JP2009138604A - Catalyst deterioration diagnosis device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve diagnosis accuracy and reliability by detecting sulfur poisoning in at least one catalyst in a catalyst deterioration diagnosis device for an internal combustion engine with an upstream catalyst and a downstream catalyst arranged in an exhaust passage. <P>SOLUTION: The last oxygen occlusion volume and the before last oxygen occlusion volume of the upstream catalyst 11 and the downstream catalyst 19 are measured, and the present oxygen occlusion volume of the downstream catalyst 19 is measured. The sulfur poisoning of the upstream catalyst and the downstream catalyst is detected on the basis of a varied amount from the before last oxygen occlusion volume to the last oxygen occlusion volume of the upstream catalyst, a varied amount from the before last oxygen occlusion volume to the last oxygen occlusion volume of the downstream catalyst, and a varied amount from the oxygen occlusion volume to the present oxygen occlusion volume of the downstream catalyst. Existence of sulfur poisoning is accurately detected by using a fact that sulfur poisoning of both catalysts is mutually different. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine.

For example, in an internal combustion engine for a vehicle, a catalyst for purifying exhaust gas is installed in the exhaust system. Some of these catalysts have an oxygen storage capacity (O ₂ storage capacity). This is because when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes larger than the stoichiometric air-fuel ratio (stoichiometric), that is, the exhaust gas becomes lean. Excess oxygen present in the gas is adsorbed and held, and when the air-fuel ratio of the catalyst inflow exhaust gas becomes smaller than the stoichiometric, that is, becomes rich, the adsorbed and held 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, such an air-fuel ratio shift can be absorbed by the oxygen storage / release action of the three-way catalyst.

  By the way, when the catalyst deteriorates, the purification efficiency of the catalyst decreases. On the other hand, there is a correlation between the degree of deterioration of the catalyst and the degree of reduction of the oxygen storage capacity because they are reactions through noble metals. Therefore, it is possible to detect that the catalyst has deteriorated by detecting that the oxygen storage capacity has decreased. In general, active air-fuel ratio control for forcibly switching the air-fuel ratio of the exhaust gas flowing into the catalyst to rich and lean is performed, and the oxygen storage capacity of the catalyst is measured as the active air-fuel ratio control is executed. A method (so-called Cmax method) for diagnosing deterioration of the ink is employed.

  On the other hand, sulfur (S) may be contained in the fuel at a relatively high concentration depending on the region of use. When such fuel is refueled, poisoning (S poisoning) in which the performance of the catalyst is degraded due to the influence of the sulfur component in the exhaust gas occurs. When S poisoning occurs, the oxygen storage / release action of the catalyst is hindered, and the oxygen storage capacity of the catalyst decreases. However, if fuel with a low sulfur concentration is refueled, the poisoning state will eventually be resolved. The catalyst performance degradation due to S poisoning is temporary and recoverable. Therefore, in the deterioration diagnosis of the catalyst, it is necessary not to mistakenly diagnose that the temporary deterioration due to the S poisoning is an irrecoverable permanent deterioration (thermal deterioration) that should be diagnosed. In particular, it is necessary to prevent a catalyst that is still normal while being in the vicinity of the boundary between normality and deterioration (criteria) from being erroneously diagnosed as deterioration.

  As a countermeasure, it is conceivable to perform a process of desorbing sulfur by transiently correcting the air-fuel ratio when the catalyst is poisoned with sulfur (see, for example, Patent Document 1), and then performing a deterioration diagnosis. Such a forced desorption of sulfur still has a problem in that it further deteriorates the catalyst.

JP-A-8-158857

  On the other hand, according to the result of earnest research by the present inventors, it has been found that when the upstream catalyst and the downstream catalyst are disposed in the exhaust passage of the internal combustion engine, the way of sulfur poisoning differs between the upstream catalyst and the downstream catalyst. . Therefore, if S poisoning of at least one catalyst can be detected by utilizing this fact, the above-described erroneous diagnosis can be prevented in advance, which is advantageous in improving diagnostic accuracy and reliability.

  Therefore, the present invention has been made in view of such circumstances, and its object is to provide sulfur poisoning of at least one catalyst when an upstream catalyst and a downstream catalyst are disposed in an exhaust passage of an internal combustion engine. An object of the present invention is to provide a catalyst deterioration diagnosis device for an internal combustion engine that can detect and improve diagnosis accuracy and reliability.

According to one aspect of the invention,
In an internal combustion engine in which an upstream catalyst and a downstream catalyst are arranged in an exhaust passage of the internal combustion engine, the apparatus diagnoses deterioration of the upstream catalyst and the downstream catalyst,
Measuring means for measuring the oxygen storage capacity of the upstream catalyst and the downstream catalyst;
When the oxygen storage capacity and the previous oxygen storage capacity of the upstream catalyst and the downstream catalyst are measured by the measuring means, and the current oxygen storage capacity of the downstream catalyst is measured, the previous time from the previous time of the upstream catalyst is measured. Of the upstream catalyst and the downstream catalyst based on the change amount of the oxygen storage capacity from the previous time to the previous time of the downstream catalyst and the change amount of the oxygen storage capacity from the previous time to the current time of the downstream catalyst. An apparatus for diagnosing catalyst deterioration in an internal combustion engine is provided, comprising: a sulfur poisoning detection unit that detects poison.

  According to the results of earnest studies by the present inventors, it has been found that the oxygen storage capacity of the catalyst is reduced by the sulfur component in the fuel due to the following two phenomena. The first phenomenon is a phenomenon in which SOx in the exhaust gas reacts with a catalyst component, that is, a noble metal supported on the catalyst, a reaction rate of the noble metal is lowered, and thus an oxygen storage capacity of the catalyst is lowered. The second phenomenon is a phenomenon in which SOx in the exhaust gas is adsorbed on the occlusion component of the catalyst, and the oxygen occlusion capacity of the catalyst is thereby lowered. In the case of the upstream catalyst, since the catalyst temperature is higher than that of the downstream catalyst, the second phenomenon, that is, the adsorption of sulfur to the storage component does not easily occur, and the decrease in the oxygen storage capacity is caused by the first phenomenon, that is, the reaction rate of the noble metal decreases. Dominant. At this time, the decrease in the oxygen storage capacity occurs immediately after the fuel is replaced from the low sulfur fuel to the high sulfur fuel (that is, immediately after the high sulfur fuel is supplied). On the other hand, in the case of the downstream catalyst, the second phenomenon occurs along with the first phenomenon, and the oxygen storage capacity decreases, and the decrease gradually proceeds after the fuel exchange. Thus, a difference is seen in the way of sulfur poisoning of both catalysts.

  Considering this difference, the present invention can suitably detect sulfur poisoning of the upstream catalyst and the downstream catalyst. For example, when the oxygen storage capacity change amount from the previous time to the previous time of the upstream catalyst and the oxygen storage capacity change amount from the previous time to the previous time of the downstream catalyst are larger than a predetermined value, there is a possibility of sulfur poisoning of both catalysts. Provisionally estimated. Then, when the oxygen storage capacity change amount from the previous time to the current time of the downstream catalyst is larger than a predetermined value, it is finally detected that both catalysts are sulfur poisoned. The second catalyst is unlikely to occur in the upstream catalyst. Therefore, once the oxygen storage capacity is reduced from the previous time to the previous time, the second phenomenon is not changed in the downstream catalyst. Since this occurs, the oxygen storage capacity further decreases even after it has decreased. By utilizing this difference, it is possible to accurately detect sulfur poisoning of the catalyst and improve diagnostic accuracy and reliability.

