JP2013253515A - Catalyst deterioration diagnosis device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect the deterioration of a catalyst with higher accuracy without installing an additional hydrogen sensor or the like for catalyst deterioration detection.SOLUTION: A catalyst deterioration diagnosis device is applied to an internal combustion engine including a catalyst installed in an exhaust path, a first sensor installed upstream of the catalyst and a second sensor installed downstream of the catalyst. In the catalyst deterioration device, when control for catalyst deterioration diagnosis is executed, an output of the first sensor and an output of the second sensor are detected while an air-fuel ratio of the internal combustion engine is controlled to reach a rich air-fuel ratio. When a detected difference between the first output and the second output exceeds a reference value, catalyst deterioration is detected.

Description

  The present invention relates to a catalyst deterioration diagnosis apparatus. More specifically, the present invention relates to a catalyst deterioration diagnosis device that detects deterioration of a catalyst disposed in an exhaust path of an internal combustion engine.

  Patent Document 1 discloses a system in which a catalyst is installed in an exhaust path of an internal combustion engine, an air-fuel ratio of exhaust gas upstream and downstream of the catalyst is detected, and air-fuel ratio control is performed based on the detected air-fuel ratio. Patent Document 1 discloses that the catalyst has a property of generating hydrogen when it is new (fresh catalyst), and this property is weakened with deterioration of the catalyst. Patent Document 1 discloses detecting information relating to the discharge of hydrogen downstream of the catalyst in order to compensate for the influence of hydrogen on air-fuel ratio control.

  Specifically, Patent Document 1 discloses a method of detecting information related to the discharge of hydrogen downstream of the catalyst, in which a hydrogen sensor is installed downstream of the catalyst and detected directly according to the output of the hydrogen sensor. Yes.

  Patent Document 1 discloses the following technique as another detection technique for information relating to the discharge of hydrogen downstream of the catalyst. That is, after the fuel cut operation, a process of releasing the oxygen stored in the catalyst by controlling the air-fuel ratio to rich is executed, and a change in sensor output at that time is detected. Here, if the catalyst is not deteriorated, hydrogen is discharged downstream of the catalyst. Therefore, if the catalyst has not deteriorated, the output of the air-fuel ratio sensor downstream of the catalyst rapidly changes to a rich output. However, this rapid change in sensor output may be due to catalyst degradation.

  For this reason, in the system of Patent Document 1, a change in sensor output is detected when the air-fuel ratio is switched from rich to lean. Based on this output change, the deterioration of the catalyst is determined regardless of the generation of hydrogen. Then, it is distinguished whether the sudden change in the sensor output detected when the air-fuel ratio is controlled to be rich is due to catalyst deterioration or hydrogen generation, and information relating to hydrogen discharge is detected.

JP 2003-120383 A JP 2007-231777 A JP 2000-008920 A JP 2001-124730 A

  Here, as described in Patent Document 1, when the catalyst is new, a large amount of hydrogen is discharged downstream of the catalyst. Using this fact, it is conceivable to perform a deterioration diagnosis of the catalyst by installing a hydrogen sensor downstream of the catalyst and monitoring the discharge of hydrogen by the hydrogen sensor. The amount of hydrogen emission has a high correlation with the deterioration of the catalyst, and it is considered that the deterioration of the catalyst can be diagnosed with high accuracy by using the amount of hydrogen emission as an indicator for the catalyst deterioration diagnosis. However, in order to perform catalyst deterioration diagnosis according to the amount of hydrogen discharged, it is necessary to accurately grasp the amount of hydrogen discharged downstream of the catalyst. However, it is not preferable to install a separate hydrogen sensor for that purpose because it leads to an increase in cost.

  The present invention aims to solve the above-mentioned problems, and has been improved so that the deterioration of the catalyst can be detected with higher accuracy without installing a separate hydrogen sensor or the like for detecting the deterioration of the catalyst. A diagnostic device is provided.

  The present invention is a catalyst deterioration diagnostic apparatus for achieving the above object, comprising a catalyst installed in the exhaust path, a first sensor installed upstream of the catalyst, and a second sensor installed downstream of the catalyst. The present invention is applied to an internal combustion engine including a sensor. The catalyst deterioration diagnosis device of the present invention includes rich air-fuel ratio control means for controlling the air-fuel ratio of the internal combustion engine to a rich air-fuel ratio that is fuel-rich than the stoichiometric air-fuel ratio, and controls the air-fuel ratio of the internal combustion engine to the rich air-fuel ratio. When the difference between the output of the first sensor and the output of the second sensor and the detected first output and the second output is larger than the reference value in the state where Deterioration detecting means for detecting deterioration is provided.

  The catalyst deterioration diagnosis apparatus of the present invention further includes means for detecting the output of the first sensor and the output of the second sensor before the catalyst activity, which is before the catalyst temperature reaches a predetermined activation temperature, Means for correcting the output characteristics of the first sensor and the output characteristics of the second sensor in accordance with the output of the first sensor and the output of the second sensor may be provided.

  The rich air-fuel ratio control means in the catalyst deterioration diagnosis device of the present invention may control the air-fuel ratio to a rich air-fuel ratio after the fuel cut operation of the internal combustion engine is executed.

  Further, the catalyst deterioration diagnosis device of the present invention can be applied to a catalyst having a function of occluding or releasing oxygen. In this case, the catalyst deterioration diagnosis device includes a lean air-fuel ratio control means for controlling the air-fuel ratio of the internal combustion engine to a lean air-fuel ratio that causes a fuel shortage than the stoichiometric air-fuel ratio; Means for detecting an oxygen storage period, which is a period until the difference between the output of the first sensor and the output of the second sensor becomes smaller than a predetermined value, may be provided. In this case, the catalyst deterioration diagnosis device further includes means for setting a reference time based on the oxygen storage time, and after the elapsed time from when the output detection means is set to the rich air-fuel ratio reaches the reference time, The output of the first sensor and the output of the second sensor may be detected.

