Combustion chamber structure of engine

Abstract

To provide a combustion chamber structure of an engine capable of suppressing combustion fluctuation and increasing heat efficiency.SOLUTION: A downwardly recessed cavity 53 is formed in a center part of a crown surface 50 of a piston 5, an upwardly recessed recess part 45 is formed in a portion of a ceiling surface 40 of a combustion chamber 6 opposite the cavity 53, and an ignition plug 8 is disposed so that an electrode part 81 at its tip faces the combustion chamber 6 from a bottom surface of the recess part 45. When a side of the combustion chamber 6 where an intake port 10 is disposed is defined as an intake side, and a side where an exhaust port 11 is disposed is defined as an exhaust side, the recess part 45 is provided with an intake-side recess part 46 located on the intake side of the ignition plug 8, and an exhaust-side recess part 47 located on the exhaust side, and a volume of the exhaust-side recess part 47 is made larger than a volume of the intake-side recess part 46.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, the flow is formed along the recess, so the center of the tumble flow is closer to the intake side than the spark plug. In other words, the spark plug is located away from the center of the tumble flow, where the intake air flow is likely to change. This can lead to variations in the formation of the initial flame around the spark plug, and therefore in the combustion state, which can result in significant combustion fluctuations between combustion cycles. Furthermore, because the initial flame spreads on the relatively fast flow outside the center, it is more likely to come into contact with the combustion chamber wall, increasing cooling loss and reducing 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 suppress combustion fluctuations and 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 that is 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 that is provided with an ignition plug that ignites a mixture of fuel and air formed in the combustion chamber, and the ceiling surface of the combustion chamber is provided with an intake port for introducing air into the combustion chamber and an exhaust port for leading out exhaust gas from the combustion chamber, and a cavity recessed downward is formed in the center of the piston crown surface. a tee is formed in the top surface of the combustion chamber, and a recess that is recessed upward is formed in the portion of the ceiling surface of the combustion chamber facing the cavity, and the ignition plug is arranged so that an electrode portion provided at its tip faces the combustion chamber from the bottom surface of the recess, and when the side of the combustion chamber where the intake port is arranged is defined as the intake side and the side where the exhaust port is arranged is defined as the exhaust side, the recess includes an intake side recess located on the intake side of the ignition plug and an exhaust side recess located on the exhaust side, and the volume of the exhaust side recess is larger than the volume of the intake side recess.

[0008] In the present invention, a cavity is formed in the piston crown, and a recess is formed in the opposite ceiling surface. Recesses are also provided on both the intake and exhaust sides of the spark plug. This allows intake air to swirl along the cavity and recess, maintaining tumble flow, even near top dead center of compression. Furthermore, the volume of the exhaust-side recess, located on the exhaust side of the spark plug, is larger than the volume of the intake-side recess, located on the intake side. This allows most of the gas riding on the tumble flow toward the exhaust side along the ceiling of the combustion chamber to be guided downward on the exhaust side of the spark plug, while minimizing the amount of gas riding on the tumble flow toward the exhaust side along the ceiling of the combustion chamber on the intake side of the spark plug. In other words, this prevents the tumble flow from the intake side toward the exhaust side from becoming excessively strong, and prevents the center of the tumble flow from shifting toward the exhaust side relative to the spark plug electrode, thereby bringing the spark plug electrode closer to the center of the tumble flow. As a result, according to the present invention, while maintaining the tumble flow up to near the top dead center of compression, the initial flame can be formed in a region near the center of the tumble flow where the intake flow is relatively small, thereby suppressing combustion fluctuations and suppressing contact between the initial flame and the combustion chamber wall surface, thereby improving thermal efficiency.

[0009] In the above configuration, preferably, the recess is formed in a shape such that, when viewed along the intake / exhaust direction, a tangent line passing through the end on the exhaust side is closer to vertical than a tangent line passing through the end on the intake side (claim 2).

