Air-fuel ratio control device and air-fuel ratio control system

Abstract

To provide an air-fuel ratio control device and an air-fuel ratio control system capable of maintaining a high purification rate by using a simple configuration even when an operating state of an internal combustion engine and a temperature of an exhaust emission control catalyst have been changed.SOLUTION: An air-fuel ratio control device 10 includes: an air-fuel ratio adjustment section 11 that vibrates an air-fuel ratio on an upstream side of exhaust gas G flowing in an exhaust emission control catalyst 56 connected to an engine 50 to the rich side and the lean side; a search section 12 changing frequency of the vibration step by step to measure a downstream side air-fuel ratio that is an air-fuel ratio in the exhaust gas G on the downstream side of the exhaust emission control catalyst 56 or a change of the downstream side air-fuel ratio to a change of the frequency; and a frequency determination section 13 determining the frequency H when a value of the downstream side air-fuel ratio or an inclination g of the downstream side air-fuel ratio relative to the frequency H reaches a predetermined threshold value g0 as optimum frequency H0.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to an air-fuel ratio control technique for an engine. [Background technology]

[0002] In general, a mixture of fuel and air supplied to an internal combustion engine is subjected to feedback control to maintain a stoichiometric air-fuel ratio at which the oxygen in the mixture reacts with the fuel in just the right amount. When a sensor installed in the exhaust system of an internal combustion engine detects a fuel-rich condition, the air-fuel ratio of the mixture is raised via the engine control unit (ECU). Conversely, when a fuel-lean condition is detected, fuel is added to lower the air-fuel ratio. In this way, through feedback control based on exhaust gas components, the air-fuel ratio is maintained approximately at the stoichiometric air-fuel ratio while fluctuating at a predetermined frequency across the stoichiometric air-fuel ratio.

[0003] Incidentally, it has been known for some time that the purification rate of an exhaust gas purification catalyst (sometimes simply referred to as "catalyst") can be improved by forcibly oscillating the air-fuel ratio. The mechanism by which the purification rate of the catalyst improves with the frequency of this oscillation is believed to be due to the involvement of oxygen active species. Another known technology improves the purification rate of a catalyst by increasing the temperature of the catalyst by oscillating the air-fuel ratio to bring more reactive components into contact with the catalyst. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 52-081438 Summary of the Invention [Problem to be solved by the invention]

[0005] However, there is a problem in that the purification rate of a catalytic converter may deviate from its maximum value when the operating state of the internal combustion engine, such as a change in the amount of exhaust gas, or the temperature of the exhaust purification catalyst changes.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide an air-fuel ratio control device and an air-fuel ratio control system that have a simple configuration and can maintain a high purification rate even when the operating state of the internal combustion engine or the temperature of the exhaust purification catalyst changes. [Means for solving the problem]

[0007] The air-fuel ratio control device according to this embodiment includes an air-fuel ratio adjusting unit that oscillates the air-fuel ratio on the upstream side of exhaust gas flowing through an exhaust purification catalyst connected to an engine between the rich side and the lean side, and a downstream air-fuel ratio that is the air-fuel ratio in the exhaust gas downstream of the exhaust purification catalyst by changing the frequency of the oscillation in stages. In number a search unit that measures a change in the downstream air-fuel ratio relative to the Downstream side the value of the air-fuel ratio or the downstream air-fuel ratio for the frequency Changes in and a frequency determination unit that determines the frequency when the gradient of the frequency reaches a predetermined threshold as the optimum frequency. [Effects of the Invention]

