Silencer

Details Silencer Silencer

1999-02-16

2001-11-06

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S171000, C422S177000, C422S179000, C422S181000, C181S264000, C181S265000

Reexamination Certificate

active

06312650

ABSTRACT:

The present invention relates to an apparatus for silencing and purification of exhaust gases, e.g. exhaust gases from internal combustion engines.
In modern vehicles, both silencers and purification devices, e.g. catalytic converters, are in many instances installed in engine exhaust systems with the aim of simultaneously reducing exhaust noise and noxious exhaust gas element emissions to acceptable levels. Under-vehicle space available for such equipment is often limited. In addition, exhaust system back-pressure should not exceed certain limits, to prevent excessive detraction from fuel economy and engine performance. Thus, the combined requirements for effective noise suppression and purification represent geometric difficulties to the exhaust system designer.
Sometimes the space available for silencers and purification devices may also be limited in the case of stationary engines, e.g. gas engines for combined heat and power generation.
Various devices have been introduced to accommodate catalytic converters in silencer casings, instead of using separate units. In most cases such combinations have presupposed a simple series connection of silencing and catalytic elements. Such an arrangement can be designed without excessive space demands, when catalysers occupy only a small fraction of the volume needed for silencers. So far, when legal limits on noxious emissions have compromised with limits to investment costs and to the technology available, catalytic bodies in many cases have been rather small, typically occupying no more than 10-20% of the silencer volume.
However, ever more stringent demands on noxious emissions tend to call for bigger purification devices in addition to engine developments towards lower cylinder emissions, thus aggravating the space requirement conflict. A particularly difficult case is emerging in relation to diesel engine emissions. In the case of gasoline engine, 3-way catalysers are state of the art and provide an effective simultaneous reduction of hydrocarbons, carbon oxide, and nitric oxides. Diesel engines, in contrast, because of high contents of oxygen and particles in the exhaust gases may require a combination of an oxidizing catalytic body, a reducing catalytic body, and a particle trap for effective reduction of all noxious components.
Another problem is that although purification devices like catalytic converters may provide some reduction of high-frequent noise, primarily due to increased flow resistance in the narrow flow channels through the monolith, converters in themselves do not in general contribute significantly to the suppression of low-frequent noise. This is unfortunate, since low-frequent noise suppression in silencers calls for big acoustic chamber volumes to become effective.
U.S. Pat. No. 5,426,269 discloses a silencer with a built-in catalytic converter.
The present invention provides a combined silencer-purification apparatus. The apparatus may be designed in a number of embodiments derived from a common principle, all very efficient, both as silencers and as purification devices. The invention allows for rather voluminous catalysers and particulate traps to be fitted in to silencer casings of a limited size, along with efficient low- and high-frequent noise suppression.
Accordingly, the invention relates to an apparatus for silencing and purification of exhaust gases comprising: an air-tight casing connected to an exhaust inlet pipe and to an exhaust outlet pipe and containing: at least two acoustic compartments, and one or more monolithic bodies through which exhaust gases, in operation of the apparatus, flow in a flow direction in longitudinal channels or porosities, and one or more pipes or channels, at least one pipe or channel penetrating one or more of the monolithic bodies and guiding exhaust gases in a flow direction which is opposite to the flow direction in the channels or porosities of the monolithic body, and at least one of the pipes or channels connecting the at least two compartments.
In the present context, the term “acoustic compartment” designates a continuous space or volume of a cross sectional area throughflowable by gas, the space being limited at a gas inlet part thereof by an inlet of a smaller cross sectional area and at a gas outlet part thereof by an outlet of smaller cross sectional area. The cross sectional area of the space is at least 1.5 times the cross sectional area of the inlet or outlet, normally at least 2 times and in most cases at least 3 times, such as at least 4 times or 5 times or often preferably at least 6, 7, 8 or 9 times the cross sectional area of the inlet or outlet; typical values are 10-20 times the cross sectional area of the inlet or outlet. In the calculation of the throughflowable cross sectional area of the continuous space, any non-throughflowable obstruction is deducted. Thus, for example, when a major part of the cross sectional area of the continuous space is occupied a wall-flow particle filter with wall cross sections occupying up to, e.g., 50% of the cross section of the filter, that 50% of the cross section is deducted.
Typical values of the diameters of the inlet and outlet pipes are 2-11 inches for vehicle applications of an apparatus according to the invention and 300 mm-1000 mm for ship applications. The casing typically has an overall length of 1-3 m for vehicle applications and up to 15 m for ship applications. However, smaller or larger devices may be preferred for some applications.
In the present context, the term “monolithic body” or “monolith” designates, as is customary in the art, a body of an overall or macroscopic monolithic appearance, often a cylindrical body, which has a structure allowing an overall axial gas flow through the body. The term “monolithic” does not rule out that the body could be made from a plurality of segments joined or arranged together. The structure allowing an overall axial gas flow through the body will depend on the construction and material of the monolith; two typical relevant monolith types are:
a monolith made from a corrugated foil wound up cylindrically so that the corrugations provide axial gas flow channels, and
a monolith made of a particulate ceramic material, e.g., silicon carbide particles sintered together, and having a honeycomb structure comprising axial channels constituted by a plurality of coextending throughgoing passages separated by common passage walls, the passages being closed at the inlet and the outlet end, alternately, Thus, in a filter body of this kind, the gas travels into the passages open at the inlet side, through the walls into the passages open at the outlet side and then out of the filter body.
The invention is based on findings that one or more flow reversals can be accommodated internally in the silencer/monolith combination in such a way that internal connecting pipes or channels can be designed to a substantial length, compared to the total length of the casing containing all sound suppression and emission reducing elements; the length of the internal connecting pipes or channels will normally be at least the length of a monolith or almost the length of a monolith and can be up to, e.g, almost the length of the casing or even longer. The invention makes it possible to provide designs with rather narrow flow areas of internal pipes or channels, such as flow areas of about the same size as the inlet and outlet pipes or even smaller, without causing excessive pressure drops, typical pressure drops being of the magnitude of 1-2 times the dynamic pressure in the inlet pipe when the monolith or monoliths is/are of the through-flow catalyst monolith type. In accordance with this last objective, it is preferable to use smooth surfaces of these internal pipes or channels, preferably without any perforations in their side walls.
The acoustic attractiveness of the above mentioned findings and objectives can be explained by the theory of silencers in which sound is reflected at changes in flow area between pipes and acoustic chambers. Such silencers act as low-pass filters, i.e. the

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