Electric heating type exhaust particulate filter

Abstract

A monolith filter (31) that constitutes an electric heating type exhaust particulate filter (17) includes a closed cell (32A) having an upstream end closed by a first sealing part (34) and an open cell (32B) having a downstream end closed by a second sealing part (35), and collects soot as gas passes through a wall (33). The monolith filter (31) is made from an electrically conductive ceramic, and the monolith filter (31) itself functions as a heating element when an electric current is passed via a pair of electrodes (37). The first sealing part (34) has a long axial length (L1) to prevent soot from accumulating on an upstream-side end part, and each electrode (37) is provided at a position overlapping the axial length (L1). Heat is generated at the upstream-side end part of the monolith filter (31) without being inhibited by soot having low resistance.

Description

Electrically heated exhaust particulate filter

[0001] The present invention relates to an exhaust particulate filter that captures exhaust particulates contained in the exhaust gas of an internal combustion engine, and more particularly to an electrically heated exhaust particulate filter in which a honeycomb monolith filter made of conductive ceramic serves as a heating element.

[0002] Honeycomb monolithic filters made of porous ceramic material are known as particulate filters used in the exhaust systems of internal combustion engines. These filters are formed, for example, in a cylindrical shape from porous ceramic material, and have numerous fine passages, or cells, axially arranged within the filter. Each cell is separated by a thin wall. Some of the cells are blocked (i.e., sealed) at the upstream end of the filter, while the remaining cells are blocked at the downstream end. Typically, cells with blocked upstream ends (called blocked cells) and cells with blocked downstream ends (called open cells) are arranged adjacent to each other, alternating. Thus, exhaust gas flowing from the upstream side into an open cell passes through the porous wall into an adjacent closed cell and then exits through the open downstream end of the closed cell. Exhaust gas particles contained in the exhaust gas are primarily captured and removed as the exhaust gas passes through the porous wall.

[0003] Patent Document 1 discloses an electrically heated exhaust particulate filter that uses a conductive ceramic such as SiC as the porous ceramic material that constitutes the monolith filter, and by passing electricity through a pair of electrodes, the monolith filter itself becomes a heating element.

[0004] However, if soot accumulates on the inner peripheral surface of the cells of the monolith filter, the electrical resistance of the soot, which is primarily composed of carbon, is lower than the electrical resistance of the ceramic material of the monolith filter, and the soot acts as a short-circuit current path, causing a large amount of current to flow, and reducing the current flowing through the monolith filter itself, which acts as a heating element.As a result, sufficient heat generation is not achieved, and some areas of low temperature are created.

[0005] Japanese Unexamined Patent Publication No. 58-119317

[0006] The present invention is an electrically heated exhaust particulate filter comprising a honeycomb-type monolith filter using a porous conductive ceramic having a large number of closed cells whose upstream ends are closed by first plugging portions and a large number of open cells whose downstream ends are closed by second plugging portions, and a pair of electrodes for passing current through the monolith filter, wherein the electrodes are provided on the outer surface of the monolith filter at a position in the axial direction of the monolith filter that at least partially overlaps with the first plugging portions so that current flows through the first plugging portions of the multiple closed cells in a direction perpendicular to the axial direction of the monolith filter.

[0007] In the length range where the first plugging portion exists at the upstream end of the closed cell, exhaust gas does not flow through the wall from the adjacent open cell, so soot does not accumulate. Because the pair of electrodes are positioned to at least partially overlap with the first plugging portion, current flows through the wall between the cells at the upstream end of the monolith filter and the first plugging portion, generating heat at the upstream end of the monolith filter. In other words, the flow of current through soot is suppressed, and the monolith filter effectively generates heat.

[0008] 1 is a diagram illustrating the configuration of a series hybrid vehicle; 2 is a diagram illustrating the configuration of an intake system and an exhaust system of an internal combustion engine; 3 is a front view of an exhaust particulate filter of an embodiment; 4 is a longitudinal sectional view of an exhaust particulate filter of an embodiment; 5 is a longitudinal sectional view of an exhaust particulate filter of a second embodiment in which a current interrupting portion is provided in the form of a partition wall; 6 is a perspective view of an exhaust particulate filter of a second embodiment; 7 is a longitudinal sectional view of an exhaust particulate filter of a third embodiment in which the lengths of the first sealing portions are set to different lengths; 8 is a longitudinal sectional view of an exhaust particulate filter of a fourth embodiment including a first sealing portion located away from the front end face;

