DK169823B1 - Muffler - Google Patents

Abstract

A silencer, preferably for exhaust gases from internal combustion engines, consisting of a container having a shell (1) formed essentially as a cylinder face, and two end bottoms (2, 3) as well as at least one admission pipe (4) and one discharge pipe (5) for exhaust gas, wherein the flow prior to entering the internal volume of the container is converted to a slot flow between an outer plate (6) and an inner plate (8), said plates being so shaped at their periphery as to form an exhaust gas discharge opening (14), which is positioned radially substantially symmetrically around the pressure node of a transverse oscillation in the container, said plates (6, 8) having a contour direction at the discharge opening (14) which is substantially perpendicular to the radial direction. Improved damping of transverse oscillations in the chamber of the silencer is hereby obtained by simple means.

Description

in DK 169823 B1

The present invention relates to silencers for reducing noise in flowing gas media, preferably for incorporation into exhaust pipes from internal combustion engines.

Mufflers for such applications will often be either of the type of reflection dampers, absorption dampers, or constitute a combination of these two types. Both are characterized by providing a broadband attenuation in the acoustic frequency spectrum. Such a broadband attenuation is usually required, since the attenuated spectrum of the motor may contain peaks, but at the same time contains a considerable measure of all frequencies within the audible range.

Both reflection and absorption dampers are based on silencing mechanisms associated with one or more chambers, typically in a container with an inlet and outlet pipe. In the reflection attenuator, the attenuating effect is obtained by reflecting sound energy at cross-sectional transitions between pipes 20 and chambers. The effect of the absorber is achieved by transmitting sound energy to a sound absorbing material, e.g. mineral wool in which vibrational energy is dissipated by internal friction in the gas and by interaction between the gas and the absorbent fibers.

25 The attenuation range in the frequency spectrum is different for the two types of attenuator. Since the effect of the absorption damper assumes standing waves in the absorbent, this gives a restriction down the spectrum. Also, the effect of the reflection attenuator 30 is limited downward in the spectrum, namely by the filter gene frequency. However, this is generally substantially lower. Thus, in many combustion engine applications, it is difficult to obtain sufficient low frequency attenuation with absorption dampers alone. This is significant as precisely the most powerful frequency in the attenuated spectrum, as a rule the ignition frequency of the motor (attributable to the cyclic process of the motor) will be relatively low. Thus, a pure reflection attenuator, or a combined reflection and absorption attenuator, will generally be needed.

5 The pure reflection attenuator has the weakness that in its attenuation spectrum, inconvenient passage frequencies appear, ie. dive in the attenuation spectrum. These dives can be attributed to standing gas swings in the chambers. In some cases, a dive may be so pronounced that it is even associated with negative attenuation at the characteristic frequency, ie. that there is amplification of this frequency.

By incorporating a sound absorber into a reflection damper, the detrimental effect of passage frequencies can be mitigated to a certain extent as standing oscillations in the chambers can be reduced by dissipation of oscillatory energy in the absorbent. However, the particular ground frequency of a chamber can still occur with a significant dive in the attenuation spectrum. The fundamental gene frequency of the chamber is often a fraction 20 higher in the frequency spectrum than the filter gene frequency.

A known method of counteracting this dive consists of extending the access pipe to the center of the first chamber.

Such a geometry is appropriate, since the fundamental frequency of a chamber has a pressure node in the center. This means that the vibrational energy emitted here can only excite the ground vibration in the chamber to a limited extent. Furthermore, excitation of all the higher-order intrinsic frequencies, which also have a pressure node in the center of the chamber 30, will be avoided.

However, if such positioning of pipe mouths to the chamber center is realized by a simple termination of the pipe, the positioning becomes somewhat blurry as the sound emission 35 occurs over a certain zone in the axial direction, a zone whose length is coupled to the pipe diameter.

DK 169823 B1 3

This problem is solved according to Danish Patent No. 128 427, by terminating the inlet pipe with a radial diffuser, from which the gas flow is conducted into the chamber in the form of a thin veil, the extent of which in the axial direction is very small and therefore makes an accurate positioning to the pressure node. possible.

The change of direction associated with the radial diffuser, from axial to radial flow, need not involve substantial irreversibility in the flow. If the geometry of the diffuser is appropriate, dissolution in the flow can be avoided.

A further advantage of utilizing radial diffusers in 15 reflection silencers according to DK Patent No. 128 427 is that the transverse plate of the diffuser reflects sound. Thereby, a noise reducing effect is obtained which is added to the reflection effect conditioned initially by the cross-sectional transition (from pipe to chamber) and to the above mentioned effect which can be obtained by pressure node positioning.