  In a rich atmosphere, a large amount of hydrogen is produced when the catalyst is functioning normally, and the amount of hydrogen produced decreases as the catalyst deteriorates. Further, in an environment where there are few rich components downstream of the catalyst, when the amount of hydrogen is small, the deviation from the sensor output on the upstream side of the catalyst tends to be large. In this regard, according to the present invention, the output difference between the first and second sensors upstream and downstream of the catalyst in a rich atmosphere is detected and compared. Thereby, the amount of hydrogen discharged downstream of the catalyst can be grasped to some extent, and deterioration of the catalyst can be diagnosed with high accuracy.

  In addition, when correcting the output characteristics of the first sensor and the second sensor before the catalyst activation, the output characteristics of the first and second sensors due to individual differences of the first and second sensors respectively. The deviation can be eliminated in advance. Therefore, the deterioration diagnosis of the catalyst can be performed with high accuracy based on the outputs of the first sensor and the second sensor.

  In the present invention, after the fuel cut operation of the internal combustion engine is executed, the air-fuel ratio is controlled to the rich air-fuel ratio and the catalyst deterioration diagnosis is performed. Reduction can be achieved.

  Further, in the present invention, a reference time for determining the timing for detecting the sensor output used for the catalyst deterioration diagnosis is set based on the detection of the oxygen storage period of the catalyst while the lean air-fuel ratio is controlled. If this is the case, it is possible to reliably prevent erroneous determination that the catalyst has deteriorated due to the output difference between the first sensor and the second sensor that occurs in a state where the catalyst exhibits oxygen storage capacity. can do. In addition, the detection timing of the sensor output is determined with reference to a reference time set based on the catalyst storage period corresponding to the state of the catalyst at that time. For this reason, it is not necessary to set the reference time longer unnecessarily, and an opportunity for diagnosis of catalyst deterioration can be further secured.

It is a schematic diagram for demonstrating the whole structure of the system in Embodiment 1 of this invention. FIG. 3 is a diagram for explaining output characteristics of an air-fuel ratio sensor on the upstream side of the catalyst and output characteristics of an air-fuel ratio sensor on the downstream side of the catalyst when the catalyst is normal / deteriorated in Embodiment 1 of the present invention. . It is a flowchart for demonstrating the control routine which a control apparatus performs in Embodiment 1 of this invention. It is a flowchart for demonstrating the control routine which a control apparatus performs in Embodiment 1 of this invention. FIG. 10 is a diagram for explaining output characteristics of an air-fuel ratio sensor on the upstream side of the catalyst and output characteristics of an air-fuel ratio sensor on the downstream side of the catalyst when the catalyst is normal / deteriorated in Embodiment 2 of the present invention. . It is a flowchart for demonstrating the control routine which a control apparatus performs in Embodiment 2 of this invention.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof is simplified or omitted.

Embodiment 1 FIG.
[Overall Configuration of System of First Embodiment]
FIG. 1 is a schematic diagram for explaining an overall configuration of a system according to Embodiment 1 of the present invention. The system of FIG. 1 is used by being mounted on a vehicle or the like. In FIG. 1, a catalyst 6 is installed in the exhaust path 4 of the internal combustion engine 2. An air-fuel ratio sensor 10 (first sensor) is installed upstream of the catalyst 6 in the exhaust path 4. An air-fuel ratio sensor 12 (second sensor) is installed downstream of the catalyst 6 in the exhaust path 4.

  For convenience, in the following embodiment, the air-fuel ratio sensor 10 on the upstream side of the catalyst 6 is also referred to as “Fr sensor 10”, and the air-fuel ratio sensor 12 on the downstream side is also referred to as “Rr sensor 12”. Further, when referring to both the Fr sensor 10 and the Rr sensor 12, they are also simply referred to as “up and down sensors”.

  The system of FIG. 1 includes a control device 14. The control device 14 comprehensively controls the entire system of the internal combustion engine 2. Various actuators are connected to the output side of the control device 14, and various sensors such as the air-fuel ratio sensors 10 and 12 are connected to the input side.

  The control device 14 receives the sensor signal, detects the air-fuel ratio of the exhaust gas, the engine speed, and other various information necessary for the operation of the internal combustion engine 2, and operates each actuator according to a predetermined control program. There are many actuators and sensors connected to the control device 14, but the description thereof is omitted in this specification.

[Operation of catalyst 6]
The catalyst 6 is a three-way catalyst. In a normally functioning state, the catalyst 6 efficiently uses the three components of HC, CO, and NOx in the exhaust when the inflowing exhaust gas air-fuel ratio is in the vicinity of the theoretical air-fuel ratio (hereinafter also referred to as “stoichiometric”). Purify.

  Further, the catalyst 6 adsorbs and stores oxygen in the exhaust gas when the exhaust gas air-fuel ratio is lean (ie, a state in which oxygen is more excessive than the stoichiometric state), and reduces lean components such as nitrogen oxides (NOx). Releases the stored oxygen when the exhaust gas air-fuel ratio is rich (that is, oxygen is insufficient compared to stoichiometry) and oxidizes rich components such as carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas To do. As a result, the exhaust gas can be purified to a gas in the vicinity of the stoichiometry.

Further, when new, the catalyst 6 generates hydrogen (H ₂ ) by reacting HC or CO with water (H ₂ O). Then, the amount of H ₂ due to this reaction decreases as the deterioration of the catalyst 6 proceeds. That is, when the catalyst 6 is new, the proportion of H ₂ contained in the exhaust gas flowing downstream of the catalyst 6 is large, and the proportion of H ₂ decreases as the deterioration of the catalyst 6 proceeds.

