Air-fuel ratio control device

Abstract

To suppress deterioration of oxygen storage capacity in an oxygen storage agent while suppressing deterioration of fuel economy.SOLUTION: An air-fuel ratio control device that controls an air-fuel ratio of exhaust gas flowing into exhaust emission control catalysts 20, 24 provided in an exhaust passage of an internal combustion engine 100 and having oxygen storage capacity includes an air-fuel ratio control section 362 for controlling an air-fuel ratio of exhaust gas flowing into the exhaust emission control catalysts. The air-fuel ratio control section alternately and repeatedly executes: rich processing for controlling the air-fuel ratio of exhaust gas flowing into the exhaust emission control catalysts to a rich air-fuel ratio richer than a theoretical air fuel ratio; and lean processing for controlling the air-fuel ratio of exhaust gas flowing into the exhaust emission control catalysts to a lean air-fuel ratio leaner than the theoretical air fuel ratio. During the rich processing, the air-fuel ratio control section executes lean pulse processing for controlling the air-fuel ratio of exhaust gas flowing into the exhaust emission control catalysts to a lean air-fuel ratio which has a lean degree higher than that of the air-fuel ratio during the lean processing over a period shorter than a period of the single lean processing.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to an air-fuel ratio control device. [Background technology]

[0002] It has been known for some time that a three-way catalyst is provided in the exhaust passage of an internal combustion engine and the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is controlled (Patent Documents 1 to 3). In particular, the device described in Patent Document 1 varies the air-fuel ratio of the exhaust gas flowing into the three-way catalyst between a lean air-fuel ratio and a rich air-fuel ratio, and controls the air-fuel ratio so that the average air-fuel ratio when the air-fuel ratio is varied in this manner becomes the stoichiometric air-fuel ratio. Patent Document 3 also discloses that secondary air is introduced into the exhaust port to create an oxygen-rich atmosphere in the exhaust gas, thereby reducing deterioration of the exhaust purification catalyst due to carbon deposition. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-090880 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-236450 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-112300 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, if there is a period during which the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst at high temperatures is set to a rich air-fuel ratio, substances containing carbon such as carbon or hydrocarbons (hereinafter referred to as "carbon-containing substances") may precipitate on the oxygen storage agent of the exhaust purification catalyst during that period. If such carbon-containing substances precipitate on the surface of the oxygen storage agent and widely cover this surface, oxygen will no longer be stored in the oxygen storage agent, and as a result, the purification performance of the exhaust purification catalyst will decrease.

[0005] Furthermore, as mentioned above, Patent Document 3 discloses that secondary air is introduced into the exhaust port to create an oxygen-rich atmosphere in the exhaust gas, thereby reducing the deterioration of the exhaust purification catalyst associated with carbon deposition. Here, if the carbon deposited on the surface of the oxygen storage agent can be used to reduce NOx, the amount of hydrocarbons (i.e., the amount of fuel) required to reduce NOx can be reduced accordingly. However, when secondary air is introduced to create an oxygen-rich atmosphere in the exhaust gas, the deposited carbon is not used in the NOx reduction reaction, which results in a deterioration in fuel economy.

[0006] In view of the above-mentioned problems, an object of the present disclosure is to suppress a decrease in the oxygen storage capacity of an oxygen storage agent while suppressing a deterioration in fuel economy. [Means for solving the problem]

[0007] The gist of the present disclosure is as follows.

[0008] (1) An air-fuel ratio control device for controlling the air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst having an oxygen storage capacity and provided in an exhaust passage of an internal combustion engine, an air-fuel ratio control unit that controls the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; the air-fuel ratio control section alternately and repeatedly executes a rich process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and a lean process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, The air-fuel ratio control unit executes lean pulse processing during the rich processing, for a period shorter than the period of one of the lean processing, to control the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the air-fuel ratio during the lean processing. (2) The air-fuel ratio control device described in (1) above, wherein the air-fuel ratio control unit sets at least one of the execution timing of the lean pulse processing, the execution period of the lean pulse processing, and the lean degree of the exhaust gas flowing into the exhaust purification catalyst during the lean pulse processing, based on the value of a deposition parameter related to the amount of deposition of carbon-containing substances on the oxygen storage agent supported on the exhaust purification catalyst. (3) The air-fuel ratio control device described in (2) above, wherein the air-fuel ratio control unit executes the lean pulse processing when the value of the deposition parameter is a value indicating that the amount of deposition of carbon-containing substances on the oxygen storage agent is equal to or greater than a predetermined reference deposition amount. (4) The air-fuel ratio control device described in (3) above, wherein the execution period of the lean pulse processing and the degree of leanness in the lean pulse processing are fixed values set so that the reference deposition amount of carbon-containing substances is completely removed from the oxygen storage agent. (5) The air-fuel ratio control device according to (2) above, wherein the air-fuel ratio control section periodically executes the lean pulse processing at a predetermined cycle. (6) The air-fuel ratio control device described in (5) above, wherein the air-fuel ratio control unit sets at least one of the execution period of the lean pulse processing and the degree of leanness in the lean pulse processing based on the value of the precipitation parameter when the lean pulse processing is executed. (7) Further comprising a deposition amount calculation unit that calculates the value of the deposition parameter, The air-fuel ratio control device according to any one of (2) to (6), wherein the deposition amount calculation unit calculates the value of the deposition parameter so as to be proportional to an accumulated value of excess reducing agent flowing into the exhaust purification catalyst when the temperature of the exhaust purification catalyst is equal to or higher than a predetermined reference temperature and the amount of oxygen stored in the exhaust purification catalyst is zero. (8) The air-fuel ratio control device described in (7) above, wherein when fuel cut control is executed to temporarily stop the supply of fuel to the internal combustion engine while the internal combustion engine is operating, the deposition amount calculation unit resets the value of the deposition parameter to a value indicating that the amount of deposition of carbon-containing substances on the oxygen storage agent is zero. (9) An estimation unit that estimates the oxygen storage amount of the exhaust purification catalyst is further provided, The air-fuel ratio control device according to any one of (1) to (8), wherein the air-fuel ratio control unit switches from the lean processing to the rich processing before the oxygen storage amount estimated by the estimation unit reaches a maximum oxygen storage amount. (10) The air-fuel ratio control device according to any one of (1) to (9) above, wherein the lean pulse processing is performed when the oxygen storage amount of the exhaust purification catalyst is zero. (11) An air-fuel ratio control device according to any one of (1) to (10) above, wherein the lean pulse processing is performed when the temperature of the exhaust purification catalyst is at a temperature at which carbon-containing substances precipitate on an oxygen storage agent supported on the exhaust purification catalyst. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to suppress a decrease in the oxygen storage capacity of the oxygen storage agent while suppressing a deterioration in fuel economy. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram that schematically shows an internal combustion engine in which an air-fuel ratio control device according to one embodiment is used. [Figure 2] FIG. 2 is a functional block diagram of the processor. [Figure 3] FIG. 3 is a time chart of the target air-fuel ratio etc. when air-fuel ratio control is performed. [Figure 4] FIG. 4 is a diagram that schematically shows how carbonaceous matter deposits in an exhaust purification catalyst. [Figure 5] FIG. 5 is a diagram that schematically illustrates the state around an oxygen storage agent when ceria is used as the oxygen storage agent. [Figure 6] FIG. 6 is a flowchart that schematically shows the flow of air-fuel ratio control that is executed in the air-fuel ratio control unit. [Figure 7]FIG. 7 is a time chart similar to FIG. 3 of the target air-fuel ratio and the like when air-fuel ratio control according to one modified example is performed. [Figure 8] FIG. 8 is a time chart of the target air-fuel ratio and the like when the air-fuel ratio control in the comparative control 1 is performed. [Figure 9] FIG. 9 is a time chart similar to FIG. 8 of the target air-fuel ratio etc. when air-fuel ratio control in comparative control 2 is performed. [Figure 10] FIG. 10 is a time chart similar to FIG. 8 of the target air-fuel ratio etc. when air-fuel ratio control in comparative control 3 is performed. [Figure 11] FIG. 11 is a graph showing the ratio of the NOx reduction rate per unit amount of deposited carbonaceous matter. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, the embodiments will be described in detail with reference to the drawings. In the following description, like components are designated by like reference numerals.

