BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to an exhaust system for a motorcycle and to a motorcycle
including such an exhaust system.
Description of the Background Art
[0002] Conventionally, various mufflers have been developed in order to reduce exhaust sounds
caused by discharges of exhaust gases from engines of automobiles (see
JP 60-37287 B, for example).
[0003] The muffler disclosed in
JP 60-37287 B has a perforated pipe and an external cylinder. The perforated pipe is inserted into
the external cylinder. A resonant chamber is formed between an outer peripheral surface
of the perforated pipe and an inner peripheral surface of the external cylinder. An
expansion chamber is formed on the downstream side of the resonant chamber within
the external cylinder. The resonant chamber and the expansion chamber are separated
by a partition plate. An internal space of the perforated pipe communicates with the
resonant chamber through holes of the perforated pipe. Furthermore, a downstream end
of the perforated pipe is opened within the expansion chamber.
[0004] An end on the downstream side of the exhaust pipe is narrowed down, to form a small-diameter
portion. The end of the exhaust pipe is fitted in the perforated pipe. The exhaust
gas discharged from the exhaust pipe is introduced into the expansion chamber through
the perforated pipe, and is further introduced into the atmosphere through a tail
pipe. At this time, the exhaust sound is reduced by the function of the resonant chamber
and the expansion chamber.
[0005] Meanwhile, mufflers are also provided in motorcycles. The mufflers used in the motorcycles
are required to reduce exhaust sounds caused by discharges of high-pressure and high-temperature
exhaust gases. Furthermore, the rotational speeds of engines of the motorcycles rise
to not less than 8000 rpm. Therefore, the mufflers used in the motorcycles are required
to bring out desired engine performances by adjusting the flows of the exhaust gases
discharged from the engines.
[0006] In the case of multiple cylinder engines, a plurality of exhaust pipes communicate
with one another, so that a pressure fluctuation in each of the exhaust pipes is absorbed
by the other exhaust pipe. Therefore, the pressure fluctuation in each of the exhaust
pipes is relatively small. In contrast, in the case of single cylinder engines, a
pressure fluctuation in an exhaust pipe is great.
[0007] In recent years, motorcycles in which exhaust systems are provided with catalysts
in order to clean up exhaust gases have been also developed. In this case, the temperatures
of the exhaust gases rise by the catalysts. This causes the sound velocities to increase,
causing high-frequency components of exhaust sounds to increase. As a result, metallic
exhaust sounds including the high-frequency components are produced when the catalysts
are employed for exhaust systems in the single cylinder engines.
[0008] Furthermore, mufflers that are thinner and shorter than those in automobiles are
required from space limitations in the motorcycles.
[0009] When the structures of mufflers for automobiles are employed for the motorcycles
having the catalysts, it is impossible to sufficiently reduce the high-frequency components
of the exhaust sounds without increasing the sizes of the mufflers.
[0010] Particularly in motorcycles including the single cylinder engines, it is desired
that the output performances of the engines are sufficiently brought out by small-sized
mufflers while low exhaust sounds that are synchronized with burning of the engines
are obtained.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide an exhaust system for a motorcycle
allowing the output performance of an engine to be sufficiently brought out while
high-frequency components of an exhaust sound is sufficiently reduced.
[0012] This object is achieved by an exhaust system according to claim 1.
- (1) According to an aspect of the present invention, an exhaust system for a motorcycle
includes an exhaust pipe, and a muffler that discharges the exhaust gas flowing out
of the exhaust pipe to the outside, in which the exhaust pipe includes a first pipe
having a downstream-end opening inserted into the muffler, and a second pipe having
an upstream-end opening and a downstream-end opening and extending toward the downstream
side from the first pipe within the muffler, a narrowed portion having an inner diameter
smaller than that of the first pipe is provided in the downstream-end opening of the
first pipe, and is inserted into the upstream-end opening of the second pipe, a first
expansion chamber that integrally encloses the outer periphery and the downstream-end
opening of the second pipe is formed within the muffler, and a plurality of holes
are formed in a distributed manner in a region, except a region at a downstream end,
of a peripheral surface of the second pipe, and the length in the axial direction
of the region at the downstream end is not less than 1/3 times the inner diameter
of the second pipe.
Embodiments of the invention concern a motorcycle including the exhaust system. In
such a motorcycle, the exhaust gas from the single cylinder engine provided in the
vehicle body flows into the exhaust pipe. At this time, the exhaust gas may be cleaned
up by a catalyst while entering a high-temperature state. This causes the sound velocity
to increase, causing the high-frequency components of the exhaust sound to increase.
