Exhaust system for engine

Abstract

To provide an exhaust system for an engine capable of suppressing lowering of a temperature of a catalyst and preventing deterioration of purification performance of the catalyst by suppressing a poisoning phenomenon of the catalyst caused by engine oil.SOLUTION: An exhaust system 1 includes: a main flow passage 3A that is formed within an exhaust pipe 3 and in which exhaust gas flows to pass through a catalyst device 4; an auxiliary flow passage 3B formed within the exhaust pipe 3 separately from the main flow passage 3A and in which exhaust gas flows; a changeover device 6 capable of switching a flow of exhaust gas within the exhaust pipe 3 to either of the main flow passage 3A or the auxiliary flow passage 3B; an electronic control unit 9 that controls the changeover device 6 on the basis of an operating state of the engine 2; and a recirculation passage 8 that connects a portion located downstream of the auxiliary flow passage 3B and an intake passage 2B of the engine 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to engine exhaust systems. [Background technology]

[0002] As a conventional technique related to the exhaust system of an engine mounted on a vehicle such as an automobile, for example, Patent Document 1 discloses an exhaust purification device having a secondary flow passage that branches off from the exhaust pipe downstream of a catalyst carrier provided in the exhaust pipe, passes around the catalyst carrier, and merges into the exhaust pipe. In this exhaust purification device, a heat storage member is provided in the secondary flow passage at a position around the catalyst carrier, and by switching the flow of exhaust in the exhaust pipe to the secondary flow passage, the reaction heat of the catalyst acts on the heat storage member through the exhaust flow, maintaining the temperature of the catalyst within an appropriate temperature range. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7063069 Summary of the Invention [Problem to be solved by the invention]

[0004] When a vehicle decelerates, engine fuel cutoff control may be performed, or in series hybrid vehicles, motoring operation (non-ignition operation) may be performed, in which the engine is driven as an electric motor using regenerative power generated during deceleration to maintain brake vacuum. Series hybrid vehicles are vehicles that store power generated by the engine in a battery and use that power to drive the motor. If engine fuel cutoff control or motoring operation as described above continues, the temperature of the exhaust gas flowing from the engine into the exhaust pipe drops, which in turn lowers the temperature of the catalyst in the exhaust pipe. This could result in a decrease in the catalyst's purification performance when the engine resumes firing operation (ignition operation).

[0005] Furthermore, during the above-mentioned engine fuel cut control or motoring operation, the negative pressure inside the engine cylinder increases, making it more likely that engine oil will flow from the lower part of the cylinder block bore or the cylinder head into the combustion chamber, resulting in a phenomenon known as oil rising and / or falling. Because typical engine oil contains phosphorus (P), sulfur (S), etc., if oil components that become mist-like in the exhaust gas flow reach the catalyst, they can cause a poisoning phenomenon that reduces catalyst activity and significantly reduces the catalyst's purification performance. However, with the conventional technology disclosed in the aforementioned Patent Document 1, it is difficult to suppress both the catalyst temperature drop and the poisoning phenomenon that can occur during the above-mentioned engine fuel cut control or motoring operation, leaving room for improvement in terms of preventing a decline in catalyst purification performance.

[0006] The present invention has been made with the above points in mind, and aims to provide an engine exhaust system that can suppress a drop in catalyst temperature and also suppress catalyst poisoning caused by engine oil, thereby preventing a decline in the catalyst's purification performance. [Means for solving the problem]

[0007] In order to achieve the above object, one aspect of the present invention provides an exhaust system equipped with a catalytic device for purifying exhaust gas in an exhaust pipe through which exhaust gas from an engine flows. The exhaust system for an engine includes: a main passage formed in the exhaust pipe for passing the exhaust gas through the catalytic device; a secondary passage formed in the exhaust pipe separately from the main passage for passing the exhaust gas; a switching device for switching the flow of the exhaust gas in the exhaust pipe to either the main passage or the secondary passage; a control device for controlling the switching device based on the operating state of the engine; and a return passage connecting a downstream portion of the secondary passage to an intake path of the engine. [Effects of the Invention]

