JP2004245843A - Oxygen concentration sensing system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To diagnose abnormalities in a limiting current type oxygen sensor with high precision and ease. <P>SOLUTION: The limiting current type oxygen sensor has an oxygen concentration sensing element, which outputs limiting current which is proportional to oxygen concentration and a heater which heats the sensing element. A CPU in a microcomputer controls the energization of the heater to activate the oxygen sensor and calculates element resistance from an applied voltage and detected current of the oxygen sensor. In a sensor abnormality diagnosis routine, the CPU in steps 301 to 307, determines the preconditions for abnormalities diagnosis; and if all the preconditions are fulfilled, the CPU conducts abnormality diagnosis in steps 308 to 311. More specifically, after determining whether the element resistance falls within a predetermined range, if it falls short of the lower limit of the predetermined range, it is determined that there are high-temperature abnormalities in the element; and if the element resistance exceeds the upper limit of the predetermined range, it is determined that there are low-temperature abnormalities in the element. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to an oxygen concentration detecting device including a limiting current type oxygen sensor including an oxygen concentration detecting element that outputs a limiting current proportional to the oxygen concentration, and a heater that heats the detecting 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 limiting current type oxygen sensor (oxygen concentration detecting device) that outputs a limiting current proportional to the oxygen concentration in the exhaust gas is used, and the microcomputer takes in the detection result by the sensor. To calculate the air-fuel ratio, and perform the air-fuel ratio feedback control based on the air-fuel ratio. Thereby, optimal 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 air-fuel ratio control system, since the control accuracy is significantly deteriorated when the reliability of the detection signal from the oxygen sensor is reduced, a technique for accurately performing abnormality diagnosis of the oxygen sensor has been demanded. . Therefore, as a conventional technology, for example, in "Air-fuel ratio control device for internal combustion engine" of Patent Document 1, a temperature sensor for detecting the temperature of an oxygen sensor (oxygen concentration detecting 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. Patent Document 2 discloses an “oxygen sensor heater control device” that controls a heater power supply so that a heater resistance value becomes a target resistance value when the heater power supply falls outside a predetermined range. It describes that the abnormality of the resistance value is detected.
JP-A-1-232143 JP-A-3-189350

  However, the above-described conventional technology has the following problems. In the former technique described above (Patent Document 1), a sensor that detects a sensor temperature is a requirement, and includes a problem that costs are increased. Further, in the latter technique (Patent Document 2), it is only determined whether or not the target resistance value is properly set, and the abnormality is determined to be abnormal in the abnormality diagnosis processing when the battery or the sensor is replaced. Limited. Therefore, the reliability of the oxygen sensor could not be actually determined.

  The present invention has been made in view of the above-mentioned conventional problems, and its object is to propose an abnormality diagnosis technique having a novel configuration, whereby the abnormality of the limiting current type oxygen sensor can be detected accurately and easily. An object of the present invention is to provide an oxygen concentration detecting device capable of making a diagnosis.

  In order to achieve the above object, an invention according to claim 1 includes an oxygen concentration detecting element provided in an exhaust system of an internal combustion engine and outputting a limiting current proportional to the oxygen concentration, and a heater for heating the detecting element. A limiting current type oxygen sensor, a heater control means for controlling energization of the heater to activate the oxygen sensor, a fuel amount increasing / decreasing means for increasing or decreasing the fuel supply amount to the internal combustion engine, and the fuel amount A sensor abnormality diagnosing means for diagnosing abnormality of the oxygen sensor according to whether or not the output value of the oxygen sensor has changed within a predetermined range when the fuel supply amount is increased or decreased by the increasing / decreasing means. And

  According to a second aspect of the present invention, in the first aspect of the present invention, there is provided a heater supply power estimating unit for estimating an integrated value of the heater supply power from the start of heater energization, wherein the sensor abnormality diagnosing unit comprises When it is determined that the integrated value of the heater supply power estimated by the supply power estimation means is equal to or more than a predetermined amount, the abnormality diagnosis of the oxygen sensor is performed.

  According to a third aspect of the present invention, there is provided a limiting current type oxygen sensor having an oxygen concentration detecting element for outputting a limiting current proportional to the oxygen concentration, and a heater for heating the detecting element, and detecting an element resistance of the oxygen sensor. Element resistance detection means, heater supply power control means for feedback-controlling the supply power to the heater to eliminate a deviation between the element resistance detected by the element resistance detection means and a target element resistance of the oxygen sensor, The gist of the invention is to provide a sensor abnormality diagnosis means for diagnosing 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.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the sensor abnormality diagnosing means includes means for setting the abnormality determination value according to an operating state of the internal combustion engine.

  According to a fifth aspect of the present invention, in the third or fourth aspect of the present invention, there is provided a power integrated value calculating means for calculating an integrated value of heater supply power from the beginning of heater energization, and the sensor abnormality diagnosing means is provided. An abnormality of the oxygen sensor is diagnosed based on whether or not the integrated value of the heater supply power calculated by the integrated power value calculation means exceeds a predetermined abnormality determination value.

  According to a sixth aspect of the present invention, in the invention of any of the third to fifth aspects, a heater initial resistance value detecting 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 value detection means is within a predetermined range for determining the cold state of the oxygen sensor.