[About the function of the vertical sensor]
The vertical sensor is a one-cell type limiting current type sensor. Each of the upper and lower sensors has a pair of electrodes of an exhaust electrode and an atmospheric electrode, and a solid electrolyte membrane sandwiched between the electrodes. A diffusion layer is formed on the exhaust electrode. When a predetermined voltage is applied to the upper and lower sensors, oxygen ions (O ²⁻ ) move in the solid electrolyte membrane according to the oxygen concentration of the exhaust gas to be detected, and a limit current corresponding to the moved oxygen concentration is generated. Flowing.

More specifically, in a state where a predetermined voltage is applied to the upper and lower sensors, when the air-fuel ratio of the exhaust gas to be detected is a lean air-fuel ratio, the oxygen on the exhaust electrode side that has passed through the diffusion layer is ionized. O ²⁻ moves through the solid electrolyte membrane from the exhaust electrode toward the atmosphere electrode. A limit current corresponding to the amount of O ²⁻ moved from the exhaust electrode to the atmosphere electrode flows through the upper and lower sensors, and the output of the upper and lower sensors corresponds to this limit current.

On the other hand, when the exhaust gas air-fuel ratio is a rich air-fuel ratio, rich components such as HC and CO and H ₂ existing in the exhaust gas pass through the diffusion layer and reach the exhaust electrode. On the other hand, oxygen is ionized on the atmosphere electrode side. O ²⁻ moves through the solid electrolyte membrane from the atmosphere electrode toward the exhaust electrode side, and reacts with HC, CO, and H 2 at the exhaust electrode. When the air-fuel ratio of the exhaust gas to be detected is a rich air-fuel ratio, a limit current corresponding to the amount of O ²⁻ that has moved through the solid electrolyte membrane by this reaction flows in the upper and lower sensors, and the output of the upper and lower sensors It depends on the limit current.

  The limit current (IL) (or an output equivalent to this) is detected as the output of the up / down sensor. In general, the output of the vertical sensor is detected, and the air-fuel ratio corresponding to the output is obtained according to a map, function, or the like that defines the relationship between the output and the air-fuel ratio. However, for simplification of description, the following embodiment will be described on the assumption that the output of the upper and lower sensors directly indicates the value of the air-fuel ratio.

By the way, H ₂ moves through the diffusion layer at a faster speed than other components that affect the output of the Rr sensor 12, rich components such as HC and CO, and O ₂ . Therefore, the output of the up / down sensor tends to shift to the rich side from the actual air / fuel ratio with respect to the rich air / fuel ratio exhaust gas in which a large amount of H ₂ exists. However, in the first embodiment, the upper and lower sensors are assumed to use an environment in which a large amount of H ₂ is present in the exhaust gas and compensated to generate a correct air-fuel ratio for the exhaust gas containing H _2. To do.

[Control of Embodiment 1]
In the control executed by the control device 14 in this system, the deterioration of the catalyst 6 based on the outputs of the air-fuel ratio sensors 10 and 12 is diagnosed and the accuracy of the deterioration diagnosis of the catalyst 6 is improved. Correction of sensor output characteristics.

<Catalyst deterioration diagnosis>
FIG. 2 is a diagram for explaining the outputs of the Fr sensor 10 and the Rr sensor 12 when the air-fuel ratio is forcibly changed from lean to rich in the first embodiment of the present invention. In FIG. 2, the horizontal axis represents time, the upper vertical axis represents the target air-fuel ratio set in the air-fuel ratio control of the internal combustion engine 2, and the lower vertical axis represents the output value of each sensor when the target air-fuel ratio is controlled. is there. In FIG. 2, line (a) is the output of the Fr sensor 10, line (b) is the output of the Rr sensor 12 when the catalyst 6 is normal, and line (c) is the Rr when the catalyst is deteriorated. The output of the sensor 12 is represented. Here, it is assumed that both the Fr sensor 10 and the Rr sensor 12 are normal. Hereinafter, the deterioration diagnosis of the catalyst 6 in the first embodiment will be described with reference to FIG.

  In the example of FIG. 2, the air-fuel ratio of the internal combustion engine 2 is switched from lean to rich at time T0. The Fr sensor 10 installed upstream of the catalyst 6 uses the exhaust gas before purification as a detection target. Therefore, as shown in FIG. 2, when the target air-fuel ratio is switched from lean to rich at time T0, a rich output is displayed at an early stage.

  On the other hand, immediately after T0 when the air-fuel ratio is switched to rich, the catalyst 6 releases the oxygen stored during the period in which the air-fuel ratio is controlled to be lean, thereby oxidizing the rich components in the exhaust gas and Purify gas into gas near stoichiometric. Accordingly, even after the air-fuel ratio is switched to rich, the exhaust gas in the vicinity of the stoichiometric exhaust is discharged downstream of the catalyst 6 while oxygen is stored in the catalyst 6 (hereinafter also referred to as “oxygen storage period”).

  Therefore, the Rr sensor 12 downstream of the catalyst 6 that detects the exhaust gas purified by the catalyst 6 is in a state where oxygen is occluded in the catalyst 6 even after the target air-fuel ratio is switched from lean to rich. The output near the stoichiometry is shown (lines (b) and (c) in FIG. 2). Eventually, when all the oxygen occluded by the catalyst 6 is released, the catalyst 6 becomes incapable of oxidizing the rich component, and the air-fuel ratio of the exhaust gas becomes substantially the same between the downstream of the catalyst 6 and the upstream of the catalyst 6.

