EP4775827A1

EP4775827A1 - Exhaust port structure for internal combustion engine

Info

Publication number: EP4775827A1
Application number: EP25224373.8A
Authority: EP
Inventors: Takuya Kanai; Ayumi OISHI
Original assignee: Suzuki Motor Corp
Current assignee: Suzuki Motor Corp
Priority date: 2025-01-09
Filing date: 2025-12-17
Publication date: 2026-07-15
Also published as: JP2026120311A

Abstract

Object

Provided is an exhaust port structure for an internal combustion engine capable of uniformizing and increasing the flow rate of exhaust gas flowing through an exhaust port and increasing the output of the internal combustion engine.

Solution

In an exhaust port structure for an internal combustion engine 1, when an imaginary plane 21 connecting an upstream end 5d of a lower surface 5m of an exhaust passage portion 5D and a downstream end 5c of the lower surface 5m of the exhaust passage portion 5D is set, the lower surface 5m of the exhaust passage portion 5D is formed with a swollen portion 5g that swells above the imaginary plane 21. The swollen portion 5g includes an increasing swollen portion 5h whose height increases with respect to the imaginary plane 21 from the upstream end 5d of the swollen portion 5g toward an apex 5j of the swollen portion 5g and a decreasing swollen portion 5i whose height decreases with respect to the imaginary plane 21 from the apex 5j toward the downstream end 5c of the swollen portion 5g. The increasing swollen portion 5h is disposed at a position opposed to the boss portion 5B in an up-down direction.

Description

Technical Field

The present invention relates to an exhaust port structure for an internal combustion engine.

Background Art

As a cylinder head for an engine including an exhaust passage for discharging exhaust gas, a cylinder head configured such that a combustion chamber is provided in a recessed manner at a lower surface of the cylinder head which is a joint surface with a cylinder block, and two curved exhaust ports are opened in the combustion chamber is known (see JP 2002-122044 A ).

Citation List

Patent Literature

Patent Document 1: JP 2002-122044 A

Summary of Invention

Technical Problem

However, in the cylinder head for an engine in the related art, a connecting portion between the exhaust port and a lower surface of the exhaust passage is formed in a shape having an angle close to a right angle, and a connecting portion between the exhaust port and an upper surface of the exhaust passage is connected by a gently curved surface (see FIG. 1 of Patent Document 1).
Therefore, the exhaust gas is separated at the lower surface of the exhaust passage to generate disturbed flow, and the flow rate of the exhaust gas in a region on a side of the lower surface of the exhaust passage becomes smaller than that in a region on a side of the upper surface of the exhaust passage. As a result, the flow rate of the exhaust gas flowing through the exhaust passage becomes uneven and the flow rate of the exhaust gas is reduced, so that the output of the internal combustion engine may be reduced.
The present invention has been made in view of the above circumstances, and an object thereof is to provide an exhaust port structure for an internal combustion engine capable of uniformizing and increasing a flow rate of exhaust gas flowing through an exhaust port, thereby increasing the output of the internal combustion engine.

Solution to Problem

According to the present invention, an exhaust port structure for an internal combustion engine includes: an exhaust port formed at a cylinder head and including an opening communicating with a combustion chamber; an exhaust valve guide supported by the cylinder head so as to be opposed to the opening; and an exhaust valve reciprocatively supported by the exhaust valve guide so as to open and close the opening. The exhaust port includes: a cylindrical portion continuous with the opening; an exhaust passage portion extending in an oblique direction from the cylindrical portion; and a boss portion projecting into the exhaust port from an upper surface of the exhaust passage portion opposed to the opening in an axial direction of the exhaust valve. When a first imaginary plane connecting an upstream end of a lower surface of the exhaust passage portion and a downstream end of the lower surface of the exhaust passage portion is set, the lower surface of the exhaust passage portion is formed with a swollen portion that is swollen above the first imaginary plane. The swollen portion includes an increasing swollen portion whose height increases with respect to the first imaginary plane from an upstream end of the swollen portion toward an apex of the swollen portion and a decreasing swollen portion whose height decreases with respect to the first imaginary plane from the apex toward a downstream end of the swollen portion. The increasing swollen portion is disposed at a position opposed to the boss portion in an up-down direction.

