Combustion chamber structure of spark ignition type internal combustion engine

Abstract

A combustion space is formed by a crown surface of a piston (1) and a combustion chamber (21) on a cylinder head side. The piston (1) has a tumble storage surface (2) recessed in a dish shape at the center, and an outer peripheral squish surface (3). The combustion chamber (21) on the cylinder head side has: an intake-side main wall surface (23) and an exhaust-side main wall surface (25) that constitute a pent-roof combustion chamber; and an intake-side squish surface (26) and an exhaust-side squish surface (27) that correspond to the outer peripheral squish surface (3) of the piston (1). The arithmetic mean roughness (Ra1) of the tumble storage surface (2), the arithmetic mean roughness (Ra2) of the outer peripheral squish surface (3), the arithmetic mean roughness (Ra3) of the main wall surfaces (23, 25), and the arithmetic mean roughness (Ra4) of the squish surfaces (26, 27) on the cylinder head side have the relationship Ra3 > Ra4 > Ra2 > Ra1.

Description

Combustion Chamber Structure of Spark-Ignition Internal Combustion Engine

[0001] This invention relates to a combustion chamber structure of a spark-ignition internal combustion engine configured by combining a cylinder head side combustion chamber and a piston crown surface.

[0002] Patent Document 1 discloses a combustion chamber structure in which a heat insulation film is formed on the central portion of the piston crown surface in order to reduce the cooling loss and knocking of an internal combustion engine. The heat insulation film is not formed on the squish portion, and mirror finishing is performed.

[0003] However, in this Patent Document 1, no consideration is given to the heat exchange between the gas and the wall surface of the cylinder head side combustion chamber.

[0004] Japanese Patent Application Laid-Open No. 2020-084765

[0005] This invention is a combustion chamber structure of a spark-ignition internal combustion engine configured by combining a cylinder head side combustion chamber and a piston crown surface, where the piston crown surface has a central piston-side combustion space forming surface that forms a combustion space with the cylinder head, and a piston-side squish surface that is outside the piston-side combustion space forming surface and forms a squish gap with the cylinder head side combustion chamber, the cylinder head side combustion chamber has a head-side combustion space forming surface recessed from the lower surface of the cylinder head so as to form a combustion space with the piston-side combustion space forming surface, and a head-side squish surface that is outside the head-side combustion space forming surface and faces the piston-side squish surface, the arithmetic mean roughness Ra1 of the piston-side combustion space forming surface, the arithmetic mean roughness Ra2 of the piston-side squish surface, the arithmetic mean roughness Ra3 of the head-side combustion space forming surface, and the arithmetic mean roughness Ra4 of the head-side squish surface are set in the relationship of Ra3 > Ra4 > Ra2 > Ra1.

[0006] In order to suppress knocking, which is a problem in a spark-ignition internal combustion engine, it is preferable to lower the in-cylinder gas temperature near top dead center of compression, so-called compression end temperature. Generally, the temperature of the wall surface of the cylinder head side combustion chamber during the combustion cycle is relatively lower than the temperature of the piston crown surface.

[0007] Furthermore, during the intake stroke, the temperature of the gas inside the cylinder is lower than that of these metal walls (the combustion chamber wall on the cylinder head side and the piston crown surface), so it is preferable to suppress the transfer of heat from the metal walls to the gas inside the cylinder (the rise in the temperature of the gas inside the cylinder). By setting the arithmetic mean roughness Ra1 of the piston-side combustion space forming surface, which is relatively hotter among the metal walls and mainly in contact with the gas inside the cylinder, to the smallest possible value (i.e., a smooth surface), the transfer of heat from the hot piston to the gas inside the cylinder is suppressed.

[0008] In the latter half of the compression stroke following the intake stroke, the temperature of the gas inside the cylinder becomes higher than the temperature of the metal walls (the combustion chamber wall on the cylinder head side and the piston crown surface). Therefore, it is preferable to promote the transfer of heat from the gas inside the cylinder to the metal walls (cooling of the gas inside the cylinder). By setting the arithmetic mean roughness Ra3 of the combustion space forming surface on the head side, which is relatively cooler among the metal walls and mainly in contact with the gas inside the cylinder, to the largest possible value (i.e., a rough surface), the transfer of heat from the gas inside the cylinder to the cooler cylinder head is promoted.

