EXHAUST ENGINE CLEANING SYSTEM

Abstract

Exhaust gas purification system of an internal combustion engine, comprising: a catalyst (20) arranged in an outlet passage and capable of storing oxygen; and an air-fuel ratio control device (31) configured to control the air-fuel ratio of incoming exhaust gas flowing into the catalyst (20), wherein the catalyst (20) comprises a precious metal and the precious metal has a property that a vapor pressure decreases at a predetermined temperature when it is oxidized, In the event that the temperature of the catalyst (20) is greater than or equal to a threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the air-fuel ratio of the incoming exhaust gas leaner than a stoichiometric air-fuel ratio, so that an oxygen storage quantity of the catalyst (20) becomes greater than or equal to an upper reference quantity, and the threshold temperature is set at a temperature at which the vapor pressure of the precious metal does not increase when local heat generation of the catalyst (20) occurs due to the supply of oxygen.

Description

AREA

[0001] The present invention relates to an exhaust gas purification system for an internal combustion engine. BACKGROUND

[0002] In the past, it was known to place a catalytic converter in the exhaust port of an internal combustion engine and to remove the harmful substances from the exhaust gas (HC, NOx, etc.) at the catalyst (e.g., JP 2018 - 9 535 A). However, if oxygen is supplied to the catalyst at a high temperature, oxidation of the precious metal can cause a deterioration of the catalyst.

[0003] In contrast, if, in the internal combustion engine described in JP 2018-9535A, the catalyst temperature is greater than or equal to a predetermined temperature when the condition for implementing a fuel cut-off control (which stops fuel injection by a fuel injector) is met, then the control to introduce EGR gas instead of fresh air into the catalyst is implemented when the fuel cut-off control is executed. It is assumed that this prevents oxygen from entering the catalyst and thus prevents catalyst degradation.

[0004] Furthermore, US patent 2015 / 0322878A1 discloses a control device for an internal combustion engine, comprising: an upstream catalyst; a downstream catalyst located further downstream in the exhaust flow direction than the upstream catalyst; a downstream air-fuel ratio sensing device provided between these catalysts; a storage quantity estimating device that estimates the oxygen storage quantity of the downstream catalyst; and an incoming exhaust air-fuel ratio control device that controls the exhaust gas's air-fuel ratio entering the upstream catalyst such that the exhaust gas's air-fuel ratio achieves a target air-fuel ratio.In a rich-run mode during normal operation, the target air-fuel ratio is set lean when the air-fuel ratio detected by the downstream air-fuel ratio sensing device is rich, and the target air-fuel ratio is set rich when the oxygen storage quantity of the upstream catalyst is equal to or greater than an upstream reference storage quantity. If the oxygen storage quantity of the downstream catalyst is equal to or less than a downstream lower limit storage quantity that is less than the maximum storage quantity, then the target air-fuel ratio is set lean, so that the air-fuel ratio of the exhaust gas flowing from the upstream catalyst becomes lean.

[0005] Further relevant prior art can be found in the following non-patent literature: CARRILLO, Cristihan [et al.]: regenerative trapping: How Pd improves the durability of Pt diesel oxidation catalysts. In: Applied Catalysis B: Environmental, Vol. 218, 2017, pp. 581-590. ISSN 0926-3373. SUMMARY [TECHNICAL PROBLEM]

[0006] However, to implement the above control, a design for recirculating the EGR gas from an exhaust port to an intake port is essential, and it is necessary to delay the start of the fuel cut-off control to effect this EGR gas recirculation. Furthermore, oxygen can flow into the catalyst even at a time other than the fuel cut-off control, for example, when the internal combustion engine is stopped. In addition, at high catalyst temperatures, the catalyst may degrade, depending on the properties of its precious metal, due to exhaust gas with a richer air-fuel ratio than the stoichiometric air-fuel ratio entering the catalyst.

[0007] Therefore, in view of the above problem, the present invention is based on the objective of providing an exhaust gas purification system for an internal combustion engine that is capable of effectively preventing deterioration of a catalyst. [SOLUTION TO THE PROBLEM]

[0008] The above problem is solved by the subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of the subsequent dependent claims.

[0009] The following are explanatory aspects of the present revelation. (1) Exhaust gas purification system of an internal combustion engine comprising: a catalyst arranged in an exhaust port and capable of storing oxygen;and an air-fuel ratio control device configured to control the air-fuel ratio of incoming exhaust gas flowing into the catalyst, wherein the catalyst comprises a noble metal and the noble metal has the property of having a vapor pressure that decreases at a predetermined temperature when oxidized, and in the event that a temperature of the catalyst is greater than or equal to a threshold temperature or a temperature increase of the catalyst is predicted, the air-fuel ratio control device is configured to make the air-fuel ratio of the incoming exhaust gas leaner than a stoichiometric air-fuel ratio, such that an oxygen storage quantity of the catalyst becomes greater than or equal to an upper reference quantity. (2) Exhaust gas purification system of an internal combustion engine according to (1), further comprising a NOx cleaning device which is arranged in the exhaust passage on a downstream side of the catalyst in an exhaust gas flow direction and is able to remove the NOx flowing out of the catalyst. (3) Exhaust gas purification system of an internal combustion engine according to (2), wherein the NOx purification device can store oxygen, and in the event that the temperature of the catalyst is greater than or equal to the threshold temperature or a temperature increase of the catalyst is predicted, the air-fuel ratio control device is configured to make the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio, so that an oxygen storage quantity of the NOx purification device becomes less than or equal to a lower reference quantity which is less than the upper reference quantity, and then to make the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the catalyst becomes greater than or equal to the upper reference quantity. (4) Exhaust gas purification system of an internal combustion engine according to (2), further comprising a fuel supply valve which supplies fuel to the exhaust passage between the catalyst and the NOx cleaning device, wherein the NOx cleaning device is capable of storing oxygen, and the air-fuel ratio control device is configured to control the fuel supply valve, and in the event that the temperature of the catalyst is greater than or equal to the threshold temperature or a temperature increase of the catalyst is predicted, supplying fuel through the fuel supply valve in such a way that the oxygen storage quantity of the NOx cleaning device becomes less than or equal to a lower reference quantity which is less than the upper reference quantity. (5) Exhaust gas purification system of an internal combustion engine comprising: a catalyst arranged in an exhaust port and capable of storing oxygen;and an air-fuel ratio control device configured to control the air-fuel ratio of incoming exhaust gas flowing into the catalyst, wherein the catalyst comprises a noble metal and the noble metal has the property of increasing its vapor pressure at a predetermined temperature when oxidized, and in the event that the temperature of the catalyst is greater than or equal to a threshold temperature or a temperature increase of the catalyst is predicted, the air-fuel ratio control device is configured to make the air-fuel ratio of the incoming exhaust gas richer than a stoichiometric air-fuel ratio, such that the oxygen storage quantity of the catalyst becomes less than or equal to a lower reference quantity. (6) Exhaust gas purification system of an internal combustion engine according to (5) or (4), further comprising an HC cleaning device which is arranged in the exhaust passage on a downstream side of the catalyst in an exhaust gas flow direction and is able to remove HC escaping from the catalyst. (7) Exhaust gas purification system of an internal combustion engine according to (6), wherein the HC purification device can store oxygen, and in the event that the temperature of the catalyst is greater than or equal to the threshold temperature or a temperature increase of the catalyst is predicted, the air-fuel ratio control device is configured to make the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio, so that an oxygen storage quantity of the HC purification device becomes greater than or equal to an upper reference quantity which is greater than the lower reference quantity, and then to make the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the catalyst becomes less than or equal to the lower reference quantity. (8) Exhaust gas purification system of an internal combustion engine according to (6), further comprising an air supply device which supplies air to the exhaust passage between the catalyst and the HC cleaning device, wherein the HC cleaning device is capable of storing oxygen, and the air-fuel ratio control device is configured to control the air supply device, and in the event that the temperature of the catalyst is greater than or equal to the threshold temperature or a temperature increase of the catalyst is predicted, supplying air through the air supply device so that the oxygen storage quantity of the HC cleaning device becomes greater than or equal to an upper reference quantity which is greater than the lower reference quantity. (9) Exhaust gas purification system of an internal combustion engine, comprising: a catalyst arranged in an exhaust passage and capable of storing oxygen; and an air-fuel ratio control device configured to control an atmosphere of the catalyst, wherein the catalyst comprises a precious metal and the precious metal has the property that a vapor pressure at a predetermined temperature in a second atmosphere from a reducing atmosphere or an oxidizing atmosphere is lower compared to a first atmosphere from an oxidizing atmosphere or a reducing atmosphere, and in the event that a temperature of the catalyst is greater than or equal to a threshold temperature or a temperature increase of the catalyst is predicted, the air-fuel ratio control device is configured to make the catalyst the second atmosphere. (10) Exhaust gas purification system of an internal combustion engine according to (9), further comprising a downstream catalyst which is arranged in the exhaust passage on a downstream side of the catalyst in an exhaust gas flow direction and is capable of storing oxygen, and in the case that the temperature of the catalyst is greater than or equal to the threshold temperature or a temperature increase of the catalyst is predicted, the air-fuel ratio control device is designed to make the downstream catalyst the first atmosphere. Advantageous effects of the invention

[0010] According to the present invention, an exhaust gas purification system for an internal combustion engine is provided which is capable of effectively preventing deterioration of a catalyst. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic view of an internal combustion engine in which an exhaust gas purification system of an internal combustion engine according to a first embodiment of the present invention is provided. Fig. Figure 2 is a view showing the cleaning characteristics of a three-way catalyst. Fig. Figure 3 is a view showing vapor pressure curves of Pd and PdO. Fig. Figure 4 is a time diagram of the temperature of a catalyst, etc., when in the first embodiment an air-fuel ratio control is carried out to prevent deterioration of a catalyst. Fig. Figure 5 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the first embodiment. Fig. Figure 6 is a schematic view of an internal combustion engine in which an exhaust gas purification system of an internal combustion engine according to a second embodiment of the present invention is provided. Fig. Figure 7 is a time diagram of the temperature of a catalyst, etc., when in the second embodiment an air-fuel ratio control is carried out to prevent deterioration of a catalyst. Fig. Figure 8 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the second embodiment. Fig. Figure 9 is a schematic view of an internal combustion engine in which an exhaust gas purification system of an internal combustion engine according to a third embodiment of the present invention is provided. Fig. Figure 10 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the third embodiment. Fig. Figure 11 are characteristic curves showing vapor pressure curves of Rh, RhO and RhO2. Fig. Figure 12 are characteristic curves showing vapor pressure curves of Pt, PtO and PtO2. Fig. Figure 13 is a time diagram of the temperature of a catalyst, etc., when in a fourth embodiment an air-fuel ratio control is carried out to prevent deterioration of a catalyst. Fig. Figure 14 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the fourth embodiment. Fig. Figure 15 is a time diagram of the temperature of a catalyst, etc., when an air-fuel ratio control is carried out to prevent deterioration of a catalyst in a fifth embodiment. Fig. Figure 16 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the fifth embodiment. Fig. Figure 17 is a schematic view of an internal combustion engine in which an exhaust gas purification system of an internal combustion engine according to a sixth embodiment of the present invention is provided. Fig. Figure 18 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the sixth embodiment. Fig. Figure 19 is a schematic view of the design of a vehicle in which an exhaust gas purification system of an internal combustion engine is provided according to a seventh embodiment of the present invention. Fig. Figure 20 is a flowchart showing a control routine of a processing operation to prevent deterioration of a catalyst in the seventh embodiment. DESCRIPTION OF EXECUTION FORMS

[0011] Embodiments of the present invention are explained in detail below with reference to the figures. It should be noted that identical components are assigned the same reference numerals in the following explanation. <Erste Ausführungsform>

[0012] First, with reference to Fig. 1 to Fig. 5 explains a first embodiment of the present invention. <Allgemeine Erläuterung einer Verbrennungskraftmaschine>

[0013] Fig. Figure 1 is a schematic view of an internal combustion engine equipped with an exhaust gas purification system of an internal combustion engine according to a first embodiment of the present invention. The Fig. The internal combustion engine shown is a spark-ignition type engine. The internal combustion engine is mounted in a vehicle.