Advantageous Effects of Invention

As described above, according to the present invention, the flow rate of the exhaust gas flowing through the exhaust port can be uniformized and increased, and the output of the internal combustion engine can be increased.

Brief Description of Drawings

FIG. 1 is diagram illustrating an exhaust port structure for an internal combustion engine according to an example of the present invention, and is a horizontal cross-sectional view of a cylinder head.
FIG. 2 is diagram illustrating the exhaust port structure for an internal combustion engine according to the example of the present invention, and is an enlarged partial horizontal cross-sectional view of the cylinder head.
FIG. 3 is a cross-sectional view taken along an arrow line III-III in FIG. 2, and illustrates a state in which an exhaust valve is attached.
FIG. 4 is an enlarged cross-sectional view of FIG. 3, and illustrates a state in which the exhaust valve is removed.
FIG. 5 is a diagram illustrating the exhaust port structure for an internal combustion engine according to the example of the present invention, and is a diagram illustrating the flow of exhaust gas inside an exhaust port.

Description of Embodiments

According to an embodiment of the present invention, an exhaust port structure for an internal combustion engine includes: an exhaust port formed at a cylinder head and including an opening communicating with a combustion chamber; an exhaust valve guide supported by the cylinder head so as to be opposed to the opening; and an exhaust valve reciprocatively supported by the exhaust valve guide so as to open and close the opening. The exhaust port includes: a cylindrical portion continuous with the opening; an exhaust passage portion extending in an oblique direction from the cylindrical portion; and a boss portion projecting into the exhaust port from an upper surface of the exhaust passage portion opposed to the opening in an axial direction of the exhaust valve. When a first imaginary plane connecting an upstream end of a lower surface of the exhaust passage portion and a downstream end of the lower surface of the exhaust passage portion is set, the lower surface of the exhaust passage portion is formed with a swollen portion that is swollen above the first imaginary plane. The swollen portion includes an increasing swollen portion whose height increases with respect to the first imaginary plane from an upstream end of the swollen portion toward an apex of the swollen portion and a decreasing swollen portion whose height decreases with respect to the first imaginary plane from the apex toward a downstream end of the swollen portion. The increasing swollen portion is disposed at a position opposed to the boss portion in an up-down direction.
With this configuration, the exhaust port structure for an internal combustion engine according to an embodiment of the present invention is capable of uniformizing and increasing the flow rate of exhaust gas flowing through the exhaust port, thereby increasing the output of the internal combustion engine.