[0010] With this configuration, most of the gas moving toward the exhaust side along the ceiling surface of the combustion chamber can be more reliably guided downward by the exhaust side portion of the recess, i.e., the exhaust-side recess.

[0011] In the above configuration, the height position of the end of the recess on the exhaust side is preferably lower than the height position of the end of the recess on the intake side (claim 3).

[0012] According to this configuration, the intake air can be guided downward more reliably in the exhaust side portion of the recess, i.e., the exhaust side recess.

[0013] In the above configuration, the cavity preferably has a shape in which the position corresponding to the electrode portion of the spark plug in the up-down direction is the lowest end (claim 4).

[0014] With this configuration, it is possible to minimize the misalignment between the center of the tumble flow and the electrode portion of the spark plug in a direction perpendicular to the vertical direction.

[0015] In the above configuration, the spark plug is preferably located closer to the exhaust side than the center of the ceiling surface of the combustion chamber (claim 5).

[0016] This configuration ensures space for installing a fuel injection valve, etc., on the intake side of the spark plug. However, if the spark plug is located on the exhaust side, the initial flame tends to come into contact with the combustion chamber wall on the exhaust side, increasing cooling loss. In contrast, in the present invention, as described above, the volume of the exhaust-side recess is increased, and the distance between the bottom of the recess on the exhaust side of the spark plug, i.e., the combustion chamber wall, and the electrode portion of the spark plug is increased. This prevents contact between the exhaust-side combustion chamber wall and the initial flame, suppressing increases in cooling loss and achieving high thermal efficiency.

[0017] In the above configuration, preferably, the recess is entirely located inside the cavity when viewed in the up-down direction (claim 6).

[0018] This configuration reliably guides most of the gas traveling along the tumble flow from the intake side to the exhaust side along the ceiling surface of the combustion chamber on the exhaust side of the spark plug into the cavity, i.e., downward. Furthermore, on the intake side of the spark plug, gas traveling upward on the tumble flow can be guided to the outer periphery of the cavity, i.e., the intake-side squish area, by colliding with the outer peripheral portion of the intake-side recess on the ceiling surface of the combustion chamber. This reduces the momentum of the gas traveling upward along the bottom surface of the cavity and then traveling along the ceiling surface of the combustion chamber from the intake side to the exhaust side. This more reliably prevents the center of the tumble flow, i.e., the center of the vertical swirling vortex, from shifting toward the exhaust side relative to the spark plug. Furthermore, by introducing more gas into the intake-side squish area, the flow toward the intake side within the combustion chamber can be strengthened during the expansion stroke. This promotes flame propagation toward the intake side, preventing unburned gas and knocking at the intake-side end of the combustion chamber. [Effects of the Invention]

[0019] As described above, the combustion chamber structure of the engine according to the present invention can suppress combustion fluctuations and 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. 2 is a schematic cross-sectional view for explaining the structure of a combustion chamber. [Figure 4] FIG. 2 is a schematic cross-sectional view for explaining the structure of a combustion chamber. [Figure 5] FIG. 2 is a plan view showing the crown surface of the piston. [Figure 6] FIG. 4 is an enlarged cross-sectional view showing an intake-side recess and an exhaust-side recess. [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. 6 is a schematic cross-sectional view showing a combustion chamber of an engine according to a second embodiment of the present invention. 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 X 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 on which the intake valve 12 is provided will be referred to as the intake side, and the side on which 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-rear direction. In addition, the central axis X of cylinder 2 will be referred to as the cylinder axis X.

[0024] Above the piston 5, a combustion chamber 6 is defined by the peripheral surface 20 of the cylinder 2, the crown surface 30 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.

[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 intake ports 10 for introducing air into the combustion chamber 6 and exhaust ports 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 at the downstream ends of the intake ports 10 and two exhaust-side openings 42 at the downstream ends of the exhaust ports 11.

