CN1682015A - Sound damper and structural design method therefor - Google Patents

Abstract

The invention relates to a silencer comprising a silencer housing (1) and at least one inlet (3) and one outlet (4) for a waste gas. At least one functional surface (8) and at least one bead (5, 6, 7) are sunk into the housing surface (2), and the beads (5, 6,7) have at least one specific irregularity in the form and/or arrangement thereof for reducing the sound radiation. The invention also relates to a method for designing a silencer.

Description

Muffler and its structural design method

The invention relates to a muffler and a method of designing the muffler according to the preambles of the independent claims.

It is already known to provide mufflers in the exhaust system of motor vehicles. The exhaust gas from the internal combustion engine is discharged into the free atmosphere through the exhaust system. This work, in addition to avoiding pollution damage due to gaseous and solid harmful substances, should also be as noiseless as possible. For reasons of environmental protection, therefore, the permissible noise emissions are increasingly restricted by legislative measures.

Furthermore, another important economic objective for an exhaust system is low price. The combination of low weight and compact dimensions, which are also important in motor vehicles, is gaining more and more significance. Depending on the vehicle and engine size, a complete exhaust system from exhaust elbow to end pipe can weigh between 20 and 45 kg. All components affect the acoustic performance of the exhaust system, of course in completely different ways. In this, not only the objective evaluation of the noise reflection of the exhaust openings and surfaces of the exhaust system, which is known to be important for the noise emission when accelerating overtaking - but also the observer's objective perception of the acoustic image plays an important role.

The acoustic behavior of the exhaust system is largely determined by the structural dimensioning of the exhaust system, in addition to the load-change noise of the engine. In particular the symmetry of the fork pipes, the number and position of the silencers and the diameter of the connecting pipes all play an important role here. In the case of poor design, the exhaust system can generate increased noise. The increasingly common multi-valve technology of transmissions leads to a marked increase in the noise level entering the exhaust system. However, the further reduction of the noise limit value for accelerating overtaking in the future will require a significant reduction in exhaust noise. These required reductions in the noise level are achieved today essentially by increasing the volume of the silencer, but this quickly reaches the limits of the available installation space. Therefore, in order to make optimum use of the installation space available in the underbody module, mufflers of shell construction are increasingly being used even in medium-sized motor vehicles. These housing silencers basically have a large flat surface, which facilitates noise reflection. Such a casing muffler is described, for example, in the engine technology magazine MTZ Annual Publication 62, 2001, Vol. 3, pp. 3 ff.

For these reasons, the surface of the exhaust system as a noise source has taken on significant significance in the design of the exhaust system in recent years. In the case of engines with high specific power and poor muffler geometry, surface noise reflections may even predominate. The noise emission and therefore the general layout of the exhaust system must be evaluated and optimally set. If this work is not done, there is a great risk that due to the poor distribution of the muffler volume and the structural configuration of the muffler, there will be significantly more volume than required, or in relation to the exhaust backpressure of the internal combustion engine of the motor vehicle and thus the power Loss tolerance becomes necessary.

The increase in volume additionally leads to an undesired increase in weight. Furthermore, such a general layout of the exhaust system significantly influences not only the noise emission via the volume distribution and the length of the connecting pipes, but also the natural resonance behavior of the mechanical vibrations via the associated mass distribution and overall mass. This affects both the vibration fatigue strength of the equipment and the noise strength in the cabin. In addition, this tendency has long been attempted to reduce the surface-to-volume ratio of the muffler due to structural space constraints.

Not only the body noise but also the noise reflections depend on the physical parameters of the structural elements of the muffler. These parameters are the bending stiffness and mass per unit length of the structural member, which are determined by the structural member geometry. Likewise, material properties such as spring modulus and thickness - which are predetermined by the applied material and therefore cannot be significantly changed - also have an influence. The damping of structural members is a basic parameter, but it is difficult to make a quantitative evaluation. Variations in certain dimensions can cause different degrees of bulk noise and reflections. An increase in the bending stiffness will therefore generally lead to a reduction in the body noise level, but generally to an increase in the reflectivity, with the body noise level mostly predominating. Even for simple geometrical shapes, the modeling and prediction of subsequent structural component behavior is difficult because these parameters interact with each other in a complex manner.