[0008] The present invention provides an air-fuel ratio control device and an air-fuel ratio control system that have a simple configuration and can maintain a high purification rate even when the operating state of the internal combustion engine or the temperature of the exhaust purification catalyst changes. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a block diagram showing an intake port fuel injection engine to which an air-fuel ratio control device according to a first embodiment of the present invention is applied. [Figure 2] FIG. 1 shows the composition of the model gas used in the verification experiment. [Figure 3]FIG. 10 is an experimental graph showing the relationship between the frequency measured by changing the catalyst performance, temperature, and volumetric velocity and the conversion rate of the catalyst. [Figure 4] FIG. 10 is an experimental graph showing the relationship between the frequency measured by changing the catalyst performance, temperature, and volumetric velocity and the conversion rate of the catalyst. [Figure 5] FIG. 10 is an experimental graph showing the relationship between the frequency measured by changing the catalyst performance, temperature, and volumetric velocity and the conversion rate of the catalyst. [Figure 6] FIG. 10 is an experimental graph showing the relationship between the frequency measured by changing the catalyst performance, temperature, and volumetric velocity and the conversion rate of the catalyst. [Figure 7] 6 is a graph showing output waveforms of the upstream and downstream air-fuel ratio sensors in a high frequency region higher than the maximum value of the purification rate. [Figure 8] FIG. 8 is a graph showing output waveforms of the upstream and downstream air-fuel ratio sensors in a low frequency region below the maximum value of the purification rate. [Figure 9] 4 is a flowchart illustrating a procedure for determining an optimum frequency and adjusting the purification rate to an optimum value. [Figure 10] 6 is a flowchart illustrating the operation of the air-fuel ratio control device according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the embodiment, the terms "upstream" and "downstream" are used based on 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."

[0011] (First embodiment) First, an air-fuel ratio control device (hereinafter simply referred to as the "control device") 10 according to a first embodiment will be outlined using a block diagram of an engine 50 and its peripheral devices in FIG. The control device 10 according to the first embodiment is suitably applied to an engine 50 in which the engine operating range changes relatively slowly, such as a series-connected hybrid engine or a stationary engine. 1, an intake pipe 52 that is open to the atmosphere via an air cleaner 51 is connected to the engine 50 via an intake manifold. Air is supplied to the fuel chamber of the engine 50 through this intake pipe 52.

[0012] An exhaust pipe 53 is connected to an exhaust port of the engine 50 via an exhaust manifold. A catalytic converter 54 is connected downstream of the exhaust pipe 53. The catalytic converter 54 houses an exhaust purification catalyst (hereinafter simply referred to as "catalyst") 56, such as a three-way catalyst, that removes CO, HC, and NOx from the exhaust gas G. The catalytic converter 54 is also provided with air-fuel ratio sensors (A / F sensors) 57, 58 on the upstream and downstream sides of the catalyst 56, respectively. The upper air-fuel ratio sensor 57, which is installed upstream of the catalyst 56, measures the air-fuel ratio of the exhaust gas G that is burned in the engine 50, exhausted, and flows into the catalyst 56. The lower air-fuel ratio sensor 58, which is installed downstream of the catalyst 56, measures the air-fuel ratio of the exhaust gas G that is purified by the catalyst 56 and flows out from the catalyst 56 (hereinafter referred to as "downstream air-fuel ratio"). In other words, the purification system for the exhaust system in the first embodiment is configured as a so-called 2 A / F sensor system.

[0013] An engine control unit (ECU) 59 is also connected to the engine 50. The ECU 59 is configured as a microcomputer equipped with a central processing unit (CPU), read-only memory (ROM), and random access memory (RAM). The CPU executes desired calculations in accordance with a control program to perform various processes and controls. The ROM stores the control program and control data to be processed by the CPU, and the RAM is mainly used as a work area for various control processes. The ECU 59 is connected via a network 60 to sensors 61 , 57 , 58 and actuators provided at various locations on the engine 50 and the catalytic converter 54 , and monitors and controls the operation of the engine 50 .

[0014] The ECU 59 is also provided with the control device 10 according to the first embodiment. The control device 10 includes an air-fuel ratio adjusting unit 11, a frequency determining unit 12, and a searching unit 13. The air-fuel ratio adjustment unit 11 determines whether the air-fuel mixture in the engine 50 is rich (fuel rich) or lean (fuel lean) based on information about the air-fuel ratio of the exhaust gas G sent to the ECU 59 from the two air-fuel ratio sensors 57, 58 described above. The air-fuel ratio adjustment unit 11 then constantly increases or decreases the amount of fuel supplied to adjust the air-fuel ratio so that the air-fuel ratio becomes the stoichiometric air-fuel ratio. This adjustment causes the intake air-fuel ratio value to fluctuate between rich and lean at a constant cycle, straddling the stoichiometric air-fuel ratio. This fluctuation will be referred to as λ oscillation hereinafter, and the oscillation frequency of this λ oscillation will be referred to as frequency H. Note that by oscillating the intake air-fuel ratio, the air-fuel ratio value of the exhaust gas G also λ oscillates at frequency H.

