection of the wire harness of the heater 33. Also includes a mode that stops running.

  When it is determined that the oxygen sensor 26 is abnormal as described above, a fail safe routine shown in FIG. 9 is executed (for example, a cycle of 32 msec). In FIG. 9, the CPU 48a first determines in step 401 whether or not the element is abnormal. Here, if an element abnormality (high temperature abnormality or low temperature abnormality) is determined according to FIG. 8, the CPU 48a proceeds to step 402 and stops air-fuel ratio feedback. Further, the CPU 48a stops energization of the heater in step 403 and turns on the warning lamp 29 indicating element abnormality in the subsequent step 404. At this time, the high temperature abnormality and the low temperature abnormality may be displayed separately.

  In the first embodiment described above, abnormality of the sensor 26 is diagnosed depending on whether or not the element resistance of the oxygen sensor 26 is within a predetermined range (steps 308 to 311 in FIG. 8). In other words, in the limiting current type oxygen sensor 26, the output characteristic is specified by the slope in the resistance dominant region (in FIG. 3, the slope of the voltage region smaller than the straight line portion parallel to the voltage axis V), that is, the element resistance. Is done. In this case, if the oxygen sensor 26 is abnormal, the element resistance is too small or too large, and the abnormality can be diagnosed accurately and easily.

  Further, in this embodiment, the element low temperature abnormality or the element high temperature abnormality of the oxygen sensor 26 is determined depending on whether the element resistance is larger or smaller than the allowable range. In other words, the element resistance of the oxygen sensor 26 being excessive means that the element temperature is too low, and an element low temperature abnormality is determined. Further, if the element resistance of the oxygen sensor 26 is too low, it means that the element temperature is too high, and an element high temperature abnormality is determined.

  Further, in the present embodiment, the temperature sensor is not required as in the prior art, so that the cost is not increased. Further, in the conventional technology, the abnormality is diagnosed mainly when the battery or the sensor is replaced. However, in this embodiment, the abnormality can always be diagnosed during normal driving of the vehicle, and the reliability of the sensor output can be determined. As a result, a highly accurate air-fuel ratio control system can be realized.

  The heater energization control method is not limited to the method of this embodiment. That is, in the above-described embodiment, the 100% duty control and the first and second heater energization controls are sequentially performed. However, for example, only the first or second heater energization control is always performed, or a predetermined after the engine is started. It may be configured such that 100% duty control is performed only for the time, and thereafter the first or second heater energization control is always performed.

(Second embodiment)
Next, the second embodiment will be described focusing on differences from the first embodiment. In this embodiment, the CPU 48a in the microcomputer 48 constitutes a heater control means, a fuel increase / decrease means, and a sensor abnormality diagnosis means. FIG. 10 shows a sensor abnormality diagnosis routine in the second embodiment.

  In FIG. 10, the CPU 48 a determines in step 501 whether a precondition for sensor abnormality diagnosis is satisfied. This precondition corresponds to steps 301 to 307 in FIG. In step 502, the CPU 48a determines whether air-fuel ratio feedback is being performed. In this case, if any of steps 501 and 502 is negatively determined, the CPU 48a ends the routine, and if both of steps 501 and 502 are positively determined, the process proceeds to step 503.

  In step 503, the CPU 48a stores the limit current Ip detected by the sensor current detection circuit 45 of FIG. 2 at that time as “Ipo”. Further, in the subsequent step 504, the CPU 48a stores the current operating conditions (intake negative pressure Pm, engine speed NE) as “Pmo” and “NEo”.

  Thereafter, in step 505, the CPU 48a increases the fuel injection amount by the injector 6 by α% (for example, 10%) (decrease may be possible), and in a subsequent step 506, determines whether or not a predetermined time has elapsed after the fuel increase. Here, the fuel increase means forcibly shifting the air-fuel ratio to the rich side. Then, when a predetermined time elapses after the fuel increase, the CPU 48a proceeds to step 507. In step 507, the CPU 48a determines whether or not the intake negative pressure Pm and the engine speed NE at that time substantially match the values “Pmo” and “NEo” (stored values in step 504) before fuel increase. . In this case, if the operating condition has changed, the CPU 48a ends this routine as it is without performing the subsequent abnormality diagnosis. On the other hand, if the operating condition has not changed, the CPU 48a proceeds to step 508 to perform subsequent abnormality diagnosis.

