JP2016132992A - Compression ignition type internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the overlap of flames derived from injection holes from an initial stage of combustion, in a compression ignition type internal combustion engine having a piston which has a top face at which a heat insulation film is formed, and an in-cylinder injector which radially injects fuel toward the top face from the plurality of injection holes.SOLUTION: A top face of a piston 14 is divided into eight regions to which injected fuel is spread. The adjacent two regions of the eight regions are partitioned by a line which passes through an intermediate part of axial lines of the adjacent two injection holes. A heat insulation film 24a is formed at one of the adjacent two regions, and a heat insulation film 24b is formed at the other. The surface roughness Ra of the heat insulation film 24a 2 μm or less, and the surface roughness Ra of the heat insulation film 24b is 4 μm or more.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a compression ignition type internal combustion engine, and more particularly to a compression ignition type internal combustion engine in which a heat insulating film is formed on the top surface of a piston.

  Conventionally, for example, Japanese Patent Application Laid-Open No. 2009-243355 discloses that the top surface of a piston of a compression ignition internal combustion engine has a lower conductivity than the base material of the piston and is lower than the base material or the base material. A technique for forming a heat insulating film including a heat insulating material having a heat capacity per unit volume substantially equal to that of the material is disclosed. Since this heat insulating film includes the heat insulating material having the above-described characteristics, the temperature of the top surface of the piston can be made to follow the gas temperature in the combustion chamber which changes every moment in the cycle of the internal combustion engine, thereby The heat loss in can be reduced.

JP 2009-243355 A JP 2012-047134 A Japanese Utility Model Publication No. 63-31242

  By the way, a general compression ignition type internal combustion engine such as the above-mentioned internal combustion engine includes an in-cylinder injector having a plurality of injection holes, and from each injection hole to the top surface of the piston in the vicinity of the compression top dead center. The fuel is injected towards. Here, the fuel injected from each injection hole diffuses radially from the central portion of the top surface of the piston toward the outer peripheral portion while entraining the air in the cylinder. In this diffusion process, the fuel is heated to a high temperature due to compression, thereby generating a plurality of flame nuclei. These flame nuclei are heated to a surrounding temperature to become a growth flame, and the combustion spreads over a wide range. This is the combustion mechanism of a compression ignition type internal combustion engine.

  However, these flame nuclei are generated on the axis of each nozzle hole and at a location where the distance from each nozzle hole is substantially equal. This is because the speed of the fuel injected from each nozzle hole is substantially equal, and the top surface of the piston has a uniform shape in the concentric direction. Accordingly, the plurality of flame nuclei described above are concentrically distributed on the top surface of the piston.

  When a plurality of flame nuclei are concentrically formed on the top surface of the piston, the growth flames derived from the respective flame nuclei overlap each other on a certain concentric circle. Of course, from the middle stage of combustion to the latter stage, overlapping of the growth flames derived from each flame kernel occurs. However, if this overlap occurs from the early stage of combustion, the overlap will continue for such a long time. If it does so, the surface temperature of the heat insulation film located directly under the overlapping part of a growth flame will become high compared with the surface temperature of the heat insulation film located directly under a non-overlapping part.

  As the surface temperature of the heat insulating film increases, the temperature difference between the heat insulating film and the piston base material increases, so that the amount of heat escaping from the surface side of the heat insulating film to the piston base material side increases. Therefore, if the growth flame overlaps at an early stage in the early stage of combustion, the heat insulating effect by the heat insulating material described above, that is, the effect of following the gas temperature in the combustion chamber (the heat insulating film rises according to the gas temperature in the combustion chamber, The effect of reducing the temperature difference of the heat insulating film and suppressing the amount of heat escape decreases. In addition, there is a problem that soot is easily generated due to the overlapping of growth flames.

  The present invention has been made to solve the above-described problems. That is, in a compression ignition type internal combustion engine including a piston having a top surface on which a heat insulating film is formed and an in-cylinder injector that injects fuel radially from the plurality of injection holes toward the top surface, flames derived from the injection holes The purpose is to suppress the overlap of the fuel from an early stage of combustion.

In order to solve the above-described problem, a first invention includes a piston having a top surface on which a heat insulating film is formed, and an in-cylinder injector that injects fuel radially from the plurality of injection holes toward the top surface. A compression ignition internal combustion engine comprising:
The top surface is divided into a plurality of regions corresponding to the number of the nozzle holes by an intermediate line passing between the axes of two adjacent nozzle holes among the nozzle holes,
Two adjacent regions of the regions are different in surface roughness.

