Diesel particulate filter

Abstract: A diesel particulate filter for filtering exhaust gases , said filter comprising a filter case (10) in which at least one filter body is disposed, said at least one filter body having filtering passages, the coarseness of which is, preferably gradually, increasingly dense in a direction from an upstream side toward a downstream side of the exhaust flow.

Description: The present invention relates to a diesel particulate filter for collecting and burning particulates contained in exhaust of a diesel engine. Since the diesel engine employs a heterogeneous combustion in which fuel is injected into air elevated in temperature by heat insulating compression to effect combustion, there existed a problem that less carbon monoxide is contained in exhaust but a large amount of nitrogen oxide (NOx) and particulates (mainly, carbon) are contained. As means for coping with the aforementioned point, a method of using a high pressure pump as a fuel pump to reduce the particulates generated and a method of mounting a filter in an exhaust flowpassage to filter particulates in the exhaust have been studied and developed. The former method uses a special pump, and the engine itself is extremely expensive. Therefore, the filtration method of the latter capable of achieving purification of exhaust at less cost has been desired. Attention has been paid to a ceramic material for a filter body, which is formed to be porous or have a coarse surface to thereby provide a large particulate adsorption ability. In the conventional particulate filter of this kind, particulates have been filtered by a single kind of a filter. The exhaust is introduced into an exhaust pipe as an exhaust valve is opened but a flow velocity and pressure of the exhaust always varies with the number of revolutions of the engine and the load. It is required that the exhaust pipe releases the exhaust at low resistance. On the other hand, the particulates contained in the exhaust of the diesel engine are extremely fine within the combustion chamber of the engine but as the particulates flow into the exhaust pipe via the exhaust valve, fine particles are gradually gathered and agglomerate to form large particles. It is said that the distribution of particle sizes is substantially a normal distribution, and the distribution state of particles is in the range of from 2 to 30µm with a particle diameter of 15µm being in the centeral value. When such particulates pass throuth a porous filter, they collide with each other in the vicinity of the inlet of the filter and the particle size thereof further grows and then the particles are collected. However, the particulates having a particle size in the range of 2 to 100µm are contained in the exhaust which has reached the filter. The conventional filter of a single material, which is fine in mesh, has been employed so that particulates of a small particle size can be collected. For this reason, the filter becomes severely clogged such that a portion in the vicinity of the outlet of the filter body is first clogged with particulates and a portion in the vicinity of the inlet is then clogged to increase a gas flow resistance of the filter and to elevate a pressure, impairing a smooth release of exhaust. The present invention has been achieved to cope with the aforementioned point. An aim of the present invention is to provide an arrangement wherein internal pressure of the filter is made as even as possible to secure a smooth flow of exhaust, a collecting efficiency of particulates is enhanced by making the flow velocity even, a rapid rise of pressure due to the collection of particulates is avoided, and an exhaust pressure of the engine is prevented from being elevated by the filter. FR-A-2 600 907 discloses a method and apparatus for disposing of particulate matter retained in an exhaust gas filter of an internal combustion engine, using an electric current source. In one embodiment a filter element comprises two cylindrical filter layers. Accordingly, the present invention provides a diesel particulate filter for filtering exhaust gases of a diesel engine which collect particulates and burn the same by resistance heating, said filter comprising a filter case in which a plurality of cylindrical filter bodies are disposed in a coaxial relationship, the exhaust flow being from the innermost to the outermost cylindrical body, the coarseness of the filtering passages of said plurality of cylindrical filter bodies being increasingly dense from the innermost to the outermost cylindrical body, such that the innermost cylindrical filter body is coarse and the outermost cylindrical filter body is fine, characterised in that said filter further comprises a bypass pipe arranged in the innermost filter body and a bypass valve disposed in one end thereof for selectively opening the bypass pipe to receive the exhaust. Exhaust released from the engine is introduced into the filter at a relatively high speed, and the flow velocity of exhaust is converted into pressure in accordance with Bernoulli's theorem due to the fluid resistance of the filter to increase the pressure of the exhaust. Since the exhaust flows at the inlet portion of the filter, the decrease in flow velocity and the increase in exhaust pressure at the inlet portion is small. The exhaust flows forward deeply within the filter. Since the flow velocity at the upstream side of an exhaust flow is high and a pressure difference before and behind the filter is small, the particulates move forward within the filter, and only the particulates having a large particle size are preferentially collected by the filter. In this manner, the exhaust passes through the filter and moves downstream. It is to be noted that granular