The high-temperature exhaust gas flows out into the first expansion chamber in the
muffler via the first pipe and the second pipe in the exhaust pipe. At this time,
the exhaust gas is extruded into the second pipe while being compressed by the narrowed
portion of the first pipe so that exhaust gas pressure (pressure of the exhaust gas)
increases. Thisprevents an unburned mixed gas from going out in an overlap period
of the engine. This results in improvements in torques generated by the engine in
a low-speed area and a medium-speed area.
In this case, a part of the exhaust gas within the second pipe expands by flowing
out into the first expansion chamber through the holes. Furthermore, the remaining
exhaust gas within the second pipe expands by flowing out into the first expansion
chamber from the downstream-end opening. In this case, the first expansion chamber
integrally and continuously encloses the outer periphery and the downstream-end opening
of the second pipe.
The plurality of holes are formed in the region, except the region at the downstream
end, of the peripheral surface of the second pipe. The length in the axial direction
of the region at the downstream end is set to not less than 1/3 times the inner diameter
of the second pipe. Thus, the pressure fluctuation of the exhaust gas flowing out
of the second pipe through the holes and the pressure fluctuation of the exhaust gas
flowing out from the downstream-end opening of the second pipe respectively have high-frequency
components of different phases. Therefore, the high-frequency components of different
phases are canceled by each other. As a result, the high-frequency components of the
exhaust sound is reduced.
Furthermore, the exhaust gas compressed by the narrowed portion of the first pipe
expands step by step in the second pipe and the first expansion chamber. This causes
a pressure wave that can be generated by rapid expansion of the exhaust gas to be
relieved. This results in inhibition of the production of a sound caused by the pressure
wave.
Furthermore, the first expansion chamber integrally encloses the outer periphery and
the downstream-end opening of the second pipe within the muffler. Therefore, it is
possible to ensure that the volume of the first expansion chamber serving as the same
pressure space is sufficiently great. As a result, it is possible to sufficiently
reduce the high-frequency components of the exhaust sound without increasing the length
and the cross-sectional area of the muffler.
These results show that the output performance of the single cylinder engine is sufficiently
brought out while the high-frequency components of the exhaust sound is sufficiently
reduced.
- (2) The second pipe may be joined to the first pipe at a position on the upstream
side of the narrowed portion.
In this case, the second pipe is reliably fixed to the first pipe in a cantilevered
state without using a supporting member. This avoids the vibration and the swing of
the second pipe being generated. This results in prevention of the production of a
sound due to the vibration or the swing of the second pipe.
- (3) The narrowed portion may include a tapered portion having an inner diameter that
gradually decreases. In this case, it is possible to prevent the production of a flow
noise due to the disturbance of the flow of the exhaust gas.
- (4) A second expansion chamber may be further formed on the downstream side of the
first expansion chamber within the muffler, the first expansion chamber and the second
expansion chamber may be separated by a partition plate, and a connecting pipe may
be provided to penetrate the partition plate.
In this case, the exhaust gas compressed by the narrowed portion of the first pipe
expands step by step in the second pipe, the first expansion chamber, and the second
expansion chamber. This causes a pressure wave that can be generated by rapid expansion
of the exhaust gas to be effectively relieved. This results in sufficient inhibition
of the production of a sound due to the pressure wave.
- (5) The volume of the first expansion chamber may be greater than the volume of the
second expansion chamber. In this case, the first expansion chamber integrally encloses
the outer periphery and the downstream-end opening of the second pipe. Thus, it is
possible to make the volume of the first expansion chamber greater than the volume
of the second expansion chamber without increasing the length and the diameter of
the muffler. Therefore, the high-frequency components of the exhaust sound can be
effectively reduced in the first expansion chamber.
- (6) In the exhaust pipe, a part of the first pipe may be held in the muffler, and
the second pipe may not be held in the muffler.
[0013] In this case, no supporting member exists on the side of the outer periphery of the
second pipe, so that the flow of the exhaust gas flowing out of the plurality of holes
of the second pipe is not disturbed. This can prevent the production of a flow noise
due to the disturbance of the flow of the exhaust gas.
[0014] According to the present invention, the output performance of the engine is sufficiently
brought out while the high-frequency components of the exhaust sound is sufficiently
reduced.