  According to the first aspect of the present invention, the heater control means controls the energization of the heater to activate the oxygen sensor. Then, when the oxygen sensor is normally activated by energizing the heater, the oxygen sensor outputs a limit current proportional to the oxygen concentration. Further, the fuel amount increasing / decreasing means increases or decreases the fuel supply amount to the internal combustion engine. The sensor abnormality diagnosis means diagnoses an abnormality of the oxygen sensor depending on whether or not the output value of the oxygen sensor has changed within a predetermined range when the fuel supply amount is increased or decreased by the fuel amount increasing / decreasing means.

  In short, for example, when the fuel supply amount to the internal combustion engine is increased, the air-fuel ratio becomes rich (the oxygen concentration decreases), and the limit current output from the oxygen sensor decreases. At this time, if the oxygen sensor is normal, the change amount of the limit current is within a predetermined range defined by the actual air-fuel ratio change amount, but if the oxygen sensor is abnormal, the change amount of the limit current is Deviates from a predetermined range. From this, the sensor abnormality can be diagnosed accurately and easily.

  According to the second aspect of the present invention, the heater supply power estimating means estimates an integrated value of the heater supply power from the start of the heater energization. The sensor abnormality diagnosis means performs an abnormality diagnosis of the oxygen sensor when it is determined that the integrated value of the heater supply power estimated by the heater supply power estimation means is equal to or more than a predetermined amount. That is, in order to more accurately diagnose the abnormality of the oxygen sensor, it is preferable to perform the abnormality diagnosis in an active state of the sensor (or a state in which the sensor is naturally activated). Therefore, in this configuration, the sensor activation state is estimated from the above-described integrated value of the heater supply power, and abnormality diagnosis is permitted in the activation state.

  According to the third aspect of the present invention, the heater supply power control means performs feedback control of the power supply to the heater so as to eliminate a deviation between the element resistance detected by the element resistance detection means and the target element resistance of the oxygen sensor. I do. The sensor abnormality diagnosis 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, if a sensor abnormality occurs, an excessive amount of heater supply power is required to maintain the element resistance (element temperature) in the active region. Therefore, the sensor abnormality can be diagnosed accurately and easily.

  According to the invention described in claim 4, 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.

  According to the invention described in claim 5, the integrated power value calculating means calculates the integrated value of the heater supply power from the beginning of the heater energization. The sensor abnormality diagnosis unit diagnoses the oxygen sensor abnormality 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 the abnormality diagnosis based on the integrated value of the heater supply power, the accuracy of the diagnosis data is increased, and accurate abnormality diagnosis can be performed.

  According to the sixth 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 unit permits the abnormality diagnosis by the sensor abnormality diagnosis unit only when the initial resistance value of the heater detected by the heater initial resistance value detection unit 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 restarting warm-up of the engine, the integrated value of the heater supply power is relatively small, and diagnosing a sensor abnormality based on the integrated value is not desirable in terms of diagnostic accuracy. Therefore, the abnormality diagnosis is limited to when the sensor is cold, so that good abnormality diagnosis can always be performed.

(First embodiment)
Hereinafter, a first embodiment in which the oxygen concentration detecting device of the present invention is embodied in an air-fuel ratio control device of an internal combustion engine for a vehicle will be described with reference to the drawings.

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

  An injector 6 is provided in an intake pipe (intake port) 2 for each cylinder in the engine 1. Fuel in the fuel tank 7 is sucked up by a fuel pump 8 and supplied to a pressure regulator 10 through a fuel filter 9. The pressure regulator 10 supplies fuel adjusted to a constant pressure to the injector 6 and returns excess fuel to the fuel tank 7. The injector 6 is opened by power supply from the battery 15 to inject fuel. Then, the fuel injected by the injector 6 is mixed with the intake air to form 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 near 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 pressure sensor 22, which detects an intake pipe pressure (intake negative pressure). The cylinder block of the engine 1 is provided with a water temperature sensor 23 for detecting the temperature of the engine cooling water.

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

  Further, a limiting current type oxygen sensor 26 is provided in the exhaust pipe 3 of the engine 1, 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 provided downstream of the oxygen sensor 26, and the converter purifies exhaust gas.

  The detection signals of the above 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 supply, starts the engine 1 in response to an ON signal of the ignition switch 28, and increases or decreases the air-fuel ratio correction coefficient based on the output signal of the oxygen sensor 26 during engine operation, thereby increasing the air-fuel ratio. Is controlled near the target air-fuel ratio (for example, the stoichiometric air-fuel ratio). Further, the ECU 40 executes a sensor abnormality diagnosis process described later to diagnose whether or not the oxygen sensor 26 is abnormal. When an abnormality occurs, the ECU 40 turns on a warning lamp 29 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 is roughly classified 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 thereof. The sensor body 32 generates a limit 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 main body 32 will be described in detail. In the sensor main body 32, an exhaust gas side electrode layer 36 is fixed to an outer surface of a solid electrolyte layer 34 formed in a cup-shaped cross section, and an atmosphere side electrode layer 37 is fixed to an inner surface. A diffusion resistance layer 35 is formed outside the exhaust gas side electrode layer 36 by a plasma spraying method or the like. The solid electrolyte layer _{34, ZrO 2, HfO 2, ThO} 2, Bi 2 O 3 or the like _{CaO, MgO, Y 2 O 3} , Yb 2 O 3 or the like of oxygen ion conductive oxide sintered obtained by solid solution as a stabilizer The diffusion resistance layer 35 is formed of a heat-resistant inorganic substance such as alumina, magnesia, silica, spinel, and mullite. Both the exhaust gas side electrode layer 36 and the atmosphere side electrode layer 37 are made of a noble metal having high catalytic activity such as platinum, and the surfaces thereof are 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 about 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 detecting element.

