Control device for internal combustion engine

Abstract

To suppress occurrence of misfire.SOLUTION: An internal combustion engine 20 to which a control device 100 is applied has cylinders 24, an exhaust passage 51, a filter 53 and a cooling water temperature sensor 93. The cooling water temperature sensor 93 detects a temperature of cooling water of the internal combustion engine 20 as a cooling water temperature TW. The control device 100 executes determination processing for determining that the internal combustion engine 20 is in a stable state when an absolute value of an index value indicating a change rate of a load of the internal combustion engine 20 is smaller than a threshold value. The control device 100 executes lean combustion processing for burning an air-fuel mixture adjusted to have an air-fuel ratio leaner than a theoretical air-fuel ratio in the cylinder 24 to burn particulate matters collected in the filter 53 on the condition that a temperature of the filter 53 is a specified temperature or higher and the internal combustion engine 20 is in the stable operating state. When the cooling water temperature TW is low, the control device 100 sets the threshold value to be used in the determination processing to be smaller than that when the cooling water temperature TW is high.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a control device for an internal combustion engine. [Background technology]

[0002] The internal combustion engine described in Patent Document 1 has a cylinder, an exhaust passage, and a catalyst. The control device for the internal combustion engine described in Patent Document 1 is capable of executing lean combustion processing. The lean combustion processing is a processing for burning an air-fuel mixture adjusted to an air-fuel ratio leaner than the theoretical air-fuel ratio in the cylinder. The control device also calculates the stability of the internal combustion engine based on the amount of fluctuation in the engine speed of the internal combustion engine. The worse the stability of the internal combustion engine, the larger the correction amount for correcting the air-fuel ratio is. When the correction amount of the air-fuel ratio exceeds a predetermined value, the control device switches the air-fuel ratio to the theoretical air-fuel ratio. In other words, when the stability of the internal combustion engine deteriorates, the control device does not execute the lean combustion processing. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-272591 Summary of the Invention [Problem to be solved by the invention]

[0004] In an internal combustion engine such as that described in Patent Document 1, when the cooling water temperature is low, the temperature of the cylinder is lower than when the cooling water temperature is high. When the cylinder temperature is low, the atomization of fuel in the cylinder is hindered, and the friction between the inner wall of the cylinder and the piston increases. Therefore, even if a mixture adjusted to the same air-fuel ratio is burned in the cylinder, misfires are likely to occur when the cylinder temperature is low. Therefore, if the lean combustion process is performed when the cylinder temperature is low, the possibility of misfire increases. [Means for solving the problem]

[0005] In order to solve the above problems, the present invention provides a control device applied to an internal combustion engine having a cylinder, an exhaust passage through which exhaust gas from the cylinder flows, a filter that collects particulate matter contained in the exhaust gas, and a coolant temperature sensor that detects the temperature of coolant flowing through a water jacket as the coolant temperature, the control device comprising: a determination process for determining that the internal combustion engine is in a stable operating state when an absolute value of an index value of a rate of change of a load of the internal combustion engine is less than a threshold value; a setting process of setting the threshold value to be smaller when the cooling water temperature detected by the cooling water temperature sensor is low than when the cooling water temperature is high; A control device for an internal combustion engine that executes a lean combustion process in which an air-fuel mixture adjusted to an air-fuel ratio leaner than the stoichiometric air-fuel ratio is combusted in the cylinder in order to combust the particulate matter trapped in the filter, under the conditions that the temperature of the filter is equal to or higher than a predetermined specified temperature and the internal combustion engine is in a stable operating state.

[0006] According to the above configuration, the threshold value used in the process of determining whether the internal combustion engine is in a stable operating state or a transient operating state is set lower when the cooling water temperature is low than when the cooling water temperature is high. Therefore, the control device can easily determine that the internal combustion engine is in a transient operating state in a situation where misfire is likely to occur due to the low cooling water temperature. And, according to the above configuration, the lean combustion process is executed on the condition that the internal combustion engine is in a stable operating state. In this way, the control device does not execute the lean combustion process in a situation where misfire is likely to occur, thereby suppressing the occurrence of misfire. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of an internal combustion engine. [Diagram 2] FIG. 2 is a flowchart showing a series of processes including the setting process. [Diagram 3] FIG. 3 is a flowchart showing a series of processes including the determination process. [Figure 4] FIG. 4 is a flowchart showing a series of processes including the lean combustion process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] (One embodiment) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a control device for an internal combustion engine will now be described with reference to the drawings. In this embodiment, an internal combustion engine 20 is mounted on a vehicle.