[0008] According to the engine exhaust system of the present invention, it is possible to suppress a decrease in catalyst temperature and to suppress catalyst poisoning caused by engine oil, thereby preventing a decrease in the purification performance of the catalyst. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing the configuration of an exhaust system according to an embodiment of the present invention, together with a vertical cross-sectional view of an exhaust pipe. [Figure 2] 2A and 2B are cross-sectional views taken along the line AA in FIG. 1, respectively corresponding to before and after switching from the main flow path to the sub-flow path. [Figure 3] FIG. 4 is a vertical cross-sectional view showing the flow of exhaust gas in the exhaust pipe in a state where the switching device is switched to the main flow path side in the embodiment. [Figure 4] FIG. 4 is a vertical cross-sectional view showing the flow of exhaust gas in the exhaust pipe in a state where the switching device is switched to the sub-flow path side in the embodiment. [Figure 5] FIG. 10 is a cross-sectional view showing a modification of the switching device related to the above embodiment. [Figure 6] FIG. 10 is a longitudinal cross-sectional view showing a modified example of the main flow path and the sub-flow path related to the above embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram showing the configuration of an exhaust system 1 according to one embodiment of the present invention, together with a vertical cross-sectional view of an exhaust pipe 3. As shown in Fig. 1, the exhaust system 1 of this embodiment is applied to a vehicle such as an automobile that uses an engine (ENG) 2 as at least a power source. In other words, the exhaust system 1 can be applied to a vehicle that uses the engine 2 as its only power source, or to a hybrid vehicle that combines the engine 2 with a power source (not shown) such as an electric motor that is different from the engine 2. The exhaust system 1 includes an exhaust pipe 3 that is connected to an exhaust path 2A of the engine 2 that is mounted on the vehicle.

[0011] The engine 2 is a well-known internal combustion engine that obtains power by burning fuel inside the engine cylinders. When the engine 2 is operating in firing mode (ignition mode) while receiving a supply of fuel, such as during vehicle acceleration, the engine 2 discharges high-temperature gases combusted inside the engine cylinders from an exhaust path 2A to an exhaust pipe 3. When the engine 2 is operating in fuel cut control or motoring mode (non-ignition mode) during vehicle deceleration, the engine 2 discharges relatively low-temperature gases remaining inside the engine cylinders from the exhaust path 2A to the exhaust pipe 3.

[0012] The exhaust pipe 3 has, for example, a substantially cylindrical shape, and a large-diameter portion 31 is formed in one longitudinal portion thereof, the diameter of which is larger than that of the remaining portion. Gas (exhaust gas) discharged from the exhaust path 2A of the engine 2 flows into the exhaust pipe 3 from one end of the exhaust pipe 3 located on the engine 2 side in the longitudinal direction, and flows toward the other end located opposite the engine 2. In the following description, the expressions "upstream side" and "downstream side" refer to the upstream side and downstream side in the flow direction of the exhaust gas within the exhaust pipe 3 (the direction of arrow F in FIG. 1).

[0013] The exhaust pipe 3 has an upstream section 32 formed in a section located upstream of the large diameter section 31, and a downstream section 33 formed in a section located downstream of the large diameter section 31. In this embodiment, the diameter of the upstream section 32 is larger than the diameter of the downstream section 33, and the diameter of the large diameter section 31 is even larger than the diameter of the upstream section 32.

[0014] The large-diameter portion 31 has a double-pipe structure in which an outer pipe 31A and an inner pipe 31B of different diameters are concentrically combined. The upstream end of the outer pipe 31A is connected by welding or the like to the downstream end of the upstream section 32 of the exhaust pipe 3. The downstream end of the outer pipe 31A is also connected by welding or the like to the upstream end of the downstream section 33 of the exhaust pipe 3. The diameter of the outer pipe 31A gradually widens from the upstream end, which is adjusted to the same diameter as the upstream section 32 of the exhaust pipe 3, toward the downstream side, then becomes roughly constant, and then gradually narrows toward the downstream side, until at the downstream end it is adjusted to the same diameter as the downstream section 33 of the exhaust pipe 3.

[0015] The diameter of the upstream end of the inner pipe 31B is narrower than the diameter of the upstream end of the outer pipe 31A. That is, the upstream end of the inner pipe 31B is disposed radially inward with a gap relative to the upstream end of the outer pipe 31A. Meanwhile, the downstream end of the inner pipe 31B is connected by welding or the like to the joint between the downstream end of the outer pipe 31A and the upstream end of the downstream section 33 of the exhaust pipe 3. The diameter of the inner pipe 31B gradually widens from the upstream end, which is narrower than the diameter of the upstream section 32 of the exhaust pipe 3, toward the downstream side, then becomes roughly constant, and then gradually narrows further toward the downstream side, until at the downstream end it is the same diameter as the downstream section 33 of the exhaust pipe 3.