  The heater 33 is housed in the atmosphere-side electrode layer 37 and heats the sensor main 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 heat energy. . The heater 33 has a heat generating capacity sufficient to activate the sensor main body 32.

  In the oxygen sensor 26 having the above configuration, the sensor main body 32 generates a concentration electromotive force at the stoichiometric air-fuel ratio point, and generates a limit current according to the oxygen concentration in the lean region from the stoichiometric 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 body 32 can detect the oxygen concentration with a linear characteristic. However, a high temperature of about 650 ° C. or more is required to activate the sensor body 32, Since the activation temperature range is narrow, the activation region cannot be controlled by heating only 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 main body 32 generates a limiting current corresponding to the CO concentration. appear.

  The voltage-current characteristics of the sensor main body 32 will be described with reference to FIG. As shown in FIG. 3, the current-voltage characteristics are such that the current flowing into the solid electrolyte layer 34 of the sensor body 32 and the voltage applied to the solid electrolyte layer 34 are proportional to the detected oxygen concentration (air-fuel ratio) of the oxygen sensor 26. Is linear. Then, when the sensor main body 32 is in the active state at the temperature T = T1, a 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 of the characteristic line L1 parallel to the voltage axis V specifies the limit current of the sensor main 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 decreases as the air-fuel ratio becomes richer.

  In this voltage-current characteristic, a voltage range smaller than a linear portion parallel to the voltage axis V is a resistance dominating region, and the slope of the characteristic line L1 in the resistance dominating region is determined by the solid electrolyte layer 34 in the sensor body 32. (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 slope decreases due to an increase in element resistance. That is, when the temperature T of the sensor 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 such a case, the straight line portion of the characteristic line L2 parallel to the voltage axis V specifies the limit current of the sensor main body 32 at T = T2, and this limit current substantially matches the limit current by the characteristic line L1.

  Then, when a positive applied voltage Vpos is applied to the solid electrolyte layer 34 of the sensor main body 32 on the characteristic line L1, the current flowing through the sensor main body 32 becomes the limit current Ipos (see point Pa in FIG. 3). When a negative applied voltage Vneg is applied to the solid electrolyte layer 34 of the sensor main body 32, the current flowing through the sensor main body 32 does not depend on the oxygen concentration, but becomes a negative temperature current Ineg which is proportional only to the temperature (FIG. 3). 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 are connected. The bias control circuit 41 includes a positive bias DC power supply 42, a negative bias DC power supply 43, and a changeover switch circuit 44. The negative electrode of the positive bias DC power supply 42 and the positive electrode of the negative bias DC power supply 43 are both connected to the exhaust gas side electrode layer 36.

  The changeover switch circuit 44 connects only the positive electrode of the positive bias DC power supply 42 to the sensor current detection circuit 45 in the first switching state, and connects the negative electrode of the negative bias DC power supply 43 in the second switching state. Only 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 main body 32, and a current in the positive direction 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 current in the negative direction flows through the solid electrolyte layer 34. In such a case, the terminal voltages of the DC power supplies 42 and 43 correspond to the 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 controls the duty of the power supplied from the battery power supply 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 a heater current Ih) is detected by a current detection resistor 50.

  The A / D converter 47 converts the detection current (Ipos, Ineg in FIG. 3), the heater current Ih, and the voltage applied to the heater 33 (hereinafter, referred to as heater voltage Vh) by the sensor current detection circuit 45 into digital signals. Output to the microcomputer 48. The microcomputer 48 includes a CPU 48a for executing various arithmetic processes, a memory 48b including a ROM and a RAM, and the like. 49 is controlled. The EFI 49 receives the various sensor signals as engine information and detects an intake air temperature Tam, an intake negative pressure Pm, a cooling water temperature Thw, an engine speed NE, a vehicle speed Vs, and the like. Then, the fuel injection by the injector 6 is controlled based on the 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 the present embodiment will be described along various control programs executed by the CPU 48a in the microcomputer 48. Hereinafter, the heater energization control, the air-fuel ratio detection process, and the sensor abnormality diagnosis process 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. 4, in step 101, the CPU 48a determines heater control flags F1 and F2 indicating the control state of the heater 33. That is, in the first embodiment, the heater control shifts in the order of the duty 100% control, the first heater energization control, and the second heater energization control in accordance with the ON operation of the ignition switch 28. 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, both the heater control flags F1 and F2 have been cleared to "0" (initial value), and the CPU 48a proceeds to step 102 to execute 100% duty control. In other words, the heater control circuit 46 of FIG. 2 is controlled at a duty of 100% 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 based on the heater voltage Vh and the heater current Ih in Step 103 (RH = Vh / Ih), and in Step 104, determines whether or not 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, the duty 100% control is continued.

