Engine air-fuel ratio control device

Abstract

To achieve both of a purification rate of exhaust air and operability by preventing unnecessary execution of air-fuel ratio oscillation control.SOLUTION: An air-fuel ratio control device of an engine includes: an exhaust sensor constituted to output a signal according to an air-fuel ratio of the exhaust gas; and a controller constituted to acquire the signal output by the exhaust sensor. The controller can switch and execute air-fuel ratio feedback control to control an air-fuel ratio of the exhaust gas flowing into an exhaust emission control catalyst to a stoichiometric equivalent value, and air-fuel ratio oscillation control to oscillate the air-fuel ratio of the exhaust gas flowing into the exhaust emission control catalyst to a rich side and a lean side with respect to the stoichiometric equivalent value. In a case when an operational state of the engine is in a first region determined according to a catalyst temperature and an exhaust flow rate, the air-fuel ratio oscillation control is executed, and in a case when it is in a second region excluding the first region, the air-fuel ratio feedback control is executed.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In an engine equipped with an exhaust purification catalyst in the exhaust passage, it is known that the exhaust purification rate of the catalyst can be improved by forcibly oscillating the air-fuel ratio of the mixture at a predetermined frequency between the rich side and the lean side relative to the stoichiometric air-fuel ratio (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 56-017533 Summary of the Invention [Problem to be solved by the invention]

[0004] However, if the air-fuel ratio oscillation at the above-mentioned predetermined frequency is always performed over the entire operating range of the engine regardless of the purification rate actually obtained by the exhaust purification catalyst, the air-fuel ratio oscillation will continue even if the necessity for it is low from the viewpoint of the purification rate. Therefore, if adjustment for the air-fuel ratio oscillation is performed by increasing or decreasing the amount of fuel supplied to the engine, the continued air-fuel ratio oscillation will have rather significant adverse effects on drivability and ride comfort, such as fluctuations in engine torque and increases in vibration and noise.

[0005] Therefore, an object of the present invention is to provide an engine air-fuel ratio control device that can improve the purification rate of an exhaust purification catalyst while avoiding unnecessary implementation of air-fuel ratio oscillation control and suppressing the adverse effects that air-fuel ratio oscillation control can have on drivability and ride comfort, such as fluctuations in engine torque and increases in vibration noise. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the air-fuel ratio control device for an engine according to the present invention is an air-fuel ratio control device for an engine that has an exhaust purification catalyst in the exhaust passage, and comprises an exhaust sensor that is installed in the exhaust passage and is configured to be able to output a signal corresponding to the air-fuel ratio of the exhaust flowing through the exhaust passage, and a controller that is configured to be able to acquire the signal output by the exhaust sensor. The controller has air-fuel ratio feedback control means that executes air-fuel ratio feedback control to control the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a value equivalent to stoichiometry based on a signal output by the exhaust sensor; air-fuel ratio oscillation control means that executes air-fuel ratio oscillation control to oscillate the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst between the rich side and the lean side with respect to the value equivalent to stoichiometry; operating range determination means that determines whether the operating state of the engine is in a predetermined first range determined according to the catalyst temperature of the exhaust purification catalyst and the exhaust flow rate, or in a second range other than the first range; and air-fuel ratio control selection execution means that executes air-fuel ratio oscillation control by the air-fuel ratio oscillation control means when the operating state of the engine is in the first range, and executes air-fuel ratio feedback control by the air-fuel ratio feedback control means when the operating state of the engine is in the second range. [Effects of the Invention]

[0007] According to the present invention, it is possible to improve the purification rate of the exhaust purification catalyst while avoiding unnecessary implementation of air-fuel ratio oscillation control, thereby suppressing the adverse effects that air-fuel ratio oscillation control may have on drivability and ride comfort, such as fluctuations in engine torque and increases in vibration noise. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram showing the overall configuration of an engine according to an embodiment of the present invention. [Figure 2] 3 is a flowchart showing the overall flow of air-fuel ratio control according to the embodiment; [Figure 3] 4 is an operating region map showing regions in which air-fuel ratio feedback control and air-fuel ratio oscillation control are performed in the air-fuel ratio control according to the embodiment; [Figure 4]3 is a flowchart showing a basic flow of air-fuel ratio oscillation control according to the embodiment; [Figure 5] 3 is a flowchart showing a basic flow of air-fuel ratio feedback control according to the embodiment; [Figure 6] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is highly active and the exhaust flow rate is low. [Figure 7] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is highly active and the exhaust flow rate is high. [Figure 8] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is low in activity and the exhaust flow rate is low. [Figure 9] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is low in activity and the exhaust flow rate is high. [Figure 10] 10 is a graph showing output waveforms of an upstream exhaust sensor and a downstream exhaust sensor in the high frequency range during air-fuel ratio oscillation control. [Figure 11] 10 is a graph showing output waveforms of an upstream exhaust sensor and a downstream exhaust sensor in a low frequency range during air-fuel ratio oscillation control. [Figure 12] 10 is a graph showing the relationship between the inlet gas temperature Tcat_in (catalyst inlet gas temperature) of the exhaust purification catalyst and the purification rate ηthc of total hydrocarbons (THC) for a plurality of frequencies of air-fuel ratio oscillation. [Figure 13] 10 is a graph showing the relationship between the inlet gas temperature Tcat_in (catalyst inlet gas temperature) of the exhaust purification catalyst and the purification rate ηnox of nitrogen oxides (NOx) for a plurality of frequencies of air-fuel ratio oscillation. [Figure 14]10 is a graph showing the relationship between the inlet gas temperature Tcat_in of the exhaust purification catalyst (catalyst inlet gas temperature) and the minimum output value of the downstream exhaust sensor (downstream minimum air-fuel ratio λr_min) for a plurality of frequencies of air-fuel ratio oscillation. [Figure 15] 10 is a graph showing the relationship between the inlet gas temperature Tcat_in of the exhaust purification catalyst (catalyst inlet gas temperature) and the maximum output value of the downstream exhaust sensor (downstream maximum air-fuel ratio λr_max) for a plurality of frequencies of air-fuel ratio oscillation. [Figure 16] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and maximum output value λr_max of the downstream exhaust sensor and the purification rate of total hydrocarbons for each of air-fuel ratio feedback control and air-fuel ratio oscillation control, and also showing the implementation region of air-fuel ratio feedback control. [Figure 17] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and the maximum output value λr_max of the downstream exhaust sensor and the nitrogen oxide purification rate for each of air-fuel ratio feedback control and air-fuel ratio oscillation control, and also showing the implementation region of air-fuel ratio feedback control. [Figure 18] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and maximum output value λr_max of the downstream exhaust sensor and the average purification rates of total hydrocarbons and nitrogen oxides for each of air-fuel ratio feedback control and air-fuel ratio oscillation control, and also showing the implementation region of air-fuel ratio feedback control. [Figure 19] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and maximum output value λr_max of the downstream exhaust sensor and the purification rate of total hydrocarbons, overlaid with respect to different exhaust flow rates. [Figure 20] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and the maximum output value λr_max of the downstream exhaust sensor and the nitrogen oxide purification rate, superimposed for different exhaust flow rates. [Figure 21] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and maximum output value λr_max of the downstream exhaust sensor and the average purification rates of total hydrocarbons and nitrogen oxides, with different exhaust flow rates superimposed. [Figure 22]10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio in air-fuel ratio oscillation control according to another embodiment of the present invention, for different catalyst temperatures (a) Tcat1 and (b) Tcat2 (>Tcat1). [Figure 23] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in the air-fuel ratio oscillation control according to the embodiment, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio, for different catalyst temperatures (a) Tcat3 (>Tcat2) and (b) Tcat4 (>Tcat3). [Figure 24] FIG. 10 is a distribution diagram showing the relationship between the minimum output value λr_min and the maximum output value λr_max of the downstream exhaust sensor and the average purification rates of total hydrocarbons and nitrogen oxides for each of the air-fuel ratio feedback control and the air-fuel ratio oscillation control according to the embodiment, and also showing the implementation region of the air-fuel ratio feedback control. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0010] FIG. 1 is a schematic diagram showing the overall configuration of an internal combustion engine (hereinafter simply referred to as "engine") E according to one embodiment of the present invention.