[0014] Referring to Fig. 1 represents a power engine body, 2 a cylinder block, 3 a piston that moves back and forth in the cylinder block 2, 4 a cylinder head that is attached to the cylinder block 2, 5 a combustion chamber formed between the piston 3 and the cylinder head 4, 6 an intake valve, 7 an intake port, 8 an exhaust valve, and 9 an exhaust port. The intake valve 6 opens and closes the intake port 7, while the exhaust valve 8 opens and closes the exhaust port 9.

[0015] As in Fig. As shown in Figure 1, a spark plug 10 is arranged in the central part of the inner wall surface of the cylinder head 4. A fuel injector 11 is arranged in the region of the inner wall surface of the cylinder head 4. The spark plug 10 is designed to generate a spark in accordance with an ignition signal. Furthermore, the fuel injector 11 injects a predetermined quantity of fuel into the combustion chamber 5 in accordance with an injection signal. In the present embodiment, gasoline with a stoichiometric air-fuel ratio of 14.6 is used as the fuel.

[0016] The inlet opening 7 in each cylinder is connected to a buffer tank 14 via a corresponding inlet channel 13. The buffer tank 14 is connected to an air purifier 16 via an inlet pipe 15. The inlet opening 7, the inlet channel 13, the buffer tank 14, the inlet pipe 15, etc., form an inlet passage that supplies air to the combustion chamber 5. Furthermore, a throttle valve 18 is arranged in the inlet pipe 15 and is actuated by a throttle valve actuator 17. The throttle valve 18 can be rotated by the throttle valve actuator 17 to change the opening range of the inlet passage.

[0017] In contrast, the exhaust port 9 in each cylinder is connected to an exhaust manifold 19. The exhaust manifold 19 has a plurality of channels that are connected to the exhaust ports 9 and a head where these channels converge. The head of the exhaust manifold 19 is connected to a housing 21, which contains a catalyst 20. The housing 21 is connected to an exhaust pipe 22. The exhaust port 9, the exhaust manifold 19, the housing 21, the exhaust pipe 22, etc., form an exhaust passage that carries away exhaust gas produced by the combustion of the air-fuel mixture in the combustion chamber 5.

[0018] Various control routines of the internal combustion engine are performed by an electronic control unit (ECU) 31. That is, the ECU 31 functions as a control device for the internal combustion engine. Outputs from various sensors located in the internal combustion engine or in the vehicle equipped with the internal combustion engine are fed into the ECU 31, and the ECU 31 controls various actuators based on the sensor outputs, etc.

[0019] The ECU 31 consists of a digital computer equipped with components interconnected by a bidirectional bus 32, such as a RAM (random access memory) 33, a ROM (read-only memory) 34, a CPU (microprocessor) 35, an input port 36, and an output port 37. It should be noted that in the present embodiment a single ECU 31 is provided, but multiple ECUs can be provided for each function.

[0020] An air flow meter 40 is arranged in the inlet pipe 15, which measures the flow rate of air flowing through the inlet pipe 15. The output of the air flow meter 40 is fed into the input port 36 via a corresponding analog-to-digital converter 38.

[0021] Furthermore, an air-fuel ratio sensor 41 is arranged on the head of the exhaust manifold 19, i.e., the upstream side of the catalyst 20 in the exhaust flow direction. This sensor detects the air-fuel ratio of the exhaust gas flowing through the interior of the exhaust manifold 19 (i.e., the exhaust gas flowing into the catalyst 20). The output of the air-fuel ratio sensor 41 is input to the input port 36 via the corresponding analog-to-digital converter 38.

[0022] Furthermore, a temperature sensor 45 is arranged in the housing 21, which contains the catalyst 20, and which detects the temperature of the catalyst 20 (bed temperature). The output of the temperature sensor 45 is input to the input port 36 via a corresponding analog-to-digital converter 38.

[0023] Furthermore, a load sensor 43, which generates an output voltage proportional to the depressor pressure of an accelerator pedal 42, is connected to the accelerator pedal 42, which is provided in the vehicle with the internal combustion engine. The output voltage of the load sensor 43 is input to the input port 36 via a corresponding analog-to-digital converter 38. The ECU 31 calculates the engine load based on the output of the load sensor 43.

[0024] Furthermore, a crankshaft angle sensor 44, which generates an output pulse each time a crankshaft rotates by a predetermined angle (for example, 10°), is connected to input port 36. This output pulse is fed into input port 36. The ECU 31 calculates the engine speed based on the output of the crankshaft angle sensor 44.

[0025] In contrast, output port 37 is connected to various actuators of the internal combustion engine via a corresponding control circuit 39. In the present embodiment, output port 37 is connected to spark plugs 10, fuel injectors 11, and a throttle valve actuator 17, and the ECU 31 controls these. Specifically, the ECU 31 controls the ignition timing of the spark plugs 10, the injection timing and injection quantities of the fuel injectors, and the opening degree of the throttle valve 18.

[0026] It should be noted that the aforementioned internal combustion engine is a naturally aspirated, gasoline-powered engine; however, the design of the internal combustion engine is not limited to the configuration described above. Therefore, the cylinder arrangement, the fuel injection mode, the design of the intake and exhaust systems, the design of the valve actuation mechanism, the presence of a turbocharger, and other specific configurations of the internal combustion engine may differ from the one described in the above. Fig. The embodiment shown in Figure 1 may differ. For example, the fuel injectors 11 can be arranged to inject fuel into the inlet openings 7. Furthermore, an embodiment can be provided to recirculate the EGR from the exhaust port to the inlet port. Additionally, a particulate filter can be provided in the exhaust port to capture particulate matter (PM) contained in the exhaust gas, etc. <Erläuterung des Katalysators>

[0027] The catalyst 20, arranged in the outlet passage, can store oxygen. For example, it is a three-way catalyst capable of simultaneously removing hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). The catalyst 20 comprises a ceramic support, a precious metal with catalytic properties, and a co-catalyst with oxygen storage capacity (for example, cerium dioxide (CeO2)). The precious metal and the co-catalyst are supported on the ceramic support.

[0028] Fig. Figure 2 shows the cleaning characteristics of a three-way catalyst. As in Fig. As shown in Figure 2, the HC, CO, and NOx cleaning rates of the catalyst 20 become extremely high when the air-fuel ratio of the exhaust gas flowing into the catalyst 20 is in the range close to the stoichiometric air-fuel ratio (cleaning window A in Figure 2). Fig. 2). Therefore, the catalyst can effectively remove HC, CO and NOx if the air-fuel ratio of the exhaust gas is maintained at the stoichiometric air-fuel ratio.

[0029] Depending on the air-fuel ratio of the exhaust gas, the catalyst 20 further stores or releases oxygen via the co-catalyst. Specifically, the catalyst 20 stores excess oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio. Conversely, the catalyst 20 releases the amount of additional oxygen required to oxidize HC and CO when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio. Consequently, even if the air-fuel ratio of the exhaust gas deviates somewhat from the stoichiometric air-fuel ratio, the air-fuel ratio on the surface of the catalyst 20 is maintained close to the stoichiometric air-fuel ratio, and HC, CO, and NOx are effectively removed at the catalyst 20. <Abgasreinigungssystem einer Verbrennungskraftmaschine>

[0030] An exhaust gas purification system for an internal combustion engine according to the first embodiment of the present invention (hereinafter referred to simply as the "exhaust gas purification system") is described below. The exhaust gas purification system is provided with a catalyst 20, an air-fuel ratio sensor 41, a temperature sensor 45, and an air-fuel ratio control device. In the present embodiment, the ECU 31 functions as the air-fuel ratio control device.

[0031] The air-fuel ratio control device controls the air-fuel ratio of the exhaust gas flowing into the catalyst 20 (hereinafter referred to as the "incoming exhaust gas"). Specifically, the air-fuel ratio control device sets a target air-fuel ratio of the incoming exhaust gas and controls the amount of fuel supplied to the combustion chambers 5 such that the air-fuel ratio of the incoming exhaust gas matches the target air-fuel ratio. For example, the air-fuel ratio control device controls the amount of fuel supplied to the combustion chambers 5 by means of feedback such that the output air-fuel ratio of the air-fuel ratio sensor 41 matches the target air-fuel ratio.Here, the “output air-fuel ratio” refers to an air-fuel ratio that corresponds to the output value of the air-fuel ratio sensor, that is, the air-fuel ratio detected by the air-fuel ratio sensor.

[0032] It should be noted that the air-fuel ratio control device can control the amount of fuel supplied to the combustion chambers 5 such that the air-fuel ratio of the incoming exhaust gas matches the target air-fuel ratio without using the air-fuel ratio sensor 41. In this case, the air-fuel ratio control device supplies the combustion chambers 5 with a quantity of fuel calculated from the intake air quantity detected by the air flow meter 40 and the target air-fuel ratio, such that the ratio of fuel to air supplied to the combustion chambers 5 matches the target air-fuel ratio.

[0033] Furthermore, the air-fuel ratio control device performs a fuel cut-off control, which stops the supply of fuel to the combustion chambers 5 when a predetermined operating condition is met. The predetermined operating condition is met, for example, when the depressor pedal 42 is zero or substantially zero (that is, the engine load is zero or substantially zero) and the engine speed is greater than or equal to a predetermined speed that is higher than the speed at idle. When the fuel cut-off control is performed, air is diverted from the intake port to the exhaust port and air is supplied to the catalyst 20.

[0034] In the present embodiment, the catalyst contains 20 palladium (Pd) as a precious metal. Fig. Figure 3 shows the vapor pressure curves of Pd and PdO. As shown from Fig. As can be seen in section 3, Pd has a higher vapor pressure than PdO at a predetermined temperature (greater than or equal to 750°C). This means that Pd has the property that its vapor pressure decreases at a predetermined temperature when it is oxidized. In other words, Pd has the property that its vapor pressure is lower at a predetermined temperature in an oxidizing atmosphere compared to a reducing atmosphere.

