Combustion chamber structure of engine

Abstract

To provide a combustion chamber structure of an engine capable of increasing heat efficiency.SOLUTION: In an engine comprising a combustion chamber 6 defined by a crown surface 50 of a piston 5, a peripheral surface 40 of a cylinder, and a pent roof-shaped ceiling surface 40, and an ignition plug 8, an intake port 10 for introducing air into the combustion chamber 6 and an exhaust port 11 for discharging exhaust gas from the combustion chamber 6 are opened in the ceiling surface 40 of the combustion chamber 6, a downwardly recessed cavity 53 is formed in the center of the crown surface 50 of the piston 5, and when a side of the combustion chamber 6 where the intake port 10 is disposed is an intake side and a side where the exhaust port 11 is disposed is an exhaust side, a curved recess part 61 recessed upward is formed in an exhaust-side corner part of the ceiling surface 40 of the combustion chamber 6.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a combustion chamber structure of an engine. [Background technology]

[0002] As disclosed in Patent Document 1 below, in engines installed in vehicles, etc., the ceiling surface of the combustion chamber is made pent roof shaped to form a tumble flow in the combustion chamber. If a tumble flow can be formed in the combustion chamber and maintained until near the top dead center of compression where combustion of the air-fuel mixture begins, combustion of the air-fuel mixture is promoted and thermal efficiency is improved.

[0003] In the combustion chamber structure of Patent Document 1, a pent roof surface that forms the upper surface of the combustion chamber is provided on the cylinder head, and an ignition plug is attached. In this structure, a recess is formed on the ceiling surface of the combustion chamber on the upstream side of the tumble flow relative to the spark plug, i.e., on the intake side in the intake / exhaust direction. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 197860 Summary of the Invention [Problem to be solved by the invention]

[0005] In the combustion chamber structure of Patent Document 1, intake air flows along a recess formed in the ceiling of the combustion chamber, maintaining tumble flow until near top dead center of compression. However, because the recess is formed on the intake side of the spark plug, the tumble flow is biased toward the intake side. This makes it easier for the initial flame to come into contact with the combustion chamber wall on the intake side, which can increase cooling loss and reduce thermal efficiency.

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a combustion chamber structure for an engine that can improve thermal efficiency. [Means for solving the problem]

[0007] In order to solve the above problems, the combustion chamber structure of the present invention is a combustion chamber structure for an engine comprising: a combustion chamber defined by a piston crown surface, a circumferential surface of a cylinder in which the piston is housed so as to be able to slide up and down, and a pent roof-shaped ceiling surface formed in a cylinder head; and an ignition plug that ignites a fuel-air mixture formed in the combustion chamber, wherein an intake port for introducing air into the combustion chamber and an exhaust port for guiding exhaust gas from the combustion chamber are opened in the ceiling surface of the combustion chamber, a cavity recessed downward is formed in the center of the piston crown surface, and the ignition plug is arranged so that an electrode portion provided at its tip faces the cavity from the ceiling surface of the combustion chamber, whereby when the side of the combustion chamber where the intake port is arranged is defined as the intake side and the side of the combustion chamber where the exhaust port is arranged is defined as the exhaust side, a recess having an upwardly curved shape is formed in the exhaust-side corner of the ceiling surface of the combustion chamber.

[0008] In the present invention, a downwardly recessed cavity is formed in the center of the piston crown. Therefore, by directing the intake air along the cavity, a tumble flow can be more reliably formed in the combustion chamber. Furthermore, in the present invention, a recess with an upwardly curved shape is formed in the exhaust-side corner of the ceiling surface of the combustion chamber. This allows the intake air flowing into the combustion chamber from the intake port to be guided closer to the exhaust side, and by directing the intake air along the recess, the intake air can be smoothly swirled, forming a tumble flow with a large swirl diameter in the combustion chamber. Therefore, the tumble flow can be maintained in the combustion chamber until near top dead center of the compression stroke, and the center of the tumble flow can be prevented from shifting toward either the intake side or the exhaust side of the combustion chamber. This prevents the initial flame from coming into contact with the combustion chamber wall due to the tumble flow, thereby improving thermal efficiency.

[0009] In the above configuration, preferably, an inner recess having an upwardly concave curved shape is formed in the ceiling surface of the combustion chamber at a position adjacent to the intake side of the recess (claim 2).

[0010] With this configuration, intake air that does not reach the recess due to its relatively low inflow velocity from the intake port into the combustion chamber can be smoothly swirled along the inner recess, thereby reliably forming a tumble flow within the combustion chamber.

[0011] In the above configuration, the inner recess is preferably shallower than the recess (claim 3).

