Engine

Abstract

To provide an engine capable of forming an excellent tumble flow to increase heat efficiency.SOLUTION: A ceiling surface 40 of a combustion chamber 6 is provided with an intake-side inclined surface 43 whose height decreases as it moves away from the center of the ceiling surface 40 toward an intake side, and an exhaust-side inclined surface 44 whose height decreases as it moves away from the center of the ceiling surface 40 toward an exhaust side. On the intake-side inclined surface 43, openings 41 of two intake ports 10 are formed side by side in a front-to-rear direction. On the exhaust side inclined surface 44, an opening 42 of an exhaust port 11 is formed. A downwardly recessed cavity 60 is formed in the center of a crown surface 50 of a piston 5. The cavity 60 is shaped to extend in the front-to-rear direction between centers X2, X2 of the openings 41 of the two intake ports 10, and an exhaust-side opening edge 61G of the cavity 60 is configured to extend linearly along the front-to-rear direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an engine. [Background technology]

[0002] In engines installed in vehicles, etc., a tumble flow is formed in the combustion chamber by making the ceiling surface of the combustion chamber pent roof type, etc. 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 mixture is promoted and thermal efficiency is improved.

[0003] For example, Patent Document 1 discloses an engine in which upwardly protruding ribs are provided on both front-to-rear ends of a cavity formed on the piston crown surface in order to form a good tumble flow in the combustion chamber near the compression top dead center. In the configuration of Patent Document 1, the squish flow generated on the outer periphery of the combustion chamber near the compression top dead center is isolated from the cavity by the ribs, thereby preventing the squish flow from adversely affecting the tumble flow in the cavity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-61901 Summary of the Invention [Problem to be solved by the invention]

[0005] The intake air flow in the combustion chamber near the compression top dead center changes depending on the intake air flow after entering the combustion chamber. Therefore, to create a good tumble flow in the combustion chamber near the compression top dead center, it is necessary to improve not only the intake air flow immediately before the compression top dead center, but also the intake air flow during the intake stroke and the first half of the compression stroke. In this respect, the configuration of Patent Document 1 leaves room for improvement.

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

[0007] In order to solve the above problems, the engine of the present invention is 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, an ignition plug that ignites a fuel-air mixture formed in the combustion chamber, two intake ports for introducing air into the combustion chamber, and an exhaust port for leading exhaust gas out of the combustion chamber, wherein the ceiling surface of the combustion chamber is sloped on the intake side such that the height decreases as the distance from the center of the ceiling surface to the intake side increases. The piston has a sloping surface and an exhaust-side sloping surface that becomes lower in height as it moves away from the center of the ceiling surface to the exhaust side, and the openings of the two intake ports are formed to be lined up in the front-to-back direction on the intake-side sloping surface, and the openings of the exhaust port are formed on the exhaust-side sloping surface, and a cavity that is concave downward is formed in the center of the crown surface of the piston, and the cavity has a shape that extends in the front-to-back direction between the centers of the openings of the two intake ports, and the exhaust-side opening edge of the cavity extends linearly along the front-to-back direction.

[0008] In the present invention, a cavity recessed downward is formed in the center of the piston crown, so that intake air can flow along the cavity to form a tumble flow in the combustion chamber.

[0009] Moreover, the cavity is formed to extend in the front-to-rear direction between the centers of the openings of the two intake ports, and the opening edge on the exhaust side of the cavity extends linearly in the front-to-rear direction. This allows the intake air to swirl evenly up to the compression top dead center, creating a good tumble flow within the cavity near the compression top dead center.

[0010] Specifically, in a combustion chamber with two intake port openings arranged side by side in the longitudinal direction, the intake air flow velocity in the middle of these openings tends to be faster than the flow velocity at the outer sides. As a result, the intake air flows unevenly between the middle and outer positions of the intake port openings during the intake stroke, causing the tumble flow to tilt at the outer positions and making the center position of the tumble flow misaligned between the middle and outer positions. When the intake air flow is uneven during the intake stroke, a biased intake air flow is formed in the combustion chamber near the top dead center of compression. In other words, good tumble flow is not formed in the combustion chamber.

