Spark plug

Abstract

A spark plug for electric ignition of the fuel-air mixture is disclosed, which spark plug is provided with a central positive electrode and an outer negative electrode facing the positive electrode via a sparking gap, and further comprises an electric insulator member for enclosing the sparking gap so that a discharge chamber is defined by the insulator member, said positive electrode and negative electrode, the discharge chamber being operable to produce plasma-like gas while discharging occurs between both said electrodes, and at least a nozzle is provided for said discharge chamber for enabling the plasma-like gas to jet into the fuel-air mixture. The spark plug can be adapted for use in a spark ignition type internal combustion engine.

Description

The present invention relates to a spark plug adapted for use in a spark ignition engine in order to ignite the fuel-air mixture by utilizing a spark discharge between the positive and negative electrodes of the spark plug.

In spark ignition engines, poor ignition and misfire often occur during operation when using lean fuel-air mixture. In order to attain reliable ignition of such lean fuel mixture, diverse investigations and improvements have conventionally been carried out. Among these investigations and improvements, a method of increasing spark discharge energy and another method of improving the shape of electrodes of a spark plug have been conducted with respect to spark plugs.

However, in a conventional spark plug in which a spark is produced in a sparking gap defined between positive and negative electrodes by applying extreme high potential between the electrodes, and where the produced spark ignites the fuel-air mixture by directly heating the mixture with the energy thereof, it is known that increasing the spark energy over a specified value (the general specified value is thirty mili joules) cannot be effective for increasing ignitability over a given extent. The reason for this fact is understood in that the spark energy is not effectively transmitted to the circumferential mixture. That is to say, as only a limited partial mixture around the sparking gap is heated up to an ignitable temperature by the spark energy, even if a small spot-like flame is initially generated by ignition of the mixture, said small spot-like flame is immediately cooled down so as to be extinguished by the circumferential unburnt mixture, or often by the electrodes themselves before the small spot-like flame becomes larger flame. Thus, the flame propagation is not obtained, and misfire occurs in the engine. In order to prevent this kind of failure to ignite, a method for making the spark jump in the high speed flow of the mixture has been practiced so that the spark energy is transmitted to the widest possible extent throughout said mixture, within a limited time. However, it is understood that this method still deteriorates the ignition ability of a spark ignition engine, since when the spark jumps in the mixture having large flow rate, the discharge between electrodes is made unstable.

In the shape of positive and negative electrodes of a conventional spark plug, investigations and improvements were made so as to satisfy the requirements for preventing the scattering of the initial spot-like flame generated by ignition, as well as for preventing the initial spot-like flame from being cooled down by the electrodes themselves. However, these two requirements are inherently contradictory, and as a result, no electrode of any shape in a conventional spark plug has been found which can satisfy both requirements. Thus, the improvements in the shape of electrodes of a spark plug still have not contributed to an increase in the ignition ability of the spark plug.

Therefore, the object of the present invention is to provide a spark plug which is capable of exhibiting an excellent ignition ability.

According to the present invention, a spark plug for electric ignition of the fuel-air mixture, which is provided with a central positive electrode and an outer negative electrode facing said positive electrode via a sparking gap, comprises an electric insulator member for enclosing the sparking gap so that a discharge chamber is defined by the electric insulator member and the positive and negative electrodes, said discharge chamber being operable to produce plasma-like gas while discharging is held between said both electrodes, and at least a nozzle is provided for said discharge chamber for enabling said plasma-like gas to jet into said fuel-air mixture.

The present invention will readily be understood from the ensuing description of embodiments with reference to the accompanying drawing wherein:

FIG. 1 is a front elevation view of a spark plug with an electric circuit system, in part cross section, according to an embodiment of the present invention;

FIG. 2 is a cross sectional view of the portion designated by an arrow P in FIG. 1;

FIG. 3 through FIG. 6 are partial cross sectional views of the other embodiments of the present invention, which respectively show only the portions corresponding to the portion P of FIG.1.

Referring to FIG. 1, a spark plug is provided with a centrally positioned positive electrode (a central positive electrode) 10, a tubular insulator 11 for electrically isolating the central positive electrode 10, a threaded part 13 for enabling the mounting of the spark plug onto an engine, a plug holder 12, with a hexagonal head 14 for locking of the spark plug, a grounded or earth electrode (a negative electrode) 15 formed as one part with the plug holder 12 and disposed to be opposite to said central electrode, and a gasket 16. It should be noted that the construction of the spark plug shown in FIG. 1 is quite similar to that of a conventional spark plug, except for the portion designated by an arrow P. Therefore, the present description will hereinafter be directed mainly to this portion designated by said arrow P.