In order to increase the bending stiffness, it is known to introduce so-called grooves into the surface of the muffler. It is also known that the grooves have different shapes, such as worm grooves or golf bag shapes. Earthworm grooves are elongated tissue structures that are shaped like an earthworm, while golf ballsicken are protruding or concave hemispherical structures in the form of golf balls. Such golf bags are known, for example, from automotive industry magazines.

In the same document DE 198 49 118 A1 an exhaust gas muffler is known which is produced from half-shells which have an essentially flat bottom. A plurality of outwardly directed elongated protrusions or grooves are provided in the bottom of the upper shell. On the one hand, these grooves strengthen the rigidity of the housing and as mentioned above are beneficial to reduce the noise excitation caused by vibration. On the other hand, the grooves form corresponding protrusions in the inner cavity of the muffler. The side of the bottom plane creates additional volume.

The object of the present invention is to provide a muffler with further improved damping properties and a method for its structural design.

This object is solved by the features of the independent claims.

According to the invention, an optimal grooved housing surface is provided, wherein the configuration and/or structural arrangement of the grooves is characterized by suitable inhomogeneities for reducing noise reflections from the sound absorber.

Preferably, the groove of the present invention generally does not have a line of symmetry in a cross-sectional plane parallel to the surface of the muffler housing. It is also preferred that the surface fraction of the individual grooves has a very wide distribution compared to the total surface of the sound absorber. Furthermore, it is advantageous to provide a broad, preferably continuous, curved profile of the groove envelope.

Further advantages and advantageous configurations of the invention can be taken independently from the claims, the description and the drawing.

The invention will be explained in detail with the aid of the accompanying drawings, wherein the accompanying drawings show:

FIG. 1 is a view from above obliquely onto a muffler with a preferred groove (Sicken), which is depicted by the same hatching;

Fig. 2 is a view obliquely from below to the muffler with the preferred groove in Fig. 1;

Figure 3 is a view of the interior of a muffler with functional surfaces dividing the muffler into segments;

Fig. 4 is a sectional view through a typical earthworm groove;

Figure 5 is the distribution of the preferred number of preferred grooves as a function of the number of grooves;

Figure 6 is the distribution of the preferred number of preferred grooves relative to the total surface of a muffler as a function of the number of grooves;

FIG. 7 is a preferred distribution of the preferred grooves relative to the surface portion of the grooves;

Fig. 8 is the comparison of the noise level under the situation of the typical groove (a ₀ , b ₀ , c ₀ ) and the groove of the present invention (a ₁ , b ₁ , c ₁ );

The function of the known grooves on a muffler housing is primarily to increase the bending stiffness of the housing and to increase the flow-guiding effect. In addition, usually the surfaces of the muffler housing are separated by functional surfaces. These partial regions of the housing surface are generally used as seats for the bottom. These subregions then each act as reflection chambers inside the muffler, in which the noise is to be damped and generally there is parallelism between the bottom seats. In order to increase the bending stiffness, a minimum number of bottoms should be provided, in which regular grooves are embedded. In contrast, the configuration and/or arrangement of the grooves according to the invention can be determined according to a preferred number of optimization methods, as a result of which the smallest possible noise reflection of the sound absorber is achieved.

In the optimization method according to the invention, a detection of the number of gas pulses in the silencer, ie a detection of the pressure field p(x,t,u(x)), is first carried out. where x represents a position on the inner surface of a muffler housing. Since the wall thickness of the muffler is small compared to the characteristic dimensions such as length and width and height, the difference between the inner and outer surface is preferably negligible for the value of x. In addition, t represents time, and u(x) represents a displacement vector, that is, a displacement vector of the outer surface of a grooved muffler relative to the outer surface of a non-grooved muffler. The parameter u(x) thus defines the groove. It is known empirically that the correlation of the displacement vector u(x) is often negligible for the pressure signal. Based on the pressure field p(x,t,u), a velocity field v(x,t,u(x)) on the outer surface of the muffler is derived. This velocity field v(x,t,u(x)) leads to the emission of noise. A hitherto optimal housing surface is characterized in that the noise emission is reduced or limited to a permissible value.

