[0Q01] The invention relates to an internal combustion engine having an ignition plug, and in particular to an internal combustion engine having an ignition plug in which a ground electrode is opposed to a center electrode in an axial direction of a plug body. 2. Description of Related Art

[0002] As described in patent publications as listed below, various ideas regarding the shape of a ground electrode of an ignition plug have been proposed. A ground electrode depicted in FIG 3 of Japanese Patent Application Publication No. 2013-098042 (JP 2013-098042 A) has a protrusion opposed to a center electrode, and an opposed surface of the ground electrode is inclined such that a distance from the center electrode increases toward the downstream side in a direction of airflow. A ground electrode depicted in FIG 1 of Japanese Patent Application Publication No. 5-159856 (JP 5-159856 A) is formed in a tapered shape such that its width is reduced toward a distal end , thereof. A ground electrode depicted in FIG. 3 of Japanese Patent Application Publication No. 2010-102864 (JP 2010-102864 A) is formed in a tapered shape such that its thickness is reduced toward a distal end thereof. A ground electrode depicted in FIG 10 of Japanese Patent Application Publication No. 2005-050612 (JP 2005-050612 A) is formed in the shape of a wedge that tapers in an axial direction of a plug body, on one side opposite to a center electrode. A ground electrode depicted in FIG 11 and FIG 12 of Japanese Patent Application Publication No. 2011-187437 (JP 2011-187437 A) is formed such that its portion opposed to a center electrode is inclined relative to a plane perpendicular to the axis of a plug body.

SUMMARY OF THE INVENTION [0003] As described in the above-identified patent publications, in the known spark-ignition internal combustion engines, there is room for improvement in the ignition performance, in particular, the ignition performance during operation in a lean-burn mode. Thus, the invention of this invention made an observation as to how a discharge spark and an initial flame change immediately after discharge. The inventor found, as a result of the observation, that the discharge spark formed between the center electrode and the ground electrode flows in radial directions in a combustion chamber, under the influence of airflow in the cylinder, and the initial flame produced from the discharge spark contacts with a ceiling surface of the combustion chamber (a surface that partially defines the combustion chamber of the cylinder head), whereby heat of the initial flame is conducted and lost to the ceiling surface, which may result in extinction of the flame.

[0004] In order to prevent deterioration of the ignition performance due to extinction of the initial flame, it is necessary to prevent the initial flame from contacting with the ceiling surface of the combustion chamber. However, no measure to prevent the initial flame from contacting with the ceiling surface of the combustion chamber has been taken in ignition plugs that have been in practical use or proposed, including the ignition plugs disclosed in the above-identified patent publications.

[0005] The invention provides an internal combustion engine in which the initial flame produced from a discharge spark of an ignition plug is less likely or unlikely to contact with a ceiling surface of a combustion chamber.

[0006] An internal combustion engine according to one aspect of the invention includes a cylinder head, and an ignition plug. The ignition plug includes a plug body and a ground electrode. The plug body is mounted in the cylinder head and provided with a center electrode. The ground electrode includes an opposed portion and a connection portion. The opposed portion is opposed to the center electrode in an axial direction of the plug body. The opposed portion includes a first surface opposed to the center electrode, and a second surface opposite to the first surface. The connection portion is configured to connect the opposed portion with a housing of the plug body. The plug body is mounted in the cylinder head such that a first direction and a second direction intersect with each other in at least a part of operating regions of the internal combustion engine. The first direction is a direction of airflow produced around the center electrode and the ground electrode at the time of ignition of the ignition plug, and the second direction is a direction from the center electrode toward the connection portion. A distance between at least a part of the second surface of the opposed portion and the plug body increases toward a downstream side of the airflow in the first direction.

