Title of Invention

INTERNAL COMBUSTION ENGINE

Abstract

To suppress an increase in intake resistance and a decrease in intake flow rate within an intake port in an internal combustion engine having two mutually opposing projecting portions within the intake port. [Solution] In an internal combustion engine which includes first and second projecting portions 24, 25 that project into an intake port 8 from a wall surface of the intake port 8, and in which the projecting portions 24, 25 extend along an intake flow direction within the intake port 8 and are provided so as to be opposed to each other as seen in a cross-section orthogonal to the intake flow direction, projecting top portions 24a, 25a of the projecting portions 24, 25 into the intake port 8 are arranged so as to be displaced from each other in the intake flow direction. [Selected Drawing] Fig. 2

Full Text

[Document Name] Specification [Title of the Invention] INTERNAL COMBUSTION ENGINE [Technical Field] [0001] The present invention relates to an internal combustion engine having an intake port that forms an intake passage leading into a combustion chamber. [Background Art] [0002] According to the related art, the above-mentioned internal combustion engine includes first and second projecting portions that project into the intake port from the wall surface of the intake port, and the projecting portions extend along an intake flow direction within the intake port and are provided so as to be opposed to each other as seen in a cross-section orthogonal to the intake flow direction (for example, see Patent Document 1). The projecting top portions of the projecting portions into the intake port are located at substantially the same position in the intake flow direction. As these projecting portions face each other to partially narrow the intake passage, a rectifying effect is given to the intake flow within the intake port and its flow rate distribution is changed to thereby achieve enhanced combustibility. [Patent Document 1] JP-A No. 2007-9747 [Disclosure of the Invention] [Problem to be Solved by the Invention] [0003] However, according to the configuration of the related art mentioned above, since the projecting top portions of the projecting portions face each other so as to narrow the intake passage, there are cases where an increase in intake resistance and a decrease in intake flow rate are caused due to excessive narrowing of the intake passage. An improvement in this respect is desired. Accordingly, it is an object of the present invention to suppress an increase in intake resistance and a decrease in intake flow rate within an intake port in an internal combustion engine having two mutually opposing projecting portions within the intake port, [Means for Solving the Problem] [0004] As means for addressing the above-mentioned problem, in an aspect of the invention in Claim 1, there is provided an internal combustion engine (for example, an engine 1 according to an embodiment) including: a cylinder head (for example, a cylinder head 2 according to an embodiment) that forms a combustion space (for example, a combustion chamber 4a according to an embodiment) in cooperation with a cylinder (for example, a cylinder body 3 according to an embodiment); an intake port (for example, an intake port 8 according to an embodiment) provided in the cylinder head and having an intake opening (for example, a combustion-chamber-side opening 8a according to an embodiment) that opens into the combustion space; an intake valve (for example, an intake valve 6 according to an embodiment) for opening and closing the intake opening; and first and second projecting portions (for example, first and second projecting portions 24, 25 according to an embodiment) that project into the intake port from a wall surface of the intake port, the projecting portions extending along an intake flow direction within the intake port and being provided so as to be opposed to each other as seen in a cross-section orthogonal to the intake flow direction, in which projecting top portions (for example, projecting top portions 24a, 25a according to an embodiment) of the projecting portions into the intake port are arranged so as to be displaced from each other in the intake flow direction. [0005] In an aspect of the invention according to Claim 2, an end portion of a valve guide (for example, a valve guide 6c according to an embodiment) through which a stem (for example, a stem 6b according to an embodiment) of the intake valve is inserted projects into the intake port, one of the projecting portions (for example, a second projecting portion 25 according to an embodiment) is provided in a wall surface of the intake port on a side where the valve guide is provided, a projecting top portion (for example, a projecting top portion 25a according to an embodiment) of the one projecting portion is provided near the projecting end portion of the valve guide and on an intake upstream side with respect to the projecting end portion, an inclined portion (for example, an upstream-side inclined portion 25b according to an embodiment) that extends at an incline with respect to the intake flow direction from a wall surface of the intake port to the projecting top portion is