JP2009121414A - Catalyst deterioration diagnosing device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To carry out precise and reliable diagnosis regardless of the sulfur concentration of fuel. <P>SOLUTION: A plurality of active air/fuel ratio control having different center air/fuel ratios is executed, thereby measuring a plurality of oxygen occlusion amount corresponding to respective center air/fuel ratios. The sulfur concentration of fuel is estimated according to the change rate of the oxygen occulusion amount to change of the center air/furl ratio. According to the estimated sulfur concentration, an oxygen occulusion capacity measuring value is corrected if necessary. The oxygen occulusion capacity measuring value is corrected to be a precise value without sulfur influence, so that the precise and reliable diagnosis can be executed. <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 supplied, poisoning (S poisoning) occurs in which sulfur components accumulate in the catalyst and the performance of the catalyst decreases. When S poisoning occurs, the oxygen storage / release reaction 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 performance degradation of the catalyst due to S poisoning is temporary. Therefore, in the deterioration diagnosis of the catalyst, it is necessary not to mistakenly diagnose the temporary deterioration due to the S poisoning as permanent 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.

  In order to solve this problem, the technique disclosed in Patent Document 1 uses the characteristic that the influence of sulfur on the oxygen storage capacity differs between when the air-fuel ratio of the exhaust gas flowing into the catalyst is lean and when it is rich. However, the deterioration of the catalyst is determined only by the oxygen storage capacity when the air-fuel ratio is lean.

JP 2006-291773 A

  By the way, according to the results of the test research by the present inventors, when the central air-fuel ratio of the active air-fuel ratio control is changed, the change rate of the oxygen storage capacity with respect to the change of the central air-fuel ratio depends on the sulfur concentration of the fuel. Turned out to change. Therefore, if this characteristic can be used to accurately measure the oxygen storage capacity without the influence of sulfur, it is very advantageous to improve diagnostic accuracy.

  Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a catalyst deterioration diagnosis device for an internal combustion engine capable of highly accurate and reliable diagnosis regardless of the sulfur concentration of the fuel. There is.

According to the present invention,
An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
Active air-fuel ratio control means for executing active air-fuel ratio control for alternately switching the air-fuel ratio of the exhaust gas flowing into the catalyst to the rich side and the lean side with a predetermined central air-fuel ratio as a boundary;
Measuring means for measuring the oxygen storage capacity of the catalyst in accordance with execution of the active air-fuel ratio control;
When the plurality of active air-fuel ratios having different central air-fuel ratios are executed by the active air-fuel ratio control means, and a plurality of oxygen storage capacities corresponding to the respective center air-fuel ratios are measured by the measurement means. An apparatus for diagnosing catalyst deterioration in an internal combustion engine is provided, comprising: a sulfur concentration estimating means for estimating a sulfur concentration of fuel based on a rate of change of the oxygen storage capacity with respect to a change in central air-fuel ratio.

  As described above, according to the results of the study by the present inventors, it has been confirmed that the rate of change of the oxygen storage capacity with respect to the change of the central air-fuel ratio is a function or correlation value of the sulfur concentration of the fuel. Therefore, in the present invention, the sulfur concentration of the fuel is estimated based on data on a plurality of pairs of central air-fuel ratio and oxygen storage capacity. By estimating the sulfur concentration of the fuel, it is possible to make a necessary correction to the measured value of the oxygen storage capacity and to make it an accurate value without the influence of sulfur. By using this for the deterioration determination, highly accurate and reliable diagnosis can be performed, and erroneous diagnosis can be prevented in advance.

  Preferably, at least one of the central air-fuel ratios is equal to stoichiometric, and at least one is a value leaner than stoichiometric.

  According to the results of the study by the present inventors, it has been confirmed that the oxygen storage capacity change rate changes significantly due to the difference in the sulfur concentration of the fuel in the central air-fuel ratio region between stoichiometric and lean. Therefore, in a plurality of active air-fuel ratio controls with different central air-fuel ratios, by making at least one of the central air-fuel ratios equal to the stoichiometric value and making at least one a value leaner than the stoichiometric value, the efficiency can be reduced with a small number of active air-fuel ratio controls In addition, the estimation of the sulfur concentration of the fuel and the catalyst deterioration diagnosis can be executed in a short time.

