Ultrasonic burner system for regenerating a filter

Abstract: An ultrasonic burner system for regenerating a filter (55) comprises an ultrasonic atomizer (23) for atomizing fuel oil into fine droplets and a combustion chamber (52) that is adapted to pass an exhaust gas created therein by the combustion of the fine droplets toward the filter (55) to regenerate the filter (55) by burning previously unburnt matter trapped thereby, wherein the fine droplets are burned in such a manner that the exhaust gas is at a higher temperature in the wall region than in the axially central part of the combustion chamber (52). Consequently, the combustion chamber (52) can be made compact, and since the temperature of the combustion gas following in the combustion chamber (52) is made higher in the wall region than in the axially central part of the combustion chamber, the temperature distribution in the filter (55) can be made uniform.

Description: The present invention relates to an ultrasonic burner system for regenerating a filter and removing particulates such as carbonaceous materials carried by the exhaust gases of a diesel engine, and more particularly to an ultrasonic burner system for burning particulates trapped by a filter. In recent years, regulations on emission of exhaust gases from vehicles driven by internal combustion engines, particularly diesel engines, are becoming more and more stringent. With respect to particulates carried by the exhaust gases, there is a trend in some states of the United States toward quantitatively restricting their level to approximately 1/3 of the present level. However, the conventional means of modifying the combustion system for an engine cannot adequately cope with such stringent control. Most efforts to remove particulates from the diesel engine exhaust gases have been directed to trapping them by a filter, usually made of a ceramic material, which is periodically regenerated by burning the particulates, the heat required for combustion being supplied either from an electric heater or combustion of a diesel fuel. Some filters may be coated with a catalyst film to lower the combustion temperature; in such a case the filter may be regenerated by the exhaust gases, without using a special heating system. In the aforementioned catalytic filter system, there are problems in that the production costs of the filter are high, and that the catalyst tends to be deactivated by sulfur compounds contained in the exhaust gases. In addition, the exhaust gases alone as the heat source may not always function effectively depending on the engine conditions. The system using an electric heater has a drawback that the power consumption by the electric heater is high, so that this system is unsuitable for automobiles that use batteries as a power source. Accordingly, to overcome such problems, the system that burns part of diesel fuel to generate heat necessary for the regeneration has been recently attracting attention in the industry. Fig. 5 illustrates a system disclosed by the Japanese Patent Laid-Open No. 43114/1985, which incorporates a system of burning particulates in accordance with the aforementioned system (3). This system is arranged as follows: A housing 7 incorporating a filter 6 made of a ceramic is connected midway in the exhaust pipe 3 of an engine 2 so as to remove particulate matter carried by the exhaust gases. When the amount of the particulate matter trapped by the filter 6 increases, the fluid resistance of the filter 6 also increases. As a result, pressure differential across the filter rises, which is detected by an exhaust gas pressure sensor 10, and a signal obtained by a pressure/electricity converter 11 is inputted to a controller 20. Subsequently, a switch 16 is closed on the basis of a judgment made by the controller 20 to connect a fuel pump 9 and an ignition device 8 to a power source battery 17. Then, fuel is injected from a fuel tank 12 through a burner 4 provided upstream of the filter 6 by means of a fuel pump 9, and, at the same time, a spark is generated at an ignition plug 5 by the ignition device 8 so as to ignite the burner 4. Thus, the high-temperature combustion gas is supplied to the filter 6 to burn the particulate matter trapped by the filter 6, thereby regenerating the filter 6. In the above-described conventional system, however, since the high-pressure fuel is injected from the burner 4, the flame reaches the filter 6 without being sufficiently mixed with the air flow, and, as shown by the line B-B in Fig. 6, as for the radial temperature distribution of the filter trap 6, the temperature of the central portion is abnormally higher than that of the outer peripheral portion. This may produce a sufficient thermal shock that can damage the filter. To overcome this problem, if the length of a combustion line 19 is set to 10 to 15 times greater than the diameter of the filter, the flame can be mixed well with the carrier gas, and the radial temperature distribution of the filter becomes uniform. However, if the diameter of the filter is set to 10 cm, the length of the combustion line 19 becomes 1 m to 1.5 m, so that there is the problem that such a particulate combustion system cannot be mounted on a vehicle having a limited space. In the case of the invention disclosed in, for instance, Japanese Utility Model Publication