In Fig. 1, a muffler according to the invention is depicted when viewed obliquely from above, with a base body comprising two half-shells and grooves in a known manner, while Fig. 2 shows the bottom of the muffler in Fig. 1 A view from the side. The groove can both extend into the interior of the muffler and extend out from the muffler. The two half-shells of the basic body defined in this preferred manner can be designed as single-walled or double-walled.

The muffler comprises a muffler housing 1 with at least one inlet 3 and one outlet 4 for exhaust gas, corresponding to which one or more common perforated pipes extending inside the muffler can be respectively assigned. One or more functional surfaces 8 and one or more recesses 5 , 6 , 7 are embedded in the housing surface 2 . For reasons of clarity, not all existing grooves are shown, but they should be easily identifiable in the drawings due to their irregular shape.

The functional surface 8 divides the muffler into three inner sections 9 , to which the end pieces of the muffler 1 are also adjoined, which are likewise provided with recesses 5 , 7 . The functional surfaces 8 are groove-shaped, but their task is primarily to ensure a bottom seat for the muffler.

FIG. 3 shows an internal view of the muffler 1 without the housing 2 , so that only the internally running pipes 11 for guiding the exhaust gases and the frame for the base 10 are visible. The positions of the bottom 10 and the functional surface 8 are correlated and define a segment 9 therebetween. The exhaust pulse in the pipe 11 is the source of the reflected noise. Depending on the housing surrounding the arrangement, more or less noise reflections will occur.

Typical grooves known in the prior art are characterized by a structural configuration with a highly symmetrical profile, such as a trapezoidal groove whose cross-section is trapezoidal or eg the earthworm groove. In the case of a typical earthworm groove it is easy to find a continuous line of symmetry whose distance to the edge of the groove is characterized in such a way that with respect to a point on the line of symmetry two opposite edge points can be found which are to The standard distances of this point on the line of symmetry are equally large, the individual surfaces being considered parallel to the housing surface 1 .

In the case of an earthworm groove depicted in Fig. 4a, the connecting line between the two edge points _R1 and _R2 and the point P on the line of symmetry SP is the tangent to the groove edge curve at these two edge points T ₁ and T ₂ generate two included angles α ₁ and β ₁ and α ₂ and β ₂ respectively. These included angles α, β are generally characterized by α≤90° (±0,5°) and β≥90° (±0,5°), and α+β=180° (±1°). Furthermore, such known grooves generally have the property that α ₁ =α ₂ (±0,5°), while this is generally not the case in the optimized grooves 5 , 6 , 7 according to the invention . Instead, the angles α, β fluctuate in the case of the worm groove according to the invention, so that α ₁ , α ₂ differ with an offset of at least ±1°, preferably ±10°. Alternatively or additionally, the angles α ₁ , and α ₂ may be different and have an offset of ±5°, preferably ±10°.

In Fig. 4b a sectional view along section A-A' of the earthworm groove 4a is shown. In this case, the inner contour IK of the worm groove has an approximately circular shape. However, in the prior art there are also triangular or similar designs. According to the invention the inner contour of a recess is undulating. For example, a variable bending radius can be set. In particular the inner profile of the cross-section can be undulating along a longitudinal line L, which is a longitudinal extension of the groove, generally arranged asymmetrically with respect to the edge curve of the groove, characterizing the Parametric curve for the groove length feature. Experience has shown that generally such inhomogeneity-softening (aufweichend) grooves account for a minimum share of the total number of grooves, which is then necessary in order to achieve an effective reduction of noise reflections, so that, for example, in an end region For example, in FIG. 4 a a recess with an inhomogeneity in the end region E would not be able to achieve the desired damping effect.