particulates are electrified and caught when they flow in a zigzag manner between fibers interiorly of the filter and tend to be gradually accumulated. However, if a filtering passage between the fibers is wide, many of the particulates flow out without contacting the fibers, and if the flow velocity of exhaust is high, the particulates are less likely to accumulate. Since the exhaust flows between the fibers having a small diameter at the downstream side of the exhaust flow within the filter, the exhaust is dispersed and the flow velocity of the exhaust decreases so that it often contacts the fibers. Since the coarseness or mesh of the filter in the downstream portion is fine, the ability to collect and accumulate particulates increases, and most of the particulates having a small particle size can be collected. In the particulate filter according to the present invention, a guide can be provided in order to realize the aforementioned action under even conditions over the whole area of the filter. The flow rate of exhaust is made even over the whole surface of the filter by the guide, and in addition, the flow velocity of the exhaust is gradually lowered to elevate the pressure evenly as the exhaust flows downstream. Thereby, the particulates in the exhaust can be effectively collected, and even with respect to the pulsation of exhaust in the exhaust pipe, it to possible to reduce a fluctuation in pressure in the filter. When a diameter of the conical guide provided at the inlet of the filter in order to guide the exhaust into the filter is modified, a flow of exhaust becomes smoother. In the particulate filter according to the present invention, a plurality of filters, such as a coarse mesh filter, a medium mesh filter, and a dense or fine mesh filter may be serially arranged in the direction of exhaust flow. Accordingly, it becomes possible to collect particulates of all particle diameters without clogging the filter for a short period of time. Further, since the filter having a coarse mesh is located at the most upstream side, it is possible not to increase the pressure of exhaust even if the particulates having a large particle size are collected. Further, since the coarse mesh filter is formed in a cylinder of a small diameter and the fine or dense mesh filter is formed in a cylinder of a large diameter, the dense mesh filter where a flow resistance per area of exhaust is high and a flow velocity of exhaust is low, will be of large area. Therefore, there is no place where exhaust pressure is locally high. Thus, a flow of exhaust, without rapid rise in pressure due to the collection, is smooth, rendering effective filtering possible. Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a sectional view showing the construction of a particulate filter according to a first embodiment of the present invention. Fig. 2 is a sectional view showing the construction of a particulate filter according to a second embodiment of the present invention. Fig. 1 is a sectional view showing a particulate filter according to a first embodiment of the present invention. As shown in Fig. 1, in a particulate filter 100 of the present embodiment, a closed-end cylindrical bypass pipe 11 having substantially the same diameter as that of an exhaust pipe 1 is installed in coaxial relationship within a central portion of the cylindrical steel filter case 10. The bypass pipe 11 is formed of porous silicon carbide (SiC) ceramic or porous metal, the bypass pipe itself having a filter function. The bypass pipe 11 has an electric conductivity, and an electric power is supplied through an electrode 12 to burn the collected particulates. A fiber filter 13 (a filter body) formed of ceramic fibers in arranged at the outer periphery of the bypass pipe 11. The fiber filter 13 comprises inner and outer tubes which are coaxially disposed and connected to each other at the downstream end, the right hand end in Figure 1. The fiber filter 13 constitutes a coarse filter at the inlet side of the filter (the left hand portion in Figure 1), and constitutes a dense filter at the portion adjacent the closed end. The left hand portion of the fiber filter 13 is formed of bold or thick ceramics fibers (diameter 10 to 20µm) having a coarse surface, and the right hand portion thereof is formed of fine or thin ceramics fibers (diameter 10 to 15µm) having a dense surface. The surface of the ceramic fibers is made coarse by employing a reheating process after forming the ceramic fibers by way of sintering. Alternatively, the fiber filter 13 may be formed such that the coarsness or mesh of the filter gradually reduces from the inlet portion to the closed end portion. A metal wire-net heater 14 for burning the collected particulates is provided on the upstream side surface or inner surface of the fiber filter 13 which receives the exhaust, the metal wire-net heater 14 being provided at the end thereof with an electrode 15. The metal wire-net heater 14 is formed from a resistance wire, for example, such as a nickel-chrome alloy, and a ceramic material is coated on the surface of the metal wire-net heater 14. The fiber filter 13 and the metal wire-net heater 14 are held in place by a porous ceramic tube 16. The metal wire-net heater 14 is internally provided with a metal guide tube 14a. The metal guide tube 14a is disposed so as to cover the outer surface of the metal wire-net heater 14. The metal guide tube 14a is provided with a suitable number of holes through which exhaust can pass. It is constructed so that when the exhaust flows through the fiber filter 13, the flow velocity is converted into pressure, and when the exhaust flows into the fiber filter 13, the metal guide tube 14a causes the exhaust not to generate large pulsations. Between