[0007] The airflow produced around the center electrode and the ground electrode at the time of ignition is separated at the opposed portion of the ground electrode to one side closer to the center electrode, and the other side opposite to or remote from the center electrode. An air-fuel mixture that flows on the above-indicated one side of the opposed portion closer to the center electrode causes a discharge spark formed between the center electrode and the ground electrode to flow downstream in the direction of the airflow. On the other hand, the air-fuel mixture flowing on the other side of the opposed portion opposite to the center electrode is guided by the inclined surface formed on the opposed portion, and is separated from the opposed portion at around a downstream end portion of the inclined surface. The air-fuel mixture flowing on the other side of the opposed portion opposite to the center electrode is accelerated when flowing along the inclined surface, and is also guided in a direction away from the air-fuel mixture , flowing on the above-indicated one side of the opposed portion closer to the center electrode. Therefore, a negative pressure developed by the separation of the mixture is considerably strong. The strong negative pressure is developed in a region on the back side of the opposed portion as viewed in the airflow direction, namely, in a region closer to the center of the combustion chamber than the position at which the discharge spark is generated. With this arrangement, the discharge spark is drawn into the negative-pressure region located closer to the center of the combustion chamber, along with the air-fuel mixture that has passed through the one side of the opposed portion closer to the center electrode, and is thus spaced apart from the ceiling surface of the combustion chamber. Consequently, the initial flame produced from the discharge spark is less likely or unlikely to contact with the ceiling surface of the combustion chamber. [0008] Preferably, the opposed portion of the ground electrode is formed in the shape of a wedge that tapers toward the upstream side in the direction of airflow. With the opposed portion thus formed in the wedge shape, turbulence of airflow that is vertically separated by the opposed portion can be reduced, and the negative pressure developed by the separation can be increased.

[0009] The opposed portion of the ground electrode may include a wall that rises from a downstream end portion of the inclined surface as viewed in the airflow direction, toward the plug body. The wall may be in parallel with the axis Of the plug body, or may be inclined relative to the axis of the plug body, provided that the angle formed by the wall and the inclined surface is such an angle as to promote separation of the air-fuel mixture.

[0010] The above-indicated second surface on the other side of the opposed portion of the ground electrode opposite to the center electrode is not necessarily a flat surface. For example, the surface may be a concaved surface formed such that the angle of inclination relative to a plane perpendicular to the axis of the plug body gradually increases toward the downstream side in the airflow direction. With the concaved surface thus formed, the angle of inclination at the downstream end portion of the opposed portion is increased as compared with the case of the flat surface, and the air-fuel mixture is more likely to separate from the opposed portion. Also, the second surface may be a convex surface formed such that the angle of inclination relative to a plane perpendicular to the axis of the plug body is gradually reduced toward the downstream side in the airflow direction. With the convex surface thus formed, the flow rate of the mixture flowing along the second surface on the other side of the opposed portion opposite to the center electrode is increased, and the negative-pressure region produced by separation of the air-fuel mixture is expanded.

[0011] In the opposed portion of the ground electrode, an end portion of the downstream side of the first surface is formed such that the distance from the plug body gradually increases toward the downstream side of the airflow in the first direction. More specifically, the opposed portion is formed with an inside inclined surface that is inclined relative to a plane perpendicular to the axis of the plug body, away from the plug body, toward the downstream side of the airflow in the first direction. With the opposed portion thus formed with the inside inclined surface, flow of the air-fuel mixture along the inside inclined surface is formed, and the discharge spark is more likely to be drawn into the negative-pressure region.