  In step 508, the CPU 48a reads the limit current Ip detected by the sensor current detection circuit 45 at that time, and calculates the current change amount ΔIp from the current value change before and after the fuel increase in step 509 (ΔIp = Ip− Ipo). Thereafter, the CPU 48a determines whether or not the current change amount ΔIp (absolute value) exceeds the first current determination value KDIL in step 510 (whether ΔIp> KDIL), and in step 511, the current change value ΔIp is determined. It is determined whether or not it is equal to or smaller than the second current determination value KDIH (whether ΔIp ≦ KDIH) (where KDIL <KDIH). Here, the allowable change range defined by “KDIL to KDIH” is set to an amount corresponding to the actual change in the air-fuel ratio due to the fuel increase.

  In this case, if the current change value ΔIp is within the range of “KDIL to KDIH”, the CPU 48a makes an affirmative decision in both steps 510 and 511. If ΔIp ≦ KDIL, the CPU 48a makes a negative determination in step 510, and determines in step 512 that the element has a low temperature abnormality. If ΔIp> KDIH, the CPU 48a makes a negative determination in step 511, and determines in step 513 that the element is hot.

  FIGS. 11A to 11C are diagrams showing output signals of the oxygen sensor 26 at the normal time, when the element low temperature is abnormal, and when the element high temperature is abnormal. In the figure, current change amounts ΔIp1, ΔIp2, and ΔIp3 indicate differences in limit current when the voltage applied to the oxygen sensor 26 changes from “Vp1” to “Vp2”. That is, the element resistance is large at the time of the element low temperature abnormality shown in FIG. 11B, and the slope of the characteristic line in the resistance dominant region is small. Therefore, “ΔIp2” is smaller than “ΔIp1” in the normal state (ΔIp2 <ΔIp1). In such a case, a negative determination is made in step 510 of FIG. 10, and it is determined that the element has a low temperature abnormality. In addition, when the temperature of the device is abnormal as shown in FIG. Therefore, “ΔIp3” becomes larger than “ΔIp1” in the normal state (ΔIp3> ΔIp1). In such a case, a negative determination is made in step 511 in FIG. 10, and it is determined that the element has a high temperature abnormality.

  As described above, according to the second embodiment, the amount of fuel supplied to the engine 1 is increased (step 505 in FIG. 10), and the output value (limit current) of the oxygen sensor 26 changes within a predetermined range when the amount of fuel increases. The abnormality of the sensor 26 is diagnosed depending on whether or not (steps 510 to 513 in FIG. 10). As a result, it is determined whether or not the enrichment of the air-fuel ratio due to the fuel increase (decrease in oxygen concentration) is normally reflected in the sensor output, and sensor abnormality can be diagnosed accurately and easily. In this embodiment, since a range is provided for the abnormality determination value at the time of abnormality diagnosis, it is possible to discriminate between element low temperature abnormality and element high temperature abnormality.

(Third embodiment)
Next, a third embodiment will be described. In the first and second embodiments, the heater 33 is open-controlled, but in the third embodiment, the heater 33 of the oxygen sensor 26 is subjected to element temperature feedback control. In this embodiment, the CPU 48a of the microcomputer 48 constitutes element resistance detection means, heater supply power control means, and sensor abnormality diagnosis means.

  FIG. 12 is a time chart of heater control in the present embodiment, and shows the operation until the oxygen sensor 26 is fully activated after the start of energization of the heater 33 when the engine is started. In the present embodiment, the heater control is divided into the control of I to IV in FIG. 12 due to the difference in its purpose and control method, and each will be described in turn. Controls I to III indicate heater control before activation of the oxygen sensor 26, and control IV indicates heater control after activation of the oxygen sensor 26.