[0012] With this configuration, intake air that flows into the combustion chamber from the intake port at a high speed is deflected by the inner recess before reaching the recess, which can prevent the swirl diameter from becoming smaller.

[0013] In the above configuration, preferably, the ceiling surface of the combustion chamber is formed with two openings of the intake port and two openings of the exhaust port, such that the openings of one intake port and one exhaust port are aligned in the intake / exhaust direction, and the openings of the other intake port and the openings of the other exhaust port are aligned in the intake / exhaust direction, and the recess is formed in an area that overlaps with the openings of the two exhaust ports when viewed along the intake / exhaust direction (Claim 4).

[0014] According to this configuration, both intake airs that flow into the combustion chamber from the two intake ports can be swirled by the recessed portions, and a larger amount of intake air can be swirled with a larger diameter.

[0015] In the above-mentioned configuration, a piston-side recessed portion having a curved shape recessed downward is formed at a corner of the piston crown surface on the exhaust side (claim 5).

[0016] With this configuration, the intake air flowing downward toward the piston crown on the exhaust side of the combustion chamber can be smoothly deflected toward the intake side by the piston-side recess, thereby more reliably maintaining tumble flow.

[0017] In the above configuration, preferably, an intake-side recess having an upwardly concave curved shape is formed in an intake-side corner of the ceiling surface of the combustion chamber (claim 6).

[0018] With this configuration, the intake air that has risen toward the intake-side corner of the ceiling surface of the combustion chamber can be smoothly deflected toward the exhaust side by the intake-side recess, thereby more reliably maintaining tumble flow. [Effects of the Invention]

[0019] As described above, the combustion chamber structure of the engine according to the present invention can improve thermal efficiency. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram showing the configuration of an engine according to an embodiment of the present invention. [Figure 2] FIG. 2 is a plan view showing the ceiling surface of the combustion chamber. [Figure 3] FIG. 3 is a schematic cross-sectional view of the cylinder head taken along line III-III in FIG. 2. [Figure 4] FIG. 4 is a schematic cross-sectional view of the cylinder head taken along line IV-IV in FIG. 2. [Figure 5] FIG. [Figure 6] FIG. 3 is a schematic cross-sectional view of the piston taken along line III-III in FIG. 2. [Figure 7] FIG. 2 is a diagram showing a schematic view of intake air flow in a combustion chamber. [Figure 8] FIG. 2 is a diagram showing a schematic view of intake air flow in a combustion chamber. [Figure 9] FIG. 2 is a diagram showing a schematic view of intake air flow in a combustion chamber. [Figure 10] FIG. 1 is a diagram showing a schematic view of an initial flame. [Figure 11] FIG. 2 is a diagram showing a schematic view of intake air flow in a combustion chamber. [Figure 12] FIG. 1 is a diagram showing a schematic view of an initial flame. [Figure 13] FIG. 2 is a diagram for explaining the effects of the present invention, and is a diagram schematically showing the intake air flow in a combustion chamber. [Figure 14] FIG. 2 is a diagram for explaining the effects of the present invention, and is a diagram schematically showing the intake air flow in a combustion chamber. [Figure 15] FIG. 10 is a plan view showing a piston crown surface according to another example of the present invention. [Figure 16] FIG. 16 is a schematic cross-sectional view of the piston taken along line XVI-XVI in FIG. 15. DETAILED DESCRIPTION OF THE INVENTION

[0021] (1) Overall structure FIG. 1 is a diagram showing the configuration of an engine according to one embodiment of the present invention, and is a diagram showing a schematic cross section of the engine. The engine 1 shown in this diagram is a spark-ignition four-stroke engine. The engine 1 is mounted on a vehicle, for example, as a power source for traveling. FIG. 2 is a schematic plan view showing a combustion chamber ceiling surface 40, which will be described later.

[0022] The engine 1 has a cylinder block 3 in which cylinders 2 are formed, a cylinder head 4 attached to the upper surface of the cylinder block 3 so as to close the cylinders 2 from above, and a piston 5 inserted into each cylinder 2 so as to be able to slide back and forth in the up and down direction. The engine may be either a single-cylinder engine or a multi-cylinder engine having multiple cylinders 2. The following description will be given of the case where the engine 1 is a single-cylinder engine having only one cylinder 2.

[0023] In engine 1, an intake valve 12 (described later) is provided on one side of a plane S passing through the central axis X1 of a cylinder 2, and an exhaust valve 13 (described later) is provided on the other side. Hereinafter, the direction perpendicular to plane S will be referred to as the intake / exhaust direction, and in this intake / exhaust direction, the side where the intake valve 12 is provided will be referred to as the intake side, and the side where the exhaust valve 13 is provided will be referred to as the exhaust side. In addition, the direction perpendicular to the up-down direction and the intake / exhaust direction will be referred to as the front-to-rear direction, and the left side of the upper side of the paper in FIG. 2 facing the intake side will be referred to as the front, and the opposite side will be referred to as the rear. In addition, the central axis X1 of cylinder 2 will be referred to as the cylinder axis X1.

