JPS5813733B2 - Driving method of three-way catalyst for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a three-way catalyst provided in an exhaust system of an internal combustion engine, which is widely known as an exhaust gas purification device for an internal combustion engine.

More specifically, in an internal combustion engine in which a three-way catalyst to which an element with oxygen storage capacity (oxygen storage substance) is added is installed in the exhaust system, the air-fuel ratio of the air-fuel mixture (supplied to the internal combustion engine) in the engine intake system By setting the air-fuel mixture concentration to be lower than the stoichiometric air-fuel ratio, that is, setting the mixture concentration to the rich side, and intermittently supplying secondary air at a certain frequency upstream of the three-way catalyst in the exhaust system, the air-fuel ratio of the exhaust gas can be adjusted. The present invention relates to a method of driving a three-way catalyst for an internal combustion engine that alternately changes the air-fuel ratio to the rich side and lean side with respect to the stoichiometric air-fuel ratio.

A three-way catalyst that simultaneously removes harmful components (CO, HC, NOx) contained in exhaust gas emitted from internal combustion engines,
Now that exhaust regulations have become stricter, they are being widely studied, and various related systems have been proposed.

This three-way catalyst has three components (CO, HC, NOx,
) can be highly purified (this is the range of air-fuel ratio that achieves a purification rate of 80% or more, hereinafter referred to as the A/F window width)
is limited to an extremely narrow range near the stoichiometric air-fuel ratio.

Therefore, the conventionally proposed exhaust gas purification system using a three-way catalyst installs an air-fuel ratio detector in the exhaust system to detect this stoichiometric air-fuel ratio, and based on the signal from the detector, mixes the air-fuel ratio in the intake system. The idea was to maintain the air-fuel ratio of the air-fuel mixture at approximately the stoichiometric air-fuel ratio while effectively operating the three-way catalyst by feedback-controlling the air-generating means (e.g., a carburetor or electronic fuel injection device). .

However, even with this air-fuel ratio feedback method, it is impossible to always maintain the air-fuel ratio at the stoichiometric air-fuel ratio, and the actual air-fuel ratio alternately changes toward the rich side and the lean side with the stoichiometric air-fuel ratio as a boundary.

The width of the fluctuation and the wave number of the fluctuation change significantly depending on the operating state of the engine.

Therefore, this air-fuel ratio feedback system could not fully utilize the three-way catalyst effectively.

Therefore, recently, catalysts have been developed that exhibit a high purification rate even if the air-fuel ratio fluctuates to some extent.

For example, as shown in JP-A-52-56216.52-56217, a three-way catalyst A/F
Techniques have been proposed to increase the window width.

Furthermore, Japanese Patent Laid-Open No. 52-27087 discloses a technique for similarly increasing the A/F window width by adding an oxygen storage substance to a catalyst.

However, these known techniques are concerned with improving the catalyst itself, and how to drive this type of three-way catalyst to operate the three-way catalyst most effectively, or how to operate the three-way catalyst most effectively. This does not refer to whether the F window width can be expanded.

On the other hand, US Pat. No. 4,024,706 discloses a method for driving a three-way catalyst in order to effectively operate the three-way catalyst.

According to this U.S. patent, the air-fuel ratio of the air-fuel mixture supplied to the engine is alternately varied between the rich side and the lean side with the stoichiometric air-fuel ratio as the boundary, thereby attempting to expand the A/F window width of the three-way catalyst. It is something.

However, the A/F window width obtained by the driving method of this US patent is 0.1 in gasoline air-fuel ratio.
8 units, and such an A/F window width requires feedback control using an air-fuel ratio detector, which is by no means a satisfactory driving method for a three-way catalyst.

Moreover, in this U.S. patent, the air-fuel ratio of the air-fuel mixture supplied to the engine (the air-fuel ratio in the intake system) is alternately varied between rich and lean sides, so as the range of variation in the air-fuel ratio increases, The engine operation becomes unstable, and the smaller the fluctuating frequency, the greater the effect, so the method of driving a three-way catalyst disclosed in this US patent has the disadvantage of adversely affecting engine operation.