[0015] Other features, elements, characteristics, and advantages of the present invention
will become more apparent from the following description of embodiments of the present
invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
Fig. 1 is a side view of a motorcycle according to an embodiment of the present invention;
Fig. 2 is a perspective view of the appearance of an exhaust device as viewed from
the top;
Fig. 3 is a perspective view of the appearance of the exhaust device as viewed from
the side;
Fig. 4 is a cross-sectional view taken along a line IV - IV of a muffler shown in
Fig. 2;
Fig. 5 is an enlarged sectional view of an exhaust pipe shown in Fig. 4;
Fig. 6 is a cross-sectional view of a muffler in a comparative example 1;
Fig. 7 is a diagram showing the respective results of measurement of the damping properties
of a muffler in an inventive example and the muffler in the comparative example 1;
and
Fig. 8 is a cross-sectional view of a muffler in a comparative example 2.
DESCRIPTION OF THE EMBODIMENTS
[0017] Referring to the drawings, embodiments of the present invention will be described
while referring to the drawings. In the following description, the upstream side and
the downstream side will be defined with the flow of an exhaust gas used as a basis.
(1) Configuration of motorcycle
[0018] Fig. 1 is a side view of a motorcycle according to an embodiment of the present invention.
[0019] A motorcycle 100 shown in Fig. 1 includes a vehicle body 80 composed of a vehicle
body frame and a frame cover. A head pipe (not shown) is provided at the front of
the vehicle body 80, and a handle 81 is provided at an upper end of the head pipe.
A front fork 82 is attached to a lower end of the head pipe. In this state, the front
fork 82 is rotatable within a predetermined angle range with the axis of the head
pipe used as its center. A front wheel 83 is rotatably supported on a lower end of
the front fork 82. A rear wheel 84 is rotatably supported at the rear of the vehicle
body 80. A single cylinder engine 50 is provided at the center of the vehicle body
80. The rear wheel 84 is rotated by a rotating force of the engine 50.
[0020] An exhaust device 10 that introduces an exhaust gas to the outside is connected to
a cylinder head 51 of the engine 50. The exhaust device 10 includes an exhaust pipe
20 and a muffler (silencer) 30. The exhaust pipe 20 extends backward from the cylinder
head 51 of the engine 50. The muffler 30 extends toward the side of the rear wheel
84 from a downstream end of the exhaust pipe 20.
(2) Configuration of exhaust device 10
[0021] Fig. 2 is a perspective view of the appearance of the exhaust device 10 as viewed
from the top. Fig. 3 is a perspective view of the appearance of the exhaust device
10 as viewed from the side.
[0022] An exhaust port connector 22 is provided at an upstream end of the exhaust pipe 20.
An opening on the upstream side of the exhaust pipe 20 is connected to an exhaust
port of the cylinder head 51 of the engine 50 shown in Fig. 1 by the exhaust port
connector 22. The downstream end 24 of the exhaust pipe 20 is inserted into an upstream
end (an inlet) of the muffler 30. The outer periphery at the upstream end of the muffler
30 is fastened by a mounting member 26 so that the muffler 30 is fixed to the exhaust
pipe 20.
[0023] The downstream end 24 of the exhaust pipe 20 is connected to an upstream end of an
exhaust pipe 40 within the muffler 30. A three way catalyst 90 is provided within
the exhaust pipe 40. Note that the exhaust pipe 20 and the exhaust pipe 40 may be
integrally formed. Alternatively, the exhaust pipe 20 and the exhaust pipe 40 may
be connected to each other through another member. Covers 31a and 31b are attached
to an outer peripheral surface of the muffler 30.
[0024] An exhaust gas generated by burning of an air-fuel mixture within the engine 50 shown
in Fig. 1 is fed into the muffler 30 through the exhaust pipe 20. The exhaust gas
is discharged into the atmosphere through the muffler 30. Thus, an exhaust path of
the engine 50 includes the exhaust port of the cylinder head 51, the exhaust pipe
20, and the muffler 30 in the order from the upstream side to the downstream side.
[0025] The muffler 30 has a muffling function for reducing an exhaust sound before discharging
the exhaust gas into the atmosphere. In the present embodiment, the inside of the
muffler 30 has a structure of a multistage expansion type, described later. That is,
the inside of the muffler 30 is partitioned into a plurality of expansion chambers,
so that the exhaust gas expands by passing through the plurality of expansion chambers.
This causes the exhaust gas to be depressurized.
(3) Internal structure of muffler 30
[0026] The internal structure of the muffler 30 will be then described while referring to
Fig. 4. Fig. 4 is a cross-sectional view taken along a line IV - IV of the muffler
30 shown in Fig. 2. Fig. 5 is an enlarged sectional view of the exhaust pipe 40 shown
in Fig. 4.