  If the heater resistance RH ≧ 2Ω 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 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, the 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, in a low load or low rotation range, a large duty is set because the amount of heat of the exhaust gas is small. It has become so. After setting the flag F1, the CPU 48a directly proceeds from step 101 to step 106, and performs the first heater energization control.

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

  If Zdc ≦ 90Ω in step 108, the CPU 48a proceeds to step 109 to operate the heater control flag F1 at “0” and operate the heater control flag F2 at “1”. Perform 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 controlled according to the engine load (for example, the 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 executes the second heater energization control. As described above, in the present embodiment, the energization of the heater 33 is controlled to be open 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 is executed by the CPU 48a, for example, every 8 msec.

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

  Further, the CPU 48a determines the activity of the oxygen sensor 26 based on whether or not the element resistance Zdc is in a predetermined activity determination range (KREL to KRHE) in step 204 (for example, KREL = 10Ω, KRH = 90Ω). That is, if KREL ≦ Zdc ≦ KREH and the determination in step 204 is affirmative, the CPU 48a determines that the sensor activation has been completed, and proceeds to the subsequent 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 area of FIG. Further, the CPU 48a reads the limit current Ip (n) detected by the sensor current detection circuit 45 in FIG. 2 in step 206, and converts the limit current Ip (n) into an air-fuel ratio (A / F) in step 207. I do. Further, the CPU 48a calculates an applied voltage Vp (n + 1) for detecting the next air-fuel ratio in step 208, and also outputs {Vp (n + 1) = f (Ip)} and sends 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 the time (n) is “16” and the air-fuel ratio (A / F) at the time (n + 1) is “15”, application of Vp (n) causes Ip ( 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 the 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 same sensor activity determination processing in steps 210 to 213 as in 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 regards that the sensor is activated and proceeds to step 206 to execute the above-described air-fuel ratio detection processing. If a negative determination is made in step 213, 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 to 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.). Is determined. In step 303, it is determined whether or not the engine speed NE is equal to or more than a predetermined determination value KNE (for example, 500 rpm). In step 304, the vehicle speed Vs is lower than a 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 greater than a predetermined determination value KCAST (for example, 20 seconds). In step 306, the battery voltage VB is reduced to a predetermined determination value KVB (for example, 13 V ) 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 performed has been cleared to "0", that is, whether or not the fuel cut has been performed.

  In this embodiment, among the preconditions described above, the integrated value of the heater supply power is estimated based on the elapsed time CAST after engine start and the battery voltage VB, and when these values are equal to or greater than a 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 the active state or is in a state where it should be activated, and the abnormality diagnosis is permitted. By setting the conditions, the abnormality diagnosis can be performed more accurately.

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

  In step 308, the CPU 48a determines whether or not the element resistance Zdc is smaller than a first resistance determination value KREL (10Ω in this embodiment). If Zdc <KREL, the process 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 risen excessively, and the CPU 48a determines in step 309 that the oxygen sensor 26 has an element high temperature abnormality. The element high temperature abnormality includes the following modes. In other words, the mode includes a mode in which the heater resistance of the oxygen sensor 26 varies to a smaller value and the current flows excessively, and a mode in which the wire harness on the ground side of the heater 33 is always ground short-circuited and the current cannot be controlled, and the current flows excessively.

  If Zdc ≧ KREL, the CPU 48a determines in step 310 whether the element resistance Zdc is equal to or greater than a second resistance determination value KREH (90Ω in the present embodiment). Proceed to 311. When the element resistance Zdc is equal to or higher than the second resistance determination value KREH, it means that the element temperature does not rise and remains low, and the CPU 48a determines in step 311 that the oxygen sensor 26 has an element low temperature abnormality. I do. The element low temperature abnormality includes the following modes. That is, a mode in which the heater resistance of the oxygen sensor 26 varies to a larger value and the current is reduced, a mode in which the heater 33 is deteriorated to increase the resistance and the current is reduced, and a mode in which the current is reduced due to the disconnection of the wire harness of the heater 33. Also includes a mode in which the flow stops.

  Then, when the element abnormality of the oxygen sensor 26 is determined 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 the element abnormality (high temperature abnormality or low temperature abnormality) is determined from FIG. 8, the CPU 48a proceeds to step 402 and stops the air-fuel ratio feedback. Further, the CPU 48a stops energizing the heater in step 403, and turns on the warning lamp 29 indicating an element abnormality in 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, the abnormality of the oxygen sensor 26 is diagnosed depending on whether the element resistance of the oxygen sensor 26 is within a predetermined range (steps 308 to 311 in FIG. 8). That is, in the limiting current type oxygen sensor 26, the output characteristic is specified by the gradient in the resistance dominant region (in FIG. 3, the gradient of the voltage region smaller than the linear portion parallel to the voltage axis V), that is, the magnitude of the element resistance. Is done. In this case, if the oxygen sensor 26 is abnormal, the element resistance becomes too low or too high, and the abnormality can be diagnosed accurately and easily.

