JPH06307229A - Exhaust gas purifying trap - Google Patents

Abstract

PURPOSE:To completely eliminate harmful components in exhaust gas to be discharged from a diesel engine at high collecting efficiency without raising pressure loss in short periods by a trap capable of being regenerated at low output. CONSTITUTION:A plurality of filter elements 11 composed of heaters 13 incorporated together with cylindrical filters 12 are provided in a trap container 10. The filters 12 are composed of cylindrical forming bodies formed by compressing a three-dimensional steel-like structural porous body made of heat resistant metal, holes as seeing the porous body in section parallel to the gas flow are formed as space parts of approximately oval spherical bodies, and the filters 13 are provided on the inside diameter side from the center of the filter wall thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying trap for collecting and removing particulates (particles) such as carbon in exhaust gas of a diesel engine.

[0002]

2. Description of the Related Art Exhaust gas from an automobile is one of the major causes of air pollution, and a technique for removing harmful components contained in the exhaust gas is extremely important. Particularly in diesel engine vehicles, the removal of particulates (particulates) mainly composed of NOx and carbon is an important issue.

In order to remove these harmful components, EG
Improvements have been made on the engine side, such as turning R, improving the fuel injection system, and improving the shape of the combustion chamber, but there is no fundamental deciding factor, and the exhaust gas is disclosed in JP-A-58-51235. It has been proposed to remove particulates by a filter installed in the passage, and this post-treatment method is considered to be the most practical at present.

By the way, as a trap for collecting particulates contained in the exhaust gas of a diesel engine, it is necessary to satisfy the following performances from the conditions of use.

First, regarding the collection performance, it is necessary that the collection efficiency of particulates is sufficient to satisfy the cleanliness of the passing exhaust gas. The particulate emission amount changes depending on the exhaust amount and load of the diesel engine, but generally it is necessary to be able to collect an average of 60% or more of the emission amount from the diesel engine.

Secondly, it is necessary that the pressure loss with respect to the exhaust gas is small. As the particulates are collected, the pressure loss when the engine exhaust passes through the trap increases, and back pressure is exerted on the engine, which causes an adverse effect. For this reason, it is said that it is usually necessary to suppress the pressure loss after collection to 30 KPa or less. Therefore, if a certain amount of particulates are trapped in the trap,
It is necessary to periodically remove and regenerate the collected matter to recover the initial pressure loss state. If the rate of increase in pressure loss with respect to the amount of collected particulates is large, the frequency of this removal and regeneration operation becomes large, which is not practical. Therefore, it is necessary for the trap not only to have a small initial pressure loss, but also to be hard to increase the pressure loss even when the exhaust gas is passed and the particulates are collected.

Thirdly, it is necessary that the above-described repetitive removal and regeneration process can be completed easily with low energy. As a particulate removal and regeneration method, a method using a light oil burner is also being considered, but considering safety and ease of control and installation in a car, the method using an electric heater is the most preferable. It is said to be promising. However, in the case of electric heater regeneration,
It is necessary to be able to reproduce with the lowest possible power so as not to put an excessive burden on the battery and the oil generator installed in the car.

Conventionally, as a filter element satisfying the above requirements, a wall-flow type honeycomb porous body of cordierite ceramics has been considered to be most practical. In this method, particulates tend to accumulate locally, and because cordierite ceramics have a low thermal conductivity, heat spots are easily formed during regeneration, which may cause the filter to melt or crack due to thermal stress. , Reliability could not be secured. Therefore, a metal filter element, which is highly reliable and does not cause cracks or the like during reproduction, is currently receiving attention.

[0009]

However, the metal filter element described above cannot have a larger exhaust gas inflow surface area than the wall flow filter made of cordierite ceramics because of its structure. Therefore, if you design the filter to obtain high collection efficiency,
Particulates are trapped only on the filter surface and clogged, resulting in a rapid increase in pressure loss and a short filter regeneration interval.

The metal filter element is preferably one having a high particulate collection efficiency and a small increase in pressure loss due to the collection. From this point of view, the three-dimensional network structure porous body made of a heat-resistant metal having continuous ventilation holes has excellent properties. As shown in FIG. 4, the three-dimensional network structure porous body is a porous body having a spherical space portion in which pocket-shaped communicating pores 1 are surrounded by a skeleton 2.
Since this three-dimensional network structure porous body has a high porosity, the gas flow resistance is extremely small, and since the skeleton forms a three-dimensional network, the particulate collection performance is excellent.

Further, in the wall flow filter of cordierite ceramics, it is possible to efficiently regenerate with a small amount of electric power by combustion propagation utilizing self-heating at the time of combustion of particulates, whereas a metal filter element is used. Does not collect as much particulate as the cordierite ceramic wall flow filter, and the combustion heat of the particulate is small,
Regeneration must be performed only with the heat generated by the electric heater. As a result, there is a problem that the amount of regenerated electric power becomes too large.

