JP2010163885A - Catalyst degradation determination device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst degradation determination device capable of detecting sulfur concentration in fuel. <P>SOLUTION: According to output reversion of a catalyst-posterior sensor 18, active air-fuel ratio control of alternately switching air-fuel ratio on the upstream of a catalyst 11 to rich and lean is performed, the oxygen storage capacity of the catalyst 11 is measured according to the execution thereof, and degradation of the catalyst 11 is determined based on the measurement value. A storage-reduction NOx catalyst 21 having a relatively small capacity is disposed between the catalyst 11 and the sensor 18, and an NOx sensor 22 is disposed on the downstream of the NOx catalyst 21. The sulfur concentration in fuel is detected based on the output of the NOx sensor during lean control. Since the output behavior of the NOx sensor during lean control is varied according to the sulfur concentration in fuel, this characteristic can be used to detect the sulfur concentration in fuel. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an apparatus for diagnosing deterioration of a catalyst, and more particularly 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 stoichiometry. However, when a three-way catalyst having oxygen storage capacity is used, the actual air-fuel ratio slightly deviates from stoichiometry depending on operating conditions. However, the 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 that switches the air-fuel ratio upstream of the catalyst alternately between rich and lean is performed, and the oxygen storage capacity of the catalyst is measured along with the execution of this active air-fuel ratio control to diagnose catalyst deterioration. The so-called method (so-called Cmax method) is employed.

  For example, Patent Document 1 discloses an apparatus for determining catalyst deterioration using the Cmax method, which determines the degree of catalyst deterioration based on the measured oxygen storage capacity and the number of times the oxygen storage capacity is calculated. .

JP 2004-176611 A

  By the way, depending on a use area etc., sulfur (S) may be contained in fuel by comparatively high concentration. 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 apparent oxygen storage capacity of the catalyst decreases. However, if the 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 catalyst deterioration diagnosis, it is necessary not to mistakenly diagnose the temporary deterioration due to the S poisoning as permanent deterioration or abnormality such as heat deterioration to be originally 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 prevent such a misdiagnosis, it is preferable to discriminate the fuel property, particularly to detect the sulfur concentration of the fuel. This is because if such discrimination or detection is performed, necessary measures can be taken when it is detected that the fuel has a high sulfur concentration, and erroneous diagnosis can be prevented.

  Accordingly, 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 capable of detecting the sulfur concentration of fuel.

According to one aspect of the invention,
An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A post-catalyst sensor that is disposed downstream of the catalyst and detects an air-fuel ratio of exhaust gas;
Active air-fuel ratio control means for executing active air-fuel ratio control for alternately switching the air-fuel ratio on the upstream side of the catalyst between rich and lean as the output of the post-catalyst sensor is inverted;
Measuring means for measuring the oxygen storage capacity of the catalyst in accordance with execution of the active air-fuel ratio control;
Determination means for determining deterioration of the catalyst based on the measured value of the oxygen storage capacity;
A relatively small capacity NOx storage reduction catalyst disposed between the catalyst and the post-catalyst sensor;
A NOx sensor that is disposed downstream of the NOx catalyst and detects the NOx concentration of the exhaust gas;
A sulfur concentration detecting means for detecting the sulfur concentration of the fuel based on the output of the NOx sensor during lean control of the catalyst upstream air-fuel ratio;
There is provided a catalyst deterioration diagnosis device characterized by comprising:

  During the lean control of the catalyst upstream air-fuel ratio, lean exhaust gas is supplied to the catalyst, but this lean exhaust gas gradually leaks from the catalyst as the oxygen storage amount of the catalyst approaches the saturation state. come. As the leakage amount increases, the NOx amount leaking from the catalyst also gradually increases. If the NOx catalyst is in a normal state, the leaked NOx is occluded by the NOx catalyst and is not discharged from the NOx catalyst. That is, if a fuel having a low sulfur concentration is used, the NOx catalyst is not poisoned with S, so that the leaked NOx is occluded and is not discharged. Therefore, the output of the NOx sensor is maintained at a low value.

  However, when a fuel with a high sulfur concentration is used, the NOx catalyst is poisoned with S, and its inherent NOx storage capacity is reduced. Therefore, the leaked NOx supplied to the NOx catalyst leaks from the NOx catalyst from a relatively early stage, and in response to this, the output of the NOx sensor changes. As described above, the output behavior of the NOx sensor during the lean control differs depending on the sulfur concentration of the fuel. Therefore, the sulfur concentration of the fuel is detected using this characteristic.

  Preferably, the sulfur concentration detection means measures the time from the start of the lean control until the output of the NOx sensor reaches a predetermined value, and when the measured time is less than the predetermined value, Detects high sulfur concentration.

  This utilizes the fact that NOx begins to leak from the NOx catalyst earlier when high sulfur fuel is used and NOx sensor output rises earlier than when low sulfur fuel is used.

  Preferably, the sulfur concentration detection means measures an area or trajectory length based on the NOx sensor output during the lean control, and the sulfur concentration of the fuel is high when the measured area or trajectory length is a predetermined value or more. Detect that.

  This utilizes the fact that the NOx sensor output changes greatly when high sulfur fuel is used compared to when low sulfur fuel is used.

  Preferably, the determination means determines the deterioration of the catalyst by comparing the measured value of the oxygen storage capacity with a predetermined deterioration determination value, and the catalyst deterioration diagnosis device uses the sulfur concentration detection means to determine the amount of fuel. When it is detected that the sulfur concentration is high, the apparatus further includes a correction unit that corrects the oxygen storage capacity measurement value or the deterioration determination value.