[0024] Above the piston 5, a combustion chamber 6 is defined by the peripheral surface 20 of the cylinder 2, a crown surface 50 of the piston 5, and a ceiling surface 40 formed on the cylinder head 4. Hereinafter, the ceiling surface 40 of the combustion chamber 6 will be referred to as the combustion chamber ceiling surface 40 where appropriate. Also, the crown surface 50 of the piston 5 will be referred to as the piston crown surface 50.

[0025] A fuel injection valve 7 that injects fuel into the combustion chamber 6 is attached to the cylinder head 4. In the first embodiment, the fuel injection valve 7 is a side-injection type fuel injection valve that injects fuel into the combustion chamber 6 from the side on the intake side. Fuel containing gasoline as a main component is supplied to the combustion chamber 6 by injection from the fuel injection valve 7. Note that the fuel injected into the combustion chamber 6 is not limited to gasoline. For example, a fuel containing a secondary component such as bioethanol in addition to gasoline may be injected into the combustion chamber 6.

[0026] The cylinder head 4 is provided with an intake port 10 for introducing air into the combustion chamber 6 and an exhaust port 11 for discharging exhaust gas generated in the combustion chamber 6. Two intake ports 10 and two exhaust ports 11 are provided. The intake ports 10 and the exhaust ports 11 open to a combustion chamber ceiling surface 40, which is provided with two intake-side openings 41 that are openings for the intake ports 10 and two exhaust-side openings 42 that are openings for the exhaust ports 11. The two intake-side openings 41 are aligned in the front-rear direction. The two exhaust-side openings 42 are aligned in the front-rear direction. The front intake-side opening 41 and the front exhaust-side opening 42 are aligned in the intake-exhaust direction. The rear intake-side opening 41 and the rear exhaust-side opening 42 are aligned in the intake-exhaust direction.

[0027] Intake valves 12 that open and close each of the intake-side openings 41 and exhaust valves 13 that open and close each of the exhaust-side openings 42 are mounted on the cylinder head 4. The intake valves 12 and exhaust valves 13 are driven to open and close in conjunction with the rotation of the crankshaft by a valve mechanism (not shown) that includes a pair of camshafts and the like that is disposed in the cylinder head 4.

[0028] An ignition plug 8 is attached to the cylinder head 4, which ignites the fuel-air mixture formed in the combustion chamber 6 by spark discharge. An electrode portion 81 for discharging the spark is provided at the tip of the spark plug 8. The spark plug 8 is attached to the cylinder head 4 so that the electrode portion 81 faces the combustion chamber 6 from the combustion chamber ceiling surface 40. The electrode portion 81 of the spark plug 8 is located between the intake valve 12 and the exhaust valve 13 in the intake / exhaust direction, and is located between two intake valves 12 and two exhaust valves 13 in the front-to-rear direction.

[0029] Ignition by a spark plug 8 causes a mixture of fuel and air supplied from a fuel injection valve 7 to combust in a combustion chamber 6. The piston 5 is pushed down by the expansion force caused by this combustion, causing it to reciprocate up and down. A crankshaft (not shown), which is the output shaft of the engine 1, is provided below the piston 5. The crankshaft is connected to the piston 5 via a connecting rod 9, and is driven to rotate around its central axis in response to the reciprocating motion (up and down movement) of the piston 5.

[0030] (2) Combustion chamber shape 3 and 4 are cross-sectional views illustrating the structure of the combustion chamber 6. FIGS. 3 and 4 are cross-sectional views of the engine near the combustion chamber 6. The cross-section of FIG. 3 corresponds to line III-III in FIG. 2, passes through the center of the combustion chamber 6, and extends along the intake / exhaust direction. The cross-section of FIG. 4 corresponds to line IV-IV in FIG. 2, passes through the centers of the rear intake-side opening 41 and the rear exhaust-side opening 42, and extends along the intake / exhaust direction. FIG. 5 is a schematic plan view of a piston crown surface 50 that defines the bottom surface of the combustion chamber 6.

[0031] The combustion chamber ceiling surface 40 is formed by a part of the underside of the cylinder head 4. The combustion chamber ceiling surface 40 is a pent roof type ceiling surface. That is, in a cross section along the intake / exhaust direction, the combustion chamber ceiling surface 40 has an intake-side inclined surface 43 whose height decreases with increasing distance from the cylinder axis X1 toward the intake side, and an exhaust-side inclined surface 44 whose height decreases with increasing distance from the cylinder axis X1 toward the exhaust side.