[0009] Furthermore, for similar reasons, it is desirable that the arithmetic mean roughness Ra2 of the piston-side squish surface and the arithmetic mean roughness Ra4 of the head-side squish surface satisfy the relationship Ra4 > Ra2. In addition, the influence on heat exchange with the gas inside the cylinder is greater on the piston-side combustion space forming surface and the head-side combustion space forming surface that form the combustion space than on the piston-side squish surface and the head-side squish surface that constitute the squish area. Therefore, it is desirable that the arithmetic mean roughness Ra1 of the piston-side combustion space forming surface and the arithmetic mean roughness Ra2 of the piston-side squish surface satisfy the relationship Ra2 > Ra1, and that the arithmetic mean roughness Ra3 of the head-side combustion space forming surface and the arithmetic mean roughness Ra4 of the head-side squish surface satisfy the relationship Ra3 > Ra4.

[0010] Therefore, by setting the relationship Ra3 > Ra4 > Ra2 > Ra1, the compression end temperature of the gas inside the cylinder can be suppressed most effectively, thereby reducing knocking.

[0011] A plan view of the piston crown of one embodiment. A plan view of the cylinder head side combustion chamber of one embodiment. A cross-sectional view along line A-A in Figure 1 showing the piston crown and cylinder head side combustion chamber combined. A cross-sectional view along line B-B in Figure 1 showing the piston crown and cylinder head side combustion chamber combined. An explanatory diagram of heat transfer from the intake stroke to the compression stroke. A cross-sectional view similar to Figure 3 showing a second embodiment.

[0012] Hereinafter, an embodiment of this invention will be described in detail with reference to the drawings. Figures 1 to 4 show a combustion chamber structure consisting of a piston 1 and a cylinder head-side combustion chamber 21 of one embodiment. The combustion chamber structure 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. In particular, the internal combustion engine equipped with the combustion chamber structure shown is an in-cylinder injection type internal combustion engine in which fuel is injected directly into the cylinder by a fuel injection valve, and combustion is performed 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.

[0013] First, the configuration of the crown surface of the piston 1 will be described based on Figure 1. In one embodiment, 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 itself 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).

[0014] 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 corresponds to the "piston-side combustion space forming surface" in the claim. As shown in Figure 3, the cross-sectional shape is a smoothly continuous curve to be 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. In one embodiment, this tumble preservation surface 2 has a rotationally symmetric shape with respect to the center line CL1 of the piston 1. That is, 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 recessed in a dish shape with respect to a smoothly continuous curved surface.

[0015] 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.

[0016] 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.

[0017] 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 1, 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 4).

[0018] Figures 3 and 4 are cross-sectional views of the main part of the piston 1 at top dead center when the piston 1 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 2, 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, and these main wall surfaces 23 and 25, which are recessed from the lower surface of the cylinder head, form a so-called pent-roof type combustion chamber that forms a combustion space with the tumble-preserving surface 2 of the piston 1. In other words, the intake side main wall surface 23 and the exhaust side main wall surface 25 correspond to the "head side combustion space forming surface" in the claim. The intake side squish surface 26 and the exhaust side squish surface 27 have a shape that follows the conical surface corresponding to the outer peripheral squish surface 3, which is the conical surface on the piston 1 side.

[0019] Therefore, as shown in Figure 3, 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 circumferential squish surface 3 on the piston 1 side. In particular, when the piston 1 descends from the top dead center position shown in Figure 3, 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. Note that at the top dead center position of the piston shown in Figure 3, the vertical distance between the squish surfaces 26 and 27 on the cylinder head side and the outer circumferential squish surface 3 on the piston 1 side is the smallest vertical distance between the combustion chamber 21 on the cylinder head side and the crown surface of the piston 1.

[0020] Furthermore, the combination of the pent-roof type combustion chamber 21, which consists of the intake-side main wall surface 23 and the exhaust-side main wall surface 25, and the tumble-preserving surface 2 on the piston 1 side, creates a combustion space that is close to a sphere, and the tumble generated inside the cylinder during the intake stroke is effectively preserved.

[0021] Here, if we denote the arithmetic mean roughness of the tumble preservation surface 2 on the piston 1 side as Ra1, the arithmetic mean roughness of the outer squish surface 3 on the piston 1 side as Ra2, the arithmetic mean roughness of the intake side main wall surface 23 and the exhaust side main wall surface 25 on the cylinder head side as Ra3, and the arithmetic mean roughness of the squish surfaces 26 and 27 on the cylinder head side as Ra4, then these surface roughnesses are set to the relationship "Ra3 > Ra4 > Ra2 > Ra1".

