JP3593751B2 - Combustion chamber structure of direct injection diesel engine - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a combustion chamber structure of a direct injection diesel engine.
[0002]
[Prior art]
In a direct injection diesel engine, a cavity for forming a combustion chamber is generally formed on a top surface of a piston. As a shape of the cavity formed on the top surface of the piston, there is a reentrant type having a shape in which the diameter is increased outward in the cylinder radial direction from the opening edge of the cavity toward the back.
[0003]
On the other hand, in a direct-injection diesel engine, it is generally practiced to improve the combustion by forming a flow of intake air toward a cavity, that is, toward a cylinder center side, that is, a squish flow, from a gap between a piston top surface and a cylinder head lower surface. Is being done.
[0004]
In a reentrant combustion chamber, that is, one having a cavity, two vortices having swirling directions opposite to each other are formed in the cavity by using the squish flow. That is, a first vortex is swirled in a predetermined direction in the cavity when the piston moves toward the top dead center, and at the end of the compression stroke, a squish flow is used to turn in a direction opposite to the predetermined direction. A second vortex is formed. The turbulence energy at the boundary between the two vortices becomes extremely large. By setting the turbulence so that the injected fuel is supplied as much as possible, smoke can be reduced.
[0005]
JP-A-59-158316 discloses a cylinder corresponding to a main cavity for forming a main combustion chamber formed on the top surface of a piston in order to prevent a vortex generated in the cavity from forming a reverse squish flow. It has been proposed to form a sub-cavity on the lower surface of the head and set the opening edge position of this sub-cavity closer to the center of the cylinder by a predetermined step difference than the opening edge position of the main cavity. That is, in the vicinity of the opening edge of the sub-cavity, a vortex is swirled in a direction opposite to the vortex formed in the main cavity, and the vortex in the main cavity becomes a reverse squish flow from the main cavity. I try to prevent it from flowing out.
[0006]
However, in the above-mentioned publication, the shape of the main cavity is such that the diameter gradually decreases from the opening edge to the back side, and one vortex swirling in a predetermined direction is formed in the main cavity. It is of a type to be formed, and has a shape different from that of a reentrant type in which two vortices are formed in a cavity.
[0007]
[Problems to be solved by the invention]
In recent years, there has been a strong demand for miniaturization of the direct injection diesel engine, that is, reduction of the displacement, and miniaturization of the direct injection diesel engine having the reentrant combustion chamber has been considered. In this case, the volume, particularly the depth, of the cavity formed on the top surface of the piston is limited by the relationship of the mounting position of the connecting rod with respect to the piston, so that there is a limit in reducing the compression ratio. That is, in the current small diesel engine, the compression ratio is generally 18 or more, but it is considered that reducing the compression ratio to around 14 to 15 is most preferable for improving fuel efficiency.
[0008]
Further, as described above, two vortices whose swirling directions are opposite to each other are generated in the cavity constituting the reentrant combustion chamber, and fuel is supplied near the boundary between the two vortices having large turbulent energy. However, in order to effectively supply fuel to the position where the turbulent energy is large, it is desired to form the vicinity of the boundary at a higher position, that is, closer to the top surface of the piston. It becomes.
[0009]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a direct injection diesel engine having a reentrant combustion chamber in which the volume of the combustion chamber can be sufficiently ensured, and the piston top surface Another object of the present invention is to provide a combustion chamber structure of a direct-injection diesel engine in which a portion having a large turbulent energy in a cavity formed at a higher position is set at a higher position so that smoke can be reduced more sufficiently.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the present invention has the following configuration. That is, as described in claim 1 of the claims,
A main cavity constituting a reentrant combustion chamber is formed on the top surface of the piston,
On the lower surface of the cylinder head, a sub cavity that forms a sub combustion chamber corresponding to the main cavity is formed,
Injection fuel from the fuel injection valve located on the cylinder center side is directed toward the inner surface near the opening edge of the main cavity,
The main cavity opening edge and the sub cavity opening edge are set to be substantially the same position in the cylinder radial position,
In the direction going outward from the cylinder axis in the cylinder radial direction, the positions of the opening edge of the sub cavity and the opening edge of the main cavity are set so as not to exceed the valve stem axis of each valve. And
A recess for the intake valve and a recess for the exhaust valve formed on the lower surface of the cylinder head are formed so as to straddle the sub-cavity,
The umbrella portion of the intake / exhaust valve in the closed state is a position retracted a predetermined distance from the lower surface of the cylinder head,
The depth of the sub-cavity is set such that the inner bottom surface of the sub-cavity is flush with the lower surface of the intake / exhaust valve in the closed state,
A ridge is formed at the center of the inner bottom surface of the main cavity,
It is like that.
