JP4281747B2 - Deterioration detection device for air-fuel ratio detection device - Google Patents

Description

  The present invention relates to an air-fuel ratio detection device, and more particularly to a deterioration detection device for an air-fuel ratio detection device for diagnosing deterioration of a downstream air-fuel ratio sensor disposed downstream of a catalyst. The present invention also relates to a deterioration detection device for an air-fuel ratio detection device that can be accurately performed.

  2. Description of the Related Art Conventionally, a configuration is known in which oxygen sensors are respectively disposed upstream and downstream of a catalyst interposed in an engine exhaust system. In such a configuration, the air-fuel ratio feedback correction coefficient is set based on the output value of the upstream oxygen sensor arranged upstream of the catalyst, and the air-fuel ratio is controlled so that the air-fuel ratio upstream of the catalyst becomes the target air-fuel ratio. is doing. Furthermore, so-called 2O2 air-fuel ratio control is known in which the air-fuel ratio is corrected by correcting the air-fuel ratio feedback correction coefficient based on the output value of the downstream oxygen sensor disposed downstream of the catalyst.

  By the way, when each oxygen sensor deteriorates in such a 2O2 air-fuel ratio control system, the responsiveness of the oxygen sensor deteriorates, so that proper air-fuel ratio control is impaired.

  In the 2O2 air-fuel ratio control system, the deterioration of the catalyst is diagnosed by comparing the outputs of both oxygen sensors provided upstream and downstream of the catalyst. Therefore, when each oxygen sensor deteriorates, the accuracy of the catalyst deterioration diagnosis using this oxygen sensor also decreases, so it is necessary to detect the deterioration of the air-fuel ratio sensor.

  At this time, since the upstream oxygen sensor is disposed upstream of the catalyst, it directly detects the oxygen concentration in the exhaust gas discharged from the engine. Therefore, when the air-fuel ratio fluctuation occurs, the upstream oxygen sensor immediately reacts to the air-fuel ratio fluctuation. Therefore, the deterioration detection of the upstream oxygen sensor can be performed relatively easily by monitoring the output of the upstream air / fuel ratio sensor when the air / fuel ratio fluctuation occurs.

  On the other hand, since the downstream oxygen sensor is provided downstream of the catalyst, it detects the air-fuel ratio in the exhaust gas after passing through the catalyst. Therefore, even if air-fuel ratio fluctuations occur, the air-fuel ratio fluctuations are smoothed by the oxidation of the catalyst, oxygen adsorption / desorption due to the reduction reaction, and the storage effect of the catalyst. Will be detected. Further, the storage effect of the catalyst changes due to deterioration. Therefore, it is difficult to detect the deterioration of the downstream oxygen sensor itself from the reaction state of the downstream oxygen sensor with respect to the air-fuel ratio fluctuation of the engine.

  In order to solve this problem, a method for detecting deterioration of a downstream air-fuel ratio sensor that is hardly affected by a catalyst has been proposed. For example, in Japanese Utility Model Publication No. 03-037949, the output of the catalyst downstream oxygen sensor with respect to the air-fuel ratio fluctuation upstream of the catalyst is detected before the catalyst is activated. In Japanese Patent Laid-Open No. 62-250351, deterioration detection is performed when the air-fuel ratio changes beyond the catalyst storage capacity, such as when the fuel is cut.

  However, as in Japanese Utility Model Publication No. 03-037949, in the method of detecting the deterioration of the oxygen sensor before the catalyst is activated, the conditions under which the deterioration can be detected are limited at the time of cold start. Similarly, in the method of detecting the deterioration of the oxygen sensor at the time of fuel cut as disclosed in JP-A-62-250351, the conditions for detecting the deterioration are limited at the time of fuel cut. In particular, in an automatic transmission vehicle, fuel cut is hardly activated when driving in an urban area, so that the frequency of detection of deterioration is reduced.

  Thus, since the execution conditions are extremely limited in both methods, the detection frequency is reduced. Furthermore, even if the execution condition is satisfied, there is a problem that it is difficult to ensure detection accuracy because it is a transient condition.

  Accordingly, an object of the present invention is to provide a failure detection device for an air-fuel ratio detection device that is not easily affected by the catalyst storage capacity and can ensure the detection frequency.

In order to achieve the above object, in the invention according to claim 1, the output change rate of the air-fuel ratio detecting means when the temperature of the solid electrolyte element is adjusted to at least two different temperatures by the temperature adjusting means is compared . Deterioration of the air-fuel ratio detection means provided downstream is detected.

  When the solid electrolyte element temperature of the air-fuel ratio detecting means changes, the abnormality of the air-fuel ratio detecting means is detected by utilizing the characteristic that the sensitivity to the exhaust gas component changes due to the difference in the solid electrolyte element temperature, that is, the activity of the electrode part. It is what I did.

  For example, if the air-fuel ratio detection means is normal, the sensitivity to exhaust gas changes with changes in the element temperature, so a difference occurs when the output waveforms are compared at different element temperatures. On the other hand, the air-fuel ratio detecting means that has deteriorated has the electrode portion deteriorated and its activity level decreased, so that the change in the output waveform is small even if the element temperature of the solid electrolyte changes. Therefore, the deterioration of the air-fuel ratio detection device can be detected by comparing the outputs of the air-fuel ratio detection means at different solid electrolyte element temperatures.

Here, the air-fuel ratio detecting means may be any means having the above characteristics and includes a linear air-fuel ratio sensor and an oxygen sensor .

  Further, as described in claim 2, the temperature adjustment means estimates the temperature of the solid electrolyte element by detecting the internal resistance of the air-fuel ratio detection means, and based on the estimated temperature, The temperature should be adjusted.

  Thereby, since the temperature adjustment of the solid electrolyte element when performing deterioration detection can be performed with high accuracy, the accuracy of deterioration detection can be improved.

<< Embodiment (1) >>
Hereinafter, an embodiment (1) of the present invention will be described with reference to FIGS.

  Hereinafter, an embodiment in which the present invention is embodied in an air-fuel ratio detection apparatus will be described with reference to the drawings. Note that the air-fuel ratio detection apparatus in the present embodiment is particularly applied to an electronically controlled gasoline injection engine mounted on an automobile. In the engine air-fuel ratio control system, the fuel injection amount to the engine is controlled to a desired air-fuel ratio based on the detection result by the air-fuel ratio detection device.

  First, a schematic configuration of the entire engine control system will be described with reference to FIG. An air cleaner 13 is provided at the most upstream portion of the intake pipe 12 of the engine (internal combustion engine) 11, and an air flow meter 14 for detecting the intake air amount is provided downstream of the air cleaner 13. A throttle valve 15 and a throttle opening sensor 16 for detecting the throttle opening are provided on the downstream side of the air flow meter 14.

