JP3799974B2 - Piston for direct-injection spark ignition internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a piston of a direct-injection spark-ignition internal combustion engine, and particularly to the crown shape of the piston.
[0002]
[Prior art]
As a piston of a conventional direct-injection spark-ignition internal combustion engine, it has a circular outer shape when viewed from above, leaving an annular flat surface portion with a minimum width on the outermost peripheral side on the piston crown surface, toward the center portion For example, a conical bulge is formed, and a concave cavity is formed near the bulge in the intake valve. With the swirl component in the cylinder, the intake valve Fuel injection in the compression stroke is performed from the fuel injection valve arranged on the side to form a stratified mixture intensively around the spark plug, realizing stratified combustion, while tumble (vertical) in the cylinder By injecting fuel during the intake stroke with the vortex component added, the fuel is diffused throughout the combustion chamber to form a homogeneous air-fuel mixture, thereby realizing homogeneous combustion (Japanese Patent Laid-Open No. 10-1999). No. 331644, JP-A-11- See No. 10470).
[0003]
[Problems to be solved by the invention]
By the way, for example, in order to maintain a predetermined compression ratio when extending the piston stroke of the current engine to increase the stroke volume (displacement), or to reduce the compression ratio of the current engine, When increasing the cavity diameter when increasing the combustion chamber volume, the flow speed of the swirl in the cylinder during stratified combustion does not change, but the flow speed of the swirl in the cavity induced by the swirl in the cylinder decreases (if the induced energy is the same) As the cavity diameter increases, the flow velocity decreases), the balance between the two swirls is lost, and inconveniences such as failure to transport the fuel to the spark plug may occur, and the stratified combustion performance may deteriorate.
[0004]
In such a case, it is necessary to adjust (for example, reduce) the flow rate of the swirl in the cylinder by reviewing the flow path of the swirl in the cylinder near the top dead center. However, the influence on the homogeneous combustion performance is also considered. When the external shape of the raised portion of the piston crown surface is a simple circle as in the prior art, there is a problem that the degree of freedom in design is small for various requirements.
[0005]
In view of such conventional problems, the present invention can improve the stratified combustion performance and the homogeneous combustion performance by reviewing the shape of the raised portion of the piston crown surface in consideration of adaptation to engines of various specifications. The purpose is to be able to.
[0006]
[Means for Solving the Problems]
For this reason, in the invention according to claim 1, the ignition plug is provided at the approximate center of the wall surface of the combustion chamber recessed in the cylinder head, the intake valve and the exhaust valve are sandwiched between the ignition plug, and the intake air It has a fuel injection valve that directly injects fuel into the cylinder on the valve side, and stratified combustion is realized by injecting fuel near the compression stroke with the swirl component applied in the cylinder, and the tumble component is injected into the cylinder. In a piston of a direct-injection spark ignition internal combustion engine that achieves homogeneous combustion by injecting fuel near the intake stroke in the applied state, leaving a flat surface on the outermost circumferential side of the piston crown, top view Has a substantially elliptical outer shape that is long in a straight line connecting the intake valve side and the exhaust valve side , and forms a ridge that rises in a mountain shape toward the center, and this ridge is viewed from above. Width in the direction perpendicular to the linear direction , To the intake valve side, in the exhaust valve side, it is formed to be narrower, and characterized by forming a concave cavity to the intake valve toward the ridge.
[0007]
The invention according to claim 2 is characterized in that an inclined surface that is more gently inclined than a mountain-shaped inclination of the raised portion is formed in a portion on the exhaust valve side in the linear direction of the raised portion.
The “linear direction connecting the intake valve side and the exhaust valve side” is hereinafter referred to as “cross flow direction” in the present specification.
[0008]
【The invention's effect】
According to the first aspect of the present invention, the ridged portion of the piston crown surface has a substantially elliptical shape that is long in the crossflow direction when viewed from above, so that the stratified combustion is performed on the intake valve side of the piston crown surface. By securing the wall of the cavity and securing the tumble-holding ridge for homogeneous combustion on the exhaust valve side of the piston crown, and reducing the width of the ridge in the direction perpendicular to the crossflow direction The flow area of the swirl in the cylinder near the top dead center can be increased, which makes it possible to adjust the flow speed of the swirl in the cylinder during stratified combustion, which improves the design freedom. can get.
