JP3538892B2 - Engine exhaust recirculation system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas recirculation device for an engine, and more particularly to a countermeasure against corrosion in a device in which at least one of an intake system and an exhaust gas recirculation system is formed of a metal material.

[0002]

2. Description of the Related Art Conventionally, an exhaust gas recirculation device that recirculates a part of exhaust gas flowing through an exhaust passage to an intake system in order to reduce the amount of NOx generated in an engine is generally known. Also,
Recently, in order to suppress the rise of combustion temperature and exhaust gas temperature under high load in a turbocharged engine, etc., low temperature E
A system in which GR gas (cold EGR) is sent to an intake system has also been proposed.

For example, as disclosed in Japanese Patent Application Laid-Open No. 5-187329, in a turbocharged engine, EGR can be performed even under a high load in which relatively low-temperature exhaust gas is recirculated and intake pressure downstream of the supercharger is high. There is known an exhaust gas recirculation device provided with an EGR passage for recirculating exhaust gas from a downstream side of a catalyst in an exhaust passage to an upstream side of a supercharger in an intake passage.

[0004]

In an engine provided with an exhaust gas recirculation device, the pH (hydrogen ion index) of exhaust gas recirculated from an exhaust passage to an intake system is not always neutral, and the intake gas is generally not charged. Since the system and the exhaust gas recirculation system are formed of a metal material, there is a problem that the metal material constituting the intake system and the exhaust gas recirculation system is corroded by the recirculation of the exhaust gas. In particular, as shown in the above publication, when exhaust gas is recirculated from the downstream of the catalyst in the exhaust passage to the upstream side of the supercharger in the intake passage, when the intake system is formed of an aluminum-based metal, Tends to occur.

The problem of corrosion due to the recirculation of exhaust gas will be specifically described with reference to FIGS.

FIG. 1 shows the pH of a gas or liquid in contact with a metal.
The relationship between the value and the amount of corrosion of the metal is shown for an aluminum-based metal and an iron-based metal. The solid line is an aluminum-based metal and the temperature is room temperature, and the dashed line is the same aluminum-based metal but the temperature is 80 °. In the case of, the dashed line indicates the case of an iron-based metal.

As shown in this figure, if the pH is substantially neutral (pH value is about 7), almost no metal is corroded, but aluminum-based metal becomes alkaline when the pH value is larger than about 8. When the acidity becomes less than about 4 or pH value, the corrosion is liable to occur. In particular, the corrosion becomes remarkable at a temperature of about 80 °. In addition, iron-based metals are easily corroded when the pH value becomes less than about five.

The exhaust gas discharged from the combustion chamber of the engine contains NOx in which nitrogen is combined with oxygen and SOx in which sulfur contained in fuel or oil is combined with oxygen. Since SOx is weakly acidic, the exhaust gas discharged from the combustion chamber is acidic. When a three-way catalyst (a catalyst having both oxidation and reduction functions) is provided in the exhaust passage, when the exhaust gas passes through the catalyst, the NOx is reduced and a part of the NOx is reduced to alkaline NH. ₃ (ammonia)
Is oxidized and changes to SO ₃ . The pH of the exhaust gas after passing through the catalyst tends to shift to the alkali side due to the influence of NH ₃ . In addition, not only the three-way catalyst but also a catalyst having at least a reducing function, for example, a NOx purification catalyst is provided, when the exhaust gas passes through the catalyst, NH ₃ is generated and the pH shifts to the alkali side.

Further, the air-fuel ratio is related to the pH of the exhaust gas. FIG. 2 shows the relationship between the air-fuel ratio, the generation amount of NOx and SOx, and the exhaust gas pH upstream and downstream of the catalyst. As shown in this figure, NOx has an air-fuel ratio (A / F) of 16
It is most frequently generated in the vicinity, and SOx increases as the air-fuel ratio becomes leaner because sulfur and oxygen are more easily combined. In relation to this, the pH of the exhaust gas upstream of the catalyst
The value becomes 7 or less (acidic) and becomes lower as the air-fuel ratio becomes leaner. Theoretical air-fuel ratio (A / F = 14.7,
At λ = 1), the pH value is about 5.6.

[0010] On the other hand, the pH of the exhaust gas downstream of the catalyst is higher than that of the upstream of the catalyst (alkaline side). In the case where the catalyst is a three-way catalyst, the pH of the exhaust gas downstream of the catalyst decreases when the lean air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio, due to a decrease in NH ₃ accompanying a decrease in the NOx reduction function. Also, when the catalyst is a NOx purification catalyst, the pH value decreases with a decrease in the amount of NOx generated when the air-fuel ratio becomes lean to a certain degree or more.
The degree of decrease in the H value is smaller than in the case of the three-way catalyst.

