JP7740573B2 - Engine silencer - Google Patents

Description

The present invention relates to an engine silencer.

In a muffler for an automobile engine, three chambers are arranged side by side, each separated in the direction of the exhaust pipe, and the exhaust pipe passes through these three chambers. A branch pipe from the exhaust pipe connects to one of the three chambers, and the chamber to which the branch pipe is connected is connected to the chamber at the other end via a communicating pipe, making the two end chambers resonance chambers. A muffler is known in which the central chamber is made into a resonance chamber or expansion chamber by providing multiple small holes in the exhaust pipe of the central chamber (Patent Document 1).

Special Publication No. 3-45208

However, with the above-mentioned conventional technology, there are cases where plane wave sound waves traveling straight through the exhaust pipe are output directly from the muffler (hereinafter also referred to as a silencer) without entering the branch pipe, which creates the problem that the desired silencing effect cannot be achieved in the resonant chambers at both ends of the three parallel chambers.

The problem that this invention aims to solve is to provide an engine silencer that can prevent a decrease in the silencing effect in the silencer's resonance chamber.

The present invention solves the above problem by using an exhaust pipe that includes a main pipe into which engine exhaust gas is introduced from a base end and a branch pipe that branches off from the main pipe at a branching section, wherein the second pipe axis direction of the main pipe at the outlet of the branching section is a direction that is along the first pipe axis direction of the main pipe at the inlet of the branching section, and the third pipe axis direction of the branch pipe at the outlet of the branching section is a direction different from the first pipe axis direction of the main pipe at the inlet of the branching section, and by connecting the tip of the main pipe to the resonance chamber of a silencer, and curving the pipe axis of the branch pipe from the base end to the tip of the branch pipe so that the angle formed between the second pipe axis direction and the pipe axis direction of the branch pipe becomes larger as it approaches the tip of the branch pipe.

According to the present invention, it is possible to prevent a decrease in the sound-absorbing effect in the resonance chamber of the silencer.

1 is a plan view showing an example of a noise suppressor according to the present invention. FIG. 2 is a plan cross-sectional view of the exhaust pipe of FIG. 1 . FIG. 2 is a plan cross-sectional view of the silencer of FIG. 1 . 2 is a plan cross-sectional view showing the flow of exhaust gas and sound waves in the silencer of FIG. 1. FIG. 2 is a cross-sectional view of a main part showing the flow of exhaust gas in the branch pipe of FIG. 1 . FIG. FIG. 10 is a plan cross-sectional view showing another example of a silencer according to the present invention. FIG. 7 is a front end view of the main part of the connection portion where the main pipe and the branch pipe in FIG. 6 are connected. FIG. 7 is a cross-sectional view of a main part showing the flow of exhaust gas in the branch pipe of FIG. 6 . FIG. 10 is a cross-sectional plan view showing still another example of an exhaust pipe according to the present invention. FIG. 2 is a plan cross-sectional view showing an example of a noise suppressor according to a comparative example of the present invention.

An embodiment of the present invention is described below based on the drawings.

Figure 1 is a plan view showing an example of a silencer 1 according to the present invention. The silencer 1 is a device that reduces exhaust noise from an internal combustion engine (hereinafter also referred to as an engine) mounted on an automobile, motorcycle, etc., and such engines include hybrid engines that combine an internal combustion engine with an electric motor. Figure 1 is a plan view of the silencer 1 mounted on an automobile. As shown in Figure 1, the silencer 1 includes an exhaust pipe 10, a silencer 20, and a tailpipe 30.

The exhaust pipe 10 is a pipe that introduces exhaust gases from an internal combustion engine, as well as exhaust sound waves, into the silencer 20. The exhaust gases and exhaust sound waves emitted from the engine are introduced to the base end of the exhaust pipe 10 via an exhaust manifold attached to the engine and an exhaust pipe routed under the floor of the vehicle. The exhaust pipe routed under the floor may be a different exhaust pipe from the exhaust pipe 10, or it may be the same exhaust pipe integrated with the exhaust pipe 10.

The exhaust pipe 10 is a circular pipe made of corrosion-resistant stainless steel (such as SUS409) or a titanium alloy. The exhaust pipe 10 is made, for example, by rolling a stainless steel or titanium alloy plate to the desired diameter. When combining multiple pipes, they are joined by, for example, welding. The exhaust pipe 10 can be bent as needed, for example, using a pipe bender. The exhaust pipe 10 is suspended from the underside of the vehicle floor by stays (not shown). The stays are fixed to the floor via rubber.