[0035] When exhaust gas with a lean air-fuel ratio flows into catalyst 20, the precious metal of catalyst 20 is oxidized, and Pd is converted to PdO. If, for example, the aforementioned fuel cut-off control is activated, oxygen is supplied to catalyst 20, and Pd is converted to PdO. However, immediately after the supply of oxygen to catalyst 20, the HC adsorbed onto the precious metal reacts with the oxygen, and local heat generation occurs within catalyst 20. This prevents further oxidation of the precious metal, and the precious metal exists as Pd.

[0036] For this reason, when oxygen is supplied to catalyst 20 at a high temperature, a local heat generation of catalyst 20 occurs, as indicated by the arrow marking in Fig. Figure 3 shows that the vapor pressure of the precious metal rises rapidly along the vapor pressure curve of Pd. As a result, the precious metal evaporates and the catalyst 20 deteriorates.

[0037] In contrast, in the low-temperature range of catalyst 20 (in the example of Fig. 3. In the region below 750°C, the slope of the vapor pressure curve of the noble metal is essentially zero. Therefore, in this temperature range, the vapor pressure of the noble metal does not increase, even if local heat generation from the catalyst 20 causes a temperature rise in the noble metal. Thus, deterioration of the catalyst 20 can be efficiently prevented by exposing the catalyst 20 to an oxidizing atmosphere before its temperature increases.

[0038] For this reason, in the present embodiment, the air-fuel ratio control device exposes the catalyst 20 to an oxygen atmosphere when the temperature of the catalyst 20 is greater than or equal to a threshold temperature. The threshold temperature is predetermined and set at a temperature at which the vapor pressure of the precious metal does not increase when local heat generation occurs in the catalyst 20 due to the supply of oxygen. For example, the threshold temperature is set at 650°C to 700°C.

[0039] For example, if the temperature of catalyst 20 is greater than or equal to the threshold temperature, the air-fuel ratio control device allows the air-fuel ratio of the incoming exhaust gas to become leaner than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of catalyst 20 becomes greater than or equal to an upper reference quantity. The upper reference quantity is predetermined and is set at a quantity greater than half the maximum oxygen storage quantity at the time of a new catalyst. For example, the upper reference quantity is set at a quantity of 2 / 3 to 4 / 5 of the maximum oxygen storage quantity at the time of a new catalyst.

[0040] Furthermore, once the oxygen storage quantity of the catalyst 20 has reached the upper reference quantity, the air-fuel ratio control device adjusts the air-fuel ratio of the incoming exhaust gas such that the oxygen storage quantity of the catalyst 20 is maintained at the upper reference quantity until the catalyst temperature becomes less than or equal to the threshold temperature. This allows the catalyst 20 to be kept in an oxidizing atmosphere when its temperature is high. <Erläuterung einer Luft-Kraftstoff-Verhältnis-Steuerung anhand eines Zeitdiagramms>

[0041] Referring to Fig. Section 4 explains in detail an air-fuel ratio control system to prevent deterioration of the catalyst 20. Fig. Figure 4 is a time diagram of a temperature of the catalyst 20, an air-fuel ratio of the incoming exhaust gas and an oxygen storage quantity of the catalyst 20 when an air-fuel ratio control is carried out to prevent deterioration of the catalyst 20 in the first embodiment.

[0042] In the illustrated example, the temperature of catalyst 20 at time t0 is lower than the threshold temperature Tth. When the temperature of catalyst 20 is lower than the threshold temperature Tth, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating conditions of the internal combustion engine.

[0043] After time t0, the temperature of the catalyst 20 rises in conjunction with an increase in engine load and reaches the threshold temperature Tth at time t1. Consequently, the air-fuel ratio control is initiated to prevent deterioration of the catalyst 20, and the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio to create an oxidizing atmosphere for the catalyst 20. Specifically, the air-fuel ratio of the incoming exhaust gas is set to a leaner air-fuel ratio AFlean, which is leaner than the stoichiometric air-fuel ratio. The lean air-fuel ratio AFlean is predetermined; for example, it is set to 16 to 17.

[0044] If the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio, the oxygen supplied to the catalyst 20 reacts with the hydrocarbons adsorbed on the noble metal of the catalyst 20, and local heat generation occurs in the catalyst 20. As a result, the temperature of the catalyst 20 rises to a greater extent.

[0045] Furthermore, if the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio, oxygen is supplied to catalyst 20 and the oxygen storage quantity of catalyst 20 increases. As a result, the oxygen storage quantity of catalyst 20 reaches the upper reference quantity Uref at time t2.

[0046] After time t2, the air-fuel ratio of the incoming exhaust gas is controlled such that the oxygen storage quantity of the catalyst 20 is maintained at the upper reference quantity Uref. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a value that is leaner than the stoichiometric air-fuel ratio, so that the HC and CO in the exhaust gas are oxidized. The leanness of the target air-fuel ratio at this time is less than the leanness of the lean air-fuel ratio AFlean. It should be noted that the "leanness" refers to the difference between the air-fuel ratio that is leaner than the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio.

[0047] Subsequently, at time t3, the temperature of the catalyst 20 drops to the threshold temperature Tth. Consequently, at time t3, the air-fuel ratio control, which prevents deterioration of the catalyst 20, ceases. For this reason, after time t3, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating state of the internal combustion engine. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0048] The following is described with reference to the flowchart of Fig. 5 an air-fuel ratio control to prevent deterioration of the catalyst 20 is explained in detail. Fig. Figure 5 is a flowchart showing a control routine for a process to prevent catalyst degradation in the first embodiment. The control routine is executed repeatedly by the ECU 31 at predetermined time intervals.

[0049] First, in step S101, the air-fuel ratio control device assesses whether the temperature of catalyst 20 is greater than or equal to the threshold temperature. For example, based on the output of temperature sensor 45, the air-fuel ratio control device assesses whether the temperature of catalyst 20 is greater than or equal to the threshold temperature.

[0050] It should be noted that the temperature sensor 45 can be located on the upstream side of the catalyst 20 in the exhaust port to detect the temperature of the incoming exhaust gas, or it can be located in the downstream side of the exhaust port of the catalyst 20 to detect the temperature of the exhaust gas exiting the catalyst 20. Furthermore, the air-fuel ratio control device can calculate the temperature of the catalyst 20 without using the temperature sensor 45, based on a predetermined state variable of the internal combustion engine (for example, the intake air quantity, engine load, etc.). In this case, the temperature sensor 45 can be omitted from the exhaust gas purification system.

[0051] If step S101 determines that the temperature of catalyst 20 is lower than the threshold temperature, the control routine terminates. However, if step S101 determines that the temperature of catalyst 20 is greater than or equal to the threshold temperature, the control routine continues with step S102.

[0052] In step S102, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio. Specifically, the air-fuel ratio control device sets the target air-fuel ratio of the incoming exhaust gas to the lean air-fuel ratio AFlean.

[0053] Next, in step S103, the air-fuel ratio control device assesses whether the oxygen storage quantity (OSA) of catalyst 20 is greater than or equal to the upper reference quantity (Uref). For example, the air-fuel ratio control device cumulatively adds the oxygen excess / deficiency relative to the stoichiometric air-fuel ratio of the incoming exhaust gas to calculate the oxygen storage quantity (OSA) of catalyst 20. The oxygen storage quantity (OSA) of catalyst 20 is calculated as a value between zero and the maximum oxygen storage quantity.

[0054] It should be noted that "the oxygen excess / deficiency relative to the stoichiometric air-fuel ratio of the incoming exhaust gas" refers to the amount of oxygen that results in excess or deficiency when attempting to adjust the air-fuel ratio of the incoming exhaust gas to the stoichiometric air-fuel ratio. If the air-fuel ratio of the incoming exhaust gas is leaner than the stoichiometric air-fuel ratio, oxygen is stored in the catalyst 20, and the oxygen excess / deficiency value becomes positive. Conversely, if the air-fuel ratio of the incoming exhaust gas is richer than the stoichiometric air-fuel ratio, oxygen is released from the catalyst 20; therefore, the oxygen excess / deficiency value becomes negative.

[0055] The oxygen excess / deficiency OED is calculated, for example, using the following formula (1) based on the output of the air-fuel ratio sensor 41 and the fuel injection quantity: OED=0.23×(AFup−14.6)×Qi

[0056] Here, 0.23 is the oxygen concentration in the air, 14.6 is the stoichiometric air-fuel ratio, Qi is the fuel injection quantity, and AFup is the output air-fuel ratio of the air-fuel ratio sensor 41.

[0057] It should be noted that the oxygen excess / deficiency OED can be calculated using the following formula (2) based on the output of the air-fuel ratio sensor 41 and the intake air quantity: OED=0.23×(AFup−14.6)×Ga / AFup

[0058] Here, 0.23 is the oxygen concentration in the air, 14.6 is the stoichiometric air-fuel ratio, Ga is the intake air volume, and AFup is the output air-fuel ratio of the air-fuel ratio sensor 41. The intake air volume Ga is measured by the air flow meter 40.

[0059] Furthermore, the oxygen excess / deficiency (OED) can be calculated based on the target air-fuel ratio of the incoming exhaust gas without using the output of the air-fuel ratio sensor 41. That is, in the formulas (1) and (2) above, the value of the target air-fuel ratio can be used instead of the output air-fuel ratio (AFup) of the air-fuel ratio sensor 41. In this case, the air-fuel ratio sensor 41 can be omitted from the exhaust gas purification system.

[0060] Furthermore, if a fuel cut-off control is activated, the oxygen storage quantity (OSA) of catalyst 20 can be corrected to the maximum oxygen storage quantity. Additionally, if the target air-fuel ratio of the incoming exhaust gas is maintained at a value richer than the stoichiometric air-fuel ratio for a period greater than or equal to a predetermined time, the oxygen storage quantity (OSA) of catalyst 20 can be corrected to zero. Such a correction makes it possible to reduce the error in calculating the oxygen storage quantity (OSA).

[0061] If, in step S103, it was determined that the oxygen storage quantity OSA of catalyst 20 is less than the upper reference quantity Uref, the control routine returns to step S102, in which the target air-fuel ratio of the incoming exhaust gas is maintained at the lean-set air-fuel ratio AFlean. Conversely, if, in step S103, the oxygen storage quantity OSA of catalyst 20 is greater than or equal to the upper reference quantity Uref, the control routine proceeds to step S104.

[0062] In step S104, the air-fuel ratio control device calculates the target air-fuel ratio (TAF) such that the oxygen storage quantity (OSA) of the catalyst 20 is maintained at the upper reference quantity (Uref). For example, the air-fuel ratio control device calculates the target air-fuel ratio (TAF) based on a predetermined state variable of the internal combustion engine (e.g., intake air quantity, engine load, etc.) such that the HC and CO in the exhaust gas are oxidized. In this case, the air-fuel ratio control device uses a map or formula to calculate the target air-fuel ratio (TAF) based on the predetermined state variable of the internal combustion engine. At this point, the target air-fuel ratio (TAF) is calculated as a value that is leaner than the stoichiometric air-fuel ratio.

