EP1456513B1 - Device and method for dampening noise in the exhaust system of an internal combustion engine - Google Patents

Description

The invention relates to a device for soundproofing in the exhaust system an internal combustion engine. Such a device serves, for example for damping one or more of the internal combustion engine or an example so operated automobile particularly critical frequencies.

In the automotive industry are numerous devices and methods for soundproofing known. It is often necessary, especially critical frequencies that z. B. lead to resonances in parts of the automobile to dampen. To do this sometimes met very complex construction measures. In particular, be often requires additional components.

From US 5,645,803 a catalyst carrier body is known, the smooth and corrugated sheet metal layers is constructed, formed between which spacers are. The catalyst carrier body according to US Pat. No. 5,645,803 can be flowed through radially and has channels whose cross-sectional area changes.

On this basis, it is an object of the invention in exhaust systems of Internal combustion engines with weaving bodies for exhaust gas purification substantially without additional components a soundproofing, especially for special critical frequencies.

This object is achieved by a honeycomb body according to the features of Claim 1, by an exhaust system according to the features of claim 5 and a method for sound damping according to the features of claim 9. Advantageous Embodiments that may occur singly or in combination are described in the respective dependent claims.

A basic construction of such honeycomb body is for example from the EP 0 245 737 B1 or EP 0 430 945 B1. But the invention can be also in other designs, eg. B. spirally wound designs, realize. Also in one direction conical designs for example from the

WO 99/56010 known. The known for honeycomb body manufacturing process can also be used for the present invention. Newer developments concerning the cell geometry have the use of microstructures in the channel walls produced, for example, from WO 90/08249 and WO 99/31362 are known. These developments can also be used for here apply present invention additionally. Generally, there are known measures for producing or improving the effectiveness of such honeycomb bodies also the present invention is applicable.

The inventive device for sound damping in an exhaust system an internal combustion engine comprises such a honeycomb body. Of the Honeycomb body has an axial length and has substantially separate, exhaust gas flow through channels. The channels are divided into at least a first subset of channels and a second subset of channels. At least the cross-sectional areas of one of the two subsets of channels change so about the axial length of the honeycomb body, that the running time of the exhaust gas in the different subsets channels is different.

It makes sense for soundproofing in the exhaust system of an internal combustion engine to use a honeycomb body, since such often, for example, in Catalysts for exhaust gas purification find use and thus already in the exhaust system an automobile are present. This allows the sound reduction in the exhaust system, without introducing additional components into the exhaust system would have to be. Thus, a structurally simple and cost-effective way given for soundproofing.

In a channel of varying cross-sectional area, the velocity of a gas stream behaves inversely proportional to the cross-sectional area. As a result, the flow of gas slows as the cross-sectional area of the channel increases over the axial length of the honeycomb body. Conversely, as the cross-sectional area of the channel decreases over the axial length of the honeycomb body, gas flow accelerates. According to the invention, an exhaust gas stream entering the honeycomb body is separated into at least two partial amounts of exhaust gas, which respectively flow through a part of a channel. If the honeycomb body has an axial length L in a main flow direction z, the transit time t (L) of a gas at a velocity v which is dependent on z can be calculated as follows:

Thus, since the velocity function v (z) is due to the change in the cross-sectional area of the channel can be influenced and the transit time of a gas through a channel depends on the one hand the length of the channel and on the other hand of the im Channel applicable speed function, the running time of an exhaust gas is in one Channel very precisely adjustable.

Applied to the two subsets channels now results in the invention Possibility to make a runtime difference between the two subsets Channels flowing through subsets produce exhaust gas. Is the exhaust gas carrier of sound waves, so you can use this difference in transit time a phase difference create channels between the sound waves in the two subsets. With a suitable choice of the transit time difference, this leads to a Attenuation of sound waves of a certain wavelength.