Referring to FIG. 2, the insulator 11 extends so as to enclose the sparking gap formed between the central plug 10 and the earth electrode 15. That is to say, a considerably small discharging chamber 17 enclosed by the central electrode 10, together with the grounded electrode 15 and the insulator 11, is formed. This discharging chamber 17 is capable of serving to prevent diffusion of energy of the spark which is produced between both electrodes 10 and 15. Thus, the energy of the high-tension spark allows the mixture within the chamber 17 to produce a plasma-like gas. This plasma-like gas jets into the associated combustion chamber (not shown) through a nozzle 18 formed in the earth electrode 15. It should be noted that said nozzle 18, which allows said plasma-like gas to jet into the combustion chamber, also allows the mixture or the mixture containing residual gas to flow into the chamber 17 from the combustion chamber.

When the spark plug of the present invention is mounted onto an engine, the spark plug operates as follows.

As is shown in FIG. 1, the electrode 15 is grounded, and the central electrode 10 is connected to a high-tension current generator H which is connected to a battery B. When electric current is applied between the electrodes 10 and 15 under a high electric voltage so as to synchronize with the ignition timing of the engine, the breakdown of the electric current takes place between both electrodes while producing a high-tension spark. A part of the energy of the high-tension spark is supplied to the mixture staying in the discharging chamber 17 so as to heat said mixture. As the discharging chamber 17 is formed as a spatially isolated chamber enclosed by the electrodes 10 and 15, and by the insulator 11, the energy supplied to the mixture within the chamber 17 is completely prevented from being diffused. Therefore, the mixture within the chamber 17 is subsequently supplied with the thermal energy of the spark, and is heated extremely rapidly. As a result, the heated mixture within the chamber 17 is altered into said plasma-like gas due to the occurrence of partial ionization in the heated mixture. It should be understood that the production of the plasma-like gas is accompanied by thermal expansion of the mixture within the chamber 17, which expansion increases the pressure in the discharging chamber 17 thereby allowing said plasma-like gas within the chamber 17 to jet into the associated combustion chamber at a quite high speed, through the nozzle 18. The jetted plasma-like gas designated by the reference G in FIG. 2, includes therein a high thermal energy together with a high kinetic energy of positive ions and electrons which are contained in said plasma-like gas. Therefore, the plasma-like gas jetting from the discharging chamber 17 can effectively supply the high thermal energy to an extensive amount of the mixture staying in the combustion chamber of an engine, so that said extensive amount of said mixture is heated and ignited. In other words, in accordance with the present invention, the energy of the spark is effectively transmitted to the extensive amount of said mixture within the combustion chamber of an engine. Also, it should be understood that the flame propagation takes place according to the present invention, since in the combustion chamber, the flame of the mixture ignited by the plasma-like gas is not cooled or extinguished by the electrodes of the spark plug or other cold elements because the flame production occurs in places far from the electrodes or other cold elements.

Further, in the spark plug of the present invention, as shown in FIGS. 1 and 2, the sparking gap itself, defined between both electrodes, is contained in the small discharge chamber 17. As a result, the spark discharge which takes place in the sparking gap is not affected by the flow velocity of the mixture in the combustion chamber. Thus, a stable discharge can be maintained over a considerably long period, and the total amount of the energy of the spark is increased.

FIGS. 3 through 6 show the other embodiments of the present invention, respectively. It should be understood that each of these embodiments is provided with a structure similar to that of the embodiment of FIG. 1 except for the portion corresponding to the portion P of FIG. 1, and therefore FIGS. 3 through 6 show only the respective portions corresponding to said portion P of FIG. 1. Also, it should be noted that in FIGS. 3 through 6 like elements or like parts are designated by the same reference numerals as in FIG. 1.

FIG. 3 shows another embodiment of a spark plug which is provided with a precombustion chamber 19 formed in front of the nozzle 18 by an extension 15a of the earth electrode 15. The precombustion chamber 19 is connected to the associated combustion chamber (not shown) of an engine through a through-hole 20 in the extension 15a. In the case of this embodiment of FIG. 3, plasma-like gas produced in the chamber 17, jets into the precombustion chamber 19 through the nozzle 18 in the same manner as described with reference to FIG. 2. As a result, the mixture staying in the precombustion chamber 19 simultaneously ignites and burns. Subsequently, in response to an increase in the internal pressure of the precombustion chamber 19, which increase takes place by a temperature raise within the chamber 19, the combustion gas within the precombustion chamber 19 together with the plasma-like gas from the nozzle 18 jet into the associated combustion chamber of an engine at a high speed through the through-hole 20 in the extension 15a. Thus, the mixture within the combustion chamber is then ignited by the combustion and plasma-like gases jetting from the through-hole 20. It should be noted that in the case of the embodiment of FIG. 3, a considerable amount of the mixture occupying the entire volume of the precombustion chamber 19 is burned and then is heated, and thus the amount of gases which jet from the through-hole 20 is naturally increased compared to the amount of only the plasma-like gas jetting from the nozzle 18. Consequently, the velocity of the flame propagation within the combustion chamber of an engine is increased further than in the case of the embodiment of FIG. 1, and a rise in the efficiency of the engine is also attained.