[0011] In the following description, the terms "upstream" and "downstream" are used in relation to the direction of normal exhaust gas flow emitted from the engine E. For example, the upstream side of the exhaust purification device refers to the upstream side of the exhaust purification device in the direction of exhaust gas flow, and the downstream side of the exhaust purification catalyst refers to the downstream side of the exhaust purification catalyst in the direction of exhaust gas flow.

[0012] In this embodiment, the engine E is mounted on a vehicle and serves as its drive source. An example of a vehicle to which the engine E can be applied is a series hybrid vehicle. However, the engine E can also be applied as a power source for driving a generator in a stationary power generation system.

[0013] The engine E includes an engine body 1 having a combustion chamber, an intake system 2, and an exhaust system 3. In this embodiment, the engine E is an in-line four-cylinder engine, but the type of the engine E, i.e., the number and arrangement of cylinders in the engine E, is not limited to this. Various types of engines can be used, such as single-cylinder, two-cylinder, six-cylinder, V-type, and horizontally opposed type.

[0014] The engine body 1 includes a cylinder block, a cylinder head, and a crankcase. A piston is inserted into the cylinder block, and the space formed between the piston crown surface and the inner surface of the cylinder head serves as a combustion chamber.

[0015] The intake system 2 includes an intake pipe 21 and an intake manifold 22, as well as an air cleaner 23 attached to the inlet of the intake pipe 21. Air from which foreign matter such as dust has been removed via the air cleaner 23 is introduced into the intake pipe 21. The intake pipe 21 is connected to a collecting section of the intake manifold 22, and the intake manifold 22 branches off from the collecting section and is connected to a side section of the cylinder head. The air that flows from the intake pipe 21 into the intake manifold 22 is distributed to each cylinder via the branching sections of the intake manifold 22.

[0016] In this embodiment, a port injection type fuel supply system is employed. The engine E is equipped with a plurality of fuel injectors 41 embedded in the cylinder head, and fuel is injected from each of the plurality of fuel injectors 41 toward the corresponding cylinder. The fuel supply system is not limited to this, and a fuel supply system other than port injection, such as a direct injection system, can also be employed.

[0017] In this embodiment, fuel injected by the fuel injector 41 is mixed with air that has passed through a branching portion of the intake manifold 22 and is introduced into the corresponding cylinder. In each cylinder, the fuel and air continue to mix, forming an air-fuel mixture. This mixture is then ignited by the spark plug 51, causing it to burn.

[0018] The exhaust system 3 includes an exhaust manifold 31 and an exhaust pipe 32, as well as a catalytic converter 33. After combustion, the exhaust gas remaining in the cylinders is discharged to a branching portion of the exhaust manifold 31. The exhaust gas is collected from the branching portion to a collecting portion in the exhaust manifold 31 and introduced into the exhaust pipe 32. The catalytic converter 33 is installed in the exhaust pipe 32, and the exhaust gas flowing through the exhaust pipe 32 is introduced into the catalytic converter 33, where harmful exhaust components including total hydrocarbons (THC) and nitrogen oxides (NOx) are purified by an exhaust purification catalyst 331 housed in the catalytic converter 33, before being released into the atmosphere. In this embodiment, the catalytic converter 33 includes a three-way catalyst as the exhaust purification catalyst 331.

[0019] In addition to the above, the engine E includes an engine controller 101 and various sensors 201-207.

[0020] The engine controller 101 is an electronic control unit configured as a microcomputer including a central processing unit (CPU), storage devices such as ROM and RAM, an input / output interface, and the like.

[0021] The engine E is equipped with an accelerator sensor 201 and an engine rotation speed sensor 202, as well as an air flow meter 203, a coolant temperature sensor 204, a catalyst temperature sensor 205, an upstream air-fuel ratio sensor 206, and a downstream air-fuel ratio sensor 207. Detection signals output from these sensors 201 to 207 are input to the engine controller 101.

[0022] The accelerator sensor 201 detects the amount of depression of the accelerator pedal by the driver as the accelerator opening APO, which is an index of the target load required for the engine E.

[0023] The engine rotation speed sensor 202 detects the rotation speed Ne of the engine E. A crank angle sensor can be used as the engine rotation speed sensor 202, and converts the elapsed time per unit crank angle or reference crank angle detected by the crank angle sensor into the rotation speed Ne.

[0024] The air flow meter 203 detects the flow rate of air introduced into the engine E as the intake air amount Qa. In this embodiment, the intake air amount Qa is referred to as an index of the flow rate of exhaust gas discharged from the combustion chamber. The air flow meter 203 constitutes a "second state sensor" according to this embodiment.

[0025] The coolant temperature sensor 204 detects the temperature Tw of the coolant flowing through a coolant passage formed in the cylinder block of the engine body 1 .

[0026] The catalyst temperature sensor 205 detects the temperature Tcat of the exhaust purification catalyst 331 provided in the catalytic converter 33 (hereinafter referred to as the "catalyst temperature"). In this embodiment, the temperature Tcat_in of the exhaust gas at the inlet of the catalytic converter 33 (hereinafter referred to as the "catalyst inlet gas temperature") is detected as the catalyst temperature Tcat. The catalyst temperature sensor 205 constitutes a "first state sensor" according to this embodiment.

