Auxiliary chamber combustion 4-stroke engine

Abstract

To suppress an increase in the size of a cylinder head and the occurrence of pre-ignition.SOLUTION: A pre-chamber combustion four-stroke engine has a pre-chamber which is in communication with a main combustion chamber via a plurality of communication holes and has an internal space in which a portion of a pre-chamber spark plug is exposed, and does not have an ignition assistance device for assisting in ignition of an air-fuel mixture. A control device controls an intake passage injection valve for injecting a fuel into an intake passage, so that, in at least a part of a low-load region, an air-fuel mixture has a first air-fuel ratio that allows treatment with a three-way catalyst after combustion or a second air-fuel ratio that is richer than the first air-fuel ratio. A cylinder head has a cooling section for accommodating a cooling medium, which receives heat from an electrode section of the pre-chamber spark plug, and a pre-chamber wall section in which the plurality of communication holes are formed. The following are each formed so as to be dispersed in a circumferential direction: a plurality of spark discharges generated in the electrode section of the pre-chamber spark plug; the plurality of communication holes; a plurality of heat paths from the electrode section of the pre-chamber spark plug to the cooling section; and a plurality of heat paths from the pre-chamber wall section to the cooling section.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a pre-combustion four-stroke engine having a main combustion chamber and a pre-combustion chamber. [Background technology]

[0002] Conventionally, a pre-chamber combustion four-stroke engine having a main combustion chamber and a pre-chamber communicating through a plurality of communication holes, as disclosed in, for example, Patent Document 1, is known. The mixture in the pre-chamber is ignited by a spark plug. The pre-chamber combustion four-stroke engine of Patent Document 1 does not have a pre-chamber fuel injection valve that injects fuel into the pre-chamber, but has an intake passage injection valve that injects fuel into the intake passage. The intake passage injection valve of Patent Document 1 is controlled so that a mixture having a stoichiometric or richer air-fuel ratio than the stoichiometric ratio is generated in the main combustion chamber. The pre-chamber combustion four-stroke engine of Patent Document 1 has a cooling jacket (cooling unit) in the cylinder head. In addition, the pre-chamber combustion four-stroke engine of Patent Document 1 has an auxiliary spark plug (ignition assistance device) that assists in ignition of the mixture in the main combustion chamber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 10,612,454 Summary of the Invention [Problem to be solved by the invention]

[0004] In a pre-chamber combustion four-stroke engine such as that described in Patent Document 1, the periphery of the multiple communication holes and the electrode portion of the spark plug become hot. These become particularly hot during high loads, and pre-ignition is likely to occur in their vicinity. Pre-ignition is a phenomenon in which the air-fuel mixture self-ignites before ignition by the spark plug. If the volume of the cooling jacket were to be increased to suppress the occurrence of pre-ignition, the cylinder head would become larger.

[0005] An object of the present invention is to provide a pre-chamber combustion four-stroke engine that can suppress the occurrence of pre-ignition while suppressing an increase in the size of the cylinder head. [Means for solving the problem]

[0006] The pre-chamber combustion four-stroke engine according to one embodiment of the present invention has the following configuration. A pre-chamber combustion four-stroke engine having a main combustion chamber to which an intake passage and an exhaust passage are connected, a throttle valve for adjusting the amount of air taken into the main combustion chamber through the intake passage, an intake passage injection valve for injecting liquid fuel, which is gasoline fuel, alcohol fuel, or gasoline-alcohol mixed fuel, into the intake passage, a pre-chamber formed in a cylinder head to have a smaller volume than the main combustion chamber, the internal space of which communicates with the internal space of the main combustion chamber through a plurality of communication holes, and in which a part of a pre-chamber spark plug is exposed, and a control device for controlling the intake passage injection valve and the pre-chamber spark plug. The control device controls the intake passage injection valve so that the mixture mixed in the intake passage and the main combustion chamber has a first air-fuel ratio that can be processed by a three-way catalyst after combustion or a second air-fuel ratio that is richer than the first air-fuel ratio, at least in a part of a low load range where the opening degree of the throttle valve is small. The pre-chamber combustion four-stroke engine does not have either a pre-chamber fuel injection valve that injects fuel into the pre-chamber, or an ignition assist device that assists ignition of the mixture in the pre-chamber or the main combustion chamber. The cylinder head has a cooling section that accommodates a cooling medium that receives heat from the electrode portion of the pre-chamber spark plug and the pre-chamber wall portion in which the multiple communication holes are formed. The electrode portion of the pre-chamber spark plug is formed so that multiple spark discharges are generated in a circumferentially distributed manner at the electrode portion, the multiple communication holes are formed in a circumferentially distributed manner, and the cylinder head is formed so that multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling section and multiple heat paths from the pre-chamber wall portion to the cooling section are each formed in a circumferentially distributed manner.

[0007] According to this configuration, liquid fuel, which is gasoline fuel, alcohol fuel, or gasoline-alcohol mixed fuel, is injected from the intake manifold injection valve into the intake manifold. In addition, in at least a part of the low load region, the mixture mixed in the intake manifold and the main combustion chamber has a first air-fuel ratio or a second air-fuel ratio richer than the first air-fuel ratio. Therefore, even at low load, there are many positions in the internal space of the pre-chamber where the mixture is easy to ignite. Therefore, when a spark plug having an electrode portion including, for example, a plurality of ground electrodes arranged in the circumferential direction or an annular ground electrode is used as the pre-chamber spark plug, multiple spark discharges can be generated in a circumferentially dispersed manner at the electrode portion of the pre-chamber spark plug at low load and high load. By generating multiple spark discharges in a circumferentially dispersed manner, heat is generated in a circumferentially dispersed manner at the electrode portion of the pre-chamber spark plug. Furthermore, the plurality of communication holes are formed in a circumferentially dispersed manner, so that heat is generated in a circumferentially dispersed manner in the sub-chamber wall portion in which the plurality of communication holes are formed. In this way, heat is generated in a circumferentially dispersed manner in the electrode portion and the pre-chamber wall portion of the pre-chamber spark plug. Furthermore, the cylinder head is formed so that multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion and multiple heat paths from the pre-chamber wall portion to the cooling portion are formed in a circumferentially dispersed manner. Therefore, heat is easily transferred from the electrode portion and the pre-chamber wall portion of the pre-chamber spark plug, which become particularly hot, to the cooling portion. In addition, if an auxiliary ignition device such as a spark plug is provided in the main combustion chamber, the auxiliary ignition device will also become hot, making it difficult for heat to transfer from a position on the auxiliary chamber wall that is close to the auxiliary ignition device.By not providing an auxiliary ignition device in the main combustion chamber, the ease of heat transfer from the auxiliary chamber wall to the cooling section can be made more uniform in the circumferential direction, making it easier for heat to transfer from the auxiliary chamber wall. In addition, if an auxiliary ignition device were provided in the pre-chamber, the auxiliary ignition device would also become hot, making it difficult for heat to transfer from the electrode of the pre-chamber spark plug close to the auxiliary ignition device. By not providing an auxiliary ignition device in the pre-chamber, the ease of heat transfer from the electrode of the pre-chamber spark plug to the cooling section can be made more uniform in the circumferential direction, making it easier for heat to transfer from the electrode of the pre-chamber spark plug. In this way, heat can be easily transferred from the electrode portion and the pre-chamber wall portion of the pre-chamber spark plug to the cooling portion, so the occurrence of pre-ignition can be suppressed while suppressing an increase in the size of the cooling portion. Moreover, since no ignition auxiliary device is provided, the increase in size of the cylinder head can be further suppressed. Therefore, the occurrence of pre-ignition can be suppressed while suppressing an increase in size of the cylinder head.

[0008] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. The base material of the sub-chamber wall has a higher melting point than the base material of the cylinder head, a higher specific heat multiplied by specific gravity than the base material of the cylinder head, and a thermal conductivity equal to or higher than that of chromium-based stainless steel.