If one wishes to attenuate sound waves of a wavelength λ, a phase velocity c and an angular frequency ω, one chooses preferably as a transit time difference between a transit time t _{1 of} the first subset of exhaust gas through the first subset of channels and a transit time t _{2 of} the second subset of exhaust gas through the second subset of channels | t 1 - t 2 | = 2 n +1 2 λ c . where n is a natural number. According to an advantageous embodiment of the honeycomb body, the first subset of channels has in each case a first input cross-sectional area and a first output cross-sectional area and the second subset of channels has a second input cross-sectional area and a second output cross-sectional area over their axial length. According to the invention, the ratio of the first input cross-sectional area to the first output cross-sectional area is different than that of the second input cross-sectional area to the second output cross-sectional area. In this case, therefore, the cross-sectional areas of the first subsets of channels and the second subset of channels change in different ways. This requires a speed change in both subsets exhaust gas flowing through the two subsets channels and thus a delay difference.

According to yet another advantageous embodiment of the honeycomb body increases Cross-sectional area of at least a subset of channels in the main flow direction z, preferably it increases monotonically and more preferably strictly monotone and / or the cross-sectional area of another subset of channels falls in the main flow direction, preferably it is monotonous and more preferably strictly monotone. Monoton means that quite a part of a channel or even the whole Channel may have the same cross-sectional area over the axial length L. at strictly monotonous gradients, this is not possible, here must be a steady increase or reduction of the cross-sectional area over the axial length. Particularly preferred is a still further advantageous embodiment of the honeycomb body, in which at least a subset of channels widens conically and / or at least one further subset of channels narrows conically. consequently can according to the invention a first subset of channels cross-sectional area over Do not change the axial length, while a second subset conical expanded or narrowed, or in other ways, the cross-sectional area in the main flow direction z increases or decreases monotonically. It is according to the invention It is also possible that a first subset of channels widens conically while a second subset of channels narrows conically. This allows a very simple constructive structure of a honeycomb body according to the invention.

According to yet another advantageous embodiment of the honeycomb body are the Subsets channels designed so that different subsets channels different Have integral of the cross-sectional areas over the axial length. This advantageously allows the channels with chambers, widenings and constrictions and so different requirements for example with regard to the required pressure losses and flow cross sections, as well structural conditions or restrictions. It is too possible to produce large runtime differences at relatively short axial lengths L

According to the inventive concept, an exhaust system of a Internal combustion engine proposed. This exhaust system has at least a honeycomb body, with the exhaust gas flowed through channels and a axial length. A flow path of a first subset of the exhaust gas is through a first subset formed channels and a flow path for a second Subset of the exhaust gas through a second subset of channels. The cross-sectional areas at least one of the two subsets channels change over the axial Length of the honeycomb body. This causes a delay difference between the two subsets of the exhaust gas. Here, too, can be so by a corresponding Sizing the subsets channels an attenuation of at least one frequency cause a sound wave in the exhaust gas. Particularly preferred here is that the first subset of channels each have a first input cross-sectional area and a first output cross-sectional area and the second subset of channels, respectively a second input cross-sectional area and a second output cross-sectional area and the ratio of the first input cross-sectional area to the first output cross-sectional area is other than that of the second input cross-sectional area.

According to an advantageous embodiment of the exhaust system, the cross-sectional area increases at least a subset of channels in the main flow direction z, preferably it rises monotonously and most preferably strictly monotone and / or fall the cross-sectional area of at least one further subset of channels in the main flow direction z, preferably monotonically, and more preferably strictly monotone. Monotone means here that the cross-sectional area of a part a channel or a whole channel, but not It can happen, for example, that a channel expands first then narrow again later. Strictly monotonous rising means that everyone Coordinate z in the main flow direction are other cross-sectional areas, which increase with increasing coordinate z, ie a steady increase takes place. The same applies to a monotonously or strictly monotone decreasing course. Particularly preferred in this context is an advantageous embodiment the exhaust system in which at least a subset of channels of at least a honeycomb body conically widened and / or at least one more subset Channels narrowed conically. Thus, it is possible for a first subset Channels dilate conically or narrowed, while a second subset channels the cross-sectional area does not change over the axial length. It is also possible, that a first subset of channels widens conically while a second subset Subset channels conically narrowed. This advantageously allows a structural simple design of the exhaust system. It's not just conical cross-sectional area changes in the main flow direction z possible, but each monotone cross-sectional area change is possible and according to the invention.

According to yet another advantageous embodiment of the exhaust system have different subsystems channels different integrals of cross-sectional areas over the axial length L on. This allows the training of example Chambers, expansions and constrictions, with which, for example achieved good sound attenuation despite existing constructive limitations can be.