[0016] Therefore, in the double-pipe structure of the large-diameter portion 31, the space radially inside the inner pipe 31B is open to both the upstream and downstream sides, and the space radially outside the inner pipe 31B and radially inside the outer pipe 31A is open to the upstream side and closed to the downstream side. In this embodiment, the space radially inside the inner pipe 31B forms part of the main flow path 3A of exhaust gas flowing through the exhaust pipe 3, and the space radially outside the inner pipe 31B and radially inside the outer pipe 31A forms the secondary flow path 3B of exhaust gas flowing through the exhaust pipe 3. In other words, in the large-diameter portion 31 of the exhaust pipe 3, the main flow path 3A and the secondary flow path 3B are separated by the common inner pipe 31B. A catalytic device 4 and a heat insulating material 5 are disposed in the space radially inside the inner pipe 31B (main flow path 3A). Note that in this embodiment, the inner pipe 31B of the large-diameter portion 31 corresponds to the "partition wall" of the present invention.

[0017] The catalytic converter 4 has a cylindrical or columnar outer shape so that it can be housed inside the inner pipe 31B, and a catalyst is supported inside the catalytic converter 4. The catalyst has the function of purifying substances such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in the exhaust gas flowing through the main flow path 3A in the exhaust pipe 3. Specific examples of such catalysts include platinum (Pt), palladium (Pd), and rhodium (Rh). However, the catalyst is not limited to these specific examples. The catalyst is supported on a sealing material (mat) or the like.

[0018] The heat insulating material 5 is disposed on the inner surface of the inner pipe 31B in a portion located around the catalytic converter 4. In this embodiment, the cylindrical heat insulating material 5 is sandwiched between the inner surface of the inner pipe 31B and the outer surface of the catalytic converter 4 while abutting against each other, and most of the outer surface of the catalytic converter 4 is covered by the heat insulating material 5. As a specific example of the heat insulating material 5, the above-mentioned catalyst-supporting sealing material (mat) or the like can be used. However, the heat insulating material 5 is not limited to the above example.

[0019] A switching device 6 is provided at the upstream end of the large-diameter portion 31 of the double-pipe structure described above, which is capable of switching the flow of exhaust gas in the exhaust pipe 3 to either the main flow path 3A or the sub-flow path 3B. The switching device 6 has, for example, one on-off valve 61 on the main flow path 3A side and multiple on-off valves 62 on the sub-flow path 3B side, and the on-off valves 61, 62 open and close in conjunction with each other to switch the flow of exhaust gas to the main flow path 3A or the sub-flow path 3B. Figure 2 is a cross-sectional view showing the cross section taken along line AA in Figure 1 before and after switching from the main flow path 3A to the sub-flow path 3B.

[0020] As shown in FIGS. 1 and 2, the on-off valve 61 on the main flow path 3A side is disposed so as to cover the upstream end of the inner pipe 31B in the large-diameter portion 31. The on-off valve 61 has a valve element 61B that can rotate around an axis 61A that is perpendicular to the flow direction of exhaust gas in the exhaust pipe 3. The valve element 61B is formed in a circular shape with a diameter that is approximately the same as the inner diameter of the inner pipe 31B. The rotation angle of the valve element 61B is controlled in accordance with a control signal output from an electronic control unit (ECU) 9, which will be described later. The on-off valve 61 can be in an open state (solid line in FIG. 1 and upper part of FIG. 2) where the valve element 61B is approximately parallel to the flow direction of exhaust gas, or in a closed state (two-dot chain line in FIG. 1 and lower part of FIG. 2) where the valve element 61B is approximately perpendicular to the flow direction of exhaust gas.

[0021] Each on-off valve 62 on the sub-flow path 3B side is disposed so as to cover the space between the upstream end of the outer pipe 31A and the upstream end of the inner pipe 31B in the large-diameter portion 31. Each on-off valve 62 has a valve element 62B that can swing about an axis 62A that is perpendicular to the flow direction of exhaust gas in the exhaust pipe 3. Each valve element 62B is formed in a sector shape that divides the annular gap between the outer pipe 31A and the inner pipe 31B equally by the number of on-off valves 62 installed, and the axis 62A is provided radially outward of the sector. The swing angle of each valve element 62B is controlled in accordance with a control signal output from the electronic control unit 9. Each on-off valve 62 can be in a closed state (solid line in FIG. 1 and upper part of FIG. 2) in which each valve element 62B is approximately perpendicular to the flow direction of exhaust gas, or in an open state (two-dot chain line in FIG. 1 and lower part of FIG. 2) in which each valve element 62B is approximately parallel to the flow direction of exhaust gas.