No. 29135/1988 in order to solve such problems, an attempt is made to allow the primary air introduced in a tangential direction and the secondary air introduced rearwardly thereof to swirl in mutually opposite directions, but this arrangement disadvantageously requires a complicated system. Accordingly, an object of the present invention is to provide a system for regenerating a filter which has a more compact combustion chamber, a uniform radial temperature distribution of the filter and a reduced length of combustion line, thereby overcoming the above-described drawbacks of the conventional art. FR-A-2527309 describes a burner system for providing a hot gaseous mixture wherein said system comprises an ultrasonic atomizer capable of atomizing fuel oil into fine droplets which is mounted at one end of the central axis of a combustion chamber, an air/fuel mixture guiding port being formed in the periphery of said ultrasonic atomizer whereby the hot exhaust gas created by the combustion of said fine droplets of fuel is passed to a discharge port. This burner is for use in small air heaters. We now provide a modified burner system of this type, for the aforesaid purpose, the system being characterised in that at least one swirling nozzle is formed in the periphery of said combustion chamber to pass air into said combustion chamber tangentially and the outlet portion of said chamber comprises a baffle and an annular combustion gas discharge port formed between an inner cylinder of the chamber wherein are provided a multiplicity of outlet holes and an outer cylinder; whereby, during use, the hot exhaust gas is at a higher temperature in the region adjacent the wall than in the axially central part of said combustion chamber and thereafter is led toward a filter provided adjacent to said burner system in the exhaust line of a diesel engine so that the filter is regenerated by burning carbonaceous material trapped in the filter. By means of the present invention, it is possible to shorten the flames in the combustion chamber and thereby to make the overall filter system compact, while preventing large, unburned fuel droplets from trickling down from the burner and, at the same time, soot from adhering to, and accumulating on, the combustion chamber walls. It is also possible to reduce a radial temperature differential in the filter, and thereby to mitigate thermal stresses which would lead eventually to damages of the filter. It is further possible to provide a wider operable load range. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view illustrating an embodiment of an ultrasonic burner system for regenerating a filter in accordance with the present invention; Fig. 2 is a side elevation of Fig. 1 as seen from right-hand side thereof; Fig. 3 is a diagram illustrating the temperature distribution of a filter in a case where the present invention is applied; Fig. 4 is a diagram illustrating a conventional filter burning system; and Fig. 5 is a diagram illustrating the temperature distribution of a conventional filter. In Fig. 1, an ultrasonic burner system for regenerating a filter in accordance with the present invention comprises a combustion chamber 21, an air supply duct 22, and an ultrasonic atomizer 23. The combustion chamber 21 comprises an outer barrel 25 lined with a refractory material 24 and an inner barrel 26 having a multiplicity of holes through which the combustion gases flow out of the combustion chamber. An air/fuel mixture guiding port 27 is provided in one side of the combustion chamber 21. In addition, as shown in detail in Fig. 2, four air swirling nozzles 29 are provided in the outer peripheral portion of the combustion chamber to guide combustion air into the combustion chamber in a tangential direction, and an ignition plug 30 is installed in face-to-face relationship with the air/fuel mixture guiding port 27. Meanwhile, a baffle 31 is disposed on the other side of the combustion chamber 21, and an annular combustion gas discharge port 32 is formed between the inner barrel 26 and the refractory material 24. The high-temperature combustion gas discharged therefrom is introduced into a filter (not shown) disposed downstream of this burner system. An air passage 33 is formed between the air supply duct 22 and the combustion chamber 21, and as a blower 36 is connected to the air passage 33 via an air pipe 35, the atmospheric air supplied from the blower 36 is introduced into the air/fuel mixture guiding port 27 and the air swirling nozzles 29. In addition, an ultrasonic atomizer 23 which is inserted and disposed in the mixture guiding port 27 is installed in the air supply duct 22. This ultrasonic atomizer 23 is arranged such that a piezoelectric transducer horn 39 is vibrated by an ultrasonic oscillator 37 so as to atomize the fuel sent from the fuel pump 40 into fine droplets. The fuel supplied to the piezoelectric transducer horn 39 is atomized by the ultrasonic vibrations after flowing in a thin film over the horn surface. Subsequently, pressure of the exhaust gases is detected by an exhaust pressure sensor 41, and an operation signal is transmitted to the ultrasonic oscillator 37, the fuel pump 40, and the ignition plug 30 on the basis of a judgment made by a controller 42. As for the aforementioned piezoelectric transducer horn 39, it is possible to