[0015] Here, the excess air ratio λ is a physical quantity that is standardized by dividing the air-fuel ratio by the stoichiometric air-fuel ratio, and is roughly the same concept as the air-fuel ratio. The excess air ratio λ defined in this way is 1 when the air-fuel ratio matches the stoichiometric air-fuel ratio. When the excess air ratio λ is less than 1, the air-fuel ratio is rich, and when the excess air ratio λ is greater than 1, the air-fuel ratio is lean. In the following description, each of the components 11 to 13 of the control device 10 will be described as monitoring and controlling this excess air ratio λ, which is a normalized air-fuel ratio, instead of the air-fuel ratio itself.

[0016] The search unit 13 calculates the upper excess air ratio λ obtained by the upper air-fuel ratio sensor 57 via the air-fuel ratio adjustment unit 11. f Based on this upper air excess ratio λ f By changing the frequency H of λ oscillation, the optimum frequency H o This is because it is estimated that the conversion rate is maximized at the optimum frequency Ho, where oxygen active species can be utilized most efficiently.

[0017] 2 to 6, a verification experiment will be described in which the relationship between the frequency H of the λ oscillation, the purification rate of the catalyst 56, and the excess air ratio λ based on the downstream air-fuel ratio was investigated while changing environmental conditions. FIG. 2 is a table showing the composition of the model gas used in this verification experiment. In this verification experiment, the excess air ratio λ of the model gas was oscillated between rich (λ=0.95) and lean (λ=1.05), and the purification rates of THC (total hydrocarbons) and NOx were investigated in relation to the frequency H of the λ oscillation. The table in Figure 2 confirms that when the excess air ratio λ is rich in accordance with the λ oscillation, the content of carbon monoxide (CO), an unburned component of fuel, is high, and when it is lean, the content of oxygen (O2) is high.

[0018] 3 to 6 are experimental graphs showing the relationship between the frequency H and the purification rate of the catalyst 56, measured by changing the performance, temperature, and volumetric velocity of the catalyst 56. In all of Figures 3 to 6, the horizontal axis is the logarithmic frequency H [Hz], the left vertical axis is the purification rate [%], and the right vertical axis is the minimum excess air ratio λ, which will be described in detail later using Figures 7 and 8. r_min Since the horizontal axis is logarithmic, 0 on the scale is 1 [Hz], -1 on the scale is 10 -1 3 to 6, the graphs represented by "■" and thin solid lines indicate the purification rate of total hydrocarbons, and the graphs represented by "●" and thin dashed lines indicate the purification rate of NOx. Also, "▲" and "△" indicate the measured values ​​of the excess air factor λ, and the thick dashed lines are linear fittings of these measured values ​​of the excess air factor λ.

[0019] FIG. 3 shows the experimental results when the catalyst performance was set to high, the flow rate of the supplied exhaust gas G was set to "low flow rate" which is 30 x 103 times the catalyst flow rate, and the temperature of the catalyst 56 was set to 450°C. FIG. 4 shows the experimental results when the temperature was changed to 350°C under the conditions of FIG. 3. FIG. 5 shows the experimental results when the flow rate was changed to "high flow rate" which is about 8 times the flow rate under the conditions of FIG. 3. FIG. 6 shows the experimental results when the catalyst performance was changed to low performance under the conditions of FIG. 3.

[0020] In any of the environmental conditions shown in Figures 3 to 6, the minimum excess air ratio λ r_min It can be seen that changes linearly with the logarithm of the frequency H, and the slope g changes discontinuously at the point where the purification efficiency is maximized. In other words, it can be seen that the purification efficiencies of both total hydrocarbons and NOx reach almost their maximum values ​​in the vicinity of frequency H where the measured value of the excess air ratio λ jumps discontinuously. In other words, it can be confirmed that the purification efficiencies of the catalyst 56 reach almost their maximum when the excess air ratio λ approaches 1.