[0009]
Therefore, for example, when the flow velocity of the swirl in the cavity decreases due to the enlargement of the cavity diameter, the flow velocity of the swirl in the cylinder can be decreased accordingly, and the balance between both swirls can be appropriately maintained. .
In addition, the intake valve side of the raised portion needs to ensure a cavity wall for stratified combustion, whereas the exhaust valve side only needs to have a width necessary to hold the tumble during homogeneous combustion. Therefore, by narrowing the width on the exhaust valve side relative to the width on the intake valve side, while ensuring the required cavity volume for performance, the flow area of the swirl in the cylinder is maximized, and the above-mentioned The effect can be maximized.
[0010]
According to the invention which concerns on Claim 2 , the rectification | straightening effect of a tumble can be heightened by making the part which hold | maintains the tumble at the time of homogeneous combustion in a protruding part into a gently inclined surface.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a plan view of a piston crown surface showing an embodiment of the present invention, and FIG. 2 is a sectional view of an engine including a piston corresponding to the AA section of FIG.
In these drawings, 1 is a cylinder head, 2 is a cylinder block, and 3 is a piston.
[0012]
In the cylinder head 1, a pent roof type combustion chamber 4 is recessed, and an ignition plug 5 is disposed in the approximate center of the wall surface of the combustion chamber 4 (slightly close to the exhaust valve 9).
The cylinder head 1 has two intake ports 6 and exhaust ports 7 facing each other with an ignition plug 5 interposed therebetween, and an intake valve 8 and an exhaust valve 9 are arranged at each opening to the combustion chamber 4. Has been. Here, a swirl control valve 10 capable of closing at least a part of one of the two intake ports 6 is provided as a swirl generating means.
[0013]
A fuel injection valve 11 is attached to the cylinder head 1 obliquely downward from the intake valve 8 side to the exhaust valve 9 side, between the two intake ports 6 and on the peripheral edge side of the intake port 6. The fuel is directly injected into the cylinder.
Next, the shape of the crown surface of the piston 3 will be described.
The crown surface of the piston 3 has a substantially elliptical outer shape 13a that is long in the crossflow direction (intake valve 8-exhaust valve 9 direction) when viewed from above, leaving an annular flat surface portion 12 on the outermost peripheral side. A raised portion 13 that rises in a mountain shape toward the portion is formed.
[0014]
In addition, referring to FIG. 3, the width W2 on the exhaust valve 9 side is made narrower than the width W1 on the intake valve 8 side with respect to the width of the substantially elliptical outer shape 13a of the raised portion 13 in the direction perpendicular to the crossflow direction. In other words, the width of the flat portion 12 on the exhaust valve 9 side is made wider than the width of the flat portion 12 on the intake valve 8 side.
Here, the outer shape of the raised portion 13 is expressed as “substantially elliptical”, but as shown by the difference in width between the intake valve 8 side and the exhaust valve 9 side, it is not a complete ellipse, but an egg shape. FIG. 3 is actually close to the mold, and one side has an arc of radius r1 centered on point a, an arc of radius r2 centered on point b, and a radius r3 centered on point c. It has a shape that connects arcs, and the opposite side is symmetrical.
[0015]
The raised portion 13 is formed with a cavity 14 that is concave and has a bottom surface 14 a lower than the flat portion 12, near the intake valve 8. That is, the cavity 14 has a center at a position offset from the center of the piston 3 toward the intake valve 8 and has a substantially circular outer shape in a top view. Therefore, the ridge line 14b at the peripheral edge of the cavity 14 is low on the intake valve 8 side and high on the exhaust valve 9 side (piston 3 center side), and the spark plug 5 is located above the peripheral edge on the piston 3 center side in the cavity 14. Is located.
[0016]
Further, the portion on the exhaust valve 9 side in the cross flow direction of the raised portion 13 (outside of the ridge line 14b on the peripheral side of the piston 3 at the center of the cavity 14) is cut off obliquely to suppress the height of the highest portion of the piston 3. At the same time, an inclined surface (tumble holding surface) 15 that is more gently inclined than the angle of the mountain-shaped inclination of the raised portion 13 is formed for the purpose of better holding the tumble during homogeneous combustion. Further, in order to make the inclined surface 15 longer and to secure the flat portion 12 on the exhaust valve 9 side, the flat portion 12 and the inclined surface 15 are connected by an inclined surface 16 that is steeper than the inclined surface 15.