provided on an intake upstream side with respect to the projecting end portion of the one projecting portion, and inclination of the inclined portion becomes steeper near the projecting top portion. [0006] In an aspect of the invention according to Claim 2, the intake port is curved toward the intake opening, the valve guide and the one projecting portion (for example, a second projecting portion 25 according to an embodiment) are provided on a curved outer peripheral side of the intake port, the other projecting portion (for example, a first projecting portion 24 according to an embodiment) is provided on a curved inner peripheral side of the intake port, and a projecting top portion (for example, a projecting top portion 24a according to an embodiment) of the other projecting portion is provided on an intake upstream side with respect to the projecting top portion (for example, a projecting top portion 25a according to an embodiment) of the one projecting portion. [Effect of the Invention] [0007] According to the aspect of the invention in Claim 1, by providing the projecting portions that extend along the direction of an intake flow within the intake port, a rectification effect is given to the intake flow within the intake port to provide increased intake flow rate, thus promoting formation of a vortex flow within the combustion space due to the intake flow for enhanced combustibility. Further, by displacing the projecting top portions of the projecting portions from each other in the intake flow direction, it is possible to suppress an increase in intake resistance due to excessive narrowing of the intake passage. According to the aspect of the invention in Claim 2, when an intake flow within the intake port passes through the vicinity of the projecting end portion of the valve guide, the intake flow is guided by the inclined portion of the one projecting portion and rides over the projecting end portion of the valve guide, thereby making it possible to suppress an increase in intake resistance around the projecting end portion of the valve guide* Further, since the inclination of the inclined portion becomes steeper near the projecting top portion of the one projecting portion, an intake flow flowing along the inclined portion can readily ride over the projecting end portion of the valve guide. [0009] According to the aspect of the invention in Claim 3, as a part of an intake flow flowing along the curved inner peripheral side of the intake port passes through the projecting top portion of the other projecting portion and then flows along the inclined portion on the intake downstream side, its direction of flow is changed toward the curved inner peripheral side of the intake port- Likewise, as a part of an intake flow flowing along the curved outer peripheral side of the intake port flows along the inclined portion on the intake upstream side with respect to the projecting top portion of the one projecting portion, its direction of flow is changed toward the curved inner peripheral side of the intake port. It is thus possible to provide increased intake flow rate on the curved inner peripheral side where an intake flow rate tends to easily decrease, thus securing a high intake flow rate by making effective use of the entire cross- section of the intake port, [Best Mode for Carrying Out the Invention] [0010] Hereinbelow, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a side view of a cylinder head 2 of a four-stroke OHC single-cylinder engine 1 used as the prime motor of a vehicle such as a motorcycle, for example. A valve chamber 4 is formed within the cylinder head 2, and a valve gear 5 for driving intake and exhaust valves 6, 7 is accommodated within the valve chamber 4* The cylinder head 2 is mounted to the distal end of a cylinder body 3, A combustion chamber 4a of the engine 1 is formed by the cylinder head 2 and the cylinder body 3. It should be noted that symbol Cl in the drawing indicates the center axis (cylinder axis) of the cylinder bore within the cylinder body 3. [0011] Intake and exhaust ports 8, 9 are formed within the cylinder head 2. Combustion-chamber-side openings 8a, 9a of the intake and exhaust ports 8, 9 are respectively opened and closed by the intake and exhaust valves 6, 7. Further, the intake and exhaust ports 8, 9 respectively have cylinder-outer-side openings 8b, 9b that open in the right outer wall and left outer wall of the cylinder head 2 in Fig. 1, The cylinder-outer-side openings 8b, 9b are each provided so as to extend along a plane substantially parallel to the cylinder axis Cl. [0012] The intake and exhaust ports 8, 9 extend from the cylinder-outer-side openings 8b, 9b toward the cylinder center side (cylinder axis Cl side) along a direction generally orthogonal to the cylinder axis Cl (direction orthogonal to the cylinder), and then their parts on the cylinder center side are curved toward the combustion chamber 4a side to reach the combustion-chamber-side openings 8a, 9a. The combustion-chamber-side openings 8a, 9a are each provided along a plane that is inclined so that their cylinder center side is located on the valve chamber 4 side (distal end side of the cylinder head 2)• [0013] The valves 6, 7 have bar-like stems 6b, 7b extended