[0063] Next, in step S105, the air-fuel ratio control device adjusts the air-fuel ratio of the incoming exhaust gas to the target air-fuel ratio TAF.

[0064] Next, in step S106, the air-fuel ratio control device assesses whether the temperature of catalyst 20 is less than or equal to the threshold temperature. If it is assessed that the temperature of catalyst 20 is higher than the threshold temperature, the control routine returns to step S104, and steps S104 and S105 are repeated. Conversely, if it is assessed that the temperature of catalyst 20 is less than or equal to the threshold temperature, the control routine terminates.

[0065] It should be noted that the air-fuel ratio control device in step S106 can assess whether the temperature of catalyst 20 is less than or equal to a predetermined temperature, which is lower than the threshold temperature. That is, the air-fuel ratio control device can control the air-fuel ratio of the incoming exhaust gas such that the oxygen storage quantity of catalyst 20 is maintained at the upper reference quantity after the oxygen storage quantity of catalyst 20 has reached the upper reference quantity and until the temperature of the catalyst becomes less than or equal to a predetermined temperature, which is lower than the threshold temperature. <Zweite Ausführungsform>

[0066] The design and control of the exhaust gas purification system in a second embodiment is, with the exception of the points explained below, fundamentally the same as the exhaust gas purification system in the first embodiment. For this reason, the second embodiment of the present invention is explained below with a focus on parts that differ from the first embodiment.

[0067] Fig. Figure 6 is a schematic view of an internal combustion engine in which the exhaust gas purification system of an internal combustion engine according to the second embodiment of the present invention is provided. In the second embodiment, the exhaust gas purification system is provided with a catalyst (upstream catalyst) 20, an air-fuel ratio sensor (upstream air-fuel ratio sensor) 41, a temperature sensor 45, an air-fuel ratio control device, a NOx cleaning device 23, and an downstream air-fuel ratio sensor 46.

[0068] The NOx cleaning device 23 is arranged in the outlet passage on the downstream side of the catalyst 20 in the exhaust gas flow direction and removes the NOx flowing out of the catalyst 20. Specifically, the NOx cleaning device 23 is housed in a downstream casing 24, which is arranged on the downstream side of the casing (the upstream casing) 21 in the exhaust gas flow direction. The casing 21 is connected to the downstream casing 24 by the exhaust pipe 22.

[0069] The downstream air-fuel ratio sensor 46 is located between the catalyst 20 and the NOx cleaning device 23 in the exhaust passage and detects the air-fuel ratio of the exhaust gas flowing out of the catalyst 20 (i.e., the exhaust gas flowing into the NOx cleaning device 23). The output of the downstream air-fuel ratio sensor 46 is fed into the input port 36 via a corresponding analog-to-digital converter 38.

[0070] If, as explained above, the catalyst 20 is exposed to an oxygen atmosphere to prevent deterioration, its cleaning capacity with respect to NOx in the exhaust gas decreases, and NOx can escape from the catalyst 20. In contrast, in the second embodiment, even if NOx escapes from the catalyst 20, the NOx cleaning device 23, which is located on the downstream side of the catalyst 20 in the direction of exhaust gas flow, can remove the NOx. Therefore, a deterioration of exhaust emissions can be prevented while simultaneously and effectively preventing deterioration of the catalyst 20.

[0071] In the present embodiment, the NOx cleaning device 23 is a three-way catalyst capable of storing oxygen and, for example, simultaneously removing HC, CO, and NOx. That is, the NOx cleaning device 23 is configured in the same way as the catalyst 20. The NOx cleaning device 23 is also referred to as an "downstream catalyst."

[0072] To increase the NOx removal capacity of an oxygen-storing NOx cleaning device 23, it is necessary to operate the NOx cleaning device 23 in a reducing atmosphere. Therefore, in the second embodiment, the air-fuel ratio control device creates a reducing atmosphere for the NOx cleaning device 23 when the temperature of the catalyst 20 is greater than or equal to the threshold temperature. This makes it possible to increase the cleaning capacity of the NOx cleaning device 23 and to prevent NOx leakage more effectively.

[0073] The exhaust gas passing through the catalyst 20 flows into the NOx cleaning device 23. For this reason, in order to reduce the oxygen storage quantity of the NOx cleaning device 23 so that the NOx cleaning device 23 becomes a reducing atmosphere, the oxygen storage quantity of the catalyst 20 must be set to zero.For this reason, when the temperature of the catalyst 20 is greater than or equal to the threshold temperature, the air-fuel ratio control device allows the air-fuel ratio of the incoming exhaust gas to become richer than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the NOx purification device 23 becomes less than or equal to a lower reference quantity. Conversely, it allows the air-fuel ratio of the incoming exhaust gas to become leaner than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the catalyst 20 becomes greater than or equal to the upper reference quantity. The lower reference quantity is predetermined and set at a quantity that is less than the upper reference quantity. That is, the lower reference quantity is set at a quantity that is less than half the maximum oxygen storage quantity at the time of a new catalyst.For example, the lower reference quantity is set to an amount of 1 / 5 to 1 / 3 of the maximum oxygen storage quantity at the time of a new catalyst. <Erläuterung einer Luft-Kraftstoff-Verhältnis-Steuerung anhand eines Zeitdiagramms>

[0074] Fig. Figure 7 is a time diagram of a temperature of the catalyst 20, an air-fuel ratio of the incoming exhaust gas, an oxygen storage quantity of the catalyst 20 and an oxygen storage quantity of the NOx cleaning device 23 when the air-fuel ratio control to prevent deterioration of the catalyst 20 is carried out in the second embodiment.

[0075] In the illustrated example, at time t0 the temperature of catalyst 20 is lower than the threshold temperature Tth. When the temperature of catalyst 20 is lower than the threshold temperature Tth, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio in accordance with the operating conditions of the internal combustion engine.

[0076] After time t0, the temperature of the catalyst 20 rises in conjunction with the increase in engine load and reaches the threshold temperature Tth at time t1. Consequently, an air-fuel ratio control is initiated to prevent deterioration of the catalyst 20, and the air-fuel ratio of the incoming exhaust gas is made richer than the stoichiometric air-fuel ratio to create a reducing atmosphere for the NOx purification device 23. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a rich air-fuel ratio AFrich, which is richer than the stoichiometric air-fuel ratio. The rich air-fuel ratio AFrich is predetermined and is set, for example, to between 12 and 13.5.

[0077] Furthermore, if the air-fuel ratio of the incoming exhaust gas is made richer than the stoichiometric air-fuel ratio, oxygen is initially released from the catalyst 20, and the oxygen storage capacity of the catalyst 20 is reduced. As a result, the oxygen storage capacity of the catalyst 20 becomes zero at time t2. As the oxygen storage capacity of the catalyst 20 decreases from time t1 to time t2, the HC and CO in the exhaust gas are removed at the catalyst 20, and the oxygen storage capacity of the NOx cleaning device 23 is essentially kept constant.

[0078] After time t2, exhaust gas with an air-fuel ratio richer than the stoichiometric air-fuel ratio flows into the NOx cleaning device 23, oxygen is released from the NOx cleaning device 23, and the oxygen storage quantity of the NOx cleaning device 23 decreases. Meanwhile, the oxygen storage quantity of the catalyst 20 is kept at zero.

[0079] Then, at time t3, the oxygen storage quantity of the NOx purification device 23 reaches the lower reference quantity Dref. Consequently, the air-fuel ratio of the incoming exhaust gas at time t3 is made leaner than the stoichiometric air-fuel ratio to create an oxidizing atmosphere for the catalyst 20. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a lean air-fuel ratio AFlean, which is leaner than the stoichiometric air-fuel ratio. The lean air-fuel ratio AFlean is predetermined; for example, it is set to 16 to 17.

[0080] If the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio, the oxygen supplied to the catalyst 20 reacts with the hydrocarbons adsorbed onto the noble metal of the catalyst 20, and local heat generation occurs within the catalyst 20. As a result, the temperature of the catalyst 20 rises to a greater extent.

[0081] Furthermore, if the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio, oxygen is supplied to catalyst 20 and the oxygen storage quantity of catalyst 20 increases. As a result, the oxygen storage quantity of catalyst 20 reaches the upper reference quantity Uref at time t4.

[0082] After time t4, the air-fuel ratio of the incoming exhaust gas is controlled such that the oxygen storage quantity of the catalyst 20 is maintained at the upper reference quantity Uref. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a value that is leaner than the stoichiometric air-fuel ratio, so that the HC and CO in the exhaust gas are oxidized. The leanness of the target air-fuel ratio at this time is less than the leanness of the lean air-fuel ratio AFlean.

[0083] Subsequently, at time t5, the temperature of the catalyst 20 drops to the threshold temperature Tth. Consequently, at time t5, the air-fuel ratio control, which prevents deterioration of the catalyst 20, ceases. For this reason, after time t5, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating state of the internal combustion engine. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0084] Fig. Figure 8 is a flowchart showing a control routine for a process to prevent catalyst degradation in a second embodiment. The control routine is executed repeatedly by the ECU 31 at predetermined time intervals.

[0085] First, in step S201, the air-fuel ratio control device assesses in the same way as in step S101 of Fig. 5. The control routine determines whether the temperature of catalyst 20 is greater than or equal to the threshold temperature. If it is determined that the temperature of catalyst 20 is lower than the threshold temperature, the control routine terminates. However, if it is determined that the temperature of catalyst 20 is greater than or equal to the threshold temperature, the control routine continues with step S202.

[0086] In step S202, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio. Specifically, the air-fuel ratio control device sets the target air-fuel ratio of the incoming exhaust gas to the rich air-fuel ratio AFrich.

[0087] Next, in step S203, the air-fuel ratio control device assesses whether the oxygen storage quantity OSAd of the NOx cleaning device 23 is less than or equal to the lower reference quantity Dref. For example, the air-fuel ratio control device cumulatively adds the oxygen excess / deficiency relative to the stoichiometric air-fuel ratio of the exhaust gas flowing into the NOx cleaning device 23 to calculate the oxygen storage quantity OSAd of the NOx cleaning device 23. The oxygen storage quantity OSAd of the NOx cleaning device 23 is calculated as a value between zero and the maximum oxygen storage quantity.

[0088] It should be noted that "the oxygen excess / deficiency relative to the stoichiometric air-fuel ratio of the exhaust gas flowing into the NOx cleaning device 23" refers to the amount of oxygen that results in excess or deficiency when attempting to adjust the air-fuel ratio of the exhaust gas flowing into the NOx cleaning device 23 to the stoichiometric air-fuel ratio. If the air-fuel ratio of the exhaust gas flowing into the NOx cleaning device 23 is leaner than the stoichiometric air-fuel ratio, oxygen is stored in the NOx cleaning device 23, and the oxygen excess / deficiency value becomes positive.If, on the other hand, the air-fuel ratio of the exhaust gas flowing into the NOx cleaning device 23 is richer than the stoichiometric air-fuel ratio, oxygen is released from the NOx cleaning device 23 and the value of the oxygen excess / deficiency becomes negative.