[0032] The two intake-side openings 41 are formed in the intake-side inclined surface 43. The two exhaust-side openings 42 are formed in the exhaust-side inclined surface 44. In the first embodiment, a fuel injection recess 43A is formed in the intake-side inclined surface 43, and the fuel injection valve 7 is attached to the cylinder head 4 so that its tip faces the combustion chamber 6 from the bottom of the fuel injection recess 43A. A plug mounting hole 43B that communicates with the combustion chamber 6 is formed in the center of the combustion chamber ceiling surface 40. The spark plug 8 is fixed to the cylinder head 4 while inserted into this plug mounting hole 43B.

[0033] An upwardly recessed first recess 61 is formed in a corner of the combustion chamber ceiling surface 40 on the exhaust side. The first recess 61 has a curved shape. Specifically, the bottom surface of the first recess 61 is curved to bulge upward. The first recess 61 is formed in a region surrounded by the periphery of the combustion chamber ceiling surface 40 and the periphery of the two exhaust-side openings 42, 42, and overlaps with the exhaust-side openings 42, 42 when viewed along the intake / exhaust direction. When viewed along the up-down direction, the first recess 61 has a generally trapezoidal shape whose front-to-rear dimension decreases toward the intake side. The first recess 61 extends from a portion of the exhaust-side periphery of the combustion chamber ceiling surface 40 that spans the centers of the two exhaust-side openings 42, 42 in the front-to-rear direction to the intake side, passing between the two exhaust-side openings 42, 42. In the intake / exhaust direction, the first recess 61 extends slightly toward the intake side beyond the exhaust-side ends of the exhaust-side openings 42, 42. The first recessed portion corresponds to the "recessed portion" of the present invention.

[0034] The combustion chamber ceiling surface 40 is formed with a second recess 62 recessed upward at a position adjacent to the intake side of the first recess 61. The second recess 62 has a curved shape. Specifically, the bottom surface of the second recess 62 curves to bulge upward. When viewed in the vertical direction, the second recess 62 has a generally triangular shape whose front-to-rear dimension decreases toward the intake side. The second recess 62 extends from the intake-side edge of the first recess 61 to the intake side, passing between the two exhaust-side openings 42, 42. In the intake / exhaust direction, the intake-side end of the second recess 62 is located near the center between the exhaust-side end of the exhaust-side openings 42, 42 and the centers X2, X2 of the exhaust-side openings 42, 42. The second recess corresponds to the "inner recess" of the present invention.

[0035] In this embodiment, the second recess 62 is shallower than the first recess 61, and the depth d2 of the second recess 62 is set to a dimension smaller than the depth d1 of the first recess 61. Specifically, the maximum distance d2 between a plane Q2 passing through the periphery of the second recess 62 and the bottom surface of the second recess 62 is set to a dimension smaller than the maximum distance d1 between a plane Q1 passing through the periphery of the first recess 61 and the bottom surface of the first recess 61. In this embodiment, the two planes Q1 and Q2 are on the same plane. Furthermore, in this embodiment, the dimension of the second recess 62 in the front-to-rear direction is shorter than the dimension of the first recess 61 in the front-to-rear direction.

[0036] An intake-side recess 63 recessed upward is formed in an intake-side corner of the combustion chamber ceiling surface 40. The intake-side recess 63 has a curved shape. Specifically, the bottom surface of the intake-side recess 63 is curved to bulge upward. The intake-side recess 63 is formed in a position facing the first recess 61, in an area surrounded by the periphery of the combustion chamber ceiling surface 40 and the periphery of the two intake-side openings 41, 41, and in an area overlapping the intake-side openings 41, 41 and the exhaust-side openings 42, 42 when viewed along the intake / exhaust direction. The intake-side recess 63 extends from a portion of the intake-side periphery of the combustion chamber ceiling surface 40 between the centers of the two intake-side openings 41, 41 in the front-to-rear direction to the exhaust side, passing between the two intake-side openings 41, 41. In the intake / exhaust direction, the intake-side recess 63 extends slightly toward the exhaust side beyond the intake-side ends of the intake-side openings 41, 41.

[0037] In this embodiment, the fuel injection recess 43A is formed in the center in the front-rear direction of the intake-side recess 63. Specifically, the center portion in the front-rear direction of the intake-side recess 63 and the portion on the intake side are recessed further upward than the remaining portion, and this protruding portion forms the fuel injection recess 43A.

