Note: Descriptions are shown in the official language in which they were submitted.
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DaimlerChrysler AG
Exhaust pipe insulated by an air gap, and method for
producing the same
The invention relates to a bent exhaust pipe insulated
by an air gap, and to a method for producing the same.
In exhaust systems of modern internal combustion
engines which are equipped with a catalytic converter,
use is increasingly being made of double-walled pipes
which have an outer pipe and an inner pipe, with an
insulating air gap being formed between the outer pipe
and the inner pipe. Pipes of this type are what are
referred to as pipes insulated by an air gap (IAG). The
air gap between the outer pipe and the inner pipe
advantageously acts as heat insulation, which, owing to
the poor transfer of heat from the inner pipe to the
external surroundings, rapidly brings the exhaust
system and the catalytic converter connected thereto to
its operating temperature.
In conjunction with units being arranged, for example
in the engine compartment of a motor vehicle, as
compactly as possible, it is endeavored in general to
design the abovementioned double-walled pipes in bent
form, so that a space-saving installation in the region
of the engine compartment can be obtained. Without
additional auxiliary means it is currently not readily
possible to bend an IAG pipe. The difficulty resides in
maintaining the required gap size between the pipe
walls . The pipe walls must not touch after the bending
and the gap size between the inner pipe and the outer
pipe must not undergo a serious change, such as, for
example, due to pipe collapse and the like.
In known bending processes, this requirement is met by
inserting additional materials in loose form which are
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placed between inner pipe and outer pipe. In this case,
use can be made, for example, of the materials sand,
steel balls, low-melting alloys, ice and the like. The
disadvantages in this regard reside firstly in the
outlay, for example the supply of power to melt ice in
order, after the IAG pipe is formed, to remove the
inserted media again from the gap. Secondly, there is a
disadvantage in the excessive soiling of the working
environment by the emerging media. In particular if
steel, balls lie around on the floor, there is a
considerable potential risk to people by them sliding on
the balls. A further disadvantage is that residues of
the additional material (in particular steel balls) may
remain in the bent pipe and, during subsequent operation,
may therefore cause malfunctions of units, such as, for
example, of the catalytic converter or a turbocharger.
Similarly, undesirable noise emissions may occur due to
remaining residual media.
The publication DE 102 Ol 594 Al describes a method for
producing a bent double-walled IAG pipe. In this pipe,
the gap between the inner pipe and the outer pipe is
completely filled by a spacer layer. If the spacer layer
is composed of a plastic or of a low-melting alloy, the
removal of the spacer layer is obtained by burning or
melting it out. However, an IAG pipe produced according
to the method of this publication is subject to the
disadvantage that the gap between the inner pipe and
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outer pipe is completely filled by the spacer layer and
accordingly a large quantity of material is to be removed
after the production of the component.
In the publication DE 44 37 380 Al, an exhaust pipe
insulated by an air gap, and a method for producing the
same are described. This exhaust pipe insulated by an
air gap has two inner pipe sections connected via a
sliding fit. During the production of this exhaust pipe,
an inner pipe is inserted into an outer pipe, with a
space between the inner pipe and the outer pipe being
filled with an essentially incompressible material,
preferably steel grit. This arrangement is then bent.
After the bending operation, the incompressible material
is removed from the space. In addition, a radial
mounting is provided between the inner pipe and the outer
pipe. This radial mounting comprises three knitted wire
elements which are distributed over the circumference,
are spaced apart from one another and may also be
designed as encircling knitted wire rings.
The publication US 3,343,250 relates to a method for
producing a double-walled pipe. For this purpose, it is
provided to arrange an inner pipe, for example, coaxially
with an outer pipe and to arrange spacers in a cavity
between these two pipes. Furthermore, a filling
material, preferably polyethylene glycol which has a low
melting point is placed into the cavity between these two
pipes in a crystalline or solid state. A bending of the
pipe to be produced then takes place. The filling
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material is subsequently melted with hot water, washed
out and therefore removed from the cavity. It is
provided that the spacers are left in their original
position and they therefore serve the purpose of
maintaining the pipe structure, with the result that the
two pipes cannot touch each other after the filling
material is removed.
The invention is based on the object of providing a bent
double-walled component and a method for producing the
same, in which the positioning of an intermediate piece
in the pipe composite structure formed from the inner
pipe and the outer pipe requires a low outlay and the
removal of the intermediate piece is readily possible.
According to the invention, the object is achieved by a
method with the features of claim l, and furthermore by a
component with the features of claim 13. Advantageous
developments of the invention are the subject matter of
the dependent claims.