Figure 2 is a plan cross-sectional view of the exhaust pipe 10 shown in Figure 1. Specifically, it is a cross-sectional view of a section perpendicular to the height direction of the vehicle, including the central axis A1-A3 (hereinafter also referred to as the pipe axis) of the exhaust pipe 10. As shown in Figure 2, the exhaust pipe 10 includes a main pipe 11 and a branch pipe 12 that branches off from the main pipe 11 at branch point B. The pipe axis of the main pipe 11 is A1, and the pipe axes of the branch pipe 12 are A2 and A3. The diameter of the exhaust pipe 10 (main pipe 11 and branch pipe 12) can be set to an appropriate value within a range that allows the engine exhaust gas to be appropriately introduced into the muffler 20.

The main pipe 11 has a base end E1 and a tip end E2. Engine exhaust gas and exhaust sound waves are introduced into the base end E1, and the tip end E2 is connected to the resonance chamber of the silencer 20 using a joining method such as welding. The main pipe 11 also has a branch section B, defined by dashed lines L1 to L3 in Figure 2. Dashed line L1 is a line parallel to the cross-sectional direction of the main pipe 11, and its position corresponds to position C1, one of the base ends of the branch pipe 12. Similarly, dashed line L2 is a line parallel to the cross-sectional direction of the main pipe 11, and its position corresponds to position C2, the other of the base ends of the branch pipe 12. Dashed line L3 is a line connecting positions C1 and C2.

The branch pipe 12 has a base end E3 at a position corresponding to dashed line L3 and a tip E4. Tip E4 is connected to the expansion chamber of the silencer 20 using a joining method such as welding. Here, assuming that dashed line L1 is the entrance of branch B and dashed lines L2 and L3 are the exits of branch B, the positional relationship between the main pipe 11 and the branch pipe 12 before and after branch B is as follows: That is, the pipe axis direction D1 (hereinafter also referred to as the first pipe axis direction D1) of the main pipe 11 at the entrance of branch B (the position of dashed line L1) and the pipe axis direction D2 (hereinafter also referred to as the second pipe axis direction D2) of the main pipe 11 at the exit of branch B (the position of dashed line L2) are parallel to each other. That is, the first pipe axis direction D1 and the second pipe axis direction D2 may be in the same direction, or they may form an angle (e.g., an angle greater than 0° and less than 10°). In contrast, the tube axis direction D3 (hereinafter also referred to as the third tube axis direction D3) of the branch pipe 12 at the outlet of the branch section B (position of the dashed line L3) is a direction different from the above-mentioned first tube axis direction D1, and the first tube axis direction D1 and the third tube axis direction D3 form a predetermined angle, for example, within a range of 10 to 90 degrees.

The tube axis A1 of the main tube 11 at the branching section B may be a straight line along the first and second tube axis directions (e.g., a straight line parallel to the tube axis directions D1 and D2). Furthermore, the tube axis A1 of the main tube 11 may be curved between the base end E1 and the entrance of the branching section B, and between the exit of the branching section B and the tip E2. The angle between the first tube axis direction D1 and the second tube axis direction D2 is assumed to be smaller than the angle between the first tube axis direction D1 and the third tube axis direction D3.

The silencer 20 is a device that lowers the temperature of the exhaust gas introduced from the exhaust pipe 10 and reduces the sound pressure (or volume) of the exhaust noise. The silencer 20 is a cylindrical, hollow molded product made of corrosion-resistant stainless steel (such as SUS409) or titanium alloy, and its cross-sectional shape can be circular, elliptical, rectangular, or other shapes. For example, the silencer 20 is created in the desired shape by pressing and welding a stainless steel plate or titanium alloy plate. The silencer 20 is suspended from the underside of the vehicle floor, for example, via stays (not shown) attached to both longitudinal and/or lateral ends. The stays are fixed to the underside of the floor via rubber.

Figure 3 is a plan cross-sectional view of the silencer 20. Specifically, it is a cross-sectional view of a section perpendicular to the height direction of the vehicle, including the pipe axes A1-A3 of the exhaust pipe 10. The silencer 20 has a resonance chamber 21 and an expansion chamber 22, and the two chambers are separated by a partition 23. The partition 23 may be provided with a hole or a connecting pipe that connects the resonance chamber 21 and the expansion chamber 22. The silencer 20 may also have one or more resonance chambers 21 and one or more expansion chambers 22, and the number and arrangement of the resonance chambers 21, expansion chambers 22, and partitions 23 can be set as appropriate within the range in which the silencer 20 can achieve the desired silencing effect.