[0021] Here, the minimum excess air ratio λ, which is the variable on the right vertical axis in Figures 3 to 6, r_min This article explains: Fig. 7 is a graph showing the output waveforms of the upstream and downstream air-fuel ratio sensors 57, 58 in a high frequency range α higher than the maximum value of the purification rate, and Fig. 8 is a graph showing the output waveforms of the upstream and downstream air-fuel ratio sensors 57, 58 in a low frequency range β lower than the maximum value of the purification rate. In the graphs of FIGS. 7 and 8, the horizontal axis represents time [s], the left vertical axis represents the gas temperature [° C.] at the inlet of the catalyst 56, and the right vertical axis represents the excess air factor λ. In the graph, the air excess ratio λ detected by the upper air-fuel ratio sensor 57 is expressed as the upper air excess ratio λ f The lower excess air ratio λ based on the downstream air-fuel ratio detected by the lower air-fuel ratio sensor 58 is represented by a thin line. r is shown by a thick line.

[0022] In the high frequency region α of Fig. 7, the lower air excess ratio λ r The vibration of the upper air excess ratio λ f It can be seen that the lower air excess ratio λ is attenuated significantly compared to the lower air excess ratio λ. This attenuation is thought to be due to the oxygen storage function (OSC function) of the catalyst 56 absorbing the λ oscillation. On the other hand, in the low frequency range β in Figure 8, r It can be seen that the λ oscillation is not attenuated. This is thought to be because the OSC function of the catalyst 56 can no longer alleviate the λ oscillation in the low frequency region β. From this result, in the high frequency region α, the lower air excess ratio λ rIn the low frequency region β, the influence of frequency H is extremely small, and therefore the value of the slope g is expected to be relatively small. r becomes smaller as the frequency H decreases, and therefore it can be expected that the value of the slope g will become relatively larger.

[0023] Therefore, in the verification experiment, the lower air excess ratio λ r The minimum air excess ratio λ is the minimum value of the oscillation of r_min The above-mentioned tendency was confirmed by reading the graphs in Figures 7 and 8 and plotting them on the graphs in Figures 3 to 6. If the frequency H is further lowered below the low frequency region β, steady rich and steady lean are repeated, and the minimum excess air ratio λ r_min is no longer affected by frequency H. It has also been confirmed that the purification rate of the catalyst 56 is low in this region.

[0024] Based on this verification experiment, the search unit 13 changes the frequency H shown in FIGS. 3 to 6 to obtain the minimum excess air ratio λ r_min Then, the search unit 13 measures the minimum air excess ratio λ when the state transitions from the high frequency region α to the low frequency region β. r_min The frequency H when the value of changes suddenly and discontinuously, i.e., the optimum frequency H o Explore. Optimal frequency H o is the minimum excess air ratio λ r_min Instead of the minimum excess air ratio λ on the vertical axis r_min The search may be performed using a gradient g obtained by dividing the change in the value of by the change in frequency H on the horizontal axis. In other words, the search may be performed using a gradient g calculated from the output value of the lower air-fuel ratio sensor 58 using the following equation (1). g=Δλ r_min / Δlog(frequency H) (1) In addition, by taking the logarithm of frequency H instead of the frequency itself, the minimum air excess ratio λ for frequency H can be calculated. r_min This allows for clearer detection of trends. The frequency determination 12 determines the frequency H at which the slope g changes suddenly as the optimal frequency H o The optimum frequency H determined by the frequency determination 12 is o The air-fuel ratio adjusting unit 11 maintains this.

[0025] As mentioned above, the minimum air excess ratio λ is used instead of the slope g. r_min The minimum excess air ratio λ is monitored. r_min The frequency H at which the purification rate becomes maximum may be estimated from a discontinuous change in . In this case, for example, when the frequency H is lowered, it is determined that the frequency range at which the purification rate becomes maximum is the frequency range at which the purification rate becomes maximum when the frequency H becomes less than a preset threshold value Ω. In addition, the estimation by the slope g and the minimum air excess ratio λ r_min The optimum frequency H o These can be combined to identify the optimum frequency H o By determining the optimum frequency H o The accuracy can be improved.