[0027] The upstream air-fuel ratio sensor 206 detects the air-fuel ratio λf of the exhaust gas flowing through the exhaust pipe 32 upstream of the catalytic converter 33, that is, the exhaust gas before flowing into the catalytic converter 33. The upstream air-fuel ratio sensor 206 constitutes a "second exhaust sensor" according to this embodiment.

[0028] The downstream air-fuel ratio sensor 207 detects the air-fuel ratio λr of the exhaust gas flowing in the exhaust pipe 32 downstream of the catalytic converter 33, that is, the exhaust gas after passing through the catalytic converter 33. The downstream exhaust sensor 207 constitutes a "first exhaust sensor" according to this embodiment.

[0029] The engine controller 101 controls the air-fuel ratio of the mixture used for combustion based on detection signals output from the various sensors 201 to 207 described above, while controlling the operating state of the engine E. The engine controller 101 constitutes the "controller" according to this embodiment.

[0030] In controlling the air-fuel ratio, the engine controller 101 switches between air-fuel ratio feedback control and air-fuel ratio oscillation control. The air-fuel ratio feedback control is a control that adjusts the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio based on a signal from an air-fuel ratio sensor, in this embodiment, the upstream air-fuel ratio sensor 206, while the air-fuel ratio oscillation control is a control that forcibly oscillates the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33 between the rich side and the lean side with respect to a value equivalent to stoichiometry. As a result of the air-fuel ratio feedback control, the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33 approaches the value equivalent to stoichiometry. The air-fuel ratio oscillation control is performed by forcibly increasing or decreasing the amount of fuel supplied to the engine E, specifically, the amount of fuel injected by the fuel injector 51.

[0031] Switching between air-fuel ratio feedback control and air-fuel ratio oscillation control depends on the operating range to which the operating state of engine E belongs. In this embodiment, the operating range of engine E is divided into a plurality of regions A and B determined according to the catalyst temperature Tcat and the exhaust flow rate Qexh (FIG. 3), and it is determined whether the operating state is in a first region A where the catalyst temperature Tcat is lower than a predetermined temperature Tcat1 or the exhaust flow rate Qexh exceeds the predetermined flow rate Qexh1, or in a region other than the first region A, that is, a second region B where the catalyst temperature Tcat is equal to or higher than the predetermined temperature Tcat1 and the exhaust flow rate Qexh is equal to or lower than the predetermined flow rate Qexh1. If the operating state of engine E is in the first region A, air-fuel ratio oscillation control is executed, and if it is in the second region B, air-fuel ratio feedback control is executed.

[0032] Here, catalyst temperature Tcat is an indicator of the activity state of exhaust purification catalyst 331, and catalyst temperature Tcat being equal to or higher than a predetermined temperature Tcat1 indicates that activation of exhaust purification catalyst 331 has progressed and that exhaust purification catalyst 331 is in a highly activated state. On the other hand, exhaust flow rate Qexh is an indicator of the amount of harmful exhaust substances that need to be purified by exhaust purification catalyst 331, and exhaust flow rate Qexh being equal to or lower than a predetermined flow rate Qexh1 indicates that the amount of harmful exhaust substances that need to be purified is relatively small and that conditions are in which processing by the highly activated exhaust purification catalyst 331 is easy. In other words, performing air-fuel ratio feedback control in operating range B means performing air-fuel ratio feedback control when activation of exhaust purification catalyst 331 has progressed and it is determined that processing of harmful exhaust components by exhaust purification catalyst 331 is easy.

[0033] FIG. 3 is an operating region map showing regions in which air-fuel ratio feedback control and air-fuel ratio oscillation control are performed. As shown in FIG. 3, when the catalyst temperature Tcat is equal to or higher than a predetermined temperature Tcat1 and the exhaust flow rate Qexh is equal to or lower than a predetermined flow rate Qexh1, the operating state of the engine E belongs to the second region B, the activation of the exhaust purification catalyst 331 progresses, and the conditions are such that processing by the exhaust purification catalyst 331 is easy, and air-fuel ratio feedback control is selected and executed. In other cases, the operating state is considered to be in the first region A, and air-fuel ratio oscillation control is selected and executed. In this embodiment, the catalyst inlet gas temperature Tcat_in is used as the catalyst temperature Tcat, and the intake air amount Qa is used as the exhaust flow rate Qexh. The intake air amount Qa is an example of a state variable that has a high correlation with the exhaust flow rate Qexh.

[0034] FIG. 2 is a flowchart showing the overall flow of air-fuel ratio control according to this embodiment.

[0035] In this embodiment, the air-fuel ratio control according to the routine shown in Fig. 2 is executed by the engine controller 101 at predetermined time intervals after the start of the engine E. When starting the air-fuel ratio control, the engine controller 101 selects the air-fuel ratio oscillation control.

[0036] In S101, the engine rotation speed Ne, the intake air amount Qa, the upstream air-fuel ratio λf, the downstream air-fuel ratio λr, etc. are read as indicators of the operating state of the engine E.

[0037] In S102, it is determined whether the intake air amount Qa exceeds a predetermined flow rate Qa1. If it exceeds the predetermined flow rate Qa1, the process proceeds to S105, and if it is equal to or less than the predetermined flow rate Qa1, the process proceeds to S103. As mentioned above, the intake air amount Qa is an example of a state variable that replaces the exhaust flow rate Qexh, and if the intake air amount Qa exceeds the predetermined flow rate Qa1, it is determined that a large amount of harmful exhaust substances that need to be purified by the exhaust purification catalyst 331 are present and that the conditions are such that processing by the exhaust purification catalyst 331 is not easy, so the process of S105 is executed without going through the processes of S103 and S104.

[0038] In S103, based on the downstream air-fuel ratio λr, the maximum value (hereinafter referred to as "downstream maximum air-fuel ratio") λr_max and the minimum value (hereinafter referred to as "downstream minimum air-fuel ratio") λr_min of the downstream air-fuel ratio λr within a predetermined time are acquired. Here, the predetermined time may be a time equivalent to one cycle of the air-fuel ratio oscillation, or may be a time longer than this. In this embodiment, in order to stably perform the next region determination, this predetermined time is set to a time longer than one cycle of the air-fuel ratio oscillation. The processing of S102 corresponds to the processing executed by the engine controller 101 as the "specific air-fuel ratio detection means" according to this embodiment.

[0039] In S104, it is determined whether the downstream maximum air-fuel ratio λr_max and the downstream minimum air-fuel ratio λr_min are equal to or less than predetermined determination values ​​SL11 and SL12, respectively. If the downstream maximum air-fuel ratio λr_max is equal to or less than the determination value SL11 and the downstream minimum air-fuel ratio λr_min is equal to or less than the determination value SL12, the operating state of the engine E is determined to be in the second region B shown in FIG. 3, and the process proceeds to S106. Otherwise, the operating state of the engine E is determined to be in the first region A, and the process proceeds to S105. The process of S104 will be described in more detail with reference to FIGS. 12 to 21. The processes of S102 to S104 correspond to the processes executed by the engine controller 101 as the "operating region determination means" according to this embodiment.