Following the inventive concept, a method for Soundproofing proposed in an exhaust system of an internal combustion engine. In this case, the exhaust system contains at least one honeycomb body of the exhaust gas permeable channels and has an axial length. A first subset of the exhaust gas is passed through a first subset of channels and a second subset of the exhaust gas is passed through a second subset of channels directed. The cross-sectional areas of at least one of the two subsets channels changes over the axial length of the honeycomb body, making a difference in the duration of the exhaust gas in the different subsets channels arises. The Subsets of the exhaust gas are behind the at least one honeycomb body again merged. This method allows according to the invention, at least to attenuate existing sound waves in the exhaust of a given frequency.

According to an advantageous embodiment of the method has the first subset Channels each have a first input cross-sectional area and a first output cross-sectional area while the second subset channels each have a second one Has input cross-sectional area and a second output cross-sectional area. The ratio of the first input cross-sectional area to the first output cross-sectional area is other than that of the second input sectional area to the second output cross-sectional area. It is particularly advantageous here that the cross-sectional area of at least a subset of channels in the main flow direction z increases, preferably increases monotonically and more preferably strictly monotone increases, while the cross-sectional area of another subset channels alternatively or additionally falls, preferably monotone and more preferably strictly monotone falls. This allows advantageously simple manner to carry out the method for soundproofing.

According to a further advantageous embodiment of the method flows through the Exhaust gas at least one honeycomb body, which channels at least a subset which widens conically and / or at least one further subset Channels narrowing conically. This simplifies the calculation and adjustment the difference in transit time.

According to yet another advantageous embodiment of the method flows through the exhaust gas has different subsets of channels, which have a different integral of Have cross-sectional area over the axial length. This allows for example the implementation of the method, for example, even under difficult geometric Conditions and limitations.

According to yet another advantageous embodiment of the method, the transit time difference of the subsets of the exhaust gas is just chosen so that when merging the at least two subsets at least partially present a destructive interference for at least one frequency. For this purpose, the transit time difference between the transit time of the first subset of exhaust gas and the transit time of the second quantity of exhaust gas for sound waves of the angular frequency ω, the wavelength λ and the phase velocity c is set precisely such that | t 1 - t 2 | = 2 n +1 2 λ c with natural numbers n holds. This requires a further phase factor in the wave equation, which is then to be represented for amplitudes A ₁ in the first subset of exhaust gas and A ₂ in the second subset of exhaust gas as follows: φ ( z ) = exp ( i ω t 1 + ikz ) [ A 1 + A 2 exp (- i (2 n +1) π)].

The amplitudes A ₁ and A ₂ are adjustable via the ratio of a first input cross-sectional area of the first subset of channels to the second input cross-sectional area of the second subset of channels. If these two amplitudes A ₁ and A _{2 are} just the same, the wave with the angular frequency ω is completely extinguished. There is destructive interference.

If the amplitudes A _{1 of} the first subset of exhaust gas and A _{2 of} the second subset of exhaust gas are not identical, damping of the sound wave and corresponding harmonics occurs in each case.

According to yet another advantageous embodiment of the method, the destructive interference just for a critical frequency. This allows the Attenuation of frequencies, e.g. for the internal combustion engine itself or are critical for the automobile powered by this. For example This may be a frequency that has resonance effects. These are usually undesirable because they represent an increased material load.

According to yet another advantageous embodiment of the method of the Delay of the subsets of the exhaust gas chosen so that when merging the at least two subsets at least partially destructive Interference for at least two frequencies is present. This allows it more advantageously Way to attenuate several critical frequencies.

Figur 1

eine schematische Zeichnung des Kanalsystems eines erfindungsgemäßen Wabenkörpers;

Figur 2

einen Ausschnitt aus der stirnseitigen Ansicht eines ersten Ausfühnmgsbeispiels eines erfindungsgemäßen Wabenkörpers;

Figur 3

eine Welllage zur Herstellung des ersten Ausführungsbeispiels eines eines erfindungsgemäßen Wabenkörpers;

Figur 4

einen Ausschnitt aus der stirnseitigen Ansicht eines zweiten Ausführungsbeispiels eines erfindungsgemäßen Wabenkörpers und

Figur 5

eine strukturierte Blechlage zur Herstellung des zweiten Ausführungsbeispiels eines erfindungsgemäßen Wabenkörpers.