[0022] The switching device 6 switches the flow of exhaust gas in the exhaust pipe 3 to the main flow path 3A by controlling the on-off valve 61 on the main flow path 3A side to an open state and the on-off valves 62 on the sub-flow path 3B side to a closed state in conjunction with each other (solid line in FIG. 1 and upper part of FIG. 2). Also, the switching device 6 switches the flow of exhaust gas in the exhaust pipe 3 to the sub-flow path 3B by controlling the on-off valve 61 on the main flow path 3A side to a closed state and the on-off valves 62 on the sub-flow path 3B side to an open state in conjunction with each other (two-dot chain line in FIG. 1 and lower part of FIG. 2).

[0023] One end of the return passage 8 is connected to a portion of the exhaust pipe 3 located downstream of the secondary passage 3B via a pressure opening and closing device 7. The other end of the return passage 8 is connected to the intake passage 2B of the engine 2. In other words, in this embodiment, the pressure opening and closing device 7 and the return passage 8 connect the portion located downstream of the secondary passage 3B to the intake passage 2B of the engine 2.

[0024] The pressure opening and closing device 7 is attached to a mounting hole 31C (FIG. 1) formed to penetrate a portion of the large-diameter portion 31 of the exhaust pipe 3 located downstream of the outer pipe 31A, while maintaining airtightness within the secondary passage 3B. The pressure opening and closing device 7 is configured to mechanically open and close in accordance with the exhaust gas pressure Pg within the secondary passage 3B, for example. Specifically, the pressure opening and closing device 7 is in a closed state when the pressure Pg is lower than a preset exhaust pressure threshold THpg, and in an open state when the pressure Pg is equal to or higher than the exhaust pressure threshold THpg.

[0025] The return path 8 forms a path for returning the exhaust gas flowing in the secondary path 3B to the intake path 2B of the engine 2 when the pressure opening and closing device 7 is in the open state. As will be described later, the exhaust gas introduced into the secondary path 3B while the engine 2 is under fuel cut control or motoring operation is continuing may contain a large amount of oil components in the form of mist, etc. For this reason, in this embodiment, a configuration is adopted in which the exhaust gas flowing in the secondary path 3B is returned to the intake path 2B of the engine 2.

[0026] The electronic control unit 9 controls the power source, such as the engine 2, in accordance with the driving conditions of the vehicle, and is capable of controlling the on-off valves 61, 62 of the switching device 6 based on the operating state of the engine 2. A sensor group 10 is connected to the electronic control unit 9, which detects various data related to the driving conditions of the vehicle and the operating state of the engine 2. The electronic control unit 9 in this embodiment acquires information related to the pressure Pe in the engine cylinders, information related to the temperature Tg of exhaust gas discharged from the engine 2 to the exhaust pipe 3, and information related to the temperature Tc of the catalytic converter 4 provided in the exhaust pipe 3, based on the detection results of the sensor group 10. The electronic control unit 9 then controls the open / close states of the on-off valves 61, 62 in conjunction with each other in accordance with the acquired various information, and also controls the operation of the spray device 11 that sprays water into the exhaust pipe 3. In this embodiment, the electronic control unit 9 has functions corresponding to the "control device," "negative pressure information acquisition means," "exhaust temperature information acquisition means," and "catalyst temperature information acquisition means" of the present invention.

[0027] The spray device 11 is disposed around the large diameter portion 31 of the exhaust pipe 3, and is configured to be able to spray water toward the outer pipe 31A of the large diameter portion 31. When the electronic control unit 9 determines that the temperature Tc of the catalytic converter 4 is equal to or higher than the catalyst temperature threshold value THtc, the spray device 11 sprays water toward the outer pipe 31A of the large diameter portion 31 in accordance with a control signal output from the electronic control unit 9.

[0028] Next, the operation of the exhaust system 1 according to this embodiment will be described with reference to Figures 3 and 4. Figure 3 shows the flow of exhaust gas in the exhaust pipe 3 when the switching device 6 is switched to the main flow path 3A side, and Figure 4 shows the flow of exhaust gas in the exhaust pipe 3 when the switching device 6 is switched to the sub-flow path 3B side.

[0029] For example, when the vehicle is accelerating or traveling at a constant speed, fuel is supplied to the engine 2 and the engine 2 performs firing operation. At this time, the pressure Pe inside the engine cylinder becomes negative during the intake stroke and becomes positive during each of the compression, combustion, expansion, and exhaust strokes. High-temperature, high-pressure exhaust gas generated during the combustion stroke passes through the expansion stroke and is discharged from the exhaust path 2A of the engine 2 to the exhaust pipe 3 during the exhaust stroke. The sensor group 10 detects various data relating to the traveling state of the vehicle and the operating state of the engine 2, and transmits the detection results to the electronic control unit 9.

