US20090000862A1

US20090000862A1 - Motorcycle exhaust system

Info

Publication number: US20090000862A1
Application number: US11/772,507
Authority: US
Inventors: Erik Buell; Ed Weston; Brian Freeh; Matthew J. Sheahan; Nick Zeidler; Michael D. Samarzja; Mike Johnson; Anthony D. Stefanelli; Paul Pedersen; Erick L. Gruber
Original assignee: Buell Motorcycle Co
Current assignee: Buell Motorcycle Co
Priority date: 2007-06-28
Filing date: 2007-07-02
Publication date: 2009-01-01
Also published as: JP2009008089A; DE102008030596A1

Abstract

An exhaust system for a motorcycle engine includes a muffler assembly. The muffler assembly includes a downstream attenuation portion adapted to be positioned substantially rearward of the motorcycle engine and having a first height, an upstream portion adapted to be positioned substantially forward of the motorcycle engine and having a second height, and an intermediate attenuation portion between the upstream portion and the downstream attenuation portion. The intermediate attenuation portion is adapted to be positioned substantially below the motorcycle engine and has a third height less than half the first height. In some embodiments, a motorcycle is provided, in which the motorcycle engine is positioned substantially within a recess of the muffler assembly to a depth greater than the third height.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No. 11/770,051 filed Jun. 28, 2007, the entire contents of which is incorporated herein.

BACKGROUND

The present invention relates to an exhaust system for a motorcycle engine. More particularly, the invention relates to a muffler assembly having a particular arrangement of parts and overall shape.

SUMMARY

In one embodiment, the invention provides a muffler assembly for a motorcycle engine. The muffler assembly includes a downstream attenuation portion adapted to be positioned substantially rearward of the motorcycle engine and having a first height, an upstream portion adapted to be positioned substantially forward of the motorcycle engine and having a second height, and an intermediate attenuation portion between the upstream portion and the downstream attenuation portion. The intermediate attenuation portion is adapted to be positioned substantially below the motorcycle engine and has a third height less than half the first height.
In another embodiment, the invention provides a motorcycle including an engine, a transmission coupled to the engine and configured to receive power from the engine, a rear wheel coupled to the transmission and configured to receive engine power through the transmission, and a muffler assembly coupled to the engine and in communication therewith to receive exhaust gases from the engine. The muffler assembly includes a downstream attenuation portion positioned between the transmission and the rear wheel and an intermediate attenuation portion positioned forwardly of the downstream attenuation portion. The intermediate attenuation portion extends along an underside of the engine.
In yet another embodiment, the invention provides a motorcycle including an engine, two wheels defining a central axis of the motorcycle, and a muffler assembly in communication with the engine to receive exhaust gases from the engine, the muffler assembly being positioned substantially along the central axis. The muffler assembly includes a downstream attenuation portion, an upstream portion, an intermediate attenuation portion positioned substantially under the engine, and a recess at least partially defined by the downstream attenuation portion and the intermediate attenuation portion. The recess has a depth greater than a height of the intermediate attenuation portion, and the engine is positioned substantially within the recess.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a motorcycle having an exhaust system embodying the present invention;

FIG. 2 is a partial cutaway perspective view of the exhaust system of FIG. 1.

FIG. 3 is a partial cutaway top view of a muffler assembly of the exhaust system shown in FIG. 1.

FIG. 4 is a partial cutaway side view of the muffler assembly of FIG. 1.

FIG. 5 is a graph representative of exhaust pressure versus crank angle illustrating the effect of the exhaust system of FIG. 1.

FIG. 6 is a graph representative of engine output versus engine speed illustrating the effect of the exhaust system.

FIG. 7 is an enlarged side view of the muffler assembly, the engine, and the transmission of FIG. 1.