  By such correction, it is possible to prevent the erroneous diagnosis by suppressing the influence of the decrease in the measured value of the oxygen storage capacity due to the S poisoning of the catalyst.

  According to the present invention, it is possible to provide an excellent catalyst deterioration diagnosis device capable of detecting the sulfur concentration of fuel.

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 the content of the active air fuel ratio control at the time of catalyst deterioration diagnosis. FIG. 4 is a time chart similar to FIG. 3 for illustrating a method for measuring the oxygen storage capacity. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a time chart which shows the change of each value at the time of catalyst degradation diagnosis and sulfur concentration detection. It is a flowchart of a catalyst deterioration diagnostic process including the 1st aspect of sulfur concentration detection. The map for calculating a correction coefficient is shown. The map for calculating a correction coefficient is shown. It is a time chart for demonstrating the area based on a NOx sensor output. It is a flowchart of the catalyst deterioration diagnosis process including the 2nd aspect of sulfur concentration detection. The map for calculating a correction coefficient is shown. It is a time chart for demonstrating the locus | trajectory length based on a NOx sensor output. It is a flowchart of a catalyst deterioration diagnostic process including the 3rd aspect of sulfur concentration detection. The map for calculating a correction coefficient is shown. It is the schematic which shows the modification of an upstream catalyst and a NOx catalyst.

  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. In the intake pipe 13, an air flow meter 5 for detecting the intake air amount (the amount of air flowing into the internal combustion engine) and an electronically controlled throttle valve 10 are incorporated 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. These exhaust ports, branch pipes and exhaust pipe 6 form an exhaust passage. The exhaust pipe 6 includes, in order from the upstream side, a catalyst composed of a three-way catalyst having an oxygen storage capacity, that is, an upstream catalyst 11, a relatively small capacity storage reduction type NOx catalyst 21, and a three-way catalyst having an oxygen storage capacity. And the downstream catalyst 19 are provided in series. An upstream side of the upstream catalyst 11 is provided with an air-fuel ratio sensor, that is, a pre-catalyst sensor 17 for detecting the air-fuel ratio of the exhaust gas. Further, an air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas, that is, a post-catalyst sensor 18, and a NOx sensor 22 for detecting the NOx concentration of the exhaust gas are provided between the NOx catalyst 21 and the downstream catalyst 19. It has been. As a result, the NOx catalyst 21 is disposed between the upstream catalyst 11 and the post-catalyst sensor 18. 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 (Z characteristic) in which the output value changes suddenly at the theoretical air-fuel ratio. The output characteristics of the pre-catalyst sensor 17 and the post-catalyst sensor 18 are shown in FIG.

  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 post-catalyst sensor 18, and the NOx sensor 22, 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. The accelerator opening sensor 15 for detecting the above 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 ECU 20 feedback-controls the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 so that the air-fuel ratio detected by the pre-catalyst sensor 17, that is, the pre-catalyst air-fuel ratio A / Ff matches the target air-fuel ratio A / Ft. . On the other hand, the catalysts 11 and 19 simultaneously purify NOx, HC and CO simultaneously with high efficiency when the air-fuel ratio of the exhaust gas flowing into the catalyst 11 and 19 is the stoichiometric air-fuel ratio (stoichiometric, for example, A / Fs = 14.6). Therefore, the ECU 20 sets the target air-fuel ratio A / Ft equal to the stoichiometric air-fuel ratio during normal operation of the internal combustion engine so that the pre-catalyst air-fuel ratio A / Ff detected by the pre-catalyst sensor 17 matches the stoichiometric air-fuel ratio. Feedback control is performed on the amount of fuel injected from the injector 12. 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 downstream catalyst 19 is configured in the same manner as the upstream catalyst 11. As shown in FIG. 2, in the catalyst 11, a coating material 31 is coated on the surface of a carrier base material (not shown), and 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, three exhaust gas components such as NOx, HC and CO are simultaneously purified even if the pre-catalyst air-fuel ratio A / Ff slightly varies from the theoretical air-fuel ratio during normal air-fuel ratio control. can do. Therefore, in normal air-fuel ratio control, it is also possible to purify exhaust gas by causing the pre-catalyst air-fuel ratio A / Ff to oscillate minutely around the theoretical air-fuel ratio and repeat the oxygen absorption and release.

  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 the present embodiment, the deterioration degree of the upstream catalyst 11 is detected by detecting the oxygen storage capacity of the upstream catalyst 11 that has a particularly large influence on the emission. Here, the oxygen storage capacity of the catalyst 11 is represented by the size of the oxygen storage capacity (OSC; O ₂ Storage Capacity, the unit is g), which is the maximum amount of oxygen that the current catalyst 11 can store.

  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 upstream of the catalyst 11, that is, the air-fuel ratio of the air-fuel mixture in the combustion chamber 3 and thus the air-fuel ratio of the exhaust gas supplied to the catalyst 11 borders a predetermined center air-fuel ratio A / Fc. Are alternately (forcedly) switched between the rich side and the lean side. The air-fuel ratio when switched to the rich side is referred to as rich air-fuel ratio A / Fr, and the air-fuel ratio when switched to the lean side is referred to as lean air-fuel ratio A / Fl. While the pre-catalyst air-fuel ratio A / Ff is controlled to be either rich or lean 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.

