JP7582281B2 - Combustion chamber structure of an internal combustion engine - Google Patents

Description

This disclosure relates to the combustion chamber structure of an internal combustion engine.

In direct injection internal combustion engines that inject fuel directly into the cavity, in internal combustion engines with one intake valve and one exhaust valve per cylinder, it is desirable to make the valve diameter as large as possible to avoid increasing pumping loss. As such an internal combustion engine, Patent Document 1 describes a direct injection diesel engine in which the center of the cavity is offset from the center of the cylinder, and the fuel injection valve is offset from the center of the cavity and installed at a specified angle with respect to the piston axis.

Patent Document 2 also proposes a method for improving the diffusion of fuel spray into the squish area during low-speed operation. The combustion chamber structure proposed in Patent Document 2 includes a gouged portion on the outer periphery of the cavity opening that is recessed to a required depth relative to the piston top surface to form a step and has a guide surface that gradually rises and rises in a concave shape as it moves radially outward, and a lip portion that is formed at a position one step lower than the piston top surface by the inner periphery of the bottom surface of the gouged portion and a combustion chamber wall portion that rises from the bottom surface of the cavity.

Figure 5 is a schematic diagram showing an example of a conventional combustion chamber structure of an internal combustion engine having a structure as described in Patent Documents 1 and 2. In the example of Figure 5, a cavity 110 is provided that is recessed downward from the top surface 6a of the piston 106. The center of the cavity 110 is offset to the right of the page from the piston central axis A1.

The cavity 110 is also formed with a step portion 121 on the outer periphery of the cavity opening, which is recessed to a predetermined depth from the top surface 6a to form a step and has a guide surface 122 that gradually rises and rises in a concave shape as it moves radially outward, and a lip portion 123 formed at a position one step lower from the top surface 6a by the inner periphery of the bottom surface of the step portion 121 and a combustion chamber wall portion that rises from the bottom surface of the cavity 110. In addition, a ball portion 111 with an arc-shaped cross section is formed on the combustion chamber wall surface between the lip portion 123 and the bottom surface of the cavity 110.

Figure 5 also shows the intake valve 3 and the fuel injector 5. The spray nozzle of the fuel injector 5 is on an axis B1 that is offset to the right of the page from the cavity center axis. Also, the axis B2 of the fuel injector 5 is inclined at an angle to the axis B1 that is parallel to the piston center axis A1.

JP 2000-199431 A JP 2014-238011 A

Figure 6 is a schematic diagram showing an example of fuel spray in a conventional combustion chamber structure. In a combustion chamber having a shape with a lip portion 123 and a step portion 121, as in the conventional example shown in Figure 6, it is common to spray fuel aimed at the lip portion 123 to facilitate mixing of the fuel and air. The fuel that collides with the lip portion 123 is divided along the step portion 121 into a portion heading toward the top surface 6a of the piston 6 and a portion heading toward the ball portion 111, and spreads out, where it is mixed with the air and combusted.

Here, in the fuel injection valve 5, which is offset from the cavity central axis and further inclined, more fuel is sprayed in the direction F1, where the nozzle angle and spray angle are gentle, than in the direction F2, where the nozzle angle and spray angle are closer to a right angle. Therefore, in the direction F1, fuel that cannot mix with the air is more likely to occur than in the direction F2. For example, fuel that travels along the step portion 121 toward the top surface 6a of the piston 106 in the direction F1 is blocked from diffusing due to interference with the bottom surface 2a of the cylinder head 2, and the spread of the spray and mixing with the air are likely to be hindered.

Figure 7 is a schematic diagram showing the diffusion of fuel sprayed toward a conventional combustion chamber structure. As shown in Figure 7, the fuel sprayed in the direction F1 in Figure 6 and whose diffusion is blocked by interference with the lower surface 2a of the cylinder head 2 becomes locally over-concentrated in region X11. This may cause a large amount of soot to be emitted. Meanwhile, air that did not mix with the fuel remains in the squish area between the top surface 6a of the piston 106 and the lower surface 2a of the cylinder head 2.

This disclosure has been made in consideration of the above problems. In other words, the purpose of this disclosure is to reduce the spread of spray and the degree of interference with the cylinder head in a combustion chamber structure with a stepped portion, and to promote the mixing of air and fuel.