Examples

Hereinafter, an exhaust port structure for an internal combustion engine according to an example of the present invention will be described with reference to the drawings.
FIGS. 1 to 5 are diagrams each illustrating the exhaust port structure for an internal combustion engine according to the example of the present invention.
First, the configuration will be described.
In FIGS. 1 to 5, up, down, front, rear, left, and right directions are defined with reference to an internal combustion engine mounted on a vehicle. The front-rear direction of the vehicle is a front-rear direction, the left-right direction of the vehicle (vehicle width direction) is a left-right direction, and the vertical direction of the vehicle (height direction) is an up-down direction.
As illustrated in FIGS. 1 and 3, an internal combustion engine 1 including four cylinders is mounted on a vehicle. As illustrated in FIG. 3, the internal combustion engine 1 includes a cylinder block 2, a cylinder head 3 provided at an upper part of the cylinder block 2, a cylinder head cover (not illustrated) provided at an upper part of the cylinder head 3, and an oil pan (not illustrated) provided at a lower part of the cylinder block 2, converts thermal energy into mechanical energy by combusting fuel, and transmits power to a transmission (not illustrated).
As illustrated in FIG. 3, the cylinder block 2 is formed with a cylinder 2B at which a piston 2A is reciprocatively provided, and a combustion chamber 10 is formed at a bottom surface of the cylinder head 3.
The combustion chamber 10 is a space surrounded by an upper surface of the piston 2A located at a top dead center, an inner wall surface of the cylinder 2B, and the bottom surface of the cylinder head 3, and the fuel is combusted in the space.
The cylinder head 3 is formed with an intake port 4, and the intake port 4 communicates with the cylinder 2B (see FIG. 1).
A pair of intake ports 4 is provided at each cylinder 2B. In the internal combustion engine 1 including the four cylinders, eight intake ports 4 are provided at the cylinder head 3. The intake port 4 introduces intake air into each cylinder 2B through an intake manifold (not illustrated).
The cylinder head 3 is formed with an exhaust port 5. A pair of exhaust ports 5 is provided at each cylinder 2B. In the internal combustion engine 1 including the four cylinders, eight exhaust ports 5 are provided at the cylinder head 3 (see FIG. 1).
An opening 5a is formed at a lower surface of the cylinder head 3, and the opening 5a communicates with the combustion chamber 10 and the exhaust port 5. Exhaust gas combusted in the combustion chamber 10 is discharged to the exhaust port 5 through the opening 5a.
The cylinder head 3 is provided with an exhaust valve 6. The exhaust valve 6 includes a valve stem 6A and a valve body 6B having an umbrella shape and provided at a lower part of the valve stem 6A.
An exhaust valve guide 7 having a tubular shape is provided at the cylinder head 3 along the up-down direction, and the exhaust valve guide 7 is disposed above the exhaust port 5 so as to be opposed to the opening 5a. The valve stem 6A of the exhaust valve 6 is slidably supported at the exhaust valve guide 7.
The cylinder head 3 is provided with a valve seat 8 having an annular shape, and the valve body 6B is contactable with a lower surface of the valve seat 8. The opening 5a of the cylinder head 3 is formed by an inner peripheral surface of the valve seat 8.
The exhaust valve 6 is moved downward by being pressed against a cam surface of an exhaust valve (not illustrated), and moved upward by a valve spring (not illustrated) when the pressing of the exhaust valve is released.
When the exhaust valve 6 is moved downward by being pressed against the cam surface of the exhaust valve, the exhaust valve 6 is separated from the valve seat 8 to open the opening 5a, thereby causing the combustion chamber 10 and the exhaust port 5 to communicate with each other.
When the pressing of the exhaust valve is released, the exhaust valve 6 is biased by the valve spring to move upward and come into contact with the lower surface of the valve seat 8, thereby closing the opening 5a and blocking the communication between the combustion chamber 10 and the exhaust port 5.
In this way, the exhaust valve 6 is attached to the exhaust valve guide 7 so as to open and close the opening 5a. In addition, the exhaust valve 6 is reciprocatively supported by the exhaust valve guide 7 having a long dimension in the up-down direction, so that the exhaust valve 6 is prevented from being inclined and the occurrence of wear is suppressed.
The exhaust port 5 includes a cylindrical portion 5C and an exhaust passage portion 5D. The cylindrical portion 5C is continuous with the opening 5a. The cylindrical portion 5C has an opening area smaller than that of the valve body 6B of the exhaust valve 6, and is disposed above the valve body 6B.