  Further, in the present 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. That is, an excessive element resistance of the oxygen sensor 26 means that the element temperature is too low, and an element low temperature abnormality is determined. If the element resistance of the oxygen sensor 26 is too small, it means that the element temperature is too high, and an element high temperature abnormality is determined.

  Further, in the present embodiment, since a temperature sensor is not required as in the related art, there is no increase in cost. Further, in the prior art, the abnormality is diagnosed mainly when the battery or the sensor is replaced. However, in the present embodiment, the abnormality can be always diagnosed during normal traveling of the vehicle, and the reliability of the sensor output can be improved. In addition to improving the air-fuel ratio, a highly accurate air-fuel ratio control system can be realized.

  Note that the heater energization control method is not limited to the method of the present 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, a configuration may be employed in which only the first or second heater energization control is always performed, or a predetermined time after the engine is started. It is also possible to adopt a configuration in which 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 the differences from the first embodiment. In this embodiment, the CPU 48a in the microcomputer 48 constitutes heater control means, fuel increase / decrease means and sensor abnormality diagnosis means. FIG. 10 shows a sensor abnormality diagnosis routine in the second embodiment.

  In FIG. 10, the CPU 48a determines in step 501 whether the precondition for the sensor abnormality diagnosis is satisfied. This precondition corresponds to steps 301 to 307 in FIG. Further, the CPU 48a determines whether or not the air-fuel ratio feedback is being performed in step 502. In this case, if any of steps 501 and 502 is negatively determined, the CPU 48a ends this routine, and if both 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 in FIG. 2 at that time as "Ipo". Further, in the subsequent step 504, the CPU 48a stores the current operating conditions (the intake negative pressure Pm and the engine speed NE) as "Pmo" and "NEo".

  Thereafter, the CPU 48a increases the fuel injection amount by the injector 6 by α% (for example, 10%) at step 505 (or may decrease the fuel injection amount), and determines at step 506 whether a predetermined time has elapsed after the fuel increase. Here, the increase in fuel means that the air-fuel ratio is forcibly shifted to the rich side. Then, when a predetermined time has elapsed 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 numerical values "Pmo" and "NEo" (the values stored in step 504) before the fuel increase. . In this case, if the operating conditions have changed, the CPU 48a terminates this routine without performing subsequent abnormality diagnosis. If the operating conditions have not changed, the CPU 48a proceeds to step 508 to perform the 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 the subsequent step 509 (ΔIp = Ip− Ipo). Thereafter, the CPU 48a determines in step 510 whether the current change amount ΔIp (absolute value) exceeds the first current determination value KDIL (whether or not ΔIp> KDIL), and in step 511, determines whether the current change value ΔIp It is determined whether or not the current is equal to or smaller than the second current determination value KDIH (ΔIp ≦ KDIH) (however, KDIL <KDIH). Here, the permissible change range defined by “KDIL to KDIH” is set by an actual air-fuel ratio change corresponding to an increase in fuel.

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

  FIGS. 11A to 11C are diagrams showing output signals of the oxygen sensor 26 in a normal state, an abnormal low-temperature element, and an abnormal high-temperature element. In the figure, the current change amounts ΔIp1, ΔIp2, and ΔIp3 indicate the differences between the limit currents 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. Therefore, “ΔIp2” is smaller than “ΔIp1” in the normal state (ΔIp2 <ΔIp1). In such a case, step 510 in FIG. 10 is negatively determined, and it is determined that the element is at a low temperature abnormality. In addition, when the temperature of the element is abnormal as shown in FIG. Therefore, “ΔIp3” is larger than “ΔIp1” in the normal state (ΔIp3> ΔIp1). In such a case, a negative determination is made in step 511 of FIG. 10 described above, and it is determined that there is an element high temperature abnormality.

  According to the second embodiment, the fuel supply amount 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 during the fuel increase. An abnormality of the sensor 26 is diagnosed according to whether or not the determination is made (steps 510 to 513 in FIG. 10). Accordingly, it is determined whether or not the air-fuel ratio enrichment (decrease in oxygen concentration) due to the increase in fuel is normally reflected in the sensor output, and the sensor abnormality can be accurately and easily diagnosed. Further, in the present embodiment, since a range is provided for the abnormality determination value at the time of abnormality diagnosis, it is possible to distinguish between an element low temperature abnormality and an element high temperature abnormality.

(Third embodiment)
Next, a third embodiment will be described. In the first and second embodiments, the heater 33 is controlled to be open. 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 detecting means, heater supply power control means, and sensor abnormality diagnosing means.

  FIG. 12 is a time chart of the heater control in the present embodiment, and FIG. 12 shows an operation from the start of energization to the heater 33 upon engine start until the oxygen sensor 26 is sufficiently activated. In the present embodiment, the heater control is divided into the control of I to IV in FIG. 12 from the difference of the purpose and the control method, and each will be described in order. 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 start of the engine, a heater voltage of 100% duty is applied to the heater 33 (hereinafter, this is referred to as full conduction control). That is, when the heater 33 and the sensor element (the sensor main body 32) are cold, the maximum power is supplied to the heater 33 in order to heat the heater 33 in a short time.