  Preferably, at least one of the central air-fuel ratios is equal to stoichiometric, at least one is a leaner value than stoichiometric, and at least one is a richer value than stoichiometric.

  When the center air-fuel ratio richer than stoichiometric is included in this way, the number of active air-fuel ratio controls increases, but the number of data increases, the range of the center air-fuel ratio is expanded, the calculation accuracy of the oxygen storage capacity change rate, and the sulfur concentration The estimation accuracy can be improved.

  Preferably, the sulfur concentration estimating means calculates a change rate of the oxygen storage capacity by a least square method based on the data of the central air-fuel ratio and the oxygen storage capacity.

  Preferably, the active air-fuel ratio control means continuously executes a plurality of active air-fuel ratio controls with different central air-fuel ratios. In this way, the diagnosis time can be shortened compared to the case where a plurality of active air-fuel ratio controls are intermittently executed.

  Preferably, when the sulfur concentration of the fuel is estimated to be high by the sulfur concentration estimating means, the oxygen storage capacity when the central air-fuel ratio is stoichiometric is corrected, and based on the corrected oxygen storage capacity, the catalyst A determination means for determining deterioration is provided.

  As a result, it is possible to determine the deterioration after correcting the oxygen storage capacity when the central air-fuel ratio is stoichiometric to a value that does not affect the sulfur, and the diagnostic accuracy and reliability can be improved.

  Preferably, the determination means determines the deterioration of the catalyst based on the oxygen storage capacity when the central air-fuel ratio is stoichiometric when the sulfur concentration estimation means estimates that the sulfur concentration of the fuel is low.

  Preferably, the active air-fuel ratio control means performs a plurality of active air-fuel ratio controls with different central air-fuel ratios when the catalyst temperature is low, and also makes the central air-fuel ratio stoichiometric when the catalyst temperature is high When the catalyst temperature is high, the determination means determines the deterioration of the catalyst based on the oxygen storage capacity measured by the measurement means along with the single active air-fuel ratio control. To do.

  The catalyst is easily influenced by sulfur when the catalyst temperature is low, and conversely, when the catalyst temperature is high, the catalyst is not easily influenced by sulfur. Therefore, in this preferred embodiment, when the catalyst temperature is high, single active air-fuel ratio control with the central air-fuel ratio as stoichiometric is performed, and deterioration of the catalyst is determined based only on the oxygen storage capacity measured thereby. In this way, the active air-fuel ratio control need not be performed a plurality of times, and the diagnosis time can be shortened.

  According to the present invention, an excellent effect is achieved that a highly accurate and reliable diagnosis is possible regardless of the sulfur concentration of the fuel.

  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, and the exhaust pipe 6 has catalysts 11, 19 made of a three-way catalyst having an oxygen storage capacity. Are attached in series. An exhaust passage is formed by the exhaust port, the branch pipe, and the exhaust pipe 6. Air-fuel ratio sensors for detecting the air-fuel ratio of exhaust gas, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 are installed on the upstream side and the downstream side of the upstream catalyst 11, respectively. 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 post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output value changes suddenly at the theoretical air-fuel ratio. The post-catalyst sensor 18 is installed between the upstream catalyst 11 and the downstream catalyst 19.

  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 internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. The opening 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 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 catalysts 11 and 19 simultaneously purify NOx, HC and CO when the air-fuel ratio A / F of the exhaust gas flowing into the catalysts 11 and 19 is a stoichiometric air-fuel ratio (stoichiometric, for example, A / Fs = 14.6). In response to this, the ECU 20 adjusts the air-fuel ratio so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11, that is, the pre-catalyst air-fuel ratio A / Ffr matches the stoichiometric air-fuel ratio during normal operation of the internal combustion engine. Control. 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.

Here, the upstream catalyst 11 to be subjected to deterioration diagnosis will be described in more detail. The following description applies to the downstream catalyst 19 as well. 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 in which a large number of particulate catalyst components 32 are dispersedly arranged. 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 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 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 CeO ₂ or zirconia. For example, if the atmosphere gas of the catalyst component 32 and the coating material 31 is richer than the stoichiometric air-fuel ratio, the oxygen stored in the oxygen storage component present 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. On the contrary, when 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, 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). 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.