[0011] In contrast, in the present invention, the opening edge on the exhaust side of the cavity extends linearly in the front-to-rear direction, so that the position where the intake air is deflected by contact with the opening edge can be made roughly the same at each point in the front-to-rear direction, aligning the center of the tumble flow. Furthermore, because the cavity extends in the front-to-rear direction between the centers of the openings of the two intake ports, both front-to-rear ends of the cavity can prevent the tumble flow from tilting at positions outside the midpoint of the intake port openings. This allows the intake air to be uniformly swirled vertically during the intake stroke, forming a good tumble flow in the combustion chamber and cavity near top dead center of compression. This, in turn, improves thermal efficiency.

[0012] In the above configuration, preferably, a sloped portion is formed extending from the opening edge on the exhaust side of the cavity to the exhaust side, and the sloped portion is inclined so that its height decreases as it approaches the cavity (claim 2).

[0013] According to this configuration, the tumble flow can be guided into the cavity by the slope portion, and the tumble flow can be prevented from being crushed in the so-called squish area on the outer periphery side of the cavity.

[0014] In the above configuration, preferably, the engine further comprises a fuel injection valve that injects fuel into the combustion chamber and a catalytic device that purifies exhaust gas discharged from the combustion chamber, and the fuel injection valve injects fuel into the combustion chamber during the compression stroke when the temperature of the catalytic device is low (claim 3).

[0015] This configuration allows a fuel-rich mixture to be formed around the spark plug when the temperature of the catalytic converter is low. Therefore, even if the mixture is ignited relatively late, combustion stability can be ensured while the exhaust gas temperature is raised, thereby activating the catalytic converter early. Here, a shallow cavity is preferable to form a fuel-rich mixture around the spark plug. On the other hand, if the cavity is shallow, the tumble flow may not be contained within the cavity. In contrast, with the above configuration, the sloped portion guides the tumble flow into the cavity. Therefore, this configuration allows a good tumble flow to be formed within the cavity, while ensuring combustion stability and activating the catalytic converter early.

[0016] In the above configuration, it is preferable that the cross section cut along the front-rear direction has a constant height (claim 4).

[0017] With this configuration, the position in the intake / exhaust direction where the intake air is deflected by contact with the bottom surface of the cavity can be made roughly the same, making the flow of intake air more uniform.

[0018] In the above configuration, upright wall portions rising upward from the bottom surface of the cavity are provided at both ends in the front-rear direction of the cavity (claim 5).

[0019] According to this configuration, the upright wall portion can more reliably prevent the tumble flow from tilting. [Effects of the Invention]

[0020] As described above, the engine of the present invention can improve thermal efficiency. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a diagram showing a 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. [Figure 4] FIG. 4 is a schematic cross-sectional view of the piston crown surface taken along line III-III in FIG. 3. [Figure 5] 4 is a schematic cross-sectional view of the piston crown surface taken along line IV-IV in FIG. 3. [Figure 6] 4 is a schematic cross-sectional view of the piston crown surface taken along line VV in FIG. 3. [Figure 7] FIG. 2 is a diagram schematically illustrating the intake air flow in the 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. 2 is a schematic plan view showing the center position of a tumble flow. [Figure 11] FIG. 1 is a diagram showing a schematic view of an initial flame. [Figure 12] FIG. 2 is a diagram schematically illustrating the intake air flow in the combustion chamber. [Figure 13] FIG. 2 is a schematic plan view showing the center position of a tumble flow. [Figure 14] FIG. 1 is a diagram showing a schematic view of an initial flame. [Figure 15] FIG. 10 is a diagram for explaining the effects of the present invention, and is a diagram schematically showing the flow of intake air near the piston crown surface. [Figure 16] FIG. 10 is a diagram for explaining the effects of the present invention, and is a diagram schematically showing the intake air flow near the cavity. [Figure 17] FIG. 1 is a diagram for explaining the effects of the present invention, and is a diagram schematically showing the intake air flow near the compression top dead center. DETAILED DESCRIPTION OF THE INVENTION

[0022] (1) Overall structure FIG. 1 is a diagram showing the configuration of an engine according to one embodiment of the present invention. The engine E has an engine body 1, and an intake passage 70 and an exhaust passage 90 connected to the engine body 1. The engine body 1 shown in this figure is a spark-ignition four-stroke engine. The engine body 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.