[0009] According to this configuration, the base material of the pre-chamber wall has a higher melting point than the base material of the cylinder head. Therefore, the heat resistance of the pre-chamber wall can be ensured. In addition, the base material of the pre-chamber wall has a higher value obtained by multiplying the specific heat and the specific gravity than the base material of the cylinder head. Here, the value obtained by multiplying the specific heat and the specific gravity represents the heat capacity per unit volume. The larger the heat capacity per unit volume, the less the temperature rises. By forming the base material of the pre-chamber wall from a material with a higher heat capacity per unit volume than the base material of the cylinder head, the temperature rise of the pre-chamber wall is suppressed, and heat is easily transferred from the high-temperature pre-chamber wall to the cooling section formed in the cylinder head. In addition, the base material of the pre-chamber wall has a thermal conductivity equal to or higher than that of chromium-based stainless steel. Therefore, heat is more easily transferred from the pre-chamber wall to the cooling section. As a result, the occurrence of pre-ignition can be further suppressed.

[0010] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. No protrusions are formed on the inner surface of the pre-chamber except for the pre-chamber spark plug, and the length of the pre-chamber spark plug in the plug axial direction in the internal space of the pre-chamber is less than twice the maximum length of the internal space of the pre-chamber in a direction perpendicular to the plug axial direction.

[0011] If a protrusion were formed on the inner surface of the pre-chamber, heat would easily accumulate in the protrusion. Since no protrusion is formed on the inner surface of the pre-chamber except for the pre-chamber spark plug, heat would easily move from the pre-chamber wall to the cooling section. In addition, the length of the internal space of the pre-chamber in the plug axial direction is less than twice the maximum length of the internal space of the pre-chamber in the direction perpendicular to the plug axial direction. Therefore, the perimeter of the pre-chamber can be made longer while maintaining the volume of the pre-chamber. This makes it possible to ensure more heat paths from the pre-chamber wall to the cooling section. Therefore, heat is more easily moved from the pre-chamber wall to the cooling section. As a result, the occurrence of pre-ignition can be further suppressed.

[0012] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. The pre-chamber wall is formed to protrude into the internal space of the main combustion chamber, and the pre-chamber is formed such that when the internal space of the pre-chamber is divided into two spaces by a plane that passes through the internal space of the pre-chamber but not through the outer surface of the pre-chamber wall and is perpendicular to the plug axial direction of the pre-chamber spark plug, the volume of the space closer to the main combustion chamber is smaller than the volume of the space farther from the main combustion chamber.

[0013] With this configuration, although the auxiliary chamber wall protrudes into the internal space of the main combustion chamber, the amount of protrusion is small. Therefore, heat is less likely to accumulate in the auxiliary chamber wall, and heat is more likely to transfer from the auxiliary chamber wall to the cooling section. As a result, the occurrence of pre-ignition can be further suppressed.

[0014] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A plane that passes through the cooling portion and is perpendicular to the plug axial direction of the pre-chamber spark plug passes through the pre-chamber spark plug.

[0015] With this configuration, since the sub-chamber spark plug is close to the cooling section, heat from the electrode portion of the sub-chamber spark plug is easily transferred to the cooling section, which results in further suppression of pre-ignition.

[0016] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. The internal space of the pre-chamber is a space surrounded by a pre-chamber member including the pre-chamber wall portion, which is separate from both the cylinder head body and the pre-chamber spark plug, and which includes a portion of the pre-chamber wall portion and is exposed to the internal space of the main combustion chamber, and the pre-chamber spark plug.

[0017] With this configuration, the pre-chamber member can be made longer in the plug axial direction of the pre-chamber spark plug while maintaining the shape and size of the pre-chamber internal space, compared to when the pre-chamber internal space is surrounded by the pre-chamber member including the pre-chamber wall, the pre-chamber spark plug, and other members (e.g., the cylinder head body). Therefore, when the pre-chamber member is made of a material that transfers heat easily, heat transfers more easily from the pre-chamber wall to the cooling section. As a result, the occurrence of pre-ignition can be further suppressed.

[0018] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A plane that passes through the cooling portion and is perpendicular to the plug axial direction of the pre-chamber spark plug passes through the pre-chamber member.

[0019] With this configuration, since the sub-chamber member is close to the cooling portion, heat can be more easily transferred from the sub-chamber wall portion to the cooling portion, which can further suppress the occurrence of pre-ignition.

[0020] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A part of the inner surface of the cooling portion is at least a part of the outer circumferential surface of the sub chamber member.

[0021] With this configuration, the cooling medium flowing through the cooling portion comes into contact with the outer circumferential surface of the sub-chamber member, which makes it easier for heat to transfer from the sub-chamber wall portion to the cooling portion, thereby making it possible to further suppress the occurrence of pre-ignition.

[0022] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A male thread formed on the pre-chamber spark plug meshes with and contacts a female thread formed on the pre-chamber member.

[0023] With this configuration, the contact area between the pre-chamber spark plug and the pre-chamber member is large. Therefore, heat is easily transferred from the pre-chamber spark plug to the pre-chamber member. Therefore, heat is easily transferred from the electrode portion of the pre-chamber spark plug to the cooling portion via the pre-chamber member. As a result, the occurrence of pre-ignition can be further suppressed.

[0024] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A plane that passes through the cooling portion and is perpendicular to the plug axial direction of the pre-chamber spark plug passes through the point where the male thread formed on the pre-chamber spark plug meshes with and contacts the female thread formed on the pre-chamber member.

[0025] With this configuration, since the sub-chamber member is close to the cooling portion, heat is more easily transferred from the sub-chamber wall to the cooling portion. Furthermore, since the screw contact portion between the sub-chamber spark plug and the sub-chamber member is close to the cooling portion, heat is more easily transferred from the electrode portion of the sub-chamber spark plug to the cooling portion via the sub-chamber member. As a result, the occurrence of pre-ignition can be further suppressed.

[0026] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. The outer peripheral surface of the sub chamber member is in contact with the cylinder head body.

[0027] With this configuration, heat is more easily transferred from the sub-chamber wall to the cylinder head body than when the outer circumferential surface of the sub-chamber member is not in contact with the cylinder head body, thereby making it possible to further suppress the occurrence of pre-ignition.

[0028] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A plane that passes through the internal space of the pre-chamber and is perpendicular to the plug axial direction of the pre-chamber spark plug passes through a portion of the outer circumferential surface of the pre-chamber member that contacts the cylinder head body.

[0029] With this configuration, the internal space of the auxiliary chamber is close to the point where the outer peripheral surface of the auxiliary chamber member contacts the cylinder head body. Therefore, the auxiliary chamber wall is close to the point where the outer peripheral surface of the auxiliary chamber member contacts the cylinder head body. Therefore, heat is easily transferred from the auxiliary chamber wall to the cylinder head body. As a result, the occurrence of pre-ignition can be further suppressed.

[0030] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. A plane that passes through the internal space of the sub-chamber and is perpendicular to the plug axial direction passes through the location where the male thread formed on the sub-chamber member meshes with and contacts the female thread formed on the cylinder head body.

[0031] With this configuration, the contact area between the sub chamber member and the cylinder head body is large, which makes it easier for heat to transfer from the sub chamber member to the cylinder head body, thereby further suppressing the occurrence of pre-ignition.

[0032] The pre-chamber combustion four-stroke engine of one embodiment of the present invention may have the following configuration. There is no main combustion chamber fuel injection valve for injecting fuel into the main combustion chamber.

[0033] With this configuration, the size of the cylinder head can be prevented from increasing, compared to when a main combustion chamber fuel injection valve is provided.

[0034] In the present invention and the embodiments, the low load range is the lower range when the range from the lowest to the highest engine load is divided into two equal parts.

