JP2008038720A - Abnormality diagnosis device for downstream side oxygen sensor of exhaust emission control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make abnormality diagnosis of an oxygen sensor on a downstream side of a catalyst without deteriorating conversion rate of exhaust gas. <P>SOLUTION: The abnormality diagnosis device for the oxygen sensor judges heat quantity given to the oxygen sensor 33 on a downstream side of the catalyst 31 based on integrated value of heater heat generation quantity and exhaust gas heat quantity, judges whether heat quantity necessary for activation of the downstream side oxygen sensor 33 is given or not, and measures a period in which output of an air fuel ratio sensor 32 on an upstream side of the catalyst 31 is continuously kept under a rich condition (continuous rich period). When output of the downstream side oxygen sensor 33 does not changes to a rich condition even if it is judged that heat quantity necessary for activation of the downstream side oxygen sensor 33 is given and it is judged that the continuous rich period exceeds a delay period in which air fuel ratio on the downstream side of the catalyst 31 (air fuel ratio of exhaust gas flowing around the downstream side oxygen sensor 33) changes to a rich condition, abnormality of the downstream side oxygen sensor 33 is judged. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an abnormality diagnosis device for a downstream oxygen sensor of an exhaust gas purification system that performs abnormality diagnosis of an oxygen sensor installed downstream of an exhaust gas purification catalyst in an exhaust passage of an internal combustion engine.

In a recent vehicle exhaust gas purification system, for example, as described in Patent Document 1 (Japanese Patent Laid-Open No. 2-37146), an upstream side of an exhaust gas purification catalyst installed in an exhaust passage of an internal combustion engine; An air-fuel ratio sensor (A / F sensor) or an oxygen sensor (O ₂ sensor) that detects the air-fuel ratio or rich / lean of the exhaust gas is installed on the downstream side, and the upstream of the catalyst based on the output of the upstream sensor. Feedback control (main feedback control) near the stoichiometric air-fuel ratio and sub feedback control for correcting main feedback control based on the output of the downstream sensor, or downstream sensor Some of them are designed to diagnose the deterioration of the catalyst based on the output of the above.

  In general, an air-fuel ratio sensor is more expensive than an oxygen sensor. Therefore, even in a system that uses an air-fuel ratio sensor as an upstream sensor, the downstream sensor requires an inexpensive oxygen sensor to meet the demand for cost reduction. A sensor is used. Of course, there is a system using an inexpensive oxygen sensor as well as an upstream sensor.

  As is well known, an oxygen sensor or an air-fuel ratio sensor cannot accurately detect the air-fuel ratio or rich / lean of exhaust gas unless the temperature is raised to the activation temperature. The sensor element is activated at an early stage by the heat. Since the main / sub feedback control described above is performed after the sensor is activated, it is necessary to determine the activity. Since the air-fuel ratio sensor includes a circuit for detecting element resistance, it is possible to determine the activity by converting the detected value of the element resistance into the element temperature, but oxygen that outputs only binary values of rich / lean. Since the sensor does not include a circuit for detecting the element resistance, it is not possible to make an activity determination based on the element resistance.

The output of the oxygen sensor is always a lean output when it is not activated, and after activation, when the air-fuel ratio of the exhaust gas changes to a rich state, there is a characteristic that changes to a rich output. After a predetermined time has elapsed from the start of the engine, the fuel injection amount is increased to forcibly change the air-fuel ratio on the downstream side of the catalyst to rich, and the downstream oxygen sensor determines whether the output of the downstream oxygen sensor changes to rich or not. Whether the sensor is active / inactive is determined.
JP-A-2-37146 (page 13 etc.)

  If the technique of the above-mentioned patent document 1 is used, after a sufficient time has elapsed from the start of the engine until the downstream oxygen sensor is activated, the fuel injection amount is increased to forcibly make the air-fuel ratio downstream of the catalyst rich. If the output of the downstream oxygen sensor does not change richly even if it is changed, it can be determined that the downstream oxygen sensor is abnormally activated. However, in order to diagnose the abnormality of the downstream oxygen sensor, if the fuel injection amount is increased and the air-fuel ratio downstream of the catalyst is forcibly changed to rich, the amount of HC emissions into the atmosphere increases and the exhaust gas There was a drawback that the purification rate of the deteriorated.

  The present invention has been made in view of such circumstances, and therefore the object thereof is the downstream side of the exhaust gas purification system capable of performing abnormality diagnosis of the downstream oxygen sensor without deteriorating the exhaust gas purification rate. An object of the present invention is to provide an oxygen sensor abnormality diagnosis device.

  In order to achieve the above object, an invention according to claim 1 is an air-fuel ratio sensor (hereinafter referred to as “upstream side”) that detects an air-fuel ratio of exhaust gas upstream of an exhaust gas purification catalyst in an exhaust passage of an internal combustion engine. In an exhaust gas purification system in which an oxygen sensor (hereinafter referred to as a “downstream oxygen sensor”) is installed on the downstream side of the catalyst and an oxygen sensor (hereinafter referred to as “downstream oxygen sensor”) is installed on the downstream side of the catalyst. The amount of heat given to the sensor is determined by the supply heat amount determination means, and the period during which the output of the upstream air-fuel ratio sensor is continuously rich (hereinafter referred to as “continuous rich period”) is measured by the continuous rich period measurement means. As a result, it is determined that the amount of heat necessary for the activation of the downstream oxygen sensor is given, and the continuous rich period measured by the continuous rich period measuring means is the downstream Even if it is determined that the delay period until the air-fuel ratio of the exhaust gas flowing around the oxygen sensor changes to the rich state has been exceeded, the abnormality of the downstream oxygen sensor occurs when the output of the downstream oxygen sensor does not change to the rich state. Judgment is made.