  Preferably, determination means for determining deterioration of the upstream catalyst and downstream catalyst based on the oxygen storage capacity measured by the measurement means, and sulfur poisoning of the upstream catalyst and downstream catalyst are detected by the sulfur poisoning detection means. And a determination prohibiting means for prohibiting the determination by the determination means. As a result, it is possible to prevent erroneous determination of deterioration based on the measured value of the oxygen storage capacity that has decreased due to sulfur poisoning.

  Preferably, the previous oxygen storage capacity and the previous oxygen storage capacity of the upstream catalyst and the downstream catalyst are values measured by the previous trip and the previous trip, respectively, with fueling interposed therebetween. Since the decrease in the oxygen storage capacity due to sulfur poisoning occurs immediately after the fuel change, i.e., refueling, it is preferable to examine the change in the oxygen storage capacity before and after refueling.

According to another aspect of the invention,
In an internal combustion engine in which an upstream catalyst and a downstream catalyst are arranged in an exhaust passage of the internal combustion engine, the apparatus diagnoses deterioration of the upstream catalyst and the downstream catalyst,
Measuring means for measuring the oxygen storage capacity of the upstream catalyst and the downstream catalyst;
When the previous oxygen storage capacity and the current oxygen storage capacity of the upstream catalyst and the downstream catalyst are measured by the measuring means, the oxygen storage capacity change amount from the previous time to the current time of the upstream catalyst, and the previous time of the downstream catalyst. And a sulfur poisoning detecting means for detecting sulfur poisoning of the upstream catalyst and the downstream catalyst based on the amount of change in oxygen storage capacity from the current time to the present time. .

  When sulfur poisoning occurs in the upstream catalyst and the downstream catalyst, the oxygen storage capacities of both catalysts, which are first measured immediately after fuel replacement, are greatly reduced. Therefore, as in this alternative embodiment, the sulfur capacities of the upstream catalyst and the downstream catalyst are also determined based on the amount of change in the oxygen storage capacity from the previous time to the current time of the upstream catalyst and the amount of change in the oxygen storage capacity from the previous time to the current time of the downstream catalyst. It is possible to detect poison.

According to yet another aspect of the invention,
An internal combustion engine in which an upstream catalyst and a downstream catalyst are disposed in an exhaust passage of the internal combustion engine, wherein the apparatus diagnoses deterioration of the upstream catalyst,
Measuring means for measuring the oxygen storage capacity of the upstream catalyst;
Sulfur poisoning detection for detecting sulfur poisoning of the upstream catalyst based on the amount of change in oxygen storage capacity from the previous time to the current time when the previous oxygen storage capacity and the current oxygen storage capacity are measured by the measuring means. An apparatus for diagnosing catalyst deterioration of an internal combustion engine is provided.

  When the upstream catalyst and the downstream catalyst are disposed in the exhaust passage of the internal combustion engine, the oxygen storage capacity of the upstream catalyst is greatly reduced immediately after the fuel replacement due to the refueling of the high sulfur fuel. Therefore, by utilizing this, the sulfur poisoning of the upstream catalyst can be detected based on the amount of change in the oxygen storage capacity from the previous time to the current time of the upstream catalyst.

  According to the present invention, when an upstream catalyst and a downstream catalyst are disposed in an exhaust passage of an internal combustion engine, sulfur poisoning of at least one of the catalysts can be detected, and diagnostic accuracy and reliability can be improved. Excellent effect is exhibited.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the configuration of the present embodiment. As shown in the figure, the internal combustion engine 1 generates power by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston 4 in the combustion chamber 3. To do. The internal combustion engine 1 is a vehicular multi-cylinder engine (only one cylinder is shown), and is a spark ignition type internal combustion engine, more specifically, a gasoline engine.

  In the cylinder head of the internal combustion engine 1, an intake valve Vi for opening and closing the intake port and an exhaust valve Ve for opening and closing the exhaust port are provided 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 air collecting chamber via a branch pipe for each cylinder. 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. An air flow meter 5 for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 in order from the upstream side. An intake passage is formed by the intake port, the surge tank 8 and the intake pipe 13.

  An injector (fuel injection valve) 12 that injects fuel into the intake passage, particularly into the intake port, 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 a branch pipe for each cylinder. The exhaust pipe 6 includes an upstream catalyst 11 made of a three-way catalyst having an oxygen storage capacity and A downstream catalyst 19 is attached in series. An exhaust passage is formed by the exhaust port, the branch pipe, and the exhaust pipe 6. The upstream catalyst 11 is attached at a position relatively close to the combustion chamber 3 so that it can be activated early using exhaust heat. On the other hand, the downstream catalyst 19 is attached at a position relatively far from the combustion chamber 3, such as under the floor of a vehicle.

A pre-catalyst sensor 17 is installed upstream of the upstream catalyst 11, an inter-catalyst sensor 18 is installed between the upstream catalyst 11 and the downstream catalyst 19, and a post-catalyst sensor 21 is installed downstream of the downstream catalyst 19. ing. These pre-catalyst sensor 17, inter-catalyst sensor 18 and post-catalyst sensor 21 are all air-fuel ratio sensors for detecting the air-fuel ratio of the exhaust gas. In particular, the pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, can continuously detect an 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 inter-catalyst sensor 18 and the post-catalyst sensor 21 are so-called O ₂ sensors, and have characteristics that the output value changes suddenly with the theoretical air-fuel ratio as a boundary.

  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, the inter-catalyst sensor 18 and the post-catalyst sensor 21, the ECU 20 includes a crank angle sensor 14 that detects the crank angle of the internal combustion engine 1, an accelerator opening, as shown in the figure. An accelerator opening sensor 15 for detecting the degree 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 throttle valve 10, the injector 12, 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 upstream catalyst 11 and the downstream catalyst 19 simultaneously purify NOx, HC and CO when the air-fuel ratio A / F of the exhaust gas flowing into each of them is a stoichiometric air-fuel ratio (stoichiometric, for example, A / Fs = 14.6). Correspondingly, during normal operation of the internal combustion engine, the ECU 20 matches the stoichiometric air-fuel ratio, that is, the air-fuel ratio of the exhaust gas discharged from the combustion chamber 3 and flowing into the upstream catalyst 11, that is, the pre-catalyst air-fuel ratio A / Ffr. Thus, the air-fuel ratio of the air-fuel mixture is controlled. Specifically, the ECU 20 sets a target air-fuel ratio A / Ft equal to the theoretical air-fuel ratio, and makes the pre-catalyst air-fuel ratio A / Ffr detected by the pre-catalyst sensor 17 coincide with the target air-fuel ratio A / Ft. The fuel injection amount injected from the injector 12 is feedback-controlled. As a result, the air-fuel ratio of the exhaust gas flowing into the catalyst 11 is kept in the vicinity of the theoretical air-fuel ratio, and the maximum purification performance is exhibited in the catalyst 11. In such air-fuel ratio feedback control, the inter-catalyst air-fuel ratio A / Fmd detected by the inter-catalyst sensor 18 matches the stoichiometric air-fuel ratio in addition to the main feedback control that matches the pre-catalyst air-fuel ratio A / Ffr to the stoichiometric air-fuel ratio. Sub feedback control is performed. This sub-feedback control is performed for the purpose of eliminating the deviation of the center air-fuel ratio due to deterioration of the pre-catalyst sensor 17 or the like.