By means of the method according to the invention, a bent
double-walled component can be produced, in which an
intermediate piece only fills part of an intermediate
- space, which is provided between the inner pipe and the
outer pipe, in the axial direction of the component. In
this case, the thickness of the intermediate piece is
essentially matched to the distance which exists between
an outer surface of the inner pipe and an inner surface
of the outer pipe. The intermediate piece is expediently
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only positioned at a location in the intermediate space
of the component where it is required for bending the
composite structure formed from the inner pipe and the
outer pipe in order to prevent a pipe collapse and the
like. Put in other words, the formation of collapsing
points, bucklings and the like during the bending of the
composite structure is prevented at the location where
the intermediate piece is situated between the inner pipe
and the outer pipe. During the bending of the component
or of the entire pipe composite structure, the
intermediate piece therefore ensures that the inner
surface of the outer pipe is adequately supported against
the outer surface of the inner pipe, so that the gap size
or the distance between the inner pipe and the outer pipe
remains essentially constant.
An essential feature according to the invention is that
the intermediate piece does not completely fill the
intermediate space between the inner pipe and the outer
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pipe. This can take place in an advantageous embodiment
by the intermediate piece being designed in the form of
a ring which is either placed onto the inner pipe or is
placed into the outer pipe before the bending of the
composite structure. If the component has a plurality
of curvatures in the axial direction, in the case of
the method according to the invention a plurality of
intermediate pieces are advantageously arranged in the
form of rings along a longitudinal axis of the
component, so that a ring is positioned in each case on
a corresponding bending point in order to prevent a
pipe collapse at this point. The same advantageous
effect can be obtained by an intermediate piece which
is designed in the form of a spiral which extends along
a longitudinal axis of the component. In comparison to
a plurality of rings, a spiral of this type affords the
advantage of further simplified handling and a
correspondingly shortened installation time.
Furthermore, the provision of an intermediate piece,
for example in the form of an individual ring or a
spiral, affords the advantage that, after an internal
combustion engine in which the double-walled component
is inserted is put into operation, the high exhaust
temperatures and the resultantly induced thermal
destruction of the intermediate piece mean that only a
small quantity of gas has to escape from the
intermediate space. There is therefore only a small if
an-y "enrichment" of the exhaust gas flow, which is
guided in the inner pipe, of the internal combustion
engine by the quantity of gas which arises because of
the burning of the intermediate piece.
According to an advantageous alternative embodiment,
the intermediate piece may also be formed from a
coating of an inner pipe which has already been
manufactured. In comparison to an installation of a
separate ring or a separate spiral, this affords
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significant advantages, with regard to a continuous
manufacturing process, in the form of a further-reduced
outlay on installation.
In an advantageous development of the invention, the
intermediate piece is produced from a plastic.
Particularly suitable for this is a plastic made from
polyethylene for which a residue-free burning is
ensured as a result of the high exhaust temperature.
Damage, for example to the catalytic converter through
which the exhaust gas flow is exclusively conducted,
due to residues of the burnt intermediate piece
therefore does not occur. In the same manner, other
plastics which, during their thermal destruction or
burning, are converted exclusively into reusable
cleavage products are also suitable for the
intermediate piece, as a result of which damage to the
catalytic converter or the like is prevented.
In what is referred to as an IAG pipe, the outer pipe
is generally of a design tight to exhaust gas. In an
advantageous development of the invention, the inner
pipe is connected to an adjacent part of the exhaust
system by means of a sliding fit. Such a sliding fit
firstly ensures compensation for thermal stresses which
arise due to the very severe differences in temperature
during operation of the internal combustion engine in
comparison to the inoperative state. Secondly, the gas
formed by the burning of the intermediate piece as a
consequence of the exhaust temperature of some hundred
degrees Celsius can readily pass through the sliding
fit from the intermediate space into the inner pipe in
order then to be output to the outside together with
the regular exhaust gas flow, for example through the
catalytic converter. The residue-free burning of the
intermediate piece therefore creates the desired
complete air gap between the inner pipe and the outer
pipe which ensures the required thermal insulation for
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the double-walled component during the further
operation of the internal combustion engine.
Further advantages and refinements of the invention
emerge from the description and the attached drawing.
It goes without saying that the features of the
invention which are mentioned above and those which
have yet to be explained below can be used not only in
the respectively stated combination but also in other
combinations or on their own without departing from the
scope of the present invention.
The invention is illustrated diagrammatically below in
the drawing with reference to an embodiment, which is
to be understood as being only by way of example, and
is explained in more detail below with reference to the
drawing.
Figure 1 shows a double-walled pipe according to the
invention in a bent state.