[0017]
In addition, the bottom surface 14a and the side surface (inner peripheral surface) in the cavity 14 are basically connected by a radius 14c, but in this embodiment, the side surface on the fuel injection valve 11 side in the cavity 14 is overlaid. And an inclined surface (spray guide surface) 17 which is formed of a conical surface having the central axis of the piston 3 as a central axis and guides the fuel spray from the fuel injection valve 11 to smoothly transport to the spark plug 5 side during stratified combustion. It is. Here, referring to FIG. 8, the inclination angle θ2 of the inclined surface 17 is set to be gentler than the inclination angle θ1 of the injection direction line of the fuel injection valve 11 (θ2 <θ1).
[0018]
Next, the operation will be described.
At the time of stratified combustion mainly in a low load region, one of the two intake ports 6 is closed by the swirl control valve 10 and sucked from one intake port 6, as shown in FIG. The swirl S1 is formed therein, and the swirl S2 is induced in the cavity 14 by the in-cylinder swirl S1. Then, these swirls S1 and S2 are maintained after the compression stroke, and fuel is injected into the cavity 14 from the fuel injection valve 11 in the latter half of the compression stroke, and the injected fuel is put on the swirl S2 in the cavity 14 and the ignition plug By transporting to the vicinity of 5, the fuel is collected around the spark plug 5 and stratified combustion is performed.
[0019]
Here, a sufficient volume of the cavity 14 is provided on the side of the intake valve 8 on the crown surface of the piston 3 by forming the raised portion 13 on the crown surface of the piston 3 into a shape having a substantially elliptical outer shape 13a that is long in the cross flow direction in a top view. And the wall of the piston 3 and the exhaust valve 9 on the crown surface of the piston 3 are secured with a bulge (inclined surface 15) for holding a tumble during homogeneous combustion, which will be described later. In particular, it is necessary to secure the wall of the cavity 14 on the intake valve 8 side of the raised portion 13, while the exhaust valve 9 side performs tumble during homogeneous combustion. Since the width required for holding is sufficient, the width W2 on the exhaust valve 9 side is made narrower than the width W1 on the intake valve 8 side of the raised portion 13, thereby securing a shape essential for performance. Above, Siri near top dead center The flow area of the dust in the swirl S1 is maximally increased, with, it becomes possible to reduce the flow rate of the cylinder swirl S1.
[0020]
Therefore, even when the flow velocity of the swirl S2 in the cavity decreases due to the enlargement of the diameter of the cavity 14, the flow velocity of the swirl S1 in the cylinder can be sufficiently reduced accordingly, and the balance between the swirls S1 and S2 can be balanced. It becomes possible to maintain appropriately, and the stratified combustion performance can be improved.
On the other hand, at the time of homogeneous combustion mainly in a high load region, the swirl control valve 10 is opened and the air is sucked from the two intake ports 6 to generate a tumble as shown in FIG. Then, fuel is injected into the cylinder from the fuel injection valve 11, and the injected fuel is placed in a tumble and diffused throughout the combustion chamber 4 to form a homogeneous mixture, thereby performing homogeneous combustion.
[0021]
Here, the portion of the raised portion 13 on the exhaust valve 9 side, in particular, the inclined surface 15 has a function of rectifying and holding the tumble as shown in FIG. 5, thereby improving the homogeneous combustion performance.
The present invention is particularly effective when the diameter of the cavity 14 is enlarged. The ratio Ra = Ac / Ab between the cavity area Ac and the bore area Ab shown in FIG. 6, and the cavity shown in FIG. When the ratio Rv = Vc / Vt between the volume Vc and the raised portion (convex portion) volume Vt is generally known, Ra = 0.28 to 0.33 and Rv = 0.7 to 1.6 are generally used. The present invention exhibits its effect particularly when Ra ≧ 0.35 and Rv ≧ 2.0.
[0022]
Next, the reason why the inclined surface 17 (see FIG. 8) is formed on the fuel injection valve 11 side in the cavity 14 will be described.
When the side surface on the fuel injection valve 11 side in the cavity 14 and the bottom surface 14a are connected by a radius 20 as shown in FIG. 10, which is a reference example, fuel spray tends to accumulate at the corner of the cavity 14 directly below the fuel injection valve 11, There is a problem that the fuel HC increases.