[0022] Preferably, the tumble preservation surface 2 on the piston 1 side has a mirror-like surface with an arithmetic mean roughness Ra (Ra1) of 0.5 μm or less. Preferably, the outer circumferential squish surface 3 on the piston 1 side has a mirror-like surface with an arithmetic mean roughness Ra (Ra2) of 0.7 μm or less. For example, the arithmetic mean roughness Ra1 of the tumble preservation surface 2 on the piston 1 side is set to 0.1 μm or less, the arithmetic mean roughness Ra2 of the outer circumferential squish surface 3 on the piston 1 side is set to 0.1 to 0.7 μm, the arithmetic mean roughness Ra3 of the main wall surfaces 23 and 25 on the cylinder head side is set to 3 μm or more, and the arithmetic mean roughness Ra4 of the squish surfaces 26 and 27 on the cylinder head side is set to 0.7 to 3 μm. The surface roughness of the valve recesses 4 and 5 and the flat surface 6 of the piston 1 can be set arbitrarily, but for example, the arithmetic mean roughness Ra is about 0.7 to 3 μm. The above figures are merely examples, and the present invention is not limited to these figures.

[0023] Each of these surface roughnesses can be obtained by an appropriate method suitable for that particular surface roughness, such as general cutting, ultra-precision cutting using a single-crystal diamond tip, or burnishing.

[0024] As described above, by varying the surface roughness, the transfer of heat between the gas inside the cylinder, the cylinder head, and the piston 1 can be optimized, and the compression end temperature of the gas inside the cylinder can be suppressed to avoid knocking.

[0025] Figure 5 is an explanatory diagram of heat transfer from the intake stroke to the compression stroke, and the curve Tgas represents the temperature of the gas inside the cylinder. The gas temperature Tgas at the beginning of the intake stroke is high because it is the temperature of the residual gas from the previous cycle, but the temperature decreases as fresh air is introduced in the intake stroke, and reaches its lowest temperature around the intake top dead center. Then it rises with compression in the compression stroke. In the figure, the line Tpi represents the temperature of the piston 1, and the line Thead represents the temperature of the cylinder head. The cylinder head is cooled by coolant, so its temperature is basically lower than that of the piston 1.

[0026] In the latter half of the intake stroke, as shown in Figure 5, the temperature Tgas of the gas inside the cylinder is lower than the temperature Thead of the cylinder head and the temperature Tpi of the piston 1. Therefore, heat moves from the cylinder head and piston 1 to the gas inside the cylinder, acting to increase the temperature Tgas of the gas inside the cylinder. Accordingly, in order to prevent the temperature Tgas of the gas inside the cylinder from rising, it is preferable to suppress the transfer of heat from the metal wall surface to the gas inside the cylinder. By setting the arithmetic mean roughness Ra1 of the tumble preservation surface 2, which is relatively hot among the metal wall surfaces surrounding the gas inside the cylinder and is in main contact with the gas inside the cylinder, to the smallest possible value (i.e., a smooth surface), the transfer of heat from the hot piston 1 to the gas inside the cylinder is suppressed. In Figure 5, arrow H1 indicates the heat transfer from the piston 1 to the gas inside the cylinder during the intake stroke, and arrow H2 indicates the heat transfer from the cylinder head to the gas inside the cylinder. As shown in the figure, the heat transfer H1 from the piston 1 to the gas inside the cylinder is greater than the heat transfer H2 from the cylinder head to the gas inside the cylinder, so it is advantageous that the surface roughness of the tumble-preserving surface 2 of the piston 1 that receives the heat transfer H1 is small.

[0027] In the latter half of the compression stroke, as shown in Figure 5, the temperature Tgas of the gas inside the cylinder is higher than the temperature Thead of the cylinder head and the temperature Tpi of the piston 1. Therefore, heat moves from the gas inside the cylinder to the cylinder head and piston 1, acting to cool the gas inside the cylinder. Consequently, in order to lower the compression end temperature of the gas inside the cylinder, it is preferable to promote the transfer of heat from the gas inside the cylinder to the metal wall surface. By setting the arithmetic mean roughness Ra3 of the main wall surfaces 23 and 25 on the cylinder head side, which are relatively cooler among the metal walls surrounding the gas inside the cylinder and are in main contact with the gas inside the cylinder, to the largest possible value (i.e., rough surface), a large amount of heat transfer from the gas inside the cylinder to the relatively cooler cylinder head side can be obtained. In Figure 5, arrow H3 indicates the heat transfer from the gas inside the cylinder to the piston 1 during the compression stroke, and arrow H4 indicates the heat transfer from the gas inside the cylinder to the cylinder head. As shown in the figure, the heat transfer H4 from the gas inside the cylinder to the cylinder head is greater than the heat transfer H3 from the gas inside the cylinder to the piston 1, so it is advantageous for the main wall surfaces 23 and 25 on the cylinder head side that receive the heat transfer H4 to have a large surface roughness.