  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 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 post-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 post-catalyst sensor 18 correspond to the values of the pre-catalyst air / fuel ratio A / Ffr and the post-catalyst air / fuel ratio A / Frr, 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 post-catalyst sensor 18 is switched from rich to lean, or from lean to rich. As shown in the figure, the output voltage of the post-catalyst sensor 18 changes suddenly at the theoretical air-fuel ratio A / Fs, and the post-catalyst air-fuel ratio A / Frr is the 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 post-catalyst air-fuel ratio A / Frr is the lean air-fuel ratio greater than the theoretical air-fuel ratio A / Fs, the output voltage becomes 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 post-catalyst sensor 18 changes from the rich value to the lean value 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 post-catalyst sensor 18 changes from the lean value to the rich side and becomes equal to the rich determination 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 post-catalyst air-fuel ratio A / Frr changes to the lean side, and when the output voltage of the post-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 using the output of the post-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 post-catalyst air-fuel ratio A / Frr does not become rich, so the output of the post-catalyst sensor 18 is not reversed. When oxygen is continuously released from the catalyst 11, all the stored oxygen is eventually released from the catalyst 11, and at that time, oxygen can no longer be released, and rich gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Frr changes to the rich side, and when the output voltage of the post-catalyst sensor 18 reaches the rich determination 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 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 accuracy, the oxygen storage capacity (in this case, oxygen storage amount) is measured in the oxygen storage cycle in which the target air-fuel ratio A / Ft is on the lean side, and the oxygen storage capacity of these oxygen storage capacities is measured. 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 (oxygen storage 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 measured. 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 (OSC2 in FIG. 5) 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 catalyst deterioration diagnosis.

  Next, estimation of the sulfur concentration of the fuel in this embodiment will be described.

  FIG. 5 shows the test results of examining the relationship between the central air-fuel ratio A / Fc of active air-fuel ratio control and the measured oxygen storage capacity OSC. In the figure, rhombus data indicates a case where the fuel sulfur concentration is low (in the case of low sulfur fuel), and square data indicates a case where the fuel sulfur concentration is high (in the case of high sulfur fuel). A low sulfur fuel having a sulfur concentration of 30 ppm and a high sulfur fuel having a sulfur concentration of 300 ppm are used. The center air-fuel ratio A / Fc is differentiated as 14.3, 14.6 (Stoichi), 14.9. The conditions other than the central air-fuel ratio and the fuel sulfur concentration are the same, and the catalyst temperature is 770 ° C. As can be seen, it is clear that the oxygen storage capacity measurement value OSC at the same central air-fuel ratio is lower in the case of high sulfur fuel than in the case of low sulfur fuel due to the influence of sulfur poisoning.

In addition to this, the rate of change of the oxygen storage capacity OSC with respect to the change of the central air-fuel ratio A / Fc is different between the case of high sulfur fuel and the case of low sulfur fuel. When the change rate of the oxygen storage capacity OSC is H, the change amount of the central air-fuel ratio A / Fc is ΔA / Fc, and the corresponding change amount of the oxygen storage capacity OSC is ΔOSC, H = ΔOSC / ΔA / Fc. Is done. For example, in the case of the high-sulfur fuel in the illustrated example, the oxygen storage capacity when the central air-fuel ratio is 14.6 (stoichiometric) is OSCh _14.6 , and the oxygen when the central air-fuel ratio is 14.9 (lean from stoichiometric) When the storage capacity is OSCh _14.9 , the oxygen storage capacity change rate H between the center air-fuel ratio of _{14.6 and 14.9} is H = (OSCh _14.9 -OSCh _14.6 ) / (14.9- 14.6). That is, the change rate H means the slope or gradient of the graph, and the oxygen storage capacity change rate H between the center air-fuel ratio of _{14.6 to 14.9} is OSCh _14.9 <OSCh _14.6 . Value. Hereinafter, the fact that the rate of change H is large in the negative direction is also simply referred to as “the rate of change H (or slope) is large”.