Here, the upstream catalyst 11 and the downstream catalyst 19 to be subjected to deterioration diagnosis will be described in more detail. Since the upstream catalyst 11 and the downstream catalyst 19 have the same configuration, the upstream catalyst 11 will be described as an example. 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 the coating material 31 is held in a state where a large number of particulate catalyst components 32 are dispersedly arranged. It is exposed inside the catalyst 11. 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 the role of a promoter that promotes the reaction at the interface between the exhaust gas and the catalyst component 32 and includes an oxygen storage component that can absorb and release oxygen according to the air-fuel ratio of the atmospheric gas. The oxygen storage component is made of, for example, cerium dioxide CeO ₂ or zirconia. For example, when the atmosphere gas of the catalyst component 32 and the coating material 31 is richer than the theoretical air-fuel ratio, oxygen stored in the oxygen storage component existing around the catalyst component 32 is released, and as a result, the released oxygen As a result, unburned components such as HC and CO are oxidized and purified. Conversely, if the atmosphere gas of the catalyst component 32 and the coating material 31 is leaner than the stoichiometric air-fuel ratio, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmosphere gas, and as a result, NOx is reduced and purified. The

  By such an oxygen absorption / release action, even if the pre-catalyst air-fuel ratio A / Ffr slightly varies from the stoichiometric air-fuel ratio during normal air-fuel ratio control, three exhaust gas components such as NOx, HC and CO are simultaneously purified. can do. Therefore, in normal air-fuel ratio control, it is also possible to purify the exhaust gas by making the pre-catalyst air-fuel ratio A / Ffr oscillate minutely around the theoretical air-fuel ratio and repeating the absorption and release of oxygen.

  By the way, in the catalyst 11 in the new state, as described above, a large number of fine particulate catalyst components 32 are uniformly distributed, and the contact probability between the exhaust gas and the catalyst component 32 is kept high. However, when the catalyst 11 deteriorates with time due to thermal stress, some of the catalyst components 32 are disappeared, and some of the catalyst components 32 are baked and solidified by exhaust heat to become a sintered state ( (See dashed line in figure). In this case, the contact probability between the exhaust gas and the catalyst component 32 is lowered, and the purification rate is lowered. 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, the degree of deterioration of the catalyst 11 and the degree of decrease in the oxygen storage capacity of the catalyst 11 are in a correlation. Therefore, in this embodiment, the degree of deterioration of the catalyst 11 is detected by detecting the oxygen storage capacity of the catalyst 11. Here, the oxygen storage capacity of the catalyst 11 is represented by the size of the oxygen storage capacity (OSC; O ₂ Strage Capacity, the unit is g), which is the maximum amount of oxygen that the current catalyst 11 can store.

  Hereinafter, the catalyst deterioration diagnosis in the present embodiment will be described. For the sake of convenience, the aspect of the deterioration diagnosis (first aspect) when limited to the upstream catalyst 11 having a large emission effect will be described here, and the aspect of the deterioration diagnosis including the downstream catalyst 19 (second aspect) will be described later. I will explain.

  The catalyst deterioration diagnosis of the present embodiment is basically based on the Cmax method described above. When the deterioration diagnosis of the catalyst 11 is performed, the active air-fuel ratio control is executed by the ECU 20. In the active air-fuel ratio control, the air-fuel ratio of the air-fuel mixture, and thus the pre-catalyst air-fuel ratio A / Ffr, is forcibly (actively) switched alternately to the rich side and the lean side with a predetermined center air-fuel ratio A / Fc as a boundary. The air-fuel ratio when changed to the rich side is referred to as rich air-fuel ratio A / Fr, and the air-fuel ratio when changed to the lean side is referred to as lean air-fuel ratio A / Fl. When the pre-catalyst air-fuel ratio A / Ffr is changed to the rich side or the lean side by this active air-fuel ratio control, the oxygen storage capacity OSC of the catalyst 11 is measured.

  The deterioration diagnosis of the catalyst 11 is executed during steady operation of the internal combustion engine 1 and when the catalyst 11 is in the active temperature range. Measurement of the temperature of the catalyst 11 (catalyst bed temperature) may be detected directly using a temperature sensor, but in the present embodiment, it is estimated from the operating state of the internal combustion engine. For example, the ECU 20 estimates the temperature Tc of the catalyst 11 using a preset map based on the intake air amount Ga detected by the air flow meter 5. It should be noted that parameters other than the intake air amount Ga, for example, the engine rotational speed Ne (rpm) may be included in the parameters used for the catalyst temperature estimation.

  In FIGS. 3A and 3B, the outputs of the pre-catalyst sensor 17 and the inter-catalyst sensor 18 when the active air-fuel ratio control is executed are indicated by solid lines. In FIG. 3A, the target air-fuel ratio A / Ft generated inside the ECU 20 is indicated by a broken line. The output values of the pre-catalyst sensor 17 and the inter-catalyst sensor 18 correspond to the values of the pre-catalyst air-fuel ratio A / Ffr and the inter-catalyst air-fuel ratio A / Fmd, respectively.

  As shown in FIG. 3A, the target air-fuel ratio A / Ft is centered on the stoichiometric air-fuel ratio (stoichiometric) A / Fs as the center air-fuel ratio, and a predetermined amplitude (rich amplitude Ar, An air-fuel ratio (rich air-fuel ratio A / Fr) separated by Ar> 0), and an air-fuel ratio (lean air-fuel ratio A / Fl) separated from it by a predetermined amplitude (lean amplitude Al, Al> 0) Forcibly and alternately. Following the switching of the target air-fuel ratio A / Ft, the pre-catalyst air-fuel ratio A / Ffr as an actual value is also switched with a slight time delay with respect to the target air-fuel ratio A / Ft. From this, it will be understood that the target air-fuel ratio A / Ft and the pre-catalyst air-fuel ratio A / Ffr are equivalent except that there is a time delay.

  In the illustrated example, the rich amplitude Ar and the lean amplitude Al are equal. For example, theoretical air fuel ratio A / Fs = 14.6, rich air fuel ratio A / Fr = 14.1, lean air fuel ratio A / Fl = 15.1, rich amplitude Ar = lean amplitude Al = 0.5. Compared with the normal air-fuel ratio control, the active air-fuel ratio control has a larger amplitude of the air-fuel ratio, that is, the values of the rich amplitude Ar and the lean amplitude Al are larger.

  By the way, the timing at which the target air-fuel ratio A / Ft is switched is the timing at which the output of the inter-catalyst sensor 18 switches from rich to lean, or from lean to rich. As shown in the figure, the output voltage of the inter-catalyst sensor 18 suddenly changes with the theoretical air-fuel ratio A / Fs as a boundary, and the inter-catalyst air-fuel ratio A / Fmd is a rich air-fuel ratio smaller than the theoretical air-fuel ratio A / Fs. When the output voltage becomes equal to or higher than the rich determination value VR, and when the inter-catalyst air-fuel ratio A / Fmd is a lean side air-fuel ratio larger than the theoretical air-fuel ratio A / Fs, the output voltage becomes equal to or lower than the lean determination value VL. Here, VR> VL, for example, VR = 0.59 (V) and VL = 0.21 (V).

  As shown in FIGS. 3A and 3B, when the output voltage of the inter-catalyst sensor 18 changes from the rich side value to the lean side and becomes equal to the lean determination value VL (time t1), the target sky The fuel ratio A / Ft is switched from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr. Thereafter, when the output voltage of the inter-catalyst sensor 18 changes from the lean side value to the rich side and becomes equal to the rich judgment value VR (time t2), the target air-fuel ratio A / Ft becomes lean from the rich air-fuel ratio A / Fr. The air-fuel ratio is switched to A / Fl.

