engine

Abstract

An engine is disclosed, which takes into account both the increase in the flow rate of intake air and the flow enhancement. The engine has a combustion chamber (20), the engine has a cylinder head (18) formed with an intake port (21) communicating with the combustion chamber (20), the intake port (21) is an inner wall portion arranged from the upstream side to the downstream side, and has a first inner wall portion (31) having a valve guide hole (33) and a second inner wall portion (32) opposite to the first inner wall portion (31), a first curved concave surface (41) for changing the flow direction of intake air is formed on the first inner wall portion (31), and a second curved concave surface (42) for changing the flow direction of intake air is formed on the second inner wall portion (32).

Description

Technical Field

[0001] The present invention relates to an engine with an air intake formed therein. Background Art

[0002] An intake port connected to the combustion chamber is formed in the cylinder head of the engine. The shape of the intake port not only affects the increase or decrease of the flow rate of the intake air, but also is the main factor affecting the strength of the tumble flow, etc. Therefore, from the perspective of increasing the intake air flow rate and strengthening the gas flow of the tumble flow, etc., various shapes of intake ports have been proposed (see Patent Documents 1 to 3).

[0003] [Prior art literature]

[0004] [Patent Document]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2003-262132

[0006] [Patent Document 2] Japanese Patent Application Publication No. 2015-190373

[0007] [Patent Document 3] Japanese Patent Application Publication No. 2016-180356 Summary of the invention

[0008] However, it is difficult to take into account both the increase in the flow rate of the intake air and the strengthening of the flow when designing the shape of the air intake port. That is, designing the air intake port to increase the flow rate of the intake air is the main reason for weakening the flow of the intake air. In addition, designing the air intake port to strengthen the flow of the intake air is the main reason for reducing the flow rate of the intake air.

[0009] An object of the present invention is to achieve both an increase in the flow rate of intake air and flow enhancement.

[0010] The engine of the present invention includes a combustion chamber, wherein the engine has a cylinder head formed with an intake port communicating with the combustion chamber, the intake port being an inner wall portion provided from an upstream side to a downstream side, including a first inner wall portion having an opening of a valve guide hole and a second inner wall portion opposite to the first inner wall portion, a first curved concave surface for changing a flow direction of intake air being formed on the first inner wall portion, and a second curved concave surface for changing a flow direction of intake air being formed on the second inner wall portion.

[0011] According to the present invention, a first curved concave surface for changing the flow direction of the intake air is formed on the first inner wall, and a second curved concave surface for changing the flow direction of the intake air is formed on the second inner wall. Thus, both the flow rate increase and flow enhancement of the intake air can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a schematic diagram showing an engine according to an embodiment of the present invention.

[0013] Figure 2 This is a cross-sectional view showing an intake port and a combustion chamber together with an intake valve.

[0014] Figure 3 It is a diagram showing the shapes of an intake port and a combustion chamber formed in a cylinder head.

[0015] Figure 4 is from Figure 3 The direction of arrow α shows the shape of the intake port and the combustion chamber.

[0016] Figure 5 is along Figure 4 Virtual plane X shows a cross-sectional view of the air intake and the combustion chamber.

[0017] Figure 6 Yes Figure 5 A partially enlarged cross-sectional view of the air intake is shown in FIG.

[0018] Figure 7 (A) is a diagram showing the relationship between each tangent line of the first curved concave surface and the center line of the valve guide hole, and (B) is a diagram showing the relationship between each tangent line of the second curved concave surface and the center line of the valve guide hole.

[0019] Figure 8 (A) to (C) are diagrams showing intake flow paths at each position of the intake port.

[0020] Fig. 9 This is a diagram schematically showing the flow of intake air passing through the air intake port of the first embodiment.

[0021] Fig.10 It is a diagram schematically showing the flow of intake air passing through the air intake port of the comparative example.

[0022] Fig.11 1 is a diagram schematically showing the flow of intake air passing through the air intake port of Reference Example 1.

