JP2013024118A - Catalyst failure diagnosis apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent wrong diagnosis caused by a sulfur component regarding an apparatus for diagnosing whether a catalyst provided to the exhaust passage of an internal combustion engine is normal or abnormal.SOLUTION: In an apparatus for diagnosing whether the catalyst provided to the exhaust passage of the internal combustion engine is normal or abnormal, a catalyst temperature Tc is acquired by detection or estimation, a cumulative sulfur amount W in the catalyst is estimated and on the basis of the estimated cumulative sulfur amount W, the threshold T1 of the catalyst temperature is determined. Then, diagnosis is permitted or inhibited in accordance with whether the acquired catalyst temperature Tc is lower than the threshold T1.

Description

  The present invention relates to a catalyst abnormality diagnosis apparatus, and more particularly to an apparatus for diagnosing whether a catalyst provided in an exhaust passage of an internal combustion engine is normal or abnormal.

  For example, in an internal combustion engine for automobiles, a catalyst for purifying exhaust gas is installed in the exhaust system. Some of these catalysts have an oxygen storage capacity (O2 storage capacity). When the air-fuel ratio of the exhaust gas flowing into the catalyst becomes larger than the stoichiometric air-fuel ratio (stoichiometric), that is, when the engine becomes lean, the catalyst having oxygen storage capacity occludes excess oxygen present in the exhaust gas. When the fuel ratio becomes smaller than stoichiometric, that is, when it becomes rich, the stored oxygen is released. For example, in a gasoline engine, air-fuel ratio control is performed so that the exhaust gas flowing into the catalyst is in the vicinity of the stoichiometric. However, if a three-way catalyst having an oxygen storage capacity is used, the actual air-fuel ratio slightly deviates from the stoichiometric depending on the operating conditions. However, such an air-fuel ratio shift can be absorbed by the oxygen storage / release action of the three-way catalyst.

  On the other hand, when the catalyst deteriorates, the purification rate of the catalyst decreases. There is a correlation between the deterioration degree of the catalyst and the reduction degree of the oxygen storage capacity. Therefore, it is possible to detect deterioration or abnormality of the catalyst by detecting a decrease in oxygen storage capacity. In general, a method (so-called Cmax method) is employed that performs active air-fuel ratio control that alternately switches the air-fuel ratio between rich and lean, measures the oxygen storage capacity of the catalyst, and diagnoses catalyst deterioration (so-called Cmax method). Reference 1).

JP 2009-36172 A

  By the way, depending on a use area etc., sulfur (S) may be contained in fuel by comparatively high concentration. When such a high sulfur fuel is supplied, the catalyst may be sulfur poisoned (S poison) due to the influence of the sulfur component in the exhaust gas. When S poisoning occurs, the activity of the catalyst is reduced and the oxygen absorption / release action of the catalyst is hindered, so that the apparent oxygen storage capacity of the catalyst is reduced. However, when the fuel with a low sulfur concentration is refueled or the catalyst is exposed to a high temperature and rich atmosphere, the poisoning state is eliminated.

  The catalyst performance degradation due to S poisoning is temporary and recoverable. Therefore, in the abnormality diagnosis of the catalyst, it is necessary not to mistakenly diagnose the temporary abnormality due to the S poisoning as an unrecoverable permanent abnormality (thermal deterioration) that should be diagnosed.

  Accordingly, the present invention has been made in view of such circumstances, and an object thereof is to provide a catalyst abnormality diagnosis device that can prevent erroneous diagnosis due to a sulfur component.

According to one aspect of the invention,
An apparatus for diagnosing whether a catalyst provided in an exhaust passage of an internal combustion engine is normal or abnormal,
Obtaining the catalyst temperature by detection or estimation, estimating the amount of sulfur accumulation in the catalyst,
Determining a threshold value of the catalyst temperature based on the estimated sulfur accumulation amount;
According to the present invention, there is provided a catalyst abnormality diagnosis device, wherein diagnosis is permitted or prohibited depending on whether or not the acquired catalyst temperature is lower than the threshold value.

  Preferably, the threshold value is higher as the estimated sulfur accumulation amount is smaller.

  Preferably, the threshold value is a temperature at which sulfur oxide begins to desorb from the coating material containing an oxygen storage component in the catalyst.

  Preferably, the catalyst abnormality diagnosis device permits diagnosis when the acquired catalyst temperature is lower than the threshold value, and prohibits diagnosis when the acquired catalyst temperature is equal to or higher than the threshold value.

  Preferably, the catalyst abnormality diagnosis device permits diagnosis when the acquired catalyst temperature is equal to or higher than another threshold value higher than the threshold value.

  Preferably, when the acquired catalyst temperature becomes less than the threshold value after prohibition of diagnosis, the catalyst abnormality diagnosis device, according to the presence or absence of a history that the acquired catalyst temperature is equal to or higher than the another threshold value, Allow or disallow diagnosis.

  Preferably, the catalyst abnormality diagnosis device permits diagnosis when there is a history that the acquired catalyst temperature is equal to or higher than the another threshold, and the acquired catalyst temperature is equal to or higher than the another threshold. Diagnosis is prohibited when there is no history.

  Preferably, the catalyst abnormality diagnosis device suppresses a temperature increase for suppressing an increase in exhaust gas temperature when the acquired catalyst temperature is lower than the threshold value and in the vicinity of the threshold value during diagnosis. Execute control.

  Preferably, the catalyst abnormality diagnosis device performs temperature increase control for increasing the temperature of the exhaust gas while the diagnosis is prohibited.

  According to this invention, the outstanding effect that the misdiagnosis by a sulfur component can be prevented is exhibited.

It is the schematic which shows the structure of embodiment of this invention. It is a schematic sectional drawing which shows the structure of a catalyst. It is a time chart of active air fuel ratio control. FIG. 4 is a time chart similar to FIG. 3 for illustrating a method for measuring the oxygen storage capacity. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a figure for demonstrating the principle of S poisoning in a catalyst. It is a graph which shows the fall rate of the oxygen storage capacity measured value in each area | region. It is a graph which shows the 1st-3rd field specified by catalyst temperature and S accumulation amount. It is a figure which shows the permission and prohibition pattern of a diagnosis. It is a figure for demonstrating temperature rising suppression control. It is a flowchart of the main routine of diagnosis. It is a flowchart regarding on / off processing of a prohibition flag. It is a flowchart of temperature rising suppression control. It is a flowchart of temperature rising control.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the configuration of this embodiment. As shown in the figure, an engine 1 that is an internal combustion engine burns a mixture of fuel and air in a combustion chamber 3 formed in a cylinder block 2 and reciprocates a piston 4 in the combustion chamber 3 to drive power. Is generated. The engine 1 of the present embodiment is a multi-cylinder engine for automobiles (only one cylinder is shown), and is a spark ignition type internal combustion engine, more specifically, a gasoline engine.