  While performing the active air-fuel ratio control that performs such an air-fuel ratio change, the oxygen storage capacity OSC of the catalyst 11 is measured as follows, and the deterioration of the catalyst 11 is determined.

  Referring to FIG. 3, the target air-fuel ratio A / Ft is set to the lean air-fuel ratio A / Fl before time t1, and the lean gas flows into the catalyst 11. At this time, the catalyst 11 continues to absorb oxygen, but when it fully absorbs oxygen, it can no longer absorb oxygen, and the lean gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the inter-catalyst air-fuel ratio A / Fmd changes to the lean side, and when the output voltage of the inter-catalyst sensor 18 reaches the lean determination value VL (t1), the target air-fuel ratio A / Ft becomes the rich air-fuel ratio A / Fr. Or reversed. In this way, the target air-fuel ratio A / Ft is reversed with the output of the inter-catalyst sensor 18 as a trigger.

  This time, rich gas flows into the catalyst 11. At this time, the oxygen stored in the catalyst 11 continues to be released from the catalyst 11. Therefore, the exhaust gas of the theoretical air-fuel ratio A / Fs flows out to the downstream side of the catalyst 11 and the inter-catalyst air-fuel ratio A / Fmd does not become rich, so the output of the inter-catalyst sensor 18 is not reversed. When oxygen is continuously released from the catalyst 11, all of the stored oxygen is eventually released from the catalyst 11, and at that time, no more oxygen can be released, and the rich gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the inter-catalyst air-fuel ratio A / Fmd changes to the rich side, and when the output voltage of the inter-catalyst sensor 18 reaches the rich judgment value VR (t2), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / Fl. Can be switched to.

  The larger the oxygen storage capacity OSC, the longer the time during which oxygen can be absorbed or released. That is, when the catalyst is not deteriorated, the inversion cycle of the target air-fuel ratio A / Ft (for example, the time from t1 to t2) becomes longer, and the inversion cycle of the target air-fuel ratio A / Ft 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 / Ffr as the actual value is slightly delayed with the rich air-fuel ratio A / Fr. Switch to Fr. From the time t11 when the pre-catalyst air-fuel ratio A / Ffr reaches the theoretical air-fuel ratio A / Fs to the time t2 when the target air-fuel ratio A / Ft next reverses, the following equation (1) An oxygen storage capacity dOSC (instantaneous value of the oxygen storage capacity) is calculated, and the oxygen storage capacity dOSC for each minute time is integrated from time t11 to time t2. Thus, the oxygen storage capacity, that is, the amount of released oxygen (OSC (1) in FIG. 4) in this oxygen release cycle is measured.

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

  Basically, the oxygen storage capacity OSC measured at one time is used and compared with a predetermined deterioration judgment value OSCs. If the oxygen storage capacity OSC exceeds the deterioration judgment value OSCs, the oxygen storage capacity is normal. If the OSC is equal to or lower than the deterioration determination value OSCs, the deterioration of the catalyst can be determined such as deterioration. However, in this embodiment, in order to improve the accuracy, the oxygen storage capacity (the stored oxygen amount in this case) is also measured in the oxygen storage cycle in which the target air-fuel ratio A / Ft is on the lean side. The average value is measured as an oxygen storage capacity of one unit related to one absorption / release cycle. Further, the absorption / release cycle is repeated a plurality of times to obtain a value of oxygen storage capacity of a plurality of units, and the average value is used as the final oxygen storage capacity measurement value.

  Regarding the measurement of the oxygen storage capacity (storage oxygen amount) in the oxygen storage cycle, as shown in FIG. 4, after the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio A / Fl at time t2, the pre-catalyst air-fuel ratio is changed. From time t21 when A / Ffr reaches the theoretical air-fuel ratio A / Fs to time t3 when the target air-fuel ratio A / Ft reverses to the rich side next, the oxygen storage capacity dOSC for each minute time is calculated from the previous equation (1). The calculated oxygen storage capacity dOSC for each minute time is integrated. Thus, the oxygen storage capacity OSC, that is, the amount of stored oxygen (OSC (2) in FIG. 4) in this oxygen absorption cycle is measured. The oxygen storage capacity OSC1 of the previous cycle and the oxygen storage capacity OSC2 of the current cycle should be approximately equal.

  Next, the deterioration of the catalyst is determined using the measured oxygen storage capacity. That is, the measured value of the oxygen storage capacity OSC is compared with a predetermined deterioration determination value OSCs. If the value of the oxygen storage capacity OSC is larger than the deterioration determination value OSCs, the catalyst is normal and the value of the oxygen storage capacity OSC is the deterioration determination value. If OSCs or less, the catalyst is determined to be deteriorated. When it is determined that the catalyst is deteriorated, it is preferable to activate a warning device such as a check lamp in order to notify the user of the fact. The above is the basic content of the deterioration diagnosis aspect (first aspect) when the upstream catalyst 11 is limited.

  The method of the first aspect can also be applied to the downstream catalyst 19 alone. In this case, the output of the pre-catalyst sensor 17 is replaced with the output of the inter-catalyst sensor 18, and the output of the inter-catalyst sensor 18 is replaced with the output of the post-catalyst sensor 21.

  Next, a deterioration diagnosis mode (second mode) including the upstream catalyst 11 and the downstream catalyst 19 will be described.

  In this second mode, the target air-fuel ratio A / Ft switching timing in the active air-fuel ratio control is coincident with the inversion timing of the post-catalyst sensor 21 on the downstream side of the downstream catalyst 19, and the upstream catalyst 11 and the downstream catalyst 19 are Oxygen is occluded or released simultaneously and the oxygen storage capacity OSC of the upstream catalyst 11 and the downstream catalyst 19 is measured simultaneously and continuously.

  FIG. 5 shows how each value changes when active air-fuel ratio control is executed. (A) is the target air-fuel ratio A / Ft, (B) is the output of the inter-catalyst sensor 18, (C) is the actual inter-catalyst air-fuel ratio A / Fmd, (D) is the output of the post-catalyst sensor 21, (E) Indicates the actual post-catalyst air-fuel ratio A / Frr.

  In the illustrated example, before the time T1, the target air-fuel ratio A / Ft (and the pre-catalyst air-fuel ratio A / Ffr) is maintained at a rich air-fuel ratio, and the oxygen stored in the upstream catalyst 11 is released to purify the exhaust gas. Then, the exhaust gas having a substantially stoichiometric air-fuel ratio flows downstream. Accordingly, during that time, the inter-catalyst air-fuel ratio A / Fmd is maintained substantially at the stoichiometric air-fuel ratio. When all the stored oxygen in the upstream catalyst 11 is released, a rich gas containing unburned components flows downstream from the upstream catalyst 11, and the output of the inter-catalyst sensor 18 is inverted from the lean side to the rich side correspondingly. In the illustrated example, the output of the inter-catalyst sensor 18 reaches the rich determination value VR at time T1.

  However, unlike the first aspect, the target air-fuel ratio A / Ft is not yet switched at this point. Then, after time T1, exhaust gas rich in air-fuel ratio flows into the downstream catalyst 19. When the rich gas flows, the downstream catalyst 19 purifies the rich gas while releasing the stored oxygen. Then, an exhaust gas having a substantially stoichiometric air-fuel ratio flows downstream. Accordingly, the post-catalyst air-fuel ratio A / Frr is maintained at a substantially stoichiometric air-fuel ratio during that time.