In addition to grooves with longitudinal symmetry, there are also grooves with rotational symmetry in the prior art, such as the already mentioned golf ball grooves. Among them, the symmetrical elements are aligned by rotating around a symmetrical axis. According to the invention, an increase in the noise damping effect is achieved when such grooves are subjected to increased inhomogeneities as required by the fluctuation of the angle of symmetry. It is known that the line of symmetry of the groove, depending on the type of groove, can also decay to a point of symmetry, for example in golf grooves, since a golf groove can have as many symmetrical surfaces as desired.

Furthermore, bilaterally symmetrical grooves are known which can be divided by a plane into two mirror-symmetrical halves. Accordingly, the inventive groove is characterized at least by the undulation of the symmetrical elements of such typical grooves.

Furthermore, in known grooves, the deviation of the orthogonal distance between two edge points, taking into account the points on the line of symmetry, is at most ±10%, preferably ±5%, of the characteristic groove width. %, particularly preferably ±1%. These offsets may arise, for example, due to manufacturing tolerances and are not suitable according to the invention.

This known groove is also characterized in that its contour is formed by straight sections and substantially uniformly curved regions, while in this characteristic case the curvature of the base body is not taken into account. The radii of the surrounding edges are thus set in such a way that they are actually only set to a few intermittent values.

In contrast to this, the configuration and/or arrangement of the grooves 5 , 6 , 7 according to the invention has purpose-required inhomogeneities. The inhomogeneity according to the purpose requirements of the grooves 5, 6, 7 can have one or more of the following property groups according to the invention:

- The configuration symmetry of the grooves 5, 6, 7 is reduced. In particular the grooves 5 , 6 , 7 are substantially free of a line of symmetry in a section plane parallel to the housing surface 2 with respect to their envelope curve in this section plane. Preferably there is no axis or line of symmetry between 10% and 90% of the grooves 5,6,7. According to the purpose, the calibration should be carried out in terms of flatness, since the groove is spatially structured and the line of symmetry of the groove is formed by locally symmetrical points according to the prior art.

- The surface proportions of the individual grooves 5 , 6 , 7 relative to the surface of the housing surface 2 have a non-linear envelope distribution, such as a Gaussian distribution or a Poisson distribution or the like.

- The peripheral curve of the grooves 5, 6, 7 has a substantially continuous curved structure. In this case, the curved portion of the peripheral edges of the grooves 5 , 6 , 7 is preferably at least 30% relative to the individual grooves. The straight-line proportion of the peripheral curve is preferably at least 10%, wherein by "straight" is preferably understood to mean at least 1 cm in length and at the same time deviations of up to 1 mm over 1 cm should be perceived. As a result, the dome of the base body of the sound absorber 1 is advantageously removed.

- The indentation depths of the grooves 5, 6, 7 relative to the housing surfaces fluctuate, in particular 80% of the surface on the bottom of the grooves have a depth of 30%, 40%, 50% of the deepest indentation depths Or a 60% change in height. The grooves 5 , 6 , 7 are not rectilinear strip grooves, in particular they do not have a line of symmetry.

Preferably, the recess is arranged in the housing surface with a purpose-required inhomogeneity in order to reduce noise reflections. At the same time this arrangement can be realized along a longitudinal or transverse direction of the muffler housing (1). Depending on the position of the recess on the housing surface, in particular the bending stiffness changes even if the geometry of the other structural parts remains unchanged.

Apart from these grooves, which have a surface that is simply associated with an irregular contour in a section parallel to the housing surface, the other grooves according to the invention have a higher-correlation topology. In FIG. 1 , a groove with an enclave is denoted by Sk, in which an ungrooved island is arranged inside a groove. These surface regions are characterized here as ungrooved, which belong to the original surface of the silencer 1 . In the simplest case a circular structure can thus be produced, but at the same time the inhomogeneity is achieved by the outer or inner contour of the groove and by the contour of the enclave or island or by both. It is also possible to provide complex shapes with more than one ungrooved enclave.

In a preferred configuration of the invention, the surface proportion of the individual grooves 5, 6, 7 fluctuates in proportion to the total surface of all grooves 5, 6, 7, especially if there are such individual grooves 5, 6, 7, which is clearly raised in its surface relative to the surface of the other grooves 5, 6, 7. This is depicted in Figure 5. Most of the grooves 5, 6, 7 therefore have a relatively small surface. In this example 25 of the 28 grooves have a surface below 100 cm ² , while a few individual grooves have significantly larger surfaces up to 250 cm ² .