[0009] <Overall structure of the internal combustion engine> First, the overall configuration of an internal combustion engine to which the control device for the internal combustion engine is applied will be described. As shown in FIG. 1, the internal combustion engine 20 has a cylinder block 21, a cylinder head 22, a plurality of pistons 23, and a plurality of cylinders 24.

[0010] The cylinder 24 is a cylindrical space defined inside the cylinder block 21. Both ends of the cylinder 24 in a direction along the central axis are open to the outside of the cylinder block 21. The piston 23 is disposed inside the cylinder 24. The piston 23 reciprocates inside the cylinder 24. The top surface of the piston 23 faces a first end in a direction along the central axis of the cylinder 24. The cylinder head 22 is connected to the cylinder block 21. The cylinder head 22 has a recess 25. The recess 25 is recessed in the outer surface of the cylinder head 22 on the cylinder block 21 side. The recess 25 faces the cylinder 24 in a direction along the central axis of the cylinder 24. The wall surface of the cylinder block 21 that defines the cylinder 24, the wall surface of the recess 25, and the top surface of the piston 23 define a combustion chamber R.

[0011] The internal combustion engine 20 has a connecting rod 26 and a crankshaft 27. The connecting rod 26 is connected to the piston 23. The connecting rod 26 extends in the opposite direction to the cylinder head 22, sandwiching the piston 23 therebetween. The crankshaft 27 is connected to the connecting rod 26. The connecting rod 26 and the crankshaft 27 convert the reciprocating linear motion of the piston 23 into rotational motion.

[0012] The internal combustion engine 20 has an intake port 28. The intake port 28 is a space defined inside the cylinder head 22. A first end of the intake port 28 opens toward the recess 25. A second end of the intake port 28 opens toward the outside of the cylinder head 22.

[0013] The internal combustion engine 20 has an exhaust port 29. The exhaust port 29 is a space defined inside the cylinder head 22. A first end of the exhaust port 29 opens toward the recess 25. A second end of the exhaust port 29 opens toward the outside of the cylinder head 22.

[0014] The internal combustion engine 20 includes an intake valve 30 and an exhaust valve 31. The intake valve 30 is a valve that opens and closes a first end of the intake port 28. The exhaust valve 31 is a valve that opens and closes a first end of the exhaust port 29.

[0015] Although FIG. 1 illustrates only one set of the combustion chamber R and the configuration related to the combustion chamber R, the internal combustion engine 20 includes a plurality of sets of these configurations. The internal combustion engine 20 has an intake passage 41 for drawing in outside air. The intake passage 41 is connected to a second end of the intake port 28. The intake passage 41 houses a throttle valve 42. The throttle valve 42 adjusts the intake air amount GA, which is the flow rate of air flowing through the intake passage 41, by changing the valve opening degree. The air drawn in through the intake passage 41 flows into the combustion chamber R via the intake port 28.

[0016] The internal combustion engine 20 is equipped with an in-cylinder injection valve 44. The in-cylinder injection valve 44 is attached to the cylinder head 22. The tip of the in-cylinder injection valve 44 is located in the combustion chamber R. The in-cylinder injection valve 44 injects fuel directly into the combustion chamber R without passing through the intake port 28.

[0017] The internal combustion engine 20 is equipped with an ignition plug 45. The ignition plug 45 is attached to the cylinder head 22. The ignition plug 45 is located between the intake port 28 and the exhaust port 29. The tip of the spark plug 45 is located near the tip of the in-cylinder injection valve 44. The spark plug 45 ignites the air-fuel mixture introduced into the combustion chamber R by a spark. That is, the spark plug 45 ignites the air-fuel mixture in the cylinder 24.

[0018] The internal combustion engine 20 includes an exhaust passage 51 through which exhaust gas generated by combustion in the combustion chamber R flows. That is, exhaust gas from the cylinder 24 flows through the exhaust passage 51. The exhaust passage 51 is also connected to a second end of the exhaust port 29.