[0030] Based on the detection results of the sensor group 10, the electronic control unit 9 determines that the engine 2 is in a firing operation state, generates a control signal to direct the flow of exhaust gas in the exhaust pipe 3 to the main passage 3A, and outputs the control signal to the switching device 6. In accordance with the control signal from the electronic control unit 9, the switching device 6 controls the on-off valve 61 on the main passage 3A side to an open state and simultaneously controls the on-off valves 62 on the sub-passage 3B side to a closed state (upper part of FIG. 2 and FIG. 3). As a result, exhaust gas flowing from the engine 2 into the exhaust pipe 3 passes through the upstream section 32 of the exhaust pipe 3 and into the main passage 3A formed in the inner pipe 31B of the large-diameter section 31, as shown by the arrows in FIG. 3, and is purified by passing through the catalytic device 4 provided in the main passage 3A. The exhaust gas purified by the catalytic device 4 passes through the downstream section 33 of the exhaust pipe 3 and is discharged outside the vehicle.

[0031] When exhaust gas passes through the catalytic converter 4, the exhaust gas reacts with the catalyst, generating heat of reaction, and the heat of reaction and the heat of the exhaust gas cause the temperature Tc of the catalytic converter 4 to rise. When the electronic control unit 9 determines, based on the detection results of the sensor group 10, that the temperature Tc of the catalytic converter 4 is equal to or higher than the catalyst temperature threshold value THtc, it generates a control signal to cause the sprayer 11 to spray water into the exhaust pipe 3, and outputs this control signal to the sprayer 11. In accordance with the control signal from the electronic control unit 9, the sprayer 11 sprays water toward the outer pipe 31A of the large-diameter portion 31 in the exhaust pipe 3. This prevents the catalyst temperature from rising excessively.

[0032] For example, when the vehicle decelerates and fuel cut control for engine 2 continues, or when a series hybrid vehicle continues to operate in motoring mode (non-ignition mode) in which engine 2 is driven as an electric motor using regenerative power during deceleration to ensure brake negative pressure, the pressure Pe inside the engine cylinders becomes negative more frequently than during the firing mode described above, and the temperature Tg of the exhaust gas discharged from exhaust path 2A of engine 2 decreases. Various data relating to such changes in the vehicle's running state and the operating state of engine 2 are detected by sensor group 10, and the detection results are transmitted to electronic control unit 9.

[0033] Based on the detection results of the sensor group 10, the electronic control unit 9 determines that the negative pressure state in the engine cylinder is increasing, or that the exhaust gas temperature Tg is equal to or lower than the exhaust temperature threshold value THtg, and generates a control signal to switch the flow of exhaust gas in the exhaust pipe 3 from the main flow path 3A to the secondary flow path 3B, and outputs the control signal to the switching device 6. In accordance with the control signal from the electronic control unit 9, the switching device 6 controls the on-off valve 61 on the main flow path 3A side to a closed state, and simultaneously controls the on-off valves 62 on the secondary flow path 3B side to an open state (the lower part of FIG. 2 and FIG. 4). As a result, exhaust gas that has flowed from the engine 2 into the exhaust pipe 3 passes through the upstream section 32 of the exhaust pipe 3 and flows into the secondary flow path 3B formed between the outer pipe 31A and inner pipe 31B of the large-diameter section 31, as shown by the arrows in FIG. 4.

[0034] During non-ignition operation of the engine 2, as described above, engine oil is likely to flow into the combustion chamber from the lower bore of the cylinder block or the cylinder head, resulting in the phenomenon of oil rising and / or falling. Therefore, the relatively low-temperature exhaust gas emitted from the engine 2 during non-ignition operation is likely to contain oil components in the form of mist. However, the on-off valve 61 on the main flow path 3A, which is controlled to a closed state, blocks the flow of low-temperature exhaust gas containing oil components into the main flow path 3A, where the catalytic converter 4 is located. This prevents low-temperature exhaust gas from passing through the catalytic converter 4, thereby suppressing a temperature drop in the catalyst. Furthermore, the provision of heat insulating material 5 around the catalytic converter 4 prevents a temperature drop in the catalyst even when low-temperature exhaust gas flows through the secondary flow path 3B.

[0035] As the engine 2 continues to operate in a non-ignition state, the exhaust gas pressure Pg in the secondary passage 3B increases over time. However, when the exhaust gas pressure Pg reaches or exceeds the exhaust pressure threshold THpg, the pressure opening and closing device 7 switches from a closed state to an open state, and the exhaust gas remaining in the secondary passage 3B is returned to the intake path 2B of the engine 2 through the pressure opening and closing device 7 and the return passage 8. This prevents the exhaust gas from flowing back into the exhaust pipe 3.