[0027] As shown in Fig. 4, the muffler 30 includes a conical head 32, a hollow cylindrical
body 34, and a bowl-shaped tail 37. The head 32 is fitted in an opening on the upstream
side of the body 34. The tail 37 is fitted in an opening on the downstream side of
the body 34.
[0028] Partition plates 36 and 38 are arranged with predetermined spacing in the order from
the upstream side at positions close to a downstream end of the body 34. Thus, an
internal space of the muffler 30 is partitioned into a first expansion chamber 70,
a second expansion chamber 72, and a third expansion chamber 74. The first expansion
chamber 70, the second expansion chamber 72, and the third expansion chamber 74 line
up in the order from the upstream side to the downstream side in the longitudinal
direction of the muffler 30.
[0029] A connecting pipe 62 penetrates the partition plate 36, a connecting pipe 64 penetrates
the partition plate 38, and a tail pipe 66 penetrates the tail 37.
[0030] The exhaust pipe 40 is inserted into the head 32. A downstream portion of the exhaust
pipe 40 extends into the first expansion chamber 70. The exhaust gas is introduced
into the first expansion chamber 70 through the exhaust pipe 40.
[0031] The exhaust pipe 40 includes a first pipe 42 on the upstream side and a cylindrical
second pipe 44 on the downstream side. The first pipe 42 has a cylindrical portion
46 on the upstream side and a narrowed portion 47 on the downstream side. The narrowed
portion 47 has a tapered portion 48a and a small-diameter portion 48b. The small-diameter
portion 48b has an outer diameter and an inner diameter respectively smaller than
the outer diameter and the inner diameter of the cylindrical portion 46. The outer
diameter and the inner diameter of the tapered portion 48a gradually decrease from
an outer diameter and an inner diameter that are the same as those of the cylindrical
portion 46 toward an outer diameter and an inner diameter that are the same as the
small-diameter portion 48b. Thus, the cross-sectional area of a flow path of the narrowed
portion 47 gradually decreases from the upstream side to the downstream side.
[0032] The inner diameter R2 of the second pipe 44 is equal to the outer diameter of the
cylindrical portion 46 of the first pipe 42. The narrowed portion 47 of the first
pipe 42 is inserted into an upstream-end opening of the first pipe 42 such that an
upstream end of the second pipe 44 is overlapped with the cylindrical portion 46 at
a position on the upstream side of the narrowed portion 47 of the first pipe 42. In
this state, an outer peripheral surface of the cylindrical portion 46 of the first
pipe 42 and an inner peripheral surface of the second pipe 44 are welded so that the
second pipe 44 is joined to the cylindrical portion 46. Thus, the second pipe 44 is
held in the first pipe 42 in a cantilevered state. Furthermore, a cylindrical clearance
is formed between the inner peripheral surface of the second pipe 44 and the narrowed
portion 47. The tapered portion 48a is opened within the second pipe 44.
[0033] The first pipe 42 is attached to an inner surface of the head 32 through a supporting
stay 35 so as to be positioned at the center of the head 32 and the body 34.
[0034] The inner diameter R0 of the small-diameter portion 48b of the narrowed portion 47
is smaller than the inner diameter R1 of the cylindrical portion 46 (R0<R1). The inner
diameter R2 of the second pipe 44 is larger than the inner diameter R0 of the small-diameter
portion 48b of the narrowed portion 47 and is smaller than the inner diameter R3 of
the first expansion chamber 70 (R0<R2<R3). This enables a pressure fluctuation created
by rapid expansion of the exhaust gas flowing out of the first pipe 42 into the first
expansion chamber 70 to be relieved.
[0035] In this case, it is preferable that the inner diameter R2 of the second pipe 44 is
not too close to the inner diameter R3 of the expansion chamber 70 and is not too
close to the inner diameter R0 of the small-diameter portion 48b. This enables a pressure
wave caused by the rapid expansion of the exhaust gas to be sufficiently relieved
by the second pipe 44.
[0036] It is preferable that the inner diameter R2 of the second pipe 44 is approximately
equal to the inner diameter R1 of the cylindrical portion 46 of the first pipe 42.
In the present embodiment, the inner diameter R2 of the second pipe 44 is equal to
the outer diameter of the cylindrical portion 46 of the first pipe 42. Therefore,
the inner diameter R2 of the second pipe 44 is approximately equal to the inner diameter
R1 of the cylindrical portion 46 of the first pipe 42. This enables the pressure wave
to be effectively relieved.