The resonance chamber 21 is a compartment that attenuates exhaust sound waves introduced from the exhaust pipe 10 by interfering with sound waves having an opposite-phase waveform. For example, sound waves introduced from the tip E2 of the main pipe 11 interfere with sound waves whose waveform has been reflected at the fixed end within the resonance chamber 21 and become the opposite phase, thereby attenuating the exhaust sound. Furthermore, the presence of the resonance chamber 21 causes the air present between the outlet of branch B (position of dashed line L2) and tip E2 shown in Figure 2 to act as a spring, attenuating sound waves of a certain frequency among the exhaust sound waves through resonance.

The shape of the resonance chamber 21 can be set using a formula for calculating the resonant frequency of Helmholtz resonance, depending on the frequency of the sound waves to be attenuated by resonance. In other words, the resonant frequency of the Helmholtz resonator, which consists of the resonance chamber 21 and the portion of the main pipe 11 from the outlet of branch B to the tip E2 (hereinafter referred to as the neck portion), is calculated from the axial length of the neck portion, the cross-sectional area of the main pipe 11, and the volume of the resonance chamber 21. Therefore, if the dimensions of the main pipe 11 are fixed, the resonance chamber 21 can be shaped appropriately within a range that ensures a volume that achieves the desired resonant frequency.

The expansion chamber 22 is a compartment that expands the exhaust gas introduced from the branch pipe 12 to reduce its pressure and temperature. The expansion chamber 22 may be divided into multiple compartments by partitions 23 to expand the exhaust gas in stages. The expansion chamber 22 may also function as a resonance chamber different from the resonance chamber 21. The exhaust gas expanded in the expansion chamber 22 is discharged into the atmosphere via a tail pipe 30 connected to the expansion chamber 22. The tail pipe 30 is made using the same material and method as the exhaust pipe 10, and is connected to the expansion chamber 22 using a joining method such as welding.

Figure 4 is a plan cross-sectional view showing the flow of exhaust gas and exhaust sound waves in the silencer 1. The cross-section shown in Figure 4 is the same as the cross-sections shown in Figures 2 and 3. First, exhaust gas introduced from the base end E1 travels straight through the main pipe 11 and remains in the resonance chamber 21. Then, after the resonance chamber 21 and the neck portion of the main pipe 11 are filled with exhaust gas, the exhaust gas is introduced from the branch pipe 12 into the expansion chamber 22 along arrow G1 shown in Figure 4. The exhaust gas expands in the expansion chamber 22, reducing its temperature and pressure, and is then discharged into the atmosphere from the tailpipe 30 along arrow G2.

On the other hand, after being introduced through the base end E1, exhaust sound waves travel straight through the main pipe 11 along arrow S1 in Figure 4. This is because the sound waves propagating through the exhaust pipe 10 are plane waves, propagating straight along the axis of the exhaust pipe 10. Some sound waves travel toward the branch pipe 12 at branch B and are introduced into the expansion chamber 22 along arrow S4. However, a relatively large proportion of sound waves travel straight through branch B and are introduced into the resonance chamber 21 along arrow S2. The sound waves introduced into the resonance chamber 21 interfere with sound waves that have reverberated in the resonance chamber 21 and become out of phase (arrow S3), reducing their sound pressure. This makes it easier for plane sound waves traveling straight through the main pipe 11 to be introduced into the resonance chamber 21 than into the branch pipe 12, thereby preventing a reduction in the silencing effect of the resonance chamber 21. The sound waves introduced into the expansion chamber 22 are then output into the atmosphere from the tailpipe 30 along arrow S5.

In contrast, in the silencer 1x according to a comparative example of the present invention shown in Figure 10, the silencing effect in the resonance chamber 21 is reduced. Figure 10 is a plan cross-sectional view showing the silencer 1x. The silencer 1x differs from the silencer 1 shown in Figure 1 in that the main pipe 11 is connected to the expansion chamber 22 and the branch pipe 12 is connected to the resonance chamber 21.