[0040] In S105, air-fuel ratio oscillation control is executed. The air-fuel ratio oscillation control follows the flowchart shown in Fig. 4. The processing shown in the flowchart in Fig. 4 corresponds to the processing executed by the engine controller 101 as "air-fuel ratio oscillation control means" according to this embodiment.

[0041] In S106, air-fuel ratio feedback control is executed. The air-fuel ratio feedback control follows the flowchart shown in Fig. 5. The process shown in the flowchart in Fig. 5 corresponds to the process executed by the engine controller 101 as the "air-fuel ratio feedback control means" according to this embodiment.

[0042] Furthermore, the processes of S105 and S106 correspond to the processes executed by the engine controller 101 as the "air-fuel ratio control selection and execution means" according to this embodiment.

[0043] FIG. 4 is a flowchart showing the basic flow of the air-fuel ratio oscillation control according to this embodiment.

[0044] In S201, it is determined whether or not setting of the control frequency Fcn has already been completed. The control frequency Fcn is an optimum frequency at which the highest purification rate (hereinafter referred to as "maximum purification rate") η can be obtained by the exhaust purification catalyst 331 when performing air-fuel ratio oscillation control, and in this embodiment, the control frequency Fcn is set for each operating range of the engine E determined according to the rotation speed and load of the engine E. If setting of the control frequency Fcn has already been completed, the process proceeds to S202, and if not yet completed, the process proceeds to S203.

[0045] In S202, the control frequency Fcn is read. In this embodiment, an operating range map is provided in which the control frequency Fcn can be assigned in accordance with the rotation speed and load of the engine E, and the corresponding control frequency Fcn is read from an operating range for which setting has already been completed. After the control frequency Fcn is read, the process proceeds to S210.

[0046] In S203, it is determined whether the value of the flag FRG is 0. If the value of the flag FRG is 0, the process proceeds to S204, and if not, the process proceeds to S206.

[0047] In S204, a reference frequency F0 is set for the frequency Frq of the air-fuel ratio oscillation. The reference frequency F0 is a value that is temporarily set when starting to specify the control frequency Fcn, and is set in advance for each operating range in the operating range map as an initial value for the frequency Frq. Alternatively, the reference frequency F0 can be simply set to 1 [Hz] across the entire operating range of the engine E.

[0048] In S205, the value of the flag FRG is set to 1.

[0049] In S206, the frequency Frq of the air-fuel ratio oscillation is decreased by a predetermined frequency ΔF. Specifically, the frequency Frq is updated to a frequency that is decreased from the current frequency Frq by the predetermined frequency ΔF (Frq=Frq-ΔF), and air-fuel ratio oscillation control is executed using the new updated frequency Frq.

[0050] In S207, the amount of change Δλr_min in the downstream minimum air-fuel ratio λr_min while the frequency Frq is decreased by ΔF (hereinafter referred to as "air-fuel ratio change amount") is calculated based on the downstream minimum air-fuel ratio λr_min.

[0051] In S208, the gradient glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq is calculated by dividing the air-fuel ratio change amount Δλr_min by ΔF. In this embodiment, the gradient glmba is the gradient of the change in the downstream minimum air-fuel ratio λr_min obtained when the frequency Frq is decreased from the reference frequency F0 by a predetermined frequency ΔF. Then, it is determined whether the absolute value of the gradient (=|glmba|) is equal to or greater than a predetermined value g01. If the gradient is equal to or greater than the predetermined value g01, in other words, if the gradient of the change in the downstream minimum air-fuel ratio λr_min when the frequency Frq is decreased increases and reaches the predetermined value g01, the process proceeds to S209. If the gradient is less than the predetermined value g01, the process bypasses the process of S209 and proceeds to S210. The processes of S206 to S208 will be described in more detail with reference to FIGS. 6 to 11.

[0052] In step S209, the control frequency Fcn is set to the frequency Frq when the gradient of the change in the downstream minimum air-fuel ratio λr_min reaches a predetermined value SL21.

[0053] The processing from S207 to S209 corresponds to the processing executed by the engine controller 101 as the "control frequency specifying means" according to this embodiment.

[0054] In S210, a fuel injection amount Qf is calculated. In air-fuel ratio oscillation control, a basic injection amount Qfb of fuel equivalent to the intake air amount Qa is calculated, and the basic injection amount Qfb is multiplied by a correction coefficient α for air-fuel ratio oscillation. Furthermore, various increase correction amounts H according to the coolant temperature Tw and the like are added to calculate the fuel injection amount Qf. Here, the correction coefficient α is set to switch between values ​​greater than 1 and less than 1 every time equivalent to half the cycle of the control frequency Fcn (=1 / (2Fcn)). By multiplying by the correction coefficient α, the air-fuel ratio of the mixture fluctuates or oscillates, for example, between 0.95 and 1.05 in terms of an excess air ratio.

[0055] In step S211, the fuel injector 41 is driven with the fuel injection amount Qf.

[0056] FIG. 5 is a flowchart showing the basic flow of air-fuel ratio feedback control according to this embodiment.

[0057] In S301, a basic injection amount Qfb is calculated. The basic injection amount Qfb is an injection amount corresponding to the amount of fuel equivalent to the intake air amount Qa.

[0058] In S302, an air-fuel ratio feedback correction amount Hqf is calculated. In this embodiment, the air-fuel ratio feedback correction amount Hqf is calculated as a function of the difference between the upstream air-fuel ratio λf detected by the upstream air-fuel ratio sensor 206 and the stoichiometric equivalent value λst (i.e., 1). The air-fuel ratio feedback correction amount Hqf is calculated as an increasing correction amount that increases the amount of fuel when the difference between the upstream air-fuel ratio λf and the stoichiometric equivalent value λst is greater than 0 and the exhaust is in an air-excess state, and as a decreasing correction amount that decreases the amount of fuel when the difference is less than 0 and the exhaust is in a fuel-excess state.

[0059] In S303, the fuel injection amount Qf is set. In the air-fuel ratio feedback control, the fuel injection amount Qf is calculated by adding the air-fuel ratio feedback correction amount Hqf to the basic injection amount Qfb, and also adding various increase correction amounts H according to the coolant temperature Tw, etc.

[0060] In S304, the fuel injector 41 is driven with the fuel injection amount Qf.

[0061] Now, the processes from S206 to S208 in FIG. 4 will be described in more detail with reference to FIGS.