  According to the results of FIG. 5, the average oxygen storage capacity change rate H (indicated by a broken line in the figure) between the center air-fuel ratio of 14.3 to 14.9 is higher in the case of high sulfur fuel than in the case of low sulfur fuel. Is bigger. Further, if the central air-fuel ratio is limited to between 14.6 and 14.9, the oxygen storage capacity change rate H is significantly larger in the case of high sulfur fuel than in the case of low sulfur fuel. Therefore, the oxygen storage capacity change rate H when the central air-fuel ratio is changed by a certain amount is a value that reflects and correlates with the sulfur concentration of the fuel, and is a value that increases as the sulfur concentration of the fuel increases. Therefore, in the present embodiment, this is utilized to estimate the sulfur concentration of the fuel based on the oxygen storage capacity change rate H.

FIG. 6 shows an example of the active air-fuel ratio control of this embodiment, and particularly shows how the target air-fuel ratio A / Ft changes. In the illustrated example, three active air-fuel ratio controls I to III in which the central air-fuel ratio A / Fc is different in the order of lean, stoichiometric, and rich are continuously executed. That is, the first oxygen storage capacity measurement value OSC _I is measured along with the first active air-fuel ratio control I in which the center air-fuel ratio A / Fc is a predetermined value (eg, 15.1) leaner than the stoichiometric value. Further, the second oxygen storage capacity measurement value OSC _II is measured along with the second active air-fuel ratio control II in which the center air-fuel ratio A / Fc is equal to the stoichiometric value (for example, 14.6). The third oxygen storage capacity measurement value OSC _III is measured along with the third active air-fuel ratio control III that is a predetermined value (eg, 14.1) richer than the stoichiometry. Then, an average oxygen storage capacity change rate H is calculated using these three central air-fuel ratios A / Fc and the three oxygen storage capacity measured values OSC _{I to} OSC _III, and based on this, the sulfur concentration of the fuel is calculated. Is estimated. In the first active air-fuel ratio control I, the value of the target air-fuel ratio A / Ft when the target air-fuel ratio A / Ft is switched to the rich side is slightly richer than stoichiometric (for example, 14.5, that is, rich Amplitude Ar = 0.6). In the third active air-fuel ratio control III, the value of the target air-fuel ratio A / Ft when the target air-fuel ratio A / Ft is switched to the lean side is slightly leaner than the stoichiometric value (for example, 14.7, ie, Lean amplitude Al = 0.6).

  As will be described later, in the present embodiment, the oxygen storage capacity measurement value is corrected as necessary based on the estimated sulfur concentration, and the final catalyst deterioration determination is made based on the corrected oxygen storage capacity measurement value. The By this correction, the measured value of oxygen storage capacity is increased and corrected to a true value having no sulfur influence. This eliminates the effect of sulfur and enables highly accurate and reliable diagnosis regardless of the sulfur concentration of the fuel.

  The example of active air-fuel ratio control shown in FIG. 6 is only one preferred example, and other modes of active air-fuel ratio control are possible. For example, the number of the central air-fuel ratio A / Fc and the active air-fuel ratio control is not limited to three, and may be any number as long as it is two or more. In particular, according to the result of FIG. 5, the difference due to the sulfur concentration in the fuel remarkably appears in the oxygen storage capacity change rate H from the stoichiometric to the lean state of the central air-fuel ratio A / Fc. Fc may be two or more stoichiometric and lean, or three or more between stoichiometric and lean, and active air-fuel ratio control may be executed for each. In this way, the number of active air-fuel ratio controls can be reduced to two, and there is a possibility that deterioration diagnosis can be performed more efficiently in a short time.

  Further, a plurality of active air-fuel ratio controls may be executed intermittently instead of continuously. However, continuous execution is advantageous because the diagnosis time can be shortened. In the example of FIG. 6, the center air-fuel ratio A / Fc is changed from the lean side to the rich side. However, the present invention is not limited to this, and the center air-fuel ratio A / Fc may be changed from the rich side to the lean side. The central air-fuel ratio A / Fc may be changed to stoichiometric, rich (or lean), lean (or rich). In the example of FIG. 6, the target air-fuel ratio A / Ft is switched only about several times in one active air-fuel ratio control. Naturally, the number of times of switching is arbitrary, and can be increased or decreased as appropriate.