[0037] Note that a length L from a downstream-end opening 49 of the narrowed portion 47
to a downstream-end opening 43 of the second pipe 44 is not particularly limited and
is determined such that a low-frequency component of the exhaust sound does not disappear
and the strength of the second pipe 44 held in a cantilevered state is ensured.
[0038] As shown in Fig. 5, the second pipe 44 includes an upstream region RU where a plurality
of holes 45 are provided and a downstream region RD where no holes are substantially
provided in the order toward the downstream side. The plurality of holes 45 are equally
spaced in the upstream region RU. The plurality of holes 45 are punching holes, for
example. The plurality of holes 45 penetrate the second pipe 44 from its outer peripheral
surface to its inner peripheral surface. In the present embodiment, the shape of each
of the holes 45 is circular.
[0039] The downstream region RD has a length D toward the upstream side from the downstream-end
opening 43 of the second pipe 44. The length D of the downstream region RD is not
less than 1/3 times the inner diameter R2 of the second pipe 44.
[0040] The downstream region RD may be provided with one or more holes so as to have an
opening ratio that is not more than one third of the opening ratio of the upstream
region RU. The opening ratio of the upstream region RU means the ratio of the sum
of the open areas of the plurality of holes 45 to the area of the upstream region
RU. The opening ratio of the downstream region RD means the ratio of the sum of the
open areas of the one or more holes to the area of the downstream region RD. In this
case, the one or more holes provided in the downstream region RD hardly affect the
effect of reducing a high-frequency components, described later. Therefore, the fact
that the downstream region RD is provided with the one or more holes so as to have
the opening ratio that is not more than one third the opening ratio of the upstream
region RU means that no holes are substantially provided in the downstream region
RD.
[0041] Although the ratio of the sum of the open areas of the plurality of holes 45 to the
cross-sectional area of the flow path of the second pipe 44 (hereinafter referred
to as an area ratio of the plurality of holes 45) is not particularly limited, it
is preferable that the area ratio of the plurality of holes 45 is not less than 0.5
nor more than 2.0, for example, approximately 1.0.
[0042] Note that the inner diameter of each of the holes 45 and the pitch between the holes
45 (the distance between the centers of the adjacent holes 45) can be adjusted as
needed such that the above-mentioned sound reducing effect can be satisfactorily attained.
In this case, the inner diameter of each of the holes 45 and the pitch between the
holes 45 can be selected so as to efficiently obtain the sound reducing effect while
restraining a pressure loss.
(4) Operation of exhaust device 10
[0043] The operation of the exhaust device 10 according to the present embodiment will be
then described.
[0044] An exhaust gas from the exhaust port of the engine 50 shown in Fig. 1 is introduced
into the exhaust pipe 40 through the exhaust pipe 20 shown in Figs. 2 and 3. At this
time, the exhaust gas is cleaned up by the three way catalyst 90 while entering a
high-temperature state. This causes the sound velocity to increase, causing a high-frequency
components of an exhaust sound to increase.
[0045] The high-temperature exhaust gas flows out into the first expansion chamber 70 in
the muffler 30 via the first pipe 42 and the second pipe 44 in the exhaust pipe 40.
At this time, the exhaust gas is extruded into the second pipe 44 while being compressed
by the narrowed portion 47 of the first pipe 42 so that exhaust gas pressure (pressure
of the exhaust gas) increases. This prevents an unburned mixed gas from going out
in an overlap period (a period during which both a suction valve and an exhaust valve
are opened) of the engine 50. This results in improvements in torques generated by
the engine 50 in a low-speed area and a medium-speed area.
[0046] The exhaust gas compressed by the narrowed portion 47 expands by flowing out into
the second pipe 44. As shown in Fig. 5, an exhaust gas E1 that is a part of the exhaust
gas within the second pipe 44 expands by flowing out into the first expansion chamber
70 through the holes 45. Furthermore, the remaining exhaust gas E2 expands by flowing
out into the first expansion chamber 70 from the downstream-end opening 43 of the
second pipe 44. In this case, the first expansion chamber 70 integrally and continuously
encloses the outer periphery and the downstream-end opening 43 of the second pipe
44.
[0047] The holes 45 are formed in the upstream region RU, except in the downstream region
RD, as described above. Therefore, the pressure fluctuation of the exhaust gas E1
flowing out of the second pipe 44 through the holes 45 and the pressure fluctuation
of the exhaust gas E2 flowing out from the downstream-end opening 43 of the second
pipe 44 respectively have high-frequency components of different phases. Thus, the
high-frequency components of the pressure fluctuation of the exhaust gas E1 and the
high-frequency components of the pressure fluctuation of the exhaust gas E2 are canceled
by each other within the same pressure space. The results of considerations given
by the inventors of the present invention show that the high-frequency components
are effectively canceled by each other when the length D of the downstream region
RD is not less than 1/3 times the inner diameter R2 of the second pipe 44. This causes
the high-frequency components of the exhaust sound to be reduced.