to the valve chamber 4 side from umbrella-shaped valve bodies 6a, 7a that conform to the combustion-chamber-side openings 8a, 9a of the ports 8, 9. The stems 6b, 7b are held by the cylinder head 2 via tubular valve guides 6c, 7c in a manner that allows reciprocating motion, [0014] Retainers 6d, 7d are mounted to the distal ends of the stems 6b, 7b of the valves 6, 7. Due to the spring forces of springs 6e, 7e provided under compression between the retainers 6d, 7d and the cylinder head 2, the valves 6, 1 are urged to the valve chamber 4 side, thus bringing their valve bodies 6a, 7a into close contact with the combustion-chamber-side openings 8a, 9a to close the combustion-chamber-side openings 8a, 9a. On the other hand, as the valves 6, 7 are stroked to the combustion chamber 4a side against the urging forces of the springs 6e, 7e, the valve bodies 6a, 7a of the valves 6, 7 separate from the combustion-chamber-side openings 8a, 9a to open the combustion-chamber-side openings 8a, 9a. The stems 6b, 7b of the valves 6, 7 are provided at an incline with respect to the cylinder axis Cl so as to form a V shape that opens to the valve chamber 4 side in Fig. 1. A camshaft 11 disposed so as to be orthogonal to the plane of Fig. 1 is provided between the extended end sides of the stems 6b, 7b. During operation of the engine 1, the camshaft 11 is driven to rotate in conjunction with a crankshaft via a chain-type transmission mechanism, for example (both not shown). It should be noted that symbol C2 in the drawing indicates the rotation center of the camshaft 11. [0016] The valves 6, 7 are actuated so as to open and close as they are pressed by intake and exhaust cams 11a, lib of the camshaft 11 via locker arms 12, 13 on the intake side and exhaust side. Locker arm shafts 14, 15 on the intake side and exhaust side parallel to the cam shaft 11 are respectively disposed on the distal end side of the cylinder head 2 with respect to the camshaft 11 and on the intake valve 6 side and exhaust valve 7 side. The longitudinally intermediate portions of the locker arms 12, 13 are swingably supported on the locker arm shafts 14, 15, On each of the locker arms 12, 13, a cam roller 16 that is provided so as to roll on the outer peripheral surface (cam surface) of each of the cams 11a, lib is supported at an end portion on the camshaft 11 side, and a tappet bolt 17 that abuts on the distal end of the stem is supported at an end portion on the stem side. r As the camshaft 11 is driven to rotate during operation of the engine 1, the locker arms 12, 13 swing in accordance with the cam patterns of the cams 11a, lib, so the valves 6, 7 reciprocate along their stems so as to open and close the combustion-chamber-side openings 8a, 9a of the ports 8, 9. [0018] Upon opening the intake valve 6, an air-fuel mixture of outside air and fuel flows into the intake port 8 from the cylinder-outer-side opening 8b as its intake-upstream-side end. This air-fuel mixture flows into the combustion chamber 4a from the combustion-chamber-side opening 8a as the intake-downstream-side end of the intake port 8. The air-fuel mixture is compressed after the intake valve 6 is closed, and ignited by an ignition plug (not shown) for combustion. The exhaust gas after the combustion is discharged to the outside of the combustion chamber 4a via the exhaust port 9 when the exhaust valve 7 is opened. [0019] Hereinbelow, the intake port 8 of the engine 1 will be described in detail with reference to Figs. 2, 3. The intake port 8 extends from the cylinder-outer-side opening 8b to the combustion-chamber-side opening 8a while having a generally constant cross-sectional area. That is, the cross-sectional area of the intake passage formed by the intake port 8 is generally constant. [0020] The cross-section shape of the intake port 8 orthogonal to the longitudinal direction in each of the vicinity of the cylinder-outer-side opening 8b and the vicinity of the combustion-chamber-side opening 8a (at either longitudinal end portion) is a perfect circle having no inwardly convex part. On the other hand, the above-mentioned cross-sectional shape of the intake port 8 between the cylinder-outer-side opening 8b and the combustion-chamber-side opening 8a (in the longitudinally intermediate portion) is a different shape having inwardly convex parts. [0021] As described above, the intake port 8 extends from the cylinder-outer-side opening 8b to the cylinder center side along a direction generally orthogonal to the cylinder, and then its part on the intake downstream side is curved to the combustion chamber 4a side before reaching the combustion-chamber-side opening 8a. Hereinafter, the curved part of the intake port 8 on the intake downstream side mentioned above will be referred to as a curved portion 22. Symbol C3 in the drawing denotes a port centerline that extends along the longitudinal direction (extending direction/intake flow direction) of the intake port 8 while passing through the center position of the cross-sectional shape of the intake port 8. [0022] More specifically, from the cylinder-outer-side opening 8b to the cylinder center side, the intake port 8 extends at a slight incline so as to be located closer to the valve chamber 4 side with