[0012] <Overall explanation of the internal combustion engine> FIG. 1 is a diagram schematically illustrating an internal combustion engine 100 in which an air-fuel ratio control device according to one embodiment is used. As shown in FIG. 1, an engine body 1 of the internal combustion engine 100 includes a cylinder block 2, a piston 3 that reciprocates within the cylinder block 2, a cylinder head 4 fixed onto the cylinder block 2, and a combustion chamber 5 formed between the piston 3 and the cylinder head 4. In this embodiment, the cylinder block 2 has multiple cylinders, and one piston 3 reciprocates within each cylinder. The cylinder head 4 has an intake port 7 formed therein, which is opened and closed by an intake valve 6. Similarly, the cylinder head 4 has an exhaust port 9 formed therein, which is opened and closed by an exhaust valve 8.

[0013] As shown in FIG. 1, a spark plug 10 is disposed in the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is disposed around the periphery of the inner wall surface of the cylinder head 4. The spark plug 10 is configured to generate a spark in response to an ignition signal. The fuel injection valve 11 injects a predetermined amount of fuel into the combustion chamber 5 in response to an injection signal. The fuel injection valve 11 may also be disposed to inject fuel into the intake port 7. In this embodiment, gasoline with a stoichiometric air-fuel ratio of 14.6 is used as the fuel. However, the internal combustion engine may use fuels other than gasoline or mixed fuels with gasoline.

[0014] The internal combustion engine 100 has a surge tank 14 connected to the intake ports 7 of each cylinder via corresponding intake manifolds 13, an intake pipe 15 connected to the surge tank 14, and an air cleaner 16 connected to the intake pipe 15. The intake ports 7, the intake manifolds 13, the surge tank 14, and the intake pipe 15 form an intake passage. A throttle valve 18 driven by a throttle valve drive actuator 17 is disposed in the intake pipe 15. The throttle valve 18 is rotated by the throttle valve drive actuator 17 to change the opening area of the intake passage.

[0015] Meanwhile, the internal combustion engine 100 has an exhaust manifold 19 connected to the exhaust ports 9 of each cylinder, an upstream casing 21 connected to the exhaust manifold 19 and housing an upstream exhaust purification catalyst (hereinafter referred to as the "upstream catalyst") 20, a first exhaust pipe 22 connected to the upstream casing 21, a downstream casing 23 connected to the first exhaust pipe 22 and housing a downstream exhaust purification catalyst (hereinafter referred to as the "downstream catalyst") 24, and a second exhaust pipe 25 connected to the downstream casing 23. The second exhaust pipe 25 communicates with the atmosphere via, for example, a muffler (not shown). The exhaust ports 9, the exhaust manifold 19, the upstream casing 21, the first exhaust pipe 22, the downstream casing 23, and the second exhaust pipe 25 form an exhaust passage. In this embodiment, the exhaust system is provided with two exhaust purification catalysts, an upstream catalyst 20 and a downstream catalyst 24, but the exhaust system may be provided with only one exhaust purification catalyst or three or more exhaust purification catalysts.

[0016] The internal combustion engine 100 also has an electronic control unit (ECU) 31. The ECU 31 has an input port 33, an output port 34, a memory 35, and a processor 36, which are interconnected via a bidirectional bus 32.

[0017] The input port 33 is connected to various sensors. An air flow meter 40 is disposed in the intake pipe 15 to detect the flow rate of air flowing through the intake pipe 15. The air flow meter 40 is connected to the input port 33 via a corresponding AD converter 37, and the output of the air flow meter 40 is input to the input port 33.

[0018] Further, an upstream air-fuel ratio sensor 41 is disposed in the exhaust manifold 19 to detect the air-fuel ratio of the exhaust gas flowing inside the exhaust manifold 19 (i.e., the exhaust gas flowing into the upstream catalyst 20). In addition, a downstream air-fuel ratio sensor 42 is disposed in the first exhaust pipe 22 to detect the air-fuel ratio of the exhaust gas flowing inside the first exhaust pipe 22 (i.e., the exhaust gas flowing out from the upstream catalyst 20 and flowing into the downstream catalyst 24). These air-fuel ratio sensors 41, 42 are connected to the input port 33 via corresponding AD converters 37, and the outputs of the air-fuel ratio sensors 41, 42 are input to the input port 33.

[0019] In this embodiment, limiting current type air-fuel ratio sensors are used as the air-fuel ratio sensors 41, 42. Therefore, the air-fuel ratio sensors 41, 42 are configured so that the output current from the air-fuel ratio sensors 41, 42 increases as the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 41, 42 increases (i.e., as the exhaust gas becomes leaner). Therefore, the air-fuel ratio corresponding to the output value of the upstream air-fuel ratio sensor 41 (hereinafter referred to as the "output air-fuel ratio") represents the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20. Furthermore, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 represents the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 24.

[0020] In this embodiment, limiting current type air-fuel ratio sensors are used as the air-fuel ratio sensors 41 and 42, but air-fuel ratio sensors other than limiting current type air-fuel ratio sensors may be used as long as the sensor's output changes according to the air-fuel ratio of the exhaust gas. An example of such an air-fuel ratio sensor is an oxygen sensor, the output of which changes rapidly near the stoichiometric air-fuel ratio without applying a voltage between the electrodes that make up the sensor.

[0021] An upstream temperature sensor 43 is disposed on the upstream catalyst 20 to detect the temperature of the upstream catalyst 20. In addition, a downstream temperature sensor 44 is disposed on the downstream catalyst 24 to detect the temperature of the downstream catalyst 24. These temperature sensors 43, 44 are connected to the input port 33 via corresponding AD converters 37, and the outputs of the temperature sensors 43, 44 are input to the input port 33.

[0022] In addition, a load sensor 46 is connected to the accelerator pedal 45, and generates an output voltage proportional to the amount of depression of the accelerator pedal 45. The load sensor 46 is connected to the input port 33 via a corresponding AD converter 37, and the output of the load sensor 46 is input to the input port 33. The crank angle sensor 47 generates an output pulse, for example, every time the crankshaft rotates 15 degrees. The crank angle sensor 47 is connected to the input port 33, and the output pulse of the crank angle sensor 47 is input to the input port 33. The processor 36 calculates the engine rotation speed from the output pulse of the crank angle sensor 47.