  In the controls II and III, the power supplied to the heater 33 is controlled so that the heater temperature is maintained at a predetermined target heater temperature (for example, 1200 ° C., which is the heater upper limit temperature) (hereinafter, this is referred to as power control). ). That is, if the element temperature is at the activation temperature (700 ° C.), the heater temperature is uniquely determined from the supplied electric power. Therefore, to maintain the heater 33 at a constant temperature, the predetermined electric power may be continuously supplied. However, when the element temperature is low, the supply power required to keep the heater temperature constant varies according to the element temperature. Generally, 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 have a relationship shown in FIG. 5).

  However, at the beginning of the power control, the element resistance is extremely large and exceeds a detectable maximum value (for example, 600Ω). Therefore, the power supplied to the heater 33 is maintained at a constant power (for example, 60 W) in the region where the element resistance cannot be detected (II control). Then, when the element temperature rises and the element resistance becomes 600Ω or less, thereafter, power corresponding to the element resistance is supplied to the heater 33 (control of III).

  In the IV control, the power supplied to the heater 33 is feedback controlled so that the element resistance becomes 30Ω (corresponding 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 48a determines whether or not the execution condition of the element temperature feedback control is satisfied in a step 601. This execution condition is satisfied if the element resistance of the oxygen sensor 26 is 30Ω or less. Further, the CPU 48a determines whether the power control execution condition is satisfied in Step 602. As the power control execution conditions, two conditions are set according to whether or not the oxygen sensor 26 (the sensor body 32 and the heater 33) is in a cold state. For example, the condition is satisfied when a predetermined time has elapsed since the start of the full conduction control (the control of I in FIG. 12), and the condition is satisfied when the heater resistance value becomes equal to or higher than the target heater resistance value when the condition is not cold. By performing the full conduction control according to the cold state of the oxygen sensor 26 in this manner, an excessive rise in the heater temperature at the time of restarting the engine is prevented.

  Therefore, if both 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 (I control). That is, a heater voltage of 100% duty 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 (II and III controls). In this case, as described above, if the element resistance is in the undetectable region (element resistance> 600Ω), the supplied power is controlled at a fixed value (control of II), and when the element resistance can be detected, the heater temperature is set to the target heater temperature. Is controlled in accordance with the element resistance (control of III).

  Thereafter, when the execution condition of the element temperature feedback control 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)
Here, “DUTY.I” is the initial value of the control duty DUTY, and “ZdcT” is the control target value (in this embodiment, DUTY.I = 20%, ZdcT = 30Ω). “GP” represents a proportional term, “GI” represents an integral term, “KP” represents a proportional constant, and “KI” represents an integral constant (in the present embodiment, KP = 4.2%, KI = 0.2% ). Note that these numerical values are experimentally obtained 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, for example, every 128 ms. 14, the CPU 48a reads the heater current Ih detected by the current detection resistor 50 of FIG. 2 in step 701, and reads the heater voltage Vh in subsequent step 702. Further, the CPU 48a calculates the heater resistance RH by dividing the heater voltage Vh by the heater current Ih in step 703 (RH = Vh / Ih), and in a subsequent step 704, multiplies the heater voltage Vh by the heater current Ih. The supply power WH is calculated (WH = Vh · Ih). Further, the CPU 48a calculates a smoothed value of the heater supply power WH (hereinafter referred to as a power average value WHAV) by a 1/64 smoothing operation in step 705 {WHAV = (63WHAVi-1 + WH) / 64}.

  The flowchart of FIG. 15 shows a sensor abnormality diagnosis routine executed by the CPU 48a at, for example, a one-second cycle. In this abnormality diagnosis routine, a sensor abnormality is determined according to the heater supply power WH required at the time of executing the element temperature feedback control. In other words, when the sensor is abnormal (for example, the sensor deteriorates with time), the heater supply power WH required to maintain the element temperature at the target value (for example, 700 ° C.) increases, and 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 start of the element temperature feedback in step 801. In a subsequent step 802, the CPU 48a determines whether or not a predetermined time KAFST (for example, 100 Second) has elapsed. Further, the CPU 48a determines in step 803 whether or not the steady operation state (for example, the idle state) has been continued for a predetermined time KSMST (for example, 5 seconds) or more. Then, if any of steps 801 to 803 is negatively determined, the CPU 48a ends the process as it is, and if all of the steps 801 to 803 are positively determined, the process proceeds to step 804.

  In step 804, the CPU 48a determines whether or not the average power 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 determines 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 an abnormality as a diagnosis process. That is, in steps 804 to 809, the diagnosis process is performed when it is determined twice (WHAV ≧ KWHAV) that the abnormality has occurred consecutively.

  As described above, in the third embodiment, the power supply 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 the element temperature of 700 ° C.) (element temperature feedback control in FIG. 13). At this time, the abnormality of the oxygen sensor 26 is diagnosed based on whether 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, it is possible to accurately and easily diagnose a sensor abnormality. Further, in this embodiment, since the abnormality diagnosis is limited to the normal operation (step 803 in FIG. 15), the influence of the exhaust gas temperature on the power supplied to the heater can be eliminated, and a proper 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 the abnormality diagnosis processing. 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 according to the engine operating state is set. Here, the heater power determination value KWHAV is set based 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 and KWHAV2 in the figure. In this case, the heater power determination value KWHAV is set to be lower as the rotation speed is higher or the load is higher, and to be higher as the rotation speed is lower or the load is lower. As described above, according to the fourth embodiment, it is possible to perform the optimum abnormality diagnosis according to the engine operating state.

