Engine air-fuel ratio control device

Abstract

To keep a high purification rate by air-fuel ratio oscillation over a wider range of an operation region of an engine.SOLUTION: An air-fuel ratio control device of an engine includes: a downstream-side air-fuel ratio sensor disposed on an exhaust passage at a downstream side with respect to an exhaust emission control catalyst; and a controller constituted to acquire an air-fuel ratio detected by the downstream-side air fuel ratio sensor. The controller executes air-fuel ratio oscillation control to oscillate an air-fuel ratio of exhaust gas flowing into the exhaust emission control catalyst to a rich side and a lean side with respect to a stoichiometric equivalent value. A downstream-side maximum air-fuel ratio and a downstream-side minimum air-fuel ratio in the air-fuel ratio oscillation formed by air-fuel ratio oscillation control are detected, on the other hand, the frequency of the air-fuel ratio oscillation is reduced from a prescribed value to specify the frequency at a time when inclination of change of difference between the downstream-side maximum air-fuel ratio and the downstream-side minimum air-fuel ratio to the frequency of air-fuel ratio oscillation reaches a prescribed value, as a control frequency. After the specification of the control frequency, the air-fuel ratio of an exhaust gas is oscillated with the control frequency.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, the optimum frequency for purifying exhaust gases by air-fuel ratio oscillation is not necessarily constant regardless of the engine's operating state, and if the oscillation were to be made to occur at the same frequency across the entire operating range, it is possible that the current frequency will deviate from the optimum frequency as the engine's operating state changes, making it impossible to obtain sufficient purification performance. The optimum frequency for air-fuel ratio oscillation may change depending on the exhaust flow rate, which is a variable that represents the engine's operating state, and also changes depending on the temperature of the exhaust purification catalyst, i.e., the activation state of the catalyst, in addition to the operating state of the engine itself.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an air-fuel ratio control device for an engine that can maintain a high purification rate through air-fuel ratio oscillation over a wider range of the engine operating range. [Means for solving the problem]

[0006] In order to achieve the above object, an air-fuel ratio control device for an engine according to the present invention is an air-fuel ratio control device for an engine having an exhaust purification catalyst in an exhaust passage, and includes: a downstream air-fuel ratio sensor installed in the exhaust passage downstream of the exhaust purification catalyst and detecting the air-fuel ratio of exhaust gas that has passed through the exhaust purification catalyst; and a controller configured to be able to acquire the air-fuel ratio detected by the downstream air-fuel ratio sensor. The controller includes air-fuel ratio oscillation control means for executing 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 of a value equivalent to stoichiometry, specific air-fuel ratio detection means for detecting a maximum air-fuel ratio and a minimum air-fuel ratio in the air-fuel ratio oscillation formed by the air-fuel ratio oscillation control based on the air-fuel ratio detected by the downstream air-fuel ratio sensor, oscillation frequency reduction means for reducing the frequency of the air-fuel ratio oscillation from a predetermined value, and control frequency identification means for identifying as a control frequency the frequency at which a slope of change that a difference between the maximum air-fuel ratio and the minimum air-fuel ratio detected by the specific air-fuel ratio detection means makes with respect to the frequency of the air-fuel ratio oscillation reaches a predetermined value. 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. [Effects of the Invention]