[0023] On the other hand, the output port 34 is connected to various actuators. Specifically, the output port 34 is connected to, for example, the spark plug 10, the fuel injector 11, and the throttle valve drive actuator 17 via corresponding drive circuits 38, and the operation of these actuators is controlled by the drive signals output from the output port 34.

[0024] The memory 35 includes, for example, a volatile semiconductor memory (for example, RAM) and a non-volatile semiconductor memory (for example, ROM). The memory 35 stores computer programs for executing various processes in the processor 36, various data used when the processor 36 executes various processes, and the like.

[0025] The processor 36 includes one or more central processing units (CPUs) and their peripheral circuits. The processor 36 may further include an arithmetic circuit such as a logic unit or a numerical operation unit. The processor 36 executes various processes based on computer programs stored in the memory 35.

[0026] 2 is a functional block diagram of the processor 36. As shown in FIG. 2, the processor 36 includes an oxygen storage amount estimation unit 361 that estimates the oxygen storage amount in the oxygen storage agent of the upstream catalyst 20 or the downstream catalyst 24, an air-fuel ratio control unit 362 that controls the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20, and a deposition amount calculation unit 363 that calculates the amount of carbon-containing substances deposited on the oxygen storage agent of the upstream catalyst 20 or the downstream catalyst 24. Each of these units of the processor 36 is a functional module realized by, for example, a computer program running on the processor 36. Alternatively, each of these units of the processor 36 may be a dedicated arithmetic circuit provided in the processor 36. Details of each of these functional blocks will be described later.

[0027] The processor 36 controls the opening of the throttle valve 18 based on the load detected by the load sensor 46 (sending a control signal to the throttle valve drive actuator 17 via the drive circuit 38, and controls the amount of air supplied to the combustion chamber 5). In addition, the processor 36 controls the amount of fuel injected from the fuel injection valve 11 so that the air-fuel ratio of the exhaust gas becomes a target air-fuel ratio (sending a control signal to the fuel injection valve 11 via the drive circuit 38). Therefore, the ECU 31 having the processor 36 functions as an air-fuel ratio control device that controls the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst.

[0028] <Configuration of exhaust purification catalyst> The exhaust purification catalysts (upstream catalyst 20 and downstream catalyst 24) are catalysts with oxygen storage capacity, and in this embodiment, are three-way catalysts. Specifically, the exhaust purification catalysts 20, 24 are three-way catalysts in which a catalytic precious metal (e.g., platinum (Pt)) with catalytic activity and an oxygen storage agent (e.g., ceria (CeO2)) with oxygen storage capacity are supported on a ceramic carrier. The three-way catalyst has the function of simultaneously purifying unburned HC, CO, and NOx when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is maintained at the stoichiometric air-fuel ratio. In addition, when a certain amount of oxygen is stored in the oxygen storage agent of the exhaust purification catalysts 20, 24, unburned HC, CO, and NOx are simultaneously purified even if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 deviates slightly from the stoichiometric air-fuel ratio to the rich side or lean side.

[0029] That is, when the oxygen storage agent of the exhaust purification catalysts 20, 24 is in a state capable of storing oxygen, that is, when the oxygen storage amount of the exhaust purification catalysts 20, 24 is less than the maximum storable oxygen amount, if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 becomes slightly leaner than the stoichiometric air-fuel ratio, excess oxygen contained in the exhaust gas is stored in the exhaust purification catalysts 20, 24. When ceria is used as the oxygen storage agent, the reaction represented by the following formula (1) occurs. The valence of the cerium ion at this time is tetravalent. Chief Operating Officer 2-X +x / 2 O2 → CeO2…(1)

[0030] In this way, oxygen is stored by the oxygen storage agents of the exhaust purification catalysts 20, 24, thereby maintaining the stoichiometric air-fuel ratio on the surfaces of the exhaust purification catalysts 20, 24. As a result, unburned HC, CO, and NOx are simultaneously purified on the surfaces of the exhaust purification catalysts 20, 24, and at this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

[0031] On the other hand, when the exhaust purification catalysts 20, 24 are in a state where they can release oxygen, that is, when the oxygen storage amount of the exhaust purification catalysts 20, 24 is greater than 0, and the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 is slightly richer than the stoichiometric air-fuel ratio, the oxygen that is insufficient to reduce the unburned HC and CO contained in the exhaust gas is released from the exhaust purification catalysts 20, 24. When ceria is used as the oxygen storage agent, the reaction represented by the following formula (2) occurs. The valence of the cerium ion at this time is trivalent. CeO2 → CeO 2-X +x / 2 O2…(2)

[0032] In this way, the stoichiometric air-fuel ratio is maintained on the surfaces of the exhaust purification catalysts 20, 24 by releasing oxygen from the oxygen storage agents of the exhaust purification catalysts 20, 24. As a result, unburned HC, CO, and NOx are simultaneously purified on the surfaces of the exhaust purification catalysts 20, 24, and at this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

[0033] More precisely, ceria is reduced by reducing species such as hydrogen to form CeO2 into CeO 2-X Therefore, rather than oxygen being released from ceria, oxygen in ceria reacts with reducing species such as hydrogen and changes to HO or the like. For ease of understanding, this specification will be described assuming that oxygen is released from ceria as shown in the above formula (2).

[0034] In this way, when a certain amount of oxygen is stored in the exhaust purification catalysts 20, 24, even if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 deviates slightly toward the rich side or lean side of the stoichiometric air-fuel ratio, unburned HC, CO, and NOx are purified at the same time, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

[0035] <Basic air-fuel ratio control> Next, a basic air-fuel ratio control performed in the air-fuel ratio control device according to this embodiment will be briefly described. In the air-fuel ratio control according to this embodiment, feedback control is performed to control the fuel injection amount from the fuel injector 11 so that the output air-fuel ratio of the upstream air-fuel ratio sensor 41 becomes the target air-fuel ratio.

[0036] In addition, in the basic air-fuel ratio control of this embodiment, the target air-fuel ratio is set based on the output air-fuel ratio of the downstream air-fuel ratio sensor 42, etc. Below, the process of setting the target air-fuel ratio in the basic air-fuel ratio control will be described with reference to Fig. 3. Fig. 3 is a time chart of the target air-fuel ratio AFT, the output air-fuel ratio AF1 of the upstream air-fuel ratio sensor 41, the oxygen storage amount OSAup of the upstream catalyst 20, the output air-fuel ratio AF2 of the downstream air-fuel ratio sensor 42, and the carbonaceous deposition amount PC when the air-fuel ratio control according to this embodiment is performed.

[0037] 3, before time t1, lean processing is performed to control the target air-fuel ratio AFT of the exhaust gas discharged from the engine body 1 to an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter referred to as "lean air-fuel ratio"). As a result, the air-fuel ratio of the exhaust gas discharged from the engine body 1 and flowing into the upstream catalyst 20 is controlled to a lean air-fuel ratio. In particular, in the lean processing in this embodiment, the target air-fuel ratio AFT is set to a first lean set air-fuel ratio AFTlean1, which is a predetermined air-fuel ratio (for example, approximately 14.65 to 15.5) that is slightly leaner than the stoichiometric air-fuel ratio.

