Outboard motor exhaust system

An outboard motor comprising a lower unit including an outer housing and an inner exhaust housing mounted within the outer housing, the inner exhaust housing defining an exhaust chamber, the upper end of the inner exhaust housing having therein spaced first and second exhaust inlets, and the inner exhaust housing also including a first exhaust pipe having an upper end communicating with the first exhaust inlet and having a lower end communicating with the exhaust chamber, a second exhaust pipe having an upper end communicating with the second exhaust inlet and having a lower end communicating with the exhaust chamber, and a connecting exhaust pipe having opposite first and second ends respectively communicating with the first and second exhaust inlets, the connecting pipe being open only at the opposite ends, and a two-stroke internal combustion engine including first, second, third and fourth cylinders and first, second, third and fourth exhaust gas ducts having respective first ends communicating respectively with the first, second, third and fourth cylinders and having respective second ends, the second ends of the first and third ducts communicating with the first exhaust inlet, and the second ends of the second and fourth ducts communicating with the second exhaust inlet.

BACKGROUND OF THE INVENTION 
The invention relates to internal combustion engines and more particularly 
to two-stroke internal combustion engines. Still more particularly, the 
invention relates to exhaust systems for such engines, especially in 
outboard motors. 
U.S. Pat. No. 5,101,626, which is assigned to the assignee hereof and which 
is incorporated herein by reference, discloses a two-stroke internal 
combustion engine including an exhaust gas discharge system which, for 
each cylinder, provides an outgoing positive acoustical pressure wave that 
arrives at the exhaust port of the previously fired cylinder prior to 
closure thereof and provides a returning negative acoustical pressure wave 
that travels in the exhaust system after substantial completion of the 
travel of the outgoing positive acoustical pressure wave and that arrives 
at the exhaust port of the originating cylinder prior to closure thereof. 
SUMMARY OF THE INVENTION 
The invention provides an outboard motor and a two-stroke internal 
combustion engine assembly incorporating at least some of the teachings of 
U.S. Pat. No. 5,101,626. 
The outboard motor comprises a lower unit including an outer housing having 
an exhaust outlet adjacent the lower end of the outer housing. The lower 
unit also includes an inner exhaust housing within the outer housing. The 
inner exhaust housing defines an exhaust chamber, and the lower end of the 
inner exhaust housing has therein an exhaust outlet communicating between 
the exhaust chamber and the exhaust outlet of the outer housing. The upper 
end of the inner exhaust housing has therein spaced first and second 
exhaust inlets. The inner exhaust housing also includes: a first exhaust 
pipe or megaphone having an upper end communicating with the first exhaust 
inlet and having a lower end communicating with the exhaust chamber; a 
second exhaust pipe or megaphone having an upper end communicating with 
the second exhaust inlet and having a lower end communicating with the 
exhaust chamber; and a connecting exhaust pipe or crossover tube which is 
preferably U-shaped, which has opposite first and second ends respectively 
communicating with the first and second exhaust inlets, and which is open 
only at its opposite ends. 
The outboard motor also comprises a two-stroke internal combustion engine 
mounted on the upper end of the outer housing. The engine is preferably a 
V-4 engine with first, second, third and fourth cylinders having 
respective first, second, third and fourth exhaust ports. The cylinders 
sequentially fire in the stated series. The engine also includes first, 
second, third and fourth exhaust gas ducts having respective first or 
upper ends communicating respectively with the first, second, third and 
fourth exhaust ports and having respective second or lower ends. The lower 
ends of the first and third ducts communicate with the first exhaust inlet 
in the inner exhaust housing, and the lower ends of the second and fourth 
ducts communicate with the second exhaust inlet in the inner exhaust 
housing. Thus, the first and third cylinders communicate via the first and 
third exhaust gas ducts with the first megaphone and with the first end of 
the crossover tube, and the second and fourth cylinders communicate via 
the second and fourth exhaust gas ducts with the second megaphone and with 
the second end of the crossover tube. The crossover tube therefore 
connects each cylinder with the previously fired cylinder. 
Preferably, the first and second megaphones and the crossover tube are 
formed independently of each other such that any one of the megaphones and 
crossover tube can be reconfigured without changing the other two. This 
allows optimization of each via testing without affecting the others. 
