ELECTRICAL IGNITION SYSTEM FOR REGENERATION OF A PARTICULATE TRAP

Abstract: An ignition system is disclosed for regenerating a particulate trap (B) used to filter the exhaust gas of an internal combustion engine, the particulate trap (B) having an entrance face (25) for receiving a gaseous flow therethrough. The system comprises flow guide means (A) effective to direct a gaseous flow to said entrance face (25) during filtration and during regeneration of the trap (B), an open cell ceramic foam body (C) extending across such flow guide means (A) and having a radially outer ring exit surface (29) in contact with the radially outer portion (25a) of the entrance face (25), and an entrance surface (30) remote from the particulate trap (B), the open cell foam body (C) being effective to siphon off an ignitable collection of particulates from the exhaust gas during filtration, and electrically energized resistance heating means (42) stationed in a radially central portion of the open cell body (C) adjacent to the entrance surface (30) effective to heat the body (C) during regeneration to a temperature effective to ignite the ignitable collection.

Description: The invention relates to an electrical ignition system for regenerating a particulate trap for the exhaust gases of an internal combustion engine and, more particularly, to an ignition device which more economically ignites the particulates to initiate regeneration. Electrical heating elements have been devised to ignite particulate collections in porous traps or filters for internal combustion engines, particularly for diesel engines. Particulates, in case of diesel exhaust emissions, is a term used herein to describe carbonaceous solids and condensable matter as defined by the U.S. Environmental Protection Agency. To date such heating elements have been either (a) embedded at or near the front face of the particulate trap (filters having a high trapping efficiency in the range of 50-90) to ignite the dense particulate collection for removal by oxidation (see U.S. patents 4,516,993 and 4,523,935), or (b) have been embedded in a support element (an element providing little or no trapping efficiency) up stream from the particulate trap to heat the gaseous flow to an adequate temperature which, in turn, ignites the front part of the particulate collection in the trap (see U.S. patents 4,544,388 and 4,427,418). JP-A-57110948 discloses an ignition system for regenerating a particulate trap for the exhaust gases of an internal combustion engine. The system comprises a flow guide means for directing a gaseous flow to the entrance face of the trap, a prefilter ceramic member extending across said guide means and electrical heater disposed at the inner side of the prefilter member. In either case the ignition temperature required is relatively high demanding that the power wattage be at a level of at least 1500 watts or more to raise state-of-the-art resistance elements to above that temperature. This results from two factors. First, the electrical resistance means only indirectly heats the particulates because the gaseous flow passing therethrough is heated directly and, in turn, heats the particulates. Particulates must be heated to a level of at least a 1000°F in order to ignite unless subjected to a catalyst which lowers the ignition temperature to the range of 800°F. Secondly, the electrical heating means heats the entire cross-sectional area of the entrance of the particulate trap which is a very extensive area requiring greater heat content. Thirdly, the particulate collection, as contained in a wall-flow particulate filter, exposes only the edges of the particulate columns to the frontal flow which reduces the effectiveness of heated gaseous flow to achieve ignition. [A wall-flow particulate trap has columns which present cells to the front face of the flow, the cells being relatively few per square inch across such face; alternate columns or cells are closed forcing the flow to penetrate laterally or sideways through the wall before being permitted to exit in an alternate cell or channel.] What is needed is an electrical ignition device which requires considerably less energy to ignite the particulate collection. Such device should provide for heating directly a small siphoned quantity of the particulates independent from the primary dence collection of particulates; such siphoned quantity is non-layered so that it can be easily heated by conduction from electrical wires adjacent to the particulates. Such device needs to be exposed to only a small portion of the area of the flow, such as 20% or less, to be effective. According to the invention there is provided an an electrical ignition system for regenerating a particulate trap (B) for the exhaust gas of an internal combustion engine, the particulate trap (B) having an entrance face (25) for receiving a gaseous flow therethrough, the system comprising a flow guide means (A) effective to direct a gaseous flow to said entrance face (25) during filtration by said trap (B) and during regeneration of said trap (B), an open cell ceramic foam body (C) extending across said flow guide means (A) having an entrance surface (30) remote from said particulate trap (B), said open cell foam