Exhaust silencing device

In an exhaust silencing device having inner tubes which are connected to an exhaust pipe connected to an engine and provided with a plurality of through holes, and an outer tube which covers the inner tubes. The occurrence of a whistling sound is prevented while avoiding the use of a sound absorbing material. Recesses are provided on the inner surface of the inner tube, which, together with the outer tube, form an annular expansion chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an exhaust silencing device having inner 
tubes which are connected to an exhaust pipe and provided with a plurality 
of through holes, and an outer tube covering the inner tubes. 
2. Description of Background Art 
An exhaust silencing device has been known, for example, as set forth in 
Japanese Utility Model (Kokoku) Publication Nos. Hei 5-5213 and Hei 
5-5220. According to the exhaust silencing device disclosed in the 
Japanese Utility Model (Kokoku) Publication No. Hei 5-5213, a sound 
absorbing material such as glass wool is filled in between the inner tubes 
and the outer tube for the purpose of deadening an exhaust sound by the 
sound absorbing function of the sound absorbing material. However, this 
type of construction for holding the sound absorbing material becomes 
complicated and results in an increase in the cost. 
In Japanese Utility Model (Kokoku) Publication No. Hei 5-5220, no sound 
absorbing material is filled in a space between the inner tubes and the 
outer tube. The space is used as an expansion chamber, whereby a 
shortcoming of the use of the sound absorbing material is obviated. 
However, production of a high-frequency component sound, a so-called 
whistling sound, cannot be prevented. 
SUMMARY AND OBJECTS OF THE INVENTION 
In view of the above-described shortcoming, it is an object of the present 
invention to provide an exhaust silencing device of simple construction 
which is capable of preventing an occurrence of the whistling sound while 
avoiding the use of the sound absorbing material. 
To accomplish the above-described object, the exhaust silencing device of 
the present invention has an inner tube provided with a plurality of 
through holes which are connected to an exhaust pipe connected to an 
engine, and an outer tube covering the inner tube. The inner tube forms an 
annular expansion chamber between the aforementioned inner tube and the 
outer tube and the inner surface of the inner tube is provided with 
recesses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter one embodiment of an exhaust silencing device according to the 
present invention will be explained with reference to the accompanying 
drawings. 
FIGS. 1 to 3 show one embodiment of the present invention wherein FIG. 1 
illustrates a saddle-type motor vehicle such as an all terrain vehicle 
(ATV) for off-road riding. A pair of right and left front wheels W.sub.F 
are installed at the front of a vehicle body frame B. A pair of right and 
left rear wheels We are suspended at the rear of the vehicle body frame B. 
The wheels W.sub.F and W.sub.R are mounted with balloon type low-pressure 
tires. A steering handlebar 1, a fuel tank 2, and a straddle-type rider's 
seat 4 in order of mention from the front are mounted on the upper part of 
the vehicle body frame. 
In the middle portion of the vehicle body frame B, below the fuel tank 2 
and the rider's seat 4, a power unit P includes an engine E and a 
transmission T which are mounted as a unit in a common casing. A driving 
power from the power unit P is transmitted to the right and left front 
wheels W.sub.F through a drive shaft 6.sub.F extending forwardly from the 
power unit P, and also to the right and left rear wheels W.sub.R through a 
drive shaft 6.sub.R extending rearwardly from the power unit P. 
An exhaust pipe 8 is connected at the front face of the cylinder head 7 of 
the engine E which extends to the rear around the left side of the engine 
E and is connected to an exhaust silencing device 9 which is fixedly 
supported at the rear part of the vehicle body frame B. 
In FIGS. 2 and 3, the exhaust silencing device 9 is provided with a 
cylindrical casing 10. As shown in FIG. 1, a pair of brackets 11, 11 are 
mounted on the casing 10 and fixedly supported on the rear part of the 
vehicle body frame B by screw members 12, 12. 
The front end of the casing 10 is closed with a front end wall 13. A rear 
end wall 14 having an exhaust port 14a which is flared backwards, is 
provided at the rear end of the casing 10. Therefore, the interior of the 
casing 10 is separated by partition walls 15, 16 and 17 secured on the 
inner surface of the casing 10 which are spaced in the axial direction. 
The casing is separated into a first chamber 18 between the front end wall 
13 and the partition wall 15, a second chamber 19 between the partition 
walls 15 and 16, a third chamber 20 between the partition walls 16 and 17, 
and a fourth chamber 21 between the partition wall 17 and the rear end 
wall 14. 
A tube section 13a connected in a gas tight manner through to the front end 
of the inner tube 22.sub.1 which is in parallel with the casing 10 is 
provided in a position offset from the axial line of the front end wall 
13. The front end of the inner tube 22.sub.1 protruding forwards from the 
tube section 13a is securely connected to the exhaust pipe 8, while the 
rear end of the inner tube 22.sub.1 extends into the interior of the 
second chamber 19. 
In the casing 10, the inner tube 22.sub.1 is coaxially covered with the 
outer tube 23. The contracted front end of the outer tube 23 is welded to 
the whole circumference of the outer surface of the inner tube 22.sub.1 in 
the vicinity of the front end wall 13, while the contracted rear end of 
the outer tube 23 is welded to the whole circumference of the outer 
surface of the inner tube 22.sub.1 in the second chamber 19. Therefore, an 
annular expansion chamber 24 is formed between the inner tube 22.sub.1 and 
the outer tube 23. A tube section 15a protrudes in a gas tight condition 
into the second chamber 19 through the outer tube 23 on the partition wall 
15. 