FIG. 4 shows another embodiment, in which the distance designated as B between the central electrode 10 and the earth electrode 15, is rendered smaller than the length A of a longitudinally extended discharge chamber 17. It should be appreciated that if the distance B is kept small, the gradient of the potential between the electrodes 10 and 15 can be large and thus, at the beginning of the discharge between both electrodes, a corona discharge is easily produced without affecting the provision of insulator member 11 between both electrodes. Therefore, dielectric breakdown between both electrodes occurs at a relatively low potential. This fact enables the high-tension current generator H to employ its remaining output energy other than that used for the above-mentioned dielectric breakdown, as ignition energy, even if the current generator H can exert only a limited constant energy on its output. Thus, the ignition energy of this embodiment can be larger than that of the embodiment shown in FIGS. 1 and 2.

FIG. 5 shows another embodiment in which a discharge space 17a has a larger volume compared to the discharge space 17 of the embodiment of FIG. 1. In the case of the embodiment of FIG. 5, the amount of the mixture occupying the discharge chamber 17a will be increased. Therefore, if the size of the nozzle 18 is kept equal to that of the nozzle 18 of the embodiment of FIG. 1, the plasma-like gas jetting from the nozzle 18 will be increased in its jetting velocity due to the increase in the amount of the mixture occupying the discharge chamber 17, so that the flame propagation within the combustion chamber of an engine will also be increased.

Finally, FIG. 6 shows another embodiment in which plural nozzles 18, 18a and 18b are provided with respect to a discharge chamber 17 having a considerably large volume. In accordance with the provision of said plural nozzles 18, 18a and 18b, the plasma-like gas jetting out of the discharge chamber 17 can contact a larger amount of the mixture within the combustion chamber than in the case of a single nozzle and as a result, uniform heating of the mixture within the combustion chamber is obtained so that a stable ignition can be attained with certainty.

It will now be understood from the foregoing description with reference to various embodiments that according to the spark plug of the present invention, the energy of the spark produced by the discharge between the electrodes of the spark plug can be effectively supplied to the mixture and therefore the ignition ability of the spark plug can easily and advantageously be raised in response to a raise in the spark energy.

It should also be noted that as the extensive amount of the mixture is heated by the plasma-like gas according to the present invention, misfire due to the conventional cooling of the flame is prevented, and as a result, the ignition always occurs even in the range of the lean mixture, which has poor ignitability.

It should also be appreciated that the structure of the spark plug is not complex compared with that of the conventional spark plug, and as a result, an increase in the manufacturing cost will not be envolved.

Further, it should be noted that the application of the spark plug according to the present invention is not limited to an internal combustion engine.

Claims

What is claimed is:

1. A spark plug for electric ignition of a fuel-air mixture, of the type having an outer metal body including a cavity provided with a restricted opening at one end, a central electrode and insulating means for supporting the central electrode in the outer body with a tip of the electrode positioned within the cavity, the outer metal body serving as an outer electrode facing the central electrode via a sparking gap defined between the central electrode and the wall of the cavity, wherein the improvement comprises:

an electric and heat insulator member lining substantially the entire inner wall of the cavity in the outer body from the tip of the central electrode to at least adjacent to the restricted opening for enclosing the sparking gap so that an insulated discharge chamber is defined by said insulator member in conjunction with said central electrode and said outer electrode, said insulated discharge chamber being small enough to be capable of maintaining a sufficiently high temperature to produce plasma-like gas when a spark discharge occurs between both said electrodes, and said restricted opening is at least not larger than the cross section of said discharge chamber to provide a throttling nozzle for enabling said plasma-like gas to jet at high velocity into said fuel-air mixture.

2. A spark plug as claimed in claim 1, further comprising a second chamber formed in said outer metal body, said second chamber being connected to said discharge chamber through said nozzle and having a through hole to connect said second chamber with fuel-air mixture outside of said spark plug.

3. A spark plug as claimed in claim 1, wherein said discharge chamber is elongated, one end of said chamber being defined by the tip of said central electrode, the opposite end being defined by a portion of said outer metal body adjacent to said nozzle, and the side wall of said chamber being defined by said insulator member, the wall thickness of said insulator member being less than the length of said elongated chamber.

4. A spark plug as claimed in claim 1, wherein the cross-sectional area of the discharge chamber in a plane perpendicular to the axis of the central electrode is no greater than the cross-sectional area of said central electrode.

5. A spark plug as claimed in claim 1, wherein the cross-sectional area of the discharge chamber in a plane perpendicular to the axis of the central electrode is substantially equal to the cross-sectional area of said central electrode.

6. A spark plug as claimed in claim 5, wherein the insulator member lining the cavity in said outer metal body extends to the inner edge of the restricted opening from said cavity.

7. A spark plug as claimed in claim 1, further comprising at least one additional nozzle opening from said discharge chamber to provide additional high speed jets of plasma-like gas for contacting a larger amount of fuel-air mixture outside the chamber.