In the silencer 1x shown in Figure 10, exhaust gas introduced from the base end E1 is guided from the main pipe 11 to the expansion chamber 22 along arrow Ga. The exhaust gas expands in the expansion chamber 22, reducing its temperature and pressure, and is then discharged into the atmosphere from the tailpipe 30 along arrow Gb. Meanwhile, exhaust sound waves are introduced from the base end E1 and then travel straight through the main pipe 11 along arrow Sa. Sound waves that travel straight through branch B along arrow Sb are introduced into the expansion chamber 22. They are then output from the tailpipe 30 into the atmosphere along arrow Sc at a sound pressure that exceeds a specified sound pressure, for example, as specified by law. The sound waves introduced into the resonance chamber 21 along arrow Sd are less than the sound waves that travel straight through branch B and are introduced into the expansion chamber 22, reducing the noise reduction effect (arrow Se) due to interference from the resonance chamber 21.

Figure 5 is a cross-sectional view of a key portion showing the flow of exhaust gas in the branch pipe 12. The cross section shown in Figure 5 is the same as the cross section shown in Figure 4. The pipe axis A2 of the branch pipe 12 shown in Figure 5 is parallel to the third pipe axis direction D3 at the base end E3. In this case, the inner surface of the branch pipe 12 is also parallel to the third pipe axis direction D3, and such a smooth, uncurved inner surface promotes the growth of exhaust gas vortices. For example, if an exhaust gas vortex W1 is generated near the base end E3, the vortex W1 is subjected to a force f1 caused by the exhaust gas flow and a force f2, which is a reaction force to force f1 exerted by the inner surface of the branch pipe 12. Because both forces f1 and f2 rotate the vortex W1 clockwise, the flow of exhaust gas promotes the rotation of the vortex W1, and the vortex W1 moves along the pipe axis A2 and grows into a larger vortex W2.

The force that promotes the rotation of the vortex is continuously applied to the generated vortex, so as shown in Figure 5, forces f1 and f2 are also applied to vortex W2, promoting the rotation of vortex W2, which then moves along the tube axis A2 and becomes a larger vortex W3. Similarly, forces f1 and f2 are also applied to vortex W3, promoting the rotation of vortex W3, which then moves along the tube axis A2 and becomes a larger vortex W4.

When vortex growth is accelerated and a large vortex is generated, a loud sound of a specific frequency is generated depending on the size and rotational speed of the vortex. The sound caused by the vortex is a high-frequency sound similar to a whistle, and the sound pressure level of only some frequencies in the high-frequency range (e.g., above 1000 Hz) is increased. If the sound pressure level of only some frequencies in the high-frequency range is increased, the passengers of the vehicle will recognize it as an abnormal sound coming from the engine. Therefore, to suppress the growth of exhaust gas vortices in the branch pipe 12 and achieve a uniform sound pressure level in the high-frequency range, a pleasant high-pitched sound like that of a jet engine (hereinafter also referred to as jet sound), an exhaust pipe 10 having a branch pipe 12a with the shape shown in Figure 6 can be used.

Figure 6 is a plan cross-sectional view of silencer 1a, and the cross-section shown in Figure 6 is the same as the cross-section shown in Figure 4. The only difference between silencer 1 and silencer 1a is that branch pipe 12 has been replaced with branch pipe 12a. The pipe axis of branch pipe 12a is A4.

The branch section B of the silencer 1a has an annular connection section where the base end E3 of the branch pipe 12a and the main pipe 11 are connected. The annular connection section is shown in Figure 7. Figure 7 is a front end view of the main section C3 where the main pipe 11 and branch pipe 12a shown in Figure 6 are connected. The left side of Figure 7 is the base end E1 side of the main pipe 11, and the right side is the tip E2 side of the main pipe 11. In the case of the main pipe 11 and branch pipe 12a shown in Figure 6, the connection section C3 has an elliptical shape as shown in Figure 7. When the connection section C3 shown in Figure 7 is viewed from the front in a direction along the third pipe axis direction D3 (e.g., the same direction as the third pipe axis direction D3), if the half of the connection section C3 on the tip E4 side of the branch pipe 12a is designated as section P, the surface formed by the inner surfaces of the main pipe 11 and branch pipe 12a corresponding to section P is a curved surface having a radius equal to or greater than a predetermined value in a cross section C4 perpendicular to the connection section C3. When cross section C4 shown in Fig. 7 is viewed from viewpoint C5, the portion corresponding to portion P shown in Fig. 7 is portion Q shown in Fig. 6. That is, in the silencer 1a, the surface formed by the inner surfaces of the main pipe 11 and the branch pipe 12a at portion Q shown in Fig. 6 is a curved surface having a radius R equal to or greater than a predetermined value.