[0089] The oxygen excess / deficiency OEDd is calculated, for example, using the following formula (3) based on the output of the downstream air-fuel ratio sensor 46 and the fuel injection quantity. OEDd=0.23×(AFdwn−14.6)×Qi

[0090] Here, 0.23 is the oxygen concentration in the air, 14.6 is the stoichiometric air-fuel ratio, Qi is the fuel injection quantity, and AFdwn is the output air-fuel ratio of the downstream air-fuel ratio sensor 46.

[0091] It should be noted that the oxygen excess / deficiency OEDd can be calculated using the following formula (4) based on the output of the downstream air-fuel ratio sensor 46 and the intake air quantity: OEDd=0.23×(AFdwn−14.6)×Ga / AFup

[0092] Here, 0.23 is the oxygen concentration in the air, 14.6 is the stoichiometric air-fuel ratio, Ga is the intake air quantity, and AFdwn is the output air-fuel ratio of the downstream air-fuel ratio sensor 46. The intake air quantity Ga is measured by the air flow meter 40.

[0093] Furthermore, the oxygen excess / deficiency OEDd can be calculated without using the output of the downstream air-fuel ratio sensor 46, based on the target air-fuel ratio of the incoming exhaust gas. That is, in the formulas (3) and (4) above, the target air-fuel ratio value can be used instead of the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 46. It should be noted that the oxygen excess / deficiency OEDd is set to zero if the oxygen storage quantity of the catalyst 20 is not zero or is at its maximum oxygen storage quantity. If the output of the downstream air-fuel ratio sensor 46 is not used, the downstream air-fuel ratio sensor 46 can be omitted from the exhaust aftertreatment system.

[0094] Furthermore, when the fuel cut-off control is activated, the oxygen storage quantity OSAd of the NOx cleaning device 23 can be corrected to the maximum oxygen storage quantity. Additionally, if the target air-fuel ratio of the incoming exhaust gas is maintained at a value richer than the stoichiometric air-fuel ratio for a period greater than or equal to a predetermined time, the oxygen storage quantity OSAd of the NOx cleaning device 23 can also be corrected to zero. Such a correction can reduce the calculation error of the oxygen storage quantity OSAd.

[0095] If, in step S203, it is determined that the oxygen storage quantity OSAd of the NOx cleaning device 23 is greater than the lower reference quantity Dref, the control routine returns to step S202, in which the target air-fuel ratio of the incoming exhaust gas is maintained at the rich air-fuel ratio AFrich. If, however, in step S203, it is determined that the oxygen storage quantity OSAd of the NOx cleaning device 23 is less than or equal to the lower reference quantity Dref, the control routine proceeds to step S204.

[0096] Steps S204 to S208 are performed in the same way as steps S102 to S106 of Fig. 5. The control routine ends after step S208.

[0097] It should be noted that the NOx purification device 23 can be a NOx-adsorbing material that adsorbs NOx. In this case, steps S202 and S203 are omitted. Furthermore, the NOx purification device 23 can be a combination of a NOx-adsorbing material and a three-way catalyst.

[0098] Furthermore, the NOx cleaning device 23 can be a NOx-storing and -reducing catalyst that stores NOx. In this case, step S203 is omitted, and in step S202, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio for a predetermined time in order to reduce the NOx stored in the NOx cleaning device 23. <Dritte Ausführungsform>

[0099] The exhaust gas purification system according to a third embodiment is, with regard to its design and control, fundamentally identical to the exhaust gas purification system according to the second embodiment, except for the points explained below. For this reason, the third embodiment of the present invention is explained below with a focus on parts that differ from the second embodiment.

[0100] Fig. Figure 9 is a schematic view of an internal combustion engine in which an exhaust gas purification system of an internal combustion engine according to the third embodiment of the present invention is provided. In the third embodiment, the exhaust gas purification system is provided with a catalyst (upstream catalyst) 20, an air-fuel ratio sensor (upstream air-fuel ratio sensor) 41, a temperature sensor 45, an air-fuel ratio control device, a NOx cleaning device 23, an downstream air-fuel ratio sensor 46, and a fuel injection valve 47.

[0101] The fuel injection valve 47 is located between the catalyst 20 and the NOx reduction device 23 in the exhaust port and supplies fuel to the exhaust port between the catalyst 20 and the NOx reduction device 23. The fuel injection valve 47 is connected to the outlet port 37. The ECU 31 controls the fuel injection valve 47. Specifically, the ECU 31 controls the injection timing and the injection quantity of the fuel injection valve 47.

[0102] Furthermore, the fuel injection valve 47 is located in the exhaust passage on the upstream side of the downstream air-fuel ratio sensor 46 in the exhaust flow direction. For this reason, the downstream air-fuel ratio sensor 46 can detect the air-fuel ratio of the exhaust gas to which fuel is added, injected from the fuel injection valve 47.

[0103] In the third embodiment, the air-fuel ratio control device controls the fuel injection valve 47 and, if the temperature of the catalyst 20 is greater than or equal to the threshold temperature, the fuel supply through the fuel injection valve 47, thereby creating a reducing atmosphere for the NOx cleaning device 23. For example, if the temperature of the catalyst 20 is greater than or equal to the threshold temperature, the air-fuel ratio control device supplies fuel through the fuel injection valve 47 such that the oxygen storage quantity of the NOx cleaning device 23 becomes less than or equal to the lower reference quantity. In this case, the oxygen storage quantity of the catalyst 20 does not need to be set to zero to create a reducing atmosphere for the NOx cleaning device 23. For this reason, the NOx cleaning device 23 can be quickly brought into a reducing atmosphere. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0104] Fig. Figure 10 is a flowchart showing a control routine of a processing operation to prevent deterioration of the catalyst in the third embodiment.

[0105] The control routine is repeatedly executed by the ECU 31 at predetermined intervals.

[0106] First, in step S301, the air-fuel ratio control device assesses in the same way as in step S201 of Fig. 8. The control routine determines whether the temperature of catalyst 20 is greater than or equal to the threshold temperature. If it is determined that the temperature of catalyst 20 is lower than the threshold temperature, the control routine terminates. However, if it is determined that the temperature of catalyst 20 is greater than or equal to the threshold temperature, the control routine continues with step S302.

[0107] In step S302, the air-fuel ratio control device uses the fuel injection valve 47 to supply fuel. As a result, fuel-laden exhaust gas, that is, exhaust gas with an air-fuel ratio richer than the stoichiometric air-fuel ratio, flows into the NOx cleaning device 23.

[0108] Next, in step S303, the air-fuel ratio control device assesses in the same way as in step S203 of Fig. 8. Whether the oxygen storage quantity OSAd of the NOx cleaning device 23 is less than or equal to the lower reference quantity Dref. If in step S303 it is determined that the oxygen storage quantity OSAd of the NOx cleaning device 23 is greater than the lower reference quantity Dref, the control routine returns to step S302, in which the fuel supply continues. If, however, in step S303 it is determined that the oxygen storage quantity OSAd of the NOx cleaning device 23 is less than or equal to the lower reference quantity Dref, the control routine continues with step S304.

[0109] Steps S304 to S308 are performed in the same way as steps S204 to S208 of Fig. 8. The control routine ends after step S308.

[0110] It should be noted that steps S304 and S305 can be performed in parallel with steps S302 and S303.

[0111] Furthermore, the NOx purification device 23 can be a NOx-adsorbing material for adsorbing NOx. In this case, steps S302 and S303 are omitted. The NOx purification device 23 can also be a combination of a NOx-adsorbing material and a three-way catalyst.

[0112] Furthermore, the NOx cleaning device 23 can be a NOx-storing and -reducing catalyst that stores NOx. In this case, step S303 is omitted, and in step S302, the air-fuel ratio control device supplies fuel through the fuel injection valve 47 for a predetermined time to reduce the NOx stored in the NOx cleaning device 23. <Vierte Ausführungsform>

[0113] The exhaust gas purification system according to a fourth embodiment is, with regard to its design and control, fundamentally identical to the exhaust gas purification system according to the first embodiment, except for the points explained below. For this reason, the fourth embodiment of the present invention is explained below with a focus on parts that differ from the first embodiment.

[0114] In the fourth embodiment, the catalyst contains 20 rhodium (Rh) or platinum (Pt) as a precious metal. Fig. Figure 11 shows the vapor pressure curves of Rh, RhO, and RhO2. As shown from... Fig. As can be seen in Figure 11, RhO has a higher vapor pressure than Rh at a given temperature. Furthermore, RhO2 has a higher vapor pressure than Rh at a given temperature. This means that Rh has the property that its vapor pressure increases at a given temperature when it is oxidized. In other words, Rh has the property that its vapor pressure decreases at a given temperature in a reducing atmosphere compared to an oxidizing atmosphere.

[0115] Fig. Figure 12 is a view showing vapor pressure curves of Pt, PtO, and PtO2. As shown from Fig. As can be seen in Figure 12, PtO has a higher vapor pressure than Pt at a given temperature. Furthermore, PtO₂ has a higher vapor pressure than Pt at a given temperature. This means that Pt has the property that its vapor pressure increases at a given temperature when it is oxidized. In other words, Pt has the property that its vapor pressure decreases at a given temperature in a reducing atmosphere compared to an oxidizing atmosphere.

[0116] When exhaust gas with a high richness of air-fuel ratio flows into catalyst 20 in a state where the noble metal of catalyst 20 is oxidized, the noble metal of catalyst 20 is reduced. However, immediately after the HC, etc. is introduced into catalyst 20, the oxygen adsorbed on the noble metal reacts with the HC, etc., and local heat generation occurs within the catalyst 20. This prevents further reduction of the noble metal.

[0117] For this reason, the local heat generation of catalyst 20 causes the vapor pressure of the noble metal to rise rapidly, in accordance with the vapor pressure curve of the oxidized noble metal, when HC, etc., is supplied to catalyst 20 at a high catalyst temperature. As a result, the noble metal evaporates and catalyst 20 deteriorates.

[0118] In contrast, in a region where the temperature of catalyst 20 is low, the slope of the vapor pressure curve of the noble metal is essentially zero. Therefore, in this temperature range, the vapor pressure of the noble metal does not increase even if local heat generation from catalyst 20 causes a temperature rise in the noble metal. Thus, deterioration of catalyst 20 can be efficiently prevented by subjecting it to a reducing atmosphere before its temperature increases.

[0119] For this reason, in this embodiment, the air-fuel ratio control device creates a reducing atmosphere around the catalyst 20 when the catalyst 20's temperature is greater than or equal to the threshold temperature. The threshold temperature is predetermined and set at a temperature at which the vapor pressure of the precious metal does not increase when local heat generation occurs in the catalyst 20 due to the addition of HC, etc.