  Hereinafter, a method for measuring the oxygen storage capacity of the upstream catalyst 11 will be described with reference to FIGS. 3 and 4. Here, it is assumed that there is no NOx catalyst 21 provided on the downstream side of the upstream catalyst 11. In FIGS. 3A and 3B, the output behavior of the pre-catalyst sensor 17 and the post-catalyst sensor 18 when the active air-fuel ratio control is executed is indicated by a solid line. In FIG. 3A, the target air-fuel ratio A / Ft generated inside the ECU 20 is indicated by a broken line. The output value of the pre-catalyst sensor 17 shown in FIG. 3A is a value converted to the pre-catalyst air-fuel ratio A / Ff. The output value of the post-catalyst sensor 18 shown in FIG. 3B is the output value itself, that is, the value of the output voltage Vr.

  As shown in FIG. 3A, the target air-fuel ratio A / Ft is centered on the theoretical air-fuel ratio A / Fs as the center air-fuel ratio, and then has a predetermined amplitude (rich amplitude Ar, Ar> 0) on the rich side. ) Separated by an air-fuel ratio (rich air-fuel ratio A / Fr) and an air-fuel ratio (lean air-fuel ratio A / Fl) separated from the air-fuel ratio by a predetermined amplitude (lean amplitude Al, Al> 0) on the lean side. And alternately. Following the switching of the target air-fuel ratio A / Ft, the pre-catalyst air-fuel ratio A / Ff 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 / Ff 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, 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.

  The target air-fuel ratio A / Ft is switched as the output of the post-catalyst sensor 18 is reversed. That is, the timing or timing at which the target air-fuel ratio A / Ft is switched is the same as the timing at which the output of the post-catalyst sensor 18 is switched from rich to lean or from lean to rich. The air / fuel ratio of the air / fuel mixture actually supplied is also switched simultaneously with the target air / fuel ratio A / Ft. As shown in the figure, the output voltage Vr of the post-catalyst sensor 18 changes suddenly at the theoretical air-fuel ratio A / Fs. In order to determine the inversion timing of the output voltage Vr, that is, the timing at which the output voltage Vr is inverted to the rich side and the timing at which the output voltage Vr is inverted to the lean side, two inversion threshold values VR and VL relating to the output voltage Vr are determined in advance. ing. Here, VR is referred to as a rich determination value, and VL is referred to as a lean determination value. VR> VL, for example, VR = 0.59 (V) and VL = 0.21 (V). When the output voltage Vr changes to the lean side, that is, decreases and reaches the lean determination value VL, the output voltage Vr is considered to have been reversed to the lean side, and the post-catalyst air-fuel ratio A / Fr detected by the post-catalyst sensor 18 Is at least leaner than the stoichiometric air-fuel ratio. On the other hand, when the output voltage Vr changes to the rich side, that is, increases and reaches the rich determination value VR, it is considered that the output voltage Vr is reversed to the rich side, and the post-catalyst air-fuel ratio A / Fr is at least greater than the stoichiometric air-fuel ratio. Judged to be rich. As shown in FIG. 5, the rich determination value VR is a value that is larger (rich side) than the stoichiometric equivalent value Vst, and the lean determination value VL is a value that is smaller (lean side) than the stoichiometric equivalent value Vst. The stoichiometric air-fuel ratio is included in a narrow region Y between the air-fuel ratios corresponding to the rich determination value VR and the lean determination value VL (this is referred to as a transition region). Basically, it is only possible to detect whether the post-catalyst air-fuel ratio A / Fr is richer or leaner than the stoichiometric air-fuel ratio from the output voltage Vr, and it is difficult to detect the absolute value of the post-catalyst air-fuel ratio A / Fr. .

  As shown in FIGS. 3A and 3B, when the output voltage Vr of the post-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 The air-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 Vr of the post-catalyst sensor 18 changes from the lean side value to the rich side and becomes equal to the rich determination value VR (time t2), the target air-fuel ratio A / Ft is changed from the rich air-fuel ratio A / Fr. The lean air-fuel ratio A / Fl is switched. Thus, whenever the output of the post-catalyst sensor 18 is reversed to the lean side or the rich side, the air-fuel ratio is actively switched to the rich side or the lean side.

  While executing this active air-fuel ratio control, 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 / Fr 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.

  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 / Fr 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 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 post-catalyst air-fuel ratio A / Fr 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 / Ff as an actual value is slightly delayed with the rich air-fuel ratio A / Ff. Switch to Fr. From the time t11 when the pre-catalyst air-fuel ratio A / Ff reaches the 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, during this rich control or in the oxygen release cycle, the oxygen storage capacity OSC as the final integrated value, that is, the amount of released oxygen (OSC1 in FIG. 4) 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).

  The oxygen storage capacity (the stored oxygen amount in this case) is similarly measured during the lean control or the oxygen storage cycle where the target air-fuel ratio A / Ft is on the lean side. The target air-fuel ratio A / Ft is alternately switched between rich and lean, and the oxygen storage capacity is measured every time rich control and lean control are alternately performed. Then, an average value OSCav of the obtained plurality of oxygen storage capacity measurement values is calculated, and the average value OSCav is set as the final oxygen storage capacity measurement value OSC.

