JP4428325B2 - Combustion chamber structure of spark ignition engine - Google Patents

Description

  The present invention relates to a structure of a combustion chamber of a spark ignition engine, and more particularly to a structure of a combustion chamber formed between a cylinder head lower surface and a piston top surface and having the cylinder head lower surface as a ceiling wall. .

  In recent years, not only economic aspects but also environmental aspects of preventing global warming, the demand for improving fuel efficiency of engines has been increasing. In order to improve fuel efficiency in a spark ignition type engine, it is only necessary to increase the combustion efficiency, and an effective means is to increase the compression ratio.

  In order to increase the compression ratio, the combustion chamber volume may be reduced with respect to the cylinder volume. As such a combustion chamber structure suitable for increasing the compression ratio, for example, a pent roof type combustion chamber structure is often used. This combustion chamber structure is formed so that the ceiling wall on the intake side and the ceiling wall on the exhaust side form a roof shape, and the combustion chamber volume is reduced while ensuring a relatively large intake and exhaust valve diameter. There is a feature that can be. Also swirl (swirl flow around the piston slide axis, transverse vortex), tumble (swirl flow in a plane parallel to the piston slide axis, vertical vortex), or squish (from the piston bore peripheral edge to the center when the piston is raised) This structure is also advantageous for generating in-cylinder flow such as a flow that extrudes into a cylinder).

  For example, Patent Documents 1 to 3 show the structure of a combustion chamber in which various in-cylinder flows are generated to improve combustion efficiency. From the illustrated cross-sectional structure, all are pent roof type combustion chambers. It is understood that there is.

  FIG. 9 shows a cross-sectional structure of a conventional general pent roof type combustion chamber. FIG. 9 shows a state where the piston 93 is at the top dead center. The combustion chamber 94 is a space surrounded by the cylinder bore 12 of the cylinder block 50, the piston top surface 97, and the ceiling wall 91 that is the lower surface of the cylinder head 10 facing the combustion chamber 94. The ceiling wall 91 is formed such that the intake side ceiling wall 91a and the exhaust side ceiling wall 91b form a roof shape.

  Near the center of the cylinder bore 12 in the radial direction, there is provided a spark plug 15 having a tip erected from the ceiling wall 91 to the combustion chamber 94.

  The intake-side ceiling wall 91a is provided with two intake ports 21 that open to the intake-side ceiling wall 91a. Each intake port 21 is provided with an intake valve 19 that opens at a predetermined intake timing. The exhaust-side ceiling wall 91b is provided with two exhaust ports 22 that open to the exhaust-side ceiling wall 91b. Each exhaust port 22 is provided with an exhaust valve 20 that opens at a predetermined exhaust timing. The surfaces of the intake valve 19 and the exhaust valve 20 facing the combustion chamber 94 form part of the intake side ceiling wall 91a and the exhaust side ceiling wall 91b, respectively.

  The intake valve 19, the exhaust valve 20, the intake port 21, and the exhaust port 22 are actually offset from the cross-sectional position shown in the drawing in the front-rear direction of the drawing. Show.

The peripheral edge portion 91d of the ceiling wall 91 is substantially flush with a mating surface with the cylinder block 50 (specifically, a mating surface with a head gasket (not shown) provided between the cylinder head 10 and the cylinder block 50). ing. This ceiling wall peripheral portion 91d is generally called a squish area.
Japanese Patent Laid-Open No. 08-254126 Japanese Patent Application Laid-Open No. 08-049546 JP 2003-184559 A

  However, even if a high compression ratio is realized by devising the structure of the combustion chamber as described above, it is not always possible to cause practically effective combustion immediately. As is well known, when the compression ratio is increased, abnormal combustion such as knocking (hereinafter referred to as knocking or the like) easily occurs. That is, in practice, the compression ratio can be increased only in a range where knocking or the like does not occur.

  However, it also means that if the occurrence of knocking or the like can be suppressed, that is, if the anti-knocking performance can be improved, the compression ratio can be further increased.

  In view of the above circumstances, an object of the present invention is to provide a combustion chamber structure of a spark ignition engine that can effectively increase the compression ratio practically by improving the anti-knock performance.

  The inventor of the present application performs in-cylinder pressure and temperature by burning at a low speed in the first main combustion period (a period in which 10% to less than 50% of the main combustion period in which 10% to 90% of the combustion mass burns) Is suppressed, and pre-ignition of unburned fuel is effectively suppressed, so that high anti-knocking performance can be obtained, and the late main combustion period (90% or more of the combustion mass in the main combustion period is 90% In the period when less than% is burned), by burning the unburned fuel at high speed and completing the combustion quickly, self-ignition with the unburned residue as the core can be suppressed, and the anti-knocking performance can also be improved, Focusing on the combustion mode (hereinafter referred to as late-stage center-of-gravity combustion) that can effectively suppress knocking without substantially extending the main combustion period of the entire combustion, repeated earnest research Found combustion chamber structure following a spark-ignition engine such late centroid combustion can be easily performed.

