Crown surface structure of piston and method for machining same

Abstract

A crown surface of a piston (1) comprises: a central tumble preservation surface (2) recessed in a dish shape; an outer peripheral squish surface (3) comprising a conical surface forming a minute squish gap between the outer peripheral squish surface (3) and a combustion chamber wall surface on a cylinder-head side; a pair of intake valve recess parts (4) and a pair of exhaust valve recess parts (5) formed so as to avoid interference with an intake valve and an exhaust valve; and a pair of flat ground faces (6) each comprising a flat surface. The tumble preservation surface (2) and the outer peripheral squish surface (3) form a rotationally symmetrical shape centered on a center line (CL1) of the piston (1), and are machined into mirror surfaces having an arithmetic average roughness (Ra) of 0.5 μｍ or less by ultraprecision cutting or burnishing.

Description

Piston crown surface structure and its processing method

[0001] This invention relates to the crown surface structure of a piston in a spark ignition internal combustion engine combined with a pent-roof type combustion chamber on the cylinder head side.

[0002] By mirror-polishing the surface of the piston crown surface that contacts the gas in the combustion chamber, the turbulent flow components near the surface are reduced, and the heat transfer amount is decreased. Therefore, it is known that the heat transfer amount lost as cooling loss from the gas to the piston in the expansion stroke is reduced, and the indicated thermal efficiency is improved.

[0003] Patent Document 1 discloses that for a piston having a central recessed portion recessed in a dish shape at the center of the crown surface, while pressing a cotton buff rotating at high speed as a polishing member against the crown surface with an appropriate load, the piston as a workpiece is reciprocated in the diametrical direction, so-called buff polishing is performed.

[0004] However, when attempting to perform mirror processing on the piston crown surface by such buff polishing, since the piston is polished with a rotating polishing member while reciprocating in the diametrical direction, the processing time for each piston becomes long, and a polishing facility different from general piston processing facilities is required, which is not preferable.

[0005] Japanese Unexamined Patent Application Publication No. 2022-079301

[0006] This invention is a crown surface structure of a piston in a spark ignition internal combustion engine combined with a pent-roof type combustion chamber on the cylinder head side, comprising a central tumble retention surface recessed in a dish shape, an outer peripheral squish surface located on the outer periphery of this tumble retention surface and forming a minute squish gap with the above pent-roof type combustion chamber, and intake valve recess portions and exhaust valve recess portions formed so as to avoid interference with the intake valve and the exhaust valve. The above tumble retention surface has a rotationally symmetric shape centered on a first center line parallel to or the same as the piston center line, the above outer peripheral squish surface has a rotationally symmetric shape centered on a second center line parallel to or the same as the first center line, and the above tumble retention surface and the above outer peripheral squish surface form a mirror surface with an arithmetic mean roughness Ra of 0.5 μm or less.

[0007] Thus, because the tumble-preserving surface and the outer squish surface to be polished to a mirror finish have a rotationally symmetric shape, a mirror finish can be achieved relatively easily by machining while the workpiece is rotating at high speed, such as by precise cutting or burnishing along the rotationally symmetric shape. A mirror finish with an arithmetic mean roughness Ra of 0.5 μm or less can be sufficiently obtained by precise cutting, and the aforementioned improvement in thermal efficiency due to the mirror finish can be achieved.

[0008] A perspective view of the piston crown surface of one embodiment. A plan view and a cross-sectional view along line A-A in Figure 2. A cross-sectional view along line A-A in Figure 2 showing the piston combined with the cylinder head side combustion chamber. A cross-sectional view along line B-B in Figure 2 showing the piston combined with the cylinder head side combustion chamber. A plan view showing an example of the cylinder head side combustion chamber. An explanatory diagram of the machining process of one embodiment. An explanatory diagram schematically showing the piston crown surface of the second embodiment.