[0023] Fig.12 It is a diagram showing simulation results of the velocity range of intake air flowing into the combustion chamber from the intake port of Reference Example 1 and the Comparative Example.

[0024] Fig.13 This is a diagram schematically showing the flow of intake air passing through the air intake port of Reference Example 2.

[0025] Fig.14 It is a diagram showing simulation results of the velocity range of intake air flowing into the combustion chamber from the intake port of Reference Example 2 and the Comparative Example.

[0026] Fig.15It is a diagram showing simulation results of the velocity range of intake air flowing into the combustion chamber from the intake port in Example 1 and the comparative example.

[0027] Fig.16 It is a diagram showing the simulation results of the intake air flow rate and the gas flow in the intake port.

[0028] Fig.17 It is a diagram showing the shapes of the intake port and the combustion chamber of the second embodiment.

[0029] Fig.18 is along Figure 4 Virtual plane X shows a cross-sectional view of the air intake and the combustion chamber.

[0030] Fig.19 It is a diagram schematically showing the flow of intake air through the air intake port.

[0031] (Explanation of Reference Numerals)

[0032] 10: engine; 18: cylinder head; 20: combustion chamber; 21: intake port; 31: first inner wall portion; 32: second inner wall portion; 33: valve guide hole; 41: first curved concave surface; 42: second curved concave surface; 50: intake port; X: virtual plane; CL1: center line; La1~La3: tangent line; Lb1~Lb3: tangent line; α1~α3: angle; β1~β3: angle. DETAILED DESCRIPTION

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, the engine shown in the drawings is a horizontally opposed engine, but the present invention is not limited thereto and can also be applied to an inline engine, a V-type engine, and the like.

[0034] [Engine structure]

[0035] Figure 1 1 is a schematic diagram showing an engine 10 according to an embodiment of the present invention. Figure 1 As shown, the engine 10 includes a cylinder block 11 provided in one cylinder group, a cylinder block 12 provided in the other cylinder group, and a crankshaft 13 supporting the pair of cylinder blocks 11 and 12. A piston 15 is accommodated in a cylinder bore 14 formed in the cylinder blocks 11 and 12, and the crankshaft 13 is connected to the piston 15 via a connecting rod 16.

[0036] A cylinder head 18 having a valve mechanism 17 is assembled to the cylinder blocks 11 and 12, and a valve cover 19 covering the valve mechanism 17 is assembled to the cylinder head 18. In addition, an intake port 21 communicating with a combustion chamber 20 is formed in the cylinder head 18, and an intake valve 22 for opening and closing the intake port 21 is assembled. In addition, an exhaust port 23 communicating with the combustion chamber 20 is formed in the cylinder head 18, and an exhaust valve 24 for opening and closing the exhaust port 23 is assembled. In addition, an intake manifold 25 for guiding intake air is connected to the intake port 21, and an exhaust manifold 26 for guiding exhaust gas is connected to the exhaust port 23.

[0037] [Air intake structure]

[0038] Hereinafter, the air intake port 21 of the first embodiment will be described. Figure 2 It is a cross-sectional view showing the intake port 21 and the combustion chamber 20 together with the intake valve 22 . Figure 3 2 is a diagram showing the shapes of the intake port 21 and the combustion chamber 20 formed in the cylinder head 18. Figure 4 is from Figure 3 The arrow α direction shows the shape of the intake port 21 and the combustion chamber 20. Figure 5 is along Figure 4 A virtual plane X shows a cross-sectional view of the air intake port 21 and the combustion chamber 20 .

[0039] In addition, Figure 2 to Figure 5 In, from Figure 1 The state shown in FIG. 1 is rotated 90° to show the air inlet 21 and the like, and the exhaust port 23 is omitted. Figure 2 to Figure 5 The upstream side and the downstream side shown in refs. 2 mean the upstream side and the downstream side in the flow direction of the intake air toward the combustion chamber 20. Figure 4 The virtual plane X described in φ is a virtual plane that includes a center line CL1 of a valve guide hole 33 described later and intersects a first curved concave surface 41 and a second curved concave surface 42 described later.