[0036] As described above, in the exhaust system 1 according to this embodiment, the main flow path 3A and the sub-flow path 3B are formed in the exhaust pipe 3, and the return flow path 8 is formed connecting the downstream portion of the sub-flow path 3B with the intake path 2B of the engine 2. The switching device 6 is controlled by the electronic control unit 9 based on the operating state of the engine 2, thereby switching the flow of exhaust gas in the exhaust pipe 3 to either the main flow path 3A or the sub-flow path 3B. With the exhaust system 1 configured in this manner, even if the engine 2 continues to operate in a non-ignition state and low-temperature exhaust gas containing oil components flows into the exhaust pipe 3, a decrease in catalyst temperature can be suppressed and poisoning of the catalyst by the oil components in the exhaust gas can be suppressed. This makes it possible to prevent a decrease in the purification performance of the catalyst.

[0037] Furthermore, in the exhaust system 1 according to this embodiment, the electronic control unit 9 acquires information regarding the negative pressure in the engine cylinders, and when it determines an increase in the negative pressure in the engine cylinders based on the acquired information, it controls the switching device 6 to switch the flow of exhaust gas in the exhaust pipe 3 from the main flow path 3A to the sub-flow path 3B. By controlling the switching device 6 in this manner, it is possible to reliably determine the oil rise and / or fall phenomenon that occurs when the engine 2 continues to operate without firing, and to switch from the main flow path 3A to the sub-flow path 3B. This makes it possible to effectively suppress catalyst poisoning caused by oil components in the exhaust gas, and reliably prevent a decrease in the purification performance of the catalyst.

[0038] Furthermore, in the exhaust system 1 according to this embodiment, the electronic control unit 9 acquires information regarding the exhaust gas temperature, and when it determines based on the acquired information that the exhaust gas temperature Tg is equal to or lower than the exhaust temperature threshold value THtg, it controls the switching device 6 so that the flow of exhaust gas in the exhaust pipe 3 is switched from the main flow path 3A to the sub-flow path 3B. By controlling the switching device 6 in this manner, it is possible to reliably determine a decrease in the exhaust gas temperature Tg caused by continued non-ignition operation of the engine 2 and switch from the main flow path 3A to the sub-flow path 3B. This makes it possible to effectively suppress a temperature decrease in the catalyst and reliably prevent a decrease in the purification performance of the catalyst.

[0039] Furthermore, in the exhaust system 1 according to this embodiment, a pressure opening and closing device 7 is provided in a portion located downstream of the secondary flow path 3B, and when the exhaust gas pressure Pg in the secondary flow path 3B reaches or exceeds the exhaust pressure threshold value THpg, the pressure opening and closing device 7 switches from a closed state to an open state. By providing such a pressure opening and closing device 7, it is possible to reduce the frequency with which exhaust gas is recirculated into the intake path 2B of the engine 2 while the engine 2 is continuing to operate without ignition. The recirculation of exhaust gas containing oil components can be a factor in degrading the performance of the engine 2, but since such a situation is less likely to occur, it is possible to suppress degradation of the performance of the engine 2.

[0040] Furthermore, in the exhaust system 1 according to this embodiment, the sub-flow passage 3B is disposed along and adjacent to the main flow passage 3A, and the main flow passage 3A and the sub-flow passage 3B are formed using the inner pipe 31B as a common partition wall. Forming the main flow passage 3A and the sub-flow passage 3B in this manner within the exhaust pipe 3 simplifies the structure of the exhaust pipe 3, improving the layout flexibility of the exhaust pipe 3. In particular, forming the main flow passage 3A and the sub-flow passage 3B in a double-pipe structure within the exhaust pipe 3 allows the main flow passage 3A and the sub-flow passage 3B to be formed within the same layout range as existing exhaust pipes while improving the thermal insulation effect, thereby increasing the design flexibility of the exhaust pipe 3. Furthermore, because the flow of exhaust gas within the sub-flow passage 3B follows the main flow passage 3A, the pressure of the exhaust gas flowing through the sub-flow passage 3B can be estimated in the same way as the pressure of the exhaust gas flowing through the main flow passage 3A. This allows for easy and accurate design of the exhaust pressure threshold THpg to prevent backflow of exhaust gas within the sub-flow passage 3B.

