HK1236281B - Ignition plug - Google Patents

Description

spark plug

Technical Field

The present invention relates to a spark plug for an internal combustion engine.

Background Art

To improve emissions in gasoline internal combustion engines, the engines can be operated with exhaust gas recirculation (EGR) or a lean air-fuel mixture. However, when using known spark plugs and ignition systems, combustion stability may become unacceptable under these conditions. One reason is that the typically small volume of gas ignited across the spark gap of known spark plugs (typically 0.8 mm) is a factor.

It has been shown that by using a corona ignition system with a larger gap, combustion stability can be improved by igniting a larger volume of gas. However, this system requires a higher voltage, which is generally problematic. For example, a higher voltage can cause the ceramic body of the spark plug to break down, arcing back in the spark plug, etc. Some known corona systems include sharp electrodes extending into the combustion chamber to ignite a larger volume of gas. However, these electrodes may become too hot under certain conditions, which may result in combustion at an inappropriate time. Moreover, due to the high voltage (typically about 100 kV) and the associated heat, electrode wear becomes a problem.

Furthermore, sparks can also occur in corona systems, damaging sharp electrodes. In conventional corona systems, sparks must be prevented as much as possible to minimize electrode wear, and when sparks occur, they must be controlled to achieve good combustion (see, for example, U.S. Patents 8,578,902 and 8,726,871). As the electrode wears, it becomes blunt and requires a higher voltage to achieve corona.

Purpose of the Invention

It is therefore an object of the present invention to provide a spark plug by which the Applicant believes that the above-mentioned disadvantages may be at least alleviated or which may provide an advantageous alternative to known spark plugs and systems.

Summary of the Invention

According to the present invention, there is provided a spark plug comprising:

- an elongated cylindrical body of electrically insulating material, the elongated cylindrical body having a first end, a second end opposite the first end, and a first face at the first end;

a first elongated electrode extending longitudinally in the body, the first electrode having a first end and a second end, the first electrode terminating at its first end at a first distance d1 from the first end of the body in a direction towards the second end of the body;

- the body defines a blind hole between the first end of the first electrode and the first end of the body; and

- a second electrode provided on an outer surface of the body, the second electrode terminating at one of: a) flush with the first face of the body; and b) a second distance d2 from the first end of the body in a direction towards the second end of the body.

The body may have a circular cross-section, and the first electrode may extend axially within the body.

The cross-section of the first electrode may decrease towards its first end such that the first electrode terminates in the blind hole at its tip.

The second electrode may be in the form of a sleeve or sheath for the body.The second electrode may extend from a region towards the second end of the body towards the first end of the body, typically extending from the second end of the body.

The first side of the body may define a hollow formation into the first side, such as a depression, recess, groove, or the like.

The present invention also extends to an ignition system comprising a spark plug as described above and a drive circuit for the spark plug.

Also within the scope of the present invention is a method of igniting a gaseous substance as defined and/or described herein.

More specifically, there is provided a method of igniting a gaseous substance in a chamber, the method comprising:

- a spark plug comprising: a body of electrically insulating material; a first electrode extending partially through the body so as to define, at its tip, a blind hole in the body together with the body, the blind hole being in communication with the chamber gas flow through a mouth in the body; and a second electrode on the outer surface of the body;

- driving the first electrode so as to pre-set the corona at the tip so that the corona plasma in the blind hole extends towards the mouth and ejects the plasma into the chamber; and

- controlling the corona plasma so as to form one of the following: a) a spark extends from the tip of the first electrode along a path surrounded by the gaseous material through the chamber to the second electrode; and b) a corona plasma extends from the tip of the first electrode along a path surrounded by the gaseous material into the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be further described by way of example only with reference to the accompanying drawings, in which:

FIG1 is a schematic cross-sectional view through a first example embodiment of a spark plug; and

FIG. 2 is a similar view of a second example embodiment of a spark plug.

DETAILED DESCRIPTION

First and second exemplary embodiments of the spark plug are indicated in Figures 1 and 2 by reference numerals 10.1 and 10.2, respectively.