[0038] Except for its outer periphery and center, the piston crown surface 50 is formed so that its height increases toward the cylinder axis X1 in a cross-sectional view along the intake / exhaust direction. Specifically, the piston crown surface 50 has an intake-side piston inclined surface 51 facing the intake-side inclined surface 43, and an exhaust-side piston inclined surface 52 facing the exhaust-side inclined surface 44. The intake-side piston inclined surface 51 is formed so that its height increases toward the cylinder axis X1 (toward the radially inner side), and the exhaust-side piston inclined surface 52 is formed so that its height increases toward the cylinder axis X1 (toward the radially inner side).

[0039] A cavity 53 recessed downward is formed in the center of the piston crown surface 50. In this embodiment, as shown in FIG. 5 , the cavity 53 is formed to extend in the front-to-rear direction in a plan view, and the front-to-rear dimension of the cavity 53 is longer than the dimension in the intake / exhaust direction. The cavity 53 is formed so that both the central portions of its periphery in the front-to-rear direction on the intake side and the exhaust side extend substantially straight in the front-to-rear direction. In this embodiment, the front portion of the periphery of the cavity 53 is curved to bulge forward, and the rear portion of the periphery is curved to bulge rearward. In this embodiment, the bottom surface of the cavity 53 is formed to be generally flat, and the peripheral wall of the cavity 53 rises from the bottom surface at a substantially right angle.

[0040] In this embodiment, since the fuel injection valve 7 is of a side injection type, a fuel introduction portion 54 for guiding fuel into the cavity 53 is formed on the piston crown surface 50. Specifically, the fuel introduction portion 54 is recessed downward and formed in a portion adjacent to the upper end of the central portion in the front-to-rear direction on the intake side of the cavity 53.

[0041] A downwardly recessed piston-side recess 56 is formed in a corner of the piston crown surface 50 on the exhaust side. The piston-side recess 56 has a curved shape. Specifically, the bottom surface of the piston-side recess 56 is curved so as to bulge downward. The piston-side recess 56 is formed in the exhaust-side portion of the exhaust-side piston inclined surface 52. The piston-side recess 56 is formed in a region extending between the centers X2, X2 of the two exhaust-side openings 42 in the front-rear direction and has a shape extending in the front-rear direction along the periphery of the piston crown surface 50. The piston-side recess 56 is formed in a position slightly inwardly spaced from the periphery of the piston crown surface 50 in the radial direction of the piston crown surface 50. In the example shown in FIG. 5 , the radial dimension of the piston-side recess 56 is set to approximately 1 / 5 of the radius of the piston crown surface 50, and the piston-side recess 56 is formed in a region closer to the exhaust side than the centers X2, X2 of the two exhaust-side openings 42, 42 in the intake / exhaust direction.

[0042] In this embodiment, a pair of intake valve recesses 59 are formed on the intake side of the piston crown surface 50, spaced apart in the front-to-rear direction, to prevent contact between the intake valve 12 and the piston 5. On the other hand, on the exhaust side, the exhaust-side piston inclined surface 52 is formed flat at a position that does not contact the exhaust valve 13, thereby preventing contact between the exhaust valve 13 and the piston 5. The piston-side recess 56 is formed on the exhaust-side piston inclined surface 52, and is recessed even larger than the recess necessary to prevent contact between the exhaust valve 13 and the piston 5.

[0043] (3) Effects, etc. With the above-described configuration, the engine according to this embodiment can generate a tumble flow with a large swirl diameter, thereby improving thermal efficiency. Details will be explained using Figs. 7 to 14.

[0044] 7 and 8 are diagrams showing a simulated intake flow in the combustion chamber 6, with the main flow of the intake air extracted from the simulation results and schematically indicated by arrows. These diagrams show the intake flow during the intake stroke and compression stroke of an engine in which the piston does not have a cavity or piston-side recess, and the combustion chamber ceiling does not have recesses 61, 62, 63, and 56. The left-hand diagrams in FIGS. 7 and 8 are cross-sectional views taken along a plane that passes through the center of the combustion chamber 6 and extends along the intake and exhaust direction. The right-hand diagrams in FIGS. 7 and 8 are cross-sectional views taken along a plane that passes near the centers of the intake valve 12 and the exhaust valve 13 and extends along the intake and exhaust direction. Hereinafter, the cross-section of the left-hand diagrams in FIGS. 7 and 8, which passes through the center of the combustion chamber 6 and extends along the intake and exhaust direction, will be referred to as the bore center cross-section. Furthermore, the cross-section of the right-hand diagrams in FIGS. 7 and 8, which passes near the centers of the intake valve 12 and the exhaust valve 13 and extends along the intake and exhaust direction, will be referred to as the valve cross-section. 7 and 8(a) are diagrams showing the state near the start of opening of the intake valve 12, FIGS. 7 and 8(b) are diagrams showing the state near the intake bottom dead center, FIGS. 7 and 8(c) are diagrams showing the state near 60° CA before compression top dead center, and FIGS. 7 and 8(c) are diagrams showing the state near 10° CA before compression top dead center.