That is, the invention according to claim 1 is formed between a cylinder head lower surface and a piston top surface, and has a combustion chamber having the cylinder head lower surface as a ceiling wall, and an ignition having a tip erected from the ceiling wall into the combustion chamber. A combustion chamber structure of a spark ignition type engine including a plug, wherein a main portion of the combustion chamber space is a first combustion space around the spark plug and a second cylinder bore peripheral portion with the piston at top dead center. formed by the combustion space, via a small gap which gap is narrowed between the piston top surface and the ceiling wall and the said first combustion space and said second combustion space are communicated, the piston crown A convex portion projecting toward the ceiling wall and a concave portion recessed relative to the convex portion are formed in the portion, and the small gap portion includes the top surface of the convex portion and the ceiling wall. And the first The firing space and the second combustion space are formed between the recess and the ceiling wall, the spark plug is provided near the center in the radial direction of the cylinder bore, and the convex portion is substantially concentric with the periphery of the piston. The protruding amount of the convex portion is relatively larger on the exhaust side than on the intake side .

  The invention according to claim 2 is the combustion chamber structure of the spark ignition type engine according to claim 1, wherein the small gap portion is in a radial direction of the cylinder bore from an intermediate point between the ignition plug and the peripheral edge of the cylinder bore. It is formed near the cylinder bore periphery.

  The invention according to claim 3 is the combustion chamber structure of the spark ignition type engine according to claim 1 or 2, wherein the spark plug is provided near the radial center of the cylinder bore, and the small gap portion is the spark plug. And the cylinder bore periphery, and the second combustion space is formed annularly on the outer peripheral side of the small gap portion.

  According to a fourth aspect of the present invention, in the combustion chamber structure of the spark ignition type engine according to any one of the first to third aspects, the spark plug is provided near the radial center of the cylinder bore, and the small gap Among the portions, the smallest gap portion having the smallest gap is formed in the middle from the spark plug to at least the cylinder bore peripheral portion on the exhaust side.

The invention according to claim 5 is the combustion chamber structure of the spark ignition type engine according to any one of claims 1 to 4 , wherein the combustion chamber has a roof shape with an intake side ceiling wall and an exhaust side ceiling wall. A gap between the ridge line portion of the pent roof and the piston top surface is larger than a gap between the surrounding ceiling wall and the piston top surface, and the ridge line of the second combustion space and the pent roof. A second spark plug having a tip erected at the portion is additionally provided.

The invention according to claim 6 is the combustion chamber structure of the spark ignition engine according to any one of claims 1 to 5, wherein the combustion chamber is a pent roof type in which the intake side ceiling wall and the exhaust side ceiling wall form a roof shape. The ceiling wall at the peripheral edge of the cylinder bore is formed to be offset from the mating surface of the cylinder head with the cylinder block toward the side away from the cylinder block.

  According to the first aspect of the present invention, as described below, the above-mentioned late center-of-gravity combustion can be easily performed. As described above, the knocking resistance can be improved by the late center of gravity combustion, and the compression ratio can be effectively increased practically. In addition, the fuel efficiency can be improved by using it.

  According to the configuration of the present invention, combustion is performed mainly in the first combustion space close to the spark plug during the first main combustion period, and combustion is performed mainly in the second combustion space at the peripheral edge of the cylinder bore during the second main combustion period. In general, combustion proceeds by flame propagation, and the flame surface (the front line of flame propagation) is substantially concentric, centering on the flame core formed near the electrode of the spark plug, pushing out unburned gas. It expands into a spherical shape.

  However, in the configuration of the present invention, a small gap portion in which the gap between the piston top surface and the combustion chamber ceiling wall is narrowed is provided between the first combustion space and the second combustion space. When the unburned gas pushed out to the flame surface passes through this small gap portion, it undergoes a kind of squeezing action. Under the influence, the flame propagation of combustion in the first combustion space is suppressed. For this reason, the combustion rate in the first main combustion period can be kept relatively low.

  When the flame surface reaches the second combustion space via the small gap portion, the flame propagation proceeds promptly because it is no longer affected by the throttling action by the small gap portion. That is, the combustion speed in the late main combustion period becomes relatively high.

  Eventually, the above-mentioned late center-of-gravity combustion is performed, in which relatively low-speed combustion is performed in the first main combustion period and relatively high-speed combustion is performed in the second main combustion period.

Further, according to the present invention, the first combustion space, the small gap portion, and the second combustion space can be formed with a simple structure in which the piston crown is provided with irregularities.

Furthermore, according to the present invention, as described below, a more uniform flame propagation speed can be obtained, and smooth combustion can be achieved.

As described above, the flame surface spreads in a substantially concentric sphere centered on the flame core formed near the electrode of the spark plug, but more strictly speaking, the propagation speed to the exhaust side (exhaust valve side) is the intake air. It is slightly higher than the propagation speed to the side (intake valve side). This is because the combustion reaction is further promoted on the high temperature exhaust side.

According to the configuration of the present invention, an annular convex portion is provided in the middle from the spark plug to the cylinder bore peripheral portion, and the protruding amount of the convex portion is set to be larger on the exhaust side than on the intake side. Therefore, the flame propagation to the exhaust side is particularly suppressed.