[0062] 6 to 9 are graphs of experimental data measured under a plurality of conditions by changing the catalyst temperature Tcat and the exhaust flow rate Qexh, showing the change in the purification rate η of the exhaust purification catalyst 331 and the downstream minimum air-fuel ratio λr_min relative to the frequency Frq of the air-fuel ratio oscillation in the air-fuel ratio oscillation control. Fig. 6 shows data when the catalyst is highly active and the exhaust flow rate is low, Fig. 7 shows data when the catalyst is highly active and the exhaust flow rate is high, Fig. 8 shows data when the catalyst is low activity and the exhaust flow rate is low, and Fig. 9 shows data when the catalyst is low activity and the exhaust flow rate is high.

[0063] 6 to 9, the horizontal axis represents the logarithm of the frequency Frq, and the vertical axis represents the purification efficiency η and the downstream minimum air-fuel ratio λr_min. log(Frq)=0 corresponds to 1 [Hz]. The open squares represent the total hydrocarbon (THC) purification efficiency ηthc, and the open circles represent the nitrogen oxide (NOx) purification efficiency ηnox. The triangles represent the downstream minimum air-fuel ratio λr_min.

[0064] Here, the downstream minimum air-fuel ratio λr_min measured in the region on the lower frequency side than the frequency at which the purification efficiency η is maximized is indicated by an open triangle, and the downstream minimum air-fuel ratio λr_min measured in the region on the higher frequency side is indicated by a filled triangle. The thick dotted line is an approximation line of the downstream minimum air-fuel ratio λr_min.

[0065] 6 to 9, it can be seen that the downstream minimum air-fuel ratio λr_min changes discontinuously and the gradient of the change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq changes near the frequency at which the purification rate η is maximized. This phenomenon can be more clearly seen by setting the frequency Frq on the horizontal axis to a logarithm log(Frq). In this embodiment, attention is focused on the frequency Frq at which the downstream minimum air-fuel ratio λr_min and its gradient show discontinuous changes, and this is called the "optimum frequency" and set as the control frequency Fcn. When taking the logarithm of the frequency Frq, it is preferable to set the predetermined value g01 to a value in the range from 0.015 to 0.025, which makes it possible to set an appropriate control frequency Fcn.

[0066] 10 and 11 are graphs showing output waveforms of the upstream air-fuel ratio sensor 206 and the downstream air-fuel ratio sensor 207 in air-fuel ratio oscillation control, with Fig. 10 showing these output waveforms in the high frequency range and Fig. 11 showing them in the low frequency range. The "high frequency range" in Fig. 10 is a part of the "range on the higher frequency side than the frequency at which the purification rate η becomes maximum" in Figs. 6 to 9, and the "low frequency range" in Fig. 11 is a part of the "range on the lower frequency side than the frequency at which the purification rate η becomes maximum." In Figs. 10 and 11, thin solid lines indicate the upstream air-fuel ratio λf, and thick solid lines indicate the downstream air-fuel ratio λr. The dotted lines indicate the catalyst inlet gas temperature Tcat_in.

[0067] Referring to Figure 10, it can be seen that in the high-frequency range, the oscillation of the downstream air-fuel ratio λr is significantly damped compared to the upstream air-fuel ratio λf. In contrast, referring to Figure 11, it can be seen that in the low-frequency range, no damping phenomenon can be confirmed in the downstream air-fuel ratio λr, and rather, the oscillation is amplified. This is presumably due to the oxygen storage capacity (OSC) of the exhaust purification catalyst 331. That is, in the high-frequency range, the oxygen storage capacity compensates for the imbalance between fuel and air that accompanies air-fuel ratio oscillation, and the air-fuel ratio of the exhaust after passing through the catalyst 331 (downstream air-fuel ratio λr) is maintained near a value equivalent to stoichiometry, whereas in the low-frequency range, the oxygen storage capacity cannot fully compensate for the imbalance between fuel and air, and fluctuations in the air-fuel ratio in the mixture are directly reflected in changes in the downstream air-fuel ratio λr. As a result, at the boundary between the region where the oxygen storage capacity is effective and the region where the oxygen storage capacity fails, the downstream minimum air-fuel ratio λr_min changes discontinuously, and the slope of the change with respect to the frequency Frq changes to an extent that can be determined by comparison with a threshold value.

[0068] Based on the above findings, the procedure for extracting the optimum frequency, that is, for specifying the control frequency Fcn, will be described below.

[0069] Air-fuel ratio oscillation control is started, and the frequency Frq of the air-fuel ratio oscillation is decreased from the reference frequency F0 in increments of a predetermined frequency ΔF. The downstream minimum air-fuel ratio λr_min is detected, and the gradient of the change in the downstream minimum air-fuel ratio λr_min detected before and after decreasing the frequency ΔF is calculated as the air-fuel ratio change amount Δλr_min. Then, the absolute value (=|Δλr_min|) of the air-fuel ratio change amount Δλr_min is compared with a judgment threshold value SL21, and the frequency Frq at which the absolute value of the air-fuel ratio change amount Δλr_min changes from less than the threshold value SL21 to equal to or greater than the threshold value SL21 is identified, and this is set as the control frequency Fcn.

[0070] As mentioned above, instead of identifying the optimum frequency by comparing the air-fuel ratio change amount Δλr_min (i.e., the slope) with the threshold value SL21, the frequency Frq at which a large change (i.e., difference) that exceeds a predetermined threshold value SL22 occurs in the downstream minimum air-fuel ratio λr_min before and after reducing the frequency ΔF may be identified as the optimum frequency and set as the control frequency Fcn.

[0071] The reference frequency F0 is a value that is initially set when performing air-fuel ratio oscillation, and may be set uniformly for the entire operating range of the engine E, or may be set to a different value for each operating range. When the reference frequency F0 is set uniformly, 1 [Hz] (i.e., logF0=0) can be simply adopted as the reference frequency F0. Furthermore, an operating range map may be set so that the control frequency Fcn can be stored for each operating range, and the identified control frequency Fcn may be stored or updated for each operating range.

[0072] The process of S104 in FIG. 2 will be described in more detail below with reference to FIGS.

[0073] Figures 12 to 15 are graphs of experimental data obtained when the frequency Frq of the air-fuel ratio oscillation was switched between a plurality of different frequencies, and show changes in the purification rate η, downstream minimum air-fuel ratio λr_min and downstream maximum air-fuel ratio λr_max relative to the catalyst inlet gas temperature Tcat_in of the exhaust purification catalyst 331. Figure 12 shows changes in the purification rate ηthc of total hydrocarbons, and Figure 13 shows changes in the purification rate ηnox of nitrogen oxides. Figure 14 shows changes in the downstream minimum air-fuel ratio λr_min, and Figure 15 shows changes in the downstream maximum air-fuel ratio λr_max.