  Further, a surge tank 17 is provided on the downstream side of the throttle valve 15, and an intake pipe pressure sensor 18 for detecting the intake pipe pressure is provided in the surge tank 17. The surge tank 17 is provided with an intake manifold 19 for introducing air into each cylinder of the engine 11, and a fuel injection valve 20 for injecting fuel is attached in the vicinity of the intake port of the intake manifold 19 of each cylinder. .

  On the other hand, in the middle of the exhaust pipe 21 (exhaust gas passage) of the engine 11, an upstream catalyst 22 and a downstream catalyst 23 that reduce harmful components (CO, HC, NOx, etc.) in the exhaust gas are installed in series. . In this case, the upstream catalyst 22 is formed with a relatively small capacity so that warm-up is completed early at the start and exhaust emission at the start is reduced. On the other hand, the downstream catalyst 23 is formed with a relatively large capacity so that the exhaust gas can be sufficiently purified even in a high load region where the amount of exhaust gas increases.

  Further, a linear air-fuel ratio sensor 24 that outputs a linear air-fuel ratio signal corresponding to the air-fuel ratio of the exhaust gas is provided on the upstream side of the upstream catalyst 22, and the downstream side of the upstream catalyst 22 and the downstream side of the downstream catalyst 23. On the side, a first oxygen sensor 25 and a second oxygen sensor 26 having so-called Z characteristics whose outputs change relatively abruptly in the vicinity of the theoretical air-fuel ratio are provided. Hereinafter, the linear air-fuel ratio sensor and the oxygen sensor are collectively referred to as an air-fuel ratio sensor. A cooling water temperature sensor 27 for detecting the cooling water temperature and a crank angle sensor 28 for detecting the engine speed NE are attached to the cylinder block of the engine 11.

  These various sensor outputs are input to an engine control circuit (hereinafter referred to as “ECU”) 29. The ECU 29 is mainly composed of a microcomputer, and executes a program stored in a built-in ROM (storage medium), for example, to feedback control the air-fuel ratio of exhaust gas.

  In the present embodiment, the air-fuel ratio of the exhaust gas is controlled by feedback control described in, for example, Japanese Patent Application Laid-Open No. 2001-193521.

  FIG. 2 shows a configuration in which the linear air-fuel ratio sensor 24 is used as the air-fuel ratio sensor on the upstream side of the catalyst and either the first oxygen sensor 25 or the second oxygen sensor 26 is used as the air-fuel ratio sensor on the downstream side of the catalyst. 6 is a flowchart of air-fuel ratio feedback control when one of them is switched and used.

  FIGS. 3 and 4 are flow charts of other air-fuel ratio feedback control when the second oxygen sensor 26 is used in addition to the linear air-fuel ratio sensor 24 and the first oxygen sensor 25 of FIG.

  First, the processing contents of the target air-fuel ratio setting program in FIG. 2 will be described. When this program is started, a downstream oxygen sensor used for setting the target air-fuel ratio λTG is selected from the first oxygen sensor 25 and the second oxygen sensor 26 in step 701.

  For example, at the time of low load operation with a small exhaust gas flow rate, the exhaust gas can be considerably purified by the upstream catalyst 22 alone. Therefore, as the downstream sensor used for setting the target air-fuel ratio λTG, the responsiveness of the air-fuel ratio control is better when the first oxygen sensor 25 is used. However, when the exhaust gas flow rate increases, the amount of exhaust gas components that pass through without being purified in the upstream catalyst 22 increases, so it is necessary to effectively use both the upstream catalyst 22 and the downstream catalyst 23 to purify the exhaust gas. is there. In this case, since it is preferable to perform air-fuel ratio feedback control in consideration of the state of the downstream catalyst 23, it is preferable to use the second oxygen sensor 26 as a downstream sensor used for setting the target air-fuel ratio λTG. .

  Further, the shorter the delay time until the change in the air-fuel ratio of the exhaust gas discharged from the engine 11 (the output change of the air-fuel ratio sensor 24 upstream of the upstream catalyst 22) appears in the output change of the first oxygen sensor 25, This means that the amount of exhaust gas component that passes through the upstream catalyst 22 without being purified is increased (that is, the purification efficiency is reduced). Therefore, when the delay time of the output change of the first oxygen sensor 25 is short, it is preferable to use the output of the second oxygen sensor 26 as a downstream sensor used for setting the target air-fuel ratio λTG.

  Therefore, the conditions for selecting the second oxygen sensor 26 as the downstream sensor used for setting the target air-fuel ratio λTG are: (1) Change in the air-fuel ratio of exhaust gas discharged from the engine 11 (change in output of the linear air-fuel ratio sensor 24) ) Is shorter than a predetermined time (or a predetermined period) until the output change of the first oxygen sensor 25 appears, or (2) the intake air amount (the exhaust gas flow rate) is a predetermined value or more. It is supposed to be.

  When either one of these two conditions (1) and (2) is satisfied, the second oxygen sensor 26 is selected, and when neither is satisfied, the first oxygen sensor 25 is selected. The second oxygen sensor 26 may be selected when both the conditions (1) and (2) are satisfied.

  In this way, after selecting the downstream sensor used for setting the target air-fuel ratio λTG, the routine proceeds to step 702, where the output voltage VOX2 of the selected oxygen sensor corresponds to the theoretical air-fuel ratio (λ = 1). Whether it is rich or lean is determined depending on whether it is higher or lower (for example, 0.45 V). Here, when lean, the routine proceeds to step 703, where it is determined whether or not the previous time was lean. If both the previous time and the current time are lean, the routine proceeds to step 704, where the rich integral amount λIR is obtained from the map according to the current intake air amount QA.

  As a map of the rich integral amount λIR, the upstream catalyst downstream sensor (first oxygen sensor) map shown in the upper column of FIG. 5A and the downstream catalyst shown in the upper column of FIG. A map for the downstream sensor (second oxygen sensor) is stored, and one of the maps is selected according to the sensor to be used. The map characteristics of these rich integral amounts λIR are set such that the rich integral amount λIR decreases as the intake air amount QA increases. In the region where the intake air amount QA is small, the map of the downstream catalyst downstream sensor The rich integration amount λIR is set to be slightly larger than that of the upstream catalyst downstream sensor map. After calculating the rich integration amount λIR, the process proceeds to step 705, the target air-fuel ratio λTG is corrected to the rich side by λIR, the rich / lean at that time is stored (step 713), and this program is terminated.