And since the flame propagation speed to the exhaust side, which tends to be high, is relatively strongly suppressed, a more uniform flame propagation speed can be obtained as a whole, and smooth combustion can be achieved.

  The inventor of the present application has confirmed that the compression ratio can be increased by 0.5 or more compared with the conventional one without deteriorating the knocking resistance performance by the combustion chamber structure of the present invention.

  According to the invention of claim 2, as described below, the initial combustion period (the period before 10% of the combustion mass burns before the main combustion period) is completed early, and the transition to the main combustion period is delayed. It can be effectively prevented.

  As described above, combustion is performed mainly in the first combustion space in the first main combustion period. Therefore, combustion is performed mainly in the first combustion space during the initial stage of the previous stage. Here, if the distance between the spark plug and the small gap portion is too short, the combustion in the initial combustion period is strongly influenced by the throttling action by the small gap portion, and the combustion speed is reduced. If the combustion speed in the initial combustion period decreases, the transition to the main combustion period is delayed, leading to a delay in the entire combustion, which is not preferable.

  Therefore, according to the configuration of the present invention, the small gap portion is formed closer to the cylinder bore periphery than the midpoint between the spark plug and the cylinder bore periphery in the radial direction of the cylinder bore. That is, since the spark plug and the small gap portion are appropriately separated from each other, it is possible to hardly affect the combustion in the initial combustion period by the throttling effect by the small gap portion.

  However, if the small gap portion is brought too close to the periphery of the cylinder bore, it is difficult to ensure a sufficient second combustion space. Therefore, it is preferable that the small gap portion is formed at an appropriate position that is closer to the cylinder bore periphery than the intermediate point between the spark plug and the cylinder bore periphery and that is spaced apart from the cylinder bore periphery to some extent. The appropriate position varies depending on engine characteristics and the like, but is generally in the range of 60 to 85% of the distance from the spark plug to the cylinder bore periphery.

  According to the invention of claim 3, as described below, more remarkable late center of gravity combustion can be realized, and the effect of improving anti-knocking performance can be further enhanced.

  According to the configuration of the present invention, the small gap portion is annularly formed between the spark plug and the cylinder bore periphery. That is, the small gap portion is provided so as to surround the first combustion space. Therefore, the narrowing action of the small gap portion can be more evenly applied to the combustion in the first combustion space, and the combustion speed reduction effect in the main combustion period can be further enhanced.

  According to the configuration of the present invention, since the second combustion space is formed in an annular shape on the outer peripheral side of the small gap portion, the flame surface reaches the second combustion space more evenly. For this reason, combustion in the late main combustion period can be performed more rapidly.

  Eventually, a sharp late-stage center-of-gravity combustion can be realized in which lower-speed combustion is performed in the first-stage main combustion period and higher-speed combustion is performed in the second-stage main combustion period.

According to the invention of claim 4, the strong throttling action by the minimum gap portion formed on the exhaust side particularly suppresses the flame propagation to the exhaust side, and a more uniform flame propagation speed can be obtained as a whole. Combustion can be achieved.

If placed subordinate to or claim 3, the transition from the first combustion space of the flame front into the second combustion space, since it is possible to more uniformly performed can be further enhanced antiknock performance improvement .

According to the fifth aspect of the present invention, the compression ratio can be easily increased by utilizing the characteristic of the pent roof type combustion chamber that the combustion chamber volume can be reduced while ensuring a relatively large intake / exhaust valve diameter. be able to.

  Further, the pent roof type combustion chamber has a feature that a gap between the ceiling ridge portion and the piston top is relatively wide. In the configuration of the present invention, since the second spark plug is provided at a location where the gap is wide, the layout can be easily performed. Even in the case where there is a concern about the delay of flame propagation to the second combustion space, the second spark plug can cause timely combustion in the second combustion space.

According to the invention of claim 6 , the second combustion space can be made wider. In the conventional pent roof type combustion chamber structure, the cylinder head (periphery ceiling portion of the combustion chamber) corresponding to the cylinder bore peripheral portion is substantially flush with the mating surface with the cylinder block (the ceiling wall peripheral portion 91d in FIG. 9). reference). On the other hand, in the configuration of the present invention, the ceiling wall of the combustion chamber is offset and formed on the side away from the cylinder block, so that the second combustion space can be widened accordingly.

  FIG. 1 is a longitudinal sectional view showing a combustion chamber structure of a spark ignition engine according to a first embodiment of the present invention. FIG. 2 is an enlarged view of the main part. 3 is a cross-sectional view taken along line III-III in FIG.

  The combustion chamber 14 of this embodiment is a pent roof type, and FIGS. 1 to 3 show a state in which the piston 13 is at the top dead center. The combustion chamber 14 is a space surrounded by the cylinder bore 12 of the cylinder block 50, the piston top surface 4, and the ceiling wall 11 that is the lower surface of the cylinder head 10 facing the combustion chamber 14. The ceiling wall 11 is formed such that the intake-side ceiling wall 11a and the exhaust-side ceiling wall 11b form a roof shape.