  Eventually, when all of the stored oxygen in the downstream catalyst 19 is released, rich gas flows out downstream of the downstream catalyst 19, and in response to this, the output of the post-catalyst sensor 21 is reversed from the lean side to the rich side. In the illustrated example, at the time T2, the output of the post-catalyst sensor 21 reaches the rich determination value VR. When the output of the post-catalyst sensor 21 is reversed to the rich side, it can be determined that both the upstream catalyst 11 and the downstream catalyst 19 have exhausted all the stored oxygen. Therefore, at the same time, the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio so that the pre-catalyst air-fuel ratio A / Ffr is reversed to lean. Thus, the target air-fuel ratio A / Ft is switched at the timing when the output of the post-catalyst sensor 21 is reversed.

  After the time T2, the air-fuel ratio lean exhaust gas flows into the upstream catalyst 11, and the upstream catalyst 11 purifies the exhaust gas while storing excess oxygen in the exhaust gas. Therefore, after a slight delay time has elapsed from time T2, the inter-catalyst air-fuel ratio A / Fmd changes to the vicinity of the theoretical air-fuel ratio. When the upstream catalyst 11 is fully filled with oxygen, lean gas flows out downstream of the upstream catalyst 11, and the output of the inter-catalyst sensor 18 is inverted from the rich side to the lean side correspondingly. In the illustrated example, the output of the inter-catalyst sensor 18 reaches the lean determination value VL at time T3.

  As described above, at this time, the target air-fuel ratio A / Ft is not yet switched. Then, after time T3, lean exhaust gas having an air-fuel ratio flows into the downstream catalyst 19. When such lean gas flows, the downstream catalyst 19 purifies the lean gas while storing excess oxygen. Therefore, the exhaust gas having a substantially stoichiometric air-fuel ratio flows downstream, and the post-catalyst air-fuel ratio A / Frr is maintained substantially at the stoichiometric air-fuel ratio.

  Eventually, when the downstream catalyst 19 is fully filled with oxygen, lean gas flows out downstream of the downstream catalyst 19, and the output of the post-catalyst sensor 21 is inverted from the rich side to the lean side accordingly. In the illustrated example, the output of the post-catalyst sensor 21 reaches the lean determination value VL at time T4. When the output of the post-catalyst sensor 21 is reversed to the lean side, it can be determined that both the upstream catalyst 11 and the downstream catalyst 19 have occluded oxygen to the full capacity. Accordingly, at the same time, the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio so that the pre-catalyst air-fuel ratio A / Ffr is inverted to be rich.

  Similarly, the rich gas flows into the upstream catalyst 11 and the upstream catalyst 11 releases the stored oxygen. When oxygen is completely released from the upstream catalyst 11, the rich gas flows out downstream of the upstream catalyst 11, and the output of the inter-catalyst sensor 18 is inverted to the rich side (time T5). At this time, the target air-fuel ratio A / Ft is not switched. Then, the rich gas flows into the downstream catalyst 19, and oxygen is released in the downstream catalyst 19. When oxygen is completely released from the downstream catalyst 19, rich gas flows out downstream of the downstream catalyst 19, and the output of the post-catalyst sensor 21 is reversed to the rich side (time T6). Simultaneously with this reversal, the target air-fuel ratio A / Ft is switched to the lean side.

  Along with such active air-fuel ratio control, the oxygen storage capacities OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst 19 are measured in the same manner as in the first mode based on the previous equation (1). That is, when oxygen is stored in the upstream catalyst 11 at times T2 to T3, as in the example shown in FIG. 4, the output of the pre-catalyst sensor 17 changes to the lean side and reaches a value corresponding to the theoretical air-fuel ratio. From the time point to the time point T3 when the output of the inter-catalyst sensor 18 is reversed to the lean side, the oxygen storage capacity dOSC for each predetermined minute time is calculated by the above equation (1), and the oxygen storage capacity dOSC for each minute time is calculated. Accumulated. At this time, as the value of the air-fuel ratio difference ΔA / F, the difference between the air-fuel ratio converted from the output of the pre-catalyst sensor 17 and the stoichiometric air-fuel ratio is used.

  Next, when oxygen is stored in the downstream catalyst 19 at time T3 to T4, from time T3 when the output of the inter-catalyst sensor 18 is inverted to the lean side, to time T4 when the output of the post-catalyst sensor 21 is inverted to the lean side. The oxygen storage capacity dOSC for each predetermined minute time is calculated by the previous equation (1), and the oxygen storage capacity dOSC for each minute time is integrated. At this time, since the lean gas flows through the upstream catalyst 11 and flows into the downstream catalyst 11, the value of the air-fuel ratio ΔA / F is the air-fuel ratio converted from the output of the pre-catalyst sensor 17 and the stoichiometric air-fuel ratio. Differences are used.

  Thereafter, when the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio, the oxygen storage capacity of the upstream catalyst 11 is measured from time T4 to T5 in the same manner, and the oxygen storage capacity of the downstream catalyst 19 is measured from time T5 to T6. Capacity is measured.

  A plurality of measurement data of the oxygen storage capacity thus measured is stored in the ECU 20 and the above-described averaging process is performed to calculate the final oxygen storage capacity measurement values OSC1 and OSC2 of the upstream catalyst 11 and the downstream catalyst 19. . These measured values OSC1 and OSC2 are respectively compared with deterioration determination values OSC1s and OSC2s set individually in advance, and the normality and deterioration of the upstream catalyst 11 and the downstream catalyst 19 are individually determined according to the magnitudes thereof. When it is determined that one of the catalysts is deteriorated, it is preferable to activate a warning device such as a check lamp in order to notify the user of the fact.

  Next, detection of sulfur poisoning of the catalyst in the present embodiment will be described.

  As described above, when a fuel having a high sulfur concentration is used, the catalyst is poisoned with sulfur, and the oxygen storage capacity of the catalyst is temporarily reduced, thereby causing a problem such as erroneous diagnosis. By the way, according to the results of intensive studies by the present inventors, it was found that the upstream catalyst 11 and the downstream catalyst 19 differ in the way of S poisoning.

  That is, there are the following two phenomena in which the oxygen storage capacity of the catalyst decreases due to the sulfur component in the fuel. The first phenomenon is a phenomenon in which SOx in the exhaust gas reacts with the catalyst component 32, that is, the noble metal supported on the catalyst, the reaction rate of the noble metal decreases, and the oxygen storage capacity of the catalyst thus decreases. The decrease in the oxygen storage capacity at this time occurs immediately after the fuel is replaced from the low sulfur fuel to the high sulfur fuel (that is, immediately after the high sulfur fuel is refueled), and the amount of the decrease is the sulfur concentration of the fuel and the catalyst. Depends on the oxygen storage capacity of The second phenomenon is a phenomenon in which SOx in the exhaust gas is adsorbed on the storage component 31 of the catalyst, and the oxygen storage capacity of the catalyst is reduced. The decrease in the oxygen storage capacity at this time gradually proceeds slowly after the fuel is replaced from the low sulfur fuel to the high sulfur fuel.

  Here, paying attention to the upstream catalyst 11, the high-temperature exhaust gas from the combustion chamber 3 is initially and constantly flowing into the upstream catalyst 11, and the catalyst temperature is high. Therefore, the second phenomenon, that is, the adsorption of sulfur content to the storage component 31 hardly occurs, and the decrease in the oxygen storage capacity of the upstream catalyst 11 is predominantly caused by the first phenomenon, that is, the reaction rate of the noble metal is decreased.

