Silencer with incorporated catalyst

Details Silencer with incorporated catalyst Silencer with incorporated catalyst

1998-01-16

2001-04-24

Ryznic, John E. (Department: 3745)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C422S176000

Reexamination Certificate

active

06220021

ABSTRACT:

The present invention discloses a silencer with a built-in catalyst which utilises a given total space optimally for simultaneous silencing and conversion of noxious exhaust gases, typically exhaust gases from prime mover internal combustion engines.
The invention utilises diffuser technology in a novel way, in that a special design of a built-in diffuser is adopted, both for sound attenuation and for even distribution of exhaust gas flow to the inlet face of a catalyser body.
As a consequence of ever more stringent environmental regulations, demands for low exhaust noise levels and for low levels of particle and noxious gas emissions to the atmosphere are increasing all the time. In addition, it is required that the flow resistance provided by silencers, catalysts, etc. be as small as possible, in order that the back-pressure to the engine can be kept as low as possible. This poses a problem to the exhaust system designer, since the available under-vehicle space is normally restricted.
A first step towards space economy, which has been adopted already, is to combine silencers and catalysts by inserting a catalysts inside the casing of a silencer. Even a simple catalysts containing canister causes some noise attenuation, by virtue of its acoustic volume or by throttling of the exhaust flow. In the case of a catalytic body with uninterrupted, straight channels of low pressure drop, however, the attenuation effect of the catalysts as such is only marginal, which can be shown by removing the catalytic body and by measuring how this influences the exhaust noise level outside the exhaust pipe system. Wall-flow catalysts, in which gases are forced to follow tortuous pathways inside the catalyst body, are more effective in suppressing noise, but such devices also cause rather high pressure drops.
In diesel engine exhaust systems accumulation of particulate matter is sometimes a problem. In catalysts particulate matter which is not converted tends to hamper the conversion process and to cause increased pressure drop. This problem, which at present receives much attention, primarily refers to the design of the catalyst as such, but should also be addressed when developing concepts for combined silencer catalysts.
Various sorts of diffusers have been utilised as flow distribution arrangements in front of catalysts and as flow elements in silencers.
In the first case these arrangements are answers to the following problem: Supposing that a catalysts is positioned close to an inlet pipe of substantially smaller diameter, how can an even flow distribution across the diameter of the catalysts be achieved? The demand for a close positioning results from an overall demand for compactness.
Many types of diffusers have been suggested as solutions to this flow distribution problem. Examples of this are: German Offenlegungsschrift no. 24 28 966, which describes a pure flow line diffuser and German Offenlegungsschrift no. 24 29 002, which describes arrangements with a plurality of flow dividing cones. The latter type of solution resembles well-known arrangements incorporating guide vanes in front of steam boiler exhaust catalysts, as well as ‘splitter’ type diffusers commonly used in ventilating ductwork. German Offenlegungsschrift no. 24 28 964 and Norwegian utlegningsskrift no. 169581 both disclose more original diffuser catalyser arrangements.
German Offenlegungsschrift no. 2 307 215 describes a diffuser-type arrangement in which a perforated, conical member is inserted into a conical end cap at the inlet to a catalyst. This arrangement divides the rather small cavity in front of the catalyst into a flow distributing first cavity with radial diffuser properties and a second, flow mixing cavity immediately in front of the catalyser.
However, none of these solutions take acoustic aspects into consideration. To an extent this is inherent in the above formulation of the catalyst flow distribution problem, according to which the space in front of the catalytic body should be minimised, thereby significantly reducing the acoustic chamber effect. Of course, the gas volume contained within the catalytic body as such may provide some acoustic chamber effect. But from a sound attenuation point of view it is less expedient to arrange the inlet pipe / chamber flow area expansion at the upstream end of the casing. The reason is that this type of geometry tends to excite the fundamental acoustic chamber resonance maximally. This mode corresponds to a wavelength twice the acoustic chamber length, with a pressure node in the middle and maximum pressure variations at each end of the chamber.
Danish patent no. 128427 discloses a type of silencer in which a radial diffuser is utilised for achieving a low pressure drop and for positioning the outflow from the inlet pipe exactly in the middle along the length axis of a chamber, which suppresses the fundamental acoustic mode of the chamber. Danish patent no. 169823 discloses how special type diffusers with narrow, axial outflows into acoustic compartments can be adopted for suppressing lateral, resonant gas vibrations, which is particularly relevant in the case of silencers with a large casing diameter compared to pipe diameters.
This last-mentioned patent in a sub-claim also describes the possibility of utilising a radial flow property of axial outflow diffusers to obtain a flow distribution effect in front of a catalyst inserted into the silencer. However, due to the narrow lateral extension of the diffuser outflow, this tends to require that the catalytic body be of a ring-type cross section. In the case of a large diameter casing this could for instance be provided for by dividing the catalytic body into several parallel elements. But in the case of long and not too wide casings, as are generally required for under-vehicle installations, much speaks in favour of retaining a simple cylinder form of the catalytic body. In such a case the rather narrow axial outflow at a considerable distance from the centerline is less expedient in providing flow to the center of the inlet face of the catalytic body.
In the present invention the silencing and flow distributing objectives are met simultaneously by utilising a novel, special type of diffuser provided with, as a minimun 2, but in general further, apertures, as can be seen from
FIG. 1
which shows a first embodiment of the invention.
Here, an acoustic compartment
4
and a catalytic body
5
are both fitted into a casing
1
, which is connected to an inlet pipe
2
and to an outlet pipe
3
. An elastic layer
5
a
holds the catalyst and protects it from undue mechanical forces. The diffuser element
6
and the juxtaposed cross-plate flange
7
, provided with apertures
8
,
10
, together constitute a pressure recovering and flow distributing cross-plate diffuser. Due to the rather big aperture
8
,
9
,
10
it is ensured that a significant proportion of the acoustic energy present in the gas is transmitted into the compartment
4
, in which sound-absorbing material
14
is inserted inside a perforated pipe
15
.


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