Air-fuel ratio control system

Abstract

Air-fuel ratio control (10), comprising: an air-fuel ratio adjuster (11) configured to cause an air-fuel ratio of exhaust gas (G) on an upstream side of an exhaust gas purification catalyst (56) connected to an engine (50) to fluctuate between a rich side and a lean side, with the exhaust gas (G) flowing through the exhaust gas purification catalyst (56); a search unit (12) configured to incrementally change a frequency (H) of the fluctuation, and to measure a downstream air-fuel ratio and change the downstream air-fuel ratio with respect to a change in the frequency (H) of the fluctuation, wherein the downstream air-fuel ratio is the air-fuel ratio of the exhaust gas (G) on a downstream side of the exhaust gas purification catalyst (56); and a frequency determination unit (13) configured to determine a frequency (H) at the time when a value of the downstream air-fuel ratio reaches a predetermined threshold (Ω) or at the time when a gradient (g) of the downstream air-fuel ratio with respect to frequency reaches a predetermined threshold (g0) as an optimal frequency (H0).

Description

Background of the invention; Field of the invention

[0001] Embodiments of the present invention relate to a method for controlling the air-fuel ratio of an engine. Description of the state of the art

[0002] In general, the mixture of fuel and air supplied to an internal combustion engine is regulated to maintain a theoretical air-fuel ratio in which oxygen and fuel react appropriately with each other in the mixture.

[0003] When sensors installed in the combustion engine's exhaust system detect a fuel-rich condition, the air-fuel ratio in the air-fuel mixture is increased by the engine control unit (ECU). Conversely, when a fuel-poor condition is detected, fuel is added to lower the air-fuel ratio. The control system described above is based on exhaust components and maintains the air-fuel mixture at approximately the theoretical air-fuel ratio, while fluctuating around this ratio at a predetermined frequency. As is known, allowing the air-fuel ratio to fluctuate improves the catalytic conversion efficiency of the exhaust aftertreatment catalyst (hereinafter referred to simply as the "catalyst").The mechanism for improving the catalytic conversion efficiency with respect to the fluctuation frequency is presumably related to the role of active oxygen species. In another known method, fluctuations in the air-fuel ratio bring more reaction components into contact with the catalyst, which increases the catalyst temperature and improves the catalytic conversion efficiency. This method relating to fluctuations in the air-fuel ratio is disclosed in the unexamined Japanese patent application JP S52-81438A.

[0004] DE 691 06 247 T2 describes a monitoring method for the effect of a motor vehicle catalytic converter with a closed control loop, in which the control loop is artificially modulated. The air-fuel ratio of the exhaust gas is measured – in the direction of flow – downstream of the catalytic converter, and it is determined whether there is a change in the air-fuel parameters before and after the modulation.

[0005] However, there is a problem insofar as the catalytic conversion efficiency may deviate from the maximum value in connection with the temperature change of the catalyst or the operating condition of the combustion engine, for example the amount of exhaust gas. REVELATION OF THE INVENTION

[0006] In view of the circumstances described above, an objective of the present invention is to provide an air-fuel ratio control and an air-fuel ratio control system which can maintain a high catalytic conversion efficiency by means of a simple configuration even when there is a temperature change of the catalyst or a change in the operating state of the internal combustion engine.

[0007] An air-fuel ratio control unit comprises an air-fuel ratio adjuster, a search unit, and a frequency determination unit. The air-fuel ratio adjuster is configured to fluctuate the air-fuel ratio of exhaust gas on the upstream side of an exhaust aftertreatment catalyst connected to an engine between a rich side (fuel-rich side) and a lean side (fuel-poor side) as the exhaust gas flows through the aftertreatment catalyst. The search unit is configured to incrementally change the frequency of the fluctuation, as well as to measure at least one value of a downstream air-fuel ratio and a change in the downstream air-fuel ratio with respect to a change in the frequency fluctuation, where the downstream air-fuel ratio is that present in the exhaust gas on the downstream side of the catalyst.The frequency determination unit is configured to determine an optimum frequency, which is a frequency at a time when a value of the downstream air-fuel ratio reaches a predetermined threshold, or at a time when a gradient of the downstream air-fuel ratio with respect to the frequency reaches a predetermined threshold. Modes of operation of the invention

