Patent Grant
7305954
The present invention relates to a plasma-jet spark plug that produces a plasma by a spark discharge to ignite an air-fuel mixture in an internal combustion engine. The present invention also relates to an ignition system using the plasma-jet spark plug.
A spark plug is widely used in an automotive internal combustion engine to ignite an air-fuel mixture by a spark discharge. In response to the recent demand for high engine output and fuel efficiency, it is desired that the spark plug increase in ignitability to show a higher ignition-limit air-fuel ratio and achieve proper lean mixture ignition and quick combustion.
One example of high-ignitability spark plug is known as a plasma-jet spark plug. The plasma-jet spark plug has a pair of center and ground electrodes defining therebetween a discharge gap and an electrical insulator surrounding the discharge gap so as to form a discharge cavity within the discharge gap. In the plasma-jet spark plug, a spark discharge is generated through the application of a high voltage between the center and ground electrodes. A phase transition of the discharge occurs by a further energy supply to eject a plasma from the discharge cavity for ignition of an air-fuel mixture in an engine combustion chamber.
The plasma can be ejected in various geometrical forms such as flame form. The plasma in flame form (occasionally referred to as “plasma flame”) advantageously extends in an ejection direction and secures a large contact area with the air-fuel mixture for high ignitability.
Japanese Laid-Open Patent Publication No. 2006-294257 discloses an ignitability improvement technique in which the configuration (shape and volume) of the discharge cavity of the plasma-jet spark plug is modified to increase the ejection length of the plasma for the purpose of improvement in ignitability.
The increase of the plasma ejection length does not, however, always contribute to ignition improvement. Further, some of the configuration modifications of the discharge cavity can cause adverse influences such as deteriorations in electrode durability.
It is therefore an object of the present invention to provide a plasma-jet spark plug capable of ejecting a plasma from a discharge cavity through a ground electrode opening in such a manner as to maximize ignition performance and obtain improvement in ignitability.
It is also an object of the present invention to provide an ignition system using the plasma-jet spark plug.
As a result of extensive research and development, it has been found by the present inventors that the ignitability of the plasma-jet spark plug depends more largely on the configuration of the ground electrode opening than the configuration of the discharge cavity. The present invention is made based on such a finding.
According to one aspect of the present invention, there is provided a plasma-jet spark plug, comprising: a metal shell; an electrical insulator retained in the metal shell and formed with an axial hole; a center electrode held in the axial hole of the electrical insulator so as to define a discharge cavity by a front end face of the center electrode and an inner circumferential surface of the axial hole in a front end part of the electrical insulator; and a ground electrode formed in a plate shape, arranged on a front end of the electric insulator and connected electrically with the metal shell, the ground electrode having an opening defining portion defining therein an opening for communication between the discharge cavity and the outside of the spark plug; the opening defining portion being located radially inside of or in contact with a first imaginary circular conical surface and including a section projecting radially inwardly from a second imaginary circular conical surface with the proviso that: the first imaginary circular conical surface has an axis coinciding with an axis of the spark plug and a vertex angle of 1200 opening toward a front of the spark plug and passing through a front edge of the axial hole of the electrical insulator; and the second imaginary circular conical surface has an axis coinciding with the axis of the spark plug and a vertex angle of 60° opening toward the front of the spark plug and passing through the front edge of the axial hole of the electrical insulator; and the radially inwardly projecting section having a volume of 0 mm3 to less than 1.5 mm3.
According to another aspect of the present invention, there is provided an ignition system, comprising: the above plasma-jet spark plug and a power source having a capacity to supply 50 to 200 mJ of energy to the spark plug.
The other objects and features of the present invention will also become understood from the following description.
The present invention will be described below in detail by way of the following first to fifth embodiments, in which like parts and portions are designated by like reference numerals.