the bypass pipe 11 and the fiber filter 13 is a rectifying pipe 17 for preventing exhaust discharged from one side from being flown to the other side and exhaust heat from being radiated. A cylindrical exhaust guide pipe 18 is arranged outside the porous ceramic tube 16, and between the exhaust guide pipe 18 and the filter case 10 is arranged a heat insulating member 19 for providing thermally separation therebetween. A trumpet-like guide pipe 20 is mounted at the inlet of the bypass pipe 11, and the guide pipe 20 is interiorly provided with a bypass valve 21 for controlling a flow of exhaust to the bypass pipe 11. The bypass valve 21 is opened and closed by an actuator 22 controlled by a controller 23. A convergent conical guide plate 24 for directing exhaust is mounted at the inlet of the fiber filter 13. In normal operation, the bypass valve 21 is closed, and accordingly, the exhaust does not pass through the bypass pipe 11. At this time, the exhaust (from the left hand side) is guided by the conical guide 24 and introduced into the fiber filter 13. The flow velocity of the exhaust is gradually lowered while passing through the flowpassage within the double cylindrical fiber filter 13, and is released outside of the fiber filter 13 while slowly filtering the particulates. In the above-described filtration process, particulates having a large particle size are collected mainly by the coarse filter portion at the inlet portion, and particulates having a small particle size are collected mainly by the dense filter portion at the closed end portion. In the case where clogging of the filter 13 occurs due to the operation of the engine for a long period of time, the bypass valve 21 is opened by the actuator 22 controlled by the controller 23 to introduce the exhaust into the bypass pipe 11, and the metal wire-net heater 14 on the filter 13 is energized to burn the particulates. At this time, the filtration of exhaust is performed by the porous cylinder of the bypass pipe 11. When a predetermined time after energizing the metal wire-net heater 14 has elapsed, the energization of the metal wire-net heater 14 is stopped and the bypass valve 21 is closed, and subsequently, electric power is supplied to the bypass pipe 11 through the electrode 12 of the bypass pipe 11 to reproduce the bypass pipe 11. Here, since a fine clearance is present in the bypass valve 21, air necessary for burning the particulates is introduced into the bypass pipe 11. While the reproducing operation of the filter 13 can be automatically performed by the controller 23, it is to be noted that it can be manually performed by an operator. While in the above embodiment, the coarseness or mesh of the fiber filter 13 has been divided into two grades, it is to be understood that the mesh of the fiber filter 13 can be divided into three grades of meshes or more, and further the grade of mesh can be continuously changed. Further, the filter 13 may comprise porous ceramics instead of ceramics fibers. Moreover, while in the aforementioned embodiment, the filter 13 is supported by a porous ceramic tube, it is to be noted that the filter 13 may be supported by a porous metal tube. Fig. 2 is a sectional view of a particulate filter according to a second embodiment of the present invention. As shown in Fig. 2, in a particulate filter 100 according to the present embodiment, a cylindrical bypass pipe 11 is arranged in a central portion interiorly of a filter case 10 made of steel, and closed-end cylindrical filters 30 to 32 are arranged outside the bypass pipe 11. More specifically, three kinds of filters, i.e. a coarse filter 30, a medium filter 31 and a dense filter 32, are concentrically installed in that order from the centre of the filter outwards. This arrangment state is called a flowpassage series arrangement. Each of the filters 30 to 32 are formed of woven fabric or non-woven fabric formed of porous ceramics fibers of silicon carbide (SiC). The coarse filter 30, the medium filter 31 and the dense filter 32, which comprise non-woven fabrics of coarse ceramics fibers (diameter 20 to 30µm), medium ceramics fibers (diameter 10 to 20µm) and fine ceramics fibers (diameter 5 to 10µm) respectively, are laminated and partly entangled. Each of the filters 30 to 32 is interposed between a wire-net heater 33 and a mesh-like cylindrical metal support body 33a. Annular electrodes 34 are formed on the opposite ends of each of the metal wire-net heaters 33. In place of the construction in which the filters 30 to 32 and the wire-net heater 33 are supported by the support body 33a, the filters 30 to 32 may be held within a cylindrical container made of porous steel or ceramic having a high rigidity. An exhaust guide pipe 18 is installed externally of the outermost dense filter 32, and an insulating sheet 35 is arranged between the exhaust guide pipe 18 and the filter case 10. In the vicinity of the inlet of the bypass pipe 11 is provided a bypass valve 21 for controlling a flow of exhaust. The bypass valve 21 is opened and closed through an arm 36 by an actuator 22. The operation of the actuator 22 is controlled by a controller 23. The controller 23 receives signals from the exhaust pressure sensor 40, the engine r.p.m. sensor 41, the idling sensor 42, and the engine load sensor 43 and supplies electric power to the metal wire-net heater 33 through electrodes 34 when the particulates are to be burned. A trumpet-like inlet guide 37 is arranged externally of an inlet portion of the bypass pipe 11. In normal operation, the bypass valve 21 is closed, and accordingly, the exhaust does not flow into the bypass pipe 11. At this time, exhaust passing from the left hand side in Figure 2 is guided by the guide 37 and introduced into the flowpassage