[0048] Furthermore, the exhaust gas compressed by the narrowed portion 47 of the first pipe
42 expands step by step in the second pipe 44 and the first expansion chamber 70.
This causes a pressure wave that can be generated by rapid expansion of the exhaust
gas to be relieved. This results in inhibition of the production of a sound due to
the pressure wave (particularly a metallic sound including the high-frequency components).
[0049] The exhaust gas within the first expansion chamber 70 expands by flowing out into
the second expansion chamber 72 through the connecting pipe 62. The exhaust gas within
the second expansion chamber 72 expands by flowing out into the third expansion chamber
74 through the connecting pipe 64. The exhaust gas within the third expansion chamber
74 is discharged to the outside through the tail pipe 66 and is released to the atmosphere.
[0050] If the distance between the downstream-end opening 43 of the second pipe 44 and the
upstream-end opening of the connecting pipe 62 is too small, a muffling effect is
reduced. If the distance between the downstream-end opening 43 of the second pipe
44 and the partition plate 36 is too small, the torque generated by the engine 50
is reduced. Therefore, the length D of the downstream region RD is preferably not
more than three times and more preferably not more than two times the inner diameter
R2 of the second pipe 44.
(5) Effects of preferred embodiment
[0051] In the motorcycle 100 according to the present embodiment, the exhaust gas introduced
into the exhaust pipe 40 from the single cylinder engine 50 through the exhaust pipe
20 is compressed by the narrowed portion 47 of the first pipe 42. This prevents the
unburned mixed gas from going out from the engine 50. This results in improvements
in torques generated by the engine 50 in the low-speed area and the medium-speed.
[0052] Furthermore, the exhaust gas compressed by the narrowed portion 47 of the first pipe
42 expands step by step in the second pipe 44, the first expansion chamber 70, the
second expansion chamber 72, and the third expansion chamber 74 so that the pressure
of the exhaust gas decreases to the atmospheric pressure step by step. This causes
the exhaust sound to be reduced.
[0053] In this case, the exhaust gas E1 within the second pipe 4 flows out into the first
expansion chamber 70 through the holes 45, and the remaining exhaust gas E2 flows
out into the first expansion chamber 70 from the downstream-end opening 43. Thus,
the high-frequency components of the pressure fluctuation of the exhaust gas E1 and
the high-frequency components of the pressure fluctuation of the exhaust gas E2 are
canceled by each other within the first expansion chamber 70 serving as the same pressure
space. This causes the high-frequency components of the pressure fluctuation increased
by the three way catalyst 90 to be reduced. Therefore, the high-frequency components
of the exhaust sound is sufficiently reduced. As a result, it is possible to obtain
an intermittent low exhaust sound that is synchronized with burning of the single
cylinder engine 50 while inhibiting the metallic sound including the high-frequency
components from being produced.
[0054] Furthermore, the second pipe 44 is fixed to the first pipe 42 in a cantilevered state
at its upstream end. Thus, no supporting member exists on the side of the outer periphery
of the second pipe 44, and the first expansion chamber 70 integrally and continuously
encloses the outer peripheral surface and the downstream-end opening 43 of the second
pipe 44. Therefore, it is possible to ensure that the volume of the first expansion
chamber 70 serving as the same pressure space is sufficiently great. As a result,
it is possible to sufficiently reduce the high-frequency components of the exhaust
sound without increasing the length and the diameter of the muffler 30. Furthermore,
this can prevent the production of a flow noise due to the disturbance of the flow
of the exhaust gas.
[0055] Since the second pipe 44 is connected to the first pipe 42 at a position on the upstream
side of the narrowed portion 47, the second pipe 44 is stably fixed to the outer peripheral
surface of the first pipe 42 with high strength. This avoids the vibration and the
swing of the second pipe 44 being produced. This results in prevention of the production
of a sound having a natural frequency of the vibration or the swing of the second
pipe 44 and a continuous sound caused by the vibration or the swing of the second
pipe 44.
(6) Other Embodiments
[0056] Although the narrowed portion 47 has the tapered portion 48a and the small-diameter
portion 48b in the above-mentioned embodiment, the narrowed portion 47 may have a
step structure unless the flow of the exhaust gas is disturbed. In such a case, exhaust
pressure is also increased by the narrowed portion 47.
