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The present invention relates to a filter for particulates from diesel engines, namely, a filter for separating fine particles consisting mainly of carbon in exhaust gases exited from diesel engines.
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Heretofore, a filter for particulates from diesel engines has been known as described in Japanese Patent Application Laid-open No. 56-129,020. This patent covers a filter made of a honeycomb structural body characterized by thin thickness of partition walls and large specific surface area per unit volume of the honeycomb structural body. By making the wall thickness of the filter quite thin as compared with conventional ceramic filters while making the effective surface area of the filter structurally large, a compact structure of the filter can be realized without incurring an increase of pressure drop, even when a fine pore filter material is used for removing fine particulates. The filter is a honeycomb structural body made of a porous ceramic material and having a number of throughholes which are alternately sealed at the inlet and outlet ends of the honeycomb structural body.
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As a diesel particulates filter, ceramic filters have been used because of their large thermal shock resistance, durability, and easiness for designing compact structures, etc. However, even such ceramic filters can not avoid accumulation of particulates thereon, which incurs high pressure drop and consequently deterioration of the performance of diesel engines.
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As a counter means for obviating such problems, a method has been practiced wherein the entire filter is heated to an elevated temperature to burn out and remove the deposited particulates. As a practical example of regenerating ceramic filters by means of burning out deposited diesel particulates, a method has been known wherein an electric heater is provided on the exhaust gas inlet side of the filter to ignite a fire and propagate the fire on the deposited particulates. The method has been widely used because of its simplicity, low cost, and high reliability.
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Requisite temperature for the method is at least 600°C, so that it requires a high electric capacity of about 2 KW. However, consumption of a battery which supplies the electric power is quite large in practice, so that the capacity of the electric heater has to be suppressed to the minimum necessary for the burning of the particulates. In addition, the filter for diesel particulates has a large heat capacity and a low heat conductivity, so that the above necessary temperature is not always ensured.
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As described above, if the electric heater has a low capacity, the temperature of a filter for diesel particulates can not be raised sufficiently, so that combustion of the accumulated particulates becomes insufficient. As a result, unburned residue of the accumulated particulates remains, so that the regeneration efficiency of the filter becomes inferior and hence the pressure drop of the filter becomes high.
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By repetition of insufficient regeneration cycle of the filter a large amount of unburned fine particulates is accumulated on the filter, which at last is burnt at a time of the regeneration of the filter emanating a large combustion heat, so that a rapid temperature increase of the filter occurs resulting in melting of the filter per se or generating of cracks on the filter by thermal shock.
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An object of the present invention is to obviate the above problems and drawbacks.
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Another object of the present invention is to provide a filter for diesel particulates which can practically and sufficiently burn accumulated particulates each time even by means of an electric heater of a small capacity.
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The present invention is a filter for separating particulates in exhaust gases emitted from diesel engines. This filter comprises a honeycomb structural body having a number of throughholes which are defined by the partition walls that separate particles. These throughholes are alternately sealed at the inlet and outlet ends of the honeycomb structural body by sealing membrs. At least the sealing members at the inlet end must be partially or wholly made of a material having an emissivity of a minimum of 0.6 at the outer surface of the inlet end of the honeycomb structural body.
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Because the filter is constructed in such a fashion that the inlet end surface of the honeycomb structural body is sealed by sealing members of the desired emissivity, the filter can efficiently absorb heat emanated from a heat source for combustion, so that the deposited particulates are satisfactorily burned.
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For a better understanding of the present invention, reference is made to the accompanying drawings, in which:
- Fig. 1 is a schematic cross-sectional view of a first embodiment of the present invention;
- Fig. 2 is schematic cross-sectional view of a second embodiment of the present invention;
- Fig. 3 is a graph of temperature increase of filters; and
- Fig. 4 is a graph illustrating regeneration efficiency of filters.
Numberings in the drawings.
- 1 ... partition wall
- 2 ... throughhole
- 3 ... sealing member at gas inlet side
- 4 ... sealing member at gas outlet side
- 5 ... electric heater
- 6 ... thin plate-shaped part at inlet side.
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Hereinafter, the present invention will be explained in more detail with reference to examples.
Example 1
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Referring to Fig. 1, a cross-section of a first embodiment of the present filter is shown, wherein the filter has partition walls 1 and throughholes 2, 2 defined by the partition walls 1 and formed in parallel to each other, thus forming a honeycomb structural body. The throughholes 2 are alternately sealed at the inlet and outlet ends by sealing members 3 and 4, so that respective throughholes 2, 2 are sealed at only one end to form a checkerboard pattern or an alternate-rows pattern of the sealing members 3 and 4 on end surface.
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The sealing members 3 and 4 plug the throughholes 2, 2 and the end surfaces of the sealing members 3 and 4 are located in the same plane as those of partition walls 1. A sealing member located at the exhaust gas inlet side of the filter is sealing member 3, and a sealing member located at the exhaust gas outlet side is sealing member 4.
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Because of this structure, when an exhaust gas is introduced to the filter, the gas stream goes in the throughhole 2 flowing around sealing members 3. Because the other end of the throughhole 2 is closed by the sealing member 4, the gas flow permeates through the porous partition walls 1 to go in the neighboring throughhole 2 and exits from the outlet side of the filter. Therefore, the porous partition walls defining the throughholes 2, 2 play a role of filtering elements which can separate particulates suspended in the exhaust gas.