Therefore, an object of the present invention is to provide a method for driving a three-way catalyst that effectively operates the three-way catalyst. A three-way catalyst to which a substance has been added is arranged, and the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set to be richer than the stoichiometric air-fuel ratio, and upstream of the three-way catalyst, the air-fuel ratio of the upstream exhaust gas is adjusted to the above-mentioned value. The exhaust gas air-fuel ratio is alternately varied to the rich side and the lean side with the stoichiometric air-fuel ratio as the boundary, and secondary air is intermittently supplied so that the fluctuating value of the exhaust gas air-fuel ratio becomes equal with the stoichiometric air-fuel ratio as the boundary, and This is a three-way catalyst driving method that effectively operates the three-way catalyst by setting the intermittent supply time of secondary air so that the fluctuation cycle to the rich side is shorter than the fluctuation cycle to the rich side.

? Before explaining embodiments of the invention, the purification action of a three-way catalyst using an oxygen storage substance will be explained. FIG. 2 shows the exhaust gas composition of an internal combustion engine with respect to changes in the air-fuel ratio of the air-fuel mixture supplied to the engine.

As is clear from the figure, 0 and C0 fluctuate rapidly near the stoichiometric air-fuel ratio at which the three-way catalyst operates most efficiently.

The reaction formula on this three-way catalyst generally consists of the following formula.

Formulas (1) to (3) are oxidation reactions, and formulas (4) to (6) are reduction reactions, and a three-way catalyst is one in which these reactions proceed simultaneously.

Currently known three-way catalysts include Pt-IRn metals, and their effective operating range (A/F window width) is extremely narrow as shown in Figure 1 (usually 0.05~
(about 0.1).

As is clear from the composition diagram in Figure 2, especially considering the change in the 02 concentration, the oxidation reactions of formulas (1) to (3) occur on the lean side, and the formulas (1) to (3) occur on the rich side, and (
The reduction reactions of 4) to (6) precede, and in the former, formulas (4) to
CO, H2, and CnHm necessary for the reaction (6) are removed, and the reduction reaction of NO is inhibited. Although the reduction reactions of formulas (4) to (6) proceed due to the Co, H2, and CnHm present in The oxidation reactions of ~(3) are inhibited.

Therefore, what prevents the acid reaction and reduction reactions in formulas (1) to (6) from proceeding simultaneously are the three components 02, CO, and H2, which change rapidly with the air-fuel ratio. These are almost the same composition ratio as the stoichiometric air-fuel ratio (CO, NO, HC, H2, 0 series)
If the change is gradual, the effective operating range (A
/F window width) should be enlarged.

However, these cannot be expected unless the combustion reaction of the internal combustion engine changes significantly to achieve a desirable exhaust composition, and in reality, this is almost impossible.

Therefore, the above-mentioned known technology attempts to perform this on a catalyst.

This was done for 02 above, and formulas (1) to (6
) in addition to the Pt-Rh catalyst metal that governs the reaction of
It does not contribute to the ternary reaction with the addition of commonly known oxygen storage substances of the lanthanide group (Ce02, Lar03, etc.) and n-type semiconductors (Cub, ZnO, Zr02, etc.) that have 2 storage (or adsorption) ability. In addition, as the oxygen partial pressure in the exhaust gas changes, oxygen is stored on the lean side where the oxygen partial pressure is high, and the stored oxygen is released on the rich side where the oxygen partial pressure is low.
In the case of a system in which the oxygen partial pressure (concentration) on the catalytic active surface changes smoothly, and the engine changes alternately from rich to lean, the atmosphere on the catalytic active surface changes to the stoichiometric air-fuel ratio in each 1/2 cycle. By approximating this, the operating range (A/F window width) of the three-way catalyst is intended to be expanded.

FIGS. 3A to 3D schematically show storage and release of oxygen in a three-way catalyst to which an oxygen storage substance is added.

Figure 3A shows γ-alumina (Al203) as a carrier;
A pellet-type three-way catalyst particle in which a Pt-Rh catalyst metal and an oxygen storage material are supported on the carrier is shown, and Fig. jB shows a schematic structure of the catalyst metal 1 and the oxygen storage material 2.

FIG. 3C shows a state in which the oxygen absorbing substance 2 has occluded (adsorbed) oxygen 023 in the exhaust gas when the exhaust gas atmosphere is in a lean state.

FIG. 3D shows a state in which the oxygen storage material 2 releases the oxygen 3 stored in the exhaust gas when the exhaust gas atmosphere becomes rich.

Therefore, even if the exhaust gas atmosphere is lean, the excess oxygen 3 is stored in the oxygen storage material 2, so the atmosphere around the catalyst metal 1 is maintained near the stoichiometric air-fuel ratio,
On the other hand, when the exhaust gas atmosphere becomes rich, the atmosphere around the catalyst metal 1 is similarly maintained near the stoichiometric air-fuel ratio due to the released oxygen.