  As shown in FIG. 4, the target air-fuel ratio A / Ft was switched to the lean air-fuel ratio A / Fl at time t2, as shown in FIG. 4, for the measurement of the oxygen storage capacity (storage oxygen amount) during lean control or in the oxygen storage cycle. Thereafter, from the time point t21 when the pre-catalyst air-fuel ratio A / Ff reaches the theoretical air-fuel ratio A / Fs to the next time point t3 when the target air-fuel ratio A / Ft reverses to the rich side, the above equation (1) is used every minute time. The oxygen storage capacity dOSC is calculated, and the oxygen storage capacity dOSC for each minute time is integrated. Thus, the oxygen storage capacity (OSC2 in FIG. 4) in this oxygen storage cycle is measured. Ideally, the oxygen storage capacity OSC1 of the oxygen release cycle and the oxygen storage capacity OSC2 of the oxygen storage cycle are substantially equal to each other.

  Next, the deterioration of the catalyst is determined based on the final oxygen storage capacity measurement value OSC. That is, the oxygen storage capacity measurement value OSC is compared with the predetermined deterioration determination value OSCs. If the oxygen storage capacity measurement value OSC is larger than the deterioration determination value OSCs, the catalyst is normal, and if the oxygen storage capacity measurement value OSC is less than or equal to the deterioration determination value OSCs. The catalyst is judged 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.

  By the way, as described above, when a fuel having a high sulfur concentration is used, the upstream catalyst 11 (and the downstream catalyst 19) is poisoned by S, not only the exhaust purification performance is deteriorated, but also less oxygen storage than originally intended at the time of deterioration diagnosis. There is a possibility that the capacity value is measured and misdiagnosed. Therefore, in the present embodiment, in order to prevent this, the sulfur concentration of the fuel can be detected.

For this sulfur concentration detection, the NOx storage reduction catalyst 21 and the NOx sensor 22 are used. The NOx storage reduction (NSR) catalyst 21 has a noble metal such as platinum Pt as a catalyst component and a NOx absorption component supported on the surface of a base material made of an oxide such as alumina Al ₂ O _3. Configured. The NOx absorption component is selected from, for example, alkali metals such as potassium K, sodium Na, lithium Li, and cesium Cs, alkaline earths such as barium Ba and calcium Ca, and rare earths such as lanthanum La, yttrium Y, and cerium Ce. Consisting of at least one.

The NOx storage reduction catalyst 21 has NOx storage capacity. When the air-fuel ratio of the exhaust gas supplied to the NOx catalyst 21 is leaner than the stoichiometric air-fuel ratio, the NOx in the exhaust gas is stored in the form of nitrate and supplied to this. When the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or richer, the stored NOx is released. This released NOx reacts with the stoichiometric air-fuel ratio or richer exhaust gas via the noble metal in the catalyst, and is reduced to N ₂ . By releasing the stored NOx in this way, the NOx catalyst 21 regains its original NOx storage capability and is regenerated (this is called NOx regeneration).

  Note that the NOx absorption / release action of the NOx catalyst 21 is possible only when the NOx catalyst 21 is in an active state, that is, within a predetermined active temperature range (for example, 280 to 550 ° C.). In the present embodiment, for example, the temperature of the NOx catalyst 21 is estimated based on the estimated temperature of the upstream catalyst 11, and it is determined whether or not the NOx catalyst 21 is in an active state.

  In particular, the NOx catalyst 21 has a relatively small capacity. The reason for this will be described in detail later. In this embodiment, the sulfur concentration of the fuel is detected using the presence or absence of S poisoning of the NOx catalyst 21. More specifically, the NOx due to S poisoning of the NOx catalyst 21 is detected. This is because NOx leaking from the NOx catalyst 21 due to the decrease in the storage capacity is detected by the NOx sensor 22 to detect that the sulfur concentration of the fuel is high. Therefore, it is preferable that the NOx catalyst 21 has only a capacity such that NOx leaks out in a short time when there is S poison. The NOx catalyst 21 does not have to have a large capacity enough to practically store NOx in the exhaust gas. The capacity of the NOx catalyst 21 is, for example, less than 1/2 of the capacity of the upstream catalyst 11, for example, 1/3 of the capacity of the upstream catalyst 11.

  FIG. 6 shows changes in values at the time of catalyst deterioration diagnosis and sulfur concentration detection. (A) shows the target air-fuel ratio A / Ft, and (B) shows the post-catalyst sensor output Vr. (C) shows the NOx concentration of the exhaust gas at a position between the upstream catalyst 11 and the NOx catalyst 21, that is, the NOx concentration of the exhaust gas discharged from the upstream catalyst 11 and supplied to the NOx catalyst 21. (D) shows the NOx sensor output I when using a low sulfur concentration fuel (low sulfur fuel), and (E) shows the NOx sensor output I when using a high sulfur concentration fuel (high sulfur fuel). Note that the NOx sensor output I increases as the NOx concentration increases.

  Hereinafter, the period from when the target air-fuel ratio A / Ft is switched to one of rich and lean to when it is switched again in the same direction is referred to as one cycle of active air-fuel ratio control. A period from when the target air-fuel ratio A / Ft is switched to one of rich and lean to when it is switched to the opposite direction is called a half cycle of active air-fuel ratio control. A half cycle starting from when the target air-fuel ratio A / Ft is switched to rich is called a rich half cycle, and a half cycle starting from when the target air-fuel ratio A / Ft is switched to lean is called a lean half cycle. In the illustrated example, for example, a period from t0 to t1 is a lean half cycle, a period from t1 to t2 is a rich half cycle, and a period from t0 to t2 is one cycle. The rich half cycle and lean half cycle are also referred to as rich control time and lean control time, respectively, and the half cycle is also referred to simply as “mountain”.