The exhaust passage portion 5D extends obliquely upward to the left from the cylindrical portion 5C, and communicates, at a downstream end 5c thereof, with an exhaust collection portion 9 (see FIGS. 1 and 2). That is, the downstream end 5c of the exhaust passage portion 5D is an inlet of the exhaust collection portion 9.
The exhaust collection portion 9 communicates with the downstream end 5c of the exhaust passage portion 5D provided at each cylinder 2B, and collects the exhaust gas discharged from the exhaust passage portion 5D provided at each cylinder 2B and discharges the collected exhaust gas to the outside of the cylinder head 3.
As illustrated in FIGS. 3 and 4, the cylinder head 3 is provided with a boss portion 5B. The boss portion 5B projects into the exhaust port 5 from an upper wall 5A of the exhaust port 5. The boss portion 5B is formed in a cylindrical shape surrounding the circumference of the lower end portion 7a of the exhaust valve guide 7.
Specifically, the boss portion 5B projects into the exhaust port 5 from an upper surface 5u of the exhaust passage portion 5D which is opposed to the opening 5a in a direction of an axis 6s of the exhaust valve 6. Hereinafter, the direction of the axis 6s of the exhaust valve 6 is referred to as an axial direction.
That is, a lower surface 5b of the boss portion 5B is located below the upper surface 5u of the exhaust passage portion 5D. The upper surface 5u of the exhaust port 5 is a lower surface of the upper wall 5A.
The boss portion 5B supports a lower end portion 7a of the exhaust valve guide 7, and the exhaust valve guide 7 extends downward from the upper surface 5u of the exhaust port 5 by an amount corresponding to the length of the boss portion 5B in the up-down direction.
That is, the exhaust valve guide 7 is formed to have a dimension in the up-down direction elongated by the length of the boss portion 5B in the up-down direction. Accordingly, the exhaust valve 6 is supported by the exhaust valve guide 7 in a long range in the up-down direction of the valve stem 6A, and thus the exhaust valve 6 can be more effectively prevented from falling.
The exhaust passage portion 5D is formed with a recessed portion 5E. The recessed portion 5E has a shape obtained by making an inner wall of the exhaust passage portion 5D upstream of the boss portion 5B swell outward. That is, the recessed portion 5E swells in a direction away from the downstream end 5c of the exhaust passage portion 5D with respect to the boss portion 5B. Here, upstream and downstream mean upstream and downstream in a direction of the exhaust gas flowing through the exhaust port 5.
As illustrated in FIG. 4, the recessed portion 5E includes a first curved surface 5e having a curvature radius R1 and extending outward from the cylindrical portion 5C and a second curved surface 5f having a curvature radius R2 larger than the curvature radius R1 of the first curved surface 5e and being curved from the first curved surface 5e toward the lower surface 5b of the boss portion 5B.
When an imaginary extension curve 23 having the curvature radius R2 which is the same as the curvature radius of the second curved surface 5f and extending from the second curved surface 5f toward the boss portion 5B is set, the imaginary extension curve 23 is in contact with the lower surface 5b of the boss portion 5B.
When an imaginary plane 21 connecting an upstream end 5d of a lower surface 5m of the exhaust passage portion 5D and the downstream end 5c of the lower surface 5m is set, the lower surface 5m of the exhaust passage portion 5D is formed with a swollen portion 5g that swells above the imaginary plane 21. The imaginary plane 21 of the present example constitutes a first imaginary plane.
The swollen portion 5g includes an increasing swollen portion 5h whose height increases with respect to the imaginary plane 21 from the upstream end 5d of the lower surface 5m of the exhaust passage portion 5D toward an apex 5j of the swollen portion 5g and a decreasing swollen portion 5i whose height decreases with respect to the imaginary plane 21 from the apex 5j toward the downstream end 5c.
Here, the upstream end 5d of the swollen portion 5g and the upstream end 5d of the lower surface 5m of the exhaust passage portion 5D are at the same position, and the downstream end 5c of the swollen portion 5g and the downstream end 5c of the lower surface 5m of the exhaust passage portion 5D are at the same position.
The boss portion 5B includes an inclined surface 5k. The inclined surface 5k is inclined with respect to the axis 6s of the exhaust valve 6, extends from the lower surface 5b of the boss portion 5B toward the downstream side of the exhaust passage portion 5D, and is continuous with the upper surface 5u of the exhaust passage portion 5D.
The inclined surface 5k is inclined so as to increase a cross-sectional area of a flow path between the lower surface 5m of the exhaust passage portion 5D and the inclined surface 5k from the upstream side to the downstream side of the exhaust passage portion 5D.