Further, in the filter having the electric heater regenerating device, when the electric current is passed through the heater to heat the filter during regeneration, the particulates trapped in the filter and the holes are heated accordingly. When the particulates reach the ignition point, they burn and start regeneration. Due to the requirement of regeneration with low power, regeneration generally needs to be completed in 5 to 10 minutes. The temperature of each part of the filter heated by the heater gradually rises, and finally reaches a constant temperature in balance with the heat radiation from each part of the filter. However, the fact that the regeneration must be completed in 5 to 10 minutes as described above means that the regeneration is completed before the temperature of each part of the filter reaches a steady state. That is, the heat generated from the heater incorporated during the temperature rising process is quickly transmitted to the filter portion, and at the same time, the heat transfer from the heater to the regeneration atmosphere gas and the amount of heat radiation due to heat radiation can be suppressed to achieve short-term regeneration. It becomes a point.

In the present invention, in consideration of various problems of the above-mentioned conventional exhaust gas purifying trap, high trapping efficiency is maintained while maintaining high performance such that pressure loss does not sharply rise in a short time due to trapping. An object of the present invention is to provide an exhaust gas purifying trap for a diesel engine, which is composed of a metal filter element incorporating a heater and which can be regenerated efficiently with a low output in a short time.

[0014]

According to the present invention, as a means for solving the above-mentioned problems, a filter element in which a heater element is integrally incorporated in a filter is mounted in a trap container installed in an exhaust system, and the filter element is A cylindrical filter made of a heat-resistant metal three-dimensional mesh structure porous body, each pore of which is seen in a cross section parallel to the exhaust gas flow is formed of a substantially elliptical sphere having a major axis and a minor axis. Is a trap for purifying exhaust gas, which is built in from the center of the wall thickness of the filter cross section to the inner diameter side.

In this case, it is preferable that the ratio of the long diameter to the short diameter of the holes is a value between 4: 1 and 1.5: 1.

The major axis of the holes may be 0.8 mm or less.

[0017]

In the exhaust gas purifying trap according to the first aspect of the present invention having the above-mentioned structure, the three-dimensional net-structure porous body forming the filter element has the respective major and minor diameters when viewed from a cross section parallel to the exhaust gas flow. Since it is a substantially ellipsoidal sphere, it has the anisotropy that the thermal conductivity is low in the direction of exhaust gas flow and high in the direction perpendicular to it. Since the heater is arranged on the exhaust gas inflow side of the filter element cross section, the amount of heat generated by energizing the heater element during regeneration is transferred to the surface near the exhaust gas inflow surface of the filter element faster. Become. A large amount of particulates are collected near the exhaust gas inflow surface, so that the particulates can be efficiently burned and regenerated.

Such a three-dimensional network structure porous body in which the pores viewed from the cross section parallel to the exhaust gas flow have a substantially elliptical shape having a major axis and a minor axis, and the three-dimensional network structure porous body is formed into a filter shape. At this time, it can be formed by compressing in one direction. Alternatively, a sheet-shaped three-dimensional network structure porous body that has been compression-molded in one direction beforehand may be molded into a filter shape. Further, the three-dimensional network structure porous body is manufactured by a method such as electroplating a resin porous body having a three-dimensional network structure, in which case the pores are formed into a substantially ellipsoidal sphere having a major axis and a minor axis. You may use the resin porous body which exists.

As defined in the second invention, the ratio of the major axis to the minor axis of the holes is 1.
It is preferably 5: 1 or more. However, if the ratio of the major axis to the minor axis of the pores exceeds 4: 1, the skeletons of the three-dimensional network structure porous body block the flow path of the exhaust gas, and the exhaust gas flow resistance becomes excessive, which is not preferable.

In the third invention, the major axis of the holes is 0.8 m.
It is preferably m or less. This is because if the length of the pores exceeds 0.8 mm, the required particulate collection efficiency cannot be secured.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of a trap for purifying exhaust gas of a diesel engine of the present invention. The filter element 11 is composed of a cylindrical filter 12 and a heater 13 integrally incorporated therein. Seven filter elements 11 were mounted in the trap container 10. In the case of this embodiment, the exhaust gas is supplied from the inner surface of the cylindrical filter 12 and is discharged from the outer surface after the particulates are removed.

FIG. 2A shows the filter element 11
3 is a perspective view including a partial cross section showing the structure of FIG. Filter 1
2 is a NiCrAl three-dimensional network structure porous body (Sumitomo Electric Industries, trade name “Celmet”, average pore diameter 400 μm) compression-molded to 1/3 in the plate thickness direction, outer diameter 48 mm, inner diameter 3
It consists of a cylindrical molded body having a length of 2 mm and a length of 190 mm.