The spark plug includes an elongated cylindrical body 12 of electrically insulating material having a first end 12.1 and a second end 12.2 opposite the first end. A first face 14 is provided at the first end. A first elongated electrode 16 extends longitudinally within the body 12. The first electrode 16 has a first end 16.1 and a second end 16.2. The first electrode terminates at its first end 16.1 at a first distance d1 from the first end 12.1 of the body in a direction toward the second end 12.2 of the body. The body thus defines a blind hole 18 extending between the first end 16.1 of the first electrode and a mouth 19 at the first end 12.1 of the body. A second electrode 20 is provided on an outer surface of the body 12, the second electrode terminating at one of: a) flush with the first face 14 of the body (as shown in FIG. 1 ); and b) a second distance d2 from the first end 12.1 of the body in a direction toward the second end 12.2 of the body (as shown in FIG. 2 ).

The body 12 is preferably circular in cross-section and may be made of a suitable ceramic material.

In the example embodiment, the second electrode 20 is in the form of a sleeve or sheath for the body 12, extending from the second end of the body towards the first end of the body, and may be grounded or connected to ground potential in use.

The first electrode 16 can have a first cross-sectional area toward its second end 16.2 and over a major portion of its length, and the first cross-sectional area can decrease toward the first end of the electrode so that the first electrode terminates at a tip 16.1 at the blind end of the hole 18, the tip 16.1 having a cross-sectional area smaller than the first cross-sectional area. The first electrode can have a circular cross-section with a sufficiently large diameter (approximately 1 mm) such that when the first electrode 16 extends through the grounded second electrode 20, the electric field at the interface between the first electrode 16 and the body 12 is below the breakdown field of the ceramic body. The tip 16.1 at the blind end of the hole 18 can have a diameter of less than 0.5 mm to generate a sufficiently large electric field in the hole 18.

1, the second electrode 20 terminates flush with the first face 14. For reasons described below, the first embodiment of the spark plug may be referred to as a spark plug.

2, the second electrode 20 ends at a distance d2 from the first end 12.1 in the direction towards the second end 12.2. Also for reasons described below, the second embodiment of the spark plug may also be referred to as a corona plug.

In use, the electrode of spark plug 10.1 is connected to a suitable drive circuit (not shown), and the spark plug extends into a chamber 22 of a cylinder (also not shown) of an internal combustion engine (also not shown). The gaseous material fills the chamber and extends into the blind hole 18. The drive circuit first generates a corona at the tip 16.1, which heats the gaseous material in the blind hole. The gaseous material in the blind hole and the corona are then ejected into the combustion chamber 22, thereby generating a long plasma 24 that is used to ignite the gaseous material in the chamber. The plasma length l can be approximately 10 mm.

The volume of the blind hole 18 must be small enough so that the temperature and pressure of the gaseous material therein can be sufficiently increased by approximately 1 mJ of energy. For example, using a hole with d1 = 1 mm and a diameter of 0.5 mm would result in a volume V = 0.2 mm ³ . If there is no heat loss in the ceramic, then air at 300 K, 20 bar, and V = 0.2 mm ³ would be raised to a temperature of approximately 650 K and a pressure of approximately 40 bar by 1 mJ of energy.

In the first embodiment where the grounded second electrode 20 extends flush with the first face 14, the ejected corona tends to grow towards the second electrode 20, thereby forming a spark. This spark is much longer than the spark in known spark plugs and spark ignition systems, as described in the introduction to this specification.

In the second embodiment 10.2 where the grounded second electrode 20 terminates at a distance d2 from the first end 12.1 of the body 12, the corona 24 tends to grow away from the spark plug 10.2 and no spark discharge is formed. The first face 14 may define depressions 26, grooves and other formations to help the corona grow away from the spark plug.

To achieve corona spray, the first electrode must be driven at a high enough voltage, at a high enough frequency, and for a long enough time. The voltage must be high enough to cause a corona to form at the tip 16.1 of the first electrode within the blind hole 18 and grow toward the mouth. The ceramic near the electrode tip 16.1 helps to generate a high enough electric field strength at the tip, enabling the corona to start at approximately 30 kV, which is much lower than the approximately 100 kV mentioned above for known corona systems.

Once the corona is formed, energy must be transferred to the gaseous material in order to heat the gas. The corona can be viewed as a resistive and capacitive load. Therefore, a sufficiently high voltage must be provided in order to transfer sufficient energy to this load. The frequency at which the voltage is provided must be high enough (>1 MHz) so that the gas can be fully heated before being injected into the combustion chamber. It must also be high enough so that the corona does not extend along the first face 14 of the ceramic body 12. At 5 MHz, the electrons will travel 1 mm in each cycle, which is the same order of magnitude as the depth d1 of the blind hole 18 and much shorter than the distance from the first electrode to any grounded metal (including the second electrode).