[0035] In the present invention and the embodiments, the air-fuel ratio, which is the mixture ratio of fuel and air, is expressed by a first air-fuel ratio, a second air-fuel ratio, and a third air-fuel ratio. The first air-fuel ratio is an air-fuel ratio that can be processed by a three-way catalyst after combustion. The first air-fuel ratio may be a stoichiometric ratio or a window of air-fuel ratios including the stoichiometric ratio. The first air-fuel ratio may be an air-fuel ratio in the vicinity of the stoichiometric ratio. The first air-fuel ratio may be a window including an air-fuel ratio in the vicinity of the stoichiometric ratio but not including the stoichiometric ratio. The second air-fuel ratio is an air-fuel ratio richer than the first air-fuel ratio. When the first air-fuel ratio is an air-fuel ratio in the vicinity of the stoichiometric ratio or a window not including the stoichiometric ratio, the second air-fuel ratio may or may not be richer than the stoichiometric ratio. The third air-fuel ratio is an air-fuel ratio leaner than the first air-fuel ratio. In the present invention and the embodiments, rich means that the mixture is rich in fuel. Lean means that the fuel in the mixture is thin. In the present invention and the embodiments, the air-fuel ratio that can be processed by a three-way catalyst after combustion is the air-fuel ratio of the mixture that can process the exhaust gas generated after the combustion of the mixture by the three-way catalyst. In the present invention and the embodiments, the control device may control the intake passage injection valve so that the mixture mixed in the intake passage and the main combustion chamber has a stoichiometric air-fuel ratio or an air-fuel ratio richer than the stoichiometric air-fuel ratio in at least a part of the low load region. The pre-chamber combustion four-stroke engine of the present invention has a catalyst arranged in the exhaust passage. The pre-chamber combustion four-stroke engine of the present invention may have a three-way catalyst arranged in the exhaust passage. The pre-chamber combustion four-stroke engine of the present invention may have a catalyst other than a three-way catalyst arranged in the exhaust passage. The pre-chamber combustion four-stroke engine of the present invention has an oxygen sensor arranged between the main combustion chamber and the catalyst to detect the oxygen concentration of the exhaust gas flowing through the exhaust passage.

[0036] In the present invention and the embodiments, the auxiliary ignition device that assists in igniting the mixture in the pre-chamber or main combustion chamber is, for example, a device that generates microwave discharge, a device that generates dielectric barrier discharge (silent discharge), or a spark plug that ignites the mixture in the main combustion chamber. In the present invention and the embodiments, when a pre-chamber combustion four-stroke engine does not have an auxiliary ignition device, it includes not only that an auxiliary ignition device separate from the pre-chamber spark plug is not provided, but also that the pre-chamber spark plug does not have the function of an auxiliary ignition device.

[0037] In the present invention and the embodiments, the auxiliary chamber having a smaller volume than the main combustion chamber means that the volume of the auxiliary chamber is smaller than the minimum volume of the main combustion chamber. The volume of the main combustion chamber changes with the movement of the piston. The volume of the auxiliary chamber is the volume of the internal space of the auxiliary chamber. In the present invention and the embodiments, the internal space of the auxiliary chamber does not include the internal space of the multiple communication holes. In the present invention and the embodiments, the inner surface of the auxiliary chamber is the surface that forms the internal space of the auxiliary chamber. In the present invention and the embodiments, the auxiliary chamber spark plug forms a part of the inner surface of the auxiliary chamber. In the present invention and the embodiments, the fact that no protrusions are formed on the inner surface of the auxiliary chamber except for the auxiliary chamber spark plug means that no protrusions are formed on the inner surface of the auxiliary chamber, or that the only protrusions formed on the inner surface of the auxiliary chamber are the protrusions due to the auxiliary chamber spark plug. In the present invention, the auxiliary chamber wall portion having multiple communication holes formed therein is a wall portion having one side exposed to the internal space of the main combustion chamber. The auxiliary chamber wall portion may be formed so as to protrude into the internal space of the main combustion chamber, or may be formed so as not to protrude. When the pre-chamber wall portion is formed so as to protrude into the internal space of the main combustion chamber, the pre-chamber wall portion has a cylindrical portion.

[0038] In the present invention and the embodiments, the cooling medium is liquid or gas. The liquid cooling medium may be, for example, water or lubricating oil. The gas cooling medium may be, for example, air. In the present invention and the embodiments, the cooling section in which the cooling medium is accommodated is at least one chamber or at least one passage. The cooling medium may flow through the cooling section. The passage through which the cooling medium flowing into the cooling section and the passage through which the cooling medium discharged from the cooling section flows may be connected to the cooling section. The cooling section may be multiple chambers or multiple passages that are not in communication with each other.

[0039] In the present invention and the embodiments, a thermal path is a path through which heat moves. In the present invention and the embodiments, the multiple thermal paths from the electrode portion of the pre-chamber spark plug to the cooling portion are not limited to being independent of each other. In other words, heat may be able to move between the multiple thermal paths. The same applies to the definition of the multiple thermal paths from the pre-chamber wall portion to the cooling portion in the present invention and the embodiments.

[0040] In the present invention and the embodiment, the electrode portion of the pre-chamber spark plug includes at least one center electrode and at least one ground electrode. The electrode portion may include, for example, a single center electrode and multiple ground electrodes or an annular ground electrode. The multiple ground electrodes may be, for example, two ground electrodes. The multiple ground electrodes may be, for example, three or more ground electrodes. When the electrode portion has a single center electrode and multiple ground electrodes, multiple discharge gaps are formed. When the electrode portion has a single center electrode and an annular ground electrode, an annular discharge gap is formed. Spark discharge occurs in the discharge gap. In the present invention and the embodiment, the plug axial direction of the pre-chamber spark plug is a direction parallel to the central axis of the pre-chamber spark plug. The plug axial direction of the pre-chamber spark plug may or may not be parallel to the central axis of the cylinder hole that forms the main combustion chamber.

[0041] In the present invention and the embodiment, "multiple spark discharges are generated in a circumferentially distributed manner" means that the positions where the spark discharges are generated are distributed in the circumferential direction. "Multiple spark discharges are generated in a circumferentially distributed manner" does not mean that multiple spark discharges are generated simultaneously at positions distributed in the circumferential direction. Multiple spark discharges may be generated simultaneously at positions distributed in the circumferential direction. In this case, at least one of the multiple spark discharges generated simultaneously becomes the starting point of ignition. The circumferential direction in the sentence "multiple spark discharges are generated in a circumferentially distributed manner" is, for example, the circumferential direction centered on a straight line parallel to the plug axial direction of the sub-chamber spark plug. An example of a case where multiple spark discharges are not generated in a circumferentially distributed manner is a case where the electrode portion has a single center electrode and a single ground electrode. Another example of a case where multiple spark discharges are not formed in a circumferentially distributed manner is a case where the electrode portion has a single center electrode, a first ground electrode that is mainly used, and a second ground electrode that is used auxiliary. Another example of a case where multiple spark discharges are not generated in a distributed manner in the circumferential direction is a case where the positions where the mixture is easily ignited are not substantially uniform in the circumferential direction due to variations in the concentration of the mixture inside the auxiliary chamber. Specific examples of a case where multiple spark discharges are not generated in a distributed manner in the circumferential direction are not limited to these.

[0042] In the present invention and the embodiments, "the multiple communication holes are formed in a distributed manner in the circumferential direction" means that the multiple communication holes are formed side by side in the circumferential direction without excessive bias. When the multiple communication holes are formed in a distributed manner in the circumferential direction, the multiple communication holes are formed side by side in the circumferential direction. "The multiple communication holes are formed in a distributed manner in the circumferential direction" does not necessarily mean that the multiple communication holes are formed at equal intervals in the circumferential direction. The circumferential direction in the phrase "the multiple communication holes are formed in a distributed manner in the circumferential direction" refers to, for example, the circumferential direction centered on a straight line parallel to the plug axial direction of the sub-chamber spark plug.