Further advantages and particularly preferred embodiments of the invention will be explained in more detail with reference to the drawing, wherein the invention is not limited to the illustrated embodiments. Show it:

FIG. 1

a schematic drawing of the channel system of a honeycomb body according to the invention;

FIG. 2

a detail of the front view of a first Ausfühnmgsbeispiels of a honeycomb body according to the invention;

FIG. 3

a corrugated sheet for producing the first embodiment of a honeycomb body according to the invention;

FIG. 4

a detail of the front view of a second embodiment of a honeycomb body according to the invention and

FIG. 5

a structured sheet metal layer for producing the second embodiment of a honeycomb body according to the invention.

FIG. 1 shows a part of a honeycomb body 1 according to the invention in longitudinal section in a schematic representation. Through an input side 2, an exhaust gas stream flows 3 into the honeycomb body 1 and leaves it through an exit side 4. The honeycomb body contains two subsets of channels that pass through the Change the channel cross-sectional area over the axial length of the channels L differ. The first subset of channels consists of expanding channels 5, the first input cross-sectional area 6, which faces one of the input side 2 and a larger first output cross-sectional area 7, that of the exit side 4 of the honeycomb body 1 faces. The second subset consists of channels tapered channels 8, one of the input side 2 facing the second Input cross-sectional area 9 and a smaller second Ausgangsquerschnittsfläche 10, which faces the output side 4 of the honeycomb body 1, have. In On the one hand, this example corresponds to the first input cross-sectional area 6 of FIG second output cross-sectional area 10 and on the other hand, the second input cross-sectional area 9 of the first output cross-sectional area 7. Thus, the ratio from the first input cross-sectional area 6 and the first output cross-sectional area 7 is the reciprocal of the ratio of the second input cross-sectional area 9 and the second output cross-sectional area 10.

In each of the channels 5 and 8, the cross-sectional area change is made strictly monotone. The number of channels in both subsets of channels is the same. A first subset of exhaust gas flows through the first subset of channels and one out the other half existing second subset exhaust the second subset Channels. In the mixing zone 11 downstream of the two subsets channels the two subsets of exhaust gas are mixed and leave the honeycomb body 1 through the output side 4th

wobei für die beiden Teilmengen Abgas die Geschwindigkeitsfunktionen v(z) jeweils unterschiedlich sind. Das heißt, die erste Teilmenge Abgas benötigt für das Durchströmen der ersten Teilmenge Kanäle die Zeit t ₁, die zweite Teilmenge Abgas für das Durchströmen der zweiten Teilmenge Kanäle die Zeit t ₂. Gilt für den durch das Durchströmen der beiden Teilmengen Kanäle 5 und 8 erhaltene Laufzeitunterschied t ₁-t ₂ zwischen den beiden Teilmengen Abgas |t 1-t 2| = 2n+12 λ c mit einer ganzen Zahl n, so erhält man einen zusätzlichen Phasenfaktor. Die gesamte Wellengleichung lässt sich dann darstellen als ϕ(z) = exp(iωt 1 + ikz)[A 1 + A 2 exp(-i(2n+1)π)] wobei ω die Kreisfrequenz der Welle und A ₁ und A ₂ die Amplituden der Wellen in der ersten und der zweiten Teilmenge Abgas angeben. Wird der Abgasstrom 3 in zwei gleiche Teilmengen Abgas aufgeteilt, gilt also A ₁ = A ₂, so löscht sich die Welle vollständig aus. Sind die Amplituden A ₁ der ersten Teilmenge Abgas und A ₂ der zweiten Teilmenge Abgas nicht identisch, so kommt es in jedem Fall zu einer Dämpfung der Schallwelle mit der Wellenlänge λ und entsprechender Oberwellen. Diesen Umstand kann man dazu ausnutzen, im Abgasstrom 3 nicht nur Schallwellen einer Wellenlänge zu dämpfen, sondern vielmehr Schallwellen mehrerer Wellenlängen. Hierzu wird der Abgasstrom nicht nur durch zwei Teilmengen Kanäle geleitet, sondern durch entsprechend mehr Teilmengen, deren Kanäle entsprechend konzipiert sein müssen.If now contains the exhaust stream 3 sound waves of wavelength λ and the phase velocity c, then the intensity of the sound waves when flowing through the output side 4 of the honeycomb body 1 is usually different from the intensity when flowing into the honeycomb body 1. Each of the two partial gas streams changes its velocity by changing the channel cross-sectional areas. For the example considered here, the velocity v of the gas in the main flow direction z has an inversely proportional relationship with the cross-sectional area through which it flows. As a result, the first subset of exhaust gas slows in the expanding channels 5, while the second subset of exhaust gas accelerates in the tapered channels 8. For each of the two subsets exhaust gas, the velocity changes continuously during the flow through the two subsets channels. For the period t _{1 of} the first subset of exhaust gas and the running time t _{2 of} the second subset of exhaust gas then applies