  In short, even in a normal operating state, if the operating state in which the output of the upstream air-fuel ratio sensor (the air-fuel ratio upstream of the catalyst) continuously becomes rich to some extent continues, the catalyst eventually purifies the rich components of the exhaust gas. The air-fuel ratio on the downstream side of the catalyst (the air-fuel ratio of the exhaust gas flowing around the downstream oxygen sensor) changes to a rich state. The present invention performs an abnormality diagnosis of the downstream oxygen sensor using such an operating state, is provided with a heat amount necessary for activation of the downstream oxygen sensor, and outputs the upstream air-fuel ratio sensor. Even when it is determined that the air-fuel ratio on the downstream side of the catalyst has changed to the rich state after the operation state in which the (air-fuel ratio on the upstream side of the catalyst) is continuously rich to some extent continues, When the output does not change from the lean state to the rich state, the downstream oxygen sensor is determined to be abnormal. In this way, it is possible to perform abnormality diagnosis of the downstream oxygen sensor without forcibly changing the air-fuel ratio on the downstream side of the catalyst to rich as in the prior art. It is possible to avoid the deterioration of the exhaust gas purification rate due to.

  In this case, when the operation state in which the output of the upstream air-fuel ratio sensor (the air-fuel ratio upstream of the catalyst) is in a rich state continues, the delay period until the air-fuel ratio on the downstream side of the catalyst changes to the rich state is exhausted. Changes depending on the rich component concentration of gas (rich component supply amount per unit time to the catalyst), the rich component adsorption state of the catalyst at the start of measurement, the flow delay of exhaust gas from the upstream air-fuel ratio sensor to the downstream oxygen sensor, etc. To do.

  In a system using an air-fuel ratio sensor as a sensor upstream of the catalyst, the rich component concentration of the exhaust gas upstream of the catalyst can be detected from the output of the upstream air-fuel ratio sensor, so the rich component integration of the output of the upstream air-fuel ratio sensor The value is data correlating with the supply amount of the rich component to the catalyst.

  In consideration of this characteristic, the rich component of the output of the upstream air-fuel ratio sensor is integrated as in claim 2, and the measurement of the continuous rich period is started after the rich component integrated value exceeds a predetermined value. It is good to make it. In this way, measurement of the delay period (continuous rich period) can be started after supplying a substantially constant amount of rich component to the catalyst, and the rich component adsorption state of the catalyst at the start of measurement is made substantially constant. be able to.

  On the other hand, when the present invention is applied to a system using an oxygen sensor as a sensor on the upstream side of the catalyst, the output of the oxygen sensor on the upstream side of the catalyst (hereinafter referred to as “upstream oxygen sensor”) is continuously output as in claim 3. And a continuous rich period measuring means for measuring a period of time during which the engine is in a rich state (hereinafter referred to as “continuous rich period”). Even if it is determined that the continuous rich period measured by the means exceeds the delay period, the downstream oxygen sensor may be determined to be abnormal when the output of the downstream oxygen sensor does not change to the rich state.

  Although the upstream oxygen sensor cannot detect the rich component concentration of the exhaust gas upstream of the catalyst, it performs air-fuel ratio feedback control based on the output of the upstream oxygen sensor, so correction to the lean side by this air-fuel ratio feedback control The amount is information reflecting the duration of the rich state. That is, it is considered that the amount of correction to the lean side by air-fuel ratio feedback control increases as the duration of the rich state increases.

  In consideration of this characteristic, as in claim 4, the correction amount to the lean side by the air-fuel ratio feedback control based on the output of the upstream oxygen sensor is integrated, and the integrated value of the correction amount to the lean side is a predetermined value. It is preferable to start measurement of the continuous rich period after exceeding. In this way, similarly to the second aspect, the measurement of the delay period (continuous rich period) can be started after the rich component adsorption state of the catalyst becomes substantially constant.

  In the inventions according to claims 1 to 4 described above, the delay period may be set to a fixed period in advance by an adaptation process or the like. In this case, a longer delay period is set in order to prevent misdiagnosis. As a result, the frequency of the abnormality diagnosis of the downstream oxygen sensor may decrease, and the detection of the abnormality of the downstream oxygen sensor may be delayed.

  As a countermeasure, the delay period is changed by taking into account that the exhaust gas flow rate (lean component supply amount per unit time to the catalyst and the exhaust gas flow rate) changes according to the operating state of the internal combustion engine, and the delay period changes. As shown in FIG. 5, the delay period may be changed according to the operating state of the internal combustion engine. In this way, since the delay period can be appropriately changed in response to the change in the exhaust gas flow rate according to the operating state of the internal combustion engine, the minimum delay required according to the operating state of the internal combustion engine. The period can be set, the frequency of the abnormality diagnosis of the downstream oxygen sensor can be increased, and the abnormality can be detected early when the abnormality of the downstream oxygen sensor occurs.