[0011]
According to the present invention, by forming the sub-cavity on the lower surface of the cylinder head, it is possible to secure a sufficient volume of the combustion chamber and reduce the compression ratio, which is preferable in terms of improving fuel efficiency. In addition, since a part of the squish flow generates a vortex in the sub-cavity, the vortex generated in the main cavity by the squish flow is moderately weak, and the turbulence energy in the main cavity is reduced. Can be moved closer to the piston top surface, and smoke can be effectively reduced.
In addition to the above, the present invention has the following various effects.
As a whole combustion chamber consisting of a main combustion chamber composed of a main cavity and a sub-combustion chamber composed of a sub-cavity, there is almost no step between the two cavities, and a partial increase in heat load and flammability caused by the step This is preferable in preventing deterioration.
This is preferable in that the stepped portion is eliminated as much as possible on the lower surface side of the cylinder head to prevent the deterioration of the flammability due to the step.
This is preferable in that the momentum of the squish flow is moderate and the above-described effects are sufficiently exhibited.
The colliding injected fuel is effectively scattered to a portion of the main cavity where the turbulent energy is large, which is more preferable in reducing smoke.
Utilizing the ridges to further promote the momentum of the vortex swirling in the direction opposite to the squish flow, that is, to increase the turbulence energy at a higher position to reduce smoke more effectively. The above is preferable.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In FIG. 1, 1 is a cylinder block, 2 is a cylinder head, and 3 is a piston. A main cavity 11 that forms a main combustion chamber is formed on the top surface of the piston 3. The main cavity 11 is of a reentrant type and has a shape whose diameter is gradually increased from the opening edge 11a toward the back. Thereby, the opening edge 11a of the main cavity 11 is formed to be a lip.
[0013]
A raised portion 12 is formed from the inner bottom surface of the main cavity 11. The raised portion 12 is formed so as to have a height that does not reach the top surface of the piston, and has a hem portion 12a whose center portion is the highest and gradually decreases toward the outside in the cylinder radial direction.
[0014]
The main cavity 11 is formed in a point-symmetrical shape about the cylinder axis, and the raised portion 12 is also formed in a point-symmetrical shape about the cylinder axis. That is, the main cavity 11 is configured such that the cross-sectional shape along the cylinder axis at an arbitrary position in the cylinder circumferential direction becomes the shape shown in FIG.
[0015]
A sub-cavity 13 forming a sub-combustion chamber is formed on the lower surface of the cylinder head 2 corresponding to the main cavity 11. The sub cavity 12 has a point-symmetrical shape about the cylinder axis. That is, the cross-sectional shape of the sub cavity 13 along the cylinder axis at an arbitrary position in the cylinder circumferential direction is set to the shape shown in FIG.
[0016]
The center of the sub-cavity 13 coincides with the center of the main cavity 11, and the opening edge 13 a of the sub-cavity 13 also substantially coincides with the opening edge 11 a of the main cavity 11. That is, the radius of the sub cavity 13 about the cylinder axis is substantially the same as the radius of the main cavity 11 about the cylinder axis.
[0017]
The fuel injection valve 14 is disposed in the cylinder head 2 at a substantially central position of the cylinder (in this embodiment, just on the cylinder axis), that is, above a central position of the raised portion 12. The fuel injection valve 14 has a plurality of (five in this embodiment) injection holes at equal intervals in the circumferential direction, and the directivity direction of each injection hole is determined by the inner surface near the opening edge 11 a of the main cavity 11. Have been. In addition, the height position of each injection hole is set so as to substantially coincide with the top surface height position of the piston 3 at the top dead center position.
[0018]
As shown in FIG. 2, the main cavity 11 has a plurality of (two in the embodiment) fuel reflections sequentially on the inner surface slightly deeper from the opening edge 11a from the opening edge 11a to the depth. The surface portions 15 and 16 are formed as a stepped shape. Each of the reflecting surface portions 15 and 16 is formed in an arc shape having a predetermined radius of curvature (in the embodiment, a concave shape toward the main cavity 11). Then, the fuel injected from the fuel injection valve 14 at a predetermined vertical width is impact-reflected on the fuel reflecting surface portions 15 and 16, and has an effect on a boundary portion (specific region) having a large turbulent energy in the main cavity 11 described later. Is scattered.