  If the current rich is reversed to the current lean, the process proceeds from step 703 to step 706, and the skip amount λSKR to the rich side is obtained according to the rich component storage amount OSTrich of the catalyst. Note that the rich component storage amount OSTrich calculation process is the same as the process described in Japanese Patent Laid-Open No. 2001-193521, and is omitted here.

  The map characteristics of FIG. 6 are set so that the rich skip amount λSKR decreases as the absolute value of the rich component storage amount OSTrich decreases. After calculating the skip amount λSKR, the routine proceeds to step 707, the target air-fuel ratio λTG is corrected to the rich side by λIR + λSKR, the rich / lean at that time is stored (step 713), and this program is terminated.

  On the other hand, in skip 702, when the output voltage VOX2 of the oxygen sensor is rich, the routine proceeds to step 708, where it is determined whether or not the previous time was also rich. If both the previous time and the current time are rich, the routine proceeds to step 709, where the lean integral amount λIL is obtained from the map shown in FIG. 5 according to the current intake air amount QA. As a map of the lean integral amount λIL, a map for the upstream catalyst downstream sensor (first oxygen sensor) shown in the lower column of FIG. 5A and a downstream catalyst shown in the lower column of FIG. A map for the downstream sensor (second oxygen sensor) is set, and one of the maps is selected according to the sensor selected as the downstream sensor.

  The map characteristics of the lean integral amount λIL in FIGS. 5A and 5B are set such that the lean integral amount λIL decreases as the intake air amount QA increases. In the region where the intake air amount QA is small, The map for the downstream catalyst downstream sensor is set so that the lean integral amount λIL is slightly larger than the map for the upstream catalyst downstream sensor. After calculating the lean integration amount λIL, the process proceeds to step 710, the target air-fuel ratio λTG is corrected to the lean side by λIL, the rich / lean at that time is stored (step 713), and this program is terminated.

  Further, if the previous leaning is performed on the lean side this time, the process proceeds from step 708 to step 711 and the skip amount λSKL to the lean side is obtained from the map shown in FIG. 6 according to the lean component storage amount OSTLean of the catalyst. . Note that the lean component storage amount OSTLean calculation process is the same as the process described in Japanese Patent Laid-Open No. 2001-193521, and is omitted here.

  The map characteristics shown in FIG. 6 are set such that the lean skip amount λSKL decreases as the lean component storage amount OSTLean decreases. Thereafter, in step 712, the target air-fuel ratio λTG is corrected to the lean side by λIL + λSKL, the rich / lean at that time is stored (step 713), and the program is terminated.

  As is apparent from the map of FIG. 6, when the rich component storage amount OSTrich and the lean component storage amount OSTLean are reduced due to deterioration of the catalysts 22 and 23, the rich skip amount λSKR and the lean skip amount λSKL are also gradually set to smaller values. Therefore, it is possible to prevent over-correction beyond the adsorption limit of the catalysts 22 and 23 to discharge harmful components.

  Next, another embodiment of the target air-fuel ratio setting process will be described with reference to the flowcharts of FIGS.

  The ECU 29 executes the target air-fuel ratio setting program of FIG. 3 and the target output voltage setting program of FIG. 4 to use the first oxygen sensor 25 as a downstream sensor used for setting the target air-fuel ratio λTG of the air-fuel ratio feedback control. When selected, the target output voltage TGOX of the first oxygen sensor 25 is changed according to the output of the second oxygen sensor 26.

  In FIG. 3, the same step numbers as in FIG. 2 are assigned to steps for executing the same processing as in FIG. In the following, differences from FIG. 2 will be mainly described.

  In the target air-fuel ratio setting program of FIG. 3, first, in step 701, downstream sensors used for setting the target air-fuel ratio λTG are the downstream oxygen sensor 25 on the upstream catalyst 22 and the oxygen sensor 26 on the downstream side of the downstream catalyst 23. Then, the process proceeds to step 714, where a target output voltage setting program TGOX shown in FIG. 4 described later is executed to set a target output voltage TGOX of the downstream sensor used for setting the target air-fuel ratio λTG.

  Thereafter, the process proceeds to step 715, where it is determined whether the selected oxygen sensor output voltage VOX2 is higher or lower than the target output voltage TGOX, whether it is rich or lean, and depending on the result, in steps 703-713, the method described above is performed. Then, the target air-fuel ratio λTG is calculated, the rich / lean at that time is stored, and this program is terminated.

  Next, the processing contents of the target output voltage setting program of FIG. 4 executed in step 714 of FIG. 3 will be described. When this program is started, first, in step 901, it is determined whether or not the first oxygen sensor 25 is selected as a downstream sensor used for setting the target air-fuel ratio λTG. If the first oxygen sensor 25 is selected as the downstream sensor used for setting the target air-fuel ratio λTG, the process proceeds to step 902 and the target output voltage TGOX using the output voltage of the second oxygen sensor 26 as a parameter. From the map, a target output voltage TGOX corresponding to the current output voltage of the second oxygen sensor 26 is calculated.

  In this case, the map of the target output voltage TGOX shows that the output voltage of the second oxygen sensor 26 (the air-fuel ratio of the outflow gas of the downstream catalyst 23) is within a predetermined range (β ≦ output voltage ≦ α) near the theoretical air-fuel ratio. The target output voltage TGOX is set so as to decrease (lean) as the output of the second oxygen sensor 26 increases (becomes rich). Further, in a region where the output of the second oxygen sensor 26 is larger than the predetermined value α, the target output voltage TGOX becomes a predetermined lower limit value (for example, 0.4 V), and the output of the second oxygen sensor 26 is higher than the predetermined value β. In the small region, the target output voltage TGOX is set to be an upper limit value (for example, 0.65 V).

  Thereby, the target output voltage TGOX of the first oxygen sensor 25 is within a range where the adsorption amount of the exhaust gas component of the downstream catalyst 23 is not more than a predetermined value or the air-fuel ratio of the exhaust gas flowing through the downstream catalyst 23 is a predetermined purification window. It is set to be within the range.

  On the other hand, when the second oxygen sensor 26 is selected as the downstream sensor used for setting the target air-fuel ratio λTG, the process proceeds from step 901 to step 903 to set the target output voltage TGOX to a predetermined value (eg, 0.45 V). Set to. The target output voltage setting program described above plays a role corresponding to the second feedback control means.