Location of the megaphones and the crossover tube in the inner exhaust 
housing permits modification of the megaphones and the crossover tube with 
minimal changes to the overall outboard motor. Also, in the preferred 
embodiment of the invention, the flow length from the first exhaust port 
to the lower end of the first megaphone is substantially equal to the flow 
length from the second exhaust port to the lower end of the second 
megaphone, and the flow length from the third exhaust port to the lower 
end of the first megaphone is substantially equal to the flow length from 
the fourth exhaust port to the lower end of the second megaphone. 
Furthermore, the flow lengths via the crossover tube from each of the 
exhaust ports to the exhaust port of the previously fired cylinder are 
preferably unequal. The unequal flow lengths provide broad band tuning. 
The exhaust system provides the outboard motor with increased low-end power 
for quick acceleration and with increased part-throttle running stability, 
due to the broad torque curve. 
Other features and advantages of the invention will become apparent to 
those skilled in the art upon review of the following detailed 
description, claims and drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An outboard motor 10 embodying the invention is illustrated in the 
drawings. The outboard motor 10 comprises (see FIG. 1) a lower unit 14 
adapted to be mounted on the transom 18 of a boat 22 for pivotal movement 
relative thereto about a generally horizontal tilt axis 26 and for pivotal 
movement relative thereto about a generally vertical steering axis 30. The 
lower unit 14 is preferably mounted on the boat 22 by a transom bracket 34 
and a swivel bracket 38. The transom bracket 34 and the swivel bracket 38 
are conventional and will not be described in greater detail. 
The lower unit 14 includes (see FIG. 2) an outer housing 42 having upper 
and lower ends and including an exhaust outlet 46 adjacent the lower end 
of the outer housing 42. A propeller shaft 50 (see FIG. 1) is rotatably 
supported by the outer housing 42 adjacent the lower end thereof. A 
propeller 54 is mounted on the propeller shaft 50 for rotation therewith. 
The propeller 54 preferably has therethrough an exhaust outlet passageway 
58 as in known in the art. The exhaust outlet 46 communicates with the 
propeller exhaust outlet passageway 58. 
The lower unit 14 also includes (see FIG. 2) an inner exhaust housing 62 
mounted within the outer housing 42. The inner exhaust housing 62 has 
upper and lower ends and defines an exhaust chamber 66 extending between 
the upper and lower ends of the inner exhaust housing 62. The lower end of 
the inner exhaust housing 62 has therein an exhaust outlet 70 
communicating between the exhaust chamber 66 and the exhaust outlet 46 of 
the outer housing 42. The exhaust chamber 66 thus communicates with the 
propeller exhaust outlet passageway 58. The upper end of the inner exhaust 
housing 62 has therein spaced first and second exhaust inlets 74 and 78, 
respectively. The inner exhaust housing 62 also includes a first 
downwardly-diverging exhaust pipe or megaphone 82 having an upper end 
communicating with the first exhaust inlet 74 and having a lower end 
communicating with the exhaust chamber 66. The inner exhaust housing 62 
also includes a second downwardly-diverging exhaust pipe or megaphone 86 
having an upper end communicating with the second exhaust inlet 78 and 
having a lower end communicating with the exhaust chamber 66. The inner 
exhaust housing 62 further includes a connecting exhaust pipe or crossover 
tube 90 having opposite first and second or (see FIG. 2) left and right 
ends respectively communicating with the first and second exhaust inlets 
74 and In the illustrated construction, the upper end of the megaphone 82 
and the left end of the crossover tube 90 define the exhaust inlet 74, and 
the upper end of the megaphone 86 and the right end of the crossover tube 
90 define the exhaust inlet 78. Alternatively, the megaphones 82 and 86 
and the crossover tube 90 could split a slight distance below the inlets 
74 and 78. The crossover tube 90 is open only at its opposite ends. 
The inner exhaust housing 62 is preferably made of metal, such as aluminum, 
and is cast as a single piece. Preferably, the megaphones 82 and 86 and 
the crossover tube 90 are formed independently of each other such that any 
one of the megaphones 82 and 86 and the crossover tube 90 can be 
reconfigured with changing the others of the megaphones 82 and 86 and the 
crossover tube 90. 