  The cylinder head of the engine 1 is provided with an intake valve Vi that opens and closes an intake port and an exhaust valve Ve that opens and closes an exhaust port for each cylinder. Each intake valve Vi and each exhaust valve Ve are opened and closed by a camshaft (not shown). A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

  The intake port of each cylinder is connected to a surge tank 8 serving as an intake manifold through an intake manifold. An intake pipe 13 that forms an intake manifold passage is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. The intake pipe 13 is provided with an air flow meter 5 for detecting the amount of air flowing into the engine, that is, the amount of intake air, and an electronically controlled throttle valve 10 in order from the upstream side. An intake passage is formed by the intake port, the intake manifold, the surge tank 8 and the intake pipe 13.

  An injector for injecting fuel into an intake passage, particularly an intake port, that is, a fuel injection valve 12 is provided for each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture. The air-fuel mixture is sucked into the combustion chamber 3 when the intake valve Vi is opened, compressed by the piston 4, and ignited and burned by the spark plug 7. It is done.

  On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 6 forming an exhaust collecting passage through an exhaust manifold. An exhaust passage is formed by the exhaust port, the exhaust manifold, and the exhaust pipe 6. The exhaust pipe 6 is provided with a catalyst composed of a three-way catalyst having oxygen storage capacity, that is, an upstream catalyst 11 and a downstream catalyst 19 in series on the upstream side and the downstream side. For example, the upstream catalyst 11 is disposed immediately after the exhaust manifold, and the downstream catalyst 19 is disposed under the floor of the vehicle.

  An air-fuel ratio sensor that detects the air-fuel ratio of the exhaust gas based on the oxygen concentration, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 is provided on the upstream side and the downstream side (front and rear) of the upstream catalyst 11, respectively. As shown in FIG. 5, the pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, can continuously detect the air-fuel ratio over a relatively wide range, and outputs a signal having a value proportional to the air-fuel ratio. On the other hand, the post-catalyst sensor 18 is a so-called O2 sensor, and has a characteristic (Z characteristic) in which the output value changes abruptly at the theoretical air-fuel ratio.

  The spark plug 7, the throttle valve 10, the injector 12, and the like described above are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 14 that detects the crank angle of the engine 1, and an accelerator opening that detects the accelerator opening, as shown in the figure. The degree sensor 15 and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the injector 12, the throttle valve 10, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel injection timing, throttle opening. Control the degree etc.

  The throttle valve 10 is provided with a throttle opening sensor (not shown), and a signal from the throttle opening sensor is sent to the ECU 20. The ECU 20 normally feedback-controls the opening of the throttle valve 10 (throttle opening) to a target throttle opening determined according to the accelerator opening.

  The ECU 20 detects the amount of intake air per unit time, that is, the amount of intake air based on the signal from the air flow meter 5. The ECU 20 detects the load of the engine 1 based on at least one of the detected accelerator opening, throttle opening, and intake air amount.

  The ECU 20 detects the crank angle itself and the rotational speed of the engine 1 based on the crank pulse signal from the crank angle sensor 14. Here, “the number of rotations” means the number of rotations per unit time and is synonymous with the rotation speed. In the present embodiment, it means rpm per minute.

  The upstream catalyst 11 and the downstream catalyst 19 simultaneously purify NOx, HC and CO, which are harmful components in the exhaust gas, when the air-fuel ratio A / F of the exhaust gas flowing into each of them is close to the stoichiometric range. The air-fuel ratio width (window) that can simultaneously purify these three with high efficiency is relatively narrow.

  Therefore, during normal operation, the air-fuel ratio feedback control is executed by the ECU 20 so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11 is controlled near the stoichiometric range. The air-fuel ratio feedback control is performed by a main air-fuel ratio control (main air-fuel ratio feedback control) that matches the exhaust air-fuel ratio detected by the pre-catalyst sensor 17 with a predetermined target air-fuel ratio, and a post-catalyst sensor 18. It consists of auxiliary air-fuel ratio control (auxiliary air-fuel ratio feedback control) that makes the detected exhaust air-fuel ratio coincide with stoichiometric. Note that air-fuel ratio feedback control in which the target air-fuel ratio is stoichiometric is called stoichiometric control.

  Here, the upstream catalyst 11 that is mainly the object of diagnosis will be described in more detail. The downstream catalyst 19 is configured in the same manner as the upstream catalyst 11. As shown in FIG. 2, in the catalyst 11, a coating material 31 is coated on the surface of a carrier base material (not shown), and a large number of particulate catalyst components 32 are supported on the coating material 31 in a dispersed manner. The catalyst 11 is exposed inside. The catalyst component 32 is mainly composed of a noble metal such as Pt or Pd, and serves as an active point for reacting exhaust gas components such as NOx, HC and CO. On the other hand, the coating material 31 plays a role of a promoter that promotes a reaction at the interface between the exhaust gas and the catalyst component 32 and includes an oxygen storage component capable of absorbing and releasing oxygen according to the air-fuel ratio of the atmospheric gas. The oxygen storage component is made of, for example, cerium dioxide CeO2 or zirconia. Note that “absorption” or “adsorption” may be used in the same meaning as “occlusion”.

  For example, if the atmospheric gas in the catalyst is leaner than the stoichiometric air-fuel ratio, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmospheric gas, and as a result, NOx is reduced and purified. On the other hand, when the atmospheric gas in the catalyst is richer than the stoichiometric air-fuel ratio, oxygen stored in the oxygen storage component is released, and the released oxygen oxidizes and purifies HC and CO.

  Due to this oxygen absorption / release action, even if the actual air-fuel ratio varies somewhat with respect to stoichiometry during normal stoichiometric control, this variation can be absorbed.

  Incidentally, in the new catalyst 11, as described above, a large number of catalyst components 32 are evenly distributed, and the contact probability between the exhaust gas and the catalyst component 32 is maintained high. However, when the catalyst 11 deteriorates, some of the catalyst components 32 are lost, and some of the catalyst components 32 are baked and solidified by exhaust heat (see broken lines in the figure). If it becomes like this, the contact probability of exhaust gas and the catalyst component 32 will fall, and it will become the cause of reducing a purification rate. In addition to this, the amount of the coating material 31 existing around the catalyst component 32, that is, the amount of the oxygen storage component decreases, and the oxygen storage capacity itself decreases.