  First, in the control of I immediately after the engine is started, a heater voltage having a duty of 100% is applied to the heater 33 (hereinafter referred to as full conduction control). That is, the maximum power is supplied to the heater 33 in order to heat the heater 33 in a short time when the heater 33 and the sensor element (sensor body 32) are cold.

  In the control of II and III, the power supplied to the heater 33 is controlled so as to keep the heater temperature at a predetermined target heater temperature (for example, 1200 ° C. which is the heater upper limit temperature) (hereinafter referred to as power control). ). In other words, if the element temperature is at the activation temperature (700 ° C.), the heater temperature is uniquely determined from the supplied power. Therefore, it is sufficient to keep supplying the predetermined power to keep the heater 33 at a constant temperature. However, when the element temperature is low, the supplied power required to keep the heater temperature constant varies depending on the element temperature. In general, the lower the element temperature, the greater the required power supply. Therefore, in the power control, the supplied power is controlled according to the element resistance (the element temperature and the element resistance are in the relationship shown in FIG. 5).

  However, at the beginning of power control, the element resistance is very large and exceeds the maximum value (eg, 600Ω) that can be detected. Therefore, the power supplied to the heater 33 is kept at a constant power (for example, 60 W) in the region where the element resistance cannot be detected (control of II). When the element temperature rises and the element resistance becomes 600Ω or less, the electric power corresponding to the element resistance is supplied to the heater 33 (control of III).

  In IV control, the power supplied to the heater 33 is feedback-controlled so as to maintain an element resistance of 30Ω (equivalent to an element temperature of 700 ° C.) in order to maintain the active state of the element (hereinafter referred to as element temperature feedback control). Call).

  The heater control routine will be described with reference to FIG. In FIG. 13, the CPU 48 a determines in step 601 whether or not the element temperature feedback control execution condition is satisfied. This execution condition is satisfied if the element resistance of the oxygen sensor 26 is 30Ω or less. In addition, the CPU 48a determines whether or not a power control execution condition is satisfied in step 602. As the power control execution conditions, two conditions are set depending on whether or not the oxygen sensor 26 (the sensor main body 32 and the heater 33) is in a cold state, and the oxygen sensor 26 is in a cold state. For example, the condition is satisfied when a predetermined time has elapsed since the start of full conduction control (control of I in FIG. 12), and is satisfied when the heater resistance value is equal to or higher than the target heater resistance value unless it is in a cold state. In this way, by executing the full conduction control according to the cold state of the oxygen sensor 26, the heater temperature is prevented from excessively rising when the engine is restarted.

  Therefore, if both the steps 601 and 602 are negatively determined at the beginning of the heater control, the CPU 48a proceeds to step 603 and executes the full conduction control of the heater 33 (control of I). That is, a 100% duty heater voltage is applied to the heater 33.

  When the power control execution condition in step 602 is satisfied, the CPU 48a proceeds to step 604 and executes power control (controls II and III). In this case, as described above, if the element resistance is in a non-detectable range (element resistance> 600Ω), the supplied power is controlled at a fixed value (control of II). When the element resistance becomes detectable, the heater temperature is set to the target heater temperature. The power supplied to the heater 33 is controlled according to the element resistance (control of III).

  Thereafter, when the element temperature feedback control execution condition in step 601 is satisfied, the CPU 48a executes element temperature feedback control in step 605 (IV control). At this time, the CPU 48a calculates the control duty DUTY of the heater voltage based on the following equations (1) to (3).

DUTY = DUTY. I + GP + GI (1)
GP = KP · (Zdc−ZdcT) (2)
GI = GI + KI · (Zdc−ZdcT) (3)
However, “DUTY.I” is an initial value of the control duty DUTY, and “ZdcT” is a control target value (DUTY.I = 20%, ZdcT = 30Ω in this embodiment). “GP” represents a proportional term, “GI” represents an integral term, “KP” represents a proportional constant, and “KI” represents an integral constant (in this embodiment, KP = 4.2%, KI = 0.2%). ). These numerical values are obtained experimentally and are changed according to the specifications of the oxygen sensor 26.

