Control device for internal combustion engines

Abstract

In internal combustion engines that perform intermittent shutdowns, the system provides more opportunities to update the index value that indicates the degree of variation in the air-fuel ratio between multiple cylinders, which is used in calculating the target air-fuel ratio. [Solution] The control device 90 is capable of performing the following: a first process of calculating an imbalance index value indicating the degree of variation in the air-fuel ratio among multiple cylinders 11; a second process of calculating a target air-fuel ratio by correcting the reference air-fuel ratio based on the imbalance index value; a third process of injecting an instructed injection amount corresponding to the target air-fuel ratio from the fuel injection valve 17 for each cylinder 11; and a fourth process of stopping the operation of the internal combustion engine 10 when the stop condition is met. If the control device 90 finds that a continuation condition indicating an abnormality regarding the variation in the air-fuel ratio among multiple cylinders 11 is met, it will repeat the first, second, and third processes, respectively, without stopping the operation of the internal combustion engine 10 until the termination condition is met, even if the stop condition for the internal combustion engine 10 is met.

Description

Technical Field

[0001] This invention relates to a control device for an internal combustion engine.

Background Art

[0002] The internal combustion engine disclosed in Patent Document 1 includes a plurality of cylinders, a fuel injection valve for each cylinder, an exhaust passage, and an air-fuel ratio sensor. The exhaust passage is connected to each cylinder. The air-fuel ratio sensor is located in the middle of the exhaust passage. The air-fuel ratio sensor detects the air-fuel ratio of the exhaust discharged from each cylinder. A series of periods during which the plurality of cylinders each encounter a combustion stroke once is called a combustion cycle. The control device for the internal combustion engine calculates an imbalance index value indicating the degree of variation in the air-fuel ratio among the plurality of cylinders based on the transition of the detection values of the air-fuel ratio sensor over a predetermined period spanning a plurality of combustion cycles. When the control device calculates the imbalance index value, it stores this imbalance index value in the memory. In each combustion cycle, the control device calculates a target air-fuel ratio based on the latest imbalance index value stored in the memory. Then, the control device calculates the indicated injection amount for the fuel injection valve based on this target air-fuel ratio.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Hybrid vehicles may be equipped with an internal combustion engine and control device as described in Patent Document 1. In hybrid vehicles, the vehicle can be driven by a generator-motor, which is a drive source separate from the internal combustion engine, so there are many opportunities for the internal combustion engine to be intermittently stopped. Suppose that after the internal combustion engine has recovered from an intermittent stop, it encounters a situation where it is intermittently stopped again before a predetermined time has elapsed for calculating the imbalance index value. In this case, the control device will stop the internal combustion engine again without updating the imbalance index value during this operating period. If such short operating periods and intermittent stops are repeated, the control device may continue to be unable to update the imbalance index value. As a result, the imbalance index value stored in memory may gradually deviate from a value that reflects the degree of variation in the air-fuel ratio between cylinders at the current time. Eventually, this deviation may become too large to ignore. If the target air-fuel ratio and, consequently, the instructed injection amount are calculated based on such a deviated imbalance index value, exhaust emissions may worsen. [Means for solving the problem]

[0005] The control device for an internal combustion engine to solve the above problem is applied to an internal combustion engine comprising an engine body having multiple cylinders and a fuel injection valve for each cylinder, and is capable of performing the following: a first process of calculating an imbalance index value indicating the degree of variation in the air-fuel ratio among the multiple cylinders based on the changes in parameters indicating the operating state of the internal combustion engine over a predetermined set period during the operation of the internal combustion engine; a second process of calculating a target air-fuel ratio by correcting a predetermined reference air-fuel ratio based on the latest imbalance index value; a third process of injecting an instructed injection amount corresponding to the latest target air-fuel ratio from each of the fuel injection valves; and a fourth process of stopping the operation of the internal combustion engine when predetermined stop conditions are met. If a predetermined continuation condition is met as a condition indicating an abnormality regarding the variation in the air-fuel ratio among the multiple cylinders, even if the stop condition is met, the first, second, and third processes are repeated without stopping the operation of the internal combustion engine until predetermined termination conditions are met. [Effects of the Invention]

[0006] The above technical concept allows for more opportunities to update the index value indicating the degree of variation in the air-fuel ratio between multiple cylinders, which is related to the calculation of the target air-fuel ratio, in an internal combustion engine that performs intermittent shutdowns. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a diagram showing the schematic configuration of the vehicle. [Figure 2] Figure 2 is a flowchart illustrating the processing steps for stopping the system. [Figure 3] Figure 3 is a flowchart illustrating the processing steps for the flag setting process. [Figure 4] Figure 4 is a diagram illustrating the details of the injection control. [Modes for carrying out the invention]

[0008] <Overall Structure> Hereinafter, one embodiment of the control device for an internal combustion engine will be described with reference to the drawings. As shown in Figure 1, the vehicle 100 comprises an internal combustion engine 10, a first MG71, a second MG72, a planetary gear mechanism 70, a drive shaft 74, and drive wheels 75. Both the first MG71 and the second MG72 are generator-motors. In other words, the vehicle 100 is a hybrid vehicle powered by the internal combustion engine 10, the first MG71, and the second MG72.