[0074] 12 to 15, open triangles connected by a two-dot chain line represent a case where the frequency Frq is 0.05 [Hz], and open squares connected by a long dotted line represent a case where the frequency Frq is 0.1 [Hz]. Furthermore, open circles connected by a one-dot chain line represent a case where the frequency Frq is 0.2 [Hz], and open triangles connected by a short dotted line represent a case where the frequency Frq is 0.5 [Hz]. Solid black circles connected by a thick solid line represent a case where the frequency Frq is 1 [Hz], which corresponds to the frequency obtained by air-fuel ratio feedback control.

[0075] 12 and 13, it can be seen that, as an overall trend, the purification efficiencies ηthc and ηnox of total hydrocarbons and nitrogen oxides increase as the catalyst inlet gas temperature Tcat_in increases, that is, as the activation of the exhaust purification catalyst 331 progresses. Furthermore, when air-fuel ratio oscillation control is performed at a frequency of 0.5 [Hz] or less, higher purification efficiencies ηthc and ηnox can be obtained particularly in the low temperature range compared to when oscillation is applied at a frequency of 1 [Hz], which corresponds to when air-fuel ratio feedback control is performed, and it can be seen that at 0.5 [Hz] and 0.2 [Hz], higher purification efficiencies ηthc and ηnox can be obtained across the entire temperature range than at 1 [Hz].

[0076] 14, it can be seen that the downstream minimum air-fuel ratio λr_min decreases as the catalyst inlet gas temperature Tcat_in increases. This corresponds to an increase in the purification rate η of the exhaust purification catalyst 331, that is, the degree of progress of activation of the catalyst 331, and is because when the air-fuel ratio fluctuates to the rich side, the exhaust purification catalyst 331 generates hydrogen and the downstream air-fuel ratio sensor 207 reacts to this hydrogen. In this way, there is a correlation between the catalyst inlet gas temperature Tcat_in and the downstream minimum air-fuel ratio λr_min, and it is possible to indirectly grasp the catalyst inlet gas temperature Tcat_in, that is, the activation state of the exhaust purification catalyst 331, from the downstream minimum air-fuel ratio λr_min. In this embodiment, the downstream minimum air-fuel ratio λr_min is used for range determination in air-fuel ratio control (S104 in FIG. 2).

[0077] Here, when the frequency is 0.05 [Hz], as with the other frequencies, the downstream minimum air-fuel ratio λr_min decreases as the catalyst inlet gas temperature Tcat_in increases (Fig. 14), but the nitrogen oxide purification rate ηnox is particularly low compared to the other frequencies. This is because the frequency of 0.05 [Hz] is too low for the oxygen storage capacity of the exhaust purification catalyst 331, and the oxygen storage capacity can no longer absorb the fluctuations in the air-fuel ratio due to the air-fuel ratio oscillation, and the air-fuel ratio in the reaction field of the catalyst 331 becomes close to repeating steady rich and steady lean.

[0078] In this way, it is not possible to determine whether the oxygen storage capacity has collapsed simply based on the fact that the downstream minimum air-fuel ratio λr_min has decreased, and it may be possible to make an erroneous determination that the activation of the exhaust purification catalyst 331 has progressed, even though in reality a sufficient purification rate η is not obtained.

[0079] Such a situation can be avoided by referring to the downstream maximum air-fuel ratio λr_max. As shown in Fig. 15, when the frequency is 0.05 [Hz], the downstream maximum air-fuel ratio λr_max maintains a relatively high value, for example, a value higher than 1, even when the catalyst inlet gas temperature Tcat_in has increased. In this embodiment, the downstream maximum air-fuel ratio λr_max is referenced in addition to the downstream minimum air-fuel ratio λr_min in the range determination (S104 in Fig. 2).

[0080] Figures 16 to 18 are distribution diagrams in which the purification efficiency η of the exhaust purification catalyst 331 is allocated to the downstream side minimum air-fuel ratio λr_min and the downstream side maximum air-fuel ratio λr_max. The purification efficiency η in the case of air-fuel ratio feedback control is shown by plotting with x marks, and the purification efficiency η in the case of air-fuel ratio oscillation control is shown by plotting with ○ marks. For each of the plots with x marks and ○ marks, the higher the concentration, the higher the purification efficiency η. Figure 16 shows the purification efficiency ηthc of total hydrocarbons, Figure 17 shows the purification efficiency ηnox of nitrogen oxides, and Figure 18 shows the average purification efficiency ηave of total hydrocarbons and nitrogen oxides.

[0081] 16 to 18, it can be seen that the purification rate η tends to be higher as one moves toward the lower left of the figure, that is, as the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max become lower. Due to this tendency, within the thick dotted line frame in Figures 16 to 18 where the downstream minimum air-fuel ratio λr_min is equal to or lower than the threshold value SL11 and the downstream maximum air-fuel ratio λr_max is equal to or lower than the threshold value SL12, a high purification rate η can be obtained by both air-fuel ratio feedback control and air-fuel ratio oscillation control. For example, when the threshold value SL11 is set to 0.965 and the threshold value SL12 is set to 1.000, it has been confirmed that a purification rate η of 80% or more can be obtained within the frame for total hydrocarbons, nitrogen oxides, and their averages. In other words, in S104 of Figure 2, by setting the determination value SL11 to, for example, 0.965 and the determination value SL12 to, for example, 1.000, it is possible to ensure a high purification rate η by air-fuel ratio feedback control. The judgment values ​​SL11 and SL12 can be set appropriately within the ranges of 0.960±0.01 and 1.000±0.01, respectively.

[0082] In this way, in the region where the downstream minimum air-fuel ratio λr_min is equal to or less than the threshold value SL11 and the downstream maximum air-fuel ratio λr_max is equal to or less than the threshold value SL12, a high purification rate η can be obtained not only by air-fuel ratio oscillation control but also by air-fuel ratio feedback control, so the air-fuel ratio oscillation control, which is accompanied by fluctuations in engine torque and increases in vibration noise, is not selected, but air-fuel ratio feedback control is selected and executed (S104 and S106 in FIG. 2).

[0083] 19 to 21 are distribution diagrams showing the purification efficiencies η obtained under different exhaust flow rates Qexh superimposed on the purification efficiencies η shown in FIGS. 16 to 18. For example, the SV value when the purification efficiencies η shown in FIGS. 16 to 18 were obtained was 26k. In FIGS. 19 to 21, in addition to the purification efficiencies η obtained when the SV value was 26k, the purification efficiencies η obtained when the SV value was 49k and 98k are superimposed. As in FIG. 16 etc., the purification efficiencies η obtained when air-fuel ratio feedback control is shown by plotting cross marks, and the purification efficiencies η when air-fuel ratio oscillation control is shown by plotting white circles. Similarly, in each of the plots with white circles and white circles, the higher the concentration, the higher the purification efficiencies η. FIG. 19 shows the purification efficiencies ηthc for total hydrocarbons, FIG. 20 shows the purification efficiencies ηnox for nitrogen oxides, and FIG. 21 shows the average purification efficiencies ηave for total hydrocarbons and nitrogen oxides.