increasing proximity to the cylinder center side with respect to the direction orthogonal to the cylinder, and is slightly curved so as to be convex to the valve chamber 4 side. Then, after the intake port 8 extends at a slight incline on its intake downstream side so as to be located closer to the combustion chamber 4a side with increasing proximity to the cylinder center side with respect to the direction orthogonal to the cylinder (the portion of the intake port 8 up to this point is referred to as an upstream portion 21), the part of the intake port 8 located on the further intake downstream side (the curved portion 22) is curved toward the combustion chamber 4a side as described above, and reaches the combustion-chamber-side opening 8a while having its radius of curvature increased (with its curve made sharper) and its inclination toward the combustion chamber 4a side made steeper. [0023] That is, the intake port 8 is generally curved as a whole with its intake downstream side directed to the combustion chamber 4a side, and varies in radius of curvature so that its curve becomes sharper on the intake downstream side. Hereinafter, the valve chamber 4 side in the intake port 8 is referred to as the curved outer peripheral side, and the side opposite to this side is referred to as the curved inner peripheral side. [0024] In this case, on the curved inner peripheral side of the upstream portion 21 of the intake port 8, there is provided a first projecting portion 24 that projects from its wall surface to the inner side of the cross section of the port. The first projecting portion 24 is provided so as to extend along the intake flow direction, and forms a projection surface of a circular arc shape that is convex toward the inner side of the port cross-section as seen in a cross-section shown in Figs. 3(b), 3(c) (as seen in a cross-section orthogonal to the intake flow direction). It should be noted that a material-removed portion 27 is formed on the curved outer peripheral side of the intake port 8 so as to secure the cross-sectional area (intake passage area) of the intake port 8 reduced by the projection of the first projecting portion 24. [0025] The first projecting portion 24 projects in the form of a gentle circular arc elongated in the intake flow direction in Fig. 2. The end portions of the first projecting portion 24 on the intake upstream side and downstream side are formed smoothly continuous to the wall surface of the intake port 8. The radius of curvature on the curved inner peripheral side of the upstream portion 21 of the intake port 8 thus becomes partially large, thereby reducing variation in curvature between the upstream portion 21 and the curved portion 22 on the curved inner peripheral side. [0026] Therefore, after a part of an intake flow flowing along the curved inner peripheral side of the intake port 8 is changed in its flow direction to the curved inner peripheral side along the first projecting portion 24, the intake flow can readily flow along the curved inner peripheral side of the curved portion 22 as well. This prevents separation of the intake flow from the wall surface of the inner peripheral side of the curved portion 22 on the intake downstream side (particularly the curved portion 22) of the intake port 8, thereby suppressing intake resistance. Further, an intake flow rate increases on the curved inner peripheral side within the intake port 8 where an intake flow rate tends to easily decrease, thereby facilitating control of an intake flow (particularly a tumble flow (vortex flow in the vertical direction of the cylinder)) flowing into the combustion chamber 4a. In this regard, a tumble flow generated within the combustion chamber 4a can be controlled not only by suppressing separation of the intake flow but also by varying the projecting height of the first projecting portion 24 and its position in the intake flow direction as appropriate to purposefully cause separation of the intake flow mentioned above. [0027] Hereinafter, the projecting top portion of the first projecting portion 24 toward the inner side of the port cross-section is denoted by symbol 24a, and inclined portions on the intake upstream side and downstream side (upstream-side inclined portion and downstream-side inclined portion) with respect to the projecting top portion 24a are respectively denoted by symbols 24b, 24c. [0028] On the other hand, provided on the curved outer peripheral side of the intake port 8 is a second projecting portion 25 that projects from the wall surface thereof toward the inner side of the port cross-section. The second projecting portion 25 is provided so as to extend along the intake flow direction, and forms a projection surface of a circular arc shape that is convex to the inner side of the port cross-section as seen in a cross-section shown in Figs. 3(c), 3(d) (as seen in a cross-section orthogonal to the intake flow direction). It should be noted that the above-described material-removed portion 27 is formed continuous to either side of the second projecting portion 25 on the curved outer peripheral side of the intake port 8. [0029] The second projecting portion 25 projects in the shape of a chevron elongated in the intake flow direction in Fig. 