| 摘要 |
A tuned exhaust gas relief system for marine propulsion systems, for example an outboard motor, includes a lower drive shaft housing coupled to a two stroke engine by a pair of intermediate stacked exhaust extension plates. The housing directs the exhaust gas downwardly to a through-the-hub exhaust propeller for exit therethrough. With the unit in reverse or idling, exhaust gases are trapped within the housing. A pair of tuned exhaust relief passageways may be formed by cavities in the mating faces of the two extension plates with a pair of inlet openings in the lower wall of the bottom plate. A baffle member may overlie the inlet openings. The passageways define constant cross-sectional area channels which terminate in exhaust openings in the rear wall of the drive shaft housing. A resonant cavity, which may be integrally formed by mating recesses in the opposed faces of the extension plates, is in close spaced relation to each idle exhaust passageway. Connecting openings are distributed along the length of the common wall between the cavity and passageway. An exhaust housing within the drive shaft housing forms an exhaust expansion chamber of high acoustical impedance between the lower ends of the exhaust pipe means and the upper wall extension plate assembly. The idle exhaust passageways have a low acoustical impedance, so that the acoustical impedance mismatch between the chamber and the passageways attenuates the high frequency sounds. The idle exhaust passageways may transmit the half wave frequency and even multiplies thereof. The resonator cavity is selected to attenuate the particular half wave frequency, or an even multiple thereof, which because of physical space requirements may be present and not attenuated by the idle exhaust passageway means.
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