  By the way, if the downstream oxygen sensor is activated even after the continuous rich period exceeds the delay period after starting the internal combustion engine, the air-fuel ratio on the downstream side of the catalyst (exhaust gas flowing around the downstream oxygen sensor). It is normal for the output of the downstream oxygen sensor to change from the lean state to the rich state at the time when the air-fuel ratio) changes to the rich state.

  Considering this point, the output of the downstream oxygen sensor changes from the lean state to the rich state even after the continuous rich period exceeds the delay period after the internal combustion engine is started. When this is detected, it may be determined that the downstream oxygen sensor is normally activated. In this way, it is possible to determine the normal activity of the downstream oxygen sensor even if the operation state in which the continuous rich period does not exceed the delay period continues for a long time, and the normal activity determination of the downstream oxygen sensor is delayed. It is possible to avoid delaying the start timing of sub-feedback control based on the output of the downstream oxygen sensor or catalyst deterioration diagnosis.

  Hereinafter, two Examples 1 and 2, which embody the best mode for carrying out the present invention, will be described.

A first embodiment of the present invention will be described with reference to FIGS.
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 11 that is an internal combustion engine, and an air flow meter 14 that detects the intake air amount is provided downstream of the air cleaner 13. A throttle valve 16 whose opening is adjusted by a motor 15 and a throttle opening sensor 17 for detecting the opening (throttle opening) of the throttle valve 16 are provided on the downstream side of the air flow meter 14.

  Further, a surge tank 18 is provided on the downstream side of the throttle valve 16, and an intake pipe pressure sensor 19 for detecting the intake pipe pressure is provided in the surge tank 18. The surge tank 18 is provided with an intake manifold 20 for introducing air into each cylinder of the engine 11, and a fuel injection valve 21 for injecting fuel is attached in the vicinity of the intake port of the intake manifold 20 of each cylinder. Yes. An ignition plug 22 is attached to the cylinder head of the engine 11 for each cylinder, and the air-fuel mixture in the cylinder is ignited by spark discharge of each ignition plug 22.

  A cooling water temperature sensor 26 that detects the cooling water temperature and a crank angle sensor 28 that outputs a pulse signal each time the crankshaft 27 of the engine 11 rotates a predetermined crank angle are attached to the cylinder block of the engine 11. Based on the output signal of the crank angle sensor 28, the crank angle and the engine speed are detected.

  On the other hand, the exhaust pipe 30 (exhaust passage) of the engine 11 is provided with a catalyst 31 such as a three-way catalyst that purifies the exhaust gas, and an air-fuel ratio sensor that detects the air-fuel ratio of the exhaust gas upstream of the catalyst 31. (Hereinafter referred to as “upstream air-fuel ratio sensor”) 32 is installed, and further, an oxygen sensor (hereinafter referred to as “downstream oxygen sensor”) 33 for detecting rich / lean exhaust gas is installed downstream of the catalyst 31. Has been. Each of the upstream air-fuel ratio sensor 32 and the downstream oxygen sensor 33 includes a heater (not shown) that heats the sensor element.

  Outputs of these various sensors are input to a control circuit (hereinafter referred to as “ECU”) 34. The ECU 34 is mainly composed of a microcomputer, and executes various engine control programs stored in a built-in ROM (storage medium), so that the fuel injection amount of the fuel injection valve 21 according to the engine operating state and the like. The ignition timing of the spark plug 22 is controlled.

  At that time, the ECU 34 executes an air-fuel ratio feedback control program (not shown) to set the air-fuel ratio of the exhaust gas upstream of the catalyst 31 based on the output of the upstream air-fuel ratio sensor 32 to the target air-fuel ratio (within the purification window of the catalyst 31). The fuel injection amount is feedback controlled (main feedback control) so that the target air fuel ratio of the main feedback control is corrected according to the output of the downstream oxygen sensor 33 (the air fuel ratio downstream of the catalyst 31). Feedback control is implemented. Alternatively, the deterioration diagnosis of the catalyst 31 may be performed based on the output of the downstream oxygen sensor 33.

  Further, the ECU 34 executes an abnormality diagnosis of the downstream oxygen sensor 33 as follows by executing a downstream oxygen sensor abnormality diagnosis program shown in FIGS.

  Even in a normal engine operation state, if the operation state in which the output of the upstream air-fuel ratio sensor 32 (the air-fuel ratio upstream of the catalyst 31) continuously becomes rich to some extent continues, the catalyst 31 eventually purifies the rich component of the exhaust gas. As a result, the air-fuel ratio downstream of the catalyst 31 (the air-fuel ratio of the exhaust gas flowing around the downstream oxygen sensor 33) changes to a rich state. Further, the output of the downstream oxygen sensor 33 is always a lean output when it is not activated, and after activation, when the air-fuel ratio downstream of the catalyst 31 changes to a rich state, it has a characteristic of changing to a rich output.

