Note: Descriptions are shown in the official language in which they were submitted.
The presen-t inven-tion relates to an exha~lst silencer havincJ
good acoustic and mechanical properties, comprisinq a prefer-
ably cylindrical casing of sheet metal, end walls of sheet
metal connected to the casing and at leas-t one sound-absorbing
porous ceramic element with at least one through-going channel
for the exhaust gases, wherein the ceramic element arranged in
said casing has a cylindrical outer surface lying close to the
inner cylindrical wall of the casing and is able to absorb
compression strains. The invention also relates to a method of
lQ manufacturing such a silencer.
Exhaust silencers containing one or more sound-absorbing porous
ceramic elements are described in co-pending Canadian patent
application No. 406,~62, owned by the present applicant, filed
on June 30, lg82, for an invention entitled "A Silencer".
~e~7~
These silencers have -turned ou-t to be e~tremely e~fective from
the view-point of sound absorption compared with conven-tional
silencers which are wholly made of sheet me-tal or include
sound-damping Eibrous material. Moreover, -the known sound-
damping fibrous material is rapidly decomposed and blown out of
-the silencer. Then, of course, the sound-damping effect is
seriously decreased.
The porous ceramic element, however, is wholly intact.
A common feature of conventional, previously known silencers is
that the casing must be made of relatively thick sheet metal,
for example 1.5 mm stainless steel sheet, resulting in a rela-
tively large mass. A further disadvantage is that the sheet
metal casing and possibly also the components located in the
casing give rise to resonant vibrations. Therefore, the
radiated sound from the outer surface will often get higher than
the exhaust sound from the outlet of the silencer.
The present invention relates to a further development of the
invention accordlng to aforesaid Canadian patent application
No. 406,462, where the excellent mechanical properties of the
porous ceramic elements described in said application are used
to bring about a new construction of an exhaust silencer, which
construction makes it possible to use a much thinner sheet metal
for the casing without decreasing the mechanical strength of the
silencer. The new construction also results in a total elimina-
tion or at least a considerable decrease of the resonant vibra-
tions.
Th~s, according to the present invention an exhaust silencer
having good acoustic and mechanical properties has been brought
about. The silencer comprises a pre-ferably cylindrical casing
of sheet metal, end walls of sheet metal connected to the casing
and at least one sound-absorbing porous ceramic element wi~h at
least one through-going channel for the exhaust gases, wherein
the ceramic element arranged in said casing has a cylindrical
outer surface lying close to the inner cylindrical ~.vall of the
casing and is able to absorb compression strains.
The silencer is characterized in that the casing and the ceramic
element to~ether form a rigid sandwich construction where the
cylindrical outer surface of the ceramic element is firmly
connected with the inner wall of the casing.
The ~irm connection is preferably achieved by means of a
shrinkage fit and/or a press fit, but it is also possible to
connect the casing and ceramic element by means of an adhesive
capable of withstanding heating. Due to the firm connection
between the casing and the ceramic element the silencer can be
considered as a unitary body or a sandwich construction with a
large mass and a large wall thickness. Therefore, the wall
thickness of the casing can be reduced. It is not necessary
that it is more than 0.3-0.5 mm. Moreover, there is no risk
that the silencer will radiate disturbing resonant vibrations.
The greatest stability of the silencer is achieved when the
compressing strain constantly acts between the casing and the
ceramic element and hence shrinkage and/or press fits are
preferred.
The invention will no\~ be described with reference to the
accompanying drawin~s, in whioh
Figure 1 is a partially cut-a~Yay view of a silencer according
to the invention and
Figure 2 is a partially, cut-away view of a further silencer
according to the invention.