In a part corresponding to the expansion chamber 24 of the inner tube 221 
is provided a plurality of through holes 25 at a space in circumferential 
and axial directions. Of these through holes 25, through holes on the rear 
side near the second chamber 19 are closed with the cylindrical cover tube 
26 coaxially enclosing the inner tube 22.sub.1. The front and rear ends of 
the cover tube 26 are connected by welding to the whole circumference of 
the outer surface of the inner tube 22.sub.1. The outer end of the 
plurality of through holes 25 provided in the portion near the rear end of 
the inner tube 22.sub.1 is closed by the cover tube 26, thereby forming 
recesses 27.sub.1 on the inner surface of the rear portion of the inner 
tube 22.sub.1. 
In a position offset from the axial line of the casing 10, a relatively 
short first connecting tube 28 is fixedly mounted through the partition 
wall 15 in parallel with the outer tube 23. Thus, the second chamber 19 is 
connected with the first chamber 18 by the first connecting tube 28. Also 
in a position offset from the axial line of the casing 10, a relatively 
long second connecting tube 29 which is in parallel with the first 
connecting tube 28 is fixedly mounted through the partition walls 15 and 
16, opening at the front end to the first chamber 18 and also opening at 
the rear end to the third chamber 20. 
A plurality of louvers 30 are provided at the central portion of the 
partition wall 16 to form a communicating hole 31 therebetween. 
In the fourth chamber 21, the rear end of a rear inner tube 33 having a 
plurality of through holes 32 is connected to the exhaust port 14a on the 
same axis as the casing 10. The contracted front end of a rear outer tube 
34 coaxially covering the rear inner tube 33 is welded to the whole outer 
peripheral surface of the rear end portion of the rear inner tube 33. The 
contracted rear end of the rear outer tube 34 is welded to the whole outer 
peripheral surface of the rear end portion of the rear inner tube 33. In 
addition, a plurality of legs 35 are fixedly installed in positions at 
spaces in the circumferential direction of the rear outer tube 34. The 
legs 35 are securely installed to the inner surface of the casing 10. A 
sound absorbing material 36 such as glass wool is filled between the rear 
inner tube 32 and the rear outer tube 34. 
At the rear end of the casing 10 is fixedly attached an insertion tube 37 
protruding into the fourth chamber 21. A sensor (not shown) for detecting 
an exhaust temperature is inserted in the insertion tube 37. 
Next, operation of the first embodiment will be explained. The exhaust gas 
from the exhaust pipe 8 passes through in the inner tube 22.sub.1, being 
led into the second chamber 19, from which the exhaust gas is fed back 
into the first chamber 18 through the first connecting tube 28. Then, the 
exhaust gas is introduced from the first chamber 18 into the third chamber 
20 by the second connecting tube 29, from which the exhaust gas then 
passes through the louvers 30, becoming a swirl flow. After being led from 
the third chamber 20 into the fourth chamber 21, the exhaust gas is 
discharged out through the rear inner tube 32 and the exhaust port 14a. 
In the exhaust silencing device 9 of the above-described construction, the 
exhaust gas is led into the inner tube 22.sub.1 communicating with the 
expansion chamber 24 through a plurality of through holes 25, thereby 
accomplishing the deadening of exhaust noise by the expansion of the 
exhaust gas. In such a construction, the occurrence of a resonance effect 
producing a whistling sound is unavoidable. It has been confirmed 
experimentally that, because of the provision of the recesses 27.sub.1 on 
the inner surface of the rear end portion of the inner tube 22.sub.1, the 
exhaust gas flow is made turbulent to cancel the resonance effect, thereby 
enabling to prevent the occurrence of the whistling sound. 
Therefore, there is no necessity to fill a sound absorbing material between 
the inner tube 22.sub.1 and the outer tube 23, thereby obviating a 
disadvantage accompanied by the filling of the sound absorbing material, 
that is, an increase in cost and further to reduce the sound of a 
high-frequency component. 
In addition, since the recesses 27.sub.1 are formed by partly covering the 
inner tube 22.sub.1 by the cover tube 26, the recesses 27.sub.1 can easily 
be formed on the inner surface of the inner tube 22.sub.1. 
It is to be noticed that, as shown in FIGS. 4 and 5 as the second 
embodiment of the present invention, a plurality of recesses swelling 
outwards may be provided at a space in the circumferential and axial 
directions in a portion near the rear end of the inner tube 22.sub.2 
which, together with the outer tube 23, forms the expansion chamber 24 as 
the second embodiment of the present invention as shown in FIGS. 4 and 5, 
thereby providing the inner surface of the inner tube 22.sub.2 with 
recesses 27.sub.2, or that, as shown in FIG. 6 as the third embodiment of 
the present invention, a plurality of annular recesses 38 swelling 
outwards may be provided at a space in the axial direction, in the rear 
end portion of an inner tube 22.sub.3 which, together with the outer tube 
23, forms the expansion chamber 24, thereby providing the inner surface of 
the inner tube 22.sub.3 with recesses 27.sub.3. 
The preferred embodiments of the present invention have been described in 
detail; however, the present invention should not be limited to the 
above-described embodiments and various changes and modifications in 
design are possible within the scope of the present invention stated in 
claims. 
For example, the recesses 27.sub.1 to 27.sub.3 may be provided in the front 
end portion of the inner tubes 22.sub.1 to 22.sub.3, and the location of 
these recesses 27.sub.1 to 27.sub.3 is not limited. 
According to the present invention, as described above, in the exhaust 
silencing device having the inner tubes which are connected to the exhaust 
pipe connected to the engine and are provided with a plurality of through 
holes, and the outer tube which covers the inner tubes, the recesses are 
provided on the inner surface of the inner tube which, together with the 
outer tube, forms the annular expansion chamber. Therefore, the sound 
absorbing material is dispensed with and the provision of the recesses 
cancels the resonance effect by disturbing the exhaust gas flow, to 
thereby prevent the occurrence of the whistling sound. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.