Example 2
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Referring to Fig. 2, a second embodiment of the present filter is shown, wherein thin plate-shaped portions or parts 6 are located on the gas inlet side sealing member 3 at the inlet side of the filter. The thin plate-shaped parts 6 are positioned on the plane of the inlet side sealing members 3 as well as the ends of the partition walls 1.
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In the sealing members shown in the above examples, the inlet side sealing members 3 in Example 1 and the thin plate-shaped parts 6 in Example 2 are made of a material of high emissivity, for improving the heat absorption of the filter. More concretely explaining, they are made of a material of an emissivity of at least 0.6. The inlet side sealing members 3 in Example 2 need not be a material of an emissivity of at least 0.6. Though the thin plate-shaped parts 6 and the inlet side sealing members 3 are shown as separate elements from each other, they may be formed as an integral element with each other, as the case may be.
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When particulates are accumulated on the filter of the above structure, radiation heat is supplied to the filter from electric heater 5 as a heat source located at the upstream of inlet side of the filter. Then, the radiation heat is efficiently absorbed through the inlet side sealing members 3 or the thin plate-shaped part 6 of the filter to elevate the temperature of the filter, so that the accumulated particulates are ensuredly, ignited and the ignited fire propagates to a wide extent and a broad range of the accumulated particulates can be burn out. Also, unburned residue of the accumulated particulates can be reduced, so that melting or breakage of the filter per se due to accumulation of unburned residues can be prevented when the regeneration work is repeated.
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Moreover, the filter can efficiently absorb the heat generated by the electric heater 5 and hence temperature elevation of the whole filter can be improved, so that the heat capacity of the electric heater 5 can be made small and consumption of a battery supplying an electric power to the heater 5 can be mitigated.
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The inlet side sealing member 3, the thin plate-shaped part 6, and the honeycomb structural body consist of cordierite material.
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Regeneration tests were effected on filters made of cordierite honeycomb structural body having a diameter of 144 mm, a length of 152 mm, a partition wall thickness of 0.4 mm, and a cell density of 15 cells/cm².
The results are shown in the following Table 1.
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Both the inlet side sealing member 3 and the outlet side sealing member 4 had a length of 10-15 mm in the axial direction of the throughholes 2.
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The outlet side sealing member 4 was formed of a well-known cordierite. Meanwhile, the inlet side sealing member 3 was made of compositions wherein desired amounts of Fe₂O₃, CoCO₃, MnO₂, etc. was mixed with 100 parts of cordierite to prepare Sample Nos. 1-3 with the design of Example 1. The thin plate-shaped parts 6 of Sample No. 4 prepared with the design of Example 2 were 1 mm thick and made of the same composition as the sealing members 3 of Sample No. 2.
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Emissivity of respective Sample Nos. 1-4 was measured by an infrared spectrometer.
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Thus prepared filters were heated by a 2 KW electric heater while supplying 25 ℓ/min of air to measure temperatures at the rear side of the sealing member 3 by means of a K-thermocouple of a diameter of 1.0 mm. The results are graphically shown in Fig. 3.
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As seen from Table 1 and Fig. 3, it is indicated that the temperature of the whole filter becomes higher in accordance with increase of the emissivity of the sealing member 3 and the thin plate-shaped part 6 and that an emissivity of at least 0.6 is necessary for obtaining a temperature of at least 600°C required for igniting accumulated particulates and propagating the fire.
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Then, the filters were mounted on a 2.8 ℓ diesel engine and operated at a condition of 2,300 rpm and a 3/4 load of the engine up to a pressure difference of the filter of 3,000 mmH₂O accumulating about 20 g of particulates. Thereafter, the filters were heated by a 2 KW electric heater, while supplying 25 ℓ/min of air. The maximum temperatures in the filters were measured during the regenerations and regeneration efficiencies were calculated as combustion rate of accumulated particulates from filter weights before and after the regeneration. Also, the filter damages were inspected after regeneration with an aid of a steromicroscope of a magnification of 20X.
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Relations between the above regeneration efficiencies and the emissivities are shown in Fig. 4. As seen from Fig. 4, the filters of prior art and referential example have very low regeneration efficiencies of around 50% because of low emissivities of the sealing members 3 as compared with the filters of the present invention. Regeneration efficiencies of the filters of the present invention become high in accordance with increase of their emissivities in a range of at least 60%.
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Damage of the filters was not seen on the whole filters at a first regeneration test, as shown in Table 1. However, after 3 cycles of accumulating diesel particulates on the filters up to a filter pressure difference of 3,000 mmH₂O and regenerating the filters, the filters of prior art and referential example of low regeneration efficiencies were found broken because of increase of the maximum temperatures in the filters during regenerations caused by increase of the amount of the accumulated particulates although the filter pressure difference before the regeneration was 3000 mmH₂O same as the first cycle. Meanwhile, the filters of the present invention did not show increase of the maximum temperatures in the filters so that no breakage was found, even after repeating the regeneration of the filters.
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As explained in detail in the foregoings, the filter of the present invention has sealing members of a high emissivity of at least 0.6 at the inlet side thereof, so that a broad range of accumulated particulates can be efficiently burned when regenerating the filter by supplying a radiation heat thereto from a heat source. Therefore, it can be prevented that the filter per se is damaged because of unburned residues of accumulated particulates during repeating the regeneration of the filter. Moreover, the regeneration can be performed with efficient use of heat, so that the capacity of the heat source can be made small or suppressed as well as the consumption of the battery.
