JP2020177855A - Spark plug - Google Patents

Abstract

To provide a spark plug capable of lowering the firing voltage with simple configuration.SOLUTION: A spark plug 100 includes a center electrode 10 extending in the axial direction, an insulation 20 covering the lateral face of the center electrode 10, a housing 30, a ground electrode 40, and a cylindrical part 50. The ground electrode 40 conducts with the housing 30, and opposes the tip 10a of the center electrode 10 with an interval in the axial direction. The cylindrical part 50 is composed of a dielectric body, and covers the lateral face at the tip 10a of the center electrode 10, and exposes the apical surface of the tip 10a facing the ground electrode 40. Discharge occurs between the center electrode 10 and the ground electrode 40, by applying a voltage with the center electrode 10 as a cathode, and the ground electrode 40 as an anode. Preferably, the tip of the cylindrical part 50 facing the ground electrode 40 is projecting farther to the ground electrode 40 side than the apical surface of the center electrode 10, or is flush with the apical surface of the center electrode 10.SELECTED DRAWING: Figure 2

Description

The present invention relates to a spark plug.

A spark plug having a center electrode and a ground electrode and igniting an air-fuel mixture in an internal combustion engine by electric discharge by applying a high voltage between the two electrodes is known. For example, in Patent Document 1, a voltage at which at least one of the surface portions of the center electrode and the ground electrode is formed of chromium carbide or carbonitride to start discharge between the electrodes (hereinafter, discharge start voltage). An ignition plug that attempts to reduce the voltage is disclosed.

Japanese Unexamined Patent Publication No. 5-114459

There is still room for improvement in reducing the discharge start voltage with a simple configuration.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a spark plug capable of lowering the discharge start voltage with a simple configuration.

In order to achieve the above object, the spark plug according to the present invention
A columnar center electrode extending in the axial direction,
An insulator that covers the side surface of the center electrode and
A housing formed from a conductive material into a tubular shape and holding the insulator,
A ground electrode that conducts with the housing and faces the tip of the center electrode at a distance in the axial direction.
A tubular portion made of a dielectric that covers the side surface of the tip portion of the center electrode and exposes the tip surface of the tip portion that faces the ground electrode is provided.
A discharge is generated between the center electrode and the ground electrode by applying a voltage with the center electrode as a cathode and the ground electrode as an anode.

The tip of the tubular portion facing the ground electrode protrudes toward the ground electrode side from the tip surface of the center electrode with a length shorter than the distance, or is flush with the tip surface. May be good.

The length in the axial direction from the tip surface of the center electrode to the tip of the tubular portion may be larger than 0 mm and 2 mm or less.

The tubular portion may be formed integrally with the insulator.

The outer peripheral surface of the center electrode, the tubular portion, and the inner peripheral surface may be in contact with each other, and the tip surface may be formed flat.

According to the present invention, the discharge start voltage can be lowered with a simple configuration.

It is a schematic diagram which shows the engine and the like including the spark plug which concerns on one Embodiment of this invention. It is a partial cross-sectional view of a spark plug. It is an enlarged sectional view of the main part of a spark plug. (A) and (b) are diagrams for explaining the discharge electrode body used in the experiment. It is a schematic diagram of an experimental circuit. It is a figure which shows the experimental result. (A) is a diagram showing the voltage and current waveforms of the discharge electrode body according to one embodiment at the time of discharge, and (b) is the voltage at the time of discharge of the discharge electrode body without a dielectric as a comparative example. It is a figure which shows the current waveform.

An embodiment of the present invention will be described with reference to the drawings.

The spark plug 100 according to the present embodiment is used in the ignition device 8 that ignites the engine 1, for example, as shown in FIG. The ignition device 8 includes a spark plug 100 and a high voltage power supply 9 that applies a high voltage to the spark plug 100. The engine 1 includes a combustion chamber 2, a piston 3, an intake valve 4, an intake pipe 5, an exhaust valve 6, and an exhaust pipe 7.