[0019] The internal combustion engine 20 is equipped with a catalyst 52. The catalyst 52 is located in the exhaust passage 51. The catalyst 52 is a three-way catalyst, and purifies the hydrocarbons, carbon monoxide, and nitrogen oxides contained in the exhaust gas. The catalyst 52 also has an oxygen storage capacity.

[0020] The internal combustion engine 20 is equipped with a filter 53. The filter 53 is located downstream of the catalyst 52 in the exhaust passage 51. The filter 53 collects particulate matter contained in the exhaust gas.

[0021] The internal combustion engine 20 is provided with a crank angle sensor 91. The crank angle sensor 91 is located in the vicinity of the crankshaft 27. The crank angle sensor 91 detects the rotation phase SC of the crankshaft 27.

[0022] The internal combustion engine 20 is equipped with an airflow meter 92. The airflow meter 92 is located upstream of the throttle valve 42 in the intake passage 41. The airflow meter 92 detects the intake air amount GA, which is the flow rate of air flowing through the intake passage 41. The airflow meter 92 also detects the intake temperature TI, which is the temperature of the air flowing through the intake passage 41.

[0023] The internal combustion engine 20 is equipped with a coolant temperature sensor 93. The coolant temperature sensor 93 detects a coolant temperature TW, which is the temperature of the coolant flowing through a water jacket of the internal combustion engine 20. The vehicle equipped with the internal combustion engine 20 is provided with an operation switch 94. The operation switch 94 is a switch that is turned on when the operation of the internal combustion engine 20 is started, and is turned off when the operation of the internal combustion engine 20 is stopped.

[0024] The internal combustion engine 20 is equipped with an exhaust gas temperature sensor 95. The exhaust gas temperature sensor 95 detects the temperature of the exhaust gas flowing into the filter 53, that is, the exhaust gas temperature TO. The internal combustion engine 20 is equipped with an air-fuel ratio sensor 96. The air-fuel ratio sensor 96 detects the exhaust air-fuel ratio AF, which is the air-fuel ratio of the exhaust gas flowing into the filter 53.

[0025] <About the control device> The vehicle equipped with the internal combustion engine 20 is provided with a control device 100. The control device 100 controls the internal combustion engine 20. The control device 100 acquires a signal indicating the rotation phase SC of the crankshaft 27 from a crank angle sensor 91. The control device 100 acquires a signal indicating the intake air amount GA from an air flow meter 92. The control device 100 acquires a signal indicating the intake air temperature TI from the air flow meter 92. The control device 100 acquires a signal indicating the coolant temperature TW from a coolant temperature sensor 93. The control device 100 acquires an ON signal for starting the operation of the internal combustion engine 20 and an OFF signal for stopping the operation of the internal combustion engine 20 from an operation switch 94. The control device 100 acquires a signal indicating the exhaust temperature TO from an exhaust temperature sensor 95. The control device 100 acquires a signal indicating the exhaust air-fuel ratio AF, which is the air-fuel ratio of the exhaust flowing into the filter 53, from an air-fuel ratio sensor 96.

[0026] The control device 100 includes a CPU 101, a peripheral circuit 102, a ROM 103, a storage device 104, and a bus 105. The bus 105 connects the CPU 101, the peripheral circuit 102, the ROM 103, and the storage device 104 so that they can communicate with each other. The peripheral circuit 102 includes a circuit that generates a clock signal that regulates internal operations, a power supply circuit, a reset circuit, and the like. The ROM 103 stores in advance various programs that the CPU 101 uses to execute various controls. The CPU 101 controls the internal combustion engine 20 by executing the various programs stored in the ROM 103.

[0027] <About the accumulation amount calculation process> The CPU 101 executes an accumulation amount calculation process for calculating a PM accumulation amount DA, which is the accumulation amount of particulate matter trapped on the filter 53. The CPU 101 repeatedly executes, at a predetermined cycle, a program for calculating the PM accumulation amount DA stored in the ROM 103. The accumulation amount calculation process is realized by the CPU 101 repeatedly executing, for example, at a predetermined cycle, a program for calculating the PM accumulation amount DA stored in the ROM 103.