[0038] When the lean processing is performed and the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20 becomes lean, the oxygen storage amount OSAup in the upstream catalyst 20 gradually increases. The oxygen storage amount OSAup in the upstream catalyst 20 is calculated by the storage amount estimation unit 361 of the processor 36 in the ECU 31.

[0039] Here, in this embodiment, the storage amount estimation unit 361 calculates the oxygen storage amount OSAup in the upstream catalyst 20 based on the amount of oxygen that is excessive or deficient when the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20 is adjusted to the stoichiometric air-fuel ratio (i.e., the amount of reducing agent (unburned HC, CO, etc.) that becomes excessive). When there is an excess of oxygen in the exhaust gas flowing into the upstream catalyst 20, the storage amount estimation unit 361 calculates the oxygen storage amount by assuming that oxygen equivalent to the excess amount of oxygen is stored in the upstream catalyst 20. Furthermore, when there is a shortage of oxygen in the exhaust gas flowing into the upstream catalyst 20 (when there is an excess of reducing agent), the storage amount estimation unit 361 calculates the oxygen storage amount by assuming that oxygen equivalent to the deficient amount of oxygen is released from the upstream catalyst 20.

[0040] Specifically, the storage amount estimation unit 361 calculates the amount of oxygen stored in or released from the upstream catalyst 20 (hereinafter referred to as "oxygen absorption / release amount") OSR based on, for example, an estimated value of the amount of intake air into the combustion chamber 5 calculated based on the output air-fuel ratio AF1 of the upstream air-fuel ratio sensor 41 and the output of the air flow meter 40, or the amount of fuel supplied from the fuel injection valve 11. The storage amount estimation unit 361 calculates the oxygen absorption / release amount OSR of the upstream catalyst 20, for example, by the following equation (3): OSR=0.23×Qi×(AF1-AFR)…(3) Here, 0.23 represents the oxygen concentration in the air, Qi represents the fuel injection amount, AF1 represents the output air-fuel ratio of the upstream air-fuel ratio sensor 41, and AFR represents the stoichiometric air-fuel ratio.

[0041] The storage amount estimation unit 361 then integrates the oxygen absorption and release amounts OSR calculated in this manner to estimate the oxygen storage amount OSAup of the upstream catalyst 20. Note that if the oxygen storage amount OSAup of the upstream catalyst 20 calculated in this manner becomes a negative value, the oxygen storage amount OSAup is maintained at zero.

[0042] In this embodiment, when the oxygen storage amount OSAup of the upstream catalyst 20 calculated in this manner reaches a predetermined switching reference value Cref (at times t1, t5, and t9), a rich processing is initiated to control the target air-fuel ratio AFT of the exhaust gas discharged from the engine body 1 to an air-fuel ratio richer than the stoichiometric air-fuel ratio (hereinafter referred to as a "rich air-fuel ratio"). As a result, the air-fuel ratio of the exhaust gas discharged from the engine body 1 and flowing into the upstream catalyst 20 is controlled to a rich air-fuel ratio. In particular, in the rich processing in this embodiment, the target air-fuel ratio AFT is set to a rich set air-fuel ratio AFTrich, which is a predetermined air-fuel ratio (for example, approximately 13.4 to 14.55) slightly richer than the stoichiometric air-fuel ratio. The switching reference value Cref is set to an amount smaller than the maximum storable oxygen amount Cmax, which is the maximum amount of oxygen that the upstream catalyst 20 can store. Therefore, in this embodiment, the air-fuel ratio control unit 362 switches from lean processing to rich processing before the oxygen storage amount of the upstream catalyst 20 estimated by the storage amount estimation unit 361 reaches near the maximum storable oxygen amount Cmax. Therefore, in this embodiment, rich processing is started before the oxygen storage amount of the upstream catalyst 20 reaches near the maximum storable oxygen amount Cmax and oxygen and NOx begin to flow out of the upstream catalyst 20.

[0043] When the rich processing is performed and the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20 becomes a rich air-fuel ratio, the oxygen storage amount OSAup in the upstream catalyst 20 gradually decreases and eventually becomes zero (times t2 and t6). When the oxygen storage amount OSAup in the upstream catalyst 20 becomes zero, unburned HC and CO in the exhaust gas are not purified by the upstream catalyst 20, and therefore exhaust gas with a rich air-fuel ratio flows out from the upstream catalyst 20. As a result, after times t2 and t6, the output air-fuel ratio of the downstream air-fuel ratio sensor 42 changes to a rich air-fuel ratio.

[0044] In this embodiment, the lean pulse processing is executed at a predetermined timing (time t3, t7) after the oxygen storage amount OSAup in the upstream catalyst 20 becomes zero. The lean pulse processing will be described in detail later.

[0045] After the lean pulse processing is completed, the rich processing is started, and the target air-fuel ratio AFT is controlled to the rich air-fuel ratio (times t4 and t8), the lean processing is started again, and then the same operation is repeated. In this way, in the basic air-fuel ratio control of this embodiment, the rich processing and the lean processing are alternately repeated. In other words, in the air-fuel ratio control of this embodiment, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is basically switched alternately between the rich air-fuel ratio and the lean air-fuel ratio.

[0046] By performing the basic air-fuel ratio control in this embodiment described above, unburned HC, CO, and the like temporarily flow out from the upstream catalyst 20 at times t2 to t4 and t6 to t8, but NOx does not generally flow out from the upstream catalyst 20. Furthermore, the unburned HC and CO that flow out from the upstream catalyst 20 are purified in the downstream catalyst 24. Furthermore, the oxygen storage amount of the downstream catalyst 24 increases to the maximum storable oxygen amount Cmax during fuel cut control in which the internal combustion engine 100 is operated without supplying fuel, and then decreases when unburned HC and CO flow out from the upstream catalyst 20 and are purified.

[0047] <Lean pulse processing> However, when basic air-fuel ratio control (control not including lean pulse processing) in which lean processing and rich processing are alternately and repeatedly executed as described above is performed, the overall average air-fuel ratio becomes slightly rich. This is because rich processing continues even when the oxygen storage amount of the upstream catalyst 20 becomes almost zero at time t2 or time t6.

[0048] Experiments by the inventors have revealed that when the overall average air-fuel ratio is slightly rich like this, unburned HC contained in the exhaust gas with a rich air-fuel ratio is dehydrogenated on the ceria, which is the oxygen storage agent, and deposits on the ceria as carbon-containing substances containing carbon such as carbon or hydrocarbons (hereinafter referred to as "carbonaceous matter"). More specifically, such deposition of carbonaceous matter on the oxygen storage agent occurs when the temperature of the exhaust purification catalysts 20, 24 is 450°C to 650°C and there is little oxygen in the exhaust gas flowing through the exhaust purification catalysts 20, 24.

[0049] Even if carbonaceous matter deposits on the ceria in this way, the deposited carbonaceous matter can be removed by performing fuel cut control, which operates the internal combustion engine 100 without supplying fuel. When fuel cut control is performed, the air supplied to the combustion chamber 5 is discharged from the combustion chamber 5 as is, so air flows into the exhaust purification catalysts 20, 24. Since the oxygen concentration in air is much higher than that of exhaust gas, the oxidizing property (reactivity) of the deposited carbonaceous matter is not very high, but if the temperature of the exhaust purification catalysts 20, 24 is high, the carbonaceous matter reacts with the oxygen in the air and is removed.