The tip of the spark plug 100 (lower end in FIG. 1) is exposed in the combustion chamber 2, and the spark plug 100 is installed, for example, along the moving direction of the piston 3. Hereinafter, the ignition of the engine 1 by the ignition device 8 will be briefly described. When the intake valve 4 opens and the piston 3 moves downward in FIG. 1, fuel (air-fuel mixture) is introduced from the intake pipe 5 into the combustion chamber 2. When the piston 3 moves to the bottom dead center, the intake valve 4 is closed, and then the piston 3 moves upward in FIG. 1, the pressure in the combustion chamber 2 is increased. At this time, a high voltage is applied from the high voltage power supply 9 between the center electrode 10 and the ground electrode 40, which will be described later, of the spark plug 100, and plasma is generated between the two electrodes, so that the air-fuel mixture burns in the combustion chamber 2. To do. When the air-fuel mixture burns, the generated exhaust gas is discharged to the outside through the exhaust valve 6 and the exhaust pipe 7, and the air-fuel mixture is introduced into the combustion chamber 2 again.

As shown in FIG. 2, the spark plug 100 includes a center electrode 10, an insulator 20 (insulator), a housing 30, a ground electrode 40, and a tubular portion 50.

The center electrode 10 is formed in a columnar shape extending in a direction along the axis AX (hereinafter, referred to as an axis direction). The center electrode 10 is formed of a conductive material (for example, a metal such as copper or a nickel alloy). The center electrode 10 is conductive with the terminal 11 located on the axis AX and the discharge conductor 12. A high voltage is applied to the center electrode 10 via a terminal 11 electrically connected to the high voltage power supply 9 and a discharge conductor 12.

The tip portion 10a of the center electrode 10 faces the ground electrode 40 described later in the axial direction. As shown in FIG. 3, the tip surface 10b of the tip portion 10a facing the ground electrode 40 is a flat surface, and is formed in a plane shape with the axis AX as the normal line, for example.

The insulator 20 prevents electric leakage from the center electrode 10 to the outside. The insulator 20 is formed of ceramic (for example, alumina) in a tubular shape that covers the outer peripheral side surfaces of the center electrode 10 and the discharge conductor 12. The terminal 11 described above is provided so as to be exposed from the upper end portion of the insulator 20 in FIG.

The housing 30 is formed in a tubular shape from a conductive material (for example, a metal such as low carbon steel), and holds the insulator 20 inside the housing 30. The housing 30 has a threaded portion 31 on the outer periphery of a portion surrounding the center electrode 10. The screw portion 31 is a portion where a screw thread (not shown) is formed. The spark plug 100 is attached to the engine 1 by tightening the screw portion 31 into a screw hole provided in the engine head. When the spark plug 100 is attached to the engine 1, the housing 30 and the ground electrode 40 described below are grounded via the engine head.

The ground electrode 40 is a plate-shaped member that conducts with the housing 30 and faces the tip portion 10a of the center electrode 10 with a gap (gap G shown in FIG. 3) in the axial direction. For example, the ground electrode 40 is formed integrally with the housing 30 and is located at the tip of a portion of the housing 30 that is bent in an L shape from the lower opening end in FIG. As shown in FIG. 3, of the ground electrode 40, the facing surface 41 facing the tip surface 10b of the center electrode 10 is formed, for example, substantially parallel to the tip surface 10b (including just parallel).

When a high voltage is applied between the high voltage power supply 9 and the center electrode 10 and the ground electrode 40 of the spark plug 100, a spark discharge occurs in the gap G between the center electrode 10 and the ground electrode 40. In the present embodiment, the high voltage power supply 9 is connected with the center electrode 10 as the cathode and the ground electrode 40 as the anode. As the high voltage power supply 9, for example, a known high-speed pulse power supply that applies a high voltage a plurality of times in a pulse shape can be used. The high-voltage power supply 9, for example, raises the voltage of a battery mounted on a vehicle such as a vehicle by a coil and applies it to the spark plug 100 under the control of a control device such as an ECU (Engine Control Unit).

The tubular portion 50 is formed of ceramic (for example, alumina), and is formed in, for example, a cylindrical shape. The tubular portion 50 according to the present embodiment is integrally formed with the insulator 20. Therefore, the tubular portion 50 according to the present embodiment is configured as the lower end portion of the insulator 20 in FIG.

The tubular portion 50 is provided so as to cover the outer peripheral side surface of the tip portion 10a of the center electrode 10 and to expose the tip surface 10b of the center electrode 10 from the opening as shown in FIG. Specifically, the tubular portion 50 is provided so as to expose only the tip surface 10b of the tip portion 10a of the center electrode 10 toward the ground electrode 40. The inner peripheral surface of the tubular portion 50 and the outer peripheral surface of the tip portion 10a of the center electrode 10 are in contact with each other.