[0009] An embodiment of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows a schematic configuration of a series hybrid vehicle as an example of a vehicle to which the present invention can be applied. The series hybrid vehicle includes a power-generating motor-generator 1 that operates primarily as a generator, an internal combustion engine 2 that serves as a power-generating internal combustion engine and drives the power-generating motor-generator 1 in response to a power demand, a traction motor-generator 4 that operates primarily as a motor to drive drive wheels 3, and a battery 5 that stores the generated power. The power obtained by the internal combustion engine 2 driving the power-generating motor-generator 1 is stored in the battery 5 via an inverter device (not shown). The traction motor-generator 4 is driven and controlled using the power from the battery 5. The power generated by the traction motor-generator 4 during regeneration is stored in the battery 5 via an inverter device (not shown).

[0010] The operation of the motor generators 1 and 4, the charging and discharging of the battery 5, and the operation of the internal combustion engine 2 are controlled by a controller 6. The controller 6 is composed of multiple controllers connected to each other so that they can communicate with each other, such as a motor controller 7 that controls the motor generators 1 and 4, an engine controller 8 that controls the internal combustion engine 2, and a battery controller 9 that manages the battery 5. Information such as the accelerator pedal position and vehicle speed (not shown) is input to the controller 6. The battery controller 9 also calculates the SOC of the battery 5 based on the voltage and current of the battery 5. When the SOC drops to a predetermined lower limit, the internal combustion engine 2 is started via the engine controller 8 to generate electricity. Such a series hybrid vehicle has two driving modes: an EV mode in which the vehicle runs on power from the battery 5 without combustion operation of the internal combustion engine 2, and an HEV mode in which the vehicle runs while generating electricity through combustion operation of the internal combustion engine 2. Even if the SOC is above the lower limit, the internal combustion engine 2 is driven and the vehicle runs in the HEV mode when the required driving force of the vehicle is relatively large. Therefore, the internal combustion engine 2 repeatedly performs combustion operation and stops of combustion operation while the main switch of the vehicle is on.

[0011] 2 shows the configuration of an intake system and an exhaust system of the internal combustion engine 2. The internal combustion engine 2 of one embodiment is a four-stroke spark-ignition gasoline engine equipped with a turbocharger 13 as a supercharger, and is, for example, a so-called direct injection type internal combustion engine in which fuel is injected directly into the cylinders by a fuel injection valve.

[0012] A compressor 15 of a turbocharger 13 is disposed in the intake passage 12 of the internal combustion engine 2, and an electronically controlled throttle valve 21 that controls the amount of intake air is disposed downstream of the compressor 15. A water-cooled intercooler 22, for example, that cools the supercharged intake air is disposed between the compressor 15 and the throttle valve 21. An air cleaner 28 is disposed upstream of the compressor 15, and a vacuum generating valve 27 that generates the vacuum required for exhaust gas recirculation is provided between the compressor 15 and the air cleaner 28. The throttle valve 21 and the vacuum generating valve 27 are both butterfly valves with circular valve bodies, and their respective opening degrees are controlled by the engine controller 8.

[0013] An exhaust turbine 14 of a turbocharger 13 is disposed in an exhaust passage 11 of the internal combustion engine 2, and an electrically heated exhaust particulate filter 17 of one embodiment, which serves as an upstream catalytic converter, and a downstream catalytic converter 18, which uses, for example, a three-way catalyst, are disposed downstream of the exhaust turbine 14. The exhaust particulate filter 17 carries, for example, a catalytic metal that serves as a three-way catalyst, and functions as a filter that captures exhaust particulates including soot, as well as a catalytic converter. An exhaust silencer 19 is provided further downstream of the downstream catalytic converter 18 in the exhaust passage 11, and the exhaust passage 11 is opened to the outside via the exhaust silencer 19.

[0014] An exhaust gas recirculation passage 24 for recirculating a portion of the exhaust gas to the intake system is provided between the exhaust passage 11 and the intake passage 12. A base end 24a of the exhaust gas recirculation passage 24 branches off from the exhaust passage 11 downstream of the exhaust turbine 14, specifically between the exhaust particulate filter 17 and the downstream catalytic converter 18. A tip end 24b is connected to the intake passage 12 at a position upstream of the compressor 15. The exhaust gas recirculation passage 24 includes, for example, a water-cooled EGR gas cooler 26 for cooling the recirculated exhaust gas, and an EGR valve 25 whose opening is variably controlled by the engine controller 8.