[0041] Furthermore, in the exhaust system 1 according to this embodiment, a heat insulating material 5 is disposed in a portion of the inner pipe 31B (partition wall) located around the catalytic device 4. By providing such a heat insulating material 5, even if low-temperature exhaust gas flows through the sub-passage 3B, the heat insulating material 5 prevents heat transfer from the catalytic device 4 on the main passage 3A side to the sub-passage 3B side, thereby further suppressing a decrease in the temperature of the catalyst.

[0042] Furthermore, in the exhaust system 1 according to this embodiment, the electronic control unit 9 acquires information about the temperature Tc of the catalytic converter 4, and when it determines based on the acquired information that the temperature Tc of the catalytic converter 4 is equal to or higher than the catalyst temperature threshold value THtc, it controls the sprayer 11 to spray water into the outer pipe 31A that forms the secondary flow passage 3B. By controlling the sprayer 11 in this way, it is possible to prevent the catalyst temperature from rising excessively, and therefore it is possible to more reliably prevent a decrease in the purification performance of the catalyst.

[0043] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and changes are possible based on the technical concept of the present invention. For example, in the above-described embodiment, an example has been described in which the flow of exhaust gas in the exhaust pipe 3 is switched from the main flow path 3A to the sub-flow path 3B when the electronic control unit 9 determines that the negative pressure state in the engine cylinders has increased or that the exhaust gas temperature Tg is equal to or lower than the exhaust temperature threshold value THtg, but the timing of switching from the main flow path 3A to the sub-flow path 3B is not limited to the above example. For example, the switching from the main flow path 3A to the sub-flow path 3B may be performed in accordance with the timing when the electronic control unit 9 starts fuel cut control or motoring operation of the engine 2.

[0044] In the above-described embodiment, the switching device 6 is configured such that the valve bodies 62B of the on-off valves 62 on the sub-flow path 3B side are formed in a sector shape by dividing the annular gap between the outer pipe 31A and the inner pipe 31B equally by the number of on-off valves 62. However, as shown in FIG. 5, for example, an annular plate member 63 may be disposed in the gap between the outer pipe 31A and the inner pipe 31B, and on-off valves 62' each made of a circular valve body 62B' rotatable about an axis 62A' may be disposed in each of a plurality of through holes 63A (six in the illustrated example) formed in the plate member 63. Note that, as with FIG. 2 described above, the upper part of FIG. 5 shows a state in which the flow of exhaust gas has been switched to the main flow path 3A, and the lower part of FIG. 5 shows a state in which the flow of exhaust gas has been switched to the sub-flow path 3B. This modified example of the switching device 6 can also achieve the same effects as those of the above-described embodiment.

[0045] In the above-described embodiment, the large-diameter portion 31 of the exhaust pipe 3 has a double-pipe structure, forming the main flow path 3A and the sub-flow path 3B. However, as shown in FIG. 6 , a longitudinally extending partition wall 34 may be provided near the center of the exhaust pipe 3 in the width direction, so that the main flow path 3A and the sub-flow path 3B are formed side-by-side. In this case, if the exhaust pipe 3 has a circular cross-sectional shape, the main flow path 3A and the sub-flow path 3B each have a semicircular cross-sectional shape. If the exhaust pipe 3 has a rectangular cross-sectional shape, the main flow path 3A and the sub-flow path 3B each have a rectangular cross-sectional shape. The main flow path 3A is equipped with an on-off valve 61, a catalytic converter 4, and a heat insulator 5, which are adapted to the cross-sectional shape of the main flow path 3A. The sub-flow path 3B is also equipped with an on-off valve 62, which is adapted to the cross-sectional shape of the main flow path 3A, at its upstream end. The downstream end of the sub-flow path 3B is sealed, and is connected to a pressure on-off device 7 and a return flow path 8. Arranging the spray device 11 on the main flow path 3A side near the catalytic converter 4 is effective in preventing the catalyst temperature from rising excessively.

[0046] In the above-described embodiment, the pressure opening and closing device 7 is provided downstream of the secondary flow path 3B. However, the arrangement of the pressure opening and closing device 7 is not limited thereto. For example, the pressure opening and closing device 7 may be provided on the return flow path 8. Furthermore, while the configuration in which the pressure opening and closing device 7 mechanically opens and closes according to the exhaust gas pressure Pg in the secondary flow path 3B has been described, the pressure opening and closing device 7 may be controlled, for example, according to a control signal from the electronic control unit 9. In this case, the electronic control unit 9 can determine, based on the detection results of the sensor group 10, that the exhaust gas pressure Pg in the secondary flow path 3B is equal to or greater than the exhaust pressure threshold value THpg, and control the pressure opening and closing device 7 from the closed state to the open state. Alternatively, the electronic control unit 9 may control the pressure opening and closing device 7 from the closed state to the open state in accordance with the timing at which the switching device 6 switches the exhaust gas flow from the main flow path 3A to the secondary flow path 3B. In addition, although the pressure opening and closing device 7 controls the flow of exhaust gas from the secondary flow path 3B to the return flow path 8, the pressure opening and closing device 7 may be omitted.