  If the catalyst 6 is normal at this time, the Rr sensor 12 starts to show a rich output at a relatively early stage as shown by the line (b) in FIG. The output and the output of the Fr sensor 10 coincide with each other. On the other hand, as shown by the line (c) in FIG. 2, when the catalyst 6 is deteriorated, the output of the Fr sensor 10 and the output of the Rr sensor 12 after the oxygen stored by the catalyst 6 is released. There is a gap between them. The output deviation of the Rr sensor 12 that occurs downstream of the normal catalyst 6 and the deteriorated catalyst 6 is considered as follows.

As described above, the catalyst 6 purifies the exhaust gas by burning the three components of HC, CO, and NOx in the exhaust gas even after all the stored oxygen is released. As a result, after the oxygen storage period, the exhaust gas air-fuel ratio is generally the same between the upstream and downstream of the catalyst 6, but the amount of HC, CO, and H ₂ contained in the exhaust gas downstream of the catalyst 6 is the upstream side. Very little compared to exhaust gas. Thus, in an environment where the amount of HC, CO, and H ₂ downstream of the catalyst 6 is extremely small, the reaction between HC, CO, and H ₂ at the exhaust electrode and O ²⁻ that has moved from the atmosphere electrode side occurs. Since it is difficult to proceed, the sensor output tends to generate an output that deviates leaner than the actual oxygen concentration.

However, when the catalyst 6 is new, H ₂ is generated in the catalyst 6 and discharged downstream. For this reason, the ratio of H ₂ contained in the exhaust gas on the downstream side increases. H ₂ has a higher diffusion speed than other rich components, and reaches the exhaust electrode through the diffusion layer at a high stage. Therefore, even in an environment where the amount of rich components downstream of the catalyst 6 is very small, the reaction between H ₂ and O ²⁻ at the exhaust electrode proceeds due to H _{2 having} a high diffusion rate. Therefore, the output of the Rr sensor 12 coincides with the output of the Fr sensor 10 upstream of the catalyst 6 installed in an exhaust gas environment with a large amount of H ₂ .

On the other hand, when the catalyst 6 deteriorates and H ₂ is not generated, the amount of H _{2 in} the exhaust gas downstream of the catalyst 6 is also reduced. For this reason, the movement of O ²⁻ to the exhaust electrode side is difficult to proceed, and the output of the Rr sensor 12 is shifted to the lean side compared to the output of the Fr sensor 10 in an environment where there is a lot of upstream H _2. It becomes.

From the above, after the exhaust gas is controlled to the rich air-fuel ratio and the oxygen storage period has elapsed, the output of the Fr sensor 10 and the Rr sensor 12 in the state where the upstream and downstream exhaust gas air-fuel ratios of the catalyst 6 are the same. The difference from the output is considered to have a certain degree of correlation with the difference in the amount of H ₂ produced by the catalyst 6. The amount of H ₂ produced in the catalyst 6 has a correlation with the degree of deterioration of the catalyst 6. Therefore, in the deterioration diagnosis of the catalyst 6 in the present embodiment, the difference (output difference) between the output of the Fr sensor 10 and the output of the Rr sensor 12 after the reference time has elapsed since switching to the rich air-fuel ratio is detected. When the output difference is equal to or greater than the reference value, the deterioration of the catalyst 6 is detected.

  It should be noted that the reference value for the output difference in this determination is the difference between the output value of the Rr sensor 12 installed downstream of the catalyst 6 whose deterioration degree is at the boundary of the allowable range and the output of the Fr sensor 10 upstream of the catalyst 6. Is obtained by simulation or the like, and may be appropriately set based on this. That is, this value is a suitable value that can be determined according to how much deterioration of the catalyst 6 is allowed.

  The reference time is a time set based on the oxygen storage period of a normal catalyst. In the present embodiment, the reference time is set to an appropriate value (for example, 5 seconds or more) in advance by simulation or the like, It is stored in the control device 14. The oxygen storage period is a period during which the catalyst 6 can store oxygen or can release oxygen, and after the air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio, lean exhaust gas is discharged downstream. This is the time set based on the period until the exhaust gas is exhausted to the downstream side after the air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio. .

<Correcting the output of the vertical sensor>
By the way, in the control of the deterioration diagnosis of the catalyst 6, the deterioration diagnosis is performed using the difference in output between the Fr sensor 10 and the Rr sensor 12 as a parameter. Therefore, in order to correctly perform the deterioration diagnosis, it is desirable that the output characteristics of the Fr sensor 10 and the output characteristics of the Rr sensor 12 match. That is, individual differences in output characteristics caused by circuit errors and deterioration with the Fr sensor 10 and Rr sensor 12 are removed in advance, and when the same exhaust gas is detected, the output of the upper and lower sensors and the response speed are the same. It is desirable to make adjustments so that

  Therefore, in the system according to the first embodiment, correction for matching the output characteristics of the Fr sensor 10 and the Rr sensor 12 is performed prior to the deterioration diagnosis of the catalyst 6. Specifically, correction for adjusting the stoichiometric point and correction before the catalyst 6 is warmed up are executed.

  A correction value for the stoichiometric point is calculated and learned so that the outputs at the stoichiometric positions of the Fr sensor 10 and the Rr sensor 12 match. The correction for the stoichiometric point corrects an output error caused by the circuits of the upper and lower sensors. As a result, the shift correction is performed so that the stoichiometric point of the Fr sensor 10 and the stoichiometric point of the Rr sensor 12 coincide.