  FIG. 14 shows a processing data calculation routine executed by the CPU 48a every 128 ms, for example. In FIG. 14, the CPU 48 a reads the heater current Ih detected by the current detection resistor 50 of FIG. 2 in step 701, and then reads the heater voltage Vh in step 702. In step 703, the CPU 48a divides the heater voltage Vh by the heater current Ih to calculate the heater resistance RH (RH = Vh / Ih). In the next step 704, the CPU 48a multiplies the heater voltage Vh and the heater current Ih. Supply power WH is calculated (WH = Vh · Ih). Further, in step 705, the CPU 48a calculates a smoothed value of the heater supply power WH (hereinafter referred to as an average power value WHAV) by 1/64 smoothing calculation {WHAV = (63 · WHAVI-1 + WH) / 64}.

  The flowchart of FIG. 15 shows a sensor abnormality diagnosis routine executed by the CPU 48a, for example, at a cycle of 1 second. In this abnormality diagnosis routine, a sensor abnormality is determined in accordance with the heater supply power WH required when the element temperature feedback control is executed. That is, when the sensor is abnormal (for example, deterioration of the sensor over time), the heater supply power WH necessary for maintaining the element temperature at the target value (for example, 700 ° C.) increases, and it is easily compared with that in the normal state. can do. Hereinafter, the abnormality diagnosis procedure will be described with reference to FIG.

  In FIG. 15, the CPU 48a determines whether or not a predetermined time KSTFB (for example, 10 seconds) has elapsed after the element temperature feedback is started in step 801, and in a subsequent step 802, the predetermined time KAFST (for example, 100) is determined after the previous abnormality diagnosis. Second) is determined. Further, in step 803, the CPU 48a determines whether or not the steady operation state (for example, the idle state) has continued for a predetermined time KSMST (for example, 5 seconds) or more. If any of steps 801 to 803 is negatively determined, the CPU 48a ends the process as it is, and if all of steps 801 to 803 are positively determined, the process proceeds to step 804.

  In step 804, the CPU 48a determines whether or not the power average value WHAV is equal to or greater than a predetermined heater power determination value KWHAV (whether WHAV ≧ KWHAV). At this time, if WHAV <KWHAV, the CPU 48a regards that there is no sensor abnormality, proceeds to step 805, clears the abnormality determination flag XELER to “0”, and ends the processing.

  If WHAV ≧ KWHAV, the CPU 48a proceeds to step 806, determines whether or not an abnormality other than the sensor is detected, and proceeds to step 807 if there is no other abnormality. Then, the CPU 48a determines whether or not the abnormality determination flag XELER has already been set to "1" in step 807. If XELER = 0, the CPU 48a sets "1" to the abnormality determination flag XELER in step 808. If XELER = 1, the CPU 48a proceeds to step 809 and turns on the warning lamp 29 indicating the occurrence of abnormality as a diagnosis process. That is, in steps 804 to 809, when it is determined that an abnormality has occurred (WHAV ≧ KWHAV) twice in succession, a diagnosis process is performed.

  As described above, in the third embodiment, the power supplied to the heater 33 is feedback-controlled so that the element resistance (element temperature) of the oxygen sensor 26 becomes the target element resistance of 30Ω (equivalent to an element temperature of 700 ° C.) (element temperature feedback control of FIG. 13). In this case, the abnormality of the oxygen sensor 26 is diagnosed based on whether or not the heater supply power exceeds a predetermined abnormality determination value (steps 804 to 809 in FIG. 15). In short, when the element temperature feedback control is executed, the element resistance (element temperature) is controlled to a desired active region even when the sensor is abnormal such as deterioration, and an excessive amount of heater supply power is required when the sensor is abnormal. . From this, sensor abnormality can be diagnosed accurately and easily. In this embodiment, since the abnormality diagnosis is limited to the steady operation (step 803 in FIG. 15), the influence of the exhaust gas temperature on the heater supply power can be eliminated, and an appropriate diagnosis result can be obtained.