Further, the flow rate of the exhaust gas is related to the pH of the exhaust gas, and the relationship between the flow rate (the exhaust flow rate or the corresponding intake air flow rate) and the pH of the exhaust gas upstream and downstream of the catalyst (at a stoichiometric air-fuel ratio). ) Is shown in FIG. As shown in this figure, when the flow rate increases, the relative exhaust gas purification rate decreases, so that the pH value of the exhaust gas downstream of the catalyst decreases and approaches the pH value of the exhaust gas upstream of the catalyst.

Considering the above tendency, when the exhaust gas is recirculated from the downstream of the catalyst having the reducing function,
When the engine is operated at the stoichiometric air-fuel ratio and the flow rate is up to a certain level, the EGR gas becomes alkaline with a pH value of about 9, so that corrosion occurs when the intake system is made of aluminum-based metal. . In particular, when an intercooler made of aluminum-based metal is provided downstream of the supercharger in an engine with a supercharger, the intake air temperature in the intercooler becomes about 80 °.
When the GR gas passes through the intercooler, corrosion of the intercooler is likely to occur.

In addition to the corrosion caused by the alkalinity of the EGR gas as described above, for example, when the exhaust gas is recirculated from the upstream of the catalyst, the EGR gas becomes relatively strongly acidic depending on the operation state and the like. When the moisture in the EGR gas is condensed, the SO ₃ combines with the water to generate H ₂ SO ₄ (sulfuric acid), and the condensed water may exhibit strong acidity. Corrosion due to acid also poses a problem when it is made of a base metal or when the exhaust gas recirculation system is made of an iron base metal.

Conventionally, measures against such a problem have not been sufficiently taken.

As a well-known technique, Japanese Patent Laid-Open Publication No.
No. 0 discloses a structure in which an oxidation catalyst is provided upstream of a NOx purification catalyst in an exhaust passage, and exhaust gas is taken out from between both catalysts and recirculated to an intake system. Although it is at first glance similar to one embodiment of the present invention described below in that exhaust gas is taken out between the catalysts,
The catalyst on the upstream side is an oxidation catalyst.
The function of adjusting the pH of the GR gas cannot be obtained.

The present invention has been made in view of the above circumstances, and in an engine in which an intake system and an exhaust gas recirculation system are formed of a metal material, it is ensured that the recirculation gas becomes alkaline or acidic and corrodes the intake system or the like. An object of the present invention is to provide an exhaust gas recirculation device for an engine that can prevent the exhaust gas recirculation.

[0017]

According to one aspect of the present invention, an exhaust gas recirculation system for recirculating a part of exhaust gas flowing through an exhaust passage of an engine to an intake system through an exhaust recirculation passage is provided. An engine in which at least one of the intake system and the exhaust gas recirculation system is formed of a metal material, a catalyst having at least a reducing function is provided in the exhaust passage.
And a pH for adjusting the pH of the recirculated gas recirculated from the exhaust passage to the intake system to a substantially neutral predetermined range.
Adjusting means is provided, and the pH adjusting means is
Upstream exhaust that guides exhaust gas from the upstream to the exhaust recirculation passage
Exhaust gas is exhausted from the gas extraction section and downstream from the catalyst.
A downstream-side exhaust gas outlet leading to the air recirculation passage;
A mixture for adjusting the mixing ratio of exhaust gas from the gas gas extraction section
Characterized in that it comprises means for adjusting proportions.
You.

In the present invention, the mixing ratio adjusting means preferably changes the mixing ratio in accordance with the flow rate of the exhaust gas (Claim 2 ). The air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is preferably changed. (Claim 3 ) is preferable to change the mixing ratio according to the above.

According to a fourth aspect of the present invention, there is provided an exhaust passage for an engine.
Part of the exhaust gas flowing through the passage is taken in through the exhaust recirculation passage
The system is equipped with an exhaust gas recirculation system that recirculates
An engine in which at least one of the flow systems is formed of a metal material
A first catalyst and a downstream portion thereof in the exhaust passage.
And a second catalyst located at
Also has a reduction function, and performs a predetermined operation in which exhaust gas recirculation is performed.
The pH of the exhaust gas is approximately neutral between the two catalysts in the range
The capacity of the first catalyst is set so that
The exhaust gas outlet that guides exhaust gas to the exhaust gas recirculation passage is
By being provided in the middle of both catalysts,
The pH of the recirculated gas returned to the intake system from the
It is characterized by comprising pH adjustment means for adjusting the surroundings
It is assumed that.