[0023] The engine body 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 vertical 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 body 1 is a single-cylinder engine having only one cylinder 2.

[0024] In the engine body 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. In this specification, the direction perpendicular to the plane S is referred to as the intake / exhaust direction, and in this intake / exhaust direction, the side where the intake valve 12 is provided is referred to as the intake side, and the side where the exhaust valve 13 is provided is referred to as the exhaust side. In addition, the direction perpendicular to the up-down direction and the intake / exhaust direction is 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 is referred to as the front, and the opposite side is referred to as the rear. In addition, the central axis X1 of a cylinder 2 is referred to as the cylinder axis X1.

[0025] Above the piston 5, a combustion chamber 6 is defined by the circumferential surface 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.

[0026] 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.

[0027] A fuel injection valve 7 that injects fuel into the combustion chamber 6 is attached to the cylinder head 4. In this embodiment, the fuel injection valve 7 is a side-injection type fuel injection valve. The fuel injection valve 7 is attached to the intake side of the cylinder head 4 and injects fuel into the combustion chamber 6 from the side of the intake side. A fuel mainly composed of gasoline 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. In this 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.

[0028] 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 of the intake ports 10 and two exhaust-side openings 42 that are openings of 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. The two intake-side openings 41 are formed on an intake-side inclined surface 43. The two exhaust-side openings 42 are formed on an exhaust-side inclined surface 44.

[0029] 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.

[0030] 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 center of 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.

[0031] Ignition by spark plug 8 causes a mixture of fuel and air supplied from fuel injection valve 7 to combust in 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 15, which is the output shaft of engine body 1, is provided below piston 5. Crankshaft 14 is connected to piston 5 via connecting rod 9, and is driven to rotate around its central axis in response to the reciprocating motion (up and down movement) of piston 5.

[0032] The intake passage 70 is connected to the engine body 1 in communication with each intake port 10. Intake air passes through the intake passage 70 and is introduced into the intake ports 10 and ultimately into the combustion chambers 6. The exhaust passage 90 is connected to the engine body 1 in communication with each exhaust port 11. Exhaust gas discharged from the combustion chambers 6 to the exhaust ports 11 passes through the exhaust passage 90 and is released to the outside of the engine E. A catalytic device 91 including a catalyst for purifying exhaust gas is provided in the exhaust passage 90. The catalyst included in the catalytic device 91 is, for example, a three-way catalyst.

[0033] In the engine E of this embodiment, when the temperature of the catalytic device 91 is low, the ignition timing is retarded to promote activation of the catalytic device 91. Specifically, when the temperature of the catalytic device 91 is low, the spark plug 8 ignites the air-fuel mixture later than the top dead center of compression. By retarding the ignition timing in this manner, the air-fuel mixture begins to burn closer to the start of opening of the exhaust valve 13. As a result, the temperature of the exhaust gas discharged to the exhaust passage 90 increases, promoting activation of the catalytic device 91. However, if the ignition timing of the spark plug 8 is delayed from the top dead center of compression, combustion stability decreases. To address this, in the engine E of this embodiment, when the temperature of the catalytic device 91 is low, the ignition timing is retarded and fuel is injected into the combustion chamber 6 from the fuel injector 7 during the compression stroke. Injecting fuel during the compression stroke stratifies the air-fuel mixture in the combustion chamber 6, forming a mixture with a high fuel concentration around the spark plug 8. In other words, this results in a stable initial flame and improved combustion stability. The engine E is provided with a control device 100 that controls the spark plugs 8 and the fuel injection valves 7, and the spark plugs 8 and the fuel injection valves 7 are driven as described above in response to commands from the control device 100.

[0034] (2) Piston shape Fig. 3 is a schematic plan view of a crown surface 50 of the piston 5 that defines the bottom surface of the combustion chamber 6. Fig. 4 is a schematic cross-sectional view taken along line IV-IV in Fig. 3. Fig. 5 is a schematic cross-sectional view taken along line VV in Fig. 3. Fig. 6 is a schematic cross-sectional view taken along line VI-VI in Fig. 3.

[0035] Except for its outer periphery and center, the crown surface 50 of the piston 5 is formed so that its height increases toward the cylinder axis X in a cross-sectional view along the intake / exhaust direction. Specifically, the crown surface 50 of the piston 5 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).

