Abstract:
A propulsion system for a boat has a powerhead or a motor with an exhaust port for exhaust gases. The exhaust gases are exhausted through the propeller hub via an exhaust housing. The exhaust housing is a walled enclosure having an inlet, an internal volume in flow communication with the inlet, and an outlet in flow communication with the internal volume. The exhaust housing further includes hollow structures for dividing a portion of the internal volume into a plurality of flow channels which extend in side-by-side relationship. The transverse dimensions of each flow channel is substantially less than the transverse dimensions of the walled enclosure. The result is that standing waves are shifted to a higher frequency range. The hollow dividing structures have internal volumes which communicate with space external to the exhaust housing via openings in the walled enclosure, which allow the admission of a cooling medium. The hollow structures increase the stiffness of the walled enclosure of the exhaust housing.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to means for suppressing noise in an outboard marine engine. In particular, the present invention relates to means for suppressing noise transmitted from the exhaust housing of an outboard engine. 
     BACKGROUND OF THE INVENTION 
     Typical marine engines are noisy, especially when being operated at higher rpm&#39;s while driving a vessel rapidly through the water. This noisy operation is extremely unattractive to occupants of the vessel, as well as to passers-by, and it is highly desirable to reduce this noise without reducing vessel efficiency. Further, regulatory bodies, in their desire to improve the environment, are imposing emission standards on marine vessels. These standards not only regulate the contents of the emissions but also apply to the noise level of the emission. It is therefore highly desirable to provide a marine engine that is noise reduction efficient without detracting from the vessel operating efficiently. 
     More general than the noise reduction is noise control. Noise control requires an understanding of the vibro-acoustic behavior of the article in question with its environment. If boundary conditions permit, approximations can be made by isolating the article from its environment. This cannot be done “simply” for an integrated structure. For example, an outboard marine engine is an integrated structure. To capture correctly the vibro-acoustic behavior of an outboard engine, the engine should be fully assembled, mounted to a boat and in the open water. For example, feedback from the added inertia of the water as the boat travels in the water could produce a narrow-band spectrum different from a steady-state condition. There is also feedback from the components of the engine, for example, the crankshaft and block can produce a phenomenon that does not exist for either part acting alone. 
     To determine the acoustic “fingerprint” for an integrated structure such as an outboard marine engine, a narrow-band analysis must be performed. This will allow identification of tones, i.e., frequency responses, of the interacting components. The components corresponding to these responses can be identified from the frequencies, i.e., based on wavelength and speed of sound. Vibro-acoustic treatments can be designed and or critically placed to attenuate or simply move a tone from one frequency to another. The effectiveness of this effort is based on the precision of the data and the methodology by which the data is acquired. 
     The precision of the data is a function of the frequencies of the data collected and of the transducer sensitivity. The frequency range of interest is a function of human hearing, i.e., 10 kHz is sufficient. For the present work, data was collected using accelerometers and microphones. Accelerometer data was collected to 5 kHz at 1 Hz bandwidth; microphone data was collected to 10 kHz at 2.5 Hz bandwidth. Acoustic intensity testing and stethoscopic probing both showed agreement that over 80% of the vibro-acoustic energy produced by a particular outboard marine engine was coming from below the interface between the engine&#39;s upper and lower motor covers, a large part of the noise being transmitted from the exhaust housing. It was further discovered that a particular tone produced inside the exhaust housing did not change frequency as the rpm of the engine was modulated. This discovery led to the realization that a standing wave was being set up inside the exhaust housing, causing the exhaust housing to vibrate at low frequency (in one case, at about 3,500 Hz). 
     Thus there is a need for a structure which can be incorporated inside an exhaust housing of an outboard marine engine to break up standing waves and reduce noise output. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved exhaust housing having means for breaking up standing acoustic waves resonating inside the exhaust housing. Such standing waves intensify and prolong the acoustic noise transmitted from the exhaust housing. A standing acoustic wave can be produced when the passage through which air, e.g., exhaust gas, flows has a dimension which equals at least one fourth the speed of sound divided by the frequency of the standing wave. 
     In accordance with the preferred embodiment of the invention, this resonant condition is eliminated by incorporating plates inside the exhaust housing. These resonance-avoiding plates are generally parallel to the direction of flow from the powerhead and are welded to the walls of the exhaust housing. (The terms “powerhead” and “motor” will be used interchangeably throughout the written description and the claims.) The resonance-avoiding plates divide the exhaust housing into multiple channels, each channel having transverse dimensions smaller than the transverse dimensions of the unmodified exhaust housing. Consequently, any standing acoustic wave in one of the channels will have a frequency higher than the frequency of a standing wave in the unmodified exhaust housing. In addition, the plates serve to increase the stiffness of the exhaust housing, changing the mode of vibration of the exhaust housing from low to high frequency. As a result, the tones produced by the vibrating exhaust housing will be moved to higher frequencies, i.e., further away from the so-called Speech Interference Level 123 (SIL123) corresponding to the frequency range from 1,000 to 3,000 hertz. 