According to the present invention, a supercharger is provided in the intake system,
The exhaust gas recirculation system sends recirculated gas upstream of the supercharger in the intake system.
With an exhaust gas recirculation passage for feeding
In the downstream intake passage, an aluminum metal
This is effective when an intercooler is provided.
Item 5). The pH adjusting means adjusts the pH value of the reflux gas from 6
It is preferable to adjust the range up to 8.
Claim 6).

[0021]

According to the present invention, the pH of the return gas is adjusted to approximately neutral by the pH adjusting means, thereby preventing the intake system or the exhaust gas recirculation system made of a metal material from being corroded by the recirculated gas. You.

For example, when the intake system is made of an aluminum-based metal, it is possible to avoid a situation in which the reflux system becomes alkaline or acidic and the intake system is corroded. Especially in the case of a turbocharged engine, when the recirculated gas is sent upstream from the supercharger in the intake system in order to recirculate the low-temperature exhaust gas at high load and prevent the exhaust gas temperature from rising, the recirculated gas becomes alkaline. And that the intake air temperature rises due to supercharging, which tends to cause corrosion.
It is sufficiently prevented by the adjusting action by the pH adjusting means.

If the pH of the reflux gas is adjusted in the range of 6 to 8, corrosion is effectively prevented.

Specifically, when the pH adjusting means is configured as described in claim 1 , the exhaust gas is acidic on the upstream side of the catalyst having the reducing function, and the exhaust gas is alkaline on the downstream side of the catalyst. In this situation, the exhaust gas upstream of the catalyst and the exhaust gas downstream of the catalyst are mixed to obtain a substantially neutral reflux gas. In this case, if the mixing ratio is changed in accordance with the flow rate of the exhaust gas, the reflux gas can be made substantially neutral in various operating states having different flow rates. Further, if the mixture ratio is changed in accordance with the air-fuel ratio of the air-fuel mixture, the recirculated gas can be made substantially neutral even if the air-fuel ratio changes depending on the operating state or the like.

Further, when the pH adjusting means is constituted as described in claim 4 , while the action of purifying NOx is sufficiently obtained by both the first and second catalysts, the pH of the first catalyst is reduced. Since the action of purifying NOx is limited to a certain extent, the exhaust gas is acidic and the second exhaust gas is upstream of the first catalyst.
Downstream of the catalyst in an alkaline state,
The exhaust gas becomes substantially neutral between the two catalysts, and is extracted as reflux gas.

[0026]

An embodiment of the present invention will be described with reference to the drawings.
FIG. 4 schematically shows an exhaust gas recirculation device provided with a pH adjusting means according to a first embodiment of the present invention.
Denotes an engine body, 2 denotes an intake passage, and 3 denotes an exhaust passage.

In the intake passage 2, an air cleaner 4, an air flow meter 5, a throttle valve 6, a supercharger 7, an intercooler 8, a surge tank 9, and a fuel injection valve 10 are arranged in this order from the upstream side. In the illustrated example, the supercharger 7 is a mechanical supercharger such as a Richolm type supercharger, and is mechanically driven by an engine output shaft via a transmission means such as a belt (not shown) and an electromagnetic clutch. It has become so.
The intake passage 2 is provided with a bypass passage 11 for bypassing the supercharger 7.
Valve 12 that opens and closes according to the engine load, etc.
Is interposed.

The intake system is thus constructed, and in this intake system, the intake pipe, the intercooler 8 and the like are formed of aluminum-based metal.

In the exhaust passage 3, a catalyst device 15 is provided in the middle thereof, and a silencer 16 is provided downstream of the exhaust passage. The catalyst device 15 is formed of a catalyst having at least a reducing function, for example, a three-way catalyst having both oxidation and reduction functions.

Between the intake passage 2 and the exhaust passage 3,
An exhaust gas recirculation system that recirculates part of the exhaust gas flowing through the exhaust passage 3 to the intake system is provided. This exhaust gas recirculation system is EGR
In this embodiment, the first EG
An R passage 21 and a second EGR passage 22 are provided.

The first EGR passage 21 feeds EGR gas (recirculated gas) upstream of the supercharger 7 in the intake system. For example, the first EGR passage 21 is provided in the intake passage 2 between the throttle valve 6 and the supercharger 7. The outlet end is connected. The second EGR passage 22 is connected to a portion of the intake system downstream of the supercharger 7, for example, an outlet end of the surge tank 9. The inlet side of each of the EGR passages 21 and 22 is connected to exhaust gas take-out portions 25A and 25B via passages 26A and 26B as described later.

The EGR passages 21 and 22 are provided with EGR valves 23 and 24, respectively. Each of the EGR valves 23, 2 is operated according to the operating state of the engine.
4, the exhaust gas recirculates from the second EGR passage 22 in a low load region where the intake pressure is low, and the first EGR passage 2 in a high load region (supercharging region) where the intake pressure is high.
Exhaust gas recirculation is performed from 1.