[0120] For example, if the temperature of catalyst 20 is greater than or equal to the threshold temperature, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of catalyst 20 becomes less than or equal to the lower reference quantity. The lower reference quantity is predetermined and is set to an amount less than half the maximum oxygen storage quantity at the time of a new catalyst. For example, the lower reference quantity is set to an amount of 1 / 5 to 1 / 3 of the maximum oxygen storage quantity at the time of a new catalyst.

[0121] Furthermore, once the oxygen storage quantity of the catalyst 20 has reached the lower reference quantity, the air-fuel ratio control device adjusts the air-fuel ratio of the incoming exhaust gas such that the oxygen storage quantity of the catalyst 20 is maintained at the lower reference quantity until the catalyst temperature becomes less than or equal to the threshold temperature. This allows the catalyst 20 to be kept in a reducing atmosphere when its temperature is high. <Erläuterung einer Luft-Kraftstoff-Verhältnis-Steuerung anhand eines Zeitdiagramms>

[0122] Fig. Figure 13 is a time diagram of the temperature of the catalyst 20, the air-fuel ratio of the incoming exhaust gas and the oxygen storage quantity of the catalyst 20 when an air-fuel ratio control is carried out to prevent deterioration of the catalyst 20 in the fourth embodiment.

[0123] In the illustrated example, the temperature of catalyst 20 at time t0 is lower than the threshold temperature Tth. When the temperature of catalyst 20 is lower than the threshold temperature Tth, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating conditions of the internal combustion engine.

[0124] After time t0, the temperature of the catalyst 20 rises in conjunction with the increase in engine load and reaches the threshold temperature Tth at time t1. Consequently, the air-fuel ratio control is initiated to prevent deterioration of the catalyst 20. To create a reducing atmosphere for the catalyst 20, the air-fuel ratio of the incoming exhaust gas is made richer than the stoichiometric air-fuel ratio. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a rich air-fuel ratio AFrich, which is richer than the stoichiometric air-fuel ratio. The rich air-fuel ratio AFrich is predetermined and, for example, set to between 12 and 13.5.

[0125] If the air-fuel ratio of the incoming exhaust gas is richer than the stoichiometric air-fuel ratio, the HC, etc. supplied to the catalyst 20 reacts with the oxygen adsorbed on the noble metal of the catalyst 20, and local heat generation occurs in the catalyst 20. As a result, the temperature of the catalyst 20 rises to a greater extent.

[0126] Furthermore, if the air-fuel ratio of the incoming exhaust gas is made richer than the stoichiometric air-fuel ratio, oxygen is released from the catalyst 20 and the oxygen storage quantity of the catalyst 20 decreases. As a result, the oxygen storage quantity of the catalyst 20 reaches the lower reference quantity Dref at time t2.

[0127] After time t2, the air-fuel ratio of the incoming exhaust gas is controlled such that the oxygen storage quantity of the catalyst 20 is maintained at the lower reference quantity Dref. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a richer value than the stoichiometric air-fuel ratio, thus reducing the NOx in the exhaust gas. At this time, the richness of the target air-fuel ratio is lower than the richness of the rich air-fuel ratio AFrich. It should be noted that the "richness" refers to the difference between the richer air-fuel ratio and the stoichiometric air-fuel ratio.

[0128] Then, at time t3, the temperature of the catalyst 20 drops to the threshold temperature Tth. Consequently, at time t3, the air-fuel ratio control to prevent deterioration of the catalyst 20 is terminated. For this reason, after time t3, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating state of the internal combustion engine. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0129] Below is an air-fuel ratio control system to prevent deterioration of the catalyst 20, with reference to the flowchart of Fig. 14 explained in detail. Fig. Figure 14 is a flowchart showing a control routine for a process to prevent catalyst degradation in the fourth embodiment. The control routine is executed repeatedly by the ECU 31 at predetermined time intervals.

[0130] First, in step S401, the air-fuel ratio control device assesses in the same way as in step S101 of Fig. 5. The control routine determines whether the temperature of catalyst 20 is greater than or equal to the threshold temperature. If it is determined that the temperature of catalyst 20 is lower than the threshold temperature, the control routine terminates. However, if it is determined that the temperature of catalyst 20 is greater than or equal to the threshold temperature, the control routine continues with step S402.

[0131] In step S402, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio. Specifically, the air-fuel ratio control device sets the target air-fuel ratio of the incoming exhaust gas to the rich air-fuel ratio AFrich.

[0132] Next, in step S403, the air-fuel ratio control device assesses whether the oxygen storage quantity OSA of catalyst 20 is less than or equal to the lower reference quantity Dref. For example, the air-fuel ratio control device cumulatively adds the oxygen excess / deficiency relative to the stoichiometric air-fuel ratio of the incoming exhaust gas, in the same manner as in step S103 of Fig. 5, in order to calculate the oxygen storage quantity OSA of the catalyst 20.

[0133] If, in step S403, it is determined that the oxygen storage quantity OSA of catalyst 20 is greater than the lower reference quantity Dref, the control routine returns to step S402, in which the target air-fuel ratio of the incoming exhaust gas is maintained at the rich air-fuel ratio AFrich. However, if, in step S403, it is determined that the oxygen storage quantity OSA of catalyst 20 is less than or equal to the lower reference quantity Uref, the control routine proceeds to step S404.

[0134] In step S404, the air-fuel ratio control device calculates the target air-fuel ratio (TAF) such that the oxygen storage quantity (OSA) of the catalyst 20 is maintained at the lower reference quantity (Dref). For example, the air-fuel ratio control device calculates the target air-fuel ratio (TAF) based on a predetermined state variable of the internal combustion engine (e.g., the intake air quantity, engine load, etc.) so that the NOx in the exhaust gas is reduced. In this case, the air-fuel ratio control device calculates the target air-fuel ratio (TAF) using a map or formula based on the predetermined state variable of the internal combustion engine. The target air-fuel ratio (TAF) is calculated as a value that is richer than the stoichiometric air-fuel ratio.

[0135] Next, in step S405, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas the target air-fuel ratio TAF.

[0136] Next, in step S406, the air-fuel ratio control device assesses whether the temperature of catalyst 20 is less than or equal to the threshold temperature. If it is assessed that the temperature of catalyst 20 is higher than the threshold temperature, the control routine returns to step S404, and steps S404 and S405 are repeated. Conversely, if it is assessed that the temperature of catalyst 20 is less than or equal to the threshold temperature, the control routine terminates.

[0137] It should be noted that the air-fuel ratio control device in step S406 can assess whether the temperature of catalyst 20 is less than or equal to a predetermined temperature that is lower than the threshold temperature. That is, once the oxygen storage quantity of catalyst 20 has reached the lower reference quantity, the air-fuel ratio control device can control the air-fuel ratio of the incoming exhaust gas such that the oxygen storage quantity of catalyst 20 is maintained at the lower reference quantity until the temperature of the catalyst becomes less than or equal to a predetermined temperature that is lower than the threshold temperature. <Fünfte Ausführungsform>

[0138] The design and control of the exhaust gas purification system in a fifth embodiment is, with the exception of the points explained below, fundamentally the same as the exhaust gas purification system in the fourth embodiment. For this reason, the fifth embodiment of the present invention is explained below with a focus on parts that differ from the fourth embodiment.

[0139] The exhaust gas purification system in the fifth embodiment is based on the in Fig. The internal combustion engine shown in Figure 6 is used. In the fifth embodiment, the exhaust gas purification system is provided with a catalyst (upstream catalyst) 20, an air-fuel ratio sensor (upstream air-fuel ratio sensor) 41, a temperature sensor 45, an air-fuel ratio control device, an HC cleaning device 23, and an downstream air-fuel ratio sensor 46.

[0140] The HC cleaning device 23 is arranged in the outlet passage on the downstream side in the exhaust gas flow direction of the catalyst 20 and removes the HC flowing out of the catalyst 20. Specifically, the HC cleaning device 23 is housed in the downstream housing 24, which is arranged on the downstream side of the housing (the upstream housing) 21 in the exhaust gas flow direction. The housing 21 is connected to the downstream housing 24 by the exhaust pipe 22.

[0141] As explained above, if the catalyst 20 is exposed to a reducing atmosphere to prevent deterioration, its cleaning capacity with respect to the HC in the exhaust gas decreases, and HC can escape from the catalyst 20. In contrast, in the second embodiment, the NOx cleaning device 23, which is located on the downstream side of the catalyst 20 in the direction of exhaust gas flow, can be used to remove the HC even if HC escapes from the catalyst 20. Therefore, a deterioration of exhaust emissions can be prevented while simultaneously and effectively preventing deterioration of the catalyst 20.

[0142] In the present embodiment, the HC cleaning device 23 is capable of storing oxygen and is, for example, a three-way catalyst that can simultaneously remove HC, CO, and NOx. That is, the HC cleaning device 23 has a similar design to the catalyst 20. The HC cleaning device 23 is also referred to as an downstream catalyst.

[0143] To increase the HC removal capacity of the oxygen-storing HC cleaning device 23, the HC cleaning device 23 must be exposed to an oxidizing atmosphere. Therefore, in the second embodiment, the air-fuel ratio control device exposes the NOx cleaning device 23 to an oxidizing atmosphere when the temperature of the catalyst 20 is greater than or equal to the threshold temperature. This makes it possible to increase the cleaning capacity of the HC cleaning device 23 and to prevent HC leakage more effectively.

[0144] Exhaust gas passing through the catalyst 20 flows into the HC cleaning device 23. Therefore, in order to increase the oxygen storage capacity of the HC cleaning device 23 to such an extent that it becomes an oxidizing atmosphere, the oxygen storage capacity of the catalyst 20 must be increased to its maximum. For this reason, when the temperature of the catalyst 20 is greater than or equal to the threshold temperature, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio, so that the oxygen storage capacity of the HC cleaning device 23 becomes greater than or equal to the upper reference quantity.It then makes the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the catalyst becomes less than or equal to the lower reference quantity. The upper reference quantity is determined beforehand and is set at a quantity greater than the lower reference quantity. That is, the upper reference quantity is set at a quantity greater than half the maximum oxygen storage quantity at the time of a new catalyst. For example, the upper reference quantity is set at a quantity of 2 / 3 to 4 / 5 of the maximum oxygen storage quantity at the time of a new catalyst. <Erläuterung einer Luft-Kraftstoff-Verhältnis-Steuerung anhand eines Zeitdiagramms>

[0145] Fig. Figure 15 is a time diagram of the temperature of the catalyst 20, the air-fuel ratio of the incoming exhaust gas, the oxygen storage quantity of the catalyst 20 and the oxygen storage quantity of the HC cleaning device 23 when the air-fuel ratio control to prevent deterioration of the catalyst 20 is carried out in the fifth embodiment.

[0146] In the illustrated example, the temperature of catalyst 20 at time t0 is lower than the threshold temperature Tth. When the temperature of catalyst 20 is lower than the threshold temperature Tth, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating conditions of the internal combustion engine.