  On the other hand, since the catalyst temperature of the downstream catalyst 19 is lower than that of the upstream catalyst 11, a second phenomenon occurs along with the first phenomenon, and the oxygen storage capacity decreases. In other words, the oxygen storage capacity of the upstream catalyst 11 rapidly decreases immediately after the fuel exchange and hardly changes thereafter, whereas the oxygen storage capacity of the downstream catalyst 19 gradually decreases even after the rapid decrease immediately after the fuel replacement. Go.

  Therefore, in the present embodiment, in view of such a difference in the S poisoning method, sulfur poisoning of the catalyst is detected as follows, and further, deterioration determination of the catalyst is prohibited.

  First, the first embodiment in which the degradation diagnosis target is limited to the upstream catalyst 11 will be described. FIG. 6 shows a change in the oxygen storage capacity OSC1 of the upstream catalyst 11 before and after the replacement when the fuel is replaced with the high sulfur fuel. “This time” is synonymous with the present time and means a reference time or period. “Last time” means “one time before this time”. “Previous times” used later means “one time before the previous time”. A “trip” is a period during which the engine (or engine system) is on. In the illustrated example, the fuel is changed during the engine off between the previous trip and the current trip. The solid line shows the actual change in oxygen storage capacity.

As shown in the figure, the oxygen storage capacity OSC1 _n-1 (previous OSC) of the upstream catalyst 11 is measured in the previous trip, and the oxygen storage capacity OSC1 _n (current OSC) of the upstream catalyst 11 is also measured in the current trip. . Then, with respect to the oxygen storage capacity OSC1 _n-1 of the previous, current oxygen storage capacity OSC1 _n immediately after refueling vary greatly, are significantly reduced and more specifically. Therefore, if the amount of change or reduction from the previous oxygen storage capacity OSC1 _n-1 to the current oxygen storage capacity OSC1 _n is calculated, it is detected that high sulfur fuel has been supplied based on this value, and further upstream It can be detected that the catalyst 11 is sulfur poisoned. Here, the oxygen storage capacity change amount (or decrease amount) ΔOSC1 is simply defined as the difference between the previous oxygen storage capacity OSC1 _n-1 and the current oxygen storage capacity OSC1 _n , that is, ΔOSC1 = OSC1 _n-1 −OSC1 _n. However, other definitions are possible, for example, ΔOSC1 = (OSC1 _n−1 −OSC1 _n ) / OSC1 _n−1 may be defined.

  The catalyst deterioration diagnosis process relating to the first embodiment will be described with reference to FIG. This process is executed by the ECU 20.

  First, in step S101, it is determined whether a precondition for starting diagnosis is satisfied. For example, the engine is in a steady operation state such that the fluctuation range of the intake air amount Ga and the engine rotational speed Ne is within a predetermined range, and the catalyst 11 and the air-fuel ratio sensors 17, 18, and 21 are at a predetermined activation temperature. If it has been reached, the precondition is satisfied. Note that the precondition is not limited to the example described here. If the precondition is not satisfied, the process is terminated. On the other hand, if the precondition is satisfied, the process proceeds to step S102.

  In step S102, the oxygen storage capacity OSC1 of the upstream catalyst 11 is measured. Here, since only the upstream catalyst 11 is to be diagnosed, it is preferable to employ the oxygen storage capacity measuring method as described in the first mode shown in FIGS. 3 and 4. However, the oxygen storage capacity measurement method as described in the second embodiment shown in FIG. 5 may be adopted, and only the oxygen storage capacity measurement value of the upstream catalyst 11 may be used.

  Next, in step S103, it is determined whether or not at least one of misfire and air-fuel ratio abnormality has been detected, specifically, whether or not there is such a detection history. That is, in the present embodiment, an abnormality detection means for detecting at least one of misfire and air-fuel ratio abnormality is provided. For example, when the fluctuation of the engine rotational speed Ne is larger than a predetermined value, or when a large rich air-fuel ratio is detected by the pre-catalyst sensor 17 or when there is a hydrogen sensor in the exhaust passage before the upstream catalyst, the hydrogen concentration in the exhaust gas is When it is detected that the value is higher than the predetermined value, it can be determined that a misfire has occurred. Also, for example, when the correction value of the main feedback air-fuel ratio control is a value that corrects the air-fuel ratio richly and is maintained at a predetermined limit value, the reason is that fuel is not sufficiently supplied due to a fuel system failure It can be determined that the air-fuel ratio abnormality has occurred.

  When inferior fuel, dissimilar fuel (for example, light oil) or the like is supplied, abnormal combustion or misfire may occur and the catalyst may melt. Accordingly, in this step S103, it is determined whether or not a factor of catalyst melting loss has occurred. When an affirmative determination is made in step S103, a normal deterioration determination is performed in step S107. That is, the oxygen storage capacity OSC1 of the upstream catalyst 11 measured in step S102 is compared with the deterioration determination value OSC1s. If OSC1> OSC1s, the upstream catalyst 11 is determined to be normal, and if OSC1 ≦ OSC1s, the upstream catalyst 11 is determined to be deteriorated. If the catalyst is melted, this is permanent deterioration that cannot be recovered. Therefore, the deterioration is determined as usual, and if the result is deterioration, the check lamp is turned on to prompt the user to replace the catalyst.

On the other hand, when a negative determination is made in step S103, the process proceeds to step S104, and the oxygen storage capacity change amount ΔOSC1 of the upstream catalyst 11 is calculated. That is, using the oxygen storage capacity OSC1 _n measured in step S102 at the time of the current process and the oxygen storage capacity OSC1 _n-1 measured in step S102 at the time of the previous process, ΔOSC1 = OSC1 _n-1 −OSC1 _n Thus, the oxygen storage capacity change amount ΔOSC1 is calculated.

  Next, the oxygen storage capacity change amount ΔOSC1 is compared with a predetermined threshold value α (see FIG. 6). When ΔOSC1 ≦ α, the process proceeds to step S107, and the deterioration determination is made based on the oxygen storage capacity measurement value OSC1 of the upstream catalyst 11 measured in step S102 as usual.

  On the other hand, when ΔOSC1> α, the oxygen storage capacity OSC1 of the upstream catalyst 11 has greatly changed (decreased), the process proceeds to step S106, where it is determined that the upstream catalyst 11 has sulfur poisoning, and at the same time, the upstream catalyst. 11 is not permitted to be determined based on the measured oxygen storage capacity value OSC1. In this way, sulfur poisoning of the upstream catalyst 11 is suitably detected, deterioration determination based on the oxygen storage capacity measurement value OSC1 greatly reduced by sulfur poisoning is prohibited, and misdiagnosis is prevented, and diagnostic accuracy and reliability are improved. Can be improved.

  Next, another catalyst deterioration diagnosis process relating to the first embodiment will be described with reference to FIG. This process is executed by the ECU 20. Note that the same steps as the processing in FIG. 7 are simply described by changing the reference numerals to the 200s, and detailed description thereof will be omitted.

  This other process is different from the above process in that step S202A is added after step S202, and step S208 is added after step S206.

  Step S202A is executed after step S202. In step S202A, it is determined whether or not the oxygen storage capacity measurement in step S202 is the first oxygen storage capacity measurement after the fuel change. Since the decrease in the oxygen storage capacity due to sulfur poisoning of the upstream catalyst 11 occurs abruptly immediately after fuel replacement, adding information on whether or not fuel replacement is performed in this way is advantageous in improving diagnostic accuracy. In this case, means for detecting whether or not fuel replacement is performed is provided. For example, the means includes a fuel tank remaining amount sensor (fuel gauge) and a sensor for detecting the opening of the fueling lid. If it is detected that the remaining amount of fuel in the fuel tank has increased or the refueling lid has been opened, it is considered that fuel replacement has been performed.