[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 electrode portion 81 of the spark plug 8, and extends along the front-rear direction. FIG. 5 is a schematic plan view of the crown surface 50 of the piston 5, which defines the bottom surface of the combustion chamber 6.

[0031] The ceiling surface 40 of the combustion chamber 6 is formed by a part of the lower surface of the cylinder head 4. The ceiling surface 40 of the combustion chamber 6 is a pent roof type ceiling surface. That is, in a cross-sectional view along the intake / exhaust direction, the ceiling surface 40 of the combustion chamber 6 has an intake-side inclined surface 43 whose height decreases with increasing distance from the cylinder axis X to the intake side, and an exhaust-side inclined surface 44 whose height decreases with increasing distance from the cylinder axis X to the exhaust side.

[0032] The two intake side openings 41 are formed in the intake side inclined surface 43. The two intake side openings 41 are aligned in the front-rear direction. The two exhaust side openings 42 are formed in the exhaust side inclined surface 44. The two exhaust side openings 42 are aligned in the front-rear direction. 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.

[0033] An upwardly recessed recess 45 is formed in the combustion chamber ceiling surface 40. The recess 45 is formed in the center of the combustion chamber ceiling surface 40, in an area surrounded by four valves 12, 13 (two intake valves 12 and two exhaust valves 13). In the first embodiment, the recess 45 is formed to have a substantially elliptical shape extending in the intake / exhaust direction in a plan view, and the dimension of the recess 45 in the intake / exhaust direction is longer than the dimension in the front-to-rear direction. A bottom surface 45A of the recess 45 is curved with its apex located near the center. In the first embodiment, the curvature of the bottom surface 45A of the recess 45 is substantially constant over its entirety.

[0034] The spark plug 8 is attached to the cylinder head 4 so that its electrode portion 81 faces the combustion chamber 6 from the bottom surface of the recessed portion 45. The spark plug 8 is provided near the top of the bottom surface 45A of the recessed portion 45. That is, in the intake / exhaust direction, the electrode portion 81 of the spark plug 8 is located near the center of the recessed portion 45, and the recessed portion 45 includes an intake-side recess 46 located on the intake side of the electrode portion 81 of the spark plug 8, and an exhaust-side recess 47 located on the exhaust side.

[0035] The recess 45 is formed so that the volume of the exhaust-side recess 47 is larger than the volume of the intake-side recess 46. Here, the volume of each recess 46, 47 is the volume of the space defined by a plane Q passing through the periphery of the recess 45 and the bottom surface of each recess 46, 47, as shown in FIG.

[0036] To achieve the above volume relationship, the dimension L1 in the intake / exhaust direction of the exhaust-side recess 47 is set to be larger than the dimension L2 in the intake / exhaust direction of the intake-side recess 46. Also, the dimension of the recess 45 in the front-to-rear direction is set to increase overall toward the exhaust side.

[0037] FIG. 6 is an enlarged cross-sectional view of the intake-side recess 46 and the exhaust-side recess 47. Note that in FIG. 6, the differences between the recesses 46, 47 are exaggerated to clarify the differences. As shown in FIG. 6, in this embodiment, the height positions of the exhaust-side end portions 47P of the recess 45 and the exhaust-side recess 47 are lower than the height positions of the intake-side end portions 46P of the recess 45 and the intake-side recess 46. Furthermore, the recess 45 is formed such that, when viewed in the front-rear direction, a tangent 47Q of the exhaust-side recess 47 passing through the exhaust-side end portion 47P of the exhaust-side recess 47 is closer to vertical than a tangent 46Q of the intake-side recess 46 passing through the intake-side end portion 46P of the intake-side recess 46. Specifically, the angle (the angle smaller than 90 degrees) between the tangent 47Q of the exhaust side recess 47, which passes through the exhaust side end 47P of the exhaust side recess 47, and the straight line extending in the vertical direction, the tangent 46Q of the intake side recess 46, which passes through the intake side end 46P of the intake side recess 46, is smaller.