[0026] It is desirable that the ECU 59 stores a map showing the operating state of the engine 50, such as a map showing the relationship between the engine speed and the operating load, for each frequency H. Then, when the operating state changes to a different map, the search unit 13 searches for the optimum frequency H. o It is desirable to start searching for the optimum frequency H for each map. By restarting the search when the operating conditions change significantly, the purification rate can be maintained at the highest value. o The valid range of the optimum frequency H may be stored in the ECU 59. When a different map is used, the optimum frequency H is set within the valid range stored for the map. o By searching for the best possible purification rate, it is possible to maximize the purification rate in a shorter time.

[0027] In addition, when the driving time or distance exceeds a certain value, such as 10,000 hours or 10,000 km, the optimum frequency H oBy correcting at such timing, the optimum frequency H o can be corrected.

[0028] Next, using the flowchart in Figure 9, the optimum frequency H o The procedure for determining the rate and adjusting the purification rate to an optimum value will be described (see FIGS. 3 to 8 as appropriate). In FIG. 9 and the following description, each step will be denoted as "S11," etc. The search unit 13 also calculates the frequency H on the horizontal axis and the minimum air excess factor λ r_min This will be explained using an example of searching using the gradient g when the vertical axis is

[0029] First, as shown in FIG. 9, the search unit 13 calculates the lower excess air ratio λ , which is the output value of the lower air-fuel ratio sensor 58. r and the upper excess air ratio λ, which is the output value of the upper air-fuel ratio sensor 57 f At this time, the search unit 13 monitors the upper excess air ratio λ output from the upper air-fuel ratio sensor 57 via the air-fuel ratio adjustment unit 11 (S11). f The search unit 13 controls the λ oscillation based on the lower air excess ratio λ. r The minimum air excess ratio λ is the minimum value of the λ oscillation. r_min Extract. Next, gradually decrease the frequency H from the standard value of 1 Hz by the specified scale on the logarithmic scale, and plot the minimum excess air ratio λ on a semi-logarithmic graph. r_min is plotted (S12).

[0030] As long as the gradient g of the graph is less than the threshold value g0, the frequency H is decreased (NO in S13, go to S12). Then, if the gradient g of the graph exceeds the threshold value g0 (YES in S13), it is determined that a transition has occurred to the low frequency region β, and the process ends (S14, END). The frequency determination 12 determines the frequency H at this time as the optimum frequency H o The air-fuel ratio adjusting unit 11 also adjusts the frequency of the λ oscillation to be H.

[0031] If the value of the threshold g0 is too large, the threshold g0 will not be able to detect the transition from the high frequency region α to the low frequency region β. Also, if the value of the threshold g0 is too small, it will erroneously diagnose that a transition has occurred when no transition has actually occurred, resulting in an incorrect detection of an inappropriate frequency H. Experiments have shown that the threshold g0 should be set to approximately 0.015 to 0.025. By setting the threshold g0 in this range, the optimum frequency H can be accurately determined. o However, it is determined at the design stage depending on the specifications of the catalyst 56 and the engine 50.

[0032] As described above, the control device 10 according to the first embodiment can maintain a high purification rate with a simple configuration even when the operating state of the internal combustion engine or the temperature of the catalyst 56 changes.

[0033] (Second embodiment) FIG. 10 is a flowchart illustrating the operation of the control device 10 according to the second embodiment. In the second embodiment, as shown in FIG. 10, the frequency determination 12 increases or decreases the frequency H of the λ oscillation by a small percentage when the slope g exceeds the threshold value g0 to obtain the optimum frequency H. o (S15). Here, the minute ratio means a ratio of 20% or less, such as 10% of the step width of the frequency H. In other words, in the second embodiment, the optimum frequency H determined in the first embodiment is o By adjusting the frequency by a small percentage, the optimum frequency H o Let's say.

[0034] If the step size of the change in frequency H is made small, torque fluctuations occur each time, which impairs the riding comfort of the passenger. In particular, if the flywheel is small, the torque fluctuations felt by the passenger will be large. Therefore, in the second embodiment, the frequency H is increased in increments to reduce the number of torque fluctuations due to the search by the search unit 13. When it is detected that the gradient g has exceeded the threshold value g0, the optimum frequency H oIt is assumed that a smaller frequency H is detected across the frequency band. Then, the frequency decision 12 determines the optimum frequency H o The optimum frequency H is calculated by adding a small percentage to the detected frequency H, which is considered to be smaller than the o Let's say. When the frequency H is increased stepwise from the low frequency region β, the optimum frequency H is obtained by subtracting a small percentage from the frequency H when the threshold value g0 is exceeded. o Let's say.