As shown in FIG. 3, the tip of the tubular portion 50 facing the ground electrode 40 protrudes toward the ground electrode 40 from the tip surface 10b of the center electrode 10. Although not shown, the tip of the tubular portion 50 facing the ground electrode 40 may be flush with the tip surface 10b of the center electrode 10. Here, if the length in the axial direction from the tip surface 10b of the center electrode 10 to the tip of the tubular portion 50 is the protrusion length L, the protrusion length L is smaller than the gap G in the present embodiment. Within the range, it is set to 0 mm or more (including 0 mm). As will be described later, the protrusion length L is preferably 0 mm or more and 2 mm or less (0 mm ≦ L ≦ 2 mm).

In the spark plug 100 according to the present embodiment configured as described above, the tubular portion 50 made of a dielectric material covers the outer peripheral side surface of the tip portion 10a of the center electrode 10 which is a cathode that emits electrons. It is possible to suppress the component of the electric field generated between them in the direction away from the axis AX and increase the component of the electric field in the axial direction. This is because the tubular portion 50 made of a dielectric material covers the outer peripheral side surface of the tip portion 10a of the center electrode 10, so that the number of electric lines of force generated in the direction perpendicular to the axis AX is reduced, and the number of electric lines of force generated in the direction perpendicular to the axis AX is reduced by that amount. This is because the density of the lines of electric force along the axial direction increases as the number of lines of electric force generated at the tip surface 10b and the triple point J described later increases. In addition, a positive charge is generated on the inner peripheral surface of the tubular portion 50 from which electrons have escaped due to secondary electron emission, and an action of strengthening the electric field also occurs between the inner peripheral surface and the tip surface 10b of the center electrode 10. .. This action also increases the axial component of the electric field. As a result, according to the spark plug 100 according to the present embodiment, the discharge start voltage can be lowered.

Further, in the spark plug 100 according to the present embodiment, the outer peripheral surface of the tip portion 10a of the center electrode 10 and the inner peripheral surface of the tubular portion 50 are in contact with each other, and the tip surface 10b of the center electrode 10 is formed flat. Therefore, the gap G to the ground electrode 40 is set in the same manner, and the triple point J (also referred to as the cathode triple junction) shown in FIG. 3 is closer to the ground electrode 40 when compared with the center electrode having a tapered tip. Can be located in. The triple point J is a position where the center electrode 10 intersects with the tubular portion 50 as an insulator and the air, and is a position where electrons are considered to be supplied. Although it is shown as dots in the figure, in reality, the spark plug 100 according to the present embodiment has an annular shape. As described above, according to the spark plug 100 according to the present embodiment, the triple point J can be relatively close to the ground electrode 40, so that the electron beam can be stably emitted. As a result, there is little variation in the discharge start voltage, and the discharge current can be increased.

(About the experiment)
From here, the experimental results for verifying the effect of the spark plug 100 described above will be described. The discharge electrode body 200 shown in FIG. 4A used in the experiment includes a first electrode 210, a second electrode 240, and a dielectric 250. The first electrode 210 is a columnar electrode extending in the axial direction and formed of copper, and corresponds to the above-mentioned center electrode 10 as a cathode. The second electrode 240 is a flat plate electrode that faces the tip of the first electrode 210 with a gap G, and corresponds to the above-mentioned ground electrode 40 as an anode. The dielectric 250 is a cylindrical ceramic pipe and corresponds to the above-mentioned tubular portion 50. As shown in FIG. 4B, in the discharge electrode body 200, the outer diameter φ1 of the first electrode 210 was set to 1 mm, and the outer diameter φ2 of the dielectric 250 was set to 2 mm (inner diameter was 1 mm).

Further, FIG. 5 shows an experimental circuit 90 for applying a high voltage between the first electrode 210 and the second electrode 240. In the experimental circuit 90, the voltage from the commercial power supply 91 (AC100 [V]) was boosted by a neon transformer 92 whose voltage can be continuously adjusted by a slider. Further, the secondary output current of the neon transformer 92 is full-wave rectified by the diode bridge 93 (schematically shown in the figure) to charge the capacitor 94 (C = 2552 [pF]) to increase the charging voltage. Then, the discharge was started between the first electrode 210 and the second electrode 240. Then, the voltage and current waveforms before and after the start of discharge were detected by the voltage probe 95 and the shunt resistor 96, and observed with an oscilloscope. The protection resistor 97 is for limiting the discharge current.