[0036] A downwardly recessed cavity 60 is formed in the center of the piston crown 50. The center of the cavity 60 is located at a position that substantially coincides with a point on the cylinder axis X1. The cavity 60 is also located in an area that overlaps with the two intake-side openings 41 and the two exhaust-side openings 42 in a plan view.

[0037] The cavity 60 has a shape that extends in the front-rear direction between the centers X2, X2 of the two intake-side openings 41, 41. That is, the dimension L1 of the cavity 60 in the front-rear direction is set to be the same as the distance L10 between the centers X2, X2 of the two intake-side openings 41, 41 in the front-rear direction. Note that the term "same dimension" here includes not only the case where they are exactly the same, but also the case where they are substantially the same with a slight difference. The dimension L2 of the cavity 60 in the intake / exhaust direction is shorter than the dimension L1 in the front-rear direction. In this embodiment, the dimension L2 of the cavity 60 in the intake / exhaust direction is set to be smaller than the distance L11 between the center X2 of the intake-side opening 41 and the center X3 of the exhaust-side opening 42 in the intake / exhaust direction and larger than half of this distance L11. In addition, in this embodiment, the aspect ratio of the vertical distance between the bottom surface 62 of the cavity 60 and the apex of the combustion chamber 6 when the piston 5 is at the top dead center position, to the dimension L2 of the cavity 60 in the intake / exhaust direction, is set to approximately 0.8, and the cavity 60 is a so-called shallow dish-shaped cavity.

[0038] The cavity 60 has a shape like a rectangle extending in the front-rear direction in a plan view with four curved corners. Accordingly, the opening edge 61 of the cavity 60 also has a shape like a rectangle extending in the front-rear direction with four curved corners. Specifically, the opening edge 61 of the cavity 60 includes a first edge 61A that forms its front end and extends linearly in the intake / exhaust direction, and a second edge 62B that forms its rear end and extends linearly in the intake / exhaust direction. The opening edge 61 of the cavity 60 includes a curved third edge 61C that extends from the exhaust-side end of the first edge 61A toward the exhaust side and rear, and a curved fourth edge 61D that extends from the intake-side end of the first edge 61A toward the intake side and rear. The opening edge 61 of the cavity 60 includes a curved fifth edge 61E extending from the exhaust side end of the second edge 61B toward the exhaust side and the front side, and a sixth edge 61F curved from the intake side end of the second edge 61B toward the intake side and the front side. The opening edge 61 of the cavity 60 includes a seventh edge 61G extending linearly in the front-to-rear direction from the rear end of the third edge 61C to the front end of the fifth edge 61E, and an eighth edge 61H extending linearly in the front-to-rear direction from the rear end of the fourth edge 61D to the front end of the sixth edge 61F.

[0039] The opening edge 61 on the exhaust side of the cavity 60 is mainly composed of the seventh edge 61G. As a result, the cavity 60 is formed so that its exhaust-side opening edge (71G) extends linearly in the front-to-rear direction. Similarly, the opening edge 61 on the exhaust side of the cavity 60 is mainly composed of the eighth edge 61H. As a result, the cavity 60 is formed so that its intake-side opening edge (71H) extends linearly in the front-to-rear direction. The seventh edge 61G and the eighth edge 61H extend straight forward and backward from the center of the cavity 60 in the front-to-rear direction. In this embodiment, the front-to-rear dimension of the seventh edge 61G and the eighth edge 61H is set to approximately 60% of the front-to-rear dimension of the cavity 60.

[0040] In this embodiment, the fuel injection valve 7 is of a side injection type and is attached to the intake side of the cylinder head 4. Accordingly, the crown surface 50 of the piston 5 is formed with a downwardly recessed fuel introduction portion 54 for guiding fuel into the cavity 53 so as to communicate with the cavity 60. The fuel introduction portion 54 communicates with the cavity 60 via the central portion in the front-rear direction of the eighth edge portion 61H, and the central portion in the front-rear direction of the eighth edge portion 61H is recessed downward.