In this way, the added oxygen storage material plays the role of adjusting the atmosphere of exhaust gas that reaches the vicinity of the catalyst metal, and even if the air-fuel ratio fluctuates slightly, the atmosphere around the catalyst metal remains close to the stoichiometric air-fuel ratio. can be kept, therefore A/
It becomes possible to widen the F window width.

The test equipment used to conduct various experiments of the present invention will be explained based on FIG. 4.

The outline of this device is shown in Figure 4, where 11 is 2.00.
0cc, 4-cycle, 6-cylinder engine, carburetor 12
It is a gasoline-powered engine equipped with.

The exhaust pipe 13 is equipped with a three-way catalyst 14 containing an oxygen storage substance.
is provided.

This three-way catalyst 14 is made of γ-alumina (γ-Al203)
A catalyst carrier containing 1.5 g/l of Pt-Rh catalyst metal and 20 g/l of cerium oxide CeO2 as an oxygen storage material.
It is a pellet type that supports 1 liter and is filled into a 2.5 liter case.

An air supply pipe 15 for supplying secondary air has an opening 1 in the exhaust pipe 13 upstream of the three-way catalyst 14 .

16 is a solenoid valve provided in this supply pipe 15, and the transmitter 1
The supply pipe 15 is opened M (ON-OFF) by the signal No. 1.

A well-known air pump 20 was used as a source of secondary air, and air from this pump 20 was guided to the solenoid valve 16 via an air pressure regulator 21 and an opening regulator 22.

Known air-fuel ratio detectors 18 and 19 were disposed upstream and downstream of the three-way catalyst 14, respectively, and the outputs of the detectors were measured with an electromagnetic oscilloscope 30.

Also, the fuel used here has a stoichiometric air-fuel ratio of 14.5.
gasoline with a value of

The operating conditions of the engine 11 are that the rotation speed is 160 Orpm,
The blowing air negative pressure is -375 mmHg.

Experiment 1 Using the device shown in Figure 4 above, fix the air-fuel ratio of the mixture generated in the carburetor 12 to 13, and turn on the solenoid valve 16 (open).
Secondary air was gradually supplied to the exhaust pipe 13 under these conditions.

The purification rate of each component of the exhaust gas at this time is shown by the broken line in FIG.

Next, the solenoid valve 16 is turned OFF (closed) and the air-fuel ratio of the air-fuel mixture in the carburetor 12 is gradually changed from 13 to 16. Indicated by a solid line in the figure.

Note that the purification curve shown by this solid line almost matches the purification curve in FIG. 1.

Comparing the solid line and the broken line in Fig. 5, it is clear that it is better to fix the air-fuel ratio of the intake system and supply secondary air to the exhaust system.
/F It can be seen that the window width has become wider.

The exhaust gas composition in this case differs depending on the fixed air-fuel ratio of the intake system, but in this case (intake system air-fuel ratio=13) it is as shown in FIG. 6.

As is clear from a comparison between FIG. 6 and FIG. 2, the exhaust gas compositions of the two are significantly different.

The most notable difference in the above composition diagram is that when the intake side fuel-fuel ratio is made variable (Fig. 2), as mentioned above, C0, 02, and H2 change significantly after the stoichiometric air-fuel ratio, and NO, The change in HC is also relatively large, but what happens when secondary air is supplied to the exhaust side (Figure q)? Since the component composition determined by the intake side is only diluted with secondary air, CO, H2,
NO and HC vary only extremely smoothly, and only <, 0 is a point that changes largely and almost linearly with respect to air-fuel ratio fluctuations.

The purification performance of the three-way catalyst using these two methods of changing the air-fuel ratio is as shown in Figure 5.
The effective area of the main catalyst (the three components of CO, HC, and NO are all 8
The range of air-fuel ratios purified by 0% or more) becomes wider, and the reason for this is thought to be the difference in exhaust gas composition.

The reaction formula (1) described above from both the composition diagrams in Figures 2 and 6
~ Based on (6), typically CO, H2・HC/0, NO/
FIG. 7 shows a comparison of the composition ratios of CO.

Is this the difference in purification rate from Figure 7 to Figure 5? The explanation is as follows.

NO/CO ratio: As the air-fuel ratio shifts to the lean side, in the case of variable intake side, NO increases, but CO, which is a reducing agent for NO, decreases, so the reduction reaction of reaction equation (4) does not progress, and the NO purification rate decreases.