The curvature of the envelope curves of the grooves 5 , 6 , 7 is lower than in the other embodiment with a wider distribution and is not characterized by a few discrete values. For example, at least 2, 3 or 4 different values of the bending radius are set. The bending radii here have different values when they lie outside a predetermined tolerance range. For example offset values are 0,5 mm, 1 mm, 2 mm, 3 mm or 5 mm, especially for grooves with one surface > 1 cm ² . Further preferably, about 30% of the grooves 5 , 6 , 7 with a surface >1.5 cm ² have a bending radius of ratio >1.5.

The surface proportion of all grooves 5 , 6 , 7 is preferably at most 50%, particularly preferably at most 30%, compared to the entire surface of the silencer 1 . The surface portion of the grooves on the individual bases 9 bounded by the functional surfaces 8 can be between approximately 3% and approximately 70%, preferably between 5% and approximately 60%, of the corresponding outer surface of the housing 1 .

The surface of the individual grooves 5 , 6 , 7 is located between 0,05% and 10%, preferably between 0,1% and 5%, of the entire surface of the muffler. This is depicted in Figure 4. The majority of the grooves, in this example approximately 25 of the 28 grooves, have a surface fraction of at most 2% of the entire surface, while a few fewer grooves have a significantly greater surface fraction of up to 5%. The absolute number of grooves can of course be different for different mufflers and be determined individually.

In FIG. 5 , the preferred distribution of the inventive grooves relative to the groove surface fraction is shown. The abscissa indicates the relative groove number S, and the ordinate indicates the associated relative surface fraction. The totality of all grooves is characterized by a surface fraction of 1 by the point S/F=1. A straight line distribution results with grooves of the same size. But preferably there is a sub-linear distribution of surface versus number of grooves, as can be seen on the curve below the line. A surface fraction of 0,1 is generated in the case of a scale of 0,4. This means that 25% of the grooves according to the invention occupy a surface fraction which is slightly less than 0,1 of the entire groove surface of the grooves according to the invention. A surface fraction of 0,6 is obtained at a value of F/S=0,85. This means that approximately 75% of the grooves occupy a surface share of 0,6.

Advantageously, the surface of an individual groove 5 , 6 , 7 has a deviation of more than 50% from the mean value relative to the entire surface of all grooves 5 , 6 , 7 .

It is also advantageous if the percentage distribution of the surface of the individual recesses 5 , 6 , 7 is non-linear relative to the entire surface of the housing surface 2 .

In a purposeful arrangement of the grooves 5, 6, 7, these grooves are mutually asymmetrical on the two oppositely facing upper and lower housings of the housing surface 2, as seen from FIGS. 1 and 2 It can be clearly seen in the comparison of the views.

In the transition region between the recess and the housing surface ( 2 ), in the case of the recess according to the invention, steps are provided which can have inhomogeneities in order to reduce the noise reflection. When the groove reaches or exceeds a certain predetermined depth, it is preferred to have a bottom substantially parallel to the housing surface (2). However, grooves with an inclined bottom are also provided, in particular for grooves of lesser depth.

FIG. 8 schematically shows the noise reflection of a muffler according to the invention as a function of engine speed. The functional surface 8 divides the muffler housing 1 into three regions. They each have grooves. Among them, the curves a ₀ and a ₁ express the noise reflection in the left area of the muffler shell 1, b ₀ and b ₁ express the noise reflection in the right area, and c ₀ and c ₁ express the noise reflection in the middle area. The noise reflections are determined at a distance from the silencer housing 1 which is sufficient to distinguish acoustically the different regions of the silencer, wherein a noise reflection occurs with a mixing frequency of around 250 Hz. This family of curves a ₀ , b ₀ , c ₀ is obtained by a muffler provided with generally customary grooves. All three curves climb monotonically with increasing average engine speed, e.g. a ₀ , b ₀ in this example from about 70 to a maximum of 100 dB between 1000 and 5000 l/min, where the middle region produces a ratio The noise reflection is about 5 dB smaller for those two end regions whose noise reflection is about the same size.