wherein for the two subsets exhaust the speed functions v (z) are each different. That is, the first subset of exhaust gas needs for the passage of the first subset of channels time t ₁ , the second subset of exhaust gas for the passage of the second subset of channels time t ₂ . Applies to the transit time difference t ₁ -t ₂ between the two subsets of exhaust gas obtained by the passage of the two subsets channels 5 and 8 t | t 1 - t 2 | = 2 n +1 2 λ c with an integer n, one obtains an additional phase factor. The entire wave equation can then be represented as φ ( z ) = exp ( i ω t 1 + ikz ) [ A 1 + A 2 exp (- i (2 n +1) π)] where ω is the angular frequency of the shaft and A ₁ and A ₂ indicate the amplitudes of the waves in the first and the second subset of exhaust gas. If the exhaust gas stream 3 is divided into two equal subsets of exhaust gas, ie A ₁ = A ₂ , the wave extinguishes completely. If the amplitudes A _{1 of} the first subset of exhaust gas and A _{2 of} the second subset of exhaust gas are not identical, damping of the sound wave with the wavelength λ and corresponding harmonics occurs in each case. This circumstance can be exploited to attenuate not only sound waves of one wavelength in the exhaust stream 3, but rather sound waves of several wavelengths. For this purpose, the exhaust gas flow is not only passed through two subsets of channels, but by correspondingly more subsets, the channels must be designed accordingly.

FIG. 2 shows a detail of an end view of the input side 2 an embodiment of a honeycomb body according to the invention 1. This has a first subset of expanding channels 5 and a second subset tapered channels 8. The widening channels 5 each have a first smaller input cross-sectional area 6, while the tapered Channels 8 each have a second larger input cross-sectional area. 9 exhibit. The cross-sectional area change over the axial length L of the honeycomb body takes place in both subsets channels in this embodiment strictly monotone. The honeycomb body is made of alternating smooth sheet metal layers 12 and corrugated sheet metal layers 13 constructed

An embodiment of a corrugated sheet metal layer 13 is shown in FIG. 3. The corrugation height this corrugated sheet metal layer 13 changes strictly monotonously in the direction the longitudinal axis, which causes the cross-sectional area of the through the corrugated Sheet metal layer 13 with an adjacent smooth sheet metal layer 12 formed channels changes strictly monotonically in the direction of the longitudinal axis. By the combination with a neighboring smooth sheet metal layer 12 arise on the one hand channels with a first input cross-sectional area 6 and on the other hand channels with a second Input cross-sectional area 9. If the honeycomb body is constructed so that the one smooth sheet metal layer 12 adjacent corrugated sheet metal layers 13 with respect to the central axis 14 are each rotated 180 ° against each other, so can advantageously, a cylindrical honeycomb body 1 are constructed, which is expanding channels 5 and tapered channels 8 has. The expanding ones Channels 5 and the tapered channels 8 alternate in layers From, the cross-sectional area of the widening channels 5 increases strictly monotonous from the first input section 6 to the first output section 7, while the Cross-sectional area of the tapered channels 8 strictly monotone of the second Input cross section 9 to the second output section 10 falls. Such a Honeycomb body 1 advantageously has due to its layered structure of alternately expanding channels 5 and tapered channels 8 over no preferential direction with respect to its longitudinal axis, so that during installation the honeycomb body 1 no installation direction must be observed.