[0050] On the other hand, when the internal combustion engine 100 is operating steadily (for example, when a vehicle equipped with the internal combustion engine 100 is running steadily at high speed), the fuel cut control is not performed for a long period of time. When the fuel cut control is not performed for such a long period of time, the carbonaceous matter deposited on the oxygen storage agent of the exhaust purification catalysts 20, 24 is not removed and therefore gradually increases.

[0051] Carbonaceous matter deposits in sequence starting from the downstream side of the exhaust purification catalysts 20, 24. FIG. 4 is a diagram that shows a schematic diagram of how carbonaceous matter deposits in the exhaust purification catalysts 20, 24. As shown in FIG. 4(A), when carbonaceous matter gradually deposits after fuel cut control, the carbonaceous matter first deposits in the rear stage of the downstream catalyst 24. Thereafter, when basic air-fuel ratio control is performed without executing fuel cut control, the carbonaceous matter deposits in the entire downstream catalyst 24 as shown in FIG. 4(B), and eventually the carbonaceous matter deposits in the rear stage of the upstream catalyst 20. In this way, the reason why carbonaceous matter deposits in sequence starting from the downstream side of the exhaust purification catalysts 20, 24 is that the oxygen contained in the exhaust gas is consumed in the upstream side and does not reach the downstream side.

[0052] FIG. 5 is a diagram showing the state around the oxygen storage agent when ceria is used as the oxygen storage agent. FIG. 5(A) shows a state in which the exhaust gas flowing into the exhaust purification catalysts 20, 24 has a rich air-fuel ratio and carbonaceous matter is deposited on the ceria, which is the oxygen storage agent. When carbonaceous matter is deposited on the ceria, the ceria is no longer able to store oxygen, thereby reducing the oxygen storage capacity of the oxygen storage agent. In the state shown in FIG. 4(A), the downstream catalyst 24 cannot absorb and release oxygen in the subsequent stages. In the state shown in FIG. 4(C), the downstream catalyst 24 as a whole and the upstream catalyst 20 cannot absorb and release oxygen in the subsequent stages. As a result, in the state shown in FIG. 4(C), the oxygen storage capacity of the exhaust system including both exhaust purification catalysts 20, 24 is low.

[0053] Here, when a rich air-fuel ratio exhaust gas continues to flow into the exhaust purification catalyst 20, 24, i.e., when the cerium ion valence is trivalent, if a relatively lean air-fuel ratio exhaust gas temporarily flows into the exhaust purification catalyst 20, 24 and oxygen is supplied to the ceria, active oxygen is released from the ceria, as shown in FIG. 5(B). The released active oxygen is adsorbed by the carbonaceous material deposited on the ceria, thereby increasing the oxidizing property (reactivity) of the carbonaceous material. In particular, in carbonaceous material having carbon-carbon double bonds, defects are formed in the double bonds due to the active oxygen, resulting in increased oxidizing property (reactivity) of the carbonaceous material. When the oxidizing property of the carbonaceous material increases in this way, the carbonaceous material can reduce and purify NOx in the exhaust gas flowing into the exhaust purification catalyst 20, 24, as shown in FIG. 5(C). This allows the carbonaceous material deposited on the ceria to be removed while purifying NOx in the exhaust gas.

[0054] Therefore, in this embodiment, as shown in FIG. 3, lean pulse processing is executed when rich processing is being performed and the oxygen storage amount OSAup in the upstream catalyst 20 is zero. Therefore, in this embodiment, as shown in FIG. 3, lean pulse processing is executed once during each rich processing period. In the lean pulse processing, the target air-fuel ratio AFT is controlled to a predetermined constant second lean set air-fuel ratio AFTlean2 (for example, approximately 15.0 to 25.0) that is leaner than the air-fuel ratio during lean processing. As a result, the air-fuel ratio of the exhaust gas discharged from the engine body 1 and flowing into the upstream catalyst 20 becomes an air-fuel ratio that is leaner than the air-fuel ratio during lean processing. Note that, in the lean pulse processing, the supply of fuel may be temporarily stopped. Therefore, the target air-fuel ratio AFT in the lean pulse processing may be an extremely large value.

[0055] Furthermore, the lean pulse processing is executed for a period shorter than the period of one lean processing (for example, from time t4 to t5). For example, the lean pulse processing is executed for a period during which combustion in the combustion chamber 5 is performed any number of times, from once to several tens of times. In particular, in this embodiment, the execution period of the lean pulse processing is set based on the amount of carbonaceous deposits PC at the start of the lean pulse processing. Specifically, the execution period of the lean pulse processing is set longer as the amount of carbonaceous deposits increases. In this embodiment, as a result of setting the execution period of the lean pulse processing in this manner, in one rich-lean cycle consisting of one rich processing and one lean processing (a cycle in which the oxygen storage amount OSAup changes from the switching reference value Cref to zero and then reaches the switching reference value Cref again; in FIG. 3, the cycle from time t1 to time t5), the excess amount of oxygen and the shortage amount of oxygen in the exhaust gas flowing into the upstream catalyst 20 become equal, and therefore the overall average air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20 becomes approximately the stoichiometric air-fuel ratio.

[0056] The carbonaceous deposition amount PC is calculated by the deposition amount calculation unit 363 of the processor 36 of the ECU 31. As described above, carbonaceous deposition occurs when the temperature of the exhaust purification catalysts 20, 24 is 450°C to 650°C and there is little oxygen in the exhaust gas flowing through the exhaust purification catalysts 20, 24. Therefore, in this embodiment, the deposition amount calculation unit 363 calculates the amount of carbonaceous deposition in both exhaust purification catalysts 20, 24 by integrating the amount of oxygen that is insufficient in the exhaust gas flowing into the upstream catalyst 20 (i.e., the amount of excess reducing agent) when the temperature of the upstream catalyst 20 detected by the upstream temperature sensor 43 and the temperature of the downstream catalyst 24 detected by the downstream temperature sensor 44 are equal to or higher than a first reference temperature (e.g., 450°C or higher) and equal to or lower than a second reference temperature (e.g., 650°C or lower) and the oxygen storage amount of the upstream catalyst 20 estimated by the storage amount estimation unit 361 is zero. In this embodiment, the temperatures of both exhaust purification catalysts 20, 24 are detected by the temperature sensors 43, 44, but if the engine load remains high for a long period of time, for example, when a vehicle equipped with the internal combustion engine 100 is steadily running on a highway, the temperatures of the exhaust purification catalysts 20, 24 will increase. Therefore, the temperatures of the exhaust purification catalysts 20, 24 may be estimated based on the output of the load sensor 46 or the like.

[0057] Note that even when the temperature of the upstream catalyst 20 is equal to or higher than the first reference temperature and equal to or lower than the second reference temperature and the oxygen storage amount of the upstream catalyst 20 is zero, not all of the inflowing reducing agent is deposited in the upstream catalyst 20. Therefore, the deposition amount calculation unit 363 may calculate the amount of carbonaceous deposition as a value obtained by multiplying the cumulative value of the oxygen amount that is deficient (i.e., the excess reducing agent amount) in the exhaust gas that flows into the upstream catalyst 20 by a predetermined coefficient that is less than 1. Therefore, the deposition amount calculation unit 363 may calculate the amount of carbonaceous deposition so that it is proportional to the cumulative value of the oxygen amount that is deficient (i.e., the excess reducing agent amount) in the exhaust gas that flows into the upstream catalyst 20.