If a muffler housing structure of the same type is provided with the grooves 5 , 6 , 7 according to the invention, the noise reflections are significantly reduced. The noise reflection according to the family of curves a ₁ , b ₁ , c ₁ is about 10 dB smaller on average than the family of curves a ₀ , b ₀ , c ₀ with known grooves. In order to produce a damping effect of a comparable noise reflection level, it is sufficient for a muffler according to the invention to have a smaller number of bottom structures than mufflers of typical embodiments. This simplifies production, with a corresponding potential for savings in terms of manufacturing costs, processing time and tooling. Preferably, the number of bottoms 9 can be reduced by at least one bottom 9 compared to an arrangement with regular grooves. On the other hand, with the same number of bases, noise reflections can be significantly reduced by means of the grooves 5 , 6 , 7 according to the invention.

Claims (15)

1. A muffler with a muffler housing (1) and at least one inlet opening (3) and at least one outlet opening (4) for exhaust gases, wherein the housing surface (2) has one or more grooves (5,6,7), characterized in that: The configuration and/or arrangement of the grooves (5, 6, 7) have suitable inhomogeneities for reducing the noise reflection. 2. The muffler according to claim 1, characterized in that: The suitable non-uniformity relates to one or more of the following characteristics of the groove: - configuration or symmetry of the grooves (5, 6, 7), - the size distribution of the surface of the individual grooves (5, 6, 7) relative to the housing surface (2), - curved distribution of the peripheral curves of the grooves (5, 6, 7), - Distribution of the grooves (5, 6, 7) on the housing surface (2). 3. The muffler according to claim 1 or 2, characterized in that: between 5% and 100% of the grooves (5, 6, 7) in a sectional plane parallel to the housing surface (2) are substantially free of symmetry with respect to their envelope curve in this sectional plane or no point of symmetry. 4. The muffler according to at least one of claims 1 to 3, characterized in that: 5% and 100% of the number of grooves (5, 6, 7) have a curved configuration with at least 2, 3, or 4 different bending radii or a substantially continuously distributed envelope curve. 5. The muffler according to at least one of the preceding claims, characterized in that: The arrangement of the grooves (5, 6, 7) is different on the upper and lower housing surface (2). 6. The muffler according to at least one of the preceding claims, characterized in that: The surface of a single groove (5, 6, 7) has a deviation of more than 50% of the mean value with respect to the total surface of all grooves (5, 6, 7). 7. The muffler according to at least one of the preceding claims, characterized in that: The percentage distribution of the surface of the individual grooves (5, 6, 7) relative to the total surface of the housing surface (2) is non-linear. 8. The muffler according to at least one of the preceding claims, characterized in that: The total surface of the grooves ( 5 , 6 , 7 ) occupies a portion of the housing surface ( 2 ) of at most 30%, 40% or 50%. 9. The muffler according to at least one of the preceding claims, characterized in that: A functional surface (8) defining a bottom (10) is provided in the housing surface (2). 10. The muffler according to claim 9, characterized in that: Between the segments ( 9 ) of the housing surface ( 2 ) in which the functional surfaces ( 8 ) are located, the portion of the recessed surface is between 5% and 60% of the corresponding surface. 11. The muffler according to claim 10, characterized in that: Set two or more segments (9). 12. The muffler according to at least one of the preceding claims, characterized in that: The surface of the individual grooves (5, 6, 7) is between 0,05% and 10% of the housing surface (2). 13. The muffler according to claim 12, characterized in that: The surface of the individual grooves (5, 6, 7) is between 0,1% and 5% of the housing surface (2). 14. The muffler according to at least one of the preceding claims, characterized in that: The number of bottoms (10) is reduced by at least one bottom (10) compared to a corresponding muffler (1) with regular grooves. 15. A method for designing a muffler according to one of the preceding claims 1-14, characterized in that: The configuration and/or structural configuration of the recesses (5, 6, 7) is determined by an optimization method according to the noise reflection of the muffler.

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