[0009] The internal combustion engine 10 inputs torque to the planetary gear mechanism 70. The planetary gear mechanism 70 distributes the torque from the internal combustion engine 10 to the first MG 71 and the drive shaft 74. The first MG 71 generates electricity according to the distributed torque. The drive shaft 74 transmits the distributed torque to the drive wheels 75. The second MG 72 inputs torque to the drive shaft 74. This torque is transmitted to the drive wheels 75. An example of a vehicle with this type of configuration is described in Japanese Patent Application Publication No. 2022-166473.

[0010] The internal combustion engine 10 comprises an engine body 10A, a crankshaft 31, and a crank angle sensor 64. The engine body 10A has four cylinders 11. Each cylinder 11 is a space partitioned within the engine body 10A. In each cylinder 11, a mixture of fuel and intake air is burned by a spark plug 19 for each cylinder 11. The crankshaft 31 rotates in accordance with the combustion of the mixture. The crank angle sensor 64 outputs a signal corresponding to the rotational position of the crankshaft 31. The engine body 10A is equipped with a water jacket 18 through which cooling water flows.

[0011] The internal combustion engine 10 includes an intake passage 15, a throttle valve 16, and a fuel injection valve 17 for each cylinder 11. The intake passage 15 is a passage for introducing intake air into each cylinder 11. The throttle valve 16 adjusts the amount of intake air. The fuel injection valve 17 injects fuel to supply fuel into the cylinder 11.

[0012] The internal combustion engine 10 includes an exhaust passage 21 and a three-way catalytic converter 22. The exhaust passage 21 is a passage through which exhaust gas discharged from each cylinder 11 flows. The exhaust passage 21 includes individual passages 21A for each cylinder 11 and a confluence passage 21B. The individual passages 21A for each cylinder 11 extend from each cylinder 11 and merge into the confluence passage 21B. The three-way catalytic converter 22 is located in the middle of the confluence passage 21B. In other words, the three-way catalytic converter 22 is located downstream of the confluence point of the multiple individual passages 21A in the exhaust passage 21. The three-way catalytic converter 22 purifies hydrocarbons, carbon monoxide, and nitrogen oxides contained in the exhaust gas.

[0013] The internal combustion engine 10 is equipped with an upstream air-fuel ratio sensor 62 and a downstream air-fuel ratio sensor 63. The upstream air-fuel ratio sensor 62 is located upstream of the three-way catalytic converter 22 in the confluence passage 21B. The air-fuel ratio of the exhaust gas flowing between the confluence point of the multiple individual passages 21A in the exhaust passage 21 and the three-way catalytic converter 22 is called the upstream air-fuel ratio AF1. The upstream air-fuel ratio AF1 can also be said to be the air-fuel ratio of the exhaust gas flowing into the three-way catalytic converter 22. The upstream air-fuel ratio sensor 62 outputs a signal relating to a voltage value corresponding to the upstream air-fuel ratio AF1. As the upstream air-fuel ratio AF1 changes from rich to lean, the output voltage of the upstream air-fuel ratio sensor 62 increases linearly in proportion to the upstream air-fuel ratio AF1. The downstream air-fuel ratio sensor 63 is located downstream of the three-way catalytic converter 22 in the confluence passage 21B. In the confluence passage 21B, the exhaust gas flowing downstream of the three-way catalyst 22 is referred to as the downstream air-fuel ratio AF2. The downstream air-fuel ratio sensor 63 outputs a signal relating to a voltage value corresponding to the downstream air-fuel ratio AF2. The downstream air-fuel ratio sensor 63 is an oxygen sensor whose output voltage changes significantly at the stoichiometric air-fuel ratio. An example of the detailed structure of the upstream air-fuel ratio sensor 62 and the downstream air-fuel ratio sensor 63 is disclosed in Patent Document 1.

[0014] Vehicle 100 is equipped with a start switch 67, an accelerator sensor 68, and a vehicle speed sensor 69. The start switch 67 is a switch that the user uses to turn the main system of vehicle 100 on or off. Hereinafter, the period from when the start switch 67 is turned on until it is turned off will be referred to as one trip. The accelerator sensor 68 outputs a signal corresponding to the amount the accelerator pedal is operated in vehicle 100. The vehicle speed sensor 69 outputs a signal corresponding to the driving speed of vehicle 100.

[0015] Vehicle 100 is equipped with a control device 90. The control device 90 is a computer equipped with a processing circuit. The processing circuit includes a CPU 91 and a memory 92. The memory 92 includes three types: RAM, ROM, and electrically rewritable non-volatile memory. In this embodiment, these three types are collectively referred to as memory 92. The memory 92 pre-stores various programs that describe the processing that the CPU 91 should execute. The memory 92 pre-stores various data necessary for the CPU 91 to execute the programs.

[0016] The control device 90 repeatedly acquires signals from various sensors installed in the vehicle 100. The CPU 91 calculates necessary parameters as needed based on the acquired signals. For example, the CPU 91 converts the signal related to the voltage value output by the upstream air-fuel ratio sensor 62 into the upstream air-fuel ratio AF1. The CPU 91 calculates the engine rotation speed, which is the rotational speed of the crankshaft 31, based on the signal from the crank angle sensor 64. For the downstream air-fuel ratio sensor 63, the CPU 91 uses the acquired voltage value directly in each process. The CPU 91 also handles signals from other sensors. Examples of other sensors include the air flow meter and the camshaft angle sensor. The air flow meter outputs a signal corresponding to the intake air volume. The camshaft angle sensor outputs a signal corresponding to the rotational position of the intake camshaft that drives the intake valves. As described in Patent Document 1, the CPU 91 can calculate the crank angle, which is the rotational angle of the crankshaft 31, by combining the signal from the crankshaft angle sensor 64 and the signal from the camshaft angle sensor. The crank angle takes values ​​from 0 to 720 degrees, with reference to a predetermined rotational position on the crankshaft 31.