2. In addition, the part of the second projecting portion 25 on the intake upstream side with respect to its projecting top portion 25a toward the inner side of the port cross-section is formed elongated in the intake flow direction, and its end portion located further on the intake upstream side is formed smoothly continuous to the wall surface of the intake port 8• The projecting top portion 25a of the second projecting portion 25 is generally located at the boundary between the upstream portion 21 and the curved portion 22. Hereinafter, inclined portions on the intake upstream side and downstream side (upstream-side inclined portion and downstream-side inclined portion) with respect to the projecting top portion 25a of the second projecting portion 25 are respectively denoted by symbols 25b, 25c. [0030] In this regard, on the curved outer peripheral side of the curved portion 22, an end portion on the combustion chamber 4a side of the valve guide 6c for the intake valve 6 projects to the inner side of the port cross-section. The valve guide 6c is arranged orthogonal to the combustion-chamber-side opening 8a, and its end portion on the combustion chamber 4a side (projecting end portion) projects so as to be directed to the center of the combustion-chamber-side opening 8a. The centerline of the valve guide 6c coincides with a tangent to the port centerline C3 at the position of the combustion-chamber-side opening 8a (a straight line passing through the center of the combustion-chamber-side opening 8a and inclined so as to be located closer to the cylinder center side with increasing proximity to the cylinder body 3 side) , [0031] The projecting end portion of the valve guide 6c extends through the second projecting portion 25, and its distal end is exposed on the downstream-side inclined portion 25c in the second projecting portion 25 which is formed relatively short with respect to the intake flow direction• The projecting top portion 25a of the second projecting portion 25 is located on the immediately intake upstream side of the distal end of the valve guide 6c, and the upstream-side inclined portion 25b that is formed relatively long with respect to the intake flow direction is provided on the intake upstream side of the projecting top portion 25a. The upstream-side inclined portion 25b is formed as a slope that gently rises up from the wall surface on the upstream side of the upstream portion 21 and reaches the projecting top portion 25a. [0032] Accordingly, a part of an intake flow flowing along the curved outer peripheral side of the intake port 8 changes its direction of flow to the curved inner peripheral side along the second projecting portion 25, and flows to the intake downstream side so as to ride over the projecting end portion of the valve guide 6c, thus suppressing resistance (reaction) to the intake flow passing through the vicinity of the projecting end portion of the valve guide 6c. [0033] Further, more specifically, the upstream-side inclined portion 25b is slightly curved so as to form an outer surface of a circular arc shape that is convex toward the valve chamber 4 side in Fig. 2, and increases in curvature so as to make its inclination steeper (make its curve shaper) near the projecting top portion 25a. [0034] Accordingly, as a part of an intake flow within the intake port 8 passes through the projecting top portion 25a of the second projecting portion 25, a directivity toward the inner side of the port cross-section and hence toward the curved inner peripheral side is imparted to the intake flow, thereby increasing the intake flow rate on the curved inner peripheral side while suppressing separation of the intake flow from the wall surface on the curved inner peripheral side. It should be noted that the projecting height of the projecting top portion 25a of the second projecting portion 25 toward the inner side of the port cross-section is larger than that of the projecting end portion of the valve guide 6c, [0035] The projecting portions 24, 25 are respectively provided on the curved inner peripheral side and outer peripheral side of the intake port 8 so as to be opposed to each other across the port centerline C3 as seen in the cross-section shown in Fig. 3(c). In this regard, as shown in Fig. 2, the projecting top portions 24a, 25a of the projecting portions 24, 25 are arranged such that their positions in the intake flow direction are displaced from each other. Specifically, the projecting top portion 24a of the first projecting portion 24 is arranged so as to be displaced to the intake upstream side in the intake flow direction with respect to the projecting top portion 25a of the second projecting portion 25 (in other words, the projecting top portion 25a of the second projecting portion 25 is displaced to the intake downstream side with respect to the projecting top portion 24a of the first projecting portion 24). This prevents excessive narrowing of the intake passage due to the provision of the projecting portions 24r 25 within the intake port 8. [0036] As shown in Fig. 4, in the ceiling-forming part of the combustion chamber 4a in the cylinder head 2, a plug insertion opening 28 toward which the electrode portion of the above-mentioned ignition