[0008] The present invention provides an air-fuel ratio control and an air-fuel ratio control system, each of which can maintain a high catalytic conversion efficiency with a simple configuration even when there is a temperature change of the catalyst or a change in the operating state of the internal combustion engine. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings show: Fig. 1 a block diagram illustrating an inlet-port fuel injection engine in which an air-fuel ratio control according to the first embodiment of the invention is applied; Fig. 2 a table illustrating the composition of the model gas used in a verification experiment; Fig. 3 an experimental graphical representation illustrating the relationship between frequency and catalyst conversion efficiency, measured by verifying the power and temperature of the catalyst and the volumetric velocity of the exhaust gas in the catalyst; Fig. 4. Another experimental graphical representation illustrating the relationship between frequency and catalytic conversion efficiency, measured by verifying the power and temperature of the catalyst and the volumetric velocity of the exhaust gas in the catalyst; Fig. 5 another experimental representation illustrating the relationship between frequency and catalytic conversion efficiency, measured by verifying the power and temperature of the catalyst and the volumetric velocity of the exhaust gas in the catalyst; Fig. 6. A further experimental graphical representation illustrating the relationship between frequency and catalytic conversion efficiency, measured by verifying the power and temperature of the catalyst and the volume velocity of the exhaust gas in the catalyst; Fig. 7 a graphical representation illustrating the output waveforms of the respective air-fuel ratio sensors on the downstream and upstream sides in a high-frequency range that is higher than a local maximum value of the catalytic conversion efficiency; Fig. 8 a graphical representation illustrating the output waveforms of the respective air-fuel ratio sensors on the upstream and downstream sides in a low frequency range that is lower than a local maximum value of the catalytic conversion efficiency; Fig. Figure 9 is a flowchart illustrating a procedure for determining the optimal frequency and adjusting the catalytic conversion efficiency to the optimal value; and Fig. 10 a flowchart illustrating the operation of the air-fuel ratio control according to the second embodiment. DETAILED DESCRIPTION

[0010] Exemplary embodiments of the invention are described below with reference to the accompanying drawings.

[0011] In the following embodiments, the terms "upstream" and "downstream" are used with reference to the normal flow direction of exhaust gas discharged from the engine. In other words, the side closer to the engine is the "upstream side," and the opposite side is the "downstream side." (First embodiment)

[0012] First, based on the block diagram of a motor 50 and its peripheral components, according to Fig. 1. A description of the air-fuel ratio control 10 (hereinafter also referred to as "control 10") according to the first embodiment. The control 10 of this embodiment is used in an engine in which the engine operating range changes relatively slowly, as is the case, for example, with a series-connected hybrid engine or a stationary engine.

[0013] As in Fig. As shown in Figure 1, the engine 50 is connected via an intake manifold to an intake pipe 52, which is open to the atmosphere via an air filter 51. The intake pipe 52 supplies air to the fuel chamber of the engine 50.

[0014] The exhaust port of the engine 50 is connected to an exhaust pipe 53 via an exhaust manifold. The downstream side of the exhaust pipe 53 is connected to a catalytic converter 54.

[0015] The catalytic converter 54 incorporates an exhaust gas purification catalyst 56 (hereinafter simply referred to as "the catalyst 56"), for example a three-way catalyst that removes CO, HC, and NOx from the exhaust gas G. Furthermore, the catalytic converter 54 is equipped with air-fuel ratio sensors (i.e., L / K sensors) 57 and 58 on the upstream and downstream sides of the catalyst 56, respectively.

[0016] The upstream air-fuel ratio sensor 57 on the upstream side of the catalyst 56 measures the air-fuel ratio of the exhaust gas G, which is produced by combustion in the engine 50 and flows into the catalyst 56. The downstream air-fuel ratio sensor 58 on the downstream side of the catalyst 56 measures the air-fuel ratio of the exhaust gas G that has been cleaned by the catalyst 56 and flows out of the catalyst 56 (hereinafter referred to as the "downstream air-fuel ratio"). That is, the exhaust system of the first embodiment is configured as a so-called 2L / K sensor system.

