JPH0914047A - Exhaust port structure of engine - Google Patents

Abstract

PURPOSE: To improve engine performance by reducing the exhaust resistance when exhaust gas is exhausted from a combustion room through an exhaust port. CONSTITUTION: An exhaust valve 14 for closing an opening port 11 to a combustion room 8 openably/closably at an exhaust port 10 is provided. A contact surface 26 for contacting the valve umbrella 15 of the exhaust valve 14 to the opening edge 24 of the opening port 11 is formed on the above opening edge 24. The one part inner periphery surface 29 of the exhaust port 10 connected to its downstream side near this end edge 27 from the end edge 27 of the downstream side of an exhaust gas 19 on the contacted surface 26 is formed in an arc shape project surface projected toward the inner side of the exhaust port 10. A tangent line 35 passing the end edge 34 of the contact surface 26 side on one part inner periphery surface 29 formed to the arc shape project surface is almost matched with a disguised extension line 30 extended along the contact surface 26.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a four-cycle engine mounted on a motorcycle or the like, and more particularly to an exhaust port structure of an engine which reduces resistance of exhaust gas passing through an exhaust port.

[0002]

2. Description of the Related Art Conventionally, there is an engine exhaust port structure shown by phantom lines in FIG. 2 and FIGS.

According to this, the engine 1 which is an internal combustion engine
Has a cylinder 2, and this cylinder 2 has a cylinder body 3
And a cylinder head 4 attached to one end of the cylinder body 3.

As shown in the side view of FIG. 4, the cylinder head 4 is attached to the upper end of the cylinder body 3 when the axis 6 of the cylinder body 3 is viewed substantially vertically. A cylinder hole 7 is formed in the cylinder body 3 on an axis 6 thereof, and a piston is slidably fitted in the cylinder hole 7 along the axis 6 and the cylinder head 4 is inserted in the cylinder hole 7. The space surrounded by and the piston is the combustion chamber 8.

An intake port (not shown) for communicating the outside of the cylinder 2 with the combustion chamber 8 is provided with the cylinder head 4.
Is formed. Further, a pair of exhaust ports 10, 10 for communicating the combustion chamber 8 inside the cylinder 2 to the outside of the cylinder 2 is formed in the cylinder head 4.

An intake valve is provided for opening and closing the opening of the intake port to the combustion chamber 8. Further, as shown in the plan view of FIG. 3, each opening 11 to each combustion port 8 in each exhaust port 10 has a circular shape when viewed from the line of sight along the axis 6 of the cylinder 2, and each opening 11 Reference numerals 11 are offset from the axis 6 to one side (arrows A and B in FIG. 3). The opposite side of the exhaust port 10 from the opening 11 extends upward from the opening 11 and then bends substantially horizontally to extend in a direction away from the axis 6 of the cylinder 2. There is. Further, the extended end sides of the respective exhaust ports 10 join together to form a joining passage 12.

As shown by the solid line in FIG. 3 and the chain double-dashed line in FIG. 4, an exhaust valve 14 is provided that closes each opening 11 from the combustion chamber 8 side so as to be openable and closable. This exhaust valve 14
Is on the common axis 16 with the opening 11, and the opening 1
1 is provided with a valve opening 15 for opening and closing, and the valve opening 15
A valve rod 17 integrally extends upward from the upper surface of the valve through the opening 11. The upper end of the valve rod 17 is cam-engaged with the valve operating mechanism together with the intake valve.

The intake valve and the exhaust valve 14 are appropriately opened and closed by the operation of the valve operating mechanism which is operated in accordance with the operation of the engine 1.

When the intake valve opens the intake port, the air-fuel mixture is drawn into the combustion chamber 8 through the intake port and burned there. Then, the thermal energy due to this combustion is converted into power and output.

Exhaust gas 19 generated by the above combustion is shown in FIG.
As indicated by the solid line, the gas is discharged to the outside of the cylinder 2 through the exhaust port 10 including the opening 11 opened by the opening operation of the exhaust valve 14 and the merging passage 12. That is, the inside of the exhaust passage 10 including the opening 11 and the confluence passage 12 is an exhaust passage.

The cylinder head 4 has the opening 11
A cylinder head body 21 in which an exhaust port 10 on the downstream side of the exhaust 19 is formed, and the cylinder head body 2
1 and a valve seat 23 that is fitted into the cylinder head body 21 to form the cylinder head body 21.
The surface of which forms the opening edge 24 of the opening 11.