A combustion chamber structure of an internal combustion engine according to an embodiment of the present disclosure includes:
A combustion chamber structure of an internal combustion engine having a combustion chamber defined by a piston, a cylinder, and a cylinder head,
a fuel injection valve provided in the cylinder head at a predetermined angle with respect to a piston central axis;
a cavity formed in a top surface of the piston and offset with respect to a central axis of the piston ;
a protrusion provided at the center of a bottom surface of the cavity so as to have a mountain-shaped cross section;
a ball portion that smoothly connects the protrusion portion and a side wall of the cavity and has an arc-shaped cross section;
a lip portion provided annularly along a circumferential direction of a side wall rising from a bottom surface of the cavity and protruding toward a center of the cavity;
a step portion provided from the lip portion toward an outer periphery of the cavity and recessed to a predetermined depth from the top surface,
a step portion central axis that is an axis passing through a center of the step portion is offset toward the piston central axis side with respect to a cavity central axis that is an axis passing through the center of the bottom surface of the cavity ,
Fuel is injected from the fuel injection valve toward the lip portion, and when it collides with the lip portion, it is split into fuel that heads toward the step portion and fuel that heads toward the ball portion, and the fuel that heads toward the step portion diffuses into a squish area near the side wall portion of the cylinder between the top surface and the cylinder head without interfering with the cylinder head.

According to the present disclosure, in a combustion chamber structure with a stepped portion, it is possible to reduce the spread of spray and the degree of interference with the cylinder head, and promote the mixing of air and fuel.

FIG. 1 is a top view of a cylinder head according to one embodiment of the present disclosure. FIG. 2 is a top view and a cross-sectional view of a piston according to an embodiment of the present disclosure and a piston according to a conventional example. FIG. 3 is a schematic diagram showing an example of fuel spray in a combustion chamber structure according to an embodiment of the present disclosure. FIG. 4 is a schematic diagram showing the diffusion state of sprayed fuel in a combustion chamber structure according to an embodiment of the present disclosure. FIG. 5 is a schematic diagram showing an example of a conventional combustion chamber structure. FIG. 6 is a schematic diagram showing an example of a fuel spray in a combustion chamber structure according to a conventional example. FIG. 7 is a schematic diagram showing the diffusion state of sprayed fuel in a combustion chamber structure according to a conventional example.

The combustion chamber structure of an internal combustion engine according to one embodiment of the present disclosure will be described below with reference to the drawings. In each drawing, identical or equivalent components are given the same reference numerals, and duplicated descriptions will be omitted as appropriate. In addition, the dimensions of each component shown in each drawing are for convenience of explanation and may differ from the actual dimensions.

First, a combustion chamber structure according to an embodiment of the present disclosure will be described with reference to Figures 1 to 3. Figure 1 is a top view of a cylinder head 2 according to an embodiment of the present disclosure. Figure 2 is a top view and a cross-sectional view of a piston 6 according to an embodiment of the present disclosure and a piston 106 according to a conventional example. Figure 3 is a schematic diagram showing an example of fuel spray in a combustion chamber structure according to an embodiment of the present disclosure.

Below, a direct injection diesel engine will be described as an example of an internal combustion engine according to one embodiment of the present disclosure, but the internal combustion engine according to the present disclosure is not limited to this. Furthermore, the internal combustion engine according to the present disclosure is not limited to one that uses diesel fuel as fuel, and may be one that uses liquefied gas fuel, such as CNG (Compressed Natural Gas) or LNG (Liquefied Natural Gas).

As shown in FIG. 3, the combustion chamber 1 of the direct injection diesel engine according to this embodiment is a space defined by a piston 6, a cylinder 7, and a cylinder head 2. As shown in FIG. 1, the cylinder head 2 has one intake valve 3, one exhaust valve 4, and one fuel injection valve 5.

As shown in FIG. 2, a cavity 10 is recessed in the top surface 6a of the piston 6, offset from the piston central axis A1. The cavity central axis D1 in the piston 6 is offset to the right from the piston central axis A1 on the page. The structure in which the cavity central axis D1 is offset to the right from the piston central axis A1 on the page is the same as that of the piston 106 of the conventional example.

The bottom surface of the cavity 10 has a mountain-shaped cross section, with a protrusion in the center of the bottom surface. From this protrusion toward the outer periphery, the bottom surface of the cavity 10 gently slopes and is connected to a side wall that rises from the bottom surface of the cavity 10. At the connection between the bottom surface of the cavity 10 and the side wall, a ball portion 11 with an arc-shaped cross section is formed.