At portion Q shown in Fig. 6, the surface formed by the inner surfaces of the main pipe 11 and the branch pipe 12a is a curved surface having a radius R equal to or greater than a predetermined value, so that the state of exhaust gas flow in the branch pipe 12a differs from the state shown in Fig. 5. Fig. 8 is a cross-sectional view of a main portion showing the flow of exhaust gas in the branch pipe 12a. The cross section shown in Fig. 8 is the same cross section as the cross section shown in Fig. 5. If a vortex W5 of exhaust gas is generated at a position close to the base end E3, a force f1 caused by the flow of exhaust gas and a force f2 acting as a reaction force to force f1 exerted by the inner surface of the branch pipe 12a are applied to the vortex W5.
However, because the inner surface of the branch pipe 12a is curved, the generated vortex W5 floats away from the inner surface of the branch pipe 12a, and the reaction force f2 from the inner surface of the branch pipe 12a is no longer applied to the vortex W5, thereby suppressing the growth of the vortex W5.

The above-mentioned effect also applies to vortices W6 and W7 generated at different positions, resulting in the generation of multiple vortices of different sizes and rotational speeds within branch pipe 12a. In this way, by suppressing the growth of vortices and generating multiple different vortices, it is possible to prevent the sound pressure level of only certain frequencies from increasing in the high-frequency range (e.g., the range above 1000 Hz). As a result, the sound pressure level can be made uniform across the high-frequency range, achieving a jet sound. The predetermined radius, shown as R in Figure 8, can be set to any value within the range that suppresses the growth of exhaust gas vortices, e.g., 5 mm or greater.

In addition to the portion P facing the flow of exhaust gas shown in Figure 7, when the connection portion C3 is viewed from the front in the direction along the third pipe axis D3, the surface formed by the inner surfaces of the main pipe 11 and the branch pipe 12a, which corresponds to the half of the connection portion C3 on the base end E3 side of the branch pipe 12a, may be a curved surface having a radius R of a predetermined value or greater in a cross section C4 perpendicular to the connection portion C3. In other words, the entire connection portion C3 may be a curved surface having a radius R of a predetermined value or greater in a cross section C4 perpendicular to the connection portion C3.

Furthermore, instead of the branch pipe 12a shown in FIG. 6, an exhaust pipe 10 having a branch pipe 12b shown in FIG. 9 may be used. The pipe axis A5 of the branch pipe 12b is curved from the base end E3 of the branch pipe 12b toward the tip E4. Specifically, the angle between the second pipe axis direction D2 and the pipe axis direction of the branch pipe 12b decreases as the pipe approaches the tip E4 of the branch pipe 12b. Alternatively, the angle between the second pipe axis direction D2 and the pipe axis direction of the branch pipe 12b may increase as the pipe approaches the tip E4 of the branch pipe 12b. In this case, the pipe axis A5 of the branch pipe 12b is curved in a direction away from the main pipe 11.

[Embodiments of the present invention]
As described above, according to this embodiment, there is provided an engine silencer 1 including an exhaust pipe 10 including a main pipe 11 into which engine exhaust gas is introduced from a base end E1 and a branch pipe 12 branching from the main pipe 11 at a branching section B, and a silencer 20 including a resonance chamber 21, wherein a second pipe axis direction D2 of the main pipe 11 at an outlet of the branching section B is a direction along a first pipe axis direction D1 of the main pipe 11 at an inlet of the branching section B, and a third pipe axis direction D3 of the branch pipe 12 at the outlet of the branching section B is a direction different from the first pipe axis direction D1, and the tip E2 of the main pipe 11 is connected to the resonance chamber 21. This makes it possible to suppress a decrease in the silencing effect of the silencer 20 in the resonance chamber 21.

Furthermore, in the silencer 1 of this embodiment, the second pipe axis direction D2 may be the same direction as the first pipe axis direction D1. This allows the exhaust sound waves to be more effectively introduced into the resonance chamber 21.

In addition, in the silencer 1 of this embodiment, the silencer 20 may include an expansion chamber 22 that expands the exhaust gas, and a tailpipe 30 connected to the expansion chamber 22 that discharges the expanded exhaust gas into the atmosphere, and the tip E4 of the branch pipe 12 may be connected to the expansion chamber 22. This allows the exhaust gas to be discharged from the silencer 1 into the atmosphere while suppressing exhaust noise.