body (C) being effective to siphon off an ignitable collection of particulates from the exhaust gas during filtration, and electrically energized resistance heating means (42) stationed in a radially central portion of the open cell body (C) adjacent said entrance surface (30) effective to heat said body (C) during regeneration to a temperature effective to ignite said ignitable particulate collection, characterised in that said foam body (C) has a radially outer ring surface (29) in contact with the radially outer portion (25a) of said entrance face (25) and a pocket (35) located radially inwardly of said ring surface to provide a separation between said body (C) and the trap (B), thereby forcing heat transfer to be through said ring surface (29). The electrical ignition system embodying the invention has an advantage that it regenerates at lower cost by heating only a central core of an omni-flow filter which has a relatively open pore characteristic to siphon only a light amount of soot during the normal filtration period of the exhaust gases. Further the electrical ignition system is effective to heat directly, by conduction, a siphoned collection of particulates useful for ignition purposes; such siphoned collection of particulates, when ignited, is used to directly heat the primary dense collection of particulates by conduction radiation and connection. Preferably the open cell ceramic foam body has a porosity which provides 4-12 cells per linial centimeter (10-30 cells per lineal inch) while the particulate trap is comprised of a wall-flow ceramic having (16-160 cells per lineal centimeter (40-400 cells per lineal inch). The foam body if preferably shaped in a frustro conical configuration having a neck to for said entrance throat at one end and an opposite end having a base perimeter defining the outer periphery of the ring surface; the neck of the foam body preferably has cast-in-place electrical resistance heating wires; the cross-sectional area of the neck is no greater than 20% of the cross-sectional area of the trap entrance face. Advantageously the ratio of the diameter of the neck of such ceramic foam body to the exit base thereof is in the range of 1/3 to 3/4. Advantageously the ceramic foam body is effective to collect 0.3-1.0 grams of soot for purposes of providing an ignitable collection. Preferably the ratio for the soot collected in the wallflow particulate trap during a given filtration period is in the ratio of 1/30 to 1/100. Preferably the resistance heating means is energized to provide 800-1100 watts of heating, the resistance heating means being supplied with an electrical current of about 20 amps at a voltage of about 45. Advantageously the ceramic foam body contains a washcoat thereon comprising a catalyst (palladium plus tungsten) for reducing the ignition temperature of said siphon particulate collection to about 204-427°C (400-800°F). The gaseous flow carried through said flow guide means is preferably exhaust gas during filtration period and air during regeneration period. The flow rate of said exhaust gas during filtration is in the range of 100-1500 cfm and the air flow during regeneration is preferably in the range of 1.5-15 cfm. Preferably the particulate trap is comprised of a wall-flow type design whereby longitudinally extending cells of said trap are alternately closed at the face thereof, the wall thickness of said trap of each of the said cells is about.025cm (.01 inch) and each of said cells have a square cross-section with a side of about.23cm (.09 inches). The invention will now be described further by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of an automotive filter trap and regeneration system embodying the principles of this invention; Figure 2 is an enlarged central sectional view of a leading portion of the filter trap and the heating means employed to ignite the particulate collection in the filter trap; Figure 3 is a sectional view taken substantially along line III-III of Figure 2. The regeneration system utilizes an electric heating assembly that heats a ceramic foam body to ignite a low density particulate collection carried thereon when in the presence of air passing thereover; the combustion of such thin collection of particulates raises the temperature of the ceramic foam body to transfer heat through radiation and through a ring contact with the front face of the particulate trap and raises the temperature of the air flow therethrough to transfer heat by connection. The leading portion of the collection of particulates in the trap is ignited by such heat transfer. The only source of energy is that supplied to the electrical resistance heating wires cast-in-place in a radially central portion of the ceramic foam body, the wires therefore being limited in size and area for heating with less wattage is required for such resistance heating. As shown in Figure 1, the apparatus for the trap and regeneration system broadly comprises a flow guide assembly A, a particulate trap B, a ceramic foam body C, an ignition assembly and a control E. Flow Guide Assembly The flow guide assembly A is comprised of a canister 10 effective to contain and support the trap B in manner so that the entire flow passes through such trap. The