On the other hand, when supplying secondary air to the exhaust side, NO
It can be seen that this is effective for NO purification on the lean side because the /CO ratio hardly changes and the sufficient condition is maintained for the equivalence ratio of NO/CO=1.

However, since the relationship between the ratios of CO, H2, and He/0 described below naturally affects this, there is a limit value on the lean side.

(If reducing agents such as CO, H2, HC, etc. react with 0 and are eliminated, NO purification will stop.) co, H2, HC/0 ratio: HC in reaction formula (3) is converted to C3H
Required to completely oxidize the amounts of CO, H2, and HC in the exhaust gas in reaction formulas (1), (2), and (3) when converted to 8 (propane) and set as C3H8 + 503 → 3CO2 + 4H20 (7). na 0
It is the ratio of the amount of 02 in the exhaust gas to the amount of 02, and a ratio of 1 corresponds to the equivalent amount.

(However, the values of CO, H2, and HC that reduce NO are ignored, so they are just reference values.) Therefore, when this ratio is greater than 1, it is in an incompletely oxidized state with many oxidized substances, and conversely, it is less than 1. Assuming a complete oxidation state with a lot of 02 when The ratio of both curves is 1 near .5
It was found that for air-fuel ratios before and after that, they intersect and diverge from each other.

This well explains the difference in purification rate between the two mentioned above, and in the former case (when changing the intake system air-fuel ratio) there is an absolute shortage of 02 necessary for CO and HC oxidation on the rich side. On the lean side, on the other hand, due to the absolute excess of 02, complete oxidation of CO and HC progresses, inhibiting the reduction reaction of NO, and reducing the NO/C
In conjunction with the graph of o, it can be seen that the NO purification conditions on the lean side are inappropriate.

On the other hand, in the latter case, when secondary air is supplied to the exhaust system, the curve is extremely smooth, and on the rich side the ratio is lower than the former, which is advantageous for the oxidation of Co and HC, and on the lean side it is advantageous for oxidation of Co and HC. Since the ratio is higher than the former, it can be seen that the absence of a large excess of 02 and the NO/CO ratio are advantageous for NO reduction.

In addition to the above, an exothermic reaction is considered to be the cause of the difference in FIG.

The oxidation reactions in reaction formulas (1) to (3) are exothermic reactions, and looking at the concentrations of the oxidized substances CO, HC, and H2 in this reaction, in the former case, the concentrations rapidly increase as the air-fuel ratio increases. In contrast, in the latter case, the concentration is maintained to a certain extent (see Figure 6), so the reaction heat on the catalyst surface is higher in the latter case, and the reaction on the catalyst decreases. It also seems to have the effect of promoting

As described above, the present inventors pursued the advantages and causes of the system of supplying secondary air to the engine exhaust system, and based on this, further conducted the following experiments.

Experiment 2 Under the same conditions as Experiment 1, with the solenoid valve 16 turned on, the amount of secondary air was fixed so that the air-fuel ratio on the exhaust side was 16.0.

If the solenoid valve 16 is turned ON and OFF in this state, the exhaust gas air-fuel ratio will repeat the cycle of 13.0 (OFF) ← 16.0 (ON), rich ← lean. Now, fix the oscillation frequency of the oscillator 17 to IHz, and set the ON-OFF time ratio as ON/OFF=Osec/lsec-0.5secC/0
．． The solenoid valve 16 is operated by changing the speed from 5sec to 1sec/Osec, and the exhaust gas (
The average) air-fuel ratio and purification performance of the three-way catalyst were measured.

This result is shown by the solid line in FIG.

The ON-OFF time ratio at the intersection of the purification curves for N0 and CO, which is the point with the highest purification rate of the three components in the figure, is ON/O.
FF=o. 45s. It was ec/0.55 sec.

In order to compare the results of Experiment 2 with the results of Experiment 1, the broken lines in FIG. 5 are shown as broken lines in FIG. 8.

Needless to say, the broken line in Figure 8 indicates that the air-fuel ratio of the intake system is 1.
3 is fixed, and the exhaust gas air-fuel ratio is gradually changed from 13 to 16 by gradually adding secondary air to the exhaust system.

On the other hand, the solid line in Fig. 8 shows that the air-fuel ratio of the intake system is fixed at 13, secondary air is intermittently supplied to the exhaust system, and the average air-fuel ratio of the exhaust gas is increased to 13 while oscillating the exhaust gas air-fuel ratio to 13016. It is a purification curve of each component when changing from to 16.