  In consideration of such circumstances, in the first embodiment, the downstream oxygen sensor is utilized using an operation state in which the output of the upstream air-fuel ratio sensor 32 (the air-fuel ratio upstream of the catalyst 31) is continuously rich. The abnormality diagnosis of 33 is performed as follows. First, the amount of heat given to the downstream oxygen sensor 33 is determined based on the integrated value of the heater heat generation amount and the exhaust heat amount (exhaust gas temperature), and whether or not the amount of heat necessary to activate the downstream oxygen sensor 33 has been given. And a period during which the output of the upstream air-fuel ratio sensor 32 is continuously rich (hereinafter referred to as “continuous rich period”) is measured. Then, it is determined that the amount of heat necessary for the activation of the downstream oxygen sensor 33 is given, and the air rich ratio in the downstream of the catalyst 31 (the air fuel ratio of the exhaust gas flowing around the downstream oxygen sensor 33) is rich during the continuous rich period. Even if it is determined that the delay period until the state changes is reached, when the output of the downstream oxygen sensor 33 does not change to the rich state, it is determined that the downstream oxygen sensor 33 is abnormal.

  In this case, when the operation state in which the output of the upstream air-fuel ratio sensor 32 (the air-fuel ratio upstream of the catalyst 31) is in the rich state continues, the delay period until the air-fuel ratio downstream of the catalyst 31 changes to the rich state. Is the rich component concentration of exhaust gas (rich component supply amount per unit time to the catalyst 31), the rich component adsorption state of the catalyst 31 at the start of delay period measurement, from the upstream air-fuel ratio sensor 32 to the downstream oxygen sensor 33 It varies depending on the flow delay of exhaust gas.

  Therefore, in the first embodiment, considering that the rich component concentration of the exhaust gas upstream of the catalyst 31 can be detected from the output of the upstream air-fuel ratio sensor 32, the rich component integrated value of the output of the upstream air-fuel ratio sensor 32 is The measurement of the continuous rich period is started after exceeding a predetermined value. In this way, measurement of the delay period (continuous rich period) can be started after the rich component adsorption state of the catalyst 31 becomes substantially constant.

  In the present invention, the delay period may be set to a certain period in advance by an adaptation process or the like, but in this case, it is necessary to set a longer delay period in order to prevent misdiagnosis. There is a possibility that the frequency of performing the abnormality diagnosis of the side oxygen sensor 33 decreases, and the detection of the abnormality of the downstream oxygen sensor 33 is delayed.

  As a countermeasure against this, in the first embodiment, it is considered that the exhaust gas flow rate (lean component supply amount per unit time to the catalyst 31 and the exhaust gas flow rate) changes according to the engine operating state and the delay period changes. Thus, the delay period is changed according to the engine operating state (engine speed, load, etc.). In this way, since the delay period can be appropriately changed in response to the change in the exhaust gas flow rate according to the engine operating state, the minimum delay period is set according to the engine operating state. Therefore, the frequency of abnormality diagnosis of the downstream oxygen sensor 33 can be increased, and the abnormality can be detected at an early stage when the abnormality of the downstream oxygen sensor 33 occurs.

  By the way, if the downstream oxygen sensor 33 is activated even after the continuous rich period exceeds the delay period after the engine is started, the air-fuel ratio on the downstream side of the catalyst 31 (the exhaust gas flowing around the downstream oxygen sensor 33). It is normal for the output of the downstream oxygen sensor 33 to change from the lean state to the rich state when the gas air-fuel ratio) changes to the rich state.

  In consideration of this point, in the first embodiment, the output of the downstream oxygen sensor 33 has changed from the lean state to the rich state even after the engine has started and before the continuous rich period exceeds the delay period. At the time of detection, it is determined that the downstream oxygen sensor 33 is normally activated. In this way, it is possible to determine the normal activity of the downstream oxygen sensor 33 even if the operation state in which the continuous rich period does not exceed the delay period continues for a long time, and the normal activity determination of the downstream oxygen sensor 33 can be performed. It is possible to avoid delaying the start timing of the sub feedback control based on the output of the downstream oxygen sensor 33 and the deterioration diagnosis of the catalyst 31 with a delay.

  The abnormality diagnosis of the downstream oxygen sensor 33 according to the first embodiment described above is executed by the ECU 34 in accordance with the downstream oxygen sensor abnormality diagnosis program shown in FIGS. This program is executed at a predetermined cycle (for example, 128 ms cycle) during engine operation. When this program is started, first, in step 101, the heater heat amount H applied to the sensor element of the downstream oxygen sensor 33 is calculated based on the heater supply power of the downstream oxygen sensor 33. At this time, the heater duty and the battery voltage are used as information on the heater supply power of the downstream oxygen sensor 33, and the heat quantity H of the heater is determined according to the current heater duty and the battery voltage with reference to a map using these parameters as parameters. calculate.

  Thereafter, the routine proceeds to step 102 where the heat quantity E of the exhaust gas given to the sensor element of the downstream oxygen sensor 33 is calculated based on the current engine operating state. At this time, the engine speed and load are used as information on the engine operating state, and a map using these parameters as parameters is given to the sensor element of the downstream oxygen sensor 33 according to the current engine speed and load. The amount of heat E of the exhaust gas is calculated.

In the next step 103, the heat quantity H and E calculated in steps 101 and 102 are added to the previous heat quantity integrated value I given to the sensor element of the downstream oxygen sensor 33, and the heat quantity integrated value I is updated. To do.
I = I + H + E
The processing of steps 101 to 103 described above plays a role as supply heat amount determination means in the claims.