Figure 2 shows various embodiments of a pipe end in
the region A of figure I.
Figure 3 shows a double-walled pipe according to the
invention with an intermediate piece in a
non-bent state.
Figure 4 shows the pipe from figure 3 in a bent state.
Figure 5 shows a double-walled pipe according to the
invention with an alternative intermediate
piece in a non-bent state.
Figure 6 shows the pipe of figure 5 in a bent state.
Figure 7 shows the double-walled pipe according to the
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invention in a state in which it is fitted in
an exhaust system by means of a sliding fit.
Figure 1 shows a lateral cross-sectional view of an
embodiment of a double-walled pipe 10 according to the
invention in a bent state. In detail, the pipe 10 has
an inner pipe 11 and an outer pipe 12 which are
arranged coaxially with each other. The outside
diameter of the inner pipe 11 is selected to be
substantially uniformly smaller along the entire axial
extent I of the pipe 10 than the inside diameter of the
outer pipe 12, with the result that an intermediate
space I3 is formed between the inner pipe 11 and the
outer pipe 12. In this case, an outer surface 14 of the
inner pipe 11 is spaced apart essentially uniformly by
a distance d from an inner surface 15 of the outer
pipe 12.
The bent state of the pipe 10 that is shown in figure 1
is expediently obtained by means of an intermediate
piece (not shown in figure 1) which is placed into the
intermediate space 13 before an appropriate bending or
forming of the pipe 10. An intermediate piece of this
type can advantageously be produced from a
thermoplastic, such as, for example, polyethylene. If,
after its completion, the pipe 10 is fitted in an
exhaust system of an internal combustion engine, the
plastic intermediate piece burns in a residue-free
manner under the effect of the high exhaust temperature
which usually assumes a value of some 100°C. As a
result, a continuous air gap is produced by the burning
of the intermediate piece in the intermediate space 13,
thus producing the desired heat insulation of the inner
pipe 11. The pipe 10 shown in figure 1 is shown in a
state in which the intermediate piece has already been
burnt in a residue-free manner.
The manner in which the pipe 10 is brought by means of
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_ g _
the intermediate piece into its bent state is explained
in detail below with reference to figures 3 to 6.
First of all, the inner pipe 11 and the outer pipe 12
are provided preferably in rectilinear form. An
intermediate piece 16 in the form of a plastic ring is
subsequently either placed onto the outer surface 14 of
the inner pipe 11 or placed into the inner surface 15
of the outer pipe 12. In order to prevent an
undesirable slipping of the plastic ring 16 during the
further manufacturing sequence, the ring preferably has
a matching size with regard to the outer surface 14 of
the inner pipe 11 or to the inner surface 15 of the
outer pipe 12.
In a next step, the inner pipe 11 and the outer pipe 12
are brought together to form a composite structure,
with the inner pipe 11 and the outer pipe 12 now being
spaced apart from each other at a defined distance d by
the ring 16. The thickness of the ring 16 is suitably
matched to the respective dimensions of the inner pipe
11 and of the outer pipe 12, so that the two pipes can
readily be brought together without jamming or the like
together.
The ring 16 is preferably produced from a
thermoplastic, such as, for example, polyethylene,
which firstly results in favorable production costs.
The ring 16 is placed into the intermediate space 13
between the inner pipe 11 and the outer pipe 12 at a
location at which the pipe 10, which is still
rectilinear, is subsequently subject to a bending or
forming process. With regard to the bending of the pipe
10, the composition of the ring 16 from a plastic
material ensures that the ring 16, owing to its
elasticity, does not provide an excessive resistance to
the deformation. Furthermore, the intermediate ring 16
is of such dimensional stability that it does not yield
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to the acting forces and accordingly is not massively
deformed.
The composite structure which is formed by the inner
pipe 11 and the outer pipe 12 being joined together can
be transferred, for example, by means of a conventional
CNC bending with suitable mandrel supports and the like
into the bent state which is shown in figure 4 in a
lateral cross-sectional view. During the bending of the
pipe 10, the intermediate ring 16 remains dimensionally
stable with regard to its thickness, so that the inner
pipe 11 remains approximately uniformly spaced apart
from the outer pipe 12 by the distance d even in the
section of the pipe 10 which is now bent. During the
course of the bending operation, the inner surface 15
of the outer pipe 12 is reliably supported with respect
to the outer surface 14 of the inner pipe 11 by the
ring 16. The intermediate ring 16 therefore contributes
to the required gap size d in the pipe 10 also being
maintained in its bent section. In the course of the
bending of the pipe 10, the intermediate ring 16
prevents an impermissibly high pipe collapse, bucklings
and the like, which would otherwise mean that a
constant gap size between the inner pipe 11 and the
outer pipe 12 would no longer be ensured.
