JPH0932560A - Combustion chamber structure of direct injection diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce smoke while securing a capacity of a combustion chamber by constituting an auxiliary cavity on a cylinder head under surface. SOLUTION: A re-entrant type main cavity 11 is formed on a top surface of a piston 3, and an auxiliary cavity 13 is formed on an under surface 2a of a cylinder head 2. A first swirl flow 21 is formed in the main cavity 11, and a second swirl flow 22 is formed by a part of a squish flow. The first swirl flow and the second swirl flow are turned in the opposite direction, and turbulent energy of a specific area 24 being its boundary becomes strong. Power of the second swirl flow is properly weakened when a third swirl flow is formed in the auxiliary cavity 13 by the squish flow, and a forming area of the specific area 24 becomes high. Injection fuel collided with the vicinity of the opening edge 11a of the main cavity 11 is scattered to the specific area 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion chamber structure for a direct injection diesel engine.

[0002]

2. Description of the Related Art In a direct injection diesel engine,
A cavity for forming a combustion chamber is generally formed on the top surface of the piston. As a shape of the cavity formed on the piston top surface, there is a reentrant type in which the diameter is expanded outward in the cylinder radial direction from the opening edge of the cavity toward the inner side.

On the other hand, in a direct injection diesel engine, from the gap between the top surface of the piston and the bottom surface of the cylinder head,
It is common practice to improve combustion by forming a flow of intake air, that is, a squish flow, toward the cavity, that is, toward the center of the cylinder.

In the case of a reentrant type combustion chamber, that is, a cavity, a squish flow is used.
Two vortexes having swirling directions opposite to each other are formed in the cavity. That is, the first vortex that is swirled in the predetermined direction is formed in the cavity when the piston moves toward the top dead center, and at the end of the compression stroke, the squish flow is used to swirl in the direction opposite to the predetermined direction. A second swirl flow is formed. Then, the turbulent energy at the boundary between the two vortexes becomes extremely large, and by setting so that the injected fuel is supplied to this large turbulence as much as possible, smoke can be reduced.

Japanese Unexamined Patent Publication No. 59-158316 corresponds to a main cavity for forming a main combustion chamber formed on the top surface of a piston in order to prevent a vortex flow generated in the cavity from becoming a reverse squish flow. Therefore, it is proposed to form a sub-cavity on the lower surface of the cylinder head and set the opening edge position of the sub-cavity to a position closer to the cylinder center side by a predetermined step than the opening edge position of the main cavity. ing. That is, in the vicinity of the opening edge of the sub-cavity, a vortex flow swirling in the opposite direction to the vortex flow formed in the main cavity is formed, and the vortex flow in the main cavity becomes a reverse squish flow from the main cavity. I try to prevent it from leaking.

However, in the above-mentioned publication, the main cavity has a shape in which the diameter gradually decreases from the opening edge toward the inner side, and the main cavity is swung in a predetermined direction. This is a type in which two vortexes are formed, and has a shape different from the reentrant type in which two vortexes are formed in the cavity.

[0007]

By the way, recently,
There is a strong demand for downsizing the direct injection diesel engine, that is, for reducing the displacement, and downsizing of the direct injection diesel engine having the reentrant combustion chamber is considered. In this case, the volume of the cavity formed on the top surface of the piston, particularly the depth, is limited by the relationship of the connecting position of the connecting rod to the piston, and 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 said that reducing the compression ratio to around 14 to 15 is the most preferable for improving fuel efficiency.

In addition, as described above, two vortex flows whose swirling directions are opposite to each other are generated in the cavity forming the reentrant combustion chamber, and near the boundary between the two vortex flows having large turbulent energy. It is preferable to supply fuel in order to reduce smoke, but in order to effectively supply fuel to the position where the turbulence energy is large, the vicinity of the boundary should be formed at a higher position, that is, closer to the piston top surface. It will be what you want.

The present invention has been made in consideration of the above circumstances, and an object thereof is to ensure a sufficient combustion chamber volume in a direct injection diesel engine having a reentrant combustion chamber. To provide a combustion chamber structure for a direct injection diesel engine in which a portion of high turbulence energy in a cavity formed on a top surface of a piston is set to a higher position so that smoke can be sufficiently reduced. is there.

[0010]

Means for Solving the Problems In order to achieve the above object, the present invention has the following configuration. That is, the main cavity forming the reentrant combustion chamber is formed on the top surface of the piston, and the bottom surface of the cylinder head is
The sub-cavity forming the sub-combustion chamber is formed corresponding to the main cavity. Preferred embodiments of the present invention based on the above configuration are as described in claims 2 and below in the claims.