[0009] Hereinafter, an embodiment of this invention will be described in detail with reference to the drawings. Figures 1 to 3 show the crown structure of a piston 1 of one embodiment. The piston 1 of this embodiment is used in a spark-ignition internal combustion engine (a so-called gasoline engine) equipped with a pair of intake valves and a pair of exhaust valves in each cylinder, and is combined with a pent-roof type combustion chamber on the cylinder head side to form a combustion space (a so-called combustion chamber) as will be described later. In particular, the internal combustion engine in which the illustrated piston 1 is used is an in-cylinder injection type internal combustion engine in which fuel is injected directly into the cylinder by a fuel injection valve, and is configured to perform combustion by utilizing the tumble flow generated in the cylinder by the shape of the intake port itself or by a tumble control valve placed in the intake port.

[0010] As shown in Figures 1 to 3, the crown surface of the piston 1 includes a dish-shaped recessed central tumble-retaining surface 2, an outer peripheral squish surface 3 located on the outer circumference of the tumble-retaining surface 2 that forms a minute squish gap between it and the combustion chamber wall on the cylinder head side, a pair of intake valve recesses 4 and a pair of exhaust valve recesses 5 formed to avoid interference with the intake valve and exhaust valve, and a pair of flat surfaces 6 that are planes perpendicular to the center line CL1 of the piston 1 (see Figure 3).

[0011] The tumble preservation surface 2 is the portion that forms a substantial combustion space with the combustion chamber wall on the cylinder head side, and as shown in Figure 3, its cross-sectional shape is a smoothly continuous curve suitable for preserving tumble flow. This tumble preservation surface 2 is formed to occupy a relatively large area in the center of the piston 1, leaving an annular outer squish surface 3 on the outer circumference of the piston 1. Here, this tumble preservation surface 2 has a rotationally symmetric shape with respect to the center line CL1 of the piston 1. In other words, the tumble preservation surface 2 is surrounded by a circular ridge line L1 that forms a perfect circle with respect to the center line CL1, and the inner circumference of this circular ridge line L1 is concave in a dish shape consisting of a smoothly continuous curved surface.

[0012] The outer squish surface 3 is formed in an annular shape between the outer circumference line L2 of the piston crown surface and the circular ridge line L1. As shown in Figure 3, this outer squish surface 3 is formed in a gentle taper shape, gradually becoming higher towards the inner circumference from the outer circumference line L2 towards the circular ridge line L1, and in particular, it has a rotationally symmetric shape around the center line CL1 of the piston 1. In other words, the outer squish surface 3 is a conical surface with a relatively large apex angle.

[0013] The pair of intake valve recesses 4 and the pair of exhaust valve recesses 5 have shapes corresponding to the valve heads of the intake and exhaust valves, which are made up of poppet valves, and are provided at positions where they intersect with the circular ridge line L1. In other words, the pair of intake valve recesses 4 and the pair of exhaust valve recesses 5 are arranged across the tumble preservation surface 2 and the outer squish surface 3.

[0014] Furthermore, the pair of flat surfaces 6 are formed in a crescent shape at two locations along the crankshaft axis of the piston 1 (in other words, at the edges facing the front-rear direction of the internal combustion engine). More specifically, as shown in Figure 2, the flat surfaces 6 are formed between the straight line L3, which corresponds to a chord perpendicular to the crankshaft axis, and the outer circumference line L2. These flat surfaces 6 are at the same height as the outer circumference line L2 and have a shape as if a part of the outer squish surface 3, which is made up of a tapered surface (conical surface), has been cut into a flat surface. Therefore, there is a slight step difference at the straight line L3 that forms the boundary between the two (see Figure 5).

[0015] As described above, the tumble preservation surface 2 and the outer squish surface 3, which have a rotationally symmetric shape around the center line CL1, are finished to a mirror surface with an arithmetic mean roughness Ra of 0.5 μm or less, preferably a mirror surface with an arithmetic mean roughness Ra of 0.3 μm or less, by cutting along the rotationally symmetric shape using a lathe or milling machine. For example, it is possible to obtain a mirror surface with an arithmetic mean roughness Ra of 0.1 μm or less by using a lathe that rotates the workpiece piston 1 at high speed around the center line CL1 and cutting with an ultra-precision machining tool using a tip made of single-crystal diamond. Alternatively, a cutting process to an arithmetic mean roughness Ra of about 0.3 μm may be performed, followed by burnishing to finish to a mirror surface with an arithmetic mean roughness Ra of 0.1 μm or less. Burnishing is a process that plastically deforms minute protrusions by pressing a hard roller against the surface of a workpiece that is rotating at high speed, thereby creating a mirror surface.