[0007] According to the present invention, it is possible to maintain a high purification rate due to air-fuel ratio oscillation over a wider range of the operating region, and further, by specifying the control frequency based on the gradient of change that the difference between the maximum and minimum values ​​of the downstream air-fuel ratio in the air-fuel ratio oscillation makes with respect to the frequency of the air-fuel ratio oscillation, it is possible to maintain a high purification rate with a simple configuration without adding any parts such as a dedicated sensor. [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] 6 is a graph showing waveforms of (a) original phase, (b) opposite phase, and (c) combined opposite phase of air-fuel ratio oscillation according to the embodiment of the present invention; [Figure 3] 4 is an explanatory diagram illustrating a schematic diagram of antiphase synthesis of air-fuel ratio oscillation according to the embodiment of the present invention; FIG. [Figure 4] 10 is a graph of experimental data showing the relationship between the frequency Frq and the purification rate η of the exhaust purification catalyst and the downstream minimum air-fuel ratio λr_min in the case of air-fuel ratio oscillation in the same phase according to the embodiment of the same invention. [Figure 5] 10 is a graph of experimental data showing the relationship between the frequency Frq and the purification rate η of the exhaust purification catalyst and the downstream minimum air-fuel ratio λr_min in the case of air-fuel ratio oscillation in antiphase synthesis according to the embodiment of the same. [Figure 6] 3 is a flowchart showing a basic flow of air-fuel ratio oscillation control according to the embodiment; [Figure 7] 4 is a flowchart showing the contents of a process for setting a control frequency Fcn in the air-fuel ratio oscillation control. [Figure 8] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in antiphase synthesis, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb 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 antiphase synthesis, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is highly active and the exhaust flow rate is low. [Figure 10] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in antiphase synthesis, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is low in activity and the exhaust flow rate is high. [Figure 11] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in antiphase synthesis, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is highly active and the exhaust flow rate is high. [Figure 12] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in the same phase, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is low in activity and the exhaust flow rate is low. [Figure 13] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in the same phase, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is highly active and the exhaust flow rate is low. [Figure 14]10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in the same phase, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is low in activity and the exhaust flow rate is high. [Figure 15] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in the same phase, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb when the catalyst is highly active and the exhaust flow rate is high. 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, engine E has a plurality of cylinders, and the plurality of cylinders are divided into a plurality of cylinder groups, and different cylinder groups are connected in parallel, and the cylinders constituting each cylinder group are connected in parallel. Specifically, engine E is an in-line four-cylinder engine, and four cylinders #1 to #4 are divided into two cylinder groups (in FIG. 1, the cylinders from the left are designated as the first cylinder #1, the second cylinder #2, the third cylinder #3, and the fourth cylinder #4), and two cylinders constituting one cylinder group, i.e., the cylinders #2 and #3 of the first group, are connected in parallel, and two cylinders constituting the other cylinder group, i.e., the cylinders #1 and #4 of the second group, are connected in parallel, and further, the two cylinder groups are connected in parallel.

[0013] 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.

[0014] The engine E includes an engine body 1 having a combustion chamber, an intake system 2, and an exhaust system 3. As mentioned above, the engine E is an in-line four-cylinder engine, but the type of the engine E, that is, 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.

[0015] 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.

[0016] 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, which 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 of the cylinders #1 to #4 via the branching sections of the intake manifold 22.

[0017] 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.

[0018] 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.

[0019] 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 cylinder is discharged to a branching portion of the exhaust manifold 31.

[0020] In this embodiment, the exhaust pipe 32 has two intermediate sections 32a, 32b and a collecting section 32c. The intermediate sections 32a, 32b are arranged in parallel with each other and connected at their downstream ends to the collecting section 32c. The branching section of the exhaust manifold 31 is such that the branching section connected to the first group of cylinders #2 and #3 is connected to the first intermediate section 32a, and the branching section connected to the second group of cylinders #1 and #4 is connected to the second intermediate section 32b. Thus, exhaust gas discharged from the first group of cylinders #2 and #3 flows from the branching section of the exhaust manifold 31 into the first intermediate section 32a of the exhaust pipe 32, and exhaust gas discharged from the second group of cylinders #1 and #4 flows from the branching section of the exhaust manifold 31 into the second intermediate section 32b of the exhaust pipe 32. The exhaust gas flowing through the first and second intermediate sections 32a, 32b then flows into the collecting section 32c of the exhaust pipe 32 and is released into the atmosphere via a muffler (not shown). The collecting portion 32c of the exhaust pipe 32 constitutes a "collecting exhaust pipe" according to this embodiment.

[0021] Furthermore, in this embodiment, two catalytic converters 33a and 33b are provided as the catalytic converter 33, with the catalytic converter 33a being installed in the first intermediate section 32a of the exhaust pipe 32 and the other catalytic converter 33b being installed in the second intermediate section 32b. The catalytic converters 33a and 33b incorporate exhaust purification catalysts 331a and 331b.

[0022] Exhaust gas emitted from the first group of cylinders #2 and #3 flows into the catalytic converter 33a in the first intermediate section 32a, where harmful exhaust components, including total hydrocarbons (THC) and nitrogen oxides (NOx), are purified by the exhaust purification catalyst 331a housed therein. On the other hand, exhaust gas emitted from the second group of cylinders #1 and #4 flows into the catalytic converter 33b in the second intermediate section 32b, where harmful exhaust components, such as total hydrocarbons, are purified by the exhaust purification catalyst 331b housed therein. In this embodiment, the catalytic converters 33a and 33b are equipped with three-way catalysts as the exhaust purification catalysts 331a and 331b. The exhaust purification catalyst 331a housed in the catalytic converter 33a in the first intermediate section 32a constitutes the "first exhaust purification catalyst" according to this embodiment, and the exhaust purification catalyst 331b housed in the catalytic converter 33b in the second intermediate section 32b constitutes the "second exhaust purification catalyst" according to this embodiment.

[0023] In addition to the above, the engine E includes an engine controller 101 and various sensors 201-207.