[0041] On the exhaust side from the third edge 61C, the fifth edge 61E, and the seventh edge 61G, sloped portions 66 are formed extending from these edges 61C, 61E, and 61G to the exhaust side. The sloped portions 66 are inclined so that their height decreases as they approach the intake side, i.e., the cavity 60. In this embodiment, sloped portions 67 are also formed on the intake side from the fourth edge 61D, the sixth edge 61F, and the eighth edge 61H, whose height decreases as they approach the cavity 60. However, the dimension of the sloped portions 67 in the intake / exhaust direction is shorter than that of the sloped portions 66. In this embodiment, the dimension of the sloped portions 66 in the intake / exhaust direction is set to approximately 1 / 4 of the dimension of the cavity 60 in the intake / exhaust direction.

[0042] As shown in Fig. 6, the bottom surface 62 of the cavity 60 is formed to have a constant height in a cross section cut along the front-to-rear direction. In other words, when the cavity 60 is cut along a plane perpendicular to the intake / exhaust direction, the bottom surface 62 of the cavity 60 is a straight line extending straight in the front-to-rear direction on all cut surfaces. In this embodiment, as shown in Fig. 4 etc., the bottom surface 62 of the cavity 60 is formed to have a substantially constant height in the intake / exhaust direction, and the bottom surface of the cavity 60 is substantially flat and perpendicular to the up-down direction.

[0043] As shown in Fig. 6, upright wall portions 63, 63 rising upward from a bottom surface 62 of the cavity 60 are provided at both ends in the front-rear direction of the cavity 60. That is, the front end and rear end of the peripheral wall of the cavity 60 extend in the up-down direction so as to be approximately perpendicular to the bottom surface 62. Also, as shown in Fig. 6, the height position of the upper ends of the upright wall portions 63, 63 is higher than the height position of the upper ends of other parts of the peripheral wall of the cavity 60. Here, the upper edge of the front upright wall portion 63 is a first edge portion 61A, and the upper edge of the rear upright wall portion 63 is a second edge portion 61B, and in this embodiment, the height positions of the first edge portion 61A and the second edge portion 61B are higher than the other edges 61C to 61H.

[0044] The piston 5 has a recess 59 formed on the intake side of the crown surface 50 thereof, which is a so-called intake valve recess, for preventing contact between the intake valve 12 and the piston 5.

[0045] (3) Effects, etc. With the above-described configuration, the engine according to this embodiment can effectively form a tumble flow in the cavity 60 near the top dead center of the compression stroke, thereby improving thermal efficiency. Details will be explained using Figs. 7 to 17.

[0046] 7 and 8 are diagrams showing a simulated intake flow in the combustion chamber 6, with the main flow portion of the intake flow extracted from the simulation results and schematically indicated by arrows. FIGS. 7 and 8 show the intake flow during the intake stroke and compression stroke of an engine in which the piston does not have a cavity 60 or a sloped portion 66. The left-hand side of each of FIGS. 7 and 8 is a cross-sectional view taken along a plane that passes through the center of the combustion chamber 6 and extends along the intake / exhaust direction. The right-hand side of each of FIGS. 7 and 8 is a cross-sectional view taken along a plane that passes near the centers of the intake valve 12 and the exhaust valve 13 and extends along the intake / exhaust direction. Hereinafter, the cross-section of the left-hand side of each of FIGS. 7 and 8, which passes through the center of the combustion chamber 6 and extends along the intake / exhaust direction, will be referred to as the bore center cross-section. Furthermore, the cross-section of the right-hand side of each of FIGS. 7 and 8, which passes near the centers of the intake valve 12 and the exhaust valve 13 and extends along the intake / 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.

[0047] Figures 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 Figure 7 is an ideal pattern, in which the flow of intake air into the combustion chamber 6 is virtually adjusted so that a tumble flow is formed in the combustion chamber 6 near the top dead center of the compression stroke. On the other hand, Figure 8 shows the result of simulating an example of a flow pattern that occurs in an actual engine. Hereinafter, the flow pattern shown in Figure 7 will be referred to as pattern A, and the flow pattern shown in Figure 8 will be referred to as pattern B.

[0048] Fig. 9 is a diagram schematically showing the intake air flow in the combustion chamber 6 in pattern A. Fig. 10 is a diagram schematically showing the intake air flow in the combustion chamber 6 in pattern A in a plan view. Fig. 11 is a diagram schematically showing the state of the initial flame near the compression top dead center in pattern A.