(Fifth embodiment)
Next, a fifth embodiment will be described. In this embodiment, the CPU 48a of the microcomputer 48 constitutes an integrated power value calculating means, a heater initial resistance value detecting means, and a sensor abnormality diagnosing means.

  FIG. 18 is a time chart of the heater control in the fifth embodiment. FIG. 18 shows the operation from the start of energization of the heater 33 upon engine start until the oxygen sensor 26 is sufficiently activated. In this embodiment, the heater control is divided into the control of I to III in FIG. 18 from the difference of the purpose and the control method (I is full conduction control, II is power control, and III is element temperature feedback control). Will be described in order.

  First, in the full conduction control (I control) immediately after the start of the engine, a heater voltage having a 100% duty is applied to the heater 33. That is, when the heater 33 and the sensor element are cold, the maximum power is supplied to the heater 33 in order to heat the heater 33 in a short time. 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 upper limit temperature of the heater). In the element temperature feedback control (control of III), the power supplied to the heater 33 is feedback-controlled so that the element resistance becomes 30Ω (corresponding to the element temperature of 700 ° C.) in order to maintain the active state of the element. In the element temperature feedback control, if the power supplied to the heater 26 exceeds the upper limit, the power supplied to the heater 33 is limited.

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

  In FIG. 19, the CPU 48a determines whether or not the ignition switch 28 is turned on (whether or not the power is on) in step 901. If the power is off, the CPU 48a ends this routine. Proceed to 902. Further, the CPU 48a determines whether or not the initial end completed flag XINIT is “0” in step 902 (the initial completed flag XINIT is initialized to “0” when the power is turned on). If there is, go to step 903; if XINIT = 1, go 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. Further, the CPU 48a sets the target power integrated value WADTG based on the initial heater resistance RHINT in step 904 according to 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 half-activated state of the oxygen sensor 26. If RHINT ≦ KRHINT, the abnormality diagnosis permission flag XWADER is set to “ Set "1".

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

  Thereafter, the CPU 48a determines in a step 908 whether or not the element temperature feedback execution flag XEFB is "1". At the beginning of the heater control (before time t1 in FIG. 18), XEFB = 0, so that a negative determination is made in step 908, 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 lower 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 equal to or less than 30Ω, and proceeds to step 910 when the element resistance Zdc is not equal to or less than 30Ω.

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

  In this case, if either of Steps 910 and 911 is negatively determined (RH <RHADP or WADD <WADTG), the CPU 48a proceeds to Step 912 to execute full conduction control (I control). 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. 20 and applies a heater voltage of 100% duty to the heater 33.

  If both steps 910 and 911 are affirmatively determined (RH ≧ RHADP and WADD ≧ WADTG), the CPU 48a proceeds to step 920 to execute power control (II control). That is, in the period from time t1 to time t2 in FIG. 18, the CPU 48a proceeds to steps 908 → 909 → 910 → 911 → 920 in FIG. 20 in order to maintain 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 affirmative determination is made in step 909, and the CPU 48a proceeds to step 915 to determine whether 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 of step 930, it is determined whether or not the heater resistance RH at that time is a numerical value exceeding 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 the element temperature feedback execution flag XEFB to “1”. 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 the power ON is equal to or longer than a predetermined time (for example, 24.5 seconds), the control duty DUTY is calculated by the following equations (4) to (7). Here, "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 the power ON is less than a predetermined time (for example, 24.5 seconds) and the air-fuel ratio> 12, the following equation (8) is obtained by using the proportional term GP and the integral term GI. Calculates the control duty DUTY.

DUTY = GP + GI / 16 + GD (8)
(C) If the elapsed time from the power ON is shorter 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 (air-fuel ratio ≦ 12), it is difficult to perform the element temperature feedback control by the PID, and the heater resistance feedback control is executed instead of the element temperature feedback control. Here, “KPA” is a constant, and “RHG” is a target heater resistance (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 48a determines in a step 941 whether or not the abnormality diagnosis permission flag XWADER is “1”. If XWADER = 0, the CPU 48a ends this routine. If XWADER = 1, the CPU 48a proceeds to step 942, and determines whether or not the integrated power value WADD is greater than or equal to a predetermined abnormality determination value KWADER (whether 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, if abnormality determination is performed twice consecutively, a diagnosis process is performed (the warning lamp 29 is turned on).

  According to the fifth embodiment, the integrated value (power integrated value WADD) of the heater supply power from the beginning of the heater energization is calculated, and whether the integrated power value WADD exceeds a predetermined abnormality determination value KWADER is determined. An abnormality of the oxygen sensor 26 is diagnosed (steps 942 to 946 in FIG. 22). In this case, by performing the abnormality diagnosis based on the integrated value of the heater supply power, the accuracy of the diagnosis data is increased, and accurate abnormality diagnosis can be performed.

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

  According to the first aspect of the present invention, when the fuel supply amount is increased or decreased, the abnormality of the oxygen sensor is diagnosed according to whether or not the output value of the oxygen sensor has changed within a predetermined range. Abnormalities can be diagnosed accurately and easily.