  Thus, there is a correlation between the degree of deterioration of the catalyst 11 and the oxygen storage capacity. Therefore, in the present embodiment, the degree of deterioration of the upstream catalyst 11 is indirectly detected by detecting the oxygen storage capacity of the upstream catalyst 11 that has a particularly large influence on the emission, thereby diagnosing whether the upstream catalyst 11 is normal or abnormal. I am going to do that. Here, the oxygen storage capacity of the catalyst 11 is represented by the size of the oxygen storage capacity (OSC; O2 Storage Capacity, the unit is g), which is the maximum amount of oxygen that the current catalyst 11 can store. Therefore, the oxygen storage capacity forms an index value (degradation index value) indicating the degree of deterioration of the catalyst 11.

  The catalyst abnormality diagnosis of this embodiment is based on the Cmax method described above. In the abnormality diagnosis, the active air-fuel ratio control is executed by the ECU 20. That is, the ECU 20 alternates the air-fuel ratio of the exhaust gas supplied to the catalyst 11, specifically, the air-fuel ratio of the air-fuel mixture in the combustion chamber 3, in a rich and lean manner, with the stoichiometric A / Fs being the central air-fuel ratio as a boundary. And switch to active.

  The active air-fuel ratio control and diagnosis are executed only when predetermined preconditions are satisfied. This precondition will be described later.

  In the present embodiment, the oxygen storage capacity of the upstream catalyst 11 is measured along with the execution of the active air-fuel ratio control, and whether the upstream catalyst 11 is normal or abnormal is diagnosed based on the measured value. Details of the diagnostic method are described below.

  3A, the broken line indicates the target air-fuel ratio A / Ft, and the solid line indicates the output of the pre-catalyst sensor 17 (however, the converted value to the pre-catalyst air-fuel ratio A / Ff). In FIG. 3B, the solid line indicates the output of the post-catalyst sensor 18 (however, the output voltage Vr).

  As shown in the figure, before the time t1, lean control for switching the air-fuel ratio to lean is executed, the target air-fuel ratio A / Ft is set to lean air-fuel ratio A / Fl (for example, 15.1), and the catalyst 11 A lean gas having an air-fuel ratio equal to the target air-fuel ratio A / Ft is supplied. At this time, the catalyst 11 continues to occlude oxygen. However, when the oxygen is occluded until it is saturated, that is, full, it can no longer occlude oxygen. As a result, the lean gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 changes to the lean side, and at the time t1 when the output voltage Vr reaches a predetermined lean determination value VL (for example, 0.21 V), the target air-fuel ratio A / Ft becomes the rich air-fuel ratio A / F. It is switched to Fr (for example, 14.1). Thus, rich control is started, and rich gas having an air-fuel ratio equal to the target air-fuel ratio A / Ft is supplied.

  When the rich gas is supplied, the catalyst 11 continues to release the stored oxygen. When the stored oxygen is eventually released from the catalyst 11, the catalyst 11 cannot release oxygen at that time, and the rich gas passes through the catalyst 11 and flows out downstream of the catalyst 11. When this happens, the output of the post-catalyst sensor 18 changes to the rich side, and at the time t2 when the output voltage Vr reaches a predetermined rich determination value VR (for example, 0.59 V), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / It is switched to Fl (for example, 15.1). As a result, the lean control is started again, and a lean gas having an air-fuel ratio equal to the target air-fuel ratio A / Ft is supplied.

  When the catalyst 11 again stores oxygen until it is full and the output voltage Vr of the post-catalyst sensor 18 reaches the lean determination value VL, the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t3. Rich control is started.

  In this way, every time the output of the post-catalyst sensor 18 is inverted, the lean control and the rich control are alternately and repeatedly executed.

  During execution of this active air-fuel ratio control, the oxygen storage capacity OSC of the catalyst 11 is measured by the following method.

  The larger the oxygen storage capacity of the catalyst 11, the longer the time during which oxygen can be stored or released. That is, when the catalyst has not deteriorated, the inversion period of the post-catalyst sensor output Vr (for example, the time from t1 to t2) becomes longer, and the inversion period becomes shorter as the deterioration of the catalyst proceeds.

  Therefore, using this fact, the oxygen storage capacity OSC is measured as follows. As shown in FIG. 4, immediately after the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t1, the pre-catalyst air-fuel ratio A / Ff as an actual value is slightly delayed with the rich air-fuel ratio A / Ff. Switch to Fr. From the time t11 when the pre-catalyst air-fuel ratio A / Ff reaches the stoichiometric A / Fs to the time t2 when the post-catalyst sensor output Vr is next reversed, the oxygen storage capacity for each predetermined calculation cycle is obtained by the following equation (1). dOSC is sequentially calculated, and the oxygen storage capacity dOSC is sequentially accumulated from time t11 to time t2. Thus, the oxygen storage capacity OSC as the final integrated value during the rich control, that is, the amount of released oxygen indicated by OSCb in FIG. 4 is measured.

  Q is the fuel injection amount. When the air-fuel ratio difference ΔA / F is multiplied by the fuel injection amount Q, the air amount that is insufficient or excessive with respect to the stoichiometry can be calculated. σ is a constant representing the proportion of oxygen contained in air (about 0.23).

  Similarly, during lean control, the oxygen storage capacity, that is, the amount of stored oxygen indicated by OSCa in FIG. 4 is measured. Each time the rich control and the lean control are alternately performed, the released oxygen amount and the stored oxygen amount are alternately measured.

  As can be seen here, “oxygen storage capacity” is a generic term for “amount of released oxygen” and “amount of stored oxygen”. The “released oxygen amount” refers to the amount of oxygen released by the catalyst during rich control, and the “occluded oxygen amount” refers to the amount of oxygen stored by the catalyst during lean control.

  If a plurality of measured values of the released oxygen amount and the occluded oxygen amount are obtained in this way, whether the catalyst is normal or abnormal is determined by the following method.

  First, the ECU 20 calculates an average value of a plurality of measured values of the released oxygen amount and the stored oxygen amount. Then, the average value is compared with a predetermined abnormality determination value α. The ECU 20 determines that the upstream catalyst 11 is normal when the average value is greater than the abnormality determination value α, and determines that the upstream catalyst 11 is abnormal when the average value is equal to or less than the abnormality determination value α. When it is determined that the upstream catalyst 11 is abnormal, it is preferable to activate a warning device (not shown) such as a check lamp in order to notify the user of the fact.