[0040] like Figure 2 to Figure 5 As shown, the intake port 21 formed in the cylinder head 18 is an inner wall portion arranged from the upstream side to the downstream side, and has a first inner wall portion 31 located on the outer side of the engine 10, that is, the valve cover 19 side, and a second inner wall portion 32 located on the inner side of the engine 10, that is, the cylinder block 11 (12) side. Figure 2 As shown in FIG. 1 , a valve guide hole 33 is opened in the first inner wall portion 31 of the intake port 21, and a valve guide 34 supporting the shaft portion 22a of the intake valve 22 is inserted into the valve guide hole 33. In this way, the intake port 21 is provided with the first inner wall portion 31 in which the valve guide hole 33 is opened and the second inner wall portion 32 facing the first inner wall portion 31. In addition, a valve seat 35 that contacts the umbrella portion 22b of the intake valve 22 is provided at the opening end of the intake port 21 facing the combustion chamber 20.

[0041] like Figure 2 , Figure 3 as well as Figure 5 As shown, a first curved concave surface 41 is formed on the first inner wall portion 31 of the intake port 21, which is recessed toward the valve cover side to expand the intake flow passage 40, and a second curved concave surface 42 is formed on the second inner wall portion 32 of the intake port 21, which is recessed toward the cylinder body side to expand the intake flow passage 40. In addition, the first curved concave surface 41 is formed on the first inner wall portion 31 closer to the downstream side than the upstream side, and the second curved concave surface 42 is formed on the second inner wall portion 32 closer to the downstream side than the upstream side. In addition, at least a portion of the first curved concave surface 41 and the second curved concave surface 42 are arranged opposite to each other. In addition, as shown in FIG. Figure 3 as well as Figure 5 As shown, the first curved concave surface 41 is an area recessed toward the valve cover side, that is, an area between the inflection point A1 and the inflection point A2 that appear on the first inner wall portion 31, and the second curved concave surface 42 is an area recessed toward the cylinder body 11 side, that is, an area between the inflection point B1 and the inflection point B2 that appear on the second inner wall portion 32.

[0042] Here, Figure 6 Yes Figure 5 FIG. 2 is a partially enlarged cross-sectional view of the air inlet 21 shown in FIG. Figure 6 , a cross-sectional view along a virtual plane X including a center line CL1 of the valve guide hole 33 is shown. Figure 7 (A) is a diagram showing the relationship between the tangent lines La1 to La3 of the first curved concave surface 41 and the center line CL1 of the valve guide hole 33. Figure 7 (B) is a diagram showing the relationship between the tangent lines Lb1 to Lb3 of the second curved concave surface 42 and the center line CL1 of the valve guide hole 33. Figure 6 as well as Figure 7 The tangent lines La1 to La3 and Lb1 to Lb3 shown in FIG. 5 are tangent lines on a virtual plane X including the center line CL1 of the valve guide hole 33 .

[0043] like Figure 6 As shown in FIG. 1 , the tangent lines La1 to La3 at the points Pa1 to Pa3 of the first curved concave surface 41 gradually incline in the direction along the center line CL1 from the upstream side to the downstream side. Figure 7 As shown in (A), the angle α3 formed by the center line CL1 and the tangent line La3 is smaller than the angle α2 formed by the center line CL1 and the tangent line La2, and the angle α2 formed by the center line CL1 and the tangent line La2 is smaller than the angle α1 formed by the center line CL1 and the tangent line La1. That is, among the angles formed by the tangent lines La1 to La3 of the first curved concave surface 41 and the center line CL1 of the valve guide hole 33, the angles α1 to α3 on the acute angle side gradually decrease from the upstream side to the downstream side.