[0015] 1 receives detection signals from a variety of sensors, not shown, directly or via other controllers, such as an air flow meter that detects the amount of intake air, an air-fuel ratio sensor that detects the exhaust air-fuel ratio, a crank angle sensor that detects the engine speed, a water temperature sensor that detects the temperature of the cooling water, a boost pressure sensor that detects the boost pressure, an accelerator position sensor that detects the amount of depression of the accelerator pedal, an atmospheric pressure sensor that detects the atmospheric pressure, and an outside air temperature sensor. Based on these detection signals and requests from the other controllers, the engine controller 8 optimally controls the fuel injection amount and injection timing, ignition timing, the opening of the throttle valve 21, the boost pressure, the EGR rate, the opening of the vacuum generating valve 27, and the like.

[0016] The engine controller 8 also controls the supply of current to the electrically heated particulate filter 17. The electrically heated particulate filter 17 is supplied with current in two main situations. One is preheating to activate the catalyst during a cold start of the internal combustion engine 2. When a start of the internal combustion engine 2 is requested due to a power generation request in a cold state, the electrically heated particulate filter 17 is supplied with current before the start of combustion operation (fuel injection and ignition), preheating the particulate filter 17, which functions as a catalytic converter. This preheating improves exhaust emissions from the initial stage of combustion operation. The other is heating to burn off soot that is performed when soot accumulates on the particulate filter 17. For example, when the vehicle ends a trip and the main switch is turned off, the particulate filter 17 is heated by supplying current to remove soot, provided that the particulate filter 17 is at a certain temperature (e.g., 400°C) or higher. Soot is generated and accumulated in large amounts when the internal combustion engine 2 is operated in a cold state or under operating conditions with low exhaust gas temperatures, and is naturally removed from the exhaust gas particulate filter 17 when the exhaust gas temperature rises due to high-load operation. The amount of soot accumulated in the exhaust gas particulate filter 17 is constantly calculated by the engine controller 8. When soot accumulates to a predetermined amount or more due to repeated low-load operation, for example, the soot is forcibly removed by energizing the filter. Note that the former preheating is performed with a target temperature of about 400°C, and the latter heating for soot removal is performed with a target temperature of about 500 to 700°C.

[0017] Next, the configuration of the electrically heated exhaust particulate filter 17, which is a key component of the present invention, will be described. Figures 3 and 4 show an example of the exhaust particulate filter 17. The exhaust particulate filter 17 of this example is configured by housing a cylindrical honeycomb monolith filter 31, as shown, in a metal case (not shown) via a buffer material. The monolith filter 31 is integrally molded using a porous ceramic material, more specifically, a conductive ceramic material such as SiC, and has a large number of fine passages, i.e., a large number of cells 32, along its axial direction. Each cell 32 is separated by a thin wall 33. Some of the cells 32 are configured as closed cells 32A that are secondarily blocked by plugging portions (referred to as first plugging portions) 34 at the upstream end of the monolith filter 31, while the remaining cells are configured as open cells 32B that are secondarily blocked by plugging portions (referred to as second plugging portions) 35 at the downstream end of the monolith filter 31. In the illustrated example, the cells 32, each having a substantially square cross section, are arranged in a grid pattern, with closed cells 32A with closed upstream ends and open cells 32B with open upstream ends alternately arranged adjacent to each other. Therefore, exhaust gas flowing into the open cells 32B from the upstream side passes through the porous walls 33 into the adjacent closed cells 32A and then flows out from the open downstream ends of the closed cells 32A. Exhaust particulate matter, primarily soot, contained in the exhaust gas is captured and removed as the exhaust gas passes through the porous walls 33. The size, number, and dimensional ratios of the cells 32 in the drawing are depicted for illustrative purposes only and are not accurate.

[0018] The plugging portions 34, 35 are made of a ceramic material, just like the monolith filter 31 itself, and are formed by filling the cells 32 as a slurry and then solidifying. At least the first plugging portion 34 is made of a conductive ceramic material such as SiC. This makes both the monolith filter 31 and the first plugging portion 34 conductive and serves as a heating element that generates heat when electricity is applied. Note that by appropriately adjusting the components of the ceramic material of the monolith filter 31 and the ceramic material of the first plugging portion 34, it is possible to adjust the resistance values ​​of each.