  First, the catalyst deterioration diagnosis in the illustrated example will be outlined. Active air-fuel ratio control is started at time t0, and the first half cycle (lean half cycle) is started. In this half cycle, the oxygen storage capacity is not measured. The reason is that the pre-start air-fuel ratio is stoichiometric, different from that during execution of active air-fuel ratio control, and the preconditions are different. The first half cycle is abandoned.

  Next, the target air-fuel ratio A / Ft is switched to rich at time t1, and the first rich half cycle is started. In this rich half cycle, the oxygen storage capacity is measured (integrated) by the method described above. At the same time as the post-catalyst sensor output Vr is inverted to rich at time t2, the first oxygen storage capacity measurement in the rich half cycle is terminated, and at the same time, the target air-fuel ratio A / Ft is switched to lean.

  Similarly, every time the target air-fuel ratio A / Ft is alternately switched between lean, rich,... At time t2, t3,..., Oxygen storage in the lean half cycle, rich half cycle,. Capacity is measured sequentially. Then, at the time t7 when the oxygen storage capacity for N cycles (N = 3 in this embodiment) is measured, the active air-fuel ratio control and the oxygen storage capacity measurement are finished, and a plurality of measured (six in this embodiment) are measured. ) Is calculated as an average value of the measured oxygen storage capacity.

  During the catalyst deterioration diagnosis, the fuel sulfur concentration is detected at the same time. Here, attention is paid to the behavior of NOx as follows. In the first lean half-cycle (t0 to t1), the upstream catalyst 11 is supplied with exhaust gas (lean gas) that is leaner than the stoichiometric gas. This lean gas becomes saturated as the oxygen storage amount of the upstream catalyst 11 approaches saturation. It gradually leaks from the upstream catalyst 11. As the leakage amount increases, the amount of NOx leaking from the upstream catalyst 11 gradually increases as shown in (C). This tendency is observed in all lean half periods (t2-t3, t4-t5, t6-t7). Note that due to the difference in the preconditions, the first lean half cycle has a larger amount of NOx leakage than the subsequent lean half cycle.

  If the NOx catalyst 21 is in a normal state, the leaked NOx is occluded by the NOx catalyst 21 and is not discharged downstream of the NOx catalyst 21. That is, as shown in (D), if a predetermined low sulfur fuel is used, the NOx catalyst 21 is not poisoned with S, and the NOx catalyst 21 has a sufficient NOx storage capacity. Therefore, the NOx catalyst 21 occludes leaked NOx and does not discharge leaked NOx. Therefore, the output I of the NOx sensor 22 on the downstream side of the NOx catalyst 21 is maintained at a low value corresponding to a low NOx concentration. Note that the output I of the NOx sensor 22 slightly rises in the first lean half cycle, which is due to the relatively large amount of leaked NOx being supplied to the NOx catalyst 21.

  The rich gas gradually leaks from the upstream catalyst 11 in the rich half cycle (t1 to t2, t3 to t4, t5 to t6) between the lean half cycles, but the leaked rich gas is supplied to the NOx catalyst 21. As a result, the occluded NOx is released from the NOx catalyst 21 and reduced. Therefore, the NOx catalyst 21 is regenerated every rich half cycle.

  By the way, as shown to (E), when a high sulfur fuel is used, the NOx catalyst 21 will be poisoned by S, and the NOx occlusion ability which it originally has will fall. Therefore, the amount of NOx that can be stored by the NOx catalyst 21 decreases, and as a result, the leaked NOx leaks from the NOx catalyst 21 from a relatively early stage. Therefore, the output I of the NOx sensor 22 starts to rise and changes from a relatively early stage.

  Thus, the behavior of the output I of the NOx sensor 22 during lean control differs between when using low-sulfur fuel and when using high-sulfur fuel, due to the presence or absence of S poisoning of the NOx catalyst 21. Therefore, in this embodiment, the sulfur concentration of the fuel is detected using this characteristic.

The first mode of sulfur concentration detection uses, as a detection parameter, a time T from the start of lean control until the NOx sensor output I reaches a predetermined value as shown in (E). That is, simultaneously with the start of the first lean half cycle (t2 to t3), the measurement of the time T and the monitoring of the NOx sensor output I are started, and at the same time, the value (start value) I ₀ of the NOx sensor output at the start. Is memorized. After that, when the NOx sensor output I reaches a value larger than the start value I ₀ by a predetermined value A, the time T at that time is stored. More specifically, after the start of the lean half cycle, the difference between the actual NOx sensor output I and the start value I ₀ , that is, the NOx sensor output difference ΔI = I−I ₀ is monitored, and this NOx sensor output difference ΔI is predetermined. When the value is equal to or greater than A, the measurement time T from the start of the lean half cycle at that time is stored.

  Next, when the stored time T is equal to or less than the predetermined value Ts, it is detected that the fuel has a high sulfur concentration, that is, it is determined that the fuel is a high sulfur fuel. On the other hand, when the time T is longer than the predetermined value Ts, it is detected that the sulfur concentration of the fuel is low, that is, it is determined that the fuel is a predetermined low sulfur fuel.

  As can be seen from the above description, this first mode utilizes the fact that NOx begins to leak from the NOx catalyst 21 and the NOx sensor output I rises earlier when using high sulfur fuel than when using low sulfur fuel. is there.

  Hereinafter, the catalyst deterioration diagnosis process including the first aspect of the sulfur concentration detection will be described with reference to FIG. The illustrated routine is repeatedly executed by the ECU 20 every predetermined sampling period τ (for example, 16 msec).