[0024] 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.

[0025] 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 first upstream air-fuel ratio sensor 205, a second 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.

[0026] 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.

[0027] 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.

[0028] The air flow meter 203 detects, as the intake air amount Qa, the flow rate of air introduced into the engine E. 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.

[0029] 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 .

[0030] The first upstream air-fuel ratio sensor 205 detects the air-fuel ratio λf1 of the exhaust gas flowing through the first intermediate section 32a of the exhaust pipe 32 that is upstream of the catalytic converter 33a, i.e., the exhaust gas before flowing into the catalytic converter 33a. The first upstream air-fuel ratio sensor 205, together with the second upstream air-fuel ratio sensor 206 described next, constitutes the "upstream air-fuel ratio sensor" according to this embodiment.

[0031] The second upstream air-fuel ratio sensor 206 detects the air-fuel ratio λf2 of the exhaust gas flowing through the second intermediate section 32b of the exhaust pipe 32, which is upstream of the catalytic converter 33b, i.e., the exhaust gas before it flows into the catalytic converter 33b.

[0032] The downstream air-fuel ratio sensor 207 detects the air-fuel ratio λr of the exhaust gas flowing through the collecting portion 32c of the exhaust pipe 32 downstream of the catalytic converters 33a and 33b, that is, the exhaust gas after passing through the catalytic converters 33a and 33b. The downstream exhaust sensor 207 constitutes the "downstream air-fuel ratio sensor" according to this embodiment.

[0033] 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.

[0034] The engine controller 101 controls the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33 (33a, 33b) by air-fuel ratio oscillation control. 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. In this embodiment, 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, and oscillating the air-fuel ratio by reversing the phase between the first group of cylinders #2, #3 and the second group of cylinders #1, #4 (i.e., by inverting the phase).

[0035] In this embodiment, when performing air-fuel ratio oscillation control, the frequency Frq when oscillating the air-fuel ratio is optimized. Specifically, the frequency Frq is changed, and the frequency at which the purification rate η of the exhaust purification catalyst 331 becomes maximum with respect to the change in frequency Frq (hereinafter, may be referred to as "optimum frequency") is extracted as a singular point, and this is set as the control frequency Fcn. After setting the control frequency Fcn, the air-fuel ratio oscillation control is performed using the control frequency Fcn.

[0036] The control frequency Fcn is set based on the detection signal output from the downstream air-fuel ratio sensor 207 while the frequency Frq is being changed, that is, the downstream air-fuel ratio λr. If the phases of the air-fuel ratio oscillations of the first group of cylinders #2, #3 and the second group of cylinders #1, #4 are the same and aligned with each other, it has been confirmed that the fluctuation of the downstream air-fuel ratio λr increases with a decrease in frequency Frq, the downstream minimum air-fuel ratio λr_min decreases, and the gradient of the change in the downstream minimum air-fuel ratio λr_min tends to change significantly in the vicinity of the frequency at which the purification rate η of the exhaust purification catalyst 331 is maximized.

[0037] Fig. 4 is a graph of experimental data showing the relationship between the frequency Frq and the purification rate η of the exhaust purification catalyst 331 and the downstream minimum air-fuel ratio λr_min in the case of in-phase air-fuel ratio oscillation. In Fig. 4, the open squares indicate the total hydrocarbon purification rate ηthc, and the open triangles indicate the nitrogen oxide purification rate ηnox. The open circles indicate the downstream minimum air-fuel ratio λr_min, and the approximate straight lines are shown by a thick solid line and a thick dotted line. The thick solid line is an approximate straight line of the downstream minimum air-fuel ratio λr_min measured in a region higher than the frequency Frq1 at which the purification rate η becomes maximum, and the thick dotted line is an approximate straight line of the downstream minimum air-fuel ratio λr_min measured in a region lower than the frequency Frq1.

[0038] 4, in the case of in-phase air-fuel ratio oscillation, it can be seen that the gradient of change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq (specifically, the logarithm log(Frq) of the frequency Frq) clearly changes in the vicinity of the optimum frequency Frq1. Therefore, it is possible to identify the optimum frequency Frq1 based on the gradient of change in the downstream minimum air-fuel ratio λr_min and set this as the control frequency Fcn.

[0039] On the other hand, in the case of air-fuel ratio oscillation due to anti-phase synthesis, the following problem occurs in the method based on the gradient of change in the downstream minimum air-fuel ratio λr_min due to the phase of the air-fuel ratio oscillation being inverted between the two cylinder groups.