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

DETAILED DESCRIPTION

FIG. 1 illustrates a motorcycle 10 having a twin-cylinder engine 14. The motorcycle 10 includes a front wheel 15, a rear wheel 16, and a transmission 17 coupled to the engine 14 to receive power from the engine. The rear wheel 16 is driven by the power of the engine 14 through the transmission 17 (and a drive member such as a belt). An air-fuel mixture is ignited in a combustion chamber (not shown) for each cylinder of the engine 14. Following combustion in a given combustion chamber, the exhaust gases (containing mixed products of the combustion reaction and some residual, un-reacted components) are expelled through an exhaust port into an exhaust system 18 of the motorcycle.
The exhaust system 18, as shown in FIGS. 1-4 and 7, includes brackets 20 for mounting to the motorcycle 10. The exhaust system 18 includes a pair of header pipes (i.e., “headers”) 22, a collector section 26, a catalytic converter 30, and a sound-muffling section 34. The headers 22 are exhaust conduits leading directly from the engine 14. The sound-muffling section 34 includes a first sound-muffling (“attenuation”) portion 34A and a second sound-muffling (“attenuation”) portion 34B. The collector section 26, the catalytic converter 30, the first sound-muffling (“attenuation”) portion 34A, and the second sound-muffling (“attenuation”) portion 34B collectively define a muffler assembly 35. In the illustrated embodiment, the muffler assembly 35 is positioned substantially along a central axis of the motorcycle 10 (defined by the front wheel 15 and the rear wheel 16). Each of the first attenuation portion 34A and the second attenuation portion 34B make up at least one fifth, or 20 percent, of the length of the muffler assembly 35 as measured along a single axis (e.g., the central axis) or individual axis segments following the general contour of the muffler assembly 35. Preferably, each of the first attenuation portion 34A and the second attenuation portion 34B make up about a third of the length of the muffler assembly 35. The portion of the muffler assembly 35 forward of the second attenuation portion 34B also makes up at least 20 percent of the length of the muffler assembly 35, and preferably, about a third of the length of the muffler assembly 35.
An upstream end 36A of each header 22 is coupled to the engine 14 to receive exhaust gases from a respective exhaust port of the engine 14. The headers 22 define exhaust flow passages that are separate from one another, each header 22 routing exhaust gases directly from an exhaust port of the engine 14 to a downstream exhaust component. A downstream end 36B of each of the headers 22 leads into an upstream end 35A of the muffler assembly 35, specifically, the collector section 26. The upstream end 35A of the muffler assembly 35 is positioned generally forward of the engine 14. The illustrated collector section 26 is an exhaust conduit defining a 2-into-1 exhaust flow passage joining the two separate exhaust flow passages of the headers 22 into a single, larger exhaust flow passage adjacent the catalytic converter 30. Therefore, exhaust gases from both combustion chambers are treated by the catalytic converter 30.
The second attenuation portion 34B defines an intermediate portion 35C of the muffler assembly 35 between the upstream end 35A and a downstream end 35B of the muffler assembly 35, along the underside of the engine 14. The first attenuation portion 34A, positioned generally at the downstream end 35B, is positioned generally rearward of the engine 14. A muffler shell or casing 35D (made up of one or more pieces) extends from the upstream end 35A to the downstream end 35B, defining an outer surface of the muffler assembly 35. Although, the casing 35D can be assembled from multiple pieces in some embodiments, it defines a generally continuous outer surface without abrupt transitions from one portion of the muffler assembly 35 to the next.
Downstream of the catalytic converter 30, exhaust gases flow through the intermediate portion 35C to the first attenuation portion 34A at the downstream end 35B. As described above, the intermediate portion 35C extends longitudinally underneath the engine 14, but alternate shaping and positioning of the exhaust components on the motorcycle 10 are optional. As described in further detail below, exhaust gases from the catalytic converter 30 are directed substantially straight through the intermediate portion 35C to the first attenuation portion 34A, and at least a portion of the exhaust gases flow back from the first attenuation portion 34A into the second attenuation portion 34B before exiting the muffler assembly 35 at a pair of outlets 35E.
Returning now to the treatment of the exhaust gases at the upstream end 35A, the catalytic converter 30 improves the emissions quality of the exhaust gases expelled from the engine 14 with the use of one or more known catalyst materials (referred to hereinafter simply as catalyst 38), which are contained within the catalytic converter 30. The catalyst 38 reacts with undesirable exhaust gas components to produce more desirable products before being exhausted to the atmosphere via outlets 35E. Specifically, nitrogen oxides (NO_x) can be converted to nitrogen (N₂) and oxygen (O₂), while carbon monoxide (CO) can be converted to carbon dioxide (CO₂).