As the heater 13, four sheath heaters having an outer diameter of 6 mm and a NiV heater of 12 V and 50 W were incorporated at a position where the center thereof was 2 mm from the inner diameter side, four at a time. The heater 13 is located on the inner diameter side with respect to the thickness center of the filter 12.

FIG. 2B is a partially enlarged perspective view of the structure of the three-dimensional network structure porous body of the filter element 11. In this porous body, as shown by an alternate long and short dash line in the figure, the pores 1 are formed by forming a substantially elliptic spherical space surrounded by a skeleton 2.

The substantially elliptic spherical hole 1 has a cross section taken along the line AA in the figure, that is, a cross section seen from an arbitrary cross section parallel to the exhaust flow, is a substantially elliptical shape having a major axis in the flow direction. It has a shape. Such holes 1 are formed by compressing in one direction when molding the filter element. In the filter element of this example, the ratio of the long diameter to the short diameter of the holes 1 was 3: 1.

The trap having the above construction was mounted on the evaluation apparatus shown in FIG. 3, and trapped for 2 hours under constant load constant speed operation of 3/4 load and 1600 rpm. Then, with the switching valve flowing most of the exhaust gas to the bypass, the built-in heater was energized for regeneration.

The outline of the illustrated evaluation apparatus is as follows.
A car 20 having a 3400 cc, 4-cylinder direct-injection diesel engine is installed in the chassis dynamometer 21,
A trap 22 was attached to the exhaust pipe. A bypass circuit 23 and a switching valve 24 are provided in the trap portion. Exhaust gas passing through the trap is a dilution tunnel 25.
Be led to. A filter type particulate concentration measuring device 26 is installed in the dilution tunnel to measure the particulate concentration in the exhaust gas.

The following is the result of measuring the purifying action of the trap of Example 1 by the above evaluation apparatus.
The initial pressure loss of this filter is 95 mmAq, the collection efficiency is 72% when collecting for 2 hours, and the pressure loss of the filter part is 320%.
It was 0 mmAq. Pressure loss is 95 mm 7 minutes after the start of regeneration
It recovered to Aq and the initial value. The amount of exhaust gas passing through the filter portion at the start of regeneration was 2 l / min, and at the end of regeneration was 35 l / min.

As a comparative example, Ni having an average pore diameter of 250 μm is used.
The three-dimensional network structure porous body made of CrAl was formed into a cylindrical shape without compression molding. The size of the filter, the incorporated heater, and the incorporated position of the heater were the same as in the example. A trap incorporating this filter element was evaluated under the same conditions as in the above-mentioned examples.

The initial pressure loss of this filter was 97 mmAq, the collection efficiency was 74% when collecting for 2 hours, and the pressure loss of the filter part was 3150 mmAq. Pressure loss recovers only to 1050 mmAq 7 minutes after the start of regeneration,
At 0 minutes, the pressure loss finally recovered to 98 mmAq, which is almost the initial value.

[0031]

As described in detail above, the exhaust gas purifying trap of the diesel engine of the present invention has a high heating efficiency by the electric heater and requires a small amount of electric power for combustion regeneration, and therefore exhaust gas purifying treatment is strict. It can be effectively used for diesel vehicles that are required and have a limited battery capacity.

[Brief description of drawings]

FIG. 1 is a sectional view of a trap according to an embodiment.

FIG. 2A is a perspective view including a partial cross section of a filter element according to an embodiment, and FIG. 2B is a partially enlarged view of the filter element.

FIG. 3 is a conceptual diagram of a collection performance evaluation device.

FIG. 4 is an enlarged view of a three-dimensional network structure porous body.

[Explanation of symbols]

 1 Pores of a three-dimensional network porous body 2 Skeleton of a three-dimensional network porous body 10 Trap vessel 11 Filter element 12 Filter 13 Heater 20 Car 21 Dynamometer 22 Trap 23 Bypass circuit 24 Switching valve 25 Dilution tunnel 26 Particulate Concentration measuring instrument

Claims (3)

[Claims] 1. A filter element in which a heater element is integrally incorporated into a filter is mounted in a trap container installed in an exhaust system, and the filter element is a cylindrical filter made of a three-dimensional mesh structure porous body of heat-resistant metal. , Each of the pores of the porous body as viewed in a cross section parallel to the exhaust gas flow is formed of a substantially elliptical sphere having a long diameter and a short diameter, and the heater element is incorporated on the inner diameter side of the wall thickness center of the filter cross section. Exhaust gas purification trap. 2. The exhaust gas purifying trap according to claim 1, wherein the ratio of the major axis to the minor axis of the holes is a value between 4: 1 and 1.5: 1. 3. The exhaust gas purifying trap according to claim 1, wherein the long diameter of the holes is 0.8 mm or less.