[0019] As shown in FIG. 4 , the axial length L1 of the first plugging portion 34 is longer than the axial length L2 of the second plugging portion 35. The axial length L2 of the second plugging portion 35 is basically the minimum length required for sealing the cells 32. In contrast, the first plugging portion 34 is intentionally set longer than the length required for sealing in order to ensure an area with low resistance where soot does not accumulate. In other words, within the range of length L1 where the first plugging portion 34 exists in the closed cell 32A, gas does not flow from the adjacent open cell 32B into the closed cell 32A. Therefore, soot does not accumulate within the open cell 32B (on the inner wall surface of the open cell 32B). In other words, soot accumulates on the inner wall surface of the open cell 32B in the range to the right of length L1 in FIG. 4 , and soot is unlikely to accumulate within the range of length L1.

[0020] A pair of electrodes 37 are provided on the outer peripheral surface of the cylindrical monolith filter 31 to pass current through the monolith filter 31, which serves as a heating element (resistor). Each electrode 37 has, for example, a rectangular shape. The pair of electrodes 37 are arranged at two diametrically opposed locations as shown in FIG. 3 so that current flows in a direction perpendicular to the axial direction. Furthermore, as shown in FIG. 4, the electrodes 37 are arranged at axial positions that at least partially overlap the length range L1 of the first plugging portions 34. In the illustrated example, the entire axial length of the electrode 37 overlaps with the first plugging portions 34, that is, within the length L1 range of the first plugging portions 34.

[0021] In the electrically heated exhaust particulate filter 17 configured as described above, when current is applied via the pair of electrodes 37, current flows in the vertical direction in FIG. 4 through the upstream end of the monolith filter 31 where the first plugging portions 34 are located. If soot with a relatively lower resistance than the material of the monolith filter 31 accumulates or adheres to the monolith filter 31, the soot can become a short-circuit current path. However, as described above, there is almost no soot accumulation within the axial length L1 of the first plugging portions 34. Therefore, current flows through the wall 33 of the monolith filter 31 and the first plugging portions 34, which are made of conductive ceramic, and heat is generated effectively without being obstructed by soot. Furthermore, since the heat-generating portion is located at the most upstream portion of the gas flow in the monolith filter 31, heat spreads downstream along the gas flow in a situation where a gas flow is present, thereby efficiently heating the monolith filter 31.

[0022] Therefore, whether the monolith filter 31 is preheated during cold start-up or heated to burn off soot, the temperature can be reliably increased while suppressing unnecessary power consumption.

[0023] 5 and 6 show a second embodiment in which a current interrupting portion 39 made of a non-conductive material is provided in a portion of the interior of the monolith filter 31 to limit the area through which current flows within the monolith filter 31. The current interrupting portion 39 is provided in the form of a partition along the diameter line in a downstream portion of the monolith filter 31 that does not axially overlap the range of the axial length L1 of the first plugging portion 34. The current interrupting portion 39 is formed, for example, from a non-conductive ceramic material. In a preferred embodiment, the plate-shaped current interrupting portion 39 is formed by machining a slit along the diameter line after molding the monolith filter 31 and filling the slit with a non-conductive ceramic slurry.

[0024] 5, by providing the current interruption unit 39 in this manner, even if a large amount of soot accumulates in the downstream portion of the monolith filter 31, the current will not flow in a detour. In other words, the flow of current is limited to only the portion upstream of the current interruption unit 39, which more reliably generates heat at the upstream end of the monolith filter 31. Note that the position and shape of the current interruption unit 39 are not limited to those shown in the illustration, and it can be provided in any suitable position and in a different form.

[0025] Next, FIG. 7 shows a monolith filter 31 according to a third embodiment in which the axial lengths of the first plugging portions 34 in the multiple closed cells 32A are set to different lengths in order to adjust the heat generation distribution.

[0026] Assuming that the current flows equally through each of the first plugging portions 34 across the plurality of first plugging portions 34, the temperature during heating will be relatively high if the axial length is short. This makes it possible to adjust the temperature distribution in each portion of the monolith filter 31.

