Patent Publication Number: US-10781732-B2

Title: Acoustic attenuator for damping pressure vibrations in an exhaust system of an engine, an acoustic attenuation system using the attenuators, and method of damping pressure vibrations in an exhaust system of an engine

Description:
TECHNICAL FIELD 
     The invention relates to an acoustic attenuator for damping pressure vibrations in an exhaust system of an engine, the acoustic attenuator comprising a body which is provided with a gas inlet and a gas outlet at opposite ends thereof, and a gas passage duct arranged between the inlet and the outlet inside the body, where in the body encloses a first resonator chamber and a second resonator chamber according to the preamble of claim  1 . 
     Invention relates also an acoustic attenuation system using the attenuators, and a method of damping pressure vibrations in an exhaust system of an engine. 
     BACKGROUND ART 
     Internal combustion engines produce considerably loud noise in connection with their exhaust gas. Pressure vibrations and noise occur in the exhaust channel and are generated when exhaust gas is discharged from the cylinders of the engine. Noise emitted through exhaust system of the engine is at least a nuisance and in most cases harmful to the environment. Therefore different kinds of attenuation devices arranged to the exhaust systems have been developed. 
     Noise occurring in the exhaust system can be reduced by using different types of damping techniques. For example, one attenuator type is a reactive attenuator and another is a resistive attenuator. 
     Reactive attenuators generally consist of a duct section or alike that interconnects with a number of larger chambers. The noise reduction mechanism of reactive attenuators is that the area discontinuity provides an impedance mismatch for the noise wave traveling along the duct. This impedance mismatch results in a reflection of part of the noise wave back toward the source or back and forth among the chambers. The reflective effect of the silencer chambers and ducts (typically referred to as resonators) essentially prevents some noise wave elements from being transmitted past the silencer. The reactive silencers are more effective at lower frequencies than at high frequencies, and are most widely used to attenuate the exhaust noise of internal combustion engines. 
     WO 2014/076355 A1 discloses an exhaust gas noise attenuator unit comprising at least two reactive attenuation chambers. A first attenuation chamber of the at least two attenuation chambers is arranged in flow connection with the duct section at a first location in longitudinal direction and a second attenuation chamber of the at least two attenuation chambers is arranged in flow connection with the duct section at a second location in longitudinal direction. 
     It is also known to arrange both reactive and resistive elements into a same attenuator unit. An example of such an element is described in WO 2005/064127 A1 that discloses a sound reduction system for reducing noise from a high power combustion engine. The sound reduction system comprises an element comprising a first reactive part, a resistive part and a second reactive part. The attenuation effect of the element in the low frequencies is mainly achieved by the reactive parts. The attenuating effect in the high frequency area of each element is mainly achieved by the resistive part. The resistive part contributes also to the attenuating effect in the low frequency area as a reflective attenuator. 
     An object of the invention is to provide an acoustic attenuator which provides efficient attenuation of noise but still allowing a space saving installation in connection with an internal combustion engine exhaust gas system. 
     DISCLOSURE OF THE INVENTION 
     Object of the invention is substantially met by an acoustic attenuator for damping pressure vibrations in an exhaust system of an engine, the acoustic attenuator comprising a body which is provided with a gas inlet and a gas outlet at opposite ends thereof, and a gas passage duct arranged between the inlet and the outlet inside the body, where in the body encloses a first resonator chamber and a second resonator chamber. 