  FIG. 7 is a configuration diagram showing an outline of the air-fuel ratio detection apparatus in the present embodiment. In FIG. 7, the ECU 29 includes a microcomputer (hereinafter referred to as a microcomputer) 120 that is the center of the internal calculation, and the microcomputer 120 communicates with a host microcomputer 116 for realizing fuel injection control, ignition control, and the like. Connected as possible. The linear air-fuel ratio sensor 24 is attached to the exhaust pipe 21 extending from the engine body of the engine 11, and the microcomputer 120 detects the output. The microcomputer 120 includes a well-known CPU, ROM, RAM, backup RAM, etc. for executing various arithmetic processes (not shown), and controls the heater control circuit 125 and the bias control circuit 140 according to a predetermined control program.

  Here, the bias command signal Vr output from the microcomputer 120 is input to the bias control circuit 140 via the D / A converter 121, the low-pass filter (LPF) 122, and the switch 160. Further, the output of the linear air-fuel ratio sensor 24 corresponding to the current air-fuel ratio (oxygen concentration) is detected, and the detected value is input to the microcomputer 120 via the A / D converter 123. Further, the heater voltage and heater current are detected by a heater control circuit 125 described later, and the detected values are input to the microcomputer 120 via the A / D converter 124.

  Further, a predetermined bias command signal Vr is applied to the element, changes between predetermined times T1 and T2 shown in FIG. 8, that is, an element voltage change ΔV and an element current ΔI are detected, and an element impedance is detected from the following equation.

Impedance = ΔV / ΔI
The detected element impedance value is input to the microcomputer 120. The element impedance has a strong correlation with the element temperature as shown in FIG. 9, and the element temperature of the air-fuel ratio sensor can be controlled by duty-controlling the heater provided in the air-fuel ratio sensor so that the element impedance becomes a predetermined value. is there.

  Similarly, the first oxygen sensor 25 and the second oxygen sensor 26 also detect the element impedance, and the first and second oxygen sensors 25 and 26 respectively have a predetermined value. The element temperature of the oxygen sensor can be controlled by duty-controlling the heater.

  As this technique, in the present embodiment, as shown in FIG. 10, a technique of PI control (proportional, integral) is adopted by the deviation between the actually detected element impedance and the target impedance calculated from the target element temperature. Thus, the element temperature of the linear A / F sensor 24 (the first oxygen sensor 25 and the second oxygen sensor 26) is controlled by this method.

  The details will be described with reference to the flowchart of FIG. In this flowchart, program processing is executed at a predetermined timing (step 400).

  First, in step 401, a deviation (Δimp) between the target impedance calculated from the target element temperature and the element impedance detected by the element impedance detection circuit is calculated. In step 402, an integral value (ΣΔimp) of the impedance deviation for performing the integration control is calculated. In step 403, the heater duty is calculated from the following equation using the deviation, the integral value, the proportional coefficient P1 and the integral coefficient I2.

Heater duty (%) = P1 × Δimp + I2 × ΣΔimp
The heater duty calculated here is input to the heater control circuit denoted by reference numeral 125 in FIG. 7, and heater control of the linear air-fuel ratio sensor 24 (first oxygen sensor 25, second oxygen sensor 26) is performed.

Here, the heater duty is a calorific value adjustment amount for controlling the temperature of the oxygen sensor element, and is based on electric power (W). In order to control the temperature at a constant level, it is desirable to control the power at a constant level. When the temperature is controlled by the heater duty, the reference voltage (in order to prevent the temperature from changing due to a difference in the supplied voltage is used. For example, correction for 13.5 v), that is, correction by power × (13.5 / voltage) ² is performed.

  In FIG. 7, the linear air-fuel ratio sensor 24 protrudes toward the inside of the exhaust pipe 21, and the sensor 24 is roughly composed of a cover, a sensor body, and a heater. The cover has a U-shaped cross section, and a plurality of small holes communicating with the inside and outside of the cover are formed on the peripheral wall. The sensor main body as the sensor element section generates a voltage corresponding to the oxygen concentration in the air-fuel ratio lean region or the unburned gas (CO, HC, H2, etc.) concentration in the air-fuel ratio rich region.

  The heater is accommodated in the atmosphere-side electrode layer, and heats the sensor body (atmosphere-side electrode layer, solid electrode layer, exhaust gas-side electrode layer) by the heat generation energy. The heater has a heat generation capacity sufficient to activate the sensor body.

  The configurations of the first oxygen sensor 25 and the second oxygen sensor 26 are the same as those described above.

  In recent years, a laminated air-fuel ratio sensor in which an element and a heater are integrated has been proposed to improve the heater performance. However, this proposal is not limited to such a sensor, and the electrode is applied to a solid electrolyte element regardless of the type. The present invention is applicable to any air-fuel ratio sensor in which is arranged.

  Next, the control operation of the present plan will be described with reference to the system block diagram shown in FIG. In particular, an embodiment will be described in the case where the present invention is applied to the first oxygen sensor 25 arranged immediately below the upstream catalyst in FIG.

  The output of the exhaust gas components (rich gas and lean gas) discharged from the engine by the first oxygen sensor (oxygen sensor) 25 is detected by the output detection circuit 203 of the ECU 29, and the air-fuel ratio control amount calculation block 204 detects the air-fuel ratio control amount. Is calculated. Here, the increase / decrease amount of the fuel injection amount is determined by comparing a target voltage (not shown) and the detection voltage. The fuel injection amount determined as the air-fuel ratio control amount is supplied to the fuel injection valve 20, and a desired fuel injection amount is injected. The impedance calculation block 202 calculates the element impedance as described in FIGS. 7 and 8, and the heater control amount calculation block 214 determines the heater control amount based on the deviation from the target impedance set in the target impedance setting block 213. The heater is controlled so that the temperature of the sensor element of the first oxygen sensor 25 becomes a desired temperature.

  Here, the target impedance is calculated by the following procedure. The operating state determination block 210 determines the operating state based on information from the crank angle sensor 28, the air flow meter 14, the throttle opening sensor 16, the cooling water temperature sensor 27, and the like indicating the operating state of the engine.

  Based on the operation state determination result, the specific gas sensitivity priority determination block 211 determines whether the exhaust gas composition discharged from the engine in the current operation condition or immediately after the operation state is mainly rich gas or lean gas. When the specific gas sensitivity priority determination block 211 determines that lean gas is mainly used in a state where NOx is likely to be generated such as during high load or acceleration, the oxygen sensor element is set so that the lean gas reactivity is improved in the target element temperature setting block 212. In order to increase the temperature, the target element temperature is set to 720 ° C., for example.

  On the contrary, the specific gas sensitivity priority determination block 211 determines that HC and CO are likely to be generated at low temperatures, low loads, deceleration, etc., and that the rich gas is the main component (or the rich gas is the main component). In this case, in the target element temperature setting block 212, the target element temperature is set to 420 ° C., for example, in order to lower the oxygen sensor element temperature so that the rich gas reactivity is improved.