[0045] 7 and 8 show different flow patterns of intake air as it flows from the intake port into the combustion chamber 6. The pattern shown in FIG. 7 is an ideal pattern, in which the flow of intake air into the combustion chamber 6 is virtually adjusted so that a good tumble flow is formed in the combustion chamber 6 near the top dead center of the compression stroke. On the other hand, FIG. 8 shows the result of simulating an example of a flow pattern that occurs in an actual engine. Hereinafter, the flow pattern shown in FIG. 7 will be referred to as pattern A, and the flow pattern shown in FIG. 8 will be referred to as pattern B.

[0046] Fig. 9 is a diagram that schematically shows the intake air flow in the combustion chamber 6 in pattern A. Fig. 10 is a diagram that schematically shows the state of the initial flame near the compression top dead center in pattern A.

[0047] In pattern A, as indicated by arrows Y1 in the left and right views of FIG. 7(a), intake air flows into the combustion chamber 6 from the intake port approximately evenly in both the bore central cross section and the valve cross section, i.e., approximately evenly in the front-to-rear direction, and the intake air flows from the intake port into the combustion chamber 6 so that it flows along the ceiling surface of the combustion chamber 6. In pattern A, as indicated by arrows Y2 in the left and right views of FIG. 7(b), the intake air vertically swirls along the wall surface of the combustion chamber 6 in approximately the same manner near the intake bottom dead center in both the bore central cross section and the valve cross section. Specifically, as indicated by arrow Y5 in FIG. 9, in pattern A, the intake air swirls along a plane perpendicular to the front-to-rear direction at each position in the front-to-rear direction. In pattern A, this vertical swirl flow, or tumble flow, is maintained even during the compression stroke, as indicated by arrows Y3 in the left and right views of FIG. 7(c). As a result, in pattern A, a tumble flow is formed in the combustion chamber 6 in the front-to-rear direction even near the compression top dead center, as indicated by arrow Y4 in the left and right views of Figure 8(d). As a result, in pattern A, an initial flame M1 is formed at a position separated from the wall surface of the combustion chamber 6, as shown in Figure 10. This prevents the initial flame M1 from being cooled by the wall surface of the combustion chamber 6, thereby improving thermal efficiency. Note that points R1 and R2 in Figure 7(c) represent the centers of the vertical swirl flows.

[0048] Fig. 11 is a diagram that schematically shows the intake air flow in the combustion chamber 6 in pattern B. Fig. 12 is a diagram that schematically shows the state of the initial flame near the compression top dead center in pattern B.

[0049] In pattern B, the intake air flows into the combustion chamber 6 toward the center in the front-to-rear direction, so that the direction of the intake air flowing into the combustion chamber 6 is more downward than in pattern A, as indicated by arrow Y11 in the left diagram of FIG. 8(a). In pattern B, the inflow velocity of the intake air into the combustion chamber 6 is lower in the valve cross section, as indicated by arrow Y12 in the right diagram of FIG. 8(a). Also, in pattern B, near the intake bottom dead center, the intake air roughly swirls vertically along the wall of the combustion chamber 6 in the bore central cross section, as indicated by arrow Y13 in the left diagram of FIG. 8(b). On the other hand, in the valve cross section, the flow of the intake air that swirls upward from the bottom of the combustion chamber 6 is weak, as indicated by arrow Y14 in the right diagram of FIG. 8(b), and sufficient vertical swirl flow is not formed. As a result, in pattern B, as shown by arrow Y19 in Figure 11, the intake air swirls along this plane, i.e., a plane perpendicular to the front-to-rear direction, in the bore center cross section, while in the outer front-to-rear region of the combustion chamber 6, the tumble flow is tilted, as shown by arrow Y20, and the intake air swirls along a plane tilted with respect to the up-and-down direction. In pattern B, as shown by arrow Y16 in the right-hand diagram of Figure 8(c), a vertical swirling flow is formed in the valve cross section as the piston rises. Meanwhile, in the bore cross section, as the intake air flows from the outer region to the inner region of the combustion chamber 6, a vertical swirling flow is formed, but the flow from the exhaust side to the intake side along the bottom surface of the combustion chamber 6 becomes stronger, as shown by arrow Y15 in the left-hand diagram of Figure 8(c). As a result, in pattern B, near the compression top dead center, a tumble flow is formed at the valve center cross section, as indicated by arrow Y18 in the right diagram of FIG. 8(d), while the tumble flow almost disappears at the bore center cross section, and a flow is formed from the exhaust side to the intake side, as indicated by arrow Y17 in the left diagram of FIG. 8(d). As a result, in pattern B, the initial flame M11 flows from the exhaust side to the intake side, as shown in FIG. 12. Because the initial flame M11 flows to the intake side in this way, in pattern B, the initial flame M11 comes into contact with the intake-side wall surface of the combustion chamber 6, increasing cooling loss. Note that points R11 and R12 in FIG. 8(c) represent the centers of the vertical swirl flows.