The temperature of the catalyst 38 affects its performance. It is necessary to warm-up, or “light off”, the catalyst 38 above a minimum threshold temperature to obtain a desired level of performance from the catalytic converter 30 to effectively alter the undesirable exhaust gas components as described above. From a cold start of the engine 14, the catalyst 38 is generally below the minimum threshold temperature, and therefore it is desirable to heat up the catalyst as quickly as possible to obtain sufficient or optimal performance. One way to get quicker light off of the catalyst 38 is to place the catalytic converter 30 close to the engine 14, which is a source of heat via the hot exhaust gases flowing through the headers 22 to the catalytic converter 30.
However, placing the catalytic converter 30 at the downstream ends 36 of the headers 22 can have an undesirable effect on the exhaust gas pressure dynamics as compared to a placement further downstream. This undesirable effect can be somewhat reduced by using multiple catalytic converters 30 in parallel. However, the use of multiple catalytic converters 30 causes an undesirable increase in catalyst light off time (in addition to increasing cost, size, and weight). Regardless of its position in the exhaust system 18, the catalyst 38 is a substantial obstruction in the flow passage and, therefore, causes a sudden increase in flow resistance at its upstream end. This causes a positive pressure exhaust wave or pulse to be reflected back towards the engine 14 through the headers 22. The dynamics of the exhaust gases coming from the engine 14 and the reflected waves moving towards the engine impacts the engine performance (i.e., horsepower and torque output).
Under certain operating conditions, a reflected exhaust pulse hinders the exhaust scavenging process as well as the ability for the cylinder to become charged with fresh intake air (which can also affect the input of fuel into the cylinder). If the exhaust wave that is reflected off the catalyst 38 arrives at either combustion chamber during valve overlap (the time that both the intake and exhaust valves are open), there is a significant performance loss due to decreased volumetric efficiency. With high exhaust gas pressure downstream of the combustion chamber, the net pressure differential that draws fresh air into the cylinder is reduced. Hence, less air and fuel fills the cylinder, and volumetric efficiency is spoiled, resulting in a “hole” in horsepower and torque output. The reduced output occurs over the range of engine speeds where the positive exhaust wave returns during valve overlap. Generally, a longer distance between the cylinders and the catalyst 38 results in power loss at lower engine speeds, and a shorter distance between the cylinders and the catalyst 38 results in power loss at higher engine speeds.
In the illustrated exhaust system 18, the catalytic converter 30 is positioned within the first half of the total exhaust gas flow length between the engine 14 and the outlets 35E. Furthermore, as shown in FIGS. 2-4, at least a portion of the collector section 26 is positioned within a resonator chamber 42, which substantially surrounds or encloses the catalytic converter 30. In the illustrated embodiment, the catalytic converter 30 is entirely circumferentially enclosed within the resonator chamber 42 along the full length of the catalytic converter 30. In some embodiments, the resonator chamber 42 does not fully surround or enclose the catalytic converter 30, but rather is adjacent to or partially surrounding the catalytic converter 30. One or more apertures or openings 46 define a perforated section 50 fluidly coupling the exhaust flow passage of the collector section 26 with the resonator chamber 42, thus providing an expansion in the flow passage at the perforated section 50. As shown in FIGS. 2-4, the openings 46 are circular in shape and are equally-spaced around the circumference of the collector section 26. The openings 46 may have other shapes and/or other orientations in other embodiments.
The resonator chamber 42 serves as a “dead end” expansion volume in that the only passageways into and out of the resonator chamber 42 are the openings 46. Thus, all the exhaust gases that enter the resonator chamber 42 through the openings 46 eventually flow out of the resonator chamber 42 through the openings 46 and subsequently pass through the catalytic converter 30. On the other hand, the exhaust gases that do not enter the resonator chamber 42 can pass directly into and through the catalytic converter 30. Flow into the catalytic converter 30 is unobstructed in that there are no physical obstructions to prevent exhaust flow straight from the headers 22 and through the catalytic converter 30, only the flow-restrictive nature of the catalytic converter 30, itself.
In the illustrated embodiment, the collector section 26 does not form a substantial length of the exhaust system 18. This is in contrast to an exhaust system with a long collector section, which typically runs from the front or alongside the engine to a location rearward of the engine. Rather, the collector section 26 of the illustrated exhaust system 18 serves to consolidate the exhaust gas flow passages of the headers 22 over a short length such that the perforated section 50 and the catalytic converter 30 are positioned at or substantially adjacent the downstream ends 36 of each of the headers 22 and within about the first 40% of the total flow length between the engine exhaust ports and the outlets 35E. For example, the length from the rear cylinder exhaust port to the perforated section 50 is about 612 millimeters, and the length from the perforated section 50 to the outlets 35E is about 950 millimeters.