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 creates a vortex flow in the sub-cavity, the vortex flow generated in the main cavity by the squish flow is considered to have an appropriate weakness, and the turbulence energy in the main cavity is reduced. It is possible to move a large portion of the piston toward the top surface of the piston, and smoke can be effectively reduced.

With the structure as described in claim 2, as a whole combustion chamber including the main combustion chamber constituted by the main cavity and the auxiliary combustion chamber constituted by the sub-cavity, a step between the cavities is formed. Is substantially eliminated, and it is preferable for preventing a partial increase in heat load and deterioration of combustibility due to steps.

With the structure as described in claim 3, the size of the sub-cavity can be made appropriate, and the effect corresponding to claim 1 can be sufficiently exhibited.

With the structure as described in claim 4, the effect corresponding to claim 1 can be obtained without unnecessarily increasing the depth of the sub-cavity.

With the structure as described in claim 5, it is preferable in that the step portion is eliminated as much as possible on the lower surface side of the cylinder head to prevent the deterioration of the combustibility due to the step.

With the structure as described in claim 6, the squish flow has a proper momentum, which is preferable for sufficiently exhibiting the effect corresponding to claim 1.

With the structure as described in claim 7, it is more preferable in terms of effectively reducing the smoke by causing the injected fuel that has collided to be effectively scattered to the portion of the main cavity where the turbulent energy is large. Will be things.

With the structure as described in claim 8, by utilizing the raised portion, the momentum of the vortex flow swirled in the direction opposite to the squish flow is further promoted, that is, the portion where the turbulent energy is large. Move it to a higher position and
This is preferable in order to more effectively reduce the noise.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below 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 forming a main combustion chamber is formed on the top surface of the piston 3. The main cavity 11 is a reentrant type, and has a shape in which the diameter gradually increases from the opening end edge 11a toward the inner side. As a result, the opening edge 11a of the main cavity 11 is formed as a lip portion.

A raised portion 12 is formed from the inner bottom surface of the main cavity 11. The raised portion 12 is formed to have a height that does not reach the top surface of the piston, has the highest central portion, and has a skirt portion 12a that gradually decreases toward the cylinder radial outer side.

The main cavity 11 is formed in a point-symmetrical shape about the cylinder axis and has a raised portion 12.
Is also formed in a point-symmetrical shape around the cylinder axis. That is, the main cavity 11 is configured such that the cross-sectional shape along the cylinder axis at any position in the cylinder circumferential direction is the shape shown in FIG.

On the lower surface of the cylinder head 2, a sub-cavity 1 corresponding to the main cavity 11 and constituting a sub-combustion chamber is formed.
3 are formed. The sub-cavity 12 has a point-symmetrical shape around the cylinder axis. That is, the sub-cavity 13 is configured such that the cross-sectional shape along the cylinder axis at any position in the cylinder circumferential direction has the shape shown in FIG.

The center of the sub-cavity 13 coincides with the center of the main cavity 11, but the opening edge 13a thereof also
The position is substantially aligned with the opening edge 11a 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.

In the cylinder head 2, the fuel injection valve 1 is provided at a substantially central position of the cylinder (just on the cylinder axis in the embodiment), that is, above the central position of the raised portion 12.
4 are provided. The fuel injection valve 14 has a plurality of (five in the embodiment) injection holes at equal intervals in the circumferential direction, and the directing direction of each injection hole is the inner surface near the opening end edge 11 a of the main cavity 11. Has been done. The height position of each injection hole is set so as to substantially coincide with the top position of the piston 3 at the top dead center position.

As shown in FIG. 2, the main cavity 11 is
A plurality of (two in the embodiment) fuel reflecting surface portions 15 and 16 are sequentially formed in a stepped shape on the inner surface slightly rearward from the opening end edge 11a from the opening end edge 11a side to the rear side. There is. 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 shape that is concave toward the main cavity 11). Then, the fuel injected from the fuel injection valve 14 with a predetermined vertical width is collided and reflected by the fuel reflection surface portions 15 and 16, and is effective in the boundary portion (specific area) in the main cavity 11 having a large turbulent energy described later. It is supposed to be scattered.

In FIG. 2, when the piston 3 rises, inside the main cavity 11, the skirt 12a of the ridge 12 extends from the central portion of the main cavity 11 through the bottom side.
A first vortex 21 is formed that is swirled in a predetermined direction along the. On the other hand, when the piston 3 rises to the vicinity of the compression top dead center, a squish flow toward the center of the cylinder is generated between the top surface of the piston 3 and the lower surface of the cylinder head 2 on the cylinder radial outer side of each cavity 11, 13.
A part of this squish flow is swirled in the main cavity 11 so as to surround the opening edge 11a thereof.
That is, the second vortex 22 swirling in the direction opposite to the first vortex is generated.