The valve stem 15 has a flat truncated cone shape whose cross-sectional area gradually decreases toward the downstream side of the exhaust gas 19 in the opening 11.

At the opening edge 24 of the opening 11, an abutting surface 26 is formed which abuts the frustoconical outer peripheral surface of the valve 15 of the exhaust valve 14 in surface contact. This contact surface 26
Has a truncated cone shape in which the opening area gradually decreases toward the downstream side of the exhaust gas 19 in the opening 11.

The alternate long and two short dashes line in FIG. 2 and the points A to D in FIG.
When viewed in the longitudinal direction of the exhaust port 10, the respective positions in the hole core 28 are shown, and the point a corresponds to the downstream end edge 27 of the exhaust 19 on the contact surface 26, and the points r-n are , Corresponding to respective positions on the downstream side from the point A.

Then, as seen from the cut surface extending along the axis 16 of the opening 11 (as seen in the side view shown in FIG. 4), the edge 27 of the abutting surface 26 is moved from the edge 27 to the vicinity of the edge 27. A part of the inner peripheral surface 29 of the exhaust port 10 connected to the downstream side extends substantially linearly, and the opening area of each part of the part of the inner peripheral surface 29 is also substantially constant (the two-dot chain line and the diagram in FIG. 2). Four
Range of I ~ Ha).

[0016]

By the way, according to the above conventional configuration, there are the following problems. .

That is, in the above-mentioned conventional structure, the intersection angle θ between the virtual extension line 30 passing through the contact surface 26 and another virtual extension line 31 extending along the partial inner peripheral surface 29 is large.

Therefore, as shown by the solid line in FIG.
The exhaust valve 14 operates to open the valve, and the valve bush 15 opens the opening 11.
When the exhaust gas 19 is discharged from the combustion chamber 8 through the opening 11 through the opening 11, when the exhaust gas 19 flowing along the contact surface 26 next flows along the partial inner peripheral surface 29. The exhaust 19 abruptly separates from the inner peripheral surface 29 and a peeling phenomenon occurs.

As a result, many vortices 32 are generated along the surface of the part of the inner peripheral surface 29, which increases exhaust resistance, which is not preferable because engine performance is deteriorated.

Further, in FIG. 4, the partial inner peripheral surface 29 is formed.
Of the above, the degree of curvature of the surface 29a on the side farther from the axis 6 of the cylinder 2 from the contact surface 26 connected to the surface 29a on the far side is the same. It is larger than the degree of curvature seen from the contact surface 26 that is continuous with the surface 29b.

Therefore, of the exhaust gas 19 flowing along the contact surface 26, the surface 29 on the far side follows the exhaust gas 19.
A portion of the exhaust gas 19a flowing along a is more rapidly separated from the partial inner circumferential surface 29 than the other portion of the exhaust gas 19b flowing along the surface 29b on the closer side, and is larger, Moreover, many eddies 32 tend to be generated.

Therefore, the exhaust gas 19 flowing through the exhaust port 10
There is also a problem that the exhaust resistance becomes large due to the non-uniform flow.

[0023]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and when exhaust gas is discharged from a combustion chamber through an exhaust port, its exhaust resistance is reduced to improve engine performance. The purpose is to

[0024]

In order to achieve the above object, an engine exhaust port structure of the present invention is a cylinder 2
The combustion chamber 8 inside the cylinder 2
From the same as above, forming an exhaust port 10 for passing the exhaust 19 toward the outside of the cylinder 2, and an exhaust valve 14 for opening and closing an opening 11 of the exhaust port 10 to the combustion chamber 8 from the side of the same combustion chamber 8 as above. A contact surface 26 for contacting the valve shaft 15 of the exhaust valve 14 is formed on the opening edge 24 of the opening 11, and the contact surface 26 is formed on the opening edge 24. In the case of a truncated cone shape that gradually decreases toward the downstream side of the above, when viewed from the cut surface extending along the axis 16 of the opening 11, from the edge 27 on the downstream side of the exhaust 19 in the contact surface 26. This edge 27
A portion of the inner peripheral surface 29 of the exhaust port 10 which is connected to the downstream side in the vicinity of the above is an arc convex surface projecting toward the inner side of the exhaust port 10, and an imaginary extension line 30 extending along the contact surface 26 and an arc. The tangent line 35 passing through the end edge 34 on the contact surface 26 side of the partially inner peripheral surface 29 that is a convex surface is substantially aligned.