The above description highlights some of the difficulties with simply taking a catalytic converter from a downstream location and moving it to a far upstream location for quicker light off. The resonator chamber 42 and the perforated section 50 of the present invention enable both quick light off and satisfactory power output of the engine 14.
When the exhaust valve (not shown) of one cylinder opens, a high pressure wave propagates down the associated header pipe 22. When this wave arrives at the perforated section 50, its pressure is dissipated by the expansion of the resonator chamber 42. A secondary wave (the remaining component of the original high pressure wave) is incident on the catalyst 38. A portion of the secondary wave of exhaust gases passes through the catalytic converter 30 and on toward the first attenuation portion 34A. The portion of the secondary wave that does not go through the catalytic converter 30 is reflected off the catalyst 38 and back toward the engine 14. Before propagating to the upstream ends 36A of the headers 22, the pressure of the reflected wave is further diminished by expansion that occurs as the reflected wave encounters the perforated section 50. Therefore, the reflected wave that eventually makes it back toward the engine 14 is dissipated through expansions at the perforated section 50 (in addition to the portion which is passed through the catalytic converter 30). In addition to dissipation, a wave cancellation effect occurs under certain operating conditions and is tuned at least in part by the number of openings 46 and the size of the volume within the resonator chamber 42. In the occurrence of wave cancellation, two waves traveling in opposite directions are incident upon one another and at least one of the waves is cancelled out. For example, a wave of fresh exhaust gases from the engine 14 can cancel the effect of a reflected wave traveling from the collector section 26 toward the engine 14.
In the twin-cylinder engine 14 of the illustrated embodiment, in which both cylinders feed the single catalytic converter 30, the reflected wave off of the catalyst 38 is split at the collector section 26 and continues up both header pipes 22. In any exhaust configuration with multiple header pipes feeding a single catalytic converter, the reflected wave off of the catalyst is split at the collector among the header pipes. Therefore, the combination of the perforated section 50 and the resonator chamber 42 can deliver particularly good performance in twin-cylinder, shared exhaust setups, such as on the motorcycle 10 of FIG. 1. Although the exhaust system 18 is shown and primarily described for operation with a 2-into-1 setup, it is also useful for single-cylinder engines, and multi-cylinder engines with separated or shared exhaust systems.
FIGS. 5 and 6 illustrate the enhanced performance afforded by features of the exhaust system 18. FIG. 5 is a computer-simulated graph representative of exhaust pressure (at the port) versus crankshaft angle of the engine 14 while operating at a relatively high engine speed, such as 8000 RPM. One pressure plot in FIG. 5 is for a baseline configuration with a catalytic converter positioned similarly to the catalytic converter 30 in the muffler assembly 35 of the illustrated exhaust system 18. The baseline configuration, which is represented by a solid line, does not include the perforated section 50 or the resonator chamber 42, but is otherwise identical to the illustrated exhaust system 18. A second pressure plot in FIG. 5, indicated by the dashed line, is for the engine 14 with the exhaust system 18, including the perforated section 50 and the resonator chamber 42. The plots on the graph of FIG. 5 illustrate the effect of a reflected exhaust wave arriving at the exhaust port during valve overlap, generally around top dead center (TDC, 360 degrees as indicated in FIG. 5). The exhaust system 18 having the perforated section 50 and the resonator chamber 42 experiences a much lower exhaust pressure during valve overlap. The comparatively high exhaust pressure during valve overlap for the baseline configuration leads to decreased volumetric efficiency and decreased engine output as described above. Due to the location of the catalytic converter 30 adjacent the downstream ends 36 of the headers 22 (i.e., short exhaust length between the engine 14 and the catalyst 38), the reflected exhaust wave is present at the exhaust port during valve overlap at these relatively high engine speeds.
FIG. 6 is a computer-simulated graph illustrating the resulting power loss for an engine operating at speeds at which a reflected exhaust wave arrives at the exhaust port during valve overlap. The solid line on the graph of FIG. 6 represents the engine 14 with the theoretical baseline configuration described above, which serves as a basis for comparison. The dashed line represents the engine 14 with the illustrated exhaust system 18, including the perforated section 50 and the resonator chamber 42. Between 5500 rpm and 9000 rpm, the perforated section 50 and the resonator chamber 42 of the exhaust system 18 allow the engine to generate between 2 and 4 more horsepower. This represents up to about a 5 percent increase in power (measured at about 6000 rpm).