FIG. 4 shows a detail of a frontal schematic view of a second embodiment of a honeycomb body 6 according to the invention Honeycomb body 6 is made of substantially smooth sheet metal layers 12 and structured Sheet metal layers 13 constructed and has a first subset of expanding Channels 5 and a second subset of channels from tapered channels 8 on. The widening channels 5 have a first input cross-sectional area 6 a. By the extension of the widening channels 5 in. Main flow direction z increases the channel cross-section in this direction. Which tapered channels 8 have a second input cross-sectional area 9. Of the Channel cross-section decreases in the direction of the main flow direction z.

FIG. 5 shows a structured sheet metal layer 13, such as a honeycomb body shown in FIG having. This structured sheet metal layer 13 is characterized by this is the structure repeat length 15, which is the distance between two adjacent structure maxima 16 is defined as being across the main flow direction z, which is identical to Longitudinal axis of the structured sheet metal layer 13 is constantly changing. this leads to two subsets of channels containing the structured sheet metal layer 13 with an adjacent, not shown substantially smooth sheet metal layer 12 forms a subset Channels consists of tapered channels 8, while the other subset Channels of expanding channels 5 is as stated above, this leads with a corresponding embodiment of the structured sheet metal layers 13 to a Attenuation of sound waves of at least one frequency.

The invention makes it possible to easily existing honeycomb body anyway in the exhaust system in addition to targeted soundproofing use.

LIST OF REFERENCE NUMBERS

1

honeycombs

2

input side

3

exhaust gas flow

4

output side

5

Expanding channel

6

First input cross-sectional area

7

First output cross-sectional area

8th

Tapered channel

9

Second input cross-sectional area

10

Second exit cross-sectional area

11

mixing zone

12

smooth sheet metal layers

13

corrugated sheet metal layers

14

central axis

15

structure repeat

16

maximum structure

A ₁

Amplitude of the sound wave in the first partial gas flow

A ₂

Amplitude of the sound wave in the second partial gas flow

c

phase velocity

L

axial channel length

λ

wavelength

n

natural number

ω

Angular frequency of the sound wave

t ₁

Runtime through the first subset of channels

t ₂

Runtime through the second subset of channels

v

speed

z

Main flow direction

Claims (18)