[0023]
Further, when the diameter of the cavity 14 is enlarged, the diameter of the cavity 14 is mainly enlarged on the fuel injection valve 11 side, so that the wall thickness between the cavity 14 and the piston ring groove 18 is insufficient, and the piston ring groove 18 If the piston ring groove 18 is lowered in order to cause problems such as abnormal wear due to an increase in the temperature of the part, or a lack of wall thickness, the clevis volume will increase, leading to an increase in unburned HC. There is a problem of doing.
[0024]
Therefore, fuel spray can be prevented from accumulating at the corner of the cavity 14 immediately below the fuel injection valve 11, and even when the diameter of the cavity 14 is large, the cavity 14 and the piston ring groove 18 are not lowered without lowering the position of the piston ring groove 18. It is necessary to be able to secure the wall thickness between.
Therefore, an inclined surface (spray guide surface) 17 is provided on the fuel injection valve 11 side in the cavity 14, and in particular, the inclined surface 17 is parallel to the injection direction line of the fuel injection valve 1 (θ2 = θ1) or from the injection direction line. In addition, the gentle inclination (θ2 <θ1) not only prevents the fuel spray from collecting in the corner of the cavity 14 immediately below the fuel injection valve 11 as shown in FIG. Helps to go to the 5th side and realizes good stratification. In this case, the effect can be further improved by setting the inclined surface 17 to a gentler inclination (θ2 <θ1) than the injection direction line of the fuel injection valve 1.
[0025]
Further, since the surface can be built up by the inclined surface 17, as is clear from comparison between FIG. 9 (H1) and FIG. 10 (H2), the cavity 14 and the piston ring groove are not lowered without lowering the position of the piston ring groove 18. A wall thickness of 18 can be secured.
Further, by making the inclined surface 17 a conical surface, it is possible to prevent the flow of the swirl S2 in the cavity from being obstructed more than when the inclined surface 17 is a flat surface, and the wall thickness between the piston ring groove 18 can be efficiently reduced. Can be secured.
[Brief description of the drawings]
FIG. 1 is a plan view of a piston crown surface showing an embodiment of the present invention. FIG. 2 is a cross-sectional view of an engine including a piston corresponding to the AA cross section of FIG. FIG. 4 is an explanatory diagram of swirl in a cylinder and in a cavity. FIG. 5 is an explanatory diagram at the time of tumble generation. FIG. 6 is an explanatory diagram of a cavity area and a bore area. FIG. ] Illustration of fuel injection valve and spray guide surface [FIG. 9] Diagram showing fuel spray guidance effect [FIG. 10] Diagram showing problems in reference example [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Cylinder head 2 Cylinder block 3 Piston 4 Pent roof type combustion chamber 5 Spark plug 6 Intake port 7 Exhaust port 8 Intake valve 9 Exhaust valve 10 Swirl control valve 11 Fuel injection valve 12 Planar part 13 Raised part 13a Substantially elliptical outline 14 Cavity 14a Bottom surface 14b Ridge line 14c R 15 Inclined surface (tumble holding surface)
16 Inclined surface 17 Inclined surface (spray guide surface)
18 Piston ring groove

Claims (2)

An ignition plug is provided at the approximate center of the wall of the combustion chamber recessed in the cylinder head, and an intake valve and an exhaust valve are provided across the ignition plug, and fuel is directly injected into the cylinder on the intake valve side. It has a fuel injection valve and realizes stratified combustion by injecting fuel near the compression stroke with a swirl component in the cylinder, and injecting fuel near the intake stroke with a tumble component in the cylinder In a direct-injection spark-ignition internal combustion engine piston that achieves homogeneous combustion by performing
On the piston crown surface, leaving a flat part on the outermost peripheral side, it has a substantially elliptical shape that is long in the linear direction connecting the intake valve side and the exhaust valve side when viewed from above, and rises in a mountain shape toward the center part Forming a raised ridge,
And this ridge is formed so that the width in the direction perpendicular to the linear direction in the top view is narrower on the exhaust valve side than on the intake valve side,
A piston of a direct-injection spark-ignition internal combustion engine, wherein a concave cavity is formed near the intake valve of the raised portion. The direct-injection spark-ignition internal combustion engine according to claim 1 , wherein an inclined surface that is more gently inclined than a mountain-shaped inclination of the raised portion is formed in a portion of the raised portion on the exhaust valve side in the linear direction . piston.

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