[0047] In addition, in the above-described embodiment, an example has been described in which the exhaust gas flowing through the secondary passage 3B is returned to the intake path 2B of the engine 2. However, in addition to this return, it is also possible to provide an oil catch tank or the like in the secondary passage 3B or the return path 8, and then send the exhaust gas that has passed through the oil catch tank to the cooling device of the engine 2, the inverter of a hybrid vehicle, the battery, etc., and use the exhaust gas to cool or keep them warm. [Explanation of symbols]

[0048] 1...Exhaust system 2...Engine (ENG) 2A...Exhaust route 2B...Intake path 3...Exhaust pipe 3A…Main flow path 3B…Subchannel 31…Thick diameter part 31A…Outer tube 31B…Inner pipe (bulk part) 31C...Mounting hole 32...Upstream 33...Downstream 34...Bulkhead 4...Catalytic device 5...Insulation material 6...Switching device 61...Main flow path side on-off valve 62, 62'... Sub-passage side on-off valve 61A, 62A, 62A'...shaft 61B, 62B, 62B'...Valve body 7...Pressure switching device 8...Reflux channel 9...Electronic control unit (ECU) 10...Sensor group 11...Spray device F...Exhaust gas flow direction Pe: Pressure inside the engine cylinder Pg: Pressure of exhaust gas flowing through the secondary passage Tc: Temperature of the catalytic converter Tg: Exhaust gas temperature

Claims

1. An exhaust system equipped with a catalytic converter for purifying exhaust gas from an engine in an exhaust pipe through which the exhaust gas flows, a main flow path formed in the exhaust pipe through which the exhaust gas flows to pass through the catalytic device; a secondary flow passage formed in the exhaust pipe separately from the main flow passage and through which the exhaust gas flows; a switching device capable of switching the flow of the exhaust gas in the exhaust pipe to either the main flow path or the sub-flow path; a control device that controls the switching device based on an operating state of the engine; a return passage connecting a downstream portion of the sub-passage to an intake passage of the engine; 1. An engine exhaust system comprising:

2. 10. The engine exhaust system of claim 1, a negative pressure information acquisition means for acquiring information about the negative pressure in the engine cylinder; An engine exhaust system characterized in that the control device controls the switching device so that the flow of exhaust gas in the exhaust pipe is switched from the main flow path to the secondary flow path when it determines an increase in the negative pressure state within the engine cylinder based on information acquired by the negative pressure information acquisition means.

3. 10. The engine exhaust system of claim 1, an exhaust gas temperature information acquisition means for acquiring information about the temperature of the exhaust gas; An engine exhaust system characterized in that the control device controls the switching device so that the flow of exhaust gas in the exhaust pipe is switched from the main flow path to the secondary flow path when it determines that the temperature of the exhaust gas is below the exhaust temperature threshold based on the information acquired by the exhaust temperature information acquisition means.

4. 10. The engine exhaust system of claim 1, An engine exhaust system comprising a pressure opening and closing device provided in a portion located downstream of the secondary flow path or in the return flow path, wherein the pressure opening and closing device is configured to switch from a closed state to an open state when the pressure of the exhaust gas in the secondary flow path becomes equal to or greater than an exhaust pressure threshold.

5. 5. An engine exhaust system according to claim 4, comprising: the secondary flow passage is disposed along and tangent to the primary flow passage, 1. An engine exhaust system, comprising: a main flow passage and a secondary flow passage, the main flow passage and the secondary flow passage having at least one common partition wall portion;

6. 6. An engine exhaust system according to claim 5, comprising:

10. An engine exhaust system, comprising: a partition wall having a heat insulating material disposed in a portion of the partition wall located around the catalytic converter;

7. 6. An engine exhaust system according to claim 5, comprising: a catalyst temperature information acquisition means for acquiring information about the temperature of the catalyst device; a spray device capable of spraying water into the exhaust pipe, An engine exhaust system characterized in that the control device controls the spray device so that water is sprayed into the piping portions forming the main flow path and / or the secondary flow path when it determines, based on information acquired by the catalyst temperature information acquisition means, that the temperature of the catalyst device is equal to or higher than a catalyst temperature threshold.

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