[0017] The engine 50 is connected to an ECU (Engine Control Unit) 59. The ECU 59 is configured as a microcomputer containing a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). The CPU performs necessary calculations according to the control programs to execute various processes and control tasks. The ROM stores the control programs and directs data to the CPU for processing, while the RAM is primarily used for different work areas during control processing.

[0018] The ECU 59 is connected via a network 60 to sensors 61, 57 and 58 as well as to actuators located at positions of the motor 50 and the catalytic converter 54 to monitor and control the operation of the motor 50.

[0019] The ECU 59 is equipped with the control unit 10 of the first embodiment. This control unit 10 includes: an air-fuel ratio adjuster 11, a search unit 12 (that is, a searcher 12); and a frequency determination unit 13 (that is, a frequency determiner 13).

[0020] Based on the exhaust gas air-fuel ratio information G supplied to the ECU 59 by the two air-fuel ratio sensors 57 and 58 described above, the air-fuel ratio adjuster 11 determines whether the mixture in the engine 50 is rich (fuel-rich) or lean (fuel-poor). The air-fuel ratio adjuster 11 adjusts the amount of fuel supplied by constantly increasing or decreasing it so that the air-fuel ratio corresponds to the theoretical air-fuel ratio. This adjustment causes the intake air air-fuel ratio to fluctuate between rich and lean above the theoretical air-fuel ratio at regular intervals. This fluctuation is referred to below as the λ-fluctuation, and the frequency of this λ-fluctuation is referred to as the frequency H.The fluctuation of the air-fuel ratio of the intake air subjects the value of the air-fuel ratio of the exhaust gas G to the λ-fluctuation with the frequency H.

[0021] There is a parameter called the air-fuel ratio λ, which is the same concept as the air-fuel ratio. It is a physical quantity standardized by dividing the actual air-fuel ratio by the theoretical air-fuel ratio. The air-fuel ratio λ, defined in this way, takes the value 1 when the actual air-fuel ratio equals the theoretical air-fuel ratio. If the air-fuel ratio λ is less than 1, the air-fuel ratio is rich. If the air-fuel ratio λ is greater than 1, the air-fuel ratio is lean.

[0022] The following description assumes that the respective components 11 to 13 of the control 10 monitor and control this excess air ratio λ, which is the standardized air-fuel ratio, instead of the air-fuel ratio.

[0023] Based on the upstream air excess ratio λ f , which is detected by the upstream air-fuel ratio sensor 57 via the air-fuel ratio adjuster 11, the search unit 12 searches for the optimal frequency H0 by changing the frequency H of the A-vary of the upstream excess air ratio λ f This is because active oxygen species can be used more efficiently at the optimal frequency H0, thus maximizing the efficiency of the catalytic conversion at the optimal frequency H0.

[0024] Based on the Fig. 2, Fig. 3, Fig. 4, Fig. 5 to Fig. 6 now describes a verification experiment to investigate the relationship between the frequency H and the λ fluctuation, the efficiency of the catalytic conversion of the catalyst 56 and the excess air ratio λ, based on the downstream air-fuel ratio, for which the ambient conditions are verified.

[0025] Fig. Figure 2 is a table illustrating the composition of the model gas used in this verification experiment. In this experiment, the air-fuel ratio λ of the model gas was varied between rich (λ = 0.95) and lean (λ = 1.05), and the efficiency of the catalytic conversion of THC (total hydrocarbon) and NOx with respect to the frequency h of the λ variation was investigated. The table follows Fig. Figure 2 shows that the amount of carbon monoxide (CO) as an unburned component of the fuel is high in the rich range, whereas the amount of oxygen (O2) is high in the lean state, consistent with the λ-fluctuation of the excess air ratio λ.