If, in its end state, the pipe 10 has a bent section at
a plurality of points, then a plurality of intermediate
rings 16 is provided before the bending of the pipe 10.
In detail, a respective intermediate ring 16 is
arranged in each section of the pipe 10 or of the
intermediate space 13 that is subsequently subject to a
bending operation. This ensures in an efficient and
simple manner that, as a result of the intermediate
ring 16 positioned in those sections of the pipe 10
which are bent, an impermissibly high pipe collapse and
the like does not occur therein.
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Instead of an individual intermediate ring 16 or a
plurality of rings, the intermediate piece may
alternatively be designed in the form of a spiral 16' .
Figure 5 shows the pipe 10 in a lateral cross-sectional
view in which an intermediate piece in the form of a
spiral is placed into the intermediate space 13. The
spiral 16' is suitable in particular for the case in
which the bent section of the pipe 10 extends over a
relatively long region in the axial direction of the
pipe 10. In this case, a pipe collapse or the like can
therefore also be reliably prevented. Furthermore, in
comparison to a plurality of individual rings, the
spiral 16' is distinguished by substantially simplified
handling, which results in a shortened installation
time and therefore in a reduction in costs. In figure
6, the pipe 10 from figure 5 is shown in a bent state.
As already explained above, as a result of the spiral
16' a constant distance d between the inner pipe 11 and
the outer pipe 12 is ensured in the bent section of the
pipe 10.
As an alternative to providing the intermediate piece
in the form of a ring or a spiral, the inner pipe 11
may also be provided with a coating in that section in
which the pipe 10 which is to be produced is in the end
bent, the thickness of which coating essentially
corresponds to the desired distance d between the inner
pipe 11 and the outer pipe 12. This coating is
expediently also composed of a plastic, which, owing to
the elasticity of this material, readily permits a
deformation in the bent section.
A substantial advantage of the invention is that the
bent pipe 10 does not need any further treatment in
order to remove the intermediate piece. In the event of
the pipe being used in an exhaust system or the like of
an internal combustion engine, the high exhaust
temperature and the correspondingly produced heating of
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the pipe 10, in particular of the inner pipe 11 in
which the exhaust gas is guided, causes the
intermediate piece, if the latter is produced from a
plastic, to be burned. With regard to undesirable
damage to a unit arranged in the exhaust system, such
as, for example, a catalytic converter, a turbocharger
or the like, it is of substantial importance that the
intermediate piece burns in a residue-free manner. A
suitable plastic which fulfils this requirement is
provided, for example, by a polyethylene. Furthermore,
the intermediate piece may also be formed by different
plastics which ensure a residue-free burning in the
same manner.
It is explained below with reference to figure 7 how
the gases which are produced by the burning of the
intermediate piece 16, 16' escape from the intermediate
space 13.
In general, an exhaust system of an internal combustion
engine is subjected to considerable temperature
fluctuations which result in a thermal expansion of the
individual components of the exhaust system. To
compensate for these temperature fluctuations and the
resultantly caused thermal stresses, it is known to
connect adjacent components or pipes of the exhaust
system to each other by means of what is referred to as
a sliding fit. In such a sliding fit, one pipe is
merely pushed into another pipe, with_the result that
thermal stresses can be compensated for by a sliding
movement of the one pipe relative to the other pipe.
Figure 7 shows a lateral cross-sectional view of part
of an exhaust system. In this case, an end section of
the inner pipe 11 is fitted to an adjacent pipe 17 by
means of a sliding fit 18.,This sliding fit 18 is
explained in detail below.
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An exhaust gas flow which is produced by an internal
combustion engine or the like is entirely guided in the
inner pipe 11. In general, it is of great importance in
the case of the double-walled pipe 12 that the outer
pipe 12 is designed to be absolutely gastight in order
to prevent exhaust gas from escaping into the
atmosphere or to prevent impermissible noise emissions
or the like. In the illustration shown in figure 7, the
direction of flow in which the exhaust gas is guided in
the inner pipe 11 runs from the left to the right and
is accordingly indicated by an arrow E. The inner pipe
11 is inserted at its end 20 into the adjacent, further
pipe 17. The pipe 17 is designed to be slightly larger
in its diameter than the inner pipe 11 and therefore
surrounds the end section 20 of the inner pipe 11. In
this case, the dimensions of the inner pipe 11 and of
the pipe 17 are suitably selected in such a manner that
a small gap s is formed in the region in which the
respective end sections of the pipes overlap. During
the operation of the internal combustion engine, the
direction of flow E has the effect that, as a result of
what is referred to as an "entraining effect", a
suction effect arises from the intermediate space
through the gap s into the interior of the pipe 17. Put
in other words, air particles are sucked into the pipe
17 from the intermediate space 13 through the gap s by
the explained suction effect and are transported by the
rest of the exhaust gas flow further to the right in
figure 7, for example in the direction of a catalytic
converter or the like.