     It is characteristic to the invention that the body is provided with a common inlet communicating with the first and the second resonator chambers and the resonator chambers are arranged to extend from the common inlet towards the opposite ends of the body. 
     This provides efficient attenuation of noise but still allowing a space saving installation in connection with an internal combustion engine exhaust gas system. The acoustic attenuator according to the invention reduces noise propagation from an internal combustion piston engine into the exhaust system by means of two resonators integrated into the same body. The two resonators are dimensioned so as to produce attenuation at a broader frequency band not obtainable with singular element. The improvement relates to resonator space separation of two resonators and utilization of common, singular connection inlet for both chambers. 
     According to an embodiment of the invention the gas passage duct is formed of a straight gas duct and the resonator chambers are arranged annularly around the duct, wherein the attenuator comprises two longitudinally spaced intermediate walls radially extending from the gas passage duct to a sleeve part of the body and wherein the common inlet is arranged longitudinally between the intermediate walls. 
     This way the structure is very versatile for adjusting its properties by only simple changes in the construction, such as changing the diameter and/or length of the sleeve part, and/or changing the position(s) of the intermediate wall(s). 
     According to an embodiment of the invention the attenuator the resonator chambers are connected with the common inlet via ports arranged to, and supported by the intermediate walls. 
     According to an embodiment of the invention the gas passage duct is formed of a straight gas duct and the resonator chambers are arranged annularly around the duct, wherein the attenuator comprises two longitudinally spaced intermediate walls radially extending from the gas passage duct to a sleeve part of the body and wherein the common inlet is arranged longitudinally between the intermediate walls and in the attenuator the resonator chambers are connected with the common inlet via ports arranged to, and supported by the intermediate walls). 
     This provides reduced back-pressure of exhaust system due to straight-thru-flow design as compared to previous singular units, resulting in higher engine or power plant system efficiency and lower emissions. 
     According to an embodiment of the invention the gas passage duct is directed parallel with a longitudinal axis of the body and the ports are arranged parallel with the longitudinal axis of the body. 
     Advantageously the port is a tubular member supported by the intermediate wall. 
     Object of the invention is substantially met by an acoustic attenuation system comprising two acoustic attenuators for damping pressure vibrations in an exhaust system of an engine, in which each of the acoustic attenuator comprising a body which is provided with a gas inlet and a gas outlet at opposite ends thereof, and a gas passage duct arranged between the inlet and the outlet inside the body, where in the body encloses a first resonator chamber and a second resonator chamber, and further the body is provided with a common inlet communicating with the first and the second resonator chambers and the resonator chambers are arranged to extend from the common inlet towards the opposite ends of the body. 
     It is characteristic to the invention that the gas passage duct has a predetermined length between the common inlet for the first and the second acoustic attenuators in the system. 
     According to an embodiment of the invention the acoustic attenuators are coupled one after the other in the exhaust system of an internal combustion engine such that the distance between the common inlet for the first and the second acoustic attenuators is determined so as to control acoustic wave phase difference between the acoustic attenuators. 
     According to an embodiment of the invention the acoustic attenuators are coupled one after the other in the exhaust system of an internal combustion engine such that the distance between the common inlet for the first and the second acoustic attenuators is determined using the formula 
             L   =       C   0       4   ·     F   GA               
wherein
 