[0044] In addition, if Figure 6As shown in FIG. 1 , the tangent lines Lb1 to Lb3 on the points Pb1 to Pb3 of the second curved concave surface 42 gradually incline in a direction perpendicular to the center line CL1 from the upstream side to the downstream side. Figure 7 As shown in (B), the angle β3 formed by the center line CL1 and the tangent line Lb3 is larger than the angle β2 formed by the center line CL1 and the tangent line Lb2, and the angle β2 formed by the center line CL1 and the tangent line Lb2 is larger than the angle β1 formed by the center line CL1 and the tangent line Lb1. That is, among the angles formed by the tangent lines Lb1 to Lb3 of the second curved concave surface 42 and the center line CL1 of the valve guide hole 33, the angles β1 to β3 on the acute angle side gradually increase from the upstream side to the downstream side.

[0045] Next, the cross-sectional area of ​​the intake flow path 40 of the intake port 21 will be described. Figure 8 (A) to (C) are diagrams showing intake flow paths Fa to Fc for each portion of the intake port 21 . Figure 8 (A) shows along Figure 3 The cross section of the intake flow passage FA on line AA, Figure 8 (B) shows along Figure 3 The cross section of the intake flow passage Fb of line BB, Figure 8 (C) shows along Figure 3 The cross section of the intake runner Fc on the CC line.

[0046] like Figure 8 As shown in (A) and (B), the cross-sectional area Ab of the intake flow passage Fb is larger than the cross-sectional area Aa of the intake flow passage Fa. Figure 8 As shown in (B) and (C), the cross-sectional area Ac of the intake flow passage Fc is smaller than the cross-sectional area Ab of the intake flow passage Fb. In this way, the inner wall portion (the first inner wall portion 31, the second inner wall portion 32) of the intake port 21 forms a curved concave surface (the first curved concave surface 41, the second curved concave surface 42), so that the cross-sectional area (flow passage cross-sectional area) of the intake flow passage 40 of the intake port 21 increases from the upstream side to the downstream side and then decreases.

[0047] [Flow of Inhaled Air]

[0048] As described above, the first curved concave surface 41 is formed on the first inner wall portion 31 of the intake port 21, and the second curved concave surface 42 is formed on the second inner wall portion 32 of the intake port 21. In this way, the first curved concave surface 41 and the second curved concave surface 42 are provided in the intake port 21, so that it is considered that the flow rate of the intake air (hereinafter referred to as the intake flow rate) can be increased, and the gas flow in the combustion chamber 20 and the cylinder bore 14 can be strengthened as described later.

[0049] Here, Fig. 9 is a diagram schematically showing the flow of intake air through the air intake port 21 of the first embodiment. Fig.10FIG. 1 is a diagram schematically showing the flow of intake air through the air intake port 100 of the comparative example. Fig. 9 As shown, in the intake port 21 of the first embodiment, as indicated by arrow F1, the intake air flowing near the first inner wall portion 31 flows toward the combustion chamber 20 while changing the flow direction along the first curved concave surface 41. In addition, as indicated by arrow F2, the intake air flowing near the second inner wall portion 32 flows toward the combustion chamber 20 while changing the flow direction along the second curved concave surface 42. Furthermore, in the intake port 21 of the first embodiment, a flow of intake air as indicated by arrow F3 is generated in the combustion chamber 20 and the cylinder bore 14. In this regard, as shown in FIG. Fig.10 As shown, in the intake port 100 of the comparative example extending straight toward the combustion chamber 20 , the intake air flows straight toward the combustion chamber 20 as indicated by the arrow F10 .