In the above case, the minimum opening area S ₁ of the exhaust port 10 on the inner peripheral surface 29 is partially compared with the reference opening area S ₀ of the exhaust port 10 on the edge 27 of the contact surface 26.
The ratio R may be 55 to 76%.

Also, as seen from the line of sight along the axis 6 of the cylinder 2, the opening 11 of the exhaust port 10 is offset from the axis 6 of the cylinder 2 to one side A, B, and the same as above.
In the case where the downstream end side of the is extended in the direction away from the axis 6, the direction is substantially opposite to the one side A, B,
The axis 36 of the exhaust port 10 on the part of the inner peripheral surface 29 may be deviated from the axis 16 of the opening 11.

Further, the cylinder head 4 has a cylinder head body 21 in which an exhaust port 10 downstream of the exhaust 19 is formed, and the cylinder head body 21.
A valve seat 2 which is fitted into the opening to form the opening 11.
3, the valve seat 23 may partially form at least a part of the inner peripheral surface 29.

[0028]

[Operation] The operation of the above configuration is as follows.

In FIG. 1, the opening 1 of the exhaust port 10
The contact surface 26 of the exhaust valve 14 with the valve bush 15 in 1 is
The end surface 27 of the contact surface 26 is smoothly connected to a part of the inner peripheral surface 29 of the exhaust port 10 connected to the downstream side thereof.

Therefore, when the exhaust gas 19 is discharged from the combustion chamber 8 through the opening 11, the exhaust gas 19 flowing along the contact surface 26 then flows along the partial inner peripheral surface 29. At this time, it is possible to prevent the part from suddenly separating from the inner peripheral surface 29. Moreover, the exhaust gas 19 flowing along the contact surface 26 then flows while drawing an arc in the exhaust port 10 corresponding to the part inner peripheral surface 29, while the part inner peripheral surface 29 also has an arc convex surface. Therefore, the exhaust gas 19 flows along the partial inner peripheral surface 29 without abruptly separating from the inner peripheral surface 29.

In the above structure, the edge 27 of the contact surface 26 is
The ratio R of the minimum opening area S ₁ of the exhaust port 10 on the part of the inner peripheral surface 29 to the reference opening area S ₀ of the exhaust port 10 at 5 to 76% may be set.

By doing so, the exhaust resistance can be effectively reduced.

In FIG. 1, of the partial inner peripheral surface 29, the degree of curvature of the surface 29a of the cylinder 2 farther from the axis 6 of the cylinder 2 as seen from the contact surface 26 which is continuous with the surface 29a of the far side is the same as the above-mentioned shaft. The surface 29b on the side closer to the heart 6 is the surface 2 on the closer side.
It is larger than the degree of curvature seen from the contact surface 26 connected to 9b.

Therefore, of the exhaust gas 19 flowing along the contact surface 26, the surface 29 on the far side follows the exhaust gas 19.
A portion of the exhaust gas 19a flowing along a is more rapidly separated from the partial inner circumferential surface 29 than the other portion of the exhaust gas 19b flowing along the surface 29b on the closer side, and is larger, Moreover, many eddies 32 tend to be generated.

Therefore, as seen from the line of sight along the axis 6 of the cylinder 2, the opening 11 of the exhaust port 10 has the opening 2
From the shaft center 6 of one side to one side A, B, and the same as above exhaust port 1
When the downstream end side of 0 extends in the direction away from the shaft center 6, the shaft center 36 of the exhaust port 10 on the inner peripheral surface 29 is partially directed in the direction substantially opposite to the one side A and B.
May be deviated from the axis 16 of the opening 11.

In this way, the arc convex surface of the surface 29a on the far side projects more toward the inside of the exhaust port 10 than the arc convex surface of the surface 29b on the near side.

Therefore, a part of the exhaust gas 19a flowing along the far side surface 29a is also prevented from being abruptly separated from the far side surface 29a, so that a non-uniform flow may occur in the exhaust gas 19 flowing through the exhaust port 10. It is suppressed, and therefore the exhaust resistance is reduced.

Further, the cylinder head 4 is provided with an exhaust port 10 on the downstream side of the exhaust 19 with respect to the opening 11, and the cylinder head main body 21.
A valve seat 2 which is fitted into the opening to form the opening 11.
3, the valve seat 23 may partially form at least a part of the inner peripheral surface 29.

By doing so, the arc convex surface is formed on a part of the valve seat 23, and the wall thickness of the valve seat 23 is correspondingly increased, and the strength and rigidity thereof are improved.

Therefore, in terms of strength, this valve seat 23
Is allowed to be increased, and the opening area of the opening 11 can be increased.