  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 upstream catalyst 11, the NOx catalyst 21, and the sensors 17, 18, and 22 are in an active state. If there is, the precondition is satisfied. Note that the precondition is not limited to the example described here. If the precondition is not satisfied, the current routine is terminated. On the other hand, if the precondition is satisfied, the routine proceeds to step S102, where active air-fuel ratio control is executed and started.

  Next, in step S103, it is determined whether or not the first half cycle of the active air-fuel ratio control has been completed. This first half cycle corresponds to the period from t0 to t1 shown in FIG. 6, and it is determined whether or not the so-called deserted mountain has ended. If not completed, the current routine is terminated, and if completed, the process proceeds to step S104.

  In step S104, it is determined whether or not the present is in the first lean half cycle. This is because in the present embodiment, the sulfur concentration is detected only in the first lean half cycle. The first lean half cycle corresponds to the period from t2 to t3 shown in FIG. If the present is not in the first lean half cycle, the oxygen storage capacity OSC is measured by the above-described method in step S113. On the other hand, if the present is in the first lean half cycle, the routine proceeds to step S105, where the oxygen storage capacity OSC is measured, and the processing for detecting the sulfur concentration is executed after step S106.

  Here, when measuring the oxygen storage capacity OSC, since the NOx catalyst 21 can also store oxygen, the total value of the oxygen storage capacity in both the upstream catalyst 11 and the NOx catalyst 21 is measured. However, the oxygen storage capacity of only the upstream catalyst 11 is calculated by subtracting the oxygen storage capacity of the NOx catalyst 21 minutes from this total value. Specifically, a value obtained by multiplying the total value by a predetermined reduction coefficient W (where 0 <W <1, for example, W = 0.9) is set as the oxygen storage capacity of the upstream catalyst 11. By doing so, it is possible to accurately obtain the oxygen storage capacity of the upstream catalyst 11 in consideration of the degree of deterioration of the upstream catalyst 11 and the NOx catalyst 21 interlocked with each other.

Now, in step S106, the first lean half cycle starting NOx sensor output value or starting value _{I 0} at the time, the difference between the actual NOx sensor output value _{I n,} i.e. NOx sensor output difference ΔI _n = _{I n} - I ₀ is monitored. Here, n means each sampling period for each sampling period τ. In step S107, a time T from the start of the first lean half cycle is measured.

In step S108, it is determined whether or not the NOx sensor output difference ΔIn is greater than or _equal to a predetermined value A. Here, as the predetermined value A, NOx sensor output difference [Delta] it _n is set to a value that can be reached in one of the lean half cycle when using high sulfur fuels. If the NOx sensor output difference [Delta] I _n is not equal to or greater than the predetermined value A, the routine is ended.

On the other hand, if the NOx sensor output difference [Delta] I _n is sufficient that the predetermined value or more A, the process proceeds to step S109, with time T at that time is stored, the time T is compared with a predetermined value Ts. When T> Ts, NOx leakage from the NOx catalyst 21 is slow, and the NOx catalyst 21 is not poisoned with S. In step S110, the fuel used is determined to be low-sulfur fuel. On the other hand, when T ≦ Ts, NOx leakage from the NOx catalyst 21 is early, and the NOx catalyst 21 is S-poisoned, and in step S111, the used fuel is determined to be a high sulfur fuel.

  If it is determined that the fuel is high-sulfur fuel, a correction coefficient α for correcting the oxygen storage capacity measurement value is calculated in the next step S112. That is, in the case of high sulfur fuel, the upstream catalyst 11 is poisoned by S, and the measured value of the oxygen storage capacity may be smaller than the true value, resulting in a false diagnosis. Therefore, the correction coefficient α for correcting the increase in the oxygen storage capacity measurement value by the amount of decrease in S poisoning is calculated. The correction coefficient α is calculated using a map or function stored in the ECU 20 in advance. In this embodiment, a map as shown in FIG. 8 is used, and a correction coefficient α greater than 1 is calculated when T ≦ Ts. The map characteristics are not limited to the illustrated example.

  Subsequent to step S110, S112 or S113, in step S114, it is determined whether or not the N cycle of active air-fuel ratio control (N = 3 in the present embodiment) has ended. If not finished, this routine is finished. If completed, the process proceeds to step S115, where the active air-fuel ratio control and the oxygen storage capacity measurement are completed, and the average value OSCav is determined from the measured values of the total 2N (6 in the present embodiment) oxygen storage capacity OSC. Is calculated.

  In step S116, the final oxygen storage capacity measurement value OSC is calculated by multiplying the average value OSCav of the oxygen storage capacity measurement values by the correction coefficient α. As a result, when it is determined in step S111 that the fuel is high-sulfur fuel, the correction coefficient α greater than 1 calculated in step S112 is multiplied to compensate for the sulfur poisoning of the upstream catalyst 11. When step S116 is reached without calculating the correction coefficient α in step S112, the reference value 1 is used as the correction coefficient α, and the average value OSCav of the oxygen storage capacity measurement value is used as it is in the final oxygen storage capacity measurement. The value OSC is used.

  Next, in step S117, the final oxygen storage capacity measurement value OSC is compared with a predetermined deterioration determination value OSCs. If OSC> OSCs, the upstream catalyst 11 is determined to be normal in step S118, and if OSC ≦ OSCs, the upstream catalyst 11 is determined to be degraded in step S119. This is the end of the routine.