[0058] As described above, the deposited carbonaceous matter is removed when fuel cut control is performed. Therefore, when fuel cut control is performed, the deposition amount calculation unit 363 resets the calculated amount of deposited carbonaceous matter to zero.

[0059] The lean pulse processing is set based on the amount of carbonaceous matter deposited PC when the lean pulse processing is started. Therefore, the lean pulse processing is executed when carbonaceous matter is deposited. Consequently, in this embodiment, the lean pulse processing is executed under conditions where carbonaceous matter deposits, for example, when the temperature of the exhaust purification catalysts 20, 24 is a temperature where carbonaceous matter deposits (for example, about 450°C to 650°C) and there is little oxygen in the exhaust gas flowing through the exhaust purification catalysts 20, 24.

[0060] As described above, in this embodiment, by performing lean pulse processing, as shown in FIGS. 5(B) and 5(C), it is possible to remove carbonaceous matter that has deposited on the oxygen storage agent of the exhaust purification catalysts 20, 24, thereby suppressing a decrease in the oxygen storage capacity of the oxygen storage agent. In addition, the removed carbonaceous matter can reduce and purify NOx in the exhaust gas. This makes it possible to reduce the amount of unburned HC and other substances required to reduce and purify NOx, thereby reducing the amount of fuel required to reduce and purify NOx, and ultimately suppressing a decrease in fuel economy. Therefore, according to this embodiment, it is possible to suppress a decrease in the oxygen storage capacity of the oxygen storage agent while suppressing a decrease in fuel economy, and ultimately suppress a decrease in emissions.

[0061] Furthermore, in this embodiment, the overall average air-fuel ratio flowing into the upstream catalyst 20 can be made approximately the stoichiometric air-fuel ratio. As a result, it is possible to suppress components such as unburned HC, NH3, and N2O in the exhaust gas flowing out from the exhaust system including both exhaust purification catalysts 20, 24.

[0062] <Explanation of the flowchart> Fig. 6 is a flowchart that schematically shows the flow of air-fuel ratio control executed by the air-fuel ratio control unit 362. In particular, Fig. 6 shows the flow of air-fuel ratio control in one rich-lean cycle consisting of one rich processing and one lean processing. Therefore, the air-fuel ratio control shown in Fig. 6 is started when the lean processing in the previous rich-lean cycle ends, or when the fuel cut control ends, etc.

[0063] 6, the air-fuel ratio control unit 362 first executes rich processing (step S11). Therefore, the air-fuel ratio control unit 362 sets the target air-fuel ratio AFT to the rich set air-fuel ratio AFTrich. At this time, the storage amount estimation unit 361 estimates the oxygen storage amount of the upstream catalyst 20 based on the output air-fuel ratio of the upstream air-fuel ratio sensor 41, the fuel injection amount from the fuel injection valve 11, etc. Furthermore, when the oxygen storage amount estimated by the storage amount estimation unit 361 becomes zero, the deposition amount calculation unit 363 calculates the carbonaceous deposition amount based on the temperature of the upstream catalyst 20 detected by the upstream temperature sensor 43, the output air-fuel ratio of the upstream air-fuel ratio sensor 41, etc.

[0064] The air-fuel ratio control unit 362 determines whether or not a condition for executing lean pulse processing is satisfied during execution of rich processing (step S12). In this embodiment, the condition for executing lean pulse processing is satisfied when a predetermined time (or a predetermined first number of combustion cycles of the internal combustion engine 100) has elapsed since the oxygen storage amount estimated by the storage amount estimation unit 361 became zero.

[0065] If it is determined in step S12 that the execution condition for the lean pulse processing is met, the air-fuel ratio control unit 362 executes the lean pulse processing (step S13). In this embodiment, the air-fuel ratio control unit 362 sets the target air-fuel ratio AFT to a second lean set air-fuel ratio AFTlean2 that is leaner than the first lean set air-fuel ratio AFTlean1 during the lean processing, over a predetermined execution period. In addition, the air-fuel ratio control unit 362 sets the execution period for the lean pulse processing based on the carbonaceous deposition amount calculated by the deposition amount calculation unit 363. When the lean pulse processing ends, the air-fuel ratio control unit 362 starts the rich processing again.

[0066] Thereafter, the air-fuel ratio control unit 362 determines whether a predetermined second time period (or a predetermined second number of combustion cycles of the internal combustion engine 100) has elapsed since the oxygen storage amount estimated by the storage amount estimation unit 361 became zero (step S14). If it is determined in step S14 that the predetermined second time period has not elapsed, steps S11 to S13 are repeated.

[0067] On the other hand, if it is determined in step S13 that the predetermined second time has elapsed, the air-fuel ratio control unit 362 executes lean processing (step S15). Therefore, the air-fuel ratio control unit 362 sets the target air-fuel ratio AFT to the first lean set air-fuel ratio AFTlean1. During the execution of the lean processing, the air-fuel ratio control unit 362 determines whether the oxygen storage amount OSAup estimated by the storage amount estimation unit 361 is equal to or greater than the switching reference value Cref (step S16). If it is determined in step S16 that the oxygen storage amount OSAup is less than the switching reference value Cref, step S15 is repeatedly executed, and the execution of the lean processing is maintained. On the other hand, if it is determined in step S16 that the oxygen storage amount OSAup is equal to or greater than the switching reference value Cref, one rich-lean cycle in the air-fuel ratio control is completed, and the operation is started again from step S11.

[0068] <Modification> In the above embodiment, the deposition amount calculation unit 363 calculates the amount of carbonaceous matter deposited on the exhaust purification catalysts 20, 24. However, the deposition amount calculation unit 363 may calculate the value of another deposition parameter that changes depending on the amount of carbonaceous matter deposited, rather than the amount of carbonaceous matter deposited. For example, since the amount of carbonaceous matter deposited is proportional to the value obtained by multiplying the flow rate of exhaust gas when the oxygen storage amount of the exhaust purification catalysts 20, 24 is zero by a value obtained by subtracting the equivalence ratio from 1, the deposition amount calculation unit 363 may calculate the value of such a parameter as a deposition parameter that represents the amount of carbonaceous matter deposited. Also, even in this case, when fuel cut control is executed, the deposition amount calculation unit resets the value of the deposition parameter to a value that represents that the amount of carbonaceous matter deposited on the oxygen storage agent is zero.

[0069] In the above embodiment, the air-fuel ratio control unit 362 executes the lean pulse processing for each rich processing. However, the air-fuel ratio control unit 362 may execute the lean pulse processing once for multiple rich processings. That is, the air-fuel ratio control unit 362 may execute the lean pulse processing periodically at a predetermined fixed cycle (once for each predetermined number of rich-lean cycles). In this case, the deposition amount calculation unit 363 calculates the current carbonaceous deposition amount by integrating the carbonaceous deposition amounts in multiple past rich processings. Then, the deposition amount calculation unit 363 sets the execution period of the lean pulse processing based on the carbonaceous deposition amount calculated in this way (the deposition amount when the lean pulse processing is executed).