[0016] In this way, the tumble preservation surface 2 and the outer squish surface 3, which are targeted for mirror polishing, have a rotationally symmetrical shape, making their machining easier. That is, the machining time for each piston 1 is shortened, and polishing equipment separate from the general piston machining equipment is not required. In both ultra-precision cutting and burnishing processes, the surfaces of the tumble preservation surface 2 and the outer squish surface 3 become machined surfaces that follow a rotationally symmetrical shape, that is, machined surfaces that have been machined in the circumferential direction.

[0017] On the other hand, the intake valve recess 4, the exhaust valve recess 5, and the flat surface 6 have not been mirror-finished and remain as machined surfaces that have undergone general cutting or other machining processes. Therefore, these surfaces are rough surfaces with an arithmetic mean roughness Ra greater than 0.5 μm, or more specifically, rough surfaces with an arithmetic mean roughness Ra of approximately 0.7 μm.

[0018] Figures 4 and 5 are cross-sectional views of the piston 1 at top dead center when the crown structure of the piston 1 described above is combined with the combustion chamber 21 on the cylinder head side. The combustion chamber 21 on the cylinder head side is generally composed of an intake side main wall surface 23 through which the valve port 22 of the intake valve opens, an exhaust side main wall surface 25 through which the valve port 24 of the exhaust valve opens, an intake side squish surface 26, and an exhaust side squish surface 27, as schematically shown in Figure 6, with a spark plug 28 and a fuel injector 29 located in the center. The intake side main wall surface 23 and the exhaust side main wall surface 25 are basically inclined planes, forming a so-called pent-roof type combustion chamber. In contrast, the intake side squish surface 26 and the exhaust side squish surface 27 have a shape that follows a conical surface corresponding to the outer peripheral squish surface 3 which is a conical surface on the piston 1 side.

[0019] Therefore, as shown in Figure 4, at the top dead center position of the piston 1, a small squish gap 30 of a certain width is formed between the squish surfaces 26 and 27 on the cylinder head side and the outer peripheral squish surface 3 on the piston 1 side. In the illustrated example, in particular, when the piston 1 descends from the top dead center position shown in Figure 4, a reverse squish action is obtained in which gas is drawn into the squish gap 30, thereby strengthening the gas flow in the combustion chamber. Here, it is desirable that the tumble preservation surface 2 on the piston 1 side is recessed such that it is at a distance of 8 mm or more (distance in the height direction) from the ignition point of the spark plug 28 at the top dead center position of the piston 1 shown in Figure 4. By forming a combustion space between the tumble preservation surface 2, which has a smooth curved surface, and the cylinder head side, the tumble generated in the cylinder during the intake stroke is preserved.

[0020] In the above embodiment, since the tumble storage surface 2 and the outer squish surface 3 are mirror-finished, as mentioned above, the turbulent component near the surface is reduced, and the amount of heat transfer is reduced. Therefore, the amount of heat transferred from the gas to the piston 1 as cooling loss during the expansion stroke is reduced, and the indicated thermal efficiency is improved. In the above embodiment, the area ratio occupied by the roughened valve recesses 4 and 5 and the flat surface 6 is small, and most of the crown surface is mirror-finished, so the effect of mirror finishing is fully obtained.

[0021] Furthermore, because the outer squish surface 3 is mirror-like, deposit adhesion and accumulation are less likely to occur, making it possible to design the squish gap 30 to be smaller.

[0022] As shown in Figure 5, in the direction along the crankshaft axis, the wall surface of the combustion chamber 21 on the cylinder head side is separated from the outer circumferential squish surface 3 of the piston 1. Therefore, these parts of the outer circumferential squish surface 3 (the parts sandwiched between one intake valve recess 4 and one exhaust valve recess 5 adjacent to each other in Figure 2) do not function as squish-forming surfaces in practice.