The first embodiment of the present invention will be first explained below with reference to
As shown in
The spark plug 100 has a ceramic insulator 10 as an electrical insulator, a center electrode 20 held in a front side of the ceramic insulator 10, a metal terminal 40 held in a rear side of the ceramic insulator 10, a metal shell 50 retaining therein the ceramic insulator 10 and a ground electrode 30 joined to a front end 59 of the metal shell 50 to define a discharge gap between the center electrode 20 and the ground electrode 30.
The ceramic insulator 10 is generally formed into a cylindrical shape with an axial cylindrical through hole 12 and made of sintered alumina. As shown in
As shown in
The center electrode 20 includes a column-shaped electrode body 21 made of nickel alloy material available under the trade name of Inconel 600 or 601, a metal core 23 made of highly thermal conductive copper material and embedded in the electrode body 21 and a disc-shaped electrode tip 25 made of precious metal and welded to a front end face of the electrode body 21 as shown in
The metal terminal 40 is fitted in the rear region 62 of the insulator through hole 12 and electrically connected with the center electrode 20 via a conductive seal material 4 of metal-glass composition and with a high-voltage cable via a plug cap for high voltage supply from the power supply unit 200 to the spark plug 100. The seal material 4 is filled between the rear end of the center electrode 20 and the front end of the metal terminal 40 within the rear region 62 of the insulator through hole 12 in such a manner as not only to establish electrical conduction between the center electrode 20 and the metal terminal 40 but to fix the center electrode 20 and the metal terminal 40 in position within the insulator through hole 12.
The metal shell 50 is generally formed into a cylindrical shape and made of iron material. As shown in
The ground electrode 30 is generally formed into a disc plate shape with an axial thickness T and made of metal material having high resistance to spark wear e.g. nickel alloy available under the trade name of Inconel 600 or 601. As shown in
On the other hand, the power supply unit 200 is connected to an electric control unit (ECU) of the engine and has a spark discharge circuit 210, a control circuit 220, a plasma discharge circuit 230, a control circuit 240 and backflow prevention diodes 201 and 202 so as to energize the spark plug 100 in response to an ignition control signal (indicative of ignition timing) from the ECU as shown in
The spark discharge circuit 210 is a capacitor discharge ignition (CDI) circuit and electrically connected with the center electrode 20 of the spark plug 100 via the diode 201 so as to place a high voltage between the electrodes 20 and 30 of the spark plug 100 and thereby induce a so-called trigger discharge phenomenon in the discharge gap. In the present embodiment, the sign of potential of the spark discharge circuit 210 and the direction of the diode 201 are set in such a manner as to allow a flow of electric current from the ground electrode 30 to the center electrode 20 during the trigger discharge phenomenon. The spark discharge circuit 210 may alternatively be of full-transistor type, point (contact) type or any other ignition circuit type.
The plasma discharge circuit 230 is electrically connected with the center electrode 20 of the spark plug 100 via the diode 202 so as to supply a high energy to the discharge gap of the spark plug 100 and thereby induce a so-called plasma discharge phenomenon in the discharge cavity 60. As shown in
The control circuits 220 and 240 receive the ignition control signal from the ECU and control the operations of the spark and plasma discharge circuits 210 and 230 at the ignition timing indicated by the ignition control signal.
Before the ignition timing, the diodes 201 and 202 are operated to prevent the backflow of power to the spark plug 100. In this state, the capacitor 231 and the high-voltage generator 233 forms a closed circuit in which the output voltage of the high-voltage generator 233 is charged to the capacitor 231.