[0012] With the internal combustion engine thus configured according to the invention, the initial flame produced from the discharge spark of the ignition plug is less likely or unlikely to contact with the ceiling surface of the combustion chamber, owing to the above-described operation, so that production of the initial flame can be promoted, and the ignition performance can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a view showing the Configuration of an ignition plug according to a first embodiment of the invention;

FIG. 2 is a view showing the mounting angle of the ignition plug according to the first embodiment of the invention;

FIG 3 is a view showing one example of the mounting direction of the ignition plug according to the first embodiment of the invention;

FIG 4 is a view showing another example of the mounting direction of the ignition plug according to the first embodiment of the invention;

FIG 5 is a view showing distribution of airflows produced around a known ignition plug;

FIG. 6 is a view showing distribution of airflows produced around the ignition plug according to the first embodiment of the invention;

FIG. 7 is a view showing conditions of a discharge spark and an initial flame produced by a known ignition plug;

FIG 8 is a view showing conditions of a discharge spark and an initial flame produced by the ignition plug according to the first embodiment of the invention;

FIG 9 is a view showing the configuration of an ignition plug according to a second embodiment of the invention;

FIG 10 is a view showing the configuration of an ignition plug according to a third embodiment of the invention;

FIG 11 is a view showing the configuration of an ignition plug according to a fourth embodiment of the invention;

FIG 12 is a view showing the configuration of an ignition plug according to a fifth embodiment of the invention;

FIG 13 is a view showing the configuration of an ignition plug according to a sixth embodiment of the invention;

FIG 14 is a view showing the configuration of an ignition plug according to a seventh embodiment of the invention; and

FIG 15 is a view showing the configuration of an ignition plug according to an eighth embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0014] Some embodiments of the invention will be described with reference to the drawings. In each of the embodiments, this invention is applied to an internal combustion engine for an automobile having an ignition plug, more particularly, a lean-burn internal combustion engine capable of operating in a homogeneous lean-burn mode. The lean-burn engine is desired to have improved ignition performance for expansion of the lean-burn limit. To this end, it is meaningful to apply this invention to the lean-burn engine. However, this invention may also be applied to an internal combustion engine that operates at the stoichiometric air/fuel ratio, and this type of engine can also provide a common advantageous effect, such as improvement of the ignition performance.

[0015] In the drawings, the same or similar reference numerals are assigned to the same or similar portions. The embodiments as described below are provided for illustrating devices or methods for embodying the technical concept of the invention, but are not intended to limit the structure, arrangement, etc. of constituent components to those as described below. This invention is not limited to the embodiments as described below, but may be modified and carried out in various forms without departing from the principle of the invention.

[0016] FIG. 1 is a view showing the configuration of an ignition plug according to a first embodiment of the invention. The ignition plug 101 according to this embodiment includes a cylindrical, metallic housing 1 mounted in a cylinder head 11, and a ceramic insulator 2 that is held inside the metallic housing 1. The metallic housing 1 and the ceramic insulator 2 constitute a plug body of the ignition plug 101.

[0017] A distal end of the ceramic insulator 2 protrudes from a surface of the cylinder head 11 which partially defines the combustion chamber, i.e., a ceiling surface 12 of the combustion chamber, into the combustion chamber. A center electrode 3 is provided on the protruding distal end of the ceramic insulator 2. Further, a center electrode chip 4 is disposed on the distal end of the center electrode 3, coaxially with the axis of the plug body, i.e., the axis CLof the ceramic insulator 2.

[0018] A ground electrode 5 extends from the metallic housing 1 into the combustion chamber. The ground electrode 5 consists of an opposed portion 81 that is opposed to the center electrode 3 in the direction of the axis CL, and a connection portion 7 that connects the opposed portion 81 to the metallic housing 1. A ground electrode chip 6 is disposed on one side of the opposed portion 81 opposed to the center electrode 3, coaxially with the axis CL of the ceramic insulator 2. A clearance formed between the ground electrode chip 6 and the center electrode chip 4 provides a discharge gap in which spark discharge takes place.

[0019] The ignition plug 101 has a feature in the configuration of the opposed portion 81. The feature is related to the direction A of airflow produced around the center electrode 3 and the ground electrode 5 at the time of ignition. Therefore, the ignition plug 101 is always mounted at a given angle to the cylinder head 11. The metallic housing 1 is formed with screws for mounting the ignition plug 101 on the cylinder head 11 so that the mounting angle of the ignition plug 101 becomes equal to a predetermined angle, or falls within a predetermined angular range.