The present invention is based on the realization that reflection silencers with radial diffusers allow for almost complete elimination of pass frequencies corresponding to standing gas oscillations axially in the chambers, but not to standing gas oscillations across them. Admittedly, in some applications this is not to be considered a serious disadvantage, namely when the built-in conditions make it natural to design the damper 30 elongated so that the standing transverse waves correspond to relatively high frequencies which can be reduced relatively effectively by means of the chambers built into lydabsorben-ter.

35 However, applications of silencers are provided, e.g. where the mounting conditions make it necessary or appropriate to choose a shorter version with a container diameter which is large in relation to the pipe diameter. Admittedly, in such cases the use of a radial diffuser will involve reflection of sound at the transverse wall of the diffuser and pressure recovery, two of the properties which, as described above, are fortunate. In contrast, positioning to the center of the chamber in the axial direction will, in principle, be appropriate, but of less importance compared to the unfortunate fact that it is not possible to accurately position the node for the, in this case, due to lower frequency, more severe ground frequency. for standing waves across the chamber.

Thus, a greater significance of transverse fluctuations for passage frequencies is found in short silencers. But even in 15 cases where the length of the muffler is slightly larger than the diameter, the ratio can be significant, namely in the case that the muffler contains several chambers, one or more of which is shorter than the diameter.

In addition, e.g. according to French Patent No. 800850, a silencer of the kind specified in the preamble of claim 1. This muffler is provided with a slit outlet along the shroud walls of the muffler, which means that the gas flow which is conducted into the chamber in the form of a thin 25 veil will be able to excite especially transverse fluctuations with the fundamental frequency and lower-frequency eigenfrequencies, with these particular frequencies being excited around .a. the wrapper of the container. Since all the symmetrical transverse oscillations of the rotations have a maximum pressure on the inside of the casing, they will thereby be excited. Of this, in principle, the infinite amount of waveforms, it will in practice be first and foremost the lower order forms, and in particular the fundamental oscillation in the transverse direction, which for this reason can give rise to bothersome passage frequencies.

35

It is therefore the object of the invention to provide a silencer which exhibits improved properties with respect to damping of transverse oscillations in the chamber. This is achieved by the features of claim 1.

Thus, since the exhaust opening for the flue gas is positioned radially substantially symmetrical about the pressure node of a transverse oscillation in the container, and so that the outer plate at the outlet opening is spaced from the envelope, it is possible to design a silencer which exhibits improved properties. with regard to attenuation of transverse oscillations in the chamber, especially as regards lower order transverse oscillations.

In accordance with claim 2, at the same time as attenuation of transverse oscillations, a reasonable damping of axial oscillations in the container is obtained.

As stated in claim 3, a diffuser effect of the media flow is obtained, and thus a low back pressure for the silencer.

By constructing the outlet opening as claimed in claim 4, an increased internal reflection of the sound waves is obtained, with the resultant improved sound attenuation.

In the claim 5 or 6, a special production-friendly and thus inexpensive embodiment of the invention is disclosed.

By incorporating a catalyst element into the silencer, as set forth in claim 8, good utilization of the total surface of the catalyst is achieved.

Particularly convenient embodiments of the invention are explained below with reference to the drawing, in which: 1 is an outline sectional view of a rotationally symmetrical embodiment of the invention; FIG. 2 shows another embodiment of the invention, in a relatively flat embodiment; FIG. 3 shows a pronounced flat design, with reverse flow direction; FIG. 4 shows a fourth embodiment with a crackless deflection of the flue gas; FIG. 5 shows an alternative embodiment of the invention; FIG. 5a shows a section at A-A in FIG. 5, FIG. 6 shows a multi-chamber embodiment of the invention.

15

FIG. 1 shows an axial section in a rotationally symmetrical embodiment of the invention. The muffler is defined here by a cylindrical casing 1 and by end bottoms 2 and 3. The gas flow is conducted into the damper from the inlet pipe 4 and is led away from the damper of the outlet pipe 5. The double deflecting element is formed by an outer plate 6, which at the contour K forms a abrupt cracking to form the cover plate 7, in addition to the curved inner plate 8 and a plurality of radial ribs 9 welded to both the cover plate 7 and the inner plate 25 8, thereby retaining the latter. Outside of the inlet tube 4 and behind the inner plate 8, sound absorber 10 and 11, respectively, are provided, protected by perforated plates 12 and 13.

30 In the figure, pressure oscillation modes for ground resonant frequencies in both longitudinal and transverse directions are plotted. This shows the positioning of the gas flow to the pressure node.

Namely, in a cylindrical chamber, the node appears approximately two-thirds of the radius, calculated from the center axis toward the inner contour of the envelope. More precisely, the location can be calculated at 0.63 times the radius. This result is obtained by solving the partial differential equation called the wave equation, which describes the rotationally symmetric, three-dimensional gas oscillation field in the chamber.