[0026] Each of the Fig. 3, Fig. 4, Fig. 5 to Fig. Figure 6 shows an experimental graphic representation illustrating the relationship between the frequency H and the catalytic conversion efficiency of the catalyst 56, measured by verifying the power, temperature and volumetric velocity of the catalyst 56.

[0027] In each of the Fig. 3, Fig. 4, Fig. 5 to Fig. 6 shows the logarithmic frequency H (Hz) on the horizontal axis, the left vertical axis denotes the catalytic conversion efficiency [%], and the right vertical axis gives the minimum excess air ratio λ. r_minthat will be explained below using the Fig. 7 and Fig. This will be explained in section 8. Since the horizontal axis is a logarithmic axis, 0 on the scale represents 1 Hz, and -1 on the scale represents 10 Hz. -1 [Hz]. In each of the Fig. 3, Fig. 4, Fig. 5 to Fig. Figure 6 shows the graph indicated by the markings “■” and the thin solid line as the catalytic conversion efficiency of the total hydrocarbon, and the graph indicated by the markings “•” and the thin dashed line as the catalytic conversion efficiency of NOx. The markings “▲” and “Δ” represent measurements of the excess air ratio λ, while the thick dashed line is a linear approximation of these measurements of the excess air ratio λ.

[0028] Fig. Figure 3 shows the experimental results under the conditions where the catalyst power is high, the temperature of the catalyst 56 is 450°C and the exhaust gas flow rate G is a “low flow rate”, which is 30x103 times the catalyst flow rate.

[0029] Fig. Figure 4 shows the experimental result under the conditions where only the temperature is changed to 350°C, while the other conditions remain unchanged. Fig. 3 remain the same.

[0030] Fig. Figure 5 shows the experimental result under conditions where only the flow rate is changed to a "high flow rate", which is about 8 times the "low flow rate", while the other conditions are the same as those according to Fig. There are 3.

[0031] Fig. Figure 6 shows the experimental result under the conditions whereby only the catalyst power is changed to a low power, while the other conditions remain unchanged. Fig. 3 are the same.

[0032] In each of the Fig. 3, Fig. 4, Fig. 5 to Fig. 6, in which the environmental conditions are changed in the manner explained above, the minimum excess air ratio λ changes. r_min linear with respect to the logarithm of the frequency H, and the gradient g changes discontinuously at the point where the catalytic conversion efficiency assumes the local maximum value.

[0033] In detail: the efficiencies of the catalytic conversion of both the total hydrocarbon and NOx reach near local maximum values ​​close to the frequency H at which the measured values ​​of the excess air ratio λ jump discontinuously. In other words: the efficiency of the catalytic conversion of catalyst 56 is almost at its local maximum when the excess air ratio λ is closest to the value 1.

[0034] The following describes the minimum excess air ratio λ. r_min described, which is the variable that is plotted on the right vertical axis in each of the Fig. 3, Fig. 4, Fig. 5 to Fig. 6 is shown.

[0035] Fig. Figure 7 is a graphical representation of output waveforms of the upstream and downstream air-fuel ratio sensors 57 and 58 in the high frequency range α, which is higher than the local maximum value of the catalytic conversion efficiency.

[0036] Fig. Figure 8 is a graphical representation of output waveforms of the upstream and downstream air-fuel ratio sensors 57 and 58 in the lower frequency range β, which is lower than the local maximum value of the catalytic conversion efficiency.

[0037] In each of the graphs according to the Fig. 7 and Fig. Figure 8 shows the horizontal axis as the time [s], the left vertical axis as the gas temperature [°C] at the inlet of the catalyst 56, and the right vertical axis as the air excess ratio λ.

[0038] In the graphical representation, the excess air ratio λ, which is detected by the upstream air-fuel ratio sensor 57, is shown as the upstream excess air ratio λ. f Indicated by the thin line. The excess air ratio λ, based on the downstream air-fuel ratio detected by the downstream air-fuel ratio sensor 58, is referred to as the downstream excess air ratio λ. r Indicated by the thick line.