(Fourth embodiment)
Next, a fourth embodiment will be described. In the fourth embodiment, a part of the third embodiment is changed to execute abnormality diagnosis processing, and FIG. 16 shows a sensor abnormality diagnosis routine in the fourth embodiment.

  In the routine of FIG. 16, the process of step 820 is executed instead of step 803 of FIG. That is, in step 820, the heater power determination value KWHAV corresponding to the engine operating state is set. Here, the heater power determination value KWHAV is set on the map shown in FIG. 17, and is set according to the engine speed NE and the engine load (intake negative pressure Pm or intake air amount GN) at that time ( For example, KWHAV1, KWHAV2 in the figure). In this case, the heater power determination value KWHAV is set to be lower as the speed is higher or higher, and higher as the speed is lower or lower. As described above, according to the fourth embodiment, an optimal abnormality diagnosis according to the engine operating state can be performed.

(5th Example)
Next, a fifth embodiment will be described. In this embodiment, the CPU 48a of the microcomputer 48 constitutes an integrated power value calculation means, a heater initial resistance value detection means, and a sensor abnormality diagnosis means.

  FIG. 18 is a time chart of heater control in the fifth embodiment. FIG. 18 shows an operation until the oxygen sensor 26 is sufficiently activated after the start of energization of the heater 33 when the engine is started. In this embodiment, the heater control is divided into the controls I to III in FIG. 18 due to the difference in purpose and control method (I is full conduction control, II is power control, and III is element temperature feedback control). Each will be described in turn.

  First, in full continuity control (I control) immediately after the engine is started, a heater voltage with 100% duty is applied to the heater 33. That is, the maximum power is supplied to the heater 33 in order to heat the heater 33 in a short time when the heater 33 and the sensor element are cold. In the power control (II control), the power supplied to the heater 33 is controlled according to the element resistance Zdc so that the heater temperature is maintained at a predetermined target heater temperature (for example, 1200 ° C. which is the heater upper limit temperature). In the element temperature feedback control (III control), the power supplied to the heater 33 is feedback controlled so that the element resistance is 30Ω (equivalent to an element temperature of 700 ° C.) in order to maintain the active state of the element. In the element temperature feedback control, when the power supplied to the heater 26 exceeds the upper limit value, the power supplied to the heater 33 is limited.

  19 and 20 show a heater control routine executed by the CPU 48a every 128 ms, for example. Hereinafter, heater control and abnormality diagnosis processing will be described according to the flowchart.

  In FIG. 19, the CPU 48a determines whether or not the ignition switch 28 is turned on in step 901 (whether or not the power is turned on). Proceed to 902. In step 902, the CPU 48a determines whether or not the initial completion flag XINIT is “0” (this initial completion flag XINIT is initialized to “0” when the power is turned on), and XINIT = 0. If so, the process proceeds to step 903, and if XINIT = 1, the process proceeds to step 908.

  In step 903, the CPU 48a stores the heater resistance RH (= Vh / Ih) obtained from the heater current Ih and the heater voltage Vh as the initial heater resistance RHINT. In step 904, the CPU 48a sets the target power integrated value WADTG based on the initial heater resistance RHINT in accordance with the relationship shown in FIG. Further, in step 905, the CPU 48a determines whether or not the initial heater resistance RHINT is equal to or less than a determination value KRHINT for determining the semi-active state of the oxygen sensor 26. If RHINT ≦ KRHINT, the abnormality diagnosis permission flag XWADER is set to “ 1 ”is set.

  Next, the CPU 48a proceeds to step 907, sets the initial completion flag XINIT to “1”, and then proceeds to step 908. That is, once the target power integrated value WADTG is obtained once the power is turned on, “NO” is determined in step 902 thereafter, and the process directly proceeds to step 908.