[0020] Referring to FIG 2, the mounting angle of the ignition plug 101 will be described in detail. FIG. 2 is a view of the ignition plug 101 as viewed in the direction of the axis CL. In FIG. 2, the above-mentioned direction A of airflow and the direction B of the connection portion 7 as seen from the center electrode 3 are indicated. The connection portion 7 is positioned relative to the center electrode 3 so that the direction B is perpendicular to the direction A. If the connection portion 7 is positioned relative to the center electrode 3 in this manner, the connection portion 7 will not interfere with airflow when it passes through the discharge gap. However, as indicated by two-dot chain lines in FIG 2, the direction B may be inclined relative to the direction A. Furthermore, if the direction B and the direction A are not parallel with each other, but intersect at a suitable angle, the airflow can pass through the discharge gap without being interrupted by the connection portion 7. In view of the width of the connection portion 7, the direction B is preferably within the range of ±45 degrees as measured from a direction perpendicular to the direction A.

[0021] Referring back to FIG 1, the configuration of the opposed portion 81 will be described. The opposed portion 81 is formed in the shape of a wedge, as viewed in a direction in which one sees the connection portion 7 located behind the center electrode 3. The opposed portion 81 has an opposed surface 812 on one side thereof opposed to the center electrode 3, and an inclined surface 811 on the other side opposite to the center electrode 3. The opposed surface 812 is formed perpendicular to the axis CL. The inclined surface 811 is formed such that the distance from the plug body to the inclined surface 811 gradually increases toward the downstream side of the airflow direction A. In other words, the inclined surface 811 is formed so as to be inclined, relative to a plane perpendicular to the axis CL, toward the center of the combustion chamber, or one side of the combustion chamber opposite to the ceiling surface 12, in a direction toward the downstream side of the airflow direction A. The angle of inclination of the inclined surface 811 relative to the opposed surface 812 may be any angle provided that an effect as will be described later can be provided, and is determined depending on the specifications and operating conditions of the internal combustion engine. As one example, the angle of inclination of the inclined surface 811 may be within the range of 20 degrees to 50 degrees. A wall 813 rises from a downstream end portion of the inclined surface 811 as viewed in the airflow direction A, toward the plug body. The wall 813 makes an acute angle with the inclined surface 811, and is connected at a right angle to an end portion of the opposed surface 812.

[0022] In the meantime, airflow formed in the combustion chamber takes various forms. In an example shown in FIG. 3, tumble flow that swirls clockwise from an intake valve IN toward an exhaust valve EX along the ceiling surface of the combustion chamber is produced. In this example, the direction A of airflow at the time of ignition is a direction from the intake valve IN toward the exhaust valve EX. Accordingly, in this example, the ignition plug 101 is mounted so that the inclined surface 811 faces the intake valve IN. In an example shown in FIG. 4, on the other hand, tumble flow that swirls counterclockwise in a direction from the intake valve IN toward the exhaust valve EX, along the bottom of the combustion chamber (the top face of the piston), is produced. In this example, the direction A of airflow at the time of ignition is a direction from the exhaust valve EX toward the intake valve IN. Accordingly, in this example, the ignition plug 101 is mounted so that the inclined surface 811 faces the exhaust valve EX.

[0023] In some internal combustion engines, the direction or shape of airflow formed in the combustion chamber changes depending on the operating region, as an effect arising from the structure of the engine, or due to the operation of a device for controlling airflow in the combustion chamber. In this type of internal combustion engine, the direction A of airflow at the time of ignition changes depending on the operating region. When the ignition plug 101 is installed in this type of engine, the mounting direction may be determined with respect to a particular operating region. The particular operating region is preferably an operating region in which high ignition performance is particularly required. Accordingly, in the case of a lean-burn engine that is selectively operated at the stoichiometric air/fuel ratio, or operated in a lean-burn mode, depending on the operating region, the mounting direction of the ignition plug 101 may be determined in accordance with the direction A of airflow in the operating region in which the engine operates in the lean-burn mode.