  Furthermore, two-point correction is performed by adding correction before the catalyst 6 is warmed up. Before the catalyst 6 is warmed up, the exhaust gas flows out downstream of the catalyst 6 without being purified. That is, if the catalyst 6 is not warmed up, the exhaust gas detected by the Fr sensor 10 upstream of the catalyst 6 is sent downstream of the catalyst 6 without being purified. Therefore, the exhaust gas that was the detection target of the Fr sensor 10 can be used as the detection target of the Rr sensor 12 as it is. Therefore, by using the timing before the catalyst 6 is warmed up, it is possible to execute correction that matches the basic output characteristics of the Fr sensor 10 and the Rr sensor 12.

  More specifically, the output of the Fr sensor 10 and the output of the Rr sensor 12 are detected before the catalyst 6 is warmed up. For example, the difference between the two outputs is averaged to obtain the output of the Fr sensor 10 and the output of the Rr sensor 12. Learning as a correction value for output. Alternatively, the responsiveness of the Fr sensor 10 and the Rr sensor 12 is detected, and a correction value is detected so that the responsiveness of both sensors coincides, and this is learned. These correction values are calculated after taking into account the delay of the volume between the Fr sensor 10 and the Rr sensor 12.

In this way, by performing the correction at the two points of the correction of the stoichiometric point and the correction based on the sensor output before the catalyst warm-up, the output characteristics of the upper and lower sensors can be matched with high accuracy. Further, by using the sensor after such correction, other factors affecting the output difference between the Fr sensor 10 and the Rr sensor 12 can be eliminated, and more accurately generated from the catalyst 6. The deterioration diagnosis of the catalyst 6 can be executed by grasping the influence of H ₂ to be performed.

  3 and 4 are flowcharts for illustrating a control routine executed by control device 14 in the embodiment of the present invention. The routine of FIG. 3 is a control routine for correcting the output of the up / down sensor, and is repeatedly executed at a constant calculation cycle during the operation of the internal combustion engine 2.

  In the routine of FIG. 3, it is first determined whether or not the correction completion flag is OFF (S10). The correction completion flag is a flag that is turned on when correction for matching the output characteristics of the Fr sensor 10 and the Rr sensor 12 is completed by the processing described later in this routine, and is initialized and turned off when the internal combustion engine 2 is stopped. . In step S10, when it is not recognized that the correction completion flag = OFF, since the sensor correction has already been completed, the current process ends as it is.

  If it is determined in step S10 that the correction completion flag is OFF, it is next determined whether or not the internal combustion engine 2 has been started and the catalyst 6 has not been warmed up (step S12). If it is determined in step S12 that the catalyst 6 is not warmed up, then the outputs of the Fr sensor 10 and the Rr sensor 12 are detected (S14).

  Next, sensor correction before warm-up is performed (S16). The sensor correction is executed by obtaining an output difference between the output of the Fr sensor 10 and the output of the Rr sensor 12, and obtaining and learning a correction value so that the outputs of the upper and lower sensors match. The correction value is obtained by smoothing the output difference between the Fr sensor 10 and the Rr sensor 12, or by averaging with the correction values obtained up to the previous time. The output of each of the upper and lower sensors is corrected by this correction value.

  If it is not recognized in step S12 that the catalyst 6 is not warmed up, or if sensor correction is performed in step S16 before the catalyst 6 is warmed up, then the stoichiometric point correction is set. When the predetermined voltage is applied, the outputs of the Fr sensor 10 and the Rr sensor are detected (S18).

  Next, stoichiometric point sensor correction is performed (S20). The stoichiometric point sensor correction is a correction that removes errors included in the upper and lower sensor circuits and makes the outputs corresponding to the stoichiometric points coincide with each other, and the correction value corresponding to the outputs of the Fr sensor 10 and the Rr sensor 12 detected in step S20. Desired. Specifically, the correction value is obtained by smoothing the output difference between the Fr sensor 10 and the Rr sensor 12, or by averaging the correction value up to the previous correction value already learned. It is done. The output of each of the upper and lower sensors is shift-corrected by this correction value.

  Thereafter, the sensor correction completion flag is turned on (S22). Thereafter, this flag is initialized and turned OFF when the internal combustion engine 2 stops. Thereafter, the current sensor correction process ends.

  Next, the control routine for the deterioration diagnosis of the catalyst 6 in FIG. 4 will be described. The routine of FIG. 4 is a routine that is repeatedly executed at a constant calculation cycle during the operation of the internal combustion engine 2. In the routine of FIG. 4, it is determined whether or not the correction completion flag is ON (S100). The correction completion flag is a flag that is turned on when correction for matching the output characteristics of the Fr sensor 10 and the Rr sensor 12 is completed after the current start of the internal combustion engine 2 by the routine of FIG. If the completion of correction is not recognized in step S100, the current process is temporarily terminated.

  If it is recognized in step S100 that the correction completion flag = ON is established, it is next determined whether or not the deterioration diagnosis completion flag is OFF (S102). The deterioration diagnosis completion flag is a flag that is turned on when the deterioration diagnosis of the catalyst 6 is completed after the internal combustion engine 2 is started by processing described later, and is a flag that is initialized and turned off when the internal combustion engine 2 is stopped. It is. In step S102, when it is not recognized that the deterioration diagnosis completion flag = OFF is established, the current process ends.

  On the other hand, if it is determined in step S102 that the deterioration diagnosis completion flag = OFF is established, it is next determined whether or not the precondition is satisfied (S104). The precondition is, for example, that the catalyst 6 and the vertical sensor are activated and the temperature of the catalyst 6 is maintained within a certain range (for example, 600 to 700 ° C.). It is a condition. More specific conditions are stored in the control device 14 in advance. If the precondition is not satisfied in step S104, the current process ends.

  On the other hand, when the establishment of the precondition is confirmed in step S104, the air-fuel ratio is then controlled to a predetermined rich air-fuel ratio (for example, A / F = 14.3 or more) (S106). Here, the target air-fuel ratio is set to a rich air-fuel ratio set in advance for diagnosis of deterioration of the catalyst 6, and feedback control is performed to this air-fuel ratio based on the output of the Fr sensor 10.