  By the way, as described above, sulfur (S) may be contained in the fuel at a relatively high concentration depending on the region of use. When such a high sulfur fuel is supplied, the catalyst may be sulfur poisoned (S poison) due to the influence of the sulfur component in the exhaust gas. When S poisoning occurs, the activity of the catalyst is reduced and the oxygen absorption / release action of the catalyst is hindered, so that the apparent oxygen storage capacity of the catalyst is reduced. However, when the fuel with a low sulfur concentration is refueled or the catalyst is exposed to a high temperature and rich atmosphere, the poisoning state is eliminated.

  The catalyst performance degradation due to S poisoning is temporary and recoverable. Therefore, in the abnormality diagnosis of the catalyst, it is necessary not to mistakenly diagnose the temporary abnormality due to the S poisoning as an unrecoverable permanent abnormality (thermal deterioration) that should be diagnosed.

  Therefore, in order to prevent such a misdiagnosis, in this embodiment, the diagnosis of the catalyst is executed by the following method.

  First, the principle of S poisoning in the catalyst will be described with reference to FIG. The figure schematically shows the state of the catalyst surface in three temperature ranges with respect to the catalyst temperature Tc. These three temperature regions are composed of a first region I in a low temperature region, a second region II in an intermediate temperature region, and a third region III in a high temperature region. In addition, the numerical value in a figure is an example and can change according to the characteristic etc. of a catalyst.

  First, in the first region I, as shown in the figure, SOx (sulfur oxide) is adsorbed on the coating material 31 containing the oxygen storage component, and S (sulfur) is adsorbed on the catalyst component 32 mainly made of noble metal. Yes. However, in this state, the amount of S adsorption on the catalyst component 32 is relatively small, and the activity of the catalyst does not decrease so much. Therefore, in this state, the measured value of the oxygen storage capacity OSC does not decrease so much as compared with the case where there is no S poisoning, and is within the allowable range.

  However, when the catalyst temperature Tc rises and shifts from the first region I to the second region II, the SOx adsorbed on the coating material 31 is desorbed and reduced. At this time, the S contained in the SOx is converted to the catalyst component 32. The catalyst component 32 is coated. Then, the state of S poisoning deteriorates, the activity of the catalyst is remarkably reduced, and the measured value of the oxygen storage capacity OSC is unacceptably lowered as compared with the case of no S poisoning.

  On the other hand, when the catalyst temperature Tc further increases and shifts from the second region II to the third region III, S covering the catalyst component 32 and SOx adsorbed on the coating material 31 are almost completely desorbed, S poisoning is almost completely eliminated. Then, compared with the case where there is no S poisoning, the measured value of the oxygen storage capacity OSC does not decrease so much and returns to the allowable range.

When S on the catalyst component 32 is desorbed, if the catalyst is in a rich atmosphere, S desorbs as H ₂ S (hydrogen sulfide) as shown in the figure. On the other hand, when the catalyst is in a lean atmosphere, S is desorbed as SOx.

  In FIG. 7, the decreasing rate of the oxygen storage capacity measured value in each area | region I-III is shown. Here, the decrease rate is based on the measured value when there is no S poison. As shown in the figure, the decrease rate is significantly large only in the second region II, which is outside the allowable range. On the other hand, the decrease rate of the first region I and the third region III is small and within an allowable range.

  Regarding the data shown in the figure, the data in the first region I is for the catalyst temperature Tc = 550 ° C., the data in the second region II is for the catalyst temperature Tc = 700 ° C., and the data in the third region III is the catalyst temperature Tc = The ones are at 800 ° C., and all are for catalysts with 0.1 (g) of S accumulated. However, these numerical values are also examples.

  FIG. 8 shows the first to third regions I to III defined by the catalyst temperature Tc (° C.) and the amount of S actually adsorbed on the catalyst, that is, the S accumulation amount W (g). The S accumulation amount W is the sum of the amount of S adhering to the catalyst component 32 and the amount of S in the SOx adsorbed on the coating material 31. In addition, the numerical value in a figure is an example and can change according to the characteristic etc. of a catalyst.

  The line a is a characteristic line that defines the boundary between the first region I and the second region II. As shown in the figure, the line a shows an oblique characteristic in which the temperature increases as the S accumulation amount W decreases, and the first region I expands toward the high temperature side as the S accumulation amount W decreases.

  On the other hand, the line b is a characteristic line that defines the boundary between the second region II and the third region III. As shown in the figure, the line b shows a constant temperature regardless of the S accumulation amount W, and is about 750 (° C.) in the illustrated example.

  As described above, in the first region I on the lower temperature side than the line a and the third region III on the higher temperature side than the line b, the measured value of the oxygen storage capacity OSC is within the allowable range even if there is S poisoning. The possibility of misdiagnosis is low. On the other hand, in the second region II, which is higher than the line a and lower than the line b, when the S poison is present, the measured value of the oxygen storage capacity OSC is out of the allowable range, and the possibility of misdiagnosis is high.

  Therefore, in the present embodiment, the catalyst temperature Tc of the upstream catalyst 11 is acquired by detection or estimation, and the S accumulation amount W in the upstream catalyst 11 is estimated. The acquired catalyst temperature is referred to as an acquired catalyst temperature, and the estimated S accumulation amount is referred to as an estimated S accumulation amount. Based on the estimated S accumulation amount W, the catalyst temperature threshold T1 is determined using the characteristic line a in FIG. 8, and the diagnosis is permitted or prohibited depending on whether or not the acquired catalyst temperature Tc is less than the threshold T1. . The threshold value T1 is referred to as a first threshold value.

  Specifically, the diagnosis is permitted when the acquired catalyst temperature Tc is lower than the first threshold value T1, and the diagnosis is prohibited when the acquired catalyst temperature Tc is equal to or higher than the first threshold value.

  The first threshold value T1 is a value on the characteristic line a. As described above, the first threshold value T1 is higher as the S accumulation amount is smaller, and the temperature at which SOx starts to desorb from the coating material 31 in the upstream catalyst 11, in other words, the adsorption of SOx to the coating material 31. It is the temperature that indicates the limit.

  Thereby, diagnosis is prohibited in the second region II where the possibility of misdiagnosis is high, and misdiagnosis can be prevented in advance. In addition, diagnosis is permitted in the first region II where the possibility of misdiagnosis is low, and the diagnosis frequency can be advantageously ensured.