[0040] 2 is a graph showing (a) the original phase, (b) the opposite phase, and (c) the combined opposite phase waveform of the air-fuel ratio oscillation according to this embodiment. It can be considered that Fig. 2(a) corresponds to the air-fuel ratio oscillation waveform of the exhaust gas that has passed through one exhaust purification catalyst 331a, Fig. 2(b) corresponds to the air-fuel ratio oscillation waveform of the exhaust gas that has passed through the other exhaust purification catalyst 331b, and Fig. 2(c) corresponds to the air-fuel ratio oscillation waveform of the exhaust gas after merging.

[0041] Fig. 3 is an explanatory diagram showing the antiphase synthesis of air-fuel ratio oscillation according to this embodiment, and is a schematic representation of the air-fuel ratio oscillation waveforms (a) to (c) shown in Fig. 2. Fig. 3(a) shows an air-fuel ratio oscillation waveform in the original phase, Fig. 3(b) shows an air-fuel ratio oscillation waveform in the antiphase, and Fig. 3(c) shows a synthesized air-fuel ratio oscillation waveform in which the original phase and the antiphase air-fuel ratio oscillation waveforms are superimposed.

[0042] 3, in the case of air-fuel ratio oscillation in antiphase synthesis, the air-fuel ratio oscillation component in the original phase and the air-fuel ratio oscillation component in the antiphase cancel each other out in the exhaust gas after merging, so the fluctuations in the downstream air-fuel ratio λr become smaller and, accordingly, the change in the downstream minimum air-fuel ratio λr_min also becomes unclear. This is because, in air-fuel ratio oscillation in antiphase synthesis, the influence of disturbances superimposed on the air-fuel ratio oscillation waveform increases relatively, and the change in the downstream minimum air-fuel ratio λr_min is lost in this disturbance.

[0043] 5 is a graph of experimental data showing the relationship between the frequency Frq and the purification rate η and the downstream minimum air-fuel ratio λr_min of the exhaust purification catalyst 331 in the case of air-fuel ratio oscillation in antiphase synthesis. The objects indicated by the plots and straight lines (solid line, dotted line) are the same as those in FIG.

[0044] As can be seen from FIG. 5, in the case of air-fuel ratio oscillation in antiphase synthesis, it can be confirmed that the continuity of the approximate straight line of the downstream minimum air-fuel ratio λr_min is lost before and after the optimum frequency Frq1. However, the change in slope is slight, and depending on the slope of the change in the downstream minimum air-fuel ratio λr_min, it is difficult to distinguish between the high-frequency side and the low-frequency side and to identify the optimum frequency Frq1.

[0045] Therefore, in this embodiment, instead of making a determination based on the downstream minimum air-fuel ratio λr_min itself, the difference between the downstream maximum air-fuel ratio λr_max and the downstream minimum air-fuel ratio λr_min, that is, the range Rlmb in which the downstream air-fuel ratio λr changes (hereinafter referred to as the "air-fuel ratio range") is adopted, and the singular point of the change with respect to the frequency Frq is extracted to identify the optimal frequency Frq1 and set the control frequency Fcn.

[0046] The following describes the air-fuel ratio control of the engine E according to this embodiment. In this embodiment, the air-fuel ratio control of the engine E is performed by performing air-fuel ratio oscillation with antiphase synthesis.

[0047] FIG. 6 is a flowchart showing the basic flow of the air-fuel ratio oscillation control according to this embodiment.

[0048] In this embodiment, the air-fuel ratio oscillation control according to the routine shown in FIG. 6 is executed by the engine controller 101 at predetermined time intervals after the engine E is started.

[0049] In S101, the engine rotation speed Ne, intake air amount Qa, etc. are read as indicators of the operating state of the engine E, and the upstream air-fuel ratios λf1, λf2 and downstream air-fuel ratio λr are also read.

[0050] In S102, 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, proceed to S104, and if not yet completed, proceed to S103.

[0051] In S103, the control frequency Fcn is set. The setting of the control frequency Fcn will be described in more detail later with reference to FIG.

[0052] In S104, the control frequency Fcn is read. In this embodiment, an operating range map is provided that is set so that 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 that has already been set. The operating range map is not limited to this, and may be one in which the control frequency Fcn is assigned in accordance with the exhaust flow rate Qexh of the engine E and the temperature Tcat of the exhaust purification catalyst 331. The operating range map constitutes the "control frequency storage means" in this embodiment.

[0053] In S105, 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.

[0054] In S106, the fuel injector 41 is driven with the fuel injection amount Qf.

[0055] The overall processing of the flowchart shown in FIG. 6 corresponds to the processing executed by the engine controller 101 as the "air-fuel ratio oscillation control means" according to this embodiment.