[0019]
In FIG. 2, when the piston 3 rises, the first cavity is swung in a predetermined direction along the skirt portion 12 a of the raised portion 12 from the central portion in the main cavity 11 through the bottom surface side. Vortex 21 is formed. On the other hand, when the piston 3 rises to the vicinity of the compression top dead center, a squish flow toward the cylinder center is generated from between the top surface of the piston 3 and the lower surface of the cylinder head 2 outside the respective cavities 11 and 13 in the cylinder radial direction, A part of the squish flow is swirled in the main cavity 11 so as to surround the opening edge 11a, that is, a second swirl 22 is swirled in the direction opposite to the first swirl. .
[0020]
Due to the squish flow, a third vortex flow 23 is generated in the subcavity 13 near the opening edge 13a. The boundary between the first vortex 21 and the second vortex 22 is a portion where the turbulence energy of the intake air is large, and the portion where the turbulence energy is large is shown as a specific region 24 surrounded by a broken line in FIG.
[0021]
As the height position of the specific area 24 is higher (closer to the top surface side of the piston 3), the fuel reflected from the reflecting surfaces 15, 16 is effectively scattered to the specific area 24. Now, assuming that the sub-cavity 13 does not exist, the momentum of the second vortex 22 becomes extremely strong, so that the specific region 24 is located at a considerably low position. On the other hand, by forming the sub-cavity 13 and generating the third vortex 23 therein, the momentum of the second vortex 22 is weakened by the amount of the third vortex 23, and the specific region 24 is formed. Will be moved to a higher position. Of course, the formation of the sub-cavity 13 is preferable in terms of improving the fuel efficiency by reducing the compression ratio.
[0022]
As described above, by moving the specific region 24 having a high intake air turbulence energy to a high position, the fuel is effectively scattered in this specific region, and smoke is effectively reduced. Will be. The fact that the momentum of the second vortex 22 is weakened means that the fuel colliding and reflected by the reflection surface portions 15 and 16 easily passes through the second vortex 22 and reaches the specific region 24. Also.
[0023]
FIG. 5 shows the effect of the smoke reduction, and it is understood that the smoke reduction at the time of a high load where the smoke is a problem is extremely effectively performed. Although the amount of generated NOx hardly changed in accordance with the change in load, the amount of NOx showed a slight decreasing tendency at a low load and a slight increasing tendency at a high load. However, since the smoke reduction effect is extremely large, the fuel injection timing is retarded (the smoke reduction effect is reduced by the amount corresponding to the retard), so that the smoke can be sufficiently reduced under a high load. It is also possible to reduce NOx.
[0024]
Here, as shown in FIG. 2, the distance (= radius) of the opening edge 11a of the main cavity 11 from the cylinder axis α is r, and the distance (= radius) of the opening edge 13a of the sub cavity 13 from the cylinder axis α. ) Is φ, and the distance of the part of the main cavity 11 farthest from the cylinder axis is c. Since the main cavity 11 is of the reentrant type, it is natural that r <c.
[0025]
In the embodiment, it is assumed that φ = r. However, as φ becomes larger than r (as the opening edge 13a is positioned radially outward of the cylinder 11a from 11a), the force of the second vortex 22 is increased. Becomes weaker (the momentum of the third vortex 23 becomes stronger). Conversely, as φ becomes smaller than r (as the opening edge 13a is positioned further inward in the cylinder radial direction than 11a), the momentum of the second vortex 22 becomes stronger (the third vortex 23 becomes larger). Momentum weakens). Therefore, although the positional relationship between the opening edges 11a and 13a in the cylinder radial direction affects the setting of the height position of the specific region 24, the opening edges 11a and 13a should be substantially at the same position in the cylinder radial direction. Accordingly, it is possible to increase the height position of the specific region 24 while obtaining a squish flow having a desired strength. However, in order to ensure that the height of the specific region 24 is sufficiently high, it is preferable that r ≦ φ.
[0026]
It is advantageous from the viewpoint of machining to make the depth D of the sub cavity 13 as shallow as possible while taking into consideration the securing of the compression ratio. Further, if the depth D is too large, the turbulent energy of the whole intake air becomes smaller than when the sub cavity 13 is not formed, which is not preferable. When the cylinder bore diameter is 2B, the range of D ≦ 0.1B is not preferable. It is preferable to set
[0027]
As shown in FIG. 3, the umbrella portion 31a of the intake / exhaust valve 31 in the valve closed state is located at a position slightly retracted from the lower surface 2a of the normal cylinder head 2, and the umbrella portion 31a is located at Satisfies the relationship d ≦ 0.1B. Therefore, by matching the depth D of the sub-cavity 13 with the retracted amount d, the inner bottom surface of the sub-cavity 13 can be flush with the lower surface of the intake / exhaust valve in the closed state.