  In the present embodiment, the diagnosis is permitted when the acquired catalyst temperature Tc is equal to or higher than another threshold value higher than the first threshold value, that is, the second threshold value T2.

  The second threshold value T1 is a value on the characteristic line b. As described above, the second threshold value T1 is constant regardless of the S accumulation amount, and is the minimum temperature at which S poisoning is almost completely eliminated.

  Thereby, diagnosis is permitted in the third region III where the possibility of misdiagnosis is low, and the diagnosis frequency can be advantageously ensured.

  Although the catalyst temperature Tc of the upstream catalyst 11 can be directly detected by a temperature sensor, in the present embodiment, the ECU 20 estimates it based on parameters (for example, the rotational speed and load) representing the engine operating state. A known method can be used as the estimation method at this time.

  The estimation of the S accumulation amount W in the upstream catalyst 11 is as follows.

  Basically, the S accumulation amount of the upstream catalyst 11 is obtained by subtracting the amount of S desorbed from the upstream catalyst 11, that is, the S desorption amount, from the amount of S adsorbed by the upstream catalyst 11, that is, the S adsorption amount. It is done. This is because the S adsorption / desorption reaction on the catalyst is considered to occur simultaneously.

  The ECU 20 sequentially accumulates the S adsorption amount W1 for each calculation cycle obtained from the following equation (2), and uses the integrated value ΣW1 as the current or current S adsorption amount.

  Csox is the SOx concentration of exhaust gas, Ms is the molecular weight of S, Ma is the molecular weight of air, A is a predetermined coefficient, and Ga is the amount of intake air. In the case of this embodiment, the SOx concentration Csox is a constant conforming value obtained by experiments. However, an SOx sensor may be provided in the exhaust passage upstream of the upstream catalyst 11, and the SOx concentration detected thereby may be Csox. The coefficient A is a variable that is determined according to the exhaust air-fuel ratio (referred to as detected air-fuel ratio) A / Ff detected by the pre-catalyst sensor 17 at the current calculation timing, and is smaller as the detected air-fuel ratio A / Ff is richer. This is because the richer the exhaust air-fuel ratio, the more S does not adsorb. The intake air amount Ga is a value detected by the air flow meter 5 at the current calculation time.

  On the other hand, the ECU 20 sequentially accumulates the S desorption amount W2 for each calculation cycle obtained from the following equation (3), and sets the accumulated value ΣW2 as the current or current S desorption amount.

  Vs is an S desorption speed, and may be a predetermined map (function) based on the S accumulation amount W at the previous calculation time, the acquired catalyst temperature tc and the detected air-fuel ratio A / Ff at the current calculation time. ). The larger the S accumulation amount W, the higher the acquired catalyst temperature tc, and the richer the detected air-fuel ratio A / Ff, the larger the S desorption amount W2 for each calculation cycle.

  Finally, the ECU 20 obtains the current S accumulation amount W by subtracting the current S desorption amount ΣW2 from the current S adsorption amount ΣW1 (W = ΣW1-ΣW2).

  The ECU 20 stores the characteristics and area divisions as shown in FIG. 8 in the form of a map. Then, the ECU 20 determines a first threshold value T1 corresponding to the estimated S accumulation amount W from the map. Basically, the ECU 20 permits diagnosis when the acquired catalyst temperature Tc is lower than the first threshold value T1 (Tc <T1), that is, when it is in the first region I. Diagnosis is prohibited when the acquired catalyst temperature Tc is equal to or higher than the first threshold value T1 and lower than the second threshold value T2 (T1 ≦ Tc <T2), that is, in the second region II. Diagnosis is permitted when the acquired catalyst temperature Tc is equal to or higher than the second threshold value T2 (T2 ≦ Tc), that is, in the third region III.

  Therefore, as shown by pattern (i) in FIG. 9, as the acquired catalyst temperature Tc increases, diagnosis is permitted until the acquired catalyst temperature Tc reaches the first threshold value T1, and the acquired catalyst temperature Tc is the first threshold. After reaching the value T1, the diagnosis is prohibited until the second threshold value T2 is reached, and after the acquired catalyst temperature Tc reaches the second threshold value T2, the diagnosis is permitted.

  On the other hand, in the present embodiment, when the acquired catalyst temperature Tc becomes lower than the first threshold value T1 after the diagnosis is prohibited, depending on the history of the acquired catalyst temperature Tc being equal to or higher than the second threshold value T2, Allow or disallow diagnosis.

  Specifically, as shown by the pattern (ii) in FIG. 9, the diagnosis is prohibited when there is no history in which the acquired catalyst temperature Tc is equal to or higher than the second threshold value T2.

  On the other hand, if there is a history that the acquired catalyst temperature Tc is equal to or higher than the second threshold T2 as shown by the pattern (iii) in FIG. 9, the diagnosis is permitted. In this case, the diagnosis is permitted even when the acquired catalyst temperature Tc is equal to or higher than the first threshold value T1 and lower than the second threshold value T2 after the diagnosis is prohibited.

  Once entering the second region II and the diagnosis is prohibited, the catalyst component 32 of the upstream catalyst 11 is covered with S, and if the diagnosis is performed in this state, a misdiagnosis may occur. Therefore, in the present embodiment, only when there is a history that the acquired catalyst temperature Tc is equal to or higher than the second threshold T2, that is, when it is considered that the S poisoning has been almost completely eliminated by entering the third region III. Allow diagnosis on the colder side. By doing so, it is possible to prevent misdiagnosis and to advantageously ensure the diagnosis frequency.

  In addition, as shown by the pattern (iii) in FIG. 9, when entering the third region III after the diagnosis is prohibited, it passes through the second region II before returning to the first region II. Diagnosis is allowed during. Even when entering the second region II from a state in which S poisoning is almost completely eliminated, there is almost no SOx or S on the coating material 31 or the catalyst component 32, and there is substantially no S coating on the catalyst component 32. This is because the possibility of diagnosis is low. However, in order to improve reliability, a modification in which diagnosis is prohibited while passing through the second region II is also possible.

  On the other hand, in the present embodiment, when the acquired catalyst temperature Tc is lower than the first threshold value T1 and in the vicinity of the first threshold value T1 during execution of the diagnosis, the temperature rise for suppressing the temperature increase of the exhaust gas. Temperature suppression control is executed.