     The broad concept of the invention is directed to a boat propulsion system having a motor with an exhaust port for exhaust gases, the exhaust gases being exhausted via a resonance-avoiding exhaust housing. The exhaust housing is a walled enclosure having an inlet, an internal volume in flow communication with the inlet, and an outlet in flow communication with the internal volume. The exhaust housing further includes hollow structures for dividing a portion of the internal volume into a plurality of flow channels which extend in side-by-side relationship. The transverse dimensions of each flow channel is substantially less than the transverse dimensions of the walled enclosure. The result is that standing waves are shifted to a higher frequency range. The hollow dividing structures have internal volumes which communicate with space external to the exhaust housing via openings in the walled enclosure, which allow the admission of a cooling medium. The hollow structures also increase the stiffness of the walled enclosure of the exhaust housing, shifting the vibration mode of the exhaust housing to higher frequencies. 
     The invention further encompasses a method of retrofitting an engine exhaust housing comprising a walled enclosure having an inlet, an internal volume in flow communication with said inlet, and an outlet in flow communication with said internal volume. The retrofitting method comprises the step of dividing a portion of the exhaust housing internal volume into a plurality of flow channels which extend in side-by-side relationship. Each of the flow channels has an inlet which is closer to the exhaust housing inlet than the flow channel outlet is and an outlet which is closer to the exhaust housing outlet than the flow channel inlet is. The dividing step is accomplished by installing a hollow structure having an opening inside the internal volume of the exhaust housing, and forming an opening in the walled enclosure at a location such that the opening of the walled enclosure is in flow communication with the opening of the hollow structure. The installing step comprises the steps of attaching rigid plates to the walled enclosure such that the stiffness of the walled enclosure is increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic showing a typical outboard marine engine to which the present invention can be applied. 
     FIG. 2 is a schematic showing the outboard marine engine of FIG. 1 with the upper motor cover removed to reveal the powerhead. 
     FIG. 3 is a schematic showing a prior art technique for exhausting gases from a powerhead of an outboard engine through the propeller. 
     FIG. 4 is a schematic showing an exploded view of a known exhaust housing assembly. 
     FIGS. 5 and 6 are schematics side and rear elevational views of an upper inner exhaust housing in accordance with the preferred embodiment of the invention. 
     FIGS. 7-9 are schematics showing sectional views of the upper inner exhaust housing of FIGS. 5 and 6, the sections being taken along lines  7 — 7 ,  8 — 8  and  9 — 9  respectively, indicated in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An outboard propulsion unit and means for mounting that propulsion unit to the stern of a boat are shown in FIG.  1 . The mounting means comprise a pair of stern brackets  2  (only one of which is visible in FIG. 1) designed to be mounted to the boat stern. A swivel bracket  4 , which supports the propulsion unit, is pivotably mounted to the stern brackets  2 . The swivel bracket  4  allows the propulsion unit to be tilted about a horizontal axis. The swivel bracket  4  rotatably supports a steering arm assembly  6  (only part of which is visible in FIG. 1) which is rigidly connected to the propulsion unit, to allow the propulsion unit to be turned about the axis of the steering arm assembly  6  for steering the boat. 
     The propulsion unit comprises a powerhead  8  (visible in FIG. 2) housed in a casing formed by an upper motor cover assembly  10  and a lower motor cover assembly  12 . The lower motor cover assembly has an oval-shaped opening that allows the steering arm assembly to penetrate the lower motor cover assembly and attach to the assembly (described below) which supports the powerhead. The upper motor cover is preferably made of acetyl butyl styrene, while the lower motor cover is preferably made of fiberglass. 
     Referring again to FIG. 1, the weight of the powerhead  8  is supported by an exhaust housing assembly  26 , which is in turn mounted to the swivel bracket  4  in a known manner. Exhaust from the powerhead flows downward through a passageway in the exhaust housing assembly. A gear case  32  is attached to the bottom of the exhaust housing assembly  26 . The gear case houses the lowermost part of the vertical drive shaft (not shown) which is coupled to the powerhead, the propeller shaft (not shown) and the gears (not shown) for converting rotation of the drive shaft into rotation of the propeller shaft. A propeller  34  is mounted on the end of the propeller shaft in conventional manner. The exhaust gases flow through the inner exhaust housing and are exhausted below the waterline through an outlet in the propeller hub  36 . This arrangement is well-known in the prior art and is generally depicted in FIG. 3, which shows a path  38  for the flow of exhaust gas from an exhaust port of the powerhead  8  to the hollow propeller hub  36 . 