[0056] FIG. 7 is a flowchart showing the process of S103 shown in FIG. 6, that is, the process of setting the control frequency Fcn.

[0057] In S201, 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 region determination described later, this predetermined time is set to a time longer than one cycle of the air-fuel ratio oscillation. The processing of S201 corresponds to the processing executed by the engine controller 101 as the "specific air-fuel ratio detection means" according to this embodiment.

[0058] In S202, 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 S203, and if not, the process proceeds to S205.

[0059] In S203, 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 set the control frequency Fcn, and is set in advance for each operating range in the operating range map as the initial value of the frequency Frq. This is not limitative, and it is also possible to simply set the reference frequency F0 to 1 [Hz] uniformly for the entire operating range of the engine E. 1 [Hz] is the frequency corresponding to log(Frq)=0 shown on the horizontal axis.

[0060] In S204, the value of the flag FRG is set to 1.

[0061] In S205, the frequency Frq of the air-fuel ratio oscillation is reduced by a predetermined frequency ΔF. Specifically, the frequency Frq is updated to a frequency reduced by the predetermined frequency ΔF from the current frequency Frq (Frq=Frq-ΔF), and air-fuel ratio oscillation control is executed using the new updated frequency Frq. The processing of S205 corresponds to the processing executed by the engine controller 101 as the "oscillation frequency reducing means" according to this embodiment.

[0062] In S206, the difference between the maximum downstream air-fuel ratio λr_max and the minimum downstream air-fuel ratio λr_min (i.e., the air-fuel ratio range) Rlmb is calculated. When the predetermined time for acquiring the maximum downstream air-fuel ratio λr_max and the minimum downstream air-fuel ratio λr_min is the time for one cycle of the air-fuel ratio oscillation, the air-fuel ratio range Rlmb corresponds to the amplitude of the air-fuel ratio oscillation. As described above, in this embodiment, this predetermined time is set to a time longer than one cycle of the air-fuel ratio oscillation.

[0063] In S207, the gradient glmb of the change in the air-fuel ratio range Rlmb with respect to the frequency Frq is calculated by dividing the air-fuel ratio range Rlmb by ΔF. In this embodiment, the gradient glmb is the gradient of the change in the air-fuel ratio range Rlmb obtained for each subtraction when the frequency Frq is decreased from the reference frequency F0 by a predetermined frequency ΔF, and takes a negative value.

[0064] In S208, it is determined whether the absolute value of the gradient glmb (=|glmb|) is equal to or greater than a predetermined value g0. If it is equal to or greater than the predetermined value g0, in other words, if the gradient of the change in the air-fuel ratio range Rlmb when the frequency Frq is decreased decreases and its absolute value reaches the predetermined value g0, the process proceeds to S209, but if it is less than the predetermined value g0, the process bypasses the processing of S209 and returns to the flowchart shown in FIG.

[0065] In S209, a control frequency Fcn is set. In this embodiment, the frequency Frq when the absolute value of the gradient glmb of the change in the air-fuel ratio range Rlmb reaches a predetermined value g0 is set as the control frequency Fcn and is stored in the corresponding operating range in the operating range map. Here, based on the upstream air-fuel ratios λf1 and λf2 detected by the first and second upstream air-fuel ratio sensors 205 and 206, the actual frequency Frq1 when the absolute value of the gradient glmb reaches the predetermined value g0 is detected and set as the control frequency Fcn. The processing of S209 corresponds to the processing executed by the engine controller 101 as the "control frequency specifying means" according to this embodiment.

[0066] The processes from S205 to S208 shown in FIG. 7 will be described in more detail below with reference to FIGS.

[0067] 8 and 9 are graphs of experimental data measured for different catalyst temperatures Tcat, showing changes in the purification rate η and air-fuel ratio range Rlmb of the exhaust purification catalyst 331 relative to the air-fuel ratio oscillation frequency Frq in air-fuel ratio oscillation control, under an operating condition in which the exhaust flow rate Qexh of the engine E is low. In this embodiment, the air-fuel ratio oscillation control is performed by performing air-fuel ratio oscillation with antiphase synthesis. The temperature of the exhaust gas at the inlet of the catalytic converter 33 can be used simply as the catalyst temperature Tcat. The amount of the exhaust flow rate Qexh can be determined, for example, from the intake air amount Qa. FIG. 8(a) shows data obtained when the catalyst temperature Tcat is 350°C, FIG. 8(b) shows data obtained when the catalyst temperature Tcat is 390°C, FIG. 9(a) shows data obtained when the catalyst temperature Tcat is 450°C, and FIG. 9(b) shows data obtained when the catalyst temperature Tcat is 490°C.