  That is, as shown in FIG. 10, the temperature rise suppression control is performed in a temperature range (ie, a temperature range lower than the first threshold value T1 from a start value T1 ′ that is lower than the first threshold value T1 by a relatively small predetermined amount ΔT). Near the first threshold value T1). As the acquired catalyst temperature Tc increases, the temperature rise suppression control is stopped (off) until the acquired catalyst temperature Tc reaches the start value T1 '. Further, after the acquired catalyst temperature Tc reaches the start value T1 ', the temperature rise suppression control is performed (ON) until the first threshold value T1 is reached. After the acquired catalyst temperature Tc reaches the first threshold value T1, the temperature rise suppression control is stopped (off).

  When such temperature rise suppression control is executed, the temperature rise of the exhaust gas supplied to the upstream catalyst 11 and thus the upstream catalyst 11 can be suppressed, and the increase in the acquired catalyst temperature Tc can be suppressed. Therefore, the acquired catalyst temperature Tc can be stopped in the first region I for as long as possible, and the diagnosis frequency can be advantageously ensured.

  On the other hand, if the acquired catalyst temperature Tc rises to the first threshold value T1 or more even if the temperature rise suppression control is executed, the diagnosis is prohibited, and the temperature rise suppression control is stopped at the same time. Is done. As a result, it is possible to prevent performing unnecessary temperature rise suppression control.

  Here, the case where the acquired catalyst temperature Tc rises to the first threshold value T1 or more even when the temperature rise suppression control is executed includes, for example, a case where the engine is accelerated relatively large. On the other hand, when it is not in such a state, there is a high possibility that the acquired catalyst temperature Tc will remain in the vicinity of the first threshold value T1 by executing the temperature rise suppression control.

  Therefore, in the present embodiment, a predetermined additional condition or weighting condition is provided, the acquired catalyst temperature Tc is in the vicinity of the first threshold value T1, and the engine load is less than a predetermined value (for example, 30%) (this is an additional condition) The temperature increase suppression control is executed only in the case of Thereby, the acquired catalyst temperature Tc can be stopped in the vicinity of the first threshold value T1 by executing the temperature rise suppression control.

  On the other hand, when any one of these conditions is not satisfied, the temperature rise suppression control is stopped. In particular, even if the acquired catalyst temperature Tc is in the vicinity of the first threshold value T1, if the engine load is not less than the predetermined value, the temperature increase suppression control is stopped. In this case, the engine load is relatively large due to engine acceleration or the like, and it is considered difficult to keep the acquired catalyst temperature Tc close to the first threshold value T1 even by executing the temperature rise suppression control. On the contrary, in this case, it is better to give up the temperature increase suppression by the temperature increase suppression control and wait for the acquired catalyst temperature Tc to rise above the second threshold value T2.

  In this way, when the acquired catalyst temperature Tc is in the vicinity of the first threshold value T1, the temperature increase suppression control is executed only under conditions that allow the temperature increase to be suppressed, so that the control can be optimized. Various other examples of additional conditions are possible.

Here, the temperature rise suppression control includes at least one of the following controls.
(A) Advance control of ignition timing. If the ignition timing is advanced, the combustion timing is advanced, so that more heat of the gas in the combustion chamber is taken away by the engine block, and an increase in the exhaust temperature is suppressed.
(B) When there is an EGR device, increase control of the EGR amount. Increasing the EGR amount slows down combustion and suppresses an increase in exhaust gas temperature.
(C) When there is an exhaust valve timing variable device, the exhaust valve timing is retarded (slow open). When the exhaust valve is opened slowly, more heat of the exhaust gas in the combustion chamber is taken away by the engine block, and an increase in the exhaust temperature is suppressed.
(D) In the case of a hybrid system or vehicle including an electric motor as another power source, increase control of the motor assist rate. As the motor assist rate is increased, the engine load is reduced, and an increase in the exhaust temperature is suppressed.
(E) When a water-cooled exhaust manifold is provided, control for increasing the cooling efficiency of the exhaust manifold. As a result, more exhaust gas heat is taken away by the exhaust manifold, and an increase in exhaust temperature is suppressed.

  In the temperature increase suppression control, the temperature increase suppression amount is feedback-corrected in accordance with at least one of the acquired catalyst temperature Tc and the engine load so that the acquired catalyst temperature Tc is maintained in the vicinity of the first threshold value T1. Also good.

  Next, a routine executed by the ECU 20 will be described. First, a main routine of diagnosis including measurement of the oxygen storage capacity OSC and determination of catalyst normal / abnormality will be described with reference to FIG. This routine is repeatedly executed by the ECU 20 every predetermined calculation cycle (for example, 16 msec). Note that a measurement flag, which will be described later, is initialized at the same time as the ignition switch (IG) is turned on and turned off.

First, in step S101, it is determined whether or not a predetermined precondition for executing diagnosis is satisfied. For example, the precondition is satisfied when the following conditions (1) to (5) are all satisfied.
(1) The engine is in a warm-up state.
(2) The upstream catalyst 11 and the downstream catalyst 19 are in a warm-up state.
(3) The pre-catalyst sensor 17 and the post-catalyst sensor 18 are in an active state.
(4) The engine is in a steady operation state.
(5) The diagnosis during the current trip is incomplete.

  The success or failure of the condition (1) is determined based on the detection value of a water temperature sensor (not shown). For example, the condition is satisfied when the detection value is 75 ° C. or higher. The success or failure of the condition (2) is determined based on the temperature of each catalyst estimated separately. The success or failure of the condition (3) is determined based on the element impedance of each sensor. The success or failure of the condition (4) is determined, for example, by whether or not the fluctuation range of the intake air amount Ga and the engine speed Ne within a predetermined period is within a predetermined range. Regarding condition (5), the diagnosis is performed once during one trip of the engine, and the diagnosis is performed only when the diagnosis during the current trip is incomplete. Other examples of the precondition are possible.

  If the precondition is not satisfied, the routine is terminated. If the precondition is satisfied, the process proceeds to step S102.

  In step S102, it is determined whether or not the prohibition flag is off. Although details will be described later with reference to FIG. 12, the prohibition flag is a flag for prohibiting diagnosis.

  When the prohibition flag is not off (on), the process proceeds to step S104, the measurement flag is turned off, and the routine is terminated. As will be understood later, the measurement flag is a flag that is turned on during measurement of the oxygen storage capacity OSC (substantially during execution of diagnosis) and turned off at other times.