[0050] Next, the increase in intake air flow rate and the enhancement of gas flow by the intake port 21 of Example 1 will be described by comparing the intake port 100 of the comparative example with the intake ports 200 and 300 of Reference Examples 1 and 2. Fig.11 is a diagram schematically showing the flow of intake air through the air intake port 200 of Reference Example 1. Fig.12 : is a diagram showing the simulation results of the velocity region of the intake air flowing into the combustion chamber 20 from the intake ports 200 and 100 of Reference Example 1 and Comparative Example. Fig.13 is a diagram schematically showing the flow of intake air through the air intake port 300 of Reference Example 2. Fig.14 : is a diagram showing the simulation results of the velocity region of the intake air flowing into the combustion chamber 20 from the intake ports 300 and 100 of Reference Example 2 and Comparative Example. Fig.15 1 is a diagram showing simulation results of the velocity range of the intake air flowing from the intake ports 21 and 100 into the combustion chamber 20 in Example 1 and the comparative example. Fig.16 1 is a diagram showing the simulation results of the intake air flow rate and the gas flow in the intake port 21 , 100 .

[0051] In addition, if Figure 5 As shown by the double-dotted line Ca, Fig.12 , Fig.14 as well as Fig.15 The circular range Ca shown in FIG. 1 is a circular range set near the valve seat 35. Figure 5 as well as Fig.12 The reference point Ca1 shown in the figure is located at the center side of the cylinder bore 14, that is, at the radial inner side. Figure 5 as well as Fig.12 The reference point Ca2 shown in the figure is located on the inner wall side of the cylinder bore 14, that is, on the radially outer side. Fig.12 , Fig.14 as well as Fig.15In FIG. 1 , the velocity boundary of the intake air flowing into the combustion chamber 20 from the intake ports 200 and 300 of reference examples 1 and 2 and the intake port 21 of embodiment 1 is indicated by a solid line, and the velocity boundary of the intake air flowing into the combustion chamber 20 from the intake port 100 of the comparative example is indicated by a dotted line. Fig.12 , Fig.14 as well as Fig.15 In FIG. 1 , the area surrounded by the solid line and the dotted line is a high flow rate area where the inhaled air flows in at a speed exceeding a predetermined speed. Fig.16 The tumble ratio recorded on the horizontal axis of is an index showing the strength of the gas flow in the cylinder bore 14, which means that the larger the tumble ratio, the stronger the gas flow, that is, the tumble. Fig.16 The vertical axis shows the effective opening area of ​​the flow path through which the intake air flows, which means that the larger the value of the effective opening area is, the greater the intake air flow rate is.

[0052] like Fig.11 As shown, the air inlet 200 of Reference Example 1 is an air inlet having the first curved concave surface 41 of the air inlet 21 described above. Fig.12 As shown by reference symbol Xa in FIG. 1 , when the air inlet 200 of reference example 1 is used, the high flow velocity region is further expanded toward the reference point Ca2 side compared with the case of using the air inlet 100 of the comparative example. This is considered to be because in the air inlet 200 having the first curved concave surface 41, as shown in FIG. Fig.11 As shown by arrow F20 in FIG. 1 , the flow direction of the intake air changes along the first curved concave surface 41, and the intake air flows toward the bottom of the cylinder bore 14, that is, the top 15a of the piston 15. As a result, when the intake air flows into the combustion chamber 20, the opening area of ​​the intake port 21 can be effectively utilized, so that the intake air can easily flow into the combustion chamber 20. As a result, in the intake port 200 of reference example 1, the intake air flow rate flowing into the combustion chamber 20 can be increased compared with the intake port 100 of the comparative example. That is, it can be considered that the flow along the first curved concave surface 41 of the intake port 200 is the flow of the intake air that helps to increase the intake air flow rate.

[0053] In addition, if Fig.13 As shown, the air inlet 300 of Reference Example 2 is an air inlet having the second curved concave surface 42 of the air inlet 21 described above. Fig.14 As shown by reference symbol Xb in FIG. 1 , when the air inlet 300 of reference example 2 is used, the high flow velocity region is further expanded toward the reference point Ca1 side compared to the case where the air inlet 100 of the comparative example is used. This is considered to be because, in the air inlet 300 having the second curved concave surface 42, Fig.13As shown by arrow F30 in FIG. 1 , the flow direction of the intake air changes along the second curved concave surface 42, and the intake air flows toward the inner wall 14a of the cylinder bore 14. As a result, the intake air can be moved along the inner wall 14a of the cylinder bore 14, thereby strengthening the flow of the intake air flowing into the combustion chamber 20. As a result, in the intake port 300 of reference example 2, the tumble flow in the combustion chamber 20 can be strengthened compared with the intake port 100 of the comparative example. That is, it can be considered that the flow along the second curved concave surface 42 of the intake port 300 is the flow of the intake air that helps to strengthen the gas flow.