As is clear from comparing the two, even if secondary air is supplied, it is better to supply it intermittently rather than gradually (continuously). It has been found that the A/F window width can be expanded by using this method.

Experiment 3 A similar experiment was conducted using 2.5 liters of a pellet-type three-way catalyst carrying 1.5 g/l of a Pi-Rh catalyst metal that does not contain the oxygen storage substance CeO instead of the three-way catalyst in Experiment 2. Ta.

The results are shown in Figure 9 with a broken part.

ON-O at the intersection of the No and CO purification curves at this time
FF time ratio is ON/OFF=0.5sec/0.5s
In ec, it was the point where Zoritzchi and Lean were at the same distance.

For comparison with the results of Experiment 2, the solid line in FIG. 8 is shown as a solid line in FIG. 9.

As is clear from the results of Experiment 3 (dashed line in Figure 9), even in the method of intermittently supplying secondary air, if no oxygen storage material is added to the three-way catalyst, harmful There is no air-fuel ratio that can purify exhaust gas with a purification rate of 80% or more when considering the components (CO, HC, No.).

Furthermore, if we compare the solid line and the broken line in Figure 9, it is clear that the three-way catalyst to which an oxygen storage substance is added has much higher purification performance than the three-way catalyst to which no oxygen storage substance is added. .

In the aforementioned Experiment 2 (solid line in Figure 8),
The supply ratio of secondary air that can purify the three harmful components in exhaust gas with the highest efficiency, that is, the purification curve (solid line) in Figure 8
The ON-OFF time ratio of the solenoid valve at the intersection of is ON-
Although the OFF ratio was 0.45/0.55, the following experiment was conducted to investigate the cause of this.

Experiment 4 Solenoid valve 16 under the same conditions as Experiments 2 and 3 above. ON-
OFF, the ON/OFF ratio of this solenoid valve = 0.570
．． The detector 1 detects the air-fuel ratio upstream and downstream of the three-way catalyst 14 when the ON/OFF ratio is 0.45/0.55 and the ON/OFF ratio is 0.45/0.55.
8 and 19.

The output waveform numbers of the detectors 18 and 19 at that time are the first
This is shown in Figures 0A and 10B.

In the figure, the horizontal axis shows time, the vertical axis shows the output (voltage) of the detector, the solid line a is the output waveform of the detector 18 at the inlet (upstream) of the three-way catalyst, and the broken line b is the oxygen storage Detector 1 at the outlet (downstream) of the three-way catalyst free of substances
The dashed line C is the output waveform of the detector 19 at the outlet of the three-way catalyst containing the oxygen storage substance CeO.

As is clear from FIGS. 10A and 10B, 3
The air-fuel ratio at the inlet of the main catalyst is the secondary air's NO-N-OFF.
(supply cut off), ON/OFF ratio = 0
．． In the case of 570.5, the amount of change in the air-fuel ratio toward the rich side (the area shown by the shaded area A in the first OA diagram) is equal to the amount of change toward the lean side (the area shown by the shaded area B in the first OA diagram). I can feel numb.

Furthermore, the change in the air-fuel ratio at the outlet of the three-way catalyst that does not contain an oxygen storage substance (indicated by the broken line b in the figure) approximately follows the change in the air-fuel ratio at the inlet (solid line a) with a certain time delay.

Next, the change in the air-fuel ratio at the outlet of the three-way catalyst to which an oxygen storage substance has been added is as shown by the dashed line C in the figure, which is quite different from the change in the air-fuel ratio at the inlet (solid line a). There is.

It is easy to see that this change is brought about by the oxygen storage and release operations of the oxygen storage material.If we examine this air-fuel ratio change (dotted chain line C) in more detail, we can see that in Figure 10A, the three-dimensional When the air-fuel ratio at the catalyst inlet switches from the rich side to the lean side (detector output is 0.5V)
The time from when the air-fuel ratio at the outlet switches from rich to lean (hereinafter referred to as the switching speed from rich to lean) is the time from when the air-fuel ratio switches from rich to lean at the entrance of the three-way catalyst. It is shorter than the time from when the air-fuel ratio at the outlet switches from the lean side to the rich side to when the air-fuel ratio at the outlet switches from the lean side to the rich side (hereinafter referred to as the switching speed from lean to rich).

That is, the switching speed from rich to lean means that the speed at which the oxygen storage substance Ce02 stores oxygen and the speed at which it releases oxygen are different.