[0084] 19 to 21, it can be seen that even under different exhaust flow rates, the purification rate η tends to be higher as one moves to the lower left of the diagram, that is, as the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max become lower. Furthermore, it has been confirmed that the region in which a high purification rate η can be obtained can be defined by the same threshold values ​​SL11 and SL12 as those employed when the SV value is 26k shown in Figures 16 to 18, and that in the region in which the downstream maximum air-fuel ratio λr_min is equal to or lower than threshold value SL11 (for example, 0.965) and the downstream maximum air-fuel ratio λr_max is equal to or lower than threshold value SL12 (for example, 1.000), a high purification rate η can be obtained by both air-fuel ratio feedback control and air-fuel ratio oscillation control.

[0085] In this way, by comparing the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max with the respective threshold values ​​SL11 and SL12, it is possible to determine the region where a high purification rate η can be obtained regardless of the exhaust flow rate Qexh. Therefore, in this embodiment, the determination based on the exhaust flow rate Qexh itself (S103 in FIG. 2) using the intake air amount Qa may be omitted. However, the intake air amount Qa is a state variable that is generally acquired for engine control, not just in this embodiment, and does not require any additional parts to acquire it. Furthermore, it is possible to determine when air-fuel ratio oscillation control should be performed using a simple method using the intake air amount Qa. Therefore, the determination based on the intake air amount Qa is effective in ensuring the stability of control.

[0086] The air-fuel ratio control device for the engine E according to this embodiment has the above-described configuration. The effects obtained by this embodiment will be described below.

[0087] First, while improving the purification rate η of the exhaust purification catalyst 331 by the air-fuel ratio oscillation control, if the operating state of the engine E is in the second region B and the required purification performance of the exhaust purification catalyst 331 can be expected without the air-fuel ratio oscillation control, switching to air-fuel ratio feedback control is performed, and for example, by controlling the air-fuel ratio of the mixture formed in the combustion chamber by controlling the fuel injector 41, etc. to the stoichiometric air-fuel ratio, it is possible to suppress the adverse effects that the air-fuel ratio oscillation control may have on drivability and ride comfort, such as fluctuations in engine torque and increases in vibration noise.

[0088] Secondly, by providing the catalyst temperature sensor 205 and the exhaust flow rate sensor (air flow meter 203 in this embodiment) to directly detect the operating state of the engine E and perform range determination regarding the operating state, it is possible to more appropriately switch between air-fuel ratio feedback control and air-fuel ratio oscillation control, and to more favorably achieve both an improvement in the purification rate η of the exhaust purification device 331 and suppression of engine torque fluctuations, etc.

[0089] On the other hand, by configuring the determination of whether the operating state of engine E is in the second region B (region determination) to be performed in an estimated manner based on the air-fuel ratio λr detected by the downstream air-fuel ratio sensor 207 while air-fuel ratio oscillation control is being executed, it is possible to appropriately switch between air-fuel ratio feedback control and air-fuel ratio oscillation control, specifically, switch from air-fuel ratio oscillation control to air-fuel ratio feedback control, while suppressing an increase in the number of parts, and thereby achieving a simpler configuration.

[0090] Thirdly, it is possible to identify a more appropriate frequency as the frequency of air-fuel ratio oscillation (control frequency Fcn) when performing air-fuel ratio oscillation control, and to execute air-fuel ratio oscillation control under the more appropriate control frequency Fcn, thereby enabling further improvement in the purification rate η of the exhaust purification catalyst 331.

[0091] Fourth, based on the output of the upstream air-fuel ratio sensor 206, that is, the air-fuel ratio λf of the exhaust before it flows into the exhaust purification catalyst 331, it becomes possible to execute air-fuel ratio feedback control with higher responsiveness.

[0092] In the above explanation, when extracting the optimum frequency or specifying the control frequency Fcn, attention is paid to the gradient of change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq or the difference between the downstream minimum air-fuel ratio λr_min before and after the decrease in frequency ΔF. The specification of the control frequency Fcn is not limited to this, and it is also possible to focus on the gradient of change in the difference between the downstream maximum air-fuel ratio λr_max and the downstream minimum air-fuel ratio λr_min (hereinafter referred to as the "air-fuel ratio range") with respect to the frequency Frq.

[0093] 4, the gradient glmbb of the change in the air-fuel ratio range (=λr_max-λr_min) with respect to the frequency Frq or its logarithm log(Frq) is calculated, and further, instead of the process of S208, the frequency when the absolute value of this gradient (=|glmbb|) reaches a predetermined value g02 is set as the optimal frequency and is set as the control frequency Fcn. When taking the logarithm of the frequency Frq, it is preferable to set the predetermined value g02 in the range from 0.015 to 0.025, which makes it possible to set an appropriate control frequency Fcn.

[0094] Figures 22 and 23 are graphs of experimental data showing the relationship between the frequency Frq of air-fuel ratio oscillation in the air-fuel ratio oscillation control according to this embodiment, the purification rate η of the exhaust purification catalyst 331, and the minimum value λr_min of the downstream air-fuel ratio, for different catalyst temperatures Tcat. Figure 22(a) shows the case of a relatively low catalyst temperature Tcat1, Figure 22(b) shows the case of a catalyst temperature Tcat2 that is higher than Tcat1, Figure 23(a) shows the case of a catalyst temperature Tcat3 that is higher than Tcat2, and Figure 23(b) shows the case of a catalyst temperature Tcat4 that is higher than Tcat3.

[0095] 22 and 23, the open squares indicate the total hydrocarbon purification efficiency ηthc, and the open triangles indicate the nitrogen oxide purification efficiency ηnox. The black circles indicate the air-fuel ratio range (=λr_max-λr_min), the thick straight lines are approximation curves of the air-fuel ratio range measured in a region on the higher frequency side than the frequency at which the purification efficiency η is maximized, and the thick dotted lines are approximation curves of the air-fuel ratio range measured in a region on the lower frequency side.

[0096] 22 and 23, it can be seen that the purification rate η increases for both total hydrocarbons and nitrogen oxides as the catalyst temperature Tcat rises, and that a high purification rate η is maintained up to a higher frequency Frq. Here, when the frequency Frq is decreased in increments of a predetermined frequency ΔF, it can be confirmed that, in the vicinity of the frequency at which the purification rate η is maximized, the approximation curve of the air-fuel ratio range (=λr_max-λr_min) shifts from a straight line to a dotted line, and the gradient glmbb of the change with respect to the frequency Frq decreases, in other words, the absolute value of the gradient (=|glmbb|) increases.

[0097] In this embodiment, the frequency at which this phenomenon occurs is extracted as a singular point by comparing it with a predetermined value g02, and set as the control frequency Fcn.