[0027] On the other hand, from the viewpoint of heat capacity, the longer the axial length, the greater the heat capacity. In one embodiment, from the viewpoint of heat capacity, the axial length of the first sealing portions 34 of the closed cells 32A located in the region where the main flow of the inflowing exhaust gas hits is longer than the axial length of the first sealing portions 34 of the closed cells 32A located in other regions. In the illustrated example, the main flow of the exhaust gas hits the center of the circular front end face, and the first sealing portions 34 are longer as they are closer to the center. As a result, for example, when the internal combustion engine 2 starts combustion operation after preheating at a cold start, the center where the main flow of the exhaust gas hits has a large heat capacity, and is therefore less likely to be cooled by the exhaust gas.

[0028] The length of the first plugging portion 34 in the multiple closed cells 32A is not limited to the illustrated example, and can be set appropriately depending on the required characteristics.

[0029] Furthermore, the heat generation position of each blocked cell 32A can be adjusted by appropriately shifting the axial position of the first plugging portions 34. Figure 8 shows a fourth embodiment in which the formation positions of the first plugging portions 34 of some blocked cells 32A are set at positions away from the front end face of the monolith filter 31. Heat generation in the monolith filter 31 due to energization occurs near the positions of the first plugging portions 34, so by adjusting the axial position of the first plugging portions 34, it is possible to appropriately adjust the temperature of each portion throughout the entire cylindrical monolith filter 31.

[0030] Even when some of the first plugging portions 34 are formed at positions away from the front end face of the monolith filter 31 in this way, it is preferable that the axial positional relationship between the pair of electrodes 37 is such that the first plugging portions 34 and the electrodes 37 at least partially overlap. If necessary, the electrodes 37 may be formed long in the axial direction. This allows current to flow between the pair of electrodes 37 through the upstream end of the monolith filter 31, reliably generating heat, as in the previous embodiments.

[0031] Although one embodiment of the present invention has been described above in detail, the present invention is not limited to the above embodiment and various modifications are possible. For example, the above embodiment has been described as an exhaust particulate filter used in an internal combustion engine for generating electricity in a series hybrid vehicle, but the present invention can also be applied to an internal combustion engine that serves as a driving source for a general vehicle. Furthermore, the present invention can also be applied to a simple exhaust particulate filter that does not support a catalytic metal. Furthermore, the first plugging portion may be made of a material that is less conductive than the monolith filter itself, or conversely, may be made of a material that is more conductive.

Claims

1. An electric heating type exhaust particulate filter comprising a honeycomb type monolith filter using a porous conductive ceramic having a large number of closed cells with the upstream end portion closed by a first plugging portion and a large number of open cells with the downstream end portion closed by a second plugging portion, and a pair of electrodes for passing an electric current through the monolith filter, wherein the electrodes are provided on the outer peripheral surface of the monolith filter at a position that at least partially overlaps with the first plugging portion as a position in the axial direction of the monolith filter so that an electric current flows in a direction orthogonal to the axial direction of the monolith filter through the first plugging portion of the plurality of closed cells.

2. The electric heating type exhaust particulate filter according to claim 1, wherein the axial length of the first plugging portion is longer than the axial length of the second plugging portion.

3. The electric heating type exhaust particulate filter according to claim 1, wherein the whole of the electrodes overlaps with the first plugging portion in the axial direction.

4. The electric heating type exhaust particulate filter according to claim 1, wherein the axial lengths of the first plugging portions in the plurality of closed cells are adjusted to different lengths for adjusting the heat generation amount distribution.

5. The electric heating type exhaust particulate filter according to claim 3, wherein the axial length of the first plugging portion of the closed cells located in the region where the main flow of the inflowing exhaust gas hits is longer than the axial length of the first plugging portion of the closed cells located in other regions.

6. The electric heating type exhaust particulate filter according to claim 1, wherein the axial formation positions of the first plugging portions in the plurality of closed cells are set at different positions for adjusting the heat generation positions, and include a first plugging portion located at a position away from the front end face of the monolith filter.

7. The electric heating type exhaust particulate filter according to claim 1, wherein a current blocking portion made of a non-conductive material is provided in a part of the interior of the monolith filter to limit the portion where an electric current flows in the monolith filter.

8. The electric heating type exhaust particulate filter according to claim 7, wherein the current blocking portion is provided in a partition wall shape in a downstream portion of the monolith filter that does not overlap with the first plugging portion as a position in the axial direction of the monolith filter.

9. The electric heating type exhaust particulate filter according to claim 1, wherein the monolith filter supports a catalytic metal.

10. The electrically heated exhaust particulate filter according to claim 9, which is used as the most upstream catalyst in the exhaust system of an internal combustion engine.

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