[0017] The CPU 91 controls the internal combustion engine 10, the first MG 71, and the second MG 71. During one trip, the CPU 91 repeatedly calculates the vehicle required torque necessary for the vehicle 100 to travel based on the operation amount of the accelerator pedal and the traveling speed of the vehicle 100. When the CPU 91 calculates the vehicle required torque, it distributes this vehicle required torque to the internal combustion engine 10, the first MG 71, and the second MG 72. Then, the CPU 91 controls each of the internal combustion engine 10, the first MG 71, and the second MG 72 based on the distributed torque.

[0018] During one trip, the CPU 91 operates or stops the operation of the internal combustion engine 10 according to the torque distributed to the internal combustion engine 10. When the CPU 91 operates the internal combustion engine 10, it performs various controls for burning the air-fuel mixture in each cylinder 11. The various controls include ignition timing control related to the spark plug 19, injection control J related to the fuel injection valve 17, and opening degree control related to the throttle valve 16. Through these various controls, the CPU 91 burns the air-fuel mixture in each cylinder 11 in order. Hereinafter, a series of periods during which each cylinder 11 undergoes a combustion stroke once is called one combustion cycle. One combustion cycle is a period from when the crank angle reaches 0 degrees to 720 degrees.

[0019] As part of the process for controlling the internal combustion engine 10, the CPU 91 can execute stop processing. When a predetermined stop condition is satisfied during the operation of the internal combustion engine 10, the CPU 91 starts the stop processing. For example, the stop condition is that the vehicle required torque is below a predetermined torque. The predetermined torque is predetermined as a value that can satisfy the vehicle state according to the user's request using only the first MG 71 and the second MG 72.

[0020] As shown in FIG. 2, when starting the stop process, the CPU 91 first performs the process of step S110. In step S110, the CPU 91 determines whether the permission flag V is on. The permission flag V is a flag indicating whether the operation of the internal combustion engine 10 can be stopped. That the permission flag V is on means permission to stop the operation of the internal combustion engine 10. That the permission flag V is off means prohibition of stopping the operation of the internal combustion engine 10. The memory 92 stores the current setting content of the permission flag V. If the permission flag V is on (S110: YES), the CPU 91 advances the process to step S120.

[0021] In step S120, the CPU 91 stops the operation of the internal combustion engine 10. Specifically, the CPU 91 stops the execution of various controls for the combustion of the air-fuel mixture. Thus, when the stop condition is satisfied during the operation of the internal combustion engine 10, the CPU 91 stops the operation of the internal combustion engine 10 on the condition that the permission flag V is on. The process of this step S120 is the fourth process. When the CPU 91 executes the process of step S120, it ends the series of processes for the stop process. After the CPU 91 stops the operation of the internal combustion engine 10, when a predetermined start condition is satisfied, it resumes the operation of the internal combustion engine 10. The start condition is, for example, that the vehicle required torque is greater than a predetermined torque.

[0022] On the other hand, in step S110, if the permission flag V is off (S110: NO), the CPU 91 advances the process to step S130. In step S130, the CPU 91 maintains the execution state of various controls related to the operation of the internal combustion engine 10. Then, the CPU 91 quickly ends the process of step S130 and thus the stop process. It can be said that when the determination in step S110 is NO, the CPU 91 quickly ends the stop process without performing any special process. Incidentally, when the determination in step S110 is NO and the stop condition is satisfied at the time when the stop process ends, the CPU 91 performs the process of step S110 again.

[0023] CPU91 is capable of executing flag setting processing. Flag setting processing is the process of switching the permission flag V on or off. CPU91 starts flag setting processing on the condition that the start switch 67 is in the ON state. Note that, in conjunction with the content of flag setting processing, permission flag V is ON when CPU91 starts flag setting processing.

[0024] As shown in Figure 3, when the CPU 91 starts the flag setting process, it first performs the process in step S210. In step S210, the CPU 91 determines whether or not the continuation condition is met. The continuation condition is predetermined as a condition that indicates an abnormality regarding the variation in the air-fuel ratio among multiple cylinders 11. In step S210, the CPU 91 performs the following process: First, the CPU 91 waits until a new imbalance index value X is calculated in the first process P1 described later. Once the calculation of the new imbalance index value X is complete, the CPU 91 refers to the first and second values ​​of the time series of imbalance index value X stored in memory 92. The first value is the latest imbalance index value X. The second value is the imbalance index value X that has been traced back a predetermined number of times from the first value. The predetermined number is, for example, 2. The CPU 91 divides the value obtained by subtracting the second value from the first value by the continuation determination period. The CPU 91 treats the obtained value as the index derivative value. The index derivative is the derivative of the change in the imbalance index value X within the continuation judgment period. The continuation judgment period is predetermined as the elapsed time from when the CPU 91 calculates the second value until when it calculates the first value. The CPU 91 can calculate the length of the continuation judgment period based on the calculation timing of each imbalance index value X stored in memory 92. In conjunction with the execution period of the first process P1 described later, the continuation judgment period changes according to the engine rotation speed. After calculating the index derivative, the CPU 91 calculates the index mean value by referring to the time series of imbalance index values ​​X stored in memory 92. The index mean value is the average of multiple imbalance index values ​​X within the continuation judgment period for which the index derivative value was calculated. After calculating the index derivative and the index mean value, the CPU 91 determines whether the continuation condition is met. The continuation condition is that the index derivative value is greater than or equal to the continuation rate of change, and the index mean value is greater than or equal to the continuation judgment value. The rate of change and the continuation judgment value are predetermined based on experiments and other factors as optimal values ​​for capturing situations where the variation in air-fuel ratio among multiple cylinders 11 is expected to increase.