5 One could immediately think that the double deflection should be associated with considerable irreversibility, ie. contribute significantly to increasing the silencer's overall flow resistance. However, a closer analysis of the flow field in the flow element shows that the dual deflection can be accomplished with a remarkable degree of loss of freedom. This lucky property can be attributed both to the rotational symmetry of the flow element and to the fact that both the center plate C and the contour K constitute stag-nation singularities in the flow field. This can be explained as follows: In many types of pipe elements, deflection of the flow causes a considerable loss of friction. This is e.g. the case of a 90 ° pipe bend, although this occurs with a center line of circular arc shape, ie. without cracking. However, when significant losses occur, this is due to the fact that secondary currents occur in the bend, ie. center axis swirls parallel to the center axis of the tube axis. Of these vertebrae, significant internal shock losses occur in the flow field. In the double-deflecting flow element of FIG. 1, the rotational symmetry causes such secondary flow phenomena to be completely avoided. Also, by appropriately designing the double deflecting element (e.g., by a geometry as shown in Fig. 1), the type of swirls which in less suitably designed diffusers (e.g. with 30 for sudden area expansion) can also be avoided occur due to flow dissolution along the contour wall of the diffuser.

Thus, although eddy formation can be avoided in the double-deflecting element, in most cases the flow will be turbulent, i.e. the smooth flow along flow lines will be overlaid by random particle movements in all directions. The mean path length of such movements characterizes the degree of turbulence in the flow. This degree of turbulence is somewhat larger in the double bending end than in straight pipe flow, which implies a somewhat greater frictional loss. However, unlike vortex flow, this loss is useful in the sense that it contributes to the resistive acoustic resistance of the element, ie. is associated with a sound attenuating effect. It can be expressed that there is a controlled turbulence degree in the double deflection element.

10

To explain the above stagnation singularities in the flow, the following must be stated: In the center C of the inner plate there is a point-shaped singularity; here the gas stands still. Along the central axis 15 of the inlet pipe, the flow is gradually slowed on its way towards C; this braking is almost completely loss-free (reversible). Flow of gas particles along lines beginning at a slightly larger radius will also slow down on the way to the inner plate, but not completely. Shortly before the inner plate, the particles will deflect to preferably radial flow direction, immediately after accelerating in the radial direction. Deflection thus occurs at low flow velocity, which helps to explain the low deflection loss.

The singularity C is also present in a radial diffuser, and the just-explained explanation for the slight deflection loss upon transition from axial to radial flow is well known to some of the specialists in flow technology. By contrast, not even described in the specialized flow literature so far, however, is the corresponding stagnation effect that occurs at the contour K of the double-deflecting flow element of FIG. 1. Here, most of the radial flow on the road up to the cover plate 7 will be reversed almost reversibly so that, at low speed, 35 of bends towards renewed axial flow, thereby also achieving an almost lossless deflection of the flow.

DK 169823 B1 9

FIG. 2 shows another distinctive flat embodiment of the invention. Here, the outer plate is omitted, the inlet end bottom 2 having the dual function of forming part of the silencer's limitation towards the surroundings, and of constituting 5 the flow conductive outer plate. Another difference from the embodiment of FIG. 1, the discharge pipe 5 is shown to be lateral to the longitudinal axis of the otherwise rotationally symmetric damper. An embodiment according to FIG. 2 can e.g. may be appropriate in the case of a damper placed under the engine of a truck with downwardly inlet pipe from the engine, the continuation of the exhaust pipe being horizontal along the undercarriage.

The embodiment can e.g. as indicated in the figure, 15 is combined with an elongated damper of known type.

FIG. 3 shows a third similarly pronounced flat embodiment according to the invention. In this embodiment, the inner plate coincides with the second end bottom 3 in such a way that the double directional change of gas flow results in blurry inflow to the chamber directly against the flow direction in the inlet pipe. Here, directional changes will result in a particularly effective sound reflection in the double-deflecting flow element. Also, the embodiment of FIG.

25 3 is almost rotationally symmetrical; the only deviation from rotational symmetry is that the outlet tube 5 is attached to the end bottom 2 at a certain radius.

The embodiment of FIG. 3 may, for example, be convenient for the placement of a silencer under a truck engine, as in FIG. 2, but where the exhaust system continuation is upwards e.g. to an opening at the height of the roof of the cab. In the upwardly directed discharge pipe, in some cases, as indicated in FIG. 3, incorporate an elongated elongated damper of known type.