The second invention is the first invention, wherein
The number of the nozzle holes is an even number of 2 or more,
The region is formed by forming a smooth surface region having a surface roughness of 2 μm or less and a rough surface region having a surface roughness of 4 μm or more every other circumferential direction of the top surface. And

The third invention is the second invention, wherein
The top surface is formed with a cavity having a frustoconical bottom portion,
The rough surface region is formed on a surface corresponding to the upper surface of the truncated cone of the bottom surface portion.

  According to the first invention, the surface roughness of two adjacent regions on the top surface on which the heat insulating film is formed is different, so that a difference is caused in the concentric mobility of the injected fuel from the adjacent injection holes. The time when the growth flames derived from each flame kernel overlap can be delayed. Therefore, it can suppress that the flame derived from each nozzle hole overlaps from the early stage of the combustion initial stage.

  According to the second invention, in the rough surface region where the surface roughness is 4 μm or more, the mobility of the injected fuel in the concentric direction is lowered, while in the smooth surface region where the surface roughness is 2 μm or less, the injected fuel is reduced. The mobility in the concentric direction can be improved.

  Generally, the surface roughness of the heat insulation film is large and is adjusted by polishing. For this reason, when the surface roughness of the heat insulating film is reduced by polishing, the time required for polishing increases in proportion to the surface roughness after processing. According to the third invention, since the rough surface region is formed on the upper surface of the truncated cone of the bottom surface portion, the time required for polishing the heat insulating film in the rough surface region can be shortened.

1 is a schematic cross-sectional view of an internal combustion engine 10 of a first embodiment. It is the top view which looked at the top face of piston 14 of Drawing 1 from the cylinder head 16 side. It is AA sectional drawing of FIG. It is the figure which showed a mode that the flame kernel produced from the injected fuel. It is AA sectional drawing of FIG. FIG. 3 is a plan view of a top surface of a piston 26 of a comparative internal combustion engine as viewed from the cylinder head 16 side. It is AA sectional drawing of FIG. It is the figure which showed a mode that the flame kernel produced from the injected fuel. It is AA sectional drawing of FIG. FIG. 10 is a diagram for illustrating a modification of the first embodiment. FIG. 6 is a plan view of a top surface of a piston 32 of an internal combustion engine according to a second embodiment as viewed from the cylinder head 16 side. It is AA sectional drawing of FIG. It is the figure which showed a mode that the flame kernel produced from the injected fuel. It is AA sectional drawing of FIG. FIG. 6 is a plan view of a top surface of a piston 34 of an internal combustion engine according to a second embodiment when viewed from the cylinder head 16 side. It is the figure which showed a mode that the flame kernel produced from the injected fuel.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure, and the overlapping description is abbreviate | omitted. The present invention is not limited to the following embodiments.

Embodiment 1 FIG.
First, a first embodiment of the present invention will be described with reference to FIGS.

  The internal combustion engine of the first embodiment is a compression ignition type internal combustion engine (specifically, a diesel engine) mounted on a moving body such as a vehicle. FIG. 1 is a schematic sectional view of an internal combustion engine 10 according to the first embodiment. As shown in FIG. 1, an internal combustion engine 10 is mounted on a cylinder bore 12 formed in a cylinder block (not shown), a piston 14 accommodated in the cylinder bore 12 so as to be movable in the vertical direction, and mounted above the cylinder block. The cylinder head 16 is provided. A combustion chamber 18 is defined by at least the wall surface of the cylinder bore 12, the top surface of the piston 14, and the bottom surface of the cylinder head 16.

  A cavity 20 is formed at the center of the top surface of the piston 14. This cavity 20 also constitutes a part of the combustion chamber 18. The cavity 20 includes a substantially truncated cone-shaped bottom surface portion 20a, a substantially cylindrical side surface portion 20b, and an intermediate portion 20c that connects the bottom surface portion 20a and the side surface portion 20b. An injector 22 for directly injecting fuel into the combustion chamber 18 is attached on the piston central axis Lp in the cylinder head 16. At the tip of the injector 22, eight nozzle holes (not shown) are formed at 45 ° intervals centered on one point on the piston central axis Lp. The fuel injection timing of the injector 22 is set near the compression top dead center, and at this timing, fuel is injected radially from the eight injection holes toward the cavity 20 (specifically, the side surface portion 20b).