The squish flow produces a third vortex 23 in the sub-cavity 13 in the vicinity of the opening edge 13a. A boundary portion between the first vortex flow 21 and the second vortex flow 22 becomes a portion where the turbulent energy of intake air is large, and the portion where the turbulent energy is large is shown as a specific region 24 surrounded by a broken line in FIG.

The higher the height of the specific area 24 (the closer it is to the top surface side of the piston 3), the more the reflecting surface portions 15, 16 are.
The fuel reflected from is effectively scattered in the specific area 24. Now, assuming that the sub-cavity 13 does not exist, the momentum of the second vortex flow 22 becomes extremely strong, so that the specific region 24 is located at a considerably low position. On the other hand, a sub-cavity 13 is formed and a third cavity is formed here.
Second swirl 22
Is weakened by the generation of the third vortex 23, and the specific region 24 is moved to a higher position. Of course, the formation of the sub-cavity 13 reduces the compression ratio, which is preferable in terms of improving fuel efficiency.

As described above, by moving the specific region 24 having a large intake turbulence energy to a high position,
The fuel is effectively scattered to the specific area, and the smoke is effectively reduced. In addition,
When the momentum of the second vortex 22 is weakened, it means that the fuel reflected and collided by the reflecting surface portions 15 and 16 does not move into the second vortex 2.
It is also easy to pass through 2 and reach the specific area 24.

The effect of this smoke reduction is shown in FIG. 5, and it is understood that the smoke reduction at a high load at which smoke becomes a problem is extremely effectively performed. In addition,
Although the amount of NOx produced hardly changed according to the change in load, it showed a slight decreasing tendency at low load and a slightly increasing tendency at high load. However, since the smoke reduction effect is extremely large, it is possible to increase the smoke reduction effect by retarding the fuel injection timing.
It is possible to reduce NOx while sufficiently reducing smoke under high load.

Here, as shown in FIG. 2, the distance of the opening edge 11a of the main cavity 11 from the cylinder axis α (=
The radius is r, the distance (= radius) from the cylinder axis α of the opening edge 13a of the sub-cavity 13 is φ, and the main cavity 1 is
Let c be the distance of 1 which is the farthest from the cylinder axis. Since the main cavity 11 is a reentrant type, naturally r <c.

In the embodiment, φ = r is set, but as φ becomes larger than r (the opening edge 13a becomes 1).
The momentum of the second vortex flow 22 becomes weaker (as it is positioned outside the cylinder radial direction than 1a) (the third vortex flow 2).
The momentum of 3 becomes stronger). On the contrary, as φ becomes smaller than r (as the opening edge 13a is located inward of the cylinder radial direction with respect to 11a), the second vortex 2
The momentum of 2 becomes stronger (the momentum of the third vortex 23 becomes weaker). Therefore, the positional relationship between the opening end edges 11a and 13a in the cylinder radial direction affects the setting of the height position of the specific region 24, but the opening end edges 11a and 13a should be substantially the same position in the cylinder radial direction. This makes it 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.

The depth D of the sub-cavity 13 should be as shallow as possible in consideration of ensuring the compression ratio, which is advantageous in terms of processing. Further, if the depth D is made too large, the turbulent energy of the entire intake air becomes smaller than that in the case where the sub-cavity 13 is not formed, which is not preferable, and when the cylinder bore diameter is 2B, the range of D ≦ 0.1B is satisfied. It is preferable to set to.

As shown in FIG. 3, the umbrella portion 31a of the intake / exhaust valve 31 in the closed state is normally at a position slightly retracted from the lower surface 2a of the cylinder head 2, and the umbrella portion 31a is provided.
Of the cylinder head 2a from the lower surface is d ≦
Satisfies the relationship of 0.1B. Therefore, by matching the depth D of the sub-cavity 13 with the amount of withdrawal d, the inner bottom surface of the sub-cavity 13 can be sucked in a closed state.
It can be flush with the lower surface of the exhaust valve.

If the area of the sub-cavity 13 is too large, a sufficient squish flow cannot be obtained, which is not preferable. FIG. 4 shows a state of the cylinder head lower surface 2a in the case of a four-valve type with two intake valves and two exhaust valves. The portion indicated by single hatching is the area S1 of the sub-cavity 13, and the double cavity is shown. The hatched portion is the area S2 for squish generation, and of the white circles, 32a is the intake valve recess and 32b is the exhaust valve recess. Then, the sub-cavity 13 is formed in each recess 3 so as to have a circular shape centered on the cylinder axis.
A sub-cavity 13 is formed straddling 2a, 32b (the outer peripheral edge of the sub-cavity 13 straddling the recesses 32a, 32b is shown by a dashed line in FIG. 4). In addition,
The squish flow generation area S2 has a valve recess 32a,
This is the area excluding the portion 32b.