  In the diagnosis execution determination block 215, whether or not the deterioration detection (diagnosis) of the first oxygen sensor 25 or the second oxygen sensor 26 is to be executed based on the operation state determination result of the operation state determination block 210 is determined. Determine.

  When it is determined that the operation state is to be executed, the oxygen sensor element temperature is controlled to a low temperature state (for example, 400 ° C.) for a predetermined period in the target element temperature setting block 212, and then the oxygen sensor element temperature is a high temperature state for a predetermined period. (For example, 700 ° C.).

  Here, the target element temperature setting block 212 prioritizes the determination result of the diagnosis execution determination block 215 over the determination result of the specific gas sensitivity priority determination block, and determines the target element temperature. In other words, when it is determined in the diagnosis execution determination block 215 that the operation state is to execute the diagnosis, the target element temperature for executing the diagnosis is set. If the diagnosis execution determination block 215 determines that the operation state is not to be executed, the target element temperature is set based on the result determined by the specific gas sensitivity priority determination block 211.

  Next, the richness and lean gas reactivity of the oxygen sensor will be described with reference to the characteristic diagrams of FIGS.

FIG. 12 shows the reactivity of the O2 sensor to carbon monoxide (CO) in nitrogen (N2). As shown in the figure, it reacts with a small amount of CO at a low element temperature, but the reactivity to low concentration CO decreases as the element temperature increases. This is because there is a temperature characteristic reactivity of CO in O2 sensor electrode, under the element cold CO (adsorbed) +1/2 O ^2- (adsorption) ⇔ CO ₂ + 2e ^- reaction is accelerated in the O ₂ Because it is deprived.

FIG. 13 shows the reactivity of the O 2 sensor when nitrogen monoxide (NO) is introduced into nitrogen (N ₂ ) and carbon monoxide (CO) atmosphere. As shown in the figure, in a high temperature state of the element, it reacts with a small amount of NO, but it does not react with low concentration NO as the element temperature decreases. This is because the reaction of CO + NO → CO ₂ + N _{2 2} NO + 4e → N ₂ + 2O ²⁻ is performed on the surface and electrodes of the O 2 sensor electrode, and in the high temperature region, combustion with rich gas (CO) and This is because the decomposition of NO at the electrode is further promoted, and the electromotive force is reduced on the low concentration side.

  Based on the target temperature set in the target element temperature setting block 212 in FIG. 11, the target impedance is set in the target impedance setting block 213 from the relationship between the element impedance and the element temperature shown in FIG. The heater control amount calculation block 214 determines the heater control amount by comparison with the element impedance detection value described above.

  Next, the diagnosis process of the first oxygen sensor 25 will be described with reference to the flowchart of FIG. The same diagnosis process is executed for the second oxygen sensor 26, but the description thereof is omitted here.

  This routine is started at a predetermined timing such as time or the number of injections (step 500). First, in step 501, a diagnosis execution condition is determined based on whether the engine speed, the intake air amount, etc. are within a predetermined range, or whether the catalyst temperature is equal to or higher than a predetermined temperature. . Here, in order to improve the accuracy of the deterioration detection, it is desirable that the execution condition of the diagnosis is a stable steady running state.

  If it is determined in step 501 that the diag execution condition is satisfied, the target element impedance is set to 2000Ω so that the element temperature of the first oxygen sensor 25 is low (eg, 400 ° C.) in step 502. Start low element temperature control.

  In step 503, it is determined whether or not the element impedance is within a predetermined range in order to detect whether or not the element temperature is a desired temperature. Here, the processing of step 502 and step 503 is repeated until the impedance falls within a predetermined range, and when the impedance falls within the predetermined range, the process proceeds to step 504.

  In step 504, the output voltage change rate of the first oxygen sensor 25 is calculated by obtaining the amount of change ΔV between the predetermined timings of the output voltage of the first oxygen sensor 25 in the low element temperature state.

ΔVl = | Vln−Vln−1 |
Here, Vln is the current value of the first oxygen sensor 25 output, and Vln-1 is the previous value of the first oxygen sensor 25 output.

  In this embodiment, the change speed is calculated without distinguishing the rich direction of the O2 sensor (change speed is a positive value) and the lean direction (change speed is a negative value), but only the specific direction of rich or lean is calculated. It may be calculated.

  In the subsequent step 505, in order to improve the accuracy of deterioration detection, the change speed is integrated for a predetermined period, and the change speed integrated value (sdloxsl) is calculated based on the following equation.

sdloxsl = ΔVln−1 + ΔVln
Here, ΔVln is the current value of the change amount ΔVl, and ΔVln−1 is the previous value of the change amount ΔVl.

  Next, in step 506, it is determined whether or not a predetermined period T3 has elapsed. The processing from step 504 to step 506 is repeated until it is determined that a predetermined period has elapsed. If it is determined in step 506 that the predetermined period has elapsed, the process proceeds to step 507.

  In step 507, the element temperature control is switched to the high element temperature control. In the present embodiment, the target impedance is set to 25Ω so that the element has a low temperature (for example, 700 ° C.).

  In the following step 508, it is determined whether or not the element impedance is within a predetermined range (15Ω ≦ element impedance ≦ 25Ω). Here, the processing in step 507 and step 508 is repeated until it is determined that the element impedance is in the predetermined range. If it is determined in step 508 that the element impedance is in the predetermined range, in step 509, the oxygen sensor voltage change rate ΔVh (= | Vhn−Vhn−1 |), 510 in high temperature, in the same manner as in the low temperature process. The O2 sensor voltage change rate integrated value sdloxsh (= ΔVhn−1 + ΔVhn) at the time of high element temperature is calculated.

  Next, in step 511, it is determined whether or not a predetermined time T5 has elapsed. If the predetermined time has not elapsed, the processing from step 509 to step 511 is repeated until the predetermined time elapses. If the predetermined time has elapsed, the process proceeds to step 512.

  In step 512, the deviation amount (delxhl) between the change rate integrated value sdloxsl at low temperature and the change rate integrated value sdloxsh at high temperature is calculated from the following equation.

deloxhl = sdloxsl-sdloxsh
In step 513, the change speed integrated value deviation amount delxhl is compared with a predetermined value set in advance. If the change rate integrated value deviation amount delxhl is smaller than a predetermined value set in advance, the process proceeds to step 514 and it is determined that the first oxygen sensor has deteriorated. On the other hand, when the change rate integrated value deviation amount delxhl is larger than a predetermined value set in advance, the process proceeds to step 515 and it is determined that the first oxygen sensor is normal.