  Next, it is determined whether or not the elapsed time since switching to the rich air-fuel ratio in step S106 has reached the reference time (S108). Here, the reference time is a time stored in the control device 14, and by determining whether or not the reference time has been exceeded, it is determined whether or not the oxygen stored in the catalyst 6 has been completely released. Will be.

  If it is determined in step S108 that elapsed time> reference time is not satisfied, the determination process in step S108 is repeatedly executed at regular intervals until elapsed time> reference time is satisfied.

If it is recognized in step S108 that the elapsed time> the reference time is established, then the output of the Fr sensor 10 and the output of the Rr sensor 12 are detected (S110). Next, it is determined whether or not an output difference (output difference) between the Fr sensor 10 and the Rr sensor 12 is larger than a reference value (S112). Here, the reference value is a value stored in the control device 14 in advance. By this determination, it is determined whether or not a certain amount of H ₂ has been discharged downstream of the catalyst 6, that is, whether or not the catalyst 6 has deteriorated.

  If the output difference between the Fr sensor 10 and the Rr sensor 12 is larger than the reference value in step S112, it is recognized that the catalyst 6 has deteriorated (S114). In this case, predetermined processing such as lighting of MIL is executed. On the other hand, when the difference between the output of the Fr sensor 10 and the output of the Rr sensor 12 is equal to or smaller than the reference value, the catalyst 6 is determined to be normal (S116). After the determination in step S114 or step S116, the deterioration diagnosis completion flag is turned on (S118), and the current process ends.

As described above, according to the first embodiment, it is possible to accurately grasp the influence of H ₂ generated by the catalyst 6 and determine the deterioration of the catalyst 6 based on the influence. Since the amount of H ₂ produced in the catalyst 6 has a strong correlation with the deterioration state of the catalyst 6, the deterioration diagnosis of the catalyst 6 can be performed with higher accuracy. The deterioration diagnosis control of the catalyst 6 can be performed using an air-fuel ratio sensor that is used for air-fuel ratio control without installing a special sensor such as a hydrogen sensor. Therefore, an increase in cost for diagnosing deterioration of the catalyst 6 can be suppressed.

  In the first embodiment, in principle, the case where the deterioration of the catalyst 6 is determined after correcting the stoichiometric point and correcting the output of the upper and lower sensors before the catalyst 6 is warmed up has been described. However, the present invention is not limited to this, and correction may be made by other means so that the output characteristics of the upper and lower sensors match. As another correction, in addition to the correction before warming up of the catalyst 6 described above, the output during the fuel cut after warming up of the catalyst 6 is detected, and the sensor output to the atmosphere is detected, so that the output of the vertical sensor The characteristics can be corrected. However, in the present embodiment, since the deterioration diagnosis of the catalyst 6 is performed based on the sensor output when the air-fuel ratio is controlled to be rich, the correction based on the output difference in the rich atmosphere is not preferable. The sensor correction is the same in the second embodiment below.

Further, as described above, the advance correction of the sensor output characteristic is for more accurately grasping the influence of H ₂ discharged from the catalyst 6, but the present invention performs such correction. However, the present invention is not limited to this, and the deterioration of the catalyst 6 may be determined without performing correction for matching the output characteristics of the upper and lower sensors. The same applies to the following second embodiment.

  In the first embodiment, the case where the correction of the upper and lower sensors and the deterioration diagnosis of the catalyst 6 are each performed once between the start and stop of the internal combustion engine 2 has been described. However, the present invention is not limited to this, and a plurality of sensor corrections and deterioration diagnosis of the catalyst 6 may be performed between the start and stop of the internal combustion engine 2. Alternatively, for example, the deterioration diagnosis of the catalyst 6 and the correction of the upper and lower sensors may be performed at a fixed timing using the accumulated use time, operation time, travel distance, intake air amount, etc. of the catalyst 6 as parameters. The same applies to the following second embodiment.

  Further, in the first embodiment, the case where the deterioration diagnosis of the catalyst 6 is performed by forcibly controlling the rich air-fuel ratio when the precondition is satisfied has been described. However, the present invention is not limited to this. For example, on the condition that the fuel cut operation is continued for a predetermined time, the deterioration of the catalyst 6 is diagnosed by switching to the rich air-fuel ratio after the fuel cut operation for a predetermined time. It is good. As a result, it is possible to effectively prevent a reduction in emission due to forced rich control.

  In the first embodiment, for the sake of convenience, the outputs of the Fr sensor 10 and the Rr sensor 12 are detected once at the timing after the reference time has elapsed since switching to the rich air-fuel ratio, and the output difference is determined according to this output difference. The case where the deterioration of the catalyst 6 is determined has been described. However, the present invention is not limited to this, and the output of the Fr sensor 10 and the output of the Rr sensor are detected a plurality of times after the elapse of the reference time, and the respective outputs of the upper and lower sensors are averaged. 6 may be used as a parameter for determining deterioration. In particular, since the Fr sensor 10 is easily affected by pulsation and gas fluctuation, the deterioration of the catalyst 6 can be determined more accurately by averaging and using a plurality of outputs in this way. The same applies to the following second embodiment.

Embodiment 2. FIG.
The system of the second embodiment has the same configuration as the system of the first embodiment. Also in the system of the second embodiment, after the target air-fuel ratio is switched to the rich air-fuel ratio, the deterioration of the catalyst 6 is detected according to the output difference between the Fr sensor 10 and the Rr sensor 12 after the reference time has elapsed. Do. The control in the second embodiment is the same as the control in the system in the first embodiment except that the control includes setting the “reference time” in the control in the first embodiment. Here, the “reference time” is a value serving as a reference for determining whether or not it is the timing for detecting the output of the up / down sensor used for the deterioration diagnosis of the catalyst 6.