[0147] After time t0, the temperature of the catalyst 20 rises in conjunction with an increase in engine load and reaches the threshold temperature Tth at time t1. Consequently, the air-fuel ratio control is initiated to prevent deterioration of the catalyst 20, and the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio to create an oxidizing atmosphere for the HC cleaning device 23. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to the lean air-fuel ratio AFlean, which is leaner than the stoichiometric air-fuel ratio. The lean air-fuel ratio AFlean is predetermined, for example, set to 16 to 17.

[0148] Furthermore, if the air-fuel ratio of the incoming exhaust gas is made leaner than the stoichiometric air-fuel ratio, oxygen is initially stored at the catalyst 20, and the oxygen storage capacity of the catalyst 20 increases. Consequently, the oxygen storage capacity of the catalyst 20 reaches its maximum at time t2. As the oxygen storage capacity of the catalyst 20 increases from time t1 to time t2, the NOx in the exhaust gas is removed at the catalyst 20, and the oxygen storage capacity of the HC cleaning device 23 is essentially kept constant.

[0149] After time t2, exhaust gas with an air-fuel ratio leaner than the stoichiometric air-fuel ratio flows into the HC cleaning device 23. Oxygen is stored in the HC cleaning device 23, and the oxygen storage capacity of the HC cleaning device 23 increases. Meanwhile, the oxygen storage capacity of the catalyst 20 is maintained at its maximum level.

[0150] Then, at time t3, the oxygen storage quantity of the HC purification device 23 reaches the upper reference quantity Uref. Consequently, the air-fuel ratio of the incoming exhaust gas at time t3 is made richer than the stoichiometric air-fuel ratio to create a reducing atmosphere for the catalyst 20. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a rich air-fuel ratio AFrich, which is richer than the stoichiometric air-fuel ratio. The rich air-fuel ratio AFrich is predetermined and, for example, set to between 12 and 13.5.

[0151] If the air-fuel ratio of the incoming exhaust gas is made richer than the stoichiometric air-fuel ratio, the HC, etc. supplied to the catalyst 20 reacts with the oxygen adsorbed on the noble metal of the catalyst 20, and local heat generation occurs in the catalyst 20. As a result, the temperature of the catalyst 20 increases to a greater extent.

[0152] Furthermore, if the air-fuel ratio of the incoming exhaust gas is made richer than the stoichiometric air-fuel ratio, oxygen is released from the catalyst 20 and the oxygen storage quantity of the catalyst 20 decreases. As a result, the oxygen storage quantity of the catalyst 20 reaches the lower reference quantity Dref at time t4.

[0153] After time t4, the air-fuel ratio of the incoming exhaust gas is controlled such that the oxygen storage quantity of the catalyst 20 is maintained at the lower reference quantity Dref. Specifically, the target air-fuel ratio of the incoming exhaust gas is set to a value that is richer than the stoichiometric air-fuel ratio, thus reducing the NOx in the exhaust gas. At this time, the richness of the target air-fuel ratio is lower than the richness of the richly set air-fuel ratio AFrich.

[0154] Then, at time t5, the temperature of the catalyst 20 drops to the threshold temperature Tth. Consequently, at time t5, the air-fuel ratio control to prevent deterioration of the catalyst 20 is terminated. For this reason, after time t5, the air-fuel ratio of the incoming exhaust gas is controlled to approach the stoichiometric air-fuel ratio, in accordance with the operating state of the internal combustion engine. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0155] Fig. Figure 16 is a flowchart showing a control routine for a process to prevent catalyst degradation in the fifth embodiment. The control routine is executed repeatedly by the ECU 31 at predetermined time intervals.

[0156] First, in step S501, the air-fuel ratio control device assesses in the same way as in step S401 of Fig. 14. The control routine determines whether the temperature of catalyst 20 is greater than or equal to the threshold temperature. If it is determined that the temperature of catalyst 20 is lower than the threshold temperature, the control routine terminates. However, if it is determined that the temperature of catalyst 20 is greater than or equal to the threshold temperature, the control routine continues with step S502.

[0157] In step S502, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio. Specifically, the air-fuel ratio control device sets the air-fuel ratio of the target air-fuel ratio of the incoming exhaust gas to a lean, fixed air-fuel ratio AFlean.

[0158] Next, in step S503, the air-fuel ratio control device assesses whether the oxygen storage quantity OSAd of the HC cleaning device 23 is greater than or equal to the upper reference quantity Uref. For example, the air-fuel ratio control device cumulatively adds the oxygen excess / deficiency relative to the stoichiometric air-fuel ratio of the exhaust gas flowing into the HC cleaning device 23, in the same manner as in step S203 of Fig. 8, in order to calculate the oxygen storage quantity OSAd of the HC cleaning device 23.

[0159] If, in step S503, it is determined that the oxygen storage quantity OSAd of the HC cleaning device 23 is less than the upper reference quantity Dref, the control routine returns to step S502, in which the target air-fuel ratio of the incoming exhaust gas is maintained at the lean air-fuel ratio AFlean. However, if, in step S503, it is determined that the oxygen storage quantity OSAd of the HC cleaning device 23 is greater than or equal to the upper reference quantity Uref, the control routine proceeds to step S504.

[0160] Steps S504 to S508 are performed in the same way as steps S402 to S406 of Fig. 14. The control routine ends after step S508.

[0161] It should be noted that the HC purification device 23 can be an HC-adsorbing material that adsorbs HC. In this case, steps S502 and S503 are omitted. Furthermore, the HC purification device 23 can be a combination of an HC-adsorbing material and a three-way catalyst. <Sechste Ausführungsform>

[0162] The exhaust gas purification system according to a sixth embodiment is, with regard to its design and control, fundamentally identical to the exhaust gas purification system according to the fifth embodiment, except for the points explained below. For this reason, the sixth embodiment of the present invention is explained below with a focus on parts that differ from the fifth embodiment.

[0163] Fig. Figure 17 is a schematic view of an internal combustion engine in which the exhaust gas purification system of an internal combustion engine according to the sixth embodiment of the present invention is provided. In the sixth embodiment, the exhaust gas purification system is provided with a catalyst (upstream catalyst) 20, an air-fuel ratio sensor (upstream air-fuel ratio sensor) 41, a temperature sensor 45, an air-fuel ratio control device, an HC cleaning device 23, an downstream air-fuel ratio sensor 46, and an air supply device 100.

[0164] The air supply device 100 includes an air supply passage 101, an air pump 102, an air switching valve 103 and a shut-off valve 104. The air pump 102, the air switching valve 103 and the shut-off valve 104 are arranged in the air supply passage 101.

[0165] The air supply port 101 connects the inlet port on the upstream side of the throttle valve 18 in the inlet flow direction and the outlet port between the catalyst 20 and the HC cleaning device 23. The air pump 102 is driven by an electric motor and pressurizes the air inside the inlet port to supply it to the outlet port. The air changeover valve 103 opens and closes the air supply port 101. The check valve 104 prevents air from flowing back from the outlet port to the inlet port.

[0166] The air pump 102 and the air changeover valve 103 are electrically connected to and controlled by the ECU 31. When air is supplied to the outlet through the air supply device 100, the air changeover valve 103 opens and the air pump 102 is driven. As a result, some of the air passing through the air cleaner 16 exits through the air supply port 101 to be supplied to the outlet. Therefore, the air supply device 100 supplies air to the outlet between the catalyst 20 and the HC cleaning device 23. It should be noted that the air supply source may be different from the inlet port.

[0167] Furthermore, the air supply passage 101 is located in the outlet passage on the upstream side of the downstream air-fuel ratio sensor 46 in the exhaust gas flow direction. For this reason, the downstream air-fuel ratio sensor 46 can detect the air-fuel ratio of the exhaust gas to which air supplied by the air supply device 100 is added.

[0168] In the sixth embodiment, the air-fuel ratio control device controls the air supply device 100. When the temperature of the catalyst 20 is greater than or equal to the threshold temperature, it uses the air supply device 100 to supply air, thereby creating an oxidizing atmosphere in the HC cleaning device 23. For example, when the temperature of the catalyst 20 is greater than or equal to the threshold temperature, the air-fuel ratio control device supplies air to the air supply device 100 such that the oxygen storage quantity of the HC cleaning device 23 becomes greater than or equal to the upper reference quantity. In this case, it is not necessary to increase the oxygen storage quantity of the catalyst 20 to the maximum oxygen storage quantity to create an oxidizing atmosphere in the HC cleaning device 23.For this reason, it is possible to make the HC cleaning device 23 an oxidizing atmosphere. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0169] Fig. Figure 18 is a flowchart showing a control routine for a process to prevent catalyst degradation in the sixth embodiment. The control routine is executed repeatedly by the ECU 31 at predetermined time intervals.

[0170] First, in step S601, the air-fuel ratio control device assesses in the same way as in step S501 of Fig. 16. The control routine determines whether the temperature of catalyst 20 is greater than or equal to the threshold temperature. If it is determined that the temperature of catalyst 20 is lower than the threshold temperature, the control routine terminates. However, if it is determined that the temperature of catalyst 20 is greater than or equal to the threshold temperature, the control routine continues with step S602.

[0171] In step S602, the air-fuel ratio control device uses the air supply device 100 to supply air. As a result, the air-containing exhaust gas, that is, exhaust gas with an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, flows into the HC cleaning device 23.

[0172] Next, in step S603, the air-fuel ratio control device assesses in the same way as in step S503 of Fig. 16. Whether the oxygen storage quantity OSAd of the HC cleaning device 23 is greater than or equal to the upper reference quantity Uref. If in step S603 it is determined that the oxygen storage quantity OSAd of the HC cleaning device 23 is less than the upper reference quantity Uref, the control routine returns to step S602, in which air continues to be supplied. If, however, in step S603 it is determined that the oxygen storage quantity OSAd of the HC cleaning device 23 is greater than or equal to the upper reference quantity Uref, the control routine continues with step S604.

[0173] Steps S604 to S608 are performed in the same way as steps S504 to S508 of Fig. 16. The control routine ends after step S608.

[0174] It should be noted that steps S604 and S605 can be performed in parallel with steps S602 and S603.

[0175] Furthermore, the HC purification device 23 can be an HC-adsorbing material that adsorbs HC. In this case, steps S602 and S603 are omitted. The HC purification device 23 can also be a combination of an HC-adsorbing material and a three-way catalyst. <Siebte Ausführungsform>

[0176] The exhaust gas purification system according to a seventh embodiment is, with regard to its design and control, fundamentally identical to the exhaust gas purification system according to the first embodiment, except for the points explained below. For this reason, the seventh embodiment of the present invention is explained below with a focus on parts that differ from the first embodiment.

[0177] Fig. Figure 19 is a schematic view of the configuration of a vehicle in which the exhaust gas purification system of an internal combustion engine according to the seventh embodiment of the present invention is provided. The vehicle is equipped with a GPS receiver 91, a map database 92, a navigation system 93, a communication module 94 and a speed sensor 95.

[0178] The GPS receiver 91 receives signals from three or more GPS satellites and determines the vehicle's current position (for example, its latitude and longitude). The GPS receiver 91 is connected to the ECU 31 via an internal network. The output of the GPS receiver 91 is sent to the ECU 31.