The pH of the EGR gas is supplied to the exhaust gas recirculation system.
There is provided a pH adjusting means for adjusting the pH to a substantially neutral predetermined range. In this embodiment, the pH adjusting means sends the exhaust gas from the upstream side of the catalyst device 15 to the EGR passage 25A, and sends the exhaust gas from the downstream side of the catalyst device 15 to the EGR passage. The downstream-side exhaust gas extraction section 25B and the exhaust gas extraction sections 25A, 25
It comprises mixing ratio adjusting means for adjusting the mixing ratio of the exhaust gas from 5B, and is specifically configured as follows.

That is, exhaust gas take-out sections 25A and 25B are opened on the upstream and downstream sides of the catalyst device 15 in the exhaust passage 3, respectively.
A and 25B are connected to passages 26A and 26B, respectively, and these passages 26A and 26B are connected to each other. The first EGR passage 21 is connected to a portion where the passages 26A and 26B are connected, and the second EGR passage 22 is connected to the passage 26A.

The upstream gas outlet 25A and the EGR
A mixing ratio adjusting valve 27 as a mixing ratio adjusting means is provided in the passage 25A between the passage connecting portion.
The mixing ratio adjusting valve 27 is operated by an actuator 28, and changes its opening degree from fully closed to fully opened. The operation of the mixing ratio control valve 27 by the actuator 28 is controlled by a control unit (ECU) 30 according to the intake air amount and the like.
As described later in detail, the gas extraction section 25 is set so that the pH of the EGR gas falls within a predetermined range of approximately neutral.
The mixing ratio of the gas from A and 25B is adjusted.

The pH adjusting means is constituted as described above.

The control unit 30 is
The air flow meter 5 and the throttle opening sensor 31,
While receiving signals from the engine speed sensor 32, the O ₂ sensor 33, and the like, the fuel injection valve 10 and the like are controlled,
The actuator 28 of the mixing ratio adjusting valve 27 is controlled.

The operation of the above-described exhaust gas recirculation device of this embodiment will be described.

In the exhaust gas recirculation system, the exhaust gas is recirculated in accordance with the operation state. In this embodiment, when the load is low, the EGR gas is sent through the second EGR passage 22 to the downstream side of the intake system. When the load is high, the EGR gas
The air is fed upstream from the supercharger 7 in the intake system through the EGR passage 21. In this way, the exhaust gas is recirculated even at the time of a high load at which the overpressure increases, thereby suppressing the knocking and the increase of the exhaust temperature at the time of the high load in addition to the action of reducing the NOx.

When the exhaust gas is recirculated as described above, the exhaust gas extracted from each of the exhaust gas extraction sections 25A and 25B provided on the upstream and downstream sides of the catalyst device 15 in the exhaust passage 3 is mixed. In addition, the mixing ratio is adjusted by the mixing ratio adjusting valve 27, so that the pH of the EGR gas is adjusted to a substantially neutral predetermined range.

That is, for example, when the engine is operated at the stoichiometric air-fuel ratio, the exhaust gas is acidic on the upstream side of the catalyst device 15 and the exhaust gas is exhausted on the downstream side of the catalyst device 15 unless the exhaust gas flow rate is significantly increased as described above. The gas becomes alkaline,
The pH of the gas obtained by mixing these gases has an intermediate value between the pH values of the exhaust gas on the upstream side and the exhaust gas on the downstream side, and a substantially neutral gas can be obtained by adjusting the mixing ratio. As described above, at least in the operating range where the exhaust gas is recirculated from the first EGR passage 21,
By adjusting H, when the EGR gas is sent upstream of the turbocharger and flows down the intake passage 2, a situation in which the intercooler 8 or the like made of aluminum-based metal corrodes is prevented.

Further, since the EGR gas is substantially neutral, corrosion of the exhaust gas recirculation system made of iron-based metal is also prevented.

It is desirable that the pH of the EGR gas is adjusted in the range of 6 to 8, as shown in FIGS. That is, in order to prevent corrosion of aluminum-based metal (and iron-based metal), as shown in FIG.
It is preferable that the lower limit of the pH value be at least about 4 according to this figure. However, as described above, the condensation of the EGR gas while passing through the exhaust gas recirculation system and the intake system as described above. Since the acidity may be increased by condensation, the pH value of the EGR gas is preferably set to 6 or more in consideration of this.