[0028] Specifically, when CPU 101 starts a program for calculating PM accumulation amount DA, CPU 101 repeatedly calculates the PM generation amount and the PM regeneration amount in an accumulation amount calculation process. CPU 101 then updates PM accumulation amount DA to calculate PM accumulation amount DA. Specifically, CPU 101 calculates the sum of the difference between the PM generation amount and the PM regeneration amount and the difference added to the value of PM accumulation amount DA before updating as the latest PM accumulation amount DA value, and updates PM accumulation amount DA.

[0029] The PM generation amount is the amount of particulate matter generated by the combustion of the air-fuel mixture in the cylinder 24. The CPU 101 calculates the PM generation amount from the intake air amount GA, the fuel injection amount, and the like. The PM regeneration amount is the amount of particulate matter burned in the filter 53. The higher the exhaust temperature TO, which is the temperature of the exhaust gas flowing into the filter 53, the higher the temperature of the filter 53. Therefore, the temperature of the filter 53 can be obtained from the temperature detected by the exhaust temperature sensor 95. The CPU 101 calculates the filter temperature TF, which is the temperature of the filter 53, using a heat balance model of the filter 53 based on the flow rate of the exhaust gas flowing into the filter 53, the exhaust temperature TO, and the temperature of the outside air. The flow rate of the exhaust gas flowing into the filter 53 can be obtained from the intake air amount GA and the fuel injection amount. The temperature of the outside air can be the intake air temperature TI detected by the air flow meter 92. When the filter temperature TF is equal to or higher than the ignition point of the particulate matter, if exhaust gas containing oxygen flows into the filter 53, the particulate matter deposited on the filter 53 begins to burn. Since oxygen is necessary for the combustion of the particulate matter, the amount of particulate matter burned in the filter 53 at this time is determined according to the amount of oxygen in the exhaust gas flowing into the filter 53. The oxygen concentration of the exhaust gas flowing into the filter 53 can be obtained from the detection result of the air-fuel ratio sensor 96. Therefore, the CPU 101 calculates the PM regeneration amount based on the exhaust temperature TO detected by the exhaust temperature sensor 95, the oxygen concentration detected by the air-fuel ratio sensor 96, i.e., the exhaust air-fuel ratio AF, the intake air amount GA, and the fuel injection amount.

[0030] <About threshold setting process> During the period when the internal combustion engine 20 is running, the CPU 101 executes a setting process for setting a threshold value TH used in a determination process described below. The CPU 101 repeatedly executes a program for calculating the threshold value TH stored in the ROM 103 at a predetermined cycle. As a result, a series of processes shown in Fig. 2 are repeatedly executed. In other words, the setting process is realized by the CPU 101 repeatedly executing a program for setting the threshold value TH stored in the ROM 103 at a predetermined cycle, for example.

[0031] Specifically, as shown in FIG. 2, when the CPU 101 starts a program for setting the threshold value TH, it first executes the process of step S11. In step S11, the CPU 101 executes a correction value calculation process. In the correction value calculation process, the CPU 101 calculates a correction value CV. The correction value CV is a negative value. When the coolant temperature TW is low, the CPU 101 sets the correction value CV to be smaller than when the coolant temperature TW is high. Specifically, the CPU 101 calculates the correction value CV to be a smaller value as the coolant temperature TW is smaller. Then, the process proceeds to step S12.

[0032] In step S12, the CPU 101 executes a setting process for setting the threshold value TH. In the setting process, the CPU 101 sets the threshold value TH by adding the correction value CV calculated in step S11 to the base threshold value THb. The base threshold value THb is a positive value. The absolute value of the base threshold value THb is greater than the absolute value when the correction value CV is minimum. That is, the CPU 101 always sets the threshold value TH to a positive value. In the setting process, the CPU 101 sets the threshold value TH to a smaller value when the cooling water temperature TW is small than when the cooling water temperature TW is large. Specifically, in the setting process, the CPU 101 sets the threshold value TH to a smaller value as the cooling water temperature TW is smaller. Then, the CPU 101 updates the value of the threshold value TH stored in the storage device 104 to the newly set value of the threshold value TH. After that, the CPU 101 ends the series of processes.

[0033] <Regarding the driving state determination process> The CPU 101 executes a determination process for determining whether the operating state of the internal combustion engine 20 is a stable operating state or a transient operating state while the internal combustion engine 20 is running. The CPU 101 repeatedly executes a program for determining the operating state of the internal combustion engine 20 stored in the ROM 103 at a predetermined cycle. As a result, a series of processes shown in Fig. 3 are repeatedly executed. In other words, the determination process is realized by the CPU 101 repeatedly executing the program for determining the operating state stored in the ROM 103 at a predetermined cycle, for example.