Turning now to the structure and exhaust flow downstream of the catalytic converter 30, a first passage 60 (e.g., pipe) shown in FIGS. 2-4 receives the exhaust gases from the catalytic converter 30. The first passage 60 is positioned within a chamber 62 of the second attenuation portion 34B. However, exhaust gases inside the first passage 60 are not allowed to communicate with the chamber 62 of the second attenuation portion 34B directly. The first passage 60 extends through first and second chambers 64, 68 of the first attenuation portion 34A and opens into a third chamber 72 of the first attenuation portion 34A. First, second, and third bulkheads 74A, 74B, 74C combine with the outer casing 35D to define the first, second, and third separate chambers 64, 68, 72 of the first attenuation portion 34A. The first passage 60 is perforated at the locations passing through the first and second chambers 64, 68 of the first attenuation portion 34A to allow communication of the exhaust gases from the first passage 60 into the first and second chambers 64, 68 and vice versa.
The flow of exhaust gases changes direction in the third chamber 72 of the first attenuation portion 34A and enters a second passage 76 (e.g., pipe). The second passage 76 passes through the first and second chambers 64, 68 of the first attenuation portion 34A and opens into the chamber 62 of the second attenuation portion 34B. The second passage 76 is perforated at the locations passing through the first and second chambers 64, 68 of the first attenuation portion 34A to allow communication of the exhaust gases from the first passage 60 into the first and second chambers 64, 68 and vice versa. The second passage 76 has a cross-sectional size approximately the same as that of the first passage 60.
Exhaust gases enter the chamber 62 of the second attenuation portion 34B, which is provided with an aperture plate 80 (FIGS. 2 and 4) dividing the chamber 62 into sub-chambers that are in communication with each other through the aperture plate 80. The chamber 62 of the second attenuation portion 34B and the aperture plate 80 function as a quarter wave tuner to cancel a particular range of sound waves generated at the engine 14. A portion of the flow of exhaust gases from the second passage 76 travels through the aperture plate 80 to an end plate 84 separating the chamber 62 of the second attenuation portion 34B from the resonator chamber 42. That portion of the flow of exhaust gases is then reflected off the end plate 84 and can act against a pulse of exhaust gases flowing in the opposite direction to cancel out all or a portion of the sound energy.
From the chamber 62 of the second attenuation portion 34B, exhaust gases exit the muffler assembly 35 through a pair of outlet passages 88 (e.g., pipes) shown in FIGS. 2-4, each of which passes through the first and second chambers 64, 68 of the first attenuation portion 34A. The outlet passages 88 are perforated at the locations passing through the first and second chambers 64, 68 of the first attenuation portion 34A to allow communication of the exhaust gases from the outlet passages 88 into the first and second chambers 64, 68 and vice versa.
The muffler assembly 35 is configured to substantially surround the engine 14 along a central axis in the longitudinal direction of the motorcycle 10 defined by the front wheel 15 and the rear wheel 16. The catalytic converter 30 and the resonator chamber 42 extend substantially upright in front of the engine 14 and the first sound attenuation portion 34A fits up between the engine 14 and the rear wheel 16 of the motorcycle 10 (FIG. 1). The muffler assembly 35 has a substantially flat lower surface 96 defined by the outer casing 35D. The first attenuation portion 34A has a height H₁measured up from the lower surface 96 to its highest point. The resonator chamber 42 has a height H₂measured up from the lower surface 96 to its highest point. The second attenuation portion 34B has a minimum height H₃less than one half the respective heights H₁, H₂of the first attenuation portion 34A and the resonator chamber 42. In some embodiments, the height H₃of the second attenuation portion is about one third (or about 33% of) the height H₁of the first attenuation portion 34A and about 40% of the height H₂of the resonator chamber 42.
Even with a small space with which to work (below the engine 14), the muffler assembly 35 is configured to take advantage of the space by utilizing the intermediate portion 35C as the second attenuation portion 34B. Likewise, where there is more ample space on the motorcycle 10 (immediately in front of and behind the engine 14), the muffler assembly 35 is also configured to take advantage of that space via the catalytic converter 30, the resonator chamber 42, and the first attenuation portion 34A. The first attenuation portion 34A extends not only behind the engine 14, but also upwards above a lower edge 17A of the transmission 17 (FIG. 7), such that the first attenuation portion 34A is positioned substantially between the transmission 17 and the rear wheel 16. In some embodiments, the engine 14 and the transmission 17 are partially or wholly contained within a shared housing (e.g., a common casting including at least an engine crankcase portion and a transmission case portion).
The muffler assembly 35 defines an indentation or recess 100, which is contoured to receive the lower part of the engine 14, substantially matching a contour of the lower edge 14A of the engine 14. The depth of the recess 100 is equal to the height H₁of the first attenuation portion 34A minus the height H₃of the second attenuation portion 34B. Thus, the engine 14 is received into the recess 100 to a depth about two times the height H₃of the second attenuation portion 34B.
Thus, the invention provides, among other things, a compact muffler assembly 35 having an attenuation portion 34B below the engine 14, and a recess 100 configured to receive a significant portion of the engine 14. Various features and advantages of the invention are set forth in the following claims.