[0025] If the CPU 91 determines that the continuation condition is not met (S210: NO), it proceeds to step S240. In step S240, the CPU 91 sets the permission flag V to ON. Then, the CPU 91 finishes the series of flag setting processes. If the start switch 67 is ON at this time, the CPU 91 executes the process in step S210 again.

[0026] On the other hand, in step S210, if the continuation condition is met (S210: YES), the CPU 91 proceeds to step S220. In step S220, the CPU 91 sets the permission flag V to off. After this, the CPU 91 proceeds to step S230.

[0027] In step S230, the CPU 91 determines whether the termination condition is met. Specifically, the CPU 91 performs the following processing: The CPU 91 calculates the index derivative value in the same manner as in step S210. The default number used in step S230 may be the same as or different from the default number used in step S210. The default number may be, for example, 1. In step S230, instead of the continuation determination period, the difference between the first value and the second value is divided by the termination determination period. The termination determination period is predetermined as the elapsed time from when the CPU 91 calculates the second value until when it calculates the first value. The CPU 91 can calculate the length of the termination determination period in the same manner as the continuation determination period. In step S230, in addition to the index derivative value, the CPU 91 calculates the adjustment derivative value, which is the derivative value of the change in the adjustment coefficient Y2 within the termination determination period. The adjustment coefficient Y2 will be described later. CPU 91 refers to the latest value of the adjustment coefficient Y2 stored in memory 92, and the value calculated before the termination period for the latest value. Then, CPU 91 calculates the adjustment derivative value by dividing the difference between these two by the termination period. After this, CPU 91 determines whether the termination condition is met. The termination condition is that the absolute value of the index derivative value is less than or equal to the index judgment value, and the absolute value of the adjustment derivative value is less than or equal to the adjustment judgment value. The index judgment value is predetermined based on experiments and other factors as a value that can be considered to indicate that the time variation of the imbalance index value X is sufficiently small. The adjustment judgment value is predetermined from a similar perspective.

[0028] If the CPU 91 determines that the termination condition is not met (S230: NO), it executes the process in step S230 again. The CPU 91 repeats the process in step S230 until the termination condition is met. When the termination condition is met (S230: YES), the CPU 91 proceeds to step S240. The content of the process in step S240 is as previously explained. If the start switch 67 switches from on to off while the CPU 91 is repeating the process in step S230, the CPU 91 proceeds to step S240 at that point.

[0029] <Details of injection control> As shown in Figure 4, the injection control J includes a first process P1, a second process P2, and a third process P3. The contents of each of these processes will be explained in order.

[0030] The CPU 91 repeatedly executes the first process P1 when operating the internal combustion engine 10. Conversely, the CPU 91 repeats the first process P1 while the internal combustion engine 10 is running. The first process P1 is a process that calculates the imbalance index value X, which is information needed for the second process P2. The imbalance index value X is an index value that indicates the degree of variation in the air-fuel ratio among multiple cylinders 11. The CPU 91 continues the first process P1 for a predetermined set period. Then, when the set period ends, the CPU 91 promptly starts the first process P1 of the next cycle. In other words, the CPU 91 performs the first process P1 for each set period. The set period is a series of periods corresponding to a predetermined number of combustion cycles. That is, the set period is not determined by an absolute amount of time, but by the change in the crank angle. And the set period changes according to the engine rotation speed. The number of set cycles is determined based on experiments and other factors as the minimum number of combustion cycles required to obtain an average characteristic of the air-fuel ratio between multiple cylinders 11, after removing noise and other unwanted elements. Depending on the setting of the number of set cycles, the execution period of one first process P1 may be, for example, 10 seconds or more. As a prerequisite for performing the first process P1, the CPU 91 repeatedly acquires the output value of the upstream air-fuel ratio sensor 62, and thus the upstream air-fuel ratio AF1, at predetermined sampling intervals. The sampling interval is determined by absolute time. The maximum engine speed that the internal combustion engine 10 can achieve is called the maximum speed. The sampling interval is sufficiently shorter than one combustion cycle when the engine speed is at the maximum speed. The output value of the upstream air-fuel ratio sensor 62 is an example of a parameter indicating the operating state of the internal combustion engine 10.