In addition, in the silencer 1 of this embodiment, the branch section B has an annular connection section C3 where the base end E3 of the branch pipe 12 and the main pipe 11 are connected, and when the connection section C3 is viewed from the front in a direction along the third pipe axis direction D3, in a portion P of the connection section C3 that is half of the connection section C3 toward the tip E4 of the branch pipe 12, the surface formed by the inner surfaces of the main pipe 11 and the branch pipe 12 may be a curved surface having a radius R of a predetermined value or greater in a cross section C4 perpendicular to the connection section C3. This suppresses the growth of vortices of exhaust gas on the inner surface of the branch pipe 12, achieving a jet sound with a uniform sound pressure level in the high frequency range.

Furthermore, in the silencer 1 of this embodiment, the pipe axis A2 of the branch pipe 12 may be curved from the base end E3 toward the tip E4 of the branch pipe 12. This suppresses the growth of exhaust gas vortices on the inner surface of the branch pipe 12, achieving a jet sound with a uniform sound pressure level in the high frequency range.

Furthermore, in the silencer 1 of this embodiment, the angle formed by the second pipe axis direction D2 and the pipe axis direction of the branch pipe 12 may increase or decrease as the distance approaches the tip E4 of the branch pipe 12. This suppresses the growth of exhaust gas vortices on the inner surface of the branch pipe 12, achieving a jet sound with a uniform sound pressure level in the high frequency range.

1, 1a...silencer 1x...silencer (comparative example)
10... exhaust pipe 11... main pipe 12, 12a, 12b... branch pipe 20... silencer 21... resonance chamber 22... expansion chamber 30... tail pipe A1, A2, A3, A4, A5... central axis (pipe axis)
B...Branch sections C1, C2...Position C3...Connection section C4...Cross section C5...Viewpoints D1, D2, D3...Tube axis direction E1, E3...Base ends E2, E4...Tip ends f1, f2...Force G1, G2, Ga, Gb...Arrows L1, L2, L3...Dotted lines P, Q...Part R...Radius S1, S2, S3, S4, S5, Sa, Sb, Sc, Sd, Se...Arrows W1, W2, W3, W4, W5, W6, W7...Vortex

Claims (6)

an exhaust pipe including a main pipe into which exhaust gas from an engine is introduced from a base end and a branch pipe branching from the main pipe at a branching portion;
a silencer including a resonance chamber;
a second pipe axis direction of the main pipe at the outlet of the branching portion is a direction along a first pipe axis direction of the main pipe at the inlet of the branching portion,
a third pipe axis direction of the branch pipe at the outlet of the branching portion is different from the first pipe axis direction,
The tip of the main pipe is connected to the resonance chamber,
The pipe axis of the branch pipe is curved from the base end to the tip end of the branch pipe,
The silencer for an engine, wherein an angle formed between the second pipe axis direction and the pipe axis direction of the branch pipe increases as the second pipe axis direction approaches the tip of the branch pipe. The engine silencer according to claim 1, wherein the second pipe axis direction is the same as the first pipe axis direction. The muffler includes an expansion chamber for expanding the exhaust gas, and a tail pipe connected to the expansion chamber for discharging the expanded exhaust gas into the atmosphere.
3. The engine silencer according to claim 1, wherein a tip of the branch pipe is connected to the expansion chamber. the branch portion has an annular connection portion at which a base end of the branch pipe and the main pipe are connected,
3. The silencer for an engine according to claim 1, wherein, when the connection portion is viewed from the front in a direction along the third pipe axis direction, in a portion of the connection portion that is a half portion toward the tip end of the branch pipe, a surface formed by inner surfaces of the main pipe and the branch pipe is a curved surface having a radius of a predetermined value or more in a cross section perpendicular to the connection portion. 3. The engine silencer according to claim 1, wherein the pipe axis of the branch pipe is curved in a direction away from the main pipe. the first tube axis direction and the second tube axis direction are the same direction,
6. The silencer for an engine according to claim 5, wherein a pipe axis of the main pipe is in the same direction as the first pipe axis direction between a base end of the main pipe and an inlet of the branching portion, and is in the same direction as the second pipe axis direction between an outlet of the branching portion and a tip end of the main pipe.