[0050] As described above, in an engine that does not have cavity 53 and each recess 61, 62, 63, 56, when the intake air that flows into combustion chamber 6 flows downward as in pattern B, a tumble flow is not properly formed in combustion chamber 6 near the top dead center of compression, and contact between the initial flame and the wall surface of combustion chamber 6 reduces thermal efficiency.

[0051] FIG. 13 is a schematic diagram illustrating the flow of intake air in a bore center cross section of the engine 1 according to the embodiment. Unlike the engines corresponding to FIGS. 7 and 8 , the engine 1 according to the embodiment has a first recess 61 formed in an exhaust-side corner of the combustion chamber ceiling surface 40. Therefore, as indicated by arrow Y31 in FIG. 13 , the intake air flowing into the combustion chamber 6 can be directed further toward the exhaust side and upward. Furthermore, the intake air flows along the first recess 61, allowing it to smoothly swirl. Therefore, a tumble flow with a large swirl diameter can be formed. In particular, in the embodiment, the relatively large longitudinal dimension of the first recess 61 allows the intake air to smoothly swirl over a wider longitudinal range. Furthermore, in the engine 1 according to the embodiment, a piston-side recess 56 is formed in an exhaust-side corner of the piston crown surface 50. Therefore, as indicated by arrow Y32 in FIG. 13 , the direction of the intake air reaching the exhaust-side portion of the piston crown surface 50 can be smoothly changed toward the intake side. Therefore, disruption of the tumble flow due to collision with the piston crown surface 50 can be suppressed. Furthermore, in the engine 1 according to the above embodiment, an intake-side recess 63 is formed in a corner on the intake side of the combustion chamber ceiling surface 40. Therefore, as shown by arrow Y33 in FIG. 13, the direction of the intake air rising toward the corner on the intake side of the combustion chamber ceiling surface 40 can be smoothly changed to the exhaust side. This prevents the tumble flow from colliding with the intake-side portion of the combustion chamber ceiling surface 40 and becoming disintegrated. In this way, the engine 1 according to the above embodiment can form a tumble flow with a large swirl diameter, that is, a vertical swirl flow that flows along the wall surface of the combustion chamber 6 with its center near the center of the combustion chamber 6 as in pattern A.

[0052] Moreover, a cavity 53 is formed in the piston crown surface 50. Therefore, it is possible to prevent the tumble flow from being crushed by the piston crown surface 50 and the combustion chamber ceiling surface 40 near the compression top dead center.

[0053] As a result of the above-described action, the engine 1 according to the above embodiment can maintain tumble flow up to near top dead center of compression, thereby promoting combustion of the air-fuel mixture and preventing the initial flame from coming into contact with the wall of the combustion chamber as it rides on the intake air flow toward the intake or exhaust side, thereby reducing cooling loss and improving thermal efficiency.

[0054] Here, when the speed of intake air flowing from the intake port 10 into the combustion chamber 6 is slow, such as when the engine load is low, the amount of intake air reaching the corners on the exhaust side of the combustion chamber 6 is kept small. In this case, there is a risk that the first recess 61 will not be able to swirl the intake air sufficiently smoothly. In contrast, according to the engine 1 according to the above embodiment, the second recess 62 is provided at a position adjacent to the first recess 61 on the intake side. Therefore, even when the speed of intake air is slow, the intake air can be smoothly swirled by being guided along the second recess 62, as shown by arrow Y41 in FIG. 14, and the tumble flow can be maintained. Furthermore, the depth d2 of the second recess 62 is set smaller than the depth d1 of the first recess 61. Therefore, the second recess 62 can prevent the high-speed intake air from starting to swirl before it reaches the first recess 61. Therefore, according to the above embodiment, a tumble flow can be appropriately formed and maintained regardless of the speed of the intake air flowing into the combustion chamber 6.