[0023] Figure 7 is an explanatory diagram showing the machining process of the piston 1 in the above embodiment. In the first step (a), the piston 1 is formed by die casting using an aluminum alloy as the material. Next, in step (b), the crown surface of the cast piston 1 is roughly machined to form the tumble preservation surface 2 and the outer squish surface 3. Then, these tumble preservation surface 2 and outer squish surface 3 are polished to a mirror finish by cutting or burnishing along the rotationally symmetrical shape. After that, in step (c), the intake valve recess 4, the exhaust valve recess 5 and the flat surface 6 are machined.

[0024] The piston crown structure of one embodiment has been described above, but in this invention, the tumble preservation surface 2 and the outer circumferential squish surface 3 may each have different centerlines and be rotationally symmetric. Figure 8 schematically shows such a second embodiment of the crown structure, where the dish-shaped recessed central tumble preservation surface 2 has a rotationally symmetric shape around the centerline CL2 which roughly corresponds to the position of the spark plug 28. The outer circumferential squish surface 3, which is provided on the outer circumference of the tumble preservation surface 2 so as to surround it, has a rotationally symmetric shape around the centerline CL1 of the piston 1, and more specifically, it has a conical surface with a relatively large apex angle. Even when the tumble preservation surface 2 and the outer circumferential squish surface 3 have offset centerlines from each other in this way, they each have a rotationally symmetric shape, so they can each be easily mirror-finished by ultra-precision cutting or burnishing. It is also possible to configure the outer circumferential squish surface 3 so that its centerline is offset from the centerline CL1 of the piston 1.

Claims

1. A piston crown structure for a spark-ignition internal combustion engine, which is combined with a pent-roof type combustion chamber on the cylinder head side, comprising: a dish-shaped recessed central tumble preservation surface; an outer circumferential squish surface located on the outer circumference of the tumble preservation surface and forming a minute squish gap between it and the pent-roof type combustion chamber; and intake valve recesses and exhaust valve recesses formed to avoid interference with the intake valve and exhaust valve, wherein the tumble preservation surface has a rotationally symmetric shape about a first center line parallel to or the same as the piston center line; the outer circumferential squish surface has a rotationally symmetric shape about a second center line parallel to or the same as the first center line; and the tumble preservation surface and the outer circumferential squish surface have a mirror surface with an arithmetic mean roughness Ra of 0.5 μm or less.

2. The piston crown structure according to claim 1, wherein the outer circumferential squish surface is continuous as a single conical surface, and flat surfaces consisting of surfaces perpendicular to the piston centerline are provided at the edges of the outer circumferential squish surface in the direction toward the front and rear of the internal combustion engine.

3. The crown surface structure of the piston according to claim 2, wherein the flat surface has an arithmetic mean roughness Ra greater than 0.5 μm.

4. The crown surface structure of the piston according to claim 1, wherein the tumble storage surface and the outer periphery squish surface have a mirror surface with an arithmetic mean roughness Ra of 0.1 μm or less.

5. The crown surface structure of a piston according to claim 1, wherein the tumble preservation surface and the outer circumferential squish surface consist of machined surfaces that conform to their rotationally symmetrical shape.

6. The piston crown structure according to claim 1, wherein the tumble preservation surface is recessed so as to be at a distance of 8 mm or more from the ignition point of the spark plug at the piston top dead center position.

7. The crown surface structure of a piston according to claim 1, wherein the first center line and the second center line are a common center line.

8. The crown surface structure of the piston according to claim 1, wherein the first center line and the second center line are offset from each other.

9. A method for machining the crown surface structure of a piston according to claim 1, wherein the tumble preservation surface and the outer circumferential squish surface are made mirror-finish by cutting or burnishing along a rotationally symmetrical shape.

10. A method for machining the crown surface structure of a piston according to claim 9, wherein, after rough machining the crown surface of the piston after casting, the tumble preservation surface and the outer circumferential squish surface are polished to a mirror finish by cutting or burnishing along a rotationally symmetrical shape, and then the intake valve recess and the exhaust valve recess are machined.

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