At the ignition timing, the control circuit 220 enables the spark discharge circuit 210 to place a high voltage energy between the electrodes 20 and 30 of the spark plug 100. Then, the spark plug 100 induces a trigger discharge phenomenon in which a spark occurs with an electrical breakdown within the discharge gap. The electrical breakdown allows a passage of electricity even through the application of a relatively small voltage. When the control circuit 240 enables the capacitor 231 of the plasma discharge circuit 230 to supply a charged voltage energy to the discharge gap of the spark plug 100 during the occurrence of the trigger discharge phenomenon, the spark plug 100 subsequently induces a plasma discharge phenomenon in which the gas inside the discharge cavity 60 becomes ionized into a plasma phase. The thus-produced high-energy plasma is ejected from the discharge cavity 60 to the engine combustion chamber through the insulator opening 14 and the ground electrode opening 31. The air-fuel mixture is ignited with such a high-energy plasma discharge and combusted through flame kernel growth in the engine combustion chamber.
The energy supply to the discharge gap is finished to insulate the discharge gap after the capacitor 231 releases its charge energy. Then, the capacitor 231 and the high-voltage generator 233 again form a closed circuit so that the capacitor 231 becomes charged with the output voltage of high-voltage generator 233. Upon receipt of the next ignition control signal from the ECU, the control circuits 220 and 240 enable the discharge circuits 210 and 230 to provide an energy supply to the spark plug 100 for plasma discharge.
Herein, the degree of growth of the plasma increases with the amount of energy supplied to the spark plug 100 (i.e. the sum of the amount of energy supplied from the spark discharge circuit 210 to induce the trigger discharge phenomenon and the amount of energy supplied from the capacitor 231 of the plasma discharge circuit 230 to induce the plasma discharge phenomenon). It is preferable to supply at least 50 mJ of energy for one plasma ejection (shot) in order to produce a sufficient and effective plasma and secure a larger contact area between the plasma and the air-fuel mixture for high ignitability. In view of the consumptions of the center and ground electrodes 20 and 30 (notably, the ground electrode 30) of the spark plug 100, it is preferable to limit the energy supply amount to 200 mJ or less. In other words, the power supply unit 200 is preferably of 50 to 200 mJ capacity, and more specifically, 140 mJ capacity. In the present embodiment, the capacitance of the capacitor 231 is set in such a manner that the total amount of energy supplied from the discharge circuits 210 and 230 to the spark plug 100 takes an appropriate value within the range of 50 to 200 mJ, and more specifically, 140 mJ.
When the plasma comes in contact with the ground electrode 30 during the growth, the ground electrode 30 absorbs heat from and quenches the plasma. The configuration (size and shape) of the opening 31 of the ground electrode 30 is thus controlled so as to reduce such a quenching effect of the ground electrode 30 and generate an effective plasma discharge for proper and assured ignition of the air-fuel mixture without causing durability deteriorations of the center and ground electrodes 20 and 30.
More specifically, the ground electrode 30 has a portion, which defines the opening 31, in its entirety or in part projecting radially inwardly from and located radially inside of or in contact with a first imaginary circular conical surface with the proviso that the first imaginary circular conical surface is the conical surface of a right circular cone having an axis coinciding with the axis O of the spark plug 100 and a vertex angle of 120° opening toward the front of the spark plug 100 and passing through (held in contact with) a front edge 11 of the opening 14 of the insulator through hole 12 as indicated by a double dashed line A in
When the opening defining portion of the ground electrode 30 is located radially inside of the first imaginary circular conical surface, this opening defining portion may include a section 35 projecting radially inwardly from and located radially inside of a second imaginary circular conical surface with the proviso that the second imaginary circular conical surface is the conical surface of a right circular cone having an axis coinciding with the axis of the spark plug 100 and a vertex angle of 60° opening toward the front of the spark plug 100 and passing through (held in contact with) the front opening edge 11 of the ceramic insulator 10 as indicated by a double dashed line B in
As the plasma grows in not only an ejection direction but also directions perpendicular to the ejection direction, the amount (volume) of contact between the plasma and the ground electrode 30 varies depending on the minimum diameter D of the opening 31 of the ground electrode 30 and the thickness T of the ground electrode 30. When the projection 35 of the ground electrode 30 is smaller in volume than 1.5 mm3, the amount of contact between the plasma and the ground electrode 30 in the early stage of the plasma growth can be decreased so that it becomes unlikely that the ground electrode 30 will absorb heat from the plasma. This makes it possible to reduce the quenching effect of the ground electrode 30 and effectively prevent the ignitability of the spark plug 100 from deteriorating due to such a quenching effect of the ground electrode 30.