The increasing swollen portion 5h is disposed at a position opposed to the boss portion 5B in the up-down direction. That is, the increasing swollen portion 5h is opposed to the lower surface 5b and the inclined surface 5k of the boss portion 5B in the up-down direction.
As illustrated in FIG. 3, the exhaust valve guide 7 is attached to the cylinder head 3 such that a lower surface 7b does not project from the lower surface 5b of the boss portion 5B to the opening 5a side.
As illustrated in FIG. 2, a groove portion 5s is formed at each of both end portions in the width direction (the front-rear direction) of the boss portion 5B, and the groove portion 5s is recessed upward with respect to the lower surface 5b of the boss portion 5B.
Next, effects of the exhaust port structure for the internal combustion engine 1 of the present example will be described.
The exhaust port structure for the internal combustion engine 1 of the present example includes the exhaust port 5 formed at the cylinder head 3 and including the opening 5a communicating with the combustion chamber 10, the exhaust valve guide 7 supported by the cylinder head 3 so as to be opposed to the opening 5a, and the exhaust valve 6 reciprocatively supported by the exhaust valve guide 7 so as to open and close the opening Sa.
The exhaust port 5 includes the cylindrical portion 5C continuous with the opening 5a, the exhaust passage portion 5D extending in an oblique direction from the cylindrical portion 5C, and the boss portion 5B projecting into the exhaust port 5 from the upper surface 5u of the exhaust passage portion 5D which is opposed to the opening 5a in the axial direction of the exhaust valve 6.
Here, when a connecting portion between the cylindrical portion 5C and the exhaust passage portion 5D is formed into a shape having an angle close to a right angle, the exhaust gas is separated at the lower surface 5m of the exhaust passage portion 5D to generate disturbed flow, and the flow rate of the exhaust gas in a region on a side of the lower surface 5m of the exhaust passage portion 5D becomes smaller than that in a region on a side of the upper surface 5u of the exhaust passage portion 5D. In this case, the flow rate of the exhaust gas flowing through the exhaust passage portion 5D becomes uneven and the flow rate of the exhaust gas is reduced, so that the output of the internal combustion engine 1 may be reduced.
In the exhaust port structure of the present example, when the imaginary plane 21 connecting the upstream end 5d of the lower surface 5m of the exhaust passage portion 5D and the downstream end 5c of the lower surface 5m of the exhaust passage portion 5D is set, the lower surface 5m of the exhaust passage portion 5D is formed with the swollen portion 5g that swells above the imaginary plane 21.
In addition, the swollen portion 5g includes the increasing swollen portion 5h whose height increases with respect to the imaginary plane 21 from the upstream end 5d of the swollen portion 5g toward the apex 5j of the swollen portion 5g and the decreasing swollen portion 5i whose height decreases with respect to the imaginary plane 21 from the apex 5j toward the downstream end 5c of the swollen portion 5g, and the increasing swollen portion 5h is disposed at a position opposed to the boss portion 5B in the up-down direction.
With this configuration, the cross-sectional area of the flow path of the exhaust passage portion 5D is prevented from being rapidly enlarged by the swollen portion 5g provided at the exhaust passage portion 5D, and the flow of the exhaust gas can be prevented from being disturbed (see exhaust gas G1 in FIG. 5).
Further, since the exhaust gas is prevented from being separated at the lower surface 5m of the exhaust passage portion 5D by the swollen portion 5g, the exhaust gas can be more effectively prevented from being disturbed (see the exhaust gas G1 in FIG. 5).
In addition, since the increasing swollen portion 5h is disposed at a position opposed to the boss portion 5B in the up-down direction, the flow velocity of the exhaust gas between the increasing swollen portion 5h and the boss portion 5B can be increased, and the exhaust gas can be easily drawn into the exhaust passage portion 5D.
Therefore, even when the flow direction of the exhaust gas changes at a connecting portion between the cylindrical portion 5C and the exhaust passage portion 5D extending obliquely upward from the cylindrical portion 5C, the occurrence of stagnation of the exhaust gas at the connecting portion can be prevented.
As a result, the flow rate of the exhaust gas flowing through the exhaust port 5 can be uniformized and increased, and the output of the internal combustion engine 1 can be increased.