Claims

1. A muffler assembly for a motorcycle engine, the muffler assembly comprising:

a downstream attenuation portion adapted to be positioned substantially rearward of the motorcycle engine and having a first height;

an upstream portion adapted to be positioned substantially forward of the motorcycle engine and having a second height; and

an intermediate attenuation portion between the upstream portion and the downstream attenuation portion, the intermediate attenuation portion being adapted to be positioned substantially below the motorcycle engine and having a third height less than half the first height.

2. The muffler assembly of claim 1, further comprising a shell encasing the upstream portion, the downstream attenuation portion, and the intermediate attenuation portion.

3. The muffler assembly of claim 1, wherein the upstream portion includes a catalytic converter.

4. The muffler assembly of claim 1, wherein the downstream attenuation portion includes first, second, and third bulkheads at least partially defining first, second, and third chambers.

5. The muffler assembly of claim 1, wherein the intermediate attenuation portion includes a chamber configured to receive exhaust gases from the downstream attenuation portion.

6. The muffler assembly of claim 1, wherein the third height is about 33 percent of the first height and about 40 percent of the second height.

7. A motorcycle comprising:

an engine;

a transmission coupled to the engine and configured to receive power from the engine;

a rear wheel coupled to the transmission and configured to receive engine power through the transmission; and

a muffler assembly coupled to the engine and in communication therewith to receive exhaust gases from the engine, the muffler assembly including

a downstream attenuation portion positioned between the transmission and the rear wheel, and

an intermediate attenuation portion positioned forwardly of the downstream attenuation portion, the intermediate attenuation portion extending along an underside of the engine.

8. The exhaust system of claim 7, further comprising an upstream portion of the muffler assembly including a catalytic converter.

9. The exhaust system of claim 8, wherein the downstream attenuation portion includes a plurality of chambers configured to receive exhaust gases from the upstream portion via a first passage.

10. The exhaust system of claim 9, wherein the intermediate attenuation portion includes a chamber configured to receive exhaust gases from the downstream attenuation portion via a second passage.

11. The exhaust system of claim 7, wherein the engine includes a crankcase, the muffler assembly being shaped to conform to a front portion, a bottom portion, and a rear portion of the crankcase.

12. The exhaust system of claim 7, wherein the intermediate attenuation portion has a height less than half of a height of the downstream attenuation portion.

13. The exhaust system of claim 7, wherein the height of the intermediate attenuation portion is about 33 percent of the height of the downstream attenuation portion.

14. A motorcycle comprising:

an engine;

two wheels defining a central axis of the motorcycle; and

a muffler assembly in communication with the engine to receive exhaust gases from the engine, the muffler assembly being positioned substantially along the central axis and including

a downstream attenuation portion,

an upstream portion,

an intermediate attenuation portion positioned substantially under the engine, the intermediate attenuation portion having a height, and

a recess at least partially defined by the downstream attenuation portion and the intermediate attenuation portion, wherein the recess has a depth greater than the height of the intermediate attenuation portion, the engine being positioned substantially within the recess.

15. The motorcycle of claim 14, further comprising a muffler shell defining a substantially continuous outer surface of the muffler assembly, the muffler shell extending from the upstream portion to the downstream attenuation portion.

16. The motorcycle of claim 15, wherein the muffler shell at least partially defines a plurality of chambers of the downstream attenuation portion and a chamber of the intermediate attenuation portion.

17. The motorcycle of claim 15, wherein the upstream portion includes a catalytic converter.

18. The motorcycle of claim 17, wherein the muffler shell at least partially defines an expansion chamber, the catalytic converter being positioned within the expansion chamber.

19. The motorcycle of claim 14, wherein the height of the intermediate attenuation portion is about half the depth of the recess.

20. The motorcycle of claim 19, wherein the recess is formed by a contour substantially matching a lower contour of the engine.