[0038] 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 X (toward the radially inner side), and the exhaust-side piston inclined surface 52 is formed so that its height increases toward the cylinder axis X (toward the radially inner side).

[0039] A downwardly recessed cavity 53 is formed in the center of the crown surface 50 of the piston 5. In the first embodiment, as shown in Fig. 5, the cavity 53 is formed to have a generally elliptical shape extending in the front-to-rear direction in a plan view, and the front-to-rear dimension WcFR of the cavity 53 is longer than the dimension Wc in the intake / exhaust direction.

[0040] The cavity 53 is formed so that its lowest end is at a position P that corresponds vertically to the electrode portion 81 of the spark plug 8. In other words, the cavity 53 is formed so that its lowest point is below the electrode portion 81 of the spark plug 8. In the first embodiment, the cavity 53 is recessed in a bowl shape and curves upward from a position below the electrode portion 81 toward the outer periphery.

[0041] The dashed lines in FIG. 5 indicate the recess 45 and the spark plug 8. As shown in FIG. 5 and other figures, the recess 45 and the cavity 53 face each other in the up-down direction. That is, the recess 45 is formed at a position facing the cavity 53. When viewed in the up-down direction, the recess 45 is formed so that its entirety is located inside the cavity 53. Specifically, when viewed in the up-down direction, the recess 45 is located near the center of the cavity 53 in the front-to-rear direction. When viewed in the up-down direction, the recess 45 is formed so that it extends in the intake and exhaust directions from a position slightly closer to the intake side than the exhaust-side edge of the cavity 53 to a position near the center between the intake-side edge of the cavity 53 and the electrode portion 81. The dimension Wp of the recess 45 in the intake and exhaust direction is set to a value smaller than the dimension Wc of the cavity 53 in the intake and exhaust direction. Therefore, in this embodiment, the relationship between the dimension Wp of the recess 45 in the intake / exhaust direction, the dimension Wc of the cavity 53 in the intake / exhaust direction, and the dimension WcFR of the cavity 53 in the front-rear direction is Wp <Wc<WcFRとなる。

[0042] (3) Effects, etc. With the above-described configuration, the engine according to the first embodiment can form a tumble flow in the combustion chamber 6 near the compression top dead center and position the center of the tumble flow near the electrode portion 81, thereby improving thermal efficiency and suppressing combustion fluctuations. Details will be explained using Figs. 7 to 12.

[0043] 7 and 8 are diagrams showing a simulation of intake air 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 air flow during the intake stroke and compression stroke of an engine without a cavity in the piston and without a recess in the ceiling of the combustion chamber. The left-hand diagrams of 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 of 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 of FIGS. 7 and 8 that 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 of FIGS. 7 and 8 that 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. Figures 7 and 8(a) are figures near the start of opening of the intake valve 12, Figures 7(b) and 8(b) are figures near the intake bottom dead center, Figures 7(c) and 8(c) are figures near 60° CA before compression top dead center, and Figures 7(d) and 8(d) are figures near 10° CA before compression top dead center.

[0044] 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.

[0045] 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.

[0046] 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 swirling 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, as shown by arrow Y4 in the left and right diagrams of Figure 8(d), tumble flow is formed in the combustion chamber 6 almost uniformly in the front-to-rear direction even near the compression top dead center. 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.

[0047] 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.

[0048] 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 Figure 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 Figure 8(d). As a result, in pattern B, an initial flame M11 flows from the exhaust side to the intake side, as shown in Figure 12. As a result, 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 Figure 8(c) represent the centers of the vertical swirl flows.

[0049] As described above, in an engine that does not have a cavity or recess, the flow state of intake air in the combustion chamber 6 near the top dead center of compression varies significantly depending on the state of intake air flowing from the intake port into the combustion chamber 6. If a tumble flow is not properly formed in the combustion chamber 6 near the top dead center of compression and a flow toward the intake side or exhaust side is formed in the combustion chamber 6 as in pattern B, the thermal efficiency decreases due to contact between the initial flame and the wall surface of the combustion chamber 6.