  Here, supplementing the results of FIG. 5, if the central air-fuel ratio A / Fc is leaner than the stoichiometric ratio as in the first active air-fuel ratio control I of FIG. 6, it is assumed that the target air-fuel ratio A / Ft is set to the rich side. However, the richness of the pre-catalyst air-fuel ratio A / Ffr with respect to stoichiometry is small, and the oxygen release time from the catalyst (Ta in FIG. 6) tends to be long. The effect of sulfur is likely to occur when oxygen is released on the rich side. In the case of high sulfur, the catalyst component (noble metal) is poisoned by sulfur, and the oxygen releasing reaction is hindered. The process ends significantly earlier (that is, the post-catalyst sensor 18 is reversed at an earlier timing). Therefore, as shown in FIG. 5, when the center air-fuel ratio A / Fc is leaner than stoichiometric, the amount of decrease in the oxygen storage capacity when the fuel is changed from low sulfur to high sulfur is relatively large. On the other hand, when the center air-fuel ratio A / Fc is richer than the stoichiometric ratio as in the third active air-fuel ratio control III in FIG. 6, the pre-catalyst air-fuel ratio A / F when the target air-fuel ratio A / Ft is set to the rich side. The richness of Ffr with respect to stoichiometry is very large, and the oxygen release time from the catalyst (Tb in FIG. 6) tends to be short. At the time of oxygen release on the rich side, the influence of sulfur does not appear so much, and the oxygen release ends quickly regardless of the sulfur concentration. Therefore, as shown in FIG. 5, when the center air-fuel ratio A / Fc is richer than stoichiometric, the amount of decrease in the oxygen storage capacity when the fuel is changed from low sulfur to high sulfur is relatively small. It is known that the influence of sulfur is less likely to occur even if the sulfur content is high when oxygen is stored on the lean side with respect to the central air-fuel ratio A / Fc. Based on the above reason, the influence or difference due to sulfur is noticeable in the oxygen storage capacity change rate H especially in the stoichiometric to lean range of the central air-fuel ratio A / Fc.

  Next, a first aspect of the deterioration diagnosis process in the present embodiment will be described with reference to FIG. This process is executed by the ECU 20.

First, in step S101, the first active air-fuel ratio control I having a predetermined value A / Fc _I (for example, 15.1) whose center air-fuel ratio is leaner than stoichiometric is executed, and accordingly, the first oxygen storage capacity is OSC _I is measured. Next, in step S102, the second active air-fuel ratio control II in which the center air-fuel ratio is equal to the stoichiometric value A / Fc _II (for example, 14.6) is executed, and accordingly, the second oxygen storage capacity OSC _II. Is measured. Further, in step S103, the third active air-fuel ratio control III in which the center air-fuel ratio is a predetermined value A / Fc _III (for example, 14.1) richer than the stoichiometric is executed, and accordingly, the third oxygen storage capacity OSC. _III is measured.

Next, in step S104, data A / Fc _I , A / Fc _II , A / Fc _III and oxygen storage capacity measurement data OSC _I corresponding to each central air-fuel ratio according to steps S101 to S103, The value of the oxygen storage capacity change rate H is calculated using OSC _I and OSC _III . Here, processing for obtaining one inclination is performed using three points of data, and various approximation methods can be adopted. In the present embodiment, this is performed using the least square method. In this case, the oxygen storage capacity change rate H is calculated by the following equation (2).

Next, in step S105, the calculated oxygen storage capacity change rate H is compared with a predetermined threshold value Hs (where Hs <0). When the oxygen storage capacity change rate H is equal to or greater than the predetermined threshold value Hs, that is, when the slope in the negative direction is small or the slope is positive in the graph as shown in FIG. In step S106, the deterioration of the catalyst is determined based on the second oxygen storage capacity OSC _II measured when the center air-fuel ratio is a value A / Fc _II equal to the stoichiometric value. That is, the second oxygen storage capacity OSC _II is compared with a predetermined deterioration judgment value OSCs, and if the second oxygen storage capacity OSC _II is larger than the deterioration judgment value OSCs, the catalyst is normal and the second oxygen storage capacity OSC _II is If the deterioration determination value OSCs or less, the catalyst is determined to be deteriorated. This process is completed.