  On the other hand, if the prohibition flag is off, the process proceeds to step S103, where the measurement flag is turned on, active air-fuel ratio control is executed in step S105, and the oxygen storage capacity OSC is measured. As described above, the oxygen storage capacity OSC here means an average value of a plurality of measured values of the released oxygen amount and the stored oxygen amount.

  In step S106, it is determined whether or not the measurement of the oxygen storage capacity OSC is completed. If not completed, the routine is terminated. If completed, the measurement flag is turned off in step S107.

  Next, in step S108, the measured value of the oxygen storage capacity OSC is compared with a predetermined abnormality determination value α. When the measured value of the oxygen storage capacity OSC is larger than the abnormality determination value α, the upstream catalyst 11 is determined to be normal in step S109. On the other hand, when the measured value of the oxygen storage capacity OSC is equal to or less than the abnormality determination value α, the upstream catalyst 11 is determined to be abnormal in step S110.

  Next, a routine for prohibition flag on / off processing relating to diagnosis permission / prohibition will be described with reference to FIG. This routine is also repeatedly executed by the ECU 20 every predetermined calculation cycle (for example, 16 msec). Note that a prohibition flag, a second region determination flag (referred to as R2 flag), and a third region determination flag (referred to as R3 flag), which will be described later, are initialized and turned off at the same time as the ignition switch (IG) is turned on.

  First, in step S201, the value of the S accumulation amount W that is always calculated in another routine is acquired.

  In step S202, first and second threshold values T1 and T2 corresponding to the acquired S accumulation amount W are calculated from the map as shown in FIG.

  Next, in step S203, the value of the catalyst temperature Tc that is constantly calculated in another routine is acquired, and the acquired catalyst temperature Tc is compared with the first threshold value T1.

If the acquired catalyst temperature Tc is less than the first threshold value T1, it is determined in step S204 whether the R2 flag is on. If the R2 flag is on, it is determined in step S205 whether the R3 flag is on. If the R3 flag is on, the prohibition flag is turned off in step S206.
If the prohibition flag is OFF, step S102 in FIG. 11 is YES, the measurement of the oxygen storage capacity OSC is permitted, and the diagnosis is permitted.

If the R2 flag is off in step S204, the process proceeds directly to step S206, and the prohibition flag is turned off.
When the R2 flag is off in step S205, the process proceeds to step S209 and the prohibition flag is turned on. When the prohibition flag is turned on, step S102 in FIG. 11 becomes no, the measurement of the oxygen storage capacity OSC is prohibited, and the diagnosis is prohibited.

  When the acquired catalyst temperature Tc is equal to or higher than the first threshold value T1 in step S203, the process proceeds to step S207, and it is determined whether or not the acquired catalyst temperature Tc is equal to or higher than the first threshold value T1 and lower than the second threshold value. The If the determination result is yes, the R2 flag is turned on in step S208, and the process proceeds to step S205. Once the R2 flag is turned on, it remains on until the next IG is turned on.

  As can be seen from the above, the R2 flag is a flag that indicates whether or not there is a history that the acquired catalyst temperature Tc is equal to or higher than the first threshold value T1 and lower than the second threshold value (that is, a history that has entered the second region II). Yes, this flag is on when there is a history, and off when there is no history.

  If the determination result in step S207 is no, this means that the acquired catalyst temperature Tc is equal to or higher than the second threshold value T2. In this case, the process proceeds to step S210, the R3 flag is turned on, and the prohibition flag is turned off in step S211. Once the R3 flag is turned on, it is kept on until the next IG is turned on.

  As can be seen from this, the R3 flag is a flag indicating whether or not there is a history in which the acquired catalyst temperature Tc is equal to or higher than the second threshold (that is, a history that has entered the third region III). This flag is turned off when there is no history.

  Next, a routine for temperature rise suppression control will be described with reference to FIG. This routine is also repeatedly executed by the ECU 20 every predetermined calculation cycle (for example, 16 msec).

  In step S301, it is determined whether the measurement flag is on. As described above, the measurement flag is turned on during the measurement of the oxygen storage capacity OSC in step S103 of FIG. If it is off, the routine ends. If it is on, the routine proceeds to step S302.

  In step S302, it is determined whether or not the acquired catalyst temperature Tc is less than the first threshold value T1 and in the vicinity of the first threshold value T1, that is, the acquired catalyst temperature Tc is equal to or higher than the start value T1 ′ and the first threshold value. It is determined whether or not the temperature is below the value T1. If the determination result is no, the routine ends. If the determination result is yes, the process proceeds to step S303.

In step S303, it is determined whether or not a predetermined additional condition is satisfied. That is, it is determined whether the engine load is less than a predetermined value (for example, 30%).
If the additional condition is satisfied, the temperature rise suppression control is executed in step S304. On the other hand, if the additional condition is not satisfied, the routine is terminated. If the temperature rise suppression control is not executed in step S304, this means that the temperature rise suppression control is substantially stopped.

  According to these routines, the following operation is realized. First, as shown in the pattern (i) of FIG. 9, when the acquired catalyst temperature Tc is first in the first region I, the R2 flag and the R3 flag are both off, so the process proceeds directly from step S204 to step S206 in FIG. The prohibit flag is turned off. Then, in FIG. 11, step S102 becomes YES, the measurement flag is turned on in step S103, and steps S105 to S110 are executed. That is, the diagnosis is permitted.

  At this time, if the acquired catalyst temperature Tc is between the start value T1 ′ and the first threshold value T1 and the additional condition is satisfied, steps S301, S302, and S303 in FIG. Temperature suppression control is executed.

  Next, when the acquired catalyst temperature Tc rises and enters the second region II, the R2 flag is turned on in step S208 of FIG. In this case, since the R3 flag is still off, the process proceeds from step S205 to step S209, and the prohibition flag is turned on. In FIG. 11, step S102 becomes no, the measurement flag is turned off in step S104, and steps S105 to S110 cannot be executed, that is, the diagnosis is prohibited. Since the measurement flag is off, step S301 in FIG. 13 is no, and the temperature rise suppression control is stopped.

  Next, when the acquired catalyst temperature Tc further rises and enters the third region III, the R3 flag is turned on in step S210 of FIG. 12, and the prohibition flag is turned off in step S211. As a result, the diagnosis is permitted.

  On the other hand, as shown in the pattern (iii) of FIG. 9, when the acquired catalyst temperature Tc decreases from the third region III and returns to the second region II, the R2 flag is turned on in step S208 of FIG. Since it is already on, the process proceeds from step S205 to step S206, the prohibition flag is turned off, and the diagnosis is permitted.