[0050] FIG. 13 is a diagram showing the intake air flow in a bore center cross section of the engine 1 according to the first embodiment. Unlike the engines shown in FIGS. 7 and 8, the engine 1 according to the first embodiment has a recess 45 formed around the electrode 81 of the spark plug 8 in the center of the combustion chamber ceiling 40, and a cavity 53 formed in the piston crown 50 at a position opposite the recess 45. Therefore, as indicated by arrow Y31 in FIG. 13, the intake air can be caused to vertically swirl around the electrode 81 of the spark plug 8 along the bottom surfaces of the cavity 53 and the recess 45 near the top dead center of the compression stroke, thereby achieving the intake air flow pattern A. In other words, regardless of the flow of intake air from the intake port into the combustion chamber 6, a tumble flow can be formed (maintained) around the electrode 81 of the spark plug 8 in the combustion chamber 6 near the top dead center of the compression stroke.

[0051] Moreover, the volume of the exhaust-side recess 47 is made larger than the volume of the intake-side recess 46. Therefore, even if intake air flows into the combustion chamber 6 from the intake port 10 in a flow pattern such as pattern B, a tumble like pattern A is formed, and the center of the tumble flow can be reliably brought close to the electrode portion 81 of the spark plug 8 inside the combustion chamber 6. Therefore, it is possible to prevent the initial flame from coming into contact with the wall surface of the combustion chamber 6 by being carried by the flow toward the intake side or exhaust side, thereby improving thermal efficiency.

[0052] A specific description will be given using FIG. 13 . Hereinafter, the area between the exhaust-side inclined surface 44, which is located radially outward of the recess 45, and the opposing exhaust-side piston inclined surface 52 will be referred to as the exhaust-side squish area 201. Furthermore, the area between the intake-side inclined surface 43, which is located radially outward of the recess 45, and the opposing intake-side piston inclined surface 51 will be referred to as the intake-side squish area 202. As indicated by arrow Y131 in FIG. 13 , in the combustion chamber 6 where a tumble flow is formed, the intake air flows from the intake side to the exhaust side along the combustion chamber ceiling surface 40. Furthermore, as indicated by arrow Y132, the intake air flows from the exhaust side to the intake side along the piston crown surface 50, and then flows upward. Thus, simply forming a tumble flow in the combustion chamber 6 makes it easier for the intake air to flow into the exhaust-side squish area 201, but makes it more difficult for the intake air to flow into the intake-side squish area 202. As a result, the flow from the intake side to the exhaust side along the combustion chamber ceiling surface 40 becomes stronger, and there is a risk that the center of the tumble flow will shift from the electrode portion 81 to the exhaust side.

[0053] In contrast, in the engine 1 according to the first embodiment, the volume of the exhaust-side recess 47 is larger than the volume of the intake-side recess 46. This increases the amount of intake air flowing downward along the exhaust-side recess 47, while reducing the amount of intake air flowing into the intake-side recess 46, i.e., the amount of intake air flowing upward. This reduces the amount of intake air flowing toward the exhaust-side squish area 201, as indicated by arrow Y32, and increases the amount of intake air flowing toward the intake-side squish area 202, as indicated by arrow Y33. Therefore, the engine 1 according to the first embodiment can weaken the flow from the intake side to the exhaust side along the combustion chamber ceiling surface 40, preventing the center of the tumble flow from shifting from the electrode portion 81 to the exhaust side. This reduces the distance between the center of the tumble flow and the electrode portion 81, preventing contact between the initial flame and the exhaust-side wall surface of the combustion chamber 6, and improving thermal efficiency.