In this way, since the storage rate and release rate of oxygen storage substances are different (the storage rate is greater than the release rate),
ON/OFF ratio of solenoid valve = 0.57o. 5, the amount of oxygen released by the oxygen storage material when the air-fuel ratio becomes rich is smaller than the amount of oxygen stored by the oxygen storage material when the air-fuel ratio becomes lean, and therefore the ternary It can be imagined that the atmosphere of the exhaust gas near the catalyst is adjusted to be leaner.

In Fig. 10A, as can be seen from the change in the air-fuel ratio at the outlet of the three-way catalyst to which the oxygen storage material has been added, the amount of change in the air-fuel ratio at the outlet toward the rich side (the change on the rich side indicated by the dashed line C) The fact that the area) is smaller than the amount of change toward the lean side (area on the lean side according to the dashed line C) indicates that the average air-fuel ratio of the exhaust gas supplied to the three-way catalyst is adjusted to the stoichiometric air-fuel ratio. However, it can be concluded that the atmosphere near the catalyst is adjusted to be leaner by the action of the oxygen storage substance.

Therefore, in order to adjust the atmosphere near the three-way catalyst to the stoichiometric air-fuel ratio, the ON/OFF ratio of the solenoid valve 16 = 0.45/
By setting the value to 0.55, the results shown in FIG. 10B could be obtained.

As shown by hatched areas C and D in Figure 10B, the amount of change in the air-fuel ratio toward the rich side (area C) at the outlet of the three-way catalyst is made equal to the amount of change toward the lean side (area D). I was able to do that.

In this way, the ON/OFF ratio of the solenoid valve = o. 1s/o.
By setting s5, it becomes possible to make the amount of oxygen stored and the amount of oxygen released almost equal, and therefore it becomes possible to adjust the atmosphere near the catalyst to the stoichiometric air-fuel ratio.

Experiment 5 ON/OFF ratio of the solenoid valve obtained in Experiment 4 above = 0.457
Using 0.55, the purification rate of each component was measured with respect to the ON/OFF frequency change of the solenoid valve.

That is, under the conditions of Experiment 2, the ON/Off state of the solenoid valve 16
Fixed F ratio = 0.4570.55, and instead of solenoid valve 1
The purification rate of each component was measured by changing the ON-OFF frequency of No. 6 from 0.5 to 10 Hz.

The results are shown in FIG. As you can see from this Figure 11, the frequency is 5Hz? Too much CO and HC,% CO
The purification rate continues to decline.

The purification rate of each component (NO, CO, HC) was increased to 80%.
In order to maintain this level, the frequency must be set between 0.5 and 5 Hz.

Of course, it is understood from the next experiment 6 that this also depends on the amount of oxygen storage material added, and in this case (CeO is used as the oxygen storage material)
It was concluded that 0.5 to 5 Hz is appropriate for 0.09/l).

Experiment 6 The amount of oxygen storage material CeO2 added in Experiment 5 was 20g.
However, it is obvious that the amount of oxygen stored differs depending on the amount added, and therefore the sum of the storage time and the release time also differs.

Therefore, here, the amount of oxygen storage material CeO2 added is 5
An experiment was conducted under the same conditions as in Experiment 5, except that the amount was varied from 97 liters to 30 g/1.

The experimental results are plotted as shown in FIG. 11 as in Experiment 5, and the frequency ranges in which each component of the exhaust gas can be purified by 80% or more are shown in FIG. 12 from each graph.

From the above experiments 5 and 6, it was found that as the amount of oxygen storage substance added increases, although qualitatively, the ON-OF of the solenoid valve that can purify each component of exhaust gas at a high purification rate (over 80%)
It can be seen that the frequency range of F shifts towards lower frequencies.

Next, the following experiment was conducted to investigate the relationship between frequency and catalyst carrier.

Experiment 7 Pi-Rh
A three-way catalyst (P
t-Rh=i. 5 g/l, CeO2 = 10 g/l), the ON/OFF ratio was set to 0.45/0.55, the oscillation frequency was fixed at 15 Hz, and the purification rate of the three components was measured.

At the same time, a similar experiment was conducted for the three-way catalyst to which CeO2 = 10 g/l was added in Experiment 6, and the purification rate was measured.

As a result of these experiments, the following values were obtained.

As can be seen from this result, in the high frequency region (e.g. 10Hz
above), pellet type catalysts are used with CO
, the purification rate of HC deteriorates, and the atmosphere above the catalyst shifts toward the rich side.

Therefore, in the high frequency range, a monolith type catalyst carrier is more preferable than a pellet type.