  Next, when the acquired catalyst temperature Tc further decreases and returns from the second region II to the first region I, both the R2 flag and the R3 flag are already on, so both steps S204 and S205 become yes and prohibited in S206. The flag is turned off and the diagnosis is permitted.

  On the other hand, as shown in the pattern (ii) of FIG. 9, when the acquired catalyst temperature Tc decreases from the second region II and returns to the first region I, the R2 flag is already on and the R3 flag is off. Thus, step S204 becomes yes and step S205 becomes no, the process proceeds to step S209, the prohibition flag is turned on, and the diagnosis is prohibited.

  During the measurement of the oxygen storage capacity OSC, the prohibition flag is switched from off to on, and the measurement may be interrupted in the middle. In this case, the measured value that has already been measured can be stored in the ECU 20 and used in the next measurement. That is, the diagnosis can be suspended and the measurement data already acquired at the next diagnosis can be reused.

  By the way, in the above-described diagnosis, it is preferable to execute temperature increase control for increasing the temperature of the exhaust gas while the diagnosis is prohibited.

  As described above, once the diagnosis is prohibited once entering the second region II, the diagnosis prohibited state is maintained unless entering the third region III thereafter. This is disadvantageous from the viewpoint of ensuring diagnosis frequency. Therefore, it is preferable to execute the temperature rise control while the diagnosis is prohibited. As a result, the catalyst temperature can be raised while diagnosis is prohibited, and entry into the third region III and early release of the diagnosis prohibited state (or early diagnosis permission) can be promoted. And it is possible to advantageously ensure the diagnosis frequency.

The temperature increase control is a control opposite to the temperature increase suppression control. In other words, the temperature rise control includes at least one of the following controls.
(A ′) Ignition timing retardation control. If the ignition timing is retarded, the combustion timing is delayed, so that the heat of the gas in the combustion chamber is not easily taken by the engine block, and the exhaust temperature rises.
(B ′) When there is an EGR device, the EGR amount reduction control. If the EGR amount is reduced, combustion becomes active and the exhaust temperature rises.
(C ′) When there is an exhaust valve timing variable device, advance (rapid opening) control of exhaust valve timing. When the exhaust valve is opened quickly, the exhaust gas is discharged in a state where the heat of the exhaust gas in the combustion chamber is not taken away by the engine block, and the exhaust temperature rises.
(D ′) In the case of a hybrid system or vehicle having an electric motor as another power source, motor assist rate reduction control. As the motor assist rate is reduced, the engine load increases and the exhaust temperature rises.
(E ′) Control that reduces the cooling efficiency of the exhaust manifold when a water-cooled exhaust manifold is provided. As a result, the heat of the exhaust gas is less likely to be taken away by the exhaust manifold, and the exhaust temperature rises.

  A routine for temperature rise control will be described with reference to FIG. This routine is also repeatedly executed by the ECU 20 every predetermined calculation cycle (for example, 16 msec).

  In step S401, it is determined whether the prohibition flag is on. If it is off, the routine ends. If it is on, the routine proceeds to step S402, where temperature rise suppression control is executed.

  As a result, the temperature increase control is executed while the diagnosis is prohibited, and it is possible to promote entry into the third region III and early release of the diagnosis prohibited state.

  Note that this temperature rise control during diagnosis prohibition can be performed not only when diagnosis is permitted but also when there is a history of entering the third region III after diagnosis prohibition, while diagnosis is prohibited in any case. . This is because at least S poisoning can be eliminated at an early stage.

  Although the embodiment of the present invention has been described in detail above, various other embodiments of the present invention are conceivable. For example, the use and type of the internal combustion engine are arbitrary, and may be other than for automobiles, or may be a direct injection type or the like. When diagnosing the catalyst, it is possible to make a diagnosis using a value other than the oxygen storage capacity. For example, the trajectory length or area of the sensor output after the catalyst can be used. The numerical values shown in the above embodiment are merely examples, and can be changed to other values.

  The present invention includes all modifications, applications, and equivalents included in the spirit of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 Internal combustion engine
5 Airflow meter 6 Exhaust pipe 11 Upstream catalyst 12 Injector 17 Pre-catalyst sensor 18 Post-catalyst sensor 20 Electronic control unit (ECU)

Claims (9)

An apparatus for diagnosing whether a catalyst provided in an exhaust passage of an internal combustion engine is normal or abnormal,
Obtaining the catalyst temperature by detection or estimation, estimating the amount of sulfur accumulation in the catalyst,
Determining a threshold value of the catalyst temperature based on the estimated sulfur accumulation amount;
Diagnosis is permitted or prohibited depending on whether or not the acquired catalyst temperature is less than the threshold value. The catalyst abnormality diagnosis device according to claim 1, wherein the threshold value is higher as the estimated sulfur accumulation amount is smaller. 3. The catalyst abnormality diagnosis device according to claim 1, wherein the threshold value is a temperature at which sulfur oxide starts to be desorbed from a coating material containing an oxygen storage component in the catalyst. The catalyst abnormality diagnosis according to any one of claims 1 to 3, wherein diagnosis is permitted when the acquired catalyst temperature is lower than the threshold value, and diagnosis is prohibited when the acquired catalyst temperature is equal to or higher than the threshold value. apparatus. The catalyst abnormality diagnosis device according to any one of claims 1 to 4, wherein diagnosis is permitted when the acquired catalyst temperature is equal to or higher than another threshold value higher than the threshold value. . When the acquired catalyst temperature becomes less than the threshold value after prohibition of diagnosis, the diagnosis is permitted or prohibited depending on whether there is a history of the acquired catalyst temperature being equal to or higher than the other threshold value. The catalyst abnormality diagnosis device according to claim 5. Diagnosis is permitted when there is a history that the acquired catalyst temperature is equal to or higher than the other threshold value, and diagnosis is prohibited when there is no history that the acquired catalyst temperature is equal to or higher than the other threshold value. The catalyst abnormality diagnosis device according to claim 6. The temperature increase suppression control for suppressing the temperature increase of the exhaust gas is executed when the acquired catalyst temperature is lower than the threshold value and in the vicinity of the threshold value during diagnosis. Item 8. The catalyst abnormality diagnosis device according to any one of Items 1 to 7. The catalyst abnormality diagnosis device according to any one of claims 1 to 8, wherein a temperature increase control for increasing the temperature of the exhaust gas is executed while the diagnosis is prohibited.

Images