Using the above discharge electrode body 200 and experimental circuit 90, the inventors of the present application have a distance D [mm] from the tip of the dielectric 250 to the second electrode 240, and a protrusion length L [mm] of the dielectric 250. The characteristics were measured using. As shown in FIG. 4A, the sum of the protrusion length L and the distance D is the gap G (G = L + D). In the range of D = 1 mm to 5 mm and L = 0 mm to 2 mm, the voltage is changed at 1 mm intervals, and the voltage is applied to the discharge electrode body 200 set for each parameter five times, and the discharge start voltage also varies. It was measured. At the same time, as a comparative example, the characteristics of the discharge electrode body (in this case, G = D) in which the dielectric 250 is not provided on the first electrode 210 were also observed in the same manner.

(Experimental result)
FIG. 6 shows the discharge voltage characteristics of the discharge electrode body 200 in which D and L are set for each parameter and the comparative example, which have been subjected to five times. FIG. 6 is obtained by plotting the average value of the maximum value and the minimum value of the five times performed for each parameter, where the horizontal axis is D [mm] and the vertical axis is the discharge start voltage V [V]. It is a graph. Here, since those having different gaps G are not subject to comparison of discharge voltage characteristics, those having the same gap G will be mainly considered below.

First, attention is paid to "without dielectric" and the discharge electrode body 200 with L = 0 as comparative examples. In the comparative example, the discharge start voltage is about 6 kV at D = 1 mm, and the discharge start voltage increases as the distance D (D = G in the comparative example) increases up to about 11 kV at D = 5 mm. On the other hand, in the discharge electrode body 200 of L = 0, the discharge start voltage is about 4 kV at D = 1 mm, and up to about 9 kV at D = 5 mm as the distance D (D = G even at L = 0) increases. Although the discharge start voltage is rising, it can be seen that the discharge start voltage is lower by about 2 kV than in the comparative example at any distance D.

Further, assuming that G = 5 mm, the discharge start voltage (Pn in the figure) at D = 5 mm with “no dielectric” and the discharge start voltage of the discharge electrode body 200 with L = 0 mm and D = 5 mm (P0 in the figure). ) And the discharge start voltage (P2 in the figure) of the discharge electrode body 200 having L = 2 mm and D = 3 mm. As can be seen from the graph, Pn of "without dielectric" shows about 11 kV, while P0 and P2 both show about 9 kV, and according to the discharge electrode body 200 according to this embodiment. The discharge start voltage is lower by about 2 kV than in the comparative example.

Further, assuming that G = 4 mm, the discharge start voltage (Qn in the figure) at D = 4 mm with “no dielectric” and the discharge start voltage of the discharge electrode body 200 with L = 0 mm and D = 4 mm (Q0 in the figure). ), The discharge start voltage of the discharge electrode body 200 having L = 1 mm and D = 3 mm (Q1 in the figure), and the discharge start voltage of the discharge electrode body 200 having L = 2 mm and D = 2 mm (Q2 in the figure). Pay attention to it. As can be seen from the graph, Qn of "without dielectric" shows a value of about 9 kV, whereas Q0, Q1 and Q2 show a value between about 7 to 8 kV, and the discharge according to this embodiment. According to the electrode body 200, the discharge start voltage is lower by about 1 to 2 kV than in the comparative example.

Further, considering the discharge start voltage characteristic in the range of 1 mm to 5 mm in D, the discharge electrode body 200 with L = 0 mm shows the lowest discharge start voltage characteristic. Further, in the comparative example of "without dielectric", the variation of the discharge start voltage is remarkable depending on the value of D, whereas the discharge electrode body 200 of L = 1 mm and L = 2 mm has the same gap G. It can be seen that the discharge start voltage characteristic is sufficiently lower than that of the comparative example, and the variation of the discharge start voltage depending on the value of D is smaller than that of the comparative example. In particular, when L = 1 mm, it has a discharge start voltage characteristic that is substantially linear with respect to the value of D, and it can be seen that the variation in the discharge start voltage depending on the value of D is well suppressed.

In consideration of the above, if the protruding length L of the tubular portion 50 of the dielectric 250 or the above-mentioned spark plug 100 is set to 0 mm or more and 2 mm or less (0 mm ≦ L ≦ 2 mm), the discharge start voltage can be satisfactorily lowered. Can be made to. Further, in particular, in order to sufficiently reduce the discharge start voltage, the protrusion length L of the dielectric 250 or the tubular portion 50 is set to 0 mm (that is, the shape in which the dielectric 250 or the tubular portion 50 does not protrude from the tip of the cathode). The tip of the cathode and the tip of the dielectric 250 or the tubular portion 50 may be set flush with each other. Further, in order to lower the discharge start voltage and obtain a discharge start voltage with less variation with respect to the value of D, the protrusion length L of the dielectric 250 or the tubular portion 50 is in a range larger than 0 mm (preferably 1 mm). The range may be set to 2 mm or less).