Experiment 8 Under the same conditions as in Experiment 1 above, the secondary air amount was fixed so that the exhaust side air-fuel ratio was 15 with the solenoid valve 16 turned on.

With this secondary air amount fixed, the solenoid valve 16 is turned on and off at a frequency of IHz, and the ON/OFF ratio at that time is 0.
45/0.55, and secondary air is intermittently supplied.

Therefore, the exhaust gas air-fuel ratio is varied between 13 and 15,
The purification rate of exhaust gas components at that time was measured.

Similarly, the exhaust side air-fuel ratio is 15.

The secondary air amount was varied to 5, 16.0, 16.5...18.0, and the exhaust gas air-fuel ratio was varied for each, and the purification rate of the exhaust gas components was measured.

The results are shown in FIG.

In this figure, the purification rate of each component of exhaust gas is No=
The region of the exhaust side air-fuel ratio of 80% or more and CO, HC = 90% or more is shown as the effective operating air-fuel ratio region.

The area in this case was approximately in the range from 16 to 16.5.

Experiment 9 In Experiment 8, the effective operating air-fuel ratio region was measured when the intake side air-fuel ratio was 13.

Here, the intake air-fuel ratio was fixed sequentially from 13.0 to 14.0, and the effective operating air-fuel ratio range for each intake air-fuel ratio was measured in the same manner as in Experiment 8.

The results of this experiment are shown in the shaded area a in FIG.

Experiment 10 Experiment 9 above was a measurement of the effective operating air-fuel ratio range for a three-way catalyst to which an oxygen storage material was added, but in this experiment, the solenoid valve of Experiment 8 was measured for a three-way catalyst to which no oxygen storage material was added. A similar experiment was conducted with the ON/OFF ratio = 0.5/0, 5, and the measurement results are shown in the shaded area b in Figure 14.
It was shown in

Experiment 11 The above experiments 9 and 10 were the results when the ON-OFF frequency of the solenoid valve 16 was IHz, but here, the effective operating air-fuel ratio was set to 2 Hz and the effective operating air-fuel ratio was changed in the same manner as experiments 9 and 10. The area was measured.

The results are shown in FIG. In the figure, the shaded area a is the effective operating air-fuel ratio area of the three-way catalyst to which an oxygen storage substance is added, and the shaded area b is the effective operating air-fuel ratio area of the three-way catalyst to which no oxygen storage substance is added.

As is clear from the above-mentioned FIGS. 13 to 15, there is almost no effective operating air-fuel ratio region in the three-way catalyst to which no oxygen storage material is added.

In addition, in a three-way catalyst containing an oxygen storage substance, there is a sufficient effective operating air-fuel ratio range between 13.0 and 14.0, so in actual vehicle engines, the intake side Even if the air-fuel ratio fluctuates, the three-way catalyst driving method of the present invention can sufficiently operate the three-way catalyst at a high purification rate.

As described above, various experiments of the present invention have shown that a method for effectively operating a three-way catalyst containing an oxygen storage material is to supply an air-fuel mixture having an air-fuel ratio smaller than the stoichiometric air-fuel ratio to the internal combustion engine, and to operate the three-way catalyst effectively. Upstream, the upstream exhaust gas air-fuel ratio is alternately varied to the rich side and the lean side with the stoichiometric air-fuel ratio as the boundary, and the fluctuating value of the exhaust gas air-fuel ratio is made equal to the stoichiometric air-fuel ratio by 2. We have found a method of intermittently supplying secondary air and setting the intermittent supply time of the secondary air so that the cycle of variation to the lean side is shorter than the cycle of variation to the rich side.

According to the method of the present invention, the exhaust gas air-fuel ratio is drastically changed from the rich side to the lean side from the stoichiometric air-fuel ratio by using a three-way catalyst added with an oxygen storage substance and an intermittent supply of secondary air. In addition, the effective operation of the three-way catalyst can simultaneously remove Co, He, and NOx contained in exhaust gas at a high purification rate by combining this with an operating method that makes the fluctuation value equal to the stoichiometric air-fuel ratio. The air-fuel ratio range (A/F window width) can be expanded.

The intermittent supply time of the secondary air is set so that the fluctuation cycle of the exhaust gas air-fuel ratio toward the lean side due to the intermittent supply of secondary air to the upstream side of the three-way catalyst is made shorter than the fluctuation cycle toward the rich side. By doing this, the amount of oxygen released by the enzyme storage material in the three-way catalyst when the exhaust gas air-fuel ratio becomes rich, and the amount of oxygen released by the enzyme storage material in the three-way catalyst when the exhaust gas air-fuel ratio becomes lean. The amount of stored oxygen can be made almost equal, and as a result, the air-fuel ratio of the exhaust gas in the atmosphere near the three-way catalyst can be set to the stoichiometric air-fuel ratio, so the three-way catalyst can operate more efficiently. .