  Next, the operation of the present embodiment will be described along the time chart of FIG.

  Here, (a) of FIG. 15 shows whether or not a diagnosis process execution condition is satisfied. FIG. 15B shows whether the element temperature control request is during normal control when the diagnosis process is not executed, or during low element temperature control or during high element temperature control when the diagnosis process is executed. FIG. 15C shows the solid electrolyte element temperature. FIG. 15D shows the first oxygen sensor output at the time of deterioration, and FIG. 15E shows the first oxygen sensor output at the time of normality. FIG. 15F shows the change rate integrated value sdloxsl during the low element temperature control, and FIG. 15G shows the change rate integrated value Sdoxsh during the high element temperature control. FIG. 15 (h) shows the change rate integrated value deviation amount delxhl. FIG. 15 (i) shows an abnormality detection flag.

  In FIG. 15, at time T1 when the diagnosis processing execution condition is satisfied, low element temperature control (low temperature control) of the first oxygen sensor element temperature is required, and a target impedance (not shown) is set large (for example, 2000Ω). Thus, the heater is controlled so that the solid electrolyte element temperature becomes 400 ° C.

  Next, after time T2 when the solid electrolyte element temperature is stabilized at a low temperature (the element impedance is in a predetermined range (1800Ω ≦ element impedance ≦ 2200Ω)), the normal oxygen sensor voltage is more reactive with rich gas (CO). Therefore, the output fluctuates greatly. On the other hand, the output of the deteriorated oxygen sensor has a small fluctuation amount because the reactivity is lowered. The change rate is calculated by calculating the output fluctuation amount of the oxygen sensor at this time for each predetermined timing. The change speed calculated in this way is integrated until time T3, and the change speed integrated value sdloxsl during low temperature control is calculated.

  Subsequently, at time T3, high element temperature control (high temperature control) of the first oxygen sensor element temperature is required, and the target impedance is set small (for example, 25Ω). Thus, the heater is controlled so that the solid electrolyte element temperature becomes 700 ° C. After time T4 when the solid electrolyte element is stable at a high temperature (the element impedance is in a predetermined range (15Ω ≦ element impedance ≦ 25Ω)), the output voltage of a normal oxygen sensor is due to rich gas (CO) as compared with that during low temperature control. Since the reactivity is lowered, the fluctuation amount is reduced. Similarly, a sensor that has deteriorated has a small fluctuation amount.

  The change rate integrated value sdloxsh during the high temperature control is calculated until the time T5 is reached as in the low temperature control.

  Then, at time T5, a change speed integrated value deviation amount delxhl, which is a deviation between the change speed integrated value sdloxsl during the low temperature control and the change speed integrated value sdloxsh during the high temperature control, is obtained. This deviation amount delxhl is large when the oxygen sensor is normal and small when it is deteriorated for the reason described above. Therefore, the presence or absence of deterioration can be determined by comparison with a predetermined determination value. In this embodiment, it is determined whether the deterioration is normal or not. However, the deterioration degree can be detected by providing a plurality of determination values. Of course, the deviation amount delxhl can also be used as an indicator of the degree of deterioration.

  In the present embodiment, detection of deterioration of the first oxygen sensor 25 has been described. However, the present invention is not limited to this, and can be used for detection of deterioration of the second oxygen sensor 26. It can also be used for the linear air-fuel ratio sensor 24.

  Next, it will be described with reference to FIGS. 16 and 17 that the diagnosis process according to the present embodiment is hardly affected by the catalyst storage capacity.

  As shown in FIG. 16, the rate of change of the oxygen sensor increases because the sensitivity of the rich gas (CO) component increases as the element temperature decreases. Therefore, it is possible to detect the degree of deterioration of the oxygen sensor based on the change rate deviation when the element temperature is high (for example, 700 ° C.) and low (for example, 400 ° C.).

  Further, in the state where the catalyst is deteriorated and the O2 storage capacity is lowered, the change rate of the oxygen sensor output becomes larger as shown in FIG. 16 than when the catalyst is normal. However, in this method, the deviation of the change speed between the high temperature control and the low temperature control of the element is calculated, and the deterioration of the oxygen sensor is determined based on this deviation. Less affected.

  FIG. 17 shows the O2 sensor change speed deviation according to the degree of deterioration of the catalyst. As described above, according to the present invention, since it is difficult to be influenced by the catalyst storage capacity, it is possible to distinguish between a normal oxygen sensor and a deteriorated oxygen sensor regardless of the catalyst purification capacity and the degree of deterioration.

<< Embodiment (2) >>
Hereinafter, an embodiment (2) of the present invention will be described with reference to FIGS.

  In the embodiment (1), the method of detecting an abnormality of the O2 sensor by comparing the sensor output fluctuation when the O2 sensor element temperature is controlled at a high temperature and a low temperature under a specific operation condition has been described. In the embodiment (2), a method for further improving the detectability will be described.

  FIG. 18 shows a schematic flowchart. First, step 1000 is activated at a predetermined timing. Next, in step 1001, a diagnosis execution condition is determined, such as whether the engine speed, the air amount, etc. are predetermined operating conditions and / or whether the catalyst temperature is equal to or higher than a predetermined temperature. Further, whether or not the sensor element temperature is stabilized is also determined as a diagnosis execution condition based on an elapsed time after execution of sensor element temperature control (not shown) or an estimated sensor element temperature value (including element impedance).

  If it is determined in step 1001 that the diagnosis execution condition is not satisfied, the process proceeds to step 1008 and the program is terminated. If it is determined in step 1001 that the diagnosis execution condition is satisfied, the process proceeds to step 1002.

  In step 1002, it is determined whether or not the low element temperature control is to be performed. If it is determined that the low element temperature control is to be performed, the process proceeds to step 1003 in order to further improve the diagnostic ability of the diagnosis, and the proportional control gain (rich side proportional gain) of the sub feedback control by the first oxygen sensor 25 is normally set. Greater than control, giving greater gas changes. In this embodiment, the normal gain is changed from 0.1 to 0.2.

  At the time of sensor low element temperature control, the reactivity of the oxygen sensor with respect to rich gas (CO) is improved, and thus a larger correction can be obtained by increasing the control gain. For this reason, when the sensor detects rich (high output), a large reduction correction is performed, so that lean gas can be supplied immediately, and the oxygen sensor reacts greatly to rich / lean. In step 1004, sensor output fluctuations are integrated.

  If it is determined in step 1002 that the low element temperature control is not performed, the process proceeds to step 1005. In step 1005, it is determined whether or not the high element temperature control is performed. In the case of the high element temperature control, the process proceeds to step 1006, and the proportional control gain (lean side proportional gain) of the sub feedback control is made larger than usual as in step 1003. In this embodiment, the normal gain is changed from 0.05 to 0.1. In step 1007, the sensor output fluctuation is integrated.