  Thereafter, the routine proceeds to step 104, where it is determined whether or not the output L (excess air ratio) of the upstream air-fuel ratio sensor 32 is in a rich state (L <1). In this case, the output L of the upstream air-fuel ratio sensor 32 becomes a value corresponding to the excess air ratio (= actual air-fuel ratio / theoretical air-fuel ratio), and the output L of the upstream air-fuel ratio sensor 32 is in a lean state (L ≧ 1). If so, the routine proceeds to step 105, where the integrated value RI of the rich component (1-L) of the output L of the upstream air-fuel ratio sensor 32 is reset to 0, and the output L of the upstream air-fuel ratio sensor 32 is continuously increased. A continuous rich period counter ICT that measures a period in which a rich state is set (hereinafter referred to as “continuous rich period”) is reset to 0, and this program ends.

On the other hand, if it is determined in step 104 that the output L (excess air ratio) of the upstream air-fuel ratio sensor 32 is in a rich state (L <1), the routine proceeds to step 106, where the upstream air-fuel ratio sensor is detected. The integrated value RI of the rich component (1-L) of 32 outputs L is updated.
RI = RI + (1-L)

Thereafter, the routine proceeds to step 107, where the delay period D is calculated based on the current engine operating state. At this time, the engine rotation speed and load are used as information on the engine operating state, and a delay period D is calculated according to the current engine rotation speed and load with reference to a map using these as parameters. Thereafter, the process proceeds to step 108, and the delay period DSM is updated by the annealing process using the previous delay period DSM and the current delay period D.
DSM = DSM × 0.9 + D × 0.1
Needless to say, the annealing coefficient may be changed as appropriate.

  Thereafter, the process proceeds to step 109 in FIG. 3 to determine whether or not the heat amount integrated value I given to the downstream oxygen sensor 33 exceeds a predetermined value It. If the heat amount integrated value I is equal to or less than the predetermined value It, It is determined that the downstream oxygen sensor 33 has not yet been activated, and the routine proceeds to step 112 where the continuous rich period counter ICT is reset to zero.

  On the other hand, if it is determined in step 109 that the integrated heat value I exceeds the predetermined value It, it is determined that the downstream oxygen sensor 33 is activated, and the process proceeds to step 110, where the upstream air-fuel ratio sensor is determined. It is determined whether or not the rich component integrated value RI of the output L of 32 has exceeded a predetermined value RIt. If the rich component integrated value RI is not more than the predetermined value RIt, the rich component supplied to the catalyst 31 has reached a predetermined amount. If not, the process proceeds to step 112, and the continuous rich period counter ICT is reset to zero.

  Thereafter, when the rich component integrated value RI of the output L of the upstream side air-fuel ratio sensor 32 exceeds the predetermined value RIt, “Yes” is determined in step 110, and the process proceeds to step 111, where the continuous rich period counter ICT is set. Counts up for a time corresponding to the program execution cycle (128 ms). Thus, when the rich component integrated value RI of the output L of the upstream air-fuel ratio sensor 32 exceeds the predetermined value RIt, the count-up operation of the continuous rich period counter ICT is started, and the output of the upstream air-fuel ratio sensor 32 is started. A continuous rich period in which L is continuously rich is measured. The processing of step 111 serves as a continuous rich period measuring means in the claims.

  Thereafter, the process proceeds to step 113 to determine whether or not the output of the downstream oxygen sensor 33 is in a rich state. If the output of the downstream oxygen sensor 33 is in a rich state, the process proceeds to step 114 and the normal determination flag is set. It is determined that the downstream oxygen sensor 33 is normally activated by setting it to “1”.

  On the other hand, if it is determined in step 113 that the output of the downstream oxygen sensor 33 is not in the rich state, the process proceeds to step 115, where it is determined whether or not the continuous rich period counted by the continuous rich period counter ICT exceeds the delay period DSM. If the continuous rich period does not exceed the delay period DSM, the program is terminated. If the continuous rich period exceeds the delay period DSM, the process proceeds to step 116 and the abnormality determination flag is set to “1”. Thus, it is determined that the downstream oxygen sensor 33 is abnormal. The processes in steps 113 to 116 described above serve as abnormality diagnosis means in the claims.

An example of the abnormality diagnosis process of the downstream oxygen sensor 33 of the first embodiment described above will be described with reference to the time chart of FIG.
After the engine is started, the continuous rich period is maintained even if the rich component integrated value RI of the output L of the upstream air-fuel ratio sensor 32 increases until the calorie integrated value I given to the downstream oxygen sensor 33 exceeds the predetermined value It. The value of the counter ICT is maintained at 0. Thereafter, after the time t1 when the calorific value integrated value I given to the downstream oxygen sensor 33 exceeds the predetermined value It, until the rich component integrated value RI of the output L of the upstream air-fuel ratio sensor 32 exceeds the predetermined value RIt. The value of the continuous rich period counter ICT is maintained at 0.

  Thereafter, at the time t2 when the rich component integrated value RI of the output L of the upstream air-fuel ratio sensor 32 exceeds a predetermined value RIt, the count-up operation of the continuous rich period counter ICT is started, and the output of the upstream air-fuel ratio sensor 32 is started. A continuous rich period in which L is continuously rich is measured. If the output of the downstream oxygen sensor 33 is not in the rich state at the time t4 when the count value (continuous rich period) of the continuous rich period counter ICT exceeds the delay period DSM, the abnormality determination flag is set to “1”. It is determined that the downstream oxygen sensor 33 is abnormal.