  According to the second aspect of the present invention, the sensor abnormality is diagnosed in an activated state of the oxygen sensor or in a state in which the sensor is supposed to be activated, so that more accurate abnormality diagnosis can be performed. According to the third aspect of the present invention, in a system for feedback-controlling the heater supply power so that the element resistance of the oxygen sensor becomes the target element resistance, it is determined whether the heater supply power exceeds a predetermined abnormality determination value. By diagnosing the abnormality of the oxygen sensor, the sensor abnormality can be accurately and easily diagnosed.

  According to the fourth aspect of the invention, it is possible to perform an optimum abnormality diagnosis according to the engine operating state. According to the fifth aspect of the present invention, the abnormality of the oxygen sensor is diagnosed in accordance with whether or not the integrated value of the heater supply power exceeds a predetermined abnormality determination value. Abnormality diagnosis becomes possible.

  According to the invention described in claim 6, 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 diagnosis accuracy of the sensor abnormality is maintained.

FIG. 1 is a schematic diagram illustrating an overall configuration of an air-fuel ratio control device according to an embodiment. The figure which shows the cross-section of an oxygen sensor, and the electrical structure in ECU. The figure which shows the voltage-current characteristic of an oxygen sensor. 9 is a flowchart illustrating a heater energization control routine. The figure which shows the relationship between element temperature and element resistance. 9 is a flowchart illustrating an air-fuel ratio detection routine. The figure which shows the current-voltage characteristic of an oxygen sensor. 9 is a flowchart illustrating a sensor abnormality diagnosis routine. 9 is a flowchart illustrating a fail-safe routine. 9 is a flowchart illustrating a sensor abnormality diagnosis routine according to a second embodiment. The figure which shows the current-voltage characteristic of an oxygen sensor classified into normal, element low temperature abnormality, and element high temperature abnormality. 9 is a time chart illustrating a heater control operation according to a third embodiment. 9 is a flowchart illustrating a heater control routine according to a third embodiment. 9 is a flowchart illustrating a processing data calculation routine. 9 is a flowchart illustrating a sensor abnormality diagnosis routine according to a third embodiment. 9 is a flowchart illustrating a sensor abnormality diagnosis routine according to a fourth embodiment. A map for searching for a heater power determination value. 9 is a time chart illustrating a heater control operation according to a fifth embodiment. 9 is a flowchart illustrating a heater control routine according to a fifth embodiment. 20 is a flowchart following FIG. 19; The figure which shows the relationship between an initial heater resistance and a target electric power integration value. 9 is a flowchart illustrating a heater abnormality diagnosis routine according to a fifth embodiment.

Explanation of reference numerals

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

Claims (6)

A limiting current type oxygen sensor that is provided in an exhaust system of an internal combustion engine and has an oxygen concentration detecting element that outputs a limiting current proportional to the oxygen concentration and a heater that heats the detecting element,
Heater control means for controlling the energization of the heater to activate the oxygen sensor;
Means for increasing or decreasing the amount of fuel supplied to the internal combustion engine,
When the fuel supply amount is increased or decreased by the fuel amount increasing / decreasing unit, a sensor abnormality diagnosing unit for diagnosing an abnormality of the oxygen sensor according to whether an output value of the oxygen sensor has changed within a predetermined range is provided. An oxygen concentration detecting device characterized by the above-mentioned. Heater supply power estimating means for estimating an integrated value of heater supply power from the start of heater energization,
2. The sensor abnormality diagnosis unit according to claim 1, wherein the sensor abnormality diagnosis unit performs abnormality diagnosis of the oxygen sensor when it is determined that the integrated value of the heater supply power estimated by the heater supply power estimation unit is equal to or more than a predetermined amount. Oxygen concentration detector. A limiting current type oxygen sensor having an oxygen concentration detecting element for outputting a limiting current proportional to the oxygen concentration and a heater for heating the detecting element,
Element resistance detection means for detecting the element resistance of the oxygen sensor,
Heater supply power control means for feedback-controlling the power supply to the heater to eliminate a deviation between the element resistance detected by the element resistance detection means and a target element resistance of the oxygen sensor,
An oxygen concentration detecting device comprising: a sensor abnormality diagnosis unit that diagnoses an abnormality of the oxygen sensor based on whether or not a heater supply power by the heater supply power control unit exceeds a predetermined abnormality determination value. The oxygen concentration detection device according to claim 3, wherein the sensor abnormality diagnosis unit includes a unit that sets the abnormality determination value according to an operation state of the internal combustion engine. Power integrated value calculating means for calculating an integrated value of heater supply power from the beginning of heater energization,
5. The sensor abnormality diagnosis unit according to claim 3, wherein the sensor abnormality diagnosis unit diagnoses the oxygen sensor abnormality based on whether an integrated value of the heater supply power calculated by the power integrated value calculation unit exceeds a predetermined abnormality determination value. Oxygen concentration detector. Heater initial resistance value detecting means for detecting an initial resistance value of the heater at the beginning of energization of the heater,
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 within a predetermined range for determining a cold state of the oxygen sensor. The oxygen concentration detecting device according to any one of claims 3 to 5, further comprising: