JP7760896B2 - Air-fuel ratio control device and air-fuel ratio control system - Google Patents

Description

Embodiments of the present invention relate to engine air-fuel ratio control technology.

Generally, 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 fuel-rich condition is detected by a sensor installed in the exhaust system of an internal combustion engine, 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.

It has long been known 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 catalyst's purification rate improves with the frequency of this oscillation is thought to be due to the involvement of oxygen activated species. Another known technology improves the catalyst's purification rate by increasing the temperature of the catalyst by oscillating the air-fuel ratio to bring more reactive components into contact with the catalyst.

Japanese Unexamined Patent Publication No. 52-081438

However, there was an issue with the purification rate of catalytic converters, in that the purification rate could deviate from its maximum value when the operating conditions of the internal combustion engine, such as changes in exhaust gas volume, or the temperature of the exhaust purification catalyst changed.

The present invention was made in consideration of these circumstances, and aims to provide an air-fuel ratio control device and air-fuel ratio control system with a simple configuration that 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.

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 the exhaust gas flowing through an exhaust purification catalyst connected to an engine between the rich side and the lean side, a searching unit that changes the frequency of the oscillation in stages to measure a downstream air-fuel ratio, which is the air-fuel ratio in the exhaust gas on the downstream side of the exhaust purification catalyst, or a change in the downstream air-fuel ratio with respect to the frequency , and a frequency determining unit that determines, as an optimum frequency, the frequency when the value of the downstream air-fuel ratio or the gradient of the change in the downstream air-fuel ratio with respect to the frequency reaches a predetermined threshold value.

The present invention provides an air-fuel ratio control device and air-fuel ratio control system that has 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.

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; FIG. 1 shows the composition of the model gas used in the verification experiment. 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. 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. 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. 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. 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. 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. 4 is a flowchart illustrating a procedure for determining an optimum frequency and adjusting the purification rate to an optimum value. 10 is a flowchart illustrating the operation of an air-fuel ratio control device according to a second embodiment.

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."

(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.

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 the "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, respectively, upstream and downstream of the catalyst 56. The upper air-fuel ratio sensor 57, located 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, located 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 of the catalyst 56 (hereinafter referred to as the "downstream air-fuel ratio"). In other words, the exhaust purification system in the first embodiment is configured as a so-called two A/F sensor system.

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 programs and control data 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 .

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 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 to the stoichiometric air-fuel ratio. This adjustment causes the intake air-fuel ratio to fluctuate between rich and lean at a constant cycle, straddling the stoichiometric air-fuel ratio. This fluctuation is hereinafter referred to as λ oscillation, and the oscillation frequency of this λ oscillation is referred to as frequency H. Note that by oscillating the intake air-fuel ratio, the air-fuel ratio of the exhaust gas G also oscillates λ at frequency H.

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 based on almost 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.

The search unit 13 searches for the optimum frequency Ho by changing the frequency H of the λ oscillation of the upper air excess ratio λ _f based on the upper air excess ratio λ _f acquired by the upper air-fuel ratio sensor 57 via the air-fuel ratio adjustment unit 11. This is because it is estimated that the purification rate becomes maximum at the optimum frequency _Ho because oxygen active species can be utilized most efficiently.

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 as a function of the λ oscillation frequency H. 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.

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 represents the frequency H [Hz] in logarithmic scale, the left vertical axis represents the purification rate [%], and the right vertical axis represents the minimum excess air factor λ _{r_min} , which will be described in detail later using Figures 7 and 8. Because the horizontal axis is displayed logarithmically, 0 on the scale represents 1 [Hz], and -1 on the scale represents 10 ^-1 [Hz]. In addition, in Figures 3 to 6, graphs represented by "■" and thin solid lines represent the purification rate of total hydrocarbons, and graphs represented by "●" and thin dashed lines represent the purification rate of NOx. Furthermore, "▲" and "△" represent 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.

Figure 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. Figure 4 shows the experimental results when the temperature was changed to 350°C under the conditions of Figure 3. Figure 5 shows the experimental results when the flow rate was changed to "high flow rate" which is approximately 8 times the flow rate under the conditions of Figure 3. Figure 6 shows the experimental results when the catalyst performance was changed to low performance under the conditions of Figure 3.

In this way, in all of Figures 3 to 6 where the environmental conditions are changed, it can be seen that the minimum excess air factor λ _{r_min} changes linearly with the logarithm of the frequency H, and the slope g changes discontinuously at the point where the purification efficiency becomes maximum.
In other words, it can be seen that the purification rates of both total hydrocarbons and NOx reach 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 rates of the catalyst 56 reach their maximum when the excess air ratio λ approaches 1.

Here, the minimum excess air ratio λ _{r_min,} which is the variable on the right vertical axis in FIGS. 3 to 6, will be described.
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.
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 excess air ratio λ detected by the upper air-fuel ratio sensor 57 is represented by a thin line as an upper excess air ratio _λf , and the excess air ratio λ based on the downstream air-fuel ratio detected by the lower air-fuel ratio sensor 58 is represented by a thick line as a lower excess air ratio _λr .

It can be seen that in the high frequency region α of Fig. 7, the oscillations of the lower air excess ratio _λr are attenuated more significantly than those of the upper air excess ratio _λf . This attenuation is thought to be due to the oxygen storage capacity (OSC) function of the catalyst 56 absorbing the λ oscillations. On the other hand, in the low frequency region β of Fig. 8, it can be seen that the oscillations of the lower air excess ratio _λr are not attenuated. This is thought to be due to the fact that the OSC function of the catalyst 56 can no longer mitigate the λ oscillations in the low frequency region β.
From this result, it can be predicted that in the high frequency region α, the effect of frequency H on the lower-side air excess factor _λr is extremely small, and therefore the value of slope g will be relatively small. Also, in the low frequency region β, it can be predicted that the lower-side air excess factor _λr will become smaller as frequency H decreases, and therefore the value of slope g will be relatively large.

Therefore, in the verification experiment, the minimum air excess ratio λ _{r_min} , which is the minimum value of the oscillation of the lower air excess ratio λ _r, was read from the graphs of FIGS. 7 and 8 and plotted on the graphs of FIGS. 3 to 6 to actually confirm the above-mentioned tendency.
If the frequency H is further lowered below the low frequency region β, the steady rich and steady lean states are repeated, and the minimum air excess ratio λ _{r_min} is again no longer affected by the frequency H. It has also been confirmed that the purification rate of the catalyst 56 is low in this region.

Based on this verification experiment, the search unit 13 successively measures the minimum air excess factor λ _{r_min} while changing the frequency H shown in Figures 3 to 6. Then, the search unit 13 searches for the frequency H at which the value of the minimum air excess factor λ _{r_min} changes suddenly and discontinuously as the state transitions from the high frequency region α to the low frequency region β, i.e., the optimal frequency _Ho .
The optimal frequency H _o may be searched for using a gradient g obtained by dividing a change in the value of the minimum excess air ratio λ _{r_min} on the vertical axis by a change in the frequency H on the horizontal axis, instead of the minimum excess air ratio λ _{r_min} . In other words, the optimal frequency H o may be searched for 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)

It should be noted that by taking the logarithm of the frequency H rather than the frequency itself, the tendency of the minimum air excess factor λ _{r — min} relative to the frequency H can be detected more clearly.
The frequency determination unit 12 determines the frequency H that causes the gradient g to change rapidly as the optimum frequency _H 0. The air-fuel ratio adjustment unit 11 maintains the optimum frequency H ₀ determined by the frequency determination unit 12.

Note that instead of the gradient g, the numerical value of the minimum air excess factor λ _{r_min} itself may be monitored as described above, and the frequency H at which the purification efficiency becomes maximum may be estimated from discontinuous changes in the minimum air excess factor λ _{r_min} . In this case, for example, when the frequency H is lowered, the frequency range at which the purification efficiency becomes maximum is determined to be the frequency range at which the purification efficiency becomes maximum when the frequency H becomes less than a preset threshold Ω.
Furthermore, the optimum frequency H _o may be determined by combining the estimation based on the gradient g and the estimation based on the minimum air excess factor λ _{r_min} itself. By combining these to determine the optimum frequency H _o , the accuracy of the optimum frequency H _o can be improved.

It is desirable that the ECU 59 stores a map showing the operating state of the engine 50 for each frequency H, such as a map showing the relationship between engine speed and operating load. It is desirable that the search unit 13 starts searching for the optimal frequency _H when the operating state changes to a different map. By restarting the search when the operating state changes significantly, the purification rate can be maintained at its highest value. The ECU 59 may also store a valid range for the optimal frequency _H for each map. When a different map is switched to, the purification rate can be maximized in a shorter time by searching for the optimal frequency _H within the valid range stored for that map.

It is also desirable to search for and update the optimum frequency H _{o when the driving time or distance exceeds a certain value, such as 10,000 hours or 10,000 km. By correcting the optimum frequency H o} at such timing, it is possible to correct the optimum frequency H _o in accordance with changes in the maximum value due to the progression of deterioration of the engine 50 and the catalyst 56.

Next, the procedure for determining the optimum frequency _H0 and adjusting the purification rate to the optimum value will be described using the flowchart in Figure 9 (see Figures 3 to 8 as appropriate). Note that in Figure 9 and the following description, each step will be denoted as "S11," etc.
Further, an example will be described in which the search unit 13 performs a search using a gradient g when the horizontal axis represents the frequency H and the vertical axis represents the minimum air excess factor λ _{r — min} .

9 , the searching unit 13 monitors the lower excess air factor _λr, which is the output value of the lower air-fuel ratio sensor 58, and the upper excess air factor _λf , which is the output value of the upper air-fuel ratio sensor 57 (S11). At this time, the searching unit 13 controls the λ oscillation based on the upper excess air factor _λf output by the upper air-fuel ratio sensor 57 via the air-fuel ratio adjusting unit 11. Then, the searching unit 13 extracts the minimum excess air factor _{λr_min} , which is the minimum value of the λ oscillation of the lower excess air factor _λr .
Next, the frequency H is gradually decreased by a predetermined number of divisions on the logarithmic scale from the standard value of 1 Hz, and the minimum air excess factor λ _{r — min} is plotted on a semi-logarithmic graph (S12).

While the gradient g of the graph is less than the threshold value _g0 , the frequency H is decreased (NO in S13, return to S12).
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 unit 12 determines the frequency H at this time as the optimum frequency _Ho . The air-fuel ratio adjustment unit 11 also adjusts the frequency H to be the frequency of λ oscillation.

If the value of the threshold _g0 is too large, the threshold _g0 will not be able to detect a transition from the high frequency region α to the low frequency region β. If the value of the threshold _g0 is too small, a transition will be erroneously diagnosed when it has not actually occurred, resulting in an inappropriate frequency H being erroneously detected. Experiments have shown that the threshold _g0 is preferably set to approximately 0.015 to 0.025. By setting the threshold _g0 within this range, the optimal frequency _H0 can be identified with high accuracy. However, this is determined at the design stage depending on the specifications of the catalyst 56 and the engine 50.

As described above, the control device 10 according to the first embodiment has 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 catalyst 56 changes.

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 vibration when the gradient g exceeds the threshold value g ₀ by a small percentage to set it as the optimum frequency H ₀ (S15).
Here, the term "tiny proportion" refers to a proportion 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 ₀ determined in the first embodiment is corrected by adding or subtracting a small proportion, and the optimum frequency H ₀ is set anew.

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 become 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 exceeds the threshold value _g0 , it is assumed that a lower frequency H beyond the optimum frequency _Ho has already been detected. The frequency determination unit 12 then sets the optimum frequency _Ho to a value obtained by adding a small percentage to the detected frequency H that is considered to be lower than the optimum frequency _Ho .
When the frequency H is increased stepwise from the low frequency region β, the optimum frequency H ₀ is obtained by subtracting a small percentage from the frequency H when the threshold value g ₀ is exceeded.

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 _Ho, and therefore a duplicated description will be omitted. Similarly, in the drawings, duplicated components are denoted by the same reference numerals and a description thereof will be omitted.

As described above, the control device 10 according to the second embodiment reduces torque fluctuations, providing a comfortable ride for the passenger.

The control device 10 according to each of the above-described embodiments has a simple configuration and can maintain a high purification rate even when the operating state of the engine 50 or the temperature of the exhaust purification catalyst 56 changes.

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.

For example, although it has been explained that the air-fuel ratio control device is more suitable for engines in which the engine operating range changes relatively slowly, engines to which the air-fuel ratio control device is applicable are not limited to engines in which the changes are slow.

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, g ₀ ...threshold, Ω...threshold, α...high frequency region, β...low frequency region, λ...air excess ratio, λ _f ...upper air excess ratio, λ _r ...lower air excess ratio, λ _{r_min} ...minimum air excess ratio.

Claims (6)

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. 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. 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 . 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. An air-fuel ratio control device according to any one of claims 1 to 4, wherein the frequency determination unit corrects the optimal frequency by adding or subtracting a predetermined percentage of the frequency width used in the search to the optimal frequency. 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.