[0068] 10 and 11 are graphs of experimental data measured for different catalyst temperatures Tcat, showing the change in the purification rate η and air-fuel ratio range Rlmb of the exhaust purification catalyst 331 relative to the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control, under an operating condition in which the exhaust flow rate Qexh of the engine E is high. Fig. 10(a) shows the data obtained when the catalyst temperature Tcat was 350°C, Fig. 10(b) shows the data obtained when the catalyst temperature Tcat was 390°C, Fig. 11(a) shows the data obtained when the catalyst temperature Tcat was 450°C, and Fig. 11(b) shows the data obtained when the catalyst temperature Tcat was 490°C.

[0069] 8 to 11, the open squares indicate the total hydrocarbon purification efficiency ηthc, and the open triangles indicate the nitrogen oxide purification efficiency ηnox. The open circles indicate the air-fuel ratio range Rlmb, and its approximate straight lines are indicated by a thick solid line and a thick dotted line. The thick solid line is the approximate straight line of the air-fuel ratio range Rlmb measured in a region higher than the frequency Frq1 at which the purification efficiency η is maximized, and the thick dotted line is the approximate straight line of the air-fuel ratio range Rlmb measured in a region lower than the frequency Frq1.

[0070] 8 to 11, it can be seen that the purification rate η of the exhaust purification catalyst 331 increases for both total hydrocarbons and nitrogen oxides as the catalyst temperature Tcat rises. This is because the activation of the exhaust purification catalyst 331 progresses as the temperature Tcat rises. It can also be seen that the purification rate η is higher when the exhaust flow rate Qexh is low than when the exhaust flow rate Qexh is high. This is because when the exhaust flow rate Qexh is high, the amount of harmful exhaust components that need to be purified by the exhaust purification catalyst 331 is large, and the amount of harmful exhaust components that pass through the exhaust purification catalyst 331 without being completely purified increases relatively.

[0071] On the other hand, in the process of decreasing the frequency Frq of the air-fuel ratio oscillation from the reference frequency F0, it can be seen that the purification rate η increases up to the optimal frequency Frq1, and then the purification rate η decreases once the frequency at which the purification rate η is maximized, that is, the optimal frequency Frq1, is passed. This is presumed to be a phenomenon related to the oxygen storage capacity of the exhaust purification catalyst 331. In the region higher than the optimal frequency Frq1, the oxygen storage capacity of the catalyst 331 functions effectively in increasing the purification rate η. However, once the optimal frequency Frq1 is passed, the period of the air-fuel ratio oscillation becomes too long compared to the oxygen storage capacity, causing the oxygen storage capacity to break down, and the fluctuations in the air-fuel ratio in the reaction field of the catalyst 331 become close to repeating steady rich and steady lean.

[0072] Looking at the air-fuel ratio range Rlmb, it is possible to see that the gradient of the change in the air-fuel ratio range Rlmb with respect to the frequency Frq changes discontinuously near the optimum frequency Frq1, regardless of the catalyst temperature Tcat. This phenomenon can be more clearly seen by setting the frequency Frq on the horizontal axis to a logarithm log(Frq). As described above, in this embodiment, the gradient of the change in the air-fuel ratio range Rlmb is discontinuous, and attention is focused on the optimum frequency Frq1, which shows a change that can be determined by comparison with a threshold value, and this is set as the control frequency Fcn. In order to set a more appropriate control frequency Fcn, it is preferable that the predetermined value g0 be a value in the range from 0.015 to 0.025.

[0073] Based on the above findings, the procedure for specifying the optimum frequency Frq1, that is, the procedure for setting the control frequency Fcn, will be described below.

[0074] 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 maximum downstream air-fuel ratio λr_max and the minimum downstream air-fuel ratio λr_min are detected, and the difference between the maximum downstream air-fuel ratio λr_max and the minimum downstream air-fuel ratio λr_min detected before and after the frequency ΔF is decreased, i.e., the gradient glmb of the change in the air-fuel ratio range Rlmb, is calculated. The absolute value of the gradient glmb is then compared with a determination threshold value g0, and a frequency Frq1 is identified when the absolute value of the gradient glmb changes from less than the threshold value g0 to equal to or greater than the threshold value g0, and this is set as the control frequency Fcn.

[0075] 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.

[0076] First, by air-fuel ratio oscillation control, it is possible to improve the purification rate η of the exhaust purification catalyst 331. Here, by identifying the frequency Frq1 that is optimal for purifying the exhaust gas by air-fuel ratio oscillation, setting this as the control frequency Fcn (=Frq1), and performing air-fuel ratio oscillation at the control frequency Fcn, it is possible to maintain a high purification rate η by air-fuel ratio oscillation over a wider range of the operating region.