[0054] Then, if Fig.15 As shown by reference numerals Xc and Xd in the figure, when the air inlet 21 of Example 1 is used, the high flow velocity region is expanded near the port side portion 35A of the valve seat 35 compared with the case where the air inlet 100 of the comparative example is used. This is because, in the air inlet 21 having the first curved concave surface 41 and the second curved concave surface 42, as shown in FIG. Fig. 9 As shown by arrows F1 and F2 in the figure, the flow direction of the intake air changes along the first curved concave surface 41 and the second curved concave surface 42, and the flow of the intake air as shown by arrow F3 is generated by the action of these flows. That is, it can be considered that in the intake port 21 of the first embodiment, the flow of the intake air that helps to increase the intake air flow rate ( Fig. 9 F1) and the flow of the intake air that helps to strengthen the gas flow ( Fig. 9 F2) interacts with each other to produce a flow of the intake air that contributes to both flow increase and flow enhancement ( Fig. 9 F3). Therefore, if Fig.16 As shown, in the air intake port 21 of the first embodiment, compared with the air intake port 100 of the comparative example, it is possible to increase the intake air flow rate and strengthen the gas flow.

[0055] As described so far, the flow of the intake air that helps increase the intake air flow rate is enhanced by providing the first curved concave surface 41, and the flow of the intake air that helps strengthen the gas flow such as tumble flow is enhanced by providing the second curved concave surface 42. Therefore, in the air intake port 21 of the first embodiment having both the first curved concave surface 41 and the second curved concave surface 42, both the flow of the intake air that helps increase the intake air flow rate and the flow of the intake air that helps strengthen the gas flow can be ensured, so that both the increase in the intake air flow rate and the flow strengthening can be achieved.

[0056] [Other embodiments]

[0057] Next, other embodiments of the present invention will be described. Fig.17 2 is a diagram showing the shapes of the intake port 50 and the combustion chamber 20 of the second embodiment. Fig.18 is along Figure 4The virtual plane X shows a cross-sectional view of the air intake port 50 and the combustion chamber 20. In addition, Fig.19 is a diagram schematically showing the flow of the intake air through the intake port 50. Figure 17 to Figure 19 In, about Figure 3 as well as Figure 5 The same parts as those shown in FIG. 1 are denoted by the same reference numerals and their description is omitted.

[0058] like Fig.17 as well as Fig.18 As shown, a first curved concave surface 41 that is recessed toward the valve cover side and expands the intake flow passage 40 is formed in the first inner wall portion 31 of the intake port 50, and a second curved concave surface 42 that is recessed toward the cylinder block side and expands the intake flow passage 40 is formed in the second inner wall portion 32 of the intake port 50. In addition, in the first inner wall portion 31 of the intake port 50, a first curved convex surface 51 that is convex toward the cylinder block side and reduces the intake flow passage 40 is formed in connection with the upstream side of the first curved concave surface 41. Moreover, in the second inner wall portion 32 of the intake port 50, a second curved convex surface 52 that is convex toward the valve cover side and reduces the intake flow passage 40 is formed in connection with the upstream side of the second curved concave surface 42. That is, in the intake port 50, an expansion portion 53 formed by the first curved concave surface 41 and the second curved concave surface 42 is formed, and a throttle portion 54 formed by the first curved convex surface 51 and the second curved convex surface 52 is formed.