[0034] As shown in FIG. 3, when the CPU 101 starts a program for determining the operating state of the internal combustion engine 20, the CPU 101 first executes the process of step S21. In step S21, the CPU 101 calculates an engine load factor KL. Specifically, the CPU 101 calculates an engine rotation speed based on a rotation phase SC of the crankshaft 27. The CPU 101 also calculates the engine load factor KL based on the engine rotation speed and the intake air amount GA. The engine load factor KL is an index value of an air filling rate in the combustion chamber R of the internal combustion engine 20. Specifically, the engine load factor KL is a ratio of an inflow air amount per one combustion cycle of one combustion chamber R to a reference inflow air amount. The reference inflow air amount is variably set according to the engine rotation speed. Therefore, the engine load factor KL becomes a larger value as the intake air amount GA becomes larger.

[0035] The CPU 101 stores the latest engine load factor KL calculated in step S21 as the engine load factor KLn. The CPU 101 also stores the engine load factor KL calculated in the series of processes one cycle before as the engine load factor KLm. After that, the CPU 101 advances the process to step S22.

[0036] In step S22, the CPU 101 calculates the rate of change ΔKL of the engine load ratio KL. The CPU 101 calculates the rate of change ΔKL of the engine load ratio KL by subtracting the engine load ratio KLm from the engine load ratio KLn. The rate of change ΔKL of the engine load ratio KL calculated in this manner becomes larger as the amount of change in the intake air amount GA becomes larger. After that, the CPU 101 advances the process to step S23.

[0037] In step S23, the CPU 101 executes a determination process to determine the operating state of the internal combustion engine 20. The CPU 101 compares the change rate ΔKL of the engine load factor KL, which is an index value indicating the change rate of the load of the internal combustion engine 20, with the threshold value TH set in the setting process described above, to determine whether the operating state of the internal combustion engine 20 is a stable operating state or a transient operating state.

[0038] When the absolute value AV of the rate of change ΔKL of the engine load ratio KL is equal to or greater than the threshold value TH (S23: YES), the CPU 101 determines that the operating state of the internal combustion engine 20 is a transient operating state. That is, when the absolute value of the amount of change in the intake air amount GA is equal to or greater than the value determined by the threshold value TH, the CPU 101 determines that the state of the internal combustion engine 20 is a transient operating state. A transient operating state is a state that is not a stable operating state. Specifically, a stable operating state is a state in which the intake air amount GA changes within a predetermined range. On the other hand, a transient operating state is a state in which the intake air amount GA changes outside the predetermined range.

[0039] When the CPU 101 determines that the operating state of the internal combustion engine 20 is a transient operating state, the CPU 101 advances the process to step S24. In step S24, the CPU 101 updates the transient operation flag F to "1." After that, the series of processes ends.

[0040] On the other hand, when the absolute value AV of the change rate ΔKL of the engine load ratio KL is less than the threshold value TH (S23: NO), the CPU 101 determines that the internal combustion engine 20 is in a stable operating state. In other words, when the change amount of the intake air amount GA is less than the value determined by the threshold value TH, the CPU 101 determines that the internal combustion engine 20 is in a stable operating state.

[0041] When the CPU 101 determines that the internal combustion engine 20 is in a stable operating state, the CPU 101 advances the process to step S25. In step S25, the CPU 101 updates the transient operation flag F to "0." After that, the process ends.

[0042] <Lean burn treatment> The CPU 101 performs filter regeneration control to burn particulate matter trapped in the filter 53. The filter regeneration control includes a temperature increase process and a lean burn process. The temperature increase process is a process for increasing the temperature of the filter 53 to a predetermined specified temperature TS or higher. As the temperature increase process, the CPU 101 first stops the spark ignition by the ignition plug 45 to stop the combustion in the cylinder 24. Then, the CPU 101 performs fuel injection from the in-cylinder injection valve 44. As a result, the mixture containing fuel is caused to flow through the exhaust passage 51 without being burned in the cylinder 24. When the unburned mixture is caused to flow through the exhaust passage 51, the mixture is burned in the catalyst 52. In this fuel injection, an amount of fuel that can be fully reacted in the catalyst 52 is injected so that the injected fuel does not pass through the catalyst 52 and is not discharged downstream.