C 0 =speed of sound in exhaust gas [m/s]
 
F GA =geometric average of adjacently successive tuning frequencies, for example the frequencies F 4  and F 2  in  FIG. 5 ; F GA =√(F 4 *F 2 )
 
According to an embodiment of the invention the resonator chambers are arranged such that the first resonator chamber of the first attenuator is tuned to attenuate a first frequency and the second resonator chamber of the first attenuator is tuned to attenuate a second frequency, and the first resonator chamber of the second attenuator is tuned to attenuate a third frequency and the second resonator chamber of the second attenuator is tuned to attenuate a fourth frequency, and resonator chambers are tuned to attenuate different frequencies and that two of the tuning frequencies closest to each other are arranged obtainable from separate acoustic attenuators.
 
     According to an embodiment of the invention the resonator chambers are arranged such that the first resonator chamber of the first attenuator is tuned to attenuate a first frequency and the second resonator chamber of the first attenuator is tuned to attenuate a second frequency, and the first resonator chamber of the second attenuator is tuned to attenuate a third frequency and the second resonator chamber of the second attenuator is tuned to attenuate a fourth frequency, and the tuning frequencies are selected so that the third frequency&gt;the second frequency&gt;the fourth frequency&gt;the first frequency. 
     According to an embodiment of the invention in the acoustic attenuation system the acoustic attenuator is an acoustic attenuator according to anyone of the claims  1 - 6 . 
     The acoustic attenuators are dimensioned and spatially separated so as to produce attenuation at a broader frequency band than obtainable with singular element. The attenuation is obtained by controlling acoustic wave phase difference between distributed elements by spatial and frequency separation. The obtained attenuation capacity is of higher amplitude and at broader frequency range than that is previously obtained and utilized in such applications. 
     Object of the invention is substantially met by a method of damping pressure vibrations in an exhaust system of an engine comprising steps of leading exhaust gas from an internal combustion engine via an exhaust gas system to an acoustic attenuator. The invention is characterized by damping the pressure vibrations of the gas by arranging the vibrating gas to communicate with two separate resonator chambers via a common inlet from a gas passage duct of the attenuator to the chambers. 
     Invention has several general benefits. Firstly the attenuator is such that it is possible to be installed close to the noise source, i.e. the engine thus reducing engine&#39;s acoustic or noise radiation and thus effecting on mechanical constructions of exhaust gas system due to generally lower vibration levels. Secondly the attenuator according to the invention requires generally only a small space. The attenuator provides also a reduced back-pressure of exhaust system due to straight-thru-flow design as compared to previous singular units, resulting in higher engine or power plant system efficiency and lower emissions. 
     In upgrade application the attenuator according to the invention may be easily installed to an existing plant simply by cutting the existing exhaust duct to install the intermediate walls provided with the ports, sleeve part and its endplates. 
     The attenuator provides also an efficient attenuation of low frequency noise, characteristic to reciprocating internal combustion engine, at broader frequency scale. 
     The attenuator provides also an efficient means of modularization of the construction and utilization of similar parts with increased manufacturability. 
     The utilization of the common inlet enables compact size and simple structure also in manufacturing point of view, while still maintaining attenuation of high amplitude and of low frequency acoustic wave. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which 
         FIG. 1  illustrates an acoustic attenuator in connection with an internal combustion piston engine according to an embodiment of the invention, 
         FIG. 2  illustrates a cross sectional view II-II of the attenuator in the  FIG. 1 , 
         FIG. 3  illustrates a cross sectional view III-III of the attenuator in the  FIG. 1 , 
         FIG. 4  illustrates an acoustic attenuation system in connection with an internal combustion piston engine according to an embodiment of the invention, and 
         FIG. 5  illustrates an exemplary effect of the acoustic attenuation system of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
       FIG. 1  depicts schematically an acoustic attenuator  10  according to an embodiment of the invention. The attenuator is adapted to attenuate exhaust gas noise of an internal combustion piston engine, and in the  FIG. 1  the attenuator is arranged to an exhaust gas system  12  of an internal combustion piston engine  14 . 