[0041]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1, 2 and 3 and 3.

The components and operations common to those of the prior art shown in FIGS. 2 to 4 are designated by the same reference numerals in the drawings, and the description thereof will be omitted.

As shown in FIG. 1, when viewed from the cut surface extending along the axis 16 of the opening 11 of the exhaust port 10, from the edge 27 of the contact surface 26 to the downstream side in the vicinity of this edge 27. A part of the inner peripheral surface 29 of the continuous exhaust port 10 is a circular arc convex surface that projects toward the inside of the exhaust port 10. Then, the virtual extension line 30 extending along the contact surface 26 and the tangent line 35 passing through the edge 34 on the contact surface 26 side of the partial inner peripheral surface 29 that is an arcuate convex surface are substantially coincident with each other. ing.

More specifically, the angle of intersection between the virtual extension line 30 and the tangent line 35 is about 0 to 2 °. For comparison with the conventional technique, a part of the inner peripheral surface 29 of the conventional technique is shown by a dashed line.

Because of the above-mentioned structure, the opening 1
The contact surface 26 of the valve bush 15 of the exhaust valve 14 in No. 1 is smoothly connected to a part of the inner peripheral surface 29 of the exhaust port 10 which is continuous from the edge 27 of the contact surface 26 to the downstream side thereof. ing.

Therefore, when the exhaust gas 19 is discharged from the combustion chamber 8 through the opening 11, the exhaust gas 19 flowing along the abutment surface 26 then flows along the partial inner peripheral surface 29. At this time, it is possible to prevent the part from suddenly separating from the inner peripheral surface 29. Moreover, the exhaust gas 19 flowing along the contact surface 26 then flows while drawing an arc in the exhaust port 10 corresponding to the part inner peripheral surface 29, while the part inner peripheral surface 29 also has an arc convex surface. Therefore, the exhaust gas 19 flows along the partial inner peripheral surface 29 without abruptly separating from the inner peripheral surface 29.

The ratio R of the minimum opening area S ₁ of the exhaust port 10 on the partial inner peripheral surface 29 to the reference opening area S ₀ of the exhaust port 10 on the edge 27 of the contact surface 26 is 55 to 76. It is said to be%. The areas S ₀ and S ₁ are values with the area of the valve rod 17 set to 0.

In the above case, the ratio R is
If (= S ₁ / S ₀ ) is less than 55%, a part of the inner peripheral surface 2
The amount of protrusion of 9 becomes excessively large, and the reference opening area S ₀ becomes too small, so that the pressure loss becomes large and the reduction of exhaust resistance is hindered.

On the other hand, when the ratio R exceeds 76%, the amount of protrusion of the inner peripheral surface 29 is insufficient, and a vortex starts to occur on the surface of the inner peripheral surface 29. Therefore, in this case, the exhaust resistance Reduction is insufficient.

Therefore, as described above, the ratio R is set to 55-55.
This is set to 76%, which makes it possible to reduce the exhaust resistance more effectively.

In FIG. 1, of the partial inner peripheral surface 29, the degree of curvature of the surface 29a on the side far from the axis 6 of the cylinder 2 as viewed from the contact surface 26 connected to the surface 29a on the far side is the same as the above-mentioned axis. The surface 29b on the side closer to the heart 6 is the surface 2 on the closer side.
It is larger than the degree of curvature seen from the contact surface 26 connected to 9b.

Therefore, of the exhaust gas 19 flowing along the contact surface 26, the surface 29 on the far side follows the exhaust gas 19.
A portion of the exhaust gas 19a flowing along a is more rapidly separated from the partial inner circumferential surface 29 than the other portion of the exhaust gas 19b flowing along the surface 29b on the closer side, and is larger, Moreover, many eddies 32 tend to be generated.

Therefore, as seen from the line of sight along the axis 6 of the cylinder 2, the opening 11 of the exhaust port 10 has the above-mentioned cylinder 2
From the shaft center 6 of one side to one side A, B, and the same as above exhaust port 1
When the downstream end side of 0 extends in the direction away from the shaft center 6, the shaft center 36 of the exhaust port 10 on the inner peripheral surface 29 is partially directed in the direction substantially opposite to the one side A and B.
Is deviated from the axis 16 of the opening 11.

Therefore, the exhaust port 1 has the arc convex surface of the far surface 29a more than the arc convex surface of the near surface 29b.
It will protrude more toward the inside of 0.