[0057] Although the shape of the plurality of holes 45 is circular in the above-mentioned
embodiment, the present invention is not limited to the same. For example, the shape
of the plurality of holes 45 may be elliptical or polygonal.
[0058] Furthermore, although the plurality of holes 45 are equally spaced in the upstream
region RU, except in the downstream region RD, the plurality of holes 45 may be randomly
arranged.
[0059] Although the first pipe 42 and the second pipe 44 respectively have circular cross
sections in the above-mentioned embodiment, the present invention is not limited to
the same. For example, the first pipe 42 and the second pipe 44 may respectively have
elliptical cross sections. In this case, the length of the long axis of an ellipse
is taken as an inner diameter.
Therefore, the length D of the downstream region RD of the second pipe 44 is set to
not less than 1/3 times the length of the long axis of the ellipse and preferably
not more than three times and more preferably not more than two times the inner diameter
R2 of the second pipe 44. Furthermore, the first pipe 42 and the second pipe 44 may
respectively have polygonal cross sections. In this case, the length of the longest
diagonal line of a polygon is taken as an inner diameter. Therefore, the length D
of the downstream region RD of the second pipe 44 is set to not less than 1/3 times
the length of the longest diagonal line of the polygon and preferably not more than
three times and more preferably not more than two times the inner diameter R2 of the
second pipe 44.
[0060] Although the three way catalyst 90 is used in the above-mentioned embodiment, the
present invention is not limited to the same. The three way catalyst 90 may be replaced
with an oxidation catalyst or a reduction catalyst, for example.
(7) Examples
[0061] The effects of respectively reducing high-frequency components of exhaust sounds
by mufflers in an inventive example and comparative examples 1 and 2 were examined.
In the inventive example, the muffler 30 shown in Fig. 4 was used.
[0062] Fig. 6 is a cross-sectional view of the muffler in the comparative example 1. The
muffler 30a shown in Fig. 6 differs from the muffler 30 shown in Fig. 4 in that the
muffler 30a shown in Fig. 6 does not have the second pipe 44. The configuration of
the other portions of the muffler 30a shown in Fig. 6 is the same as the configuration
of the muffler 30 shown in Fig. 4.
[0063] In the muffler 30 in the inventive example and the muffler 30a in the comparative
example 1, the inner diameter R0 of the narrowed portion 47 is 16.1 mm, and the inner
diameter R1 of the first pipe 42 is 23.6 mm. Furthermore, the inner diameter and the
length of the connecting pipe 62 are respectively 28.6 mm and 50 mm, the inner diameter
and the length of the connecting pipe 64 are respectively 22.2 mm and 50 mm, and the
inner diameter and the length of the tail pipe 66 are respectively 22.2 mm and 30
mm.
[0064] In the muffler 30 in the inventive example, the inner diameter R2 of the second pipe
44 is 27.4 mm, the inner diameter of the holes 45 is 5 mm, and the pitch between the
holes 45 is 7.5 mm. The diameter of the holes 45 is 5 mm, and the length L from the
downstream-end opening 49 of the narrowed portion 47 to the downstream-end opening
43 of the second pipe 44 is 42 mm. The length D of the downstream region RD is 9.5
mm.
[0065] The respective damping properties of the exhaust sounds by the muffler 30 in the
inventive example and the muffler 30a in the comparative example 1 were measured.
[0066] Fig. 7 is a diagram showing the respective results of the measurement of the damping
properties of the muffler 30 in the inventive example and the muffler 30a in the comparative
example 1. In Fig. 7, the horizontal axis represents a frequency (Hz), and the vertical
axis represents a sound pressure level (dB). In the same frequency, the lower the
sound pressure level is, the lower a noise value is.
[0067] In Fig. 7, the damping properties of the muffler 30 in the inventive example is indicated
by a thick solid line L0, and the damping properties of the muffler 30s in the comparative
example 1 is indicated by a thin solid line L1.
[0068] In regions indicated by arrows B1 and B2, the sound pressure levels of high-frequency
components in the muffler 30 in the inventive example were made sufficiently lower
than the sound pressure levels thereof in the muffler 30a in the comparative example
1. The respective damping properties of a low-frequency component of less than 2000
Hz in the muffler 30 in the inventive example and the muffler 30a in the comparative
example 1 were substantially equal.
[0069] Thus, the use of the muffler 30 in the inventive example caused the sound pressure
levels of high-frequency components of not less than 2000 Hz to be sufficiently reduced.