  Even before the time t4 when the count value of the continuous rich period counter ICT (continuous rich period) exceeds the delay period DSM, the output of the downstream oxygen sensor 33 exceeds the rich / lean determination value and changes to the rich state. At the time t3 when this is detected, the normality determination flag is set to "1", and it is determined that the downstream oxygen sensor 33 is normally activated.

  According to the first embodiment described above, the abnormality diagnosis of the downstream oxygen sensor 33 is performed using the operation state in which the output of the upstream air-fuel ratio sensor 32 (the air-fuel ratio upstream of the catalyst 31) is continuously rich. Therefore, it is possible to perform abnormality diagnosis of the downstream oxygen sensor 33 without forcibly changing the air-fuel ratio downstream of the catalyst 31 to rich as in the prior art, and the downstream oxygen sensor The deterioration of the exhaust gas purification rate due to the 33 abnormality diagnosis can be avoided.

  Moreover, in the first embodiment, it is considered that the delay period DSM changes due to the change in the exhaust gas flow rate (the lean component supply amount per unit time and the flow rate of the exhaust gas) according to the engine operating state. Since the delay period DSM is changed according to the engine operating state (engine speed, load), the delay period DSM is appropriately changed corresponding to the change of the exhaust gas flow rate according to the engine operating state. Can be made. As a result, the minimum required delay period DSM can be set according to the engine operating state, the frequency of performing the abnormality diagnosis of the downstream oxygen sensor 33 can be increased, and when an abnormality occurs in the downstream oxygen sensor 33 The abnormality can be detected early.

  Further, in the first embodiment, the output of the downstream oxygen sensor 33 changes from the lean state to the rich state even before the count value of the continuous rich period counter ICT (continuous rich period) exceeds the delay period DSM after the engine is started. Since it is determined that the downstream oxygen sensor 33 is normally activated when it is detected that the change has occurred, the count value (continuous rich period) of the continuous rich period counter ICT is equal to the delay period DSM after the engine is started. Sub-feedback control based on the output of the downstream oxygen sensor 33 after the normal activity determination of the downstream oxygen sensor 33 is delayed and the normal activity determination of the downstream oxygen sensor 33 is delayed. In addition, it is possible to avoid delaying the start timing of the deterioration diagnosis of the catalyst 31.

  In the first embodiment, the air-fuel ratio sensor 32 is installed on the upstream side of the catalyst 31, but in the second embodiment of the present invention shown in FIGS. 5 to 7, the rich / lean exhaust gas is provided on the upstream side of the catalyst 31. An oxygen sensor to detect is installed. An oxygen sensor 33 is set on the downstream side of the catalyst 31 as in the first embodiment. Other configurations are the same as those of the first embodiment.

  In the second embodiment, a period during which the output of the oxygen sensor upstream of the catalyst 31 (hereinafter referred to as “upstream oxygen sensor”) is continuously rich (hereinafter referred to as “continuous rich period”) is determined by the continuous rich period counter ICT. Even if it is determined that the amount of heat necessary for activation of the downstream oxygen sensor 33 is measured and the count value (continuous rich period) of the continuous rich period counter ICT exceeds the delay period DSM, the downstream oxygen sensor When the output of 33 does not change to the rich state, it is determined that the downstream oxygen sensor 33 is abnormal.

  Although the upstream oxygen sensor cannot detect the rich component concentration of the exhaust gas upstream of the catalyst 31, as shown in FIG. 7, the air-fuel ratio feedback control (hereinafter referred to as “feedback” is set to “F” based on the output of the upstream oxygen sensor. / B "), and the F / B correction coefficient F is changed according to the rich / lean change in the output of the upstream oxygen sensor, so that the F / B correction amount to the lean side (1-F) Is information reflecting the duration of the rich state. In other words, it is considered that the F / B correction amount (1-F) toward the lean side increases as the duration of the rich state increases.

  Taking this characteristic into consideration, as in the second embodiment, the F / B correction amount (1-F) to the lean side by the air-fuel ratio F / B control based on the output of the upstream oxygen sensor is integrated, and the lean The count-up operation of the continuous rich period counter ICT (measurement of the continuous rich period) is started after the integrated value FI of the F / B correction amount (1-F) to the side exceeds the predetermined value FIt. In this way, as in the first embodiment, measurement of the delay period (continuous rich period) can be started after the rich component adsorption state of the catalyst 31 becomes substantially constant.

  The abnormality diagnosis of the downstream oxygen sensor 33 according to the second embodiment described above is executed by the ECU 34 in accordance with the downstream oxygen sensor abnormality diagnosis program shown in FIGS. The downstream oxygen sensor abnormality diagnosis program of FIGS. 5 and 6 is obtained by changing the processing of steps 104 to 106 and 110 of the program of FIGS. 2 and 3 to the processing of steps 104a to 106a and 110a. The processing of the steps is the same.