  A heat insulating film 24 is formed on the entire top surface of the piston 14. The heat insulating film 24 is composed of an alumite film. The alumite film is obtained by anodizing the base material (aluminum alloy) of the piston 14, and has innumerable pores formed in the course of the anodizing process. By having such a porous structure, the alumite film functions as a heat insulating film having a low thermal conductivity and a low heat capacity per unit volume compared to the piston base material. According to the alumite film, the temperature of the top surface of the piston 14 can be made to follow the gas temperature in the combustion chamber 18 that changes every cycle in the cycle of the internal combustion engine 10, thereby reducing heat loss in the combustion chamber 18. .

  In the internal combustion engine 10 of the first embodiment, a swirl flow is generated in the combustion chamber 18. Here, the swirl ratio SR of the swirl flow (referred to as the ratio between the rotational speed of the swirl flow and the engine rotational speed; the same shall apply hereinafter) varies depending on the engine operating state. A swirl flow having a swirl ratio, that is, a swirl flow having a maximum swirl ratio SR of 1.0 is generated. The method for generating the swirl flow can be a conventionally known method. For example, a swirl flow can be generated in the combustion chamber 18 by the intake air flowing into the combustion chamber 18 from the intake port by arranging an intake port (not shown) formed in the cylinder head 16 along the circumferential direction of the cylinder bore 12. .

  FIG. 2 is a plan view of the top surface of the piston 14 of FIG. 1 as viewed from the cylinder head 16 side. In several drawings of this application including this figure, the injected fuel from each injection hole of the injector 22 is shown. Shown as “spray”. As shown in FIG. 2, the top surface of the piston 14 is divided into eight regions where the injected fuel diffuses. Two adjacent regions of the eight regions are separated by a line passing through the middle of the axis of two adjacent nozzle holes, the central angle of each region is 45 °, and the intermediate line from the axis of the nozzle hole The angle up to half of that is 22.5 °. This is because the swirl flow generated in the combustion chamber 18 has a low swirl ratio, and the injected fuel is hardly flowed by the swirl flow.

  Further, as shown in FIG. 2, a heat insulating film 24a is formed in one of two adjacent regions, and a heat insulating film 24b is formed in the other. That is, every two heat insulating films 24 a and 24 b are formed in the circumferential direction of the top surface of the piston 14. However, the heat insulating film 24a is not formed in the region immediately below the injector 22, and only the heat insulating film 24b is formed. FIG. 3 is a schematic vertical cross-sectional view of the vicinity of the top surface of the piston 14, and corresponds to the AA cross section of FIG. 2. As shown in FIG. 3, the heat insulating film 24b is formed in a region immediately below the injector 22, that is, a surface corresponding to the top surface of the truncated cone of the bottom surface portion 20a and a part of the surface corresponding to the side surface of the truncated cone.

  Here, the surface roughness Ra (referring to the arithmetic average roughness measured in accordance with JIS B601 (2001). The same shall apply hereinafter) of the heat insulating film 24a is 2 μm or less (preferably an average of 0.5 to 2 μm). The surface roughness Ra of the heat insulating film 24b is 4 μm or more (preferably 4 to 8 μm on average). The surface roughness Ra of the heat insulating films 24a and 24b is adjusted as follows. That is, (i) the above-described alumite film (average surface roughness Ra: 4 to 8 μm) is formed on the entire top surface of the piston 14. (ii) The region corresponding to the heat insulating film 24b is not polished, and only the region corresponding to the heat insulating film 24a is polished to reduce the surface roughness Ra of the region to an average of 2 μm or less.

  Returning to FIG. 3, the description will be continued. As shown in FIG. 3, the injected fuel collides with the side surface portion 20 b and diffuses both inside (that is, the intermediate portion 20 c and the bottom surface portion 20 a side) and outside of the cavity 20. In this diffusion process, the fuel is heated to a high temperature by compression to generate a flame nucleus, and further, the flame nucleus is heated to a surrounding temperature to become a growth flame. FIG. 4 is a diagram showing how flame nuclei are generated from the injected fuel. As shown in FIG. 4, two flame nuclei are generated on the axis of each nozzle hole. However, when viewed in the concentric direction, the positions of the flame nuclei generated in the region where the heat insulating film 24a is formed are different from the positions of the flame nuclei generated in the region where the heat insulating film 24b is formed.

FIG. 5 corresponds to the AA cross section of FIG. As shown in FIG. 5, in the region where the heat insulating film 24a is formed, the distance between the _two flame nuclei fk ₁ and fk ₂ on the axis of the same nozzle hole is large. In contrast, in the region where the heat insulating film 24b is formed, the distance between the two flame nuclei fk ₃ and fk ₄ is small. The reason for this is that the surface roughness Ra of the heat insulating film 24a and the heat insulating film 24b is different, and thus the concentric mobility of the injected fuel after the collision with the side surface portion 20b until the generation of the flame nuclei fk _{1 to} fk ₄ occurs This is because a difference occurs. In other words, in the region where the heat insulating film 24a is formed, the mobility of the injected fuel in the concentric direction is higher than in the region where the heat insulating film 24b is formed.

