Abstract:
Disclosed is an exhaust system ( 100 ) for an internal combustion engine, comprising a first exhaust train that is provided with a penetrable first muffler ( 120 ), especially a rear muffler, and at least one second exhaust train which encompasses a second penetrable muffler ( 122 ), particularly a rear muffler. The structure of the first muffler ( 120 ) differs from that of the second muffler ( 122 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/EP2004/010109), filed Sep. 10, 2004, and which claims priority to German Patent Application No. 103 46 479.4, filed Oct. 2, 2003. The disclosures of these applications are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to an exhaust system for an internal combustion engine comprising a first exhaust train including a flow-permeable first muffler, in particular a rear muffler, and at least one second exhaust train parallel thereto and including a flow-permeable second muffler, in particular a rear muffler, wherein the first muffler and the second muffler have a mutually deviating structure. 
     BACKGROUND OF THE INVENTION 
     Exhaust systems of this type are in particular used in series production in motor vehicles to influence the noise emission of the internal combustion engine. DE 197 43 446 A1, for example, describes an exhaust system for an internal combustion engine which ensures good sound absorption over the whole frequency range with respect to a reduced noise level of motor vehicle engines and nevertheless does not allow the exhaust pressure to increase too highly even at a high internal combustion engine speed. For this purpose, two parallel mufflers are provided which are tuned to different frequencies. Both mufflers are operable as flow-permeated reflection mufflers, on the one hand; on the other hand, a discharge of the exhaust flow from the muffler can be prevented by closing the outlet pipe of a muffler by means of an exhaust flap so that it then acts as a Helmholtz resonator. 
     The exhaust system described by DE 197 43 446 A1 comprises an exhaust train which is made in parallel sectionally. In accordance with the embodiments, the exhaust flow is supplied to the two mufflers at the inlet side via a common forking exhaust pipe and/or is merged and led away via a common exhaust pipe at the outset side or, respectively, a double-fork-like double junction is provided as a cross-position. 
     With an arrangement of this type, the typical sound of an internal combustion engine comprising at least two cylinder banks, such as a V8, is disadvantageously lost. In particular, the vibrations of odd orders are lost; the ignition order (4th order of vibrations) and its harmonic dominate. Attributes such as force and power can only be represented by volume so that a conflict is present in this respect with the comfort demand made on the drive. 
     The crankshaft of multi-cylinder internal combustion engines is produced by arranging the crank throws of the individual cylinders in rows next to one another. When fixing the throw sequence, it is important to observe an ignition sequence which is as smooth as possible, a mass balancing and the rotary oscillations. The changing cylinder pressures which act on the exhaust system in dependence on the throw sequence and on the ignition sequence via the outer structure-borne route make a decisive contribution to the characteristic sound of the internal combustion engine. 
     With V8 internal combustion engines, for example, a customary ignition sequence is 1-5-4-8-6-3-7-2, with the cylinders 1-4 being associated with one cylinder bank and the cylinders 5-8 being associated with the other cylinder bank. On the basis of this ignition sequence, significant frequency proportions arise in the oscillations of odd orders since, for example, 270° crankshaft lie between cylinders four and three on the one cylinder bank and only 90° crankshaft between cylinders two and one. The excitation spectra on the exhaust side of both cylinder banks are in turn identical, but phase shifted, to one another. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide an initially named exhaust system by means of which the typically forceful and powerful sound of an internal combustion engine with a plurality of cylinder banks, such as a V8 internal combustion engine, can be represented without excessive volume and thus while satisfying a high comfort demand. The significant frequency portions in the oscillations of odd orders should in particular be maintained. 
     The object is satisfied by the claimed features wherein, the first muffler comprises an inlet pipe and an outlet pipe, with the outlet pipe having a small length, and the second muffler comprises an inlet pipe and an outlet pipe, with the outlet pipe having a large length. “Small length” and “large length” in this context relate primarily to the ratio of the lengths among one another, with one length being much larger than the other. In this process, the acoustically effective length, which is determined by the place of exhaust inlet and outlet, is decisive. 
     Advantageous embodiments and further developments are the subject matter of the dependent claims. 
     It is of particular advantage for the outlet pipe of the first muffler to have at least approximately twice the length of the outlet pipe of the second muffler. 
     Expediently, the outlet pipe of the second muffler has an at least slightly larger diameter than the outlet pipe of the first muffler so that back pressure differences due to the longer outlet pipe are avoided. 
     In accordance with a particularly preferred embodiment, the first muffler has an internal structure which is divided into three part spaces by means of two metal separating sheets, with the first metal separating sheet being perforated and the second metal separating sheet being intact. An exchange of gas between the first and second part spaces is possible through the separating wall; the separating wall to the third part space is sealingly closed. The inlet pipe of the first muffler expediently opens into the first part space at the inlet side. The outlet pipe advantageously leads through the second and third part spaces, starting from the first part space on the inlet side, with the outlet pipe being able to be acted on by flow both from the first part space and from the first parts space through the second part space. The use of a resonator adjoining the inlet pipe and extending into the second and third part spaces has proved to be very advantageous. 