The cavity 10 also has a step 21. The step central axis C1 is offset toward the piston central axis A1 with respect to the cavity central axis D1. This is a major difference between the piston 6 according to this embodiment and the piston 106 according to the conventional example.

Here, the "piston central axis" is, for example, an axis parallel to the direction in which the piston reciprocates and passes through the center of the top surface of the piston (or a cross section perpendicular to the direction in which the piston reciprocates). The "cavity central axis" is, for example, an axis parallel to the piston central axis and passes through the center of the bottom surface of the cavity. In the example of FIG. 2, the cavity central axis D1 is an axis parallel to the piston central axis A1 and passes through the center of the protrusion provided in the center of the bottom surface of the cavity 10. The "step central axis" is, for example, an axis parallel to the piston central axis and passes through the center of the step. In the example of FIG. 2, the step central axis C1 is an axis parallel to the piston central axis A1 and passes through the center of the circle that forms the outer periphery of the step portion 21 (the boundary between the guide surfaces 22a, 22b and the top surface 6a).

In the example of FIG. 2, the step central axis C1 is offset to be in the same position as the piston central axis A1, but this is not limited thereto, and the step central axis C1 may be offset toward the piston central axis A1 with respect to the cavity central axis D1. It is preferable that the distance d by which the step central axis C1 is offset is, for example, less than twice the distance between the cavity central axis D1 and the piston central axis A1.

In the following description, step portion 21a, guide surface 22a, and lip portion 23a are portions located in the direction from cavity central axis D1 toward step portion central axis C1 (leftward on the page). Step portion 21b, guide surface 22b, and lip portion 23b are portions located in the direction from cavity central axis D1 toward the opposite side of step portion central axis C1 (rightward on the page).

Note that these names are used for convenience of explanation and do not limit the present disclosure. Step portions 21a and 21b are formed circumferentially around step portion central axis C1 and are connected to each other as a single component. Similarly, guide surfaces 22a and 22b are formed circumferentially around step portion central axis C1 and are connected to each other as a single component. Similarly, lip portions 23a and 23b are formed circumferentially around cavity central axis D1 and are connected to each other as a single component.

The lip portions 23a and 23b are provided in an annular shape along the circumferential direction of the side wall rising from the bottom surface of the cavity 10. The lip portions 23a and 23b protrude toward the center of the cavity 10. The fuel injection valve 5 sprays fuel toward the lip portions 23a and 23b to diffuse the fuel.

The step portions 21a and 21b are provided from the lip portions 23a and 23b toward the outer periphery of the cavity 10, and are recessed a predetermined depth from the top surface 6a. The step portions 21a and 21b also have guide surfaces 22a and 22b that gently connect the step portions 21a and 21b to the top surface 6a. As can be seen from FIG. 2, because the step portion central axis C1 is offset as described above, the step portion 21a is wider than the step portion 21b.

As shown in FIG. 3, the tip of the fuel injection valve 5 is located on an axis B1 that is offset a predetermined distance from the piston central axis A1. The fuel injection valve 5 is also mounted in the cylinder head 2 at a predetermined angle θ from the axis B1 that is parallel to the piston central axis A1. In this case, more fuel is usually sprayed in the direction F1, where the nozzle angle and spray angle are gentle, than in the direction F2, where the nozzle angle and spray angle are closer to a right angle.

Here, in the combustion chamber structure according to this embodiment, the step portion 21a located on the direction F1 side is wider than the step portion 21b on the direction F2 side. That is, the combustion chamber structure according to this embodiment has sufficient space on the step portion 21a side to allow the fuel to diffuse without interfering with the lower surface 2a of the cylinder head 2. Therefore, even if a large amount of fuel is sprayed on the direction F1 side, the fuel that travels along the step portion 21a toward the top surface 6a of the piston 6 is more likely to spread the spray without interfering with the lower surface 2a of the cylinder head 2, and is less likely to be hindered from mixing with the air. In addition, since less fuel is sprayed on the step portion 21b side than on the step portion 21a side, problems such as interference with the cylinder head 2 are less likely to occur.

Figure 4 is a schematic diagram showing the diffusion of sprayed fuel in a combustion chamber structure according to one embodiment of the present disclosure. As shown in Figure 4, in the combustion chamber structure according to this embodiment, the step portion 21a side is sufficiently wide. Therefore, in squish areas such as region X1, air and fuel can be sufficiently mixed, and the formation of a locally excessively rich mixture can be suppressed. This can also reduce soot and improve thermal efficiency.