[0031] The CPU 91 calculates one imbalance index value X for each first process P1. The method for calculating the imbalance index value X is described below. During the execution of the first process P1, the CPU 91 repeatedly calculates a basic index value. The basic index value is the absolute value of the difference between two consecutive upstream air-fuel ratios AF1 divided by the sampling interval of the output value of the upstream air-fuel ratio sensor 62. The CPU 91 calculates multiple basic index values ​​during one combustion cycle. The CPU 91 divides the cumulative value of these multiple basic index values ​​calculated during one combustion cycle by the number of cumulative steps. The number of cumulative steps is the number of basic index values ​​calculated by the CPU 91 during one combustion cycle. The CPU 91 treats the value obtained by dividing the cumulative value of multiple basic index values ​​by the number of cumulative steps as an intermediate generated value. The CPU 91 calculates these intermediate generated values ​​for each of the set number of combustion cycles. Once the CPU 91 has calculated the set number of intermediate generated values, it divides the cumulative value of these set number of intermediate generated values ​​by the set number. The CPU 91 then stores the obtained value in memory 92 as the latest imbalance index value X. An example of how to calculate the imbalance index value X is described in the section "Acquisition of Air-Fuel Ratio Imbalance Index Value" of Patent Document 1. In this way, in the first process P1, the CPU 91 calculates the imbalance index value X based on the change in the output value of the upstream air-fuel ratio sensor 62 during the set period. As can be seen from the calculation method of the basic index value that forms the basis of the imbalance index value X, the value of the imbalance index value X increases as the degree of variation in the air-fuel ratio among the multiple cylinders 11 increases. Memory 92 stores the change in the imbalance index value X in chronological order along with the calculation time of the imbalance index value X.

[0032] As shown in Figure 4, the CPU 91 repeatedly executes the second process P2 when operating the internal combustion engine 10. Conversely, the CPU 91 repeats the second process P2 while the internal combustion engine 10 is running. The second process P2 is a process for calculating the target air-fuel ratio Y3, which is information required for the third process P3. The CPU 91 performs the second process P2 at predetermined execution intervals, such as one combustion cycle. For example, the CPU 91 promptly performs the second process P2 at the start of each combustion cycle. In a single second process P2, the CPU 91 sequentially performs the correction amount calculation routine P2A, the adjustment coefficient calculation routine P2B, and the target calculation routine P2C.

[0033] In the correction amount calculation routine P2A, CPU91 calculates the rich correction amount Y1. The rich correction amount Y1 is a correction value used in the target calculation routine P2C to reduce the reference air-fuel ratio AFK. In this embodiment, the reference air-fuel ratio AFK is predetermined as the stoichiometric air-fuel ratio. However, the reference air-fuel ratio AFK is not limited to the stoichiometric air-fuel ratio. When calculating the rich correction amount Y1, CPU91 refers to the latest intake air volume, the latest imbalance index value X stored in memory 92, and the correction map stored in memory 92. The correction map represents the relationship between the intake air volume, the imbalance index value X, and the rich correction amount Y1. The rich correction amount Y1 takes a value of zero or greater. The correction map basically has the following characteristics: When viewed with the same intake air volume, the larger the imbalance index value X, the larger the value of the rich correction amount Y1. When viewed with the same imbalance index value X, the larger the intake air volume, the larger the value of the rich correction amount Y1. The CPU 91 applies the latest intake air volume and the latest imbalance index value X to this correction map to calculate a rich correction amount Y1 corresponding to the current engine operating state. The CPU 91 then stores the obtained value in memory 92 as the latest rich correction amount Y1. After calculating the rich correction amount Y1, the CPU 91 terminates the correction amount calculation routine P2A. The CPU 91 may also treat the value obtained by multiplying the rich correction amount Y1 calculated based on the correction map by an appropriate correction coefficient as the final rich correction amount Y1. An example of a method for calculating the rich correction amount Y1 is described in the section "Determination of Imbalance Rich Correction Amount" in Patent Document 1.

[0034] In the adjustment coefficient calculation routine P2B, the CPU 91 calculates the adjustment coefficient Y2. The adjustment coefficient Y2 is an example of an air-fuel ratio adjustment value. The adjustment coefficient Y2 is a coefficient used to adjust the degree to which the reference air-fuel ratio AFK is reduced in the target calculation routine P2C. In calculating the adjustment coefficient Y2, the CPU 91 compares the latest output value of the downstream air-fuel ratio sensor 63 with a reference value. The reference value is predetermined as the value output by the downstream air-fuel ratio sensor 63 when the air-fuel ratio detected by the downstream air-fuel ratio sensor 63 is the reference air-fuel ratio AFK. In other words, the reference value in this embodiment is a value corresponding to the stoichiometric air-fuel ratio. If the output value of the downstream air-fuel ratio sensor 63 is greater than the reference value, the CPU 91 subtracts a positive first predetermined value from the previous value of the adjustment coefficient Y2 stored in memory 92. Then, the CPU 91 stores the obtained value in memory 92 as a new adjustment coefficient. On the other hand, if the output value of the downstream air-fuel ratio sensor 63 is smaller than the reference value, the CPU 91 adds a first predetermined value to the previous value of the adjustment coefficient Y2 stored in memory 92. The CPU 91 then stores the obtained value in memory 92 as the new adjustment coefficient Y2. If the output value of the downstream air-fuel ratio sensor 63 and the reference value are the same, the CPU 91 stores the previous value of the adjustment coefficient Y2 stored in memory 92 as the new adjustment coefficient Y2. In this way, the CPU 91 updates the adjustment coefficient Y2 based on the comparison result between the output value of the downstream air-fuel ratio sensor 63 and the reference value. Memory 92 stores the changes in the adjustment coefficient Y2 in chronological order along with the calculation time of the adjustment coefficient Y2. For example, the air-fuel ratio adjustment value has upper and lower limits set so that it is a positive value of 1 or less. When the CPU 91 calculates a new air-fuel ratio adjustment value, it terminates the adjustment coefficient calculation routine P2B. The adjustment coefficient Y2 corresponds to the "fourth reflection rate" in Patent Document 1. An example of a method for calculating the adjustment coefficient Y2 is described in the section "Calculation of correction amount for imbalance-rich correction amount" in Patent Document 1.