  6 to 9 are views for explaining the top surface of a piston of a comparative internal combustion engine. The top view and cross-sectional view of the top surface of the piston 26 shown in FIGS. 6 to 9 are basically the same as the top view and cross-sectional view of the top surface of the piston 14 shown in FIGS. However, the internal combustion engine for comparison is different from the internal combustion engine 10 in that the surface roughness Ra of the heat insulating film 28 (see FIG. 6) formed on the top surface of the piston 26 is uniform (4 μm or more).

When the surface roughness Ra of the heat insulating film 28 is uniform, there is almost no difference in the mobility of the injected fuel from each nozzle hole (see FIG. 7). Therefore, two flame nuclei generated on the axis of each nozzle hole are distributed concentrically (see FIG. 8). This is because the interval between two flame nuclei fk ₅ and fk ₆ on the axis of one nozzle hole becomes equal to the interval between two flame nuclei fk ₇ and fk ₈ on the axis of another nozzle hole ( (See FIG. 9). When the flame nuclei are distributed concentrically, the growth flames derived from the respective flame nuclei overlap each other on the circumference of a certain concentric circle, and the above-described problem, that is, the effect of following the gas temperature in the combustion chamber 18 is reduced, or There arises a problem that cocoons are easily generated.

  In this regard, according to the internal combustion engine 10 described above, since the heat insulation films 24a and the heat insulation films 24b are formed every other in the circumferential direction of the top surface of the piston 14, concentric circles of the injected fuel from the adjacent injection holes. Differences in direction mobility can be made. Therefore, the time when the above-mentioned growth flame overlaps can be delayed. Therefore, since the bias of the surface temperature in the concentric direction of the heat insulating film 24 can be reduced, it is possible to suppress a decrease in the effect of following the gas temperature in the combustion chamber 18. Moreover, the production amount of soot can also be suppressed.

  By the way, in the said Embodiment 1, the heat insulation film | membrane 24 was comprised from the alumite film | membrane. However, a sealing film that closes the pores may be formed on the surface of the alumite film. Also, instead of the anodized film, ceramic sprayed film (including metal bond layer sprayed film for stress relaxation) such as zirconia, yttria, alumina, silica, titania, etc., which has lower thermal conductivity than the piston base material may be provided. it can. Also, instead of an anodized film, a film having a lower thermal conductivity than the piston base material and a lower heat capacity per unit volume (for example, hollow beads made by sintering ceramic particles having a hollow structure and baked and hardened with a ceramic adhesive as a binder) A film) can also be provided.

  Further, in the first embodiment, the injector 22 has eight injection holes, and the heat insulating film 24 having two types of surface roughness (that is, in the eight regions on the top surface of the piston 14 corresponding to these injection holes). Insulating films 24a and 24b) were formed. However, the number of nozzle holes is not limited to eight, and may be two or more and seven or less, or nine or more. Moreover, the surface roughness of the heat insulating film 24 may be three or more, and the surface roughness may be different in each of the eight regions. FIG. 10 is a diagram for explaining a modification of the first embodiment. The top surface of the piston 30 shown in FIG. 10 is divided into three regions, and the heat insulating films 24a and 24b having the above-described surface roughness are formed in two of these regions. In the remaining one region, a heat insulating film 24c having a surface roughness Ra intermediate between the heat insulating film 24a and the heat insulating film 24b is formed. As described above, if the combination of the surface roughness of the heat insulating film can cause a difference in the concentric mobility of the injected fuel from the adjacent nozzle holes of the injector having the plurality of nozzle holes, the above embodiment One modification can be used. Note that this modification can be similarly applied to embodiments described later.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to FIGS.

  11 to 14 are views for explaining the top surface of the piston of the internal combustion engine of the second embodiment. The top view and cross-sectional view of the top surface of the piston 32 shown in FIGS. 11 to 14 are basically the same as the top view and cross-sectional view of the top surface of the piston 14 shown in FIGS. However, the internal combustion engine of the second embodiment differs from the internal combustion engine 10 of the first embodiment in that the area of the heat insulating film 24a formed in the region immediately below the injector 22 is large.

