Method for Reducing Noise of a High Power Combustion Engine

Abstract

A sound reduction system for reducing noise from a high power combustion engine is supplied by means of a method. The sound reduction system comprises a plurality of elements and attenuating devices placed in an elongated channel. During design of the sound reduction system one makes use of a particular suitable attenuating element with a first reactive part, a second reactive part and a third reactive part. Such a module, which is less sensitive to position in the channel, cost effective to manufacture and cost effective to model, is combined with single attenuating devices. The method enables a user to meet the requirements on sound reduction and keeping construction costs down, by using an iterative step-by-step approach. Such an approach is unknown according to traditional methods. An advantage of the method is that it enables an accurate acoustic model of the complete exhaust system, not only in the low frequency area and in the upper frequency area, but also in the intermediate frequency area. The method provides efficient modeling of an exhaust system and enables that a desired noise level close to the outlet of the exhaust system is met.

1 . A method for supplying a system for sound attenuation of noise relating to an exhaust system ( 1 ) of exhaust gases from a high power combustion engine ( 4 ), such as the exhaust system ( 1 ) at a ship ( 2 ) or power plant, characterized in that the method comprises the steps of: 
 adding ( 7 ) to a model of the exhaust system, by means of a computing device ( 13 ), a plurality of elements ( 20   a,    20   b ) where each element comprises a first reactive part ( 21 ), a resistive part ( 22 ) and a second reactive part ( 23 );    inserting ( 8 ) into the model, by means of the computing device ( 13 ), at least one single attenuating device ( 44 ,  45 );    calculating ( 9 ), by means of the computing device ( 13 ), an attenuating effect of the elements ( 20   a,    20   b ) and an attenuating effect of the at least one single attenuating device ( 44 ,  45 ) relating to a sound pressure level of the high power combustion engine ( 4 );    repeating ( 10 ) the inserting and calculating step, until sufficient attenuation is achieved;    assembling ( 11 ) the system for sound attenuation, such that it comprises a plurality of real-world elements and at least one real-world single attenuating device mounted as channel parts along the exhaust system, wherein a measured noise level at the close vicinity of the outlet is below a desired noise level.    
   
   
       2 . A method according to  claim 1  characterized in that a contribution to an estimated attenuated effect comprises a band of frequencies corresponding to intermediate frequencies ( 60 ) of an element ( 20 ).  
   
   
       3 . A method according to  claim 2  characterized in that the contribution to the estimated attenuated effect from intermediate frequencies of an element ( 20 ) are calculated by use of four-pole theory and by use of power flow models.  
   
   
       4 . A method according to  claim 1  or  3  characterized in that the at least one single reactive attenuating device ( 45 ) is positioned at an odd number of a quarter of a wavelength from a distinct impedance, such as an area increase ( 46 ), where the wavelength is the single attenuating device's tuned frequency.  
   
   
       5 . A method according to  claim 4  with the additional step of calculating a pressure drop along the exhaust system ( 1 ).  
   
   
       6 . A method according to any previous claim characterized in that the minimum length of the exhaust system is 8 meters, the effect of the combustion engine ( 4 ) is greater than 500 kW.  
   
   
       7 . A method according to  claim 6  where the exhaust system ( 1 ) comprises a heat exchanger or boiler ( 5 ,  42 ), which reduces the temperature of the exhaust gas in the exhaust system ( 1 ) and therefore the wavelength of the sound decreases after the heat exchanger or boiler ( 5 ,  42 ), and the at least one single attenuating device is positioned in an odd number of a quarter of a wavelength from the outlet of the heat exchanger or boiler ( 5 ,  42 ), where the wavelength is the single attenuating device's tuned frequency.  
   
   
       8 . The use of the method according to  claim 1 .  
   
   
       9 . A computer program stored on a media loadable into the memory of a computing device ( 13 ) characterized in that the computer program is capable of executing any of the steps according to  claim 1 .  
   
   
       10 . An elongated exhaust system of exhaust gases from a high power combustion engine ( 4 ) characterized that a system for sound attenuation ( 47 ) of noise is supplied to the exhaust system according the method of  claim 1.