[0043] In the present invention and the embodiments, "multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion are formed in a circumferentially dispersed manner" means that the amount of heat moving from the electrode portion of the pre-chamber spark plug to the cooling portion is dispersed in the circumferential direction. In other words, it means that the degree of ease of heat movement from the electrode portion of the pre-chamber spark plug to the cooling portion is approximately uniform in the circumferential direction. The circumferential direction in the sentence "multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion are formed in a circumferentially dispersed manner" is, for example, the circumferential direction centered on a straight line parallel to the plug axial direction of the pre-chamber spark plug. In the present invention and the embodiments, "multiple heat paths from the pre-chamber wall portion to the cooling portion are formed in a circumferentially dispersed manner" means that the amount of heat moving from the pre-chamber wall portion to the cooling portion is dispersed in the circumferential direction. In other words, it means that the degree of ease of heat movement from the pre-chamber wall portion to the cooling portion is approximately uniform in the circumferential direction. The circumferential direction in the sentence "multiple heat paths from the pre-chamber wall portion to the cooling portion are formed in a circumferentially dispersed manner" is, for example, the circumferential direction centered on a straight line parallel to the plug axial direction of the pre-chamber spark plug. This may be the same as the circumferential direction in which the multiple communication holes are arranged. The electrode portion is formed so that a plurality of spark discharges occurring at the electrode portion are distributed in the circumferential direction. That is, the heat generated at the electrode portion is distributed in the circumferential direction. Therefore, in order to form a plurality of heat paths from the electrode portion of the sub-chamber spark plug to the cooling portion in a distributed manner in the circumferential direction, the cooling portion and the portion between the sub-chamber spark plug and the cooling portion in the cylinder head are important. Also, the plurality of communication holes formed in the sub-chamber wall portion are formed in a distributed manner in the circumferential direction. That is, the heat generated at the sub-chamber wall portion is distributed in the circumferential direction. Therefore, in order to form a plurality of heat paths from the sub-chamber wall portion to the cooling portion in a distributed manner in the circumferential direction, the cooling portion and the portion between the sub-chamber wall portion and the cooling portion in the cylinder head are important. For example, when the cooling portion is formed in an annular shape, the plurality of heat paths from the electrode portion of the sub-chamber spark plug to the cooling portion and the plurality of heat paths from the sub-chamber wall portion to the cooling portion are each likely to be formed in a distributed manner in the circumferential direction. For example, when the structure (shape and material) of the portion between the pre-chamber spark plug and the cooling portion in the cylinder head is substantially uniform in the circumferential direction, multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion are easily formed in a dispersed manner in the circumferential direction. Also, when the structure (shape and material) of the portion between the pre-chamber wall portion and the cooling portion in the cylinder head is substantially uniform in the circumferential direction, multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion are easily formed in a dispersed manner in the circumferential direction. An example of a case where multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion are not formed in a dispersed manner in the circumferential direction is a case where the cooling portion is formed only in an area of ​​about half the circumference. This example may be an example of a case where multiple heat paths from the pre-chamber wall portion to the cooling portion are not formed in a dispersed manner in the circumferential direction. Another example of a case where multiple heat paths from the pre-chamber wall portion to the cooling portion are not formed in a dispersed manner in the circumferential direction is a case where the material from the pre-chamber wall portion to the cooling portion is different between the half of the circumference and the remaining half of the circumference. Note that specific examples of cases in which multiple heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion are not formed in a circumferentially dispersed manner, and specific examples of cases in which multiple heat paths from the pre-chamber wall portion to the cooling portion are not formed in a circumferentially dispersed manner, are not limited to those mentioned above.

[0044] In the present invention and the embodiments, when the pre-chamber wall is made up of multiple parts made of different materials, the base material of the pre-chamber wall is the material of the part that occupies the largest volume of the multiple parts. Note that in the present invention and the embodiments, the pre-chamber wall does not have to be made up of multiple parts made of different materials. In the present invention and the embodiments, when the cylinder head is composed of a plurality of parts made of different materials, the base material of the cylinder head is the material of the part that occupies the largest volume of the plurality of parts. When the base material of the pre-chamber wall part and the base material of the cylinder head are different, the part that occupies the largest volume of the plurality of parts made of different materials that constitute the cylinder head does not include the pre-chamber wall part. Note that in the present invention and the embodiments, the cylinder head includes the pre-chamber wall part. In the present invention and the embodiments, the cylinder head main body, a part of which is exposed to the internal space of the main combustion chamber, does not include the pre-chamber wall part. The base material of the cylinder head main body is the same as the base material of the cylinder head. The cylinder head main body may be composed of a plurality of parts made of different materials, or it does not have to be composed of a plurality of parts made of different materials.

[0045] In the present invention and the embodiments, the thermal conductivity of the base material of the pre-chamber wall is equal to or higher than that of chromium-based stainless steel means that the thermal conductivity of the base material of the pre-chamber wall is equal to or higher than that of chromium-based stainless steel under the temperature conditions of the main combustion chamber and the pre-chamber when the pre-chamber combustion four-stroke engine is in operation. The temperatures of the main combustion chamber and the pre-chamber when the pre-chamber combustion four-stroke engine is in operation are, for example, about 850 to 1000°C at the highest and about 500 to 600°C at the lowest.

[0046] In the present invention and the embodiments, the length of the internal space of the pre-chamber in the plug axial direction is the length of the internal space of the pre-chamber in the plug axial direction between one end and the other end of the internal space of the pre-chamber in the plug axial direction. In other words, it is the distance between a plane that passes through one end of the internal space of the pre-chamber in the plug axial direction and is perpendicular to the plug axial direction, and a plane that passes through the other end of the internal space of the pre-chamber in the plug axial direction and is perpendicular to the plug axial direction. In the present invention and the embodiments, the definition of the length of one direction perpendicular to the plug axial direction of the internal space of the pre-chamber is the same as above. In the present invention and the embodiments, the maximum length of the internal space of the pre-chamber in the direction perpendicular to the plug axial direction is the maximum length of the internal space of the pre-chamber in multiple directions perpendicular to the plug axial direction.

[0047] In the present invention and the embodiments, the phrase "a plane passing through the cooling part and perpendicular to the plug axial direction of the pre-chamber spark plug passes through the pre-chamber spark plug" does not mean that all planes passing through the cooling part and perpendicular to the plug axial direction of the pre-chamber spark plug pass through the pre-chamber spark plug, but means that any plane passing through the cooling part and perpendicular to the plug axial direction of the pre-chamber spark plug passes through the pre-chamber spark plug. In the present invention and the embodiments, the sentences "a plane passing through the cooling part and perpendicular to the plug axial direction of the pre-chamber spark plug passes through the pre-chamber member", "a plane passing through the cooling part and perpendicular to the plug axial direction of the pre-chamber spark plug passes through a portion where the male thread formed on the pre-chamber spark plug meshes with and comes into contact with the female thread formed on the pre-chamber member", and "a plane passing through the internal space of the pre-chamber passes through a portion where the outer circumferential surface of the pre-chamber member comes into contact with the cylinder head main body" are also interpreted in the same manner as above.

[0048] In the present invention and the embodiments, "the auxiliary chamber member is separate from the cylinder head body, a part of which is exposed to the internal space of the main combustion chamber" means that the auxiliary chamber member is separated from the cylinder head body, or the auxiliary chamber member is in separable contact with the cylinder head body. In the present invention and the embodiments, the cylinder head body may be composed of one inseparable member, or may be composed of multiple separable members, each of which is partially exposed to the internal space of the main combustion chamber. When the cylinder head body is composed of multiple separable members, the cylinder head body does not include any member that is not partially exposed to the internal space of the main combustion chamber. In the present invention and the embodiments, "the sub-chamber member is separate from the sub-chamber spark plug" means that the sub-chamber member is separate from the sub-chamber spark plug, or the sub-chamber member is in separable contact with the sub-chamber spark plug. The sub-chamber member does not include a part of the electrode portion of the sub-chamber spark plug (e.g., the ground electrode).