[0043] In this way, CPU 101 injects fuel to generate heat in catalyst 52. Then, CPU 101 transfers the heat generated in catalyst 52 to the downstream side using the exhaust gas flowing through exhaust passage 51 as a medium. By transferring the heat generated in catalyst 52 to filter 53 in this way, when the temperature of filter 53 reaches or exceeds the ignition point of particulate matter, it becomes possible to burn the particulate matter accumulated on filter 53.

[0044] The lean burn process is a process in which an air-fuel mixture adjusted to an air-fuel ratio leaner than the theoretical air-fuel ratio is burned in each cylinder 24. The lean burn process is executed on the condition that the filter temperature TF is equal to or higher than a specified temperature TS. The specified temperature TS is a temperature equal to or higher than the ignition point of particulate matter.

[0045] When performing the above-mentioned temperature increase process and lean burn process, the CPU 101 executes a program stored in the ROM 103 for performing filter regeneration control for regenerating the filter 53. The CPU 101 executes the program when the PM accumulation amount DA calculated by the above-mentioned accumulation amount calculation process becomes equal to or greater than a predetermined specified amount SA. This causes a series of processes shown in Fig. 4 to be performed. In other words, the temperature increase process and lean burn process are realized by the CPU 101 executing a program stored in the ROM 103 for performing filter regeneration control.

[0046] As shown in FIG. 4, when the CPU 101 executes a program for performing filter regeneration control, it first performs the process of step S31. In step S31, the CPU 101 determines whether the operating state of the internal combustion engine 20 is a stable operating state or a transient operating state. Specifically, the CPU 101 determines whether the transient operation flag F is "1" or "0." If the transient operation flag F is "0", that is, if the operating state of the internal combustion engine 20 is a stable operating state (S31: YES), the CPU 101 advances the process to step S32.

[0047] In step S32, CPU 101 executes the temperature increasing process for a predetermined period of time. Note that the predetermined period of time is a period of time sufficient to raise the temperature of filter 53 to a temperature equal to or higher than the ignition point of particulate matter. After that, CPU 101 advances the process to step S33.

[0048] In step S33, the CPU 101 starts the lean burn process. In this embodiment, the CPU 101 starts the lean burn process after performing the temperature increase process, which satisfies the condition that the temperature of the filter 53 is equal to or higher than the specified temperature TS. Then, the CPU 101 advances the process to step S34.

[0049] In step S34, the CPU 101 determines whether or not a stop condition for the lean burn processing is satisfied. The stop condition for the lean burn processing is that one or more conditions selected from the following conditions are satisfied. One of the stop conditions for the lean burn processing is that the filter temperature TF is lower than a specified temperature TS. Another stop condition for the lean burn processing is that the PM accumulation amount DA is lower than a specified amount SA. Another stop condition for the lean burn processing is that the transient operation flag F is "1", that is, that the operating state of the internal combustion engine 20 is a transient operating state.

[0050] If the stop condition of the lean burn processing is not satisfied (S34: NO), the CPU 101 repeats the process of step S34. Therefore, the CPU 101 continues the execution of the lean burn processing until the stop condition of the lean burn processing is satisfied.

[0051] If the lean burn processing stop condition is satisfied (S34: YES), the CPU 101 proceeds to step S35. In step S35, the CPU 101 stops the execution of the lean burn processing. After that, the CPU 101 ends the current series of processing. Here, while step S34 is being repeatedly executed, it is assumed that the transient operation flag F becomes "1" by the above-mentioned operating state determination processing. In this case, the CPU 101 stops the execution of the lean burn processing even if the PM accumulation amount DA is equal to or greater than the specified amount SA.

[0052] On the other hand, in step S31, when the transient operation flag F is "1", that is, when the operating state of the internal combustion engine 20 is a transient operating state (S31: NO), the CPU 101 ends the current series of processes. That is, in the series of processes, when the operating state of the internal combustion engine 20 is a transient operating state, the CPU 101 does not execute the temperature increase process and the lean combustion process.