  If a negative determination is made in step S202A, the routine proceeds to step S207 and a normal deterioration determination is performed. On the other hand, if an affirmative determination is made in step S202A, steps S203 to S208 are performed and whether or not high sulfur fuel is supplied. As a result, the presence or absence of sulfur poisoning of the upstream catalyst 11 is detected. That is, in this other process, the sulfur poisoning detection of the upstream catalyst 11 is executed only at the time of the first oxygen storage capacity measurement after the fuel exchange.

  Further, after the sulfur poisoning of the upstream catalyst 11 is detected in step S206 and the deterioration determination of the upstream catalyst 11 is prohibited, the process proceeds to step S208 where the check lamp is turned on, and at the same time a diagnostic code to that effect is recorded in the ECU 20. Is done. As a result, the user can be prompted to supply the low sulfur fuel again. Further, at a later maintenance stage, the maintenance staff can recognize that the upstream catalyst 11 is temporarily poisoned with sulfur and that no catalyst replacement is required.

  Next, a description will be given of a second embodiment where the deterioration diagnosis target is both the upstream catalyst 11 and the downstream catalyst 19. 9 and 10 show changes in the oxygen storage capacities of the upstream catalyst 11 and the downstream catalyst 19 before and after fuel replacement, respectively. In the illustrated example, the fuel is changed between the previous trip and the previous trip.

As shown in the figure, the oxygen storage capacities of the upstream catalyst 11 and the downstream catalyst 19 are measured in the last trip, the previous trip, and the current trip, respectively. In the case of the upstream catalyst 11 shown in FIG. 9, the oxygen storage capacity OSC1 _n-1 of the previous time immediately after the fuel change is greatly reduced with respect to the oxygen storage capacity OSC1 _n-2 of the previous time, as shown in FIG. . However, in the case of the upstream catalyst 11, since the second phenomenon, that is, the sulfur adsorption to the oxygen storage component 31 is difficult to occur, since then, there is almost no further decrease, and therefore, with respect to the previous oxygen storage capacity OSC1 _n-1. The current oxygen storage capacity OSC1 _n is almost unchanged.

On the other hand, in the case of the downstream catalyst 19 shown in FIG. 10, the previous oxygen storage capacity OSC2 _n-1 immediately after the fuel change is greatly reduced as compared to the oxygen storage capacity OSC2 _n-2 of the last time, but further decrease is seen after this decrease. Therefore, the current oxygen storage capacity OSC2 _n is lower than the previous oxygen storage capacity OSC2 _n-1 . The amount of decrease between the previous time and the current time is smaller than the amount of decrease between the previous time and the previous time, and the oxygen storage capacity OSC2 of the downstream catalyst 19 tends to decrease greatly immediately after the fuel change, and continues to decrease gradually thereafter. The reason for this is that, as described above, in the downstream catalyst 19, when high sulfur fuel is supplied, in addition to the first phenomenon, that is, the reaction rate of the noble metal is decreased, the second phenomenon, that is, adsorption of the sulfur content on the oxygen storage component 31. Because it happens. Since the sulfur content is gradually adsorbed, the oxygen storage capacity gradually decreases. Considering the above oxygen storage capacity change characteristics, the catalyst deterioration diagnosis is executed as follows.

  Hereinafter, the catalyst deterioration diagnosis process according to the second embodiment will be described with reference to FIG. This process is executed by the ECU 20.

  First, in step S301, as in step S101, it is determined whether a precondition for starting diagnosis is satisfied. If the precondition is not satisfied, the process is terminated. If the precondition is satisfied, the process proceeds to step S302.

  In step S302, the oxygen storage capacity OSC1 of the upstream catalyst 11 is measured, and in the next step S303, the oxygen storage capacity OSC2 of the downstream catalyst 19 is measured. Here, since the diagnosis target is the upstream catalyst 11 and the downstream catalyst 19, the oxygen storage capacity measuring method as described in the second mode shown in FIG. 5 is adopted.

In the next step S304, the oxygen storage capacity change amount ΔOSC1 of the upstream catalyst 11 is calculated, and in the next step S305, the oxygen storage capacity change amount ΔOSC2 of the downstream catalyst 19 is calculated. That is, the oxygen storage capacity change amount ΔOSC1 = OSC1 _n−1 −OSC1 _n of the upstream catalyst 11 from the time of the previous processing to the time of the current processing is calculated by the same method as in step S104, and then from the time of the previous processing. The oxygen storage capacity change amount ΔOSC2 = OSC2 _n-1 −OSC2 _n of the downstream catalyst 19 until the current processing is calculated.

  Thereafter, in step S306, the oxygen storage capacity change amount ΔOSC1 of the upstream catalyst 11 is compared with a predetermined threshold value α (see FIG. 9). When ΔOSC1> α, the routine proceeds to step S307, where the oxygen storage capacity change amount ΔOSC2 of the downstream catalyst 19 is compared with a predetermined threshold value β (see FIG. 10). When ΔOSC2> β, the routine proceeds to step S308, where it is determined that the upstream catalyst 11 and the downstream catalyst 19 are sulfur poisoned, and at the same time, the deterioration determination based on the oxygen storage capacity measurement values OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst 19 is performed. It is forbidden. Thus, the sulfur poisoning of the upstream catalyst 11 and the downstream catalyst 19 is suitably detected, and the deterioration determination of both catalysts based on the oxygen storage capacity measurement value greatly reduced by sulfur poisoning is prohibited, thereby preventing erroneous diagnosis and diagnosis. Accuracy and reliability can be improved.

  On the other hand, when ΔOSC1 ≦ α in step S306, or when ΔOSC2 ≦ β in step S307, the process proceeds to step S309, and normal deterioration determination for the upstream catalyst 11 and the downstream catalyst 19 is made. That is, the oxygen storage capacity measurement values OSC1 and OSC2 of the upstream catalyst 11 and the downstream catalyst 19 measured in steps S102 and S103 are compared with the deterioration determination values OSC1s and OSC2s, respectively, and the measured value is larger than the deterioration determination value. A catalyst that is normal and whose measured value is equal to or less than the deterioration determination value is determined to be deteriorated. Here, for example, ΔOSC1> α for the upstream catalyst 11, but for the downstream catalyst 19, even when ΔOSC2 ≦ β, a normal deterioration determination is made in step S309. In this case, there is a high possibility of melting of the upstream catalyst 11, and this is determined as deterioration at the time of determining deterioration in step S309. Therefore, it is possible to detect permanent deterioration due to melting of one catalyst.

  As can be seen from the above description, in the diagnosis process, the presence or absence of sulfur poisoning is detected for both catalysts based on the oxygen storage capacity measurement values at two consecutive measurement timings. On the other hand, in another catalyst deterioration diagnosis process described below, the presence or absence of sulfur poisoning is detected for both catalysts based on the oxygen storage capacity measurement values at three consecutive measurement timings.

  12 and 13 are flowcharts of another catalyst deterioration diagnosis process relating to the second embodiment. This process is executed by the ECU 20. The process of FIG. 12 is executed one time before the process of FIG. 13, and if associated with the examples of FIGS. 9 and 10, for example, the process of FIG. 12 is executed in the previous trip shown in FIGS. The process of FIG. 13 is executed in the current trip shown in FIGS. 9 and 10.