[0098] Setting the control frequency Fcn based on the air-fuel ratio range (=λr_max-λr_min) is not limited to engines in which multiple cylinders are connected in parallel as shown in Fig. 1, but can also be applied to engines in which multiple cylinders are divided into multiple cylinder groups and different cylinder groups are connected in parallel. For example, an engine in which four cylinders are divided into two cylinder groups, different cylinder groups are connected in parallel, and the cylinders that make up the cylinder groups are connected in parallel.

[0099] Setting the control frequency Fcn based on the air-fuel ratio range is suitably applicable to such an engine when the phases of the air-fuel ratio oscillations are set to be opposite to each other between different cylinder groups (hereinafter referred to as "opposite phase setting").

[0100] FIG. 24 corresponds to FIG. 21 and is a distribution diagram showing the relationship between the minimum output value λr_min and maximum output value λr_max of the downstream exhaust sensor and the average purification rates of total hydrocarbons and nitrogen oxides, for each of the air-fuel ratio feedback control according to this embodiment and the air-fuel ratio oscillation control using opposite phase setting.

[0101] As in the previous embodiment, the purification rate η of the exhaust purification catalyst 331 tends to increase toward the lower left of the diagram, that is, as the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max become lower, regardless of whether air-fuel ratio feedback control or air-fuel ratio oscillation control by reverse phase setting is used. Whether or not a high purification rate η is in a region where it can be obtained not only by air-fuel ratio oscillation control but also by air-fuel ratio feedback control can be determined by a threshold value SL21 related to the downstream minimum air-fuel ratio λr_min and a threshold value SL22 related to the downstream maximum air-fuel ratio λr_max, and adoptable threshold values ​​SL21 and SL22 are, for example, 0.970 and 0.970.

[0102] Furthermore, in the above explanation, the catalyst inlet gas temperature Tcat_in is used as the catalyst temperature Tcat of the exhaust purification catalyst 331, but the catalyst temperature Tcat is not limited to this, and it is also possible to obtain it directly by installing a temperature sensor such as a thermocouple in the bed portion of the exhaust purification catalyst 331. In this case, for example, when the catalyst temperature obtained by the temperature sensor is equal to or higher than a predetermined temperature and the intake air amount Qa is equal to or lower than a predetermined flow rate, air-fuel ratio feedback control may be selected and executed. [Explanation of symbols]

[0103] E...engine, 1...engine body, 2...intake system, 21...intake pipe, 22...intake manifold, 23...air cleaner, 3...exhaust system, 31...exhaust manifold, 32...exhaust pipe, 33...catalytic converter, 331...exhaust purification catalyst, 41...fuel injector, 51...spark plug, 101...engine controller, 201...accelerator sensor, 202...engine rotation speed sensor, 203...air flow meter, 204...coolant temperature sensor, 205...catalyst temperature sensor, 206...upstream air-fuel ratio sensor, 207...downstream air-fuel ratio sensor.

Claims

1. An air-fuel ratio control device for an engine equipped with an exhaust purification catalyst in an exhaust passage, an exhaust sensor that is installed in the exhaust passage and is configured to be able to output a signal corresponding to the air-fuel ratio of the exhaust gas flowing through the exhaust passage; a controller configured to acquire a signal output by the exhaust sensor, The controller an air-fuel ratio feedback control means for executing air-fuel ratio feedback control to control the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a value equivalent to stoichiometry based on a signal output by the exhaust sensor; an air-fuel ratio oscillation control means for executing air-fuel ratio oscillation control that oscillates the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst between a rich side and a lean side with respect to the stoichiometric equivalent value; an operating range determination means for determining whether the operating state of the engine is in a predetermined first range determined in accordance with the catalyst temperature of the exhaust purification catalyst and the exhaust flow rate, or in a second range other than the first range; and air-fuel ratio control selection and execution means for executing the air-fuel ratio oscillation control by the air-fuel ratio oscillation control means when the operating state of the engine is in the first region, and for executing the air-fuel ratio feedback control by the air-fuel ratio feedback control means when the operating state of the engine is in the second region.

2. a first state sensor configured to be able to detect an inlet gas temperature of the exhaust purification catalyst or a state variable correlated therewith; a second state sensor configured to detect an exhaust flow rate or a state variable correlated therewith; 2. The engine air-fuel ratio control device according to claim 1, wherein the operating range determination means determines that the operating state of the engine is in the second range when an inlet gas temperature of the exhaust purification catalyst is equal to or higher than a predetermined temperature and an exhaust flow rate is equal to or lower than a predetermined flow rate, and otherwise determines that the operating state of the engine is in the first range.

3. the exhaust sensor includes a first exhaust sensor disposed in the exhaust passage downstream of the exhaust purification catalyst, the controller further includes specific air-fuel ratio detection means for detecting maximum and minimum values ​​of the air-fuel ratio of the exhaust gas in the air-fuel ratio oscillation control based on the air-fuel ratio detected by the first exhaust sensor, 2. The air-fuel ratio control device for an engine according to claim 1, wherein the operating range determination means determines that the operating state of the engine is in the second range when a minimum value of the air-fuel ratio detected by the specific air-fuel ratio detection means is equal to or less than a predetermined first determination value and a maximum value of the detected air-fuel ratio is equal to or less than a predetermined second determination value.

4. the first determination value is a value within a range of 0.96±0.01, 4. The engine air-fuel ratio control device according to claim 3, wherein the second determination value is a value within a range of 1.00±0.

01.

5. the exhaust sensor includes a first exhaust sensor disposed in the exhaust passage downstream of the exhaust purification catalyst, the controller further comprises control frequency specifying means for specifying, based on the air-fuel ratio detected by the first exhaust sensor, as a control frequency, the frequency at which a gradient of change in the minimum value of the air-fuel ratio of the exhaust gas or a difference between the maximum and minimum values ​​of the air-fuel ratio of the exhaust gas in the air-fuel ratio oscillation control, relative to the logarithm of the frequency of air-fuel ratio oscillation, reaches a predetermined value; 5. The engine air-fuel ratio control device according to claim 1, wherein the air-fuel ratio oscillation control means oscillates the air-fuel ratio of the exhaust gas at the control frequency after the control frequency is specified by the control frequency specifying means.

6. 6. The engine air-fuel ratio control device according to claim 5, wherein the control frequency specifying means specifies, as the control frequency, the frequency at which the absolute value of the gradient reaches a preset value in a range from 0.015 to 0.

025.

7. the exhaust sensor includes a second exhaust sensor disposed in the exhaust passage upstream of the exhaust purification catalyst, 7. The engine air-fuel ratio control device according to claim 1, wherein the air-fuel ratio feedback control means executes the air-fuel ratio feedback control based on a signal output by the second exhaust sensor.

Images