Figure 1 illustrates in perspective a silencer according to
the invention and shows a partially cut-a~-av casing 1 macle,
preferably, of stainless sheet metal. The casing is of circular-
cy~indrical configuration and the thickness of the sheet metal
in the illustrated embodiment is about 0.5 mm. To the open ends
of the casing 1, walls 2 and 3 are connected from whioh exhaust
stubs 4 and 5 extend. The walls are preferably made of a thicker
sheet metal and are welded to the casing 1. Inside the casing 1
a ceramic body 6 having a circular-cylindrical outer surface is
arranged, which body is preferably ground. The ceramic body 6,
which f-orms a sound-absorbing element, is provided with a central
through-going channel 7 for the exhaust ~ases. The ceramic
element 6 comprises a foamed ceramic material and has been
moulded in a m~ould for obtaining a high accuracy to size and,
as before mentioned, the cylindrical surface of the element is
preferably ground to the desired dimensions, in order to obtain
optimum abutment against the inner wall of the casing 1.
The ceramic element, which is highly porous and has a compression
strength of at least 40 kN/m2 and preferably at least 400 kN/m2,
has an outer diameter which, at room temperature, exceeds the
inner diameter of the casing 1. The casing 1 is therefore
heated to a temperature, for example 400-600 C, sufficiently
high to enable the ceramic element 6 to be pressed into the
casing 1, said element being held, for example, at room tempera-
tlJre .
The casing 1 is preferably brought -to a temperaturc corresponding
to a calculated highest operational temperalure. Ot` course, the
casing may be heated to much higher temperatures than those
mentioned, when it is desired to obtain ~ery high pressure forces
between the inner wall of the casing 1 ancl the ou-ter surface of
the ceramic element 6. ~lhen the ceramic element 6 has been
placed in a desired position in the heated casing 1, the casing
is allowed to cool and thus to shrin~ onto the element 6. The
end walls 2 and 3 are then ~elded -firmly to the casing. The
resul-tant unit, comprising the casincl 1 anc! the element 6,
becomes particularly rigid, which pre\ents -the casing from gene-
rating oscillations and, as before mentioned, the casing can
also be made very thin. The above mentioned temperature has
only been selected by way of example and may var~ in dependence
on the co-efficient of expansion of the sheet metal and the
ceramic material respectively, or in dependence on the tempera-
ture of the ceramic when the element 6 is pressed into the
casing 1. The described embodiment where the ceramic elemen-t
is pressed into the heated casing, creating a combined shrinkage
and press fit, may naturally be replaced with a pure shrinkage
fit, in which the casing 1 is heated and caused to expand to an
extent such that the ceramic element 6 can be inserted into the
casing without requiring an appreciable force to do so, or with
a pure press fit, in which the ceramic elemen-t is pressed into
the casing without prior heating thereof.
Irrespective of the method used, one important criterion is
that there is obtained such a JOint tha-t pressure forces remain
between the cylindrical outer surface of the ceramic body 6 and
inner wall of the casing 1, even at maximum operational tempera-
tures, e.g. operational temperatures of 600 C.
Figure 2 illustrates a sllencer comprising a plurality of
ceramic elements, for example elements 8, 9 and 10, all of which
are shrinkage-~itted in the metal casing 1.
Each cylindrical element 6, 8, 9 and 10 is ground to an outer
diameter which, at low temperatures by which is meant here
normal room temperature, i.e. about 20C, exceeds the inner
diameter of the casing 1 at the same temperature. In certain
cases, the cylindrical elements can be moulded to such precise
dimensions as to obviate the need of separately working the
outer surfaces of said element.
The shrinkage and/or press fits described above can be replaced
with an adhesive joint, where a continuous thin layer of
adhesive is applied to the entire outer cylindrical surface of
the ceramic element. The element will have a diameter so
adapted to the inner diameter of the casing that at least some
force is required to press the element into the casing. Any
suitable temperature-resistant binding agent can be used, such
as an aluminium phosphate binder, for example.
The invention is not limited to the embodiments shown, since
these can be modified in different ways within the scope of the
present invention. Thus, it is not necessary to have a cylin-
drical casing, since a casing with another geometrical shape
can also be used. Of course, the outer surface of the ceramic
element must always have the same shape as the inner wall of the
casing.
A