Therefore, it was found that the use of the muffler 30 in the inventive example could
inhibit the production of a metallic exhaust sound including the high-frequency components.
[0070] Fig. 8 is a cross-sectional view of the muffler in the comparative example 2. The
muffler 30b shown in Fig. 8 differs from the muffler 30 shown in Fig. 4 in the following
points. In the muffler 30b shown in Fig. 8, the plurality of holes 45 are equally
formed in the whole of the second pipe 44. The configuration of the other portions
of the muffler 30a shown in Fig. 8 is the same as the configuration of the muffler
30 shown in Fig. 4.
[0071] Exhaust sounds in cases where the muffler 30 in the inventive example and the muffler
30a in the comparative example 2 were used were compared. The use of the muffler 30
in the inventive example sufficiently inhibited the production of a metallic sound
including the high-frequency components. On the other hand, the use of the muffler
30b in the comparative example 2 caused a metallic sound including the high-frequency
components to be produced.
[0072] The results showed that the formation of the plurality of holes 45 in the upstream
region RU, except in the downstream region RD, could inhibit the production of the
metallic sound including the high-frequency components.
(8) Correspondences between constituent elements in claims and parts in the embodiments
[0073] In the following paragraphs, non-limiting examples of correspondences between various
elements recited in the claims below and those described above with respect to various
embodiments of the present invention are explained.
[0074] In the embodiments described above, the vehicle body 80 is an example of a vehicle
body, the engine 50 is an example of an engine, the exhaust pipes 20 and 40 are examples
of an exhaust pipe, the three way catalyst 90 is an example of a catalyst, the muffler
30 is an example of a muffler, the first pipe 42 is an example of a first pipe, the
second pipe 44 is an example of a second pipe, the narrowed portion 47 is an example
of a narrowed portion, the first expansion chamber 70 is an example of a first expansion
chamber, and the holes 45 is an example of a hole.
[0075] Furthermore, the second expansion chamber 72 is an example of a second expansion
chamber, the partition plate 36 is an example of a partition plate, the connecting
pipe 36 is an example of a connecting pipe, and the tapered portion 48a is an example
of a tapered portion.
[0076] As each of various elements recited in the claims, various other elements having
configurations or functions described in the claims can be also used.
[0077] While embodiments of the present invention have been described above on the basis
of motorcycles having a catalyst, it is noted the inventive exhaust system may also
be used in motorcycles having no catalyst.
1. An exhaust system for a motorcycle comprising:
an exhaust pipe; and
a muffler that discharges the exhaust gas flowing out of the exhaust pipe to the outside,
wherein the exhaust pipe includes
a first pipe having a downstream-end opening inserted into the muffler, and
a second pipe having an upstream-end opening and a downstream-end opening and extending
toward the downstream side from the first pipe within the muffler,
wherein a narrowed portion having an inner diameter smaller than that of the first
pipe is provided in the downstream-end opening of the first pipe, and is inserted
into the upstream-end opening of the second pipe,
wherein a first expansion chamber that integrally encloses the outer periphery and
the downstream-end opening of the second pipe is formed within the muffler, and
wherein a plurality of holes are formed in a distributed manner in a region, except
a region at a downstream end, of a peripheral surface of the second pipe, and the
length in the axial direction of the region at the downstream end is not less than
1/3 times the inner diameter of the second pipe.
2. The exhaust system according to claim 1, wherein the second pipe is joined to the
first pipe at a position on the upstream side of the narrowed portion.
3. The exhaust system according to claim 1 or 2, wherein the narrowed portion includes
a tapered portion having an inner diameter that gradually decreases.
4. The exhaust system according to any one of claims 1 to 3, wherein a second expansion
chamber is further formed on the downstream side of the first expansion chamber within
the muffler, the first expansion chamber and the second expansion chamber are separated
by a partition plate, and a connecting pipe is provided to penetrate the partition
plate.
5. The exhaust system according to claim 4, wherein the volume of the first expansion
chamber is greater than the volume of the second expansion chamber.
6. The exhaust system according to any one of claims 1 to 5, wherein in the exhaust pipe,
a part of the first pipe is held in the muffler, and the second pipe is not held in
the muffler.
7. A motorcycle, comprising:
a vehicle body;
a single cylinder engine provided in the vehicle body;
an exhaust pipe into which an exhaust gas from the engine flows;
an exhaust system according to any one of claims 1 to 6, wherein an exhaust gas from
the engine flows into the exhaust pipe.
8. The motorcycle of claim 7, further comprising a catalyst provided in the exhaust pipe.