[0077] Furthermore, by specifying and setting the control frequency Fcn based on the gradient glmb of change of the difference between the maximum value λr_max and the minimum value λr_min of the downstream air-fuel ratio λr in the air-fuel ratio oscillation with respect to the frequency Frq of the air-fuel ratio oscillation, it is possible to achieve this without adding any parts such as a dedicated sensor, and it is possible to maintain a high purification rate η with a simple configuration.

[0078] Secondly, the air-fuel ratio oscillation control according to this embodiment can be suitably applied to an engine E in which a plurality of cylinders #1 to #4 are divided into two cylinder groups, and exhaust purification catalysts 331a, 331b are provided for each cylinder group. By arranging the downstream air-fuel ratio sensor 207 at the collecting section 32c of the exhaust pipe 32, the number of air-fuel ratio sensors can be reduced compared to the case in which a downstream air-fuel ratio sensor is provided for each of the exhaust purification catalysts 331a, 331b, and control can also be implemented more easily.

[0079] Here, by oscillating the exhaust air-fuel ratio in opposite phases for cylinders #2 and #3 of the first group and cylinders #1 and #4 of the second group, in other words, by setting the phases of the air-fuel ratio oscillation to be inverted for cylinders #2 and #3 of the first group and cylinders #1 and #4 of the second group, it is possible to cancel out the fluctuations in engine torque that accompany the air-fuel ratio oscillation and suppress the adverse effects that air-fuel ratio oscillation control can have on drivability and ride comfort, such as engine torque fluctuations and increased vibration and noise.

[0080] Third, in identifying and setting the control frequency Fn, by detecting the actual frequency Frq of the air-fuel ratio oscillation based on the upstream air-fuel ratios λf1, λf2 detected by the first and second upstream air-fuel ratio sensors 205, 206, it is possible to accurately identify the control frequency Fcn and appropriately set the control frequency Fcn.

[0081] Fourth, by setting the control frequency Fcn based on the gradient glmb of the change that the air-fuel ratio range Rlmb makes with respect to the logarithm of the air-fuel ratio oscillation frequency Frq, it is possible to easily determine discontinuous changes that occur in the gradient glmb and to easily and appropriately set the control frequency Fcn.

[0082] Fifth, an operating range map is set in the engine controller 101, and the control frequency Fcn can be stored in the operating range map for each operating range of the engine E. This makes it possible to set a more appropriate control frequency Fcn in accordance with the operating state of the engine E when transitioning to a different operating range, and to maintain a high purification rate η through air-fuel ratio oscillation control. On the other hand, in an operating range where setting of the control frequency Fcn has already been completed, an appropriate control frequency Fcn can be quickly set by reading from the operating range map, and it is possible to suppress deterioration of the purification rate η until setting of the control frequency Fcn is completed.

[0083] In the above description, the air-fuel ratio oscillation control is performed by inverting the phase of the air-fuel ratio oscillation between the first group of cylinders #2 and #3 and the second group of cylinders #1 and #4. However, the air-fuel ratio oscillation control is not limited to this. For example, the phases of the air-fuel ratio oscillation between the first group of cylinders #2 and #3 and the second group of cylinders #1 and #4 may be aligned rather than being inverted. In this case, the control frequency Fcn can be determined and set based on the gradient glmb of the change in the air-fuel ratio range Rlmb, as in the case of air-fuel ratio oscillation with antiphase synthesis.

[0084] Figures 12 to 15 are graphs of experimental data measured when air-fuel ratio oscillation control is performed by aligning the phases of the air-fuel ratio oscillations between the first group of cylinders #2, #3 and the second group of cylinders #1, #4. Figures 12 and 13 show the changes in the purification rate η and air-fuel ratio range Rlmb of the exhaust purification catalyst 331 relative to the frequency Frq of the air-fuel ratio oscillation, obtained at a relatively low catalyst temperature Tcat, when the engine E is operating in a state where the exhaust flow rate Qexh is low. Figures 14 and 15 show the changes in the purification rate η and air-fuel ratio range Rlmb of the exhaust purification catalyst 331 relative to the frequency Frq of the air-fuel ratio oscillation, obtained at a relatively high catalyst temperature Tcat, when the engine E is operating in a state where the exhaust flow rate Qexh is high.

[0085] Figures 12(a) and 14(a) show the results obtained when the catalyst temperature Tcat was 350°C, and Figures 12(b) and 14(b) show the results obtained when the catalyst temperature Tcat was 390°C. Furthermore, Figures 13(a) and 15(a) show the results obtained when the catalyst temperature Tcat was 450°C, and Figures 13(b) and 15(b) show the results obtained when the catalyst temperature Tcat was 490°C.