FIG. 7 shows an example of the observed waveform by the above experiment. The horizontal axis represents the observation time with the start of discharge as 0 [ms], the left vertical axis represents the voltage applied between the electrodes [V], and the right vertical axis represents the discharge current [A]. FIG. 7 (a) is an observation waveform relating to the discharge electrode body 200 set to D = 5 mm and L = 2 mm, and FIG. 7 (b) shows a comparison in which the first electrode 210 is not provided with the dielectric 250. It is an observation waveform concerning the electrode body for discharge as an example. Since the gap G is different between FIGS. 7 (a) and 7 (b), both figures do not explain the effect of reducing the discharge voltage, but merely show an example of the behavior of the discharge voltage and the discharge current. is there. In the comparative example, as shown in FIG. 7B, the discharge was started when the applied voltage was about −10 kV, and the continuous discharge was maintained thereafter. The discharge current was about 10 mA at the peak value. On the other hand, in the discharge electrode body 200 according to the embodiment, as shown in FIG. 7A, the discharge is started when the applied voltage is about -11 kV, the discharge is stopped after about 5 ms, and the capacitor 94 is charged. , Discharge at about -10 kV and repeat. The peak value of the discharge current was about 10 mA. Since the discharge current is limited by the protection resistor 97, the upper limit can be changed by changing the capacity of the power supply side including the capacitor 94.

The present invention is not limited to the above embodiments and drawings. Changes (including deletion of components) can be made as appropriate without changing the gist of the present invention. An example of modification will be described below.

The shape of the center electrode 10 or the first electrode 210 (cathode), the shape of the ground electrode 40 or the second electrode 240 (anode), the shape of the tubular portion 50 or the dielectric 250, the gap G, the voltage applied between the electrodes, etc. , It is optional as long as it can generate a discharge between the electrodes and exert the above-mentioned effect. For example, in the above, the example in which the cathode is formed in a columnar shape is shown, but the present invention is not limited to this, and the cathode may have a square tubular shape or a shape having a tapered tip as long as it has a shape along the axial direction. However, as described above, from the viewpoint of bringing the triple point J relatively close to the anode, it is preferable that the tip of the cathode has a flat surface. The flat surface is not limited to the surface perpendicular to the axis AX, and may be inclined with respect to the axis AX. Further, the shape of the anode is arbitrary as long as it faces the anode in the axial direction. It is considered that the anode preferably has an area that overlaps with the anode when viewed from the axial direction.

Further, the spark plug 100 and the ignition device 8 are not limited to the ignition of the engine 1, but can also be used for the ignition of oil, a gas fan heater, a gas stove, and the like. Further, the discharge electrode body 200 is not limited to the ignition of the air-fuel mixture, and may be any one used to generate a discharge in the air.

Further, in the above spark plug 100, an example in which the tubular portion 50 is integrally formed with the insulator 20 is shown, but it is possible to prevent electric leakage from the center electrode 10 to the outside and reduce the discharge voltage. As long as it can be done, the insulator 20 and the tubular portion 50 can be separated.

In the above, the case where the tip of the tubular portion 50 protrudes toward the ground electrode 40 side from the tip surface 10b of the center electrode 10 and the case where it is flush with the tip surface 10b (L = 0 mm) have been described. The center electrode 10 may protrude toward the ground electrode 40 from the tip of the tubular portion 50 as long as the discharge start voltage can be lowered as compared with the case where the tubular portion 50 is not provided. If the tubular portion 50 sufficiently covers the outer peripheral side surface of the tip portion 10a of the center electrode 10, the number of electric lines of force generated in the direction perpendicular to the axis AX can be reduced, and the tip of the center electrode 10 can be reduced by that amount. This is because it is considered that the number of electric lines of force generated on the surface 10b can be increased.