[0049] In pattern A, as indicated by arrow Y1 in the left and right views of FIG. 7(a), the intake air flows into the combustion chamber 6 from the intake port approximately evenly at both the bore center cross section and the valve cross section, i.e., approximately evenly in the longitudinal direction. In pattern A, as indicated by arrow 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 approximately equally at both the bore center cross section and the valve cross section near the intake bottom dead center, as indicated by arrow Y2 in the left and right views of FIG. 7(b). Specifically, in pattern A, the intake air swirls along a plane perpendicular to the longitudinal direction at each longitudinal position, as indicated by arrow Y5 in FIG. 9. Furthermore, as indicated by the dashed lines in FIG. 7(c) and FIG. 10, the centers R1 and R2 of the swirling flow (Y5) at each longitudinal position are formed at approximately the same position in the intake / exhaust direction. In pattern A, this vertical swirling flow, or tumble flow, is maintained even during the compression stroke, as indicated by arrow Y3 in the left and right views of FIG. 7(c). As a result, in pattern A, a tumble flow is formed almost uniformly in the front-to-rear direction in the combustion chamber 6 even near the compression top dead center, as indicated by arrow Y4 in the left and right diagrams 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 11. This prevents the initial flame M1 from being cooled by the wall surface of the combustion chamber 6, thereby improving thermal efficiency.

[0050] Fig. 12 is a diagram schematically showing the intake air flow in the combustion chamber 6 in pattern B. Fig. 13 is a diagram schematically showing, in plan view, the intake air flow in the combustion chamber 6 in pattern A. Fig. 14 is a diagram schematically showing the state of the initial flame near the compression top dead center in pattern B.

[0051] As described above, if the intake air flows into the combustion chamber 6 from the intake ports approximately evenly in the longitudinal direction, a good tumble flow can be formed up to near the top dead center of compression. However, when two intake ports 10 open to the combustion chamber ceiling surface 40, the intake air flows from each intake port 10 merge at a midpoint between the two intake ports 10. Pattern B is such an intake air flow pattern. In pattern B, the intake air flow velocity at the bore center cross section is faster, as indicated by arrow Y11 in the left side of Figure 8(a), and the intake air flow velocity at the valve cross section is slower, as indicated by arrow Y12 in the right side of Figure 8(a). Furthermore, as indicated by arrow Y11 in the left side of Figure 8(a), the direction of some of the intake air at the bore center cross section is more downward than in pattern A.

[0052] In pattern B, near the intake bottom dead center, the intake air swirls roughly 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). Meanwhile, in the valve cross section, the flow of intake air spiraling upward from the bottom of the combustion chamber 6 weakens, as indicated by arrow Y14 in the right diagram of FIG. 8(b), and a sufficient vertical swirl flow is not formed. As a result, in the bore central cross section, the intake air swirls along this plane, i.e., a plane perpendicular to the front-to-rear direction, as indicated by arrow Y19 in FIG. 12. Meanwhile, in the region outside the bore central cross section in the front-to-rear direction, the swirl flow is tilted, and the intake air swirls along a plane tilted relative to the up-and-down direction, as indicated by arrow Y20. In pattern B, as the piston rises, a vertical swirl flow is formed in the valve cross section, as indicated by arrow Y16 in the right diagram of FIG. 8(c). On the other hand, in the bore cross section, intake air flows from the outer region to the inner region of the combustion chamber 6, so although a vertical swirling flow is formed, the flow from the exhaust side to the intake side along the bottom surface of the combustion chamber 6 is stronger, as indicated by arrow Y15 in the left diagram of FIG. 8(c). Furthermore, as indicated by the dashed lines in FIG. 8(c) and FIG. 13, the center R11 of the swirling flow (Y19) in the bore central cross section is shifted toward the exhaust side relative to the center R12 of the swirling flow (Y20) in the valve cross section. In other words, the position where the swirling flow of intake air is formed is shifted in the front-to-rear direction. As a result, in Pattern B, near the compression top dead center, a tumble flow is formed in the valve central cross section, as indicated by arrow Y18 in the right diagram of FIG. 8(d). However, in the bore central cross section, the tumble flow almost disappears, and a flow from the exhaust side to the intake side is formed, 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. 14. As the initial flame M11 flows toward the intake side in this manner, in pattern B, the initial flame M11 comes into contact with the wall surface of the intake side of the combustion chamber 6, increasing the cooling loss.