  Steps S401 to S407 and S409 in the process of FIG. 12 are the same as steps S301 to S307 and S309 in the process of FIG. Further, the content of step S408 in the process of FIG. 12 is replaced from that in step S308 of the process in FIG. In step S408, the sulfur poisoning detection temporary flag is set instead of executing the sulfur poisoning determination and the prohibition of the deterioration determination. That is, here, regarding the upstream catalyst 11 and the downstream catalyst 19, when the oxygen storage capacity change amounts ΔOSC1 and ΔOSC2 from the previous time to the previous time shown in FIGS. 9 and 10 are larger than the predetermined values α and β, the possibility of sulfur poisoning is tentative. Estimated. The deterioration determination is suspended, that is, substantially prohibited.

  Next, the process of FIG. 13 is executed. Steps S501 to S503 are the same as steps S301 to S303 in the process of FIG. After step S503, step S504 is executed, and it is determined whether or not the sulfur poisoning detection temporary flag is set. When the flag is not set, the process proceeds to step S509, and the normal deterioration determination for the upstream catalyst 11 and the downstream catalyst 19 is made as in step S309.

  When the flag is set, the process proceeds to step S505, and the oxygen storage capacity change amount ΔOSC2 between the previous processing and the current processing of the downstream catalyst 19 (from the previous time to the current time shown in FIGS. 9 and 10). Is calculated). In step S506, the oxygen storage capacity change amount ΔOSC2 is compared with a predetermined threshold value γ (see FIG. 10). When ΔOSC2 ≦ γ, the process proceeds to step S509, and the normal deterioration determination for the upstream catalyst 11 and the downstream catalyst 19 is made.

  On the other hand, when ΔOSC2> γ, the process proceeds to step S507, where it is finally determined that the upstream catalyst 11 and the downstream catalyst 19 are sulfur poisoned, and at the same time, deterioration determination for the upstream catalyst 11 and the downstream catalyst 19 is prohibited. In step S508, the sulfur poisoning detection temporary flag is cleared, and the process ends.

  As described above, when a large decrease in the oxygen storage capacity of both catalysts is detected in the process of FIG. 12, it is confirmed whether or not the oxygen storage capacity of the downstream catalyst is further decreased by the process of FIG. Since sulfur poisoning detection is performed, the detection accuracy can be improved, the diagnostic accuracy and reliability can be improved, and erroneous diagnosis can be prevented more reliably.

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 form of the internal combustion engine are arbitrary, and may be other than for vehicles, for example, a direct injection type or the like. A wide air-fuel ratio sensor similar to the pre-catalyst sensor may be used for the post-catalyst sensor, and an O ₂ sensor similar to the post-catalyst sensor may be used for the pre-catalyst sensor. A wide range of sensors that detect the exhaust air-fuel ratio, including these wide-range air-fuel ratio sensors and O ₂ sensors, are referred to as air-fuel ratio sensors. The present invention is applicable to any catalyst having an oxygen storage capacity in addition to a three-way catalyst.

  6, 9, and 10 show an example in which the oxygen storage capacity is measured only once per trip. However, the present invention is not limited to this, and oxygen storage is performed multiple times per trip or once per multiple trips. The present invention can also be applied to measuring the capacity, and the above-described deterioration diagnosis processes can also be applied.

  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.

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 regarding the 1st mode of catalyst degradation diagnosis. FIG. 4 is a time chart similar to FIG. 3 for illustrating a method for measuring the oxygen storage capacity. It is a time chart regarding the 2nd mode of catalyst degradation diagnosis. It is a graph which shows the change before and behind fuel replacement | exchange of the oxygen storage capacity of an upstream catalyst regarding 1st Example. It is a flowchart which shows the catalyst deterioration diagnostic process regarding 1st Example. It is a flowchart which shows another catalyst deterioration diagnostic process regarding 1st Example. It is a graph which shows the change before and behind fuel replacement | exchange of the oxygen storage capacity of an upstream catalyst regarding 2nd Example. It is a graph which shows the change before and behind fuel replacement | exchange of the oxygen storage capacity of a downstream catalyst regarding 2nd Example. It is a flowchart which shows the catalyst deterioration diagnostic process regarding 2nd Example. It is a flowchart which shows another catalyst deterioration diagnostic process regarding 2nd Example. It is a flowchart which shows another catalyst deterioration diagnostic process regarding 2nd Example.

Explanation of symbols

1 Internal combustion engine 6 Exhaust pipe 11 Upstream catalyst 12 Injector 17 Pre-catalyst sensor 18 Inter-catalyst sensor 19 Downstream catalyst 20 Electronic control unit (ECU)
21 Post-catalyst sensor OSC Oxygen storage capacity OSC1 Upstream catalyst oxygen storage capacity OSC2 Downstream catalyst oxygen storage capacity ΔOSC1 Upstream catalyst oxygen storage capacity change ΔOSC2 Downstream catalyst oxygen storage capacity change

Claims (5)

In an internal combustion engine in which an upstream catalyst and a downstream catalyst are arranged in an exhaust passage of the internal combustion engine, the apparatus diagnoses deterioration of the upstream catalyst and the downstream catalyst,
Measuring means for measuring the oxygen storage capacity of the upstream catalyst and the downstream catalyst;
When the oxygen storage capacity and the previous oxygen storage capacity of the upstream catalyst and the downstream catalyst are measured by the measuring means, and the current oxygen storage capacity of the downstream catalyst is measured, the previous time from the previous time of the upstream catalyst is measured. Of the upstream catalyst and the downstream catalyst based on the change amount of the oxygen storage capacity from the previous time to the previous time of the downstream catalyst and the change amount of the oxygen storage capacity from the previous time to the current time of the downstream catalyst. An apparatus for diagnosing catalyst deterioration in an internal combustion engine, comprising: a sulfur poisoning detection unit that detects poison. Determining means for determining deterioration of the upstream catalyst and downstream catalyst based on the oxygen storage capacity measured by the measuring means;
The internal combustion engine according to claim 1, further comprising: a determination prohibiting unit that prohibits determination by the determination unit when sulfur poisoning of the upstream catalyst and the downstream catalyst is detected by the sulfur poisoning detection unit. Catalyst deterioration diagnosis device. The preceding oxygen storage capacity and the previous oxygen storage capacity of the upstream catalyst and the downstream catalyst are values measured by a previous trip and a previous trip, respectively, with fueling interposed therebetween. 3. A catalyst deterioration diagnosis device for an internal combustion engine according to 2. In an internal combustion engine in which an upstream catalyst and a downstream catalyst are arranged in an exhaust passage of the internal combustion engine, the apparatus diagnoses deterioration of the upstream catalyst and the downstream catalyst,
Measuring means for measuring the oxygen storage capacity of the upstream catalyst and the downstream catalyst;
When the previous oxygen storage capacity and the current oxygen storage capacity of the upstream catalyst and the downstream catalyst are measured by the measuring means, the oxygen storage capacity change amount from the previous time to the current time of the upstream catalyst, and the previous time of the downstream catalyst. And a sulfur poisoning detection unit for detecting sulfur poisoning of the upstream catalyst and the downstream catalyst based on the amount of change in oxygen storage capacity from the current time to the present time. An internal combustion engine in which an upstream catalyst and a downstream catalyst are disposed in an exhaust passage of the internal combustion engine, wherein the apparatus diagnoses deterioration of the upstream catalyst,
Measuring means for measuring the oxygen storage capacity of the upstream catalyst;
Sulfur poisoning detection for detecting sulfur poisoning of the upstream catalyst based on the amount of change in oxygen storage capacity from the previous time to the current time when the previous oxygen storage capacity and the current oxygen storage capacity are measured by the measuring means. And a catalyst deterioration diagnosis device for an internal combustion engine.

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