[0049] In the present invention and the embodiments, when the number of a certain component is not clearly specified (i.e., when translated into English, it is expressed in the singular form), the number of this component may be one or more. In the present invention and the embodiments, the components whose number is not clearly specified include, for example, the main combustion chamber, the intake passage, the exhaust passage, the throttle valve, the intake passage injection valve, the sub-chamber, and the sub-chamber spark plug. The pre-chamber combustion four-stroke engine of the present invention and the embodiments may have a single main combustion chamber or multiple main combustion chambers. That is, the pre-chamber combustion four-stroke engine of the present invention and the embodiments may be a single-cylinder engine unit or multiple-cylinder engine unit. The number of pre-chambers and pre-chamber spark plugs is the same as the number of main combustion chambers. The number of intake passage injection valves may be the same as the number of main combustion chambers or may be more than that. The number of throttle valves may be the same as the number of main combustion chambers or may be less than that. The intake passage may be shaped to branch into two or more. The number of intake passages connected to one main combustion chamber is one. One intake passage with a branched shape may be connected to multiple main combustion chambers. The exhaust passage may be shaped to branch into two or more. The number of exhaust passages connected to one main combustion chamber is one. One exhaust passage with a branched shape may be connected to multiple main combustion chambers.

[0050] The pre-chamber combustion four-stroke engine of the present invention and the embodiments can be mounted on a saddle-type vehicle that is lighter than an automobile and requires a lightweight and compact engine. A saddle-type vehicle refers to any vehicle on which a driver rides astride a saddle. Saddle-type vehicles include motorcycles, scooters, motor tricycles, four-wheeled buggies (ATVs: All Terrain Vehicles), snowmobiles, and personal watercraft. The pre-chamber combustion four-stroke engine of the present invention and the embodiments can also be mounted on a work vehicle that requires a lightweight and compact engine. It goes without saying that the pre-chamber combustion four-stroke engine of the present invention and the embodiments can be mounted on an automobile. The product on which the pre-chamber combustion four-stroke engine of the present invention and the embodiments is mounted is not limited to a specific product. When the pre-chamber combustion four-stroke engine according to one embodiment of the present invention is mounted on a product, it may be mounted so that the central axis of the cylinder bore is at an angle of 0 degrees to 45 degrees or less with respect to the vertical, or at an angle of 45 degrees to 90 degrees.

[0051] As used herein and in the embodiments, the words "including," "having," "comprising," and their derivatives are intended to encompass the listed items and equivalents thereof as well as additional items.

[0052] Unless otherwise defined, all terms (including technical and scientific terms) used in the present invention and the embodiments have the same meaning as commonly understood by those skilled in the art to which the present invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted to have a meaning consistent with the meaning in the context of the relevant technology and this disclosure, and should not be interpreted in an idealized or overly formal sense.

[0053] In the present invention and the embodiments, the term "may" is non-exclusive. "May" means "may, but is not limited to." In the present invention and the embodiments, the configuration described as "may" at least achieves the above-mentioned effect obtained by the configuration of claim 1.

[0054] Before describing the embodiments of the invention in detail, it is to be understood that the invention is not limited to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments other than those described below. The invention is also capable of embodiments with various modifications to the embodiments described below. Effect of the Invention

[0055] According to the pre-chamber combustion four-stroke engine of the present invention, it is possible to suppress the occurrence of pre-ignition while suppressing an increase in the size of the cylinder head. [Brief description of the drawings]

[0056] [Figure 1] 1(a) to 1(f) are schematic diagrams of a pre-chamber combustion four-stroke engine according to a first embodiment of the present invention. [Diagram 2]FIG. 2(a) is a schematic diagram of a pre-chamber combustion four-stroke engine according to a third embodiment of the present invention, and FIG. 2(b) is a schematic diagram of a pre-chamber combustion four-stroke engine according to a fourth embodiment of the present invention. [Diagram 3] 3(a) to 3(c) are schematic diagrams of three examples of a pre-chamber combustion four-stroke engine according to a fifth embodiment of the present invention. [Figure 4] 4(a) to 4(e) are schematic diagrams of five examples of a pre-chamber combustion four-stroke engine according to a fifth embodiment of the present invention. [Diagram 5] 5(a) and 5(b) are schematic diagrams of two examples of a pre-chamber combustion four-stroke engine according to a fifth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] Hereinafter, a pre-chamber combustion four-stroke engine according to one embodiment of the present invention will be described with reference to the drawings. Note that the embodiment described below is merely an example. The present invention should not be construed as being limited in any way by the embodiment described below.

[0058] First Embodiment A pre-combustion four-stroke engine 1 according to a first embodiment of the present invention will be described with reference to Figs. 1(a) to 1(f). Fig. 1(b) shows an example of a part of a cross section taken along line AA in Fig. 1(a). Figs. 1(c) and 1(d) show two examples of a part of a cross section taken along line BB in Fig. 1(a). Figs. 1(e) and 1(f) show two examples of a part of a cross section taken along line CC in Fig. 1(a). The pre-combustion four-stroke engine 1 according to the first embodiment has at least one main combustion chamber 2. An intake passage 5 and an exhaust passage 6 are connected to the main combustion chamber 2. The main combustion chamber 2 is formed by a cylinder head 10, a cylinder hole 11, and a piston 12. The intake passage 5 includes a passage formed inside the cylinder head 10 and a passage connected to this passage. The exhaust passage 6 includes a passage formed inside the cylinder head 10 and a passage connected to this passage. The pre-combustion four-stroke engine 1 has at least one throttle valve 7. The throttle valve 7 adjusts the amount of air that passes through the intake passage 5 and is drawn into the main combustion chamber 2. The pre-chamber combustion four-stroke engine 1 has at least one intake passage injection valve 8. The intake passage injection valve 8 injects liquid fuel, which is gasoline fuel, alcohol fuel, or gasoline-alcohol mixed fuel, into the intake passage 5. The pre-chamber combustion four-stroke engine 1 has at least one pre-chamber 20. The internal space of the pre-chamber 20 communicates with the internal space of the main combustion chamber 2 through a plurality of communication holes 21. The volume of the pre-chamber 20 is formed to be smaller than the volume of the main combustion chamber 2. A portion of the pre-chamber spark plug 23 is exposed to the internal space of the pre-chamber 20. The pre-chamber 20 is formed in the cylinder head 10. The plurality of communication holes 21 are formed in a pre-chamber wall portion 22 of the cylinder head 10. The pre-chamber wall portion 22 has one side exposed to the internal space of the main combustion chamber 2. The relationship between the position of the central axis C11 of the cylinder bore 11 and the position of the central axis C23 of the pre-chamber spark plug 23 is not limited to the positional relationship shown in Figures 1(a) and 1(b). A direction parallel to the central axis C23 of the pre-chamber spark plug 23 is defined as a plug axis direction DP. In Figures 1(a) and 1(b), the plug axis direction DP is parallel to the central axis C11 of the cylinder bore 11, but the plug axis direction DP does not have to be parallel to the central axis C11 of the cylinder bore 11. The shape of the internal space of the pre-chamber 20 is not limited to the shape shown in Figures 1(a) and 1(b).The pre-chamber combustion four-stroke engine 1 has a control device 70 that controls at least one intake passage injector 8 and at least one pre-chamber spark plug 23. The control device 70 controls the intake passage injector 8 so that the mixture mixed in the intake passage 5 and the main combustion chamber 2 has a first air-fuel ratio that can be processed by a three-way catalyst after combustion or a second air-fuel ratio that is richer than the first air-fuel ratio, at least in a part of the low load range where the opening degree of the throttle valve 7 is small. For example, the control device 70 may control the intake passage injector 8 so that the mixture mixed in the intake passage 5 and the main combustion chamber 2 has a first air-fuel ratio that can be processed by a three-way catalyst after combustion, at least in a part of the low load range. The pre-chamber combustion four-stroke engine 1 may or may not have a three-way catalyst. The pre-chamber combustion four-stroke engine 1 has neither a pre-chamber fuel injector that injects fuel into the pre-chamber 20 nor an ignition assist device that assists in ignition of the mixture in the pre-chamber 20 or the main combustion chamber 2.