     The acoustic attenuator comprises a body  16  which is provided with an inlet  18  and an outlet  20  for the exhaust gas to enter and exit the acoustic attenuator. The body  16  is generally an elongated structure which is rotationally symmetrical in respect to its central axis  22 . The inlet  18  and the outlet  20  are arranged at opposite ends of the body  16 , on the central axis  22 . The inlet  18  and the outlet are of equal cross sectional area (diameter when being tubular) and the inlet and the outlet are connected with each other by a gas passage duct  24  extending through the body  16  along the central axis  22 . The gas passage is a gas passage duct arranged its centre line to coincide with the central axis  22  of the body  16 . 
     The body  16  is provided with a sleeve part  26  enclosing the gas passage duct  24  over a length in the direction of the central axis  22 . There is an annular gap arranged between the sleeve part  26  and the gas passage duct which is closed by end plates  25  at the ends of the sleeve part  26  by end parts  28 . The way a closed resonator space is arranged into the annular gap. 
     The cross sectional area of the sleeve part  26  is greater than the cross sectional area of the gas passage duct. Specifically when the attenuator is of circular cross section, the diameter of the sleeve part  26  is greater than the diameter of the gas passage duct  24  and the sleeve part and the gas passage duct are arranged coaxially. 
     The body  16  is further provided with two intermediate walls  30 ,  30 ′. The intermediate walls  30 , 30 ′ are arranged to extend radially from the gas passage duct  24  to the sleeve part  26  and circumscribe the gas passage duct  24  forming a gas tight wall to the annular gap between the sleeve part  26  and the gas passage duct. In other words the intermediate wall is an annular plate- or flange-like structure closing the gap between the sleeve part  26  and the gas passage duct. This way there are two closed resonator chambers  36 ,  38  arranged into the annular gap between respective intermediate wall  30  and the end plate  25 . The intermediate walls  30 ,  30 ′ are arranged at a distance from each other in the longitudinal direction, i.e. in the direction of the central axis  22 . There is an opening  32  arranged to the gas passage duct  24 , which opening  32  is located in longitudinal direction between the two intermediate walls  30 ,  30 ′. The intermediate walls act also as a support structure of the body part  16 . 
     The space bordered by the sleeve part  26 , the intermediate walls  30 ,  30 ′ and the wall of the gas passage duct  24 , together with the opening  32  in the gas passage duct  24  forms a common inlet  34  for the gas passage duct such that the gas passage duct is in fluid communication with the first  36  and the second  38  resonator chamber via the common inlet  34  in the body. The resonator chambers  36 , 38  are arranged to extend in the longitudinal direction from the common inlet towards the opposite ends of the body. 
     The attenuator is provided with at least one port  40  which are arranged in, and supported by each intermediate wall  30 , 30 ′ which port opens a communication between the resonator chamber  36 , 38  and the common inlet  34 , i.e. the common inlet  34  is arranged in fluid communication with the resonator chamber  36 , 38  via the port  40 . The ports  40  are tubular members having a central axis  42 . The ports  40  and their central axes  42  are arranged parallel with the longitudinal axis of the body  16 . The diameter and length of the port tube  40  is dimensioned individually based on the desired attenuation effect of the attenuator. In the attenuator of the invention the precise tuning is straightforward by changing the dimensions of the tubular port. This way the tuning can be adjusted also without changing the dimensions of the body part, which is advantageous in practise. 
     The distance between the intermediate walls is dimensioned to suit manufacturing process. The minimum distance is defined by wave motion physics to allow efficient connection from main duct into chambers via the tubular ports. 
       FIGS. 2 and 3  depicts the cross sectional views II-II and III-III in the  FIG. 1 . As can be seen there may be provided one or more parallel tubular ports  40  in connection with each of the resonator chamber  36 , 38 . The opening  32  in the gas passage duct  24  is formed by removing a segment  42  from the wall of the gas passage duct. The segment is arranged such that there is a solid wall portion of the gas passage duct  24  extending over the distance between the intermediate walls  30 ,  30 ′ circumscribing or covering partially the gas passage duct in circumferential direction. 
     The solid wall portion  44  is an optional feature which has a benefit of closing out a stagnant gas volume between the intermediate walls, to reduce gas accumulation. However, this is not essential for acoustic performance of the attenuator. Additionally the attenuator  10  may be provided with a closing plate  45  extending radially between the solid wall portion and the sleeve part  26  of the body  16 , and extending longitudinally between the intermediate walls  30 , 30 ′. This is shown with dotted lines in the figures indicating the optional nature of the feature 