  In the present embodiment, the proportional gain on the rich side or lean side is greatly changed according to the sensor high element temperature control, in order to bring out the respective gas reaction characteristics more remarkably. However, in order to improve the detectability, it is not always necessary to change each of these gains, and it is only necessary to increase the proportional gain of the sub-feedback control regardless of the temperature control when executing the diagnosis. Alternatively, the proportional gain of the sub feedback control may be changed so that only the reactivity on the rich side or the reactivity on the lean side is used.

  Next, referring to FIG. 19, the abnormality determination of the first oxygen sensor 25 will be described. Although the first oxygen sensor 25 has been described here, the same applies to the second oxygen sensor 26 as well.

  First, when step 1100 is activated at a predetermined timing, whether or not normal / abnormal of the first oxygen sensor 25 may be determined is determined in the subsequent step 1101. This is determined based on whether or not the sensor output fluctuation integration shown in FIG. 18 is performed for a predetermined time and whether or not each of the sensor high element temperature control and the low element temperature control is performed.

  If it is determined in step 1101 that the diagnosis determination condition is satisfied, the process proceeds to step 1102. In step 1102, a ratio pdloxs (= sdloxsl / sdloxsh) between the sensor output fluctuation integration (sdloxsl) at the time of sensor low element temperature control and the sensor output fluctuation integration (sdloxsh) at the time of high element temperature control is calculated. As a result, it is possible to eliminate deterioration with time such as catalyst deterioration and to stably determine sensor deterioration.

  Next, proceeding to step 1103, it is determined whether or not the sensor output fluctuation integration ratio pdloxs is equal to or less than a predetermined value. Here, if it is equal to or less than the predetermined value, it is determined that the reactivity of the sensor electrode at the low temperature and high temperature of the sensor element is impaired, and the process proceeds to Step 1104. In step 1104, a first oxygen sensor abnormality flag is set. On the other hand, if it is determined in step 1103 that the sensor output fluctuation integration ratio pdloxs is greater than the predetermined value, the process proceeds to step 1105. Then, the first oxygen sensor normal flag is set.

  In FIG. 18, the proportional gain of the sub-feedback control is changed at or above the stoichiometric (0.45 v) of the oxygen sensor, but in the modified example of FIG. 20, it is slightly rich (0.55 v) or less than the stoichiometric. The proportional gain is changed below lean (0.35v). As a result, it is possible to easily carry out normal determination when reacting richer or leaner than usual, and to prevent erroneous determination of abnormality. In the following, changes to FIG. 18 will be described.

  In this modification, when it is determined in step 1002 of FIG. 20 that the low element temperature control is being performed, the process proceeds to step 1020, and it is determined whether or not the first oxygen sensor output is greater than 0.55V. If it is determined that the voltage is greater than 0.55 V, the process proceeds to step 1003 and the same processing as in FIG. 18 is performed. On the other hand, if it is determined in step 1020 that the first oxygen sensor output is 0.55 V or less, the process proceeds to step 1021 where the rich proportional gain is set to 0.1 and the lean proportional gain is set to 0.05. Set and proceed to step 1004.

  Similarly, when it is determined in step 1005 that the high element temperature control is being performed, in the following step 1022, it is determined whether or not the first oxygen sensor output is less than 0.35V. If it is determined that the voltage is less than 0.35 V, the process proceeds to step 1006 and the same processing as in FIG. 18 is performed. On the other hand, when the first oxygen sensor output is 0.35 V or more, the routine proceeds to step 1023, where the rich proportional gain is set to 0.1 and the lean proportional gain is set to 0.05.

  Next, the operation of the second embodiment will be described with reference to the time chart of FIG.

  In FIG. 21, FIG. 21 (a) shows the vehicle speed. FIG. 21B shows the diagnostic effective conditions. FIG. 21C shows the element temperature control request, and FIG. 21D shows the element temperature. FIG. 21 (e) shows a proportional gain request for sub-feedback. FIG. 21 (f) shows a first oxygen sensor output at the time of deterioration, and FIG. 21 (g) shows a first oxygen sensor output at a normal time. FIG. 21 (h) shows the output integrated value sdloxsl during the low element temperature control, FIG. 21 (i) shows the output integrated value sdloxsh during the high element temperature control, and FIG. 21 (j) shows the output integrated ratio pdloxs. ing. FIG. 21 (k) shows an abnormality detection flag.

  In FIG. 21, the diagnosis execution condition is satisfied at time T1 when the acceleration travel is shifted to the steady travel, and the diagnosis execution permission flag is turned ON. At this time, sensor low element temperature control is required, and the sensor element temperature is lowered by setting a large target impedance (not shown) of the first oxygen sensor. As a result, the element temperature decreases to 400 ° C.

  Next, at time T2 when the element temperature is stabilized, the sub feedback gain request requests a high gain in order to set a large proportional gain of the sub feedback control. At this time, the oxygen sensor output reacts due to rich gas (CO), so the output increases. However, because the proportional gain is large, correction to the lean side (correction correction of the injection amount) works greatly, and the oxygen sensor output also reaches the lean side. Works great.

  Here, when the electrode of the oxygen sensor is deteriorated, the reactivity is decreased, so that the output of the oxygen sensor at the time of deterioration shown in the figure is shown. It will fluctuate much more like the oxygen sensor output. The oxygen sensor output fluctuations at this time are integrated, and the output integrated value at low temperature is calculated. In this manner, the O2 sensor output integration at the time of element low temperature is completed within a predetermined time from time T2 to T3, and then sensor high element temperature control is executed.

  However, since the diagnosis execution condition is not satisfied at time T4, the sensor high element temperature control is returned to the normal temperature control. After that, when the diagnosis execution condition is satisfied again at time T5, high element temperature control is started, and a request is made to increase the sub feedback gain at time T6 when the sensor element temperature is stable at a high temperature, and the proportional gain is set to be large. The

  Then, the integrated value of the oxygen sensor output fluctuation at the high temperature of the sensor element is calculated for a predetermined time from time T6 to T7. At time T7, since integrated values of oxygen sensor output fluctuations at the time of sensor element low temperature and high temperature are respectively calculated, the ratio of oxygen sensor output fluctuation integrated values at the time of sensor element low temperature and high temperature is calculated.

  When the sensor electrode is normal, the output fluctuation integration ratio becomes larger than a predetermined value, but when the electrode is deteriorated, the output fluctuation integration ratio becomes small. As described above, the deterioration of the sensor electrode can be detected by comparing the determination value stored in advance with the output fluctuation integration ratio.