[0024] Next, the operation and effect of the opposed portion 81 configured as described above will be described, based on comparison with a known ignition plug.

[0025] FIG. 5 shows a simulation result of distribution of airflow produced around a known ignition plug 201. Each arrow in FIG 5 indicates the direction and magnitude of airflow observed at its location. In the known, general ignition plug 201, an opposed portion 210 of a ground electrode has a rectangular shape in cross section, and a surface 211 of the opposed portion 210 opposite to a center electrode is perpendicular to the axis of the plug body. In the known ignition plug 201, too, a negative-pressure region CI is produced downstream of the opposed portion 210 as viewed in the airflow direction, due to separation of the air-fuel mixture. However, the negative-pressure region CI is small, and the negative pressure developed in this region is not so large.

[0026] On the other hand, according to the ignition plug 101 of this embodiment, a simulation result as shown in FIG. 6 is obtained with respect to distribution of airflow produced around the ignition plug 101. In the ignition plug 101 according to this embodiment, the air-fuel mixture flowing on the side of the opposed portion 81 opposite to the center electrode is accelerated when flowing along the inclined surface, 811, and is also guided in a direction away from the air-fuel mixture flowing on the side of the opposed portion 81 closer to the center electrode. As a result, the negative pressure developed when the air-fuel mixture is separated from the opposed portion 81 becomes considerably strong. It is understood from the simulation result shown in FIG 6 that a negative-pressure region C2 that is apparently larger than that of the known one is produced in a region on the back side of the opposed portion 81 as viewed in the direction A of airflow, namely, in a region closer to the center of the combustion chamber than the position at which a discharge spark is produced.

[0027] FIG. 7 shows conditions of a discharge spark and an initial flame produced by the known ignition plug 201. In the known ignition plug 201, the discharge spark Dl formed between the center electrode chip 4 and the ground electrode chip 6 is caused to flow downstream in the airflow direction A, by the air-fuel mixture flowing on the side of the opposed portion 210 closer to the center electrode 3. As described above, in the known ignition plug 201, too, the negative-pressure region CI (see FIG. 5) is produced downstream of the opposed portion 210. However, the negative pressure developed in this region CI is small, and is not so strong as to attract or draw the discharge spark Dl along with the air-fuel mixture. Therefore, the discharge spark Dl extends as it is downstream in the airflow direction, and the initial flame El generated from the discharge spark Dl is brought into contact with the ceiling surface 12 of the combustion chamber. With the initial flame El thus contacting with the ceiling surface 12 of the combustion chamber having a low temperature, a failure in ignition due to extinction of the initial flame El may take place.

[0028] In the case of the ignition plug 101 according to this embodiment, conditions of a discharge spark and an initial flame produced by the ignition plug 101 are those as shown in FIG 8. Namely, the discharge spark D2 formed between the center electrode chip 4 and the ground electrode chip 6 is drawn into the strong negative-pressure region C2 (see FIG. 6) produced downstream of the opposed portion 81, along with the air-fuel mixture that passes through one side of the opposed portion 81 closer to the center electrode. As a result, the discharge spark D2 is spaced apart from the ceiling surface 12 of the combustion chamber, and the initial flame E2 generated from the discharge spark D2 is prevented from contacting with the ceiling surface 12 of the combustion chamber, or is at least made less likely to contact with the ceiling surface. Since the initial flame E2 is made less likely or unlikely to contact with the ceiling surface 12, generation of the initial flame E2 is promoted, and the ignition performance is improved. Thus, the ignition plug 101 according to this embodiment makes it possible to expand the lean-burn limit of the lean-burn internal combustion engine, and operate the engine at even leaner air-fuel ratios;