       FIG. 4  shows an acoustic attenuation system  100  comprising two acoustic attenuator  10 . 1 , 10 . 2  as is shown in the  FIGS. 1 to 3 . The acoustic attenuators  10 . 1 , 10 . 2  are coupled one after the other in the exhaust system  12  of an engine such that there is a predetermined distance L of the gas passage duct  24  between the common inlet  34  for the first and the second acoustic attenuators in the system  100 . The attenuators  10 . 1 , 10 . 2  are dimensioned and longitudinally separated so as to produce attenuation at a broader frequency band than obtainable with singular element. The attenuation by the acoustic attenuators  10 . 1 , 10 . 2  coupled one after the other in series in the gas passage duct  24  is obtained by controlling acoustic wave phase difference between distributed elements by spatial and frequency separation. The obtained attenuation capacity is of higher amplitude and at broader frequency range than that is previously obtained and utilized in such applications. 
     The attenuators  10 . 1 ,  10 . 2  are each provided with two resonator chambers  36 . 1 , 38 . 1 ; 36 . 2 , 38 . 2  as is disclosed in the  FIG. 1 , The chambers are tuned to attenuate noise i.e. vibration in the following manner. The first resonator chamber  36 . 1  of the first attenuator  10 . 1  is tuned to attenuate as a center frequency a first frequency F 1  and the second resonator chamber  38 . 1  of the first attenuator  10 . 1  is tuned to attenuate as a center frequency a second frequency F 2 , and respectively the first resonator chamber  36 . 2  of the second attenuator  10 . 2  is tuned to attenuate as a center frequency a third frequency F 3  and the second resonator chamber  38 . 2  of the second attenuator  10 . 2  is tuned to attenuate as a center frequency a fourth frequency F 4 , The tuning frequencies are selected so that the third frequency F 3 &gt;the second frequency F 2 &gt;the fourth frequency F 4 &gt;the first frequency F 1 . This way the attenuators are utilized in optimized manner. In practise the frequency means a certain range having it attenuation performance above a certain limit. 
     When considering the system in relation to the gas flow direction, which is shown by an arrow A, the resonator chambers are arranged in the following order: the first resonator chamber  36 . 1  of the first attenuator  10 . 1 , the second resonator chamber  38 . 1  of the first attenuator  10 . 1 , the first resonator chamber  36 . 2  of the second attenuator  10 . 2  and the second resonator chamber  38 . 2  of the second attenuator  10 . 2 . 
     In the  FIG. 5  there is shown an example of the combined effect of the system  100  in terms of transmission loss. The transmission loss is defined as the difference between the power incident on the acoustic attenuator and that transmitted downstream from the attenuator into an anechoic termination. There are four peaks of transmission loss which represent the center tuning F 1  of the first resonator chamber  36 . 1  of the first acoustic attenuator, the center tuning F 4  of the second resonator chamber  38 . 2  of the second acoustic attenuator, the center tuning F 2  of the second resonator chamber  36 . 2  of the first acoustic attenuator, and the center tuning F 3  of the first resonator chamber  38 . 1  of the second acoustic attenuator. Typical tuning frequencies suitable for a large internal combustion piston engine are for example as follows: F 1 =12.5 Hz, F 2 =25 Hz, F 3 =37.5 Hz, F 4 =20 Hz. It is advantageous to maximize the ratios F 2 /F 1  and F 3 /F 4 . 
     According to an embodiment of the invention the resonator chambers are tuned to attenuate different frequencies and the frequencies are selected so that two of the tuning frequencies closest to each other are arranged in connection with or obtainable from separate acoustic attenuators  10 . 1 , 10 . 2 . 
     Now, by means of the combined effect of the predetermined distance L of the gas passage duct  24  between the common inlet  34  for the first and the second acoustic attenuators in the system  100 , and the first  10 . 1  and the second attenuator  10 . 2  it is possible increase the bottom value  39 ′ of the transmission loss curve at about 23 Hz considerably to the point  39 , between the adjacently successive tuning frequencies F 4  and F 2 . Additionally the combined peak of frequencies F 4 +F 4  is widened. In the  FIG. 5  the solid line bottom  39 ′ shows the transmission loss obtained by separate attenuator while the dotted line indicates the effect of the tuned system of two attenuators  10 . 1 , 10 . 2  and the gas passage duct  24  having a predetermined length L between the two attenuators  10 . 1 , 10 . 2 . This shows clearly how the transmission loss of higher level is expanded over wider range of frequency. 
     The system  100  forms a band cut filter, in which the attenuation obtained by tuned, distributed attenuators utilizing acoustic phase control between the attenuators. As an example, the system is dimensioned so that the distance between the common inlet for the first and the second acoustic attenuators is determined using the formula 
             L   =       C   0       4   ·     F   GA               
wherein
 
C 0 =speed of sound in exhaust gas [m/s]=500 m/s
 
F GA =geometric average of adjacently successive tuning frequencies, for example the frequencies F 4 =20 Hz and F 2 =25 Hz
 
and thus L=5.6 m.
 
This way an anti-resonance is provided in the gas passage duct  24 , which is adjusted to be between the adjacent successive tuning frequencies. This enhances the operation or technical effects of the adjacent resonators.
 
     While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.