[0035] In the target calculation routine P2C, CPU91 calculates the target air-fuel ratio Y3. CPU91 calculates the target air-fuel ratio Y3 based on the latest rich correction amount Y1, the latest adjustment coefficient Y2, and the reference air-fuel ratio AFK. In calculating the target air-fuel ratio Y3, CPU91 first multiplies the rich correction amount Y1 by the adjustment coefficient Y2. Then, CPU91 treats the obtained value as the adjusted correction amount. In relation to the definitions of the rich correction amount Y1 and the adjustment coefficient Y2, the adjusted correction amount is a positive value. After calculating the adjusted correction amount, CPU91 subtracts the adjusted correction amount from the reference air-fuel ratio AFK. Furthermore, CPU91 subtracts the subfeedback correction amount and the starting correction amount from the obtained value. Then, CPU91 stores the obtained value as the latest target air-fuel ratio Y3 in memory 92. Finally, CPU91 terminates the target calculation routine P2C. The subfeedback correction amount is the sum of the outputs of the proportional, integral, and differential elements, which are inputs obtained by subtracting the output value of the downstream air-fuel ratio sensor 63 from the reference value. The starting correction amount is set based on the temperature of the cooling water in the engine body 10A at engine startup. The CPU 91 calculates these subfeedback correction amounts and starting correction amounts in a separate processing routine as part of the second processing P2. An example of a method for calculating the target air-fuel ratio Y3 is described in the "Determination of Target Air-Fuel Ratio" section of Patent Document 1. An example of a method for calculating the subfeedback correction amount is described in the "Calculation of Subfeedback Amount" section of Patent Document 1. An example of a method for calculating the starting correction amount is described in the "Determination of Starting Correction Amount" section of Patent Document 1.

[0036] The following can be said about the second process P2. As mentioned above, the rich correction amount Y1 and thus the adjusted correction amount are positive values. Therefore, calculating the adjusted correction amount from the reference air-fuel ratio AFK in the target calculation routine P2C is equivalent to reducing the reference air-fuel ratio AFK. In other words, when the CPU 91 calculates the target air-fuel ratio Y3, it reduces the reference air-fuel ratio AFK using the rich correction amount Y1. As mentioned above, in the correction map, the rich correction amount Y1 takes a larger value as the imbalance index value X increases. In other words, the CPU 91 calculates the target air-fuel ratio Y3 such that the degree to which the reference air-fuel ratio AFK is reduced increases as the imbalance index value X increases. Furthermore, when the CPU 91 reduces the reference air-fuel ratio AFK, it uses the value obtained by multiplying the rich correction amount Y1 by the adjustment coefficient Y2. In other words, the CPU 91 adjusts the degree to which the reference air-fuel ratio AFK is reduced using the adjustment coefficient Y2. Then, CPU91 calculates the target air-fuel ratio Y3 by changing the degree to which it reduces the reference air-fuel ratio AFK according to the latest adjustment coefficient Y2.

[0037] As shown in Figure 4, the CPU 91 repeatedly executes the third process P3 when operating the internal combustion engine 10. Conversely, the CPU 91 repeats the third process P3 while the internal combustion engine 10 is running. The CPU 91 performs the third process P3 for each combustion cycle.

[0038] In a single third processing step P3, the CPU 91 performs the following injection processing routine once for each cylinder 11. Any of the four cylinders 11 will be referred to as the target cylinder. The contents of the injection processing routine will be explained below using this target cylinder as an example. When the target cylinder reaches a predetermined crank angle before the intake top dead center, the CPU 91 starts the injection processing routine targeting the target cylinder. When the CPU 91 starts the injection processing routine, it first calculates the current amount of intake air in the target cylinder. The amount of intake air in the cylinder is the amount of intake air that fills one cylinder 11. The CPU 91 calculates the amount of intake air in the cylinder based on the latest intake air amount and the latest engine rotation speed. After calculating the amount of intake air in the cylinder, the CPU 91 refers to the latest target air-fuel ratio Y3 stored in memory 92. Then, the CPU 91 divides the amount of intake air in the cylinder by the target air-fuel ratio Y3. The CPU 91 then treats the obtained value as the basic injection amount. The CPU 91 calculates the basic injection amount and then applies feedback correction to this basic injection amount. Specifically, the CPU 91 multiplies the basic injection amount by the main feedback coefficient, which is calculated in a processing routine separate from the injection processing routine, and its learned value, the main learned value. The CPU 91 then treats the resulting value as the instructed injection amount. The main feedback coefficient and the main learned value are correction values ​​used to feedback-correct the fuel injection amount from the fuel injector 17 so that the upstream air-fuel ratio AF1 matches the target air-fuel ratio Y3. By multiplying the basic injection amount by these correction values, any excess or deficiency in the amount of fuel supplied to the cylinder 11 in order to match the upstream air-fuel ratio AF1 to the target air-fuel ratio Y3 is compensated for. An example of how to calculate the main feedback coefficient and the main learned value is described in the "Main Feedback Control" section of Patent Document 1. An example of how to calculate the instructed injection amount is described in the "Fuel Injection Amount Control" section of Patent Document 1. Once the CPU 91 calculates the instructed injection amount, it injects this instructed injection amount into the fuel injector 17 at a predetermined injection timing crank angle. The CPU 91 terminates the injection process routine once fuel injection by the fuel injector 17 is complete. The CPU 91 performs this injection process routine for each cylinder 11 within one combustion cycle.In other words, in the third process P3, the CPU 91 injects a specified amount of fuel from each fuel injector 17 within one combustion cycle, according to the latest target air-fuel ratio Y3. In this embodiment, the specified injection amount for each fuel injector 17 is the same. As a result of the CPU 91 repeating the third process P3, fuel is supplied to each cylinder 11 with each combustion cycle.