  Near the center of the cylinder bore 12 in the radial direction, there is provided a spark plug 15 having a tip erected from the ceiling wall 11 to the combustion chamber 14.

  The intake-side ceiling wall 11a is provided with two intake ports 21 that open to the intake-side ceiling wall 11a. Each intake port 21 is provided with an intake valve 19 that opens at a predetermined intake timing. The exhaust-side ceiling wall 11b is provided with two exhaust ports 22 that open to the exhaust-side ceiling wall 11b. Each exhaust port 22 is provided with an exhaust valve 20 that opens at a predetermined exhaust timing. The surfaces of the intake valve 19 and the exhaust valve 20 facing the combustion chamber 14 form part of the intake-side ceiling wall 11a and the exhaust-side ceiling wall 11b, respectively.

  The intake valve 19, exhaust valve 20, intake port 21, and exhaust port 22 shown in FIGS. 1 and 2 are actually offset from the cross-sectional position shown in the drawing in the front-rear direction of the drawing. In FIG. 3, the same cross section is shown (see FIG. 3).

  As shown in FIG. 2, the ceiling wall peripheral part 11 d which is the peripheral part of the ceiling wall 11 is a mating surface with the cylinder block 50 (specifically, an unillustrated illustration provided between the cylinder head 10 and the cylinder block 50). It is offset from the mating surface with the head gasket) on the side away from the cylinder block 50.

  The combustion chamber 14 is formed by the first combustion space 14a around the spark plug 15 and the second combustion space 14b around the cylinder bore 12 with the piston 13 at the top dead center. ing. The first combustion space 14a and the second combustion space 14b are communicated with each other via a small gap portion 5 in which a gap between the piston top surface 4 and the ceiling wall 11 is narrowed.

  Here, the shape of the piston 13, particularly the shape of the crown portion will be described. FIG. 4 is a perspective view of the piston 13. In the following description, the vertical direction of the piston 13 is the vertical direction in the illustrated state. That is, the side closer to the ceiling wall 11 in the assembled state is the upper side.

  The piston crown portion 13 a is provided with a convex portion 6 that protrudes upward in an annular shape that is substantially concentric with the outer periphery of the piston 13. And the recessed part which was relatively immersed with respect to the convex part 6 is formed in the inner peripheral side and outer peripheral side of the convex part 6. FIG. That is, a central recess 7 is formed on the inner peripheral side of the convex portion 6, and a peripheral recess 8 is formed on the outer peripheral side.

  The detailed shape of the convex portion 6 is such that the intake side and the exhaust side of the annular body projecting upward with a predetermined height and width are scraped off by a flat slope from the inner peripheral side upper side to the outer peripheral side lower side, respectively. It has a shape. Each surface corresponding to the cut-off face forms an intake side convex portion top surface 9a and an exhaust side convex portion top surface 9b.

  In FIG. 4, the cross section of the convex part 6 in the cross-sectional position shown in FIG. 2 is shown by the intake side convex part cross section 6a and the exhaust side convex part cross section 6b, respectively. As is apparent from the comparison, the convex portion 6 has a shape such that the intake side is cut off more than the exhaust side. Therefore, when compared at a position separated from the center of the piston crown 13a by the same distance in the left-right direction in FIG. 2, the height of the exhaust-side convex top surface 9b is higher than the height of the intake-side convex top surface 9a. .

  The portion other than the intake side convex portion top surface 9a or the exhaust side convex portion top surface 9b of the convex portion top surface 9 which is the upper end surface of the convex portion 6 is a substantially flat and flat convex top surface 9c. . The average radius of the convex flat top surface 9 c is slightly larger than half of the average radius of the piston 13.

  The center-side concave portion 7 is a portion that is relatively immersed with respect to the convex portion 6 on the inner peripheral side of the convex portion 6. The central concave portion 7 has a bowl-shaped wall surface that is gently curved to the central flat portion.

  The peripheral-side concave portion 8 is a portion that is relatively immersed with respect to the convex portion 6 on the outer peripheral side of the convex portion 6. The peripheral side concave portion 8 has a substantially horizontal annular shape.

  Next, the detailed structure of the combustion chamber 14 will be described again with reference to FIG. The first combustion space 14 a is formed between the central recess 7 of the piston 13 and the ceiling wall 11. The second combustion space 14b is formed in an annular shape between the peripheral recess 8 of the piston 13 and the ceiling wall 11 (specifically, the ceiling wall peripheral part 11d).

  The small gap portion 5 that communicates the first combustion space 14 a and the second combustion space 14 b is formed in an annular shape between the convex top surface 9 of the piston 13 and the ceiling wall 11. As described above, since the average radius of the convex top surface 9c is slightly larger than half of the average radius of the piston 13, the small gap portion 5 is intermediate between the spark plug 15 and the cylinder bore periphery in the radial direction of the cylinder bore 12. It is formed closer to the periphery of the cylinder bore than the point. The optimum position varies depending on engine characteristics and the like, but is generally in the range of 60 to 85% of the distance from the spark plug 15 to the cylinder bore periphery.