     The components of a known exhaust housing assembly  26  are shown in the exploded view of FIG.  4 . The assembly comprises an outer exhaust housing  40  which is attached to the swivel bracket (item  4  in FIGS. 1 and 2) via a pair of lower rubber mounts  42  (only one of which is shown in FIG.  4 ). The outer exhaust housing  40  supports the powerhead via an exhaust housing adapter  44 , on which the powerhead sits. The steering arm assembly (item  6  in FIG. 1) is coupled to an upper rubber mount assembly  46 , which is installed within a recess in the exhaust housing adapter  44 . 
     The exhaust housing assembly  26  further comprises an inner exhaust housing which is supported inside the outer exhaust housing. The inner exhaust housing has an inlet at the top which is in flow communication with the exhaust port of the powerhead, and an outlet at the bottom which is in flow communication with the hollow propeller hub. The inner exhaust housing comprises an upper inner exhaust housing  48  and a lower inner exhaust housing  50 . The outlet at the bottom of the upper inner exhaust housing  48  is connected to the inlet at the top of the lower inner exhaust housing  50 , the interface being sealed by a pair of exhaust housing seals  52 . Other components shown in FIG. 4 are as follows: item  54  is a spray deflector; item  56  is a seal placed between the gear case and the lower inner exhaust housing  50 ; item  58  is a gasket placed between the adapter  44  and the powerhead; item  60  is a water plate which directs water and exhaust into the exhaust section; item  62  is a gasket placed between the adapter  44  and the water plate  60 ; and item  64  is a gasket placed between the upper inner exhaust housing  48  and the water plate  60 . The adapter  44 , the outer exhaust housing  40  and the inner exhaust housing  48 ,  50  are preferably made of aluminum. 
     During operation of the prior art engine depicted in FIGS. 1,  2  and  4 , an undesirable near-SIL123 frequency noise component is associated with maintenance of a standing acoustic wave inside the upper inner exhaust housing  48 . In accordance with the preferred embodiment of the invention, that near-SIL123 standing wave can be eliminated by modifying the upper inner exhaust housing as described below with reference to FIGS. 5-9. 
     FIGS. 5 and 6 are side and rear elevational views of an upper inner exhaust housing  48 ′ in accordance with the preferred embodiment of the invention. The only novel feature visible in FIG. 5 is the recess  66  (described in detail below), while the only novel feature visible in FIG. 6 is the channel  68  (also described in detail below). Otherwise the external appearance of the upper inner exhaust housing  48 ′ is unchanged from that of the upper inner exhaust housing  48  shown in FIG.  4 . 
     The structural features incorporated in the preferred embodiment of the invention are best seen in the sectional views of FIGS. 7-9, each section being taken along a respective horizontal plane through the upper inner exhaust housing as indicated by lines  7 — 7 ,  8 — 8  and  9 — 9  in FIG.  5 . 
     Referring to FIG. 7, the upper inner exhaust housing  48 ′ comprises a front wall  70 , a rear wall  72 , a port side wall  74  and a starboard side wall  76 . These walls form a walled enclosure having an exhaust inlet at the top (in flow communication with the exhaust port of the powerhead) and an exhaust outlet at the bottom (in flow communication with the hollow propeller hub). The upper inner exhaust housing  48 ′ is attached to the water plate (item  60  in FIG. 4) via flange  77 . As best seen in FIG. 8, a circular opening  78  allows a path of least resistance at idle for exhaust gases. 