[0029] FIG 9 shows the configuration of an ignition plug 102 according to a second embodiment of the invention, in particular, the configuration of an opposed portion 82 as a characteristic portion of the ignition plug 102. As in the first embodiment, the opposed portion 82 has an opposed surface 822, an inclined surface 821, and a wall 823. The second embodiment is different from the first embodiment in the angle of the wall. The wall 823 of this embodiment makes an obtuse angle with the opposed surface 822, and makes an acute angle with the inclined surface 821. Thus, the angle formed by the inclined surface 821 and the wall 823 is more acute or smaller than that of the first embodiment. With the opposed portion 82 thus configured, separation of the air-fuel mixture is promoted at a downstream end portion of the inclined surface 821.

[0030] FIG 10 shows the configuration of an ignition plug 103 according to a third embodiment of the invention, in particular, the configuration of an opposed portion 83 as a characteristic portion of the ignition plug 103. As in the first embodiment, the opposed portion 83 has an opposed surface 832, an inclined surface 831, and a wall 833. The third embodiment is different from the first embodiment in the shape of the inclined surface. The inclined surface 811 of the first embodiment is a flat surface, whereas the inclined surface 831 of this embodiment is a concaved surface that is formed such that the angle of inclination relative to a plane perpendicular to the axis CL gradually increases toward the downstream side in the airflow direction A. With the inclined surface 831 thus formed as the concaved surface, the angle formed by the inclined surface 831 and the wall 833 in a downstream end portion of the inclined surface 831 is more acute or smaller than that of the first embodiment. With the opposed portion 83 thus configured, separation of the air-fuel mixture is promoted at the downstream end portion of the inclined surface 831.

[0031] FIG 11 shows the configuration of an ignition plug 104 according to a fourth embodiment of the invention, in particular, the configuration of an opposed portion 84 as a characteristic portion of the ignition plug 104. The opposed portion 84 has an inside inclined surface 844, in addition to an opposed surface 842, an inclined surface 841, and a wall 843. The inside inclined surface 844 is an inclined surface that is formed on one side of the opposed portion 84 which is opposed to the center electrode 3, and is located downstream of the opposed surface 842 in the airflow direction A, so as to connect the opposed surface 842 with the wall 843. The inside inclined surface 844 is formed so as to be inclined, relative to the opposed surface 842, to the center side of the combustion chamber, or the side opposite to the ceiling surface 12 of the combustion chamber, toward the downstream side in the direction A of airflow. With the inside inclined surface 844 thus formed on the side opposed to the center electrode 3, flow of the air-fuel mixture directed from a discharge gap to a negative-pressure region along the inside inclined surface 844 is formed, so that a discharge spark is more likely to be drawn into the negative-pressure region.

[0032] FIG. 12 shows the configuration of an ignition plug 105 according to a fifth embodiment of the invention, in particular, the configuration of an opposed portion 85 as a characteristic portion of the ignition plug 105. The opposed portion 85 has an inclined surface 851 on one side opposite to the center electrode 3, and an opposed surface 852 and an inside inclined surface 854 on the other side opposed to the center electrode 3. An end portion of the inclined surface 851 and an end portion of the inside inclined surface 854 are connected to each other by a wall 853. Like the inclined surface 831 of the third embodiment, the inclined surface 851 is a concaved surface formed such that the angle of inclination relative to a plane perpendicular to the axis CL gradually increases toward the downstream side in the airflow direction A. The inside inclined surface 854 is formed so as to be inclined, relative to the opposed surface 852, to the center side of the combustion chamber, toward the downstream side in the airflow direction A. While the inside inclined surface 844 of the fourth embodiment is a flat surface, the inside inclined surface 854 of this embodiment is a curved convex surface. However, there is no difference in operation between these inside inclined surfaces. The opposed portion 85 of this embodiment has both the feature of the opposed portion 83 of the third embodiment, and the feature of the opposed portion 84 of the fourth embodiment, to thus provide the effects owing to the features of these opposed portions.