As an example of the manner of adjustment by the mixing ratio adjusting valve 27, when the engine is operated at the stoichiometric air-fuel ratio, at least in the operating range where the exhaust gas is recirculated from the first EGR passage 21, the above-mentioned operation is performed. The opening of the mixing ratio control valve 27 is changed as shown in FIG. 5 according to the intake flow rate detected by the air flow meter 5 so as to correspond to the relationship between the pH and the flow rate of the exhaust gas shown in FIG. In other words, on the relatively low flow rate side where the pH of the exhaust gas taken out from the downstream side of the catalyst becomes alkaline at about 9, the mixing ratio control valve 2
When the valve 7 is substantially fully opened, the mixing ratio of the exhaust gas from the upstream side of the catalyst is relatively increased. Then, as the flow rate increases and the pH of the exhaust gas from the downstream side of the catalyst decreases, the opening ratio of the mixing ratio control valve 27 is reduced, so that the mixing ratio of the exhaust gas from the upstream side of the catalyst decreases, and The flow rate Q at which the pH of the exhaust gas from the downstream side becomes 7
Above a, the mixing ratio control valve 27 is fully closed.

In this way, the pH of the EGR gas is properly adjusted, and the higher the flow rate, the higher the proportion of exhaust gas extracted from the downstream side of the catalyst, which is lower in temperature than the upstream side of the catalyst. The effect of lowering the EGR gas temperature on the flow side to suppress the rise in combustion temperature and exhaust gas temperature is also enhanced.

When the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is changed in accordance with the operation state, the EGR gas is made substantially neutral in accordance with the tendency shown in FIG. The opening of the mixing ratio adjusting valve 27 may be changed according to the air-fuel ratio.

In the above embodiment, the first exhaust gas recirculation system
Although the EGR passage 21 and the second EGR passage 22 are provided, the second EGR passage 22 may be omitted, and only the EGR passage 21 for sending the EGR gas upstream of the supercharger in the intake system may be provided.

In the above embodiment, of the passages 26A and 26B connected to the exhaust gas extraction sections 25A and 25B, the passage 26A connected to the upstream exhaust gas extraction section 25A is used.
Is provided with a mixing ratio adjusting valve 27, but a mixing ratio adjusting valve may be provided in a passage 26B connected to the downstream side exhaust gas extracting portion 25B. In this case, the cross-sectional area of the passage 26B is made larger than the cross-sectional area of the passage 26A, the opening of the mixing ratio control valve is reduced in the low flow rate operation region, and the mixing ratio is reduced in the high flow rate operation region. What is necessary is just to enlarge the opening degree of a control valve.

FIG. 6 shows another embodiment of the pH adjusting means.
In this embodiment, the catalyst device 30 provided in the exhaust passage 3
Is divided into a first catalyst 31 and a second catalyst 32 located downstream of the first catalyst 31. Both of the catalysts 31, 32 are catalysts having a reducing function, for example, three-way catalysts. The capacity of the first catalyst 31 is set so that the pH of the exhaust gas is substantially neutral between the two catalysts 31 and 32, that is, the reducing function of the first catalyst 31 is limited to a certain extent. The shortage of the reducing function is compensated for by the second catalyst 32, so that the reducing function required by the two catalysts 31, 32 is achieved.

An exhaust gas take-out unit 3 is provided between the above two catalysts.
The EGR passage 34 is connected to the exhaust gas take-out portion 33.

Even when the pH adjusting means is configured as described above, the pH of the EGR gas is adjusted to a substantially neutral predetermined range,
Corrosion of the intake system and the like is prevented. That is, as shown in FIG.
Is the pH value of the exhaust gas on the upstream side and the p value of the exhaust gas on the downstream side.
H value, and by appropriately setting the capacity of the first catalyst 31, the catalyst 3 can be used in most of the operating range.
The pH of the exhaust gas at the intermediate position between 1 and 32 can be made substantially neutral. Then, the substantially neutral exhaust gas in this portion is converted into EGR gas from the exhaust
The gas is returned to the intake system through the GR passage 34, and corrosion of the exhaust gas recirculation system and the intake system is prevented.

FIG. 8 shows still another embodiment of the pH adjusting means. In this embodiment, the main catalyst device 41
Is provided, and an auxiliary catalyst device 42 is provided at a position upstream of the engine body 1 and close to the engine body 1, and an exhaust gas take-out portion 43 is provided between the two catalyst devices 41 and 42. Although both of the catalyst devices 41 and 42 are made of a catalyst having a reducing function, for example, a three-way catalyst, the capacity of the auxiliary catalyst device 42 is made smaller than that of the main catalyst device 41 so as to function effectively at the time of starting or the like. ing.