  In this example, the sulfur concentration is detected only in the first lean half cycle. However, the present invention is not limited to this, and the sulfur concentration may be detected in any lean half cycle. For example, the sulfur concentration may be detected only in the other half (for example, the second or third) lean half cycle, or the sulfur concentration may be detected in a plurality of lean half cycles. In the latter case, the detection result may be the final detection result only when a plurality of detection results are the same, or the final detection result is determined to be high sulfur if it is determined that high-sulfur fuel is used once. It is good also as fuel use.

Further, instead of the difference ΔI between the actual NOx sensor output value I and the start value I ₀ , the actual NOx sensor output value I itself may be used.

  Instead of correcting the oxygen storage capacity measurement value OSC, the deterioration determination value OSCs may be corrected. In this case, when it is determined in step S111 that high sulfur fuel is used, a correction coefficient β smaller than 1 is calculated in step S112 from, for example, a map as shown in FIG. Instead of correcting the average value OSCav in step S116, OSCs is replaced with β · OSCs in step S117. As a result, the deterioration determination value OSCs is corrected to a smaller value so as to compensate for the S poisoning amount of the upstream catalyst 11, thereby preventing erroneous diagnosis that causes the upstream catalyst 11 to deteriorate.

  Alternatively, the diagnosis may be stopped immediately when high sulfur fuel is detected in order to prevent erroneous diagnosis.

Next, a second aspect of sulfur concentration detection will be described. In this aspect, an area based on the NOx sensor output I during lean control is used as a detection parameter. That is, as shown in FIG. 10, the NOx sensor output difference ΔI _n = I _n −I ₀ is integrated every sampling cycle τ from the start of the first lean half cycle (t2 to t3). The integrated value ShigumaderutaI _n Because corresponds to the area shown by hatching, it referred to the integrated value ShigumaderutaI _n both areas. Then, the integrated value or area ShigumaderutaI _n is when a predetermined value or more B is determined that the high-sulfur fuel, the accumulated value to the area ShigumaderutaI _n is when is less than the predetermined value B is judged to low sulfur fuels. This second aspect utilizes the fact that the NOx sensor output I changes more greatly when using high sulfur fuel than when using low sulfur fuel.

  FIG. 11 shows a routine of the catalyst deterioration diagnosis process including the second mode of detecting the sulfur concentration. In this routine, the only difference from the routine according to the first aspect shown in FIG. 7 is that steps S206 and S207 are executed instead of steps S106 to S109. Will be replaced with the 200 series, and detailed description will be omitted.

Step S206 is accumulated the NOx sensor output difference ΔI _n = _{I n} -I ₀ In the integrated value to the area ShigumaderutaI _n in this step S207 is equal to or less than the predetermined value B is determined. When the integrated value ShigumaderutaI _n is less than the predetermined value B, it is determined that low sulfur fuel in step S210. On the other hand, when the integrated value ΣΔIn is greater than or _equal to the predetermined value B, it is determined in step S211 that the fuel is high sulfur fuel. In this case, in the next step S212, a correction coefficient α for correcting the oxygen storage capacity measurement value is calculated using a map or a function. In the present embodiment, a map as shown in FIG. 12 is used, and when ΣΔI _n ≧ B, a correction coefficient α greater than 1 is calculated. However, the characteristics of this map are not limited to the illustrated example.

Although the integrated value ShigumaderutaI _n that varies with time in this example determines that the high sulfur fuel at the time of reaching a predetermined value or more B, once determined the integrated value ShigumaderutaI _n over the entire duration of the lean half cycle, the integrated value ShigumaderutaI _{If n} reaches the predetermined value B or more, it may be determined that the fuel is high sulfur. It may be used NOx sensor output value _{I n} itself instead of the difference [Delta] I _n the starting value _{I 0} of the NOx sensor output value _{I n.} In addition, the sulfur concentration can be detected in an arbitrary lean half cycle, the deterioration determination value OSCs may be corrected instead of correcting the oxygen storage capacity measurement value, and the diagnosis can be stopped immediately when determining high sulfur fuel. The good point is the same as described above.

Next, a third mode of sulfur concentration detection will be described. In this aspect, the trajectory length based on the NOx sensor output I during lean control is used as a detection parameter. That is, as shown in FIG. 13, the difference from the start of the lean half-period of the first (t2 to t3) for each sampling period tau, and NOx sensor output _{I n-1} of this NOx sensor output _{I n} and the previous ΔI _n ′ = I _n −I _n−1 is integrated. Since this integrated value ΣΔI _n ′ corresponds to the locus length or amount of change of the NOx sensor output I as shown in the figure, the integrated value ΣΔI _n ′ is also referred to as a locus length. Next, when the integrated value or trajectory length ΣΔI _n ′ is greater than or equal to the predetermined value C, it is determined that the fuel is high sulfur, and when the integrated value or trajectory length ΣΔI _n ′ is less than the predetermined value C, it is determined that the fuel is low sulfur. This third aspect also utilizes the fact that the NOx sensor output I changes more greatly when using high sulfur fuel than when using low sulfur fuel.

  FIG. 14 shows a routine of the catalyst deterioration diagnosis process including the third mode of detecting the sulfur concentration. This routine differs from the routine according to the first mode shown in FIG. 7 only in that steps S306 and 307 are executed in place of steps S106 to S109 as in the routine according to the second mode. Since the remainder is the same, the step number is replaced with the 300s and detailed description is omitted.