[0059] As shown in FIG. 1(a) and FIG. 1(b), the cylinder head 10 has a cooling section 16 in which a cooling medium (not shown) that receives heat from the electrode section 24 and the sub-chamber wall section 22 of the sub-chamber spark plug 23 is accommodated. In FIG. 1(b), the cooling section 16 is annular, but the cooling section 16 does not have to be annular. The electrode section 24 of the sub-chamber spark plug 23 is formed so that a plurality of spark discharges 33 are generated in the electrode section 24 in a circumferentially distributed manner. The plurality of communication holes 21 are formed in a circumferentially distributed manner. The cylinder head 10 is formed so that a plurality of heat paths 14 from the electrode section 24 of the sub-chamber spark plug 23 to the cooling section 16, and a plurality of heat paths 15 from the sub-chamber wall section 22 to the cooling section 16 are each formed in a circumferentially distributed manner. For example, the plurality of spark discharges 33, the plurality of communication holes 21, the plurality of heat paths 14, and the plurality of heat paths 15 may each be formed in a circumferentially distributed manner centered on the central axis C23 of the sub-chamber spark plug 23. The heat path 14 shown in FIG. 1(a) is merely one example of the heat path 14 from the electrode portion 24 of the pre-chamber spark plug 23 to the cooling portion 16. The heat path 15 shown in FIG. 1(a) is merely one example of the heat path 15 from the pre-chamber wall portion 22 to the cooling portion 16. The two spark discharges 33 shown in FIG. 1(c) are merely one example of the multiple spark discharges 33 that are distributed in the circumferential direction on the electrode portion 24 of the pre-chamber spark plug 23. The multiple spark discharges 33 shown in FIG. 1(d) are merely one example of the multiple spark discharges 33 that are distributed in the circumferential direction on the electrode portion 24 of the pre-chamber spark plug 23. The configuration of the electrode portion 24 is not limited to the configuration shown in FIG. 1(c) and FIG. 1(d). The electrode portion 24 may have a single center electrode 30 and multiple ground electrodes 31, for example, as shown in FIG. 1(c). The electrode portion 24 may have a single center electrode 30 and an annular ground electrode 31, for example, as shown in FIG. 1(d). The ground electrodes 31 are configured such that spaces are formed between the ground electrodes 31. The ground electrodes 31 are spaced apart from the center electrode 30 in a direction perpendicular to the axial direction DP. The ground electrodes 31 are not aligned with the center electrode 30 in the axial direction DP. An inner circumferential end of the annular ground electrode 31 is spaced apart from the center electrode 30 in a direction perpendicular to the axial direction DP.Between the single center electrode 30 and the ground electrodes 31 or the annular ground electrode 31, multiple or annular discharge gaps are formed in a direction perpendicular to the plug axial direction DP. The number of the ground electrodes 31 may be, for example, two. The number, position, shape, and size of the communication holes 21 are not limited to those shown in Figures 1(e) and 1(f). The number of the communication holes 21 may be, for example, three or more.

[0060] According to the configuration of the first embodiment, heat is easily transferred from the electrode portion 24 of the pre-chamber spark plug 23 and the pre-chamber wall portion 22 to the cooling portion 16, so the occurrence of pre-ignition can be suppressed while suppressing an increase in size of the cooling portion 16. Moreover, because no ignition auxiliary device is provided, the increase in size of the cylinder head 10 can be further suppressed. Therefore, the occurrence of pre-ignition can be suppressed while suppressing an increase in size of the cylinder head 10.

[0061] The pre-chamber combustion four-stroke engine 1 in Fig. 1(a) is formed so that any plane passing through the cooling portion 16 and perpendicular to the plug axis direction DP passes through the pre-chamber spark plug 23. This plane is, for example, a plane overlapping with line A-A in Fig. 1(a). In Fig. 1(a), the pre-chamber combustion four-stroke engine 1 is formed so that any plane passing through the cooling portion 16 and perpendicular to the plug axis direction DP does not pass through the internal space of the pre-chamber 20. In the first embodiment, the pre-chamber combustion four-stroke engine 1 may be formed so that any plane passing through the cooling portion 16 and perpendicular to the plug axis direction DP passes through the internal space of the pre-chamber 20.

[0062] The pre-chamber combustion four-stroke engine 1 of the first embodiment does not need to have a main combustion chamber fuel injection valve that injects fuel into the main combustion chamber 2. The pre-chamber combustion four-stroke engine 1 of the first embodiment does not need to have either a supercharger or a turbocharger. In other words, the pre-chamber combustion four-stroke engine 1 may be of a naturally aspirated type. The pre-chamber combustion four-stroke engine 1 of the first embodiment does not need to have an external exhaust gas recirculation device that includes an external exhaust gas recirculation passage that bypasses the main combustion chamber 2 and connects the exhaust passage 6 and the intake passage 5.

[0063] <Second embodiment> A pre-chamber combustion four-stroke engine 1 according to a second embodiment of the present invention will be described. The second embodiment has the configuration of the first embodiment. In the second embodiment, the melting point of the base material of the pre-chamber wall portion 22 is higher than that of the base material of the cylinder head 10. The value obtained by multiplying the specific heat and the specific gravity of the base material of the pre-chamber wall portion 22 is higher than the value obtained by multiplying the specific heat and the specific gravity of the base material of the cylinder head 10. The thermal conductivity of the base material of the pre-chamber wall portion 22 is equal to or higher than that of chromium-based stainless steel. The base material of the cylinder head 10 is, for example, aluminum or an aluminum alloy. When the base material of the cylinder head 10 is aluminum or an aluminum alloy, the base material of the pre-chamber wall portion 22 may be, for example, a material shown in Examples 1 to 4 in Table 1 below. The base material of the pre-chamber wall portion 22 may be a chromium zirconium copper alloy as in Example 1. The base material of the pre-chamber wall portion 22 may be a chromium copper alloy. Comparative Examples 1 to 3 shown in Table 1 are examples of materials that cannot be used as the base material for the sub-chamber wall portion 22 when the base material for the cylinder head 10 is aluminum or an aluminum alloy.

[0064] [Table 1]

[0065] The pre-chamber wall 22 may be made of only the base material. The pre-chamber wall 22 may be made of the base material and a material other than the base material. For example, the pre-chamber wall 22 may have a coating layer made of a material different from the base material on at least a part of the outer surface of the pre-chamber wall 22. The thermal conductivity of the coating layer is preferably higher than the thermal conductivity of the pre-chamber wall 22.

[0066] <Third embodiment> A pre-chamber combustion four-stroke engine 1 according to a third embodiment of the present invention will be described with reference to Fig. 2(a). The pre-chamber combustion four-stroke engine 1 according to the third embodiment has the following configuration in addition to the configuration according to the first or second embodiment. No protrusions are formed on the inner surface of the pre-chamber 20 except for the pre-chamber spark plug 23. The larger of the length L1 of the internal space of the pre-chamber 20 in the plug axis direction DP and the maximum length L2 of the internal space of the pre-chamber 20 in the direction perpendicular to the plug axis direction DP is less than twice the smaller length. In Fig. 2(a), the length L2 is greater than the length L1, but the length L1 may be greater than the length L2.