[0053] <Operation of the above embodiment> According to the above embodiment, the CPU 101 executes the lean combustion process when the transient operation flag F is "0", i.e., when the internal combustion engine 20 is in a stable operating state. In other words, the CPU 101 does not execute the lean combustion process when the transient operation flag F is "1", i.e., when the internal combustion engine 20 is in a transient operating state.

[0054] Here, the CPU 101 judges the operating state of the internal combustion engine 20 by a judgment process. In the judgment process, the operating state of the internal combustion engine 20 is judged depending on whether or not the absolute value AV of the change rate ΔKL of the engine load factor KL is equal to or greater than a threshold value TH. In the setting process, the threshold value TH is calculated as a smaller value as the cooling water temperature TW is smaller. Therefore, in the judgment process, even if the change rate ΔKL is the same, the smaller the cooling water temperature TW is, the more likely it is that the CPU 101 will judge that the operating state of the internal combustion engine 20 is a transient operating state. As a result, when the cooling water temperature TW is low, the CPU 101 avoids execution of the lean burn process even when the change rate ΔKL of the engine load factor KL is correspondingly small.

[0055] <Effects of the above embodiment> (1) According to the above embodiment, the threshold value TH used in the process of determining whether the internal combustion engine 20 is in a stable operating state or a transient operating state is set smaller when the cooling water temperature TW is low than when the cooling water temperature TW is high. Therefore, in a situation where misfire is likely to occur due to the low cooling water temperature TW, the control device 100 is more likely to determine that the internal combustion engine 20 is not in a stable operating state, i.e., is in a transient operating state. In this way, the control device 100 does not execute the lean combustion process in a situation where misfire is likely to occur, and therefore it is possible to suppress the occurrence of misfire.

[0056] (2) According to the above embodiment, in the determination process, the engine load factor KL is used to determine whether the absolute value of the rate of change of the intake air amount GA is equal to or greater than a threshold value. In general, in the internal combustion engine 20, the intake air amount GA changes with a certain time delay after the engine load factor KL changes. Therefore, by monitoring the change in the intake air amount GA via the engine load factor KL, the change in the intake air amount GA can be detected at an earlier timing. Therefore, by predicting the change in the intake air amount GA, the operating state of the internal combustion engine 20 can be determined at an earlier timing.

[0057] (3) According to the above embodiment, in the filter regeneration control, the CPU 101 does not execute the temperature increase process when the operating state of the internal combustion engine 20 is in a transient operating state. Therefore, in a situation where the lean burn process is not executed, the temperature increase process is not executed unnecessarily.

[0058] (Other embodiments) The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other to the extent that no technical contradiction occurs.

[0059] The method of setting the threshold value TH in the setting process is not limited to the example of the above embodiment. In other words, it is not limited to the method of calculating the threshold value TH by adding the correction value CV to the base threshold value THb. For example, the threshold value TH may be calculated by multiplying the base threshold value THb by a variable corresponding to the coolant temperature TW. Also, for example, the threshold value TH may be calculated directly from the coolant temperature TW without using the base threshold value THb and the correction value CV. In this case, a map of the threshold value TH corresponding to the coolant temperature TW is provided, and the threshold value TH may be calculated based on the map as a smaller value when the coolant temperature TW is low than when the coolant temperature TW is high.

[0060] In the process of determining the operating state of the internal combustion engine 20, the rate of change of the intake air amount GA may be used instead of the rate of change ΔKL of the engine load factor KL. In this case, a value according to the intake air amount GA may be set as the threshold value. Also, in the process of determining the operating state of the internal combustion engine 20, the amount of fluctuation of the engine rotation speed per unit time may be used instead of or in addition to the rate of change ΔKL of the engine load factor KL. In other words, any index value that reflects whether the load of the internal combustion engine 20 is constant within a certain range may be adopted as the index value used in the process of determining the operating state of the internal combustion engine 20.

[0061] In the regeneration control, the CPU 101 may perform the process of step S31 after step S32. That is, the CPU 101 may perform the temperature increase process regardless of the operating state of the internal combustion engine 20. At least, the CPU 101 may execute the lean combustion process on the condition that the operating state of the internal combustion engine 20 is stable.

[0062] In the above embodiment, the CPU 101 assumes that the filter temperature TF is equal to or higher than the specified temperature TS after performing the temperature increase process in the regeneration control. However, the CPU 101 may end the temperature increase process on the condition that the filter temperature TF is equal to or higher than the specified temperature TS, and then perform the lean burn process. In this way, the lean burn process may be performed only when the filter temperature TF is equal to or higher than the specified temperature TS.