     Pursuant to the particularly preferred embodiment, the second muffler has an internal structure divided into three part spaces by means of two metal separating sheets, with the first metal separating sheet being intact and the second metal separating sheet being perforated. An exchange of gas between the second and third part spaces is possible through the separating wall; the separating wall to the first part space is sealingly closed. The inlet pipe of the second muffler expediently extends through the first and second part spaces at the inlet side and opens into the third part space. The outlet pipe advantageously leads through the second part space into the first part space on the inlet side, starting from the third part space, and back through the second and third part spaces in an arcuate curve, with the outlet pipe being able to be acted on by flow through the second part space, at the inlet side both from the third part space and from the third part space. The use of a resonator which connects the third part space to the first part space has proved to be very advantageous. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention which is to be particularly preferred is explained in more detail in the following with reference to the Figures, wherein: 
         FIG. 1  illustrates a double-pass exhaust system for a V8 internal combustion engine with a rear muffler; 
         FIG. 2  illustrates a first rear muffler with a short outlet pipe; and 
         FIG. 3  illustrates a second rear muffler with a long outlet pipe. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
       FIG. 1  shows a double-pass exhaust system  100  comprising a first exhaust train  102  and a second exhaust train  104  for a V8 internal combustion engine not shown in more detail here. The internal combustion engine comprises two banks with four cylinders each, with a first cylinder bank being formed with the cylinders one to four and a second cylinder bank being formed with the cylinders five to eight. In operation of the internal combustion engine, ignition takes place in the order 1-5-4-8-6-3-7-2. The outlets of the first cylinder bank open into the manifold  106  of the first exhaust train  102 ; the outlets of the second cylinder bank open into the manifold  108  of the second exhaust train  104 . Directly adjoining the manifolds  106 ,  108 , each exhaust train  102 ,  104  comprises a catalytic converter  110 ,  112  close to the engine for post-treatment of the exhaust, with it being a case of conventional 3-way catalytic converters in the present case. 
     In the operation of the internal combustion engine, a changing cylinder pressure is generated by the periodic combustion processes in the cylinders and thus a (gas) oscillation is stimulated which can be perceived as sound. The sound in particular continues via the outer structure-borne route through the cylinder outlets and the whole exhaust system up to the tail pipes  124 ,  126 . The sound is decisively influenced by the mufflers arranged in each exhaust train  102 ,  104 . In the present case, each exhaust train  102 ,  104  comprises pre-mufflers  114 ,  116  made as absorptive attenuators and middle mufflers  118  as well as rear mufflers  124 ,  126 . 
     The exhaust system  100  is made completely in a double pass design, substantially without cross-positions. It has, however, proved to be advantageous optionally to provide one or more minimal cross-positions so that a softer sound can be achieved without any real gas exchange and an adjustment can be made in this respect. In the present case, a minimal cross-position of this type is provided in the region of the middle muffler  118 , with an internal separation taking place inside the common middle muffler  118 . The front region of the middle muffler  118  is associated with the one exhaust train, the rear region with the other one. 
     The rear mufflers  124 ,  126  of the exhaust system  100 , unlike the otherwise symmetrical structure of the exhaust system  100 , have a different structure with the aim of achieving an amplification of the oscillations of odd orders in the distant field at the outside and in the inner noise by a change in the phase relationship of the individual openings. Details of the rear mufflers  124 ,  126  are shown and described in  FIGS. 2 and 3 . 
     The rear muffler  120  of the first exhaust train  102  is shown in two views in  FIG. 2  and is designated by  200 . The rear muffler  200  is formed in the manner of a pot from two half shells  202  and  204  which are sealingly connected to one another and enclose an inner space. The inner space is divided into three part spaces  210 ,  212  and  214  by a perforated separating wall  206  and an intact separating wall  208 , with the part space  212  being filled with an absorptive material  228 . The inlet pipe  216  opens into the first part space  210 ; a resonator  226  adjoins which extends through the second part space  212  into the third part space  214 . The outlet pipe  224  extends from the first part space  210  through the second part space  212  and the third part space  214 . The flow through the rear muffler  200  takes place starting from the inlet pipe  216  which opens via holes  218  into the first part space  210  via holes  220  in the outlet pipe  224 , on the one hand, and through the perforated separating wall  206  and the second part space  212  via holes  222  in the outlet pipe  224 , on the other hand. 
     The rear muffler  122  of the second exhaust train  104  is shown in two views in  FIG. 3  and is designated by  300 . The rear muffler  300  is formed in the manner of a pot from two half shells  302  and  304  which are sealingly connected to one another and enclose an inner space. The inner space is divided into three part spaces  310 ,  312  and  314  by an intact separating wall  306  and an perforated separating wall  308 , with the part space  312  being filled with an absorptive material  328 . The inlet tube  316  extends through the first part space  310  and the second part space  312  and opens into the third part space  314 . To achieve a softer transition, the pipe  316  open at the end is additionally provided with holes  318  in the end region. The outlet pipe  324  extends—masked by the inlet pipe  316  in the present view—from the third part space  314  through the second part space  312  into the first part space  310  and back in an arcuate curve through the second part space  312  and the third part space  314 . The flow through the rear muffler  200  takes place starting from the inlet pipe  316  which opens into the third part space  314  through the outlet pipe  324 , on the one hand, and through the perforated separating wall  308  and the second part space  312  via holes in the outlet pipe  324  not visible here, on the other hand. A resonator  326  connects the third part space  314  to the first part space  310 . 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.