  The above-described method detects a diagnosis by using sub-feedback control that corrects air-fuel ratio feedback control (hereinafter referred to as main feedback control) by a pre-catalyst air-fuel ratio sensor based on the oxygen sensor output after the catalyst. As an example, a method using main feedback control will be described with reference to FIG.

  In FIG. 22, the determination of the sensor element temperature control in step 1002 and step 1005 is the same as that shown in FIG. 18, but the target feedback ratio of the main feedback control is changed to increase the proportional gain of the sub feedback control. It has been replaced. That is, in steps 1030 and 1031, the target air-fuel ratio for main feedback control is set to a slightly rich (14.5), and in steps 1032 and 1033, the target air-fuel ratio for main feedback control is set to a slightly lean (14.7). Set.

  In this way, when the sensor element temperature is controlled to a low temperature, the reactivity is improved by the rich gas (CO), so that an effect is obtained by controlling the exhaust gas on the rich side. On the other hand, when the sensor element temperature is controlled to be high, the effect is improved by controlling the exhaust gas on the lean side.

  Here, the air-fuel ratio after the catalyst is set to be slightly rich in step 1031, and the air-fuel ratio after the catalyst is set to be weak lean in step 1033. By detecting the fluctuation of the O2 sensor due to the sub feedback control, We are conducting a diagnosis.

  However, the same effect can be obtained even when the sub-feedback control is stopped and a minute air-fuel ratio fluctuation is given every predetermined time in the main feedback control.

  As shown in FIG. 23, the integration of the sensor output fluctuation is greatly influenced by the fluctuation of the pre-catalyst air-fuel ratio. As described above, when the diagnosis is executed only in a stable operating state, it is not affected by the air-fuel ratio fluctuation before the catalyst, but this influence must be eliminated in order to improve the detection frequency.

  This embodiment will be described with reference to FIG. Since the basic configuration is the same as in FIG. 19, the description will focus on the differences. If it is determined in step 1101 that the diagnosis determination condition is satisfied, in step 1120, the ratio of the integration of the pre-catalyst air-fuel ratio fluctuation and the integration of the post-catalyst air-fuel ratio fluctuation (oxygen sensor output fluctuation) is determined as the sensor element low Calculation is performed for control and high temperature control. This eliminates the influence of the pre-catalyst air-fuel ratio fluctuation.

  In the next step 1121, the ratio kdloxsl of the integrated value of the pre-catalyst air-fuel ratio fluctuation and the integrated value of the post-catalyst air-fuel ratio fluctuation calculated at step 1120, the integrated value of the pre-catalyst air-fuel ratio fluctuation at the high temperature and the post-catalyst value. A ratio pdloxs (= kdloxsl / kdloxsh) with the ratio kdloxsh of the integrated values of air-fuel ratio fluctuations is calculated. Next, the routine proceeds to step 1103, where normality or abnormality of the first oxygen sensor is determined as described in FIG.

  Although the present invention has been described using the integrated value of the oxygen sensor output fluctuation, the diagnosis can be executed even with the change rate (ΔV), amplitude, and frequency of the O 2 sensor per time. However, the change rate per time (ΔV) has a characteristic that the reaction rate of the O 2 sensor increases as the air amount increases as shown in FIG. 25, and it is necessary to correct the change rate according to the air amount.

1 is a schematic configuration diagram of an embodiment of the present invention. It is a flowchart of the target air fuel ratio setting process of 1st Embodiment. It is a flowchart of the target air fuel ratio setting process of the modification in 1st Embodiment. It is a flowchart of the target output voltage setting process of the 1st oxygen sensor of the modification in 1st Embodiment. 6 is a map for setting a rich integration amount and a lean integration amount in the first embodiment. It is a map for setting the skip amount of the first embodiment. It is a schematic block diagram of the air fuel ratio and impedance detection apparatus of 1st Embodiment. It is a time chart at the time of impedance detection. It is an impedance characteristic view of an oxygen sensor. It is a flowchart of heater control of the oxygen sensor of the first embodiment. It is a block diagram of control of the element temperature of an oxygen sensor. It is a CO reaction characteristic figure of an oxygen sensor. It is a NO reaction characteristic view of an oxygen sensor. It is a flowchart of an oxygen sensor degradation detection process. It is a time chart which shows the action | operation at the time of oxygen sensor degradation detection. It is a characteristic view which shows an oxygen sensor deterioration detection principle. It is a characteristic view which shows the deterioration detection margin of an oxygen sensor. It is a flowchart performed by ECU of 2nd Embodiment. It is a flowchart which shows the deterioration detection process of the oxygen sensor of 2nd Embodiment. It is a flowchart performed by ECU of the modification of 2nd Embodiment. It is a time chart which shows the action | operation of 2nd Embodiment. It is a flowchart performed by ECU of the modification of 2nd Embodiment. It is a correlation diagram showing the relationship between the pre-catalyst air-fuel ratio fluctuation and the sensor output fluctuation integrated value. It is a flowchart performed by ECU of the modification of 2nd Embodiment. It is a correlation diagram which shows the relationship between intake air amount and sensor output fluctuation | variation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Engine 14 Air flow meter 16 Throttle opening sensor 18 Intake pipe pressure sensor 22 Upstream side catalyst 23 Downstream side catalyst 24 Linear air-fuel ratio sensor (limit current type air-fuel ratio sensor)
25 First oxygen sensor (oxygen sensor)
26 Second oxygen sensor (oxygen sensor)
27 Cooling water temperature sensor 28 Crank angle sensor 29 Engine control circuit (ECU)

Claims (2)

An air-fuel ratio detecting means installed downstream of the catalyst for treating exhaust gas from the engine and having an electrode disposed on the solid electrolyte element, and detecting the air-fuel ratio in the exhaust gas from the engine;
Temperature adjusting means for adjusting the temperature of the solid electrolyte element in the air-fuel ratio detecting means to a predetermined temperature;
Air-fuel ratio detection deterioration detection for detecting deterioration of the air-fuel ratio detection means by comparing output change rates of the air-fuel ratio detection means when the temperature of the solid electrolyte element is adjusted to at least two different temperatures by the temperature adjustment means And a deterioration detection device for an air-fuel ratio detection device.   The temperature adjusting means estimates the temperature of the solid electrolyte element by detecting an internal resistance of the air-fuel ratio detecting means, and adjusts the temperature of the solid electrolyte element based on the estimated temperature. Item 2. The deterioration detection device of the air-fuel ratio detection device according to Item 1.

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