In order to avoid the contact between the plasma and the ground electrode 30 in the early stage of the plasma growth and prevent the spark plug 100 from deteriorating in ignitability due to the quenching effect of the ground electrode 30 more assuredly, the opening defining portion of the ground electrode 30 is preferably kept from contact with a third imaginary circular conical surface with the proviso that the third imaginary circular conical surface is the conical surface of a right circular cone having an axis coinciding with the axis of the spark plug 100 and a vertex angle of 30° opening toward the front of the spark plug 100 and passing through (held in contact with) the front opening edge 11 of the ceramic insulator 10 as indicated by a double dashed line C in
Further, the minimum diameter D of the opening 31 of the ground electrode 31 is preferably made larger than or equal to the thickness T of the ground electrode 31. The plasma radiates from its center to its peripheral edge and becomes higher in temperature as closer to the center and lower in temperature as closer to the peripheral edge. It is very likely that, upon contact between the plasma and the ground electrode 30, the ground electrode 30 will absorb a larger amount of heat from the high-temperature center area of the plasma (located on an around the axis O the spark plug 100) than from the low-temperature peripheral edge area of the plasma. In view of the quenching effect of the ground electrode 30, it is thus desirable that the center area of the plasma does not come into contact with the ground electrode 30 even if the peripheral edge area of the plasma comes into contact with the ground electrode 30. As mentioned above, the amount (volume) of contact between the plasma and the ground electrode 30 varies depending on the minimum diameter D of the opening 31 of the ground electrode 30 and the thickness T of the ground electrode 30. In the case where the diameter D of the opening 31 of the ground electrode 30 is held constant, the amount of contact between the plasma and the ground electrode 30 increases with the thickness T of the ground electrode 30. When the minimum diameter D of the opening 31 of the ground electrode 31 is larger than or equal to the thickness T of the ground electrode 31, the contact between the center area of the plasma and the ground electrode 30 can be avoided or minimized. This makes it possible to reduce the quenching effect of the ground electrode 30 and secure high ignitability of the spark plug 100 effectively. This also makes it possible to avoid the durability of the ground electrode 30 from becoming low due to a decrease in the ground electrode thickness T.
In the case where the minimum diameter D of the ground electrode opening 31 decreases with the diameter R of the cavity opening edge 11 for miniaturization of the spark plug 100, the ground electrode 30 becomes located nearer to the center area of the plasma and thus likely to absorb heat from the plasma. Even in this case, the ignitability deterioration of the spark plug 100 can be prevented effectively by setting the above relationship of D≧T between the minimum opening diameter D and thickness T of the ground electrode 30.
With the above opening configuration of the ground electrode 30, the spark plug 100 becomes able to reduce the quenching effect of the ground electrode 30, produce an effective plasma, without a substantial increase in the voltage required for the spark discharge, and attain proper and assured ignition of the air-fuel mixture. It is therefore possible for the spark plug 100 to attain both of high ignitability and durability.
The second embodiment of the present invention will be next explained below with reference to
The third embodiment of the present invention will be explained below with reference to
The fourth embodiment of the present invention will be explained below with reference to
Finally, the fifth embodiment of the present invention will be explained below with reference to
The present invention will be described in more detail with reference to the following examples. It should be however noted that the following examples are only illustrative and not intended to limit the invention thereto.