1. A honeycomb body (1) for an exhaust system of an internal combustion engine, the honeycomb body (1) having passages (5, 8) which are substantially separate from one another and through which exhaust gas can flow substantially in a longitudinal direction of the honeycomb body (1) and having an axial length (L), characterized in that the honeycomb body (1) has at least a first subset of passages (5) and a second subset of passages (8), and in that at least the cross-sectional areas (6, 7, 9, 10) of one of the two subsets of passages (5, 8) change over the axial length (L) of the honeycomb body (1), so that the transient time of the exhaust gas (3) is different in the different subsets of passages (5, 8).
2. The honeycomb body (1) as claimed in claim 1, characterized in that the first subset of passages (5) in each case has a first entry cross-sectional area (6) and a first exit cross-sectional area (7), and the second subset of passages (8) in each case has a second entry cross-sectional area (9) and a second exit cross-sectional area (10), and the ratio of the first entry cross-sectional area (6) to the first exit cross-sectional area (6) is different than the ratio of the second entry cross-sectional area (9) to the second exit cross-sectional area (10).
3. The honeycomb body (1) as claimed in one of the preceding claims, characterized in that the cross-sectional area of at least one subset of passages (5, 8) increases, preferably increases in monotone fashion and particularly preferably increases in strictly monotone fashion, in the main direction of flow (z) and/or the cross-sectional area of at least one other subset of passages (5, 8) decreases, preferably decreases in monotone fashion and particularly preferably decreases in strictly monotone fashion, in the main direction of flow (z).
4. The honeycomb body (1) as claimed in one of the preceding claims, characterized in that at least one subset of passages (5, 8) widens conically and/or at least one further subset of passages (5, 8) narrows conically.
5. The honeycomb body as claimed in one of claims 1 or 2, characterized in that the integrals of the cross-sectional areas over the axial length (L) differ for different subsets of passages.
6. An exhaust system of an internal combustion engine, having at least one honeycomb body (1), which has passages (5, 8) through which the exhaust gas (3) can flow substantially in a longitudinal direction of the honeycomb body (1) and an axial length (L), characterized in that a flow path for a first partial quantity of the exhaust gas is formed by a first subset of passages (5), and a flow path for a second partial quantity of the exhaust gas is formed by a second subset of passages (8), the cross-sectional areas (6, 7, 9, 10) of at least one of the two subsets of passages (5, 8) changing over the axial length (L) of the honeycomb body (1), so that the transient time of the exhaust gas (3) is different in the different subsets of passages (5, 8).
7. The exhaust system as claimed in claim 6, characterized in that the first subset of passages (5) in each case has a first entry cross-sectional area (6) and a first exit cross-sectional area (7), and the second subset of passages (8) in each case has a second entry cross-sectional area (9) and a second exit cross-sectional area (10), and the ratio of the first entry cross-sectional area (6) to the first exit cross-sectional area (6) is different than the ratio of the second entry cross-sectional area (9) to the second exit cross-sectional area (10).
8. The exhaust system as claimed in claim 6 or 7, characterized in that the cross-sectional area of at least one subset of passages (5, 8) increases, preferably increases in monotone fashion and particularly preferably increases in strictly monotone fashion, in the main direction of flow (z) and/or the cross-sectional area of at least one other subset of passages (5, 8) decreases, preferably decreases in monotone fashion and particularly preferably decreases in strictly monotone fashion, in the main direction of flow (z).
9. The exhaust system as claimed in one of claims 6 to 8, characterized in that at least one subset of passages (5, 8) of the at least one honeycomb body (1) widens conically and/or at least one further subset of passages (5, 8) narrows conically.
10. The exhaust system as claimed in one of claims 6 or 7, characterized in that the integrals of the cross-sectional areas over the axial length (L) differ for different subsets of passages.
11. A method for muffling sound in an exhaust system of an internal combustion engine, the exhaust system including at least one honeycomb body (1) which has passages (5, 8) through which the exhaust gas (3) can flow substantially in a longitudinal direction of the honeycomb body (1) and an axial length (L), characterized in that a first partial quantity of the exhaust gas is passed through a first subset of passages (5) and a second partial quantity of the exhaust gas is passed through a second subset of passages (8), the cross-sectional areas (6, 7, 9, 10) of at least one of the two subsets of passages (5, 8) changing over the axial length (L) of the honeycomb body (1), resulting in a difference in the transient time of the exhaust gas (3) in the different subsets of passages (5, 8), and in that the partial quantities of the exhaust gas are combined again downstream of the at least one honeycomb body (1).
12. The method as claimed in claim 11, characterized in that the first subset of passages (5) in each case has a first entry cross-sectional area (6) and a first exit cross-sectional area (7), and the second subset of passages (8) in each case has a second entry cross-sectional area (9) and a second exit cross-sectional area (10), and the ratio of the first entry cross-sectional area (6) to the first exit cross-sectional area (7) is different than the ratio of the second entry cross-sectional area (9) to the second exit cross-sectional area (10).
13. The method as claimed in one of claims 11 or 12, characterized in that the cross-sectional area of at least one subset of passages (5, 8) increases, preferably increases in monotone fashion and particularly preferably increases in strictly monotone fashion, in the main direction of flow (z) and/or the cross-sectional area of at least one other subset of passages (5, 8) decreases, preferably decreases in monotone fashion and particularly preferably decreases in strictly monotone fashion, in the main direction of flow (z).
14. The method as claimed in one of claims 11 to 13, characterized in that the exhaust gas flows through at least one honeycomb body (1) which has at least one subset of passages (5, 8) which widen conically and/or at least one further subset of passages (5, 8) which narrow conically.
15. The method as claimed in one of claims 11 or 12, characterized in that the integrals of the cross-sectional areas over the axial length differ for different subsets of passages.
16. The method as claimed in one of claims 12 to 15, characterized in that the difference in the transient time of the partial quantities of the exhaust gas is selected in such a way that when the at least two partial quantities are combined there is at least in part a destructive interference for at least one frequency.
17. The method as claimed in claim 16, characterized in that the destructive interference occurs for a critical frequency.
18. The method as claimed in one of claims 12 to 15, characterized in that a difference in the transient time of the partial quantities of the exhaust gas is selected in such a way that when the at least two partial quantities are combined there is at least in part a destructive interference for at least two frequencies.

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