[0059] In this way, the first curved concave surface 41 and the second curved concave surface 42 are also formed in the intake port 50 having the throttle portion 54, so that the intake air can be guided toward the combustion chamber 20 as in the intake port 21 of the first embodiment. Fig.19 As shown, in the intake port 50 of the second embodiment, as shown by arrow F1, the intake air flowing near the first inner wall portion 31 flows toward the combustion chamber 20 while changing the flow direction along the first curved concave surface 41. In addition, as shown by arrow F2, the intake air flowing near the second inner wall portion 32 flows toward the combustion chamber 20 while changing the flow direction along the second curved concave surface 42. That is, it can be considered that the flow of the intake air that helps increase the intake flow rate is the same as the intake port 21 of the first embodiment. Fig.19 F1) and the flow of the intake air that helps to strengthen the gas flow ( Fig.19 F2) interacts with each other to produce a flow of the intake air that contributes to both flow increase and flow enhancement ( Fig.19 That is, the air intake port 50 of the second embodiment can also take into account both the increase in the flow rate of the intake air and the flow enhancement. Fig.16 As shown, when the air intake port 50 of Example 2 is used, it is possible to increase the intake air flow rate and strengthen the gas flow, compared with the case where the air intake port 100 of the comparative example is used.

[0060] The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the invention. Figure 8 The cross-sectional shape of the air inlet 21 shown in FIG. 1 is a quadrilateral with rounded corners, but it is not limited thereto and can obviously be other cross-sectional shapes. For example, the first curved concave surface 41 and the second curved concave surface 42 can also be formed for the air inlet 21 or 50 having a circular or elliptical cross-sectional shape. Figure 4 In the example shown in FIG. 2 , two intake ports 21 separated from each other are connected to the combustion chamber 20 . However, the present invention is not limited to this, and one intake port including a port portion branched into two may be connected to the combustion chamber 20 .

Claims

1. An engine comprising a combustion chamber, wherein: The engine has a cylinder head formed with an intake port communicating with the combustion chamber, The air inlet is provided as an inner wall portion extending from the upstream side to the downstream side, and includes a first inner wall portion having a valve guide hole opened therein and a second inner wall portion opposite to the first inner wall portion. A first curved concave surface for changing the flow direction of the intake air is formed on the first inner wall portion. A second curved concave surface for changing the flow direction of the intake air is formed on the second inner wall portion. On a virtual plane including the center line of the valve guide hole, the acute angle of the angle formed by the tangent line of the first curved concave surface and the center line of the valve guide hole decreases from the upstream side to the downstream side, and the acute angle of the angle formed by the tangent line of the second curved concave surface and the center line of the valve guide hole increases from the upstream side to the downstream side, The intake flow passage of the air intake port includes, from the upstream side to the downstream side, a first cross-section in the shape of a quadrilateral with rounded corners, a second cross-section in the shape of a quadrilateral with rounded corners and having a larger cross-sectional area than the first cross-sectional area, and a third cross-sectional area in the shape of a quadrilateral with rounded corners, in the area of ​​the first inner wall portion where the first curved concave surface is formed and the second inner wall portion where the second curved concave surface is formed.

2. The engine according to claim 1, wherein: The first curved concave surface is formed on the first inner wall portion closer to the downstream side than the upstream side. The second curved concave surface is formed on the second inner wall portion closer to the downstream side than the upstream side.

3. The engine according to claim 1 or 2, wherein: By further providing the throttle portion at the air inlet, the flow passage cross-sectional area of ​​the air inlet increases from the upstream side to the downstream side and then decreases. The throttling portion is relatively provided with a first curved convex surface and a second curved convex surface, In the first inner wall portion, the first curved convex surface is formed to be connected to the first curved concave surface at the upstream side of the first curved concave surface and to reduce the intake flow passage. In the second inner wall portion, the second curved convex surface is formed to be connected to the second curved concave surface at the upstream side of the second curved concave surface, and narrows the intake flow path.

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