  Specifically, the small gap portion 5 includes a small gap portion 5a, a minimum gap portion 5b, a small gap portion 5c, and a small gap portion 5d (see FIG. 3).

  The small gap portion 5a is a gap between the intake side convex portion top surface 9a of the piston 13 and the ceiling wall 11 facing the same. The minimum gap portion 5b is a gap between the exhaust-side convex top surface 9b of the piston 13 and the ceiling wall 11 facing the same. As described above, the exhaust-side convex top surface 9b is positioned higher than the intake-side convex top surface 9a (for comparison, in the vicinity of the intake-side convex top surface 9a in FIG. Therefore, the minimum gap 5b is narrower than the small gap 5a. The minimum gap 5b is narrower than the other small gaps 5c and 5d, and is the smallest gap among the small gaps 5.

  The small gap portion 5c is a gap between the convex flat top surface 9c of the piston 13 and the ceiling wall 11 facing this. The small gap portion 5c is narrower as the ceiling wall 11 is lower, and becomes wider as the ceiling wall 11 is higher, that is, closer to the ridgeline portion 11c (see FIG. 3). The small gap portion 5 d is a gap between the ridge line portion 11 c and the convex flat top surface 9 c and is the widest gap among the small gap portions 5.

  Next, the operation of the spark ignition engine having the combustion chamber structure of this embodiment will be described.

  First, in the intake stroke, the intake valve 19 is opened and the piston 13 is lowered. Accordingly, the air-fuel mixture is sucked into the combustion chamber 14 from the intake port 21 by negative pressure.

  In the subsequent compression stroke, the intake valve 19 is closed and the piston 13 is raised. Along with this, the air-fuel mixture in the combustion chamber 14 is compressed, and the temperature and pressure rise. At the end of the compression stroke, that is, when the piston 13 rises to near the top dead center shown in FIG. 2, a spark is blown from the electrode of the spark plug 15. The spark ignites the air-fuel mixture near the electrode of the spark plug 15 to form a flame kernel.

  In the subsequent expansion stroke, the combustion proceeds while the flame surface of the flame kernel expands into a substantially spherical shape. The piston 13 is pushed down by the in-cylinder pressure rapidly increased by the combustion. The force that pushes down the piston 13 becomes the rotational driving force of the output shaft (crankshaft) not shown through a connecting rod etc. not shown.

  In the subsequent exhaust stroke, the exhaust valve 20 opens and the piston 13 starts to rise. Burned gas (exhaust gas) is pushed out from the exhaust port 22 by the rise of the piston 13 and is discharged.

  The engine is continuously operated by repeating the above four strokes including intake, compression, expansion, and exhaust (four-cycle engine). In the case of a multi-cylinder engine, by setting each stroke to be shifted for each cylinder, the engine can be made smoother and less susceptible to vibration and noise.

  Next, the combustion performed in the expansion stroke will be described in detail. This combustion takes the form of combustion referred to herein as late centroid combustion. In short, the late center-of-gravity combustion has a relatively low combustion rate in the first main combustion period (a period in which 10% or more and less than 50% of the combustion mass burns), and the latter main combustion period (50% or more of the combustion mass). This is a combustion mode having a relatively high combustion rate in a period during which less than 90% is burned). The late center-of-gravity type combustion is closely related to the combustion chamber structure of this embodiment, and is a combustion mode that can be achieved by this combustion chamber structure.

  The late center of gravity combustion will be described in relation to the combustion chamber structure. First, when a spark is blown from the electrode of the spark plug 15, the surrounding air-fuel mixture is ignited and a flame kernel is formed. Then, the flame surface (the forefront of flame propagation) spreads in a substantially concentric spherical shape. That is, combustion is performed in the first combustion space 14a. At that time, the flame surface expands by pushing out unburned gas.

  However, a small gap portion 5 is provided outside the first combustion space 14a. Therefore, when the unburned gas pushed out to the flame surface passes through the small gap part 5, it receives a kind of squeezing action. Under the influence, flame propagation is suppressed. For this reason, the combustion speed in the 1st combustion space 14a is restrained comparatively low.

  When the flame surface reaches the second combustion space 14b via the small gap portion 5, flame propagation proceeds promptly because it is no longer affected by the throttling action by the small gap portion 5. That is, the combustion speed in the second combustion space 14b is relatively high.

  In this way, relatively late combustion is performed mainly in the first main combustion period in the first combustion space 14a, and relatively high speed combustion is performed mainly in the second main combustion period in the second combustion space 14b. Center-of-gravity combustion is performed.

  In this embodiment, the ignition plug 15 is provided near the center of the cylinder bore 12 in the radial direction, the small gap portion 5 is formed in an annular shape between the ignition plug 15 and the cylinder bore periphery, and the second combustion space 14b is formed in the small gap. Since it is formed in an annular shape on the outer peripheral side of the part 5, the narrowing action of the small gap part 5 extends to the combustion in the first combustion space 14a more evenly, and the combustion speed reduction effect is further enhanced.

  Furthermore, since the second combustion space 14b is formed in an annular shape on the outer peripheral side of the small gap portion 5, the flame surface reaches the second combustion space 14b more evenly. For this reason, the combustion in the second combustion space 14b can be performed more rapidly.