  Thereafter, in step 908, the CPU 48a determines whether the element temperature feedback execution flag XEFB is “1”. At the beginning of the heater control (before time t1 in FIG. 18), since XEFB = 0, the determination in step 908 is negative, and the CPU 48a proceeds to step 909. In step 909, the CPU 48a determines whether or not the element resistance Zdc of the oxygen sensor 26 is equal to or less than a value 30Ω (corresponding to an element temperature of 700 ° C.) corresponding to the element temperature feedback execution temperature. Then, the CPU 48a proceeds to step 915 when the element resistance Zdc is 30Ω or less, and proceeds to step 910 when the element resistance Zdc is not 30Ω or less.

  In step 910, the CPU 48a determines whether or not the heater resistance RH at that time is equal to or greater than the heater resistance learning value RHADP. The heater resistance learning value RHADP is a value obtained by learning the heater resistance value at the target heater temperature (1200 ° C.) at the time of power control so as to eliminate the variation due to each product or a change with time. In step 911, the CPU 48a determines whether or not the power integrated value WADD is equal to or greater than the target power integrated value WADTG (the numerical value in step 904). The integrated power value WADD is obtained by a calculation routine (not shown), and is a numerical value obtained by sequentially integrating, for example, heater supply power WH (= Vh · Ih) detected every 128 ms (WADD = WADDi−1 + WH). .

  In this case, if any of Steps 910 and 911 is negatively determined (RH <RHADP or WADD <WADTG), the CPU 48a proceeds to Step 912 and executes full conduction control (control of I). That is, before the time t1 in FIG. 18, the CPU 48 a proceeds in the order of steps 908 → 909 → 910 (→ 911) → 912 in FIG.

  If both steps 910 and 911 are positively determined (RH ≧ RHADP and WADD ≧ WADTG), the CPU 48a proceeds to step 920 and executes power control (control of II). That is, at the time t1 to t2 in FIG. 18, the CPU 48a proceeds in the order of steps 908 → 909 → 910 → 911 → 920 in FIG. 20, and supplies the heater 33 to the heater 33 according to the element resistance so as to keep the heater temperature at the target heater temperature. Control the power supply. In this step 920, “1” is set to the power control execution flag XEWAT.

  At time t2 in FIG. 18, the determination at step 909 is affirmative, and the CPU 48a proceeds to step 915 to determine whether or not the power control execution flag XEWAT is “1”. If XEWAT = 1, the CPU 48a proceeds to step 930 to execute a heater resistance learning process, and then proceeds to step 950. If XEWAT = 0, the CPU 48a proceeds directly to step 950. In the heater resistance learning process in step 930, it is determined whether or not the heater resistance RH at that time exceeds a heater resistance learning value RHADP + α% (for example, α = 2%). The heater resistance learning value RHADP is updated to the heater resistance RH at that time.

  Thereafter, the CPU 48a executes a heater abnormality determination routine described later in step 940, and further executes element temperature feedback control in step 950. In this case, the CPU 48a resets the power control execution flag XEWAT to “0” and sets “1” to the element temperature feedback execution flag XEFB. The CPU 48a sets the control duty DUTY of the heater control circuit 46 in the following three modes (A) to (C).

  (A) When the elapsed time from power ON is a predetermined time (for example, 24.5 seconds) or more, the control duty DUTY is calculated by the following equations (4) to (7). However, “ZdcT” is a control target value, “GP” is a proportional term, “GI” is an integral term, “GD” is a differential term GD, “KP” is a proportional constant, “KI” is an integral constant, “KD” "Represents a differential constant.

DUTY = GP + GI / 16 + GD (4)
GP = KP · (Zdc−ZdcT) (5)
GI = GIi-1 + KI. (Zdc-ZdcT) (6)
GD = KD. (Zdci-Zdci-1) (7)
(B) When the elapsed time from power ON is less than a predetermined time (for example, 24.5 seconds) and the air-fuel ratio> 12, the following equation (8) is used by using the proportional term GP and the integral term GI. To calculate the control duty DUTY.