plug is faced is provided at a location that avoids interference with the combustion-chamber-side openings 8a, 9a of the ports 8, 9, In the engine 1 according to the present application, the positions and shapes of the projecting portions 24, 25 are set so that the above-mentioned air-fuel mixture (intake flow) gathers around the plug insertion opening 28, thereby achieving optimization of the tumble flow and swirl flow (vortex flow in the transverse direction of the cylinder) of the intake flow. [0037] By suppressing an intake resistance and increasing an intake flow rate within the intake port 8 as described above, as shown in Fig. 5, in the engine 1 according to the present application which has the projecting portions 24, 25 provided in the intake port 8, the engine output at full throttle (WOT (Wide Open Throttle) output) is increased throughout the entire engine speed range in comparison to an engine of the related art that does not have the projecting portions 24, 25. Further, by actively controlling the intake flow within the combustion chamber 4a as described above, the engine 1 according to the present invention also makes it possible to enhance the combustibility of an air-fuel mixture to achieve improved fuel economy performance. [0038] It should be noted that as shown in Fig. 1, a projecting portion 29 corresponding to the second projecting portion 25 is provided around the projecting end portion of the valve guide 6c on the curved outer peripheral side of the exhaust port 9, thereby suppressing a vortex flow on the intake downstream side of the projecting end portion of the valve guide 6c to suppress exhaust resistance. [0039] As described above, the engine 1 according to the above-mentioned embodiment includes: the cylinder head 2 that forms the combustion chamber 4a in cooperation with the cylinder body 3; the intake port 8 provided in the cylinder head 2 and having the combustion-chamber-side opening 8a that opens into the combustion chamber 4a; the intake valve 6 for opening and closing the combustion-chamber-side opening 8a; and the first and second projecting portions 24, 25 that project into the intake port 8 from the wall surface of the intake port 8. The projecting portions 24, 25 extend along an intake flow direction within the intake port 8 and are provided so as to be opposed to each other as seen in a cross-section orthogonal to the intake flow direction. The projecting top portions 24a, 25a of the projecting portions 24, 25 into the intake port 8 are arranged so as to be displaced from each other in the intake flow direction. [0040] According to this configuration, by providing the projecting portions 24, 25 that extend along the direction of an intake flow within the intake port 8, a rectification effect is given to the intake flow within the intake port 8 to provide increased intake flow rate, thus promoting formation of a vortex flow within the combustion chamber 4a due to the intake flow for enhanced combustibility. Further, by displacing the projecting top portions 24a, 25a of the projecting portions 24, 25 from each other in the intake flow direction, it is possible to suppress an increase in intake resistance due to excessive narrowing of the intake passage. [0041] Further, in the engine 1 mentioned above, an end portion of the valve guide 6c through which the stem 6b of the intake valve 6 is inserted projects into the intake port 8, the second projecting portion 25 is provided in the wall surface of the intake port 8 on the side where the valve guide 6c is provided, the projecting top portion 25a of the second projecting portion 25 is provided near the projecting end portion of the valve guide 6c and on the intake upstream side with respect to the projecting end portion, the upstream-side inclined portion 25b that extends at an incline with respect to the intake flow direction from the wall surface of the intake port 8 to the projecting top portion 25a is provided on the intake upstream side with respect to the projecting end portion 25a of the second projecting portion 25, and inclination of the upstream-side inclined portion 25b becomes steeper near the projecting top portion 25a. [0042] According to this configuration, when an intake flow within the intake port 8 passes through the vicinity of the projecting end portion of the valve guide 6c, the intake flow is guided by the upstream-side inclined portion 25b of the second projecting portion 25 and rides over the projecting end portion of the valve guide 6c, thereby making it possible to suppress an increase in intake resistance around the projecting end portion of the valve guide 6c, Further, since the inclination of the upstream-side inclined portion 25b becomes steeper near the projecting top portion 25a of the second projecting portion 25, an intake flow flowing along the upstream-side inclined portion 25b can readily ride over the projecting end portion of the valve guide 6c. [0043] Further, in the engine 1 mentioned above, the. intake port 8 is curved toward the combustion-chamber-side opening 8a, the valve guide 6c and the second projecting portion 25 are provided on the curved outer peripheral side of the intake port 8, the first projecting portion 24 is provided on the curved inner