  As a result, a sharp late-stage center-of-gravity type combustion is realized in which lower-speed combustion is performed in the first main combustion period and higher-speed combustion is performed in the second main combustion period.

  By the way, as described above, the flame surface spreads in a substantially concentric sphere centered on the flame core formed in the vicinity of the electrode of the spark plug 15, but more strictly, the propagation speed to the exhaust side is directed to the intake side. It is slightly higher than the propagation speed. This is because the combustion reaction is further promoted on the high temperature exhaust side. In this embodiment, the gas flow to the exhaust side is more tightly constricted than the others by the minimum gap 5b, so that the flame propagation speed to the exhaust side, which tends to be high, is relatively strongly suppressed. As a result, a more uniform flame propagation speed can be obtained as a whole, and smooth combustion can be achieved. Further, the transition from the first combustion space 14a to the second combustion space 14b on the flame surface can be performed more evenly.

  FIG. 5 is a characteristic diagram showing combustion characteristics in the latter-stage center-of-gravity combustion according to this embodiment. The horizontal axis shows the crank angle (° CA), and the vertical axis shows the combustion mass ratio (%). The combustion mass ratio is an integrated value of the fuel burned up to the crank angle point when the entire mass of the burned fuel is 100% (non-dimensional).

As shown, the combustion mass proportion refers to the area of less than 10% and the initial combustion region 81, the period of the initial combustion period theta _0. A region where the combustion mass ratio is 10% or more and less than 90% is referred to as a main combustion region 80. The main combustion region 80 is divided into the first and second periods with 50% as a boundary. The region where the combustion mass ratio is 10% or more and less than 50% is called the first-term main combustion region 80a, and the region where 50% or more and less than 90% is the latter main component. This is referred to as a combustion region 80b. The period of the first main combustion region 80a is referred to as the _first main combustion period θ1, and the period of the second main combustion region 80b is referred to as the _second main combustion period θ2.

  FIG. 5 shows the combustion characteristic T1 of this embodiment, and also shows a conventional general combustion characteristic T1 'for comparison. FIG. 5 shows combustion characteristics in the high-load operation state at an engine rotation speed of 1500 rpm.

In the combustion characteristic T1 of the present embodiment, the initial combustion period θ ₀ is ignition timing to about 3 ° CA, the _first main combustion period θ ₁ is about 3 to about 13 ° CA, and the _second main combustion period θ ₂ is about 13 to about 20. ° CA. On the other hand, in the conventional combustion characteristic T1 ′, the initial combustion period θ ₀ ′ is about ignition timing to about 4 ° CA, the _first main combustion period θ ₁ ′ is about 4 to about 13 ° CA, and the _second main combustion period θ ₂ ′ is about 13 to about 21 ° CA.

That is, in the combustion characteristic T1 of this embodiment, compared with the conventional combustion characteristic T1 ′, the initial combustion period θ ₀ is shortened by about 1 ° CA, the first main combustion period θ ₁ is extended by about 1 ° CA, and the second main combustion period. The period θ ₂ is shortened by about 1 ° CA. This is mainly the first combustion space 14a Combustion year main combustion period theta ₁ in burn rate relatively low place, the late main combustion period theta ₂ in combustion rate mainly combusted in the second combustion space 14b is performed It shows that it is relatively high. That is, it turns out that it is the latter term center of gravity type combustion.

Further, both the combustion in the initial combustion period θ ₀ and the _first main combustion period θ ₁ are mainly combustion in the first combustion space 14a, but the initial combustion period θ ₀ is rather shortened. This is because the small gap portion 5 is provided at an appropriate position that is not too close to the spark plug 15 (specifically, an appropriate position within the range of 60 to 85% of the distance from the spark plug 15 to the periphery of the cylinder bore). effect of throttling by section 5 indicates that does not extend up to the initial combustion period theta _0.

  FIG. 6 is a characteristic diagram showing the combustion characteristics shown in FIG. 5 from another viewpoint. The horizontal axis shows the crank angle (° CA), and the vertical axis shows the heat generation rate (%). Here, the heat generation rate is a differential value of the heat generation rate in FIG. 5, and the total heat generation amount due to combustion is defined as 100% (non-dimensionalization), and indicates the ratio of the heat generation amount at the time of the crank angle. Is.

FIG. 6 shows the combustion characteristic T2 of the present embodiment, and also shows a conventional general combustion characteristic T2 ′ for comparison. Compared with the characteristic T2 ', as hallmark characteristic T2, that it has a moderate shelf T2a of inclination in the previous year main combustion period theta _1, and in the later main combustion period theta ₂ of the maximum heat generation rate This is the point where the maximum value is large. These two points characterize the late center of gravity combustion.

Referring to ledge T2a, which shows that after transition from the initial combustion period theta ₀ in previous period main combustion period theta _1, increase of the heat generation rate is temporarily reduced. This is considered to by an aperture effect due to the small gap 5, because the combustion speed in the previous period main combustion period theta ₁ is relatively low.