  As shown in FIG. 11, the heat insulating film 24a is not formed in the region immediately below the injector 22, and only the heat insulating film 24b is formed. FIG. 12 is a schematic vertical cross-sectional view of the combustion chamber 18, and corresponds to a cross section AA in FIG. As shown in FIG. 12, the heat insulating film 24b is formed in a region immediately below the injector 22, that is, a surface corresponding to the upper surface of the truncated cone of the bottom surface portion 20a.

  As described in the first embodiment, the surface roughness Ra of the heat insulating film 24a is 2 μm or less, and the surface roughness Ra is realized by polishing a region corresponding to the heat insulating film 24a. Therefore, the larger the polishing area in the region directly under the injector 22, the more time is required for the polishing process. Therefore, from the viewpoint of shortening the processing time, it is desirable that the area of the heat insulating film 24a is narrow.

  However, the mobility of the injected fuel in the inner direction of the cavity 20 after the collision with the side surface portion 20b is improved as the surface roughness Ra of the heat insulating film 24 formed on the inner side becomes smaller. Considering this point, in the second embodiment, the area of the heat insulating film 24a formed in the region immediately below the injector 22 is increased. Although it is possible to form the entire surface corresponding to the top surface of the truncated cone of the bottom surface portion 20a with the heat insulating film 24a, the surface roughness of this surface hardly affects the mobility of the injected fuel. In consideration of this point, in the second embodiment, this surface is not polished and the polishing time is reduced.

FIG. 13 shows how flame nuclei are generated from the injected fuel, and FIG. 14 corresponds to the AA cross section of FIG. As can be seen by comparing FIG. 13 and FIG. 4 or FIG. 14 and FIG. 5, according to the second embodiment, flame nuclei generated inside the cavity 20 and in the region where the heat insulating film 24a is formed are It can be distributed more centrally than the portion 20a. As can be seen from the comparison between FIG. 14 and FIG. 5 in particular, the interval between the two flame nuclei fk ₉ and fk ₁₀ in FIG. 14 can be made larger than the interval between the _two flame nuclei fk ₁ and fk ₂ in FIG. . As described above, according to the second embodiment, a larger difference can be caused by the mobility of the injected fuel from the adjacent nozzle holes in the concentric direction as compared with the first embodiment. Accordingly, it is possible to further delay the time when the above-described growth flames overlap.

Embodiment 3 FIG.
Next, a third embodiment of the present invention will be described with reference to FIGS.

  15 to 16 are views for explaining the top surface of the piston of the internal combustion engine of the third embodiment. The plan view of the top surface of the piston 34 shown in FIGS. 15 and 16 is basically the same as the plan view of the top surface of the piston 14 shown in FIGS. 2 and 4. However, in the third embodiment, a swirl flow having a normal swirl ratio, that is, a swirl flow having a maximum swirl ratio SR of 2.0 is generated in the combustion chamber 18 and the positions where the heat insulating films 24a and 24b are formed. The internal combustion engine 10 of the first embodiment is different from the internal combustion engine 10 of the first embodiment in that it is offset in the swirl direction from a line passing through the middle of the axis of two adjacent nozzle holes.

  When a swirl flow having a normal swirl ratio is generated in the combustion chamber 18, the injected fuel flows in the swirl direction shown in FIG. Considering this point, in the third embodiment, the formation positions of the heat insulating films 24a and 24b are offset by 2.5 ° from the intermediate line described above. Therefore, according to the third embodiment, an effect similar to that of the first embodiment can be obtained even in an engine configuration in which the injected fuel flows in the swirl direction.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 14, 26, 30, 32, 34 Piston 18 Combustion chamber 20 Cavity 22 Injector 24a, 24b, 24c, 28 Thermal insulation film

Claims (3)

A compression ignition internal combustion engine comprising: a piston having a top surface on which a heat insulating film is formed; and a cylinder injector that injects fuel radially from a plurality of injection holes toward the top surface,
The top surface is divided into a plurality of regions corresponding to the number of the nozzle holes by an intermediate line passing between the axes of two adjacent nozzle holes among the nozzle holes,
2. A compression ignition type internal combustion engine, wherein two adjacent regions of the region have different surface roughness. The number of the nozzle holes is an even number of 2 or more,
The region is formed by forming a smooth surface region having a surface roughness of 2 μm or less and a rough surface region having a surface roughness of 4 μm or more every other circumferential direction of the top surface. The compression ignition internal combustion engine according to claim 1. The top surface is formed with a cavity having a frustoconical bottom portion,
The compression ignition type internal combustion engine according to claim 2, wherein the rough surface region is formed on a surface corresponding to the upper surface of the truncated cone of the bottom surface portion.

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