DUTY = GP + GI / 16 + GD (8)
(C) When the elapsed time from power ON is less than a predetermined time (for example, 24.5 seconds) and the air-fuel ratio ≦ 12, the control duty DUTY is calculated by the following equation (9). That is, in this case (when the air-fuel ratio ≦ 12), the element temperature feedback control by PID is difficult, and the heater resistance feedback control is executed instead of the element temperature feedback control. However, “KPA” is a constant, and “RHG” is a target heater resistance (equivalent to 2.1Ω = 1020 ° C.).

DUTY = HDUTYi-1 + KPA. (RHG-RH) (9)
Next, the heater abnormality diagnosis routine in step 940 of FIG. 20 will be described with reference to FIG.

  In FIG. 22, the CPU 48 a determines whether or not the abnormality diagnosis permission flag XWADER is “1” in step 941, and if XWADER = 0, this routine is ended as it is. If XWADER = 1, the CPU 48a proceeds to step 942 to determine whether or not the power integrated value WAADD is equal to or greater than a predetermined abnormality determination value KWADER (WADD ≧ KWADER). If WADD <KWADER, the CPU 48a proceeds to step 943 and clears the abnormality determination flag XELER to “0”.

  If WADD ≧ KWADER, the CPU 48a proceeds to step 944 to determine whether or not the abnormality determination flag XELER is already “1”. In this case, in steps 944 to 946, a diagnosis process is performed when the abnormality determination is made twice (the warning lamp 29 is lit).

  According to the fifth embodiment, an integrated value (power integrated value WADD) of heater supply power from the beginning of heater energization is calculated, and depending on whether or not the integrated power value WADD exceeds a predetermined abnormality determination value KWADDER. An abnormality of the oxygen sensor 26 is diagnosed (steps 942 to 946 in FIG. 22). In this case, by performing abnormality diagnosis based on the integrated value of the heater supply power, the accuracy of the diagnosis data is increased and accurate abnormality diagnosis is possible.

  In this embodiment, the initial resistance value of the heater at the beginning of heater energization is detected (step 903 in FIG. 19), and the initial resistance value of the heater is within a predetermined range for determining the cold state of the oxygen sensor 26. Only in the case (only when step 905 in FIG. 19 is YES), the sensor abnormality diagnosis is permitted (flag operation at 906 in FIG. 19). That is, for example, when heater energization is started at the time of warm-up restart of the engine, the integrated value of the heater supply power is relatively small, and it is not desirable in terms of diagnosis accuracy to diagnose sensor abnormality using the integrated value. Therefore, the abnormality diagnosis is limited to when the sensor is cold, so that a good abnormality diagnosis can always be performed.

  According to the first aspect of the present invention, in the system in which the heater supply power is feedback controlled so that the element resistance of the oxygen sensor becomes the target element resistance, whether the heater supply power exceeds a predetermined abnormality determination value or not is determined. By diagnosing the abnormality of the oxygen sensor, the sensor abnormality can be diagnosed accurately and easily.

  According to the second aspect of the present invention, it is possible to perform an optimal abnormality diagnosis according to the engine operating state. According to the invention described in claim 3, since the abnormality of the oxygen sensor is diagnosed according to whether or not the integrated value of the heater supply power exceeds a predetermined abnormality determination value, the accuracy of the diagnosis data is increased and the accuracy is increased. Abnormal diagnosis is possible.