  In the downstream oxygen sensor abnormality diagnosis program of FIGS. 5 and 6, in steps 101 to 103, the integrated heat value I given to the sensor element of the downstream oxygen sensor 33 is calculated by the same method as in the first embodiment. Thereafter, the process proceeds to step 104a, where it is determined whether or not the output of the upstream oxygen sensor is in a rich state. If the output of the upstream oxygen sensor is in a lean state, the process proceeds to step 105a and the output of the upstream oxygen sensor. The integrated value FI of the F / B correction amount (1-F) to the lean side by the air-fuel ratio F / B control based on the above is reset to 0, and the continuous rich period counter ICT is reset to 0, and this program ends. To do.

On the other hand, if it is determined in step 104a that the output of the upstream oxygen sensor is in a rich state, the process proceeds to step 106a, where the air-fuel ratio F / B control based on the output of the upstream oxygen sensor goes to the lean side. The integrated value FI of the F / B correction amount (1-F) is updated.
FI = FI + (1-F)

  Thereafter, in steps 107 and 108, the delay period DSM is calculated by the annealing process in the same manner as in the first embodiment, and then the process proceeds to step 109 in FIG. 6 to integrate the amount of heat given to the downstream oxygen sensor 33. It is determined whether or not the value I exceeds the predetermined value It. If the calorific value integrated value I is equal to or less than the predetermined value It, it is determined that the downstream oxygen sensor 33 has not yet been activated, and the routine proceeds to step 112, where The rich period counter ICT is reset to zero.

  On the other hand, if it is determined in step 109 that the heat integrated value I exceeds the predetermined value It, it is determined that the downstream oxygen sensor 33 is activated, and the process proceeds to step 110a, where the F to the lean side is determined. It is determined whether or not the / B correction amount integrated value FI exceeds the predetermined value FIt. If the F / B correction amount integrated value FI to the lean side is equal to or less than the predetermined value FIt, a rich component to be supplied to the catalyst 31 is present. It is determined that the fixed amount has not been reached, and the routine proceeds to step 112 where the continuous rich period counter ICT is reset to zero.

  Thereafter, when the F / B correction amount integrated value FI to the lean side exceeds the predetermined value FIt, “Yes” is determined in Step 110a, and the process proceeds to Step 111, where the continuous rich period counter ICT is executed. Counts up for a time corresponding to the period (128 ms). As a result, when the F / B correction amount integrated value FI to the lean side exceeds the predetermined value FIt, the count-up operation of the continuous rich period counter ICT is started, and the output L of the upstream oxygen sensor is continuously increased. A continuous rich period in which the rich state is reached is measured.

  Thereafter, the process proceeds to step 113 to determine whether or not the output of the downstream oxygen sensor 33 is in a rich state. If the output of the downstream oxygen sensor 33 is in a rich state, the process proceeds to step 114 and the normal determination flag is set. It is determined that the downstream oxygen sensor 33 is normally activated by setting it to “1”.

  On the other hand, if it is determined in step 113 that the output of the downstream oxygen sensor 33 is not in the rich state, the process proceeds to step 115, where it is determined whether or not the continuous rich period counted by the continuous rich period counter ICT exceeds the delay period DSM. If the continuous rich period does not exceed the delay period DSM, the program is terminated. If the continuous rich period exceeds the delay period DSM, the process proceeds to step 116 and the abnormality determination flag is set to “1”. Thus, it is determined that the downstream oxygen sensor 33 is abnormal.

An example of the abnormality diagnosis process of the downstream oxygen sensor 33 of the second embodiment described above will be described with reference to the time chart of FIG.
After the engine is started, the F / B correction amount to the lean side by the air-fuel ratio F / B control based on the output of the upstream oxygen sensor until the heat amount integrated value I given to the downstream oxygen sensor 33 exceeds the predetermined value It Even if the integrated value FI of (1-F) increases, the value of the continuous rich period counter ICT is maintained at 0. Thereafter, after the time t1 when the calorie integrated value I given to the downstream oxygen sensor 33 exceeds the predetermined value It, F / B correction to the lean side by the air-fuel ratio F / B control based on the output of the upstream oxygen sensor. Until the integrated value FI of the quantity (1-F) exceeds the predetermined value FIt, the value of the continuous rich period counter ICT is maintained at zero.

  Thereafter, at the time t2 when the integrated value FI of the lean side F / B correction amount (1-F) exceeds the predetermined value FIt, the count-up operation of the continuous rich period counter ICT is started, and the upstream oxygen sensor The continuous rich period during which the output is continuously rich is measured. If the output of the downstream oxygen sensor 33 is not in the rich state at the time t4 when the count value (continuous rich period) of the continuous rich period counter ICT exceeds the delay period DSM, the abnormality determination flag is set to “1”. It is determined that the downstream oxygen sensor 33 is abnormal.

  Even before the time t4 when the count value of the continuous rich period counter ICT (continuous rich period) exceeds the delay period DSM, the output of the downstream oxygen sensor 33 exceeds the rich / lean determination value and changes to the rich state. At the time t3 when this is detected, the normality determination flag is set to "1", and it is determined that the downstream oxygen sensor 33 is normally activated.

  According to the second embodiment described above, in the system in which the oxygen sensor is installed on the upstream side of the catalyst 31, the operating state in which the output of the upstream oxygen sensor (the air-fuel ratio on the upstream side of the catalyst 31) is continuously rich. Thus, the abnormality diagnosis of the downstream oxygen sensor 33 can be performed without forcibly changing the air-fuel ratio downstream of the catalyst 31 to rich as in the prior art. Therefore, it is possible to avoid the deterioration of the exhaust gas purification rate due to the abnormality diagnosis of the downstream oxygen sensor 33. In addition, the same effects as those of the first embodiment can be obtained.