[0054] Furthermore, if the amount of intake air flowing into the intake-side squish area 202 increases, the flow toward the intake-side squish area 202 during the expansion stroke, i.e., the flow toward the intake side, also becomes stronger. This also prevents the flame from flowing toward the exhaust side and coming into contact with the exhaust-side wall surface of the combustion chamber 6, thereby improving thermal efficiency.

[0055] In particular, in the above embodiment, the recess 45 is formed so that the tangent 47Q of the exhaust-side recess 47 passing through the exhaust-side end 47P of the exhaust-side recess 47 is closer to vertical than the tangent 46Q of the intake-side recess 46 passing through the intake-side end 46P of the intake-side recess 46. This allows the intake air along the exhaust-side recess 47 to be directed more downward, while reducing the amount of intake air toward the intake-side recess 46. Furthermore, in the above embodiment, the height position of the exhaust-side end 47P of the exhaust-side recess 47 is lower than the height position of the intake-side end 46P of the intake-side recess 46. This allows the amount of intake air flowing downward from the exhaust-side recess 47 to be increased, while reducing the amount of intake air flowing toward the intake-side recess 46. This more reliably prevents the center of the tumble flow from shifting toward the exhaust side from the electrode portion 81.

[0056] Here, the flow velocity near the center of the tumble flow is relatively slow. Therefore, if the center of the tumble flow can be brought closer to the electrode part 81, the thermal efficiency can be increased as described above, and the influence of the intake air flow on the formation of the initial flame can be minimized, thereby suppressing combustion fluctuations.

[0057] Furthermore, in the engine 1 according to the above embodiment, the cavity 53 is formed so that its lowest end is located at a position that corresponds vertically to the electrode portion 81 of the spark plug 8. This makes it possible to minimize the deviation between the center of the tumble flow along the cavity 53 and the electrode portion 81 in a direction perpendicular to the vertical direction. In other words, it is possible to more reliably bring the center of the tumble flow and the electrode portion 81 closer to each other.

[0058] Furthermore, in the engine 1 according to the above embodiment, the entire recess 45 is located inside the cavity 53 when viewed in the up-down direction. This ensures that intake air flowing downward along the bottom surface of the exhaust-side recess 47 can be reliably introduced into the cavity 53, thereby reducing the amount of intake air flowing into the exhaust-side squish area 201. Furthermore, intake air flowing toward the intake-side recess 46 near the intake-side end of the piston crown surface 50 can be introduced into the intake-side squish area 202 by colliding with the portion of the intake-side inclined surface 43 that is on the outer periphery of the intake-side recess. This further weakens the flow from the intake side to the exhaust side along the combustion chamber ceiling surface 40, thereby reliably preventing the center of the tumble flow from shifting away from the electrode portion 81 toward the exhaust side.

[0059] (4) Variations In the above embodiment, the spark plug 8 is provided near the center of the combustion chamber 6, but the position of the spark plug 8 is not limited to this. For example, as shown in Fig. 14, the spark plug 8 may be located closer to the exhaust side than the center of the combustion chamber 6.

[0060] Figure 14 is a view corresponding to Figure 13 and is a cross-sectional view showing the combustion chamber 66 of the engine 101 according to the second embodiment of the present invention, in which the ignition plug 8 is located closer to the exhaust side than the center of the combustion chamber 6.

[0061] As in the first embodiment, in the second embodiment, a recess 45 is formed in the ceiling surface 40 of the combustion chamber 6. The recess 45 includes an intake-side recess 46 located on the intake side of the electrode portion 81 of the spark plug 8 and an exhaust-side recess 47 located on the exhaust side. The recess 45 is formed so that the volume of the exhaust-side recess 47 is larger than the volume of the intake-side recess 46. A cavity 53 is formed in the crown surface 50 of the piston 5 at a position opposite the recess 45. The cavity 53 is formed so that its lowest end is located at a position corresponding to the electrode portion 81 of the spark plug 8 in the vertical direction. However, in the second embodiment, since the electrode portion 81 is located on the exhaust side of the center of the combustion chamber ceiling surface 40, i.e., the cylinder axis X, the bottom surface of the cavity 53 on the exhaust side is formed approximately horizontal, and the bottom surface of the cavity 53 on the intake side is curved so that it is positioned upward as it approaches the intake side.