[0028]
Making the area of the sub-cavity 13 too large is not preferable because a sufficient squish flow cannot be obtained. FIG. 4 shows the state of the cylinder head lower surface 2a in the case of a four-valve type having two intake valves and two exhaust valves. The portion indicated by single hatching is the area S1 of the sub-cavity 13, and The area indicated by hatching is the area S2 for squish generation, and among the white circles, 32a is an intake valve recess and 32b is an exhaust valve recess. The sub-cavity 13 is formed over the recesses 32a and 32b so as to be circular around the cylinder axis (the outer peripheral edge of the sub-cavity 13 extending over the recesses 32a and 32b). This is indicated by a dashed line in FIG. 4). The squish flow generation area S2 is an area excluding the valve recesses 32a and 32b.
[0029]
6, the cylinder bore diameter (diameter) is 2B, the diameter of the sub cavity 13 is 2φ, and the relationship of the ratio of the area S2 for squish flow generation to the sectional area S0 (= B × B × π) of the entire cylinder is shown in FIG. Is shown. As is apparent from FIG. 6, the area for squish flow generation decreases as the area occupied by the sub-cavity 13 in the cross-sectional area of the cylinder bore increases, but when φ / B exceeds 0.8, a sufficient squish flow generation area is obtained. Is difficult to secure. Therefore, it is preferable to set φ / B to be 0.8 or less (φ ≦ 0.8B). In consideration of the relationship of r ≦ φ described above, it is preferable to set r ≦ φ ≦ 0.8B. In the above-described setting, in the case of the four-valve type, as is apparent from FIG. 6, the opening edge 13a of the sub-cavity 13 is positioned radially inside the valve stem (axial line) of the intake / exhaust valve in the cylinder radial direction. Thereby, a sufficient area for squish flow generation can be secured.
[0030]
Although the embodiment has been described above, the raised portion 12 may not be provided (the bottom surface of the main cavity 11 is flat). However, the formation of the raised portion 12 is preferable in that the first swirl 21 is sufficiently strengthened and the position of the specific region 24 is increased.
[Brief description of the drawings]
FIG. 1 is a side sectional view showing an embodiment of the present invention.
FIG. 2 is an enlarged sectional view of a main part of FIG.
FIG. 3 is a cross-sectional view illustrating a height positional relationship between an intake / exhaust valve and a lower surface of a cylinder head.
FIG. 4 is a diagram showing a lower surface of a cylinder head.
FIG. 5 is a view showing the effect of reducing smoke.
FIG. 6 is a diagram for explaining the setting of the area of the sub-cavity.
[Explanation of symbols]
1: Cylinder block 2: Cylinder head 2a: Cylinder head lower surface 3: Piston 11: Main cavity 11a: Opening edge 12 of main cavity 12: Raised portion 12a: Foot 13: Subcavity 13a: Opening edge 14 of subcavity: Fuel injection valve 15: Injected fuel collision reflector 16: Injected fuel collision reflector 21: First vortex 22: Second vortex 23: Third vortex 31: Intake / exhaust valve 31a: Inlet / exhaust valve head 32a : Recess for intake valve 32b: Recess for exhaust valve

Claims (1)

A main cavity constituting a reentrant combustion chamber is formed on the top surface of the piston,
On the lower surface of the cylinder head, a sub cavity that forms a sub combustion chamber corresponding to the main cavity is formed ,
Injection fuel from the fuel injection valve located on the cylinder center side is directed toward the inner surface near the opening edge of the main cavity,
The main cavity opening edge and the sub cavity opening edge are set to be substantially the same position in the cylinder radial position,
In the direction going outward from the cylinder axis in the cylinder radial direction, the positions of the opening edge of the sub cavity and the opening edge of the main cavity are set so as not to exceed the valve stem axis of each valve. And
A recess for the intake valve and a recess for the exhaust valve formed on the lower surface of the cylinder head are formed so as to straddle the sub-cavity,
The umbrella portion of the intake / exhaust valve in the closed state is a position retracted a predetermined distance from the lower surface of the cylinder head,
The depth of the sub-cavity is set such that the inner bottom surface of the sub-cavity is flush with the lower surface of the intake / exhaust valve in the closed state,
A ridge is formed at the center of the inner bottom surface of the main cavity,
A combustion chamber structure of a direct injection diesel engine.

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