Further, a bypass passage 45 for bypassing the auxiliary catalyst device 42 and a control valve 46 for opening and closing the passage 45 are provided. When the exhaust gas temperature is low, the control valve 46 is closed, and when the exhaust gas temperature is high, the control valve 46 is opened and the exhaust gas flows to the bypass passage 45 so that the heat load of the auxiliary catalyst device 42 is reduced. It has become.

The exhaust gas take-out portion 43 is provided at the junction of the bypass passage 45 with the exhaust passage 3 downstream of the auxiliary catalyst device 42.
The GR passage 44 is connected.

Also in the case of this embodiment, the action of adjusting the pH of the EGR gas to approximately neutral is obtained by extracting the exhaust gas from between the two catalysts 41 and 42 having a reducing function and recirculating the exhaust gas. . In particular, the bypass passage 4
5 and the control valve 46 can be used to adjust the pH.

FIG. 9 shows still another embodiment of the pH adjusting means. In this embodiment, the exhaust passage 3 is provided with a main catalyst device 51 made of a catalyst having a reducing function such as a three-way catalyst, and an auxiliary catalyst device 52 located on the upstream side thereof. Downstream exhaust gas extraction unit 53
Is provided, and an EGR passage 54 is connected to the exhaust gas extracting portion 53. A secondary air passage 55 is connected to the upstream side of the main catalyst device 51, and a secondary air adjustment valve 56 is provided in the secondary air passage 55.

In this embodiment, the secondary air control valve 56 is controlled by a control means (not shown),
The amount of secondary air supplied upstream of the main catalyst device 51 is adjusted according to the intake flow rate (exhaust flow rate) as shown in FIG.
Thereby, the pH of the EGR gas is adjusted to be approximately neutral.

That is, when the engine is operated at the stoichiometric air-fuel ratio, as described above, the pH of the exhaust gas downstream of the catalyst increases at a relatively low flow rate side, while the secondary air is supplied upstream of the catalyst. Since the exhaust gas passing through the catalyst is similar to the case where the air-fuel ratio is made lean, the pH of the exhaust gas downstream of the catalyst tends to decrease (see FIG. 2). Accordingly, in correspondence with the relationship between the pH and the flow rate of the exhaust gas downstream of the catalyst shown in FIG. 3, the secondary air amount is increased on the relatively low flow rate side as shown in FIG. The secondary air supply is stopped at a flow rate Qa or more at which the amount of air is reduced and the pH of the exhaust gas from the downstream side of the catalyst becomes 7 so that the pH of the exhaust gas downstream of the catalyst can be made substantially neutral. .

Then, the main catalyst device 5
The substantially neutral exhaust gas downstream of 1 is taken out and recirculated to the intake system, thereby preventing the exhaust gas recirculation system and the intake system from corroding.

In this embodiment, as shown by a two-dot chain line in FIG. 9, a passage 57 for taking out the exhaust gas from the upstream of the catalyst and mixing it with the exhaust gas from the downstream of the catalyst is provided.
A valve 58 for adjusting the mixing ratio of the exhaust gas from the upstream of the catalyst is provided, and the adjustment of the mixing ratio and the adjustment of the secondary air amount are used in combination to adjust the pH of the EGR gas to a substantially neutral predetermined range. May be adjusted. In this way, a function of adjusting the pH can be sufficiently provided for a wide range of change in the exhaust gas flow rate or the like.

FIG. 11 shows still another embodiment of the pH adjusting means. In this embodiment, even if the air-fuel ratio is lean, N
A NOx purification catalyst 61 for maintaining the Ox purification function is provided in the exhaust passage, and an exhaust gas extraction unit 62 is provided downstream of the NOx purification catalyst 61. An EGR passage 63 is connected to the exhaust gas extraction unit 62, and the control unit 30 ,
The fuel injection valve 1 is controlled so that the air-fuel ratio falls within a predetermined range on the lean side.
The fuel injection amount from 0 is controlled.

According to this embodiment, the pH of the EGR gas
Is a predetermined range that is substantially neutral.

That is, as shown in FIG. 2, the pH value of the exhaust gas downstream of the catalyst having the reducing function becomes high (alkaline) near the stoichiometric air-fuel ratio and decreases when the air-fuel ratio becomes lean. However, especially when the NOx purification catalyst 61 is used, even if the air-fuel ratio becomes lean, NO
Since the action of reducing x is maintained, the degree of decrease in the pH value on the lean side becomes slower as compared with the three-way catalyst.
If the air-fuel ratio is within the predetermined range on the lean side, NOx
The pH value of the exhaust gas downstream of the purification catalyst 61 is in a preferable range of approximately neutral (6 to 8).