[0062] In the second embodiment, the recess 45 and the cavity 53 are configured as described above, so that a tumble flow can be formed near the top dead center of compression, and the center of the tumble flow can be positioned near the electrode portion 81 of the spark plug 8, thereby improving thermal efficiency and suppressing combustion fluctuations. According to the second embodiment, the spark plug 8 is positioned closer to the exhaust side than the center X of the combustion chamber 6, thereby ensuring space for installing a fuel injection valve or the like on the intake side of the spark plug 8. Positioning the spark plug 8 on the exhaust side could increase cooling loss due to contact between the exhaust-side wall surface of the combustion chamber 6 and the initial flame. However, the large volume of the exhaust-side recess 57 increases the distance between the bottom surface of the exhaust-side recess 57, i.e., the exhaust-side wall surface of the combustion chamber 6, and the electrode portion 81. Therefore, contact between the exhaust-side wall surface of the combustion chamber 6 and the initial flame can be suppressed, suppressing an increase in cooling loss and achieving high thermal efficiency.

[0063] In the first embodiment, the recess 45 has a generally elliptical shape extending in the intake / exhaust direction in a plan view, but the specific shape of the recess 45 is not limited to this. For example, the recess 45 may be formed to have a spherical crown shape.

[0064] Furthermore, in the above embodiment, the curvature of the bottom surface of the recess 45 is substantially constant, but the curvature of the bottom surface of the recess 45 does not have to be constant.

[0065] Furthermore, the specific shape of cavity 53 is not limited to that described above. Furthermore, fuel injection valve 7 is not limited to a side injection type. Furthermore, the engine may be a multi-cylinder engine. [Explanation of symbols]

[0066] 1 engine 5 pistons 6 Combustion chamber 8 Spark plugs 40 (Combustion chamber) ceiling surface 45 recess 46 Intake side recess 47 Exhaust side recess 50 (piston) crown surface 53 Cavity 81 (Spark plug) electrode

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, an upwardly recessed portion is formed in a portion of a ceiling surface of the combustion chamber facing the cavity, The spark plug is disposed so that an electrode portion provided at a tip thereof faces the combustion chamber from the bottom surface of the recess, When the side of the combustion chamber where the intake port is disposed is defined as the intake side and the side of the combustion chamber where the exhaust port is disposed is defined as the exhaust side, the recessed portion includes an intake-side recessed portion located on the intake side with respect to the spark plug and an exhaust-side recessed portion located on the exhaust side, A combustion chamber structure for an engine, characterized in that the volume of the exhaust-side recess is larger than the volume of the intake-side recess.

2. The combustion chamber structure of the engine according to claim 1, A combustion chamber structure for an engine, characterized in that the recess is formed in a shape such that, when viewed along the intake and exhaust direction, a tangent line passing through the exhaust side end is closer to vertical than a tangent line passing through the intake side end.

3. The combustion chamber structure of the engine according to claim 1, A combustion chamber structure for an engine, characterized in that the height position of the exhaust side end of the recess is lower than the height position of the intake side end of the recess.

4. The combustion chamber structure of the engine according to claim 1, 1. A combustion chamber structure for an engine, wherein the cavity has a shape in which its lowest end is located at a position corresponding to the electrode portion of the spark plug in the vertical direction.

5. The combustion chamber structure of the engine according to claim 4, 1. A combustion chamber structure for an engine, wherein the spark plug is located on the exhaust side of the center of the ceiling surface of the combustion chamber.

6. The combustion chamber structure of the engine according to any one of claims 1 to 5, A combustion chamber structure for an engine, characterized in that, when viewed in the up-down direction, the recess is entirely located inside the cavity.

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