[0039] In the high frequency range α according to Fig. 7 will be the fluctuation of the downstream excess air ratio λ r strongly dampened compared to the upstream excess air ratio λ f The cause of this damping (i.e., the drop) is that the OSC function (the oxygen storage capacity function) of catalyst 56 absorbs the λ fluctuation. In the low frequency range β according to Fig. 8 is the fluctuation of the downstream excess air ratio λ r not damped; this is probably because the OSC function of catalyst 56 is no longer able to suppress the λ fluctuation in the low frequency range β.

[0040] From this result, it can be expected that the influence of the frequency H on the downstream air excess ratio λ r In the high-frequency range α, the coefficient of the gradient α is extremely low, and therefore the value of the gradient g in the high-frequency range α is relatively small. Similarly, in the low-frequency range β, the downstream air excess ratio λ is expected to be low. r As the frequency H decreases, the value of the gradient g becomes relatively larger.

[0041] To verify the trend explained above, the graphs are analyzed according to the Fig. 7 and Fig. 8 and according to the representation of the graphs in the Fig. 3 and Fig. 6 in the verification experiment the minimum excess air ratio λ r_min , which is the local minimum value of the fluctuation of the downstream excess air ratio λ r is being recorded.

[0042] The minimum excess air ratio λ r_min The system becomes insensitive again with respect to the lower frequency H when the lower frequency H is lower than the lower frequency range β during alternating, repeating steady rich and steady lean states. It was found that the catalytic conversion efficiency of catalyst 56 is low in this range.

[0043] Based on this verification experiment, the search unit 12 sequentially measures the minimum excess air ratio λ. r_min , while the frequency H is changed, as in the Fig. 3, Fig. 4, Fig. 5 to Fig. Figure 6 shows that the search unit 12 uses this method to search for the optimal frequency H0, which is the frequency at which the state transitions from the high frequency range α to the low frequency range β and the value of the minimum excess air ratio λ r_min jumps discontinuously.

[0044] Instead of the minimum excess air ratio λ r_min To make use of this, the optimal frequency H0 can also be found by using the gradient g, which is obtained by dividing the change in the value of the minimum air excess ratio λ. r_min on the vertical axis by changing the frequency H on the horizontal axis. In other words, the search can be carried out using the gradient g, which is calculated by the following expression 1 according to the output value of the downstream air-fuel ratio sensor 58. g=Δλr_min / Δlog(Frequency H)

[0045] The tendency of the minimum air-excess ratio λ r_min Regarding the frequency H, it can be demonstrated more clearly by using the logarithm of the frequency H instead of the frequency itself.

[0046] The frequency determination unit 13 determines the frequency H at which the gradient g changes rapidly as the optimal frequency H0. The air-fuel ratio adjuster 11 sets the frequency of the λ fluctuation as the optimal frequency H0 and maintains this value, determined by the frequency determination unit 13.

[0047] Instead of using the gradient g, the value of the minimum air excess ratio λ can also be used. r_min even in the manner described above, and the frequency H at which the catalytic conversion efficiency is maximized can be determined from the discontinuous change of the minimum excess air ratio λ r_minestimate. In this case, for example, in the process of gradually reducing the frequency H, the frequency at which the catalytic conversion efficiency is maximized can be determined when the minimum excess air ratio λ r_min becomes smaller than the preset threshold value Ω.

[0048] The estimation from the gradient g and the estimation from the minimum air excess ratio λ r_min These estimates can be combined to find the optimal frequency H0. Combining these estimates allows for a more precise determination of the optimal frequency H0.

[0049] It is preferred that the ECU 59 has a map representing the operating state of the motor 50 for each frequency H. Such a map can specify the relationship between the motor speed and the operating load, and the search unit 12 begins searching for the optimal frequency H0 by switching to another map that specifies different drive conditions. Repeating the search with the frequency of significant changes in the drive conditions allows the catalytic conversion efficiency to be kept constant at the highest possible value. The ECU 59 can store a reasonable range for the optimal frequency H0 for each map, thus enabling the catalytic conversion efficiency to reach its highest value in less time.