DK 169823 B1 10

FIG. 4 shows a fourth embodiment according to the invention. This embodiment differs from those described above in that the cover plate constitutes a crack-free continuation of the outer plate 6 so that the contour K lapses. Thereby, the sound-reflective effect of the double-deflecting pipe element is reduced somewhat. But for example. in the case of large soot gas flows, this design may be motivated to prevent soot accumulation in the corner present in the double-deflecting flow element when performing with the contour K.

FIG. 5 and 5a show an embodiment of the invention, wherein the double-deflecting flow element has a fork shape which allows a two-part tangential veil upon inflow to a silencer chamber. In all of the variants described above, the veil flow is axial. Design with tangential veil may be appropriate in cases where the built-in conditions indicate that the access pipe must be inserted transversely of the casing 1, and not via the end floor 2.

20

The tangential veil flow in the embodiment according to FIG.

5 implies a slightly poorer position for the pressure node for transverse oscillations in the chamber, compared to axial blur designs. The reason for this is that the slurry, which has a length of the order of sometimes the slurry width, in the middle follows a cylinder surface having a somewhat varying radius, calculated in relation to a cylinder surface concentric with the scoop. However, when the shroud diameter is not too small, this deterioration of the positioning does not have much significance. It can be reduced without exaggerating the slightly tangential outflow to the chamber (as shown in FIG.

5) so that the center of the veil (calculated in the axial direction of the supply pipe) lies on the cylinder surface of the pressure node in the chamber.

11 DK 169823 B1

FIG. 6 shows an example of a two-chamber embodiment. The last of the chambers here utilizes a flow element of the same type as in FIG. 3, while in the first chamber there is a further variant of a flow element 5 according to the invention. Here, a ring channel (15) is inserted between the inlet pipe (4) and the outlet opening (14). This variant allows for axially flowing veil flow to the first chamber, although the axis of the supply pipe is transverse to the container axis (as in Fig. 5). In the annular channel, a peripheral flow takes place whereby the flue gas can be distributed evenly around the periphery without the major loss of total pressure before the axial inflow to the gap at the outlet opening.

The examples shown so far of embodiments of the invention are constructed with circular cylindrical sheaths and with a considerable degree of rotational symmetry. However, the basic ideas of the invention are not bound to the circular cylinder shape. The scourge can e.g. be conical, or elliptical.

This may e.g. be applicable in applications where the built-in conditions indicate that the muffler should have a flat shape.

In addition, it is possible to incorporate a catalyst or a heat exchanger such that these are located between the outlet opening (14) and the outlet pipe (5). Hereby the geometry of the outlet opening (14) causes a particularly good efficiency of the catalyst or heat exchanger, as these are provided with a uniform flow of flue gases 30 over the entire active surface.

35

Claims (9)

1. Silencer preferably for flue gases from internal combustion engines, consisting of a container with wrapper (1) formed essentially as a cylinder surface, and two end bottoms (2, 3) and at least one inlet pipe (4) and one outlet pipe (5). ) for flue gas, where the flow prior to the inflow to the inner volume of the container is converted into a gap flow 10 between an outer plate (6) and an inner plate (8), and where the plates (6, 8) are formed at their periphery to form a discharge opening (14) for the flue gas having a contour direction substantially perpendicular to the radial direction, characterized in that the outlet opening (14) is positioned radially substantially symmetrically about the pressure node for a transverse oscillation in the container, and thus the outer plate (6) at the outlet opening (14) is located at a distance from the shroud (1). Silencer according to claim 1, characterized in that the outlet opening (14) is positioned axially around the pressure node for an axial oscillation in the container. Silencer according to claim 1 or 2, characterized in that the total cross-sectional area of the outlet opening is larger than the cross-sectional area of the inlet pipe. Silencer according to one or more of the preceding claims, characterized in that the outer plate (6) at a point between the supply pipe (4) and the outlet opening (14) has an abrupt change of direction in the contour of the plate (6), the cover plate (7). Silencer according to one or more of the preceding claims, characterized in that the outer plate is formed wholly or partially by the container end bottom at the inlet pipe. 13 DK 169823 B1 Silencer according to one or more of claims 1 to 4, characterized in that the inner plate is formed wholly or partially by the container bottom towards the supply pipe. 5 Silencer according to one or more of the preceding claims, characterized in that a ring channel (15) is connected between the inlet pipe (4) and the outlet opening (14) to which the inlet pipe (4) is connected. 10 Silencer according to one or more of the preceding claims, characterized in that a catalyst element is built into the container, between the outlet opening and the outlet pipe. 15 Silencer according to one or more of the preceding claims, characterized in that the container sleeve (1) is a circular cylindrical tube. Silencer according to one or more of claims 1-8, characterized in that the container sleeve (1) is an elliptical tube. 25 30 35