between the bypass pipe 11 and the coarse filter 30. The exhaust flows through the coarse filter 30, the medium filter 31 and the dense filter 32 in that order as indicated by the arrows, and the exhaust is eliminated gradually of fine particulates and eventually flows out at the outlet on the right hand side in Figure 2. In the case where operation is continued for a long period of time, clogging will occur in the filters 30 to 32. In the case where the clogged state is detected by an exhaust pressure sensor 40, and the idling state in which an exhaust flow rate of the engine is small is detected by the idling sensor 42, the bypass valve 21 in opened by the controller 23, and electric power is supplied to the metal wire-net heater 33 to execute the combustion of particulates. After a predetermined time has elapsed, the supply of electric power is stopped and the bypass valve 21 is closed. It is to be noted that the burning operation for the particulates is automatically performed by the controller 23 in such a way that exhaust pressure is monitored by the controller 23 and a judgement is made so that when a predetermined pressure is reached clogging is deemed to have occurred. Alternatively, alarm means for warning of clogging can be provided, in for example a driver's cab, so that the combustion may be manually executed. While in the foregoing, a preferable embodiment has been described, it is to be noted that the present invention is not limited to the aforementioned embodiment, but various changes can be made without departing from the scope of the present invention. For example, while in the above embodiment the filters have been arranged in a three-layer configuration, it is to be noted that the filters 30 to 32 may be arranged in a configuration comprising two, four or more layers. Further, in place of the construction in which the filters 30 to 32 are formed of woven fabrics of ceramics fibers, they may be formed of woven fabrics of metal fibers or formed of a composite or laminate of woven fabrics of ceramics fibers and woven fabrics of metal fibers. The diesel particulate filter according to the present invention comprises a filter body having a coarse meshed portion at the upstream side and a dense or fine meshed portion at the downstream side of the exhaust flow. Therefore, a flow of exhaust is not greatly decelerated at the inlet surface of the filter, and the flow of exhaust is gradually decelerated over the full length of the filter. In other words, a change in pressure of exhaust is gradual, and the flow of exhaust is smooth. Therefore, the provision of a particulate filter according to the present invention permits an engine from being adversely affected and enables the effective filtering of particulates. In the diesel particulate filter according to the present invention, the coarse-mesh filter at the upstream side of the exhaust flow and the fine-mesh filter at the downstream side of the exhaust flow are arranged in series. Therefore, it is possible to collect effectively all of the particulates over the range of particle sizes from large to small while avoiding the occurrence of clogging in a short period of time. Further, in the case where the coarse-mesh filter is formed as a cylinder of a small diameter and the fine-mesh filter is formed as a cylinder of a large diameter, an area of the finemesh filter having a high fluid resistance per area is so large or wide that there is no portion where exhaust pressure becomes locally high so as to to enable filtering with a smooth flow of exhaust.

Claims: A diesel particulate filter for filtering exhaust gases of a diesel engine which collects particulates and burns the same by resistance heating, said filter comprising a filter case (10) in which a plurality of cylindrical filter bodies (30,31,32) are disposed in a coaxial relationship, the exhaust flow being from the innermost (30) to the outermost (32) cylindrical body, the coarseness of the filtering passages of said plurality of cylindrical filter bodies (30,31,32) being increasingly dense from the innermost (30) to the outermost (32) cylindrical body, such that the innermost cylindrical filter body is coarse and the outermost cylindrical filter body is fine, characterised in that said filter further comprises a bypass pipe (11) arranged in the innermost filter body and a bypass valve (21) disposed in one end thereof for selectively opening the bypass pipe (11) to receive the exhaust. The diesel particulate filter of claim 1, wherein the coarseness of the filtering passages is gradually increasingly dense. A diesel particulate filter according to claim 1, wherein said at least one filter body comprises a laminated fabric sheet, and the coarseness of the filtering passages varies in a stepwise or continuous manner. A diesel particulate filter according to any one of claims 1 to 3, wherein the cylindrical filter bodies comprise a woven fabric or non-woven fabric of ceramics fibers, the innermost filter body being constituted by thick ceramics fibers and the outermost filter body being constituted by thin ceramics fibers. A diesel particulate filter according to any one of claims 1 to 4, wherein the surfaces of the cylindrical filter bodies are covered with a metallic wire net-like heater (33). A diesel particulate filter according to any one of claims 1 to 5, wherein the other end of the bypass pipe (11) is closed, and said bypass pipe (11) includes s sub-filter having a metallic wire net-like heater which can be energized when said sub-filter becomes clogged with particulates.

Inventors: KAWAMURA HIDEO, KAWAMURA, HIDEO
Applicants: ISUZU CERAMICS RES INST, ISUZU CERAMICS RESEARCH INSTITUTE CO., LTD.
Application year: 1995
Main classification: F01N3
IPC codes: F01N3, F02B3