[0179] Map database 92 stores map information. This information includes the road's position, its characteristics (e.g., whether it's a curve or straight, the radius of curvature, the road gradient, etc.), the road type, speed limits, and other road information. Map database 92 is connected to ECU 31 via an internal network. ECU 31 receives map information from map database 92.

[0180] The navigation system 93 determines the vehicle's route to the destination based on the current position of the vehicle as determined by the GPS receiver 91, map information from the map database 92, input from the driver, etc. The navigation system 93 is connected to the ECU 31 via an internal network. The route determined by the navigation system 93 is transmitted to the ECU 31. It should be noted that the GPS receiver 91 and the map database 92 may be integrated into the navigation system 93.

[0181] The Communication Module 94 is a device that enables communication between the vehicle and its exterior. The Communication Module 94 includes, for example, a Data Communication Module (DCM), a short-range wireless communication module, etc. The vehicle communicates with an external server via the Data Communication Module. Furthermore, the vehicle communicates with roadside equipment via a short-range wireless communication module and with other vehicles via vehicle-to-vehicle communication. The Communication Module 94 is connected to the ECU 31 via an internal network. Information received by the Communication Module 94 is transmitted to the ECU 31.

[0182] The speed sensor 95 detects the vehicle's speed. The speed sensor 95 is connected to the ECU 31 via an internal network. The output of the speed sensor 95 is transmitted to the ECU 31.

[0183] Furthermore, the outputs of the air flow meter 40, the air-fuel ratio sensor 41, the temperature sensor 45, the crank angle sensor 44 and the load sensor 43 are entered into the ECU 31.

[0184] As explained above, if the catalyst contains Pd as the precious metal, deterioration of the catalyst can be efficiently prevented by exposing it to an oxidizing atmosphere before its temperature rises. Therefore, if the temperature increase of the catalyst can be predicted, it is preferably exposed to an oxygen atmosphere when such an increase is anticipated. This reliably prevents deterioration of the catalyst, even if its temperature rises rapidly.

[0185] For this reason, the air-fuel ratio control device, in the present embodiment, exposes the catalyst 20 to an oxygen atmosphere when a temperature increase of the catalyst 20 is predicted. For example, when a temperature increase of the catalyst 20 is predicted, the air-fuel ratio control device makes the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the catalyst 20 becomes greater than or equal to an upper reference quantity. <Verarbeitung zum Verhindern einer Verschlechterung des Katalysators>

[0186] Fig. Figure 20 is a flowchart showing a control routine for a process to prevent catalyst degradation in the seventh embodiment. The control routine is executed repeatedly by the ECU 31 at predetermined time intervals.

[0187] First, in step S701, the air-fuel ratio control device assesses, based on the output of the GPS receiver 91, on map information stored in a map database 92, on a route determined by the navigation system 93, on information received by the communication module 94, on an output from the speed sensor 95, etc., whether a temperature increase of the catalyst 20 is predicted.

[0188] For example, if the route determined by the navigation system 93 includes a section where the power demanded by the internal combustion engine increases (for example, an uphill road, etc.), a temperature increase of the catalyst is predicted in the section preceding that section. Furthermore, even if the route is not determined by the navigation system 93, a temperature increase of the catalyst 20 can be predicted based on the speed of a vehicle ahead, traffic volume, or the gradient of nearby roads, etc.

[0189] A temperature increase of the catalyst 20 is also predicted if the depressor pedal 42 is greater than or equal to a predetermined value, that is, if the output of the load sensor 43 is greater than or equal to a predetermined value. Furthermore, a temperature increase of the catalyst 20 is predicted if control is performed to bring the vehicle speed to a specific target speed (so-called "cruise control"), provided that the value of the target speed minus the current speed is greater than or equal to a predetermined value.

[0190] If step S701 determines that a temperature increase of catalyst 20 is not predicted, the control routine ends. However, if step S701 determines that a temperature increase of catalyst 20 is predicted, the control routine proceeds to step S702.

[0191] Steps S702 to S706 are performed in the same way as steps S102 to S106 of Fig. 5. The control routine ends after step S706.

[0192] Preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments and can be corrected and modified in various ways within the wording of the claims. For example, the precious metal of the catalyst 20 can be a different type of precious metal, provided that it has the property that the vapor pressure at a predetermined temperature is lower in a second atmosphere consisting of a reducing or oxidizing atmosphere compared to a first atmosphere consisting of an oxidizing or reducing atmosphere.

[0193] Furthermore, the exhaust gas purification system can be equipped with the air supply device 100, which supplies air to the exhaust port on the upstream side of the catalyst 20. The air-fuel ratio control device can supply air through the air supply device 100 to make the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio. Furthermore, the exhaust gas purification system can be equipped with the fuel injection valve 47, which supplies fuel to the exhaust port on the upstream side of the catalyst 20. The air-fuel ratio control device can supply fuel through the fuel injection valve 47 to make the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio.

[0194] Furthermore, the aforementioned embodiments can be implemented in any combination. For example, if the seventh embodiment is combined with the second to sixth embodiments, the air-fuel ratio control device in step S201 of Fig. 8, Step S301 of Fig. 10, Step S401 of Fig. 14, Step S501 of Fig. 16 and step S601 of Fig. 18 in the same way as in step S701 of Fig. 20, whether a temperature increase of the catalyst 20 is predicted.

Claims

[1] Exhaust gas purification system of an internal combustion engine, comprising: a catalyst (20) arranged in an outlet passage and capable of storing oxygen; and an air-fuel ratio control device (31) configured to control the air-fuel ratio of incoming exhaust gas flowing into the catalyst (20), wherein the catalyst (20) comprises a precious metal and the precious metal has a property that a vapor pressure decreases at a predetermined temperature when it is oxidized, In the event that the temperature of the catalyst (20) is greater than or equal to a threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the air-fuel ratio of the incoming exhaust gas leaner than a stoichiometric air-fuel ratio, so that an oxygen storage quantity of the catalyst (20) becomes greater than or equal to an upper reference quantity, and the threshold temperature is set at a temperature at which the vapor pressure of the precious metal does not increase when local heat generation of the catalyst (20) occurs due to the supply of oxygen. [2] Exhaust gas purification system of the internal combustion engine claim 1, further comprising a NOx cleaning device (23) which is arranged in the exhaust passage on a downstream side of the catalyst (20) in an exhaust gas flow direction and is able to remove the NOx flowing out of the catalyst (20). [3] Exhaust gas purification system of the internal combustion engine according to claim 2, where the NOx purification device (23) can store oxygen, and In the event that the temperature of the catalyst (20) is greater than or equal to the threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio, so that an oxygen storage quantity of the NOx cleaning device (23) becomes less than or equal to a lower reference quantity which is less than the upper reference quantity, and then to make the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio, so that the oxygen storage quantity of the catalyst (20) becomes greater than or equal to the upper reference quantity. [4] Exhaust gas purification system of the internal combustion engine according to claim 2, further comprising a fuel injection valve (47) which supplies fuel to the exhaust passage between the catalyst (20) and the NOx purification device (23), wherein the NOx purification device (23) can store oxygen, and the air-fuel ratio control device (31) is designed to control the fuel supply valve (47) and, in the event that the temperature of the catalyst (20) is greater than or equal to the threshold temperature or a temperature increase of the catalyst (20) is predicted, to supply fuel through the fuel supply valve (47) in such a way that an oxygen storage quantity of the NOx cleaning device (23) becomes less than or equal to a lower reference quantity which is less than the upper reference quantity. [5] Exhaust gas purification system of an internal combustion engine, comprising: a catalyst (20) arranged in an outlet passage and capable of storing oxygen; and an air-fuel ratio control device (31) configured to control the air-fuel ratio of incoming exhaust gas flowing into the catalyst (20), wherein the catalyst (20) comprises a precious metal and the precious metal has a property that a vapor pressure increases at a predetermined temperature when it is oxidized, In the event that the temperature of the catalyst (20) is greater than or equal to a threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the air-fuel ratio of the incoming exhaust gas richer than a stoichiometric air-fuel ratio, so that an oxygen storage quantity of the catalyst (20) becomes less than or equal to a lower reference quantity, and the threshold temperature is set at a temperature at which the vapor pressure of the precious metal does not increase when local heat generation of the catalyst (20) occurs due to an input of HC. [6] Exhaust gas purification system of the internal combustion engine according to claim 5, further comprising an HC cleaning device (23) which is arranged in the exhaust passage on a downstream side of the catalyst (20) in an exhaust gas flow direction and is able to remove HC flowing out of the catalyst (20). [7] Exhaust gas purification system of the internal combustion engine according to claim 6, wherein the HC purification device (23) can store oxygen, and in the event that the temperature of the catalyst (20) is greater than or equal to the threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the air-fuel ratio of the incoming exhaust gas leaner than the stoichiometric air-fuel ratio, so that an oxygen storage quantity of the HC purification device (23) becomes greater than or equal to an upper reference quantity which is greater than the lower reference quantity, and then to make the air-fuel ratio of the incoming exhaust gas richer than the stoichiometric air-fuel ratio, so that an oxygen storage quantity of the catalyst (20) becomes less than or equal to the lower reference quantity. [8] Exhaust gas purification system of the internal combustion engine according to claim 6, further comprising an air supply device (100) which supplies air to the outlet passage between the catalyst (20) and the HC cleaning device (23), wherein the HC cleaning device (23) can store oxygen, and the air-fuel ratio control device (31) is designed to control the air supply device (100) and, in the event that the temperature of the catalyst (20) is greater than or equal to the threshold temperature or a temperature increase of the catalyst (20) is predicted, to supply air through the air supply device (100) so that an oxygen storage quantity of the HC cleaning device (23) becomes greater than or equal to an upper reference quantity which is greater than the lower reference quantity. [9] Exhaust gas purification system of an internal combustion engine, comprising: a catalyst (20) arranged in an outlet passage and capable of storing oxygen; and an air-fuel ratio control device (31) configured to control an atmosphere of the catalyst (20), wherein the catalyst (20) comprises a precious metal and the precious metal has a property that a vapor pressure at a predetermined temperature in a second atmosphere from a reducing atmosphere or an oxidizing atmosphere is lower compared to a first atmosphere from an oxidizing atmosphere or a reducing atmosphere, In the event that the temperature of the catalyst (20) is greater than or equal to a threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the catalyst (20) subject to the second atmosphere, and the threshold temperature is set at a temperature at which the vapor pressure of the precious metal does not increase when local heat generation of the catalyst (20) occurs due to an input of oxygen or HC. [10] Exhaust gas purification system of the internal combustion engine according to claim 9, further comprising a downstream catalyst (23) which is arranged in the exhaust passage on a downstream side of the catalyst (20) in an exhaust gas flow direction and is capable of storing oxygen, and in the case that the temperature of the catalyst (20) is greater than or equal to the threshold temperature or a temperature increase of the catalyst (20) is predicted, the air-fuel ratio control device (31) is configured to make the downstream catalyst (23) the first atmosphere.

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