[0055] (4) Variations In the above embodiment, the piston-side recess 56 is formed at a position spaced from the periphery of the piston crown surface 50. However, the piston-side recess may include the periphery of the piston crown surface. FIG. 15 is a plan view showing an example of a piston 105 in which a piston-side recess 156 including the periphery of the piston crown surface 150 is formed. FIG. 16 is a cross-sectional view taken along line XVI-XVI in FIG. 15. In the piston 150 shown in FIGS. 15 and 16, front-rear end portions 156A, 156A of the piston-side recess 156 include the periphery of the piston crown surface 150 of the piston 105 and extend from this periphery to the intake side. The front-rear end portions 156A, 156A of the piston-side recess 156 are lower than the remaining periphery of the piston 105. Furthermore, the intake-side portion of the front-rear end portion of the piston-side recess 156 functions as an exhaust valve recess. 15 and 16, the piston-side recess 156 includes a portion formed by connecting the exhaust valve recesses in the front-rear direction and a portion formed to be recessed further downward on the exhaust side than this portion. With this configuration, in the piston 105 shown in FIGS. 15 and 16, the piston-side recess 156 is recessed deeper relative to the exhaust-side piston inclined surface than in the embodiment shown in FIG. 6. The specific shape of the cavity is not limited to the shape according to the above embodiment. As shown in FIG. 15, the cavity 153 may be formed to have a generally elliptical shape in plan view. As shown in FIG. 16, the cavity 153 may be formed to have a generally spherical crown shape.

[0056] In the above embodiment, the spark plug 8 is provided in the center of the combustion chamber 6, but the position of the spark plug 8 is not limited to this.

[0057] Additionally, the second recess 62, the intake side recess 63, and the piston side recess 56 may be omitted.

[0058] Furthermore, the relationship between the depths of the second recess 62 and the first recess 61 is not limited to the above. For example, these depths may be set to the same dimension.

[0059] As mentioned above, the engine may be a multi-cylinder engine.Furthermore, the fuel injection valve 7 is not limited to a side injection type. [Explanation of symbols]

[0060] 1 engine 5 pistons 6 Combustion chamber 8 Spark plugs 10 Intake port 11 Exhaust port 40 (Combustion chamber) ceiling surface 41 Intake port opening 42 Exhaust port opening 50 (piston) crown surface 53 Cavity 56 Piston side recess 61 First recess (recess) 62 Second recess (inner recess) 63 Intake side recess

Claims

1. A combustion chamber structure for an engine, comprising: a combustion chamber defined by a piston crown surface, a circumferential surface of a cylinder in which the piston is housed so as to be able to slide up and down, and a pent roof-shaped ceiling surface formed on a cylinder head; and an ignition plug that ignites a mixture of fuel and air formed in the combustion chamber, An intake port for introducing air into the combustion chamber and an exhaust port for discharging exhaust gas from the combustion chamber are opened on a ceiling surface of the combustion chamber, A downwardly recessed cavity is formed in the center of the crown surface of the piston, The spark plug is disposed so that an electrode portion provided at a tip thereof faces the cavity from a ceiling surface of the combustion chamber, A combustion chamber structure for an engine, characterized in that, when the side of the combustion chamber where the intake port is arranged is the intake side and the side where the exhaust port is arranged is the exhaust side, a recess having an upwardly concave, curved shape is formed in the corner of the exhaust side of the ceiling surface of the combustion chamber.

2. 2. The combustion chamber structure of the engine according to claim 1, A combustion chamber structure for an engine, characterized in that an inner recess having an upwardly concave, curved shape is formed in a position adjacent to the intake side of the recess on the ceiling surface of the combustion chamber.

3. The combustion chamber structure of the engine according to claim 2, A combustion chamber structure for an engine, characterized in that the inner recess is shallower than the recess.

4. The combustion chamber structure of the engine according to claim 1, two openings of the intake ports and two openings of the exhaust ports are formed on a ceiling surface of the combustion chamber, such that the openings of one of the intake ports and the openings of one of the exhaust ports are aligned in the intake / exhaust direction, and the openings of the other intake port and the openings of the other exhaust port are aligned in the intake / exhaust direction, 10. A combustion chamber structure for an engine, wherein the recess is formed in a region that overlaps with the openings of the two exhaust ports when viewed along the intake / exhaust direction.

5. The combustion chamber structure of the engine according to claim 1, A combustion chamber structure for an engine, characterized in that a piston-side recess having a downwardly curved shape is formed at a corner of the exhaust side of the crown surface of the piston.

6. The combustion chamber structure of the engine according to any one of claims 1 to 5, An engine combustion chamber structure characterized in that an intake side recess having an upwardly concave curved shape is formed at an intake side corner of the ceiling surface of the combustion chamber.

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