The spark plug 100 described above includes a tubular portion 50 made of a dielectric material that covers the side surface of the tip portion 10a of the center electrode 10 and exposes the tip end surface 10b of the tip portion 10a facing the ground electrode 40. The tubular portion 50 that covers the side surface of the tip portion 10a of the center electrode 10 is not only a mode that covers the entire outer peripheral side surface of the tip portion 10a of the center electrode 10, but also the outer circumference of the tip portion 10a of the center electrode 10. A mode in which a part of the side surface on the front end surface 10b side is exposed is also included. That is, the center electrode 10 may protrude toward the ground electrode 40 from the tip of the tubular portion 50 as long as the discharge start voltage can be lowered as compared with the case where the tubular portion 50 is not provided. The shorter the length when the center electrode 10 protrudes toward the ground electrode 40 from the tip of the tubular portion 50, the better, but for example, within 1 mm (preferably within 0.5 mm, more preferably 0.1 mm). Even if the center electrode 10 protrudes toward the ground electrode 40 from the tip of the tubular portion 50 within the range (within), the discharge start voltage can be lowered as compared with the case where the tubular portion 50 is not provided. it is conceivable that.

In the spark plug 100, the tip of the tubular portion 50 facing the ground electrode 40 protrudes toward the ground electrode 40 side from the tip surface 10b of the center electrode 10 with a length (L <G) shorter than the gap G. , It is preferable that it is flush with the tip surface 10b (L = 0 mm). Since this is done, the discharge start voltage can be lowered as described above. Further, since it is only necessary to devise the shape of the tubular portion 50, the configuration is simple. If the discharge start voltage is lowered, it is considered that the combustion efficiency can be improved. In particular, when L = 0 mm, the discharge start voltage can be satisfactorily lowered.

Further, in the spark plug 100, the length (protruding length L) in the axial direction from the tip surface 10b of the center electrode 10 to the tip of the tubular portion 50 may be larger than 0 mm and 2 mm or less. By doing so, not only can the discharge start voltage be satisfactorily lowered, but it is also possible to suppress the occurrence of variation in the discharge start voltage even when the above-mentioned distance D is changed.

Further, in the spark plug 100, the tubular portion 50 may be formed integrally with the insulator 20 (insulator). By doing so, the discharge start voltage can be lowered only by providing the tubular portion 50 having a characteristic shape as described above on the conventionally existing insulator for insulation.

Further, in the spark plug 100, the outer peripheral surface of the center electrode 10 and the inner peripheral surface of the tubular portion 50 may be in contact with each other, and the tip surface 10b may be formed flat. In this way, as described above, the triple point J can be relatively close to the ground electrode 40, so that the electron beam can be stably emitted from the cathode.

In the discharge electrode body 200 described above, the tip of the dielectric 250 facing the second electrode 240 has a length shorter than the gap G on the second electrode 240 side of the tip surface of the first electrode 210 (L <G). ), Or is flush with the tip surface of the first electrode 210 (L = 0 mm). Since this is done, the discharge start voltage can be lowered as described above.

In the above description, in order to facilitate the understanding of the present invention, the description of known technical matters has been omitted as appropriate.

1 ... Engine, 8 ... Ignition device, 9 ... High voltage power supply 100 ... Spark plug 10 ... Center electrode, 10a ... Tip, 10b ... Tip surface 20 ... Insulator 30 ... Housing 40 ... Ground electrode 50 ... Cylindrical part AX ... Axis line, J ... Triple point 200 ... Discharge electrode body, 210 ... First electrode, 240 ... Second electrode, 250 ... Dielectric 90 ... Experimental circuit

Claims (5)

A columnar center electrode extending in the axial direction,
An insulator that covers the side surface of the center electrode and
A housing formed from a conductive material into a tubular shape and holding the insulator,
A ground electrode that conducts with the housing and faces the tip of the center electrode at a distance in the axial direction.
A tubular portion made of a dielectric that covers the side surface of the tip portion of the center electrode and exposes the tip surface of the tip portion that faces the ground electrode is provided.
A discharge is generated between the center electrode and the ground electrode by applying a voltage with the center electrode as a cathode and the ground electrode as an anode.
Spark plug. The tip of the tubular portion facing the ground electrode protrudes toward the ground electrode side from the tip surface of the center electrode with a length shorter than the distance, or is flush with the tip surface.
The spark plug according to claim 1. The length in the axial direction from the tip surface of the center electrode to the tip of the tubular portion is greater than 0 mm and 2 mm or less.
The spark plug according to claim 2. The tubular portion is integrally formed with the insulator.
The spark plug according to any one of claims 1 to 3. The outer peripheral surface of the center electrode and the inner peripheral surface of the tubular portion are in contact with each other, and the tip surface is formed flat.
The spark plug according to any one of claims 1 to 4.