Therefore, by setting the air-fuel ratio to a predetermined range on the lean side and extracting exhaust gas from the downstream of the NOx purification catalyst 61, substantially neutral EGR gas is obtained, and the exhaust gas recirculation system and the intake system are controlled. Corrosion is prevented. In addition, since the air-fuel ratio is made lean and the NOx purifying catalyst 61 is used, fuel efficiency is improved and NOx is further reduced.

Further, when the pH adjusting means of this embodiment is applied to a supercharged engine, the air-fuel ratio of the air-fuel mixture is made lean even in the supercharging region, and at least in the supercharging region, the NOx
The EGR gas extracted from the downstream of the purification catalyst 61 is converted into EGR gas.
What is necessary is just to feed it upstream from the supercharger of an intake system through the passage 63. By doing so, in a high load region (supercharging region), relatively low-temperature EGR gas from downstream of the NOx purification catalyst 61 is fed into the intake system, and knocking and an increase in exhaust gas temperature are suppressed. Like E
The GR gas becomes substantially neutral, thereby preventing corrosion of the intake system and the like, as well as improving fuel efficiency and reducing NOx.

Further, as another embodiment of the pH adjusting means,
Although an illustration of the exhaust gas recirculation device is omitted, an exhaust gas extraction unit is provided downstream of the catalyst in an exhaust passage provided with a catalyst having a reducing function, and the control unit controls the fuel injection amount, whereby FIG. As shown, the air-fuel ratio may be controlled in accordance with the flow rate (the exhaust flow rate or the corresponding intake flow rate), whereby the pH of the exhaust gas downstream of the catalyst may be adjusted to be approximately neutral.

That is, as shown in FIGS. 2 and 3, the pH value of the exhaust gas downstream of the catalyst tends to be high near the stoichiometric air-fuel ratio and to decrease on the lean side, and high on the low flow rate side. Since the air-fuel ratio tends to decrease as the flow rate increases, as shown in FIG. 12, the air-fuel ratio is made lean on the low flow rate side, and the air-fuel ratio is changed to the rich side to about the stoichiometric air-fuel ratio as the flow rate increases. The pH value of the exhaust gas downstream of the catalyst can be substantially neutral. Therefore, by extracting the exhaust gas from the downstream of the catalyst while controlling the air-fuel ratio in this way, the pH of the EGR gas can be made substantially neutral.

Incidentally, such control of the air-fuel ratio according to the flow rate may be used in combination with the pH adjusting means of the various embodiments.

[0069]

As described above, the present invention relates to an engine in which at least one of the intake system and the exhaust gas recirculation system is made of a metal material, such as an engine in which the intake system is made of an aluminum-based metal or an exhaust gas recirculation system. Is formed of an iron-based metal, the engine is provided with a pH adjusting means for adjusting the pH of the recirculated gas returning from the exhaust passage to the intake system to a substantially neutral predetermined range. Alternatively, it is possible to prevent the intake system and the exhaust gas recirculation system from being corroded due to the acidity, thereby improving the durability of the intake system and the like.

In particular, a supercharger is provided in the intake system, and the recirculated gas is supplied upstream of the supercharger in the intake system in order to suppress a rise in exhaust gas temperature by recirculating low-temperature exhaust gas in a high load region. When the exhaust gas recirculation system is formed so as to be fed into the exhaust gas, the pH of the recirculated gas is made substantially neutral by the pH adjusting means, so that the corrosion of the turbocharger and equipment provided in the intake system downstream thereof is prevented. It can be effectively prevented.

Specifically, the exhaust gas is taken out from the upstream side and the downstream side of the catalyst having a reducing function provided in the exhaust passage, and these are mixed together.
By so adjusting the mixture, the exhaust gas in the catalyst upstream acidic, in a situation where the catalyst downstream in the exhaust gas is made alkaline, it is possible to obtain a recirculated gas of effectively substantially neutral.

Further, the first and second catalysts, each having a reducing function, are provided in the exhaust passage, and the capacity of the first catalyst is set so that the pH of the exhaust gas becomes substantially neutral between the two catalysts. In addition, even if an exhaust gas outlet is provided between the two catalysts, a substantially neutral reflux gas can be obtained effectively.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of corrosion of an aluminum-based metal and an iron-based metal and a pH value.

FIG. 2 is a diagram showing the relationship between the pH value of exhaust gas upstream and downstream of the catalyst and the air-fuel ratio of the air-fuel mixture when a catalyst having a reducing function is provided in the exhaust passage.

FIG. 3 is a diagram showing the relationship between the pH value of exhaust gas upstream of the catalyst and the flow rate of exhaust gas when a catalyst having a reducing function is provided in the exhaust passage.

FIG. 4 is a schematic diagram of the entire exhaust gas recirculation device according to one embodiment of the present invention.