On the other hand, if the oxygen storage capacity change rate H is less than the predetermined threshold value Hs in step S105, that is, if the slope in the negative direction is large in the graph as shown in FIG. 5, it is estimated that the fuel sulfur concentration is high. In step S107, the deterioration of the catalyst is determined after correcting the second oxygen storage capacity OSC _II . The correction is performed, for example, by multiplying the second oxygen storage capacity OSC _II by a predetermined correction coefficient α. The correction coefficient α is calculated based on the oxygen storage capacity change rate H according to a map or function stored in the ECU 20 in advance. The correction coefficient α is a value larger than 1, and a larger value is obtained as the oxygen storage capacity change rate H increases in the negative direction. By this correction, the second oxygen storage capacity OSC _II is increased and corrected so as to eliminate the influence of sulfur, and the correction is increased and increased as the sulfur concentration of the fuel is higher. When the corrected second oxygen storage capacity OSC _II ′ (= α × OSC _II ) is obtained in this way, this value is compared with the deterioration determination value OSCs to determine the deterioration of the catalyst in the same manner as described above. This process is completed. Note that other correction methods are possible. For example, the second oxygen storage capacity OSC _II may be increased and corrected by adding a predetermined correction amount.

  FIG. 8 shows the method for determining the sulfur concentration performed in step S105. In the case of low sulfur fuel, even if the central air-fuel ratio is changed, the change in the oxygen storage capacity is small, so the oxygen storage capacity change rate H is near zero. On the other hand, in the case of high sulfur fuel, the oxygen storage capacity tends to decrease as the central air-fuel ratio is changed to the lean side, so the oxygen storage capacity change rate H takes a relatively large negative value corresponding to the sulfur concentration. . Therefore, the negative threshold value Hs for determining whether or not the oxygen storage capacity measurement value needs to be corrected is set in advance through consideration of variations and the like through experiments and the like. By comparing the threshold value Hs set in this way with the actual oxygen storage capacity change rate H, it is quickly determined that high-sulfur fuel is being used, and correction is made to eliminate the influence of sulfur, thereby making a false diagnosis. Can be prevented.

  Next, a second aspect of the deterioration diagnosis process in the present embodiment will be described with reference to FIG. In addition, about the step similar to the said 1st aspect, detailed description is abbreviate | omitted only by changing a code | symbol to 200 series. In this second mode, step S201A is added before step S201. If a negative determination is made in step S201A, the process proceeds to step S201. If an affirmative determination is made in step S201A, the process proceeds to newly added step S208. It has come to go.

  In step S201A, the catalyst temperature Tc as an estimated value is compared with a predetermined threshold value Tcs. The threshold value Tcs is set to a relatively high value. That is, the catalyst is susceptible to sulfur (sulfur poisoning) when the catalyst temperature is low, and conversely, when the catalyst temperature is high (eg, 600 ° C. or higher), the catalyst is susceptible to sulfur. It ’s hard. Therefore, in this second aspect, only when such a catalyst temperature is low, calculation of the oxygen storage capacity change rate H, estimation of the sulfur concentration, correction of the oxygen storage capacity, etc. are performed. The deterioration determination of the catalyst is made based only on the measured oxygen storage capacity when the air-fuel ratio is stoichiometric. In this way, when the catalyst temperature is high, only one (one time) active air-fuel ratio control is required, and it is not necessary to perform the active air-fuel ratio control a plurality of times, and the diagnosis time can be shortened.

  In step S201A, when the catalyst temperature Tc is a low temperature equal to or lower than the threshold value Tcs, steps S201 to S207 are executed to determine the deterioration of the catalyst as in the first mode.

On the other hand, if the catalyst temperature Tc is a threshold Tcs greater high temperature, the process proceeds to step S208, the second active air-fuel ratio control II is executed center air-fuel ratio is equal to A / Fc _II to the stoichiometric, to Accordingly, the second oxygen storage capacity OSC _II is measured. In step S206, the deterioration of the catalyst is determined based on the second oxygen storage capacity OSC _II . This process is completed.