[0086] 12 to 15, the plots and straight lines (solid lines or dotted lines) indicate the same objects as those in FIGS. 8 to 11.

[0087] As shown in FIGS. 12 to 15, the method of identifying the optimal frequency Frq1 from the gradient glmb of the change in the air-fuel ratio range Rlmb and setting the control frequency Fcn can also be applied to the case where air-fuel ratio oscillation control is performed by aligning the phases of the air-fuel ratio oscillations between different cylinder groups.

[0088] Furthermore, in the above description, the air-fuel ratio oscillation control is applied to an in-line four-cylinder engine, with the four cylinders #1 to #4 divided into two cylinder groups, and the phase of the air-fuel ratio oscillation is set to be opposite between the two cylinder groups. However, the engine to which the air-fuel ratio oscillation control according to this embodiment can be applied is not limited to this. For example, the air-fuel ratio oscillation control can be applied to an in-line engine in which multiple cylinders are connected in parallel without being divided into cylinder groups, and can also be applied to a V-type engine, not limited to an in-line engine. Furthermore, when applied to a V-type engine, the multiple cylinders aligned on the left and right banks can be divided into two cylinder groups for each bank, and the phase of the air-fuel ratio oscillation can be set to be opposite or the same between the two cylinder groups. [Explanation of symbols]

[0089] 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, 32a, 32b...middle section of exhaust pipe, 32c...exhaust pipe assembly, 33 (33a, 33b)...catalytic converter, 331 (331a, 331b)...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, 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, a downstream air-fuel ratio sensor that is installed in the exhaust passage downstream of the exhaust purification catalyst and detects the air-fuel ratio of the exhaust gas that has passed through the exhaust purification catalyst; a controller configured to acquire the air-fuel ratio detected by the downstream air-fuel ratio sensor, The controller 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 a value equivalent to stoichiometry; a specific air-fuel ratio detection means for detecting a maximum air-fuel ratio and a minimum air-fuel ratio in the air-fuel ratio oscillation generated by the air-fuel ratio oscillation control based on the air-fuel ratio detected by the downstream air-fuel ratio sensor; an oscillation frequency reducing means for reducing the frequency of the air-fuel ratio oscillation from a predetermined value; a control frequency specifying means for specifying, as a control frequency, the frequency at which a gradient of change in the difference between the maximum air-fuel ratio and the minimum air-fuel ratio detected by the specific air-fuel ratio detecting means with respect to the frequency of the air-fuel ratio oscillation reaches a predetermined value; The air-fuel ratio control device for an engine, 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 identification means identifies the control frequency.

2. It has multiple cylinders, a first exhaust purification catalyst configured to be able to introduce exhaust gas discharged from a first group of cylinders among the plurality of cylinders; a second exhaust gas purification catalyst configured to be able to introduce exhaust gas discharged from a second group of cylinders different from the first group among the plurality of cylinders; a collecting exhaust pipe that joins together exhaust gas that has passed through the first exhaust purification catalyst and exhaust gas that has passed through the second exhaust purification catalyst, 2. The air-fuel ratio control device for an engine according to claim 1, wherein the downstream air-fuel ratio sensor is installed in the exhaust manifold and detects the air-fuel ratio of the exhaust gas flowing through the exhaust manifold.

3. 3. The engine air-fuel ratio control device according to claim 2, wherein said air-fuel ratio oscillation control means oscillates the air-fuel ratio of the exhaust gas in opposite phases between said first group of cylinders and said second group of cylinders.

4. an upstream air-fuel ratio sensor that is installed in the exhaust passage upstream of the exhaust purification catalyst and detects the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst; 4. The air-fuel ratio control device for an engine according to claim 1, wherein the control frequency specifying means detects an actual frequency of the air-fuel ratio oscillation and specifies the control frequency based on the air-fuel ratio detected by the upstream air-fuel ratio sensor.

5. 5. The air-fuel ratio control device for an engine according to claim 1, wherein the control frequency specifying means specifies, as the control frequency, the frequency at which a slope of a change in a logarithm of a frequency of the air-fuel ratio oscillation, which is a difference between the maximum air-fuel ratio and the minimum air-fuel ratio, reaches a predetermined value.

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 controller further includes a control frequency storage means for storing the control frequency identified by the control frequency identification means in association with an operating state of the engine; 7. 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 stored in the control frequency storage means when the engine is operated next time or later in an operating state in which the control frequency has been specified by the control frequency specifying means.

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