FIG. 5 is a diagram showing a change in an opening degree of a mixing ratio adjusting valve in the device shown in FIG. 4 according to an intake flow rate.

FIG. 6 is a schematic view of a main part showing another embodiment of the pH adjusting means in the exhaust gas recirculation device.

FIG. 7 is a diagram showing the pH of exhaust gas when the pH adjusting means shown in FIG. 6 is used.

FIG. 8 is a schematic view of a main part showing still another embodiment of the pH adjusting means in the exhaust gas recirculation device.

FIG. 9 is a schematic view of a main part showing still another embodiment of the pH adjusting means in the exhaust gas recirculation device.

10 is a diagram showing a change in a secondary air amount according to an intake air flow rate when the pH adjusting means shown in FIG. 9 is used.

FIG. 11 is a schematic view of a main part showing still another embodiment of the pH adjusting means in the exhaust gas recirculation device.

FIG. 12 is a diagram showing a change in the air-fuel ratio according to the intake flow rate when the air-fuel ratio is controlled according to the intake flow rate as still another embodiment of the pH adjusting means in the exhaust gas recirculation device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine main body 2 Intake passage 3 Exhaust passage 7 Supercharger 15, 30, 41, 42, 51, 52, 61 Catalyst device 21, 22, 34, 44, 54, 63 EGR passage 25A, 25B, 33, 43, 53 , 62 Exhaust gas outlet 55 Secondary air passage

──────────────────────────────────────────────────続 き Continued on the front page (72) Noriyuki Iwata, Inventor 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside (72) Inventor Kenya Ishii 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda (56) References JP-A-6-50134 (JP, A) JP-A-3-74561 (JP, A) JP-A-48-8640 (JP, A) JP-A-5-30453 (JP, A U) Japanese Patent Publication No. 5-41810 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) F02M 25/07 570 F02M 25/07 550 F02M 25/07 580 F01N 3/22 301

Claims (6)

(57) [Claims] 1. An exhaust gas recirculation system for recirculating a part of exhaust gas flowing through an exhaust passage of an engine to an intake system via an exhaust gas recirculation passage, wherein at least one of the intake system and the exhaust gas recirculation system is formed of a metal material. in it are engine is less when
When a catalyst having a reducing function is provided in the exhaust passage,
In both cases, a pH adjusting means is provided for adjusting the pH of the recirculated gas recirculated from the exhaust passage to the intake system to a substantially neutral predetermined range.
The pH adjusting means is provided from the upstream side of the catalyst.
Upstream exhaust gas extraction section that guides exhaust gas to the exhaust gas recirculation passage
Exhaust gas from the downstream side of the catalyst to the exhaust gas recirculation passage.
Downstream exhaust gas extraction section to guide and both exhaust gas extraction
Mixing ratio adjusting means for adjusting the mixing ratio of exhaust gas from the section
An exhaust gas recirculation device for an engine, comprising: a stage ; Wherein the mixing ratio adjusting means, according to claim 1 in accordance with the flow rate of the exhaust gas is to change the mixing ratio
An exhaust gas recirculation device for the engine as described in the above. Wherein the mixing ratio adjusting means, exhaust gas recirculation system according to claim 1 or 2, wherein the engine is to change the mixing ratio in accordance with the air-fuel ratio of the mixture supplied to the combustion chamber. 4. An exhaust gas flowing through an exhaust passage of an engine.
Exhaust gas recirculation that partially recirculates to the intake system via the exhaust gas recirculation passage
System, and at least one of the intake system and the exhaust gas recirculation system
Is formed of a metal material, a first catalyst and a second catalyst located downstream of the first catalyst are provided in the exhaust passage, and both catalysts have at least a reducing function, and the exhaust gas recirculation. The capacity of the first catalyst is set so that the pH of the exhaust gas is substantially neutral between the two catalysts in a predetermined operation range in which the exhaust gas is extracted. By being provided between the two catalysts, it is recirculated from the exhaust passage to the intake system.
An exhaust gas recirculation device for an engine, comprising a pH adjusting means for adjusting the pH of the recirculated gas to a substantially neutral predetermined range . 5. An exhaust gas recirculation system in which a supercharger is provided in an intake system.
Sends the recirculated gas upstream of the supercharger in the intake system
It has an exhaust gas recirculation passage and intake air downstream of the turbocharger.
In the passage, an intercool made of aluminum-based metal
5. The engine according to claim 1, further comprising:
Exhaust gas recirculation device. 6. The pH adjusting means adjusts the pH value of the reflux gas to 6
6. The method according to claim 1, wherein the distance is adjusted to a range from 8 to 8.
An exhaust gas recirculation device for an engine according to any one of the above.