Here, for example, when the boss portion 5B includes a flat surface which is parallel to the axis 6s of the exhaust valve 6 and extends from the lower surface 5b of the boss portion 5B toward the downstream side of the exhaust passage portion 5D to be continuous with the upper surface 5u of the exhaust passage portion 5D and the flat surface is opposed to the increasing swollen portion 5h in the up-down direction, the cross-sectional area of the flow path between the flat surface and the increasing swollen portion 5h is rapidly expanded, and thus the flow of the exhaust gas may be disturbed.
According to the exhaust port structure for the internal combustion engine 1 of the present example, the boss portion 5B includes the inclined surface 5k which is inclined with respect to the axis 6s of the exhaust valve 6 and extends from the lower end portion of the boss portion 5B toward the downstream side of the exhaust passage portion 5D to be continuous with the upper surface 5u of the exhaust passage portion 5D, and the inclined surface 5k is inclined so as to increase the cross-sectional area of the flow path between the increasing swollen portion 5h and the inclined surface 5k from the upstream side toward the downstream side of the exhaust passage portion 5D.
With this configuration, the cross-sectional area of the flow path between the inclined surface 5k and the increasing swollen portion 5h can be prevented from being rapidly expanded, and the flow velocity of the exhaust gas flowing between the inclined surface 5k and the increasing swollen portion 5h can be prevented from being rapidly changed. As a result, the exhaust gas can smoothly flow inside the exhaust passage portion 5D.
Further, according to the exhaust port structure for the internal combustion engine 1 of the present example, the exhaust passage portion 5D includes the recessed portion 5E formed by making an inner wall of the exhaust passage portion 5D upstream of the boss portion 5B swell outward.
The recessed portion 5E includes the first curved surface 5e having the curvature radius R1 and extending outward from the cylindrical portion 5C and the second curved surface 5f having the curvature radius R2 larger than the curvature radius R1 of the first curved surface 5e and being curved from the first curved surface 5e toward the lower surface 5b of the boss portion 5B.
With this configuration, the volume of the exhaust port 5 can be increased by expanding the exhaust port 5 by the first curved surface 5e of the recessed portion 5E.
In addition, the exhaust gas having flowed into the exhaust port 5 from the opening 5a can be guided to the lower surface 5b of the boss portion 5B by the second curved surface 5f having the curvature radius R2 larger than the curvature radius R1 of the first curved surface 5e and being curved toward the lower surface 5b of the boss portion 5B.
With this configuration, the exhaust gas guided to the lower surface 5b of the boss portion 5B can smoothly flow to the exhaust passage portion 5D along the lower surface 5b of the boss portion 5B. As a result, the ventilation resistance of the exhaust port 5 can be reduced, and the output of the internal combustion engine 1 can be increased.
FIG. 5 illustrates a state in which exhaust gas G1 discharged from the opening 5a to the exhaust port 5 (the exhaust gas G1 in an upper region out of the exhaust gas G1 indicated by solid lines) is discharged from a central portion in the width direction of the second curved surface 5f to the exhaust passage portion 5D through the lower surface 5b of the boss portion 5B.
FIG. 5 also illustrates a state in which exhaust gas G2 discharged from the opening 5a to the exhaust port 5 (indicated by solid lines in FIG. 2 and indicated by dotted lines in FIG. 5) is discharged from both side portions in the width direction of the second curved surface 5f to the exhaust passage portion 5D through the groove portions 5s.
In addition, according to the exhaust port structure for the internal combustion engine 1 of the present example, the exhaust valve guide 7 is attached to the cylinder head 3 such that the lower surface 7b does not project from the lower surface 5b of the boss portion 5B to the opening 5a side.
Therefore, even when the exhaust valve guide 7 is extended downward and the lower end portion of the exhaust valve guide 7 is supported at the boss portion 5B, the exhaust gas discharged from the opening 5a to the exhaust port 5 can be prevented from colliding with the lower end portion of the exhaust valve guide 7 as well as the boss portion 5B, and can smoothly flow to the exhaust passage portion 5D.
As a result, the ventilation resistance of the exhaust port 5 can be more effectively reduced, and the output of the internal combustion engine 1 can be more effectively increased.
While the example of the present invention has been disclosed, it will be apparent to those skilled in the art that modifications may be made without departing from the scope of the present invention. All such modifications and equivalents are intended to be included in the following claims.