In FIG. 6, assuming that the cylinder bore diameter (diameter) is 2B and the diameter of the sub-cavity 13 is 2φ, the relationship of the ratio of the squish flow generation area S2 to the sectional area S0 (= B × B × π) of the entire cylinder is shown. It is shown in FIG. As is clear from FIG. 6, the larger the area occupied by the sub-cavity 13 in the cylinder bore cross-sectional area, the smaller the squish flow generation area. However, if φ / B exceeds 0.8, a sufficient squish flow generation area is obtained. Will be difficult to secure. Therefore, φ / B is 0.8 or less (φ ≦ 0.
8B) is preferably set. R mentioned above
Considering the relationship of ≦ φ, it is preferable to set r ≦ φ ≦ 0.8B. In addition, in the above-mentioned setting, as is clear from FIG.
By locating the opening edge 13a of the sub-cavity 13 inside the valve stem (axis line) of the intake / exhaust valve in the cylinder radial direction, a sufficient area for squish flow generation can be secured.

Although the embodiment has been described above, the raised portion 1
2 is not necessary (the bottom surface of the main cavity 11 is flat). However, it is preferable to form the raised portion 12 in order to sufficiently strengthen the first vortex 21 and raise the position of the specific region 24.

[Brief description of 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 showing 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 diagram showing a smoke reduction effect.

FIG. 6 is a diagram for explaining area setting of a sub-cavity.

[Explanation of symbols]

 1: Cylinder Block 2: Cylinder Head 2a: Cylinder Head Lower Surface 3: Piston 11: Main Cavity 11a: Main Cavity Opening Edge 12: Ridge 12a: Bottom 13: Sub Cavity 13a: Sub Cavity Opening Edge 14: Fuel injection valve 15: Collision reflection part of injection fuel 16: Collision reflection part of injection fuel 21: First vortex flow 22: Second vortex flow 23: Third vortex flow 31: Intake / exhaust valve 31a: Intake / exhaust valve head portion 32a : Recess for intake valve 32b: Recess for exhaust valve

Claims (8)

[Claims] 1. A main cavity forming a reentrant type combustion chamber is formed on a top surface of a piston, and a sub cavity forming a sub combustion chamber corresponding to the main cavity is formed on a lower surface of a cylinder head. Combustion chamber structure of a direct-injection diesel engine. 2. The straightening mechanism according to claim 1, wherein the opening edge of the main cavity and the opening edge of the sub-cavity are set to be substantially at the same position in the cylinder radial direction. Jet type
Zel engine combustion chamber structure. 3. The distance from the cylinder axis to the outside in the cylinder radial direction according to claim 1 or 2, wherein the distance to the opening edge of the sub-cavity is φ, and the distance to the opening edge of the main cavity is A combustion chamber structure for a direct-injection diesel engine, wherein r ≦ φ ≦ 0.8B is set when the distance is r and the cylinder diameter is 2B. 4. The method according to claim 1, wherein when the depth of the sub-cavity is D and the cylinder diameter is 2B, D ≦ 0.1B is set. Combustion chamber structure for direct injection diesel engine. 5. The cap of the intake / exhaust valve in a closed state according to claim 4, wherein the umbrella portion is retracted a predetermined distance from the bottom surface of the cylinder head, and the depth of the sub-cavity is within the sub-cavity. A combustion chamber structure for a direct injection diesel engine, wherein a bottom surface is set to be flush with a lower surface of the intake / exhaust valve in the closed state. 6. A four-valve system of two intake valves and two exhaust valves according to any one of claims 1 to 5, wherein the opening of the sub-cavity is in the direction from the cylinder axis toward the cylinder radial outer side. The positions of the end edge and the opening end edge of the main cavity are set so as not to exceed the valve stem axis of each of the valves.
Combustion chamber structure of direct injection diesel engine characterized by 7. The fuel injection system according to claim 1, wherein the fuel injected 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. A plurality of arc-shaped portions are formed on the inner surface near the opening edge of the main cavity so as to be stepped from the opening edge side of the main cavity toward the inner side. Combustion chamber structure of injection diesel engine. 8. The combustion for a direct injection diesel engine according to claim 1, wherein a raised portion is formed at a central portion of an inner bottom surface of the main cavity. Room structure.