The outboard motor 10 also comprises (see FIG. 1) a two-stroke internal 
combustion engine 94 (shown schematically in the drawings) having a lower 
end mounted on the upper end of the outer housing 42. The lower end of the 
engine 94 is also secured to the upper end of the inner housing 62 by 
suitable means (not shown). The engine 94 is drivingly connected to the 
propeller shaft 50 via a conventional drive train 98. The engine 94 is 
preferably a V-4 engine including (see FIG. 2) first, second, third and 
fourth cylinders 101, 102, 103 and 104, respectively, which sequentially 
fire in the stated series. The first and third cylinders 101 and 103 are 
located in one cylinder bank, and the second and fourth cylinders 102 and 
104 are located in the other cylinder bank. The cylinders 101, 102, 103 
and 104 preferably fire at an even firing interval of ninety degrees. It 
should be understood, however, that the invention is also applicable to 
two-stroke internal combustion engines having different numbers of 
cylinders, different firing intervals and different configurations. 
The cylinders 101, 102, 103 and 104 include respective first, second, third 
and fourth exhaust ports 111, 112, 113 and 114. The engine 94 also 
includes first, second, third and fourth exhaust gas ducts 121, 122, 123 
and 124 having respective first or upper ends communicating respectively 
with the exhaust ports 111, 112, 113 and 114 and having respective second 
or lower ends. The lower ends of the first and third ducts 121 and 123 
communicate with the first exhaust inlet 74 in the inner housing and the 
lower ends of the second and fourth ducts 122 and 124 communicate with the 
second exhaust inlet 78 in the inner housing. In the illustrated 
construction, the ducts 121 and 123 actually merge into a single duct that 
communicates with the exhaust inlet 74, and the ducts 122 and 124 actually 
merge into a single duct that communicates with the exhaust inlet 78. 
Thus, the first and third exhaust ports 111 and 113 communicate with the 
megaphone 82 and with the left end of the crossover tube 90 and the second 
and the fourth exhaust ports 112 and 114 communicate with the megaphone 86 
and with the right end of the crossover tube 90. 
The flow length from the first exhaust port 111 to the lower end of the 
megaphone 82 is preferably substantially equal to the flow length from the 
second exhaust port 112 to the lower end of the megaphone 86, and the flow 
length from the third exhaust port 113 to the lower end of the megaphone 
82 is preferably substantially equal to the flow length from the fourth 
exhaust port 114 to the lower end of the megaphone 86. Also, in the 
preferred embodiment, the flow lengths via the crossover tube 90 from each 
of the exhaust ports 111, 112, 113 and 114 to the exhaust port of the 
previously fired cylinder are not equal. Instead, these flow lengths are 
selected to provide broad band tuning. The actual flow lengths must be 
determined on a case-by-case basis. One of ordinary skill in the art can 
easily do this following the teachings of this application and of U.S. 
Pat. No. 5,101,626. It should be understood that some of these flow 
lengths (via the crossover tube 90) could alternatively be equal, 
depending on the tuning characteristics of a particular outboard motor. 
The exhaust gas ducts 121, 122, 123 and 124 should have sufficient 
cross-sectional area so as not to restrict flow but should be small enough 
to retain satisfactory sonic pulse energy for good exhaust port plugging 
as described in U.S. Pat. No. 5,101,626. 
During operation of the outboard motor 10, each megaphone 82 or 86 allows 
exhaust gas from the engine 94 to exit into the atmosphere. Also, when the 
expanding acoustical pulse from an originating cylinder reaches the lower 
end of the associated megaphone, the pulse reflects back to the 
originating exhaust port as a negative pressure wave that assists in 
scavenging. The acoustical pulse created by the blow-down cycle of the 
originating cylinder travels in the crossover tube 90 and, with a high 
degree of energy retention, enters the other bank of cylinders where the 
pulse plugs the exhaust port of the previously fired cylinder. 
Except for any differences described above, the exhaust system of the 
outboard motor 10 operates in the same manner as the exhaust system 
disclosed in U.S. Pat. No. 5,101,626. 
Various features of the invention are set forth in the following claims.