Energy must be supplied to the spark plug long enough (typically >100 us) to heat the plasma inside the hole and then further heat the plasma after it is ejected into the combustion chamber.

One aspect of the present invention therefore relates to a method of operating a spark plug 10.1, 10.2 having an electrode 16.1 inside a bore 18 in order to generate a corona or spark longer than 5 mm in a combustion chamber, wherein the corona or spark plasma is surrounded by a gaseous substance and is not shielded by any electrode. There is also no electrode extending into the combustion chamber that could become a hot spot.

The driving circuit and related method for driving the spark plug are identified and described in the applicant's co-pending international patent application "Ignition system for internal combustion engine and method for driving a spark plug of an ignition system", the content of which is incorporated herein by reference.

The larger volume of gas produced for ignition can improve combustion stability under high EGR and lean mixture conditions. It is believed that the present invention can be used to improve combustion stability and can help engine manufacturers meet mandatory emission standards.

Claims (10)

1. A spark plug (10.1, 10.2), comprising: - An elongated cylindrical body (12) of an electrical insulating material, the elongated cylindrical body having a first end (12.1), a second end (12.2) opposite to the first end, and a first surface (14) at the first end; - An elongated first electrode (16) extending longitudinally in the body, the first electrode having a first end (16.1) and a second end (16.2), the first electrode terminating at its first end (16.1) at a first distance d1 from the first end (12.1) of the body along the direction toward the second end (12.2) of the body; - An inner wall of the body defines a hole (18) extending from the mouth (19) at the first end (12.1) of the body and extending to the outside of the first end (16.1) of the first electrode (16), the first end of the body axially facing the second end (12.2) of the body, the first end (16.1) of the first electrode being completely located within the hole (18), the hole having a constant diameter along the direction of extension of the hole, the inner wall and the first end (16.1) of the first electrode together forming a blind hole having a constant diameter, the blind hole extending from the mouth (19) and terminating at the first end (16.1) of the first electrode; and - A second electrode (20) is provided on the outer surface of the body and terminates at one of the following locations: a) flush with the first surface of the body to form a spark plug that generates a spark between the first end of the first electrode and the second electrode when in use; or b) at a second distance d2 from the first end of the body in a direction toward the second end of the body to form a corona plug that generates a corona when in use, the corona growing from the first end of the first electrode. 2. The spark plug according to claim 1, wherein: the body has a circular cross-section, and the first electrode and the blind hole extend centrally and axially in the body. 3. The spark plug according to claim 1 or 2, wherein: the first electrode includes a first portion toward a second end (12.2) of the first electrode, the first portion having a first cross-section that decreases toward the first end (16.1) of the first electrode, the first electrode terminating at a tip of the first end (16.1) of the first electrode, the tip having a second cross-section that is smaller than the first cross-section. 4. The spark plug according to claim 3, wherein the first cross-section and the second cross-section are circular. 5. The spark plug according to claim 1 or 2, wherein: the second electrode is in the form of a sleeve or sheath for the body. 6. The spark plug according to claim 1 or 2, wherein: a first surface of the body defines a recess. 7. The spark plug according to claim 1 or 2, wherein: a first surface of the body defines a recess. 8. The spark plug according to claim 1 or 2, wherein: the first surface of the body defines a groove. 9. An ignition system comprising: a spark plug according to any one of claims 1 to 8; and a drive circuit for the spark plug. 10. A method for igniting a gaseous substance in a chamber, the method comprising: -Using the spark plug as described in any one of claims 1-8; - Drive the first electrode to pre-position a corona at the tip, causing the corona plasma in the blind hole to extend toward the mouth and eject the corona plasma into the chamber; and -a) The second electrode of the spark plug terminates flush with the first surface, controlling the corona plasma to form a spark that extends from the first end of the first electrode through the chamber surrounded by gaseous material to the second electrode; or -b) The second electrode of the spark plug terminates at a second distance d2 from the first end of the body in the direction toward the second end of the body, and controls the corona plasma to extend from the first end of the first electrode along the passage surrounded by gaseous material into the chamber.