Then, the point at which the maximum value of the maximum heat generation rate in the later main combustion period theta ₂ is increased, relatively large residual unburned fuel in the high-speed in the second combustion space 14b that sufficient volume is ensured This is thought to be due to combustion.

  As described above, according to the combustion chamber structure of the present embodiment, it is possible to easily perform the late center-of-gravity type combustion with a simple structure. As described above, by causing the late-stage center-of-gravity combustion, the anti-knocking performance can be improved and the compression ratio can be effectively increased practically. The inventor of the present application has confirmed that the compression ratio can be increased by 0.5 or more when, for example, the same level of anti-knocking performance as that of the conventional structure is secured. And by raising a compression ratio, combustion efficiency can be improved and a fuel consumption can be improved.

  Next, a second embodiment according to the present invention will be described with reference to FIGS.

  FIG. 7 is a cross-sectional view of the combustion chamber structure of the second embodiment at a position corresponding to the line III-III in FIG. FIG. 8 is an explanatory diagram of swirl and flame propagation, (a) is a plan view of the intake side ceiling wall viewed from the piston side, and (b) and (c) are the intake port shapes of the swirl generation intake system. FIG.

  The main difference of this embodiment from the first embodiment is that a second spark plug 15a and a third spark plug 15b are provided in addition to the spark plug 15 (see FIGS. 7 and 8A). The swirl generation intake system 23 is provided (see FIG. 8A).

  As shown in FIG. 7, the second spark plug 15a and the third spark plug 15b have tips at the ends of the mating part of the pent roof type intake-side ceiling wall 11a and the exhaust-side ceiling wall 11b, that is, the ridge line part 11c. 2 It is provided to face the combustion space 14b. The pent roof type combustion chamber 14 has a structure in which the gap between the ceiling wall 11 and the piston top surface 4 in the vicinity of the ridge portion 11c is relatively wide. Therefore, the second and third spark plugs 15a and 15b can be laid out relatively easily.

  The swirl generating intake system 23 is a known intake system for causing a strong swirl (a swirling flow around a piston sliding axis, a lateral vortex). In this embodiment, the intake valves 19 are provided at two locations per cylinder, and the intake ports (primary side and secondary side) corresponding to each are provided. A straight port 21a shown in FIG. 8C is provided on the primary side, and a high flow port 21b shown in FIG. 8B is provided on the secondary side. The straight port 21a opens into the combustion chamber 14 at a relatively shallow angle (an angle closer to the vertical with respect to the piston sliding axis) than the high flow port 21b.

  Next, the operation of the spark ignition engine having the combustion chamber structure of this embodiment will be described. However, portions overlapping with those of the first embodiment are omitted as appropriate.

  First, in the intake stroke, intake that causes a strong swirl is performed by the swirl generation intake system 23. Specifically, in the straight port 21a on the primary side, the intake valve 19 is opened wide (state of FIG. 8C), and in the high flow port 21b on the secondary side, the intake valve 19 is closed or opened small (see FIG. b)). By doing so, a strong clockwise swirl 75 (shown schematically in the figure) is generated in the state of FIG.

  Much of the swirl is preserved in the subsequent compression stroke. At the end of the compression stroke, sparks are blown from the respective electrodes of the spark plug 15, the second spark plug 15a, and the third spark plug 15b. The sparks may be skipped at the same time, or may be skipped at an appropriate time. The spark ignites the air-fuel mixture in the vicinity of the electrodes of the spark plug 15, the second spark plug 15a, and the third spark plug 15b, thereby forming a flame kernel.

  In the subsequent expansion stroke, the combustion proceeds while the flame surface of the flame kernel expands into a substantially spherical shape. FIG. 8A schematically shows the state by a two-dot chain line flame propagation isochron 70. However, this flame propagation isochron 70 is a case where the effect of the swirl 75 described below is not taken into consideration. The flame propagation isochron 70 is denser on the intake side than on the exhaust side. This indicates that the flame propagation speed toward the intake side is slower than the flame propagation speed toward the exhaust side. That is, in this state, there is a concern about a delay in flame propagation to the second combustion space 14b on the intake side.

  Here, focusing on the second spark plug 15 a and the swirl 75 near the intake valve 19, the second spark plug 15 a is provided immediately upstream of the swirl of the intake valve 19. By doing so, the flame from the second spark plug 15a is quickly transmitted to the intake side on the swirl 75. Therefore, the flame propagation delay to the intake side second combustion space 14b is effectively suppressed, and combustion in the intake side second combustion space 14b is performed in a timely manner.

  Thus, according to the present embodiment, even when there is a concern about the delay of flame propagation to the intake-side second combustion space 14b, combustion in the intake-side second combustion space 14b is performed in a timely manner. be able to. Therefore, effective late center-of-gravity combustion can be performed.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this embodiment, You may perform a various deformation | transformation within a claim.

  For example, although each said embodiment has shown the case where this invention is applied to a 4-cycle engine, you may apply to other than that, for example, a 2-cycle engine.

  In the first embodiment, the shape of the combustion chamber 14 is preferably a pent roof type, but other than that, for example, a hemispherical type (dome type), a multispherical type, or the like may be used.