canister has a leading transition or entrance section 11 and an exit transition section 12, the transition sections respectively being connected at station 14 to a tubular entrance passage 15 and at station 13 to an exit tube 9. The flow guide assembly is of the bypass type, that is, the exhaust flow from the engine, conveyed by exhaust pipe 16, is allowed to enter the entrance section 11 byway of a passage 17 during normal filtering operation; during regeneration, the exhaust flow is bypassed through a channel 18 by closing passage 17 by use of diverter valve 20 (the valve is moved from its first position, closing channel 18 and allowing flow through passage 17, to a position opening channel 18 and closing passage 17). The exhaust is bypassed to converge with the exit passage 9 at station 19. The diverter valve assembly may be of the flapper type actuated by vacuum motor 40 to move the flapper valve from a normally biased position, to an actuated position. The vacuum motor is electrically actuated under a control E. Filter Trap The filter trap B has a monolithic ceramic honeycomb cell structure 26 supported and contained in the metallic canister 10, the front portion of the canister guiding the flow of exhaust gases from channel 17 through the front face 25 of the ceramic honeycomb cell structure. The honeycomb cell structure may be similar to that used for carrying a catalyst material for conversion of gases from a gasoline engine. The monolithic structure contains parallel aligned channels constituting the honeycomb cell structure. The ends of the channels are alternately blocked with high temperature ceramic cement at the front and the rear so that all of the inlet flow of gas must pass through the porous longitudinally extending side walls of the channels before exiting through a rear open channel of the filter trap. This type of monolithic ceramic structure provides very high filtration surface area per unit of volume. For example, 1950 cubic cm filter trap of this type with 16 cells per square cm and a .043 cm wall thickness will provide approximately 1970 square inches of surface area; the filtering surface area per unit volume for such a filter trap would be about 16.6 square inches per cubic inch. The channels are all preferably aligned with the direction of the flow of 17 through the trap. When particulates collect on the trap they will nest within the porosity of the walls spaced along the direction of flow. Thus, there can be a generally uniform distribution of particulates as they are collected along the length of the trap. Preferably the monolithic structure has either an oval or a rectangular cross-section with a large frontal face of 103-213 sq.cms (16-33 square inches). The axes of the frontal face preferably have a dimension of 10-12.5 cm in one direction and 18-20 cm in the other. The typical side wall thickness is about.025 cm and the typical cell diameter for each of the channels extending longitudinally thereof is about.23 cm. Ceramic Form Body The open cell ceramic foam body C is formed as a truncated cone with an exit ring surface 29 at the base of the cone and an entrance throat at the top of the cone. The cone top defines the throat as a neck 32 presenting an entrance surface 30; the exit or trailing portion of the truncated cone provides ring surface 29 which is in intimate contact with the outer portion 25a of the entrance face of the filter trap B. The ring surface 29 is defined by the outer periphery of the cone base shape and by a pocket 35 at the central portion of the trailing surface. The ratio of the entrance surface 30 to the exit surface 29 is in the range of 1/3-3/4. The open cell ceramic foam body C is positioned tightly against portion 25a of the front face of the filter by way of support straps 36 which extend between the entrance portion at 14 of the transition section 11 and the periphery of the neck 32 of the open cell body B. Exhaust flow will enter the transition section 11 and most flow will preferentially pass through the entrance neck 32 of the open cell body while the remainder of the flow will pass around the throat and enter the tapered section 42 of the cone shape. The pocket tends to setup an insulating space which encourages the flow to exit by passing through the ring surface 29 of the ceramic open cell body. The open cell body is preferentially coated with a washcoat of palladium and tungsten or fine gamma aluminum to provide a catalytic coating substance to reduce the ignition temperature of contained particulates to the range of 204-427°C (400-800°F) from that which would normally be in the range of 538-649°C (1000-1200°F). The open cell ceramic foam body is of the omni-cell/type; that is, the cells are not aligned in any particular direction thus promoting porosity that is random like that in a sponge. Typically the average cell diameter of such open cell body is about.23-.33 cm and such porosity promotes collection of.6-.10 grams of soot during a typical filtration cycle. This is in stark contrast to the amount of particulates that would be collected by the particulate trap or filter during the same period and subjected to the same exhaust gas; the later collects in the range of about 28-35 grams of soot. The open