Therefore, a portion of the exhaust gas 19a flowing along the far side surface 29a is also prevented from being abruptly separated from the far side surface 29a, so that a non-uniform flow may occur in the exhaust gas 19 flowing through the exhaust port 10. It is suppressed, and therefore the exhaust resistance is reduced.

Further, the cylinder head 4 has a cylinder head main body 21 in which an exhaust port 10 downstream of the exhaust 11 is formed, and the cylinder head main body 21.
A valve seat 2 which is fitted into the opening to form the opening 11.
3), at least a part of the partial inner peripheral surface 29 is formed by a part of the valve seat 23.

For this reason, a circular arc convex surface is formed on a part of the valve seat 23, and the wall thickness of the valve seat 23 is correspondingly increased, and the strength and rigidity thereof are improved.

Therefore, in terms of strength, this valve seat 23
Since it is permissible to increase the diameter of the opening 11,
The opening area of the exhaust gas is increased, whereby the pressure loss of the exhaust gas 19 passing through the opening portion 11 is reduced, that is, the exhaust resistance is further reduced.

[0059]

According to the present invention, the cut surface extending along the axis of the opening to the combustion chamber in the exhaust port,
A part of the inner peripheral surface of the exhaust port, which is continuous from the downstream edge of the exhaust gas in the contact surface to the downstream side in the vicinity of the edge, is a circular arc convex surface projecting toward the internal side of the exhaust port, An imaginary extension line extending along the tangent surface and a tangent line passing through the edge of the partial inner peripheral surface, which is an arcuate convex surface, on the contact surface side are substantially coincident with each other.

For this reason, the contact surface of the exhaust port opening with the valve sill of the exhaust valve is smoothly connected to a part of the inner peripheral surface of the same exhaust port connected to the downstream side from the edge of this contact surface. Has been.

Therefore, when the exhaust gas is discharged from the combustion chamber through the opening, the exhaust gas flowing along the contact surface is
Next, when flowing along the part of the inner peripheral surface, the part of the inner peripheral surface is prevented from suddenly leaving. Moreover, the exhaust gas flowing along the abutting surface then flows while drawing an arc in the exhaust port on a part of the inner peripheral surface, while the part inner peripheral surface is also an arc convex surface. Will flow along the part of the inner peripheral surface without abruptly separating from it.

As a result, the generation of vortices along the surface of the inner peripheral surface is suppressed, so that the exhaust resistance is reduced and the engine performance is improved.

In the above structure, the ratio of the minimum opening area of the exhaust port on a part of the inner peripheral surface to the reference opening area of the exhaust port on the edge of the contact surface may be 55 to 76%.

By the way, according to the invention of claim 1,
Since the arc convex surface is formed on a part of the inner peripheral surface of the exhaust port, the minimum opening area of the exhaust port is smaller than the reference opening area.

In this case, in view of the experimental results, when the above ratio is less than 55%, the protrusion amount of a part of the inner peripheral surface becomes excessively large, the reference opening area becomes too small, and therefore the pressure loss becomes large and the exhaust gas The reduction in resistance is hindered.

On the other hand, when the above ratio exceeds 76%, the protrusion amount of a part of the inner peripheral surface is insufficient, and a vortex starts to be generated on the surface of the part of the inner peripheral surface. Therefore, in this case, the exhaust resistance is reduced. Will be insufficient.

Therefore, as described above, the ratio is 55 to 7
6%, so that the exhaust resistance is more effectively reduced and the engine performance is improved.

By the way, of the partial inner peripheral surface, the degree of curvature of the surface farther from the shaft center of the cylinder as seen from the contact surface connected to this far side surface is the same as the surface close to the shaft center. It is larger than the degree of curvature seen from the contact surface that is continuous with the near surface.

Therefore, of the exhaust gas flowing along the contact surface, a part of the exhaust gas flowing along the surface on the far side subsequently to the exhaust gas flows along the surface on the near side. The part is more rapidly separated from the inner peripheral surface than the exhaust of the part, and larger and more swirls are likely to occur.

Therefore, as viewed along the axis of the cylinder, the opening of the exhaust port is offset from the axis of the cylinder to one side, and the downstream end of the exhaust port is separated from the axis of the cylinder. When extended, the axis of the exhaust port on the inner peripheral surface may be offset from the axis of the opening in a direction substantially opposite to the one side.

In this way, the arc convex surface of the far side surface projects more toward the inside of the exhaust port than the arc convex surface of the near side surface.