[0053] As explained in the above pattern B, in an engine in which two intake ports 10 open to the combustion chamber ceiling surface 40, if there is no cavity 60 or slope portion 66, tumble flow is not properly formed in the combustion chamber 6 near the top dead center of compression, and contact between the initial flame and the wall surface of the combustion chamber 6 results in low thermal efficiency.

[0054] In contrast, in the present invention, a cavity 60 recessed downward is formed in the center of the piston crown surface 50. Therefore, by making the intake air flow along the cavity 60, a tumble flow can be formed in the combustion chamber.

[0055] Moreover, the cavity 60 is formed to extend in the front-rear direction between the centers X2, X2 of the two intake-side openings 41, 41. Also, the cavity 60 is formed so that its seventh edge 61G extends linearly in the front-rear direction. Therefore, as in the above-described pattern A, the intake air can be caused to vertically swirl evenly in the front-rear direction during the intake stroke, thereby improving thermal efficiency.

[0056] Specifically, during the intake stroke, the intake air descends through the exhaust-side portion of the combustion chamber 6, then collides with the piston crown surface 50 and is deflected toward the intake side. At this time, if a cavity is formed in the piston crown surface 50, the intake air separates and deflects near the opening edge of the cavity. Therefore, in the above embodiment, the seventh edge portion 61G constituting the exhaust-side opening edge of the cavity 60 extends linearly in the front-rear direction, so that the position where the direction of the intake air changes from downward to toward the intake side can be made approximately the same at each portion in the front-rear direction, as shown in FIG. 15. Therefore, the center position of the swirling flow of the intake air can be made the same in the front-rear direction.

[0057] As described above, the swirling flow is likely to incline in a region outside the bore center cross section in the front-rear direction. The bore center cross section is a cross section passing through the centers of the two intake-side openings 41. In other words, the swirling flow is likely to incline in a region outside the center of the two intake-side openings 41 in the front-rear direction, as indicated by the dashed line Y102 in FIG. 16 . In contrast, in the above embodiment, the cavity 60 is formed to extend between the centers X2, X2 of the two intake-side openings 41, 41, and the front and rear ends of the cavity 60 are positioned approximately at the same positions as the centers X2, X2 of the two intake-side openings 41, 41 in the front-rear direction. Therefore, as indicated by the arrow Y101 in FIG. 16 , the front and rear ends of the cavity 60 can prevent the swirling flow passing through the centers X2, X2 of the two intake-side openings 41, 41, and formed outside the center of the two intake-side openings 41 in the front-rear direction from inclining. This allows the intake air to swirl along a plane perpendicular to the front-rear direction at each position in the front-rear direction. In particular, the main flow of intake air passes through the centers X2, X2 of the two intake-side openings 41, 41. Therefore, by restricting the tilt of a portion of the main flow of intake air, the entire intake air can be swirled more evenly along the plane.

[0058] As described above, according to the embodiment, a uniform tumble flow can be formed in the combustion chamber 6 until the compression top dead center is reached. Therefore, a good tumble flow can be formed in the cavity 60 near the compression top dead center, thereby improving the thermal efficiency.

[0059] In addition, in the above embodiment, the bottom surface 62 of the cavity 60 is formed as a flat surface with a constant height at each portion in the front-to-rear direction. Therefore, the position at which the intake air is deflected by contact with the bottom surface of the cavity 60 can be made roughly the same at each portion in the front-to-rear direction. Therefore, the center position of the swirling flow of the intake air can be made more uniform in the front-to-rear direction.

[0060] In the above embodiment, the upright wall portions 63 are provided at both ends in the front-rear direction of the cavity 60. Therefore, these upright wall portions 63 can reliably prevent the swirling flow formed outside the center of the two intake-side openings 41 in the front-rear direction from tilting.