cell foam body is effective to siphon off an ignitable collection of particulates from the exhaust gas during filtration. The pocket 35 located radially inwardly of the ring surface 29 provides a separation between the body and trap thereby forcing heat transfer to be through the ring surface. The open cell body has its cells defined to be in the range of 10-30 cells per lineal inch whereas the cells of the particulate trap are in the range of 40-400 CPI. Ignition Assembly The ignition assembly ignites the siphoned collection of particulates in the open cell body by use of a much smaller energy supply. To this end, electrical resistance wires 42 are cast-in-place or embedded within a radially centralized portion of the open cell body adjacent to the entrance surface 30. The electrical resistance wires 42 when energized are effective to heat the body C during regeneration to a temperature to ignite the siphoned collection. The wires are here designed for a power supply of 20 amps and 45 volts from an alternator of the automobile, and deliver 800-1100 watts of heating. During energization of the electrical heating wires 42, the exhaust flow is bypassed around the filter trap B and open cell body C by operation of valve 20. A pump 43 is actuated to provide a flow of oxygen carrying gas, such as air, at a low flow rate of 1.5 to 10 cfm through the body C. This flow rate contrasts sharply with the normal flow rate of exhaust gas which fluxuates in the range of 100-1500 cfm. Control The control E is a device described in detail in U.S. patent 4,538,411 and is comprised of two pressure sensor/transducers 50 and 51, Sensor/transducer 51 is located to sense the back pressure immediately upstream of the front of the filter trap, which pressure correlates with the degree of particulate collection in the filter or contamination thereof. The other sensor/transducer 50 is placed in the ceramic open pore body C. When the particulate loading (and trap back pressure) reaches a preset trigger condition, the regeneration system is turned on when the air pump, valve 20, and wires 42 are energized.

Claims: 1. An electrical ignition system for regenerating a particulate trap (B) for the exhaust gas of an internal combustion engine, the particulate trap (B) having an entrance face (25) for receiving a gaseous flow therethrough, the system comprising a flow guide means (A) effective to direct a gaseous flow to said entrance face (25) during filtration by said trap (B) and during regeneration of said trap (B), an open cell ceramic foam body (C) extending across said flow guide means (A) having an entrance surface (30) remote from said particulate trap (B), said open cell foam body (C) being effective to siphon off an ignitable collection of particulates from the exhaust gas during filtration, and electrically energized resistance heating means (42) stationed in a radially central portion of the open cell body (C) adjacent said entrance surface (30) effective to heat said body (C) during regeneration to a temperature effective to ignite said ignitable particulate collection, characterised in that said foam body (C) has a radially outer ring surface (29) in contact with the radially outer portion (25a) of said entrance face (25) and a pocket (35) located radially inwardly of said ring surface to provide a separation between said body (C) and the trap (B), thereby forcing heat transfer to be through said ring surface (29). 2. A system as claimed in Claim 1, in which said open cell ceramic foam body has cells numbering 4-12 cells per lineal cm (10-30 cells per lineal inch). 3. A system as claimed in Claim 1 or 2, in which said open cell ceramic foam body is configured as a frustrum of a conical shape, said shape having a neck as an entrance and the base perimeter of said cone defining the outer periphery of said ring surface. 4. A system as claimed in Claim 3, in which said surface area of said neck is no greater than 20% of the surface area of the trap entrance face. 5. A system as claimed in any one of the preceding claims, in which the ratio of the diameter of said entrance of said open cell foam body to the diameter of the exit surface of said foam body is in the range of 1/3-3/4. 6. A system as claimed in any one of the preceding claims, in which said open cell ceramic foam body has a washcoat thereon containing catalyst effective to reduce the ignition temperature of said siphoned particulate collection to 204-427°C (400-800°F). 7. A system as claimed in Claim 6, in which said catalyst is comprised of palladium and tungsten. 8. A system as claimed in any one of the preceding claims, in which said gaseous flow during regeneration is comprised of air and is at a flow rate of 1.5 to 10 cfm. 9. A system as claimed in any one of the preceding claims, in which said particulate trap is comprised of a monolithic ceramic honeycomb structure contained in a canister. 10. A system as claimed in any one of the preceding claims, in which the ratio of soot collected by said open cell foam body, in comparison to the amount of soot collected by said filter trap, is about 1-30.

Inventors: RAO, V. DURGA NAGESWAR
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1987
Main classification: F01N3
IPC codes: F02B1, F01N3