[0070] Furthermore, in the above embodiment, the deposition amount calculation unit 363 sets the execution period of the lean pulse processing based on the amount of carbonaceous deposition. However, the deposition amount calculation unit 363 may set the lean degree of the target air-fuel ratio AFT during the lean pulse processing based on the amount of carbonaceous deposition instead of or in addition to the execution period of the lean pulse processing. In this case, the deposition amount calculation unit 363 sets the lean degree of the target air-fuel ratio AFT during the lean pulse processing so that the lean degree of the target air-fuel ratio AFT during the lean pulse processing increases as the amount of carbonaceous deposition increases. Even in this case, the target air-fuel ratio in the lean pulse processing is set to an air-fuel ratio leaner than the first lean set air-fuel ratio AFTlean1 in the lean processing. Therefore, the deposition amount calculation unit 363 sets at least one of the execution period of the lean pulse processing and the lean degree in the lean pulse processing based on the amount of carbonaceous deposition when the lean pulse processing is executed.

[0071] In addition, in the above embodiment, the lean pulse processing is performed periodically at a fixed cycle, and the execution period of the lean pulse processing or the lean degree in the lean pulse processing is set based on the amount of carbonaceous matter deposited when the lean pulse processing is performed. However, the execution period of the lean pulse processing and the lean degree in the lean pulse processing may be set to fixed values, and the execution timing of the lean pulse processing may be set based on the amount of carbonaceous matter deposited.

[0072] FIG. 7 is a time chart similar to FIG. 3 of the target air-fuel ratio AFT and the like when air-fuel ratio control according to one modified example is performed.

[0073] In the example shown in Fig. 7, as in the example shown in Fig. 3, lean processing is performed before time t1. Then, at time t1, when the oxygen storage amount OSAup in the upstream catalyst 20 reaches the switching reference value Cref, rich processing is started. Thereafter, at time t2, the oxygen storage amount OSAup in the upstream catalyst 20 becomes zero, and the amount of carbonaceous matter deposited in the upstream catalyst 20 gradually increases. Thereafter, at time t3, which is a certain amount of time (driving cycle) after the oxygen storage amount OSAup in the upstream catalyst 20 becomes zero, lean processing is started again. In this way, basic air-fuel ratio control in which rich processing and lean processing are alternately repeated is also performed in this modified example.

[0074] As a result of performing such basic air-fuel ratio control, at time t6, the carbonaceous deposition amount PC calculated by the deposition amount calculation unit 363 reaches the reference deposition amount PCref. Here, the reference deposition amount PCref is a predetermined arbitrary constant deposition amount, and is, for example, an amount such that if the deposition amount increases beyond this amount, the exhaust emissions flowing out from the exhaust system including the exhaust purification catalysts 20, 24 will increase sharply.

[0075] In this manner, when the amount of carbonaceous matter deposited PC becomes equal to or greater than the reference amount of carbonaceous matter deposited PCref at time t6, the lean pulse process is started. Therefore, in this modified example, the execution timing of the lean pulse process is set based on the amount of carbonaceous matter deposited PC.

[0076] Moreover, the execution period of the lean pulse processing and the lean degree in the lean pulse processing at this time are preset fixed values. In particular, in this modified example, the execution period of the lean pulse processing and the lean degree in the lean pulse processing are set so that the carbonaceous matter of the reference deposition amount PCref can be removed from the upstream side catalyst 20. Note that, in this modified example, the execution period of the lean pulse processing and the lean degree in the lean pulse processing are preset fixed values, but they may be changed based on the temperature of the exhaust purification catalysts 20, 24, etc.

[0077] As described above, in the above embodiment and its modified examples, at least one of the timing of executing the lean pulse processing, the execution period of the lean pulse processing, and the degree of leanness of the exhaust gas flowing into the exhaust purification catalyst during the lean pulse processing is set based on the amount of carbonaceous matter deposited on the oxygen storage agent supported on the exhaust purification catalyst 20, 24.

[0078] <Verification of effectiveness> As described above, according to the above embodiment and its modified examples, the carbonaceous matter deposited on the oxygen storage agent can be removed, thereby suppressing a decrease in the oxygen storage capacity of the oxygen storage agent. In addition, the removed carbonaceous matter can reduce and purify NOx in the exhaust gas. This suppresses the deterioration of exhaust emissions. The effect of suppressing the deterioration of exhaust emissions was compared with a case where an air-fuel ratio control different from the air-fuel ratio control according to the present embodiment and its modified examples was performed.

[0079] FIG. 8 is a time chart of the target air-fuel ratio AFT, the output air-fuel ratio AF1 of the upstream air-fuel ratio sensor 41, and the oxygen storage amount OSAup of the upstream catalyst 20 when air-fuel ratio control is performed in comparative control 1. As shown in FIG. 8, in comparative control 1, when the oxygen storage amount OSAup of the upstream catalyst 20 reaches the maximum storable oxygen amount Cmax (times t1 and t3), the air-fuel ratio control is switched from lean processing to rich processing. Also, in comparative control 1, when the oxygen storage amount OSAup of the upstream catalyst 20 becomes zero (times t2 and t4), the air-fuel ratio control is switched from rich processing to lean processing. Therefore, in comparative control 1, in one rich-lean cycle, the excess amount of oxygen and the shortage amount of oxygen in the exhaust gas flowing into the upstream catalyst 20 are equal, and therefore the overall average air-fuel ratio becomes the stoichiometric air-fuel ratio.

[0080] FIG. 9 is a time chart similar to FIG. 8 showing the target air-fuel ratio AFT and other parameters when air-fuel ratio control is performed in comparative control 2. As shown in FIG. 9, in comparative control 2, when the oxygen storage amount OSAup of the upstream catalyst 20 reaches a switching reference value Cref, which is less than the maximum storable oxygen amount Cmax (times t1 and t4), the air-fuel ratio control is switched from lean processing to rich processing. Also, in comparative control 2, when a predetermined period of time has elapsed (times t3 and t6) after the oxygen storage amount OSAup of the upstream catalyst 20 becomes zero (times t2 and t5), the air-fuel ratio control is switched from rich processing to lean processing. Therefore, in comparative control 2, during one rich-lean cycle, the excess amount of oxygen in the exhaust gas flowing into the upstream catalyst 20 is less than the shortage of oxygen, and therefore the overall average air-fuel ratio becomes a rich air-fuel ratio.

[0081] 10 is a time chart similar to FIG. 8 of the target air-fuel ratio AFT, etc., when the air-fuel ratio control is performed in the comparative control 3. As shown in FIG. 10, in the comparative control 3, as in the comparative control 2, when the oxygen storage amount OSAup of the upstream catalyst 20 reaches the switching reference value Cref (at times t1, t4, t7, t 10 , t 13 ), the air-fuel ratio control is switched from lean processing to rich processing. Also, in the comparative control 3, when a predetermined period has elapsed after the oxygen storage amount OSAup of the upstream catalyst 20 becomes zero (times t3, t6, t9, t 12 ), the air-fuel ratio control is switched from rich processing to lean processing. However, in comparative control 3, the target air-fuel ratio AFT in the lean processing is set to an air-fuel ratio that is leaner than the first lean set air-fuel ratio, and the target air-fuel ratio AFT in the rich processing is set to an air-fuel ratio that is richer than the rich set air-fuel ratio.