[0061] 8(c), the center of the swirling flow of intake air tends to shift toward the exhaust side during the compression stroke. In contrast, in the above embodiment, sloped portions 66 are provided that extend toward the exhaust side from the third edge portion 61C, the fifth edge portion 61E, and the seventh edge portion 61G and that slope downward in height toward the cavity 60. Therefore, as shown by arrow Y103 in FIG. 17, near the compression top dead center, the tumble flow that is biased toward the exhaust side can be guided into the cavity 60 by the sloped portions 66 and retained within the cavity 60. Therefore, the tumble flow can be prevented from collapsing in the so-called squish area, which is located on the outer periphery of the cavity 60.

[0062] Furthermore, since the slope portion 66 can keep the tumble flow within the cavity 60, the depth of the cavity 60 can be made shallower according to the above embodiment. Making the cavity 60 shallower can suppress fuel diffusion and reliably increase the fuel concentration around the spark plug 8 when fuel is injected during the compression stroke. Therefore, according to the above embodiment, when the temperature of the catalytic device 71 is low and the ignition timing is retarded and fuel is injected during the compression stroke, the activity of the catalytic device 91 can be promoted while reliably improving combustion stability.

[0063] (4) Variations In the above embodiment, the cavity 60 has a shape like a rectangle extending in the front-to-rear direction in a plan view with the four corners curved. However, the cavity 60 may have any shape as long as the opening edge on the exhaust side extends linearly in the front-to-rear direction and extends in the front-to-rear direction between the centers X2, X2 of the two intake side openings 41, 41, and other shapes are not limited to those in the above embodiment.

[0064] In the above embodiment, the bottom surface 62 of the cavity 60 is described as being flat, but the specific shape of the bottom surface 62 is not limited to this. For example, the bottom surface 62 of the cavity 60 may be formed so as to be curved when viewed along the front-to-rear direction.

[0065] 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.

[0066] Furthermore, the slope portion 66 and the inclined portion 67 provided around the cavity 60 in the above embodiment may be omitted.

[0067] Furthermore, 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]

[0068] 1 engine 5 pistons 6 Combustion chamber 7 Fuel injection valve 8 Spark plugs 10 Intake port 11 Exhaust port 40 (Combustion chamber) ceiling surface 41 Intake side opening (intake port opening) 43 Intake side inclined surface 44 Exhaust side inclined surface 50 (piston) crown surface 60 cavities 61 Cavity opening edge 61G Cavity exhaust side opening edge 62 (cavity) bottom 63 Vertical wall 66 Slope section 80 Exhaust passage 81 Catalytic Converter

Claims

1. 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; an ignition plug for igniting a mixture of fuel and air formed in the combustion chamber; two intake ports for introducing air into the combustion chamber; and an exhaust port for discharging exhaust gas from the combustion chamber, the ceiling surface of the combustion chamber has an intake-side inclined surface whose height decreases as it moves away from the center of the ceiling surface toward the intake side, and an exhaust-side inclined surface whose height decreases as it moves away from the center of the ceiling surface toward the exhaust side, The intake-side inclined surface has openings of the two intake ports formed so as to be aligned in the front-rear direction, an opening of the exhaust port is formed on the exhaust-side inclined surface, A downwardly recessed cavity is formed in the center of the crown surface of the piston, the cavity has a shape extending in the front-rear direction between centers of openings of the two intake ports, An engine characterized in that an opening edge on the exhaust side of the cavity extends linearly along the front-to-rear direction.

2. 2. The engine of claim 1, a sloped portion extending from an opening edge on the exhaust side of the cavity to the exhaust side is formed on a crown surface of the piston, The engine, wherein the slope portion is inclined so that its height decreases as it approaches the cavity.

3. 3. The engine of claim 2, The engine further includes a fuel injection valve that injects fuel into the combustion chamber, and a catalytic converter that purifies exhaust gas discharged from the combustion chamber, The engine is characterized in that the fuel injection valve injects fuel into the combustion chamber during a compression stroke when the temperature of the catalyst device is low.

4. 2. The engine of claim 1, An engine characterized in that the bottom surface of the cavity is formed to have a constant height in a cross section cut along the front-rear direction.

5. In the engine according to any one of claims 1 to 4, An engine characterized in that the cavity is provided at both ends in the front-rear direction with upright wall portions rising upward from the bottom surface of the cavity.

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