  According to the fourth aspect of the present invention, the sensor abnormality diagnosis is permitted only when the initial resistance value of the heater is equal to or less than a predetermined value indicating the cold state of the oxygen sensor. When the integrated value of the supplied power is relatively small, the sensor abnormality is prohibited and the diagnostic accuracy of the sensor abnormality is maintained.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the whole structure of the air fuel ratio control apparatus in an Example. The figure which shows the cross-sectional structure of an oxygen sensor, and the electrical structure in ECU. The figure which shows the voltage-current characteristic of an oxygen sensor. The flowchart which shows a heater energization control routine. The figure which shows the relationship between element temperature and element resistance. The flowchart which shows an air fuel ratio detection routine. The figure which shows the current-voltage characteristic of an oxygen sensor. The flowchart which shows a sensor abnormality diagnosis routine. The flowchart which shows a fail safe routine. The flowchart which shows the sensor abnormality diagnosis routine of 2nd Example. The figure which shows the current-voltage characteristic of an oxygen sensor divided into normal, element low temperature abnormality, and element high temperature abnormality. The time chart which shows the heater control operation | movement of 3rd Example. The flowchart which shows the heater control routine of 3rd Example. The flowchart which shows a process data calculation routine. The flowchart which shows the sensor abnormality diagnosis routine of 3rd Example. The flowchart which shows the sensor abnormality diagnosis routine of 4th Example. A map for searching for a heater power judgment value. The time chart which shows the heater control operation | movement of 5th Example. The flowchart which shows the heater control routine of 5th Example. The flowchart following FIG. The figure which shows the relationship between initial heater resistance and target electric power integrated value. The flowchart which shows the heater abnormality diagnosis routine of 5th Example.

Explanation of symbols

26. Oxygen sensor (limit current type oxygen sensor),
33 ... heater,
34 ... a solid electrolyte layer as an oxygen concentration detection element,
48a... CPU as heater control means, element resistance detection means, sensor abnormality diagnosis means, heater supply power estimation means, fuel increase / decrease means, heater supply power control means, integrated power value calculation means, heater initial resistance value detection means.

Claims (4)

A limiting current type oxygen sensor having an oxygen concentration detecting element that outputs a limiting current proportional to the oxygen concentration and a heater that heats the detecting element;
Element resistance detection means for detecting element resistance of the oxygen sensor;
Heater supply power control means for feedback control of power supplied to the heater in order to eliminate a deviation between the element resistance detected by the element resistance detection means and the target element resistance of the oxygen sensor;
Sensor abnormality diagnosing means for diagnosing abnormality of the oxygen sensor according to whether or not heater supply power by the heater supply power control means exceeds a predetermined abnormality determination value ;
Power integrated value calculating means for calculating an integrated value of heater supply power from the beginning of heater energization,
The sensor abnormality diagnosing means diagnoses an abnormality of the oxygen sensor based on whether or not the integrated value of heater supply power calculated by the integrated power value calculating means exceeds a predetermined abnormality determination value. Detection device . Heater initial resistance value detection means for detecting an initial resistance value of the heater at the beginning of heater energization;
Diagnosis permission means for permitting abnormality diagnosis by the sensor abnormality diagnosis means only when the initial resistance value of the heater detected by the heater initial resistance value detection means is in a predetermined range for determining the cold state of the oxygen sensor; The oxygen concentration detection apparatus according to claim 1 , comprising: A limiting current type oxygen sensor having an oxygen concentration detecting element that outputs a limiting current proportional to the oxygen concentration and a heater that heats the detecting element;
Element resistance detection means for detecting element resistance of the oxygen sensor;
Heater supply power control means for feedback control of power supplied to the heater in order to eliminate a deviation between the element resistance detected by the element resistance detection means and the target element resistance of the oxygen sensor;
Sensor abnormality diagnosing means for diagnosing abnormality of the oxygen sensor according to whether or not heater supply power by the heater supply power control means exceeds a predetermined abnormality determination value;
Heater initial resistance value detection means for detecting an initial resistance value of the heater at the beginning of heater energization;
Diagnosis permission means for permitting abnormality diagnosis by the sensor abnormality diagnosis means only when the initial resistance value of the heater detected by the heater initial resistance value detection means is in a predetermined range for determining the cold state of the oxygen sensor; An oxygen concentration detection device comprising: The oxygen concentration detection device according to any one of claims 1 to 3, wherein the sensor abnormality diagnosis unit includes a unit that sets the abnormality determination value according to an engine speed and an engine load .

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