[0067] <Fourth embodiment> A pre-chamber combustion four-stroke engine 1 according to a fourth embodiment of the present invention will be described with reference to FIG. 2(b). The fourth embodiment has at least one of the configurations of the first to third embodiments. In the fourth embodiment, the pre-chamber wall 22 is formed so as to protrude into the internal space of the main combustion chamber 2. Furthermore, in the fourth embodiment, the pre-chamber 20 is formed so that the protruding amount of the pre-chamber wall 22 is smaller than the volume of the pre-chamber 20. Specifically, the pre-chamber 20 is formed so that when the internal space of the pre-chamber 20 is divided into two spaces by any plane S1 that does not pass through the outer surface of the pre-chamber wall 22, passes through the internal space of the pre-chamber 20, and is perpendicular to the plug axis direction DP, the volume of the space closer to the main combustion chamber 2 of the two spaces is smaller than the volume of the space farther from the main combustion chamber 2 of the two spaces. The plane S1 shown in FIG. 2(b) is merely one example of the plane S1 that does not pass through the outer surface of the pre-chamber wall 22, passes through the internal space of the pre-chamber 20, and is perpendicular to the plug axis direction DP. The outer surface of the auxiliary chamber wall 22 is the surface exposed to the main combustion chamber 2. When the inner space of the auxiliary chamber 20 is divided into two spaces by any plane S1 that does not pass through the outer surface of the auxiliary chamber wall 22, passes through the inner space of the auxiliary chamber 20, and is perpendicular to the plug axis direction DP, the auxiliary chamber 20 formed so that the volume of the space closer to the main combustion chamber 2 is smaller than the volume of the space farther from the main combustion chamber 2 means that the auxiliary chamber 20 is not the auxiliary chamber 20 described below. The auxiliary chamber 20 is the auxiliary chamber 20 in which the following relationship is established for all planes that do not pass through the outer surface of the auxiliary chamber wall 22, pass through the inner space of the auxiliary chamber 20, and are perpendicular to the plug axis direction DP. The relationship is a relationship in which, when the inner space of the auxiliary chamber 20 is divided into two spaces by a plane, the volume of the space closer to the main combustion chamber 2 is equal to or larger than the volume of the space farther from the main combustion chamber 2.

[0068] <Fifth embodiment> The pre-chamber combustion four-stroke engine 1 according to the fifth embodiment of the present invention will be described with reference to Figs. 3(a) to 3(c), 4(a) to 4(e), 5(a) and 5(b). Figs. 3(a) to 3(c) show three examples of the fifth embodiment. Figs. 4(a) to 4(e) show five examples of the fifth embodiment. Figs. 5(a) and 5(b) show two examples of the fifth embodiment. The fifth embodiment has at least one of the configurations of the first to fourth embodiments. In the fifth embodiment, the inner surface of the pre-chamber 20 is formed by the pre-chamber member 25 including the pre-chamber wall portion 22 and the pre-chamber spark plug 23. That is, the internal space of the pre-chamber 20 is a space surrounded by the pre-chamber member 25 and the pre-chamber spark plug 23. The outer peripheral surface of the pre-chamber spark plug 23 and the inner peripheral surface of the pre-chamber member 25 are in contact with each other. The pre-chamber member 25 is separate from the cylinder head body 13, a part of which is exposed to the internal space of the main combustion chamber 2, and is also separate from the pre-chamber spark plug 23. The cylinder head body 13 may be composed of a single inseparable member, or may be composed of multiple separable members, each of which has a part exposed to the internal space of the main combustion chamber 2. When the cylinder head body 13 is composed of multiple separable members, the cylinder head body 13 does not include any member that is not partially exposed to the internal space of the main combustion chamber 2.

[0069] For example, as shown in FIG. 3(a) and FIG. 3(b), the outer peripheral surface of the sub-chamber member 25 may be in contact with the cylinder head body 13. For example, as shown in FIG. 3(b), the male thread 41 formed on the sub-chamber member 25 may be in meshing contact with the female thread 40 formed on the cylinder head body 13. When the outer peripheral surface of the sub-chamber member 25 is in contact with the cylinder head body 13, as shown in FIG. 3(a) and FIG. 3(b), any plane S2 passing through the internal space of the sub-chamber 20 and perpendicular to the plug axis direction DP may pass through a portion of the outer peripheral surface of the sub-chamber member 25 that contacts the cylinder head body 13. The plane S2 passing through the contact portion may pass through a portion of the sub-chamber member 25 that is in meshing contact with the female thread 40 formed on the cylinder head body 13, as shown in FIG. 3(b). The plane S2 passing through the contact portion may pass through a portion other than the threaded portion (male thread 41 and female thread 40) of the sub-chamber member 25 and the cylinder head body 13, as shown in FIG. 3(b). The outer peripheral surface of the sub-chamber member 25 shown in Fig. 3(c) may be in contact with the cylinder head body 13 at a location not shown. For example, the outer peripheral surface of the sub-chamber member 25 shown in Fig. 3(c) may be in contact with the cylinder head body 13 so that any plane (not shown) that passes through the sub-chamber spark plug 23 and is perpendicular to the plug axis direction DP but does not pass through the internal space of the sub-chamber 20 passes through a location where the outer peripheral surface of the sub-chamber member 25 contacts the cylinder head body 13. In this case, the contact portion between the outer peripheral surface of the sub-chamber member 25 and the cylinder head body 13 may be a threaded portion (male thread 41 and female thread 40) or may not be a threaded portion.

[0070] The outer peripheral surface of the sub chamber member 25 does not have to be in contact with the cylinder head body 13. For example, the outer peripheral surface of the sub chamber member 25 shown in Fig. 3(c) does not have to be in contact with the cylinder head body. For example, the sub chamber member 25 may be connected to the cylinder head body via a member other than the cylinder head body 13 (for example, a cylinder head cover).

[0071] For example, as shown in Fig. 4(a) to Fig. 4(d), the sub-chamber member 25 and the cooling portion 16 may be formed so that any plane S3 that passes through the cooling portion 16 and is perpendicular to the plug axial direction DP passes through the sub-chamber member 25. When the plane S3 passes through the sub-chamber member 25, a part of the inner surface of the cooling portion 16 may be at least a part of the outer circumferential surface of the sub-chamber member 25, as shown in Fig. 4(a) to Fig. 4(c). When the plane S3 passes through the sub-chamber member 25, the inner surface of the cooling portion 16 may not include a part of the outer circumferential surface of the sub-chamber member 25, as shown in Fig. 4(d). Also, as shown in Fig. 4(e), the sub-chamber member 25 and the cooling portion 16 may be formed so that any plane S3 that passes through the cooling portion 16 and is perpendicular to the plug axial direction DP does not pass through the sub-chamber member 25. The relationship between the sub-chamber member 25 and the cylinder head body 13 shown in Fig. 4(a) to Fig. 4(d) is not limited to the same relationship as that in Fig. 3(a), and may be any of the relationships described above.

[0072] In the case where a part of the inner surface of the cooling part 16 is at least a part of the outer circumferential surface of the sub-chamber member 25, the sub-chamber member 25 may have at least one heat dissipation part 26 protruding into the internal space of the cooling part 16, as shown in FIG. 4(b), for example. The heat dissipation part 26 may be annular or may not be annular. For example, the heat dissipation part 26 may be an arc shape larger than a semicircle, or may not be an arc shape. The annular heat dissipation part 26 may be formed so as to divide the internal space of the cooling part 16 into a plurality of spaces. The sub-chamber member 25 may have a plurality of heat dissipation parts 26 aligned in the plug axial direction DP.