[0033] FIG 13 shows the configuration of an ignition plug 106 according to a sixth embodiment of the invention, in particular, the configuration of an opposed portion 86 as a characteristic portion of the ignition plug 106. The opposed portion 86 has a flat surface 865 and an inclined surface 861 on one side opposite to the center electrode 3, and an opposed surface 862 on the other side opposed to the center electrode 3. The flat surface 865 is located upstream of the inclined surface 861 as viewed in the airflow direction A, and the inclined surface 861 extends from an end portion of the flat surface 865. An end portion of the inclined surface 861 is connected to an end portion of the opposed surface 862 by a wall 863. In this embodiment, the inclined surface 861 is not formed over the entire area of the above-indicated one side of the opposed portion 86 opposite to the center electrode 3, but is limited to a part thereof. However, the effect of promoting separation of the air-fuel mixture and producing a strong negative pressure can also be obtained by the inclined surface 861 having, a relatively small width as measured in the airflow direction A.

[0034] FIG. 14 shows the configuration of an ignition plug 107 according to a seventh embodiment of the invention, in particular, the configuration of an opposed portion

87 as a characteristic portion of the ignition plug 107. As in the first embodiment, the opposed portion 87 has an opposed surface 872, an inclined surface 871, and a wall 873. The seventh embodiment is different from the first embodiment in the shape of the inclined surface. The inclined surface 871 of this embodiment is a convex surface that is formed such that the angle of inclination relative to a plane perpendicular to the axis CL is gradually reduced toward the downstream side in the airflow direction A. Since the , inclined surface 871 is the convex surface as described above, the length of a flow path of the air-fuel mixture flowing along the inclined surface 871 is larger than that of the first embodiment. Therefore, the air-fuel mixture flowing on the side opposite to the center electrode 3 is accelerated while flowing along the inclined surface 871, to thus provide a high flow rate when the mixture reaches a downstream end portion of the inclined surface 831. Therefore, separation of the air-fuel mixture is promoted, and a negative-pressure region produced by separation of the air-fuel mixture is expanded.

[0035] FIG 15 shows the configuration of an ignition plug 108 according to an eighth embodiment of the invention, in particular, the configuration of an opposed portion

88 as a characteristic portion of the ignition plug 108. The opposed portion 88 has an opposed surface 882 and an inside inclined surface 884 on one side opposed to the center electrode 3, and has an inclined surface 881, a wall 883, and a second inclined surface 885 on the other side opposite to the center electrode 3. The second inclined surface 885 is located downstream of the inclined surface 881 as viewed in the airflow direction A, and the inclined surface 881 and the second inclined surface 885 having different levels form a discontinuous surface. A step between the inclined surface 881 and the second inclined surface 885 is connected by the wall 883. Also, an end portion of the second inclined surface 885 is connected to an end portion of the inside inclined surface 884 by a second wall 886. Further, a through-hole 887 that extends from the inside inclined surface 884 to the second inclined surface 885 is formed through the opposed portion 88. With this arrangement, the air-fuel mixture is separated from a downstream end portion of the inclined surface 881, so that a negative-pressure region is formed in a space below the second inclined surface 885 located downstream of the inclined surface 881. The negative pressure developed in the negative-pressure region acts on the side opposed to the center electrode 3 via the through-hole 887, and draws a discharge spark toward the center of the combustion chamber, along with the air-fuel mixture passed through a discharge gap. Since the inside inclined surface 884 is formed on the side of the opposed portion 88 which is opposed to the center electrode 3, the thus drawn discharge spark is prevented from interfering with the opposed portion 88.

[0036] The embodiments as described above may be combined as appropriate. For example, the concaved surface of the third embodiment may be employed in the second, sixth and eighth embodiments. Also, the convex surface of the seventh embodiment may be employed in the second, fourth, sixth and eighth embodiments. Further, the inside inclined surface of the fourth embodiment may be employed in the second and sixth embodiments.