Although one embodiment of the present disclosure has been described above in detail, the present invention is not limited to the above-described embodiment, and suitable modifications and improvements are possible. The present invention is defined by the claims, and includes all modifications within the meaning and scope of the claims.

[Additional Notes]
As described above, the present specification discloses the following:
(1) A combustion chamber structure of an internal combustion engine having a combustion chamber defined by a piston, a cylinder, and a cylinder head,
a fuel injection valve provided in the cylinder head at a predetermined angle with respect to a piston central axis;
A cavity is provided on a top surface of the piston, offset from a central axis of the piston;
a lip portion provided annularly along the circumferential direction of a side wall rising from a bottom surface of the cavity and protruding toward the center of the cavity;
a step portion provided from the lip portion toward an outer periphery of the cavity and recessed to a predetermined depth from the top surface,
The central axis of the step portion is offset toward the central axis of the piston with respect to the central axis of the cavity.
Combustion chamber structure of an internal combustion engine.
According to this combustion chamber structure, in a combustion chamber structure provided with a step portion, the spread of the spray and the degree of interference with the cylinder head can be reduced, and the mixing of air and fuel can be promoted.
Specifically, by offsetting the center axis of the step portion toward the center axis of the piston from the center axis of the cavity, the step portion can be made wider in the offset direction. Therefore, air and fuel can be mixed sufficiently in the squish area in the offset direction, and the formation of a locally rich mixture can be suppressed. This can also be expected to reduce soot and improve thermal efficiency.

(2) A distance by which the central axis of the step portion is offset is equal to or less than twice the distance between the central axis of the cavity and the central axis of the piston.
A combustion chamber structure for an internal combustion engine as described in (1) above.
This combustion chamber structure can realize an appropriate combustion chamber structure, specifically, it can reduce the risk that the top surface of the piston in the offset direction becomes too narrow, causing a problem in mechanical strength, or that the step on the opposite side of the offset direction becomes too narrow, causing a problem in mixing fuel and air at the step on the opposite side.

(3) The central axis of the step portion is offset so as to be positioned on the central axis of the piston.
A combustion chamber structure for an internal combustion engine according to (1) or (2) above.
With this combustion chamber structure, a space appropriate for the amount of fuel sprayed can be secured both at the step portion in the offset direction and at the step portion opposite the offset direction, thereby further promoting the mixing of air and fuel.

1: Combustion chamber 2: Cylinder head 2a: Cylinder head (underside)
3: Intake valve 4: Exhaust valve 5: Fuel injection valve 6, 106: Piston 6a: Top surface (of piston) 7: Cylinder 11, 111: Ball portion 21a, 21b, 121: Step portion 22a, 22b, 122: Guide surface 23a, 23b, 123: Lip portion A1: Piston central axis B1: Axis C1: Step portion central axis D1: Cavity central axis X1, X11: Area

Claims (3)

A combustion chamber structure of an internal combustion engine having a combustion chamber defined by a piston, a cylinder, and a cylinder head,
a fuel injection valve provided in the cylinder head at a predetermined angle with respect to a piston central axis;
a cavity formed in a top surface of the piston and offset with respect to a central axis of the piston ;
a protrusion provided at the center of a bottom surface of the cavity so as to have a mountain-shaped cross section;
a ball portion that smoothly connects the protrusion portion and a side wall of the cavity and has an arc-shaped cross section;
a lip portion provided annularly along a circumferential direction of a side wall rising from a bottom surface of the cavity and protruding toward a center of the cavity;
a step portion provided from the lip portion toward an outer periphery of the cavity and recessed to a predetermined depth from the top surface,
a step portion central axis that is an axis passing through a center of the step portion is offset toward the piston central axis side with respect to a cavity central axis that is an axis passing through the center of the bottom surface of the cavity ,
The fuel is injected from the fuel injection valve toward the lip portion, and when the fuel collides with the lip portion, the fuel is divided into a portion heading toward the step portion and a portion heading toward the ball portion, and the portion heading toward the step portion diffuses into a squish area, which is in the vicinity of a side wall portion of the cylinder between the top surface and the cylinder head, without interfering with the cylinder head.
Combustion chamber structure of an internal combustion engine. 2. The combustion chamber structure for an internal combustion engine according to claim 1, wherein a distance by which the central axis of the stepped portion is offset is equal to or less than twice the distance between the central axis of the cavity and the central axis of the piston. The step central axis is offset to be located on the piston central axis.
A combustion chamber structure for an internal combustion engine according to claim 1 or 2.

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