use any of those that are disclosed in, for instance, the specification of Japanese Patents Laid-Open Nos. JPA60-222552, JPA61-138558 JPA 61-138559, JPA61-259780, JPA61-259781 and JPA62-140667 or JPU63-6074, JPA62-289260, JPA63-38193, JPA63-38810, JPA63-194765, JPU63-168065 and JPA63-259153. A description will now be given of the operation of the ultrasonic burner system of the present invention having the above-described arrangement. Part of the combustion air introduced from the blower 36 into the air passage 33 inside the air supply duct 22 via the air pipe 35 is sent to the air swirling nozzles 29, the remainder being sent to the air/fuel mixture guiding port 27. Subsequently, the air flowing into the air/fuel mixture guiding port 27 flows into the combustion chamber 21 while it is being mixed with the atomized fuel produced by the piezoelectric transducer horn 39 of the ultrasonic atomizer 23 to form an air-fuel mixture having an appropriate mixing ratio. On the other hand, the air flowing tangentially from the air swirling nozzles 29 flows into the combustion chamber 21 in the form of a strong swirling current, and the air-fuel mixture flowing into the combustion chamber through the mixture guiding port 27 is carried in the direction of the ignition plug 30 by means of the kinetic energy of this swirling current so as to burn the fuel. As a result of a combustion experiment conducted by the present inventors by using a conventional pressure injection-type burner system, it was found that a strong swirling current which is produced at the time when the combustion air introduced into the air passage 33 passes through the air swirling nozzles 29 moves inside the combustion chamber 21 toward the discharge port 32, and a flame having a length of several hundred millimeters or thereabouts is formed in the axially central portion of the combustion chamber by this swirling current. On the other hand, in the case of the above-described ultrasonic burner system for regenerating a filter in accordance with the present invention, since the initial velocity of atomized fuel is slow, atomized fuel can be easily carried by the strong swirling current from the air swirling nozzles 29, and the fuel and the air are mixed well, so that the long flame having a length of several hundred millimeters or thereabouts, which is created in the conventional case, is not generated, and a short flame is formed. Moreover, since the combustion gas which flows in a swirling manner in the space of the combustion chamber defined by the inner barrel 26 and the baffle 31 is dispersed through the multiplicity of holes and flows out of the inner barrel 26, the long flame as in the conventional case is not produced. In addition, since the refractory material 24 is heated to a high temperature by the combustion gas dispersed and flowing out of the inner barrel 26, complete combustion in the combustion chamber 21 can be ensured, and the stabilization of combustion and high-load combustion can be realized. As a result, in a case where the ultrasonic burner system for regenerating a filter in accordance with the present invention is mounted in the exhaust line 3 of an internal combustion engine as shown in Fig. 4, the radial temperature distribution in the filter 5, i.e., the temperature difference between the central portion and the outer peripheral portion thereof, is very small, as shown by the line A-A in Fig. 3, with the result that the problem of breakage of the filter 6 due to the thermal stresses created therein can be greatly mitigated. In this case, even if the length of the combustion passage 19 is made very short, the radial temperature differential in the filter 6 becomes very small, so that the overall system can be made compact.

Claims: An ultrasonic burner system for regenerating a filter (55) wherein said system comprises an ultrasonic atomizer (23) capable of atomizing fuel oil into fine droplets which is mounted at one end of the central axis of a combustion chamber (21), an air/fuel mixture guiding port (27) being formed in the periphery of said ultrasonic atomizer (23) whereby the hot exhaust gas created by the combustion of said fine droplets of fuel is passed to a discharge port (32); characterised in that at least one air swirling nozzle (29) is formed in the periphery of said combustion chamber (21) to pass air into said combustion chamber (21) tangentially and the outlet portion of said chamber comprises a baffle (31) and an annular combustion gas discharge port (32) formed between an inner cylinder (26) of the chamber wherein are provided a multiplicity of outlet holes and an outer cylinder (25); whereby, during use, the hot exhaust gas is at a higher temperature in the region adjacent the wall than in the axially central part of said combustion chamber (21) and thereafter is led toward the filter (55) provided adjacent to said burner system in the exhaust line (51) of a diesel engine so that the filter is regenerated by burning carbonaceous material trapped in the filter. A burner system as claimed in Claim 1, wherein the outer cylinder (25) is lined with refractory material (24).

Inventors: HIRABAYASHI HIDEO C O TOA NENR, HIRABAYASHI, HIDEO C/O TOA NENRYO KOGYO K.K.
Applicants: TOA NENRYO KOGYO KK, TOA NENRYO KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: F01N3
IPC codes: F01N3