[0050] The search for and updating of the optimal frequency H0 is preferably implemented as frequently as the operating time or mileage exceeds a certain value, for example, 10,000 hours and 10,000 km. Updating the optimal frequency H0 at such a time interval allows the optimal frequency H0 to take into account or follow the change in the local maximum value caused by the aging of the engine 50 and the catalyst 56.

[0051] The following describes the procedure for determining the optimal frequency H0 and adjusting the catalytic conversion efficiency to the optimal value based on the flowchart according to Fig. 9 with reference to the Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 to Fig. 8 explained.

[0052] The following description refers to the case where the search unit 12 seeks the optimal value using the gradient g of the graph, in which the horizontal axis corresponds to the frequency H and the vertical axis to the minimum air-excess ratio λ. r_min corresponds.

[0053] As in Fig. As shown in Figure 9, the search unit 12 monitors both the downstream air excess ratio λ in step S11. r , which is output by the downstream air-fuel ratio sensor 58, as well as the upstream excess air ratio λ f , which is output by the upstream air-fuel ratio sensor 57. At this time, the search unit 12 causes the air-fuel ratio adjuster 11 to adjust the λ-vary based on the upstream excess air ratio λ. rThe search unit 12 controls the air-fuel ratio output by the upstream air-fuel ratio sensor 57. Furthermore, the search unit 12 extracts the minimum excess air ratio λ. r_min , which is the local minimum value of the λ-variance of the upstream excess air ratio λ r it.

[0054] In the next step S12, the frequency H is gradually reduced by a predetermined logarithmic scale starting from the standard value of 1 Hz, and the minimum excess air ratio λ r_min The process is recorded on a semi-logarithmic graph. Processing of step S12 continues while the gradient g of the graph is equal to or less than the threshold g0.

[0055] If the result of the determination as to whether the gradient g exceeds the threshold g0 in step 13 is positive (that is, if the gradient g becomes greater than the threshold g0), processing proceeds to step S14, in which the processing of step S12 is halted. The frequency determination unit 13 determines the frequency H at this time (that is, at the time when the gradient g exceeds the threshold g0) as the optimal frequency H0. The air-fuel ratio adjuster 11 sets the frequency in accordance with the frequency H of the λ fluctuation.

[0056] If the threshold value g0 is too large, even if a transition from the high frequency range α to the low frequency range β occurs, this transition cannot be detected using the threshold value g0. If the threshold value g0 is too small, a transition will be erroneously detected before it actually occurs, and consequently, an incorrect frequency H will be erroneously detected. Experimentally, it was found that this threshold value g0 preferably lies in the range of approximately 0.015 to 0.025. Setting the threshold value g0 within this range allows for the precise determination of the optimal frequency H0. Note that the threshold value g0 is determined during the design stage depending on the specifications of the catalyst 56 or the specifications of the engine 50.

[0057] With the control unit 10 of the first embodiment according to the above description, a high catalytic conversion efficiency can be maintained with a simple design even when the temperature of the catalyst or the operating condition of the combustion engine is subject to changes. (Second embodiment)

[0058] Fig. Figure 10 is a flowchart illustrating the operation of the controller 10 according to the second embodiment.

[0059] In the second embodiment according to Fig. In step S15, the frequency determination unit 13 sets the optimum frequency H0 to a value that is increased or decreased by a small percentage with respect to the frequency H of the λ fluctuation at the time when the gradient g exceeds the threshold g0.

[0060] The small percentage mentioned above means a ratio of 20% or less of the delta (that is, increment or decrement) to the time of increasing or decreasing the frequency H, for example, 10% of the delta of the frequency H. That is to say, in the second embodiment, the optimal frequency H0 is updated in step S15 by increasing or decreasing the small percentage relative to the optimal frequency H0, as determined in the first embodiment.

[0061] Small deltas of frequency H cause torque fluctuations each time, which negatively impacts the driving comfort for the occupant. In particular, small flywheels cause larger torque fluctuations, which the occupant finds unpleasant.