Since a configuration is adopted in which the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is set on the rich side, there is no need to vary the air-fuel ratio on the intake side of the internal combustion engine, and the internal combustion engine can always be operated in a stable state.

[Brief explanation of the drawing]

Figure 1 is a graph showing the purification rate for each component of a typical three-way catalyst with respect to the air-fuel ratio. Figure 2 is the exhaust gas when the air-fuel ratio of the mixture supplied to the engine is varied from 13 to 16. Graphs showing changes in composition, Figures 3A to 3D,
A schematic diagram showing the oxygen storage and release operations of a three-way catalyst to which an oxygen storage substance has been added. Figure 4 is a schematic diagram showing the apparatus used in the experiment of the present invention.
Next, gradually add air to increase the exhaust gas air-fuel ratio from 13 to 16.
Figure 6 is a graph showing the purification rate of each exhaust gas component when the air-fuel ratio of the mixture supplied to the engine is changed to 1.
Figure 7 is a graph showing the change in exhaust gas composition when the exhaust gas air-fuel ratio is changed from 13 to 16 by gradually supplying secondary air to the exhaust system with the air-fuel ratio fixed at 3. and a graph showing the component ratio of each exhaust gas component with respect to air-fuel ratio changes based on changes in exhaust gas composition in Fig. 6, and Fig. 8 shows a graph showing the composition ratio of each exhaust gas component with respect to air-fuel ratio changes based on changes in exhaust gas composition. Figure 9 is a graph showing the purification rate of each exhaust gas component by the three-way catalyst with oxygen storage material added when secondary air is intermittently supplied to the air and the average exhaust gas air-fuel ratio is varied from 13 to 16. 10A is a graph showing the purification rate of each exhaust gas component by a three-way catalyst without adding an oxygen storage substance when secondary air is intermittently supplied as in the case of Fig. 8.
10B and 10B are waveform diagrams showing the output of the air-fuel ratio detector at the inlet and outlet of the three-way catalyst, and FIG. 11 shows the purification rate of each exhaust gas component with respect to frequency changes of intermittent supply of secondary air. The graph, Figure 12 is a graph showing the frequency change at which the purification rate of each exhaust gas component is 80% or more when the amount of oxygen storage material added is changed, and Figure 13 is the graph showing the frequency change at which the purification rate of each exhaust gas component is 80% or more. A graph showing the purification rate of gas components,
FIGS. 14 and 15 are graphs showing the effective operating air-fuel ratio range of the exhaust-side air-fuel ratio with respect to the intake-side air-fuel ratio.

Claims (1)

[Claims] 1. An air-fuel mixture having an air-fuel ratio richer than the stoichiometric air-fuel ratio is supplied to an internal combustion engine, and is installed in the exhaust system of the internal combustion engine so that the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio. Upstream of a three-way catalyst to which an oxygen storage substance is added which has the ability to store oxygen when the side is on the rich side and release the stored oxygen when the side is rich, secondary air is supplied intermittently so that the fluctuating value of the exhaust gas air-fuel ratio becomes equal to the stoichiometric air-fuel ratio, and the fluctuating cycle to the lean side is performed. A method for driving a three-way catalyst for an internal combustion engine, characterized in that the intermittent supply time of secondary air is set to be shorter than the variation cycle to the rich side. 2 When cerium oxide CeO2 is used as the oxygen storage material and the amount added is 209/l with respect to the catalyst carrier, the secondary air is intermittently supplied at a certain frequency between 0.5 Hz and 5 Hz. A method for driving a three-way catalyst for an internal combustion engine according to claim 2, characterized in that: supplying a three-way catalyst for an internal combustion engine. 3. When cerium oxide Ce02 is used as the oxygen storage material and the amount added is 10 g/l to the catalyst carrier, the secondary air is intermittently supplied at a certain frequency between 5 Hz and 10 Hz. A method for driving a three-way catalyst for an internal combustion engine according to claim 2, characterized in that: 4. The internal combustion engine according to claim 1, wherein a monolith type carrier is used as a catalyst carrier of the three-way catalyst when the secondary air is intermittently supplied at a high frequency of 10 Hz or more. How to drive a three-way catalyst for engines.