[0073] When any plane S3 passing through the cooling part 16 passes through the sub-chamber member 25 (for example, Figs. 4(a) to 4(d)), any plane S3 passing through the cooling part 16 may pass through a location where the inner peripheral surface of the sub-chamber member 25 contacts the outer peripheral surface of the sub-chamber spark plug 23. When any plane S3 passing through the cooling part 16 passes through the sub-chamber member 25, for example, as shown in Figs. 5(a) and 5(b), any plane S3 passing through the cooling part 16 may pass through a location where the female thread 42 formed on the sub-chamber member 25 meshes with and contacts the male thread 43 formed on the sub-chamber spark plug 23. The cooling part 16 in Fig. 5(a) is the same as the cooling part 16 in Fig. 4(a), but may be the same as the cooling part 16 in Fig. 4(b). The cooling part 16 in Fig. 5(b) is the same as the cooling part 16 in Fig. 4(d), but may be the same as the cooling part 16 in Fig. 4(c). 5(a) and 5(b) and the cylinder head main body 13 is not limited to the same relationship as in Fig. 3(a), and may be any of the relationships described above. When any plane S3 passing through the cooling portion 16 passes through the sub-chamber member 25, any plane S3 passing through the cooling portion 16 does not have to pass through a location where the inner peripheral surface of the sub-chamber member 25 contacts the outer peripheral surface of the sub-chamber spark plug 23.

[0074] The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the claims. For example, the pre-chamber combustion four-stroke engine of the present invention may have a supercharger or a turbocharger. The pre-chamber combustion four-stroke engine may have a main combustion chamber fuel injection valve that injects fuel into the main combustion chamber. [Explanation of symbols]

[0075] 1: pre-chamber combustion four-stroke engine, 2: main combustion chamber, 5: intake passage, 6: exhaust passage, 7: throttle valve, 8: intake passage injection valve, 10: cylinder head, 11: cylinder bore, 13: cylinder head body, 14: heat path from electrode of pre-chamber spark plug to cooling section, 15: multiple heat paths from pre-chamber wall to cooling section, 16: cooling section, 20: pre-chamber, 21: communication hole, 22: pre-chamber wall, 23: pre-chamber spark plug, 24: electrode, 25: pre-chamber member, 33: spark discharge, 40: female thread of cylinder head body, 41: male thread of pre-chamber member, 42: female thread of pre-chamber member, 43: male thread of pre-chamber spark plug, 70: control device, DP: plug axial direction, L1: length of internal space of pre-chamber in plug axial direction, L2: maximum length of internal space of pre-chamber in a direction perpendicular to the plug axial direction, S1, S2, S3: plane

Claims

1. a main combustion chamber to which an intake passage and an exhaust passage are connected; a throttle valve for adjusting the amount of air taken into the main combustion chamber through the intake passage; an intake passage injection valve that injects a liquid fuel, which is gasoline fuel, alcohol fuel, or a gasoline-alcohol mixed fuel, into the intake passage; a sub-chamber formed in the cylinder head so as to have a smaller volume than the main combustion chamber, the internal space of which communicates with the internal space of the main combustion chamber via a plurality of communication holes, and a portion of the sub-chamber spark plug being exposed in the internal space; a control device for controlling the intake manifold injection valve and the pre-chamber combustion four-stroke engine, the control device controls the intake manifold injection valve so that the air-fuel mixture mixed in the intake manifold and the main combustion chamber has a first air-fuel ratio that can be processed by a three-way catalyst after combustion, or a second air-fuel ratio that is richer than the first air-fuel ratio, in at least a part of a low load region where an opening degree of the throttle valve is small; The pre-chamber combustion four-stroke engine does not have either a pre-chamber fuel injection valve that injects fuel into the pre-chamber, or an ignition assist device that assists in ignition of the mixture in the pre-chamber or the main combustion chamber, the cylinder head has a cooling portion that accommodates a cooling medium that receives heat from an electrode portion of the sub-chamber spark plug and a sub-chamber wall portion in which the plurality of communication holes are formed, an internal space of the pre-chamber is a space surrounded by a pre-chamber member that is separate from both a cylinder head body, a part of which is exposed to the internal space of the main combustion chamber, and the pre-chamber spark plug, and that includes the pre-chamber wall portion in which the plurality of communication holes are formed, and that has an inner circumferential surface that contacts an outer circumferential surface of the pre-chamber spark plug, and the pre-chamber spark plug; The electrode portion of the sub-chamber spark plug is formed so that a plurality of spark discharges are generated in a circumferentially dispersed manner at the electrode portion, The plurality of communication holes are formed in a circumferentially dispersed manner, 1. A pre-chamber combustion four-stroke engine, comprising: a cylinder head configured such that a plurality of heat paths from the electrode portion of the pre-chamber spark plug to the cooling portion, and a plurality of heat paths from the pre-chamber wall portion to the cooling portion are formed in a circumferentially dispersed manner.

2. 2. The pre-chamber combustion four-stroke engine according to claim 1, wherein the base material of the pre-chamber wall has a higher melting point than the base material of the cylinder head, a higher value obtained by multiplying the specific heat and specific gravity than the base material of the cylinder head, and a thermal conductivity equal to or higher than that of chromium-based stainless steel.

3. 3. The pre-chamber combustion four-stroke engine according to claim 1, wherein no protrusions are formed on an inner surface of the pre-chamber except for the pre-chamber spark plug, and a length of the pre-chamber spark plug in the plug axial direction in the internal space of the pre-chamber is less than twice the maximum length of the internal space of the pre-chamber in a direction perpendicular to the plug axial direction.

4. The auxiliary chamber wall portion is formed so as to protrude into the internal space of the main combustion chamber, 4. The pre-chamber combustion four-stroke engine according to claim 1, wherein the pre-chamber is formed such that, when the internal space of the pre-chamber is divided into two spaces by any plane that passes through the internal space of the pre-chamber without passing through an outer surface of the pre-chamber wall and is perpendicular to a plug axial direction of the pre-chamber spark plug, the volume of the space closer to the main combustion chamber of the two spaces is smaller than the volume of the space farther from the main combustion chamber of the two spaces.

5. 5. The pre-chamber combustion four-stroke engine according to claim 1, wherein a plane passing through the cooling portion and perpendicular to a plug axial direction of the pre-chamber spark plug passes through the pre-chamber spark plug.

6. 6. The pre-chamber combustion four-stroke engine according to claim 1, wherein a plane passing through the cooling portion and perpendicular to a plug axial direction of the pre-chamber spark plug passes through the pre-chamber member.

7. 7. The pre-chamber combustion four-stroke engine according to claim 6, wherein a part of the inner surface of the cooling portion is at least a part of the outer circumferential surface of the pre-chamber member.

8. 8. The pre-chamber combustion four-stroke engine according to claim 1, wherein a male thread formed on the pre-chamber spark plug is in meshing contact with a female thread formed on the pre-chamber member.

9. 9. The pre-chamber combustion four-stroke engine according to claim 8, wherein a plane passing through the cooling portion and perpendicular to a plug axial direction of the pre-chamber spark plug passes through a location where the male thread formed on the pre-chamber spark plug meshes with and contacts the female thread formed on the pre-chamber member.

10. 10. The pre-chamber combustion four-stroke engine according to claim 1, wherein an outer circumferential surface of the pre-chamber member is in contact with the cylinder head body.

11. 11. The pre-chamber combustion four-stroke engine according to claim 10, wherein a plane passing through an internal space of the pre-chamber and perpendicular to a plug axial direction of the pre-chamber spark plug passes through a portion of the outer circumferential surface of the pre-chamber member that contacts the cylinder head body.

12. 12. The pre-chamber combustion four-stroke engine according to claim 11, wherein a plane which passes through an internal space of the pre-chamber and is perpendicular to the plug axial direction passes through a position where the male thread formed on the pre-chamber member meshes with and contacts the female thread formed on the cylinder head body.

13. 13. The pre-chamber combustion four-stroke engine according to claim 1, characterized in that it does not have a main combustion chamber fuel injection valve that injects fuel into the main combustion chamber.

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