[0062] For this reason, in the second embodiment, a fairly large delta for the frequency H is used to reduce the frequency of torque fluctuations caused by the search processing of the search unit 12. If it is determined that the gradient g exceeds the threshold g0, it is assumed that the frequency H has exceeded the optimal frequency H0 and is lower than the determined optimal frequency H0. Therefore, the frequency determination unit 13 adjusts the optimal frequency H0 by increasing the small percentage of the detected frequency H that is assumed to be lower than the optimal frequency H0.

[0063] In the case of gradually increasing the frequency H from the low-frequency range β, the optimal frequency H0 is set by subtracting the small percentage from the frequency H at the time when the gradient g exceeds the threshold g0.

[0064] The second embodiment is the same as the first, except that the search unit 12 sets the optimal frequency H0 by correcting the detected frequency H by a small percentage at the end of the search process. Therefore, a repeated description is unnecessary. In each figure, identical reference numerals denote identical or equivalent components, so their description is also omitted.

[0065] As explained above, according to the second embodiment, the control unit 10 can suppress torque fluctuations, thereby increasing the driving comfort for the person in the vehicle.

[0066] By means of the control 10 of each embodiment according to the above description, a high catalytic conversion efficiency can be maintained with the aid of a simple configuration even when the temperature of the exhaust gas purification catalyst 56 or the operating condition of the engine 50 changes.

[0067] While certain embodiments have been described, these serve only as examples and are not intended to limit the scope of protection of the invention. These embodiments can be implemented in various other aspects, including omissions, substitutions, modifications, and combinations of exemplary embodiments, without deviating from the scope of protection of the invention. These embodiments and their modifications are included in the scope of protection of the invention and in the fundamental concept of the invention; they are encompassed by the claims and their equivalents.

[0068] Although, for example, the control 10 has been described as being more suitable for motors where a change in the motor operating range takes place relatively slowly, the application of the control 10 is not limited to such motors.

Claims

[1] Air-fuel ratio control (10), comprising: an air-fuel ratio adjuster (11) configured to cause an air-fuel ratio of exhaust gas (G) on an upstream side of an exhaust gas purification catalyst (56) connected to an engine (50) to fluctuate between a rich side and a lean side, with the exhaust gas (G) flowing through the exhaust gas purification catalyst (56); a search unit (12) configured to incrementally change a frequency (H) of the fluctuation, and to measure a downstream air-fuel ratio and change the downstream air-fuel ratio with respect to a change in the frequency (H) of the fluctuation, wherein the downstream air-fuel ratio is the air-fuel ratio of the exhaust gas (G) on a downstream side of the exhaust gas purification catalyst (56); and a frequency determination unit (13) configured to determine a frequency (H) at the time when a value of the downstream air-fuel ratio reaches a predetermined threshold (Ω) or at the time when a gradient (g) of the downstream air-fuel ratio with respect to frequency reaches a predetermined threshold (g0) as an optimal frequency (H0). [2] Air-fuel ratio control (10) according to claim 1, wherein the gradient (g) is a rate of change of the air-fuel ratio with respect to the logarithm of the frequency. [3] Air-fuel ratio control (10) according to claim 2, wherein the threshold value (g0) is in a range of 0.015 to 0.

025. [4] Air-fuel ratio control (10) according to any one of claims 1 to 3, in which the air-fuel ratio adjuster (11) has a plurality of maps for each frequency (H), each of which specifies a relationship between engine speed and operating load, wherein the search unit (12) is configured to perform a new search for the optimal frequency (H0) responding to a shift to another card for a different driving condition. [5] Air-fuel ratio control (10) according to any one of claims 1 to 4, in which the frequency determination unit (13) is configured to correct the optimal frequency (H0) by raising or lowering a predetermined rate of the frequency width used in searching with respect to the optimal frequency (H0). [6] Air-fuel ratio control system, comprising: the air-fuel ratio control (10) according to any one of claims 1 to 5; the engine (50); the exhaust gas purification catalyst (56); and at least two sensors (57, 58) located on an upstream or are located on a downstream side of the exhaust gas purification catalyst (56) and demonstrate air-fuel ratios.

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