Therefore, a portion of the exhaust gas flowing along the surface on the far side is prevented from being abruptly separated from the surface on the far side, and the occurrence of uneven flow in the exhaust gas flowing through the exhaust port is suppressed. Exhaust resistance is reduced, and as a result, improved engine performance is achieved.

Further, according to the present invention, as described above, since the opening of the exhaust port is displaced to the one side from the axial center of the cylinder, the side of the opening near the axial center. Since the part of is located near the center of the combustion chamber, it is desirable to discharge more exhaust gas therethrough.

Therefore, as described above, the axial center of the exhaust port on a part of the inner peripheral surface is offset from the axial center of the opening in a direction substantially opposite to the one side direction.

Therefore, the exhaust of the exhaust gas from the combustion chamber through the exhaust port is made smooth, and the engine performance is further improved.

Further, the cylinder head is composed of a cylinder head body in which an exhaust port on the downstream side of the exhaust from the opening is formed, and a valve seat fitted into the cylinder head body to form the opening. In some cases, a portion of the valve seat may form at least a portion of the inner peripheral surface.

By doing so, the arc convex surface is formed on a part of the valve seat, and the wall thickness of the valve seat is correspondingly increased, and the strength and rigidity of the valve seat are improved.

Therefore, in terms of strength, it is allowed to increase the diameter of the valve seat, and the opening area of the opening can be increased. As a result, the pressure loss of the exhaust gas passing through the opening can be reduced. That is, the exhaust resistance is further reduced.

[Brief description of the drawings]

FIG. 1 is a side sectional view showing an embodiment of the present invention, which is shown in FIG.
FIG. 1 is a sectional view taken along the line -1.

FIG. 2 is a graph diagram comparing a conventional technique with an embodiment of the present invention.

FIG. 3 is a plan sectional view commonly used in a conventional technique and an embodiment of the present invention.

FIG. 4 is a diagram corresponding to FIG. 1 showing a conventional technique.

[Explanation of symbols]

1 Engine 2 Cylinder 3 Cylinder Body 4 Cylinder Head 6 Shaft Center 8 Combustion Chamber 10 Exhaust Port 11 Opening Port 14 Exhaust Valve 15 Valve Opening 16 Shaft Center 17 Valve Rod 19 Exhaust 19a Partial Exhaust 19b Other Exhaust 21 Cylinder Head Main body 23 Valve seat 24 Opening edge 26 Abutment surface 27 End edge 29 Partial inner peripheral surface 29a Far side surface 29b Near side surface 30 Virtual extension line 34 End edge 35 Tangent line 36 Shaft center S ₀ Reference opening area S ₁ Minimum opening Area R ratio

Claims (4)

[Claims] 1. A cylinder head of a cylinder is formed with an exhaust port through which exhaust gas passes from a combustion chamber inside the cylinder toward the outside of the cylinder, and an opening at the exhaust port to the combustion chamber is formed in the combustion chamber. An exhaust valve that can be opened and closed from the side is provided, and an abutment surface for abutting the valve stem of the exhaust valve on the opening edge of the opening is formed on the opening edge.
In the exhaust port structure of the engine having a truncated cone shape whose opening area gradually decreases toward the downstream side of the exhaust gas, the contact surface is seen as a cut surface extending along the axis of the opening. In the exhaust gas, a part of the inner peripheral surface of the exhaust port connected to the downstream side from the downstream edge of the exhaust in the vicinity of this edge is an arc convex surface projecting toward the inner side of the exhaust port, and along the contact surface. An exhaust port structure for an engine, wherein a virtual extension line extending in parallel with a tangent line passing through an edge on the contact surface side of the partial inner peripheral surface which is a circular arc convex surface is substantially aligned. 2. The engine according to claim 1, wherein the ratio of the minimum opening area of the exhaust port on a part of the inner peripheral surface is 55% to 76% with respect to the reference opening area of the exhaust port on the edge of the contact surface. Exhaust port structure. 3. An opening of the exhaust port is offset to one side from the axial center of the cylinder when viewed along the axial center of the cylinder, and a downstream end side of the exhaust port is separated from the axial center. In the extended engine exhaust port structure, the exhaust port axial center on a part of the inner peripheral surface is offset from the axial center of the opening in a direction substantially opposite to the one side direction. Exhaust port structure of the described engine. 4. The cylinder head is composed of a cylinder head main body having an exhaust port downstream of the opening from the exhaust, and a valve seat fitted to the cylinder head main body to form the opening. The exhaust port structure for an engine according to any one of claims 1 to 3, wherein at least a part of the inner peripheral surface of the part is formed by a part of the valve seat.