[0063] In the above embodiment, the CPU 101 executes the temperature increase process and the lean burn process as a series of processes as the filter regeneration control, but the temperature increase process and the lean burn process may be realized by executing separate programs. For example, the ROM 103 stores a program for performing the temperature increase process and a program for performing the lean burn process. In this case, the CPU 101 may execute a program for performing the temperature increase process, for example, on the condition that the PM accumulation amount DA is equal to or greater than the specified amount SA. Furthermore, the CPU 101 may execute a program for performing the lean burn process, for example, on the condition that the filter temperature TF is equal to or greater than the specified temperature TS and the operating state of the internal combustion engine 20 is a stable operating state. In this case, if the filter temperature TF is already equal to or greater than the specified temperature TS without executing the temperature increase process, the CPU 101 may perform the lean burn process without executing the temperature increase process.

[0064] As the temperature increase process, the CPU 101 may perform, for example, a dither control process. The dither control process is a control process in which some of the cylinders 24 of the internal combustion engine 20 are made richer than the theoretical air-fuel ratio to become rich burn cylinders, and the remaining cylinders 24 are made leaner than the theoretical air-fuel ratio to become lean burn cylinders. When this dither control process is performed, the catalyst 52 promotes the reaction between the unburned fuel components and incompletely burned components in the exhaust discharged from the rich burn cylinders and the oxygen in the exhaust discharged from the lean burn cylinders. As a result, the catalyst 52 is heated. When the catalyst 52 is heated in this manner, the temperature of the exhaust passing through the catalyst 52 increases. Then, the exhaust with the increased temperature flows into the filter 53 located downstream of the catalyst 52, and the temperature of the filter 53 increases. When the dither control process is performed as the temperature increase process, it is desirable that the CPU 101 executes the temperature increase process under one of the conditions that the operating state of the internal combustion engine 20 is stable, as in the above embodiment. This makes it easier to prevent misfires in lean burn cylinders during dither control processing.

[0065] The configuration of the control device 100 is not limited to the example of the above embodiment. The control device 100 may be configured as one or more processors that execute various processes according to a computer program (software). The control device 100 may be configured as one or more dedicated hardware circuits, such as an application specific integrated circuit (ASIC), that execute at least a part of the various processes, or a circuitry including a combination thereof. The processor includes a CPU 101 and memories such as a RAM and a ROM 103. The memory stores program codes or instructions configured to cause the CPU 101 to execute processes. The memory, i.e., computer-readable medium, includes any available medium that can be accessed by a general-purpose or special-purpose computer.

[0066] The configuration of the internal combustion engine 20 is not limited to the example of the above embodiment. The internal combustion engine 20 may include the cylinder 24, the exhaust passage 51, the filter 53, and the coolant temperature sensor 93. For example, the internal combustion engine 20 may include only one cylinder 24. [Explanation of symbols]

[0067] 20…Internal combustion engine 24…cylinder 51...Exhaust passage 53…Filter 93...Coolant temperature sensor 100...Control device AV: Absolute value GA…Intake air volume TH: Threshold TS…Specified temperature TW…Cooling water temperature ΔKL…rate of change

Claims

[Claim 1] A control device applied to an internal combustion engine having a cylinder, an exhaust passage through which exhaust gas from the cylinder flows, a filter that collects particulate matter contained in the exhaust gas, and a cooling water temperature sensor that detects a temperature of cooling water flowing through a water jacket as a cooling water temperature, a determination process for determining that the internal combustion engine is in a stable operating state when an absolute value of an index value indicating a rate of change of a load of the internal combustion engine is less than a threshold value; a setting process of setting the threshold value to be smaller when the cooling water temperature detected by the cooling water temperature sensor is low than when the cooling water temperature is high; a lean combustion process in which an air-fuel mixture adjusted to an air-fuel ratio leaner than a theoretical air-fuel ratio is burned in the cylinder in order to combust the particulate matter trapped in the filter, under the conditions that the temperature of the filter is equal to or higher than a predetermined specified temperature and the internal combustion engine is in a stable operating state; Run A control device for an internal combustion engine.

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