[0039] <Effects and Effects of the Embodiment> (1) If the conditions for continuing operation of the internal combustion engine 10 are met (S210: YES), the CPU 91 turns off the permission flag V (S220). If the conditions for stopping the internal combustion engine 10 are met under these circumstances, the CPU 91 continues to operate the internal combustion engine 10 without stopping its operation until the termination conditions are met (S130). The CPU 91 then repeats the first process P1, the second process P2, and the third process P3. With this configuration, when there is a sign that the variation in air-fuel ratio among multiple cylinders 11 is increasing, the CPU 91 updates the imbalance index value X without delay, even in situations where the internal combustion engine 10 should be intermittently stopped. Therefore, the imbalance index value X always reflects the current variation in air-fuel ratio among the cylinders 11. As a result, the CPU 91 can always set the optimal target air-fuel ratio Y3 from the perspective of suppressing exhaust emissions.

[0040] (2) If the time average value of the imbalance index value X is large to a certain extent, and the time rate of change of the imbalance index value X is also large to a certain extent, there is a high possibility that the variation in air-fuel ratios among the multiple cylinders 11 will increase thereafter. Therefore, adopting the continuation condition of this embodiment is suitable for detecting signs that the variation in air-fuel ratios among the cylinders 11 will increase.

[0041] (3) The average value of the actual air-fuel ratios in multiple cylinders 11 is referred to as the true mean air-fuel ratio. As described in Patent Document 1, in conjunction with the structure of the upstream air-fuel ratio sensor 62, the upstream air-fuel ratio sensor 62 may show a richer value than the true mean air-fuel ratio when the variation in air-fuel ratios among multiple cylinders 11 becomes large. On the other hand, when the CPU 91 performs feedback correction so that the upstream air-fuel ratio AF1 matches the target air-fuel ratio Y3, it corrects the fuel injection amount to eliminate the richer air-fuel ratio shown by the upstream air-fuel ratio sensor 62. As a result, the true mean air-fuel ratio may become leaner than the reference air-fuel ratio AFK. This leaner state is referred to as lean miscorrection. To compensate for this lean miscorrection, the CPU 91 corrects the reference air-fuel ratio AFK based on the imbalance index value X to set the target air-fuel ratio Y3. If the update of the imbalance index value X is delayed due to the intermittent stopping of the internal combustion engine 10, the situation in which such lean miscorrection cannot be compensated for will continue. As described in (1), the CPU 91 of this embodiment updates the imbalance index value X without delaying its update. Therefore, the CPU 91 can avoid lean miscorrection even in situations in which the internal combustion engine 10 is repeatedly stopped intermittently.

[0042] (4) As described in Patent Document 1, the difference between the output value of the downstream air-fuel ratio sensor 63 and the reference value reflects the degree of excess or deficiency of the reduction correction related to the calculation of the target air-fuel ratio Y3. Therefore, as in the CPU 91 of this embodiment, by adjusting the degree of reduction correction related to the target air-fuel ratio Y3 with the adjustment coefficient Y2, the target air-fuel ratio Y3 corresponding to the imbalance index value X can be set to the optimal value for suppressing exhaust emissions.

[0043] Suppose a foreign object clogs one fuel injector 17. Then, the amount of fuel supplied by this fuel injector 17 to cylinder 11 will be less than the instructed injection amount, and therefore less than the amount of fuel supplied to the other cylinders 11. For example, suppose that this situation causes the variation in air-fuel ratio among multiple cylinders 11 to increase over time. After this, as the degree of this variation gradually stabilizes, the fluctuation of the imbalance index value X gradually decreases. Furthermore, as the adjustment coefficient Y2 converges to an optimal value, the output value of the downstream air-fuel ratio sensor 63 approaches the reference value. Through this series of processes, it takes a certain amount of time for the output value of the downstream air-fuel ratio sensor 63 to nearly match the reference value. If intermittent shutdowns and short operating periods are repeated for the internal combustion engine 10, it may remain impossible to converge the adjustment coefficient Y2 to the value necessary to bring the output value of the downstream air-fuel ratio sensor 63 closer to the reference value. In this respect, the CPU 91 of this embodiment updates the adjustment coefficient Y2 without delaying the update of the adjustment coefficient Y2 along with the imbalance index value X. Therefore, the CPU 91 can quickly set the adjustment coefficient Y2 to the optimal value even under conditions in which the internal combustion engine 10 is repeatedly stopped intermittently.

[0044] <Example of changes> The above embodiment can be implemented with the following modifications. The above embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0045] The unbalance index value X and its calculation method are not limited to the examples of the above embodiment. The unbalance index value X only needs to indicate the degree of variation in the air-fuel ratio among the multiple cylinders 11. When calculating the unbalance index value X, parameters that indicate engine operating conditions other than the upstream air-fuel ratio AF1, such as engine rotational speed, may be used.