     In accordance with the preferred embodiment of the invention, the internal volume of the upper inner exhaust housing  48 ′ is divided into four flow channels  82 ,  84 ,  86  and  88  by a cruciform structure, each member of the cruciform structure being attached at its distal end to a respective wall of the walled enclosure. As best seen in FIG. 8, the cruciform structure comprises a first pair of opposing, but mutually diverging, plates  90  and  92 , which extend from the front wall  70  to the rear wall  72 , and from an upper elevation to a lower elevation, the distance between the upper and lower elevations being less than the full height of the upper inner exhaust housing  48 ′. The opposing plates  90  and  92  are generally disposed with mirror symmetry on opposite sides of a mid-plane  94  of the upper inner exhaust housing  48 ′. The distance between the opposing plates  90  and  92  in a vertical plane perpendicular to the mid-plane increases linearly in the downward direction from the upper elevation to the lower elevation. Also, the distance between the opposing plates  90  and  92  in a horizontal plane (i.e., the plane of the paper) perpendicular to the mid-plane increases linearly in the forward direction from a central zone to the front wall  70  and also increases linearly in the rearward direction from the central zone to the rear wall  72 . The upper edges of plates  90  and  92  are connected by a top strip  94  (see FIG. 7) and the lower edges of the plates  90  and  92  are connected by a bottom strip  96  (see FIG. 9) to form a cooling channel  68  (see FIG. 6) which is open at both ends, i.e., which communicates with respective openings in the front and rear walls of the upper inner exhaust housing  48 ′. During outboard engine operation, this cooling channel is filled with water to cool plates  90  and  92 , thereby preventing damage to plates  90  and  92  due to excessive heat from the powerhead. The cooling channel  68  communicates with the water-filled space between the inner and outer exhaust housings, as previously described. The divergence (i.e., non-parallelism) of opposing plates  90  and  92  increases the stiffness of the upper inner exhaust housing  48 ′ and also increases the volume of cooling water which can fill channel  68 . 
     Returning to FIG. 8, the preferred embodiment of upper inner exhaust housing  48 ′ further comprises a second pair of opposing and diverging plates  100  and  102 , which extend from plate  92  to the port side wall  74 , and a third pair of opposing and diverging plates  104  and  106  which extend from plate  90  to the starboard side wall  76 . The second and third pairs, like the first pair, of plates are generally parallel to the direction of the powerhead exhaust gas flow down through the upper inner exhaust housing  48 ′. The plates  100 ,  102 ,  104  and  106  have the same height as plates  90  and  92 , and extend between the same upper and lower elevations. The opposing plates  100  and  102  are generally disposed with mirror symmetry on opposite sides of a vertical plane  108  which is perpendicular to the mid-plane  94 , while the opposing plates  106  and  106  are generally disposed with mirror symmetry on opposite sides of the same vertical plane  106 . The distance between opposing plates  100  and  102  in a vertical plane parallel to the mid-plane  94  increases linearly in the downward direction from the upper elevation to the lower elevation. The same is true for the opposing plates  104  and  106  on the starboard side. The upper edges of plates  104  and  106  are connected by a top strip  110  and the lower edges of plates  104  and  106  are connected by a bottom strip  112  to form a recess  65  which communicates with an opening in the starboard side wall  76 . Similarly, the upper edges of plates  100  and  102  are connected by a top strip  114  and the lower edges of plates  100  and  102  are connected by a bottom strip  116  to form recess  66  (see FIG. 5) which communicates with an opening in the port side wall  74 . For ease of manufacture, the recesses  65  and  66  are not in flow communication with the channel  68 , but optionally, the recesses could be in flow communication with the channel via openings (not shown). Preferably the distance between opposing plates  100  and  102  in a horizontal plane perpendicular to mid-plane  94  increases linearly in the port direction from plate  92  to the port side wall  74 , while the distance between plates  104  and  106  in a horizontal plane perpendicular to mid-plane  94  increases linearly in the starboard direction from plate  90  to the starboard side wall  76 . Again, the divergence in the opposing plates of the second and third pairs increases the stiffness of the upper inner exhaust housing  48 ′ and also increases the volume of cooling water which may enter recesses  65  and  66  to cool the plates. 
     The three pairs of opposing plates  90 / 92 ,  100 / 102  and  104 / 106  divide the main inner volume of the upper inner exhaust housing into four separate channels  82 ,  84 ,  86  and  88 , as shown in FIG.  7 . Each flow channel has transverse dimensions which are less than the transverse dimensions of the unmodified upper inner exhaust housing, thereby increasing the frequencies of standing acoustic waves inside the upper inner exhaust housing and adding stiffness to the upper inner exhaust housing. The result is a reduction in the near-SIL 123  frequency noise being transmitted from the upper inner exhaust housing during engine operation. 
     It is advantageous to manufacture exhaust housings in accordance with the teaching disclosed herein. Moreover, existing exhaust housings can be retrofitted to incorporate the novel structural features of the invention. At a minimum, the retrofit method comprises the steps of installing a hollow structure having an opening inside the exhaust housing, and forming an opening in exhaust housing wall at a location such that the latter opening is in flow communication with the opening of the hollow structure. In particular, the retrofitting can be performed by welding rigid plates to the walls of the exhaust housing such that the stiffness of the walled enclosure is increased. 
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the essential scope thereof. Therefore it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.