[0028] Therefore, from the viewpoint of heat transfer from the intake stroke to the compression stroke, it is necessary that Ra3 > Ra1. Also, for the same reason, it is desirable that the arithmetic mean roughness Ra2 of the outer circumferential squish surface 3 on the piston 1 side and the arithmetic mean roughness Ra4 of the squish surfaces 26 and 27 on the cylinder head side satisfy the relationship Ra4 > Ra2. Furthermore, the influence on heat exchange with the gas inside the cylinder is greater on the tumble-preserving surface 2 and the main wall surfaces 23 and 25 on the cylinder head side that form the combustion space than on the outer circumferential squish surface 3 on the piston 1 side and the squish surfaces 26 and 27 on the cylinder head side that constitute the squish gap 30. Therefore, it is desirable that the arithmetic mean roughness Ra1 of the tumble-preserving surface 2 and the arithmetic mean roughness Ra2 of the outer circumferential squish surface 3 satisfy the relationship Ra2 > Ra1, and that the arithmetic mean roughness Ra3 of the main wall surfaces 23 and 25 on the cylinder head side and the arithmetic mean roughness Ra4 of the squish surfaces 26 and 27 on the cylinder head side satisfy the relationship Ra3 > Ra4. By doing so, finishing processes such as mirror polishing can be minimized.

[0029] The combustion chamber structure of one embodiment has been described above, but in this invention, the specific shape and structure of the piston crown surface and the combustion chamber 21 on the cylinder head side may be anything. In the above embodiment, the piston-side combustion space forming surface is a circular tumble preservation surface 2, but it may be a flatter shape, or an elliptical or oblong concave surface. Furthermore, it can also be applied to the combustion chamber structure of a port injection type internal combustion engine that injects fuel toward the intake port.

[0030] Regarding the squish surface, in the above embodiment, the outer circumferential squish surface 3 on the piston 1 side and the squish surfaces 26 and 27 on the cylinder head side are conical surfaces inclined with respect to a reference horizontal plane perpendicular to the cylinder centerline, but the present invention is not limited to this. Figure 6 shows a second embodiment in which the outer circumferential squish surface 3 on the piston 1 side and the squish surfaces 26 and 27 on the cylinder head side are parallel to a reference horizontal plane perpendicular to the cylinder centerline.

Claims

A combustion chamber structure for a spark-ignition internal combustion engine, which is formed by combining the cylinder head side combustion chamber and the piston crown surface, The piston crown surface has a central piston-side combustion space forming surface that forms a combustion space with the cylinder head, and a piston-side squish surface located outside this piston-side combustion space forming surface that forms a squish gap with the cylinder head-side combustion chamber. The cylinder head side combustion chamber has a head side combustion space forming surface recessed from the lower surface of the cylinder head so as to form a combustion space with the piston side combustion space forming surface, and a head side squish surface located outside this head side combustion space forming surface and facing the piston side squish surface. The arithmetic mean roughness Ra1 of the piston-side combustion space forming surface, the arithmetic mean roughness Ra2 of the piston-side squish surface, the arithmetic mean roughness Ra3 of the head-side combustion space forming surface, and the arithmetic mean roughness Ra4 of the head-side squish surface are, Ra3>Ra4>Ra2>Ra1 The combustion chamber structure of a spark-ignition internal combustion engine, set to the following relationship. 　 The piston-side combustion space forming surface described above consists of a dish-shaped recessed tumble preservation surface. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1. 　 The piston-side combustion space forming surface has a mirror-like surface with an arithmetic mean roughness (Ra) of 0.5 μm or less. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1. 　 The piston-side squish surface described above has a mirror-like surface with an arithmetic mean roughness (Ra) of 0.7 μm or less. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 3. 　 The piston-side squish surface and the head-side squish surface are inclined with respect to a reference horizontal plane perpendicular to the cylinder centerline. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1. 　 The piston-side squish surface and the head-side squish surface are parallel to a reference horizontal plane perpendicular to the cylinder centerline. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1. At the piston's top dead center position, the vertical distance between the piston-side squish surface and the head-side squish surface is the smallest vertical distance between the cylinder head-side combustion chamber and the piston crown surface. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1. 　 The piston crown surface further includes intake valve recesses and exhaust valve recesses formed to avoid interference with the intake valve and exhaust valve. The roughness of the wall surfaces of the intake valve recess and exhaust valve recess is arbitrary. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1. 　 The piston crown surface further has a flat surface consisting of a plane perpendicular to the piston centerline, The level of roughness in this scene is arbitrary. The combustion chamber structure of a spark-ignition internal combustion engine according to claim 1.

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