[0046] The content of the second process P2 is not limited to the examples of the above embodiment. The second process P2 may be any process that calculates the target air-fuel ratio Y3 by correcting the reference air-fuel ratio AFK based on the imbalance index value X. For example, the reference air-fuel ratio AFK may be increased. When calculating the target air-fuel ratio Y3, one or more of the rich correction amount Y1, adjustment coefficient Y2, subfeedback correction amount, and starting correction amount may be eliminated. The calculation of parameters not used for correction may be omitted.

[0047] The content of the third process P3 is not limited to the example of the above embodiment. The third process P3 only needs to involve injecting a specified injection amount from each fuel injector 17 according to the target air-fuel ratio Y3. When calculating the specified injection amount, it is not essential to apply feedback correction so that the upstream air-fuel ratio AF1 matches the target air-fuel ratio Y3. The specified injection amount may be changed for each cylinder 11.

[0048] The continuation condition is not limited to the examples of the embodiments described above. The continuation condition only needs to indicate an increase in the variation in the air-fuel ratio among the multiple cylinders 11. The continuation condition may also be defined by a parameter other than the imbalance index value X.

[0049] The termination conditions are not limited to the examples of the embodiments described above. The termination conditions only need to be such that the imbalance index value X is updated to some extent. The subfeedback correction amount may be treated as the air-fuel ratio adjustment value. Furthermore, the derivative value of the subfeedback correction amount may be compared with a dedicated adjustment judgment value as the termination condition. The termination conditions may be determined by either the imbalance index value X or the air-fuel ratio adjustment value alone, or by parameters other than these.

[0050] The overall configuration of the internal combustion engine 10 is not limited to the examples of the above embodiments. The internal combustion engine 10 may have multiple cylinders 11 and a fuel injection valve 17 for each cylinder 11. The number of cylinders 11 is not limited to four. The upstream air-fuel ratio sensor 62 and the downstream air-fuel ratio sensor 63 may be of the same type.

[0051] The overall configuration of the vehicle 100 is not limited to the examples of the above embodiments. One or more generator motors may be eliminated from the vehicle 100. Applying the control device 90 to the vehicle 100, which performs automatic stopping and automatic restarting of the internal combustion engine 10, is suitable for updating the imbalance index value X. [Explanation of symbols]

[0052] 10...Internal combustion engine 10A...Engine body 11...Cylinder 17...Fuel injector 90...Control device

Claims

1. This invention is applied to an internal combustion engine comprising an engine body having multiple cylinders and a fuel injection valve for each of the cylinders. During the operation of the aforementioned internal combustion engine, A first process that calculates an imbalance index value indicating the degree of variation in the air-fuel ratio between multiple cylinders, based on the changes in parameters indicating the operating state of the internal combustion engine during a predetermined set period, A second process involves calculating a target air-fuel ratio by correcting a predetermined reference air-fuel ratio based on the latest imbalance index value, A third process in which the instructed injection amount corresponding to the latest target air-fuel ratio is injected from each of the fuel injection valves, A fourth process is performed, which stops the operation of the internal combustion engine when predetermined stopping conditions are met. If a predetermined continuation condition is met, which indicates an abnormality in the air-fuel ratio variation between multiple cylinders, even if the stop condition is met, the first, second, and third processes will be repeated without stopping the operation of the internal combustion engine until the predetermined termination condition is met. Control device for internal combustion engines.

2. The continuation condition is that the differential value of the trend of the imbalance index value within a predetermined continuation judgment period is greater than or equal to a predetermined continuation rate of change, and the average value of the imbalance index value within the continuation judgment period is greater than or equal to a predetermined continuation judgment value. A control device for an internal combustion engine according to claim 1.

3. The internal combustion engine comprises a three-way catalytic converter located downstream of the confluence point of the individual passages extending from each of the cylinders in the exhaust passage through which the exhaust gases from the multiple cylinders flow, and an upstream air-fuel ratio sensor that outputs a value corresponding to the air-fuel ratio of the exhaust gas flowing between the confluence point and the three-way catalytic converter in the exhaust passage. In the first process, the imbalance index value is calculated based on the change in the output value of the upstream air-fuel ratio sensor, such that the value increases as the degree of variation in the air-fuel ratio among the multiple cylinders increases. In the second process, the target air-fuel ratio is calculated such that the larger the imbalance index value, the greater the degree to which the reference air-fuel ratio is reduced. In the third process, the instructed injection amount is calculated by applying feedback correction so that the air-fuel ratio of the exhaust gas flowing into the three-way catalyst matches the target air-fuel ratio. The termination condition is that the absolute value of the derivative of the trend of the imbalance index value within a predetermined termination period is less than or equal to a predetermined index determination value. A control device for an internal combustion engine according to claim 1 or 2.

4. The internal combustion engine is equipped with a downstream air-fuel ratio sensor that outputs a value corresponding to the air-fuel ratio of the exhaust gas flowing downstream of the three-way catalyst in the exhaust passage. In the second process, an air-fuel ratio adjustment value is calculated based on the comparison result between the output value of the downstream air-fuel ratio sensor and a predetermined reference value, which adjusts the degree to which the reference air-fuel ratio is reduced. The target air-fuel ratio is then calculated by changing the degree to which the reference air-fuel ratio is reduced according to the calculated air-fuel ratio adjustment value. The termination conditions are that, in addition to the absolute value of the differential of the trend of the imbalance index value during the termination determination period being less than or equal to the index determination value, the absolute value of the differential of the trend of the air-fuel ratio adjustment value during the termination determination period being less than or equal to a predetermined adjustment determination value. The control device for an internal combustion engine according to claim 3.

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