  FIG. 5 is a timing chart for explaining the control in the second embodiment of the present invention, in the case where control is performed to switch the air-fuel ratio between a predetermined rich air-fuel ratio and a lean air-fuel ratio at regular intervals. It is a figure for demonstrating the output of Fr sensor 10, and the output change of Rr sensor 12. In FIG. 5, the horizontal axis represents time, the vertical axis upper side represents the target air-fuel ratio, and the lower vertical axis represents the output value of each sensor when the target air-fuel ratio is controlled. In FIG. 5, line (a) is the output of the Fr sensor 10, line (b) is the output of the Rr sensor 12 when the catalyst 6 is functioning normally, and line (c) is the deterioration of the catalyst 6. The output of the Rr sensor 12 is shown. Here, the Fr sensor 10 and the Rr sensor 12 are both normal.

  In the second embodiment, the control device 14 sets the target air-fuel ratio of the internal combustion engine 2 to a rich air-fuel ratio (for example, 14.1) and a lean air-fuel ratio (for example, for example) at a predetermined cycle (for example, 5 seconds or more). Execute active control to switch to 15.1).

  During this active control, for example, in the example shown in FIG. 5, after the time for releasing most of the oxygen stored by the catalyst 6 (oxygen storage period), the air-fuel ratio becomes rich at the time T1 (for example, 14.1). ) To a lean air-fuel ratio (for example, 15.1). At this time, the air-fuel ratio of the exhaust gas immediately changes to lean on the upstream side of the catalyst 6, although there is a delay corresponding to the volume.

  On the other hand, when switching from rich to lean at time T1, the catalyst 6 reduces and purifies the lean component of the exhaust gas. While the catalyst 6 occludes oxygen, the lean air-fuel ratio exhaust gas is purified in the vicinity of the stoichiometric gas, so that the exhaust gas in the vicinity of the stoichiometric gas flows out downstream of the catalyst 6. Accordingly, during this time, the output of the Rr sensor 12 indicates a value near the stoichiometric value.

Eventually, the catalyst 6 becomes unable to reduce the lean component, and the exhaust gas having the same air-fuel ratio as the upstream side of the catalyst 6 starts to flow out. When the exhaust gas air-fuel ratio is lean, H ₂ is not generated. For this reason, even if the catalyst 6 is normal or deteriorated, the outputs of the upper and lower sensors coincide after the catalyst 6 becomes unable to store oxygen.

  Therefore, after the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio, the period until the output of the Rr sensor 12 substantially matches the output of the Fr sensor 10 can be detected as the oxygen storage period TOSC. This period is a period during which the lean component can be reduced to store oxygen, and conversely, it can be considered as a period during which the stored oxygen can be released to oxidize the rich component. That is, if the oxygen storage period TOSC has elapsed, the catalyst 6 cannot oxidize the rich component, and it is considered that the exhaust gas air-fuel ratio from the upstream to the downstream of the catalyst 6 does not change (or is very small). .

  Therefore, in the second embodiment, the oxygen storage period TOSC is detected while the target air-fuel ratio is controlled to the lean air-fuel ratio, and the reference time is set according to the oxygen storage period TOSC. Thereafter, at the detection timing T3 after the elapsed time starting from the time T2 when the rich air-fuel ratio is switched to the set reference time, the output of the upper and lower sensors is detected, and the deterioration of the catalyst 6 is diagnosed. The method for diagnosing deterioration of the catalyst 6 is the same as that in the first embodiment.

  The oxygen storage period TOSC changes according to the deterioration state of the catalyst 6, but in the second embodiment, the oxygen storage period TOSC is detected while being controlled to the lean air-fuel ratio, and is detected. Based on the oxygen storage period TOSC, the output of the upper and lower sensors for diagnosing deterioration of the catalyst 6 is detected. Therefore, the deterioration diagnosis of the catalyst 6 can be performed at an early stage after the air-fuel ratio of the exhaust gas upstream and downstream of the catalyst 6 becomes the same. Thereby, the opportunity of execution of the deterioration diagnosis of the catalyst 6 can be increased.

  FIG. 6 is a flowchart for illustrating a control routine executed by control device 14 in the second embodiment of the present invention. The routine of FIG. 6 is a routine that is repeatedly executed at a constant calculation cycle instead of the routine of FIG. 4 during the operation of the internal combustion engine 2. After the process of S104 of the routine of FIG. The routine is the same as the routine of FIG. 4 except that the processes of S202 to S208 are included.

  Specifically, after the precondition is satisfied in step S104, active control is first executed (S202). The active control is control for switching the target air-fuel ratio between a predetermined rich target air-fuel ratio and a lean target air-fuel ratio at a constant cycle. The rich target air-fuel ratio and the lean target air-fuel ratio are stored in the control device 14.

  Next, it is determined whether the target air-fuel ratio is a lean air-fuel ratio (S204). In step S204, when it is not recognized that the air-fuel ratio is lean, the process returns to the process of S204 again, and the determination process of S204 is repeated until it is recognized that the air-fuel ratio is lean.

  If it is determined in step S204 that the air-fuel ratio is lean, then the output of the Fr sensor 10 and the output of the Rr sensor 12 are detected (S206). Thereafter, it is determined whether or not the difference (output difference) between the output of the Rr sensor 12 and the output of the Fr sensor 10 is smaller than a predetermined value (S208). Here, the predetermined value is a small value of an error level that is recognized that the outputs of the Rr sensor 12 and the Fr sensor 10 substantially match, and is set in advance and stored in the control device 14.