A test sample of the spark plug 100 was produced with the following dimensions: D=1.0 mm, T=1.0 mm, R=0.5 mm and L=2.0 mm where D was the minimum diameter of the opening 31 of the ground electrode 30; T was the axial thickness of the ground electrode 30; R was the diameter of the discharge cavity 60 (the diameter of the insulator opening 14 at the front opening edge 11); and L was the depth of the discharge cavity 60 (the distance between the front end face 16 of the ceramic insulator 10 and the front end face 26 of the center electrode 20 along the plug axis direction). The test sample was then subjected to ignitability test. The ignitability test was conducted by mounting the test sample in a pressure chamber, charging the chamber with a mixture of air and C3H8 fuel gas (air-fuel ratio: 22) to a pressure of 0.05 MPa, activating the test sample by means of a CDI-circuit power source and monitoring the pressure in the chamber with a pressure sensor to judge the success or failure of ignition of the air-fuel mixture. The output of the power source was varied from 30 to 70 mJ by using various power coils. The ignition probability of the test sample was determined by performing the above series of process steps 100 times at each energy level. The test results are indicated in
Test samples of the spark plug 100 were produced in the same manner as in Experiment 1 and subjected to durability test. In each of the test samples, the ground electrode 30 was made of Ir-5Pt alloy. The durability test was conducted by charging a pressure chamber with N2 gas to a pressure of 0.4 MPa, mounting the test sample in the pressure chamber, activating the test sample by means of a CDI-circuit power source to cause a continuous discharge at 60 Hz for 200 hours and measuring the amount of consumption of the ground electrode 30 during the continuous discharge. The output of the power source was varied from sample to sample. The test results are indicated in
Three test samples of the spark plug 100 were produced with the following dimensions: T=1.0 mm, R=0.5 mm and L=2.0 mm. In these three test samples, the opening 31 of the ground electrode 30 was formed in such a manner that the opening defining portion of the ground electrode 30 was in contact with an imaginary circular surface line having a vertex angle of 110°, 115° and 120°. A test sample of comparative spark plug was produced under the same conditions as above except that the opening defining portion of the ground electrode was in contact with an imaginary circular conical surface line having a vertex angle of 125°. Each of the test samples was then subjected to discharge test. The discharge test was conducted by charging a pressure chamber with N2 gas to a pressure of 0.4 MPa, mounting the test sample in the pressure chamber and activating the test sample by means of a power source of 140-mJ capacity to measure a discharge voltage required for the test sample to cause a continuous discharge for 200 hours. The test results are indicated in
Three test samples of the spark plug 100 were produced in such a manner that the projection 35 of the ground electrode 30 had a volume of 0.9 mm3 to less than 1.5 mm3. Test samples of comparative spark plugs were produced under the same conditions as above except that the projection of the ground electrode had a volume of 1.5 mm3 to 1.9 mm3. Each of the test samples was subjected to ignitability test. The ignitability test was conducted in the same manner as in Experiment 1, thereby determine the ignition probability of the test sample. The test results are indicated in
Test samples (sample numbers 5-1 to 5-6) of the spark plugs 100 were produced with different dimensions. The dimensions of the test samples are indicated in TABLE. Each of the test samples was subjected to ignitability test. The ignitability test was conducted in the same manner as in Experiment 1 except that the air-fuel ratio of the air-C3H8 mixture was set to 23, i.e., higher than that of Experiment 4, thereby determining the ignition probability of the test sample under more severe conditions. The test results are indicated in TABLE. The test sample had an ignition probability of 100% even under severe conditions when the ground electrode projection 35 had a volume of less than 1.5 mm3 and was kept from contact with the third imaginary circular conical surface. It has been thus shown that the spark plug 100 can be prevented from ignitability deterioration more assuredly by being kept from contact with the third imaginary circular conical surface.