[0035] In the second embodiment, the frequency H detected in the operation procedure of the search unit 13 is corrected by a small percentage to obtain the optimum frequency H o Other than that, the second embodiment is the same as the first embodiment, and therefore a duplicated description will be omitted. Similarly, in the drawings, duplicated configurations will be assigned the same reference numerals and a description will be omitted.

[0036] As described above, the control device 10 according to the second embodiment can provide a comfortable ride to the passenger with small torque fluctuations.

[0037] According to the control device 10 of each of the above-described embodiments, a high purification rate can be maintained with a simple configuration even when the operating state of the engine 50 or the temperature of the exhaust purification catalyst 56 changes.

[0038] Although an embodiment of the present invention has been described, this embodiment is presented as an example and is not intended to limit the scope of the invention. The embodiments may be embodied in various other forms, and various omissions, substitutions, modifications, and combinations may be made without departing from the spirit of the invention. The embodiments and their modifications are included in the scope of the inventions and their equivalents as defined in the claims, as well as in the scope and spirit of the inventions.

[0039] For example, although it has been explained that the air-fuel ratio control device is more suitable for an engine in which the engine operating range changes relatively slowly, the engine to which the air-fuel ratio control device is applied is not limited to an engine in which the change occurs slowly. [Explanation of symbols]

[0040] 10...control device, 11...air-fuel ratio adjustment unit, 12...frequency determination, 13...search unit, 50...engine, 51...air cleaner, 52...intake pipe, 53...exhaust pipe, 54...catalytic converter, 56...exhaust purification catalyst (catalyst), 57...upper air-fuel ratio sensor, 58...lower air-fuel ratio sensor, 59...ECU, 60...network, 61...sensor, G...exhaust gas, H...frequency, H o ...optimum frequency, g...slope, g0...threshold, Ω...threshold, α...high frequency range, β...low frequency range, λ...excess air ratio, λ f …Upper air excess ratio, λ r …lower air excess ratio, λ r_min ...Minimum excess air ratio.

Claims

1. an air-fuel ratio adjusting unit that oscillates an air-fuel ratio between a rich side and a lean side on the upstream side of exhaust gas flowing through an exhaust purification catalyst connected to the engine; a searching unit that measures a downstream air-fuel ratio, which is an air-fuel ratio in the exhaust gas downstream of the exhaust purification catalyst, or a change in the downstream air-fuel ratio with respect to the frequency by changing the frequency of the vibration in a stepwise manner; a frequency determination unit that determines, as an optimal frequency, the frequency when the value of the downstream air-fuel ratio or a gradient of change in the downstream air-fuel ratio with respect to the frequency reaches a predetermined threshold value.

2. 2. The air-fuel ratio control device according to claim 1, wherein the gradient is a rate of change of the downstream air-fuel ratio with respect to the logarithm of the frequency.

3. the threshold value is in the range of 0.015 to 0.025; 3. The air-fuel ratio control device according to claim 2, wherein the frequency determiner determines, as the optimal frequency, the frequency at which an absolute value of a gradient at which the downstream air-fuel ratio decreases with respect to a logarithm of the frequency reaches the threshold value when lowering the frequency of the vibration.

4. a map showing a relationship between the rotation speed and the operating load of the internal combustion engine for each of the frequencies; 4. The air-fuel ratio control device according to claim 1, wherein the search unit performs the search again when the map is changed to a different map.

5. 5. The air-fuel ratio control device according to claim 1, wherein the frequency determination unit corrects the optimum frequency by adding or subtracting a predetermined proportion of the frequency width used in the search to or from the optimum frequency.

6. An air-fuel ratio control device according to any one of claims 1 to 5; the engine; the exhaust purification catalyst; and two or more air-fuel ratio sensors arranged upstream and downstream of the exhaust purification catalyst to detect the air-fuel ratio.

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