  If it is not determined in step S208 that the output difference <predetermined value is established, the outputs of the Fr sensor 10 and the Rr sensor 12 are detected again in step S206, and the determination in S208 is performed based on the outputs. In step S208, the processes in steps S206 and S208 are repeated until output difference <predetermined to be established.

  If it is determined in step S208 that output difference <predetermined value is established, then a reference time is calculated (S210). Here, the reference time is a time for determining the timing for detecting the outputs of the Fr sensor 10 and the Rr sensor 12 in the catalyst 6 deterioration diagnosis. The reference time is preliminarily set in the control device 14 based on the time from the time when the target air-fuel ratio is switched from rich to lean in the active control until the time when the output difference <the predetermined value is recognized in step S208. It is calculated by a set predetermined calculation method.

  Next, it is determined whether or not the target air-fuel ratio is a rich air-fuel ratio (S212). If it is not determined in step S212 that rich control is being performed, the determination in step S212 is executed again. The determination processing in step S212 is repeatedly executed at a constant period until it is recognized that rich control is being performed.

  If it is determined in step S212 that the rich control is being performed, the process proceeds to step S108, and it is determined whether or not the elapsed time from the point of time when the current state is switched from lean to rich has exceeded the reference time. In step S <b> 108, when the elapsed time> the reference time is not established, the determination process of step S <b> 108 is repeatedly executed until the elapsed time reaches the reference time.

  On the other hand, if it is determined in step S108 that the elapsed time> the reference time is established, it can be determined that all the oxygen stored in the catalyst 6 has been released after switching to the rich air-fuel ratio. Therefore, thereafter, the deterioration determination of the catalyst 6 is executed in accordance with the processing of steps S110 to S112.

As described above, according to the second embodiment, the detection timing of the sensor output for determining the deterioration of the catalyst 6 can be set according to the current oxygen storage capacity of the catalyst 6. Thereby, the deviation produced between the output of the Fr sensor 10 and the output of the Rr sensor 12 is caused by the upstream and downstream of the catalyst 6 caused by the catalyst 6 functioning normally and exhibiting the oxygen storage capacity. Can be distinguished from the difference in the exhaust gas air-fuel ratio or the difference in the concentration of H ₂ upstream and downstream of the catalyst 6 after releasing the oxygen stored in the catalyst 6, An erroneous determination in the deterioration diagnosis of the catalyst 6 can be suppressed. In addition, a reference time can be set for each catalyst 6 each time. Therefore, the necessary and sufficient reference time according to the state of the catalyst 6 at that time can be set, so that an opportunity for determining deterioration of the catalyst 6 can be ensured more reliably.

  In the second embodiment, the reference time is based on the period from the time when the lean air-fuel ratio is switched to the time when the output difference between the output of the Rr sensor 12 and the Fr sensor 10 becomes smaller than a predetermined value. The case of calculating has been described. However, the reference time setting method is not limited to this in the present invention. For example, the output of the Rr sensor 12 becomes larger than a predetermined lean output corresponding to the target air-fuel ratio at that time from the air-fuel ratio switching time T1. It may be set based on the time until. Also, the calculation method of the reference time may be set according to the obtained oxygen storage time. Further, for example, the target air-fuel ratio in active control may be divided into a predetermined range, and a reference time may be set for each range by a map or the like.

  In the above embodiment, when referring to the number of each element, quantity, quantity, range, etc., the reference is made unless otherwise specified or the number is clearly specified in principle. The invention is not limited to the numbers. The structure and the like described in this embodiment are not necessarily indispensable for the present invention unless otherwise specified or clearly specified in principle.

2 Internal combustion engine 6 Catalyst 10 Air-fuel ratio sensor (Fr sensor)
12 Air-fuel ratio sensor (Rr sensor)
14 Control device

Claims (4)

Applied to an internal combustion engine comprising a catalyst installed in an exhaust path, a first sensor installed upstream of the catalyst, and a second sensor installed downstream of the catalyst;
Rich air-fuel ratio control means for controlling the air-fuel ratio of the internal combustion engine to a rich air-fuel ratio in which the fuel is greater than the stoichiometric air-fuel ratio;
Output detection means for detecting the output of the first sensor and the output of the second sensor in a state where the air-fuel ratio of the internal combustion engine is controlled to the rich air-fuel ratio;
Deterioration detecting means for detecting deterioration of the catalyst when a difference between the first output and the second output is larger than a reference value;
A catalyst deterioration diagnosis device comprising: Means for detecting the output of the first sensor and the output of the second sensor before the catalyst activation, which is before the catalyst temperature reaches a predetermined activation temperature;
Means for correcting the output characteristics of the first sensor and the output characteristics of the second sensor in accordance with the output of the first sensor and the output of the second sensor before the catalyst activity;
The catalyst deterioration diagnosis apparatus according to claim 1, further comprising:   3. The catalyst deterioration diagnosis device according to claim 1, wherein the rich air-fuel ratio control unit controls the air-fuel ratio to the rich air-fuel ratio after the fuel cut operation of the internal combustion engine is executed. The catalyst has a function of occluding or releasing oxygen,
Lean air-fuel ratio control means for controlling the air-fuel ratio of the internal combustion engine to a lean air-fuel ratio that causes a fuel shortage than the stoichiometric air-fuel ratio;
Means for detecting an oxygen storage period, which is a period from when the lean air-fuel ratio is set until the difference between the output of the first sensor and the output of the second sensor becomes smaller than a predetermined value;
Means for setting a reference time based on the oxygen storage time,
The output detection means detects an output of the first sensor and an output of the second sensor after an elapsed time from the time when the rich air-fuel ratio is made reaches the reference time. Item 4. The catalyst deterioration diagnosis device according to any one of Items 1 to 3.

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