In step S306, the difference between the previous and current NOx sensor outputs ΔI _n ′ = I _n −I _n−1 is integrated, and in step S307, whether or not the current integrated value or the locus length ΣΔI _n ′ is less than the predetermined value C. Is judged. When the integrated value ΣΔI _n ′ is less than the predetermined value C, it is determined at step S310 that the fuel is low sulfur. On the other hand, when the integrated value ΣΔI _n ′ is greater than or equal to the predetermined value C, it is determined in step S311 that the fuel is high sulfur. In this case, in the next step S312, a correction coefficient α for correcting the oxygen storage capacity measurement value is calculated using a map or a function. In the present embodiment used a map as shown in FIG. 15, greater than one correction coefficient α is calculated when the ΣΔI _{n '≧} C. The characteristics of this map are not limited to the illustrated example.

Same manner as in the second embodiment, although the integrated value ΣΔI _{n 'which} changes with time in this example determines that the high sulfur fuel at the time of reaching a predetermined value or more C, the integrated value over the whole period of the lean half cycle ΣΔI _n ′ is once obtained, and if the integrated value ΣΔI _n ′ reaches a predetermined value C or more, it may be determined that the fuel is high sulfur. In addition, the sulfur concentration can be detected in an arbitrary lean half cycle, the deterioration determination value OSCs may be corrected instead of correcting the oxygen storage capacity measurement value, and the diagnosis can be stopped immediately when determining high sulfur fuel. The good point is the same as described above.

By the way, as the upstream catalyst 11 deteriorates, the air-fuel ratio switching period and the inversion period of the post-catalyst sensor output Vr are shortened, the rising start timing of the NOx sensor output I is advanced, and the NOx sensor output I over the entire lean half period. May reduce the area and trajectory length based on. Therefore, in order to enable accurate detection of the sulfur concentration even in this case, the predetermined values Ts, B, and C of the detection parameters (T, ΣΔI _n , ΣΔI _n ′) are changed according to the degree of deterioration of the upstream catalyst 11. preferable. As a parameter representing the degree of deterioration of the upstream catalyst 11, an actual measured value of the oxygen storage capacity OSC can be used. As for the time T, it is preferable to decrease the predetermined value Ts as the measured value of the oxygen storage capacity OSC is smaller (that is, the degree of catalyst deterioration is larger). Regarding also the area ShigumaderutaI _n and the locus length ΣΔI _{n ',} as the measured value of the oxygen storage capacity OSC is low, it is preferable to reduce the predetermined value B, and C. This makes it possible to execute appropriate sulfur concentration detection that also considers the degree of catalyst deterioration.

  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. In the above embodiment, the upstream catalyst 11 and the NOx catalyst 21 are arranged separately from each other with a predetermined interval therebetween. However, as shown in FIG. 16, they may be integrated and arranged adjacent to each other. In this case, the three-way catalyst 11 can be formed in the front part including at least the first half of the carrier base material, and the NOx catalyst 21 can be formed in the remaining rear part.

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

1 internal combustion engine 6 exhaust pipe 11 upstream catalyst 12 injector 17 pre-catalyst sensor 18 post-catalyst sensor 19 downstream catalyst 20 electronic control unit (ECU)
21 NOx storage reduction catalyst 22 NOx sensor OSC Oxygen storage capacity OSCs Degradation judgment value Vr Post-catalyst sensor output VR Rich judgment value VL Lean judgment value A / Ft Target air-fuel ratio I NOx sensor output T Time ΣΔI _n Integrated value (area)
ΣΔI _n 'integrated value (trajectory length)

Claims (4)

An apparatus for diagnosing deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A post-catalyst sensor that is disposed downstream of the catalyst and detects an air-fuel ratio of exhaust gas;
Active air-fuel ratio control means for executing active air-fuel ratio control for alternately switching the air-fuel ratio on the upstream side of the catalyst between rich and lean as the output of the post-catalyst sensor is inverted;
Measuring means for measuring the oxygen storage capacity of the catalyst in accordance with execution of the active air-fuel ratio control;
Determination means for determining deterioration of the catalyst based on the measured value of the oxygen storage capacity;
A relatively small capacity NOx storage reduction catalyst disposed between the catalyst and the post-catalyst sensor;
A NOx sensor that is disposed downstream of the NOx catalyst and detects the NOx concentration of the exhaust gas;
A sulfur concentration detecting means for detecting the sulfur concentration of the fuel based on the output of the NOx sensor during lean control of the catalyst upstream air-fuel ratio;
A catalyst deterioration diagnosis device comprising: The sulfur concentration detection means measures the time from the start of the lean control until the output of the NOx sensor reaches a predetermined value, and when the measured time is less than the predetermined value, the sulfur concentration of the fuel is The catalyst deterioration diagnosis device according to claim 1, wherein the catalyst deterioration diagnosis device is detected. The sulfur concentration detection means measures an area or trajectory length based on the NOx sensor output during the lean control, and detects that the sulfur concentration of the fuel is high when the measured area or trajectory length is a predetermined value or more. The catalyst deterioration diagnosis device according to claim 1, wherein: The determination means determines the deterioration of the catalyst by comparing the measured oxygen storage capacity value with a predetermined deterioration determination value;
The correction means which corrects the oxygen storage capacity measurement value or the deterioration determination value when the sulfur concentration detection means detects that the sulfur concentration of the fuel is high. The catalyst deterioration diagnosis device according to claim 1.

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