[0082] Table 1 shows a comparison of the flow rates of various components flowing out from an exhaust system including the exhaust purification catalysts 20, 24 when the air-fuel ratio control according to the embodiment described above (control shown in FIG. 3), the air-fuel ratio control according to the modified example (control shown in FIG. 7), and comparative controls 1 to 3 are performed. In Table 1, "Control 1" indicates the air-fuel ratio control according to the embodiment described above, and "Control 2" indicates the air-fuel ratio control according to the modified example. The flow rates of HC and NOx are expressed as ratios when the flow rate in comparative control 1 is set to 1. In addition, the transient mode in the figure is an operation mode that includes fuel cut control, and the steady mode is an operation mode in which the internal combustion engine 100 is operated at a constant high speed without including fuel cut control.

[0083] [Table 1]

[0084] As shown in Table 1, when comparative controls 2 and 3 are performed, NOx emissions in transient mode can be suppressed compared to when comparative control 1 is performed. However, when comparative controls 2 and 3 are performed, NOx emissions increase in steady mode compared to transient mode. In contrast, when present controls 1 and 2 are performed, NOx emissions can be suppressed in steady mode to the same extent as in transient mode. In addition, with present controls 1 and 2, the average air-fuel ratio ultimately becomes approximately the stoichiometric air-fuel ratio, so HC emissions can also be suppressed.

[0085] In addition, the NOx reduction rate per unit amount of deposited carbonaceous matter was compared between Comparative Control 2 and Present Control 2. FIG. 11 is a graph showing the ratio of the NOx reduction rate at 500°C per unit amount of deposited carbonaceous matter. FIG. 11 shows the NOx reduction rate by the oxygen storage agent, i.e., the NOx reduction rate when oxygen generated by dissociation of NO at the precious metal supported on the exhaust purification catalyst is absorbed into the oxygen storage agent and NOx reduction occurs, when this rate is set to 1. As shown in FIG. 11, it can be seen that the NOx reduction rate in Present Control 2 is significantly faster than the NOx reduction rate in Comparative Control 2. As a result of this faster NOx reduction rate by the carbonaceous matter, it is believed that Present Control 2 can suppress NOx emissions even in steady mode, as shown in Table 1.

[0086] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the claims. [Explanation of symbols]

[0087] 1. Main body of the engine 20 Upstream catalyst 24 downstream catalyst 31 ECU 40 Air flow meter 41 Upstream air-fuel ratio sensor 42 Downstream air-fuel ratio sensor 43 Upstream temperature sensor 44 Downstream temperature sensor

Claims

1. An air-fuel ratio control device that controls the air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst having oxygen storage capacity that is provided in an exhaust passage of an internal combustion engine, an air-fuel ratio control unit that controls the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; the air-fuel ratio control section alternately and repeatedly executes a rich process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and a lean process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio control unit executes, during the rich processing, a lean pulse processing for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the air-fuel ratio during the lean processing, for a period shorter than a period of one of the lean processings, The air-fuel ratio control unit sets at least one of the execution timing of the lean pulse processing, the execution period of the lean pulse processing, and the lean degree of the exhaust gas flowing into the exhaust purification catalyst during the lean pulse processing, based on the value of a deposition parameter related to the amount of deposition of carbon-containing substances on an oxygen storage agent supported on the exhaust purification catalyst.

2. 2. The air-fuel ratio control device according to claim 1, wherein the air-fuel ratio control unit executes the lean pulse processing when the value of the deposition parameter is a value indicating that an amount of carbon-containing substances deposited on the oxygen storage agent is equal to or greater than a predetermined reference deposition amount.

3. 3. The air-fuel ratio control device according to claim 2, wherein the execution period of the lean pulse processing and the lean degree in the lean pulse processing are fixed values set so that the reference deposition amount of carbon-containing substances is completely removed from the oxygen storage agent.

4. 2. The air-fuel ratio control device according to claim 1, wherein the air-fuel ratio control section periodically executes the lean pulse processing at a predetermined cycle.

5. 5. The air-fuel ratio control device according to claim 4, wherein the air-fuel ratio control unit sets at least one of an execution period of the lean pulse processing and a lean degree in the lean pulse processing based on a value of the precipitation parameter when the lean pulse processing is executed.

6. Further comprising a deposition amount calculation unit that calculates the value of the deposition parameter, The air-fuel ratio control device according to any one of claims 1 to 5, wherein the deposition amount calculation unit calculates the value of the deposition parameter so as to be proportional to an integrated value of excess reducing agent flowing into the exhaust purification catalyst when the temperature of the exhaust purification catalyst is equal to or higher than a predetermined reference temperature and the oxygen storage amount in the exhaust purification catalyst is zero.

7. 7. The air-fuel ratio control device according to claim 6, wherein when fuel cut control is executed to temporarily stop the supply of fuel to the internal combustion engine while the internal combustion engine is in operation, the deposition amount calculation unit resets the value of the deposition parameter to a value that indicates that the amount of deposition of carbon-containing substances on the oxygen storage agent is zero.

8. An air-fuel ratio control device that controls the air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst having oxygen storage capacity provided in an exhaust passage of an internal combustion engine, an air-fuel ratio control unit that controls the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; the air-fuel ratio control section alternately and repeatedly executes a rich process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and a lean process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio control unit executes, during the rich processing, a lean pulse processing for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the air-fuel ratio during the lean processing, for a period shorter than a period of one of the lean processings, further comprising an estimation unit that estimates an oxygen storage amount of the exhaust purification catalyst, The air-fuel ratio control device is configured such that the air-fuel ratio control unit switches from the lean processing to the rich processing before the oxygen storage amount estimated by the estimation unit reaches a maximum oxygen storage amount.

9. An air-fuel ratio control device that controls the air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst having oxygen storage capacity provided in an exhaust passage of an internal combustion engine, an air-fuel ratio control unit that controls the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; the air-fuel ratio control section alternately and repeatedly executes a rich process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and a lean process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio control unit executes, during the rich processing, a lean pulse processing for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the air-fuel ratio during the lean processing, for a period shorter than a period of one of the lean processings, An air-fuel ratio control device, wherein the lean pulse processing is performed when the oxygen storage amount of the exhaust purification catalyst is zero.

10. An air-fuel ratio control device for controlling the air-fuel ratio of exhaust gas flowing into an exhaust purification catalyst having oxygen storage capacity provided in an exhaust passage of an internal combustion engine, an air-fuel ratio control unit that controls the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; the air-fuel ratio control section alternately and repeatedly executes a rich process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio that is richer than the stoichiometric air-fuel ratio, and a lean process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio control unit executes, during the rich processing, a lean pulse processing for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio that is leaner than the air-fuel ratio during the lean processing, for a period shorter than a period of one of the lean processings, An air-fuel ratio control device, wherein the lean pulse processing is performed when the temperature of the exhaust purification catalyst is at a temperature at which carbon-containing substances are precipitated on an oxygen storage agent carried on the exhaust purification catalyst.

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