In general, the ignitability of a spark plug to an air-fuel mixture largely decreases as the air-fuel ratio of the air-fuel mixture increases by 1 in a lean range (higher than the stoichiometric air-fuel ratio value). For example, in the case of an ordinary spark plug with a center electrode diameter of 2.5 mm and a discharge gap size of 0.8 mm, it is known that this ordinary spark plug is able to ignite an air-gasoline mixture of lean ratio but needs drastic design changes to decrease the center electrode diameter to 0.8 mm and increase the discharge gap size to 1.2 mm in order to maintain its ignitability when the air-gasoline ratio increases by one higher from the lean ratio value. However, the ignitability of the spark plug 100 can be maintained, without such drastic design changes, according to the first embodiment of the present invention.
Three test samples of the spark plug 100 were produced with the following dimensions: D=1.0 mm, T=0.5 mm, 1.0 mm and 1.5 mm and R=0.5 mm. Each of the test samples was subjected to ignitability test. The ignitability test was conducted in the same manner as in Experiment 1, thereby determining the ignition probability of the test sample. The test results are indicated in
As described above, it is possible in the first to fifth embodiments of the present invention to reduce the quenching effect of the ground electrode 30, 330, 350, 370, 390 on the plasma growth and prevent the ignitability of the spark plug 100, 320, 340, 360, 380 from deteriorating due to such an quenching effect by controlling the configuration of the opening 31, 331, 351-352, 371-372, 391 of the ground electrode 30, 330, 350, 370, 390 adequately.
The entire contents of Japanese Patent Application No. 2006-078710 (filed on Mar. 22, 2006) and No. 2007-052148 (filed on Mar. 2, 2007) are herein incorporated by reference.
Although the present invention has been described with reference to the above-specific embodiments of the invention, the invention is not limited to the these exemplary embodiments. Various modification and variation of the embodiments described above will occur to those skilled in the art in light of the above teaching.
For example, the discharge circuits 210 and 230 may be controlled directly by the ECU although the control circuits 220 and 240 are provided in the power supply unit 200 independently of and separately from the ECU in the above embodiments.
The power source and circuit configurations of the power supply unit 200 may be modified to allow a passage of electricity from the center electrode 20 to the ground electrode 30 (330, 350, 370, 390) e.g. by generating a positive-polarity voltage from the high-voltage generator 233 and by reversing the directions of the diodes 201 and 202. It is however desirable to design the power supply unit 200 in such a manner as to allow the passage of electricity from the ground electrode 30 (330, 350, 370, 390) to the center electrode 20 as in the above-mentioned embodiment, in view of the consumption of the center electrode 20, because the electrode tip 25 of the center electrode 20 is relatively small as compared to the ground electrode 30 (330, 350, 370, 390).
The front region 61 of the insulator through hole 12, which defines the cavity 60, is not necessarily made smaller in diameter than the electrode holding region 15 of the insulator through hole 12. The diameter R of the front hole region 61 may alternatively be made equal to or larger than that of the electrode holding region 15.
The ground electrode 30, 330, 350, 370, 390 is not necessarily held in contact with the ceramic insulator 10 although the ground electrode 30, 330, 350, 370, 390 is joined to the metal shell 50 with the rear end face of the ground electrode 30, 330, 350, 370, 390 held in contact with the front end face 16 of the ceramic insulator 10 in the above embodiments. The ground electrode 30, 330, 350, 370, 390 may not be held in contact with the ceramic insulator 10 as long as the quenching effect of the ground electrode 30, 330, 350, 370, 390 on the plasma can be limited effectively by controlling the configuration of the ground electrode opening 31, 331, 351-352, 371-372, 391 as specified above.
The scope of the invention is defined with reference to the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-078710 | Mar 2006 | JP | national |
| 2007-052148 | Mar 2007 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6611084 | Teramura et al. | Aug 2003 | B2 |
| 6973820 | Watarai et al. | Dec 2005 | B2 |
| 7234429 | Abe et al. | Jun 2007 | B2 |
| 20070114901 | Nagasawa et al. | May 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 57-15377 | Jan 1982 | JP |
| 2-72577 | Mar 1990 | JP |
| 2006-294257 | Oct 2006 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20070221156 A1 | Sep 2007 | US |