peripheral side of the intake port 8, and the projecting top portion 24a of the first projecting portion 24 is provided on the intake upstream side with respect to the projecting top portion 25a of the second projecting portion 25, [0044] According to this configuration, as a part of an intake flow flowing along the curved inner peripheral side of the intake port 8 passes through the projecting top portion 24a of the first projecting portion 24 and then flows along the downstream-side inclined portion 24c, its direction of flow is changed toward the curved inner peripheral side of the intake port 8. Likewise, as a part of an intake flow flowing along the curved outer peripheral side of the intake port 8 flows along the upstream-side inclined portion 25b of the second projecting portion, its direction of flow is changed toward the curved inner peripheral side of the intake port 8. It is thus possible to provide increased intake flow rate on the curved inner peripheral side where an intake flow rate tends to easily decrease, thus securing a high intake flow rate by making effective use of the entire cross-section of the intake port 8. [0045] It is to be understood that the present invention is not limited to the above-mentioned embodiment- For example, the present invention may be applied to a four-valve engine or DOC engine, or further a parallel or V multi-cylinder engine. It is needless to mention that the configuration according to the above-mentioned embodiment is an illustrative example of the present invention, and various modifications are possible without departing from the scope of the present invention. [Brief Description of the Drawings] [0046] [Fig, 1] Fig. 1 is a longitudinal sectional view of a cylinder head of an engine according to an embodiment of the present invention. [Fig. 2] Fig. 2 is an enlarged view of a portion around an intake port in Fig. 1. [Figs. 3] Fig. 3(a) is a sectional view taken along the line A-A of Fig. 2, Fig. 3(b) is a sectional view taken along the line B-B of Fig. 2, Fig. 3(c) is a sectional view taken along the line C-C of Fig. 2, Fig. 3(d) is a sectional view taken along the line D-D of Fig. 2, and Fig. 3(e) is a sectional view taken along the line E-E of Fig. 2. [Fig. 4] Fig. 4 is an explanatory plan view of the vicinity of the engine as seen in the cylinder axis direction. [Fig. 5] Fig. 5 is a graph showing variation in engine output at full throttle with respect to engine speed in the engine and an engine of the related art. [Description of Reference Numerals] [0047] 1..- Engine (internal combustion engine) 2... Cylinder head 3... Cylinder head body (cylinder) 4a,.. Combustion chamber (combustion space) 6... Intake valve 6b... Stem 6c... Valve guide 8... Intake port 8a..• Combustion-chamber-side opening (intake opening) 24... First projecting portion (the other projecting portion) 24a... Projecting top portion 25... Second projecting portion (one projecting portion) 25a... Projecting top portion 25b... Upstream-side inclined portion [Document Name] Scope of Claims [Claim 1] An internal combustion engine comprising: a cylinder head that forms a combustion space in cooperation with a cylinder; an intake port provided in the cylinder head and having an intake opening that opens into the combustion space; an intake valve for opening and closing the intake opening; and first and second projecting portions that project into the intake port from a wall surface of the intake port, the projecting portions extending along an intake flow direction within the intake port and being provided so as to be opposed to each other as seen in a cross-section orthogonal to the intake flow direction, wherein projecting top portions of the projecting portions into the intake port are arranged so as to be displaced from each other in the intake flow direction. [Claim 2] The internal combustion engine according to Claim 1, wherein an end portion of a valve guide through which a stem of the intake valve is inserted projects into the intake port, one of the projecting portions is provided in a wall surface of the intake port on a side where the valve guide is provided, a projecting top portion of the one projecting portion is provided near the projecting end portion of the valve guide and on an intake upstream side with respect to the projecting end portion, an inclined portion that extends at an incline with respect to the intake flow direction from a wall surface of the intake port to the projecting top portion is provided on an intake upstream side with respect to the projecting end portion of the one projecting portion, and inclination of the inclined portion becomes steeper near the projecting top portion. [Claim 3] The internal combustion engine according to Claim 2, wherein the intake port is curved toward the intake opening, the valve guide and the one projecting portion are provided on a curved outer peripheral side of the intake port, the other projecting portion is provided on a curved inner peripheral side of the intake port, and a projecting top portion of the other projecting portion is provided on an intake upstream side with respect to the projecting top portion of the one projecting portion*

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