Regeneration system for an exhaust gas cleaning device

Details Regeneration system for an exhaust gas cleaning device Regeneration system for an exhaust gas cleaning device

2002-07-25

2003-05-20

Smith, Duane S. (Department: 1724)

Gas separation: processes

Filtering

With cleaning of filter

C055S282300, C055S523000, C055SDIG001, C055SDIG003, C095S283000

Reexamination Certificate

active

06565630

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a regeneration system for an exhaust gas cleaning device, and more particularly to a regeneration system comprising an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine and provided with an exhaust gas cleaning honeycomb filter for catching particulates included in the exhaust gas and a heating means for the exhaust gas cleaning honeycomb filter when a fuel containing a fuel additive for mitigating particulates included in the exhaust gas is used as a fuel for the internal combustion engine.
2. Description of Related Art
In the internal combustion engine such as diesel engine or the like, particulates (e.g. soot or unburned portion of fuel) are included in the exhaust gas. Particularly, the discharge amount of particulates becomes large in a diesel engine using a gas oil as a fuel or a direct-injection type gasoline engine recently coming into wide use. Therefore, it is well-known to remove the particulates by an exhaust gas cleaning device disposed in an exhaust emission path of the internal combustion engine and provided with an exhaust gas cleaning honeycomb filter.
As the exhaust gas cleaning honeycomb filter is generally used a cordierite filter
32
of a honeycomb structure as shown by a diagrammatically section view in FIG.
1
. In such a conventional cordierite filter
32
are included a plurality of exhaust gas flowing channels
33
extending in parallel to a longitudinal direction thereof, wherein these channels
33
are alternately plugged at either upstream side or downstream side for the exhaust gas of their ends with plugging members
33
a
to form a checker pattern.
As shown in
FIG. 1
, an exhaust gas Gin emitted from a diesel engine (not shown) flows into the cordierite filter
32
through the exhaust emission path
11
, at where particulates included in the exhaust gas are filtered off on surfaces of cell walls constituting the exhaust gas flowing channels
33
. Then, the cleaned exhaust gas Gout passed through cordierite filter
32
again passes through the exhaust emission path
11
and is discharged out to the outside of the vehicle.
It is known that pressure loss &Dgr;P is produced when the exhaust gas Gin passes through the filter
32
. The pressure loss &Dgr;P is represented by the following equation (1).
&Dgr;P=&Dgr;P
1
+&Dgr;P
2
+&Dgr;P
3
+&Dgr;P
4
  (1)
wherein
&Dgr;P
1
is a resistance produced due to the narrowing of an opening portion in the exhaust gas flowing channel
33
when the exhaust gas flows into the channel
33
through the exhaust emission path
11
;
&Dgr;P
2
is a resistance produced in the flowing of the exhaust gas through the exhaust gas flowing channel
33
;
&Dgr;P
3
is a resistance produced in the passing through a wall of the exhaust gas flowing channel
33
;
&Dgr;P
4
is a resistance produced when the exhaust gas passes through particulates deposited on the surface of the exhaust gas flowing channel
33
.
In this case, the resistances &Dgr;P
1
, &Dgr;P
2
, &Dgr;P
3
are dependent upon a cell structure constituting the filter
32
, respectively and are a constant value &Dgr;P
1
not depended upon the lapse of time coming into problem in the deposition of the particulates and the like (hereinafter “&Dgr;P
1
+&Dgr;P
2
+&Dgr;P
3
” is called as “initial pressure loss”). For this end, a greater part of the pressure loss &Dgr;P is determined by the resistance &Dgr;P
4
produced when the exhaust gas passes through the particulates deposited on the cell walls. The resistance &Dgr;P
4
is usually 2-3 times the initial pressure loss &Dgr;Pi at a deposited state of the particulates.
In
FIG. 2
is shown a relation among cell structure, typical dimensions, geometrical surface area and opening ratio in the filter. The cell structure Cs (mil/cpi) is represented by thickness of cell wall dc (mil=milli inch) to cell number Nc per square inch (cpi=cells per square inch), and the geometrical surface area fs (cm
2
/cm
3
) is an area passing the exhaust gas per unit volume (filtering area). Moreover, the cell wall thickness dc is shown by unit of mm in FIG.
2
.
As seen from
FIG. 2
, the pressure loss &Dgr;P produced in the checkered honeycomb filter for cleaning the exhaust gas is small as the cell number Nc and geometrical surface area fs in the filter become large. And also, the opening ratio &agr; (%) is a ratio of total opening area of the exhaust gas flowing channels occupied in the sectional area of the filter. As shown in
FIG. 2
, a limit not creating cracks (crack limit) is large as the opening ratio &agr; becomes small.
On the other hand, a mechanical strength of the filter, i.e. bending strength S* of the filter is approximately equal to product of strength S of a filter made of porous material and relative density &rgr;* as mentioned below. When the nature of the porous material constituting the filter is represented by density &rgr; and strength S, the bending strength S* of the filter and the relative density &rgr;* are as follows:
&rgr;*=&agr;×&rgr;  (2)
S*≅&rgr;*×S  (3)
That is, the strength is high as the opening ratio a becomes small.
Further, the regeneration of the filter is carried out by burning the particulates according to the following reaction equation (4):
C+O
2
→CO
2
+Q(heat quantity)  (4),
so that the strength of the filter against the thermal stress becomes important. Particularly, when the filter is made from a ceramic material, brittle rupture is caused by thermal stress to create cracks. Such a cracking phenomenon is apt to be created as heat quantity produced in the regeneration or the amount of the particulates deposited to be burnt becomes large. Moreover, the unburned portion of fuel constituting the particulate is an organic compound, so that it is burned by heating the filter. As mentioned below, a crack limit preventing the occurrence of the cracking phenomenon is proportional to the opening ratio &agr; and is closely related to the thickness dc of the cell wall as seen from FIG.
2
. If the opening ratio &agr; is same, as the thickness dc of the cell wall becomes thick, the crack limit is high.
Therefore, the exhaust gas cleaning filter having good properties is preferably made of a material having a large crack limit, an excellent strength against thermal stress and a small pressure loss.
Recently, fuel previously including a fuel additive, or a device dropwise adding a fuel additive to a fuel is developed for controlling the amount of the particulate produced in the exhaust gas and the use thereof is increasing. Such a fuel additive has an effect of preventing the formation of soot in the burning of the fuel.
However, the formation of the particulate can not completely be controlled even by using such a fuel additive and hence the particulate is formed in the exhaust gas. Therefore, it is indispensable to use the exhaust gas cleaning filter.
In the conventional technique, the cordierite filter is generally adopted as a checkered honeycomb filter for cleaning the exhaust gas as previously mentioned. However, there is a problem that the amount of the particulate to be treated in one regeneration of the cordierite filter has a limit because the maximum service temperature in the filter is low. In this case, a large pressure loss is caused in the filter due to the deposition of the particulate, so that the combustion efficiency of the internal combustion engine lowers to degrade the fuel consumption.
And also, there is proposed a technique for regenerating the exhaust gas cleaning device by burning the particulate caught on the cordierite filter through a heating means for the filter. However, when such a greater amount of the particulate caught on the filter is burnt out by the heating means at once, a large change of the pressure loss is caused in the burning of the particulate in accordance with the heat conduction efficiency of the heating means, which gi

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