  In the second embodiment, the third spark plug 15b is not necessarily provided. Since the third spark plug 15b is provided immediately upstream of the swirl of the exhaust valve 20, the flame can be propagated on the swirl 75 directed from the vicinity of the third spark plug 15b toward the exhaust side. Therefore, there is an advantage that the flame can be propagated more promptly to the exhaust side. In addition, when selecting any one of the 2nd spark plug 15a and the 3rd spark plug 15b as an spark plug provided in addition to the spark plug 15, it is desirable to select the 2nd spark plug 15a. Normally, flame propagation is slower toward the intake side than toward the exhaust side, and the second spark plug 15a can be expected to have a great effect of effectively suppressing the flame propagation delay. is there.

It is a longitudinal cross-sectional view which shows the combustion chamber structure of the spark ignition type engine which concerns on 1st Embodiment of this invention. It is an enlarged view of the principal part of FIG. It is the III-III sectional view taken on the line of FIG. It is a perspective view of the piston shown in FIG. It is a characteristic view which shows the combustion characteristic in the latter term gravity center type combustion of 1st Embodiment. It is the characteristic view which differentiated the combustion characteristic shown in FIG. It is sectional drawing in the position equivalent to the III-III line of FIG. 1 of the combustion chamber structure which concerns on 2nd Embodiment of this invention. It is explanatory drawing about the swirl and flame propagation in 2nd Embodiment, (a) is the top view which looked at the intake side ceiling wall from the piston side, (b) and (c) are the intake port shape of a swirl production | generation intake system FIG. It is a longitudinal cross-sectional view of the conventional common pent roof type | mold combustion chamber.

4 Piston top surface 5 Small gap part 5b Minimum gap part 6 Convex part 7 Center side concave part (concave part)
8 Peripheral recess (recess)
9 Convex top 10 Cylinder head 11 Ceiling wall (Cylinder head bottom)
11a Intake side ceiling wall 11b Exhaust side ceiling wall 11c Ridge part 11d Ceiling wall peripheral part (ceiling wall in cylinder bore peripheral part)
12 Cylinder bore 13 Piston 13a Piston crown 14 Combustion chamber 14a First combustion space 14b Second combustion space 15 Spark plug 15a Second spark plug 19 Intake valve 23 Swirl generating intake system 50 Cylinder block 75 Swirl

Claims (6)

A combustion chamber formed between the lower surface of the cylinder head and the top surface of the piston and having the lower surface of the cylinder head as a ceiling wall;
A combustion chamber structure of a spark ignition engine including a spark plug having a tip erected from the ceiling wall into the combustion chamber;
In a state where the piston is at the top dead center, a main part of the combustion chamber space is formed by a first combustion space around the spark plug and a second combustion space around the cylinder bore,
The first combustion space and the second combustion space are communicated with each other through a small gap portion in which a gap between the piston top surface and the ceiling wall is narrowed ,
On the crown portion of the piston, a convex portion that protrudes toward the ceiling wall and a concave portion that is relatively recessed with respect to the convex portion are formed,
The small gap portion is formed between the top surface of the convex portion and the ceiling wall,
The first combustion space and the second combustion space are formed between the recess and the ceiling wall,
The spark plug is provided near the radial center of the cylinder bore;
The convex portion is formed in an annular shape that is substantially concentric with the peripheral edge of the piston,
The combustion chamber structure of a spark ignition engine characterized in that the protruding amount of the convex portion is relatively larger on the exhaust side than on the intake side .   2. The spark ignition engine according to claim 1, wherein the small gap portion is formed closer to the cylinder bore periphery than an intermediate point between the spark plug and the cylinder bore periphery in the radial direction of the cylinder bore. Combustion chamber structure. The spark plug is provided near the radial center of the cylinder bore;
The small gap portion is formed in an annular shape between the spark plug and the cylinder bore periphery,
The combustion chamber structure for a spark ignition engine according to claim 1 or 2, wherein the second combustion space is formed in an annular shape on the outer peripheral side of the small gap portion. The spark plug is provided near the radial center of the cylinder bore;
4. The minimum gap portion having the narrowest gap among the small gap portions is formed in the middle from the spark plug to at least the peripheral edge portion of the cylinder bore on the exhaust side. The combustion chamber structure of the spark ignition engine according to item 1. The combustion chamber is a pent roof type in which an intake side ceiling wall and an exhaust side ceiling wall form a roof shape,
  The gap between the ridge portion of the pent roof and the top surface of the piston is larger than the gap between the surrounding ceiling wall and the top surface of the piston,
  5. The spark ignition type according to claim 1, further comprising a second spark plug having a tip disposed adjacent to a ridge line portion of the second combustion space and the pent roof. 6. Engine combustion chamber structure. The combustion chamber is a pent roof type in which the intake side ceiling wall and the exhaust side ceiling wall form a roof shape.
And
  6. The spark according to claim 1, wherein the ceiling wall at the peripheral edge of the cylinder bore is offset from the mating surface of the cylinder head with the cylinder block toward the side away from the cylinder block. The combustion chamber structure of an ignition engine.

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