Reference Signs List

1 Internal combustion engine
3 Cylinder head
5 Exhaust port
5a Opening
5B Boss portion
5b Lower surface (lower surface of boss portion)
5C Cylindrical portion
5D Exhaust passage portion
5c Downstream end (downstream end of lower surface of exhaust passage portion)
5d Upstream end (upstream end of upper surface of exhaust passage portion)
5E Recessed portion
5e First curved surface
5f Second curved surface
5g Swollen portion
5h Increasing swollen portion
5i Decreasing swollen portion
5j Apex (apex of swollen portion)
5k Inclined surface
5m Lower surface (lower surface of exhaust passage portion)
5u Upper surface (upper surface of exhaust passage portion)
6 Exhaust valve
6s Axis (axis of exhaust valve)
7 Exhaust valve guide
7b Lower surface (lower surface of exhaust valve guide)
10 Combustion chamber
21 Imaginary plane (first imaginary plane)
R1 Curvature radius of first curved surface
R2 Curvature radius of second curved surface

Claims

An exhaust port structure for an internal combustion engine, comprising:
an exhaust port formed at a cylinder head and including an opening communicating with a combustion chamber;

an exhaust valve guide supported by the cylinder head so as to be opposed to the opening; and

an exhaust valve reciprocatively supported by the exhaust valve guide so as to open and close the opening,

wherein

the exhaust port includes

a cylindrical portion continuous with the opening,

an exhaust passage portion extending in an oblique direction from the cylindrical portion, and

a boss portion projecting into the exhaust port from an upper surface of the exhaust passage portion opposed to the opening in an axial direction of the exhaust valve,

when a first imaginary plane connecting an upstream end of a lower surface of the exhaust passage portion and a downstream end of the lower surface of the exhaust passage portion is set, the lower surface of the exhaust passage portion is formed with a swollen portion that is swollen above the first imaginary plane,

the swollen portion includes

an increasing swollen portion whose height increases with respect to the first imaginary plane from an upstream end of the swollen portion toward an apex of the swollen portion, and

a decreasing swollen portion whose height decreases with respect to the first imaginary plane from the apex toward a downstream end of the swollen portion,
and

the increasing swollen portion is disposed at a position opposed to the boss portion in an up-down direction.
The exhaust port structure for an internal combustion engine according to claim 1, wherein
the boss portion includes an inclined surface which is inclined with respect to an axis of the exhaust valve and extends from a lower surface of the boss portion toward a downstream side of the exhaust passage portion to be continuous with the upper surface of the exhaust passage portion, and

the inclined surface is inclined so as to increase a cross-sectional area of a flow path between the increasing swollen portion and the inclined surface from an upstream side toward the downstream side of the exhaust passage portion.
The exhaust port structure for an internal combustion engine according to claim 1 or 2, wherein
the exhaust passage portion includes a recessed portion formed by making an inner wall of the exhaust passage portion on an upstream side of the boss portion swell outward, and

the recessed portion includes

a first curved surface extending outward from the cylindrical portion, and

a second curved surface having a curvature radius larger than a curvature radius of the first curved surface and being curved from the first curved surface toward a lower surface of the boss portion.
The exhaust port structure for an internal combustion engine according to claim 3, wherein the exhaust valve guide is attached to the cylinder head such that a lower surface of the exhaust valve guide does not project from a lower end portion of the boss portion to the opening side.