It is a schematic block diagram of the whole engine control system in Example 1 of this invention. It is the flowchart (the 1) which shows the flow of a process of the downstream oxygen sensor abnormality diagnosis program of Example 1. It is a flowchart (the 2) which shows the flow of a process of the downstream oxygen sensor abnormality diagnosis program of Example 1. 3 is a time chart illustrating an example of a downstream oxygen sensor abnormality diagnosis process according to the first embodiment. It is a flowchart (the 1) which shows the flow of a process of the downstream oxygen sensor abnormality diagnosis program of Example 2. It is a flowchart (the 2) which shows the flow of a process of the downstream oxygen sensor abnormality diagnosis program of Example 2. 6 is a time chart illustrating an example of a downstream oxygen sensor abnormality diagnosis process according to a second embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Engine (internal combustion engine), 12 ... Intake pipe, 16 ... Throttle valve, 21 ... Fuel injection valve, 22 ... Spark plug, 30 ... Exhaust pipe (exhaust passage), 31 ... Catalyst, 32 ... Upstream air-fuel ratio sensor, 33 ... downstream oxygen sensor, 34 ... ECU (supplied heat quantity determination means, continuous rich period measurement means, abnormality diagnosis means)

Claims (6)

An air-fuel ratio sensor (hereinafter referred to as an “upstream air-fuel ratio sensor”) for detecting the air-fuel ratio of exhaust gas is installed upstream of the exhaust gas purification catalyst in the exhaust passage of the internal combustion engine, and downstream of the catalyst In an exhaust gas purification system in which an oxygen sensor for detecting rich / lean exhaust gas (hereinafter referred to as “downstream oxygen sensor”) is installed,
A supply heat amount determination means for determining the amount of heat given to the downstream oxygen sensor during operation of the internal combustion engine;
Continuous rich period measuring means for measuring a period in which the output of the upstream air-fuel ratio sensor is continuously rich (hereinafter referred to as “continuous rich period”);
Based on the determination result of the supply heat amount determination means, it is determined that the amount of heat necessary for activation of the downstream oxygen sensor is given, and the continuous rich period measured by the continuous rich period measurement means is the periphery of the downstream oxygen sensor If the output of the downstream oxygen sensor does not change to the rich state even if it is determined that the delay period until the air-fuel ratio of the exhaust gas flowing through the gas changes to the rich state is determined, it is determined that the downstream oxygen sensor is abnormal. An abnormality diagnosis device for an oxygen sensor downstream of an exhaust gas purification system, comprising: an abnormality diagnosis means.   The continuous rich period measuring means integrates rich components of the output of the upstream air-fuel ratio sensor, and starts measuring the continuous rich period after the rich component integrated value exceeds a predetermined value. Item 6. An abnormality diagnosis device for an oxygen sensor downstream of the exhaust gas purification system according to Item 1. An exhaust gas purification system in which oxygen sensors for detecting rich / lean exhaust gas are installed on an upstream side and a downstream side of an exhaust gas purification catalyst in an exhaust passage of an internal combustion engine, respectively, during operation of the internal combustion engine Supply heat amount determination means for determining the amount of heat given to the oxygen sensor on the downstream side of the catalyst (hereinafter referred to as “downstream oxygen sensor”);
Continuous rich period measuring means for measuring a period during which the output of the oxygen sensor upstream of the catalyst (hereinafter referred to as “upstream oxygen sensor”) is continuously rich (hereinafter referred to as “continuous rich period”);
Based on the determination result of the supply heat amount determination means, it is determined that the amount of heat necessary for activation of the downstream oxygen sensor is given, and the continuous rich period measured by the continuous rich period measurement means is the periphery of the downstream oxygen sensor If the output of the downstream oxygen sensor does not change to the rich state even if it is determined that the delay period until the air-fuel ratio of the exhaust gas flowing through the gas changes to the rich state is determined, it is determined that the downstream oxygen sensor is abnormal. An abnormality diagnosis device for an oxygen sensor downstream of an exhaust gas purification system, comprising: an abnormality diagnosis means.   The continuous rich period measuring means integrates the correction amount to the lean side by the air-fuel ratio feedback control based on the output of the upstream oxygen sensor, and the integrated value of the correction amount to the lean side exceeds the predetermined value and then The abnormality diagnosis device for an oxygen sensor downstream of an exhaust gas purification system according to claim 3, wherein measurement of a continuous rich period is started.   The abnormality diagnosis device for the downstream oxygen sensor of the exhaust gas purification system according to any one of claims 1 to 4, wherein the abnormality diagnosis unit changes the delay period according to an operating state of the internal combustion engine.   The abnormality diagnosing means detects when the downstream oxygen sensor output has changed from a lean state to a rich state even before the continuous rich period exceeds the delay period after the internal combustion engine is started. The abnormality diagnosis device for a downstream oxygen sensor of an exhaust gas purification system according to any one of claims 1 to 5, wherein the downstream oxygen sensor is determined to be normally activated.

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