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 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 as the post-catalyst sensor, and an O ₂ sensor similar to the post-catalyst sensor may be used as 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.

  Regarding the estimation of the sulfur concentration, in the above-described embodiment, the estimation is made in two stages of low sulfur and high sulfur. Of course, finer estimation can be performed. For example, based on the actually measured oxygen storage capacity change rate H, the absolute value of the sulfur concentration may be estimated from a map or function obtained experimentally in advance.

  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 for demonstrating 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 relationship between the central air fuel ratio of active air fuel ratio control, and oxygen storage capacity. It is a time chart which shows the mode of the change of the target air fuel ratio in the active air fuel ratio control of this embodiment. It is a flowchart which shows the 1st aspect of a deterioration diagnostic process. It is a graph for demonstrating the method of sulfur concentration determination. It is a flowchart which shows the 2nd aspect of a deterioration diagnostic process.

Explanation of symbols

1 internal combustion engine 6 exhaust pipe 11 upstream catalyst 12 injector 17 pre-catalyst sensor 18 post-catalyst sensor 19 downstream catalyst 20 electronic control unit (ECU)
A / Fc Center air-fuel ratio OSC Oxygen storage capacity H Rate of change α Correction factor Tc Catalyst temperature

Claims (8)

An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
Active air-fuel ratio control means for executing active air-fuel ratio control for alternately switching the air-fuel ratio of the exhaust gas flowing into the catalyst to the rich side and the lean side with a predetermined central air-fuel ratio as a boundary;
Measuring means for measuring the oxygen storage capacity of the catalyst in accordance with execution of the active air-fuel ratio control;
When the plurality of active air-fuel ratios having different central air-fuel ratios are executed by the active air-fuel ratio control means, and a plurality of oxygen storage capacities corresponding to the respective center air-fuel ratios are measured by the measurement means. An apparatus for diagnosing catalyst deterioration in an internal combustion engine, comprising: a sulfur concentration estimating means for estimating a sulfur concentration of fuel based on a rate of change of the oxygen storage capacity with respect to a change in central air-fuel ratio. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to claim 1, wherein at least one of the central air-fuel ratios is equal to stoichiometric, and at least one of the values is leaner than stoichiometric. 2. The catalyst deterioration diagnosis device for an internal combustion engine according to claim 1, wherein at least one of the central air-fuel ratios is equal to stoichiometric, at least one is a value leaner than stoichiometric, and at least one is a richer value than stoichiometric. . The said sulfur concentration estimation means calculates the change rate of the said oxygen storage capacity by the least squares method based on the data of the said central air fuel ratio and the said oxygen storage capacity. A catalyst deterioration diagnosis device for an internal combustion engine. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the active air-fuel ratio control means continuously executes a plurality of active air-fuel ratio controls with different central air-fuel ratios. When the sulfur concentration of the fuel is estimated to be high by the sulfur concentration estimating means, the oxygen storage capacity when the central air-fuel ratio is stoichiometric is corrected, and the deterioration of the catalyst is determined based on the corrected oxygen storage capacity An apparatus for diagnosing catalyst deterioration of an internal combustion engine according to any one of claims 1 to 5, further comprising a determination unit that performs the determination. The determination means determines deterioration of the catalyst based on an oxygen storage capacity when the central air-fuel ratio is stoichiometric when the sulfur concentration estimation means estimates that the sulfur concentration of the fuel is low. The catalyst deterioration diagnosis device for an internal combustion engine according to claim 6. The active air-fuel ratio control means executes a plurality of active air-fuel ratio controls with different central air-fuel ratios when the catalyst temperature is low, and a single active air-fuel ratio that makes the central air-fuel ratio stoichiometric when the catalyst temperature is high. Execute the fuel ratio control
The determination means determines deterioration of the catalyst based on the oxygen storage capacity measured by the measurement means in association with the single active air-fuel ratio control when the catalyst temperature is high. Or a catalyst deterioration diagnosis device for an internal combustion engine according to claim 7.

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