By means of the above-explained suction effect on the
end section 20 of the inner pipe 11, it is ensured that
a gas which arises as a result of the burning of the
intermediate piece 16, 16' in the intermediate space 13
can escape in a suitable manner through the gap s which
is formed in the sliding fit 18, with the result that
this gas is transported away together with the rest of
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the exhaust gas flow. Accordingly, by means of the
operation of the internal combustion engine, a burning
of the intermediate piece 16, 16' is brought about as a
consequence of the high exhaust temperatures without,
for example, an additional supply of power in order to
melt the intermediate piece, or similar special
measures.
With reference to figure 2, various embodiments are
explained in which the respective end section of the
pipe 10 can be formed. In figures 2a to 2c, possible
arrangements of that end section of the pipe 10 which
is shown by A in figure 1 are explained in detail.
According to the embodiment of figure 2a, the end
section of the pipe 10 can be designed in an "open"
variant, in which there is no metallic contact between
the inner pipe 11 and the outer pipe 12. Accordingly,
an adjacent part of the exhaust gas train is to be
designed in the same manner, with the result that a
sealing contact is produced between the respective pipe
points.
According to an alternative embodiment shown in figure
2b, the inner pipe 11 is increased in diameter or
"turned out in a tulip-shaped manner" at its end, as a
result of which contact is produced between the inner
pipe 11 and the outer pipe 12.
In the further alternative embodiment shown in figure
2c, the outer pipe 12 is reduced in its diameter, so
that contact is produced between the two pipes in the
same manner as in the embodiment of figure 2b. In both
embodiments of figure 2b and figure 2c, a welding point
can additionally also be placed at the contact point of
the two pipes in order to prevent a displacement of the
pipe ends due to the bending operation.
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The residue-free burning of the plastic intermediate
piece is explained once again below.
During operation of the internal combustion engine, the
temperature of the exhaust gas guided in the inner pipe
11 readily reaches a value of 500°C and above, with at
least the inner pipe approximately assuming the same
temperature. Since the plastic intermediate piece, as
explained above, is in contact with the inner pipe,
during operation of the internal combustion engine,
when the hot exhaust gas flows through the inner pipe,
the plastic is burned, since the exhaust temperature
mentioned is substantially greater than the destruction
temperature of the plastic. In the case of a
polyethylene, the polymeric material is converted into
its low-molecular cleavage products, i.e. carbon and
hydrogen. These low-molecular cleavage products are
completely harmless to a catalytic converter, and so
the gases of the burned plastic, as explained above,
can readily be carried away together with the regular
exhaust gas flow through the inner pipe 11 and
subsequently through the catalytic converter (not
shown) to the outside. Other plastics, in particular
thermoplastics, which, when they are being burned,
merely release low-molecular cleavage products which
are completely harmless with regard to a catalytic
converter or the like are also suitable in the same
manner.
The method according to the invention for producing a
bent double-walled component is distinguished in that a
precisely defined positioning of an inner pipe relative
to an outer pipe can be obtained in a simple manner by
an intermediate ring, a plurality of intermediate
rings, a spiral or a corresponding coating of the inner
pipe, this intermediate piece/these intermediate pieces
effectively preventing a pipe collapse or the like
during the bending of the composite structure formed
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from the inner pipe and the outer pipe. The
intermediate piece is expediently formed from a
plastic, so that, during operation of an internal
combustion engine, no special measures have to be taken
in order to remove the intermediate piece, since the
latter burns in a residue-free manner as a consequence
of the high exhaust temperatures. By providing just one
ring or a plurality of individual rings or a spiral, it
is advantageously ensured that only a small quantity of
material of the intermediate piece is to be burned, as
a result of which the regular exhaust gas flow which is
guided within the inner pipe is only slightly
"enriched" by the gas produced by the burning of the
intermediate piece. Moreover, as a result of the
quantity of material only being small, a very rapid
burning and therefore a speedy removal of the
intermediate piece come about, after which, in a
correspondingly rapid manner, a desired continuous
insulating air gap is produced between the inner pipe
and the outer pipe.
